study on structural transformation of molybdenum oxide catalyst using in-situ xrd technique for

Chapter 1, Introduction _

1

CHAPTER 1

INTRODUCTION


2

1.0 Introduction

1.1 Overview of Malaysia’s Petrochemical Industry

Oil and Gas sector is a key component in Malaysia‘s economic growth. About 30%

of the manufacturing income and roughly 8% of annual gross of domestic product comes

from the oil and gas sector (Kok, 2009). As at 2010, the petroleum and petrochemical

industry investments in Malaysia totals up to RM58 billion. Malaysia comprises the

world‘s 14th

largest natural gas reserves and the 23rd

largest crude oil reserves (MIDA,

2011). As of January 2010, Malaysia held 83 trillion cubic feet (Tcf) of proven natural gas

reserves, mostly in offshore Sarawak (EIA, 2010).

Malaysia‘s potential as an investment location for petrochemical industries is

distinguished by the presence of world renowned petrochemical companies‘ such as Shell,

BASF, Dow Chemical, BP, Toray, Mitsubishi, Idemitsu, Polyplastics and Eastman

Chemicals. Many of these petrochemical companies are in collaboration with PETRONAS,

Malaysia‘s national petroleum company. The industries rapid growth is attributed to the oil

and gas feedstock availability, infrastructure development, strong supporting services, cost

competitiveness, strategic location within ASEAN and major markets in the Far East

(AHK, 2010; MIDA, 2004).

The adequate supply of petrochemical feedstock are made certain by the six gas

processing plants in Kerteh, Terengganu and Malaysia‘s Peninsular Gas Utilisation (PGU)

pipelines that channel gas to industries around Malaysia. Table 1.1 shows the location of

the Oil Refineries in Malaysia (MIDA, 2004; PETRONAS, 2011).


3

Table 1.1: Location of Oil Refineries in Malaysia

Oil Refineries Location

Petronas Penapisan (Terengganu) Sdn Bhd Kertih, Terengganu

Petronas Penapisan (Melaka) Sdn Bhd Tangga Batu, Melaka

Malaysia Refining Company Sdn Bhd Tangga Batu, Melaka

Shell Refining Company (FOM) Bhd Port Dickson, Negeri Sembilan

Esso (Malaysia) Bhd Port Dickson, Negeri Sembilan

Source:(MIDA, 2004)

By continuing to improve the incentives and policies, Malaysia is on the right track

to remain as a competitive location for services and manufacturing activities.

Petrochemicals are targeted in the manufacturing sector (resource based) in the Third

Industrial Master Plan (IMP3), 2006-2020, for future development. Among the strategic

measures include expanding and enhancing value-added existing capacities and broadening

the range of petrochemicals produced. Others include enhancing linkages with downstream

industries, in particular plastics and oleo chemicals, intensifying the development of

technologies in materials and product applications and improving chemical process

technologies and the application of catalysts to increase yields (AHK, 2010; MITI, 2006).

1.2 Propene and Its Derivatives

Propene, also known as propene is the second most important starting product in the

petrochemical industry after ethylene. It is one of the key building block petrochemicals

utilized as feedstock for various polymers and intermediates. The main usage for propene is

for the production of polypropylene (PP), which accounts for nearly two-thirds of global

propene consumption. Other propene derivatives include acrylonitrile, propylene oxide,


4

cumene, acrylic acid, oxo alcohols, isopropyl alcohol, and oligomers as seen in Figure 1.1

(ICIS, 2010; Nexant, 2009)

Source: (Nexant, 2009)

Figure1.1: Global Propene Demand by Derivative

PP has made an impact in a wide range of consumer and industrial products as it is

one of the most versatile bulk polymers due to good mechanical and chemical properties.

Among the application of PP are electronic and electric appliances, packaging, pipes, wire,

cables, toys, and tapes. Acrylonitrile, the second largest derivative is used in various

elastomeric polymers and fibre applications. Acrylic fibres are used in clothing, home

furnishing and bedding such as socks, sports wear, carpets, upholstery, and blankets.

Acrylonitrile are also used as a chemical intermediate in the production of nitrile rubber,

acrylonitrile-butadiene-styrene (ABS)/styrene acrylonitrile (SAN), acrylamide and carbon

fibers (ICIS, 2010; Wakefield, 2007).


5

Propylene Oxide is a starting material to make propylene glycol which is used in

antifreeze, aircraft de-icing fluids, unsaturated polyester resins, propylene glycol ethers, and

polyether polyols (ICIS, 2010). Cumene is the main feedstock for the production of phenol

and acetone (ICIS, 2010; Zakoshansky, 2009). Acrylic acid is used in the production of

acrylic esters and resins used in paints, coating and printing applications. Oxo-alcohol and

Isopropanol are also used in resins, paints and adhesives application (ICIS, 2010).

1.3 Propene Production Technology

The primary source (88%) of propylene is as a by-product of ethylene production in

steam cracking of liquid feedstock‘s (naphtha and LPGs) and from off gas produced in

refinery fluid catalytic cracking (FCC) streams. Steam cracking is carried out at high

temperature and the predominant co-product is propene. Typically the propene to ethylene

ratio varies from 0.4:1 to 0.75:1. By altering the feedstock choice and cracking severity

propene output can be enhanced. Coke formation and deposition is a major problem since it

reduces products yield and energy efficiency of the process (Chan et al., 1998; ICIS, 2010;

Nexant, 2010).

Propene are also recovered from FCC operations which cracks heavy gas oils by

breaking the carbon bonds in large molecules into multiple smaller molecules (Veoliawater,

2009). However, refinery propene needs to be purified. Among the companies using this

technology are Kellogg Brown & Root (SUPERFLEX and ACO process) and Honeywell

UOP (UOP PetroFCC and UOP RxPro) (ICIS, 2010; Nexant, 2009; Tallman & Eng, 2008;

UOP, 2011).


6

Other routes leading to propene production technology are;

i. Propane Dehydrogenation (PDH) – Converts propane to propene at high

temperatures (500-800 ºC) in resulting in an endothermic equilibrium reaction. Used

commercially and readily available from licensors such as Sud-Chemie (CATOFIN

process) and UOP (C3 Oleflex Process). Problems include high capital cost due to

high price of propane, and needs continuous supply of propane and coke deposition

on catalyst bed due to its high temperature reaction (ICIS, 2010; Nexant, 2009; Süd-

Chemie, 2011; Tallman and Eng, 2008; UOP, 2011).

ii. Olefin Metathesis – Catalytic conversion of olefins (ethylene and 2-butene) to

propene. Can be added to steam crackers to boost propene production via the cracking

exchange reaction. This technology is used by companies such as BASF (BASF-Fina

cracker) and ABB Lummus Global (Philip triolefin process). Problems associated

with this technology include the need for large C4 streams and is not economically not

favourable as ethylene production as feedstock needs to be improved, thus increasing

total capital expenditure (ICIS, 2010; Mol, 2004; Nexant, 2009; Tallman and Eng,

2008).

iii. Olefin Interconversion – Catalytic conversion of C4 and C5 olefins into propene and

ethylene using fixed or fluidized bed reactor. Compatible with FCC and ethylene

crackers and does not consume ethylene. Lurgi and ExxonMobil (MOI Technology)

are among few companies that are using this technology (ICIS, 2010; Nexant, 2009).


7

iv. Methanol to Olefins (MTO) Conversion – Natural Gas (Methane) has to be

converted to methanol, and then it‘s converted to propene. This technology increases

the propene production and catalyst consumption is low. UOP Advanced MTO

Process combines MTO with cracking process uses alternative feedstock such as coal.

High capital requirement depends on the price of natural gas feed (ICIS, 2010;

Nexant, 2009; UOP, 2011).

v. Deep catalytic cracking (DCC) – Produces light olefins from vacuum gas oil and de-

asphalted oils using fluid catalytic cracking principles. Developed by Sinopec

(CHINA) and employed by Stone & Webster (ICIS, 2010; Nexant, 2009).

1.4 Propene Market and Demand

Most of the world‘s propene monomer are exported by Japan, Malaysia, South

Korea, Taiwan, Canada and US, while among the importers are West Europe, US,

Colombia, Egypt, China, South Korea, Taiwan and Indonesia. Propene demand is

anticipated to grow an estimated 5% annually from 2007-2015, which accumulates to more

than 100 million tons by 2015. Propene consumption by region is shown in Figure 1.2

(Eramo, 2005; Nexant, 2009).

However, propene supplies are constrained by co-product production from steam

crackers and FCC refinery operations. Steam cracker extension and its additions are unable

to keep up with the propene demand growth. Propene produced by propane

dehydrogenation (PDH) is significantly lower as compared to other production technology

even though PDH are more economically favourable. Therefore propene demand continues


8

CH3CH3CH3CH2+ O2

CO + CO2

+ H2O

to outpace ethylene demand and there is increasing interest in developing alternatives

sources of propene without adversely affecting ethylene availability (ICIS, 2010; Nexant,

2009).

Source: (Nexant, 2009)

Figure 1.2: Global Propene Consumption Trends

1.5 Oxidative dehydrogenation (ODH) of propane

Catalytic Oxidative dehydrogenation of propane is an attractive alternative to

accommodate the world propane demand although this technology has not been

commercialised. The mechanism of propane ODH reaction is shown in Figure 1.3.

Propane Propene

Figure 1.3: Propane ODH Reaction Network


9

This synthesis route appears to be far from realization because of some difficulties.

For instance, propene oxidizes more easily than propane, hence reducing selectivity rapidly

with conversion. At temperatures above 700 ºC, propane cracking increases, thus producing

a variety of other products. Selective oxidation catalysts are needed to increase propene

selectivity at temperatures below 700 ºC. Catalytic ODH reaction of propane requires

oxidant molecules that transform the eliminated hydrogen from propane to water thus

converting the highly endothermic reaction into an exothermic process. The reaction

temperature decreases hence reducing deactivation caused by coking as well as side

reactions such as parallel or consecutive oxidation reactions giving carbon monoxide (CO)

or carbon dioxide (CO2) as non-selective products (Bhasin et al., 2001; Khan et al., 2010;

Meunier et al., 1997; Nexant, 2010).

Over the years a variety of catalysts have been studied to improve the ODH reaction

efficiency. However, existing ODH catalyst have limited activity and poor selectivity. The

most studied catalytic systems for ODH reaction are transition metal oxides catalysts such

as molybdenum-based systems and vanadium-based systems with supports such as

niobium, magnesium and nickel (Bhasin et al., 2001; Khan et al., 2010; Meunier et al.,

1997; Nexant, 2010).


10

1.6 Molybdenum Based-Catalyst

Molybdenum based-catalyst use in petrochemical industry is extensive. The various

application and reaction of the catalyst are shown in the Table 1.2. Molybdates are mostly

used as selective oxidation catalyst (IMAO, 1998, 2005)

Table 1.2: Molybdenum compounds in catalysis

Source: (IMAO, 2005)

In propane ODH reaction, molybdenum is implicated in the activation of C-H bond

of propane and the water formation. Molybdenum can exist between oxidation states

transitions of +4, +5 and +6. The capability to do so creates high structural diversity of the

mixed transition metal oxides. Vanadium also has rich coordination geometries and

oxidizing state ranging from +3 to +5. By incorporating vanadium as a support into the

Catalyst Application Reaction Importance

Sulfided Co-Mo or

Ni-Mo on alumina

Hydrotreating,

hydrodesulfurisation

Remove sulphur

from crude petroleum

Oil and petroleum

refining

Bi-Mo oxides Propene selective

oxidation, ammoxiation

Synthesis acrolein,

acrylonitrile

Making polymers

and plastics

Mo-V oxides Acrolein oxidation Synthesis acrylic

acid

Making polymers

and plastics

Fe-Mo oxides Methanol oxidation Synthesis

formaldehyde Making formalin, polymers, resins

Mo oxide on

alumina Olefin metathesis

Propene to ethene

and butene Olefin synthesis

Mo complexes Epoxidation Olefin to epoxide Polyether

synthesis

Heteropolyacids-

phosphomolybdate Propene hydration Propene to alcohol

Alcohols

synthesis


11

molybdenum oxide system, the structural motif can be enhanced (Cavalleri et al., 2007;

Khan et al., 2010; Rödel et al., 2007).

In acquiring an enhanced catalyst system performance, structural-activity

relationship understanding is necessary. This can be done by gaining an insight of the

kinetics of structural evolution under variation condition through in-situ studies such as in-

situ X-Ray Diffraction (XRD) and in-situ Differential Scanning Calorimetry (DSC)

(Ressler et al., 2000).

1.7 Project Design Motivation

This project was designed based on the fact that propene demand had outpaced

ethylene demand therefore an alternative technology is needed to increase production of

propene. Catalytic propane oxydehydrogenation (ODH) reaction is a viable technique with

great potential. ODH transforms the highly endothermic dehydrogenation (DH) reaction

into an exothermic process, hence reducing coke formation and side reaction products.

Although ODH is thermodynamically favourable, selectivity to propene is low due to low

reactivity of propane. Subsequent reaction also can occur which leads to prooane cracking

products and combustible products (CO, CO2). Suitable catalyst such as molybdenum-

based-catalyst is needed to activate the propane methyl C-H bond as enormous energy is

needed. One of the key ways to improve on the above limitation is to optimise and control

the molybdenum phase structure, particularly during catalyst precursor activation stage. In

order to enhance catalytic performance, studies of structure-activity relationship were

essential to optimise the catalyst system striving for improved activity and selectivity to

propene.


12

1.8 Objectives

The objectives of this research project are:

1) to develop methodology for the synthesis of molybdenum oxides based-catalyst

precursors for ODH reaction.

2) to study the structural dynamics of the said precursor during activation and to conduct

preliminary reactivity studies using in-situ techniques.

Chapter 2, Literature Review _

13

CHAPTER 2

LITERATURE REVIEW


14

2.0 Literature Review

2.1 Selective Oxidation

Light alkanes (C2-C4) are of natural abundance and the best utilization is to convert

them to desired petrochemical products and feed stocks for valuable chemicals. To fulfil the

worldwide demand, great focus is given to selective oxidation studies which is not only

economical but also of lower environmental impact. The key points in selective oxidation

reactions are the activation of the highly stable C-H bond of light alkanes, further oxidation

suppression of formed products to undesired products and also to reduce C-C bond

breaking to CO and CO2 as alkenes are less stable than alkanes (D‘Ippolito et al., 2008;

Lin, 2001; Vitry et al.,2004).

2.1.1 Propane Selective Oxidation

Various studies have been carried out on the selective oxidation of propane. Figure

2.1 shows the propane oxidation pathways and the standard reaction enthalpies. Propane as

a saturated hydrocarbon is less reactive as compared to the selective oxidation products.

This is due to the bond strength of C-H especially at the methyl group as requires

significant amount of energy to activate it. Temperature control is essential to avoid total

oxidation to CO and CO2 and also to avoid cracking of the C-C bond of propane to give

lower alkanes. To overcome this limitation, catalyst is needed not only to selectively

activate the propane C-H bond but also to hinder C-C bond breaking. (Lin, 2001).


15

In a review by Bettahar et al. (1996), propane conversions includes direct oxidation

or via two step reaction (via propylene) to propanol, acrolein, acrylic acid, acetaldehyde,

methanol and formaldehyde. A great deal of attention has been given to catalytic selective

oxidation. Equally the catalytic oxidative dehydrogenations of propane studies are done to

lower the propene cost compared to the non-catalytic and non-oxidative processes as the

demand propene derivatives increases.

Source: (Lin, 2001)

Figure 2.1: Propane reaction pathways and reaction enthalpies


16

2.2 Oxydehydrogenation of Propane

Catalytic Oxydehydrogenation occurs with the existence of molecular oxygen

(hydrogen acceptor) over heterogeneous catalyst in the reaction medium. This reaction

process helps to overcome the problems of dehydrogenation that is subjected to

thermodynamics limitations (endothermic reaction) and rapid degenerative catalyst

(coking). The catalytic ODH of propane is a better choice reaction as it is an exothermic

reaction (H= -117 kJ/mol). This is because with the presence of oxygen in the reaction

which leads to water formation. However, many problems still exist such as reaction

mixtures flammability, selectivity control where propene selectivity is small due to the high

reactivity of propene towards further oxidation giving undesired oxygenated by-products

and carbon dioxides. For example propane is more thermodynamically favourable to

convert to 2-propanol (H= -168kJ mol-1

). (Bettahar et al., 1996; Cavani & Trifirò, 1995;

Lin, 2001; Viparelli et al., 1999; Vitry et al., 2004; Watson & Ozkan, 2003).

Catalyst development has to be designed to be capable in activating only C-H bonds

of light alkane molecules under oxygen flow. Focuses have been given to vanadium mixed

oxides and catalysts based on molybdates, ferrites and antimonates for light alkanes ODH

reaction. Studies are done to determine the correlation of catalysts structure to catalytic

activity and selectivity. The control of catalyst phase composition and structure in large

scale preparation are also investigated widely. The nature of support is another important

variable in determining the catalytic properties of supported catalyst (Mamedov & Cortés

Corberán, 1995).


17

Buyevskaya & Baerns (1998) have summarized three mechanisms that may be

involved in oxidative dehydrogenation of alkanes on oxides based on the surface species

character which is redox system, adsorbed oxygen state and via strongly bound lattice

oxygen. For the redox system at temperature less than 500 ºC, propene are selective but low

propane conversion observed even at high degree of catalyst. Adsorbed oxygen of rare

earth oxides on reaction surface leads to propylene, COx and significant amount of

ethylene, methane and C4-hydrocarbons caused by C-C cleavage. For propane activation on

lewis acid sites using B2O3/Al2O3, the oxygen participates in product formation leading to

propyl radicals and significant selectivity‘s to olefins and oxygenates.

Liu et al. (2010) also has discussed the mechanism of propane ODH. Propane

conversion to propene (C3H8) is by eliminating two hydrogen atoms on the active oxygen

from the metal oxide catalyst. Consecutively, the propene reacts with the adsorbed/lattice

oxygen which is supplied by the neighbouring electrophilic surface to produce majority

COx thus decreasing the selectivity of propene. Addition of anion doped metal oxide such

as fluoride enhances oxygen mobility and insulated the neighbouring electrophilic activity

oxygen, leading to propene deep oxidation termination.

The selectivity of propane ODH needs control of the active oxygen abundance and

also the adsorption/desorption quality to avoid re-adsorption of the produced propylene.

The surface of the oxides used has a combined influence of redox and acid-base properties.

