usy,coking and deactivation ,paweewan,1999

8/13/2019 USY,Coking and Deactivation ,Paweewan,1999

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Applied Catalysis A: General 185 (1999) 259268

Coking and deactivation duringn-hexane cracking in

ultrastable zeolite Y

Boontham Paweewan, Patrick J. Barrie , Lynn F. GladdenDepartment of Chemical Engineering, University of Cambridge, Pembroke Street, Cambridge CB2 3RA, UK

Received 25 February 1999; received in revised form 5 May 1999; accepted 6 May 1999

Abstract

A multi-technique approach has been used to investigate coke formation and the effects of coke formation duringn-hexane

cracking on ultrastable zeolite Y. The product analysis reveals that propane and propene are the major products, and this

suggests that the reaction initiation step may be direct protonation at very strong Brnsted acid sites. The coke formed does

not change in composition significantly over the course of the reaction. Infrared spectroscopy reveals that the vast majority

of Brnsted and Lewis acid sites are still present in the catalyst, even when the catalyst activity has become low. Diffusion

measurements using pulsed field gradient (PFG) NMR show that the mobility of adsorbed n-butane or n-hexane is not affected

by the presence of the coke, and so shows that pore blockage is not a significant factor. Based on the experimental results,

it is argued that selectivesite poisoning of a few very strong acid sites is the main deactivation mechanism for this reaction

under the conditions employed. 1999 Elsevier Science B.V. All rights reserved.

Keywords:Coking; Deactivation; Cracking; Zeolite Y; Poisoning, selective site

1. Introduction

Catalytic cracking over zeolite-based catalysts is an

important reaction in the petrochemical industry. It

is generally accepted that the initial cracking activity

is principally due to the Brnsted acidity of the zeo-

lite, and the reaction mechanism has been investigated

extensively [14]. However, the reaction is a com-

plex one and results from one zeolite-reactant system

are not necessarily directly applicable to another. The

catalyst may have a range of different Brnsted acid

sites present with varying degrees of acidity, and the

Lewis acid sites present may also have an influence.

Characterisation of acid sites in zeolites has thus been

Corresponding author. Fax: +44-1223-334796.

studied using a wide variety of different techniques[510]. The combined influence of the range of differ-

ent strength acid sites and pore diffusion has also been

considered [11]. The situation is complicated by the

fact that catalyst deactivation takes place during the

cracking reaction. The major cause of deactivation un-

der laboratory conditions is the formation of carbona-

ceous residues known as coke within the catalyst. A

wide variety of analytical techniques have been used

to study both the formation of coke and its influence

on deactivation [1217]. Despite previous work, there

is still no generally accepted model for coke forma-

tion and, more importantly, for predicting the effects

of the coke on catalyst activity and selectivity [18].Thus, an important research topic is the identification

of the mechanism as to how coke causes deactiva-

0926-860X/99/$ see front matter 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 9 2 6 - 8 6 0 X ( 9 9 ) 0 0 1 4 3 - X


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260 B. Paweewan et al. / Applied Catalysis A: General 185 (1999) 259268

tion during a particular cracking reaction on a specific

zeolite.

While n-hexane and n-heptane cracking on zeo-

lite Y have previously been studied by several groups

of workers [13,17,1927], there remain differences

in interpretation of the coke formation and deacti-

vation mechanism for this reaction. Coke can affect

the catalytic activity of zeolites through both site poi-

soning and pore blocking mechanisms, and it is im-

portant to be able to distinguish between the two.

Further, the coke formed may not necessarily be dis-

tributed homogeneously throughout the zeolite parti-

cles. For instance, a recent paper found that a large

decrease in activity occurred even though only few

acid sites were poisoned, and it was proposed that the

reaction was diffusion limited [25]. For a diffusion-

limited reaction, n-hexane cracking and coke deposi-

tion will occur primarily near the external surface of

the zeolite crystallites, with little reaction at the par-

ticle interior. Other workers have suggested that theinitial deactivation for this reaction is based on site

poisoning [20,21], while others have found that the

coke limits access of the reactant to the active sites

[13,17,23].

