Hierarchical zeolites for catalytic hydrocarbon conversion

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Hierarchical Zeolites for Catalytic

Hydrocarbon Conversion

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de

Technische Universiteit Eindhoven, op gezag van de

rector magnificus, prof.dr.ir. F.P.T. Baaijens, voor een

commissie aangewezen door het College voor

Promoties in het openbaar te verdedigen

op woensdag 28 oktober 2015 om 16.00 uur

door

Christiaan Herman Lucien Tempelman

geboren te Zaltbommel

Dit proefschrift is goedgekeurd door de promotor:

prof.dr.ir. E.J.M. Hensen

Copyright © 2015, Christiaan Tempelman

Hierarchical zeolites for catalytic hydrocarbon conversion

A catalogue record is available from the Eindhoven University of Technology Library

ISBN: 978-90-386-3938-3

The work described in this thesis has been carried out at the Schuit Institute of Catalalysis

within the Laboratory of Inorganic Chemistry and Catalysis of Eindhoven University of

Technology in The Netherlands. This work received financial support by the European

Community through the NEXT-GTL project (NMP3-LA-2009-229183).

Cover design: Ilse Weisfelt

Printed at Gildeprint

Contents

Chapter 1 General introduction…………….……………………………...

1

Chapter 2 Desilication and silylation of Mo/HZSM-5 for methane

dehydroaromatization…………………………………………..

17

Chapter 3 Activation of Mo/HZSM-5 for methane aromatization………...

51

Chapter 4 On the deactivation of Mo/HZSM-5 in the methane

dehydroaromatization reaction…………………………………

73

Chapter 5 One-step synthesis of nano-crystalline MCM-22………………

95

Chapter 6 Effect of zeolite crystalline domain size on the methane

aromatization performance of Mo/HZSM-5……………...…….

123

Chapter 7 Texture, acidity and fluid catalytic cracking performance of

hierarchical faujasite zeolite prepared by an amphiphilic

organosilane…………………………………………………….

137

Summary ………………………………………………………………….. 165

List of publications ………………………………………………………………….. 171

Acknowledgements …………………………………………………………………. 173

Curriculum Vitea ………………………………………………………………….. 177

Chapter 1

1

General introduction

1. lntroduction

1.1 Catalysis

Catalysis already played an important role in the lives of early humans. One can for

instance consider the fermentation of sugars into ethanol. The term catalysis was introduced

in 1835 by the Swedish scientist Jöns Jacob Berzelius [1]. It is a combination of the Greek

words κατά (kata, “down”) and λύω (luō, “loose”). Berzelius was the first to investigate and

report catalytic reactions in an organized manner and he can be considered the founder of

modern-day catalysis. The translation of academic catalysis research into industrial

applications started at the end of the nineteenth century. Rapid economic growth and the

increasing world population demanded the large scale production of base chemicals. The use

of catalytic processes made this economically and practically possible. The main enabling

aspect of catalysis was the increase of the rate of chemical reactions under practical

conditions. Other driving forces boosting development of catalytic processes were the

increasing need for mobility, increasing living standards and, less positively, both world wars.

During these periods large scale processes have been developed: the production of ammonia

for fertilizers (Haber-Bosch process), nitric acid production for explosives (Ostwald process)

and fuel production and processing (Fischer-Tropsch process, fluid catalytic cracking (FCC),

etc). Nowadays, catalysts are the key assets of the chemical industry, enabling approximately

85% of all industrial chemical processes [2]. From the 1970s onwards, emission control

became an important aspect in catalysis research. A very significant example in this field is

the three-way monolith catalytic converter of automotive exhaust gasses to reduce CO, NO

and hydrocarbon emissions [2-4]. Without catalysis, the current living standards of our

society would not have been achieved.

To explain the principle of catalytic action, let us consider the reaction between

molecules A and B (Fig. 1.1) to form molecule AB. In a typical chemical reaction, one has to

Chapter 1

2

overcome an energy barrier that often requires a substantial amount of energy. Very often, the

high temperatures required to overcome this uncatalyzed barrier will lead to undesired side

reactions. A successful catalyst will lower the energy barrier for the overall reaction by

providing an alternative reaction path in which a catalyst is involved. In the first step of a

catalytic reaction (adsorption), the reactants form bonds with the catalyst surface, lowering

the free energy. Subsequently, the adsorbed reactants may form bonds with each other to form

the desired product. To close the catalytic cycle, the products desorb from the catalyst surface.

Fig. 1.1. Schematic representation of a catalytic reaction between molecule A and B to form

molecule AB [2].

Catalysts come in great diversity in respect to composition, form, size and shape. One of the

largest adopters of catalysis is Nature. Almost all biological processes occur with the help of

enzymes. Examples are the build-up of proteins and DNA. Also the breakdown of ethanol in

the body by alcohol dehydrogenase and the conversion of CO2 and H2O to sugars by

chlorophyll in plants are important biochemical processes involving enzymes as catalysts. The

term heterogeneous catalysis describes a catalytic reaction in which the reactants are in a

different phase than the catalyst. Usually, reactants are in the gas or liquid phase and the

catalyst is a solid. In a homogeneous catalytic process, reactants and catalyst are in the same

phase. Typically, a heterogeneous catalyst has a high active surface area, provided by

stabilized of the active phase in the form of small particles on a relatively inert material that

Chapter 1

3

acts as a support. The reactivity of the active phase resides in the coordinative unsaturation of

the surface atoms.

An important class of heterogeneous catalysts are zeolites. Zeolites are crystalline

aluminosilicates characterized by a well-defined microporous structure; the pores have

dimensions close to those of molecules. In addition, zeolites have a tunable chemical

composition by which virtue the acidity can be controlled to some degree. The term zeolite

was first mentioned by the Swedish mineralist Axel Fredrik Cronstedt. It is a combination of

the Greek words ζέω (zéō), meaning "to boil" and λίθος (líthos), meaning "stone" [5]. Barrer

succeeded to synthesize zeolites under hydrothermal conditions, and with this a range of new

structures that did not occur in nature became available [6]. Currently, 218 zeolite frameworks

are known in the IZA database [7]. However, only a small number of these are actually used

in industrial catalytic processes. The largest application of zeolites is in laundry detergents,

which comprises an annual volume of 1.44 million metric tons [2]. As catalysts, zeolites

mainly find application as solid acids. Their high Brønsted acidity arises when a framework

oxygen atom is neighbored by Si and Al cations. The bridging oxygen atom has a -1 charge.

Compensating the negative charge with a proton results in a highly ionic bond, providing the

material its strong Brønsted acidity [8]. In addition to strong acidity, two other properties are

important to explain the widespread application of zeolites [9]. The first one is the shape

selectivity exterted by the presence of the acid sites in micropores [10, 11], with dimensions

in the range of molecules such as hydrocarbons. The second one is their outstanding thermal

and hydrothermal stability [12] that makes them applicable under harsh industrial conditions.

For instance, ZSM-5 with the MFI topology is widely used in the chemical industry. It is used

as an additive in the FCC process next to faujasite zeolite, and also in the isomerization of n-

alkanes into branched isomers to increase the octane number of gasoline. The topology of

ZSM-5 is such that a threedimensional pore system is obtained that is comprised of straight

(along the b-axis, Fig. 1.2a) and sinusoidal (along the a-axis, Fig. 1.2b) channels that

intersecting at the so-named channel intersections. The pores consist of 10-membered rings,

so that the pore diameter is 5.5 Å.

Other important zeolite types are MCM-22 (MWW topology) and zeolite Y (FAU

topology). The first one is an important catalyst for the alkylation of benzene with olefins [13],

while the second is the dominant acid component in FCC cracking catalysts [14]. The

micropore system of MWW consists of two separate two-dimensional channel systems. One

system consists of straight 10-membered rings (Fig. 1.2c, indicated in blue) [15, 16]. The

Chapter 1

4

second pore system is created when two cups located at the surface of adjacent MWW layers

are connected to form a super cage. These large ellipsoidal cages are typically 7.1 Å in

diameter and 18.2 Å in height [16]. The large cages are connected with each other through 10-

membered ring windows (Fig. 1.2c, indicated in red). The structure of the FAU family to

which zeolite Y belongs is built from sodalite cages connected to each other through a double

4-ring (Fig. 1.2d). In this configuration a supercage is formed with a diameter of 11.6 Å

[17,18]. The supercages are accessible through 12-membered windows (indicated in red)

having a diameter of 7.4 Å [17,18].

Fig. 1.2. Zeolite structures of a) ZSM-5 (MFI) viewed along the c-axis, b) ZSM-5 (MFI)

viewed along the a-axis, c) MCM-22 (MWW) and d) zeolite Y (FAU).

Most of the applications of zeolites in catalysis aim to valorize crude oil into fuels and

chemicals. Today, the production and use of fossil resources is debated because of climate

change issues. Burning fossil fuels releases vast amounts of CO2 into the atmosphere, causing

global warming [19]. Energy security is another driver to explore the use of alternative

feedstock. The largest oil reserves are located in politically unstable regions making the crude

oil supply chain uncertain. A final concern relates to the end of cheap oil; at the rate of current

consumption there is enough conventional oil for about half a century [20]. All these factors

form drivers for the search for alternative energy and chemical resources. Although during the

last decade new technologies have been discovered and are being developed, the road from

discovery to industrial application may take decades [21]. It is most desirable to convert our

current society that relies on the use of fossil resources to a carbon-neutral society based on

the use of renewable energy from the sun. To overcome long lead-times for novel renewable

energy technologies, there is a great need for transition technologies that are preferably based

on less-polluting feedstock. The option that is currently gaining much in attention is the use of

natural gas for the production of fuels and chemical [22]. Methane, the main component of

natural gas, is the cleanest of all fossil resources and it is widely available.

Chapter 1

5

1.2 Methane as feedstock

Historically, natural gas has been mainly used for heating and electrical power

generation. It can be efficiently distributed using pipelines to the end consumers. Transport

via pipelines over longer distances is also feasible, although the capital investments are

typically higher than the investments for compressed or liquefied natural gas transport.

Especially, LNG technology is rapidly becoming more important for natural gas transport

over long distances. As an alternative, the direct conversion of methane into valuable products

has gained significant interest. Methane has been used for a long time as the primary source of

hydrogen for such processes as methanol and ammonia synthesis. The use of natural gas for

the production of chemicals has certain advantages as compared to petroleum oil. The

composition of natural gas is more uniform and methane possesses the highest hydrogen to

carbon (H/C) ratio of all fossil resources. The total estimated natural gas reserves exceed that

of petroleum with a factor 10, especially when the shale gas deposits are also taken into

account [23, 24]. The extraction of natural gas from impermeable shale rock formation

recently took off with the development of horizontal drilling and hydraulic fracturing

techniques [24, 25]. This development has already dramatically changed energy scenarios and

energy markets. Despite environmental concerns about these new gas exploration techniques,

shale gas production is already fully implemented in the U.S. Other countries such as China,

Australia, South Africa and the United Kingdom are exploring the possibility of shale gas

production. As a result of the cheap shale gas production in the U.S., natural gas prices have

plummeted, making it a competitive carbon feedstock next to petroleum [26].

The catalytic conversion of cheap methane into valuable chemicals is one of the

interesting routes to decrease our dependency on petroleum. At present, Gas-to-liquid (GTL)

processes are already contributing to the conversion of methane via synthesis gas (a mixture

of CO and H2) into fuels and chemicals (Fig. 1.3). Despite the scale of these activities of Shell

and Sasol in Qatar, Nigeria and South Africa, the contribution of liquid fuels from natural gas

to satisfy our hunger for liquid fuels for mobility is still very small. The recent paper by Wood

et al. [27] presents a complete overview of both established GTL processes and those under

development. All currently commercially operated processes are based on the indirect

conversion of methane. First, methane is transformed into synthesis gas by partial oxidation,

steam reforming or autothermal reforming. The resulting syngas can then be converted into

methanol, which in turn can be used for the production of dimethyl ether (DME), light olefins

Chapter 1

6

or gasoline. Currently, China is rapidly expanding its methanol-to-olefins (MTO) capacity. A

significant fraction of the syngas for methanol production is derived from coal, although it is

projected that a number of new natural gas based plants will be built in the U.S. to produce

methanol for MTO plants at China’s east coast. Alternatively, syngas can also be directly

converted into hydrocarbons using the Fischer-Tropsch Synthesis (FTS) process [28-31]. In

FTS, a range of hydrocarbons is produced; the wax fraction is converted in a downstream

hydrocracking step to naphtha, diesel and lubricants. Due to the large investments required for

Fischer-Tropsch type GTL processes, operating this technology is only economically viable

close to very large natural gas fields. Investments in liquefaction of natural gas are also very

high so that such terminals can only be built close to large fields to be economically viable.

Monetizing on smaller fields can be done by compressed natural gas technology. However,

the majority of natural gas fields are too small to be exploited with current technology.

Fig. 1.3. Main routes from natural gas to value-added products: MDA (methane dehydro-

aromatization), POX (partial oxidation), OCM (oxidative coupling of methane), MTG

(methanol to gasoline), MTO (methanol to olefins), and FTS (Fischer–Tropsch synthesis).

The main problem with current GTL processes is the methane conversion step that

requires large capital investments (e.g., the oxygen plant for auto thermal reforming). The

economics of processes that directly convert methane into liquids would be much better,

because it would avoid the syngas generation step. An inherent problem with such direct

methane conversion routes to, for instance, methanol lies in the high reaction temperatures

needed to active the strong C-H bonds in methane. The formed products are highly reactive at

Chapter 1

7

such temperatures, often leading to undesired by-products and catalyst deactivation.

Activation of methane can be done under oxidative or non-oxidative conditions [28-31]. In

the oxidative routes, oxygen is the preferred oxidant. An example is the oxidative coupling of

methane (OCM) to ethane and ethylene. The process has not been applied commercially,

because of low product yields and the difficulty in heat management of the process. Another

oxidative route is the partial oxidation of methane to produce methanol or formaldehyde; such

approaches suffer from the higher reactivity of the product than the reactant so that reasonable

selectivities can be achieved at modest conversion.

Without an oxygen source, methane can be converted into aromatics and hydrogen. The

main process under investigation is the dehydroaromatization of methane, which was first

reported by Wang et al. in 1993 [32]. If it could be done in an economic manner, methane

dehydroaromatization (MDA) could convert methane into liquid benzene and hydrogen [33].

However, the unfavorable thermodynamic equilibrium of the methane benzene + hydrogen

reaction demands high reaction temperatures (>650°C) to attain reasonable methane

conversion [34, 35]. As a consequence of extensive coke deposition, catalyst stability is a

major problem. Usually, the process is operated at atmospheric pressure in a fixed-bed reactor

using Mo-modified zeolites. The most often used zeolite is Mo/HZSM-5 [36].

1.3 General aspects of the MDA catalyst

Since the early work by Wang et al. [32] many studies and reviews [37-42] have been

published on molybdenum modified zeolites as promising MDA catalysts. Large efforts have

been made to improve benzene selectivity and catalyst lifetime. Mo/zeolite catalysts are prone

to deactivation due to the formation of carbon species at the zeolite surface Also, substantial

research efforts has been dedicated to elucidate the reaction mechanism. The most accepted

reaction mechanism follows a cascade of reactions [41]. Initially, methane reacts with the Mo-

oxide to form the MoCx species. These species activate methane and catalyze the formation of

the C-C bonds, mainly resulting in ethylene. In the last step, the olefins aromatize at the

Brønsted acid sites to form benzene and heavier aromatics.

As mentioned, the first step in the MDA reaction is the activation of methane by breaking

one of the stable C-H bonds. Breaking the C-H bond requires high temperatures (>650°C) and,

at such conditions, the resulting CH3 specie is susceptible to further C-H bond breaking

leading to formation of coke. Theoretical calculations suggested that transition metal oxides

Chapter 1

8

are able to activate a single C-H bond without breaking the remaining C-H bonds [43, 44].

Several potential metals have been proposed including Zn, W, Re, Cu, Ni, Fe, Mn, Cr, V, Ga

and Pt [38, 39, 42]. Extensive screening revealed molybdenum to be the preferred metal-oxide

component for the activation and oligomerization of methane (see Table 1.1). In order to

obtain the active phase, the Mo-oxide is carburized upon reaction with methane to form

molybdenum carbide (MoCx) which is believed to be the active species. The formation

mechanism and properties of these MoCx species will be discussed in section 1.5.

Introducing the metal oxide onto the zeolite support has certain advantages. It improves

particle dispersion and, therefore, the catalytic activity. Furthermore, the Brønsted acidic

properties of the zeolite also contribute to the spreading of Mo over the surface and diffusion

into the micropores [45, 46]. The support also plays an important role in the stabilization of

the MoCx species and it enhances the proximity to the acid sites needed for the aromatization

step.

The aromatization step comprises transformation of the olefins over the Brønsted acid

sites into aromatic compounds, mainly benzene. Efforts have been made to correlate the acid

properties of zeolite to the performance in the MDA reaction. The influence of the Si/Al ratio

and Al distribution has been investigated [47-51]. The catalyst based on ZSM-5 as zeolite

support is the benchmark. The high selectivity towards benzene shown for Mo/ZSM-5 [37, 41,

42] originates from the micropore channel dimensions (5.4-5.6 Å) close to the kinetic

diameter of benzene. In this way, the micropores sterically hinder formation of larger

(poly)aromatic molecules. In addition to ZSM-5, other zeolite structures have been

investigated [38-40]. In particular, MCM-22 (MWW topology) showed promising catalytic

performance. activity of Mo/MCM-22 was higher and it was less prone to deactivation than

ZSM-5. The improved catalytic properties of the Mo/MCM-22 catalyst were attributed to the

more suitable topology. The micropores of MCM-22 are able to accumulate more carbon

compared to ZSM-5 before losing shape selectivity [52]. However, due to patent issues,

material costs and industrial relevance, the Mo/ZSM-5 catalyst is still considered to be the

benchmark for MDA and most of the research is focused on it.

Chapter 1

9

Table 1.1. Comparison MDA activities of different metals supported on HZSM-5 zeolite.

Adapted from [39].

Active

metals

Reaction conditions CH4

conversion

(%)

Selectivity (%)

T (ºC) Flow (ml·gcat-

1·h-1) Benzene Naphthalene

Mo 730 1500 16.7 60.4 8.1

Zn 700 1500 1.0 69.9 n.d.2

W 800 1500 13.3 52.0 n.d.2

Re 750 1440 9.3 52.0 0

Co-Ga 700 1500 12.8 66.5 7.2

Fe 750 8001 4.1 73.4 16.1

Mn 800 1600 6.9 75.6 11.9

V 750 8001 3.2 32.6 6.3

Cr 750 8001 1.1 72.0 3.7

1GHSV / h

-1;

b Not determined.

1.4 Catalyst preparation

The final performance of the catalyst in MDA strongly depends on the Mo introduction

procedure [9, 43]. A wide range of methods have been explored in the past, predominantly

related to the preparation of Mo/ZSM-5. These reports concluded that catalysts prepared by

incipient wetness impregnation procedure or solid state exchange showed the best catalytic

performance in MDA [39]. In the incipient wetness procedure, the zeolite is impregnated with

an aqueous ammonium heptamolybdate (AHM) solution. After impregnation, bulky

molybdate anions [Mo7O246-

] are deposited at the external surface of the zeolite. To

decompose the AHM precursor into smaller metal-oxide species and improve the dispersion

of the molybdate species, a calcination step is performed. Xu et al. studied the influence of

calcination after impregnation in more detail using FT-IR spectroscopy and differential

thermal analysis [53]. They found that AHM starts to decompose into MoO3 crystallites at

514 K. However these crystallites are large and remain at the external surface of the zeolite.

Increasing the temperature further to approximately 773 K, leads to degradation of the metal-

oxide into smaller fragments. These smaller fragments spread over the external surface of the

zeolite and are small enough to diffuse into the micropores. The group of Iglesia [54-56]

found the mechanism of metal oxide spreading and micropore diffusion in the solid state

Chapter 1

10

exchange procedure of H-ZSM-5 and MoO3 to be comparable with that of the incipient

wetness procedure. It was also found that the Mo species interact with the Brønsted acid sites

of the zeolite forming strong Mo-O-Al bonds. The exchange of Mo with the acidic BAS

protons leads to a decrease in acidity. Therefore, introducing large amounts of Mo can lead to

severe reduction of BAS concentration and, possibly, decrease of the catalytic activity.

Scheme 1 presents the most accepted mechanism on the formation Mo-oxo species exchanged

to the BAS. In the first step a small MoO3 fragment reacts with the BAS proton and forms a

MoO2-(OH)+ specie connected to the BAS. In the next step either monomeric (MoO2

2-) or

dimeric (Mo2O52+

) species are formed, depending on the BAS concentration. High BAS

concentration leads to the formation of predominantly monomeric Mo-oxo species (scheme 1,

equation 2). At lower BAS concentrations the dimeric Mo-oxo species and are formed when

two MoO2-(OH)+ species are in proximity (Scheme 1.1, equation 1). Extreme calcination

temperatures can lead to BAS hydrolyzation forming framework defects. The framework

defects can react with the Mo-oxo species leading to Al extraction and the formation of

Al2(MoO4)3 species.

Scheme 1.1. Interactions between MoOx species and Brønsted acid sites of HZSM-5. Adapted

from [39].

1.5 The active Mo phase in MDA

It is generally accepted that the MoCx particles are responsible for the activation and

oligomerization of methane in the MDA reaction. To obtain the active MoCx phase the MoOx

>773K MoO

3 +

O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

+ O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

+ H2O

O2 O

2

>773K

O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

+ O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

O2

>773K O

MoO2OH

Al Si

O

H

Al Si

+O2

773-793K+ H2O (g)O

Mo

O

O O

SiAl Al Si

+ H2O

O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

+ O

H

Al Si

MoO3 +O2

>773K O

MoO2OH

Al Si

+O

MoO2OH

Al Si

O2

>773K + H2O (g)

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

O2

>773K O

H

Al Si

+O

H

Al Si

O2

>773K

O

SiAl+ H2O (g)

Al Si

O2

MoO3

“Extraframework“ Al2O3 or Al2(MoO4)3

(Reversible, no loss of crystalinity)(Reversible, no loss of crystalinity)

++ O

H

Al Si

+O

H

Al Si

O2

>773K

O

SiAl+ H2O (g)

Al Si

O2

MoO3

“Extraframework“ Al2O3 or Al2(MoO4)3

(Reversible, no loss of crystalinity)(Reversible, no loss of crystalinity)

+O

2

>773K + H

2O “Extraframework”

Al2O

3 or Al

2(MoO

4)

3

(1)

(2)

(3)

Chapter 1

11

species have to be carburized by reaction with methane. The carburization leads to an

induction period towards the formation of hydrocarbon products during the early stages of

reaction. The exact place and the nature of these carbide species, however, are still not fully

understood. Studies by the groups of Solymosi and Lunsford [57, 58] proposed that highly

dispersed Mo2C species at the external surface of the zeolite are responsible for methane

activation. Later, EXAFS studies by the group of Iglesia suggested that the active MoCx

particles are formed upon carburization of dimeric Mo-oxo species located inside the micro

pores [54, 56, 59] (Scheme 1.2). In their proposed mechanism, a part of the BAS are

regenerated upon the formation of these MoCx clusters. In this way, the BAS are in close

proximity to the MoCx phase. It possibly explains the better performance of the bi-functional

catalyst compared to catalytic systems in which the metal carbide and acidic component are

separated.

Scheme 1.2. Reaction of exchanged of MoOx/H-ZSM5 with CH4. Adapted from [43].

Monitoring the size of the MoCx clusters with time on stream, the particles were observed

to increase to approximately 0.6 nm in size (ca. 10 Mo atoms) [43], close to the diameter of

the ZSM-5 pores. A small fraction of these MoCx clusters were seen to agglomerate into

larger particles and exceed the zeolite pore dimensions forcing them to migrate to channel

intersections, crystalline defects and the external surface. The resistance to agglomeration of

the main fraction of MoCx particles was attributed to several factors including the low vapor

pressure, high melting point and the strong interaction of MoCx species with the zeolite

framework [60]. They showed that highly dispersed Mo-oxo species that strongly interact

with the zeolite surface were difficult to completely reduce to Mo2C. Instead, the carburized

metal-oxide phase was only partially reduced into MoCxOy particles [61, 62].

1.6 Catalyst deactivation

Poor catalyst stability is the major hurdle to be overcome in the further development of

the MDA reaction into an industrial process. Catalyst deactivation is attributed to the high

reaction temperatures required for C-H bond activation, which favors formation of coke; it

+ CH4 O

O

O

O

Si Si

Al

SiAl

Si

SiSi

+ COx + H2OO

H

Al Si

+

Si

O

Mo

Al

O

O

Al

O

Mo

Si

O

O

O

Chapter 1

12

results in the deposition of carbonaceous products and blockage of the micropores [38].

Another problem is sublimation of some Mo-oxide phases [63]. Some researchers also

attribute deactivation to the transformation of the η-Mo3C2 phase towards less active α-MoC1-

x and β-Mo2C phases upon interaction with carbonaceous deposits [37]. Coke formation is

widely regarded as the main contributor to catalyst deactivation [11, 64-66]. Brønsted acid

sites (BAS) located at the external surface are thought to cause extensive carbon deposition

[67], with the lack of shape selectivity of the micropores explaining the different products

mixture formed from benzene and toluene formed in the micropores. The polynuclear

aromatics tend to cover the external surface and block access to the micropores.

Several papers have characterized the carbonaceous deposits by temperature programmed

oxidation and thermogravic analysis methods to the spent samples. Such characterization

provides quantitative and qualitative information on the carbonaceous deposits composition

[65-69]. The different types of coke are typically distinguished by their combustion

temperature. Usually, two types of carbon species can be identified. An amorphous type of

carbon formed in the proximity of Mo can be oxidized at lower temperatures, the alternative

interpretation being that it pertains to oligomeric species in the micropores. A polyaromatic

type of carbon can only be oxidized at higher temperatures.

1.7 Catalyst regeneration

Since it appears that coke formation is an intrinsic property of the MDA reaction over

Mo-modified zeolites, it is likely that a viable industrial process would need to involve

periodic regeneration to remove the carbonaceous deposits. Several regeneration methods

have been extensively explored [45], [70, 71]. It has turned out to be a challenge to control the

high exothermicity of the coke oxidation, which will lead to catalyst damage [70].

Sublimation of Mo-oxides formed upon regeneration also negatively affects performance of

the regenerated catalyst. Typically, oxidation of the coke in air (above 723 K to remove hard

coke) leads to decreased activity after consecutive regeneration cycles. In order to prevent

catalyst damage, regeneration should preferably be carried out at lower temperatures. A

promising example is found in the study by Ma et al. [70], who showed the possibility to

remove carbon deposits using a mixture of 2 % NO in air). In such mixture, carbon could be

fully removed at 623 K and eight regeneration cycles could be carried out without activity

loss. The regenerability depends also on the degree of coking. Ismagilov et al. [72] showed

that it is possible to regain full catalytic activity when regeneration was applied after a period

Chapter 1

13

of 6 h on stream for five consecutive regeneration cycles. However, longer periods on stream

(15 h and 20 h) led to irreversible damage of the catalyst and decrease in activity after each

regeneration cycle. This most likely relates to the nature and amount of the different coke

species.

1.8 Scope of the thesis

The valorization of methane into aromatics has already been investigated in academic and

industrial research laboratories. Poor catalyst stability hinders commercial application of this

process. This thesis investigates the deactivation of Mo-containing zeolite catalysts in the

MDA reaction. Methods to improve catalyst performance, stability and selectivity are also

investigated. Chapter 2 discusses ways to improve its performance in MDA. One strategy to

improve catalytic performance is the introduction of mesopores in the acidic zeolite support to

decrease the size of the micropore domains; in this way diffusion can be enhanced and the

negative effects of carbon deposition can be alleviated. The alternative strategy involves the

chemical modification of the external zeolite surface with silica to decrease its activity

towards formation of coke. The catalyst performance is compared to the benchmark

Mo/ZSM-5. To this end, the Mo introduction procedure into zeolite ZSM-5 has been

optimized by screening several adaptable parameters. Chapter 2 showed the beneficial effect

of smaller micropore domains in the zeolite crystal on the catalytic stability in MDA. Chapter

3 covers the influence of the gas atmosphere during the pre-heating step prior to the MDA

reaction. Therefore, the benchmark Mo/HZSM-5 and its silylated counterpart were pre-treated

in inert, oxidizing and carburizing conditions. It was found that pre-carburization of the

catalyst led to the best catalytic performance. The chemical properties of the catalyst after

precarburization was further characterized and compared to that of the fresh catalyst. In

Chapter 4, detailed analysis of activated and spent Mo/HZSM-5 samples during the MDA

reaction is carried out to identify the main reasons for the deactivation. Although formation of

carbon is argued to be the main reason for the poor catalyst stability, relatively little is known

about the nature and location of the carbon deposits that deactivate the catalyst. Chapter 5

focuses on MCM-22 as an alternative zeolite component in the MDA catalyst. In this chapter,

a method is proposed to obtain nano-crystalline MCM-22 in a one-pot synthesis approach by

partial delamination of the zeolite crystallites by adding an organo-silane molecule to the

synthesis gel. In this approach the thickness of the MCM-22 crystals can be decreased in a

simpler manner than the procedure that leads to delaminated MCM-22 (ITQ-2). The prepared

Chapter 1

14

material is extensively characterized and compared to conventional MCM-22 and ITQ-2 in

the MDA reaction, and also in liquid phase benzene alkylation with propylene. In Chapter 6

the influence of the micropore domain of the ZSM-5 zeolite support is studied in more detail.

Therefore MFI supports with varying micropore domain size have been prepared ranging

from 10 µm to several nanometers. After introducing molybdenum the catalytic performance

was tested in MDA. Evaluation of the catalytic performance revealed improved stability and

selectivity upon hierarchical structuring of the ZSM-5 support. In Chapter 7 the potential of

mesopore-containing faujasite in FCC is investigated. Using an organosilane molecule,

mesopores are introduced during Y zeolite synthesis. The various preparation methods were

scaled up for the catalytic evaluation in FCC. A mesoporous Y zeolite is compared with bulk

zeolite Y in a procedure that involves binding of the zeolite in kaolin, accelerated steam

deactivation and fluid catalytic cracking of vacuum gas oil. The residual acidity of the zeolites

and composite catalysts is determined and the influence of interconnected mesopores on the

performance in catalytic FCC is discussed. The main findings and results of the thesis are

summarized in the Summary.

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Chapter 2

17

Desilication and silylation of Mo/HZSM-5 for methane

dehydroaromatization

Summary

The influence of mesoporosity and silylation on the physico-chemical and catalytic properties

of Mo/HZSM-5 in methane dehydroaromatization was investigated. The zeolites were

characterized by XRD, 27

Al and 95

Mo NMR, UV–Vis, UV Raman and pyridine IR

spectroscopy and TEM. Base-desilicated mesoporous and bulk HZSM-5 zeolites with

comparable Brønsted acidity were employed as acidic supports. Mo loading was optimized to

minimize loss of acidity. Surface silylation of Mo/HZSM-5 resulted in improved Mo-oxide

dispersion. More intensive silylation led to decreased Mo-oxide dispersion because of

increased hydrophobicity. High methane conversion rates were associated with small Mo-

oxide precursor particles. Silylation of the external surface of Mo/HZSM-5 led to higher

methane conversion and less coke formation. On contrary, silylation of HZSM-5 prior to Mo

introduction had a negative effect on the performance. Post-synthesis silylation of Mo/HZSM-

5 affected the Mo-oxide phase. The amount of hard coke decreased with increasing silylation

degree due to deactivation of acid sites at the external surface. It also decreased naphthalene

yield. Methane conversion and aromatics selectivity were lower for mesoporous Mo/HZSM-5

compared with bulk Mo/HZSM-5. Although the initial Mo-oxide dispersion was higher, the

different nature of the mesopore surface resulted in rapid formation of large Mo-carbide

particles with higher coke selectivity. Silylation slightly improved activity and selectivity to

benzene.

This chapter is published in Microporous Mesoporous Mater. 203 (2015) 259-273.

Catalyst performance

Chapter 2

18

2.1 Introduction

Increasing crude oil prices related to the depletion of petroleum reserves are a strong

driver for the search of alternative feedstocks to produce fuels and chemicals. Natural gas is a

viable feedstock to facilitate the transition to a carbon-neutral energy supply. It is abundant

and the cleanest of all fossil resources. Currently, the price of shale gas is very low in certain

regions. The proved global natural gas reserves are estimated at ca. 180 trillion cubic metres

according BP’s statistical review of world energy [1]. A large share of these reserves is

located in remote areas and it is often produced as “associated” gas during the production of

crude oil [2]. The remote location makes exploitation of this associated gas economically

unfeasible [2] and [3]. Technologies to employ natural gas reserves are already available, e.g.,

by transport in liquefied form and by conversion into transportation fuels via the syngas

platform (a mixture of CO and H2). There is, however, also great interest in direct conversion

of methane into liquid chemicals. Direct methane oxidation to methanol remains a great

unsolved challenge. A more promising approach involves non-oxidative methane

dehydroaromatization (MDA), first described in 1993 by Wang et al. [4]. In this process,

methane is converted into aromatics, predominantly benzene, and hydrogen.

The preferred catalyst for MDA is the bifunctional Mo/HZSM-5 zeolite. At the start of

the reaction, the Mo-oxide (MoOx) phase reacts with CH4 to form molybdenum carbides

(MoCx) [5]. The MoCx phase is believed to activate methane and provide sites for C-C

coupling into ethylene [6]. The zeolitic Brønsted acid sites convert ethylene into benzene and

other aromatic molecules. A major drawback of this reaction is the low stability of

Mo/HZSM-5. The high temperatures employed, typically 973 K or higher required to activate

the strong C-H bonds of methane, result in extensive formation of carbonaceous deposits that

block the micropores and deactivate the catalyst [7-11]. The adverse effects of coking can be

lowered by introducing mesopores that reduce diffusion pathways and improve the efficiency

factor of the zeolite crystals [12, 13]. Previous studies have shown the beneficial effect on the

MDA reaction of (i) mesopore introduction by silicon extraction from HZSM-5 [13] and (ii)

carbon black templating in HZSM-5[14] and HMCM-22 [15].

Another way to improve the stability is to reduce the rate of carbon formation. The

Brønsted acid sites (BAS) located at the external surface have been implicated in the rapid

formation of large carbonaceous deposits [17], mostly polynuclear aromatic molecules too

large desorb from the zeolite surface [14]. These carbon deposits eventually block the

Chapter 2

19

micropore entrances and deactivate the catalyst [18-20]. Deactivation of the external BAS in

the parent catalyst might, therefore, be a strategy to suppress formation of this type of

coke [17-19].

In this study, we investigated the influence of mesopore introduction and external surface

BAS deactivation on the catalytic MDA performance of Mo/HZSM-5. We employed

desilication for generation of mesopores because of its low cost and facile optimization. We

first optimized the Mo introduction method for a bulk microporous zeolite, before applying

the optimized approach to the desilicated zeolite. The catalytic performance of the resulting

mesoporous Mo/HZSM-5 zeolite is compared with that of conventional Mo/HZSM-5 zeolite

with similar acidity. The second approach is to deactivate the BAS located at the external

surface by silylation with the aim to reduce coke formation. In this study, particular attention

is given to silylation before and after Mo introduction. The catalysts were extensively

characterized by FTIR, 27

Al MAS NMR, UV-Raman and UV–Vis spectroscopy. Also, spent

catalysts were characterized for changes in their textural properties. The influence of

desilication and silylation on the methane dehydroaromatization performance is discussed.

2.2 Experimental methods

2.2.1. Synthesis of materials

The parent zeolite (HZSM-5) was obtained in the ammonium form from Albemarle

Catalysts. Zeolites with different Si/Al ratios (19.4, 28.2 and 43.6 as determined by ICP

analysis) were used to optimize the desilication procedure. The zeolites are denoted as ZSM-

5(20), ZSM-5(30) and ZSM-5(40), respectively. For the introduction of mesopores in HZSM-

5, we optimized the base-catalyzed desilication procedure described by Groen et al. [20] by

varying the NaOH concentration and the temperature and time of base leaching (Table 2.1).

Typically, 1.66 g of dried ZSM-5 was suspended in a 50 ml NaOH solution. After desilication

the hot liquor was filtered and washed with copious amounts of deionized water. The

desilicated samples were exchanged three times with a 1 M NH4NO3 solution for 4 h at 353 K

followed by drying overnight at 383 K. To convert the ammonium form to the proton form,

the dried zeolite was calcined at 523 K for 4 h. The zeolites are denoted as HZSM-5 and

HZSM-5(meso) for the parent and desilicated samples, respectively.

Molybdenum oxide was introduced onto the zeolites by physical mixing with MoO3 or

incipient wetness impregnation with an aqueous solution of ammonium heptamolybdate

Chapter 2

20

tetrahydrate (AHM, Merck). For physical mixing, MoO3 and the zeolite were thoroughly

grinded in a mortar. For the incipient wetness impregnation procedure, the dried zeolite was

impregnated with a solution of appropriate concentration AHM. After impregnation the

samples were dried for 1 h. The targeted Mo content was 4 wt%. The Mo-loaded zeolites

were calcined in artificial air at varying temperatures and dwell times. The heating rate was

1.5 K/min. Mo-modified zeolites are denoted as Mo/HZSM-5 and Mo/HZSM-5(meso).

For silylation, the method described by Zheng et al. was used [21]. Typically, 2 g of

zeolite was dried overnight at 373 K and then dispersed in 50 ml n-hexane. To this

suspension, 0.3 ml tetraethylorthosilicate (TEOS, Merck) was added and stirred for 1 h under

reflux. The amount of TEOS corresponded to 0.4 wt% based on the amount of zeolite in the

suspension. Thereafter, the catalyst was filtered and dried overnight at 373 K. The samples

underwent a two-step calcination process in artificial air. The first step consisted of heating

the sample at a rate of a 2 K/min to 393 K followed by an isothermal period of 2 h. In the

second step, the temperature was further increased to 773 K at a rate of 0.2 K/min, followed

by an isothermal period for 4 h. Single and triple silylated treated Mo/HZSM-5 are

abbreviated as Mo/HZSM-5(Mo,Si1) and Mo/HZSM-5(Mo,Si3), respectively. Mo/HZSM-5

silylated before Mo introduction is referred to as Mo/HZSM-5(Si1,Mo). The same naming

method is used for the mesoporous zeolites, e.g., Mo/HZSM-5(meso,Mo,Si3).