The important requirements for selective propane ODH are weak Lewis acid centres,

intermediate reducibility and oxygen mobility. The oxygen needs to be able to bind

strongly enough to the surface to comprise attenuated oxidizing strength but weak enough


18

to oxidize the reactant molecule selectively. By using transition metal oxides catalyst, the

species of interest are M=O, M-O-M and M-O-support bonds where M is the transition

metal. The characteristic of the active oxygen depends on the transition metal loading,

support, dispersion and addition of modifiers (Watson & Ozkan, 2003). Among the

catalytic systems identified for the ODH reaction are metal molybdates, vanadates and

niobium pentoxides (Vitry et al., 2004).

2.3 Catalyst

Catalyst as defined by Wilhelm Ostwald is described as a compound that increases

the rate of a chemical reaction, but which is not consumed by the reaction. Key issues

associated with catalyst are kinetics, mechanism of catalyst reaction, elementary reaction,

reactants adsorption on solid surface, reactivity of surface and materials, enzymes and

organometallics synthesis and structure (Murzin & Salmi, 2005). Catalysts are used widely

in the productions of industrial chemicals, fine chemicals, pharmaceuticals, and to avoid

and clean up pollutants. The effectiveness of a catalyst is evaluated based on its catalytic

activity, reaction selectivity, catalyst stability, and the catalyst regenerability (Clark &

Rhodes, 2000; Heinemann, 1997). The different catalysts used are homogeneous catalyst

dissolved in the reaction solution, heterogeneous catalyst in the form of porous solids and

biological catalyst in the form of enzymes (Murzin & Salmi, 2005).

2.3.1 Heterogeneous catalyst

Heterogeneous catalyst is a solid catalyst getting into contact with a gaseous/liquid

phase reactant medium in which the catalyst is insoluble. This reaction is alternatively

known as contact catalysis. The majority of catalytic process taken place in the


19

petrochemical industry uses solid catalysts with gaseous substrates. The advantages using

heterogeneous catalyst over homogeneous catalyst are that solid catalysts are less corrosive,

a vast variety of temperature and pressure can be applied to accommodate strong

exothermic and endothermic reactions, and it is easier and cheaper to separate the substrates

and products from catalysts. The catalytic mechanism using heterogeneous catalyst

involves consecutive substrate diffusion, substrate adsorption, surface reaction, product

desorption and product diffusion (Clark & Rhodes, 2000).

In designing a new selective heterogeneous oxidation catalyst, seven fundamental

principals highlight the importance to it. The seven principles are lattice oxygen, metal–

oxygen bond strength, host structure, redox, multifunctional active sites, site isolation, and

phase cooperation. Graselli (2002) describes the principals as the seven pillars of selective

heterogeneous oxidation catalysis. By applying the seven principles and achieving proper

knowledge of the structural, surface and dynamic properties of the metal oxide catalysts on

atomic levels, a new, more efficient and environmentally friendlier selective oxidation

catalyst.

2.3.2 Catalyst Support

A catalyst support or carrier are used to higly disperse catalyst small particles to

enhance catalytic activity. Supported metals helps in handling thus minimises metal loss.

By using support, incorporation of modifiers such as promoters is also easier. Mostly used

supports are silica and alumina because of the oxide capability to become microporous

(Bond, 2005b). To synthesize a supported catalyst, 2 stages are involved. Firstly, support

material is dispersed using impregnation, co-precipitation, deposition or adsorption from


20

solution and the next step is calcination or reduction by thermal treatment. Supported

catalysts is more effectively utilised in precious-metal catalyst as compared to bulk-metal

system. However, in base-metal catalysts, support is used to improve catalyst stability

(Acres et al., 1981).

2.3.3 Nanostructured Catalyst

Nanostructured catalysts are used in a wide array of applications such as

hydrogenation, oxidation and photocatalysis. The size of the deposited metal particles and

their distribution on sizes and surface area are essential to be determined as it affects the

catalytic activity (Sergeev, 2006b). Nanostructured catalysts are usually developed by

modifying chemical and physical properties by size effects. This can occur either through

nano-sized bulk particles or via a nonporous matrix hosting the catalytic species.

Nanostructured bulk catalyst features inherently different adsorption and surface reactivity

which leads to extraordinary surface defects, morphology electronic sites. Commonly used

nanostructured catalysts are carbon nanotubes, rare earth oxides, nobel metals and

transition metal oxides (Sergeev, 2006a, 2006b).

2.3.4 Bulk Catalyst

Bulk materials are samples that contain nanoscale grains with appropriate geometry

and not particulates, thin films or nanoscale wires (Koch, 2009). Bulk nanostructured

materials also known as consolidated materials are defined as bulk solids with nanoscale or

partly nanoscale microstructures (Koch, 1999). Bulk metals mostly display the

characteristic physical properties of metallic state such as hardness, ductility, strength,

lustre, malleability, high electrical and thermal conductivity (Bond, 2005a). Bulk properties


21

that are affected when the microstructure is nanoscale are the mechanical properties and the

ferromagnetic materials (Koch, 2009).

The synthesis and assembly can be done by 2 different techniques. The first is

―bottom-up‖ assembly where the nanostructured building blocks are formed first then

consolidated into the final bulk material. Examples are gas condensation, and

powder/aerosol compaction including electrodeposition. This approach are applied mostly

for nanopowders and structural composite materials production and applied for catalysts,

films, coatings, cosmetics, electronic devices, paints, lubricants, rocket fuels and

reinforcements for nanocomposites.

The other technique is the ―top-down‖ which starts with a bulk solid then

structurally decompose to obtain nanostructures. Examples are mechanical attrition (ball

milling) and lithography/etching. In ball milling technique, the powdered particles of all

elements are refined to nanoscale grain size. The particulates of nano building blocks are

then subsequently assembled into a new bulk material (Hu & Shaw, 1999; Koch, 2009).

However lately, it is learnt that ―bottom-up‖ research are to replace ―top-down‖, a strategic

move in nanoscience (Astruc, 2008).

2.4 Catalyst Synthesis

Factors contributing to catalytic performances are crystalline structure, purity,

surface enrichment, particle size, morphology, preparation method and thermal treatment.

Catalyst components self organization in a structure is important in creating new active

sites on a catalyst. Inorganic soft synthesis methods such as micro emulsion method, sol-gel


22

method, and hydrothermal method are needed to be employed to gain and increase catalytic

activity (Oshihara et al., 2001).

2.4.1 Hydrothermal

Hydrothermal method is commonly used to synthesise transition metal oxides such

as TiO2 and ZrO2. This method also known as solvothermal method is done by heating the

reaction mixture containing the solution of catalyst components above the boiling point of

water using a closed system such as autoclave and it is exposed to steam at high pressures.

The reaction can be done in water (hydrothermal) or in solvent such as ethanol

(solvothermal). Powders acquired from hydrothermal synthesis have varied microstructure,

morphology and phase composition as determined by parameters such as temperature,

pressure, reaction time, concentration and pH solution. By increasing the reaction

temperature, accelerated crystal growth and better crystalline material with narrow particle

size distribution are obtained (Kolen‘ko et al., 2003; Yu et al., 2007).

2.4.2 Sol-Gel

Sol gel process is used to produce different types of materials such as powders,

films, fibres and monoliths. This method can be used to prepare pure dense, stoichiometric,

equiaxed, nanostructured and consistent particle size of metal oxides such as TiO2. Sol gel

process occur first with the formation of sol which is a liquid suspension of solid particles

(1nm-1micron) produced by precursor hydrolysis or partial condensation. Next, it is

followed by formation of a gel which is a dysphasic material with solid encapsulating

solvent produced by destabilizing a solution of preformed sols. Controlled parameters in

this process are solvent, precursors, catalysts, temperature, pH, additives, and mechanical


23

agitation to influence the kinetics, reaction growth, hydrolysis and condensation reaction.

Advantages are high purity can be maintained, physical characteristics (pore size

distribution & pore volume) changing ability, capable of varying compositional

homogeneity at molecular level and at low temperatures and also capable to introduce

several components in a single step. (Ko, 1997; Yu et al., 2007)

2.4.3 Impregnation

This technique is used to introduce the support into the catalyst system. There are

two types of impregnation method; dry impregnation and wet impregnation. In dry

impregnation, appropriate amount of liquid solution is used to fill up the pore volume of the

support material. The maximum loading of the support is determined by the catalyst

precursor solubility and the support pore volume. Dry impregnation can also be used when

several catalyst precursors are available simultaneously in the impregnating solution and

this is known as co-impregnation. In wet impregnation, support is dipped into an excess

quantity of catalyst precursor solution. Different profiles of the active phase over the

support are obtained depending on the process condition such as pH, temperature, and

percentage of support loading (Mul & Moulijn, 2005).

2.4.4 Precipitation

Precipitation method is technically more demanding than other methods since

having to deal with product separation. This method used especially for catalyst support

material. Among advantages of this process are, the process flexibility and very pure

material can also be obtained. Problems occurring in this method are the constant product

quality maintenance throughout the precipitation process and solid nucleation making it


24

hard to study the crystallization and precipitation process of solution. Precipitate production

from homogeneous solution involves the formation of a nucleus which is governed by the

free energy of the solution agglomerates (Schüth & Unger, 1997). This is based on the

control of parameters such as temperature, pH, reactant concentration, and time which is

correlated with factors such as supersaturation, nucleation and growth rates, surface energy,

and diffusion coefficients of the precipitate (Schüth & Unger, 1997; Yu et al. 2007).

Rodriguez-Paez et al. (2001) reported that controlled precipitation method allows

the control of the different stages of ceramic powder processing, has reproducible

properties and also larger quantities of ceramic powder can be obtained. Zinc oxide

nanoparticles agglomerates with different morphologies have been obtained by controlling

different parameters of the precipitation process such as solution concentration, pH and

washing medium.

Similar observation was reported by Oliveira et al. (2003), by precipitating zinc

oxide particles at room temperature. The precipitate formation mechanism involves the

nucleation of ZnO particles inside the matrix and then the particles aggregate into star-like

particles. Addition of additives such as sodium sulphate or sodium dodecyl sulphate leads

to synthesis of smaller particles and particles morphology also are affected. This maybe

caused by the adsorption of additives on crystallites which limits the nanocrystallite

aggregation into bigger particles.


25

As reported by Abd Hamid et al. (2003), by controlling the precipitation of

molybdenum oxide in aqueous solution, structurally complex solids can be synthesized.

The solid precipitate nucleation can be determined by the pH response immediately before

precipitation. At low rate of addition, several buffering equilibria of polyoxomolybdates are

attained. However at fast addition, buffering equilibrium is not reached leading to reaction

sequence delay thus at lower pH. It is found that at different pH levels, different

distributions of molecular species are obtained. Nanostructuring can be achieved from the

transition result from catalyst preparation to catalyst synthesis thus the surface morphology

(roughness) and the geometric surface area of the unsupported oxide can be optimized.

However, it is still recommended to precipitate at a constant pH and the mother liquor

should be removed quickly to preserve the initial structure during thermal treatments.

2.4.4.1 Co-precipitation

Co-precipitation is described as simultaneous precipitation of more than one

component with products solubility differing possibility. This method is best used for

homogeneous distribution of catalyst components or to synthesise precursors with definite

stoichiometry which will be converted to active catalyst hence good dispersion. This

method can also be used to incorporate support material by co-precipitating metal ions with

support ions to produce an uniform dispersion of active component throughout the surface

and the bulk when catalyst is calcined Co-precipitation technique is mostly used to

synthesise mixed metal oxides even at low temperature and the morphology and particle

size can be controlled to achieve nanostructuring (Acres et al., 1981; Schüth & Unger,

1997).


26

Blangenois et al. (2004) has described the effects of precipitation parameters such

as pH, temperature, stirring and addition rate in the preparation of vanadium-aluminium

oxide catalysts which is a potential propane ODH reaction catalyst. The co-precipitation pH

strongly influences texture, surface area, chemical composition, reducibility and the nature

of vanadium oxide species in V-Al-Ox catalyst hence the activity of the catalyst. The

reducibility are affected by the degree of polymerization and the state of coordination, V=O

group position in the chain and the interaction of the vanadate support. At higher pH,

polymerization degree of the tetrahedral vanadium species ([VOx]nn-

) become low.

Catalytic activity also are significantly affected by temperature whereby the lower the

reduction temperature, the higher the activity of the catalyst for propane ODH reaction.

2.5 Transition Metal Oxides (TMO)

Transition metal especially in nanoparticles form are important in catalysis as the

properties are similar to metal surface activation and nanoscale catalysis thus improving

heterogeneous catalysis selectivity and efficiency. The most active catalysts are only a few

nanaometers of diameters in size. Nanoparticle catalysts also are selective, efficient, and

recyclable which is suitable as a green catalyst. Transition metal nanoparticles are able to

self assembly via nucleation to form clusters stabilized by ligands, surfactants and polymers

(Astruc, 2008).

In order to attain high metal surface area, the transition metal catalysts are dispersed

onto oxide support. Supported transition metal oxides such as vanadium pentoxide (V2O5),

molybdenum trioxide (MoO3) and chromic acid (Cr2O3) are studied as catalysis for ODH

reactions (Shiju & Guliants, 2009). A study conducted by Bell (1995), has shown the


27

transition metal catalysts activity and selectivity, are significantly affected by metal oxide

moieties presence on the catalyst surface. The effectiveness of the oxides promoter is better

in correlation with higher Lewis acidity. When the metal surface is covered by half oxide

monolayer, the concentration of oxide perimeter is maximized thus promoter is more

effective.

Witko & Tokarz-Sobieraj (2004) have investigated the surface oxygen existence in

transition metal oxides catalyst and used Density Functional Theory (DFT) calculation to

examine the catalytic behaviour and correlating with the electronic properties of different

surface O atoms in several V-O-X systems. Different surface oxygen and local vacancies

reveals different possible sources. Different catalytic properties are influenced by the

different electronic sites resulting from structurally non-equivalent surface oxygen sites.

Adsorbed molecular oxygen can replace the surface oxygen which occupies specific lattice

site, thus big amount of oxygen species are incorporated into oxide surface which may

impact the catalytic behaviour of the V-O/Mo-O systems.

2.5.1 Mixed Metal Oxide (MMO)

An example of MMO is Mo-V-Te(Sb)-Nb-O and Mo-V-Nb-Te catalyst which have

been studied extensively for propane oxidation and ammoxidation The performance of

these catalysts expecially to acrylic acid is reported to be significantly better than other

multi-component metal oxide. In a structural view, Mo and V are essential to form the

desired orthorhombic network which gives the highest conversion of propane and propene

and catalyst containing Te has a better selectivity to arylic acid suggesting the importance

in oxygenated products formation (Vitry et al., 2004).


28

For ODH reaction, a study conducted by Li et al. (2010) shows that Ni containing

oxide system are active and selective especially at low temperature reaction. CeNbNiO

nanosize catalysts were prepared by modified sol-gel method and uses citric acid as the

ligand. Pure NiO catalyst exhibits low propane conversion and selectivity. Propane

conversion increases above 350 ºC but propene was not produced. 1.5CeNbNiO catalyst

demonstrates higher propane conversion and selectivity at lower temperature (250 ºC). Nb

holds back the reduction of bulk NiO and also decreases the surface acidity and catalyst

stoichiometric oxygen thus increasing propene selectivity. Ce helps to improve the bulk

nonstoichiometric oxygen reducibility and this enhances the catalyst activity for low

temperature propane conversion.

2.5.2 Molybdenum Oxides

The rate determining step of ODH reaction is the C-H bond breaking. Molybdenum

oxides are identified in activating C-H bonds of alkanes including light olefins and

catalysing selective partial oxidation which actually requires extremely high temperatures.

Structural complexity, the ability to occur at +4 to +6 oxidation states and oxygen

coordination geometry variation of molybdenum oxides give rise to good catalytic

performances. MoxOy units are comprising with structural arrangements of shared corners,

edges and faces forming tetrahedral, octahedral, pentagonal bipyramids and square

pyramids (Cavalleri et al., 2007). For example, Bismuth molybdates and vanadomolybdates

are normally used in propane partial oxidation and ammoxidation (Cavani & Trifirò, 1995).


29

Divalent metal molybdates (AMoO4) supported on SiO2 are studied by Stern &

Graselli (1997) for propane ODH reaction and the reaction has been catalytic not gas phase

radical initiated reaction. The reaction is also known to be first order in propane which

involves the C-H bond breaking. Highest propene yield obtained with NiMoO4/SiO4 and

Ni0.5Co0.5MoO4/SiO2. Binary Ni-Co molybdate are a more stable catalytic system due to its

lesser sensitivity to molybdenum level. Redox elements addition such as V, Fe, Ce and Cr

improves the molybdates activity thus allowing reaction at lower temperature (<560 ºC)

and improving product yield.

Harlin et al. (1999) prepared molybdenum oxide impregnated on alumina support

for dehydrogenation reaction of n-butane. It is identified that the active oxidation state of

molybdenum was either Mo(V) or Mo(IV) which were formed by Mo(VI) reduction by

hydrogen of the n-butane. Selectivity increases with addition of magnesium via slow

oxidative dehydrogenation reaction.

Chen et al. (2001) have examined the effects of structure and properties of Al2O3

supported MoO3 catalyst on propane ODH reaction. The catalyst were prepared by

impregnating -Al2O3 into molybdenum solution. The structure of MoOx species dispersed

on Al2O3 depends highly on Mo surface density. Two-dimensional MoOx oligomers formed

preferentially for Mo surface densities lower than polymolybdate monolayers. At this

surface density, the propane ODH rates increases with Mo surface density due to the higher

reducibility of larger MoOx domain. Higher surface density decreases the propane ODH

reaction due to the formation of the three-dimensional MoO3 which makes Mo species

inaccessible. However, the presence of Mo-O-Al surface sites helps the adsorption of


30

propene and also the combustion of alkoxide intermediates resulting in undesired COx

products.

To obtain a better understanding of MoOx catalytic activity, Watson & Ozkan

(2003) investigated the induced effects of low-level alkali promotion. This is done by

doping the MoOx-based-catalyst with potassium and correlating with the adsorption and

reactivity of propane and propene. The catalysts were prepared by sol-gel/co-precipitation

technique. Addition of potassium will decrease the amount of lattice oxygen. This leads to

the inhibition of desorbable oxygen species that maybe related with the propane or propene

unselective conversion. However, alkali addition also decreases the reducibility of the

catalyst that may cause the propane activation to be suppressed. Potassium not only affects

the propene transformation over MoOx catalysts, but also modifies the catalyst interaction

with propane therefore altering propane activation pathway.

High purity of hexagonal MoO3 can be done by using precipitation method

followed by hydrothermal treatment. Concentrated acid, in this study HCl once again was

used to precipitate the desired structure of the material. Based on the characterization

results of XRD diffractograms and DTA profiling, metastable h-MoO3 are formed at 300-

350 ºC. At temperature higher than 450 ºC, thermodynamically stable -MoO3 are obtained

(Song et al., 2007).