In a previous paper, we adopted a multi-technique

approach to investigate coke formation and the deac-

tivation mechanism during the conversion of ethene

on ultrastable zeolite Y at 773 K [28]. It was found

that both site poisoning and pore blockage played

significant roles. In this paper, we report results

on n-hexane cracking in ultrastable zeolite Y using

the same range of techniques. The products were

analysed using gas chromatography, and the chem-ical nature of the coke formed, and its influence

on the number of acid sites and molecular trans-

port, were studied using infrared and NMR spec-

troscopic methods. The results shed new light on

the deactivation mechanism for this particular reac-

tion, and resolve some of the contradictions in the

literature.

2. Experimental

The sample of steamed zeolite Y, denoted HUSY,

used in this work came from the same batch as thatused in our study on coke formation during ethene

conversion on ultrastable zeolite Y [28]. The sample

has been well characterised previously [29] and has

a framework Si/Al ratio of 4.15 (based on unit cell

parameter) and a bulk Si/Al ratio of 2.65 from X-

ray fluorescence measurements. Coke was generated

by passing nitrogen at 50 ml/min through a satura-

tor containing n-hexane at 303 K, and then on to a

fixed bed reactor containing 500 mg of calcined cat-

alyst at 773 K. The products from the reaction were

analysed using a Phillips Pye 4500 gas chromato-

graph (GC) equipped with a flame ionisation detec-

tor and a Porapak Q column. After a specified time

on stream, the saturator was bypassed, and the re-

actor cooled down to room temperature under the

nitrogen atmosphere. Coked samples were obtained

in this way after 5, 10, 15, 25, 65, 125, 185 and

245 min of time on stream. The integrity of the coked

samples was checked by powder X-ray diffraction,

and the samples were then examined by a variety of

techniques.

One portion of the coked sample after 125 minon stream was checked for catalytic activity for n-

dodecane cracking. In this experiment, n-dodecane

was fed at a rate of 0.1 ml/min using a peristaltic

pump in a flow of nitrogen at 50 ml/min to the re-

actor at 773 K, and the light products were analysed

using a gas chromatograph as above. Negligible ther-

mal cracking was observed when the catalyst was

absent.

The BET surface area was obtained by analysing

nitrogen adsorption at 77 K in a conventional volu-

metric apparatus (Micromeritics ASAP 2000). Each

coked sample was pre-treated by heating to 673K

at 105

Torr for 12h in order to remove all wa-ter prior to starting the BET adsorption isotherm

experiment.

The weight percent of coke present in the samples

was estimated by thermogravimetric analysis (TGA).

The sample was heated to 973 K at a rate of 10 K/min

in an inert gas to remove water, and the weight loss at

that temperature when the gas was switched to air was

measured.

Infrared spectra were obtained on a Nicolet Magna

750 FT-IR spectrometer equipped with a Spectra Tech

0030-102 diffuse reflectance assembly. Each coked

sample was heated in a helium gas flow to 673 K to

remove water present in the pore space before record-ing the spectrum at elevated temperature. Infrared

spectra were also obtained on pyridine adsorbed within


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B. Paweewan et al. / Applied Catalysis A: General 185 (1999) 259268 261

the catalyst. For these measurements, the sample were

exposed to pyridine vapour at 573 K for 10 min, and

then left in a flow of helium gas at that temperature

for 5 min, before recording the spectrum at room tem-

perature. The spectra are plotted assuming the model

of Kubelka and Munk for diffuse reflectance [30],

with the KubelkaMunk function being analogous to

absorbance [31].13C solid state NMR spectra were obtained at

50.32 MHz on the coked samples using cross-

polarisation (CP), magic-angle spinning (MAS) and

high-power proton decoupling on a Bruker MSL-

200 spectrometer. Spectra were acquired overnight

using a contact time of 1 ms and a recycle delay

between scans of 1 s. The sample spinning rate was

4 kHz, and spinning sidebands were suppressed us-

ing the five -pulse TOSS method [32]. Chemical

shifts are quoted relative to external tetramethylsilane

(TMS).