2.2.2. Characterization

The Mo and Al contents of the samples were determined by inductively coupled plasma

optical emission spectroscopy (ICP-OES) on a Spectro CIROS CCD spectrometer equipped

with a free-running 27.12 MHz generator at 1400 W. Prior to analysis, samples were digested

in a mixture of HF/HNO3/H2O (1:1:1).

XRD patterns were recorded on a Bruker D4 Endeavor powder diffraction system using

Cu Kα radiation with a scanning speed 0.01° sec−1

in the 2θ range 5–60°. XRD crystallinities

were determined using the Bruker TOPAS 3.0 software.

Infrared spectra for the determination of IR crystallinity were recorded on a Nicolet Avatar

360 spectrometer with a KBr pellet (1 mg of zeolite in 100 mg of KBr). Zeolite crystallinity

was estimated from the ratio of the intensities of bands at 450 cm−1

and 550 cm−1

[22].

Argon sorption isotherms were measured at 87 K on a Micromeritics ASAP2020 system

in static measurement mode. The samples were outgassed at 623 K for 8 h prior to the

Chapter 2

21

sorption measurements. The Brunauer–Emmett–Teller (BET) equation was used to calculate

the specific surface area (SBET) from the adsorption data obtained (p/p0 = 0.05–0.25). The

mesopore volume (Vmeso) and mesopore size distribution were calculated using the Barrett–

Joyner–Halenda (BJH) method on the adsorption branch of the isotherm. The micropore area

(Smicro) and micropore volume (Vmicro) were calculated from the t-plot curve with the thickness

range being 3.5 and 5.4 Å [23].

Infrared spectra were recorded in the 4000–400 cm−1

range using a Bruker Vertex 70v

apparatus. Samples were pressed into a self-supporting wafer with a density of about

10 mg/cm2. To remove adsorbed water the sample was evacuated for 2 h at 773 K. After

evacuation the sample was cooled to 323 K followed by recording of the background

spectrum. The total concentration of the Brønsted acid sites was determined by measuring IR

spectra of adsorbed pyridine. Pyridine adsorption was carried out on the dehydrated zeolite

wafer at 423 K. After saturation was reached, the sample was evacuated at 423 K for 2 h and a

spectrum was recorded. The amount of Brønsted acid sites on the external surface was

determined by similar procedures using 2,4,6-collidine as the base. The spectra were

deconvoluted by standard procedures and for quantification the extinction coefficients

reported by Datka et al. [24] and Nesterenko et al. [25] were used for pyridine and 2,4,6-

collidine, respectively.

The surface composition of the samples was analyzed by X-ray Photoelectron Spectroscopy

(XPS) using a Thermo Scientific K-Alpha equipped with a monochromatic small-spot X-ray

source and a 180° double focusing hemispherical analyzer with a 128-channel detector.

UV–Vis spectra were recorded on a Shimadzu UV-2401 PC spectrometer in diffuse-

reflectance mode with a 60 mm integrating sphere. BaSO4 was used as the reference.

UV Raman spectra were recorded with a Jobin–Yvon T64000 triple stage spectrograph

with spectral resolution of 2 cm−1

. The laser line at 244 nm of a Lexel 95-SHG laser was used

as exciting source with an output of 20 mW. The power of the laser on the sample was about

2 mW. The excitation laser line at 325 nm was produced by a Kimmon He–Cd laser. The

power of the laser on the sample was 4 mW.

Magic angle spinning (MAS) 27

Al single pulse NMR spectra were recorded on a Bruker

Avance DMX-500 NMR spectrometer equipped with a 2.5 mm MAS probe head operating at

an 27

Al NMR resonance frequency of 130.3 MHz. The 27

Al chemical shift is referred to a

saturated Al(NO3)3 solution. In a typical experiment 10 mg of well-hydrated sample was

packed in a 2.5 mm zirconia rotor. The MAS sample rotation speed was 20 kHz. Single-pulse

Chapter 2

22

excitation was used with a 18° pulse of 1 μs and an interscan delay of 1 s. The relaxation time

was 1 s and the pulse length was 1 s.

95Mo NMR measurements were carried out on a Agilent 850 NMR spectrometer at

a 95

Mo NMR frequency of 55 MHz. MAS NMR spectra of 95

Mo-enriched Mo/HZSM-5

zeolites were recorded using a 4-mm MAS probehead and a sample rotation rate of 8 or

16 kHz. Reference spectra of MoO3 and Al2(MoO4)3 were recorded with a MAS rate of

40 kHz by use of a 1.6 mm MAS probehead. To obtain the strongest possible 95

Mo NMR

signal, spinning sideband double-frequency sweep (SSDFS) signal enhancement was used

prior to the observation pulse. To remove probe ringing effects from the spectra, the free

induction decays were left-shifted 10 points (20 μs at a spectral width of 500 kHz) prior to

applying the Fourier transformation.

Transmission electron micrographs were obtained with a FEI Tecnai 20 transmission

electron microscope (TEM) at an electron acceleration voltage of 200 kV. Typically, a small

amount of sample was suspended in ethanol, sonicated and dispersed over a Cu grid with a

holey carbon film. Scanning electron microscopy (SEM) was performed using a Philips

environmental FEIXL-30 ESEM FEG in high-vacuum mode at low voltage.

Weight-loss curves of spent catalysts after 12 h on stream in methane aromatization were

measured by thermogravimetric analysis (TGA) using a Mettler Toledo TGA/DSC 1

apparatus. Samples were heated in uncovered alumina crucibles at a rate of 5 K/min to

1023 K in a He/O2 mixture containing 33.3 vol% O2.

2.2.3. Catalytic activity measurements

An amount of 0.5 g of catalyst was introduced in a tubular quartz reactor with a length of

490 mm and an internal diameter of 4.0 mm. The catalyst was supported on a quartz wool

plug in the isothermal zone of the oven. All gases were fed using thermal mass controllers.

The temperature was increased at a rate of 5 K/min to 973 K in a He gas flow of 25 ml/min.

The reaction was started by switching the reactor feed to a N2/CH4(5 vol% N2, internal

standard) mixture at a WHSV of 1710 ml CH4/gcat h. Products were analyzed by an online

Interscience CompactGC gas chromatograph equipped with three analysis channels for

separate analysis of light gases (Molsieve 5A, TCD), light hydrocarbons (Al2O3/KCl, TCD)

and aromatics (Rtx-1, FID).

Chapter 2

23

2.3. Results and discussion

2.3.1. Preparation of hierarchical HZSM-5

To introduce mesoporosity in HZSM-5, the desilication procedure of Groen et al. was

employed [20]. We optimized this procedure in order to obtain a hierarchical HZSM-5 zeolite

with a Si/Al ratio similar to that of a conventional bulk HZSM-5 (Si/Al ≈ 20) for proper

comparison of catalytic performance in the MDA reaction. We also aimed to retain the high

crystallinity and Brønsted acidity of the parent sample as much as possible. HZSM-5 zeolites

with varying Si/Al ratios were treated with NaOH solutions at different conditions. Table 2.1

summarizes the most important physico-chemical properties as a function of the treatment

procedure. All parent zeolites already contain a small amount of mesopores. These mesopores

are predominantly related to interparticle voids. NaOH treatment increased the mesopore

volume. It is also seen that the mesopore volume increased with decreasing Al content of the

starting zeolite. For instance, treatment of HZSM-5(20) with 0.6 M NaOH for 0.5 h at 358 K

did not generate additional mesoporosity as compared with the parent zeolite, whereas

substantial mesoporosity was introduced in HZSM-5(40), concomitant with a strong decrease

of the XRD crystallinity. These trends are consistent with those reported by Groen et al. [20].

From Table 1, we selected treatment of treatment of HZSM-5(30) with a 0.2 M NaOH

solution for 0.5 h at 338 K as a suitable method to prepare the desired hierarchical starting

zeolite. This HZSM-5(meso) zeolite has a relatively high mesopore volume of 0.27 cm3/g and

its crystallinity loss due to the base treatment is minor. The physico-chemical properties of the

material prepared following the optimized desilication procedure (ZSM-5(meso)) were

characterized in more detail and are listed in Table 2.2.

Chapter 2

24

Table 2.1. Physico-chemical properties of parent and desilicated ZSM-5 zeolites.

Sample Si/Alparent CNaOH

(M)

Temperature

(K)

time

(h)

CRXRD1

(%)

Vmeso

(cm3/g)

Si/Alfinal

ZSM-5 21.3 - - - 100 0.03 21.3

ZSM-5(meso) 21.3 0.2 338 0.5 106 0.03 -

21.3 0.2 338 2.0 105 0.04 -

21.3 0.2 358 0.5 106 0.04 -

21.3 0.6 338 0.5 90 0.04 -

21.3 0.6 338 2.0 50 0.43 -

21.3 0.6 358 0.5 45 - -

ZSM-5 28.2 - - - 100 0.08 28.2

ZSM-5(meso) 28.2 0.2 338 0.5 89 0.27 19.5

28.2 0.2 338 2.0 83 0.36 17.2

28.2 0.2 358 0.5 83 - -

28.2 0.6 338 0.5 87 0.17 12.6

28.2 0.6 338 2.0 70 0.24 -

ZSM-5 43.6 - - - 100 0.11 43.6

ZSM-5(meso) 43.6 0.2 338 0.5 97 0.24 35.5

43.6 0.2 338 2.0 78 0.49 -

43.6 0.2 358 0.5 88 0.38 29.7

43.6 0.2 358 2.0 83 - -

43.6 0.6 338 0.5 22 0.9 9.4 1 Crystallinity.

Table 2.2. Physico-chemical properties of ZSM-5(meso) prepared under optimized

desilication conditions and the untreated ZSM-5 counterparts.

Sample Si/Alfinal

Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

CRXRD1

(%)

Particle

size2

(μm)

ZSM-5 21.3 0.13 0.03 141 15 100 0.5

ZSM-5(meso) 19.5 0.14 0.31 147 114 89 0.8

ZSM-5 28.2 0.13 0.06 195 74 100 1.5 1 Crystallinity.

2 Particle size determined by SEM analysis.

2.3.2. Optimization Mo loading procedure

Mo was loaded onto the parent HZSM-5(20) zeolite by incipient wetness impregnation of

an AHM solution and by physical mixing with MoO3. The aim was to obtain a highly

dispersed Mo-oxide precursor phase and, at the same time, limit the extraction of framework

Al (FAl). The calcination temperature, calcination time and the Mo precursor (AHM vs.

MoO3) were varied. The Al distribution in the Mo-containing zeolites was investigated

by 27

Al MAS NMR spectroscopy (Fig. 2.1). The spectra contain four bands at chemical shifts

Chapter 2

25

of 55 ppm, 14 ppm, 0 ppm and −11 ppm. The feature at 55 ppm is due to FAl. The peak

around 0 ppm originates from extraframework Al (EFAl) atoms in octahedral coordination.

The signals at 14 and −11 ppm are due to Al2(MoO4)3, which are also part of the EFAl

phase [27-30]. The relative contributions of the various Al species obtained by deconvolution

of the NMR spectra are presented in Table 2.3. In HZSM-5, Al is predominantly present as

FAl. After impregnation with AHM and drying, a small amount of Al2(MoO4)3 was observed.

This phase was not observed upon physical mixing of MoO3 with the zeolite. All of these

samples contained no or at most a very small amount of Al2(MoO4)3, as long as the

calcination temperature was lower than 823 K. Above this temperature, the amount of

Al2(MoO4)3 increased significantly, and at the same time, the amount of EFAl species

characterized by the 0 ppm feature increased. The NMR spectra also show that the amount of

FAl species decreased, indicating that dealumination of the framework occurred [26].

Fig. 2.1 27

Al MAS NMR spectra of (left) AHM impregnated HZSM-5 with a) HZSM-5, b)

Mo/HZSM-5 (0 h, 298 K), c) Mo/HZSM-5 (5 h, 773 K), d) Mo/HZSM-5 (8 h, 773 K), e)

Mo/HZSM-5 (5 h, 823 K), f) Mo/HZSM-5 (5 h, 873 K) and g) Mo/HZSM-5 (5 h, 973 K). 27

Al MAS NMR spectra of (right) of MoO3 physical mixed HZSM-5 with a) HZSM-5,

Mo/HZSM-5 (0 h, 298 K), c) Mo/HZSM-5 (5 h, 773 K), d) Mo/HZSM-5 (5 h, 823 K), e)

Mo/HZSM-5 (5 h, 873 K) and f) Mo/HZSM-5 (5 h, 973 K). The information in between the

parentheses represents calcination time and calcination temperature.

Table 2.3 also summarizes the crystallinities of the Mo-containing zeolites as determined

by XRD and IR. The parent HZSM-5(20) zeolite is highly crystalline. Impregnation with an

100 50 0 -50

Inte

nsit

y (

a.u

.)

Chemical shift (ppm)

100 50 0 -50

Chemical shift (ppm)

Chapter 2

26

AHM solution and drying decreased the crystallinity. Physical mixing of HZSM-5 with

MoO3 did not affect the crystallinity when the calcination temperature was below 873 K.

Above this temperature, the crystallinity decreased. Together with the NMR data, we

conclude that extraction of Al by Mo-oxide resulting in Al2(MoO4)3 is the likely reason for

partial structural collapse of the zeolites.

Table 2.3. Physico-chemical properties of the Mo/ZSM-5 zeolite catalysts.

Sample Mo

introduction1

Tcalc

(K)

tcalc

(h)

CRXRD

(%)

CIR

(%)

AlIV

(%)

AlVI

(%)

AlAl2Mo2O4

(%)

ZSM-5 - - - 100 100 94 6 0

- 973 5 97 102 - - -

Mo/ZSM-5 IMP - - 80 97 82 16 3

IMP 773 2 81 101 - - -

IMP 773 5 81 93 70 21 9

IMP 773 8 81 93 73 20 7

IMP 823 5 78 110 82 14 4

IMP 873 2 49 74 - - -

IMP 873 5 46 60 53 29 18

IMP 873 8 47 55 - - -

IMP 973 2 23 55 - - -

IMP 973 5 33 53 43 23 34

IMP 973 8 28 49 - - -

Mo/ZSM-5 PM - - 102 102 95 5 0

PM 773 2 86 98 - - -

PM 773 5 77 96 72 18 10

PM 773 8 67 95 - - -

PM 823 5 82 98 79 20 1

PM 873 2 52 77 - - -

PM 873 5 32 51 49 29 22

PM 873 8 24 39 - - -

PM 973 2 28 49 - - -

PM 973 5 20 37 40 23 37

PM 973 8 22 40 - - -

ZSM-5(Meso) - - - 89 - 78 22 -

Mo/ZSM-5(Meso) IMP 823 5 89 - 66 25 9 1IMP = impregnation with an AHM solution, PM = physical mixing of MoO3 with zeolite.

Transmission electron micrographs of the impregnated sample calcined at 973 K points to

partial destruction of the zeolite (Fig. 2.2c) and formation of mesopores. Some of the cavities

created in the zeolite crystal are large enough to be seen in SEM images (Fig. 2.2f). Prolonged

calcination did not further change the morphology. The TEM images also point to better

dispersion of the Mo-oxide phase in the impregnated samples compared with the physically

Chapter 2

27

mixed samples (Fig. 2.2b). TEM images of the physically mixed sample show oval-shaped

particles with a typical size of ∼10 nm on the external zeolite surface.

Fig. 2.2. Transmission electron microscopy micrographs of Mo/HZSM-5 prepared by MoO3

(a) Mo/HZSM-5 (5 h, 773 K) and AHM impregnation (b) Mo/HZSM-5 (5 h, 773 K), (c)

Mo/HZSM-5 (5 h, 873 K), (d) Mo/HZSM-5 (5 h, 973K). Scanning electron micrographs of

(e) Mo/HZSM-5 (5 hours, 773K) and (f) Mo/HZSM-5 (5 h, 973K). The information in

between the parentheses represent calcination time and calcination temperature.

From all these observations, we selected AHM impregnation followed by calcination at

823 K for 5 h as the preferred method for the preparation of the Mo/HZSM-5 and applied it to

HZSM-5(meso). The XRD crystallinity of the parent HZSM-5(meso) was not affected by

introduction of Mo. However, the NMR spectrum of Mo/HZSM-5(meso) shown in Fig.

1 reveals that greater amounts of EFAl including Al2(MoO4)3formed upon calcination at

823 K compared with Mo/ZSM-5. Deconvolution of the relevant spectrum confirms this

(Table 2.3).

We also investigated the state of Mo as a function of the Mo precursor for HZSM-5 and

HZSM-5(meso) by 95

Mo MAS NMR spectroscopy. Fig. 2.3 shows NMR spectra of

Mo/HZSM-5 and Mo/HZSM-5(meso) with Mo introduced in two different ways. Spectra of

bulk MoO3 and Al2(MoO4)3 are included for comparison. The NMR detectable isotope 95

Mo

has a quadrupolar spin 5/2 and a low gyromagnetic ratio. To obtain the highest possible

signal, 95

Mo-enriched samples were prepared using 95

Mo-labeled MoO3 as the primary Mo

source. In addition, the 95

Mo NMR was run at ultrahigh magnetic field (20 T) and a specific

Chapter 2

28

pulse sequence (SSDFS) was used to enhance the NMR signal. All co-calcined mixed

Mo/HZSM-5 samples (Fig. 2.3b and c) show the same lineshape as bulk MoO3 (Fig. 2.3a).

This lineshape results from the second-order quadrupolar line-broadening, which cannot be

completely averaged by magic-angle spinning. The quadrupolar MAS NMR lineshape

observed is consistent with the one observed by Hu et al. [30], apart from a minor signal at

−73 ppm in the spectra of bulk MoO3 and co-calcined microporous Mo/HZSM5 (Figs. 2.3a

and b). This minor signal may reflect a Mo impurity in MoO3 with higher oxygen

coordination symmetry, because its chemical shift equals the isotropic shift (−73 ppm) of the

main signal component without the quadrupolar effect. The minor signal is not visible for co-

calcined mesoporous Mo/HZSM-5 (Fig. 3c), but this sample shows broad weak spectral

features in the range where one expects Al2(MoO4)3 features (Fig. 2.3d, [32]), which points to

formation of Al-O-Mo species at the internal surface of the mesopores. The spectra of the

Mo-impregnated zeolites are entirely different. For the impregnated microporous Mo/HZSM-

5 (Fig. 2.3e) a signal is observed around 0 ppm, which is practically the position of

(NH4)2MoO4 dissolved in water. It thus appears that impregnation followed by calcination

yields different MoOx species than the bulk-like MoO3 species resulting from physical mixing

and co-calcination. The signal is relatively narrow, suggesting fairly monodispersed Mo

species with highly symmetric oxygen coordination. In contrast, the 95

Mo MAS NMR signal

of the impregnated mesoporous Mo/HZSM-5 zeolites is extremely broad. This is indicative of

a broad variation of MoOx species with low oxygen coordination symmetry attached to the

internal surface of the mesopores.

Chapter 2

29

Fig 2.3. (thick lines) 95

Mo MAS NMR spectra of (a) bulk MoO3, (b,c) 5 wt.% 95

MoO3

physically mixed and co-calcined with (b) HZSM-5 and (c) HZSM-5(meso), (d) bulk

Al2(MoO4)3, (e,f) (NH4)2MoO4 -impregnated and calcined (e) microporous and (f) HZSM-

5(meso). The thin-line spectrum inserted below (a) reflects the simulated lineshape for a 95

Mo

spin with quadrupole-tensor asymmetry parameter h = 0.3. Spectra (a) and (b) contain an

additional signal at ~70 ppm, and spectrum (c) indicates the possible presence of weak signals

in the range of aluminium molybate (d). The shift range in (f) is larger than in (a-e). Sample

rotation rate: (a and d) 40 kHz, (b, c, e, f) 8 kHz; spinning sidebands are marked with a star.

2.3.3. Silylation

In order to lower the acidity of the external zeolite surface, its hydroxyl groups were

silylated. We employed the method described by Zheng et al. using TEOS [4]. The effect of

silylation was first investigated by inspection of the hydroxyl stretching regions of the IR

spectra (Fig. 2.4). Bands around 3610 cm−1

, 3665 cm−1

and 3740–3745 cm−1

are attributed to

bridging hydroxyl groups (denoted as BAS, Brønsted acid sites, further on), extraframework

hydroxyl groups and silanol groups, respectively. Comparison of the spectra of HZSM-5 and

HZSM-5(meso) shows that the mesoporous zeolite contains much more silanol groups. Also,

a more intense background extending to lower wavenumbers due to hydrogen-bonded silanol

groups is evident for this sample. On top of that, HZSM-5(meso) also contains a small

100 0 -100 -200 -300 -400 -500

95Mo NMR shift (ppm)

1000 500 0 -500 -1000

Chapter 2

30

amount of EFAl-related OH species, which are not present in HZSM-5. The introduction of

Mo by AHM impregnation and calcination led to relatively small changes in the IR spectra. A

small loss of BAS is noted. Silylation affected the IR spectra more substantially in the sense

that the silanol density strongly decreased. The decrease was more pronounced for

Mo/HZSM-5. These changes in the silanol band intensity suggest that the introduced Mo

reacted with the silanol groups. The reaction of Mo-oxides with silanol groups on silica upon

calcination has been described before [32]. It leads to dispersion of Mo-oxides into smaller

clusters such as tetrahedral di-oxo, pentahedral mono-oxo as well as polymeric species [33,

34]. It also causes a further decrease of the concentration of BAS. Repetitive silylation and

calcination had a different effect and resulted in a higher concentration of BAS, possibly due

to reinsertion of earlier formed EFAl into the zeolite framework under the influence of the

additional silicon species. When the parent zeolites were first silylated and then co-calcined

with physically admixed MoO3, the spectra were very similar to those of the two starting

materials.

Fig. 2.4. Infrared spectra in the hydroxyl region of (left) bulk HZSM-5 zeolite and (right)

mesoporous HZSM-5 zeolite with (a) the parent zeolite, (b) Mo/HZSM-5, (c) Mo/HZSM-

5(Mo,Si1), (d) Mo/HZSM-5(Mo,Si3) and (e) Mo/HZSM-5(Si1,Mo).

3800 3600 3400 3200

Ab

so

rban

ce (

a.u

.)

Wavenumber (cm-1)

3800 3600 3400 3200

Wavenumber cm-1

Chapter 2

31

2.3.4. Catalyst characterization

The resulting two sets of catalysts were characterized in more detail so as to obtain an

overview of the changes brought about by Mo introduction and silylation. The BAS and

Lewis acid sites (LAS) contents were determined by IR spectroscopy of adsorbed pyridine.

The presence of external acid sites was probed by 2,4,6-trimethylpyridine. In the latter case,

the absorption bands related to 2,4,6-trimethylpyridine coordination to LAS (1633 cm−1

) and

BAS (1638 cm−1

) overlap and attempts to deconvolute these spectra did not result in

meaningful results. Therefore, we present only the values for the total acid site densities on

the external surface. The results are collected in Table 2.4. The total acidity of HZSM-5 is

consistent with the total Al content. The mesoporous zeolite have slightly less BAS than

HZSM-5. The bulk BAS concentration is lowered, which can be attributed to the introduction

of mesopores. Kox et al. [35] have shown that mesopore introduction by silicon extraction

results in siliceous mesopore walls. The amount of external acid sites in HZSM-5(meso) is

only slightly higher than in HZSM-5, despite the much higher combined mesopore and

external surface area.

Chapter 2

32

Table 2.4. Textural and physical properties of the prepared silylated catalysts.

Sample Vmic

(cm3/g)

Vmeso

(cm3/g)

Smic

(m2/g)

Smeso

(m2/g)

Cryst.XRD1

(%)

Si/Al

(XPS)

Si/Mo

(XPS)

Al

(wt%)

Mo

(wt%)

NBAS

(μmol/g)

NLAS

(μmol/g)

Nacid,ext.

(μmol/g)

HZSM-5 0.13 0.03 141 15 100 22 ∞ 2.2 ∞ 679 214 61

Mo/HZSM-5 0.07 0.07 113 49 86 49 3 2.1 4.12 533 272 51

Mo/HZSM-5(Mo,Si1) 0.04 0.02 123 15 90 58 6 2.0 3.53 314 127 21

Mo/HZSM-5(Mo,Si3) 0.09 0.02 149 14 95 30 15 1.7 3.22 659 117 34

Mo/HZSM-5(Si1,Mo) 0.11 0.02 255 18 96 31 112 1.9 1.96 639 118 25

HZSM-5(Si1) 0.13 0.02 247 16 - - - - - 697 339 11

HZSM-5(Meso) 0.14 0.31 147 114 89 15 ∞ 2.2 ∞ 544 182 66

Mo/HZSM-5(Meso) 0.07 0.23 69 105 89 20 55 2.0 3.97 471 211 71

Mo/HZSM-5(Meso,Mo,Si1) 0.13 0.34 90 106 90 30 39 2.0 3.57 356 239 69

Mo/HZSM-5(Meso,Mo,Si3) 0.13 0.21 132 79 94 23 26 1.5 3.06 375 144 43

Mo/HZSM-5(Meso,Si1,Mo) 0.11 0.19 120 74 92 16 ∞ 1.7 4.35 554 164 66 1 Parent HZSM-5(30) was used as reference zeolite to determine the XRD crystallinity of HZSM-5(meso) and the zeolite materials derived from

HZSM-5(meso).

Chapter 2

33

Modification of the parent HZSM-5 and HZSM-5(meso) with Mo decreased the BAS

content due to the exchange of protons with Mo and/or the extraction of framework Al [36].

At the same time, the LAS concentration increased. The Lewis acidity originates from EFAl,

partly in the form of Al2(MoO4)3, but likely also from Mo-oxo species [37]. Single and triple

silylation of Mo/HZSM-5 zeolites strongly affected the acidity. Silylation resulted in strong

decrease of BAS and LAS concentrations for Mo/HZSM-5. For Mo/HZSM-5(meso), the

same decrease in the BAS concentration is observed, yet the LAS concentration slightly

increased. This is due to the higher dispersion of Mo-oxo species when deposited on HZSM-

5(meso). Triple silylation recovers a significant part of the BAS in Mo/HZSM-5(Mo,Si3).

Such behavior was not observed for Mo/HZSM-5(meso,Mo,Si3). The results for the samples

that were silylated prior to introduction of Mo are different. Silylation did not lower the

internal acidity, and in this case the external acidity was substantially lowered. The same

effect is seen when silylation was done on HZSM-5 (HZSM-5(Si1)). It suggests that the less

pronounced changes in the external acidity content upon silylation for the Mo-containing

zeolites is caused by the Lewis acidity of the dispersed Mo-oxide phase.

The textural properties determined by Ar physisorption are also reported in Table 2.4.

Introduction of Mo led to smaller micropore volumes of HZSM-5 and HZSM-5(meso). The

micropore volume of Mo/HZSM-5 decreased further upon silylation. Silylation of the parent

HZSM-5 itself did not decrease the micropore volume in line with literature data [21]. Thus,

the presence of the Mo-oxide phase in Mo/HZSM-5 affects the silylation process. Together

with the changes in the concentration of BAS upon silylation for HZSM-5 and Mo/HZSM-5,

we infer that silylation of the internal zeolite surface is more substantial for the Mo-containing

sample. This possibly relates to the lower silanol density at the external surface after Mo

introduction. The presence of the additional mesoporosity induced by the Mo deposition and

calcination processes (Table 2.4) may also facilitate the diffusion of the silylating agent inside

the zeolite crystals. Repetitive silylation resulted in a higher micropore volume and a decrease

of the mesopore volume. A minor decrease in micropore volume was observed for Mo/ZSM-

5(Si1,Mo). All of these changes are trendwise similar but less pronounced for the mesoporous

HZSM-5 zeolite.

The wide-angle XRD patterns (Fig. 2.5) recorded for most of the zeolites were similar to

the pattern of HZSM-5(20). The patterns of Mo/HZSM-5(Si1,Mo) and Mo/HZSM-

5(meso,Si1,Mo) contain additional reflections at 12.8°, 25.7° and 38.9°, which belong to bulk

MoO3. This result points to the low dispersion of the Mo-oxide phase after physical mixing

Chapter 2

34

with MoO3. In all cases, the relatively small changes in the intensities of the XRD reflections

belonging to MFI show that crystallinity was only affected slightly upon Mo introduction

(Table 2.4).

Fig. 2.5. Wide angle XRD patterns of (left) bulk HZSM-5 zeolite and (right) mesoporous

HZSM-5 zeolite with (a) the parent zeolite, (b) Mo/HZSM-5, (c) Mo/HZSM-5(Mo,Si1), (d)

Mo/HZSM-5(Mo,Si3) and (e) Mo/HZSM-5(Si1,Mo).

Fig. 2.6 shows the 27

Al MAS NMR spectra of these zeolite catalysts. The samples

derived from the microporous ZSM-5 zeolite contained mainly FAl species with a small

fraction present as EFAl. None of these samples contained Al2(MoO4)3. The FAl peak is seen

to decrease upon introduction of Mo-oxide and silylation. Concomitantly, the tetrahedral Al

feature becomes broader. Tessonier et al. ascribed this behavior to the changing chemical

environment of the Al species upon exchange of Mo-oxo species with the BAS [26]. The

NMR spectrum of Mo/ZSM-5(Si1,Mo) is similar to that of the parent HZSM-5, indicating

that Mo did not affect the BAS as is clearly visible for the other samples. Al2(MoO4)3 was

only present in the Mo/HZSM-5(meso) samples. In all Mo-containing samples prepared by

impregnation, the FAl signal is much weaker and broadened upon Mo introduction and

further silylation. All this indicates that a portion of the BAS was replaced by Mo-oxo

species, resulting in perturbation of the symmetry around the tetrahedral Al species, making

them partially NMR-invisible.

10 20 30 40 50 60

cp

s (

a.u

.)

Angle (2)

10 20 30 40 50 60

cp

s (

a.u

.)

Angle (2)

Chapter 2

35

Fig. 2.6. 27

Al-MAS-NMR of (left) bulk HZSM-5 zeolite and (right) mesoporous HZSM-5

zeolite with (a) the parent zeolite, (b) Mo/HZSM-5, (c) Mo/HZSM-5(Mo,Si1), (d)

Mo/HZSM-5(Mo,Si3) and (e) Mo/HZSM-5(Si1,Mo).

XPS was used to study the external region of the zeolite crystals. Table 2.4 lists the

atomic Si/Al and Si/Mo ratios. The bulk Mo and Al contents are also tabulated here. It was

found that the Si/Al ratio in the surface region increased upon introduction of Mo. This

suggests that the Al present in the surface region preferentially interacts with the Mo-oxo

phase. For both series, triple silylation resulted in a decrease of the Si/Al ratio. These changes

were less pronounced for the sample that were first silylated. The changes in the Si/Mo ratios

upon silylation point to loss of Mo-oxide dispersion. The sample with the lowest Mo-oxide

dispersion has the highest XPS Si/Mo ratio. Note that the changes in Al and Mo content show

the expected trend following deposition of Si by the silylation steps. The Mo content of

Mo/ZSM-5(Si1,Mo) is lower than expected.

The nature of the Mo-oxide phase was characterized by UV–Vis and UV Raman

spectroscopy. The UV–Vis spectra shown in Fig. 2.7 contain bands characteristic for different

forms of Mo-oxide [38]. The absorption bands between 200 and 400 nm are assigned to

ligand to metal CT transitions (O2−

→ Mo6+

). CT bands in the range 210–250 nm and around

280 nm have typically been assigned to isolated monomeric Mo-oxo species [39, 40]. These

bands are related to tetrahedral Mo species, while those at higher wavelengths (300–330 nm)

are due to isolated Mo-oxo centers in octahedral geometry [41]. Jezlorowski and Knözinger

100 50 0 -50

Chemical shift ppm

100 50 0 -50

Inte

nsit

y (

a.u

.)

Chemical shift (ppm)

Chapter 2

36

contended that the band around 280 nm is more likely due to Mo-O-Mo containing

structures [42]. The tail into the 400–450 nm region is indicative for the presence of larger

molybdate clusters or MoO3crystallites [43]. Fig. 2.7 shows that single silylation of

Mo/HZSM-5 resulted in an overall increase of the intensity of the UV–Vis spectrum. This

sample contains more tetrahedral Mo-oxo species. Triple silylation resulted in Mo/ZSM-

5(Mo,Si3), whose spectrum contains less intense bands of highly dispersed Mo species and

more intense bands in the 400–450 nm range. This points to agglomeration of the Mo-oxide

phase, consistent with the XPS Si/Mo trend. The spectrum of Mo/ZSM-5(Si1,Mo), obtained

by silylation prior to Mo introduction, looks very different. The comparatively low intensities

in the molybdate region below 330 nm and the intense feature around 400–450 nm show that

Mo is mainly present as MoO3. This is due to the hydrophobic nature of the external surface

of the silylated HZSM-5(Si1) zeolite. The spectral trends for the mesoporous zeolites were

very similar and are therefore not discussed in detail.

Fig. 2.7. UV–Vis of (left) bulk HZSM-5 zeolite and (right) mesoporous HZSM-5 zeolite with

Mo/HZSM-5 (solid line), Mo/HZSM-5(Mo,Si1) (dashed line), Mo/HZSM-5(Mo,Si3) (dotted

line) and Mo/HZSM-5(Si1,Mo) (dashed-dotted line).

The zeolites were also investigated by UV Raman spectroscopy. The spectra obtained by

excitation with 244 and 325 nm lasers are shown in Fig. 2.8 and Fig. 2.9, respectively. Due to

the resonance Raman effect, the use of specific laser excitation lines enhances the spectral

features of highly dispersed, isolated tetrahedral molybdates (244 nm) and polymeric

200 300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Ab

so

rban

ce

Wavenumber cm-1

200 300 400 500 600 700 800

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Wavenumber cm-1

Chapter 2

37

octahedral molybdates (325 nm). It should also be noted that the use of the 244 nm laser will

probe more of the surface region of the zeolite crystals than the use of the 325 nm laser,

whose light will penetrate deeper into the crystals [44]. For comparison, the Raman spectra of

bulk MoO3and Al2(MoO4)3 were also recorded.

Raman spectra for the microporous zeolites recorded upon excitation with the 244 nm

laser are shown in Fig. 2.8. The spectrum of HZSM-5 shows typical bands of MFI

zeolite [45]. Modification of HZSM-5 with Mo (Mo/HZSM-5) and silylation (Mo/HZSM-

5(Si1,Mo) and Mo/HZSM-5(Si3,Mo)) resulted in substantial spectral changes. The peaks

characteristic for HZSM-5 are not clearly observed anymore. Instead, new molybdenyl

stretching mode bands (960–970 cm−1

and 980–995 cm−1

[46-48]) are present due to small

Mo-oxo species [49-52]. The decreased intensity of the HZSM-5 bands and appearance of

Mo-oxo bands points to the coverage of the zeolite external surface with a Mo-oxide phase. It

is worthwhile to discuss the differences in the spectra following various treatments. The

spectrum of Mo/HZSM-5 contains a pronounced feature at 970 cm−1

due to the Mo=O stretch

of dimeric Mo structures inside the micropores [53] with a shoulder at 953 cm−1

(highly

dispersed octahedral molybdenum surface species [54]) on top of the broad 960–

970 cm−1

band. The peak at 970 cm−1

is not visible for Mo/HZSM-5(Mo,Si1) and Mo/HZSM-

5(Mo,Si3). The shoulder at 953 cm−1

has lower intensity for Mo/HZSM-5(Mo,Si1) and is

absent for Mo/HZSM-5(Mo,Si3). Bands due to aluminum molybdates are not observed. The

small feature at 360 cm−1

is a Mo-O bending mode [39, 55]. The subtle changes upon

silylation point to Mo-oxide agglomeration. The spectrum of Mo/HZSM-5(Si1,Mo) is very

different and resembles the spectrum of HZSM-5. This is caused by the low dispersion of the

MoO3 phase, so that Raman spectroscopy mainly probes the surface of the zeolite crystals.

The spectra for HZSM-5(meso) and thereof derived Mo-containing samples are collected

in Fig. 2.8. Different from the spectra for bulk HZSM-5, these contain a shoulder at

1025 cm−1

indicative of the presence of Al2(MoO4)3 [56], and a shoulder at 953 cm−1

of highly

dispersed octahedral molybdenum species. Upon single silylation of Mo/HZSM-5(meso,Mo)

a new feature at 849 cm−1

appeared due to β-MoO3 [57]. This is an intermediate phase during

transformation of amorphous Mo-oxides towards thermodynamically more stable α-

MoO3[57]. After further silylation the β-MoO3 peak was not observed anymore.

Chapter 2

38

Fig. 2.8. UV-Raman spectra recorded with a 244 nm laser of (left) bulk HZSM-5 and (right)

mesoporous HZSM-5 with (a) HZSM-5 (b) Mo/HZSM-5, (c) Mo/HZSM-5(Mo,Si1), (d)

Mo/HZSM-5(Mo,Si3), (e) Mo/HZSM-5(Si1,Mo), (f) bulk MoO3 and (g) bulk Al2(MoO4)3.

The spectra recorded with a 325 nm excitation laser are shown in Fig. 2.9. Due to

fluorescence interference, no clear features could be observed for Mo/HZSM-5(Si1,Mo). The

spectrum of Mo/HZSM-5 strongly resembles the spectrum of amorphous Mo-oxide

phases [57]. It is characterized by weak and broad bands at ∼860 cm−1

and ∼960 cm−1

. After

single and triple silylation of Mo/HZSM-5 peaks became visible at 280, 336 and 995 cm−1

.

They point to the formation of a microcrystalline α-MoO3 phase upon silylation [53, 57]. The

band at 820 cm−1

can be ascribed to α-MoO3 in close contact with the support surface[53, 57].

The spectrum still contains weak bands around 860 and 960 cm−1

due to amorphous Mo-

oxides. The spectra for the HZSM-5(meso)-derived samples are given in Fig. 2.9. The

findings are very similar with those for the microporous zeolite, albeit that the agglomeration

of dispersed Mo-oxo species into α-MoO3 was less pronounced for the mesoporous

Mo/HZSM-5 set. The latter is consistent with the observation of the intermediate β-

MoO3 phase in Fig. 2.8.

200 400 600 800 1000

Inte

nsit

y (

a.u

.)