31

2.5.2.1 Polyoxomolybdates

The advance development in the X-ray crystallographic enables crystal structure

determination with highly complicated compounds in a reasonable time. Polyoxometalates

can be classified according to the structure for example, isopoly and heteropoly vanadates,

molybdates and tungstates (Bielański et al., 2003).

Cronin et al. (2000) have described some fundamental principles in controlling the

growth of polyoxomolybdates from discrete molybdenum oxide synthons. [Mo8] is mostly

the building blocks of many polyoxometalates structure. [Mo8] unit itself is built up by a

central MoO7 pentagonal bipyramid sharing edges with five MoO6 octahedra and two

MoO6 sharing corners with pentagon atoms. To allow self aggregation, large molecular

fragments must be functionalized with groups that allow linking through certain reactions.

The linking of the giant-spherical clusters into a proper solid-state layer structure can also

occur at room temperature. Therefore, it is concluded that the nanostructured building

blocks can be isolated according to the stability. The nanostructured cavities and well

defined properties make it possible to construct materials based on characteristic synthons.

Bielański et al. (2003) have compared the thermal behaviour of hydrated ring

polyoxomolybdates. In the thermal analysis using TG/DTG and DTA two stages of

dehydration are observed. Between room temperature and 200 ºC, crystallization water is

released and from in-situ FTIR, no large destruction of polyoxomolybdates occurs. In the

second stage of heating, strongly bonded water is released which can be observed from the

TG/DTG thermogram curve as there is a fast decrease of weight along with exothermic

peaks. The exothermic peaks observed are caused by the heat evolution of the


32

recrystallization of strongly disordered dehydrated reaction into a new solid crystalline

phase.

Polymolybdates were obtained from self-assembly processes controlled by

oxidation-reduction reactions and also from MoVI

condensation-polymerisation at pH lower

than 3. Isopolymolybdate [Mo36O112(OH2)16]8-

have been found in pH 1 solution. A new

crystalline solid consisting of open framework was synthesized using (bipy) as the organic

component. The assembly of the [Mo36O112(OH2)16]8-

and H2bipy2+

builds up a mesoporous

H-bonded organic-inorganic hybrid material that has large cavities and exhibits water

sorption behaviour and still maintains its crystal properties (Atencio et al., 2004).

2.5.3 Vanadium Oxides

Vanadium systems such as vanadium oxides, vanadium-antimony, vanadium-

molybdenum and vanadium-phosphorus are constantly studied in olefins dehydrogenation

since are able to activate propane at lower temperature due to the metalloradical character

(Stern & Graselli, 1997). Vanadium pentoxide (V2O5) catalyst is not very promising for

olefins ODH reaction but by spreading the oxides onto support, a more complex structure

will lead to a more selective catalyst. However the selectivity are not higher than 40%, even

at low temperature (350-450 ºC) (Cavani & Trifirò, 1995).

Viparelli et al. (1999) have reported that vanadium supported on TiO2 have high

activity and low selectivity in propane ODH reaction but addition of niobium enhances the

catalytic performances at low V/Nb ratio. The catalytic activities were correlated with the

redox and acid properties. Propane can be activated by V/Ti binary catalysts since redox


33

sites increases as vanadium content increases. However, by increasing vanadium content,

the propene selectivity are lowered due to consecutive CO formation. 6Nb/Ti catalysts are

more selective but have low activity due to lower redox sites. By combining vanadium and

niobium species on TiO2 surface affects the catalytic properties in propane ODH reaction.

Vanadium and niobium interaction in ternary catalysts changes the acidity of the sample

surface creating different active centres as compared to the binary catalyst. Increasing

surface acidity has a promoting effect on propene selectivity.

The catalytic activity in propane ODH reaction is influenced by the reducibility and

vanadium species surface structure. Supported V2O5 on Al2O3 and TiO2 prepared by wet

impregnation disperses the vanadium species highly. The most active catalyst is V2O5/TiO2

while the most selective in propene is V2O5/Al2O3. Propene selectivity also increases by

doping the catalyst with alkali metals but catalytic activity is reduced (Lemonidou et al.,

2000).

The structural influence of active sites in Me-V-O (Me=Mg, Zn, Pb) on propane

ODH catalytic performance was studied by Rybarczyk et al. (2001). The most active

catalysts are Mg1V4 and Mg9V1 having the lowest and highest fraction of V(IV) in total

vanadium content. Propane ODH is catalysed by both valence states of vanadium, V(V)

and V(IV). However, due to the lower oxidation potemtial, V(V) is more active but less

selective than V(IV). Vanadium sites in octahedral or square pyramidal coordination are

more active but less selective than VO4 tetrahedra. Metal ions catalytic properties (Mg(II),

Zn(II), Pd(II)) are influenced by oxidation potential, acid-base properties, crystal size and

structural disorder. It is concluded that for propane ODH reaction, the promising catalyst is


34

supported vanadia catalyst with highly dispersed, tetrahedrally coordinated, V5+

sites for

higher selectivity and to achieve higher activities, high surface area supports with low

surface acidity catalyst is needed.

Ballarini et al. (2004) also have investigated the catalytic activity of supported

Vanadium oxides for ODH reaction but under co-feed and cyclic, redox-decoupling

conditions. In this study, the supported catalysts over alumina, titania, alumina-titania cogel

and silica were prepared by ion exchange, wet impregnation and co-gelation. The

mechanism on still oxidized catalysts under cyclic condition is either ODH or DH which is

followed by hydrogen combustion and has higher conversion in short reaction times. In co-

feed reaction, the catalysts are less selective as it is DH mechanism. Under cyclic condition,

vanadium oxide are highly dispersed when silica is used leading to higher propane

conversion and propane selectivity.

Khan et al. (2010) have recently reported that Vanadium oxides based materials

shows potential as ODH catalyst by investigating the effect of heterometallic centre (Mn,

Co, Fe) of the linkers in the catalyst clusters of framework. Catalyst interconnected with Fe

is however is the most active but least selective. Mn containing catalyst is the least active in

propane conversion. The best performance was exhibited by catalyst with Co linker with

the highest propane conversion and selectivity at 350 ºC. The catalysts are all active at low

temperature as they are easily reduced as indicated by TPR analysis. When treated under

400 ºC in 20% O2/Ar, DRIFTS studies shows structural changes of catalysts framework

with the broadening of V=O stretching bands. However, more work has to be put into the

material design and development for commercialization.


35

Vanadia supported on SBA-15 have shown promising catalytic activity in the ODH

of propane. Vanadia dispersion role on SBA-15 for propane ODH reaction was investigated

by Gruene et al. (2010). Mesoporous silica SBA-15 was synthesised using automated

laboratory reactor. Incipient wetness impregnation and grafting/ion-exchange are used to

disperse the vanadia on silica. Structural characterization shows mixture of monomeric and

oligomeric tetrahedral (VOx)n species depending on the loading quantity. However, at

higher vanadia loading (13.6 V/nm2), tetrahedral (VOx)n along with substantial amount of

three-dimensional, bulk-like V2O5 exists. At higher vanadia loading, the activation energy

changes with reaction condition unlike lower loading reflecting structural changes between

amorphous, bulk-like vanadia and two-dimensional highly dispersed vanadia spesies. It is

therefore concluded that polymeric species rather than monomeric species are more

important in propane activation.

2.5.4 Molybdenum Vanadium Oxides

An example is molybdenum with vanadium as the other basic catalyst component is

reactive in reactions such as propane dehydrogenation (Cavalleri et al., 2007). Various

combinations of molybdenum and vanadium oxides have been studied for ethane ODH.

The catalysts however are rarely reproducible though the optimum atomic ratio of V/Mo

seems to be between 0.25 and 0.5 which gives ethylene selectivity around 80% for low

conversion. The VMoO catalyst consists of several highly crystalline materials. Both mixed

oxides phases Mo4V6O25 and Mo6V9O40 obtained are not the most active and the surface

composition is close to bulk material (Mamedov & Cortés Corberán, 1995).


36

Meunier et al. (1997) had studied the molybdenum loading effect in ODH catalyst.

The catalytic performance of supported molybdena on oxides such as niobia, zirconia,

alumina, silica, magnesia and titania are compared for propane ODH reaction. TiO2 is the

most effective support to disperse the molybdenum. Titania coverage has to be higher than

monolayer coverage which is around 2.4 monolayers coverage to prevent any side reaction

from bare support. Catalytic activity increased with vanadium addition but less selective at

isoconversion. Addition of niobium leads to slight decrease in activity but selectivity is

improved. Therefore mixture of molybdenum, vanadium and niobium oxides supported on

titania (Mo+V+Nb)/TiO2 catalyst is more selective and the activity is good but still less

efficient as compared to NiMoO4. Modification by adding alkalies or varying the atomic

ratio of Mo/V/Nb might further improve the activity.

As reported by Cindric et al. (1999), polyoxoanions surfaces are similar to

heterogeneous metal oxides, thus new catalyst can be designed from organic derivatives of

polyoxoanions study. Polyoxomolybdates containing oxygen and nitrogen donating ligands

have been extensively prepared and characterised. Even though many types of

polyoxometalates exist, pH, range, temperature and metal ions concentration strongly

influence the formation by crystallisation caused by insolubility. Molybdovanates

(NH4)6[Mo6V2O24(C2O4)2].6H2O and (NH4)4[H2Mo2V2O12(C2O4)2].2H2O are prepared

where different species were obtained by successive precipitation at different concentration

of metal ions and pH but solubility dependant. The first molybdovanate anion consists of

6MoO6 and 2VO6 edge-sharing octahedral with two bidentate oxalate ligands bonded at V

ions, octahedral coordination of all metal ions. The bond lengths of M-O differ depending

on the metal ions repulsion. The second molybdate anion is centrosymmetric tetranuclear


37

molybdovante anion (M4O16)n-

consisting of two MoO6 and two VO6 edge-sharing

octahedral with two oxalate ligands bonded to the vanadium ions. However,

molybdovanadate anions are interconnected by H-bonds through NH4+ ions and water

molecules.

Mougin et al. (2000) synthesised Mo-V-O catalyst that has a metastable hexagonal

phase. This is done using soft chemistry method by precipitating Mo-V precursor using

65% nitric acid and varying the AMV and AHM composition while maintaining at pH1.

Proper crystallised hexagonal h-MoO3 with needle like structure and a hexagonal cross

section are obtained. Two criteria‘s that influences the structural formation are the

vanadium insertion filling the molybdenum vacancies of the h-MoO3 structure and

followed by vanadium substituting molybdenum following the formula (NH4)xVxMo1-XO3.

The addition of vanadium not only affected the morphology but also the lattice parameters

of the crystal structure. Based on the solubility limit, the final composition of the solid

solution is (NH4)0.13V0.13Mo0.87O3. Beyond the limit, during precipitation (V9Mo6O40) is

formed due to the decreasing vanadium content in the hexagonal structure.

Molybdenum oxide based-catalyst such as Mo5O14 usually incorporates transition

metals such as tungsten (W) and vanadium (V) which acts as a promoter to stabilize the

structure. Mixed oxides such as (Mo0.92V0.08)5O14 and (Mo0.75W0.25)5O14 are close to

industrial catalysts. In (MoVW)5O14, V and W promoting effect may be able to localize the

oxygen defects as V5+

prefers five-fold coordination while the redox stable W confines the

lattice deformations. Based on Raman studies, catalyst action may be determined by the

metal oxygen bond where the shorter M-O bond, the more basic is the oxygen functionality.


38

Promoters (V and W) stabilize the intermediate Mo oxides with proper M-O bonds for

selective C-H activation (Dieterle et al., 2001).

Three series of Vanadium molybdenum mixed oxides (VxMoyOz, x+y=1) were

prepared by Adams et al. (2004) for partial oxidation of acrolein to acrylic acid. The

methods used are pure oxide melting, crystallisation and spray drying which are all

followed by calcination. The samples acquired are mostly hexagonal h-(V,Mo)O3 and

(V,Mo)2O5 phases which has similar structure to vanadium pentoxide. Temperature

programmed reduction (TPR) was used to study the catalytic activity and selectivity. The

most promising composition of vanadium to molybdenum ratio is 3:7 at low temperature.

At temperature higher than 500 ºC, the catalyst decomposes into thermodynamically stable

phase and has no significant catalytic activity.

Similar studies have been conducted by Kunert et al. (2004) to investigate the

correlation of drying method with structural composition and catalytic performance. The

synthesized of Mo/V mixed oxides were dried via spray drying and crystallisation. From

TPR studies, the spray dried samples are more active. This is due to the fact that spray dried

samples yield the desirable hexagonal MoO3-type structure but crystallisation method

yields V2O5-type structure. Spray drying technique has many advantages as compared to

crystallisation. Among them is spray drying technique exerts influence on structural aspects

during the sample preparation thus enabling a promising understanding on the genesis of

desired, mostly metastable structure parts. Other advantages are spray drying can be used

for continuous preparation as it is rapid method, no loss of specific metal ion through


39

filtration, metal ions homogeneous distribution and also reproducible structurally

depending on the exact parameter variations.

Mo and V interaction on alumina for propane ODH was studied by Bañares and

Khatib. (2004). It is possible to disperse Mo and V on alumina by controlling the Mo and V

loading. During redox cycle, Mo-V-(Al)-O phase stability is dependent on the Mo + V

coverage because of the oxygen mobility. Overall, the alumina support is shared by Mo and

V thus contributes to the reaction with its own reactivity. No distinctive cooperation was

observed between the dispersed V and Mo oxides in the reaction but vanadium dominated

the catalytic performance.

The catalytic activity of MoOx catalyst doped with vanadium has been studied by

Haddad et al. (2009) for the ODH of ethane to ethylene reaction. MoOx is less active at

relatively low temperatures. By doping with a small amount of vanadium, catalytic activity

is enhanced and even better when phosphorus is added. This is because of Mo(VI) is

stabilized by the vanadium content in Mo5O14 and in MoO3. The calcination of the catalyst

creates well dispersed phosphate groups and also improves Mo and V species interactions.

These synergetic effects make Mo11VPOx catalytic performance for ethane ODH the best.

A different approach was taken by Sidochuk et al. (2010) in preparing vanadium

and molybdenum composition by mechanochemical technique of V2O5/(NH4)2Mo2O7

(V/Mo = 0.7/0.3) composition in air, ethanol, and water followed by thermal treatment

from 300 ºC to 700 ºC. The catalyst precursor was subjected to thermal analysis and the

data were correlated with powder XRD. After mechanochemically treated, the sample was


40

heated at 300 ºC and 350 ºC and ammonium hexavanadate is obtained. By heating to

400ºC, reflectance intrinsic to vanadium pentoxide and orthorhombic molybdenum trioxide

appears while ammonium hexavanadate disappears completely. At 450ºC, V2MoO8 appears

along with V2O5 and MoO3 reflection. Continuous heating leads to the growth of V2MoO8

reflection intensity. This final phase have enough high specific surface areas and

considerable mesopore and macropore volumes which is useful as a catalyst for

hydrocarbon oxidation catalysts.

2.6 Structure-Activity Relationship

Molybdenum-oxide based clusters provide a molecular model for catalytically

active metal-oxides used in especially industries. In order to exploit and control the solid-

state structures incorporating metal clusters, designed synthesis helps and also new

properties emergence can be predicted that may form due to the structural size and

complexity. Figure 2.2 shows the {Mo36} polyhedral cluster contains {Mo8} unit as one of

the building blocks (Cronin et al., 2000).

Source: (Cronin et al., 2000).

Figure 2.2: Polyhedra representations of {Mo36} and a {Mo8} unit.


41

Abd Hamid et al. (2003) have described how protonation of polyoxomolybdates,

reoligomerisation to precursors, and polyoxomolybdates hydrolysis can form either

supramolecular (Mo36O112) structure of hexagonal phase (h-MoO3) depending on the

reaction temperature. The polarity of O-H bond of the terminal oxygen species at

uncoordinated corners of MoO6 octahedra are the main reason for these transformation as

well as the occurrence of stable intermediates hence differently reactive to protonation.

Restructuring or condensation also is influenced by the structural motif steric constraints

variation caused by the increasing octahedra per polyoxomolybdate.

The source of molybdenum to synthesis hexagonal MoO3 sometimes contains

monovalent cations such as NH4+, K

+, Rb

+, Cs

+ where the cations exists in the tunnels of

the hexagonal structure as can be seen in Figure 2.3 (Mougin et al., 2000).

Source: (Mougin et al., 2000)

Figure 2.3: h-MoO3 (xy) projection


42

In Molybdenum Vanadium Oxides, the cation presence in hexagonal structure

tunnels is essential in order for the hexagonal h-MoO3 type structure to exist with needle

morphology. The cation nature also affects the lattice parameter but only in the a

parameter. When vanadium is introduced, the molybdenum vacancies are filled and a

increases. When all vacancies are filled, vanadium atoms substitutes molybdenum and

ammonium cation in the tunnels increases and a decreases. Finally the system is not

monophased and vanadium content in hexagonal needles decreases to form V9Mo6O40 and

parameters a and c increases. Vanadium also shows a stabilizing effect of the solid oxide

(Mougin et al., 2000).

It is hard to prepare pure oxides of Mo5O14 as the synthesis usually yields Mo4O11,

Mo17O47 and also MoO2. The Mo5O14 structure is deduced from the MoO3. The pentagonal

tunnels formed by the variation of an ordered array of rotational axes are incorporated into

the framework. Transition metal cation occupies the formed pentagonal tunnels in each

rotation group which consists of four corner-sharing octahedral. Therefore, the Mo5O14

structure are portrayed as a network consisting of MoO6 polyhedra and MoO7 pentagonal

bipyramids connected mutually by corner sharing and edges (Dieterle et al., 2001).

Mo5O14 structure as seen in Figure 2.4 is the best final product formed under

reduced oxygen partial structure by the grouping of oligo anions mixtures generated in the

solution. The active phase is metastable unti crystallization and the oxidative

decomposition under high oxygen partial pressure forms binary oxide phase (Knobl et al.,

2003).


43

Source: (Dieterle, et al., 2001)

Figure 2.4: Crystal structure of Mo5O14

2.7 In-situ Structural Technique Studies

The reduction of MoO3 mechanism and the existence of molybdenum suboxides

were determined by in-situ structural studies using a combination of in-situ XRD (long-

range order) and in-situ XAS (short-range order). The experiments were conducted under

different H2 partial pressure with isothermal and temperature-programmed reduction

conditions while the elucidating phase composition and the evolution with time were

observed. At temperature below 698 K, MoO3 reduction only elucidate MoO2 while

reduction above 773 K (with 10% H2), Mo metal is the final product in a two step reduction

process via MoO2 and also forming Mo4O11 above 698 K. Overall results obtained

highlight the importance of the effect of reactant concentration, reaction temperature and

reaction time on reaction products (Ressler et al, 2000).