Diffusion measurements on n-butane and n-hexaneadsorbed within the fresh zeolite and the coked sam-

ples were made using pulsed field gradient (PFG)

NMR spectroscopy [3335]. For the diffusion mea-

surements, eithern-butane orn-hexane were adsorbed

into each sample at a loading of approximately 10

molecules per unit cell. The 1H NMR spectra were

acquired at 200.13 MHz. After measuring the T1 and

T2 nuclear relaxation times to aid choice of the ac-

quisition parameters, diffusivities were measured us-

ing the PFGLED pulse sequence. This uses a stim-

ulated echo and minimises any errors due to eddy

current effects [36]. For single-component diffusion,

the signal intensity is expected to follow the formln(I/I0) =2g22D( /3)whereIis the observedintensity, I0 the intensity in the absence of gradient

pulses, the gyromagnetic ratio of1H,gis the applied

field gradient amplitude, is the length of the gradi-

ent pulse, the interval between the gradient pulses,

andD the diffusion coefficient [3436]. For measure-

ments on adsorbedn-butane, was varied while main-

tainingg at 1 T/m and at 10 ms. For measurements

on adsorbedn-hexane, was again varied, with g and

fixed at values of 2 T/m and 160 ms, respectively.

Due to the presence of both intercrystalline and in-

tracrystalline diffusion, it was necessary to analyse the

experimental data assuming the presence of two dis-tinct diffusion components using a least-squares fitting

routine.

Fig. 1. Plot of n-hexane conversion (circles, left axis) and coke

content (squares, right axis) as a function of time on stream.

3. Results and discussion

3.1. The reaction

A typical plot ofn-hexane conversion against time

on stream, together with the coke loading, is shown

in Fig. 1. It is known that significant deactivation oc-

curs during the first minute of time on stream [22]. It

can be seen that the activity continues to drop rapidly

during the first 60 min or so of the reaction, and the

conversion becomes close to zero after 180 min. The

line shown in Fig. 1 for the conversion uses the ex-

ponential decay equation X= X0exp(kdt), whereX is the conversion at time t, X0 conversion at zero

time,kdthe deactivation rate constant andtthe time on

stream.The product selectivities during the reaction are

shown in Fig. 2. The major products are the C3 hy-

drocarbons propene and propane (not resolved by the

gas chromatograph column used), with the other prod-

ucts being in the C1C4 range. During the first 60

minutes, an increasing proportion of the products are

C1 and C3 species, while the relative amount of C4

species decreases. The proportion of C2 species re-

mains roughly constant.

The generally accepted theory of the cracking of

alkanes on a solid with acid sites is that cracking pro-

ceeds via formation of a tricoordinated carbenium ion

as an intermediate, which then undergoes -scissionto form an alkene fragment and a carbenium ion frag-

ment [37,38]. However, it has been suggested that for-


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Fig. 2. Plot showing product selectivity as a function of time on

stream.

mation of primary carbenium ions is unlikely to oc-

cur, and that the intermediate may in fact be a pro-

tonated cyclopropane species [39,40]. Direct protona-

tion of hydrocarbons to form pentacoordinated carbo-

nium ions provides an alternative cracking mechanism

and this has been suggested to occur when very strongBrnsted acid sites are present at high temperatures;

this route is expected to be significant when the reac-

tant is difficult to crack [39,41]. A combination of the

mechanisms has been proposed: initial formation of

a pentacoordinated ion will be followed by protolytic

fission to give an alkene and an adsorbed carbenium

ion that will then react further [3,42]. Bimolecular re-

actions, involving hydrogen transfer between the reac-

tant and adsorbed carbenium ions, may also then occur

in a chain mechanism to produce a range of products

[3]. Coke may be formed within the catalyst from bi-

molecular interactions between adsorbed species [3].

The main area of uncertainty in the proposed reactionmechanisms is the initiation step in which the first re-

active ion is formed. Reaction at Lewis acid sites has

been proposed as an alternative initiation step for the

cracking mechanisms [4345], but other groups have

found that Lewis acid sites do not play a significant

role during the cracking of light hydrocarbons [46].