Wavenumber (cm-1)

200 400 600 800 1000

Wavenumber (cm-1)

Chapter 2

39

Fig. 2.9. UV-Raman spectra recorded with a 325 nm laser of (left) bulk HZSM-5 and (right)

mesoporous HZSM-5 with (a) HZSM-5 (b) Mo/HZSM-5, (c) Mo/HZSM-5(Mo,Si1), (d)

Mo/HZSM-5(Mo,Si3), (e) Mo/HZSM-5(Si1,Mo), (f) bulk MoO3 and (g) bulk Al2(MoO4)3.

Careful analysis of UV–Vis and UV Raman spectra of Mo/HZSM-5 and Mo/HZSM-

5(meso) highlights specific changes of the Mo-oxide phase during silylation. The main effect

is transformation of small Mo-oxo species into larger crystallites upon extensive silylation.

The silylating agent competes with the Mo species for interaction with the zeolite surface. The

data show that the Mo-oxide phase is initially better dispersed in Mo/HZSM-5(meso)

compared with Mo/HZSM-5. This relates to the higher external (mesopore) surface area. The

higher dispersion also retards formation of α-MoO3 in comparison to the microporous zeolite

samples. As a consequence, the samples contain some β-MoO3. Silylation prior to Mo

introduction resulted in poor dispersion of the Mo-oxide phase.

The morphology of the Mo phase in the microporous and mesoporous sample series was

analyzed by TEM. The micrograph of Mo/HZSM-5 (Fig. 2.10a) revealed the presence of dark

oval-shaped spots at the crystal surface due to the Mo-oxide phase. After single silylation

treatment of Mo/HZSM-5 (Mo/HZSM-5(Mo,Si1)), it is difficult to observe these particles

(Fig. 2.10b). It points to the redispersion Mo-oxide into small particles. Inspection of the

micrographs after triple silylation (Mo/HZSM-5(Mo,Si3)) shows that larger particles were

formed (Fig. 2.10c). These particles are similar in size as those in Mo/HZSM-5. The

agglomeration of the initially small MoO3 particles is probably related to the weak interaction

200 400 600 800 1000

In

ten

sit

y (

a.u

.)

Wavenumber (cm-1)

200 400 600 800 1000

Wavenumber (cm-1)

Chapter 2

40

between the Mo phase and the zeolite surface after desilication. The micrograph of

Mo/HZSM-5(Si1,Mo) contains a large particle at the zeolite surface. Formation of relatively

large MoO3 crystallites is consistent with the XRD patterns. The TEM micrographs recorded

for the mesoporous sample series show the presence of large, agglomerated MoO3 particles.

The trends upon silylation are similar to those observed for the microporous sample. For

instance, single silylation of mesoporous Mo/HZSM-5 led to redispersion of the oval-shaped

Mo-oxide into smaller particles (inset in Fig. 2.10f). The Mo dispersion in Mo/HZSM-

5(Si1,Mo) was low (Fig. 10h), which agrees with the XRD findings.

Fig. 2.10. Transmission electron microscopy micrographs of bulk HZSM-5 zeolite of (a)

Mo/HZSM-5, (b) Mo/HZSM-5(Mo,Si1), (c) Mo/HZSM-5(Mo,Si3) and (d) Mo/HZSM-5. The

TEM micrographs of mesoporous HZSM-5 zeolite of (a) Mo/HZSM-5, (b) Mo/HZSM-

5(Mo,Si1), (c) Mo/HZSM-5(Mo,Si3) and (d) Mo/HZSM-5.

2.3.5. Catalytic activity measurements

The effect of the silylation treatment on the catalytic performance in the MDA was then

studied. The results for the catalysts derived from bulk HZSM-5 are shown in Fig. 2.11. For

all samples, the methane conversion rate and benzene selectivity decreased with time on

stream (Fig. 2.11a). The methane conversion rates were highest for Mo/HZSM-5(Mo,Si1) and

Mo/HZSM-5(Mo,Si3). The deactivation rate was similar for Mo/HZSM-5 and Mo/HZSM-

5(Mo,Si1). Compared with these two samples, triple silylated Mo/HZSM-5(Mo,Si3)

deactivated at a slightly lower rate. The highest benzene selectivities were observed for

Chapter 2

41

Mo/HZSM-5(Mo,Si1) and Mo/HZSM-5(Mo,Si3) with values of ∼80 and ∼70 wt%,

respectively. The total aromatics selectivity including toluene, xylenes and naphthalene for

these two materials were 85 wt% and 81 wt%, respectively. These selectivities were

substantially higher than the selectivities for Mo/ZSM-5 (benzene and total aromatics

selectivities of 50 wt% and 60 wt%, respectively). On the contrary, the methane conversion

rate and benzene/aromatics selectivity for Mo/HZSM-5(Si1,Mo) were lower compared to

Mo/HZSM-5. The selectivity trends displayed in Fig. 2.11b and c further show that the

benzene selectivity decreased with time on stream concomitant with an increase of the

ethylene selectivity. This points to deactivation of BAS. Notably, for Mo/HZSM-5 and

Mo/HZSM-5(Si1,Mo) the aromatics product distribution shifted from benzene towards

naphthalene after prolonged reaction. In comparison, the changes in aromatics product

distribution with time on stream for Mo/HZSM-5(Mo,Si1) and Mo/HZSM-5(Mo,Si3) were

much smaller and less naphthalene was formed.

Chapter 2

42

Fig. 2.11. Methane aromatization reaction rate (a) and selectivities to benzene (open symbols) and naphthalene (closed symbols) (b) and

ethylene (open symbols) and coke (closed symbols) (c) over bulk HZSM-5 with (■) Mo/HZSM-5, (●) Mo/HZSM-5(Mo,Si1), (▲)

Mo/HZSM-5(Mo,Si3) and ( ) Mo/HZSM-5(Si1,Mo).

0 2 4 6 8 10

0

2

4

6

8

10

12

14

CH

4 r

eacti

on

rate

(m

mo

l/h

.gc

at)

Time on stream (h)

0 2 4 6 8 10

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

0 2 4 6 8 10

0

20

40

60

80

100

Time on stream (h)

0

10

20

30

40

Chapter 2

43

Fig. 2.12. Methane aromatization reaction rate (a) and selectivities to benzene (open symbols) and naphthalene (closed symbols) (b) and

ethylene (open symbols) and coke (closed symbols) (c) over mesoporous HZSM-5 with (■) Mo/HZSM-5, (●) Mo/HZSM-5(Mo,Si1), (▲)

Mo/HZSM-5(Mo,Si3) and ( ) Mo/HZSM-5(Si1,Mo).

0 2 4 6 8 10

0

20

40

60

80

100

Time on stream (h)

0

5

10

15

20

25

30

0 2 4 6 8 10

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

0 2 4 6 8 10

0

2

4

6

8

10

12

14

CH

4 r

eacti

on

rate

(m

mo

l/h

.gc

at)

Time on stream (h)

Chapter 2

44

The reactivity trends for the catalysts based on HZSM-5(meso) were similar, albeit that

the overall methane conversion rates of these catalysts were lower and the deactivation was

also more severe (Fig. 2.12). Similar to the bulk Mo/HZSM-5 catalysts silylation of

Mo/HZSM-5(meso) improved the benzene selectivity. However, the magnitude of this effect

was smaller for the mesoporous than for the microporous zeolites. The mesoporous sample

silylated prior to Mo introduction also exhibited higher benzene selectivity than the

Mo/HZSM-5(meso) reference. All of the Mo/HZSM-5(meso) catalysts had lower methane

conversion rates and produced less benzene and more coke than their microporous analogs.

The nature of the carbonaceous deposits formed during reaction was characterized by

oxidation by TGA. Spent samples were retrieved after 12 h on stream in the MDA reaction

(Fig. 2.13). The TG curves contain two main features and a small negative feature around

∼700 K, which is attributed to the adsorption of oxygen on molybdenum carbide species and

their subsequent oxidation [39, 58, 59]. The main oxidation feature around ∼750 K belongs to

aliphatic hydrocarbon species, likely formed in the proximity of Mo (soft coke) [39, 58, 59].

The other feature, which occurs around 840 K, represents hard coke and is likely the result of

polynuclear aromatic species, whose formation is catalyzed by BAS [39, 58, 59]. The results

of deconvolution of these TG curves are given in Table 2.5. Consistent with the selectivity

differences observed in Fig. 2.11 and Fig. 2.12, spent Mo/HZSM-5(meso) zeolites contained

more coke than spent Mo/HZSM-5 zeolites. For Mo/HZSM-5, the amount of coke decreased

with increasing silylation degree. Most notably, it resulted in a strong decrease of the hard

coke, while the amount of soft coke varied much less. The textural properties of these spent

zeolites (Table 2.5) show that, except for Mo/HZSM-5(Mo,Si3), all of the spent Mo/HZSM-5

catalysts did not contain accessible micropores anymore. Spent Mo/HZSM-5(Mo,Si3)

contained much less coke, presumably because of deactivation of the external BAS.

Consistent with this, some of the micropores in the spent sample are still accessible. The spent

Mo/HZSM-5(meso) catalysts contained more coke than their microporous counterparts. Spent

Mo/HZSM-5(meso,Mo,Si1) and Mo/HZSM-5(meso,Mo,Si3) contained also substantially

more soft coke. The lower amount of hard coke, likely due to the weak acidity of the

mesopore walls, is also evident from the less significant decrease of micropore accessibility

for spent Mo/HZSM-5(meso) samples.

Chapter 2

45

Fig. 2.13. TGA weightloss curves of (left) bulk HZSM-5 zeolite and (right) mesoporous

HZSM-5 of (a) Mo/HZSM-5, (b) Mo/HZSM-5(Mo,Si1), (c) Mo/HZSM-5(Mo,Si3) and (d)

Mo/HZSM-5(Si1,Mo).

Table 2.5. Textural properties and coke content (C) of spent Mo/ZSM-5 catalysts after 12 h

on stream in the MDA reaction.

Sample Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

Ctotal

(g/gcat)

Csoft

(g/gcat)

Chard

(g/gcat)

Mo/HZSM-5 0 0.02 0 8 0.13 0.022 0.108

Mo/HZSM-5(Mo,Si1) 0 0.02 0 8 0.10 0.024 0.076

Mo/HZSM-5(Mo,Si3) 0.11 0.02 149 15 0.03 0.030 0

Mo/HZSM-5(Si1,Mo) 0 0.02 0 9 0.10 0.031 0.079

Mo/HZSM-5(meso) 0.05 0.16 21 91 0.19 0.095 0.095

Mo/HZSM-5(meso,Mo,Si1) 0.06 0.11 35 65 0.21 0.187 0.023

Mo/HZSM-5(meso,Mo,Si3) 0.0 0.31 0 179 0.20 0.152 0.048

Mo/HZSM-5(meso,Si1,Mo) 0.05 0.10 45 51 0.16 0.088 0.072

2.3.6. General discussion

One of the goals of the present study was to determine the influence of mesoporosity in

ZSM-5 on the performance of Mo/HZSM-5 catalysts in the dehydroaromatization of methane.

Base leaching was employed to obtain a hierarchical HZSM-5 zeolite with a Si/Al ratio of 20,

comparable in acidity with the acidity of a conventional HZSM-5 zeolite. These two zeolites

formed the basis for the synthesis, characterization and catalytic activity measurements of

Mo/HZSM-5. Silylation of the samples before and after Mo loading was employed as a

method to deactivate the external surface.

600 700 800 900 1000

Weig

htl

oss (

mg

/K)

Temperature (K)

600 700 800 900 1000

Temperature (K)

Chapter 2

46

During Mo modification of HZSM-5 (AHM impregnation vs. mixing with MoO3) the

final physico-chemical properties were strongly influenced by the calcination step. When the

samples were calcined above 823 K in artificial air, it was always seen that extensive

Al2(MoO4)3 formation took place, concomitant with partial destruction of the zeolite

framework. Calcination at lower temperatures prevented dislodging of Al from the framework

in Mo/HZSM-5, when the starting zeolite had only micropores. The use of incipient wetness

impregnation with AHM and calcination at 823 K for 5 h resulted in Mo/HZSM-5 without

Al2(MoO4)3formation and minor loss of Brønsted acidity. The small loss of acidity is likely

due to partial ion-exchange with mobile Mo-oxo complexes [60]. Mo-oxide was mainly

present at the external surface as an amorphous oxide phase in isolated or slightly aggregated

forms. Preparation of Mo/HZSM-5 from mesoporous HZSM-5 led to formation of a greater

amount of Al2(MoO4)3. This is attributed to a higher fraction of molybdates entering the

micropores during impregnation or subsequent as a consequence of the increased

mesopore/external surface area to micropore surface area ratio of the desilicated zeolite.

Comparatively, Mo/HZSM-5 prepared by physical mixing of MoO3 with HZSM-5 always led

to low dispersion of the MoO3phase and there was little influence on the acidity as long as the

calcination temperature did not exceed 823 K.

Upon silylation significant changes were seen in the textural properties, the acidity and

the nature of the Mo-oxide phase. Consecutive silylation steps led to further changes. When

the silylation was done in the conventional manner, that is before Mo loading, it was found

that AHM wetness impregnation was not effective due to the considerable hydrophobicity of

the zeolite support. Therefore, we employed physical mixing with MoO3. Expectedly, the

result was very similar with results for the sample prepared by physical mixing of the starting

zeolite with MoO3. That is, the Mo-oxide dispersion was low and the Mo loading did not

affect the acidity.

A single silylation step of Mo/HZSM-5 resulted in improved dispersion of the Mo-oxide

phase. The increased Mo spreading resulted in a higher fraction of isolated Mo species, a

decrease of the concentration of internal and external BAS as a result of their reaction with

Mo-oxo species and minor decrease of the microporosity. For the mesoporous Mo/HZSM-5

zeolite similar but less pronounced changes were seen. When the silylation procedure was

repeated another two times on the same sample (3 times in total), the samples exhibited very

different properties. More complete silylation resulted in a substantial decrease of the Mo-

oxide dispersion as evidenced by UV–Vis and UV Raman spectroscopy. In addition, it was

Chapter 2

47

seen that, compared with single-silylated Mo/HZSM-5, the Brønsted acidity increased. 27

Al

NMR spectra show an increase of framework Al species. It is speculated that more intensive

silylation led to migration of Mo species from the micropores to the external surface and also

to partial removal of charge-compensating extraframework Al species. All this is consistent

with the increase of the micropore volume. Again, these differences were trendwise similar

for the mesoporous HZSM-5 zeolite, but less pronounced.

Next, we discuss the influence of these changes on the MDA activity. Comparing the

bulk HZSM-5 derived catalysts, it is seen that the poor Mo dispersion in Mo/HZSM-

5(Si1,Mo) is the cause of the relatively low methane conversion rates. The coke selectivity of

this silylated catalyst was also higher than of the other ones. High coke selectivity has been

related to large MoCx particles by Solymosi et al. [61], but can also be interpreted in this

particular case in terms of high Brønsted acidity on the external surface because of the low

Mo dispersion. The improved Mo dispersion in Mo/HZSM-5(Mo,Si1) and Mo/HZSM-

5(Mo,Si3) resulted in higher initial methane conversion rates. These silylated catalysts also

produced much less coke. With increasing silylation degree the amount of hard coke

decreased and it was nearly absent in spent Mo/HZSM-5(Mo,Si3). An important finding is

that the silylated samples displayed much lower rate of naphthalene formation than the non-

silylated ones. We explain this by deactivation of the external BAS, which can catalyze

formation of polynuclear aromatic hydrocarbons.

It was found that a higher proportion of the Mo species interacted with the acid sites in

the micropores of the mesoporous Mo/HZSM-5 zeolites. Although the initial dispersion of the

Mo-oxide phase in mesoporous Mo/HZSM-5 was high, pretreatment in He followed by

carburization resulted in sintering of the Mo-carbide particles, likely because of the siliceous

nature of the mesopore surface. The low dispersion of the Mo-carbide phase resulted in lower

methane conversion rates [62, 63]. These catalysts also produced more coke deposits than the

microporous ones. Analysis of the spent catalysts shows that these deposits were mainly of

the soft coke type. It appears that the coke was predominantly formed on the larger Mo-

carbide particles. The lower hard coke content is consistent with the lower acidity of the

mesopore/external surface.

Chapter 2

48

2.4. Conclusions

The influence of mesopores generated by zeolite desilication and silylation on the

catalytic performance of Mo/HZSM-5 for the dehydroaromatization of methane was

investigated. The base desilication procedure for HZSM-5 was optimized to have a mesopore-

containing ZSM-5 starting material comparable in acidic properties to a bulk HZSM-5 zeolite

(Si/Al = 20). The physico-chemical properties of Mo-modified HZSM-5 (AHM impregnation

or physical mixing with MoO3) were strongly influenced by the calcination step. The

optimum calcination temperature for high Mo dispersion without extensive

Al2(MoO4)3 formation and loss of Brønsted acidity was around 823 K. Mo/HZSM-5 prepared

by physical mixing of MoO3 had low MoO3 dispersion and acidity was not affected upon

calcination at 823 K. Silylation also led to significant changes. A single silylation step of

Mo/HZSM-5 resulted in improved Mo-oxide dispersion, a small loss of the internal and

external acidity and loss of external silanol groups. Repeated silylation and calcination led to

decreased Mo-oxide dispersion because of the high hydrophobicity. Brønsted acidity was

increased upon repetitive silylation which was attributed to removal of extraframework

aluminum exchanged to BAS. For methane dehydroaromatization high dispersion of the Mo-

oxide precursor is beneficial for high methane conversion rate and low rate of coke formation.

High Mo-oxide dispersion of single-silylated Mo/HZSM-5 led to higher methane conversion

rates and lower coke selectivity. Increasing the silylation degree resulted in less hard coke

formation, because the acid sites at the external surface become deactivated. The naphthalene

yield is lowest for the repeatedly silylated catalysts. Introducing mesoporosity in bulk HZSM-

5 does not improve the catalytic performance in methane dehydroaromatization. Both activity

and aromatics selectivity are lower than for bulk Mo/HZSM-5. Also, the effect of silylation is

less favorable for mesoporous Mo/HZSM-5. Characterization of fresh mesoporous

Mo/HZSM-5 showed an improved Mo-oxide spreading. However, when activating the

mesoporous catalyst in He followed by carburization larger Mo-carbide particles are formed,

which results in higher propensity to coke formation. Although mesoporosity does not

improve the catalytic performance of Mo/HZSM-5 for methane dehydroaromatization,

silylation of the Mo/HZSM-5 catalyst improves activity and selectivity to the desired benzene

product. Silylation prior to Mo introduction into bulk ZSM-5 was detrimental to the catalytic

performance.

Chapter 2

49

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Chapter 3

51

Activation of Mo/HZSM-5 for methane aromatization

Summary

The effect of pretreatment of Mo/HZSM-5 at 973 K in inert (He), oxidizing (artificial air) and

carburizing (a CH4/He mixture) atmosphere on its performance in the non-oxidative methane

dehydroaromatization (MDA) was investigated. The influence of post-synthesis silylation to

deactivate external acid sites was also studied. Precarburization resulted in increased

aromatics selectivity and improved catalyst stability. The benzene selectivity was highest for

the silylated Mo/HZSM-5 catalyst (benzene + naphthalene selectivity after 1 h on stream was

close to 100%). Deactivation of the precarburized zeolites was less pronounced than of the

zeolites heated in air or He. During heating in air or He, a larger fraction of the Mo-oxo

species diffused into the micropores than during heating in methane. Carburization of the

Mo-oxide species in the micropores during the MDA reaction resulted in Mo-carbide

particles that contribute to pore blocking, making the Brønsted acid sites inaccessible. The

formation of Mo-carbides during heating in methane resulted in a less mobile Mo phase. It is

argued that the presence of Mo-carbide particles in the micropores contributes to rapid

catalyst deactivation in addition to the formation of hard coke on the external surface.

This work is published in Chin. J. Catal. 36 (2015) 829-837.

Chapter 3

52

3.1 Introduction

Because petroleum oil reserves are dwindling, the identification of alternative feedstocks

for fuels and chemical production is necessary. Natural gas is increasingly considered as a

feedstock for energy, fuels, and chemicals, because it is abundant and the cleanest of all fossil

resources. In the 2013 BP Statistical Review of World Energy, it was reported that proven

natural gas reserves amount to approximately 180 trillion m3 [1]. Large amounts of these

natural gas reserves are located in remote areas, which makes their valorization costly. In

addition to direct liquefaction, there are several options for the large‐scale conversion of

natural gas to liquefied fuels via the syngas platform (a mixture of H2 and CO). These include

methanol synthesis, dimethyl ether synthesis, and Fischer‐Tropsch synthesis [2–4]. Facilities

that produce syngas are capital intensive; only large‐scale syngas production is cost effective

[5]. A long‐term goal of the chemical industry is therefore to develop alternative routes for

directly converting methane to easily transportable liquid intermediates that can serve as

platforms for fuels and chemicals. The direct oxidation of methane to methanol is a significant

scientific challenge. Non‐oxidative methane dehydroaromatization (MDA), which converts

methane to aromatics, mainly benzene, and hydrogen, is more promising than oxidative

approaches.

MDA was first described by Wang et al. [6], and has since been widely investigated by

industry and academia. Catalyst screening has shown that Mo/HZSM‐5 is the preferred

catalyst for MDA [7–9]. The molybdenum oxide phase is carburized by CH4 to a

molybdenum carbide (MoCx) phase. Although the exact nature of this carbide phase has not

been determined, its function is to convert methane to ethylene [10,11], which is then

oligomerized and cyclized to aromatic compounds at the Brønsted acid sites (BAS) of the

zeolite. The harsh process conditions, with typical temperatures of 973 K and above,

necessary for the activation of methane [12] result in poor catalyst stability; industrial

applications of this reaction are therefore challenging. The poor catalyst stability is mainly

caused by the formation of carbonaceous deposits, which block the zeolite micropores [13].

The BAS located at the external surface are considered to be involved in the formation of

large amounts of carbonaceous deposits [14]. These BAS catalyze the formation of polycyclic

aromatic hydrocarbons [15], which are deposited on the external surface and eventually block

the entrances to the micropores [16–24]. The results of our comprehensive study of catalyst

deactivation in MDA suggested that the formation of a polycyclic carbon layer around the

Chapter 3

53

zeolite crystal with progressive time on stream was the main reason [25]. It has also been

reported that deactivation of the external surface BAS suppresses the formation of coke

during MDA [21–24]. The pretreatment procedure used to activate the Mo/HZSM‐5 catalyst

strongly influences the catalytic performance.

The pretreatment gas directly affects the state of the Mo phase and indirectly affects the

state of the zeolite at the start of the reaction [26–28]. Although a significant number of

studies include data on catalyst activation [7, 26–34], comprehensive studies are lacking;

comparisons among available studies is greatly hampered by the often very different reaction

conditions and differences among catalyst samples. Two studies [28, 33] have directly

compared the influence of the pretreatment procedure, but they were conducted at lower

weight hourly space velocity (WHSV) values than those typically used in MDA. In the

present study, we systematically compared the influence of the gas (inert He, oxidizing

artificial air, and reducing/carburizing CH4) used for catalyst activation on the catalytic MDA

performance. As we showed previously, catalyst silylation significantly affects the MDA

performance [24], therefore we determined the influence of catalyst pretreatment on fresh and

silylated samples. We showed that precarburization resulted in improved catalytic

performance (improved catalyst stability and increased benzene selectivity) for silylated and

non‐silylated Mo/ZSM‐5. We determined the detailed physicochemical properties of the

fresh, carburized, and spent catalysts.

3.2 Experimental

3.2.1 Synthesis

The parent NH4ZSM‐5 was obtained from Akzo‐Nobel (now Albemarle Catalysts). The

parent zeolite had a Si/Al ratio of 19.4, determined by inductively coupled plasma optical

emission spectroscopy (ICP‐OES). Mo was introduced by incipient wetness impregnation

with an aqueous solution of ammonium heptamolybdate tetrahydrate (Merck) of appropriate

concentration. Prior to impregnation, the zeolite was dried overnightat 383 K. After

impregnation, the samples were dried for 1 h. The target Mo content was 4 wt%. The Mo‐

containing zeolites were calcined in artificial air at 823 K for 5 h after heating at a rate of 1.5

K/min. The parent ZSM‐5 zeolite is denoted by HZSM‐5. The zeolite after Mo introduction is

denoted byMo/HZSM‐5.

A method based on that reported by Zheng et al. [35] was used to silylate the external

surfaces of the zeolites. Typically, Mo/HZSM‐5 (2 g) was dried overnight at 373 K and

Chapter 3

54

dispersed in n‐hexane (50 mL). Tetraethyl orthosilicate (TEOS, 0.3 mL; Merck) was added

and the suspension was stirred for 1 h under reflux. The amount of TEOS corresponded to 0.4

wt% of the zeolite. After silylation, the zeolite was filtered, dried overnight at 373 K, and

calcined in artificial air. The catalyst was first heated at a rate of 2 K/min to 393 K, followed

by an isothermal period of 2 h. The temperature was then further increased to 773 K at a rate

of 0.2 K/min. The catalyst was kept at this temperature for 4 h. The silylated Mo/HZSM‐5

zeolite is denoted by Mo/HZSM‐5(Si).

Prior to the MDA reaction, the catalyst was heated to 973 K at a rate of 10 K/min in He,

artificial air, or an 80%/20% (v/v) CH4/He flow. In addition to activity testing, another set of

samples was prepared by rapidly cooling the catalyst after pretreatment in a He flow. The

catalyst was transported under exclusion of air into a N2‐filled glove‐box. The activation

procedure is indicated by adding the gas type as a suffix to the sample name.

3.2.2 Characterization

The Mo and Al contents of the samples were determined by ICP‐OES, using a Spectro

CIROS CCD spectrometer equipped with a free‐running 27.12 MHz generator running at

1400 W. Prior to analysis, the samples were dissolved in a mixture of HF/HNO3/H2O (1:1:1).

Ar sorption isotherms were measured at 87 K, using a Micromeritics ASAP2020 system

in static measurement mode. The samples were outgassed at 623 K for 8 h prior to the

sorption measurements. BET equation was used to calculate the specific surface area (SBET)

from the adsorption data obtained (p/p0 = 0.05–0.25). The mesopore volume (Vmeso) and the

mesopore size distribution were determined, using the BJH method, from the adsorption

branch of the isotherm. The micropore area (Smicro) and the micropore volume (Vmicro) were

determined using the t‐plot method, at a thickness range between 3.5 and 5.4 Å [36].

Infrared (IR) spectra were recorded in the 4000–400 cm−1

range using a Bruker Vertex

70V spectrometer. Samples were pressed into a self‐supporting wafer of typical density 8

mg/cm2. Adsorbed water was removed by evacuating the sample at 773 K for 2 h. After

evacuation, the sample was cooled to 323 K, and then the background spectrum was recorded.

The total concentration of BAS was determined by IR spectroscopy of adsorbed pyridine. The

dehydrated zeolite wafer was cooled to 323 K and exposed to pyridine until the bands related

to pyridine were saturated. The sample was then evacuated at 423 K for 2 h and the spectrum

Chapter 3

55

was recorded. The spectra were deconvoluted by standard procedures, and the extinction

coefficient values reported by Datka et al. [37] were used for quantification.

Ultraviolet (UV) Raman spectra were recorded using a Jobin‐Yvon T64000 triple‐stage

instrument with a spectral resolution of 2 cm−1

. The excitation laser line at 325 nm was

produced by a Kimmon He‐Cd laser. The power of the laser on the sample was 4 mW.

Magic angle spinning (MAS) 27

Al single‐pulse nuclear magnetic resonance (NMR)

spectroscopy was performed using a Bruker Avance DMX‐500 NMR spectrometer equipped

with a 2.5 mm MAS probe head operated at an 27

Al NMR resonance frequency of 130.3 MHz.

The 27

Al chemical shift was referenced to saturated Al(NO3)3 solution. In a typical

experiment, a well‐hydrated sample (10 mg) was packed in a 2.5‐mm zirconia rotor. The

MAS sample rotation speed was 20 kHz. Single‐pulse excitation was used, with a 18° pulse of

1 μs and an interscan delay of 1 s. The relaxation time was 1 s and the pulse length was 1 s.

Transmission electron microscopy (TEM; FEI Tecnai 20) was performed at an electron‐

accelerating voltage of 200 kV. Typically, a small amount of sample was suspended in

ethanol, sonicated, and dispersed over a Cu grid with a holey carbon film.

Weight‐loss curves were determined for the spent catalysts after 12 h on stream in MDA.

These measurements were carried out using a Mettler Toledo TGA/DSC 1 instrument.

Samples were heated to 1023 K in uncovered alumina crucibles at a rate of 5 K/min in a 33/67

(v/v) O2/He flow. Catalytic H/D exchange between C6H6 and C6D6 was performed using a

ten‐channel parallel microflow reactor. Typically, the zeolite material (50 mg) was loaded

into a quartz tubular reactor with an internal diameter of 4.0 mm. The zeolites were first

pelletized and sieved to 125–250 μm mesh. The ten quartz tubes were then placed in the

paallel setup. Before reaction, the samples were heated to 723 K at a rate of 5 K/min,

followed by an isothermal period of 6 h, to remove water. The reactor was then cooled to 303

K. The reaction was started by switching the reactor feed to a gas flow containing a 90/10

(v/v) C6H6/C6D6 mixture. The effluent products were analyzed using mass spectrometry. The

rate of H/D exchange between benzene and perdeuterobenzene was determined as a function

of temperature.

Chapter 3

56

3.2.3 Catalytic activity measurements

The zeolite catalyst (0.5 g) was placed in a tubular quartz reactor of length 490 mm and

internal diameter 4.0 mm. The catalyst was supported on quartz wool in the isothermal zone

of the oven. All gases were fed using thermal mass flow controllers. The temperature was

increased at a rate of 5 K/min to 973 K in a 25 mL/min flow of He, CH4/He, or artificial air.

The reaction was started by switching the reactor feed to a 5/95 (v/v) N2/CH4 mixture (N2

was used as the internal standard) at a WHSV of of 1.22 h-1

(1710 ml CH4/gcat.h at STP

conditions). The products were analyzed using an online gas chromatograph (Interscience

Compact GC) equipped with three channels for separate analyses of light gases [Molsieve 5A,

thermal conductivity detector (TCD)], light hydrocarbons [Al2O3/KCl, flame ionization

detector (FID)], and aromatics (Rtx‐1, TCD). N2 was used as an internal standard to close the

carbon balance and account for the amount of carbon deposits formed during the reaction. The

carbon formation rate rcoke was determined based on the components ethylene, ethane,

propylene, propane, benzene, toluene, and naphthalene (eq. 3.2). The methane conversion

(XCH4) and benzene selectivity (SC6H6) were calculated using eqs. (3.1) and (3.3).

blanksaromatics

blanksslightgasse

TCD

CH

TCD

CH

TCD

N

TCD

N

TCD

product

TCD

product

In

Naromatic

A

A

f

A

f

A

r

4

4

2

2

2

(3.1)

in

iiCHCoke MWrrr4

(3.2)

100% 6666

66

in

products

HCHC

HC

MWr

MWrwtS (3.3)

Chapter 3

57

Symbols

4CHr = reaction rate of CH4

ir = formation rate of compound i

In

i = molar flow rate of compound i into the reactor Out

i = molar flow rate of compound i out of the reactor TCD

if = TCD sensitivity factor of compound i FID

if = FID sensitivity factor of compound i

iA = measured peak area of compound i in the chromatogram

MWi = molecular weight compound i

3.3 Results and discussion

3.3.1 Catalytic activity measurements

Figures 3.1 and 3.2 show the time‐on‐stream reaction data for Mo/HZSM‐5 and

Mo/HZSM‐5(Si) pretreated with three different gases (CH4, He, and O2) at 973 K. In all

cases, the methane conversion rate decreased with time on stream. The rates of deactivation of

the two zeolite samples activated in CH4 were lower than those of the samples heated in air

and He. The reaction data show that initially few hydrocarbons were present in the effluent

stream; this observation is in line with previous reports [29, 38–40]. Methane carburizes the

molybdenum oxide phase during the initial stages of the reaction. As low hydrocarbon

selectivity is also seen for the precarburized sample, we can conclude that carburization of the

molybdenum oxide precursor during catalyst activation in methane is not complete. After this

initial phase, the hydrocarbon selectivity increased. The main hydrocarbon product was

benzene, and naphthalene was the main aromatic side product. The highest benzene

selectivity was observed after 1 h on stream; the highest benzene selectivity was 65 wt% for

the Mo/HZSM‐5 samples and 80 wt% for the Mo/HZSM‐5(Si) samples. In line with

previously reported data [21–24], the maximum aromatic selectivities (benzene + naphthalene

selectivity) were higher for the silylated catalysts. It can also be seen that the combined

selectivities for benzene and naphthalene were close to 100% for Mo/HZSM‐5(Si,CH4/He).

After 1 h on stream, the benzene selectivities of the catalysts pretreated in air and He

decreased rapidly. In comparison, the decrease in the benzene selectivity was less pronounced

for the precarburized Mo/HZSM‐5 and Mo/HZSM‐5(Si) catalysts. For the precarburized

catalysts, the benzene selectivities started to decrease more pronouncedly only after 4 h on

stream. The total amounts of aromatics produced were the highest for the precarburized

Chapter 3

58

catalysts. In all cases, the decrease in aromatic selectivity was accompanied by a substantial

increase in ethylene selectivity, and a smaller increase in naphthalene selectivity. This is

probably the result of deactivation of the acid sites in the micropores; this leads to higher

selectivity for the intermediate product ethylene, which is formed on the molybdenum carbide

phase from methane. The high naphthalene selectivity is related to the conversion of ethylene

on BAS on the external surface.

Chapter 3

59

Fig. 3.1. MDA reaction data: (a) methane reaction rate, (b) benzene (closed symbols) and naphthalene (open symbols) selectivities and (c)

ethylene selectivity for (■) Mo/HZSM-5(air), (●) Mo/HZSM-5(He) and (▲) Mo/HZSM-5(CH4/He).

0 2 4 6 8 10

0

2

4

6

8

10

12

14

CH

4 r

eacti

on

rate

(m

mo

l/h

.gcat)

Time on stream (h)

0 2 4 6 8 10

0

10

20

30

40

50

60

70

80

90

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

0 2 4 6 8 10

0

10

20

30

40

50

60

Eth

yle

ne s

ele

cti

vit

y (

wt%

)

Time on stream (h)

Chapter 3

60

Fig. 3.2. MDA reaction data: (a) methane reaction rate, (b) benzene (closed symbols) and naphthalene (open symbols) selectivities and (c)

ethylene selectivity for (■) Mo/HZSM-5(Si,air), (●) Mo/HZSM-5(Si,He) and (▲) Mo/HZSM-5(Si,CH4/He).

0 2 4 6 8 10

0

2

4

6

8

10

12

14

CH

4 r

eacti

on

rate

(m

mo

l/h

.gcat)

Time on stream (h)

0 2 4 6 8 10

0

10

20

30

40

50

60

70

80

90

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

0 2 4 6 8 10

0

10

20

30

40

50

60

Eth

yle

ne s

ele

ctv

ity (

wt%

)

Time on stream (h)

Chapter 3

61

3.3.2 Catalyst characterization

The textural properties of the fresh, pretreated, and spent catalysts were determined using

Ar physisorption measurements. The results are shown in Table 3.1. The introduction of Mo

into the parent zeolite led to a decrease in the micropore volume (Vmicro). This is probably

because of diffusion of part of the molybdenum oxide phase into the HZSM‐5 micropores

during calcination. The increase in the mesopore volume (Vmeso) is presumably the result of

the damage caused by Al extraction from the zeolite framework [24,41]. The micropore

volume of the silylated Mo/HZSM‐5(Si) catalyst was smaller than that of the parent

Mo/HZSM‐5 zeolite. It should be stressed that silylation was performed after the Mo‐loading

step, which is more beneficial for the MDA reaction than the usual approach in which HZSM‐

5 is silylated before Mo introduction [24]. We previously showed that silylation and

calcination of Mo/HZMS‐5 result in higher molybdenum oxide dispersion over the external

surface and an increase in the fraction of Mo entering the micropores [24]. Carburization in

methane led to small decreases in the pore volumes of Mo/HZSM‐5 and Mo/HZSM‐5(Si).

The carburization step led to the formation of different types of carbonaceous species in the

catalysts. These species were investigated using thermogravimetric analysis (TGA). The TG

curves of the carburized and spent Mo/HZSM‐5 and Mo/HZSM‐5(Si) catalysts are shown in

Fig. 3.3. The curves display three main oxidation peaks. The main peak at 700 K is

characteristic of oxidation of molybdenum carbide (CMoC) [42]. The second peak, at a higher

temperature (~750 K), is related to coke species formed in the proximity of molybdenum

carbide [42]. This is usually amorphous polyolefinic coke and is referred to as soft coke

(Csoft). The small peak at 840 K is related to polycyclic aromatics, referred to here as hard

coke (Chard) [19,40]. The quantitative data obtained by deconvolution of these traces are

collected in Table 1. The TG curves for the precarburized Mo/HZSM‐5 and Mo/HZSM‐5(Si)

catalysts contain all three types of coke. Carburization of Mo/HZSM‐5(Si) led to a larger

amount of carbon species associated with the Mo phase and soft coke compared with

carburization of Mo/HZSM‐5. This difference is in line with higher molybdenum oxide

dispersion in the silylated catalyst. The decrease in the micropore volume was slightly higher

for the silylated catalyst.

Chapter 3

62

Table 3.1. Textural properties of the fresh and spent Mo/HZSM-5 and Mo/HZSM-5(Si) and the chemical nature of the carbon deposits.

Sample Activation TOS1

(h)

Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

SBET

(m2/g)

Ctotal

(wt%)

CMoC

(wt%)

Csoft

(wt%)

Chard

(wt%)

HZSM-5 - 0 0.12 0.02 246 10 277 - - - -

Mo/HZSM-5 - 0 0.10 0.06 182 44 272 - - - -

CH4/He 0 0.09 0.03 171 22 228 3.3 0.9 0.9 1.5

CH4/He 12 0 0.02 3 12 12 13.5 0 2.7 10.8

Air 12 0.02 0.02 32 14 59 11.7 0 3.4 8.3

He 12 0.02 0.02 27 14 61 11.1 0 3.6 7.5

Mo/HZSM-5(Si)

- 0 0.07 0.01 129 6 143 - - - -

CH4/He 0 0.05 0.02 97 12 130 4.5 1.2 1.8 1.5

CH4/He 12 0.03 0.02 64 13 90 9.5 0.5 2.4 6.6

Air 12 0.02 0.02 46 14 72 7.7 0 3.7 4.0

He 12 0.04 0.03 76 15 109 9.7 0 4.1 5.6 1 Time On Stream (TOS) in MDA.