44

Schlogl et al. (2001) has demonstrated the importance of in-situ analysis in

understanding the atomistic details of chemical reaction. The solid state phase

transformation of the reduced and oxidised forms of molybdenum oxides takes place

rapidly that the suboxides are unidentified using ex-situ analysis. The reaction rate, active

structure and the electronic properties of the catalyst especially when it is in bulk can be

identified by in-situ studies

Topsøe (2003) have described the importance of in-situ characterization in the

research and development of heterogeneous catalysts. Before new and improved

characterization techniques were introduced, many problems exists such as lacking surface-

sensitive techniques that provides spectroscopic information at pressure relevant to the

catalysis and also the difficulty of obtaining a specified understanding of structural insight

in nanostructure complexes which often presents in heterogeneous catalysts. These

problems are known as ‗materials gap‘ and ‗pressure gap‘. In-situ studies are described as a

technique that gives detailed structural and chemical atomic scale insight in complex

heterogeneous catalyst. In-situ studies have been used to explain studies of individual

adsorption/desorption process, catalytic behaviour in a controlled environment after

quenching reaction and also catalytic performance under high pressure. Combination of

several in-situ techniques along with theoretical calculations and surface science studies are

essential to have a complete understanding of the catalyst behaviour.

The structural phase formation and transformation kinetics in molybdenum during

ion implantation and post-annealing treatment were studied using in-situ XRD by Bohne et

al. (2005). Ion implantation technique is used to incorporate a controllable concentration of


45

oxygen in host matrix to synthesize molybdenum oxide with thin buried layers.

Combination of XRD technique and ion implantation provides In-situ observation during

implantation at different temperatures and also obtains kinetic information of structural

phase formations. This gives an understanding of the nucleation and growth process during

implantation and/or post annealing treatment. Implantation temperature effects the crystal

growth kinetics and oxide phase formation. At 160 ºC, MoO3 and/or Mo4O11 precipitates

were formed while at 700 ºC, the precipitate grain grew to coalescence and finally to the

buried MoO2 layer. During annealing, a continuous transformation from MoO3 to MoO2 is

observed by in-situ XRD from 600 ºC to 700 ºC.

The techniques for in-situ XRD powder diffraction studies of hydrothermal and

solvothermal synthesis have matured and it is now possible to design experiments and to

study the crystallization of microporous materials under various conditions such as the

crystallization process of zeolite. XRD Diffractogram mainly provides information about

crystalline materials thus it is an efficient way of studying crystallization kinetics.

However, important processes, as reactions occurring in solution, gel formation stages and

nucleation, cannot be directly probed by this technique. This is one of the reasons for the

current emphasis on combined in-situ studies using complementary techniques

simultaneously in the same experiment, e.g. XRD/DTA, SAXS/WAXS or XRD/DLS.

Considerable challenges still remain in interpreting the results and elucidating nucleation

and crystallization mechanisms in detail (Norby, 2006).


46

Rodel et al. (2007) employed in-situ XRD and in-situ XAS techniques combined

with gas phase analysis to explore the structural evolution of single phase Mo5O14-type

materials which is (MoVW)5O14 and (MoV)5O14 under different reaction condition. These

techniques can provide an insight to the long-range order of materials and also the local

structure around the metal centres in the mixed oxides. The bulk properties of the two oxide

system appears to be different under reducing (propene), oxidizing (oxygen), atmosphere

and isothermal redox condition. From in-situ XRD analysis, both catalysts were initially

heated under helium to 773 K and the Mo5O14-type structure appears stable. Under

reducing condition (10% propene) at 773 K, both catalysts are transformed to monoclinic

MoO2-type structures. However the lattice constants of (MoV) dioxide have an increased

cell volume with a-axis expansion, b-axis and c-axis shortening. Temperature-programmed

XRD shows the reduction of (MoVW)5O14 starts at 723 K while (MoV)5O14 is at 673 K.

Under oxidizing condition (20% oxygen) at 773 K, the (MoVW) dioxide is re-oxidized to

the initial structure but (MoV) re-oxidized to orthorhombic MoO3-type structure. These

findings were backed up by in-situ XAS analysis. Therefore, the structure stabilizing effect

of tungsten was determined and also structure-directing effect towards re-oxidation to

Mo5O14-type structure.

Chapter 3, Methodology _

47

CHAPTER 3

METHODOLOGY


48

3.0 Methodology

3.1 Chemicals and Gases

All of the chemicals and gases used in the catalyst preparation, activation and

reactivity studies are listed in Table 3.1.

Table 3.1: List of Chemicals & Gases Used

No Chemicals/Gases Supplier Description

1 Propane MOX

2 Purified Argon MOX 99.99%

3 Purified Helium MOX 99.99%

4 Purified Oxygen MOX 99.8%

5 Synthetic Air MOX 21% Oxygen in Nitrogen

6

Ammonium heptamolybdate

tetrahydrate

((NH4)6Mo7O24 . 4 H2O)

Merck

99%

7 Ammonium Metavanadate

(NH4VO3) Fluka 99%

8 Nitric Acid

(HNO3)

Friendemann

Schmidt 65%

9 Oxalic acid dihydrate

(COOH)2.2H2O Sigma Aldrich 99%

10 Vanadium (V) Oxide

(V2O5) Sigma Aldrich 98+%


49

3.2 Synthesis of Mo Based-Catalyst Precursors

3.2.1 Synthesis of MoOx Catalyst

All the synthesis was done using controlled precipitation method to obtain

reproducible properties of the synthesized materials. For Molybdenum Oxides (MoOx)

precursor, the method used follows the controlled precipitation technique established by

Abd Hamid et al. (2003). In this study, the method is further optimized by adding more

variations to the controlled parameters. This was done to establish a set of relationship

between the parameters and investigate the effects of each variable on the molybdenum

oxide structure. Metal molybdate source used was Ammonium Heptamolybdate

Tetrahydrate (AHM), while Nitric Acid (HNO3) was used as the precipitating agent. Nitric

Acid (HNO3) was used since it is readily available and will not poison the catalyst as it is

thermally decomposed. The parameters varied were temperature (30 °C and 50 °C),

molybdate solution concentration (0.07 M, 0.10 M and 0.14 M), precipitating agent

concentration (1 M, 2 M, and 5 M), and rate of addition (1 mL/min, 3 mL/min, and 5

mL/min). The varying parameters of the experiments are shown in Table 3.2.

The precipitating agent (HNO3) was added to 200 mL AHM solution with a fine

control of the rate of addition using an autotitrator (Mettler Toledo DL50). The solution

was stirred and the pH changes were monitored throughout the titration process as the

termination point was set at pH 1. Equation (3.1) shows the reaction for this process.

OHNHMoOHOMoNH 24324764 3676 _ _ _ _ _ _ _ _ _ _ (3.1)

All precipitate obtained were vacuum filtered using a Buchner flask and vacuum pump at

-20 bar. The precipitates were then dried using vacuum dessicator for 3 days at 30 °C.

.


50

Table 3.2: MoOx precursors synthesized using various conditions

No Sample No Cation [MoO4]

2-

Mol/l

[H]+

Mol/l

Temperature

°C

Rate of

Addition

mL/min

1 M014 NH4+ 0.07 1 30 1

2 M033 NH4+ 0.10 1 30 1

3 M064 NH4+ 0.10 1 30 3

4 M065 NH4+ 0.10 1 30 5

5 M035 NH4+ 0.14 1 30 1

6 M043 NH4+ 0.10 2 30 1

7 M021 NH4+ 0.10 5 30 1

8 M039 NH4+ 0.10 1 50 1

3.2.2 Synthesis of MoVOx Catalyst

For the synthesis of Molybdenum Vanadium Oxides (MoVOx) precursor, two

methods with different vanadium sources were used to achieve the desired catalyst

structure. The first synthesis method was achieved by modifying the formula of the method

established by Rodel et al. (2007) to obtain desired catalytic phase. Firstly Vanadyl oxalate

(VO(C2O4)) solution was prepared by dissolving the Vanadium (V) Oxide in Oxalic acid.

This was then added to a solution of Ammonium Heptamolybdate (AHM) with the rate of

addition of 1 mL/min using an autotitrator at 80 °C. After stirring at this temperature for

one hour, the solution was spray-dried by atomizing with compressed air at 6 bar and dried

with hot air at 200 °C using a mini spray-dryer (Buchi). The spray dried powder was then

collected for activation.


51

The second method used was by mixing AHM and Ammonium Metavanadate

(AMV) (Adams et al., 2004). The synthesis was done in a precipitation reactor (LabMax) at

30 ºC. 200 mL of AMV solution varying from 10-90 % loading of vanadium content was

added to 200 mL AHM solution and the pH values were maintained at 5 by adding

appropriate amounts of nitric acid during the synthesis. The solution was then stirred for 90

min at 80 ºC. The final precursor solution was then spray dried by atomizing with

compressed air at 6 bar and dried with hot air at 200 °C. The samples with varying

parameters were shown in Table 3.3.

3.2.2.1 MoVOx Activation

The MoVOx spray dried precursors were calcined under different environments.

Samples from the first method using vanadyl were calcined under static air using muffle

furnace (Barnstead Thermolyne) at 500 ºC for 4 hours and another batch under Helium

using temperature-programmed-Reduction (TPR1100) pre-treatment port at 500 ºC for

4 hours.

Table 3.3: MoVOx precursors synthesized using various conditions

Sample

No

Vanadium

Source

[VO]2+

Mol/l

[VO3]4-

Mol/l

[MoO4]2-

Mol/l

Temperature

°C Activation

M038 Vanadyl Oxalate 0.1 - 0.05 80 Helium



M044 AMV 0.01 0.1 30 Nitrogen





52

However, the obtained spray dried powders of MoVOx precursor synthesized using

Vanadates were calcined under Nitrogen using temperature-programmed-Reduction (TPR)

pretreatment port at 500 ºC for 4 hours.

3.3 Structural and Elemental Characterization

3.3.1 X-Ray Diffractogram (XRD)

The synthesized catalyst precursors were subjected to structural and compound

analysis using Bruker‘s XRD. The diffractograms were obtained using a Theta/2theta

goniometer and a Scintillation counter detector. For MoOx precursors, the data sets were

collected in reflection geometry in the range of 2° ≤ 2θ ≤ 60° with a step size of Δ2θ =

0.02° while for MoVOx precursors, the data sets were collected in in the range of 2° ≤ 2θ ≤

80° with a step size of Δ2θ = 0.02°. Phase analysis was done and phase purity was

determined using EVA software version 2002.

3.3.2 Scanning Electron Microscope (SEM) Imaging

FEI quanta 200F Field Emission Scanning Electron Microscope (FESEM) was used

to investigate the microstructure and surface structural defects of the precursors and the

catalyst. The morphology observation was carried out under low vacuum and accelerating

voltage of 5.0 HV. The images were captured under magnifications between ranges of 1000

to 60000.


53

3.3.3 Energy Dispersive X-ray (EDX)

Spot analysis was done using Energy Dispersive X-ray (EDX) with INCA energy

400 to quantitatively analyze the local metal content. The weight percentage of elements in

the precursors and catalyst were determined. Elemental mapping were done to verify the

metal homogeneous distribution in the MoVOx precursors and catalyst.

3.4 Thermal Analysis

3.4.1 Thermogravimetric (TG)

The calcination temperature were determined by a Mettler Toledo TGA/SDTA851e

analyser coupled with a Thermostar Mass Spectrometer (MS) to analyze the evolved

decomposition gas as it measures the weight changes with respect to temperature. MoOx

precursors were heated from 30 °C to 700 °C while for MoVOx precursors were heated

from 30 °C to 500 °C at a heating rate of 5 °C/min under synthetic air flow rate at 50

mL/min. The thermograms were evaluated and the weight loss percentages were calculated

using the STARe software (V9.00).

3.4.2 Differential Scanning Calorimetry (DSC)

The crystallization and melting properties of the precursors were analyzed using

Differential Scanning Calorimetry (DSC) technique. The DSC instrument used was a

Mettler Toledo DSC822e equipped with a measuring cell with ceramic sensors which

measures the heat flow to detect endothermic and exothermic effects (Wagner, 2009). All

samples were heated from 30 °C to 500 °C at a heating rate of 10 °C/min under synthetic

air flow rate at 50 mL/min. The heat flow/energy of samples was calculated and evaluated

using STARe software V.800.


54

3.5 Reactivity Studies

3.5.1 In-situ XRD

The structural phase changes with temperature were monitored through an in-situ

X-Ray Diffractometer (Bruker) (Figure 3.1) which was equipped with Flowbus mass flow

controllers and a Thermostar mass spectrometer (MS). Selected synthesised samples

(M033, M039, and M038) were heated from 30 °C to 500 °C at a heating rate of 5 °C/min

under Helium and synthetic air at 100mL/min. The diffractograms were obtained using a

Theta/theta goniometer and a position sensitive detector (PSD). For MoOx precursors, the

diffractogram data sets were collected using a position sensitive detector (PSD) in

reflection geometry in the range of 2° ≤ 2θ ≤ 60° with a step size of Δ2θ = 0.02° while for

MoVOx precursors, the data sets were collected in in the range of 2° ≤ 2θ ≤ 80°. The XRD

diffractograms were obtained at 50 °C & 25 °C intervals.

Figure 3.1: In-situ X-Ray Diffractometer (XRD)

X-ray Source PSD Detector

Reaction cell


55

3.5.2 In-situ DSC

The energy changes of the catalyst precursors throughout the reaction were

investigated using High Pressure Differential Scanning Calorimeter, Model HPDSC827e

(Mettler Toledo) (Figure 3.2) attached to mass flow controllers (Bronkhorst) and a mass

spectrometer (Thermostar). Selected samples (M033, M039, and M038) were heated from

30 °C to 500 °C at a heating rate of 5 °C/min under inert (Argon) at 50mL/min. For the

propane ODH reaction, gas mixture of propane: oxygen: inert of 46:2:2 was mixed by the

mass flow controllers(MFC) and flowed at 50 mL/min into the chamber as the samples

were heated from 30 °C to 500 °C at 5 °C/min. The heat/energy required to transforms the

catalyst precursor structurally and the active phases were determined.

Figure 3.2: In-situ Differential Scanning Calorimeter (DSC)

Furnace

Mass Flow

Controller

HPDSC

Chapter 4, Results and Discussion _

56

CHAPTER 4

RESULTS AND

DISCUSSION


57

0 20 40 60 80 100

1

2

3

4

5

6

M033 (30°C)

M039 (50°C)

pH

Vol of HNO3

0 20 40 60 80 100

-0.12

-0.08

-0.04

0.00

M033 (30°C)

M039 (50°C)

Fir

st

De

riv

ati

ve

pH

Vol of HNO3

4.0 Results and Discussion

PART A

4.1 Synthesis and Characterization of MoOx Catalyst

4.1.1 Titration Curves

Table 3.2 shows a summary of the samples synthesized and their experimental

conditions. The differences from varying temperature of the AHM solution were

investigated by looking at samples M033 and M039. The samples were synthesized at

30 ºC (M033) and 50 ºC (M039) while maintaining all other parameters as constant.

Figure 4.1: Titration of 0.10 M AHM with 1.0 M HNO3 at different temperature

Figure 4.1 shows the titration curve and pH first derivatives comparison for M033

and M039. Based on the titration curve for M039, the starting pH is lower where the curve

starts at pH 5, implying that heating the AHM solution increases the solubility and

dissociation of AHM in water giving rise to H+ ions thus increasing acidity (Zhang et al.,

2011). The pH drop for M039 was less steep at the beginning of the titration and becomes

almost parallel to M033 at around pH 4.4 to pH 2.6. Based on the first derivative curve,


58

M033 exhibits 2 inflection points. At the first minimum point at 49.4 mL corresponding to

pH 2.4, no obvious precipitate was observed but a cloudy suspension was noticed.

However, at the maximum inflection point at 71.9 mL which corresponds to pH 1.7,

spontaneous white precipitate was observed. The building blocks consisting of {MoO7}

units were formed at the first inflection point. As protonation continues, solution

supersaturation was reached and the particles nucleation takes place. The bridging of

oxygen atoms networks of the MoO7 leads to the nuclei growth of nuclei and assembles

into a new bulk material of polyoxymolybdate (Cronin et al., 2000, Hu and Shaw, 1999).

M039 titration curve had a minimum point at 51.6 mL corresponding to pH 1.9 but

did not exhibit a maximum point as compared to M033. When heated, the solution

supersaturation decreases as {MoO7} unit particles solubility was increased in the solution

and therefore no spontaneous precipitation was observed (Feng et al, 2007a). Significantly

lesser amount of acid was needed for M039 synthesis to reach the end point. Only 74 mL of

HNO3 is needed for M039 to reach the final pH compare to M033 where 100 mL of HNO3

is needed. This coincides with the increase of proton consumption when temperature was

raised (Duc et al., 2008). The titrated solution was then further heated to 70 ºC and fine

white precipitates were observed. This was because temperature changes the MoO7

solubility which then affects the morphology of the structure (Feng et al. 2007a).

The second experiment was conducted by varying the molybdate source (AHM)

concentration. Figure 4.2 shows the titration curve and pH first derivatives comparison of

samples M014, M033, and M035 with AHM concentrations of 0.07 M, 0.10 M, and 0.14 M

respectively. Similar pattern of titration curve is observed for all three samples.


59

0 20 40 60 80 100 120 140

1

2

3

4

5

6

M014 (0.07M)

M033 (0.10M)

M035 (0.14M)

pH

Vol of HNO3

-20 0 20 40 60 80 100 120 140 160

-0.12

-0.08

-0.04

0.00

M014 (0.07M)

M033 (0.10M)

M035 (0.14M)

Fir

st

De

riv

ati

ve

pH

Vol of HNO3

Figure 4.2: Titration of AHM at different concentration with 1.0 M HNO3 at 1 mL/min

All three titration curves starts at pH 5.3 and as acid was added M014 curve shows

steeper drop in pH followed by M033 and M035. The minimum point of the derivative

curve gradually shifts as the concentration increases from 34.6 mL corresponding to pH 2.8

for M014, 50.2 mL corresponds to pH 2.4 for M033 and to 70.2 mL and 78.6 mL

corresponding to pH 2.5 and pH 2.1 respectively for M035. This was because more acid

was needed to reach the buffering equilibrium. The two minimum points observed in M035

titration curve indicates heterogeneous nucleation process takes place at different pH. The

unstable colloidal distribution of the precipitate consisting of MoOx species cannot be

maintained and the fragment interlinks and progressively grew forming larger units (Yu et

al., 2007) As Supersaturation was reached spontaneous precipitation creating white

precipitate were observed at the titration curve maximum points. These points also

gradually shift from 55.5 mL corresponding to pH 1.9 for M014, 72.1 mL corresponding to

pH 1.7 for M033 and at 90.3 mL corresponding to pH 1.8 for M035.