Thermal cracking has been suggested as a possible

initiation step during the cracking of heavy hydrocar-

bons [47].

The product distribution is shown in Fig. 2 and is

similar to that obtained recently in the literature under

conditions favouring monomolecular cracking [27]. It

is worth observing that -scission of the most stable

C6 carbenium ions that can be formed (in principle

at Lewis acid sites or by bimolecular hydrogen trans-

fer reactions) would give C2 and C4 species, and that

these are only observed in relatively small quantities

in the reaction under investigation. The observation of

C3 species as dominant products in Fig. 2 thus sug-

gests that the major mechanism occurring in this cat-

alyst is in fact direct protonation of one of the mid-

dle carbons of the n-hexane molecule at very strongBrnsted acid sites to form a non-classical pentacoor-

dinate carbonium ion. The protonation would then be

followed by fission into propane and a C3 carbenium

ion. The carbenium ion may react further, or lose a

proton to form propene. Direct protonation followed

by fissionis also a sourceof the C2 and C4 products, as

is the carbenium ion mechanism. It is, of course, pos-

sible that C4 species can react further in the catalyst

(e.g. to give C1 and C3 species). Thus, the increasing

selectivity towards methane as the reaction proceeds

is due to transformation of products rather than direct

protolytic cracking. These results are consistent with

some recent studies on n-hexane cracking in dealu-minated zeolite Y [24] and for n-hexane cracking in

ZSM-5 zeolite [4850].

The possibility that initiation of the reaction may

occur only at a few very strong active sites, which

is suggested by the observed product distribution, is

discussed further below when the deactivation mech-

anism is considered.

3.2. The chemical nature of the coke formed

The total amount of coke formed was determined

by thermogravimetric analysis and is shown in Fig. 1.It can be seen that it only takes a small amount of coke

to have a large effect on catalytic activity. It can also be


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Fig. 3. 13C MAS NMR spectra of coked samples after a time on stream of: (top) 25min; (middle) 125 min; (bottom) 185 min.

seen that coke continues to form even after 120 min re-

action time when the catalyst activity is low. However,

the amount of coke formed during n-hexane crack-

ing is significantly less than that formed during ethene

oligomerisation on the same catalyst under identical

conditions [28]. In order to probe the chemical iden-

tity of the coke formed, both 13C solid state NMR

spectroscopy and infrared spectroscopy were used.13C solid state NMR spectroscopy has been used

previously to characterise coke formed in zeolite cata-

lysts [28,5155]. In this work, we achieved high qual-

ity13 C spectra using the cross-polarisation (CP) tech-

nique at natural abundance without any chemical treat-

ment. Spectra are shown in Fig. 3. Two major spec-

tral regions can be identified: the asymmetric peak at

130 ppm is associated with aromatic carbon environ-

ments (though polyalkenes also resonate in this re-

gion), while the peaks at 1050 ppm are associatedwith saturated aliphatic carbon species. One frequently

used parameter in coal chemistry is the aromaticity,


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Fig. 6. FTIR measurements of adsorbed pyridine on HUSY after

a time on stream of: (a) 0 min (fresh catalyst); (b) 65 min; (c)

245min.

acid sites in the smaller sodalite cages, respectively

[62,63]. There may also be a contribution from extra-framework AlOH groups in this region. It can be

seen that there is no reduction in the number of acid

groups detected with increasing time on stream, even

after four hours reaction time. This indicates that the

majority of active centres are not removed during de-

activation by coking under the reaction conditions em-

ployed.

Some workers have reported the presence of an

OH stretching band at 3600 cm1 in zeolite Y af-ter chemical dealumination, or after steam dealumi-

nation followed by acid leaching, and this band has

been assigned to particularly strong Brnsted acid sites

[6469]. The intensity of this band has been foundto decrease during n-hexane cracking, and so it may

well be these sites that are removed during the cok-

ing reaction [68,69]. However, this band is only rarely

observed for steam dealuminated zeolite Y samples

(unless acid leaching is also performed), and it is not

resolved in the spectra shown in Fig. 5. No firm con-

clusion can therefore be drawn from our FTIR spectra

alone about whether any particularly strong acid sites

are affected by coking.