Chapter 3

63

Fig. 3.3. TGA weight-loss curves after 12 hours of MDA reaction of non-silylated (left) and

silylated Mo/HZSM-5 (right) pretreated in (a) He, (b) air or (c) a CH4/He mixture. Spectrum

(d) corresponds to the catalyst pretreated in CH4/He which was subsequently recovered from

the reactor after activation (5x magnified).

During MDA, the micropore volumes of the zeolite catalysts decreased substantially. The

decreases in the micropore volumes of the non‐silylated catalysts were greater than those of

the silylated ones. This difference is consistent with the smaller amount of carbonaceous

species formed in the silylated samples during the reaction. TGA of the spent samples shows

that more hard coke was formed on the non‐silylated catalysts. This hard coke is usually

associated with the formation of polycyclic aromatics on the external surface of the zeolite

[14, 15]. Silylation effectively reduces the external acidity [14, 15] and, accordingly, the

formation of hard coke [24]. The lower residual micropore volume of the spent non‐silylated

samples can therefore be correlated with high coverage of the external surface by polycyclic

aromatics, which block the micropore entrances. Despite the lower rate of deactivation in

MDA, the TG data show that the precarburized spent samples always contained more coke,

especially in the form of hard coke, than did the samples activated in He or air. The carbon

speciations for the spent samples pretreated in He and air were similar.

27Al MAS NMR spectroscopy was used to determine the Al speciation in the samples.

The NMR spectra are shown in Fig. 3.4. The dominant feature, at 55 ppm, is related to

framework Al (FAl). The small peak at 0 ppm is attributed to extraframework Al (EFAl). The

600 700 800 900 1000

Weig

ht-

loss (

mg

/K)

Temperature (K)

600 700 800 900 1000

Temperature (K)

Chapter 3

64

feature at 14 ppm also arises from EFAl, in the form of Al2(MoO4)3. The increased EFAl

concentration and the formation of aluminum molybdates observed for the Mo‐modified

ZSM‐5 zeolites suggest dealumination of the framework. The FAl peaks for the Mo‐modified

zeolites are broader than that for the parent HZSM‐5 zeolite. Tessonier et al. [42] attributed

this broadening to a change in the symmetry around the Al tetrahedra following exchange of

protons with molybdenum oxide species. The resonance area of the FAl peak was lower for

the Mo‐modified samples, probably because of asymmetric Al coordination when Mo is

present at the exchange sites. On carburization, the intensity of the FAl peak decreased, which

indicates further dealumination. This is probably related to the high temperatures of the

carburization step. The decrease in the intensity of the FAl peak was stronger when

Mo/HZSM‐5 or Mo/HZSM‐5(Si) was treated in He or air. This suggests a higher degree of

proton exchange with molybdenum oxide species at the BAS than in activation by

precarburization. A portion of the EFAl is present in the form of Al2(MoO4)3, indicated by the

small peak at 14 ppm. The NMR data suggest that more FAl species are retained after

precarburization than after pretreatment in He or air.

Fig. 3.4. 27

Al MAS NMR spectra of non-silylated (left) and silylated Mo/HZSM-5 (right)

showing (a) parent ZSM-5 (b) fresh Mo/HZSM-5 and Mo/HZSM-5 pretreated in (c) He, (d)

air and (e) CH4/He mixture.

Before discussing the acidities of the most active precarburized samples in detail, we

describe the Fourier‐transform (FT)IR spectra of the activated samples. Because carbon

100 50 0 -50

Chemical shift (ppm)

100 50 0 -50

Chemical shift (ppm)

Chapter 3

65

formation resulted in blackening of the sample, FTIR experiments were only performed on

the zeolites after activation in air or He at 973 K. The hydroxyl stretching region of the

spectra contain bands at 3745, 3665, and 3610 cm−1

; these are attributed to silanols,

extraframework hydroxyls, and bridging hydroxyl, respectively. As expected, modification of

the parent zeolite with Mo led to a decrease in the intensity of the bridging hydroxyl band.

This indicates a lower BAS density as a result of proton exchange with molybdenum oxide

species. The silylated Mo/HZSM‐5 contained fewer acid sites. Pretreatment of the parent and

silylated Mo/HZSM‐5 in He or air led to the nearly complete disappearance of all hydroxyl

features, including the BAS. The acid site contents of these zeolites were also investigated

using pyridine adsorption measurements (Table 3.2). Modification of the parent zeolite with

Mo followed by silylation led to a decrease in the BAS content. After activation in He or air,

the BAS content decreased further. The BAS densities for these two activated zeolites are

higher than those suggested by theintensity of the bridging hydroxyl band (Fig. 3.5). This

discrepancy can be explained by the close proximity of molybdenum oxide species to the

bridging hydroxyl groups, which leads to perturbation of these hydroxyls.

Fig. 3.5. FTIR spectra of non-silylated (left) and silylated Mo/HZSM-5 (right) showing (a)

Mo/HZSM-5 pretreated in He, (b) Mo/HZSM-5 pretreated in air, (c) fresh Mo/HZSM-5 and

(d) parent ZSM-5.

3800 3600 3400 3200

Ab

so

rban

ce (

a.u

.)

Wavenumber (cm-1)

3800 3600 3400 3200

Wavenumber (cm-1)

Chapter 3

66

Table 3.2. Acidity of fresh and air- and He-activated Mo/HZSM-5 and Mo/HZSM-5(Si) as

measured by pyridine IR measurements. The air and He pretreated catalysts were activated at

973 K.

Sample Pretreatment

gas used

BAS

(mmol/gcat)

LAS

(mmol/gcat)

HZSM-5 - 0.679 0.284

Mo/HZSM-5 -

He

Air

0.615

0.229

0.361

0.136

0.055

0.197

Mo/HZSM-5(Si) -

He

Air

0.530

0.278

0.239

0.060

0.148

0.145

The samples obtained by precarburization were investigated in more detail, because they

performed substantially better than did the catalysts activated in air or He. FTIR spectroscopy

of adsorbed pyridine was unsuccessful, because of the presence of carbonaceous deposits,

therefore the Brӧnsted acidity was investigated by H/D exchange between benzene and

deuterated benzene. Haw and coworkers characterized the acid sites in faujasite zeolites using

1H NMR spectroscopy to track their H/D exchange with perdeuterobenzene [43]. Poduval et

al. [44] used the IR spectra of H/D exchanged faujasite zeolites to determine the strengths and

numbers of strong acid sites. Here, we determined the rate of H/D exchange between benzene

and d6‐benzene in a fixed bed reactor. A benzene to d6‐benzene ratio of 10:1 was used in the

feed, and the rate of the reaction was determined by measuring the conversion of benzene to

d1‐benzene. The obtained reaction rates are listed in Table 3.3. The exchange rates of the Mo‐

modified zeolites were slightly lower than that of the parent HZSM‐5 zeolite. This is because

of exchange of some of the protons at exchange positions with molybdenum oxide species.

Carburization further decreased the reaction rate. The decrease in the acidity was the strongest

for the silylated sample. After activation, only 28% of the original acidity of the HZSM‐5 was

retained.

The external surfaces of the zeolite crystals were examined using X‐ray photoelectron

spectroscopy (XPS). The surface Si/Al and Si/Mo ratios for the various catalysts are listed in

Table 3.3. The Si/Al ratios of the Mo‐modified zeolites before and after precarburization were

similar to the values for the parent HZSM‐5. The Si/Mo ratios increased on precarburization

of Mo/HZSM‐5 and Mo/HZSM‐5(Si). This increase suggests agglomeration of the Mo phase

during formation of the carbide phase. The agglomeration extents were similar for the two

Chapter 3

67

samples. Elemental analysis also shows that there was only a small loss of Mo during the

activation step in methane.

Table 3.3. Characterization of the elemental composition and benzene H/D exchange reaction

rates of Mo/HZSM-5 and Mo/HZSM-5(Si) before and after precarburization.

Sample Pretreatment

gas used

Al1

(wt%)

Mo1

(wt%)

Si/Al2

(XPS)

Si/Mo2

(XPS)

rC6H63

(mmol/min)

Relative

H/D

exchange

rate4

HZSM-5 - 2.2 ∞ 22 ∞ 1.65 1.00

Mo/HZSM-5 - 1.9 3.4 27 3.5 1.30 0.79

CH4/He 1.9 3.2 24 8.1 0.79 0.48

Mo/HZSM-5(Si) - 1.9 3.4 28 3.2 1.35 0.82

CH4/He 1.8 3.2 28 8.1 0.47 0.28 1Determined by ICP-OES;

2Determined by XPS experiments;

3Determined by catalytic H/D exchange

reaction; 4Remaining C6H6 reaction rate relative to that of parent HZSM-5.

The nature of the Mo phase was further investigated using UV Raman spectroscopy.

The sample was excited with a 325 nm laser. The spectra are shown in Fig. 3.6. The spectrum

of Mo/HZSM‐5 has a broad absorption band extending over the 600–1000 cm−1

region, with

more clearly defined bands at 860 and 960 cm−1

. This spectrum resembles that of amorphous

molybdenum oxide [45]. The weak band at 820 cm−1

is attributed to microcrystalline α‐MoO3

embedded in an amorphous molybdenum oxide matrix. The 380 cm−1

band characteristic of

HZSM‐5 is not observed for Mo/HZSM‐5, because the molybdenum oxide phase covers a

substantial part of the zeolite surface. The spectrum of Mo/HZSM‐5(Si) is different from that

of Mo/HZSM‐5. The former spectrum contains bands at 280, 336, 820, and 995 cm−1

; these

show the presence of a much greater proportion of microcrystalline α‐MoO3. This spectrum

also contains some features attributable to amorphous molybdenum oxide. Bands from

molybdenum oxides are not observed after carburization of Mo/HZSM‐5, indicating that the

particle surfaces were completely converted to molybdenum carbide. The weak peaks at 336

and 995 cm−1

indicate that a small amount of α‐MoO3 is retained in Mo/HZSM‐5(Si,CH4/He).

The less extensive carburization in the silylated sample may be the result of higher dispersion

of the initial molybdenum oxide phase [24].

Chapter 3

68

Fig. 3.6. UV-Raman spectra of (a) parent HZSM-5, (b) Mo/HZSM-5, (c) Mo/HZSM-

5(CH4/He), (d) Mo/HZSM-5(Si), (e) Mo/HZSM-5(Si,CH4/He), (f) bulk MoO3 and (g) bulk

Al2(MoO4)3.

TEM images of Mo/HZSM‐5 (Fig. 3.7a) show that the molybdenum oxide phase is

present as small particles with typical diameters of 1 nm. After precarburization, these

particles become more clearly visible, because they are larger (Fig. 3.7b). This indicates that

sintering occurred during the conversion of the molybdenum oxide particles to molybdenum

carbide particles. This is in line with the XPS results. The images suggest that the particle size

distribution also broadened during carburization. The TEM images confirm that the

molybdenum oxide particles (Fig. 3.7c) are significantly smaller in silylated Mo/HZSM‐5

than in non‐silylated Mo/HZSM‐5, in line with the discussion above. Similarly, carburization

of Mo/HZSM‐5(Si) led to sintering of these small particles to large molybdenum carbide

particles (Fig. 3.7d).

Extensive characterization of the activated and spent catalysts indicated that pretreatment

in air or He led to a larger fraction of molybdenum oxide species diffusing into the

micropores than did activation in methane. This is supported by the 27

Al NMR and FTIR

spectra. The mobility of molybdenum oxide species at high temperature, probably in the

partially reduced form (MoO2) [46], explains the decreased acidity. Although inspection of

the hydroxyl region of the IR spectra indicates that after heating in air or He there are few

acid sites left, the pyridine IR results show that the BAS density for these two samples is

200 400 600 800 1000 1200

Inte

nsit

y (

a.u

.)

Wavenumber (cm-1)

Chapter 3

69

similar to that of the sample activated in methane. Accordingly, we suggest that the higher

concentration of molybdenum oxide species in the micropores after He or air pretreatment

results in faster deactivation, because the molybdenum carbide particles formed during MDA

block the micropores. When the catalyst is activated in methane, less of the molybdenum

oxide phase ends up in the micropores, because of the conversion of molybdenum oxides to

molybdenum carbides. It has been reported that molybdenum carbides are less mobile than

molybdenum oxides [47]. All these data provide a satisfactory explanation for the lower

deactivation rate and improved stability in benzene selectivity of the precarburized catalysts.

The conclusion that fewer BAS are accessible in the micropores is also supported by the

higher ethylene selectivities of the He‐ and air‐pretreated catalysts when the MDA activity

decreases. The carbon speciation determined by TGA suggests that the formation in the

micropores of soft coke from molybdenum carbides may explain the rapid deactivation of the

He‐ or air‐pretreated samples. Our explanation for the improved catalyst stability after

pretreatment in methane is different from the previous claim that molybdenum carbides

obtained by precarburization are more stable under MDA conditions than those formed during

the MDA reaction [32, 48, 49].

Fig. 3.7. TEM micrographs of (a) Mo/HZSM-5, (b) Mo/HZSM-5(CH4/He), (c) Mo/HZSM-

5(Si) and (d) Mo/HZSM-5(Si,CH4/He).

Chapter 3

70

3.4 Conclusions

The effects of pretreatment of (silylated) Mo/HZSM‐5 in various gas atmospheres (artificial

air, He, or a CH4/He mixture) at 973 K on their catalytic performances in MDA were

investigated. Precarburization in methane gave catalysts with the highest aromatic selectivities

and the lowest rates of catalyst deactivation. The benzene selectivity was the highest for the

silylated Mo/HZSM‐5 catalyst. Deactivation of the precarburized catalysts was less

pronounced in the MDA reaction than for catalysts heated in air or He. This is because a

greater amount of Mo diffuses into the zeolite micropores in the form of mobile molybdenum

oxide species during heating in air or He than in heating in methane. Carburization of the

molybdenum oxide particles present in the micropores resulted in molybdenum carbide

particles, which contributed to pore blocking, making the BAS inaccessible. The deactivation

can also be partly attributed to the formation of soft coke in the micropores, and is probably

associated with the presence of molybdenum carbides. The more rapid formation of

molybdenum carbides during heating in methane decreased the amount of mobile

molybdenum oxide species and their diffusion into the micropores.

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Chapter 3

72

Chapter 4

73

On the deactivation of Mo/HZSM-5 in the methane

dehydroaromatization reaction

Summary

The deactivation of Mo/HZSM-5 during the non-oxidative methane aromatization (MDA)

reaction that yields benzene and hydrogen was investigated. Catalysts were recovered from

the reactor after pre-activation and after increasing time on stream in methane. The physico-

chemical properties of the spent catalysts were characterized in detail by Ar physisorption, 27

Al MAS NMR and X-ray photoelectron spectroscopy. The nature of the carbon deposits was

determined by UV Raman spectroscopy and TGA, and the size and location of the Mo-carbide

particles by TEM and STEM-HAADF. The results show that the main cause for catalyst

deactivation is the formation of a carbonaceous layer at the external zeolite surface. This

layer is made up from polyaromatic hydrocarbons and decreases the accessibility of the

Brønsted acid sites in the micropores. At the same time, the decreased interaction of the Mo-

carbide particles with the external zeolite surface results in their sintering. The lower Mo-

carbide dispersion decreases methane conversion rates. The decreased accessibility of the

Brønsted acid sites shifts the selectivity from benzene to unsaturated intermediates formed on

the Mo-carbide particles. Silylation of the external surface mainly results in lower rate of

coke formation at the external surface, slowing down catalyst deactivation.

This chapter is published in Appl. Catal. B: Environ. 176 (2015) 731-739.

Chapter 4

74

4.1 Introduction

Dwindling fossil resources and concerns about the effect of their combustion on our

climate add urgency to the replacement of these non-renewable resources by renewable ones.

For example, to surpass petroleum as the main resource for aromatics scientists explore the

use of biomass as alternative feedstock [1]. A general problem is that the conversion

technologies for renewable resources such as biomass and solar energy are often at an early

stage of development. Usually, the development from the discovery of a novel energy

technology to large-scale commercial implementation takes decades [2]. Accordingly,

transition technologies based on relatively clean feedstock such as natural gas are increasingly

considered to fill the gap between conventional oil and coal based and future sustainable

processes. In energy scenarios, the abundance of natural gas is evident with estimated reserves

of 180 trillion ton cubic meters [3]. New technologies to extract natural gas from shale rock

are already leading to drastic changes in the energy and chemicals industry at the global scale

[1, 4]. On the other hand, a significant fraction of the natural gas reserves is located in

remotely located fields. Because of the small size of many of these fields, investments in

capital intensive transportation via pipelines or liquefaction are often not justified. For similar

reasons, it is common practice to flare associated gas from oil production in order to meet

safety and environmental legislations. Chemical conversion towards high-value fuels and

chemicals would be an alternative to make recovery of these natural gas resources

economically more attractive.

Currently, catalytic steam and autothermal reforming and gasification are common

technologies to convert natural gas into synthesis gas; synthesis gas serves as the platform for

the manufacture of a wide range of chemicals. The CAPEX and OPEX for the syngas

production step are very high, so that it is only profitable to construct large plants.

Accordingly, it remains a strong desire of the chemical industry to develop a simple process to

upgrade natural gas to liquids. One such process may be the direct aromatization of methane

to benzene under non-oxidative conditions (methane dehydroaromatization, MDA). Benzene

is an attractive intermediate, because it can be more easily transported than natural gas. In

addition, the increasing use of ethane from wet shale gas instead of naphtha to produce

ethylene is putting pressure on the aromatics supply. Direct methane to aromatics conversion

would be very desirable in this context [1].

Methane dehydroaromatization was first discussed by Wang et al. in 1993 [5].

Chapter 4

75

Mo/HZSM-5, the most common catalytic material for this reaction, is a bifunctional catalyst.

The molybdenum carbide phase is formed during carburization of the initial molybdenum

oxide phase. It converts methane into ethylene, while Brønsted acid sites in the shape-

selective micropores of HZSM-5 zeolite convert ethylene to benzene and other aromatics. The

process is typically operated at temperatures higher than 873 K because of the low reactivity

of methane. Thermodynamic equilibrium limits the reaction. In principle, higher reaction

temperatures than the most frequently reported one (973 K) result in higher benzene yield, but

also lead to rapid carbon laydown on the catalyst. The formation of coke, which causes

relatively fast deactivation of the catalyst, is the main challenge in the development of a

commercial process for methane dehydroaromatization [6].

Several studies have attempted to elucidate the reasons for the rapid deactivation of

Mo/HZSM-5 catalysts during the MDA reaction [7-17]. Extensive formation of polyaromatics

hydrocarbon carbon deposits was identified as the main reason for catalyst deactivation. The

formation of this type of carbon is assumed to take place at the Brønsted acid sites (BAS)

located on the external surface of the zeolite crystals [13, 14]. Some authors suggested that

such carbon species may eventually block the micropore apertures [7, 8]; this would explain

the increased formation of ethylene at the expense of benzene [9]. Deactivation of the external

surface BAS by silylation has been shown to decrease to some extent the formation rate of

such unwanted carbon species, but catalyst deactivation cannot be completely prevented in

this way [15-17]. Therefore, the coverage of the BAS inside the micropores by carbonaceous

species is also considered to contribute to catalyst deactivation [10]. We have recently

reported that silylation after Mo introduction yields better MDA catalysts than Mo

introduction after zeolite silylation [17].

Improvements of the MDA process may involve improved catalyst technology but also

reactor engineering approaches to cope with rapid catalyst deactivation. Such developments

will strongly hinge on better insight in the deactivation of the catalyst. In the present study, we

have investigated in detail the different stages in the life of a Mo/HZSM-5 catalyst during

methane dehydroaromatization. The precursor activated and deactivated catalysts after

different times on stream were extensively characterized for their physical and chemical

properties. The obtained results are captured in a model that describes deactivation during the

MDA reaction.

Chapter 4

76

4.2 Experimental

4.2.1 Catalyst synthesis

The proton form of ZSM-5 (Alsi-Penta) was obtained from Süd-Chemie (now Clarian).

The starting material had a Si/Al ratio of 15 as determined from ICP-AES elemental analysis.

For Mo loading, the zeolite was impregnated (incipient wetness impregnation) with an

aqueous solution of ammonium heptamolybdate tetrahydrate (AHM, Merck). The target Mo

content was 6 wt%. After impregnation, the material was dried for 1 h at room temperature

followed by calcination in artificial air at 823 K for 5 h after heating to the final temperature

at a rate of 1.5 K/min. Molybdenum modified zeolites are denoted by Mo/HZSM-5.

A portion of the molybdenum modified zeolite was silylated following a method adapted

from Ding et al. [14]. Typically, 2 g of zeolite was dried overnight at 373 K and then

dispersed in 50 ml n-hexane. To the suspension, an amount of 0.3 ml tetraethyl orthosilicate

(TEOS, Merck) was added and stirred for 1 h under reflux. The amount of TEOS added

corresponds to 0.4 wt% based on the amount of zeolite in the suspension. The treated zeolite

was then filtered off and dried overnight at 373 K. The resulting zeolite was calcined by a

two-step procedure in artificial air. The first step consisted of heating the sample at a rate of a

2 K/min to 393 K followed by an isothermal period of 2 h. In the second step the temperature

was increased to 773 K at a rate of 0.2 K/min followed by an isothermal period for 4 h. The

silylated Mo/HZSM-5 is denoted as Mo/HZSM-5(Si).

4.2.2 Characterization

The Mo and Al content of the zeolites was determined by inductively coupled plasma optical

emission spectroscopy (ICP-OES, Spectro CIROS CCD spectrometer). Prior to ICP

measurements, the zeolite samples were dissolved in a mixture of HF/HNO3/H2O (1:1:1).

UV Raman spectra were recorded with a Jobin–Yvon T64000 triple stage spectrograph

with a spectral resolution of 2 cm−1

. The 244 nm line at of a Lexel 95-SHG laser was used as

the excitation source. The power of the laser on the sample was about 2 mW.

Magic angle spinning (MAS) 27

Al single pulse NMR spectra were recorded on a Bruker

Avance DMX-500 NMR spectrometer (11.7 T; the Al resonance frequency at this field is

130.3 MHz). A 2.5 mm MAS probe head was used. The 27

Al chemical shift was referenced to

a saturated Al(NO3)3 solution. In a typical experiment, 10 mg of well-hydrated sample was

packed in a 2.5 mm zirconia rotor. The MAS sample rotation speed was 20 kHz. 27

Al NMR

Chapter 4

77

spectra were recorded with a single pulse sequence with 18 o pulse duration of 1 μs and a

interscan delay of 1 s.

Argon sorption isotherms were measured at 87 K on a Micromeritics ASAP2020 system

in static measurement mode. The samples were outgassed at 623 K for 8 h prior to the

sorption measurements. The Brunauer–Emmett–Teller (BET) equation was used to calculate

the specific surface area (SBET) in the pressure range p/p0 = 0.05–0.25. The mesopore volume

(Vmeso) and mesopore size distribution were calculated using the Barrett–Joyner–Halenda

(BJH) method on the adsorption branch of the isotherm. The micropore area (Smicro) and

micropore volume (Vmicro) were calculated from the t-plot curve using the thickness range

between 3.5 and 5.4 Å [19].

Transmission electron micrographs were obtained with a FEI Tecnai 20 instrument at an

electron acceleration voltage of 200 kV. Typically, a small amount of sample was suspended

in ethanol, sonicated and dispersed over a Cu grid with a holey carbon film.

Weight-loss curves were obtained from thermogravimetric analysis (TGA) using a

Mettler Toledo TGA/DSC 1 apparatus. Samples were heated in uncovered alumina crucibles

at a rate of 5 K/min to 1023 K in a 2/1 (v/v) He/O2 flow.

Isotopic H/D exchange between C6H6 and C6D6 was carried out in a 10-channel parallel

microflow reactor setup. Typically, 50 mg of zeolite was loaded in each quartz tubular reactor

with an internal diameter of 4.0 mm. Zeolites were first pelletized and then crushed and

sieved in a 125-250 μm mesh fraction. The ten quartz tubes with the catalyst contained

between quartz wool plugs were then placed in the parallel reactor setup. Before reaction

samples were dehydrated in a He flow to 723 K at a rate of 5 K/min followed by an

isothermal period of 6 h. After cooling to 303 K, the reaction was started by switching the

reactor feed to the gas flow containing a 90/10 (v/v) C6H6/C6D6 mixture. The effluent

products were analyzed by mass spectrometry. The rate of H/D exchange was determined as a

function of temperature.

4.2.3 Catalytic activity measurements

An amount of 0.5 g of catalyst was introduced in a tubular quartz reactor with a length of

490 mm and an internal diameter of 4.0 mm. The catalyst was supported on quartz wool in the

isothermal zone of the oven. All gases were fed using thermal mass controllers. The catalyst

was activated by increasing the temperature at a rate of 10 K/min to 973 K in a (80/20) (v/v)

Chapter 4

78

CH4/He gas flow of 25 ml/min. The reaction was started by switching the reactor feed to a

N2/CH4 mixture (5 vol% N2 in CH4) at a WHSV of 1.22 h-1

(1710 ml CH4/gcat.h at STP

conditions). Products were analyzed by an online gas chromatograph (Interscience

CompactGC) equipped with three analysis channels for analysis of light gasses (Molsieve 5A,

TCD), aromatics (Al2O3/KCl, TCD) and light hydrocarbons (Rtx-1, FID).

Nitrogen was used as the internal standard in order to determine the methane conversion

(XCH4) and the methane conversion rate (rCH4). The weight-based selectivities and reaction

rates of the products (rproduct) were determined using response factors for the various

compounds (main products are benzene, toluene, naphthalene, ethylene, ethane, propylene,

propane). The coke selectivity (Scoke) and the rate of coke formation (rcoke) were determined

by mass balance considerations.

To follow the deactivation process, samples were recovered after different reaction times

by rapidly cooling the reactor under flowing He. The Mo/HZSM-5 samples were then

recovered from the reactor in air. This procedure was carried out after activation of the

catalyst in CH4/He and activation in CH4/He followed by reaction for 5 min, 2 h, 5 h and 10 h.

The corresponding samples are denoted by Mo/HZSM-5(act), Mo/HZSM-5(0.08h),

Mo/HZSM-5(2h), Mo/HZSM-5(5h) and Mo/HZSM-5(10h).

4.3 Results and discussion

4.3.1 Catalytic activity measurements

Catalytic performance data for Mo/HZSM-5 and Mo/HZSM-5(Si) catalysts in the MDA

reaction are shown in Fig. 4.1. The trends in catalytic activity and selectivity with time on

stream were similar for both catalysts. In line with earlier reports [20], the methane

conversion rate gradually decreased. After 10 h on stream, nearly all catalytic activity was lost.

During reaction, the benzene formation rate strongly decreased. At the same time, the

ethylene selectivity increased. These changes point to rapid deactivation of the BAS that

convert ethylene to aromatics. It is also seen that, at the initial stages of the reaction, the

benzene selectivity was low and the coke selectivity was as high as 50 wt%. In this induction

period, part of the methane in the feed is used to convert the Mo-oxide phase into Mo-carbide

[20]. This implies that the Mo-oxide precursor was not totally converted to Mo-carbides

during heating in CH4/He. Concomitant with Mo-carbide formation, the coke selectivity

decreased and the benzene selectivity increased. The highest benzene selectivity was observed

Chapter 4

79

after a reaction time of 1-2 h. The benzene selectivity was higher for Mo/HZSM-5(Si) (75

wt%) than for Mo/HZSM-5 (64 wt%). After 2 h, the benzene selectivity decreased, first

relatively slowly, and then more rapidly after 5 h. Especially during the rapid decrease of

methane conversion, the ethylene and coke selectivity strongly increased. The coke selectivity

for Mo/HZSM-5(Si) was lower compared with its non-silylated counterpart. This difference is

in agreement with the coke content of the spent catalysts recovered after 10 h on stream

(Table 4.1). The silylated sample contained less hard coke as determined by TGA. This hard

coke is made up from polyaromatic hydrocarbons (PAHs).

Table 4.1. Carbon analysis of the spent Mo/HZSM-5 and Mo/HZSM-5(Si) catalysts

determined by TGA.

Sample Ctotal

(wt%)

CMoC

(wt%)

Csoft

(wt%)

Chard

(wt%)

HZSM-5 - - - -

Mo/HZSM-5 - - - -

Mo/HZSM-5(act) 3.7 0.9 2.8 0

Mo/HZSM-5(0.08h) 3.7 0.7 3.0 0

Mo/HZSM-5(2h) 7.8 0 4.0 3.8

Mo/HZSM-5(5h) 11.3 0 4.1 7.2

Mo/HZSM-5(10h) 14.2 0 3.8 10.4

Mo/HZSM-5(Si) - - - -

Mo/HZSM-5(Si,act) 4.7 4.1 0.6 0

Mo/HZSM-5(Si,2h) 9.3 0 3.3 6.0

Mo/HZSM-5(Si,10h) 12.3 0 2.5 9.8

Chapter 4

80

Fig. 4.1. Activity data for Mo/HZSM-5 (squares) and Mo/HZSM-5(Si) (circles) for methane dehydroaromatization showing (a) the CH4

reaction rate, (b) benzene selectivity and (c) coke (closed symbols) and olefin selectivity (open symbols) as a function of the reaction time.

0 2 4 6 8 10

0

2

4

6

8

10

12

14

16

18

20

CH

4 r

eacti

on

rate

(m

mo

l/h

.gc

at)

Time on stream (h)

0 2 4 6 8 10

0

20

40

60

80

100

Ben

zen

e s

ele

cti

vit

y (

wt%

)

Time on stream (h)

0 2 4 6 8 10

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

Chapter 4

81

4.3.2 Catalyst characterization

The XRD patterns of the fresh catalysts show that the introduction of Mo did not strongly

affect the crystallinity of the zeolites (Table 4.2). There are no indications for the presence of

large MoO3 crystallites in the diffractograms. The textural properties of the zeolite catalysts

are listed in Table 4.2. The decrease in the micropore volume upon introduction of Mo

indicates that a fraction the Mo phase is located in the zeolite micropores [20]. Activation in a

mixture of CH4/He at 973 K led to a decrease of the micropore volume, which may be due to

formation of Mo-carbide species and the deposition of carbonaceous species close to these

Mo-carbides [10]. The catalysts recovered directly after activation and after 5 min and 2 h on

stream all had comparable micropore volume (Vmicro ≈ 0.07 cm3/g). This suggests that the rate

of carbon deposition during the first 2 h of the reaction was relatively low. Samples recovered

after 5 h and 10 h had much lower micropore volumes of 0.04 cm3/g and 0 cm

3/g,

respectively. The decrease shows that substantial amounts of coke formed inside the

micropores or at the external surface, blocking the micropore entrances during the second

stage of the reaction. We verified for spent Mo/HZSM-5 that the complete loss in micropore

volume was not due to the amorphization of the zeolite; after a reaction time of 10 h, the

zeolite crystallinity remained at 64%.

Table 4.2. Textural properties of fresh and spent Mo/HZSM-5 and Mo/HZSM-5(Si).

Sample Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

SBET

(m2/g)

XRD crystallinity

(%)

HZSM-5 0.13 0.02 246 10 251 100

Mo/HZSM-5 0.10 0.03 204 14 244 93

Mo/HZSM-5(act) 0.07 0.04 130 18 164 -

Mo/HZSM-5(0.08h) 0.07 0.03 135 19 179 -

Mo/HZSM-5(2h) 0.07 0.04 140 22 196 -

Mo/HZSM-5(5h) 0.04 0.023 87 12 115 -

Mo/HZSM-5(10h) 0 0.02 3 7 8 -

Mo/HZSM-5(Si) 0.06 0.02 131 11 162 90

Mo/HZSM-5(Si,Act) 0.05 0.03 105 15 137 -

Mo/HZSM-5(Si,2h) 0.03 0.03 6 18 104 -

Mo/HZSM-5(Si,10h) 0.00 0.01 5 5 4 -

The Al speciation in the fresh and spent zeolites was characterized by 27

Al MAS NMR

spectroscopy. The weight-normalized 27

Al MAS NMR spectrum of HZSM-5 (Fig. 4.2)

contains a dominant feature at δ = 55 ppm (δ, chemical shift) due to framework Al (FAl)

Chapter 4

82

atoms and a smaller feature at δ = 0 ppm due to extraframework Al (EFAl) species.

Modification of HZSM-5 with Mo (Mo/HZSM-5) led to a decrease and broadening of the FAl

signal caused by the proximity of cationic Mo-oxo complexes that replace the protons [21].

Two weak features at δ = -11 ppm and δ = 14 ppm are due to EFAl species in the form of

Al2(MoO4). The NMR spectra of the spent samples show a decreasing FAl content for

increasing MDA reaction time. Although the weak shoulder at δ = 30 ppm due to distorted

four-coordinated or five-coordinated Al species [22] increased slightly, the decrease in FAl

content is not paralleled by an increase in EFAl content. The increasing amount of NMR-

invisible Al may be related to framework damage occurring during the MDA reaction. It is

however at odds with the remaining crystallinity of the spent Mo/HZSM-5(10h) sample.

Another explanation is that rehydration of the spent samples before the 27

Al NMR

measurements may be incomplete because of the decreased micropore accessibility. The

resulting asymmetric Al coordination environments can in principle also explain the

decreased NMR-visibility of the Al atoms.

Fig. 4.2. 27

Al MAS NMR spectra of the parent, activated and spent Mo/HZSM-5 catalysts.

Conventional characterization of the surface acidity by IR spectroscopy of the activated and

spent samples was not possible, because the samples were black due to carbon laydown. We

used the low-temperature isotopic exchange reaction between perdeuterobenzene and benzene

to probe the Brønsted acidity of the activated and spent Mo/HZSM-5 catalysts. The isotopic

exchange reaction between C6H6 and C6D6 can be catalyzed by BAS at relatively low

150 100 50 0 -50

(ppm)

Chapter 4

83

temperatures [23]. This reaction has been earlier successfully employed in order to determine

the concentration of BAS of zeolites, clays and amorphous silica-alumina [24-26]. We used

the rate of C6H5D formation at a temperature of 313 K as a measure for the number of BAS.

This isotopomer is the main reaction product of isotopic exchange reaction of C6H6, because

C6H6 was present in tenfold excess to C6D6 in the reactant feed mixture. The absolute reaction

rates and the relative reaction rates compared with the parent HZSM-5 zeolite are given in

Table 4.3. The reaction rate of Mo/HZSM-5 was only slightly lower than that of the parent

HZSM-5 zeolite, consistent with the relatively small decrease in the bridging hydroxyl

density upon Mo introduction as probed by IR spectroscopy [17, 27, 28]. Activation in

methane resulted in a significant decrease of the Brønsted acidity. Although it is usually

assumed that some BAS are regenerated by the carburization of cationic Mo-oxo complexes,

we found that the overall acidity decreased further during the carburization step. Together

with the textural data, we conclude that blockage of the micropores was the main cause of the

decreased acidity. The H/D exchange data show that the accessible acidity was lower after

prolonged reaction. After 5 min on stream, already half of the acid sites of the fresh

Mo/HZSM-5 were not involved in the H/D exchange reaction anymore. The acidity gradually

decreased for prolonged reaction times and, after 10 h on stream, the accessible acidity was

nearly completely lost. The acidity decrease strongly correlates with the loss in micropore

volume as determined by Ar physisorption.

Table 4.3. Physico-chemical properties of fresh and spent Mo/HZSM-5 and Mo/HZSM-5(Si).

Sample Mo/Al1 IFAl

2

(%)

rH/D3

(mmol/min.gcat)

normalized rH/D 4

HZSM-5 - 100 1.27 1

Mo/HZSM-5 0.39 53 1.18 0.92

Mo/HZSM-5(act) 0.39 37 0.79 0.62

Mo/HZSM-5(0.08h) 0.39 40 0.61 0.46

Mo/HZSM-5(2h) 0.39 34 0.36 0.28

Mo/HZSM-5(5h) 0.39 23 0.14 0.11

Mo/HZSM-5(10h) 0.39 11 0 0

Mo/HZSM-5(Si) - 42 - -

Mo/HZSM-5(Si,act) - 23 - -

Mo/HZSM-5(Si,2h) - 14 - -

Mo/HZSM-5(Si,10h) - 15 - - 1 Atomic Mo/Al ratio as determined by ICP-OES analysis;

2 FAl concentration references to HZSM-5

as determined by 27

Al MAS NMR spectroscopy; 3 Isotopic exchange rate.; Relative isotopic exchange

rate normalized to HZSM-5.

Chapter 4

84

The Mo speciation in the external surface region of the zeolite crystals was investigated

by XPS. Table 4.4 collects the XPS results including reference binding energies for various

Mo species taken from literature [29]. It is difficult to discern between metallic Mo and highly

dispersed Mo2C (Mo2Csmall), because these species have nearly similar binding energies [30].

Wang et al. mentioned that carburization of MoO3 to Mo2C particles is thermodynamically

favored over full reduction of Mo species to metallic Mo under MDA conditions [30]. The

Mo speciation is given in Table 4.4. The parent Mo/HZSM-5 zeolite mainly comprised MoO3.

Upon activation in methane, MoO3 was almost completely converted into highly dispersed

Mo2C. Small amounts of MoO2 and large Mo2C particle (Mo2Clarge) were also observed. After

5 min of reaction (Mo/HZSM-5(0.08)), the oxidation degree of the Mo phase was higher than

directly after carburization. A likely explanation is that the replacement of the diluted

methane feed used for pre-carburization by the pure methane reactant feed led to a much

higher carburization rate of the remaining oxides and formation of water. The formation of

water may result in re-oxidation of some of the Mo-carbides at the surface. After prolonged

reaction, the amount of low-dispersed Mo2C particles has increased at the expense of highly

dispersed Mo2C particles, pointing to slow sintering of the initially highly dispersed Mo2C

particles during the MDA reaction. The XPS data show that, at the same time, the Si/C and

Al/C ratios in the surface region decreased (Table 4.4). We interpret this in terms of the

formation of a carbonaceous layer around the zeolite that separates the Mo-phase from the

zeolite surface.