60

0 20 40 60 80 100 120

1

2

3

4

5

6

pH

Vol of HNO3

M033 (1mL/min)

M064 (3mL/min)

M065 (5mL/min)

0 20 40 60 80 100 120

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

0.02

M033 (1mL/min)

M064 (3mL/min)

M065 (5mL/min)

Fir

st

De

riv

ati

ve

pH

Vol of HNO3

The third controlled precipitation experiment was conducted by varying the rate of

addition of the precipitating agent (HNO3). Figure 4.3 shows the titration curve and pH first

derivatives comparison for M033, M064 and M065 with titration rate of 1 mL/min, 3

mL/min, and 5 mL/min respectively.

Figure 4.3: Titration of 0.10 M AHM with 1.0 M HNO3 at different rate of addition

All three pH first derivatives curves exhibit one minimum point and one maximum

point. The pH curves become less steeper indicating that the continuous addition of acid

was consumed for the precipitation therefore contributes lesser to the pH changes (Behrens

et al., 2011). Almost the same amount of acid was needed to reach the first inflection point

for all curves. For M033 the minimum was observed at 49.4 mL corresponding to pH 2.4

while for M064 the minimum was observed at 51.5 mL corresponding to pH 2.3 and for

M065 the minimum was observed at 51.9 mL corresponding to pH 2.6. The maximum

inflection point however differs for all curves. Buffering equilibrium was not reached

during fast addition (5 mL/min) resulting in delay of reaction sequence. Therefore, M065

had the smallest maximum point followed by M064 and M033 (Abd Hamid et al., 2003).


61

0 20 40 60 80 100

1

2

3

4

5

6

M033 (1.0M)

M043 (2.0M)

M021 (5.0M)

pH

Vol of HNO3

0 20 40 60 80 100

-0.4

-0.3

-0.2

-0.1

0.0

0.1 M033 (1.0M)

M043 (2.0M)

M021 (5.0M)

Vol of HNO3

Fir

st

De

riv

ati

ve

pH

The last experiment was conducted by varying the precipitating agent concentration

which in this experiment is Nitric Acid (HNO3) at 1.0 M, 2.0 M and 5.0 M.

Figure 4.4: Titration of 0.10 M AHM with HNO3 at different concentration

Figure 4.4 shows the titration curve and pH first derivatives comparison of M033,

M043 and M021. The higher the concentration of HNO3 used, the lesser amount of acid

needed to reach the termination point at pH1. The titration curve of M021 was the steepest

followed by M043 and M033. Based on the derivatives, sharp pH change was observed for

M021 which causes huge error in the curve where the inflection point is uncertain.

However for titration using lower acid concentration, small but unambiguous pH change

was observed.


62

4.1.2 Structural and Elemental Characterization

4.1.2.1 X-Ray Diffractogram (XRD) Analysis

Structural and phase purity was determined by XRD Analysis. For MoOx based

precursors, two phases was observed, ‗supramolecular‘ phase and hexagonal phase.

Figure 4.5: XRD Diffractograms of MoOx showing ‗Supramolecular‘ structure peak

characteristics.

Figure 4.5 shows the XRD patterns of some of the samples consisting of

supramolecular structures. The diffractograms appear to have a similar pattern where a

sharp high peak appear at around 7° and low intensity peaks around 11°-12°. This is

consistent with the ‗supramolecular‘ structure of isopolyanion (Mo36O112) properties where

high intensity peaks are observed at lower angle (7°) and peaks are roughly resolved with

low intensity at slightly higher diffraction angle (11°-12°).

0 10 20 30 40 50 60

Supramolecular

M021

M043

M064

M065

M033

In

ten

sit

y (

a.u

)

2

M014


63

Table 4.1: X-Ray Data of M014 (MoOx) Table 4.2: X-Ray Data of M033 (MoOx)



2 Intensity (%) dexp

6.916 100 12.7718

9.717 42 9.0949

11.848 38 7.4636

12.979 32 6.8157

19.647 31 4.5149

25.866 32 3.4417

26.427 28 3.3699

* All interplanar distances were reported in Angstrom (Å)


6.904 100 12.7929

9.247 34 9.5565

9.712 36 9.0999

11.320 36 7.8103

11.843 52 7.4666


6.916 100 12.7714

9.492 29 9.3097

11.836 44 7.4709

12.06 21 7.3328

12.936 28 6.8378

19.577 27 4.5310

26.25 21 3.3923


4.639 25 19.0342

6.995 100 12.6267

8.398 65 10.5208

10.739 35 8.2316

11.788 54 7.5015

12.624 25 7.0063


4.586 24 19.2531

6.010 26 14.6930

6.950 100 12.7079

8.345 48 10.5870

9.341 66 9.4603

11.823 52 7.4793


5.998 38 14.7229

6.943 100 12.7211

7.258 52 12.1701

8.374 66 10.5501

9.334 61 9.4674

11.253 67 7.8568

11.987 55 7.3775

25.844 48 3.4446

27.122 48 3.2852

http://en.wikipedia.org/wiki/%C3%85


64

The lower intensity peaks observed at higher angle indicates the synthesized

molybdenum oxides contain nanostructured building blocks (Abd Hamid et al., 2003).

Diffractograms of samples synthesized using lower concentration of AHM appears to be

more crystallized. The intensity of the prominent peak at 7º is different with M033 having

the highest intensity among all the other supramolecular structured catalyst indicating a

boost of oxygen influence on the structure (Bohne et al., 2005). The supramolecular

structure involving a 36-molybdate ion was self-assembled, involving an intricate system of

interlinking components of two 18-molybdate sub units via four common oxygen atoms.

The units of {Mo}18 were constructed from repetitive arrangement of sixteen {MoO6}

pseudo edge sharing octahedra and two {MoO7} distorted pentagonal bipyramids (Atencio

et al., 2004; Hu and Shaw, 1999; Paulat-Boschen, 1979). The powdered structure was

compacted as it was needed to form bulk materials and therefore protecting the

microstructures of nanoscale structure (Koch, 1999).

Table 4.1-4.6 shows some experimental data of the prominent XRD peaks of M014,

M033, M064, M065, M043 and M021 respectively. The interplanar spacing (d value) of the

most prominent peak at nearly 7º is slightly different. The interplanar distances of

molybdenum oxides samples synthesized using lower molybdate concentration are larger.

The interplanar spacing differences were also observed by comparing the precipitating

agent (HNO3) concentration where at higher concentration at 5 M (M021), the d value is

larger as compared to 1 M HNO3 (M033) and 2 M HNO3 (M043) which are similar. The

large interplanar spacing observed indicates the molybdenum oxide catalysts have smaller

crystallite size consistent with the nanostructural units (Ahmad and Bhattacharya, 2009).

Titration processes involving higher acid concentration creates precipitate faster thus lesser


65

0 10 20 30 40 50 60

Hexagonal

Supramolecular

M035

Inte

nsi

ty (

a.u

)

2

M039

time to agglomerate therefore smaller crystallite size (Song et al., 2007). However, as can

be seen in Figure 4.4, higher acidic concentration leads to indecisive particles structures

and can be seen in the diffractogram as there were more unidentified peaks compared to the

others.

Figure 4.6: XRD Diffractograms of MoOx showing hexagonal structure peak

characteristics.

Figure 4.6 shows the diffractogram obtained for both M035 and M039 and the

peaks exhibit high crystallinity. The diffractogram obtained for M039 which was

synthesized at 50 ºC has hexagonal phase properties and matches to the compound of

Ammonium Molybdenum Oxide Hydrate, (NH4)0.15MoO3.0.5H2O (PDF-File 29-0115)

with unit cell parameter a = 6.09 Å., b = 6.09 Å., c = 9.14 Å although the intensity of the

sample was higher as compared to the reference material indicating higher degree of

crystallization (Dieterle et al., 2001). As reported in Table 4.7, the interplanar spacing of


66

the M039 is smaller compared to the reference and this can be seen in the diffractogram

with the peaks shifting slightly. The sample peaks are shifted to higher angles relative to the

reference molybdenum oxides as the unit cell volume is smaller. The small peak shifts

suggests internal stress due to stacking faults of particles and this can be seen in the SEM

images which will be discussed later ( Abrishami et al., 2011; Ungár, 2004).

M035 XRD analysis shows characteristics of mixed phases of ‗supramolecular‘ and

hexagonal structure which matches Ammonium Molybdenum Oxide, (NH3 (MoO3)3) (PDF-

File 78-1027) which belongs to space group P63/m (176) with unit cell parameter a =

10.568 Å, b = 10.568 Å, and c = 3.726 Å. The supramolecular characteristics exist as

described earlier at lower angle 7º and 11º - 12º in the diffractogram but with a much lower

intensity. As reported in Table 4.7, the interplanar spacing of the sample is smaller

compared to the reference and this can be seen in the diffractogram with the peaks shifting

slightly to higher angle indicating sample particle unit cell volume to be smaller (Abrishami

et al., 2011; Keijser et al., 1991). This shows molybdate concentration does influence phase

structure of molybdenum oxide.

The diffraction peak for M035 with the highest intensity was at 9.77º angle indexed

at (110) plane as compared for M039 was at 26.03º angle indexed at (102) plane. This trend

can be attributed as the synthesized material still hold crystallized water which was

supported by the diffractogram match to Ammonium Molybdenum Oxide Hydrate. For

M039, there was also crystallized water (which will be discussed in thermal analysis) but

was possibly held in the supramolecular phase structure (Ilkenhans et al., 1995).


67

Table 4.7: X-Ray Data of M039 (MoOx)

2 Intensity (%) hkl dexp dref Phase

9.920 55 001 8.9092 9.1400 Hexagonal

17.027 12 100 5.2032 5.2741 Hexagonal

19.641 25 002 4.5162 4.5700 Hexagonal

26.030 100 102 3.4204 3.4538 Hexagonal

29.574 55 110 3.0181 3.0450 Hexagonal

35.640 33 112 2.5171 2.5340 Hexagonal

45.642 15 210 1.9861 1.9934 Hexagonal

49.103 16 - 1.8539 1.8601 Hexagonal

56.249 19 302 1.6341 1.6408 Hexagonal

Table 4.8: X-Ray Data of M035 (MoOx)


6.971 31 - 12.6709 - Supramolecular

9.779 100 100 9.0378 9.1522 Hexagonal

16.906 17 110 5.2401 5.2840 Hexagonal

19.521 34 200 4.5438 4.5761 Hexagonal

25.899 82 101 3.4374 3.4510 Hexagonal

29.458 26 111 3.0297 3.0451 Hexagonal

35.514 18 121 2.5258 2.5351 Hexagonal

45.535 13 410 1.9905 1.9972 Hexagonal




68

Table 4.9: Crystallite size of MoOx catalyst precursors

Sample 2 d value (Å) FWHM Crystallite size (nm)

M014 6.932 12.7413 0.238 6.095

M033 6.923 12.7580 0.103 14.085

M064 6.985 12.6450 0.113 12.839

M065 6.956 12.6971 0.098 14.803

M035 9.781 9.0358 0.119 12.213

M043 6.916 12.7703 0.129 11.246

M021 6.954 12.7007 0.169 8.584

M039 26.031 3.4203 0.157 12.978

The X-ray crystallite size resembles the average of the smallest undistorted volumes

in the crystal. Table 4.9 shows the size differences of all samples calculated using the

Scherrer equation. The Full Width at Half Maximum (FWHM) values varies based on the

degree of crystallization and also the intensity of the diffractograms peak. Overall, the

crystallite sizes do vary indicating the titration parameters do play a major role in

controlling the degree of crystallization as suggested above. M033 synthesized at 30ºC has

a larger crystallite size compared to M039 at 50 ºC as temperature alters the morphology.

The crystallite size of M014 which is shown in Table 4.9 is smaller compared to M033 and

M035 which were prepared with higher molybdate concentration. This shows the

crystallinity of the molybdenum oxide particles increases with molybdate concentration,

hence larger crystallite size (Mahajan et al. 2008). The rate of precipitating agent addition

does not influence greatly on the crystallite size of the molybdenum oxide particles as the

sizes are very similar to each other. The crystallite size of both hexagonal structures of

M039 and M035 are similar.



69

4.1.2.2 Scanning Electron Microscope (SEM) Imaging

The effects of all the synthesis parameters on the morphology of the particles

structure were observed by SEM imaging. Figure 4.7 shows the SEM images for M014

which was synthesized using lower molybdate concentration (0.07M) M014 at 1000, 8000

and 15000 times magnification At lower magnification (1000 X), the image shows

aggregates clusters of particles. At higher magnification (15000 X) it is observed that there

are long blocks with smooth edges and no particular shape. Similar observation was seen in

Figure 4.8 for M033 but at lower maginification (1000 X), M033 shows large fraction of

needle-like agglomerates (Wagner et al., 2004). This result coincides with the XRD results

where the most crystalline peak among all the ‗supramolecular‘ structures was in M033.

Figure 4.9 and Figure 4.10 shows the catalyst structure SEM images for M064 and

M065 synthesized with fast addition of precipitating agent (HNO3) at 3 mL/min and 5

mL/min respectively. Both catalyst particles exhibits undefined particle structures, where

both do not have smooth edges and appears to have structural collapse at higher

magnification (15000 X). This is because the nanoparticles nucleation takes place quickly

due to the fast addition. Equilibrium is not reached at supersaturation point thus leading to

incomplete catalyst structure.

The morphology of M021 seen is Figure 4.12 also have undefined particle structure

as the nuclei growth of the catalyst becomes rapid due to the high precipitating agent

concentration (5 M). The morphology of M043 catalyst structure as can be seen in Figure

4.11 is similar to the morphology observed for M014 (Figure 4.7) where the cross section


70

average length is the same at (2.9 0.9) µm. The structure similarity also can be seen from

the XRD diffractograms of both catalysts (Figure 4.5).

Figure 4.14 shows the SEM image for M039 at 1000, 8000 and 15000 times

magnification. These images show clear hexagonal structure further confirming the XRD

analysis. The lower magnification image shows a cluster of the long rods with hexagonal

cross section (Song et al., 2007).

Figure 4.13 shows the SEM images for M035. The XRD analysis implies a phase

mixture of supramolecular structure and hexagonal. Here at lower magnification (1000X),

the agglomerates are like long blocks with no particular shape similar to Figure 4.8(a) but at

higher magnification (Figure 4.13 (b)), the hexagonal cross section of the rods are clearly

seen. However, at 15000 times magnification (Figure 4.13 (c)), hexagonal plates are

observed and appears to be larger than M039. This is in good agreement with the influence

of supersaturation on the morphology of the precipitate. At lower supersaturation, the

particles formed are small and nicely shaped while at higher supersaturation level larger

particles are formed but in a controlled way (Chow and Kurihara, 2002; Yu et al., 2007).

71

a) b) c)

Figure 4.7: SEM Imaging of M014 (Supramolecular structure)

a) b) c)


72

a) b) c)


a) b) c)


73

a) b) c)


a) b) c)


74

a) b) c)

Figure 4.13: SEM Imaging of M035 (Mixed Hexagonal and Supramolecular structure)

a) b) c)

Figure 4.14: SEM Imaging of M039 (Hexagonal structure)


75

4.1.2.3 Energy Dispersive X-ray (EDX)

Table 4.10: EDX Analysis of MoOx catalyst precursors

Sample Elements (Weight %)

Molybdenum (Mo) Oxygen (O)

M014 (supramolecular) 57.77 42.23




M035 (hexagonal & supramolecular) 58.39 41.61



M039 (hexagonal) 53.51 46.49

Table 4.10 shows the morphological differences in terms of weight percentage

composition of molybdenum and oxygen in the synthesised MoOx catalyst precursor

samples. The molybdenum to oxygen bulk composition ratio of all samples appears to be

similar except for M021 catalyst precursor supporting the argument presented by XRD and

titration curve where the synthesis creates undefined particle structure therefore disrupts the

oxygen dispersion on the supramolecular framework.


76

4.1.3 Catalytic Thermal Analysis

Three catalyst samples of MoOx were chosen for thermal analysis due to the

structural diversity. Catalyst M033 (supramolecular), M035 (hexagonal) and M039

(hexagonal and supramolecular mixed phase structure) were subjected to

Thermogravimetric Analysis (TGA) accompanied with Mass Spectrometer (MS) and the

results were correlated with Differential Scanning Calorimetry (DSC) results during

temperature programmed analysis.

Figure 4.15 shows the thermogram of M033 catalyst (supramolecular) where the

analysis were done under synthetic air with flow rate at 50 mL/min while heated from 30

°C to 500 °C at a heating rate of 5 °C/min. Based on the evaluation, four mass changes

recorded and the rate of changes can also be seen clearly from the DTG curve. The mass

losses were correlated with the Mass Spectroscopy analysis in Figure 4.16 and linked to

DSC analysis in Figure 4.17.

Figure 4.15: TG/DTG Analysis of M033 from 30 °C to 500 °C

100 200 300 400 500

88

90

92

94

96

98

100

-0.08

-0.06

-0.04

-0.02

0.00

We

igh

t/ %

Temperature/C

DT

G


77

100 200 300 400 500

m/e=44

m/e=16

m/e=17

m/e=18

m/e=43

Temperature/C

Figure 4.16: MS Evaluation of M033 from 30 °C to 500 °C

Figure 4.17: DSC Analysis of M033 from 30 °C to 500 °C

100 200 300 400 500-4

-3

-2

-1

0

Temperature/C

mW


78

The first mass loss of 3.20% was recorded at temperature below 95 °C. At this

point, loose water was eliminated which can be identified in the MS graph as mass number

(m/e), 16 and 17. Since the DSC822e is a heat flux system, this reaction is considered

endothermic (Gabbott, 2008). Another two endothermic peaks were observed at the second

mass loss range at 95 °C to 215 °C. This temperature range recorded the highest mass loss

of 4.18% as the structure melting intensifies with crystallized water desorption. At the third

mass loss of 2.29% at temperature 215 °C to 320 °C, fragments of nitrogen dioxides

(m/e=44), and ammonium nitrates (m/e=17,44) are released as the catalyst restructures

from metastable hexagonal molybdates (h-MoO3) as can be seen from the exothermic peak.