One method to quantify Brnsted and Lewis acid

sites is by using pyridine adsorption in conjunction

with infrared spectroscopy. FTIR spectra on pyridine

within the fresh and coked catalysts are shown in Fig.6. The band at 1546 cm1 is characteristic of pyridinebonded to a Brnsted acid site, while that at 1453 cm1

is characteristic of pyridine bonded to a Lewis acid

site [5]. Other bands in this spectral region are due

to other vibrational modes for pyridine which overlap

and so do not easily distinguish between the different

acid sites present. For the coked sample after 65 min

reaction time, there is no significant reduction in signal

intensity of either of the two characteristic bands, and

adsorbed pyridine can still easily be observed in the

FTIR spectra of the sample that had 245 min time on

stream.

The BET surface area of the coked samples was

found to remain high, being 610 m2/g for the fresh

catalyst, and 518 m2/g after 125 min on stream. How-

ever, the small size of the nitrogen molecule means

that the BET surface area is not necessarily a good

indicator of pore blockage or restricted diffusion for

larger molecules.

In order to shed more light on the deactivation

mechanism, self-diffusion measurements on adsor-

bates within the coked catalyst were performed usingPFG NMR to see whether the coke is influencing

molecular transport within the zeolite pore space.

n-Butane was used as a probe molecule, as was also

the case in our previous work [28], due to its diffu-

sivity and favourable NMR relaxation characteristics

making the PFG NMR experiment straightforward.

Results are shown in Fig. 7. For single-component

diffusion, the plot of ln(I/I0) against2g22( /3)

is expected to be linear with a gradient of D, where

D is the diffusion coefficient [3335]. However, the

log attenuation plots in Fig. 7 show a curve. This

Fig. 7. PFG NMR intensity results for n-butane adsorbed in zeolite

HUSY after a time on stream of: (a) 0 min (fresh catalyst); (b)

65min; (c) 185 min.


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is because the length-scale being probed by the

PFG NMR measurement, which is of the order of6D, is greater than the particle size (0.5 1m

for the sample used) [34,35]. Hence both interpar-

ticle and intraparticle effects are being observed.

The initial slope depends on the interparticle dif-

fusivity and the relative weighting of how long the

adsorbed molecule spends in each region, while the

final slope gives the intraparticle diffusivity only [35].

Thus, the curves may be fitted using a least-squares

fitting routine in order to obtain the intraparticle

diffusivity.

The slower diffusion components, corresponding to

intraparticle diffusivity, for the fresh sample and those

after 65 min and 185 min of reaction time were found

to be 2.6 109, 2.2 109 and 1.6 109 m2/s(all 0.2 109 m2/s), respectively. This is aboutthe same value, 2 109 m2/s, found by PFG NMRfor n-butane in zeolite NaX [70]. Thus, there is little

change in the diffusivity of n-butane, even after 4 hon stream. This may be contrasted with our previous

work on ethene oligomerisation which showed a sig-

nificant drop in n-butane intracrystalline diffusivity

with increasing time on stream [28].

It was also possible to measure the diffusion con-

stant of the reactant itself by performing PFG NMR

experiments on n-hexane adsorbed within the coked

zeolite. However, the NMR relaxation times of ad-

sorbed n-hexane are smaller than those for n-butane,

and this results in slightly more experimental scatter

in the data points. As was the case with the measure-

ments on n-butane, a two-component fit is necessary

to fit the experimental data as shown in Fig. 8. It can beseen that the signal attenuation does not depend on the

time on stream and fitting a single line to the data gives

the average intraparticle diffusivity ofn-hexane within

ultrastable zeolite Y to be 4.5 (0.5) 1012 m2/s.