Chapter 4

85

Table 4.4. Mo and C speciation in surface region of spent Mo/HZSM-5 and Mo/HZSM-5(Si)

catalysts as determined by XPS.1

Sample MoO3

(%)

MoO2

(%)

Mo2Csmall

(%)

Mo2Clarge

(%)

Atomic ratios

Si/Al Al/C Si/C

Mo/HZSM-5 100 0 0 0 18.5 0.43 1.64

Mo/HZSM-5(act) 1 15 65 18 19.3 0.23 0.92

Mo/HZSM-5(0.08h) 16 32 27 25 19.4 0.13 0.53

Mo/HZSM-5(2h) 11 30 23 37 20.8 0.08 0.35

Mo/HZSM-5(5h) 10 30 5 55 19.3 0.05 0.19

Mo/HZSM-5(10h) 7 27 11 55 22.3 0.02 0.09

Mo/HZSM-5(Si) 100 0 0 0

Mo/HZSM-5(Si,act) 33 39 4 23 22.2 0.12 0.52

Mo/HZSM-5(Si,2h) 15 35 19 31 22.1 0.09 0.43

Mo/HZSM-5(Si,10h) 1 29 11 60 22.0 0.004 0.14 1 Binding energy of Mo species from Ref. 29: MoO3: 232.7 eV; MoO2: 229.8 eV; Mo2Csmall: 227.6 eV;

Mo2Clarge: 228.0 eV.

The nature of the Mo phase and carbonaceous deposits in the fresh, activated and spent

catalysts was investigated by UV Raman spectroscopy (Fig. 4.3). The Raman signal upon 244

nm excitation mostly derives from the species present at the external surface of the zeolite

crystals [12]. The Raman spectrum of HZSM-5 shows bands that are typical for MFI zeolite

[18]. The most prominent band at 380 cm-1

can be assigned to the double-five-ring vibration

of the MFI framework. In Mo/HZSM-5, the intensity of this band is very low, which is caused

by the presence of Mo-oxo species at the external zeolite surface [31]. Compared with

HZSM-5, the spectrum of Mo/HZSM-5 contains additional bands at 280 cm-1

, 336 cm-1

, 821

cm-1

and 995 cm-1

, which can be attributed to α-MoO3 [32-34,8]. Upon activation in methane,

the α-MoO3 signals disappeared as a result of carburization of the Mo-oxides [35]. New broad

bands appear at ~1400 cm-1

and ~1600 cm-1

. A closer look at this region (right panel of Fig. 3)

shows that the band at 1600 cm-1

shifted towards lower wavenumbers for prolonged reaction

times. Li et al. have assigned Raman bands in this region to various types of carbon [37].

PAHs and graphitic carbon give rise to bands at 1595 cm-1

and 1585 cm-1

, respectively [37].

Accordingly, we attribute the spectral changes to the formation of increasing amounts of

PAHs and graphitic carbon during the MDA reaction. The shoulders visible at 1610 cm-1

and

1560 cm-1

relate to adsorbed naphthalene molecules and to conjugated olefinic species,

respectively [37]. The broad band at 1385 cm-1

is typical for coke formed from olefinic

precursors. The band originally positioned at 1385 cm-1

, which shifted towards 1365 cm-1

after prolonged reaction; is characteristic for graphitic carbon [37]. In summary, the Raman

Chapter 4

86

spectra point to the formation of significant amounts of PAHs and graphitic carbon in the

spent samples.

Fig. 4.3. UV Raman spectra (λexcitation = 244 nm) of the parent, activated and spent Mo/HZSM-

5 catalysts.

TGA was employed to characterize the carbonaceous deposits on the spent catalysts (Fig.

5.4). Three types of carbon were distinguished as a function of the calcination temperature

during TGA in artificial air. These include relatively light carbonaceous species associated

with Mo-carbides (CMoC, ~693 K), soft coke (Csoft, ~753 K) and hard coke (Chard, ~813 K).

Soft coke is thought to be amorphous in nature and likely formed in the proximity of Mo-

carbide particles [38]. Hard coke is mainly comprised of PAHs formed by reactions of olefins

on BAS located at the external surface of the zeolite. Table 4.1 lists the results of

deconvolution of the TGA curves in Fig. 4.4. The catalysts recovered after the activation step

and after 5 min reaction (Mo/HZSM-5(0.08)) contained about 25% CMoC and 75% Csoft and

very little hard coke. The total amount of carbon was similar for these samples. Longer

reaction times led to a significant increase of the total carbon content, almost exclusively in

the form of hard coke.

1200 1400 1600

Inte

nsit

y (

a.u

.)

Wavenumber (cm-1)

400 800 1200 1600

Inte

nsit

y (

a.u

.)

Wavenumber (cm-1)

Chapter 4

87

Fig. 4.4. TGA weight-loss curves of spent Molybdenum modified ZSM-5.

We used transmission electron microscopy to study the dispersion of the Mo-oxide/carbide

phase in more detail. Bright-field TEM images are shown in Fig. 4.5; Fig. 4.6 reports

HAADF-STEM images. For fresh Mo/HZSM-5, a few large MoO3 particles with sizes up to

100 nm are visible at the external zeolite surface. As the XRD patterns did not contain

evidence for such large MoO3 particles, we conclude that the amount of such large particles is

relatively small. Activation in methane converted the Mo-oxide particles to small Mo2C

particles (Fig. 4.5b). Only a relatively small amount of Mo2C particles larger than 10 nm are

visible in the EM images. Close inspection of the images reveals that an amorphous carbon

layer has formed at the external surface of the zeolite crystals. Some of the Mo2C particles are

separated from the zeolite crystal by this carbonaceous layer. After activation, also some

carbon nanotubes are visible in line with earlier findings [39]. The TEM images of the

catalyst recovered after 2 h of reaction (Fig. 4.5c) showed a much lower density of small-

sized Mo2C particles. Clearly, the Mo-carbide particles sinter during the MDA reaction. After

2 h, the amorphous carbon layer covers a significant fraction of the external zeolite surface.

Compared with Mo/HZSM-5(act), more Mo2C particles were seen to be separated from the

zeolite surface by this carbonaceous layer. After 10 h on stream, the carbonaceous layer was

much thicker; it is difficult to discern the external surface of the zeolite (Fig. 4.5d). In this

sample, the Mo2C phase was present as relatively large particles. The images taken in STEM-

HAADF mode (Fig. 4.6) serve to illustrate the gradual transformation of the Mo-carbide

600 800 1000

Weig

ht

loss r

ate

(m

g/K

)

Temperature (K)

Chapter 4

88

phase from highly dispersed particles in the activated catalyst towards large agglomerated

particles after 10 h reaction. It supports the conclusion that the dispersion of the Mo-oxide

phase is initially very high. Upon carburization, they slowly sinter during the MDA reaction;

this process is accelerated by the detachment of these particles from the external zeolite

surface support, which is caused by the carbonaceous layer.

Fig. 4.5. Transmission electron micrographs of (a) Mo/HZSM-5, (b) Mo/HZSM-5 (act), (c)

Mo/HZSM-5 (2h) and (d) Mo/HZSM-5 (10h).

20 nm

100 nm 20 nm

Small Mo2C

Large Mo2C

Carbon nanotube

Carbon deposits + Mo2C

Carbon deposits

Large Mo2C

20 nm

Small Mo2C

Large Mo2C

Carbon deposits

b

c d

200 nm

a

Chapter 4

89

Fig. 4.6. STEM-HAADF micrographs of (a) Mo/HZSM-5, (b) Mo/HZSM-5 (act), (c)

Mo/HZSM-5 (2h) and (d) Mo/HZSM-5 (10h).

We also characterized some of the spent Mo/HZSM-5(Si) catalysts. The results were

qualitatively similar to those obtained for the spent Mo/HZSM-5 samples. A gradual decrease

in the micropore volume (Table 4.2) was observed with reaction time. The micropore volume

of the sample retrieved after 10 h reaction was also negligible for the silylated zeolite catalyst.

The FAl content as determined by NMR spectroscopy also decreased with the progressing

reaction. For silylated Mo/HZSM-5, XPS data point out the formation of a carbonaceous layer

around the zeolite crystals. TGA confirms that PAHs are the main compounds in the

carbonaceous layer formed around the zeolite crystals. Although the general trends are

similar, some subtle differences can be noted between Mo/HZSM-5 and Mo/HZSM-5(Si).

The micropore volumes for the spent Mo/HZSM-5(Si) catalysts were lower than those of the

Mo/HZSM-5 analogues. This is due to the improved spreading of the Mo phase upon

silylation [17]. Activated Mo/HZSM-5(Si) contained more MoO3 and MoO2 in comparison

with Mo/HZSM-5(act). This difference suggests slower carburization of the Mo-oxide

precursor in silylated Mo/HZSM-5, possibly due to the increased interaction of the Mo-oxide

0.5 μm

50 nm

0.2 μm0.2 μm

50 nm

a b

c d

Chapter 4

90

precursor with the zeolite surface. TGA of the carbonaceous deposits revealed a relative large

amount of soft coke after activation of Mo/HZSM-5(Si)). Combined with the XPS and

textural analysis data, these findings support the conclusion that the Mo dispersion upon the

silylation treatment of Mo/HZSM-5 was improved. The overall carbon content of the spent

Mo/HZSM-5(Si) after 10 h was lower compared to that of the non-silylated analogue. The

initial total carbon content of Mo/HZSM-5(Si) was higher than that of Mo/HZSM-5. We

believe that the amount of carbon formed due to undesired side-reactions in Mo/HZSM-5(Si,2

h) is overestimated by the TGA analysis. This is caused by the encapsulation of MoCx with a

layer of carbonaceous deposits. As these deposits have to be oxidized (at higher temperature)

before the MoCx particles can be oxidized, the TGA curves cannot be used to determine the

content of the Mo-carbide particles. In this way, the amount of coke is overestimated and the

amount of MoCx is underestimated. Subtracting the initial CMoC content from the Ctotal in the

spent catalysts recovered after 2 h in the MDA reaction indicates that less carbon deposits

were formed on Mo/HZSM-5(Si,2 h) (9.3–4.1 = 5.2wt%) compared with Mo/HZSM-5(2h)

(7.8–0.9 = 6.9wt%).

4.3.3 Deactivation mechanism

This study investigated the deactivation of the Mo/HZSM-5 catalyst in the bifunctional

MDA reaction. It is generally assumed that the Mo-carbide phase converts methane into

ethylene and hydrogen. The olefins are then reacted on the BAS to aromatic compounds. The

main focus of the present investigation was on the changes of the catalyst upon activation and

during reaction that lead to catalyst deactivation. An important finding of the present study is

that a thick carbonaceous layer is formed at the external zeolite surface. This layer blocks the

access of the olefinic intermediates to the acid sites. In addition, it leads to the detachment of

the Mo-carbide particles from the zeolite surface, thereby accelerating their sintering. While

the lower accessibility of the acid sites mainly affects the product distribution, the sintering of

the Mo-carbide phase leads to decreased methane conversion rates. During catalyst

preparation, Mo modification of the parent zeolite (Fig. 4.7a) leads to the migration of a small

fraction of mobile Mo-oxo species into the micropore space during the calcination step (Fig.

4.7b). Most of the Mo-oxo species remain at the external surface, predominantly in the form

of highly dispersed particles, but also as some larger MoO3 crystallites. Upon activation in

methane, the Mo-oxide carburizes (Fig. 4.7c). The resulting Mo-carbide (MoCx) particles are

Chapter 4

91

mainly present in highly dispersed form at the external surface and inside the micropores.

During the carburization process, the micropore volume became lower, which may be

attributed to the growth of the Mo-carbide species in the micropores as well as the formation

of amorphous polyolefinic (soft coke) associated with Mo-carbides. At the external surface, a

small amount of PAHs (hard coke) forms. The BAS located at the external surface are most

likely involved in the formation of these hard coke deposits. We speculate that also silanol

groups may be implicated in oligomerization reactions of unsaturated intermediates at the

very high temperatures used for the MDA reaction.

During the first 2 h of reaction, the micropore volume did not substantially change. After

2 h, part of the highly dispersed Mo2C particles at the external surface agglomerated into

larger particles as evidenced by TEM measurements (Fig. 4.7d). Agglomeration of the MoCx

phase may be attributed to the decreased interaction of the particles with the external zeolite

surface. This is caused by the formation of a carbon layer at the external surface that separates

the MoCx particles from the zeolite external surface as seen in the TEM images. It has been

reported that large MoCx particles are undesired, because they exhibit high selectivity towards

coke and, accordingly, accelerate deactivation [39]. Based on our results, we argue that the

loss in MoCx dispersion is the main reason for the decreased methane conversion rate

observed in the catalytic performance data.

After 2 h, the PAHs layer rapidly grows over the external zeolite surface (Figs. 4.7e and

4.7f) and, in this way, lowers the micropore volume. Consequently, the amount of BAS

accessible to olefinic intermediates is decreased. As the formation of hard coke correlates well

with the loss in micropore volume, we attribute the decrease in aromatics selectivity with time

on stream to the decreased accessibility of the shape-selective BAS located in the micropores.

Agglomeration of MoCx particles and formation of soft coke inside the micropores may also

contribute to the decreasing pore volume. After 10 h on stream, the thickness of the

polyaromatic carbon layer has grown to 20 nm (Fig. 4.7f). It leads to the nearly complete

inaccessibility of the micropores.

Chapter 4

92

Fig. 4.7. Schematic representation of the catalysts physical state after various times on stream. The figures represent (a) parent ZSM-5, (b)

Mo/HZSM-5, (c) Mo/HZSM-5 (act), (d) Mo/HZSM-5 (2h), (e) Mo/HZSM-5 (5h) and (f) Mo/HZSM-5 (10h).

a b c

d e f

zeolite

MoO3

MoCx

hard coke

soft coke

Chapter 4

93

4.4 Conclusions

The deactivation of Mo/HZSM-5 catalysis in the MDA reaction was investigated. The

parent zeolite, the Mo-modified zeolite catalyst and activated and spent catalysts were

characterized in detail for their textural properties, the dispersion and location of the Mo-

oxide/carbide phase and the nature of the carbonaceous deposits. After carburization of the

Mo-oxide phase, optimum performance in terms of benzene yield is reached. The formation

of a carbonaceous layer consisting of PAHs at the external surface decreases the accessibility

of the BAS in the zeolite micropores. Simultaneously, this carbonaceous layer lowers the

interaction of the Mo-carbide particles with the external zeolite surface. As a consequence, the

Mo-carbide particles sinter, which explains the decreased methane conversion rate. The lower

Brønsted acidity shifts the selectivity from benzene to unsaturated olefinic intermediates. The

hard coke layer at the external surface is due to acid sites that can be partially deactivated by

silylation.

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Chapter 5

95

One-step synthesis of nano-crystalline MCM-22

Summary

Nano-crystalline MCM-22 zeolite was synthesized in a one-pot procedure by the use of an

organosilane (dimethyl-octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride, TPOAC) in

the zeolite synthesis gel. This crystal growth inhibition procedure introduced mesopores in

the MCM-22 crystallites. The lower mechanical stability of the nano-crystalline MCM-22

zeolite compared with bulk MCM-22 can be countered to some extent by pillaring. The

increased external surface of the microporous zeolite domains resulted in increased

accessibility of the Brønsted acid sites, as followed from the better performance in liquid-

phase benzene alkylation with propylene as compared with bulk MCM-22. The increased

accessibility of the internal acid sites in Mo-loaded hierarchical MCM-22 was also evident

from the improved benzene selectivity during methane aromatization. Silylation of

hierarchical Mo/MCM-22 was detrimental for the catalytic performance in MDA. The nano-

crystalline MCM-22 has physico-chemical and catalytic properties intermediate between

those of MCM-22 and ITQ-2 with the benefit over ITQ-2 that it can be synthesized in a single

step.

This chapter has been accepted for publication in Microporous Mesoporous Mater. (2015)

Chapter 5

96

5.1 Introduction

Since the first report in the early 1990s [1], MCM-22 zeolite has attracted considerable

attention of the catalysis community. MCM-22 is a medium-pore zeolite, prepared at typical

Si/Al ratios between 10 and 20. The MWW pore topology endows promising catalytic

properties to MCM-22 zeolites. For instance, MCM-22 exhibits shape selectivity properties in

the trans-alkylation of toluene to p-xylene [2]. Its potential for the hydroisomerization of n-

alkanes into branched alkanes [3, 4] and the alkylation of benzene to cumene and

ethylbenzene [5, 6, 7] has also been demonstrated. MCM-22 has also been explored as an

acidic support in methane dehydroaromatization [8, 9].

As-synthesized MCM-22 consists of MWW layers that are kept together by hydrogen

bonds between the silanol groups that terminate the layers. Calcination of this precursor leads

to condensation of these silanol groups, resulting in a rigid crystal. The micropore system of

MCM-22 is made up from two separate two-dimensional channel systems [1]. The micropore

channels in the [001] direction consist of straight 10-membered rings with a diameter of

typically 5.6 Å [1, 10]. The second pore system is created when two cups located at the

surface of adjacent MWW layers are connected to form a supercage. These large ellipsoidal

cages are typically 7.1 Å in diameter and 18.2 Å in height [10]. The large cages are connected

with each other through 10-membered ring windows.

MCM-22 is employed at the industrial scale for the liquid-phase alkylation of benzene to

cumene and ethylbenzene [11-13]. The application of a solid catalyst such as MCM-22 for

benzene alkylation is important, because it can replace environmentally stressing AlCl3 [14].

Sastre et al. have shown that benzene does not enter the micropores of MCM-22 under liquid-

phase conditions, which has been related to the slightly twisted micropore entrances [15].

Accordingly, it has been assumed that alkylation takes mainly place over Brønsted acid sites

(BAS) located at the external surface of MCM-22. In agreement with this, increasing the

external surface by delamination improves the catalytic performance in alkylation reactions

[5, 16]. Delaminated MCM-22 (ITQ-2) consists of single MWW layers and contains a high

concentration of surface BAS, which are accessible to relatively bulky molecules [5]. The

preparation of ITQ-2 involves a number of delicate steps, including swelling of as-synthesized

MCM-22 by a surfactant, delamination by ultra-sonication and hydrolysis [17].

One of the other possible uses of MCM-22 zeolite is as the acidic component in

bifunctional catalysts for non-oxidative methane dehydroaromatization (MDA). This reaction

Chapter 5

97

converts methane into benzene and hydrogen. The most studied catalyst for this reaction is

Mo/ZSM-5 [18], although also other zeolite types - mainly medium pore structures - have

been tested. Several studies report that MCM-22 zeolite performs better than ZSM-5 [19-21].

MDA catalysts are comprised of an acidic zeolite and Mo-oxide. The Mo-oxide precursor

supported on the zeolite is rapidly converted into Mo-carbide under reaction conditions. These

Mo-carbide particles convert methane into a C2-intermediate, presumably ethylene, which is

further converted over BAS into benzene, toluene and naphthalene. The main challenge to

further develop the MDA technology is to overcome the rapid deactivation of the catalyst by

coke formation [22-26]. Decreasing the crystal size has been shown to lower the deactivation

rate for Mo/ZSM-5 and Mo/MCM-22 catalysts [27-29].

A large number of methods to synthesize hierarchically structured zeolites have been

described along the last decade. A number of reviews categorize and discuss these approaches

[30-32]. Broadly speaking, two strategies can be followed, namely bottom-up and top-down

approaches. In top-down approaches, mesopores are introduced after the zeolite has been

synthesized, and it is usually achieved by extracting either Al or Si atoms from the zeolite

framework. Common methods for demetallation include steaming [33-40], acid-leaching [30-

46] and base treatment [47, 48]. In bottom-up approaches, the mesopores are introduced in the

framework during the synthesis. This approach usually involves the use of a space-filling

molecule (mesoporogens). For example, mesoporous voids can be introduced by the simple

addition of carbon beads to the synthesis gel of MFI [49-51]. Choi et al. were the first to

report the introduction of intra-crystalline mesoporosity using an amphiphilic organosilane

surfactant molecule (dimethyl-octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride,

TPOAC) [41]. Recently, Carvalho et al. have prepared nano-crystalline ZSM-12 by this

approach [42]. In contrast to MFI for which several direct routes for hierarchical structuring

have been explored, only one route has been reported for the preparation of hierarchical

MCM-22. This route involves the addition of carbon black pearls to the synthesis gel [29, 52].

In this study, we report about a one-pot synthesis approach of nano-crystalline MCM-22

by use of TPOAC as a crystal growth inhibitor. The addition of TPOAC impedes the

crystallite growth, in a similar manner as has been shown for nano-crystalline ZSM-12 [42].

The mechanical stability of this nano-crystalline MCM-22 zeolite can be improved by

pillaring. The resulting zeolite material shows improved performance in the liquid-phase

alkylation of benzene and the aromatization of methane compared to catalysts based on bulk

MCM-22.

Chapter 5

98

5.2 Experimental

5.2.1 Synthesis

A literature procedure was employed for the preparation of bulk MCM-22 [53]. This

recipe was modified to generate mesoporosity by adding an organosilane. In a typical

synthesis, an amount of 5.85 g silica gel (Sigma Aldrich) was mixed with 2.97 g of

hexamethylene imine (HMI) in a round-bottom flask. In a second round-bottom flask, 0.385 g

sodium hydroxide and 0.48 g sodium aluminate were dissolved in 30 ml water. The latter

solution was added to the first one and the mixture was stirred overnight at room temperature.

Then, dimethyl-octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride (60 wt% in

methanol, ABCR) was added and the stirring was continued for 4 h. The resulting gel was

transferred to a Teflon-lined stainless-steel autoclave and kept at 423 K for 7 days under

rotation (60 rpm). The Si/Al ratio of the final synthesis gel was 16.3. The HMI/TPOAC ratio

in the synthesis gel was varied (HMI/TPOAC = 6, 12 and 120). The resulting solids are

denoted as MCM-22(x) with x being the HMI/TPOAC ratio. MCM-22 prepared without

TPOAC in an otherwise similar synthesis gel served as the reference. The organics were

removed from the as-synthesized materials using calcination in artificial air (20/80 (v/v)

O2/He) at 623 K for 6 h using a heating rate of 1 K/min.

Silica pillars were introduced in MCM-22 by impregnating 1 g of the non-calcined

MCM-22(12) with 4 g of tetrapropylammonium hydroxide solution (40 wt% TPOAH). The

suspension was stirred for 16 h at room temperature. To 1 g of the resulting zeolite/TPOAH

mixture, 5 g of tetraethylorthosilicate (TEOS) was added under vigorous stirring in Ar

atmosphere. This mixture was stirred at 351 K for 25 h. This procedure was followed by

addition of HCl until the pH was 1. The resulting mixture was stirred for 6 h at 313 K. The

solid was recovered by filtration and washed with copious amounts of water. The material was

dried overnight at 373 K and finally calcined. Calcination was done by heating in N2

atmosphere to 723 K followed by exposure to artificial air (20/80 (v/v) O2/He) at 823 K for 12

h. The pillared sample was denoted as MCM-22(12)-pillared.

Another reference sample was ITQ-2. This delaminated zeolite was prepared following

the procedure outlined by Corma and co-workers [54]. As-synthesized MCM-22 was used as

the starting material for the delamination procedure. To 1 g of the MCM-22 precursor 3.9 ml

of CTAB solution (29 wt%, CTAB) was added. After dispersion 1.2 g TPAOH (40 wt% in

water) was added. The final mixture was heated for 16 h at 353 K. After this, the mixture was

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99

ultrasonicated for 1 h. Finally, the pH was adjusted to 2 and stirred for 1 h and subsequently

filtered. To remove the organic constituents the solid was calcined in artificial air at 823 K for

6 h.

To obtain the proton form, the zeolites were ion-exchanged in a 1 M NH4NO3 solution

for 4 h followed by filtration. This procedure was repeated two times. After drying overnight

at 383 K, the solids were calcined at 723 K for 4 h in an artificial air flow.

For methane dehydroaromatization, molybdenum was loaded onto the MCM-22 zeolites. For

this purpose, the dried zeolite was impregnated with a solution of appropriate concentration

ammonium heptamolybdate tetrahydrate (AHM, Merck). After impregnation the samples

were dried for 1 h. The targeted Mo content was 4 wt%. The Mo-loaded zeolites were

calcined in artificial air after heating to 823 K at a rate of 1.5 K/min. The final temperature

and the dwell time at this temperature were varied. Zeolites modified with molybdenum are

denoted by the prefix “Mo/”.

A portion of the Mo-modified zeolites was silylated by a procedure described in the

literature [55]. For this purpose, 2 g of zeolite was dried at 373 K overnight and then

dispersed in 50 ml n-hexane. To this suspension, 0.3 ml tetraethylorthosilicate (TEOS, Merck)

was added under stirring and, subsequently, refluxed for 1 h. The amount of TEOS

corresponded to 0.4 wt% based on the amount of zeolite in the suspension. Thereafter, the

catalyst was filtered and dried at 373 K overnight. Finally, the zeolite was calcined in two

steps. The first step consisted of heating the sample at a rate of 2 K/min to 393 K followed by

an isothermal period of 2 h. The second step further increased the temperature to 773 K at a

rate of 0.2 K/min followed by an isothermal period for 4 h. The silylation treatment was

denoted by adding “Si” to the catalyst notation, e.g., Mo/MCM-22(12, Si).

5.2.2 Characterization

XRD patterns were recorded on a Bruker D4 Endeavor machine using Cu Kα radiation.

Diffraction patterns were measured in the 5° ≤ 2Ө ≤ 60° range using a step size of 0.1°.

Ar sorption isotherms were measured at 87 K on a Micromeritics ASAP2020 system in static

measurement mode. The samples were outgassed at 623 K for 8 h prior to the sorption

measurements. The Brunauer–Emmett–Teller (BET) equation was used to determine the

surface area (SBET) from the adsorption data in the p/p0 = 0.05–0.25 range. The mesopore

volume (Vmeso) and mesopore size distribution were determined by applying the Barrett–

Chapter 5

100

Joyner–Halenda (BJH) method to the adsorption branch of the isotherm. The micropore area

(Smicro) and micropore volume (Vmicro) were calculated by the NLDFT method (Ar at 87 K on

oxides as the model, assuming cylindrical pores, without regularization).

Infrared spectra were recorded in the 4000-400 cm-1

range using a Bruker Vertex 70v

apparatus. Samples were pressed into a self-supporting wafer with a density of about 10

mg/cm2. To remove adsorbed water the sample was evacuated for 2 h at 773 K. After

evacuation the sample was cooled to 323 K followed by recording of the background

spectrum. The BAS concentration was determined by measuring IR spectra of adsorbed

pyridine. Pyridine adsorption was carried out on the dehydrated zeolite wafer at 423 K. After

saturation was reached following exposure to gaseous pyridine, the sample was evacuated at

523 K for 2 h and a spectrum was recorded. The evacuation step was repeated at 623 K and

673 K. The acidity of the samples was determined by deconvolution of the weight normalized

spectra according standard procedures and the obtained values are expressed in arbitrary units.

Magic-angle spinning (MAS) nuclear magnetic resonance (NMR) measurements were

performed on a 11.7 T Bruker DMX500 NMR spectrometer operating at a frequency of 500

MHz for 1H measurements. The

27Al MAS NMR spectra were measured using a Bruker 2.5-

mm MAS probehead spinning at 20 kHz. The 1H and

29Si MAS NMR measurements were

carried out using a 4-mm MAS probehead at sample rotation rates of 12.5 kHz for 1H and 10

KHz for 29

Si NMR measurements, respectively.

Quantitative 1H NMR spectra were recorded with a Hahn-echo p1-τ1-p2-τ2-aq pulse

sequence with a 90o pulse with p1 = 5 μs and a 180º pulse with p2 = 10 μs. The interscan

delay was set at 120 s. Quantitative 29

Si NMR spectra were recorded using a high power

proton decoupling direct excitation (DE) pulse sequence with a 45 o pulse duration of 2.5 μs

and an interscan delay of 160 s. 1H-

29Si cross-polarization (CP) spectra were obtained using

an interscan delay of 3 s and a contact time of 3 ms. 27

Al NMR spectra were recorded with a

single pulse sequence with a 18o pulse duration of 1 μs and an interscan delay of 1 s.

1H and

29Si NMR shifts were referred to tetramethylsilane (TMS), while saturated Al(NO3)3 solution

was used for 27

Al NMR shift calibration. For 1H MAS NMR measurements, the zeolites were

first dehydrated at a temperature of 723 K at vacuum lower than 10-5

mbar for 6 h. The

dehydrated zeolites were placed into the 4 mm MAS NMR zirconia rotor under inert

conditions. The deconvolution of the NMR spectra was done using DMfit2011.

Transmission electron micrographs were obtained with a FEI Tecnai 20 transmission

electron microscope (TEM) at an electron acceleration voltage of 200 kV. Typically, a small

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101

amount of sample was suspended in ethanol, sonicated and dispersed over a Cu grid with a

holey carbon film. Scanning electron microscopy (SEM) was performed using a Philips

environmental FEIXL-30 ESEM FEG in high-vacuum mode at low voltage.

5.2.3 Catalytic activity measurements

The catalytic activity in the alkylation of benzene with propylene was evaluated in a

high-pressure stainless-steel reactor at 398 K and 3.5 MPa. For these measurements, the

zeolites were pelletized, crushed and sieved in a 0.25–0.42 mm fraction, and diluted with

silicon carbide to a total bed volume of 3.6 ml. The molar benzene/propylene ratio in the feed

was 3.5. The weight-hourly space velocity (WHSV) was 25 h-1

to the olefin in the feed.

Samples were analyzed as a function of time on stream by online gas chromatography (5%

phenyl–95% dimethylpolysiloxane column, length 30 m, internal diameter of 0.25 mm, 1 μm

thick stationary phase film).

The catalytic performance of Mo-loaded zeolites in methane dehydroaromatization was

measured in a fixed-bed reactor at 973 K, 0.1 MPa and a contact time of 16 g cat·h/mol CH4

(GHSV = 1500 h-1

). The catalyst weight was 0.5 g, and was diluted with silicon carbide to

achieve a bed volume of 2.8 cm3. We verified that the diluent was not catalytically active.

Prior to reaction, the catalysts were heated from room temperature to 973 K at a heating rate

of 10 K/min in a methane/nitrogen mixture (80 vol% methane). After reaching the reaction

temperature, the reactor was purged with N2 for 0.5 h. Finally, the feed was switched to

methane. The reactor outlet was analyzed by a two-channel online gas chromatograph. N2

used as internal standard, H2, CO, CO2 and CH4 were separated over HayeSep N (0.5 m),

HayeSep Q (1.5 m) and 13X molecular sieve (1.2 m length) columns (TCD). In the second

channel, hydrocarbons were separated over a pre-column (CP-Wax capillary column, 5.0 m

length and 0.32 mm inner diameter). The light hydrocarbon products were further separated in

a CP-Porabond Q (25 m length and 0.32 mm inner diameter) and detected by a FID. The

aromatics were detected by FID after separation over a CP-Wax column (1.0 m length and

0.32 mm inner diameter).

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102

5.3 Results and discussion

5.3.1 Catalyst preparation

Acid catalyst preparation and physico-chemical properties

The XRD patterns of the as-prepared MCM-22 zeolites are shown in Fig. 5.1. MCM-22,

MCM-22(12) and MCM-22(120) have the MWW structure [53], but the diffraction peaks are

less intense and broader for the materials prepared at lower HMI/TPOAC ratio. The material

prepared at a HMI/TPOAC ratio of 6 was almost completely X-ray amorphous. The ITQ-2

reference material was obtained by exposing as-synthesized MCM-22 (MCM-22(p)) to a

delamination step followed by calcination. The XRD pattern of ITQ-2 matches the one given

in the literature [56]. Table 1 shows that the Si/Al ratios of the MCM-22 materials prepared

with and without TPOAC are similar. The textural properties of the crystalline materials were

determined by Ar physisorption and the results are listed in Table 5.1. The micropore volume

of MCM-22 was 0.15 cm3/g, in good agreement with literature values [57]. The use of

TPOAC led to a small decrease in the micropore volume. The mesopore volume increased

with decreasing HMI/TPOAC ratio. The zeolite prepared at a HMI/TPOAC ratio of 12 had

the largest mesopore volume (Vmeso = 0.3 cm3/g). The higher surface area of ITQ-2 compared

with bulk MCM-22 (414 m2/g vs. 117 m

2/g) indicates that MCM-22(p) was delaminated to a

significant extent. Still, the surface area of our ITQ-2 is lower compared with some of the

values reported before for this material starting from laminar precursors prepared with higher

Si/Al ratios [58]. The lower surface area of our ITQ-2 material is likely due to the difficulty of

delaminating MCM-22(p) with relatively high Al content (Si/Al ratio ~15) [58].

Chapter 5

103

Fig. 5.1. Wide angle XRD reflection patters of calcined (a) MCM-22, (b) MCM-22(120) (c)

MCM-22(12), (d) MCM-22(6), (d) MCM-22(12)-pillared and ITQ-2.

10 20 30 40 50 60

Inte

nsit

y (

a.u

)

Angle (°)

Chapter 5

104

Table 5.1. Physico-chemical properties and elemental composition of the materials before and after applying external pressure.

Sample HMI/TPOAC Si/Al1

Si/Al2 Pressure

3

(106

N/m2)

Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

SBET

(m2/g)

ΔVmeso

(%)

MCM-22 ∞ 16.3 19.2 0 0.15 0.08 332 54 449 -

∞ 16.3 19.2 16 0.10 0.09 191 54 308 +12

MCM-22(120) 120 16.3 - 0 0.14 0.18 263 95 370 -

MCM-22(12) 12 16.3 20.6 0 0.13 0.31 184 122 359 -

12 16.3 20.6 16 0.04 0.18 94 94 219 -40

MCM-22(12)-pillared 12 16.3 21.5 0 0.05 0.32 126 182 406 -

12 16.3 21.5 16 0.05 0.26 118 145 347 -16

ITQ-2 ∞ 16.3 23.7 0 0.14 0.26 294 160 708 - 1 Ratio of Si/Al in the synthesis gel;

2 Determined from ICP-AES elemental analysis;

3 Pressure was applied for 60 s.

Chapter 5

105

In general, the mechanical stability of hierarchical zeolites is lower than that of bulk zeolites

and exerting mechanical forces usually reduces the beneficial mesoporosity. Pillaring is an

established procedure to increase the mechanical stability of such hierarchical zeolites [59]. In

the present work, we applied this procedure to MCM-22(12) because of its favorable textural

properties. Comparison of the XRD patterns before and after pillaring (Fig. 5.1) shows that

the crystallinity was not affected. Ar physisorption data point to the substantial decrease of the

micropore volume due to pillaring, suggesting that part of the micropores were blocked due to

deposition of TEOS in the micropores. The textural properties were also determined after

pressing bulk MCM-22, MCM-22(12) and pillared MCM-22 (Fig. 5.2). Comparison of the Ar

sorption isotherms before and after pressing the samples shows that the decrease of the

mesopore volume upon mechanical stress was lower for MCM-22(12)-pillared than for

MCM-22(12). This is particularly evident from the decrease of the hysteresis loop. On

contrary, the isotherm of bulk MCM-22 is hardly affected by the pressing procedure. All this

is also evident from the textural data derived from the Ar physisorption isotherms listed in

Table 5.1. It is interesting to note that the mechanical stability test led to a decrease of the

micropore volume of MCM-22(12) and MCM-22, but not for the pillared zeolites.

Fig. 5.2. Ar physisorption isotherms of (square) MCM-22, (circle) MCM-22(12), (triangle)

MCM-22(12)-pillared and (diamond) ITQ-2. Isotherms with open symbols correspond to

samples measured after exposing to external mechanical force. The isotherms are presented in

a stacked fashion for clarity. The Y offsets are progressively increased with 100 cm3/g for

each subsequent sample.

0.0 0.2 0.4 0.6 0.8 1.0

Vo

lum

e a

dso

rbed

(cm

3/g

.ST

P)

Relative pressure

100 cm3/g

Chapter 5

106

The morphology of these zeolites was investigated by electron microscopy. Representative

SEM images are shown in Fig. 5.3. The primary crystals of MCM-22 show the well-known

platelet morphology of MWW zeolite arranged in larger spherical secondary particles. The

crystal size of these platelets is several nanometers. From SEM, the morphology of MCM-

22(12) and MCM-22(12)-pillared appears to be more open. The morphology of ITQ-2 is also

platelet-like, arranged in a more aligned fashion, likely due to the single layer morphology at

the nano-scale.

Fig. 5.3. SEM micrographs of (a) MCM-22, (b) MCM-22(12), (c) MCM-22(12)-pillared and

(d) ITQ-2.

Fig. 5.4 depicts representative TEM images of the same materials. The thickness of the

MWW crystallites for MCM-22(12) (Fig. 5.4b), which are in the range of 30-40 nm, are seen

to be similar to MCM-22 (Fig. 5.4a). More detailed inspection of the TEM images of MCM-

22 shows that the zeolite crystals are aligned forming a supra-crystallite structure of

approximately 100 nm. The crystallites of MCM-22(12) seem to be organized in a more

random fashion. The addition of TPOAC possibly causes the zeolite crystals to become

separated, which is likely due to the hydrophobic nature of TPOAC grafted at the zeolite

crystal surface. The random organization of the agglomerated crystallites results in interstitial

voids that are in part responsible for the mesoporosity of the material. Furthermore, some

crystallites show a decreased crystal size (inset Fig. 5.4b) pointing to the inhibiting effect of

a b

c

d

Chapter 5

107

TPOAC on crystal growth. The TEM images show that some of the MWW layers of MCM-

22(12) are separated due to the presence of TPOAC. In this way, additional interstitial voids

were created, which contribute to the mesopore volume. The morphology of MCM-22(12)-

pillared (Fig. 5.4c) is similar to that of MCM-22(12). The TEM image for ITQ-2 (Fig. 5.4d)

confirms that this material contains single MWW layers.

Fig. 5.4. TEM micrographs of (a) MCM-22, (b) MCM-22(12), (c) MCM-22(12)-pillared and

(d) ITQ-2.

29Si MAS-NMR spectroscopy was employed to distinguish between the terminal silanol

and framework silicate species. The amount of terminal silanol groups can be related to the

external surface area. First, 1H→

29Si cross-polarization (CP) measurements were conducted to

prove the presence of Q3 (Si(OSi)3OH and Si(1Al)) and Q2 sites (Si(OSi)2(OH)2), next to the

predominant Q4 sites (SiO4). The Q2 sites are characterized by a peak at -93 ppm, while Q4

species are identified by overlapping peaks in the range of -104 to -120 ppm [60]. Q3 species

including Si(OSi)3OH and Si(1Al) are identified by peaks at -98 ppm and -101 ppm,

respectively [60]. To quantify the various silicate species, the spectra were measured in high-

power decoupling (hp-dec) mode (Fig. 5). The results of deconvolution of these spectra are

given in Table 2. Hierarchical MCM-22 contains more Q3 species than bulk MCM-22 zeolite.