The existence of ammonium fragments such as NH3, N2, and NO2 proves the existence of

ammonium in the hexagonal structure tunnels (Mougin et al., 2000).

The final weight loss was 1.6% at 320 °C to 500 °C is where the remaining

ammonium and nitrates corresponding to mass number 16 and 17, are evolved. At this

range, exothermic peak designating crystallization is observed as the catalyst decomposes

into the stable orthorhombic phase (o-MoO3) at 430 °C (Mougin et al., 2000). No more

changes is recorded after this temperature suggesting that 430 °C is the most

thermodynamically temperature. This transformation trend will be studied more deeply

using in-situ XRD in the following Part C.


79

100 200 300 400 500

94

96

98

100

-0.08

-0.06

-0.04

-0.02

0.00

We

igh

t /

%

Temperature/C

D

TG

100 200 300 400 500

m/e=19

m/e=20

m/e=17

m/e=44

m/e=18

Temperature/C

m/e=43


Figure 4.19: MS Evaluation of M039 from 30 °C to 500 °C


80

.


Two mass losses of M039 catalyst (hexagonal structure) were recorded and

displayed in the thermogram at Figure 4.18. The first weight loss was 3.28% at temperature

below 235 °C and according to the mass spectroscopy evaluation (Figure 4.19), adsorbed

water (m/e=18) was released as the hexagonal structure starts melting and was seen as the

endothermic peak in Figure 4.20. The second mass loss was 2.80% recorded at temperature

from 235 °C to 500 °C. The fragments of nitrates, ammonium and oxides (m/e=17, 18, 44)

were evolved at this point. As all the ammonium ions were released from the metastable

hexagonal molybdates (h-MoO3) tunnels, the catalysts decomposed into its

thermodynamically stable state which was seen as sharp exothermic DSC peak in Figure

4.20. Unlike described by Song et al. (2007) the catalyst recrystallizes into the

thermodynamically stable orthorhombic MoO3 at 400 °C and not 450 °C (Mougin et al.,

2000; Song et al., 2007). No more change takes place after 420 °C. Hence this temperature

is used as the reference temperature or the activation of the catalyst.

100 200 300 400 500

-3

-2

-1

0

1

2

mW

Temperature/C


81


Figure 4.22: MS Analysis of M035 from 30 °C to 500 °C

100 200 300 400

93

94

95

96

97

98

99

100

101

-0.05

-0.04

-0.03

-0.02

-0.01

0.00

0.01

We

igh

t/ %

Temperature/C

DT

G

100 200 300 400 500

m/e=19

m/e=20

m/e=17

m/e=44

m/e=18

m/e=43

Temperature/C


82


Figure 4.21 thermogram displays two major mass losses for M035 catalyst

(hexagonal and supramolecular mixed phase structure) similar to sample M039. The first

loss is at temperature below 225 °C with mass loss of 3.06%. Based on the mass

spectroscopy analysis (Figure 4.22), desorption of crystallized water (m/e=17,18) takes

place at this temperature range which were also accompanied by the shallow broad

endothermic DSC peak in Figure 4.23 indicating moisture loss. The second mass loss was

at 225 °C - 500 °C with 3.51%. At this point, fragments of nitrates, ammonium and oxides

(m/e=17, 18, 44) were evolved at this point. The metastable hexagonal molybdates (h-

MoO3) decomposes into its thermodynamically stable state of orthorhombic molybdenum

oxides (o-MoO3). However, the DSC peak in Figure 4.23 was not as sharp as Figure 4.20.

During crystal structure rearrangement, the catalyst undergoes degradation as the material

did not reach the energy state equilibrium, thus resulting in non-uniform crystallization

(Gabbott, 2008).

100 200 300 400 500

-3.0

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

mW

Temperature/C


83

PART B

4.2 Synthesis and Characterization of MoVOx Catalyst

4.2.1 Structural and Elemental Characterization of MoVOx

4.2.1.1 X-Ray Diffractogram (XRD) Analysis

(a)

(b)

Figure 4.24: XRD Diffractograms of MoVOx spray dried precursors synthesized

using (a) vanadyl oxalate (b) ammonium metavanadate

0 20 40 60 80

Inte

nsity

(a.u

)

2

M040

M042

M038

0 20 40 60 80

M056

M045

M047Inte

nsity

(a.u

)

2

M044


84

The first method which is using vanadyl oxalate as the vanadium source shows that

all spray dried precursors samples exhibits similar XRD pattern which is amorphous phase

as can be seen in Figure 4.24 (a). The amorphous material was obtained since the solute in

the precursor solution was in a distorted phase; there was not enough time to form

crystallization as it is dried rapidly. The highest reflection which was the amorphous hump

observed between 8º and 15º shows poorly crystalline oxides and a second broad shorter

hump observed around 24º 2 angles. This suggests that the precursors may be

nanostructured (nanocrystalline) material (Knobl et al., 2003). The halo intensity of all

oxide precursors were the same indicating modification on the concentration ratio of

molybdenum and vanadium does not affect the structural properties.

The other method in synthesizing MoVOx precursors was done using ammonium

metavanadate and was later calcined under Nitrogen flow using TPR pretreatment port.

Similar XRD diffractogram patterns of the spray dried precursors were observed in Figure

4.24(b) as compared to Figure 4.24(a). Vanadium loading at 10% (M044) and 20% (M047)

does not affect the intensity of the highest intensity amorphous halo which is between 7 º

and 14 º. However, higher vanadium loading at 50% (M045) and 70% (M056) increases the

intensity of the broad peak. This was because molybdenum and vanadium atomic ratio is

affected as more vanadium was deposited in precursor oxide thus increasing the intensity of

the amorphous halo.


85

0 20 40 60 80

Hexagonal

Orthorhombic

(iii)

(ii)

Inte

nsi

ty (

a.u

)

2

(i)

* Tetragonal

***

**

**

*

****

0 20 40 60 80

Orthorhombic

Hexagonal

Monoclinic

(iii)

(ii)

Inte

nsi

ty (

a.u

)

2

(i)

* Tetragonal

*****

*

*

**

Figure 4.25: XRD Diffractograms of M038 before and after calcined (i) spray Dried

(ii) calcined under static air (iii) calcined under Helium.

Figure 4.26: XRD Diffractograms of M042 before and after calcined (i) spray Dried

(ii) calcined under static air (iii) calcined under Helium


86

The spray dried amorphous precursor of M038 and M042 were activated under two

conditions. The first activation condition was under static air using a muffle furnace at

500 °C. The XRD diffractogram observed in Figure 4.25 showed the changes of the spray

dried M038 amorphous phase into crystalline peaks after activation under both conditions.

Figure 4.25(ii) shows the transformation into highly crystalline structure as can be observed

by the rapid increase of intensity at 10°. The XRD diffractogram exhibits mixed phase with

hexagonal structure matching Vanadium Molybdenum Oxide, (V0.12Mo0.88) O2.94, PDF File

81-2414, which belongs to space group P63 (173) with unit cell parameter a = 10.593 Å, b

= 10.593 Å, and c = 3.6944 Å. The other phase that exist is orthorhombic phase that

correlates to the thermodynamically stable Molybdenum Oxide, (MoO3), PDF-File 65-

2421, which belongs to space group Pnma (62) with unit cell parameter a = 13.825 Å, b =

3.694 Å, and c = 3.954 Å. This was because of exposure to oxidizing medium such as air

thus causing the complex hexagonal oxides breakdown to the o-MoO3 phase (Knobl et al.,

2003).

Table 4.11 shows the X-ray data of M038 calcined in air. The hexagonal phase was

more prominent in the agglomerated crystal structure. The interplanar spacing of the

hexagonal reference material of Vanadium Molybdenum Oxide is smaller but the

orthorhombic reference material of Molybdenum Oxide is larger compare to M038

interplanar spacing. This indicates that the hexagonal Vanadium Molybdenum Oxide

crystallite size is smaller than the reference material but the crystallite size of the

orthorhombic phase that exists is larger than the reference material. This may have been

due to the incorporation of Vanadium into the framework. Highest intensity was recorded

at 9.6º at (100) plane.


87

Table 4.11: X-Ray Data of M038 (calcined in air)


9.651 100 100 9.1574 9.1738 Hexagonal

12.768 14 200 6.9278 6.9125 Orthorhombic

16.760 13 110 5.2856 5.2965 Hexagonal

19.347 18 200 4.5842 4.5869 Hexagonal

23.367 12 101 3.8038 3.8016 Orthorhombic

25.691 62 210 3.4648 3.4674 Hexagonal

27.392 18 210 3.2534 3.2580 Orthorhombic

29.441 28 111 3.0315 3.0301 Hexagonal

33.825 8 220 2.6479 2.6483 Hexagonal

35.457 19 121 2.52966 2.5283 Hexagonal

Table 4.12: X-Ray Data of M038 (calcined in He)


7.719 26 200 11.4433 11.4195 Tetragonal

8.657 21 210 10.2056 10.2139 Tetragonal

12.255 15 310 7.2165 7.2223 Tetragonal

16.459 21 330 5.3815 5.3832 Tetragonal

22.263 100 001 3.9899 3.9900 Tetragonal

23.372 43 600 3.8031 3.8065 Tetragonal

24.951 71 540 3.5659 3.5669 Tetragonal

26.146 31 630 3.4056 3.4046 Tetragonal

27.612 17 550 3.2279 3.2299 Tetragonal

28.146 17 640 3.1679 3.1672 Tetragonal

31.565 36 740 2.8321 2.8328 Tetragonal

33.711 18 541 2.6566 2.6592 Tetragonal




88

The M038 precursor which was activated under Helium flow using TPR

pretreatment port, the diffractogram obtained (iii) is very different from (ii). The

diffractogram is also crystalline and matches Vanadium Molybdenum Oxide (V0.07Mo0.93)5

O14, PDF File 31-1437 corresponding to the tetragonal phase with unit cell parameter a =

22.839 Å, b = 22.839 Å, and c = 3.99 Å. Between 8º and 15º angle as observed in the spray

dried amorphous diffractogram, the broad halo transforms to sharp peaks corresponding to

the Mo5O14 reflections of (210) and (310). The second halo around 24º transforms sharp

peaks which correspond to plane (540) also corresponding to the tetragonal phase as

displayed in Table 4.14 (Knobl et al., 2003; Zenkovets et al., 2007). The interplanar

spacing of M038 catalyst activated under Helium is smaller when the focus is on prominent

peaks.

The highest intensity of the catalyst diffractogram is recorded at (001) plane which

was the reflection for Mo5O14 therefore was in good agreement with the literature (Knobl et

al., 2003). The tetragonal structure of Mo5O14 was regarded to be a suitable catalyst

because of the structure and channel network that can accommodate heteroatom dopants

and oxygen for reaction. The structure does not have edge sharing octahedra but instead

clusters of octahedra around a fivefold bipyramid containing Mo5+

and V4+

atoms which are

interconnected by a corner-sharing octahedra network consisting of [Mo8O26]4-

(Knobl et

al., 2003; Werner et al., 1997). In the tetrahedra V4+

containing groups have oxygen ions

which were known to be easier to remove from the lattice during reaction thus promoting

the oxygen-containing products. The stability of this catalyst with oxygen vacancies was

essential for high dehydrogenation selectivity (Mamedov & Cortés Corberán, 1995). The

dynamics of this structural transformation will be shown in in-situ XRD analysis in part C.


89

Table 4.13: X-Ray Data of M042 (calcined in air)


12.809 100 001 6.9056 6.8917 Monoclinic

23.406 20 10-1 3.7976 3.7559 Monoclinic

25.738 81 210 3.4585 3.4674 Hexagonal

27.409 45 011 3.2514 3.2510 Monoclinic

33.792 10 220 2.6504 3.6483 Hexagonal

39.013 37 400 2.3069 2.2935 Hexagonal

Table 4.14: X-Ray Data of M042 (calcined in He)


7.752 16 200 11.3954 11.4195 Tetragonal

8.684 20 210 10.1746 10.2139 Tetragonal

22.300 100 001 3.9833 3.9900 Tetragonal

23.385 24 600 3.8010 3.8065 Tetragonal

24.972 84 540 3.5629 5.6685 Tetragonal

26.263 31 630 3.3907 3.4046 Tetragonal

27.632 20 550 3.2257 3.2299 Tetragonal

28.204 21 450 3.1615 3.1618 Orthorhombic

28.450 21 630 3.1348 3.1428 Orthorhombic

31.582 41 740 2.8306 2.8328 Tetragonal

33.724 26 750 2.6556 2.6550 Tetragonal

36.975 19 711 2.4292 2.4198 Orthorhombic




90

Figure 4.26 shows the diffractogram changes of the spray dried M042 (i) when

calcined using two different conditions. Figure 4.26 (ii) shows that when calcined in static

air the diffractogram obtained reveal highly crystalline structure of mixed phases but is

different as compared to M038 calcined under air diffractogram. The diffractogram also

displays hexagonal structure matching Vanadium Molybdenum Oxide, (V0.12Mo0.88) O2.94,

PDF File 81-2414. However, the other phase that exist was the monoclinic phase that

attributes to Molybdenum Oxide, (MoO3), PDF File 47-1320, which belongs to space

group P21/m(11) with unit cell parameter a = 3.954 Å, b = 3.687 Å, and c = 7.095 Å.

Table 4.13 shows the X-Ray data of M042 (calcined in air). The most prominent

peak with the highest intensity was recorded at 12.8º 2 angle and unlike M038 (calcined in

air), it is at monoclinic (001) plane. The interplanar spacing of the existing monoclinic

phase is larger compare to the reference but for the hexagonal phase that was present, the d

values are smaller as compared to the PDF-File reference material values.

M042 catalyst activation under Helium flow gives the same result as M038

(calcined He). The crystalline diffractogram mostly matches Vanadium Molybdenum

Oxide (V0.07Mo0.93)5 O14, PDF File 31-1437 corresponding to the tetragonal phase.

However the diffractogram obtained also reveals orthorhombic phase matching

Molybdenum Oxide (Mo17O47), PDF File 71-0566 corresponding with unit cell parameter a

= 21.531 Å, b = 19.534 Å, and c = 4.001 Å belonging to space group Pba2 (32). These

traces were of partial collapse of (MoV)5O14 oxide phase and this was not observed in

M038 catalyst precursor which uses higher concentration of both vanadyl and molybdate


91

0 20 40 60 80

Orthorhombic

Monoclinic

Triclinic

Unknown

*

*

*

M056 (70%V)

M045 (50%V)

M047 (20%V)

Inte

ns

ity

(a

.u)

2

M044 (10%V)

*

*

salt (Knobl et al., 2003). For both phases, as in Table 4.14, the d values of the catalyst are

smaller compared to the reference material.

Figure 4.27: XRD Diffractograms of MoVOx synthesized using ammonium

metavanadate calcined under Nitrogen

Figure 4.27 shows the diffractograms of M044, M047, M045 and M056 (Vanadium

varies from 10%-70%) after activation under Nitrogen with a rate of 5 ºC/min. M044

(10%V), M047 (20%V) and M045 (50%) all shows similar diffractogram with varying

intensity. All the diffractograms shows mixed phase of monoclinic phase of Vanadium

Molybdenum Oxide, (MoV2O8), PDF-File 20-1377 with space group C2 (5) and unit cell

parameter a = 19.398 Å, b = 3.629 Å, and c = 4.117 Å. The other phase coexisting is

orthorhombic phase of Molybdite,syn (MoO3), PDF-File 89-5108 with space group Pbnm

(62) and unit cell parameter a = 3.962 Å, b = 13.855 Å, and c = 3.701 Å.


92

Table 4.15: X-Ray Data of M044 (calcined in Nitrogen)


12.739 25 020 6.9433 6.9275 Orthorhombic

21.003 18 - 4.2264 - -

22.159 35 001 4.0085 4.1169 Monoclinic

23.350 57 110 3.8065 3.8093 Orthorhombic

25.696 41 040 3.4641 3.4638 Orthorhombic

27.356 100 021 3.2575 3.2644 Orthorhombic

33.641 30 111 2.7187 2.6945 Monoclinic

33.820 22 510 2.6483 2.6502 Monoclinic

38.960 15 060 2.3099 2.3092 Orthorhombic

49.286 25 002 1.8474 1.8531 Orthorhombic



12.771 29 020 6.9263 6.9275 Orthorhombic

21.002 12 - 4.2265 - -

22.191 28 001 4.0027 4.1169 Monoclinic

23.393 52 101 3.7997 3.8016 Orthorhombic

25.680 40 040 3.4662 3.4638 Orthorhombic

27.342 100 021 3.2593 3.2644 Orthorhombic

32.914 31 111 2.7191 2.6945 Monoclinic

33.659 28 510 2.6606 2.6502 Monoclinic

38.958 24 060 2.3100 2.3092 Orthorhombic

49.290 29 020 1.8473 1.8470 Orthorhombic




93



12.801 31 020 6.9097 6.9275 Orthorhombic

21.003 26 - 4.2264 - -

22.236 51 001 3.9947 4.1169 Monoclinic

23.418 63 110 3.7957 3.8093 Orthorhombic

25.703 43 040 3.4633 3.4638 Orthorhombic

27.379 100 021 3.2549 3.2644 Orthorhombic

32.899 31 111 2.7203 2.6945 Monoclinic

33.651 33 510 2.6612 2.6502 Monoclinic

39.013 21 060 2.3069 2.3092 Orthorhombic

49.306 24 002 1.8467 1.8505 Orthorhombic



9.695 31 200 9.1156 9.6988 Monoclinic

22.289 100 001 3.9853 4.1169 Monoclinic

23.470 26 -201 3.7874 3.7978 Monoclinic

25.052 39 001 3.5517 3.5570 Triclinic

25.743 35 -110 3.4579 3.3771 Triclinic

29.539 26 200 3.0216 3.0236 Triclinic

31.692 33 - 2.8211 - -

45.341 19 202 1.9986 2.0112 Monoclinic




94

Table 4.15 – 4.17 shows some of the experimental data of all three diffractograms.

As observed, the most prominent peak of all diffractograms with the highest intensity was

at 27.4º corresponding to (021) plane of the orthorhombic structure. At this plane, the

interplanar spacing of all three synthesized catalyst is smaller as compared to the reference

material. The monoclinic phase existence contradicts the findings by Kunert et al. (2004),

where only hexagonal phase was detected in the spray dried material.