3.4. Discussion of the deactivation mechanism

The infrared spectroscopic results clearly show that

removal of Brnsted and Lewis acid sites does not

occur to a significant extent, and thus that uniform site

poisoning is not responsible for the deactivation of ul-

trastable zeolite Y duringn-hexane cracking. Further,

it is clear that the deactivation is not due to the poreblockage mechanism, as the diffusion rate of the re-

actant through the bulk particle is unaffected by the

Fig. 8. PFG NMR intensity results for n-hexane adsorbed in zeolite

HUSY after a time on stream of 0 min (circles), 65 min (squares),

and 125 min (diamonds).

presence of coke. It should be noted that these obser-

vations are consistent with those of some other work-

ers [25]. It is also clear that the deactivation mech-anism during n-hexane cracking on ultrastable zeo-

lite Y is very different to the one that takes places

during ethene conversion, in which both uniform site

poisoning and pore blockage effects were observed

[28].

The analysis of the product distribution discussed

above indicates that the initiation step in the n-hexane

cracking reaction is direct protonation of the reac-

tant by a very strong Brnsted acid site. It is possi-

ble that only a relatively small number of such active

sites are present in ultrastable zeolite Y. For instance,

they may only be present at defects, or at Brnsted

sites where an adsorbed molecule can simultaneouslyinteract with a Lewis acid site. This leads to the pos-

sibility that selectivesite poisoning is the main deac-

tivation mechanism: the few highly active sites would

be removed, resulting in considerable loss of activ-

ity, but the majority of the acid sites would be unaf-

fected and diffusion within the pore space would also

be unaffected. Another possible explanation, as pro-

posed by Hopkins et al. [25], is that the reaction is

diffusion limited with pore mouth poisoning as the

main deactivation mechanism. In this case, virtually

all the reaction would take place near the external

surface of the crystallites, and only this region would

become coked. Again transport and the number ofactive sites present in the bulk solid would be barely

affected.


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In order to distinguish between the selective site poi-

soning and the pore mouth poisoning models, a sample

of coked zeolite Y (deactivated by 125 min on stream

for then-hexane cracking reaction) was tested for cat-

alytic activity in the cracking of n-dodecane. If pore

mouth poisoning had been responsible for the deacti-

vation, it would be expected that n-dodecane conver-

sion would be very low (other than a small amount of

thermal cracking) as the reaction would be more dif-

fusion limited than the n-hexane cracking case. On the

other hand, if selective site poisoning was responsi-

ble, it would be expected that the coked catalyst would

be highly active, as n-dodecane would be cracked

on relatively low strength acid sites compared to n-

hexane. It was found that the coked sample was highly

active, giving a conversion to light products com-

parable to a fresh catalyst sample. This observation

confirms that the selective site poisoning mechanism

must be the one responsible for deactivation during

n-hexane cracking.

4. Conclusions

Coking and deactivation during n-hexane cracking

on ultrastable zeolite Y has been studied using a va-

riety of techniques. The product distribution suggests

that the initiation step is direct protonation at very

strong Brnsted acid sites, and it may be the removal of

these that is the principal cause of deactivation for this

reaction. The coke formed has an aromaticity of about

7880%, and does not become highly polyaromaticeven after four hours on stream under the conditions

employed. It is found that the bulk of the acid sites

present in the catalyst are not removed, even though

the catalyst activity drops significantly, while diffusion

measurements show that pore blockage is not a fac-

tor. A selective site poisoning model for deactivation

under these conditions is thus proposed. It should be

noted that the deactivation mechanism is very different

to that observed during ethene conversion on the same

catalyst. This work highlights the need for a multi-

technique approach to the study of coking and deac-

tivation in zeolite catalysts. Caution is needed when

drawing conclusions on the deactivation mechanismfrom one reaction to another, and from one zeolite to

another, as the initiation step may be different.

Acknowledgements

We thank the Cambridge Overseas Trust and the

Cambridge Thai Foundation for funding Boontham

Paweewans studentship. We are grateful to Dr. Mick

Mantle and Dr. Sunil Ashtekar for their experimental

help with the NMR and the infrared measurements.

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