50 nm

20 nm

0.2 µm

50 nm

a b

c d

Chapter 5

108

As the Al bulk content is nearly the same for all MCM-22 zeolites, the higher Q3

concentration can be attributed to the higher silanol content and the larger external surface

area. The larger external surface area is due to the inhibited crystal growth upon addition of

TPOAC. It is consistent with the increasing fraction of Q2+Q3 species.

Chapter 5

109

Fig. 5.5. 29

Si MAS NMR spectra measured in hp-dec mode of (a) MCM-22, (b) MCM-22(12), (c) MCM-22(12)-pillared and (d) ITQ-2.

-80 -100 -120 -140

(ppm)

-80 -100 -120 -140

Inte

nsit

y (

a.u

.)

(ppm)

-80 -100 -120 -140

(ppm)

-80 -100 -120 -140

(ppm)

Chapter 5

110

Acidity characterization

The Al coordination in the MCM-22 zeolites was determined by 27

Al MAS NMR

spectroscopy. The spectra are shown in Fig. 5.6. The two peaks at 55 ppm (FAl1) and 48 ppm

(FAl2) in the spectrum of MCM-22 indicate the presence of two types of framework Al

species in line with the literature [60, 61]. The FAl1 species are located inside the micropores,

while the FAl2 species reside at the external surface and/or the large cavities [61]. Another

less pronounced feature at 0 ppm is due to extraframework aluminum (EFAl). The Al

speciation was quantified by deconvolution of the 27

Al MAS NMR spectra (Table 5.2). The

fraction of EFAl in MCM-22(12) is 30%, which is significantly higher than the EFAl content

(21%) for MCM-22. After pillaring, MCM-22(12)-pillared contains a similar amount of

EFAl. The addition of TPOAC led to a decrease in the FAl2 content for MCM-22(12) and

MCM-22(12)-pillared, which we attribute to the change of the amount of FAl species at the

external surface. The removal of surface Al is also evident when the as-synthesized MCM-22

was delaminated to obtain ITQ-2. This phenomenon has already been reported for the

synthesis of ITQ-2 [54, 58].

Fig. 5.6. 27

Al MAS NMR spectra of (a) MCM-22, (b) MCM-22(12), (c) MCM-22(12)-

pillared and (d) ITQ-2.

The 1H MAS NMR spectra help to characterize the various hydroxyl groups (Fig. 5.7). The

bands at 1.9 ppm and 2.6 ppm are assigned to terminal hydroxyls associated with silanol

100 50 0 -50

Inte

nsit

y (

a.u

.)

(ppm)

Chapter 5

111

groups and EFAL species, respectively [62]. The protons of the Brønsted acid sites (BAS)

give rise to the 4.0 ppm peak in the 1H NMR spectrum. Internal silanols are characterized by

the peak at 6.1 ppm. The content of the various hydroxyl species as determined from these

NMR spectra are listed in Table 5.2. Comparing the quantitative data reveals the BAS content

in MCM-22(12) to be lower than in MCM-22. After pillaring of MCM-22(12) the BAS

concentration increased, indicated by the increase of the corresponding NMR signal. Further

inspection of the NMR spectra suggests a decrease in EFAl content after pillarization of

MCM-22(12), which is confirmed by the quantitative data. These observations are

inconsistent with the findings from the 27

Al MAS NMR experiments. Seemingly, a fraction of

the EFAl is invisible to 27

Al NMR [59, 63-66]. The larger BAS concentration of MCM-

22(12)-pillared compared with MCM-22(12) is most likely due to leaching of the EFAl

present upon acid treatment. In this way, part of the EFAl that was compensating the negative

framework charge is removed, thus increasing the amount of BAS. The removal of FAl

during delamination explains the lower BAS content of ITQ-2 as compared with the parent

MCM-22. These findings are in agreement with the trends observed by 27

Al MAS NMR

spectroscopy and literature [58].

The acidic properties of the zeolites were characterized in more detail using FTIR

spectroscopy. The spectra in Fig. 5.8 contain two main absorption bands at 3612 cm-1

(BAS)

and 3749 cm-1

(silanol groups). MCM-22(12) and MCM-22(12)-pillared contain more silanol

groups than MCM-22, while the reverse holds for the BAS content. The silanol and BAS

contents of ITQ-2 were comparable with MCM-22(12). The acidity of MCM-22(12)-pillared

was larger than that of ITQ-2 and MCM-22(12), albeit lower than that of MCM-22. These

findings are consistent with the results from 1H MAS NMR results. Acid site quantification

by FTIR spectroscopy after exposure of pyridine to the parent zeolites (Table 5.3) trends with

the observations from the FTIR spectra and the 1H MAS NMR results.

Chapter 5

112

Fig. 5.7. 1H MAS NMR spectra of (a) MCM-22, (b) MCM-22(12), (c) MCM-22(12)-pillared and (d) ITQ-2.

Table 5.2. Relative contributions of Si, Al and H species in the MWW zeolites as determined by deconvolution of 29

Si, 27

Al and 1H MAS

NMR spectra.

Nucleus 29

Si NMR 27

Al NMR 1H NMR

Sample Q4

(%)

Q3

(%)

Q2

(%)

FAl1

(%)

FAl2

(%)

EFAl

(%)

SiOHext

(a.u)

SiOHint

(a.u)

Si(OH)Al

(a.u)

EFAlOH

(a.u)

MCM-22 92 7 1 67 12 21 25 22 44 7

MCM-22(12) 84 15 1 62 8 30 49 13 26 17

MCM-22(12)-pillared 82 16 2 61 7 32 63 29 39 10

ITQ-2 79 19 2 73 6 21 52 19 36 14

0 2 4 6 8 10

Inte

nsit

y (

a.u

.)

(ppm)

0 2 4 6 8 10

(ppm)

0 2 4 6 8 10

(ppm)

0 2 4 6 8 10

(ppm)

Chapter 5

113

Fig. 5.8. FTIR spectra of (a) MCM-22, (b) MCM-22(12), (c) MCM-22(12)-pillared and (d)

ITQ-2 (samples dehydrated in vacuo at 723 K).

Catalytic activity measurements: benzene alkylation with propene

Fig. 5.9 depicts the time on stream behavior (Fig. 5.9a) and the selectivity (Fig. 5.9b) of

the various MCM-22 zeolites and ITQ-2 in the liquid-phase alkylation of benzene with

propylene. All catalysts initially show high activity, but deactivate with time on stream. ITQ-2

showed the highest stability in this reaction. The lower rate of deactivation of ITQ-2 as

compared with MCM-22 is in line with previous reports [5]. The nano-crystalline MCM-

22(12) exhibited better stability than the other MCM-22 materials, although the rate of

deactivation is higher as compared with ITQ-2. The higher catalytic stability of ITQ-2

compared with bulk MCM-22 has been explained by the larger amount of BAS accessible to

benzene at the external zeolite surface [5, 67, 68]. The deactivation of MCM-22 is usually

attributed to carbonaceous deposits that cover these BAS at the external surface [69-71]. We

attribute the lower rate of deactivation of MCM-22(12) compared with MCM-22 to the

decreased crystallite size; the decreased crystallite size implies a higher external surface area

and improved accessibility of benzene to the BAS at the external surface. Moreover, the

decrease of the diffusion path length will allow desorption of undesired products (oligomers,

multiple alkylated aromatics) before they are converted into bulkier coke species. The

stability towards deactivation of pillared MCM-22(12)-pillared was the lowest among the

MCM-22 zeolites. This may be correlated with the higher acidity of this sample. Such

3800 3600 3400 3200

Ab

so

rban

ce (

a.u

.)

Wavenumber (cm-1)

Chapter 5

114

deactivation has also been reported by the group of Corma [69], who showed that the

selective deactivation of external BAS with 2,6-di-tert-butyl pyridine (DTPBPy) led to rapid

decrease of the alkylation activity with time on stream. The explanation of low external BAS

content of MCM-22(12)-pillared is supported by the lower selectivity to diisopropylbenzene

(Fig. 5.9b) compared with MCM-22, MCM-22(12) and ITQ-2. The selectivity to multi-

alkylated benzenes products is higher for MWW zeolites with a higher external surface area

[67, 71]. The lower external acidity of MCM-22(12)-pillared due to silica deposition explains

the lower selectivity to diisopropylbenzene.

Fig. 5.9. Catalytic performance in benzene alkylation (T = 398 K; 3.5 MPa; 1/3.5 (mol/mol)

Benzene/propylene): (a) propylene conversion and (b) cumene (closed symbols) and

diisopropylbenzene (open symbols) selectivities of Beta (■), MCM-22 (●), MCM-22(12) (▲)

and MCM-22(12)-pillared (♦) and ITQ-2 (▼).

5.3.2 Preparation, characterization and catalytic testing of bifunctional Mo/zeolites.

The various MCM-22 zeolites were also used as acid supports for the preparation of Mo-

modified zeolite catalysts for the MDA reaction. The Mo was introduced by incipient wetness

impregnation followed by calcination. The targeted Mo content was 4 wt%.

Characterization of bifunctional Mo/zeolites catalysts

The acidity of the Mo-modified zeolites was determined by pyridine adsorbed FTIR (see

Table 5.3). Modification of the parent materials with Mo led to a decrease in acidity for all

0 60 120 180 240 300

0

20

40

60

80

100S

ele

cti

vit

y (

wt%

)

Time on stream (min)

0 60 120 180 240 300

0

10

20

30

40

50

60

70

80

90

100

Pro

pyle

ne c

on

vers

ion

(%

)

Time on stream (min)

Chapter 5

115

samples. The decrease in acidity is due to exchange of the protons associated to the BAS with

Mo-oxo species. Silylation led to further decrease of the acidity in agreement with an earlier

report for Mo/ZSM-5 [71]. The acidity decrease in this case can be attributed to the improved

spreading of Mo over the zeolite and to the deactivation of the external acid sites by the

silylation treatment.

Catalytic activity measurements: methane dehydroaromatization

The time on stream behavior of the Mo/zeolites is shown in Fig. 5.10. While the methane

conversion decreased with time on stream (Fig. 5.10a), the benzene selectivity exhibits an

optimum around 4 h. The initially low benzene selectivity relates to the conversion of Mo-

oxide in the precursor material into Mo-carbide. The highest benzene selectivity (55 wt%)

was observed for Mo/MCM-22(12) after 4 h on stream. The benzene selectivities for

Mo/MCM-22 and Mo/MCM-22(12)-pillared were lower (~40 wt%). Mo/ITQ-2 showed the

lowest benzene selectivity (~30 wt%). The benzene selectivity inversely correlates with the

coke selectivity. Several reports have discussed the formation of coke at the external surface

BAS [72]. Deactivation of these external surface BAS by the addition of a small amount of

silica has been shown to improve benzene selectivity [71]. Therefore, we evaluated to what

extent the performance of these materials can be improved by silylation with TEOS. The

methane conversion as a function of time on stream did not change upon silylation. Silylation

also did not affect the selectivities (Figs. 5.11b and 5.11c) for Mo/MCM-22(12), Mo/MCM-

22(12)-pillared and Mo/ITQ-2 upon silylation. However, the benzene selectivity of

Mo/MCM-22 strongly improved by silylation. We surmise that the less pronounced effect of

silylation on nano-crystalline MCM-22 and ITQ-2 is due to their much higher silanol content.

We also evaluated the usefulness of an earlier developed reaction/regeneration cycle

procedure for the MDA reaction [78]. A typical reaction/regeneration cycle consisted of

reaction for 1.5 h at 973 K, followed by an oxidation step in artificial air at 773 K. Cooling

was done in inert atmosphere but the reaction temperature was recovered by heating in the

feed mixture. The results are shown in Fig. 5.12 (each data point represents catalytic

performance 70 min after the regeneration cycle). These data point out the increased stability

and benzene selectivity of silylated Mo/MCM-22, consistent with the data in Fig. 5.10 and

5.11. Both silylated and non-silylated Mo/MCM-22 displayed increased benzene selectivity

with each consecutive regeneration cycle. The methane conversion increased during the first 3

reaction/regeneration cycles. Thereafter, a small decrease in the methane conversion can be

Chapter 5

116

noted. Acidity characterization (Table 5.3) on the calcined Mo/MCM-22 and Mo/MCM-

22(Si) catalysts after exposure of 12 consecutive reaction cycles reveals a substantial loss in

acidity (~-60%) compared to the fresh catalyst. The reason for this is not clear, but possibly

the loss of the integrity of the zeolite structure, ineffective removal of refractory coke or

agglomeration of the Mo-phase during the oxidative regeneration step may provide an

explanation [79].

Chapter 5

117

Fig. 5.10. Catalytic performance in methane dehydroaromatization (T = 973 K; 0.1 MPa; 5/95 (v/v) N2/CH4): (a) methane conversion, (b)

benzene (closed symbols) and naphthalene (open symbols) selectivities and (c) coke (closed symbols) and olefin (open symbols) selectivities

of MCM-22 (■), MCM-22(12) (●), MCM-22(12)-pillared (▲) and ITQ-2 (♦).

0 2 4 6 8

0

5

10

15

20

25

CH

4 c

on

vers

ion

(%

)

Time on stream (h)

0 2 4 6 8

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)Time on stream (h)

0 2 4 6 8

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

Chapter 5

118

Fig. 5.11. Catalytic performance in methane dehydroaromatization (T = 973 K; 0.1 MPa; 5/95 (v/v) N2/CH4): (a) methane conversion, (b)

benzene (closed symbols) and naphthalene (open symbols) selectivities and (c) coke (closed symbols) and olefin (open symbols) selectivities

of silylated MCM-22 (■), MCM-22(12) (●), MCM-22(12)-pillared (▲) and ITQ-2 (♦).

0 2 4 6 8

0

5

10

15

20

25

CH

4 c

on

vers

ion

(%

)

Time on stream (h)

0 2 4 6 8

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)Time on stream (h)

0 2 4 6 8

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)

Time on stream (h)

Chapter 5

119

Fig. 5.12. Catalytic performance in methane dehydroaromatization with intermediate regeneration ((T = 973 K; 0.1 MPa; 5/95 (v/v) N2/CH4);

each point represents a time on stream of 70 min): (a) methane conversion, (b) benzene (square) and naphthalene (circle) selectivities and (c)

coke (square) and olefin (circle) selectivities of non-silylated (closed symbols) and silylated (open symbols) MCM-22.

0 2 4 6 8 10 12

0

2

4

6

8

10

12

Meth

an

e c

on

vers

ion

(%

)

Regeneration cycle

0 2 4 6 8 10 12

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)Regeneration cycle

0 2 4 6 8 10 12

0

20

40

60

80

100

Sele

cti

vit

y (

wt%

)

Regeneration cycle

Chapter 5

120

5.4 Conclusions

A one-pot synthesis procedure for the preparation of nano-crystalline MCM-22 was

developed. It involves the addition of an amphiphilic organosilane to the synthesis gel. The

total Brønsted acidity of this nano-crystalline MCM-22 zeolite is lower than that of bulk

MCM-22. Nevertheless, nano-crystalline MCM-22 shows higher catalytic performance in the

liquid-phase alkylation of benzene with propylene due to the increased accessibility of the

Brønsted acid sites. The low mechanical stability of the hierarchical material was improved

by pillaring as followed from textural analysis before and after exerting mechanical forces by

pelletizing the powdered zeolite. The addition of silica during the pillaring with TEOS led to a

substantial decrease of the acidity and the catalytic alkylation performance. The shorter

diffusion pathways through the MCM-22 with reduced crystal size also led to an improved

benzene selectivity in the methane aromatization reaction. External surface modification of

the hierarchical Mo/MCM-22 catalyst following a silylation treatment was detrimental to the

catalytic performance of this catalyst in MDA.

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Chapter 6

123

Effect of zeolite crystalline domain size on the methane

aromatization performance of Mo/HZSM-5

Summary

The influence of the micropore domain size on the catalytic performance of Mo/HZSM-5

catalysts in the methane dehydroaromatization reaction is evaluated. For this purpose,

zeolites with varying micropore domain sizes were prepared using different hierarchical

structuring approaches. These include extraction of silica and the use of mesoporogens. The

particle size of bulk ZSM-5 zeolite was also compared by using bulk ZSM-5 zeolites with

average crystal sizes of 10 µm and 1 µm. In this way, the micropore domain size of the zeolite

supports was varied between several nm’s and 10 µm. The structure and texture of the

materials was characterized by XRD, Ar physisorption and electron microscopy. After Mo

introduction, the catalytic performance of these Mo/ZSM-5 zeolites was determined in the

dehydroaromatization of methane was at 973 K. Catalysts derived from bulk ZSM-5 crystals

are prone to rapid deactivation due to blocking of the micropores with carbonaceous

deposits. Larger crystals deactivate more rapidly. Hierarchical structuring was beneficial in

decreasing the rate of catalyst deactivation and improved the benzene formation rate.

10 μm 1 nm 1 μm 100 nm 10 nm

20 nm 30 nm 50 nm

9.66 μm

2 μm

ZSM-5 crystalline micropore domain size

Chapter 6

124

6.1 Introduction

The use of methane, the main constituent of natural gas, as a feedstock for the production

of fuels and chemicals is considered an interesting alternative to petroleum as carbon source.

A promising route is the valorization of methane in the methane dehydroaromatization

(MDA) reaction. In this process methane is converted to benzene and hydrogen. The most

studied catalyst for this reaction is the bifunctional Mo/ZSM-5 catalyst. Under reaction

conditions, the Mo-oxide supported on the zeolite is first rapidly converted into Mo-carbide

species [1]. The Mo-carbide species facilitate oligomerization of methane into olefins [2]. The

produced olefins, presumably ethylene, are then aromatized to benzene over the Brønsted acid

sites (BAS) in the zeolite [3]. The main challenge for industrial application of the MDA

process is rapid catalyst deactivation. As discussed in Chapter 4, the formation of a carbon

layer at the external surface is considered the main reason for catalyst deactivation. The

formation of a carbon layer leads to micropore blockage thereby making the BAS inside the

micropores inaccessible.

In Chapter 2, it was reported that the rate of deactivation in MDA was lower for

hierarchical Mo/HZSM-5 with mesopores than for bulk Mo/HZSM-5. It is speculated that the

presence of mesopores and the corresponding decrease of the micropore domain size of the

MFI zeolite slows down the negative effect of micropore blocking with carbonaceous

deposits. Carbon deposition is one of the major causes of catalyst deactivation in the MDA

reaction. The benefit of decreasing the micropore domain size of zeolites for the catalytic

performance has been demonstrated before. Examples include the prolonged life of

hierarchical Fe/ZSM-5 for the benzene-to-phenol [4] and SSZ-13 for the methanol-to-olefins

[5] reactions. Similar to the MDA reaction, catalyst deactivation in these two reactions can be

largely attributed to coke deposition in the micropore space. The improved lifetime is due to

the more efficient use of the zeolite crystal.

In recent years, the development of novel methods for the preparation of hierarchical

structured zeolites has gained momentum, resulting in a variety of synthesis procedures to

decrease the micropore domain size. Several papers have reviewed the plethora of preparation

methods now available to synthesize such hierarchial zeolites [6-8]. Two synthesis strategies

can be distinguished, namely bottom-up and the top-down approaches. Top-down approaches

use post-synthesis treatment of the zeolite to extract either Al or Si atoms from the zeolite

framework. Often used methods for metal extraction are steaming [9-16], acid-leaching [14-

Chapter 6

125

20] and base treatment [21, 22]. Bottom-up approaches make use of void-filling mesopore

templates or mesoporogen. The early bottom-up routes [23-25] involved addition of space-

filling hard templates such as carbon beads to the synthesis gel of MFI, creating mesoporous

voids. A problem of this approach which is shared with some of the top-down approaches is

that the micropore domains remain relatively large. Recently, Ryoo et al. [26] showed the

possibility to prepare MFI with highly interconnected micropores and mesopores by using an

organosilane surfactant molecule. An alternative approach [27] to prepare MFI with highly

interconnected mesopores is to create a matrix of agglomerated protozeolite crystals. In this

way, the interstitial voids formed between the agglomerated nano-crystallites form the

mesopores. Ryoo et al. also reported the synthesis of MFI nanosheets [28]. The zeolite in this

case crystallizes in the form of sheets with crystal dimension of only a few nm in the direction

of the straight channels.

In this chapter we investigate the role of the micropore domain size of the zeolite support

on the performance of the Mo/HZSM-5 catalyst in the MDA reaction. To this end, we

prepared various mesoporous ZSM-5 zeolites. The size of the microporous domains vary

from several nm to 10 µm. After modifying the zeolites with Mo, the catalysts were evaluated

for their performance in the MDA reaction. The differences in performance are discussed in

terms of morphology and texture.

6.2 Experimental methods

6.2.1. Synthesis of zeolites

Reference ZSM-5

NH4ZSM-5 with Si/Al = 19.4 was obtained from AkzoNobel Catalysts. The parent zeolite

was converted to the proton form by calcination at 723 K using a heating rate of 1 K/min.

This sample is denoted as HZSM-5-ref.

Large ZSM-5 crystals

A basic solution was prepared by dissolving 11.268 g NaOH and 0.876 g KOH in 585.9 ml

water. Then, 266.1 ml Ludox AS was added dropwise under vigorous stirring followed by

further stirring for 1 h. To this suspension, first 100.16 g TPABr was added followed by

addition of 134 ml of an aqueous solution containing 5.13 g NaAlO2. Finally, a solution of

26.9 g ammonium carbonate dissolved in 134 ml water was added to the synthesis gel. The

Chapter 6

126

suspension was aged overnight, transferred to a Teflon-lined autoclave and heated for 10 days

at 443 K under stirring. The organic constituents were removed by calcination. For this, the

material was heated in artificial air at a rate of 1 K/min to 823 K followed by an isothermal

period of 6 h. The final calcined zeolite is denoted as HZSM-5-large.

Desilicated ZSM-5

For desilication, an optimized method described earlier in Chapter 2 was followed. In a

Teflon beaker, 2 g NH4ZSM-5 (AkzoNobel) with a Si/Al of 30 was suspended in 50 ml of an

aqueous NaOH solution (0.2 M). The suspension was stirred vigorously at 338 K for 0.5 h.

The hot suspension was filtered off and washed with copious amounts of water. The

desilicated ZSM-5 zeolite is denoted as HZSM-5-des.

Organosilane templated mesoporous ZSM-5

Tetrapropyloctadecyl ammonium (trimethoxy) silane (TPOAC, 60 wt% in MeOH, ABCR)

was used as the mesoporogen to synthesize hierarchical ZSM-5. A solution (A) was prepared

by dissolving 0.21 g sodium aluminate (NaAlO2), 4.2 g tetrapropyl ammonium bromide

(TPABr) and 1.2 g NaOH in 202.5 ml water. A second solution (B) was prepared by mixing

12.855 g TEOS with 2.07 g TPOAC solution in methanol. Mixture B was added dropwise to

solution A. After 2 h of aging, the resulting gel was transferred into a 125 ml Teflon-lined

autoclave. The autoclave was heated for 5 days at 423 K under rotation. Then, the suspension

was filtered and the recovered solid was washed with copious amounts of water. The sample

was finally calcined at 823 K (HZSM-5-org).

Small ZSM-5 crystals

A solution A was prepared by adding 13.3 ml TEOS to 3.9 ml tetrapropyl ammonium

hydroxide (TPAOH, 40 wt% in water). A second solution B was prepared by dissolving 0.245

g NaAlO2 in 23.49 ml water. After dissolving, solution B was added to solution A. The

resulting mixture was refluxed for 20 h at 363 K under stirring. To this mixture 2.07 g of

phenylammonium trimethoxy silane (PHAPTMS, ABCR) was added and was kept refluxing

for another 6 h at 363 K. After this period, the resulting gel was transferred to a Teflon lined

autoclave and heated at 443 K for 5 days. The solid was recovered by filtration followed by

washing with copious amounts of water and then calcined at 823 K at a rate of 1 K/min in

artificial air for 6 h (HZSM-5-nano).

Chapter 6

127

Nanosheet ZSM-5

A template solution A was prepared by dissolving 1.4 g of C22-6-3 template (bromide form) in

7.8 ml of water at 333 K. To solution A, 0.21 g of NaOH was added followed by stirring for

another 4 h at 323 K. A second solution B was prepared by mixing 0.12 g Al(OH)3 and 5.54

ml TEOS for 1 h in 10 ml water. After cooling solution A to room temperature, solution B

was added dropwise under vigorous stirring. The resulting suspension was stirred for 1 h in an

open vessel at room temperature. The final gel was transferred to a Teflon-lined stainless steel

autoclave. The autoclave was heated under rotation at 443 K for 5 days. The product was

recovered by filtration. The residue was first washed with copious amounts of demineralized

water followed by washing with ethanol. The product was dried overnight at 383 K. The

organics were removed by calcination in air at 823 K (HZSM-5-sheet).

Mo loading

Prior to Mo loading, the calcined zeolites were exchanged three times with an aqueous 1 M

NH4NO3 solution at 353 K for 4 h. The sample was then dried overnight at 383 and calcined

at 723 K for 4 h in artificial air. Mo was loaded onto the zeolites by impregnation with an

aqueous solution of ammonium heptamolybdate tetrahydrate (AHM, Merck). The targeted

Mo content was 4 wt%. After drying for 1 h at ambient, the Mo-modified zeolites were

calcined in artificial air at 823 K for 5 h. The heating rate was 1.5 K/min. The Mo-containing

zeolites are denoted by the prefix “Mo/”.

6.2.2. Characterization

The Mo and Al content of the Mo-modified zeolites was determined by inductively coupled

plasma optical emission spectroscopy (ICP-OES) using a Spectro CIROS CCD spectrometer

equipped with a free-running 27.12 MHz generator at 1400 W. Prior to analysis, samples were

digested in a mixture of HF/HNO3/H2O (1:1:1).

XRD patterns were recorded on a Bruker D4 Endeavor powder diffraction system using

Cu Kα radiation with a scanning speed 0.01 ° sec−1

in the 2θ range 5–60 °. The zeolite

crystallinity was determined using the Bruker TOPAS 3.0 program.

Ar sorption isotherms were measured at 87 K on a Micromeritics ASAP2020 system in

static mode. The samples were outgassed at 623 K for 8 h prior to the sorption measurements;

samples were transferred to the measuring port using TranSeals. The Brunauer–Emmett–

Teller (BET) equation was used to calculate the specific surface area (SBET) from the

Chapter 6

128

adsorption data in the p/p0 range of 0.05–0.25. The mesopore volume (Vmeso) and mesopore

size distribution were calculated using the Barrett–Joyner–Halenda (BJH) method applied to

the adsorption branch of the isotherm. The micropore area (Smicro) and micropore volume

(Vmicro) were calculated using the t-plot method using a thickness range of 3.5-5.4 Å [30].

Transmission electron micrographs were obtained with a FEI Tecnai 20 transmission

electron microscope (TEM) at an electron acceleration voltage of 200 kV. Typically, a small

amount of sample was suspended in ethanol, sonicated and dispersed over a Cu grid with a

holey carbon film. Scanning electron microscopy (SEM) was performed using a Philips

environmental FEIXL-30 ESEM FEG in high-vacuum mode at low voltage.

6.2.3. Catalytic activity measurements

An amount of 0.5 g of catalyst was introduced in a tubular quartz reactor with a length of

490 mm and an internal diameter of 4.0 mm. The length of the catalyst bed is 50 mm. The

catalyst was supported on a quartz wool plug in the isothermal zone of the oven. All gases

were fed using thermal mass controllers. Prior to reaction, the temperature was increased at a

rate of 5 K/min to 973 K in a He gas flow of 25 ml/min. The reaction was started by

switching the reactor feed to a mixture of CH4/N2 (5 vol% N2, used as internal standard)

mixture at a WHSV of 1.22 h-1

. This corresponds to a space velocity of 1710 ml CH4/gcat h.

Products were analyzed by an online Interscience CompactGC gas chromatograph equipped

with three analysis channels for separate analysis of light gases (Molsieve 5A, TCD), light

hydrocarbons (Al2O3/KCl, FID) and aromatics (Rtx-1, TCD).

6.3. Results and discussion

The XRD patterns of the starting zeolites are collected in Fig. 6.1. All samples have the

zeolite MFI structure and they do not contain indications for the presence of impurity phases.

The absence of a broad reflection around 23 º shows that the samples contain little amorphous

silica. The diffraction lines of HZSM-5-des, HZSM-5-org HZSM-5-nano and HZSM-5-sheet

are broader than those of the bulk HZSM-5 zeolites. This broadening is due to the smaller size

of the coherent scattering domains [31, 32].

Chapter 6

129

Fig. 6.1. XRD reflection patterns of (a) HZSM-5-ref, (b) HZSM-5-des, (c) HZSM-5-large, (d)

HZSM-5-org, (e) HZSM-5-nano and (f) HZSM-5-sheet.

The textural properties of the zeolites were measured by Ar physisorption. The corresponding

data determined by analysis of the adsorption branches of the isotherms are listed in Table 6.1.

The micropore volume of some of the nanocrystalline zeolites is slightly lower as compared

with those of the bulk zeolites. The micropore volume of HZSM-5-sheet is substantially lower.

This is in part due to the problem of using the t-plot method to analyze the micropore volume

for these sheet-like zeolites [34]. As expected, the bulk HZSM-5-large and HZSM-5-ref

zeolites do not contain mesopores. HZSM-5-des obtained by base leaching contains a

considerable amount of mesopores. HZSM-5-nano and HZSM-5-org also contain mesopores,

but in smaller amount than HZSM-5-des. The large mesopore volume in HZSM-5-des is due

to the presence of relatively large mesopores formed due to ill-controlled extraction of Si

from the zeolite framework [33]. Compared with the other zeolites, the nanosheet ZSM-5

sample has the largest mesopore volume. The mesopores in HZSM-5-sheet are mainly located

between the zeolite sheets [32].

10 20 30 40 50 60

Inte

nsit

y (

a.u

)

Angle (°)

Chapter 6

130

Table 6.1. Physico-chemical properties of the prepared ZSM-5 zeolite supports.

Sample Al1

(Wt%) Mo

1

(wt%)

Vmicro (cm

3/g)

Vmeso (cm

3/g)

Smicro (m

2/g)

Smeso (m

2/g)

SBET (m

2/g)

NBAS2

(μmol/g)

HZSM-5-large 1.0 3.04 0.15 0 265 2 445 554

HZSM-5-ref 1.9 4.12 0.14 0 257 16 432 679

HZSM-5-des 1.9 3.97 0.14 0.28 236 97 457 544

HZSM-5-nano 1.3 3.5 0.12 0.1 292 81 495 -

HZSM-5-org 3.2 4.5 0.11 0.12 129 67 333 1292

HZSM-5-sheet - - 0.02 0.76 89 390 570 280[29]

1 Elemental bulk content as determined by ICP-OES analysis.

2 Determined by infrared spectroscopy after desorption of pyridine at 423 K.

The crystal size and morphology of the zeolites were analyzed by TEM. It can be appreciated

that the set of zeolites covers a range of crystalline domain sizes from several nanometers to

tens of microns. The zeolite particles in HZSM-5-large (Fig. 6.2a) and HZSM-5-ref (Fig.

6.2b) have typical sizes of 10 μm and 1 μm, respectively. The crystals of HZSM-5-des (Fig.

6.2c), HZSM-5-org (Fig. 6.2d) and HZSM-5-nano (Fig. 6.2e) are substantially smaller. The

average micropore domain size of HZSM-5-des is ~100 nm. HZSM-5-org and HZSM-5-nano

comprise crystalline particles of about 10-30 nm. From the EM images the thickness of the

HZSM-5-sheet zeolite is estimated several nanometers.

The concentration of BAS of the zeolite samples was determined by IR spectroscopy of

adsorbed pyridine and the results are listed in Table 6.1. The desilicated ZSM-5 (HZSM-5-

des) material contains less BAS than HZSM-5-ref. Based on the data in Chapter 2, we can

ascribe this to the formation of some EFAl species. Large crystal ZSM-5 (HZSM-5-large)

zeolite contains a similar amount of BAS as HZSM-5-des; the lower acidity compared with

HZSM-5-ref is due to the lower Al content (Table 6.1). The acidity of HZSM-5-sheet sample

was substantially lower than of the other zeolites. The lower acidity of the nanosheet samples

has been discussed before in literature [32]. The acidity of HZSM-5-org is the highest which

is due to the high Al content of the material.

Chapter 6

131

Fig. 6.2. Electron Microscopy micrographs of (a) HZSM-5-large, (b) HZSM-5-ref, (c)

HZSM-5-des, (d) HZSM-5-org, (e) HZSM-5-nano and (f) HZSM-5-sheet.

6.3.2 Catalytic activity measurements

Prior to catalytic testing in MDA, the zeolites were loaded with approximately 4 wt% of Mo.

The bulk content was determined by ICP-OES analysis (Table 6.1). Evaluation of the catalytic

performance in the MDA reaction shows that Mo/HZSM-5-org has the highest methane

conversion rate (Fig. 6.3). The methane conversion rates for Mo/HZSM-5-large, Mo/HZSM-

5-nano and Mo/HZSM-5-des were slightly lower than that of the reference Mo/HZSM-5-ref

catalyst. The activity of Mo/HZSM-5-sheet was very low. These activity differences seem to

vary with the acidity; a larger concentration of BAS results in a higher activity, which is in

keeping with earlier reports [36, 37]. The relation between acidity and methane reactivity,

however, is not well understood. Tessonier et al. suggested that increased acidity improves the

Mo dispersion to explain the increased methane activity [38]. The benzene selectivities are

presented in Fig. 6.3b. The lower initial benzene selectivity of the Mo/HZMS-5-des,

Mo/HZSM-5-nano and Mo/HZSM-5-sheet samples compared with the reference trends with

the lower acidity of these samples. The aromatization step is considered to take place at the

BAS located inside the micropores. In line with this, the initial benzene selectivity of the large

50 nm

30 nm 20 nm 20 nm

a b c

d e f

9.66 μm

Chapter 6

132

bulk Mo/HZSM-5 (Mo/HZSM-5-large) is also lower compared with that of Mo/HZSM-5-ref.

The lower initial benzene selectivity for the large ZSM-5 crystals may also be due to the

longer diffusion lengths that will result in more secondary coking reactions. The high initial

benzene selectivity of Mo/HZSM-5-org is attributed to high acidity. The improved stability in

benzene formation of the small micropore domain sized catalysts (Mo/HZSM-5-des,

Mo/HZSM-5-nano, Mo/HZSM-5-org and Mo/HZSM-5-sheet) can be ascribed to the

improved accessibility of the benzene selective BAS located in the zeolites micropore system.

In this way, the detrimental effect of carbon laydown at the catalysts external surface with

progressive time on stream is suppressed. In Chapter 4 we attributed deactivation in benzene

formation to the growth of a carbon layer at the external surface of the zeolite surface. Such

carbon layer blocks the micropores and, thus, the benzene selective BAS. Furthermore,

decreasing the micropore domain size improves the diffusion of products out of the zeolite

crystals and thereby limiting secondary reactions leading to the formation of carbon. In

Chapter 3 it was suggested that the formation of carbon inside the micropores contributes to

some extent to catalyst deactivation. However, the effect of such intracrystalline carbon

formation on catalyst deactivation was argued to be smalled compared with the effect of

carbon formation at the external surface.

Fig. 6.3. Methane reaction rate (a) and benzene selectivity (b) of spent Mo/HZSM-5-ref

(square), Mo/HZSM-5-des (circle), Mo/HZSM-5-large (triangle), Mo/HZSM-5-org

(diamond), Mo/HZSM-5-nano (pentagonal) and Mo/HZSM-5-sheet (star).

0 2 4 6 8 10

0

2

4

6

8

10

12

14

Acti

vit

y

mm

ol/

gcat.h

Time on stream (h)

0 2 4 6 8 10

0

20

40

60

80

100

Ben

zen

e s

ele

cti

vit

y (

wt%

)

Time on stream (h)

Chapter 6

133

The nature of the carbonaceous deposits on the spent catalysts was characterized by TG

analysis. The weight-loss curves shown in Fig. 6.4 contain various features. The peak at ~

750 K relates to carbon formed in the proximity of molybdenum [39-41] and is referred to as

soft coke (Csoft). Carbon with a poly-aromatic nature [39-41] is most likely formed at the

external surface BAS and is indicated by a peak at ~ 850 K. This coke is typically assumed to

be of poly-aromatic nature and, accordingly, named hard coke (Chard). The weight-loss curves

were deconvoluted into contributions of the various C types and the results listed in Table 6.2.

These data indicate that the carbon content is lower for the bulk MFI materials (Mo/HZSM-5-

large and Mo/HZSM-5-ref) compared with the hierarchical structured materials. The larger

amount of Csoft in spent hierarchical structured catalysts Mo/HZSM-5-nano, Mo/HZSM-5-des

and Mo/HZSM-5-sheet suggests the presence of less well dispersed Mo-oxo phase. These

large metal-oxide particles lead to the formation of large Mo2C particles, which are known to

produce more soft coke nature. The substantial Csoft content in the spent Mo/HZSM-5-sheet

sample indicates that the Mo phase is poorly dispersed in the nano-layered sample. Analysis

of the carbon deposits in spent Mo/HZSM-5-org revealed a high concentration of Chard and

suggests a large amount of external surface BAS, which is in line with the large bulk BAS

concentration in Mo/HZSM-5-org (Table 6.2).

Fig. 6.4. TGA weight loss curves of spent (a) Mo/HZSM-5-ref, (b) Mo/HZSM-5-des, (c)

Mo/HZSM-5-large, (d) Mo/HZSM-5-org, (e) Mo/HZSM-5-nano and (f) Mo/HZSM-5-sheet.

600 700 800 900 1000

Weig

ht

loss (

mg

/K)

Temperature (K)

Chapter 6

134

Table 6.2. Coke content on the spent catalyst after 12 h on stream in the MDA reaction.