M056 (70%) however exhibits a different diffractogram compare to samples

containing lower amount of Vanadium. The diffractogram shows mixed phase but mostly

matches Vanadium Molybdenum Oxide (V0.95 Mo0.97O5), PDF-File 77-0649, attributing to

a Triclinic system with space group P1 (1) and unit cell parameter a = 6.334 Å, b = 4.0463

Å, and c = 3.7255 Å. The other phase that coexists is the monoclinic phase of Vanadium

Molybdenum Oxide, (MoV2O8), PDF-File 20-1377 which is the same phase in the other

three synthesized catalyst. Higher dispersion of vanadium provides ‗site-isolation‘ effect

which is important to avoid olefins transforming to neighboring oxidized sites (Ballarini et

al., 2004). Table 4.18 shows the X-ray data of M056 (calcined). The prominent peak with

the highest intensity unlike the other catalyst is at 22.2º at plane (001) of the monoclinic

phase. The shifting to lower angle can be deduced as the catalyst having a larger unit cell

volume (Abrishami et al., 2011; Keijser et al., 1991).


95

Table 4.19: Crystallite size of MoVOx samples

Sample 2 d value FWHM Crystallite size

(nm)

M038 (calcined in air) 9.651 9.1567 0.095 15.297

M038 (calcined in He) 22.269 3.9888 0.143 10.321

M042 (calcined in air) 12.806 6.9072 0.097 15.022

M042 (calcined in He) 22.283 3.9864 0.115 12.834

M044 (calcined in Nitrogen) 27.346 3.2587 0.140 10.645




Table 4.19 displays the different crystallite sizes of all the activated catalysts. As

observed, the size which depends on the degree of crystallization varies with different

conditions used not only in synthesizing but also in catalyst activation. The calcined

catalysts were therefore made of nanostructure crystallite which is stabilized by amorphous

matrix by spray drying process (Li et al., 2010). The smallest crystallite size observed is in

sample M056_calc which were synthesized with 70% Vanadium salt precursor. Higher

vanadium loading decreases the crystallite size with more bonding interaction between

Mo-V. For samples synthesized using vanadyl source, the smallest particle size as shown

are in M038 (calcined Helium). Smaller crystallite size shows that under controlled flow of

He, nanostructuring were more refined as compared to the calcination under static air.


96

4.2.1.2 Scanning Electron Microscope (SEM) Imaging

Figure 4.28 shows the SEM images of M038 (a, b, c) and M042 (d, e, f) spray dried

catalyst morphology at 8000, 15000 and 30000 times magnification. At lower

magnification, the images shows clusters of spherical, smooth ball like particles with no

discrete features which is consistent with any kind of spray dried precursors (Kunert et al.,

2004). At higher magnification, the ball like structure appears to be spherical particles

smooth surface areas indicating the effect of drying process on surface texture (Endres et

al., 2007).

The morphology of M038 catalytic structures after calcination was shown in Figure

4.29. Figure 4.29 (a, b, c) were images after calcination under static air. Here as can be seen

at the lower magnification, the ball like structure appears to be decomposed and no longer

has a smooth surface. At higher magnification, new crystallite structure emerges and looks

like to be made of rough looking hexagonal slices. This confirms the XRD data analysis as

shown in Figure 4.25. The hexagonal plates cross section length measure at an average of

(1.1 0.4) µm.

Figure 4.29 (d, e, f) were images for M038 catalyst precursor after calcination under

Helium. At lower magnification, the ball like structure appears to be uneven but less ‗flaky‘

unlike images (a,b). At higher magnification, a new finely dispersed phase which appears

to be compiling finer crystallite. The newly formed particles morphology have tetragonal

cross sections coexisting with some other morphology indicating mixed phase and further

confirming the XRD analysis. The length of the tetragonal cross section averages around

(0.5 0.1) µm which was half the size of M038 (b) (Sidorchuk et al., 2010).


97

The morphology of M042 catalytic structures after calcination was shown in Figure

4.30. Figure 4.30 (a, b, c) were images after calcination under static air. Here as can be seen

at the lower magnification (a) the ball like shape seems to have decomposed thus appear to

be made out of aggregates of flat plates. At higher magnification (b), it can be seen clearly

that the flat plates were hexagonal crystallite. The cross section length of the hexagonal

plates measures at an average of (2.0 0.5) µm. There was also a different morphology that

can be seen in the SEM images and this was related to the orthorhombic structure as

discussed in the XRD diffractogram in Figure 4.26.

Figure 4.30 (d,e,f) were images of M042 after calcination under Helium flow.

Similar to M038, the spray dried balls particulates do not exhibit any sorts of plate at lower

magnification. At higher magnification, a mixed phase of tetragonal cross sections

coexisting with some other morphology presumably orthorhombic phase as analyzed from

the XRD data (Table 4.15). The length of the tetragonal cross section averages around

(0.39 0.08) µm.

Figure 4.31 shows the SEM images of M044 before (a,b,c) and after (d,e,f)

calcination under Nitrogen flow. Before calcination, the SEM images appears to be ball

like structures just like the other spray dried catalyst precursors but the ball structures are

not smooth and appears to be made out of clusters of plates. The diameter of the balls were

in the range of 1-10 micrometer while the cross section length of the plates averages around

(0.8 0.2) µm. However after calcination, there seems to have cracking of plates, creating

polycrystalline solid rod like structure with cross sections diameter averages at (0.21

0.07) µm (Petkov, 2008).


98

Figure 4.32 shows the SEM images of M056 before (a,b,c) and after (d,e,f)

calcination under Nitrogen flow. Before calcination, the SEM images appear to be ball like

structures just like the other spray dried catalyst precursors. After calcination,

polycrystalline needles are formed with the ball like structure still remains intact although

decomposition had taken place. As it is also displayed in M044, these were deduced to be

the thermodynamically stable oxides of monoclinic phase of Vanadium Molybdenum

Oxide, (MoV2O8) (Adams et al., 2004).

99

a) b) c)

d) e) f)

Figure 4.28: SEM imaging for M038 (a,b,c) and M042 (d,e,f) before calcination

100

a) b) c)

d) e) f)

Figure 4.29: SEM imaging for M038 after calcination (a,b,c) in air & calcination (d,e,f) in Helium

101

a) b) c)

d) e) f)

Figure 4.30: SEM imaging for M042 after calcination (a,b,c) in air & calcination (d,e,f) in Helium

102

a) b) c)

d) e) f)

Figure 4.31: SEM imaging for M044 before (a,b,c) & after calcination in Nitrogen (d,e,f)

103

a) b) c)

d) e) f)

Figure 4.32: SEM imaging for M056 before (a,b,c) & after calcination in Nitrogen (d,e,f)


104

4.2.1.3 Energy Dispersive X-ray (EDX)

Table 4.20: EDX Analysis of MoVOx catalyst precursors

Sample Elements (Weight %)

Molybdenum (Mo) Oxygen (O) Vanadium (V)

M038 52.67 45.21 2.12

M038 (calcair) 56.04 41.73 2.23

M038 (calcHe) 64.27 33.16 2.57

M042 45.16 51.59 3.24

M042 (calcair) 55.71 40.27 4.03

M042 (calcHe) 60.96 34.95 4.09

M044 56.18 43.31 0.50

M044 (calc) 55.20 43.18 1.62

M056 52.01 45.31 2.68

M056 (calc) 54.61 42.82 2.57

Table 4.20 shows the weight percentage composition of Molybdenum, Oxygen and

Vanadium in the synthesised MoVOx catalyst precursor samples. The vanadium

composition increases coinciding with an increase of vanadium addition. Calcination

affects the elemental composition with small changes (Knobl, et al., 2003)

Based on the elemental mapping analysis in Figure 4.33 (M038) and 4.34 (M042),

by employing the method involving vanadyl oxalate as the Vanadium source and after

calcination under helium flow, the elements appears to be homogenously dispersed in the

bulk catalyst.

105

Figure 4.33: Elemental Mapping for MoVOx, M038 (calcination in Helium)

106

Figure 4.34: Elemental Mapping for MoVOx, M042 (calcination in Helium)


107

4.2.2 Catalytic Thermal Analysis

Four catalyst precursors of MoVOx (M038, M042, M044 and M056(70 %V))

varied in synthesis method and structural properties were chosen for thermal analysis. The

catalyst subjected to Thermogravimetric (TG) Analysis accompanied with Mass

Spectrometer (MS) and the results were correlated with Differential Scanning Calorimetry

(DSC) results during temperature programmed analysis.

Figure 4.35 shows the thermogram of M038 where the analysis was done under air

with flow rate at 50ml/min while the sample was heated from 30 °C to 500 °C. Four steps

of mass loss were recorded. First mass loss of 5.45% was recorded at below 170 °C.

Based on the MS evaluation in Figure 4.36, water (m/e=16, 17 & 18) is released at this

point. Broad endothermic peak is observed from the DSC curve in Figure 4.37. As the

melting process continues, a second endothermic peak is observed which was associated

with a mass loss of 2.71% in the thermogram and at this section desorption of crystallized

water (m/e=16, 17 & 18) takes place. The third mass loss was 4.98% at temperature 230 °C

to 285 °C. DSC curve shows endothermic and exothermic effect with fragments of water

(m/e=16,17), carbon dioxide (m/e=28), ammonia (m/e=17,16) and nitrogen oxides

(m/e=29) released from the catalyst. The oxidation of ammonia and the reduction of

vanadium and molybdenum precursors generate the nitrogen oxides fragments while carbon

dioxide formation results from the decomposition of vanadyl oxalate used as the starting

material (Knobl et al., 2003). The final mass loss of 7.3% was recorded at 285 °C to 500

°C. At this section, the remaining fragments of water, oxides and ammonium were released

as crystallization process to the thermodynamically stable tetragonal phase of the catalyst

was correlated with the exothermic peak observed in the DSC curve.


108

100 200 300 400 500

80

85

90

95

100

-0.15

-0.10

-0.05

0.00

0.05

0.10

0.15

Temperature/C

We

igh

t/ %

DT

G

100 200 300 400 500

Temperature/C

m/e=29

m/e=16

m/e=14

m/e=17

m/e=18

m/e=28

Figure 4.35: TG/DTG Analysis of M038 from 30 °C to 500 °C.

Figure 4.36: MS Evaluation of M038 from 30 °C to 500 °C.


109

Figure 4.37: DSC Analysis of M038 from 30 °C to 500 °C.

100 200 300 400 500

-4

-3

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mW


110



100 200 300 400 500

80

85

90

95

100

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igh

t /

%

Temperature/C

100 200 300 400 500

m/e=46m/e=44

m/e=43

m/e=17

m/e=45

m/e=29

Temperature/C


111


Five steps of mass loss were observed from the Figure 4.38 thermogram. The first

mass loss of 3.68% was recorded at temperature below 150 °C. Based on the MS evaluation

in Figure 4.39, Nitrogen (m/e=29) and loose water (m/e=17) were evolved at this point

which associates with the broad endothermic peak displayed in DSC curve (Figure 4.40).

The second mass loss of 4.05% in temperature range of 150 °C involves the elimination of

crystallized water (m/e=17) which was correlated with the endothermic effect observed.

Third mass loss of 7.00% and fourth mass loss of 3.29% at temperature range of 230 °C to

300 °C and from 300 °C to 370 °C were accompanied with a endothermic and exothermic

effect respectively. Similar to M038, based on the MS evaluation, ammonia oxidation along

with the vanadium and molybdenum reduction releases nitrogen oxides (m/e=44,46) and

ammonium fragments (m/e=17,29) while the decomposition of vanadyl oxalate releases

carbon dioxide (m/e=43,45). The final mass loss is associated with the crystallization

process of the catalyst as it decomposes to the thermodynamically stable tetragonal and

orthorhombic phase which is correlated to the DSC exothermic peak.

100 200 300 400 500

-3.5

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112

100 200 300 400 500

80

85

90

95

100

-0.0003

-0.0002

-0.0001

0.0000

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W

eig

ht/

%

Temperature/C

D

TG



100 200 300 400 500

m/e=16

m/e=17

m/e=18

m/e=20

m/e=39

m/e=40

m/e=19

Temperature/C


113

100 200 300 400 500

-5

-4

-3

-2

-1

0

1

mW

Temperature/C


Six phases of mass loss were observed in the thermogram of M044 (10 %V) in

Figure 4.41. At temperature below 130 °C, 1.84% mass loss were recorded along with the

release of water (m/e=18) which can be correlated to the broad endothermic peak in Figure

4.43 at this temperature range. The second mass loss of 3.17% from temperature 130 °C to

195 °C eludes crystallized water (m/e=16,17,18) according to the MS evaluation in Figure

4.42. At this range also, endothermic effect was observed in the DSC Curve. The following

three mass losses were 7.82%, 5.02% and 0.89% at temperature 195 °C to 275 °C, 275 °C

to 345 °C and 345 °C to 390 °C respectively. At all three temperature range, fragments of

oxides (m/e=39, 40), nitrates (m/e=18, 19) and ammonium (m/e=16, 17) were released and

endothermic effects were observed. The final mass loss of 1.43% at temperature 390 °C to

500 °C attributes to an exothermic peak at 420 °C as the catalyst decomposes to the

thermodynamically stable molybdenum oxides (MoO3) and Vanadium Molybdenum Oxide

(V2MoO8) which is irreversible (Adams et al., 2004).


114



100 200 300 400 500

75

80

85

90

95

100

-0.20

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DT

G

Temperature/ C

100 200 300 400 500

m/e=16m/e=44

m/e=43

m/e=17

m/e=18

Temperature/C


115

100 200 300 400 500

-4

-3

-2

-1

0

1

mW

Temperature/C


Figure 4.44 shows the thermogram of M056 (70 %V). Five steps of mass loss were

observed. Similarly to M044, the first mass loss of 3.30% at temperature below 135 °C and

based on the MS evaluation in Figure 4.45, this loss is attributed to the release of water

(m/e=17, 18) and can be confirmed by the broad endothermic effect in Figure 4.46 DSC

peak. The second mass loss of 6.65% from temperature 135 °C to 250 °C corresponds to

the release of crystallized water (m/e=17,18), ammonium (m/e=16) and nitrates

(m/e=43,44) At this temperature two endothermic effects were observed from the DSC

Analysis. Similar results were obtained at the third mass loss of 4.69% at temperature 250

°C to 300 °C with ion fragments from the decomposition of oxides (m/e=16) and

ammonium nitrates (m/e=43,44) that was formed during synthesis, which correlates to an

endothermic effect (Mougin et al., 2000). The following loss of 5.09% also involved the

removal of nitrogen oxides and the remaining ammonium and towards the end of this range


116

an exothermic peak is observed indicating restructuring process in the bulk structure mixed

oxide starts to occur (Kunert et al., 2004). This crystallization process takes place at

temperature of 455 °C to 500 °C with mass loss of 0.54%, as the catalyst is decomposed to

the thermodynamically stable phase consisting of mixed phase mostly of monoclinic and

triclinic phase as discussed based on the XRD Data in Table 4.18.


117

PART C

4.3 Reactivity Studies

4.3.1 In-situ X-Ray Diffractogram (XRD) Analysis

Selected catalyst precursors were subjected to activation using the experimental

obtained from thermal analysis of those samples. Form MoOx precursors, M033 and M039

were chosen. This was done to show comparison of the two different crystalline phase

structure of supramolecular and hexagonal. While for MoVOx precursors, M038 were

chosen based on the calcination analysis seeing that at 500 °C, M038 appears to be more

pure phase as compared to the other samples. All samples were activated under Helium at

100ml/min. The first experiment (M033) were conducted from 50 °C to 500 °C at a heating

rate of 5 °C/min and the diffractograms were obtained using a Position-Sensitive Detector

(PSD) at every 50 °C until 200 °C and every 25 °C from 200 °C until 500 °C.

Figure 4.47 shows the XRD of MoOx precursor (M033) under in-situ activation

program. The final temperature was set at 500 °C, determined based on the thermal

analysis. When heated above 250 °C, the removal of water causes the transformation of

phase to a metastable hexagonal phase and reaches the optimal metastable hexagonal phase

at 300 °C. The phase matches Molybdenum Oxide, (MoO3) (PDF-File 21-0569) which

belongs to space group P(0) with unit cell parameter a = 10.531 Å, b = 10.531 Å, and c =

14.876 Å although the diffractogram shifts a little to the lower angle indicating a larger

hexagonal unit cell. Table 4.21 shows the interplanar spacing of the metastable hexagonal

phase at 300 °C. By comparison, the d values of the sample were larger than the reference


118

material and this was consistent with the sample having larger unit cell volume (Keijser et

al., 1991).

Figure 4.47: In-situ XRD of MoOx precursor (M033) activation from

50 °C-500 °C under Helium Gas

As the sample was heated continuously, at 375 °C the intensity of the peak at 10°

plane (100) reduces drastically and eventually disappears at 425 °C. The intensity of the

other prominent metastable peaks at 25° and 29° correlated to plane (210) and (300)

respectively were also reduced while new peak was observed at 27° when the temperature

reaches 375 °C. The catalyst phase changes to the final thermodynamically stable

orthorhombic phase (o-MoO3) at above 425 °C although the diffractogram shows a few

10 20 30 40 50 60

500C

475C

450C

425C

400C

375C

350C

325C

300C

275C

250C

225C

200C

150C

100C

50C

Inte

ns

ity

(a

.u)

2

Hexagonal (PDF 21-059)

Orthorhombic (PDF 89-5108)


119

unidentified peaks. X-Ray data (Table 4.22) at 500 °C shows the orthorhombic phase

matches Molybdite, syn (MoO3), (PDF-File 89-5108) which belongs to space group

Pbnm(62) with unit cell volume a = 3.962 Å, b = 13.855 Å, c = 3.701 Å although there are

few unidentified peaks.

Table 4.21: X-Ray Data of M033 Activation (300 °C)


9.591 100 100 9.2145 9.1201 Hexagonal

16.684 18 110 5.3094 5.2655 Hexagonal

19.294 27 200 4.5967 4.5601 Hexagonal

25.617 93 210 3.4746 3.4471 Hexagonal

29.283 57 300 3.0474 3.0400 Hexagonal

30.877 10 204 2.8936 2.8821 Hexagonal

33.760 11 220 2.6528 2.6328 Hexagonal

35.319 34 310 2.5392 2.5295 Hexagonal



12.561 17 020 7.0412 6.9275 Orthorhombic

23.249 34 110 3.8229 3.8093 Orthorhombic

23.433 26 - 3.7932 -

25.394 35 040 3.5046 3.4638 Orthorhombic

27.221 100 021 3.2735 3.2644 Orthorhombic

32.735 16 101 2.7336 2.7046 Orthorhombic

33.489 23 111 2.6737 2.6545 Orthorhombic




120

Similar as the phenomena observed around 300 ºC, the diffractogram shifts to lower

angle as compared to the reference material and the interplanar spacing as shown in Table

4.22 were bigger compared to the reference material indicating larger unit cell volume. The

arrangement to thermodynamically stable phase of o-MoO3 was facilitated by the oxygen

vacancies in the oxygen deficient intermediate metastable h-MoO3 phase (Giebeler et al.,

2010). The precursor of molybdenum oxides were therefore presumed to grow

topotactically along the (110) plane in reflection to orthorhombic molybdenum oxides.