Sample Ctotal

(g/gcat) Csoft

(g/gcat)

Chard

(g/gcat)

Mo/HZSM-5-large 0.08 0.01 0.07

Mo/HZSM-5-ref 0.13 0.02 0.11

Mo/HZSM-5-des 0.19 0.10 0.10

Mo/HZSM-5-org 0.21 - 0.21

Mo/HZSM-5-nano 0.12 0.06 0.06

Mo/HZSM-5-sheet 0.19 0.14 0.05

6.4 Conclusions

In this chapter we investigated the effect of the micropore domain size of the ZSM-5 zeolite

component in the MDA catalyst on the final catalytic performance in MDA. To this end, a set

of ZSM-5 zeolites was prepared varying in micropore domain size. Characterization of the

prepared zeolite materials revealed the crystalline domain sizes to vary from 10 μm to several

nm’s. Evaluating the performance of the Mo modified zeolites in the MDA reaction revealed

a beneficial effect of a decreased crystalline domain size on the stability in benzene formation.

We argue that the better stability of the hierarchical catalysts in MDA is due to improved

accessibility of the BAS located inside the micropores. Hierarchical structuring decreases the

adverse effect of micropore blockage. The results from catalytic testing indicate a relation

between the Brønsted acidity of the zeolite support and the methane reaction rate.

6.5 References

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[3] D. Ma, Y. Shu, M. Cheng, Y. Xu, X. Bao, J. Catal. 194 (2000) 105-114.

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Catal. Today, 168 (2011) 96-111. [5] L. Wu, E.J.M. Hensen, Catal. Today, 235 (2014) 160-168. [6] S. van Donk, A. H. Janssen, J. H. Bitter and K. P. de Jong, Catal. Rev. Sci. Eng., 45 (2003)

297–319. [7] Javier Pérez-Ramírez, C.H. Christensen, K. Egeblad, C.H. Christensen, J.C. Groen, Chem. Soc.

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[15] C. Choi-Feng, J.B. Hall, B.J. Huggins, R.A. Beyerlein, J. Catal. 140 (1993) 395. [16] R.A. Beyerlein, C. Choi-Feng, J.B. Hall, B.J. Huggins, G.J. Ray, Top. Catal. 4 (1997) 27. [17] G.R. Meima, CATTECH 2 (1998), 5. [18] P. Hudec, J. Novansky, S. Silhar, T.N. Trung, M. Zubek, J. Madar, J. Ads. Sci. Tech. 3 (1986)

159. [19] B.L. Meyers, T.H. Fleisch, G.J. Ray, J.T. Miller, J.B. Hall, J. Catal. 110 (1988) 110, 82. [20] M. Tromp, J.A. van Bokhoven, M.T. Garriga Oostenbrink, J.H. Bitter, K.P. de Jong, D.C.

Koningsberger, J. Catal. 190 (2000) 209. [21] J.C. Groen, W. Zhu, S. Brouwer, S.J. Huynink, F. Kapteijn, J.A. Moulijn, J. Pérez-Ramírez, J.

Am. Chem. Soc. 129 (2007) 355–360. [22] J. C. Groen, L. A. A. Peffer, J. A. Moulijn and J. Pérez-Ramírez, J. Mater. Chem., 16 (2006)

2121-2131. [23] C. J. H. Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt and A. Carlsson, J. Am. Chem. Soc., 122

(2000) 7116–7117. [24] K. Zhu, K. Egeblad and C. H. Christensen, Eur. J. Inorg. Chem., 2007, 3955–3960 [25] A..H. Janssen, I. Schmidt, C.J.H. Jacobsen, A.J. Koster, K.P. De Jong, Microporous

Mesoporous Mater., 65 (2003) 59-75. [26] M. Choi, H. S. Cho, R. Srivastava, C. Venkatesan, D.-H. Choi, R. Ryoo, Nat. Mater., 5 (2006)

718–723. [27] D.P. Serrano, J. Aguado, J.M. Escola, J.M. Rodríguez, A. Peral, Chem. Mater. 18 (2006) 2462–

2464. [28] K. Na, C. Jo, J. Kim, K. Cho, J. Jeng, Y. Seo, R.J. Messinger, B.F. Chmelka, R. Ryoo, Science,

333 (2011) 328–332. [29] S.Z. Chen, K. Huddersman, D. Keir, L.V.C. Rees 8 (1988) 106-109. [30] B.C. Lippens, J.H. de Boer, J. Catal. 4 (1965) 319–323. [31] K. Na, W. Park, Y. Seo, R. Ryoo, Chem. Mater. 23 (2011) 1273-1279.

[32] X.Zhu, L.Wu, P.C.M.M. Magusin, B. Mezari, E.J.M. Hensen, J. Catal., 327 (2015) 10-21.

[33] J.C. Groen, L.A.A. Peffer, J.A. Moulijn, J. Pérez-Ramírez, Chem. Eur. J., 11 (2005) 4983-4994.

[34] M.A. Camblor, A. Corma, S. Valencia, Microporous Mesoporous Mater., 25 (1998) 59-74.

[35] E. Verheyen, C. Jo, M. Kurttepeli, G. Vanbutsele, E. Gobechiya, T.I. Korányi., S. Bals, G. van

Tendeloo, R. Ryoo, C.E.A. Kirschhock, J.A. Martens, J. Catal. 300 (2013) 70-80.

[36] J. Shu, A. Adnot, B.P.A. Grandjean, Ind. Eng. Chem. Res. 38 (1999) 3860-3867.

[37] M. Marczewski, H. Marczewska, K. Mazowiecka, React. Kinet. Catal. Lett. 54 (1995) 81-86.

[38] J-P. Tessonier, B. Louis, S. Rigolet, M.J. Ledoux, C. Pham-Huu, App. Catal. A, 336 (2008) 79-

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[39] C.C. Williams, J.G. Ekerdt, J.M. Jehng, F.D. Hardcastle, A.M. Turek, I.E. Wachs, J. Phys.

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Chapter 6

136

Chapter 7

137

Texture, acidity and fluid catalytic cracking performance

of hierarchical faujasite zeolite prepared by an

amphiphilic organosilane

Summary

Mesoporous zeolite Y was synthesized by using an amphiphilic organosilane. The texture and

the acidity of the mesoporous zeolite samples were compared with a microporous faujasite

reference zeolite. The synthesis of the most suitable mesoporous zeolite Y was scaled up in

order to prepare composite catalysts that could be tested for fluid catalytic cracking.

Composite catalysts were prepared by spray-drying the zeolite with Kaolin as filler and an

alumina sol as binder. The acidic properties of these composite FCC catalysts prepared from

conventional and mesoporous faujasite zeolites were compared. While IR spectroscopy after

H/D exchange with deuterated benzene indicates that strong bridging hydroxyl groups are

present in the freshly prepared composite catalysts, these zeolitic Brønsted acid sites are not

observed anymore in the lab-deactivated composite catalysts. These samples contain a

significant number of weaker Brønsted acid sites. The strength of the acid sites in the

composite catalysts is comparable with the acidity of amorphous silica-alumina. The

composite catalysts show excellent catalytic performance in the fluid catalytic cracking of

vacuum gas oil. These data indicate that relatively weak acid sites are responsible for the

FCC activity. The well-embedded mesoporosity in the parent hierarchical zeolite crystals

results in increased diesel and decreased gasoline and coke yield.

This chapter is accepted for publication in Fuel Process. Techn. (2015) In press.

Chapter 7

138

7.1 Introduction

Fluidized catalytic cracking (FCC) is one of the key processes in the refinery industry,

converting heavy gas oils obtained from atmospheric and vacuum distillation into lighter

products [1-4]. The products from FCC units are valuable for the gasoline and diesel pool.

Currently, more than 400 FCC units are operated worldwide, converting approximately 10

million barrels heavy feedstock per day [1]. The success of this process is based on its

simplicity, relatively low construction and operation costs and the flexibility in processing

various types of heavy feedstock [4]. Zeolite Y has been the main acid component of FCC

catalysts. Composite FCC catalysts used in industry further comprise clays, binders and other

additives [5]. Zeolite Y is the preferred zeolite component because of its relatively large pore

openings (7.4 Å), strong Brønsted acidity and good (hydro)thermal stability [6, 7]. Another

important aspect is that it can be synthesized at low cost without organic structure-directing

agents.

Under FCC conditions, the pore openings of 7.4 Å impose diffusion limitations and limit

the conversion of the larger hydrocarbon molecules in the feed. One approach to overcome

this would be to prepare zeolites with larger pores, but such zeolites are usually not very

stable and their synthesis requires expensive organic templates [8-11]. Another approach to

ease diffusion of large molecules in zeolites is the fabrication of hierarchical zeolites that

contain mesopores well-interconnected with the micropore network in the zeolite [12-14].

One distinguishes bottom-up and top-down approaches. In top-down approaches, usually Si

[15-18] or Al [19-24] atoms are extracted from the zeolite framework. In bottom-up

approaches, the mesoporous zeolite is formed in one step, usually by adding to the synthesis

gel a second template as void filling spheres such as carbon black particles or in the form of

organic molecules that act as mesoporogen during the formation of the zeolite [24].

Hierarchization has most frequently been applied in the synthesis of ZSM-5 and BEA

zeolites [12, 14, 24, 25]. It is well-known that mesopores are created in zeolite Y crystals

during steam treatment as employed to convert low-acidic freshly prepared zeolite Y into

strongly acidic ultrastabilized Y zeolites, which are the main acid component in

hydrocracking operations [18]. Usually, the mesopores are not uniformly distributed over the

zeolite crystals and, sometimes, they are also not connected to the external surface [26]. De

Jong and co-workers investigated how steam treatment followed by acid and base leaching

steps improves the mesopore interconnectivity, which is useful to limit secondary cracking

Chapter 7

139

reactions [27]. The benefit of mesoporosity on the cracking performance of vacuum gas oil

and bulky model molecules has been well established [28-32].

Mesoporous Y zeolite can also be obtained using surfactant templates [33-36, 37-40].

Garcia-Martinez et al. reported about the scale-up of a surfactant-templated process to prepare

mesoporous Y zeolite; composite catalysts based on such hierarchical zeolite Y showed

improved yield of valuable gasoline and light cycle oil (LCO) products over bottoms and coke

in FCC catalyst evaluation [41]. Another versatile method to introduce mesopores in zeolites

involves the use of organosilanes that covalently bind to the growing zeolite surface [33-37].

This approach was first described by the Ryoo group in the preparation of mesoporous ZSM-5

[33]. The organosilane dimethyl-octadecyl-(3-trimethoxysilylpropyl)-ammonium chloride

(TPOAC) has also been used in the preparation of hierarchical zeolite Y [34-36]. For instance,

Fu et al. reported improved catalytic performance in the hydrodesulfurization of 4,6-

dimethyldibenzothiophene using a TPOAC-templated mesoporous Y zeolite support [34].

Another example is the benefit of mesopores in zeolite Y in the aldol condensation of n-

butanol with benzaldehyde [35]. Jin et al. showed that replacing a small portion of bulk

zeolite Y with hierarchical zeolite Y led to a shift in the FCC selectivity from coke to gasoline

and dry gas products [36].

In the present study, we investigated the feasibility of using TPOAC in the direct

synthesis of hierarchical zeolite Y for use in FCC composite catalyst. We first optimized the

synthesis of mesoporous Y zeolite at small (gram) scale. The most promising mesoporous

zeolite was scaled up. For comparison, a bulk zeolite Y was prepared and its Al content was

lowered by substitution of framework Al by Si. The acidic properties of the scaled-up

zeolites, the composite catalysts derived thereof and the lab-deactivated and regenerated FCC

catalysts were characterized in detail. The FCC performance of two lab-deactivated composite

catalysts was evaluated in an Advanced Catalytic Evaluation (ACE) unit.

7.2 Experimental

7.2.1 Zeolite synthesis

For the synthesis of reference bulk zeolite Y, a seed gel was prepared by dissolving 4.04 g

NaOH and 2.0 g NaAlO2 in 19.97 g water. Then, 22.80 g sodium silicate solution (Prolabo,

25.5 – 26.5% SiO2) was added dropwise under vigorous stirring. The resulting seed gel (gel

A) was aged overnight at room temperature. In a second round bottom flask, a feedstock gel

Chapter 7

140

(gel B) was prepared. After dissolving 0.04 g NaOH and 3.31 g NaAlO2 in 33.19 g water,

35.56 g sodium silicate (26 wt% in water, Prolabo) was added dropwise under vigorous

stirring. The Si/Al ratio of the feedstock gel was varied between 2.5 and 5.0 by adjusting the

amount of sodium aluminate. To prepare the final synthesis gel, an amount of 4.46 g of the

aged seed gel A was added to the feedstock gel B under vigorous stirring and was stirred for

another hour. The resulting gel was transferred into a 125 mL Teflon-lined stainless-steel

autoclave and heated in a static oven at 373 K for 24 h.

The gel for obtaining mesoporous zeolite Y was prepared in the same way as described

above. Prior to the hydrothermal step, dimethyl-octadecyl-(3-trimethoxysilylpropyl)-

ammonium chloride (TPOAC, ABCR, 60 wt% in methanol) was added dropwise to the

synthesis gel; this gel was further stirred for 4 h. The Si/TPOAC ratio was varied between 10

and 125. The gels were then hydrothermally treated at 373 K for 72 h. The solid materials

were recovered by filtration of the suspension, followed by washing with copious amounts of

water. To remove TPOAC, the solids were calcined in artificial air (20/80 (v%/v%) O2/He).

The materials are denoted by FAU(x, y) with x being the SiO2/TPOAC ratio (∞, 125, 45, 20,

10) and y the Si/Al ratio (2.5, 3.5, 5.0) in the starting gel.

A portion of the mesoporous zeolite Y prepared with an SiO2/TPOAC ratio of 45 and an

initial Si/Al ratio of 5.0 in the synthesis gel (Y(45,5.0)) was treated with ammonium

hexafluorosilicate (AHFS). The zeolite was first ion-exchanged four times with 1 M KNO3,

followed by four exchange cycles with 1 M NH4NO3 under reflux. After drying the zeolite

overnight, 10 g of zeolite was slurried in 100 ml of 3.4 M ammonium acetate at 348 K. An

amount of 135 ml of 0.1622 M AHFS was added dropwise over a period of 2 h. The final

slurry was stirred overnight at 348 K. The solid was recovered by filtration and washed with 1

L of hot (363 K) demineralized water. The washed solid was dried in a vacuum oven at 293 K.

The synthesis of several zeolite materials was scaled up by increasing the reactant amount

by a factor of 16. The hydrothermal synthesis was done in an 1.5 L Teflon-lined autoclave. At

this scale, a standard zeolite Y was synthesized at a Si/Al ratio of 2.5 and a mesoporous

zeolite Y at a Si/Al ratio of 5.0 in the presence of TPOAC (SiO2/TPOAC = 45). The Si/Al

ratio of the microporous zeolite Y was increased by AHFS treatment according to the

procedure outline above. The proton forms of these materials were obtained by suspending 1

g of calcined zeolite in 10 ml 1 M NH4NO3 for 4 h at 353 K. The ion-exchange was repeated

twice. The final step was calcination in artificial air at 723 K for 4 h. The calcined zeolites are

denoted as FAU(∞, 4.1)-large and FAU(45, 2.9)-large, reflecting the final Si/Al ratios as

Chapter 7

141

determined by ICP analysis.

Composite catalysts were prepared by spray-drying the zeolite with Kaolin as filler and

alumina sol as the binder. The resulting catalyst composite consisting of 35 wt % zeolite, 50

wt % Kaolin and 15 wt % alumina were subjected to steam-calcination to simulate catalyst

regeneration. The composite catalysts were steamed at 1023 K for 4 h using 100 % steam

followed by calcination at 873 K for 1 h. The deactivated catalyst was then sieved to obtain

particles in the range 38−212 μm and calcined at 873 K for 2 h.

7.2.2 Characterization

Elemental analysis was done by inductively coupled plasma optical emission

spectroscopy (ICP-OES) on a Spectro CIROS CCD spectrometer equipped with a free-

running 27.12 MHz generator at 1400 W. Zeolite samples were dissolved in a mixture of

HF/HNO3/H2O (1:1:1).

XRD patterns were recorded on a Bruker D4 Endeavor powder diffraction system using

Cu Kα radiation with a scanning speed 0.01 °·sec−1

in the 2θ range 5-60 °. The crystallinity

was determined according the standardized procedure ASTM D 3906-80. To this end, the

intensities of the 15.7 °, 18.8 °, 20.5 °, 23.8 °, 27.2 ° and 34.3 ° 2θ reflections (corresponding

to the [331], [511], [440], [533], [642] and [555] hkl planes) were taken after background

subtraction and related to the intensities of the highly crystalline sample FAU(∞, 2.5)

prepared in this work FAU(∞, 2.5).

Ar physisorption isotherms were measured at 87 K on a Micromeritics ASAP2020 system

in static measurement mode. The samples were outgassed at 623 K for 8 h prior to the

sorption measurements. The Brunauer–Emmett–Teller (BET) equation was used to calculate

the specific surface area (SBET) in the pressure range p/p0 = 0.05–0.25. The mesopore volume

(Vmeso) and mesopore size distribution were calculated using the Barrett–Joyner–Halenda

(BJH) method on the adsorption branch of the isotherm. The micropore area (Smicro) and

micropore volume (Vmicro) were calculated from the t-plot curve using the thickness range

between 3.5 and 5.4 Å [42].

Nuclear Magnetic Resonance (NMR) measurements were performed on a 11.7 Tesla

Bruker DMX500 NMR spectrometer operating at 500 MHz for 1H, 99 MHz for

29Si and 132

MHz for 27

Al. The 27

Al MAS NMR was done using a Bruker 2.5-mm MAS probehead

spinning at 20 kHz; 27

Al NMR spectra were recorded with a single pulse sequence with a 18°

pulse duration of 1 μs and a interscan delay of 1 s. A saturated Al(NO3)3 solution was used for

Chapter 7

142

27Al NMR shift calibration. The

1H and

29Si MAS NMR measurements were carried out using

a 4-mm MAS probehead with sample rotation rates of 12.5 kHz for 1H and 10 KHz for

29Si

NMR measurements. 1

H and 29

Si NMR shifts were referred to Tetramethylsilane (TMS).

Quantitative 29

Si NMR spectra were recorded using a high power proton decoupling direct

excitation (HP-DEC) pulse sequence with a 45 o pulse duration of 2.5 μs and an interscan

delay of 160 s. For 1H MAS NMR measurements, the zeolites were first dehydrated at a

temperature of 723 K at a pressure lower than 10−5

mbar for 6 h. The dehydrated zeolites were

placed into the 4-mm MAS NMR zirconia rotor under inert conditions and transferred to the

NMR probehead. Quantitative 1H NMR spectra were recorded with a Hahn-echo pulse

sequence p1-τ1-p2-τ2-aq with a 90o pulse p1 = 5 μs and a 180º p2 = 10 μs and an interscan

delay of 120 s.

Infrared spectra were recorded in the 4000-400 cm−1

range using a Bruker Vertex 70v

apparatus. Samples were pressed into a self-supporting wafer with a density of about 10

mg/cm2. To remove physisorbed water, the sample was evacuated for 2 h at 773 K at a

pressure lower than 2 x 10−6

mbar. After evacuation, the sample was cooled to 323 K; then, a

background spectrum was recorded. The total concentration of the Brønsted acid sites was

determined by monitoring the H/D exchange reaction with d6-benzene (C6D6, Sigma Aldrich)

following a literature procedure [43]. C6D6 was kept in a glass ampoule connected to an

evacuated gas supply system. C6D6 was dosed into the cell with a computer controlled

pneumatic valve, delivering a dose of 0.33 mmol C6D6. The sample was exposed for various

times to the probe, followed by evacuation for 1 h. The sequence was repeated to record the

spectra of partially exchanged samples with exposure times of 30 min, 30 min and 60 min at

303 K; 30 min at 323 K, 30 min at 373 K, and 30 min at 523 K. The total concentration of the

Brønsted acid sites was determined by IR spectroscopy of adsorbed pyridine. Pyridine

adsorption was carried out on the dehydrated zeolite wafer at 373 K. After saturation, the

sample was evacuated at 423 K for 1 h and a spectrum was recorded. This desorption step was

repeated at 573 K and 723 K. After each desorption step a spectrum was recorded at 423 K.

The spectra were deconvoluted and the acidity was quantified using the extinction coefficient

values reported by Datka [44].

Transmission electron micrographs were obtained with a FEI Tecnai 20 transmission

electron microscope (TEM) at an electron acceleration voltage of 200 kV. Typically, a small

amount of sample was suspended in ethanol, sonicated and dispersed over a Cu grid with a

holey carbon film. Scanning electron microscopy (SEM) was performed using a Philips

Chapter 7

143

environmental FEIXL-30 ESEM FEG in high-vacuum mode at low voltage.

7.2.3 Catalytic activity measurements

The Brønsted acidity of some of the materials was evaluated by catalytic activity

measurements in the bifunctional hydroconversion of n-heptane [43, 45, 46]. For this purpose,

the zeolites were loaded with 0.4 wt% Pd following incipient wetness impregnation with an

aqueous solution of Pd(NH3)4(NO3)2. The impregnated materials were calcined at 573 K and

sieved in a 250-500 μm mesh fraction. Prior to catalytic testing, the catalyst was reduced in a

H2 of 100 ml/min at 713 K and 30 bar. The reaction temperature was lowered with 0.2 K/min

from 713 K to 473 K. The product stream was analyzed by online gas chromatography.

The catalytic activity in fluid catalytc cracking was evaluated in an Advanced Catalytic

Evaluation unit (ACE, Kayser Technology, USA). The reaction temperature was 803 K. The

feedstock was a vacuum gas oil (VGO) obtained from a PetroChina refinery in Dalian. An

amount of 9 g composite catalyst was weighed into the reactor; the amount of VGO feed was

1.5 g. The contact time was 90 s. Gas products were analyzed using an online refinery gas

analyzer (M/s AC Analyticals). The boiling point distributions of the liquid products were

analyzed using a simulated distillation gas chromatograph (M/s AC Analyticals). Coke

deposited on the catalyst was burnt in a catalyst regeneration step and quantified using an

online CO2 analyzer.

7.3 Results and discussion

7.3.1 Optimization and scale-up of the synthesis procedure

We first varied the crystallization time for zeolite Y in the presence of TPOAC. The XRD

patterns of the obtained materials are shown in Fig. 7.1. Without TPOAC, highly crystalline

zeolite Y was obtained in 24 h. The optimal hydrothermal synthesis time in the presence of

TPOAC was 72 h. A shorter crystallization time resulted in a higher fraction of amorphous

silica; longer crystallization times led to the formation of zeolite P. The formation of zeolite P

as a competitive phase during zeolite Y synthesis has been reported before [47]. The need for

longer crystallization times in the presence of TPOAC in the synthesis gel is in line with

results of other studies [35, 48]. Fig. 7.2 shows the XRD patterns of the calcined zeolites

synthesized at different gel SiO2/TPOAC ratios. The XRD pattern of FAU(∞,2.5) is similar to

the one reported for crystalline zeolite Y [49, 50]. FAU(125,2.5) and FAU(45,2.5) also have

Chapter 7

144

the FAU structure, but their crystallinities are lower (Table 7.1). The materials prepared at

Si/TPOAC < 45 did not crystallize under the given conditions; the broad reflection observed

for these materials around 2θ = 23° shows that mainly amorphous silica was formed.

Fig. 7.1. XRD patterns of calcined zeolites: (a) FAU(∞,2.5), (b) FAU(125,2.5), (c)

FAU(45,2.5), (d) FAU(20,2.5) and (e) FAU(10,2.5).

Fig. 7.2. XRD patterns of calcined zeolites: (a) FAU(∞,2.5), (b) FAU(125,2.5), (c)

FAU(45,2.5), (d) FAU(20,2.5) and (e) FAU(10,2.5).

10 20 30 40 50 60

Inte

nsit

y (

a.u

.)

Angle (°)

10 20 30 40 50 60

Inte

nsit

y (

a.u

.)

Angle (°)

Chapter 7

145

The textural properties of the crystalline zeolites were investigated by Ar physisorption.

Fig. 7.3 shows the isotherms for FAU(∞, 2.5), FAU(45, 2.5) and FAU(125, 2.5). The

hysteresis loops in the relative pressure region of 0.4-0.8 evidence that FAU(45, 2.5) and

FAU(125, 2.5) contain mesopores. The textural data are collected in Table 7.1. FAU (∞, 2.5)

contains only micropores; the textural data agree with reported values for zeolite Y [51]. The

two crystalline mesoporous zeolites, FAU(45,2.5) and FAU(125,2.5), possessed a significant

amount of mesopores. The mesopore volume increased with increasing TPOAC content in the

synthesis gel. The lower micropore volume as compared with FAU (∞, 2.5) is most likely due

to the decreased crystallization degree, which was also apparent from the XRD patterns.

Chapter 7

146

Table 7.1. Textural properties of the zeolite Y materials.

Sample SiO2/TPOAC Si/Al1

Si/Al2

Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

SBET

(m2/g)

CXRD3

(%)

FAU(∞, 2.5) ∞ 2.5 2.05 0.28 0.02 571 15 612 100

FAU(∞, 3.5) ∞ 3.5 2.51 0.23 0 404 4 463 98

FAU(125, 2.5) 125 2.5 2.14 0.22 0.11 447 94 639 84

FAU(45, 2.5) 45 2.5 2.58 0.16 0.15 325 143 566 79

FAU(45, 3.5) 45 3.5 3.04 0.26 0.11 512 72 619 67

FAU(45, 5.0) 45 5.0 3.50 0.16 0.10 207 50 420 78 1 Si/Al ratio in the synthesis gel;

2 Determined by ICP elemental analysis;

3 Relative XRD crystallinity determined according the ASTMD

3906-80 standard procedure.

Chapter 7

147

Fig. 7.3. Ar physisorption isotherms of calcined zeolites: (a) FAU(∞, 2.5), (b) FAU(125, 2.5)

and (c) FAU(45, 2.5). The isotherms are presented in a stacked fashion for clarity. The Y

offsets are progressively increased with 100 cm3/g for each subsequent sample.

FAU(∞, 2.5) and FAU(45, 2.5) were treated with AHFS with the aim to increase the

framework Si/Al ratio. FAU(45, 2.5) was chosen because of its relatively large mesopore

volume and its good crystallinity. The framework structure of microporous FAU(∞, 2.5) was

largely retained during the AHFS treatment (Fig. 7.4), whereas mesoporous FAU(45, 2.5)

zeolite was found to collapse when treated in the same manner.

0.0 0.2 0.4 0.6 0.8 1.0

Vo

lum

e a

dso

rbed

cm

3/g

.ST

P

Relative pressure

a

c

b

0.0 0.2 0.4 0.6 0.8 1.0

Vo

lum

e a

dso

rbed

cm

3/g

.ST

P

Relative pressure

Chapter 7

148

Fig. 7.4. (left) XRD patterns of calcined zeolites: (a) FAU(∞,2.5), (b) FAU(∞,3.5) (c)

FAU(∞,5.0) and (d) FAU(∞,2.5) after treatment with AHFS; (right) XRD patterns of calcined

zeolites: (a) FAU(45,2.5), (b) FAU(45,3.5), (c) FAU(45,5.0) and (d) FAU(45,5.0) after

treatement with AHFS.

It was also attempted to increase the framework Si/Al ratio by lowering the Al content of

the synthesis gel. For the conventional synthesis without TPOAC, crystalline zeolites were

obtained at Si/Al ratios of 2.5 and 3.5 (Fig. 7.4). The XRD pattern of FAU(∞, 5.0) shows that

crystallization was not possible at this Si/Al ratio, in line with literature [52]. On contrary,

when zeolite Y was synthesized at Si/Al ratios of 3.5 and 5.0 in the presence of TPOAC,

highly crystalline materials were obtained as follows from the XRD patterns in Fig. 7.4.

However, the bulk Al content of the resulting samples was higher than targeted. For FAU(45,

3.5) and FAU(45, 5.0), the Si/Al ratios were 3.04 and 3.50, respectively, as determined by

elemental analysis. Textural analysis of FAU(45, 5.0) shows that this material combines

micropores and mesopores. The TEM images of FAU(45, 5.0) also reveal that the mesopores

are well integrated into the primary zeolite particles (Fig. 6c).

As larger quantities of zeolites were required for the ACE test, the synthesis of FAU(45,

5.0) was scaled-up. The resulting zeolite is denoted as FAU(45, 5.0)-large. For comparison,

the synthesis of microporous FAU(∞, 2.5) was also carried out at the same scale; this zeolite

was then treated with AHFS to decrease the framework Al content. This reference zeolite is

denoted as FAU(∞, 4.1)-large. The XRD patterns of the zeolites in Fig. 7.5 (the two bottom

10 20 30 40 50 60

Inte

nsit

y (

a.u

.)

Angle (°)

10 20 30 40 50 60In

ten

sit

y (

a.u

.)

Angle (°)

Chapter 7

149

patterns) point out the higher crystallinity of calcined FAU(∞, 4.1)-large as compared with

FAU(45, 2.9)-large. This is in accordance with the data in Table 7.3. The crystallinity of

FAU(∞, 4.1)-large was slightly lower compared to that of the FAU(∞, 2.5) zeolite suggesting

scaling-up of the synthesis procedure to hamper the crystallization process. The bulk Si/Al

ratio of FAU(∞, 4.1)-large after the AHFS treatment was 4.1. The final Si/Al ratio of FAU(45,

2.9)-large was lower (Si/Al = 2.9) compared with FAU(∞, 4.1)-large. Ar physisorption data

confirm that FAU(45, 2.9)-large contained mesopores; these mesopores are also visible in the

TEM image shown in Fig. 7.6d. The diameter of the mesopores is approximately 5 nm. The

TEM images of the microporous zeolite do not contain evidence for such mesopores (Fig.

7.6b), in keeping with the low mesopore volume of this sample. FAU(∞, 4.1)-large (Fig. 7.6e)

is made up from cubic crystals. The morphology of FAU(45, 2.9)-large is different; the SEM

image in Fig. 7.6f shows that, in addition to cubic crystals, also thin sheet-like crystals are

present, which are intergrown with the larger zeolite crystals.

Table 7.2. Textural and acidic properties of the large-scale conventional and mesoporous Y

zeolites, FCC composite catalysts before and after lab-deactivation and after ACE and

regeneration by calcination.

Sample Vmicro

(cm3/g)

Vmeso

(cm3/g)

Smicro

(m2/g)

Smeso

(m2/g)

SBET

(m2/g)

CXRD1

(%)

FAU(∞, 4.1)-large 0.24 0.04 475 24 560 93

FCC(micro, fresh) 0.07 0.06 139 28 192 23

FCC(micro, steamed) 0.03 0.06 54 32 96 15

FCC(micro, regenerated) 0.02 0.08 47 43 103 15

FAU(45, 2.9)-large 0.14 0.11 286 41 383 69

FCC(meso, fresh) 0.03 0.04 55 32 127 13

FCC(meso, steamed) 0.006 0.04 14 22 37 5

FCC(meso, regenerated) 0.004 0.05 12 28 46 5 1 Relative XRD crystallinity determined according the ASTM D 3906-80 standard procedure.

Chapter 7

150

Fig. 7.5. XRD patterns of zeolites and composite catalysts: (left) (a) FAU(∞, 4.1), (b) FCC

(micro), (c) FCC(micro, steamed) and (d) FCC(micro, regenerated); (right) (a) FAU(45, 2.9),

(b) FCC(meso), (c) FCC(meso, steamed), (d) FCC(meso, regenerated).

Fig. 7.6. TEM micrographs of calcined zeolites: (a) FAU(∞, 2.5), (b) FAU(∞, 4.1)-large, (c)

FAU(45, 5.0), d) FAU(45, 5.0)-large, and SEM images of e) FAU(∞, 2.5)-large and f)

FAU(45, 5.0)-large.

10 20 30 40 50 60

Inte

nsit

y (

a.u

.)

Angle (°)

10 20 30 40 50 60

Inte

nsit

y (

a.u

.)

Angle(°)

10 20 30 40 50 60In

ten

sit

y (

a.u

.)

Angle (°)

20 nm

20 nm

20 nm

20 nm

b a c

d e f

Chapter 7

151

7.3.2 NMR spectroscopy

27Al MAS NMR spectroscopy was used to determine the Al coordination of the zeolites.

NMR spectra are shown in Fig. 7.7. The spectra are dominated by a feature at δ = 55 ppm (δ,

chemical shift), corresponding to Al in the zeolite framework (FAl). A second smaller feature

at δ = 0 ppm is due to extraframework Al (EFAl). Both zeolite samples contain most of the Al

in the framework, but small amounts of EFAl were also present. The presence of EFAl trends

with the lower crystallinity of these materials as determined from XRD experiments.

Fig. 7.7. 27

Al MAS NMR spectra of (a) FAU(∞, 4.1)-large and (b) FAU(45, 2.9)-large.

The framework Si/Al ratio (Si/AlFW) of the zeolites was determined by 29

Si MAS NMR

spectroscopy. The spectra shown in Fig. 7.8 contain features due to Si species in different

coordination environments (denoted as Si(nAl) with n indicating the number of Al atoms in

the next-nearest-neighbour (NNN) positions). Peaks at δ = -90 ppm, δ = -95 ppm and δ = -101

ppm are attributed to Si(3Al), Si(2Al) and Si(1Al) species [53, 54]. Si(0Al) species in the

zeolite framework give rise to the feature at δ = -107 ppm [53, 54]. The results of the

deconvolution of these spectra are given in Table 7.3. The estimated framework Si/Al ratios

are 4.8 and 3.5 for FAU(∞, 4.1)-large and FAU(45, 2.9)-large, respectively. Both samples

contain less Al in the framework than the bulk Al content and this is consistent with the

presence of EFAl.

Table 7.3. Physico-chemical properties of the zeolites prepared at larger scale.

Sample Si(3Al)1

(%)

Si(2Al)1

(%)

Si(1Al)1

(%)

Si(0Al)1

(%)

Si/AlFW1 Si/Albulk

2

FAU(∞, 4.1)-large 1.8 14.6 48.3 35.3 4.8 4.05

FAU(45, 2.9)-large 7.2 26.5 39.5 26.8 3.5 2.90 1Determined from

29Si MAS NMR spectra;

2Bulk Si/Al ratio determined by ICP-OES analysis.

Chapter 7

152

Fig. 7.8. 29

Si MAS NMR spectra of (a) FAU(∞, 4.1)-large and (b) FAU(45, 2,9)-large.

The hydroxyl groups in the zeolites were characterized by 1H MAS NMR spectroscopy

(Fig. 7.9). Peaks at δ = 4.6 ppm and δ = 4.0 ppm represent BAS located in the sodalite cages

and supercages, respectively. The peak at δ = 2.6 ppm is related to hydroxyl groups associated

with EFAl. Silanol groups are identified by the signal at δ = 1.9 ppm. The content of the

various hydroxyl groups was quantified by deconvolution of the 1H NMR spectra and the

results are listed in Table 7.4. FAU(∞, 4.1)-large contains more BAS than FAU(45, 2.9)-large.

For FAU(45, 2.9)-large, the amount of BAS in the sodalite cages is lower than the amount of

BAS in the supercages. FAU(45, 2.9)-large zeolite has a larger content of hydroxyls groups

related to EFAl and silanols. On contrary, the amount of BAS in the sodalite and supercages is

similar for FAU(∞, 4.1)-large.

Chapter 7

153

Fig. 7.9.

1H MAS NMR spectra of (a) FAU(∞, 4.1)-large and (b) FAU(45, 2.9)-large.

Table 7.4. Concentration of OH groups determined by deconvolution of 1H MAS NMR

spectra.

Sample FAU(∞, 4.1)-large FAU(45, 2.9)-large

Chemical shift

(ppm)

µmol/g Chemical shift

(ppm)

µmol/g

BAS

sodalite cages 4.6 519 4.6 83

Supercages 4.0 502 3.8 138

AlOH 2.6 276 1.8 190

SiOH 1.9 149 2.4 128

7.3.3 FTIR spectroscopy of H/D exchange with C6D6

To quantify the BAS, the selective H/D exchange of the bridging hydroxyl groups with

deuterated benzene was monitored by FTIR spectroscopy. The relevant FTIR spectra of

FAU(∞, 4.1)-large and FAU(45, 2.9)-large are shown in Figs. 7.10 and 7.11. The spectra of

the dehydrated zeolites contain features at 3550 cm−1

and 3631 cm−1

in the OH stretching

region due to bridging hydroxyl groups in the sodalite cages and the supercages, respectively

[44]. The spectra also contain features of silanol groups (3745 cm−1

) and aluminol groups

associated with EFAl species (3670 cm−1

) [43]. The intensity of the BAS bands is higher for

FAU(∞, 4.1)-large as compared with FAU(45, 2.9)-large. After exposure to C6D6, the

intensities of the 3550 cm−1

and 3631 cm−1

bands are lower. The new bands at 2630 cm−1

and

2680 cm−1

are due to deuteroxyl groups corresponding to BAS in sodalite cages and

supercages, respectively, after selective exchange of the acidic protons with deuterium [43].

Chapter 7

154

Inspection of the spectrum in the OD stretching region indicates that FAU(∞, 4.1)-large

contains less BAS in the sodalite cages than in the supercages, in line with the difference

noted by 1H NMR spectroscopy. The corresponding spectrum of FAU(∞, 4.1)-large points to

nearly equivalent amounts of BAS in sodalite cages and supercages for this sample. The H/D

exchange occurs at higher rate for FAU(∞, 4.1)-large as compared with FAU(45, 2.9)-large.

The difference relates to the higher framework Al content of the latter zeolite [43]. Not all of

the BAS in these zeolites could be exchanged following extensive H/D exchange at 523 K in

contrast to earlier results for zeolite Y [43]. The reason for this difference is not clear, but it

might mean that some parts of the crystals are not accessible to C6D6. The BAS content of the

two zeolites was estimated by deconvolution of the OD stretching region after H/D exchange

at 523 K according to established procedures [43]. The data are reported in Table 7.5. The

BAS concentrations are 1.51 mmol/g and 0.65 mmol/g for FAU(∞, 4.1)-large and FAU(45,

2.9)-large, respectively. The BAS concentrations are significantly lower compared with the

theoretical values based on the framework Si/Al ratios determined from the 29

Si MAS NMR

data. Decreased crystallinity due to the presence of TPOAC in the gel and, in case of the

microporous zeolite, the AHFS treatment may explain this. EFAl species may also partially

compensate the negative framework negative charge instead of protons. The acidity of

FAU(45, 2.9)-large is lower than that of FAU(∞, 4.1)-large; this difference is in keeping with

the 1H MAS NMR data.

Chapter 7

155

Table 7.5. Results of acidity characterization of zeolite component and FCC composite catalysts.

Sample NBAS1

(μmol/g)

T40%2

NBAS-4233

(μmol/g)

NBAS-5733

(μmol/g)

NBAS-7233

(μmol/g)

NLAS-4233

(μmol/g)

NLAS-5733

(μmol/g)

NLAS-7233

(μmol/g)

FAU(∞, 4.1)-large 1512 - - - - - - -

FCC(micro,fresh) 340 - - - - - - -

FCC(micro,steamed) 0 592 100 60 21 154 94 46

FCC(micro,regenerated) 0 - 74 49 17 48 26 8.7

FAU(45, 2.9)-large 648 - - - - - - -

FCC(meso,fresh) 286 - - - - - - -

FCC(meso,steamed) 0 609 30 15 3.5 118 64 29

FCC(meso,regenerated) 0 - 40 22 5.8 68 36 14 1 Concentration of BAS determined by H/D exchange FTIR at 523 K [31];

2 Temperature required to reach n-heptane conversion of 40%;

3 Concentration

of BAS and LAS determined after evacuation for 1 h at 423 K, 573 and 723 K.

Chapter 7

156

Fig. 7.10. FTIR spectra showing the (left) OH and (right) OD stretching regions of FAU(∞, 4.1)-large recorded after exposure to d6-benzene

at different times and temperatures.

3800 3700 3600 3500 3400 3300 3200

0.0

0.1

0.2

0.3

0.4

Ab

so

rban

ce

Wavenumber cm-1

2800 2700 2600 2500 2400

0.0

0.1

0.2

0.3

Wavenumber (cm-1)

Activated 303 K, 1800 sec 303 K, 1800 sec 303 K, 3600 sec 323 K, 1800 sec 373 K, 1800 sec 523 K, 1800 sec

303 K, 1800 sec 303 K, 1800 sec 303 K, 3600 sec 323 K, 1800 sec 373 K, 1800 sec 523 K, 1800 sec

Chapter 7

157

Fig. 7.11. FTIR spectra showing the (left) OH and (right) OD stretching regions of FAU(45, 2.9)-large recorded after exposure to d6-benzene

at different times and temperatures.

3800 3700 3600 3500 3400 3300 3200

0.00

0.05

0.10

0.15

0.20

Ab

so

rban

ce

Wavenumber (cm-1)

2800 2700 2600 2500 2400

0.00

0.05

0.10

Wavenumber (cm-1)

Activated 303 K, 1800 sec 303 K, 1800 sec 303 K, 3600 sec 323 K, 1800 sec 373 K, 1800 sec 523 K, 1800 sec

303 K, 1800 sec 303 K, 1800 sec 303 K, 3600 sec 323 K, 1800 sec 373 K, 1800 sec 523 K, 1800 sec

Chapter 7

158

7.3.4 Composite catalyst characterization

Composite catalysts were prepared by spray-drying the zeolite with Kaolin as filler and

alumina sol as the binder. The resulting catalysts were deactivated by steaming at 1023 K for

4 h using 100 % steam followed by calcination at 873 K for 1 h. After FCC performance

evaluation, the catalysts were regenerated in air at 823 K. The physical properties of the

freshly prepared, lab-deactivated and regenerated composite catalysts were determined by

XRD and Ar physisorption. The contribution of the zeolite component in the composite

catalysts is clearly visible in the XRD patterns (Fig. 7.5). The additional broad feature in these

patterns around 23° is due to amorphous silica originating from the kaolin component. The

textural data of the composite catalysts listed in Table 7.2 point out the lower micropore

volume as compared with the parent zeolites. The decrease in micropore volume of the

composite catalysts trends well with the XRD crystallinity. The decrease is only slightly

higher than the zeolite content; this suggests that the procedure to obtain the composite

catalysts has only slightly damaged the zeolite structure. The BAS content of the composite

FCC(meso, fresh) catalyst as measured by H/D exchange FTIR is nearly proportional to the

zeolite content of the composite catalyst. For FCC(micro, fresh), the BAS density is slightly

lower than the zeolite content.

As customary in FCC catalyst evaluation, the composite catalysts were subjected to a

steam-calcination treatment that simulates the deactivation in the FCC regenerator. Fig. 7.5

shows that this treatment led to a significant decrease of the zeolite crystallinity. The stronger

decrease of the crystallinity of mesoporous Y zeolite points out its lower hydrothermal

stability, which can be linked to the higher Al content. The lower stability is also evident from

the stronger decrease of the micropore volume during the steam treatment step. After FCC

evaluation and regeneration at 773 K, the XRD patterns were almost unchanged. This shows

that the accelerated deactivation treatment yields representative equilibrium catalysts. This

conclusion based on XRD is underpinned by the textural data. According to H/D exchange

FTIR, the deactivated and regenerated composite catalysts do not contain BAS. The low BAS

content of the steamed and regenerated catalysts is also indicated by the absence of features at

3550 cm-1

and 3631 cm-1

in their corresponding FTIR spectra after dehydration (Fig. 7.12).

We estimate that the detection limit of the H/D exchange FTIR method is 0.002 mmol/g.

Chapter 7

159

Fig. 7.12. FTIR spectra showing the OH stretching regions of zeolites and composite

catalysts: (left) (a) FAU(∞, 4.1)-large (b) FCC(micro), (c) FCC(micro,steamed), (d)

FCC(micro, regenerated); (right) (a) FAU(45, 2.9)-large (b) FCC(meso), (c) FCC(meso,

steamed) and (d) FCC(meso, regenerated).

The acidity of the steam-deactivated and regenerated composite catalysts was also probed

by FTIR spectroscopy of adsorbed pyridine. While H/D exchange FTIR mainly probes strong

zeolite acidity, pyridine probes a broader range of BAS [55]. BAS and LAS contents

determined by deconvolution of the FTIR spectra as function of the evacuation temperature

are collected in Table 7.5. Both zeolites contain only a small amount of BAS as represented

by the values after evacuation at 423 K. The Brønsted and Lewis acidities were higher in the

FCC(micro) composite catalysts as compared with the FCC(meso) ones. The regenerated

composite catalysts had nearly similar acidities as the parent samples before FCC catalyst

activation. The amount of pyridine after evacuation at 723 K represents strong BAS. The

amount of such strong BAS in the steam-deactivated and regenerated zeolites is very low; it is

much lower than the amount of strong BAS in amorphous silica-aluminas [55]. H/D exchange

FTIR method is able to probe this small amount of strong BAS in amorphous silica-aluminas.

Thus, we speculate that the low acidity in the composite catalysts is mainly due to an

amorphous silica-alumina phase, which is likely closely integrated in the zeolite material. The

number of strong zeolite BAS in the composite catalysts is too low to be titrated by H/D

exchange FTIR. Overall, the Brønsted acidity of FCC(micro) is higher than that of

FCC(meso). Also, the FCC(micro) composite zeolites contain more Lewis acid sites.

3800 3700 3600 3500 3400 3300 3200

Ab

so

rban

ce

Wavenumber (cm-1)

3800 3700 3600 3500 3400 3300 3200

Wavenumber (cm-1)

Chapter 7

160

We further determined the acidity of the composite catalysts by measuring their catalytic

performance in the bifunctional hydroisomerization of n-heptane. For this purpose, we loaded

the catalysts with 0.4 wt% Pd. At this Pd content, the isomerization of hydrocarbons is limited

by the Brønsted acidity of the catalyst [43]. The acidity of zeolites, clays and silica-alumina in

terms of hydroisomerization activity correlates with the concentration of strong BAS of

zeolitic strength titrated by the H/D exchange FTIR method [43]. The temperature required

for a conversion of 40% (T40) is used to measure the acidity. The T40 values for FCC(micro)

and FCC(meso) were 592 and 609 K. These values are typical for amorphous silica-alumina

samples, which contain only few zeolite-like BAS [43, 56]. Thus, we speculate that only very

few zeolite-type acid sites remain due to the extensive steam-deactivation procedure.

7.3.5 Catalytic activity measurements

The lab-deactivated composite catalysts were then evaluated for their FCC catalytic

performance in an ACE testing unit using a VGO feed. Relevant data about the VGO are

given in Table 7.6. It is mainly composed of saturated and aromatic compounds. The sulfur

content is relatively low for VGO. In addition, the VGO contains small amounts of Ni and V.

The ACE test was carried out at a catalyst-to-oil ratio of 6 and a contact time of 90 s. The

temperature was 803 K. The products were analyzed by established techniques and grouped

into dry gas, gasoline, light cycle oil (LCO), bottoms and coke product classes. Table 7.7

shows that both composite zeolite catalysts can achieve high conversion of the feed. The

FCC(micro)-based catalyst exhibited a slightly higher VGO conversion than the catalyst

based on the FCC(meso) zeolite, which is most likely due to the higher acidity of the former

composite catalyst [57, 58]. We cannot draw firm conclusions about the influence of the

mesopores on the feed conversion because of the acidity difference of the parent zeolite

components. FCC(meso) shows significantly higher LCO (diesel) yield at nearly similar

gasoline yield. The combined yield of less valuable coke and LPG products is lower

compared with the reference. Although the bottoms yield is higher than for the microporous

reference catalysts, this will not be a significant drawback in practice because this fraction can

be recycled to the riser. By comparison with yield-conversion data in fluid catalytic cracking

[59,60], the higher diesel and lower gasoline yield can be reasonably linked to the improved

diffusion due to presence of mesopores that limits secondary cracking reactions of LCO and

bottoms [32].

Chapter 7

161

Table 7.6. VGO composition and properties used as feedstock for FCC catalyst evaluation.

Density (293 K)

(g/cm3)

0.93

Viscosity (373 K)

(mPa•s) 13.84

Carbon residue (wt.%) 4.19

C (wt.%) 86.76

H (wt.%) 11.63

S (wt.%) 0.40

N (wt.%) 0.69

Saturates (wt.%)

Aromatics (wt.%)

Resins (wt.%)

Asphaltenes (wt.%)

64.68

28.44

6.68

0.21

Ni (ppm)

V (ppm)

4.9

3.7

Table 7.7. Product distribution after FCC catalyst evaluation of composite catalysts (VGO, T

= 803 K, catalyst-to-oil ratio = 1.7, contact time 90 s).

Composite

catalyst dry gas

(wt%) LPG

(wt%)

gasoline

(wt%) LCO

(wt%) coke

(wt%) bottoms

(wt%) conversion

(wt%) FCC(micro) 3.2 16.7 39.6 22.9 10.7 6.9 93

FCC(meso) 2.3 12.7 36.3 29.3 8.1 11.3 89

7.4 Conclusions

The synthesis of mesoporous faujasite zeolite using an amphiphilic organosilane was

optimized. Using TPOAC, longer crystallization times were needed as compared with

conventional zeolite Y synthesis. The TPOAC-modified syntheses gave mesoporous zeolites

with appreciable mesoporosity, yet decreased crystallinity. While it was possible to selectively

remove Al from the framework of well-crystallized microporous zeolite Y by treatment with

AHFS, such treatment led to the collapse of the mesoporous Y zeolites. Two zeolites prepared

at larger scale, namely microporous Y zeolite followed by AHFS treatment and mesoporous Y

zeolite were used to prepare composite FCC catalysts by spray-drying the zeolite with Kaolin

as filler and alumina sol as binder. Infrared spectroscopy after H/D exchange shows the

presence of strong bridging hydroxyl groups in the freshly prepared composite catalysts in

amounts that are in keeping with the zeolite content. After accelerated steaming, these strong

zeolitic Brønsted acid sites are not observed anymore. These samples contain a significant

Chapter 7

162

number of weaker Brønsted acid sites. The strength of the acid sites in the composite catalysts

is comparable with the acidity of amorphous silica-alumina. The composite catalysts show

excellent catalytic performance in the fluid catalytic cracking of a vacuum gas oil. The

catalytic data indicate that the relatively weak acid sites are responsible for the FCC activity.

The well-embedded mesoporosity in the parent zeolite crystals results in higher diesel and

lower gasoline yield.

7.5 References

[1] C.I.C. Pinheiro, J.L. Fernandes, L. Domingues, A.J.S. Chambel, I. Graça, N.M.C. Oliveira,

H.S. Cerqueira, F Ramôa Ribeiro, Ind. Eng. Chem. Res. 51 (2012) 1-29.

[2] W. Vermeiren, J.-P. Gilson, Top. Catal. 52 (2009) 1131-1161.

[3] H.S. Cerquira, G. Caeiro, l. Costa, F. Ramôa Ribeiro, J. Mol. Catal. A 292 (2008) 1-13.

[4] A.A. Avidan, Oil Gas J. 90 (1992) 59-67.

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Summary

165

Hierarchical zeolites for catalytic hydrocarbon conversion

Currently, our society is largely fueled by crude oil, coal and gas. The depletion of these fossil

reserves, geo-political issues and environmental concerns are main drivers to look for other

carbon feedstocks to secure the fuels and chemicals demand in the future. The growing

concern about climate change also drives the search for renewable resources. As the lead time

for the development of a novel technology from invention to industrial use at a level that it

impacts the economy takes several decades, it is important to improve the efficiency of the

current industrial processes that convert fossil feedstock into fuels and chemicals. In addition,

it is necessary to use as clean as possible fossil feedstock. Natural gas with methane as the

main component has the highest H/C ratio of all fossil fuels and is accordingly considered as

the most important feedstock for the midterm when novel low-carbon technologies are being

developed. A further advantage of natural gas is that large reserves are available. However,

many of these reserves are stranded so that it is difficult to monetize them. Transport via

pipelines or in liquefied form either by liquefaction or after conversion into hydrocarbons via

the Fischer-Tropsch process are very costly. Accordingly, several approaches are under

investigation to convert methane directly into liquids. With methane oxidation typically

yielding only very low methanol yields, the option of aromatization of methane into benzene

and hydrogen has drawn most attention from industry and academia. In methane

dehydroaromatization (MDA), methane is directly converted into high value and easy to

handle liquid aromatic compounds, mainly benzene. Poor stability of the MDA catalyst has

hampered implementation of this reaction in the industry.

In Chapter 2, the influence of mesopores created by base desilication and silylation on the

catalytic performance of Mo/HZSM-5 in MDA was investigated. The desilication procedure

of HZSM-5 was optimized to have a mesoporous material with acidic sites similar to those in

bulk HZSM-5. The calcination procedure used during Mo introduction was shown to have a

strong influence on the physical properties of the final Mo/HZSM-5 material. It was shown

that impregnation of the zeolite with an aqueous ammonium heptamolybdate (AHM) solution

followed by calcination at 823 K for 5 h was optimal. The material obtained in this way

Summary

166

contained highly dispersed Mo-oxide phase and had a high zeolite crystallinity with only

small amounts of extraframework aluminum. The Mo/HZSM-5 material prepared upon

physical mixing with MoO3 had a low Mo-oxide dispersion after calcination at 823 K.

Evaluation of the catalytic performance of bulk and mesoporous Mo/HZSM-5 in MDA

showed that the mesoporous zeolite catalyst exhibited better stability. This improvement is

attributed to the smaller micropore domains of the zeolite support decreasing the negative

effect of micropore blockage by carbon deposition. Characterization of fresh mesoporous

Mo/HZSM-5 showed an improved Mo-oxide spreading over the surface. The methane

conversion rate and aromatics selectivity of mesoporous Mo/HZSM-5 was lower compared to

bulk Mo/HZSM-5. These difference are due to the formation of larger Mo-carbide (MoCx)

particles in mesoporous Mo/HZSM-5 when pretreating the catalysts in He followed by

exposure to methane-rich reaction conditions. The large MoCx particles formed in this way

display higher propensity to coke formation. In an alternative approach, a post-synthesis

silylation treatment was employed. Silylation selectively deactivates the Brønsted acid sites

(BAS) located at the external surface. Such BAS are suspected to form polyaromatic carbon

species during the MDA reaction leading to micropore blockage and, with this, catalyst

deactivation. The silylation of Mo/HZSM-5 led to an increase of the benzene selectivity and

methane conversion rate for both bulk and mesoporous Mo/HZSM-5 compared to their non-

silylated analogues. Post-mortem analysis of used catalysts showed smaller amounts of

aromatic hard coke in the silylated catalysts, which is in keeping with the lower rate of carbon

formation. The higher methane conversion rate after catalyst silylation was attributed to the

improved spreading of the Mo-oxide phase. It was found that silylation of the zeolite before

modification by Mo was less preferred as compared with silylation of the Mo/HZSM-5 zeolite.

The hydrophobicity of the zeolite after silylation led to poor Mo-oxide dispersion upon

modification of HZSM-5 with Mo, resulting in low methane conversion rate and benzene

selectivity in MDA.

In Chapter 3, the influence of the gas used during the pre-treatment of the catalyst on the

MDA reaction was investigated. Silylated and non-silylated Mo/HZSM-5 were exposed to

inert (He), oxidizing (artificial air) and carburizing (CH4 diluted in He) gases.

Characterization of the catalysts after such pretreatment steps showed that a larger fraction of

the Mo species was present in the micropores after pretreatment in He or air than after

precarburization in methane. The lower diffusivity of Mo species into the micropore channels

upon precarburization in methane was attributed to faster formation of immobile Mo carbides.

Summary

167

Catalytic evaluation revealed the highest catalytic stability and benzene selectivity for the

precarburized catalysts. The lower stability and benzene selectivity of air and He pretreated

catalysts was attributed to the greater amount of Mo-oxide species in the micropores for these

catalysts after pretreatment. The carburization of Mo-oxide particles inside the micropores

results in MoCx particles, which partially block the zeolite pores. A larger part of the

micropores is blocked after carburization for He and air pretreated catalysts, making a larger

part of the BAS inaccessible, than for precarburized catalysts.

Chapter 4 presents a comprehensive study to understand catalyst deactivation in the

MDA reaction. Catalysts were recovered after precarburization and after various times on

stream. These recovered catalysts were thoroughly characterized. With progressing reaction, a

poly-aromatic layer grows over the external surface of the catalyst. The formation of this layer

is detrimental for the catalyst performance in two ways. Firstly, the carbon layer blocks the

micropores, decreasing the accessibility to the BAS located inside the micropores. As a result,

the benzene formation decreases. Secondly, the carbon layer forms a barrier between the

initially highly dispersed MoCx particles and the zeolite surface. Due to lower interactions of

the Mo phase with the zeolite support, the MoCx particles agglomerate into larger particles,

resulting in a decreased methane conversion rate. Besides, the large MoCx particles are also

more susceptible to coke formation. The formation of the polyaromatic layer is likely due to

acid sites located at the external surface. As discussed in Chapter 2, these surface BAS can be

partially deactivated by a silylation treatment, which surpresses to some degree the growth of

the aromatic layer at the external surface.

In Chapter 5, a novel method for the preparation of nano-crystalline MCM-22 is

presented. To decrease the MCM-22 crystallite size an organosilane molecule (octadecyl-(3-

trimethoxysilylpropyl)-ammonium chloride, TPOAC) was added to the synthesis gel. When

grafted to the zeolite surface, the hydrophobic tail limits the growth of the MCM-22 crystal

and thus the average zeolite crystal size. The catalytic performance of the nano-crystalline

MCM-22 was evaluated in the MDA reaction as well as in liquid phase benzene alkylation.

Although the nano-crystalline MCM-22 had a lower Brønsted acidity than bulk MCM-22,

nano-crystalline MCM-2 showed improved performance in the MDA reaction compared with

conventional MCM-22. This is attributed to the smaller micropore domain size. In liquid

phase benzene alkylation, nano-crystalline MCM-22 showed catalytic performance

intermediate between bulk MCM-22 and ITQ-2. The improved performance of nano-

crystalline MCM-22 compared with bulk MCM-22 was attributed to the increased

Summary

168

accessibility of the zeolite BAS to benzene.

In Chapter 6, a brief study of the influence of the micropore domain size of the ZSM-5

zeolite support on the performance in the MDA reaction is presented. To this end, a series of

zeolites was prepared varying in micropore domain size. After Mo modification the various

catalysts were screened in the MDA reaction. The catalytic evaluation showed improved

benzene selectivities and catalytic stabilities for the hierarchical structured zeolites containing

smaller micropore domain sizes compared to their bulk analogues. We conclude that the

better stability of the hierarchical catalysts in the MDA reaction relates to the improved

accessibility of the BAS located inside the micropores. Furthermore, hierarchical structuring

decreases the adverse effect of micropore blockage.

Chapter 7 presents a study on the use of an organosilane molecule (octadecyl-(3-

trimethoxysilylpropyl)-ammonium chloride, TPOAC) as mesoporogen for the preparation of

hierarchical zeolite Y as potential acid component in FCC catalysts. The use of TPOAC in the

synthesis gel led to the formation of interconnected mesopores in the final zeolite crystals.

After optimization at the labscale, the synthesis of mesoporous and microporous zeolite Y

was scaled up with the purpose to prepare a bindered FCC catalyst, which could be evaluated

for its FCC performance in an Advanced Catalytic Evaluation (ACE) unit. A part of the

framework aluminum in the microporous material was selectively exchanged with silicon

following treatment with AHFS. Such treatment with the mesoporous material was not

possible, because it led to amorphization of the zeolite. The zeolites were used to prepare FCC

catalysts by spray-drying with Kaolin as filler and alumina sol as binder. Acidity

characterization by H/D exchange showed the presence of strong BAS in the freshly bindered

zeolites. After accelerated deactivation of the FCC catalysts by steam calcination, no strong

BAS were observed anymore. The lab-deactivated catalyst contained a substantial amount of

weaker BAS. The acid strength of these acid sites are comparable to those in amorphous silica

aluminas. Evaluation of these lab-deactivated FCC catalysts in an ACE unit using a vacuum

gas oil fluid catalytic showed high conversion rates and product distributions typical for

zeolite Y based cracking catalysts. These findings suggest that the residual weak acid sites are

responsible to catalyze the FCC reactions. The well-embedded mesoporosity resulted in

higher diesel and lower gasoline yield.

One of the major issues in the catalytic conversion of hydrocarbons is the coke formation

resulting in catalyst deactivation in processes such as dehydroaromatization of methane and

FCC of heavy oils. In general, catalyst stability largely determines the economic viability of a

Summary

169

process. Poor catalyst stability due to coke formation in the MDA reaction is an illustrative

example, where catalyst deactivation poses such a challenge that large scale implementation

in the chemical industry is hampered. To tackle deactivation problems, combining catalyst

development with advanced reaction engineering solutions appear important. To identify

better MDA catalyst formulations and process conditions, it is essential to understand better

the reaction mechanism, the exact state of the Mo component [1], the the role of the zeolite

support, the need for acid sites in the catalytic reaction and/or in stabilizing certain Mo states.

Two approaches must be considered. The design of better models with more controlled and

preferably homogeneous speciation of the Mo component to be able to better follow the

evolution of the Mo species and the acid sites during the MDA reaction. The other one is to

use advanced techniques to better investigate the catalysts during activation and operation.

For instance, high resolution electron microscopy and electron tomography can help to

establish the size and location of the Mo phase in these catalysts. A re-investigation of the Mo

phase by X-ray absorption spectroscopy starting from well-defined models will also be

helpful to resolve the location and structure of the catalytic sites that activate methane. Whilst

deactivation of the Mo/HZSM-5 seems to be a fact, it is worthwhile to investigate in more

detail the possibility to regenerate the catalysts by oxidation or reduction reactions. Oxidation

helps to remove the coke built up on the catalyst but has the drawback that oxidation of the

Mo-carbide leads to sublimation of the resulting Mo-oxide. Hydrogenation helps to reduce the

coke content also, but only as long as no large condensed aromatic products are formed.

Combinations of these approaches can also be considered.

Possible opportunities for catalyst improvement evolve from the rapid development in the

field of zeolite science. New topologies are reported frequently and routes for the hierarchical

structuring of zeolites to decrease micropore domain size are intensively investigated.

Tailoring the properties of the zeolite support could lower the rate of formation of

hydrocarbon species that deactivate the catalyst. The potential of new zeolite topologies and

procedures to introduce mesopores are not limited to the MDA reaction. Although processes

like FCC of heavy oil fractions are well established, there is still room for improvements. For

instance, Rive technology has shown the promise of hierarchically structured fuajasite zeolite

at the commercial scale [2]. The introduction of well interconnected mesopores in faujasite

improves the accessibility of the acid sites to bulky molecules present in heavy feedstocks

such that the conversion can be improved. Although many of the procedures used to generate

mesoporosity in zeolite require organic molecules as mesoporogens, our increasing

understanding about zeolite growth [3] may help to control the formation of advanced porous

solids with controlled porosity at all length scales and ideally placed active sites.

Summary

170

References

[1] J. Gao, Y. Zheng, J.M. Jehng, Y. Tang, I.E. Wachs, S.G. Podkolzin, Science 348 (6235) 686-690.

[2] K. Li, J. Valla, J. Garcia-Martinez, ChemCatChem. 6 (2014) 46-66.

[3] A.I. Lupulescu, J.D. Rimer, Science 344 (2014) 729-732.

List of publications

171

Journal publications

C.H.L. Tempelman, V.O. de Rodrigues, E.H.R. van Eck, P.C.M.M. Magusin, E.J.M

Hensen, Desilication and silylation of Mo/HZSM-5 for methane dehydroaromatization,

Microporous Mesoporous Mater. 203 (2015) 259-273.

2

C.H.L. Tempelman, X. Zhu, X., E.J.M. Hensen, Activation of Mo/HZSM-5 for methane

aromatization. Chin. J. Catal., 36 (2015) 829-837.

C.H.L. Tempelman, E.J.M. Hensen, On the deactivation of Mo/HZSM-5 in methane

dehydroaromatization. Applied Catal. B, 176 (2015) 731-739.

4 C.H.L. Tempelman, X. Zhu, K. Gudun, B. Mezari, B. Shen, E.J.M. Hensen, Texture,

acidity and fluid catalytic cracking performance of hierarchical faujasite zeolite prepared

by an amphiphilic organosilane. Fuel Process. Technol. (2015), in press.

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C.H.L. Tempelman, M. T. Portilla, M.E. Martínez-Amero, B. Mezari, N.G.R. de

Caluwé, C. Martínez, E.J.M. Hensen, One-step synthesis of nano-crystalline MCM-22,

Microporous Mesoporous Mater. (2015), in press.

The author also contributed to the following publication outside scope of this thesis:

e

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A.J.J. Koekkoek, C.H.L. Tempelman, V. Degirmenci, M. Guo, Z. Feng, C. Li, E.J.M.

Hensen, Hierarchical zeolites prepared by organosilane templating: a study of the

synthesis mechanism and catalytic activity, Catal. Today 168 (2011) 96-111.

Contribution to book publication

t

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M.T. Portilla, C.H.L. Tempelman, C. Martinez, E.J.M. Hensen, New trends in catalyst

design for methane dehydroaromatization. In G. Centi (Ed.), Recent Advances in Gas to

Liquid Technologies (2015).

Conferences

C.H.L. Tempelman, V.O. de Rodrigrues, E.J.M. Hensen, Optimizing zeolites for non-

oxidative dehydrogenation of methane to benzene: Effect of mesopore introduction and

silylation, Europacat XI. 1 – 6 September 2013, Lyon, France. [Oral]

2

C.H.L. Tempelman, N.G.R. de Caluwé, B. Mezari, E.J.M. Hensen, One-pot synthesis of

mesoporous MCM-22 for benzene alkylation, 17th International Zeolite Conference (IZC

17). 7 – 12 July 2013, Moscow, Russia. [Poster]

List of publications

172

C.H.L. Tempelman, E.J.M. Hensen, An exploratory study on the deactivation of methane

aromatization catalysts: Opportunities for improved catalyst performance, 2nd

International Conference on Materials for Energy (EnMat II). 12 – 16 May 2013,

Karlsruhe, Germany. [Oral]

4 C.H.L Tempelman, V.O. de Rodrigues, E.J.M. Hensen, Optimizing zeolites for non-

oxidative dehydrogenation of methane to benzene: Effect of mesopore introduction and

silylation, 14th Netherlands' Catalysis and Chemistry Conference (NCCC XIV). 11 – 13

March 2013, Noordwijkerhout, The Netherlands. [Oral]

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C.H.L. Tempelman, V.O. de Rodrigues, E.J.M. Hensen, Developing hierarchical zeolites

for MDA catalysis, Summer School Energy and Materials from the Sun. 20 – 23 June

2011, Rolduc Abbey, The Netherlands. [Poster]

V.O. de Rodrigues, C.H.L. Tempelman, P.C.M.M. Magusin, E.J.M. Hensen, Methane

dehydroaromatization: the influence of hierarchical structuring, 12th Netherlands'

Catalysis and Chemistry Conference (NCCC XII). 28 February – 2 March 2011,

Noordwijkerhout, The Netherlands. [Oral]

Acknowledgements

173

The long road has come to an end. The ride was sometimes rough and bumpy, but it made me

grow as a scientist and as a person. It was a privilege to work with people from so many

different backgrounds and to make a lot of new friends. Needless to say, without the help of

many people this thesis would never be possible. Probably I will forget some people in these

acknowledgements and therefore I apologize on beforehand.

First of all I would like to thank Emiel. You introduced me in the world of zeolite

catalysis when I was starting as a VKO student at the university. You showed me the

complexity of heterogeneous catalysis and thaught me to look critical at thing. Thank you for

all the advice and support during the project, but also the freedom to explore the broad world

of zeolite catalysis. It gave me the chance to evolve not only as a chemical engineer and

scientist, but also personally. Especially the opportunity to work in a European project with so

many partners and the possibility do a part of the research in Valencia were eye-opening

experiences. I am also grateful to you for giving me the chance to work in the CatchBio

project.

Also, I would like to thank the commitee members prof. Rutger van Santen, prof. Freek

Kapteijn, dr. Pieter Magusin, prof. Bert Weckhuysen and prof. Volker Hessel who took the

time to read and evaluate the thesis and the suggestions they gave me to further improve this

thesis.

Thank you Cristina, Teresa and Belene for fascilitating the benzene alkylation

experiments in Valencia and the scientific discussions during the project. Your hospitality

made the stay in Spain a great experience, which I will never forget!

My master graduation project with you Arjan was a perfect introduction to the IMC

group. You taught me the first steps as a scientist who came in handy the rest of the project.

You are a great teacher, a good scientist and a nice collegue to work with.

After working the first year of PhD with Victor I was somewhat spoiled. You generated a

great amount of data which largely contributed to the scientific output of the project. So

special thanks to you for kick-starting the project, it helped me during the rest of my PhD.

During the project I worked with several students who did a great part of the work and

initiated new ideas. Good luck Arno, Robin Broos and Robin Willems finishing your PhD

projects and Kristina and Niek with your careers.

Acknowledgements

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Of course very important, and I guess this is true for everybody in the IMC group, is the

help I got from Emma. Your knowledge about the university rules has prevented major

disasters. I really admire your to-the-point acting and your willingness to help people. Several

times I promised you to devote a special chapter to you in the thesis, but unfortunately I had

to keep it limited to these few lines due to budget problems ;). I really enjoyed working with

you Brahim. Your knowledge on zeolites was not only limited to your expertise on NMR. I

like your persistence to understand “weird” findings and your willingness to help other

people. Discussing the results during the project and putting scientific weight to them resulted

in new insides. Johan and Tiny, also known as the “Buurman and Buurman” of IMC, you guys

are an essential part of the IMC group. The way you solve technical issues is similar to that of

a formula 1 pitstop crew; fast and good. I learned a lot of new skills, which are also useful at

home. Adelheid, I am thankful for the help during the ICP sessions, but foremost I remember

the Inorganic Chemistry lab practice course we supervised. Besides the new chemistry tricks I

learned, we talked a lot about things outside chemistry. What I really appreciated is that I

could always talk to you about personal matters which were bothering me.

Alessandro, I enjoyed our separate coffee breaks we had all over the campus. During our

private coffee sessions we had lively discussions about everything. The obviousness of you to

help people without anything in return is heart striking. I often had to laugh about your typical

“Alessandroiaanse” way of explaining thing; wordy but clear. William thanks for the fruitful

collaboration on the Sn-Beta project. But even more for teaching me politics, how to grow

peppers, the country of Austria and its inhabitants, how to speak proper German and more of

these wisdoms from Lage Zwaluwe. And of course the walk to the Jan Linders during the

lunch breaks. You are a nice person to work with and I admire your patience with people,

even when they are shouting at you. Nikolay and georgy, you showed me that Russians are

pleasant people to hang out with and that you can even be friends with them. Nikolay, it is a

shame that we never succeeded in winning the NIOK catalysis football cup with the Bulls of

Hensen. A likely reason is that we always peaked too late, the third half. Good luck with the

MDA project! Georgy, thanks for showing me the nightlife in Eindhoven and surroundings

and providing bed and breakfast when needed. Oh yeah, and the infamous stay in Lyon where

we cannot talk about. I hope you have a great time in Japan. Lara, I enjoyed the conversations

we had over a beer or a coffee. I hope you will finish your thesis soon! Evgeny P., you

encouraged me to get out of my comfort zone which boosted my confidence in new social

environments. I will remember traveling to the Next-GTL meetings in the so-called “flying

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death-traps”. I had a lot of fun during these meetings. It was an absolute pleasure to work with

you during the CatchBio project. I like your non-conventional way of looking at things, it is

very refreshing. Volkan, I learned a lot from you during the graduation project and the first

years of PhD. Although we only met a few times after you left IMC, these moments were

legendary. During the last year I missed complaining to you, Aysegul, in our beloved misery

corner. I am looking forward to the next time we meet. Thanks to the other IMC group

members with who I shared good times during and after work: Andrey, Anton, Aleksander,

Arno, Abdul, Burcu, Chao Chao, Douglas, Evgeny U., Esther, Giulia, Jan-Philip, Jan, Juan,

Freke, Giulia, Gabriela, Guanna, Ivo, James, Kaituo, Lennart, Lingqian, Lei Lei, Long Chen,

Long Fei, Lu, Maarten, Robert, Robin, Sami, Tamas, Tobias, Wilburt, Xaoming, Xiaochun,

Xuefang, Yibin ….

Besides my friends of IMC I have to acknowledge some of my friends from home, which

are named here in no particular order. Thanks to you Kees; you brought me relieve in the

darkest times with your sense of humor. And you Marica for being even more dark than

Kees... ough! Together with little Boris you formed a family to go to when I felt absolutely

miserable. I also have to thank you Levijn. You are my friend since… highschool? You are

always there, whenever I am angry, anxious or sad, but above all you bring joy! Veerle,

besides being a good friend I learned from you that doing things without too much thinking

can be quite useful to get further in life. I am confident you will be Holland’s most famous

tattoo artist. Geert thank you for being... you! You and Daniëlle give sanity to this world with

your upright no-nonsense views. Therefore this need-to-keep-it-short-acknowledgement. Esgo

and Roland “Roel Driesvink” Gieles, the jamming sessions at the Tammer we had are

unforgetable and were a great relieve during times of stress. Roel, I am still waiting for your

book of memoires when you were sailing to the old Indies! Cor, you are a true friend without

agenda and politics. Naturally, I have to mention Jos, the most charismatic guy from Leiden.

The nice thing about you is that once you start talking you never stop. The sound waves you

produce in this way can be very relaxing and are often saturated with humor. And don’t forget

your voice imitation of a screaming pig, it is legendary. Nooks, Robin, Magriet, Githa and Eté

you always provide me a warm welcome to your home. Thank you for caring about me

whenever I needed it. Kwiest, Henriette, Nout, Sharon, Mik and Nomi; thanks for all the

Sunday mornings we were running with or without hangover and the holidays in Ireland.

Daniel, looking at you how you cope with your setbacks in live reminds me to put things in

perspective when things are tough. I respect you for your perservance and positivity. I am sure

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you will conquer the world with your plates and build your imperium as a designer. Peer,

Neel, Lara and Freek, thank you for all the meals you cooked for me. It prevented me from

starving. And of course the rest of gang: Clemens, Dennis, Helma, Ilse, Krista, Marcel,

Marlieke, Remo, Silvie, Stefan, Thijs and Vince. You guys are the best and are special to me!

Judith, Cecile, Mathias, Jos, Marthe, Hugo and Merel, for almost 6 years I enjoyed being at

your home. It felt like I was a part of the family. Unfortunately, things went different but I will

always cherish that period.

Last but not least, my family. Thank you grandparents, aunt Luus, uncle Dik, aunt Marion

and uncle Gerard for always being supportive. It didn’t matter if it was school, sports or

music. A special thanks to uncle Herman who always believed in me. You took and still take

us everywhere we want and I relish the discussions we have during the long hikes we make.

Mirjam and Ivo, the fact that I can always come to your place to eat, drink beers, sport, work

in the garden, watch Dotje and Jaggers fight, complain, watch TV series but above all enjoy

your company is an enrichment to my life. Linda, thank you for entering my life during the

last bits of writing. The process became way lighter and made me start to enjoy things again.

And then finally my loving mom and dad! Although you pushed me the most of all people

you also always believed in my abilities. You taught me to respect myself and others and to do

what you think is right. Those lessons turned out to be very useful in life.

Curriculum vitea

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Christiaan Tempelman was born on 02 September 1984 in Zaltbommel. After finishing HAVO in

2002 at Scholengemeenschap Cambium in Zaltbommel, The Netherlands, he studied Chemistry at the

Fontys Hogescholen in Eindhoven. After receiving his bachelor degree, he continued his master

studies Chemical Engineering at the Eindhoven University of Technology. He graduated in 2010

within the Inorganic Materials Chemistry (IMC) group on “Synthesis mechanism of mesoporous

ZSM-5” supervised by prof. dr. ir. E.J.M. Hensen. He continued to work in the same group by starting

a PhD project in 2010. His PhD study concerned the conversion of hydrocarbons over zeolite catalysts

with the main focus on the methane dehydro-aromatization reaction to directly convert low-value

methane to high-value benzene. The project was part of the Next-GTL consortium project funded by

the European Union. The results are presented in this dissertation and have been published as 5

research papers in peer reviewed international scientific journals. The findings in this work have been

presented at several national and international conferences. In 2013, he worked 6 weeks at the Instituto

Tecnología Químíca (ITQ) in Valencia as a visiting researcher. From March 2014 he started to work

as a researcher within the IMC group on the catalytic conversion of glucose based biomass towards

renewable fuels and chemicals. As of September 2015 he is employed at DAF Trucks N.V where he

works as development engineer on heavy-duty diesel after treatment systems.