Table 4.23: Crystallite size of M033 after in-situ XRD activation

Sample 2 d value FWHM Crystallite size (nm)

M033 (50 ºC) 12.116 7.2991 0.234 6.223

M033 (300 ºC) 9.589 9.2157 0.095 15.296

M033 (500 ºC) 27.203 3.2755 0.159 9.370

Table 4.23 shows the crystallite size of the catalyst precursor upon activation

progression. As can be seen, the highest intensity peak changes when the precursor is

heated. The crystallite size also changes drastically where the metastable hexagonal phase

having the largest crystallite size while the hexagonal phase obtained at 50 °C had the

smallest crystallite size according to the Scherrer equation. As the sample was heated,

sample hydration creates a structure directing effect, nanostructuring happens at the

metastable hexagonal phase and the crystal structure growth increases the crystallite size at

300 °C. According to the Wagner et al. (2004), at 500 °C, nanostructuring is lost because of

the o-MoO3 large crystal growth. However, as shown in Table 4.23, the crystallite size

decreases suggesting structural degradation of the large o-MoO3 catalyst which may be

attributed to the unidentified peaks.


121

The second experiment using MoOx precursor (M039) were conducted from 50 °C

to 500 °C at a heating rate of 5 °C/min and the diffractograms were obtained using a

Position-Sensitive Detector (PSD) at every 50 °C until 200 °C and every 20 °C from

200 °C until 500 °C. The XRD diffractograms of M039 under in-situ activation were shown

in Figure 4.48. The hexagonal Molybdenum Oxide phase obtained at 50 °C becomes more

crystallised as the catalyst precursor was heated to higher temperature. X-ray data obtained

for diffractogram at 240 °C matches Molybdenum Oxide, (MoO3) (PDF-File 21-0569)

corresponding to hexagonal phase same as M033 catalyst activation. This shows as the

temperature increases, all the ammonium ions were released to form metastable hexagonal

molybdates (h-MoO3) as can be seen from the thermal analysis.

Figure 4.48: In-situ XRD of MoOx precursor (M039) activation from


10 20 30 40 50 60

500C

480C

460C

440C

420C

400C

380C

360C

340C

320C

300C

280C

260C

240C

220C

200C

150C

100C

50C

Inte

ns

ity

(a

.u)

2

Hexagonal (PDF 21-0569)

Orthorhombic (PDF 89-5108)


122



9.591 48 100 9.2139 9.1201 Hexagonal

16.701 19 110 5.3040 5.2655 Hexagonal

19.333 26 200 4.5875 4.5601 Hexagonal

25.713 100 210 3.4619 3.4471 Hexagonal

29.264 46 300 3.0493 3.0400 Hexagonal

35.343 28 310 2.5376 2.5295 Hexagonal

43.024 12 320 2.1007 2.0923 Hexagonal

45.353 16 410 1.9981 1.9902 Hexagonal

46.522 14 404 1.9505 1.9438 Hexagonal

48.837 11 008 1.8634 1.8595 Hexagonal



9.671 14 - 9.1382 - -

12.403 26 020 7.1309 6.9275 Orthorhombic

23.138 47 110 3.8409 3.8093 Orthorhombic

25.074 24 040 3.5486 3.4638 Orthorhombic

27.159 100 021 3.2808 3.2644 Orthorhombic

33.029 12 101 2.7099 2.7046 Orthorhombic

33.603 21 111 2.6649 2.6545 Orthorhombic

38.108 16 131 2.3596 2.3338 Orthorhombic

45.564 14 200 1.9893 1.9810 Orthorhombic

49.250 19 002 1.8487 1.8505 Orthorhombic

55.150 12 112 1.6640 1.6645 Orthorhombic




123

Similar to M033, according to Table 4.24 the interplanar spacing of the catalyst was

larger compare to the reference material, thus having larger unit cell volume and decreasing

crystallite size (Ahmad and Bhattacharya, 2009). Heating the sample above 340 °C reduces

the intensity of the h-MoO3 peak at 26° plane (210). Continuous heating leads to the

formation of peak at 27° plane (021) and eventually the intensity increased with

temperature. Above 400 °C, The catalyst phase changes to the final thermodynamically

stable orthorhombic phase (o-MoO3). X-ray data at 500 °C (Table 4.25) shows the

orthorhombic phase matches Molybdite, syn (MoO3), (PDF-File 89-5108) same as M033

catalyst. The interplanar spacing as seen in Table 4.25 were also larger compared to the

reference material indicating larger unit cell volume and smaller crystallite size.



M039 (50 ºC) 25.690 3.4649 0.137 10.841

M039 (240 ºC) 25.708 3.4625 0.147 10.104

M039 (500 ºC) 27.160 3.2807 0.171 8.712

The crystallite sizes of the M039 catalyst during activation were calculated using

Scherrer equation (Table 4.26). The crystallite size decreases as the catalyst reaches its

thermodynamically stable o-MoO3 phase at 500 ºC. As the hexagonal structure morphology

of molybdenum oxides did not change at 240 ºC, the crystallite size is similar as the starting

material. However, similarly to M033 catalyst, at 500 ºC structural degradation occur thus

creating smaller crystal structure which may also be correlated to the unknown diffraction

peaks. This occurrence indicates that the bulk o-MoO3 catalyst were not stable thus forming

crystallized MoO2 at higher temperature (Dieterle et al., 2001; Ressler et al., 2000).


124

10 20 30 40 50 60 70 80 90

500C

450C

400C

350C

300C

250C

200C

150C

100C

50C

Inte

ns

ity

(a

.u)

2

Tetragonal(PDF 31-1437)

The third experiment were conducted for MoVOx precursor sample, where M038

were heated from 50 °C to 500 °C under helium at a heating rate of 5 °C/min and the

diffractograms were obtained using a Position-Sensitive Detector (PSD) at every 50 °C.

The XRD diffractograms of M038 under in-situ activation were shown in Figure 4.49. The

amorphous phase of M038 starts to change at 150 °C with the increase of the highest

reflection of the amorphous halo at 12° and 26° region. These broad amorphous peaks

intensities grew with the precursor‘s degree of crystallinity. The precursor therefore grew in

a perpendicular direction along (001) plane which is the reflection of nanocrystalline

Mo5O14 (Knobl et al., 2003).

Figure 4.49: In-situ XRD of MoVOx precursor (M038) activation from



125

These reflections continue to increase drastically until 400 °C. At this point, it is

presumed the water is removed leading to the vanadium expulsion into secondary structure

which corresponds to an intermediate amorphous form (Ilkenhans et al., 1995). At 400 °C,

the diffractogram has hump that is broad and this is presided to be the nanocrystalline phase

of Mo5O14 (Giebeler et al., 2010). Heating the precursor to 450 °C, a sharp reflection is

observed at 27° which was in good agreement with the domain growth in basal plane

(Zenkovets et al., 2007).

The final thermodynamically stable phase obtained is Vanadium Molybdenum

Oxide (V0.07Mo0.93)5 O14, PDF File 31-1437 corresponding to the tetragonal phase with unit

cell parameter a = 22.839 Å, b = 22.839 Å, and c = 3.99 Å. However unlike the ex-situ

XRD analysis during M038 calcination under helium, the prominent peaks of the

diffractogram obtained has as higher intensity suggesting a higher degree of crystallization.

This catalyst can be deduced to having a very high structural stability as temperature does

not affect the particles bulk structure which is Mo5O14 (Zenkovets, et al., 2007). The

interplanar spacing was larger indicating smaller refined crystallite size due to the

confinement of lattice deformations of V5+

five fold coordination (Dieterle et al., 2001)

According to the Scherrer equation which was calculated using the FWHM value

(Table 4.28), the crystallite size is smaller at the nanocrsystalline phase at 450 °C as

compared to the final tetragonal phase structure. This supports the previous argument that

the tetragonal phase growth starts from the nanocrystalline phase at 400 °C and at 500 °C

the crystallite size increases in accordance with particle growth. The crystallite size of the


126

nanocrystalline phase at 450 °C is also smaller compared to the MoOx samples showing

that Vanadium addition induces nanocrystallite particles.



16.458 20 330 5.3820 5.3832 Tetragonal

22.017 100 001 4.0340 3.9900 Tetragonal

23.336 28 600 3.8089 3.8065 Tetragonal

23.661 15 610 3.7573 3.7547 Tetragonal

24.920 49 540 3.5703 3.5669 Tetragonal

26.105 61 630 3.4107 3.4046 Tetragonal

27.574 18 550 3.2323 3.2299 Tetragonal

28.128 16 640 3.1699 3.1672 Tetragonal

28.395 18 - 3.1407 - -

31.515 34 740 2.8365 2.8328 Tetragonal

33.485 18 621 2.6740 2.6774 Tetragonal

36.829 19 721 2.4385 2.4662 Tetragonal



M038 (450 ºC) 26.099 3.4115 0.219 6.787

M038 (500 ºC) 22.009 4.0354 0.118 12.502




127

100 200 300 400 500

-16

-14

-12

-10

-8

-6

-4

-2

0

2

mW

Temperature/C

4.3.2 In-situ Differential Scanning Calorimetry (DSC) Analysis

Figure 4.50: In-situ DSC of M033 activation from 30 °C -500 °C

Figure 4.51: MS of In-situ DSC of M033 activation from 30 °C -500 °C

100 200 300 400 500

Temperature/C

m/e=17

m/e=19

m/e=18


128

In-situ DSC analysis was carried out for selected samples (M033 and M038) using

50 ml gas mixture of propane: oxygen: inert (Ar) of 46:2:2. The gases were mixed by the

mass flow controllers and flowed at 50 ml/min into the chamber as the samples were heated

from 30 °C to 500 °C at 5 °C/min. Figure 4.50 shows the in-situ DSC peaks for M033

during the propane ODH reaction. Four phases of catalyst structural transformation was

observed.

Comparing with the structural transformation trend using in-situ XRD, the first

phase was in the region of temperature less than 120 °C. DSC curve in Figure 4.50 shows

endothermic effect with water desorption (m/e=18,19) as shown in Mass evaluation in

Figure 4.51. The second phase at temperature range 120 °C to 250 °C also demonstrates

endothermic effect with the elimination of crystalline water (m/e=18,19). There is a rather

shallow exotherm observed in the third section of the DSC peak at temperature 250 °C to

375 °C. Here, the metastable hexagonal molybdenum oxide catalyst were oxidized and the

structural phase transition is irreversible (Gabbott, 2008; Werner et al., 1997).

However, no changes in MS were detected unlike the ex-situ DSC results in this

temperature range which eluded NH3, N2, and NO2. Ammonium (m/e=17) was only

detectable in the final phase transition of M033 at temperature of 375 °C to 500 °C. The

exothermic effect shows the crystallization to the final thermodynamically stable

orthorhombic phase of molybdenum oxides. Fragments of propene were not detectable

throughout the reaction suggesting either the reaction condition was not suitable or the

catalyst was not effective.


129

100 200 300 400 500

-40

-30

-20

-10

0

mW

Temperature/C

Figure 4.52: In-situ DSC of M038 activation from 30 °C -500 °C

Figure 4.53: MS of In-situ DSC of M038 activation from 30 °C -500 °C

100 200 300 400 500

Temperature/C

m/e=33

m/e=32

m/e=58

m/e=57

m/e=56

m/e=17

m/e=18


130

The same reaction condition was applied for catalyst precursor M038. Five regions

of intrinsic structural transformation were observed in Figure 4.52. A broad endothermic

effect was observed at temperature below 100 °C that correlates to the release of water

(m/e=18) which is shown in the mass evaluation in Figure 4.53. A small exothermic effect

was observed in the second temperature range of 100 °C – 350 °C. Fragments of

ammonium (m/e=17) and crystallized water (m/e=18) are eluded. However, fragments of

oxygenates (m/e=56,57,58) also exist indicating propane/propene oxidation process may

have occurred.

At the third temperature range of 350 °C to 400 °C, only water (m/e=18) was

detected from the MS which may have resulted as a side product of the oxidative

dehydrogenation reaction. This however did not have any effect in the catalyst structure as

shown in the DSC peak. An increase of lower hydrocarbon fragments (m/e=56,57,58) and

water (m/e=18) was observed from the MS at the temperature range of 400 °C to 450 °C.

At this section also, an exothermic peak was observed which supports the fact of

nanocrystalline phase growth. At the final temperature range of 450 °C to 500 °C, no

structural changes was observed while the MS continues to show that fragments of

hydrocarbon (m/e=57,58) and water (m/e=18) are released. Thus, this nanocrystalline phase

also is the most reactive.

Chapter 5, Conclusion _

131

CHAPTER 5

CONCLUSION


132

5.0 Conclusion

Propylene demand has increased drastically over the past years outpacing the

ethylene demand. Catalytic propane oxydehydrogenation (ODH) provides a better

alternative compared to the dehydrogenation process by reducing the reaction temperature

thus thermodynamically more favorable. The design of ODH reaction catalyst required

fundamental understanding of the structural-activity relationship to gain an insight of the

catalytic facilitated reaction mechanism. Molybdenum and vanadium based-catalyst system

are known to activate the C-H bond of propane which is the reaction mechanism rate

determining step hence making them the most suitable catalytic material for ODH reaction.

A variety of samples have been successfully synthesized for MoOx and MoVOx

based-catalyst via controlled precipitation method. For MoOx based-catalyst, the titration

parameters varied were temperature (30 ºC, 50 ºC), rate of addition (1 mL/min, 3 mL/min,

and 5 mL/min), molybdate solution concentration (0.07 M, 0.10 M and 0.14 M) and

precipitating agent (HNO3) concentration (1 M, 2 M, and 5 M). Temperature played a

major role in establishing the solution supersaturation forming the catalyst structure. At

30 ºC, precipitate formed exhibited supramolecular structure (Mo36O112) properties.

From the XRD analysis, high intensity peaks are observed at lower angle (7°) and

peaks are poorly resolved with low intensity at higher angle (10°-12°). The growths of the

catalytic structure were induced by protonation where Mo7O24 acts as a nucleus creating the

polyoxomolybdates. The bulk structural arrangement of the corner sharing pentagonal

channels of Mo36O112 distorted state gives rise to active lattice oxygens and may facilitate


133

the migration of lattice oxygen in the lattice, which is suitable for achieving high and stable

oxidation activity. This is achieved without applying heat to the preparation. At 50 ºC, no

spontaneous precipitation is observed and upon further heating to higher temperature,

supersaturation is reached and the precipitate displayed clear hexagonal phase structure

(h-MoOx), regardless of concentration used. The crystallite size calculated using the

Scherrer equation show evidence of the crystallite particles being nanostructured. The

properties of these catalysts were also analyzed using SEM and EDX analysis.

Thermal Analysis using TG-MS correlated with the DSC were also done. For the

supramolecular structure, endothermic effect was observed from as the catalyst restructures

to the metastable hexagonal molybdates (h-MoO3). At 430 °C, exothermic peak designating

crystallization is observed as the catalyst decomposes into the stable orthorhombic phase

(o-MoO3). For the hexagonal structure, the catalyst recrystallizes from the metastable

hexagonal phase into the thermodynamically stable orthorhombic MoO3 at 400 °C.

In binary oxides, the addition of vanadium promotes catalytic activity thus the need

to increase interactions of Mo and V species. In the synthesis of MoVOx based-catalyst,

amorphous phase was observed for all spray dried precursors. Highly crystalline hexagonal

phase (MoV2O8) and tetragonal phase [(MoV)5O14] were obtained after activation under air

and inert respectively when vanadyl was used as the vanadium source. Mo5O14 with the

tetragonal structure contains the pentagonal ring channels also. Vanadium behaves as

structural promoters thus stabilizing the Mo5O14 phase. The distortion of the dioxide phases

of Mo5O14 increases with increasing Vanadium content. When vanadates were used as the

vanadium source, different phases were observed with the increasing vanadium loading


134

where at 10% vanadium, mostly orthorhombic phase was observed form the XRD

diffractogram while at 70% vanadium loading, mixed phase of monoclinic and triclinic

were obtained. All the particles obtained had displayed nanocrystalline properties and had

been verified in the material characterization using SEM, EDX and XRF which also

showed homogeneous dispersion of the loading metals which provides the ‗site isolation‘

effect.

Thermal Analysis using TG-MS correlated with the DSC were also done for

MoVOx catalyst. For MoVOx with tetragonal structure (Mo5O14), endothermic effect were

observed until around 400 ºC where the amorphous phase transforms into the

thermodynamically stable tetragonal phase accompanied with an exothermic peak showing

crystallization process. However, for MoVOx with monoclinic structure (70% V),

crystallisation to the thermodynamically stable mixed phase of monoclinic and triclinic

phase happened only at 450 ºC.

Temperature programmed activation using in-situ XRD were used to study the

dynamics of structural transformation of selected synthesized MoOx bulk catalyst

precursor. The transformation for MoOx precursor took place from supramolecular to

metastable hexagonal phase at 275 ºC. The structure finally transforms to the stable

orthorhombic phase at 450 ºC. For MoVOx catalyst precursor, transformation was from

amorphous to nanocrystalline at 400 ºC. The crystal growth along the (001) plane indicates

the reflection of nanocrystalline Mo5O14. At 500 ºC, the thermodynamically stable

tetragonal phase was achieved with high degree of crystallization.


135

By incorporating the in-situ XRD data analysis, the reactivity of the catalyst was

also studied using a recently developed technique, in-situ DSC. For MoOx catalyst

precursor, fragments of propene were not observed thus no catalytic activity was observed

as the catalyst structural morphology transforms. However, the catalyst oxidation did take

place around 275 ºC. For MoVOx catalyst precursor, catalytic activity was observed at the

nanocrystalline phase region at 450 ºC as the catalyst structure transformed with

temperature. Fragments of olefins were also detectable. Hence, the most reactive region is

the nanocrystalline phase but the reaction mechanism remains unclear.

The reaction parameters also play a major role in obtaining the highest activity

besides the catalyst. In future works, parameters such as inert gas flow rate, hydrocarbon

and oxygen ratio can be varied. Moreover, by interfacing a Gas chromatograpy (GC) to the

in-situ XRD and in-situ DSC instrumental setup, the percentage of olefins eluded can be

measured giving a more cohesive activity comparison to the structural-activity relationship

of the catalyst.

study on structural transformation of molybdenum oxide catalyst using in-situ xrd technique for

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