kinetics of toulene disproportionation over unmodified and modified zsm-5 zeolites

7/29/2019 Kinetics of Toulene Disproportionation Over Unmodified and Modified ZSM-5 Zeolites

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Ind . Eng. Chem. Res . 1993,32,49-55

Kinetics of Toluene Disproportionation over Unmodified and ModifiedZSM-5 Zeolites

Maria A. Uguina,* Jos6 L. Sotelo, and David P. SerranoChemical Engineering Dep art men t, Faculty of Chemistry, Complutense Uniuersity,28040 Madrid, Spain

49

The kinet ics of to luene disproport ionat ion has been s tudied over both unmodif iedand Si-Mgmodified ZSM -5 catalysts usingan in tegral reactor a nd taking into account th e revers ioi l ity of thereaction. It has been found th at xylene dealkylation is the major secondary reaction, whereas toluenedealky lation can be co nsidered negligible.For th e unmodified ZSM-5, heterogeneous models basedin t he alkyl-transfer mechanism allowed the experimental resultsto be fit bet ter than first and secondpseudohomogeneous models, th ebest concordance being obtaine d when toluene adsorption isassumedas he rate-limiting step. T he corresponding kinetic equatio nhas been further appliedto the modifiedzeolite (SiM g/ZSM -5), leadingto th e develop ment of a kinetic model which includes the effect ofthe toluene an d p-xylene int racrysta l l ine diffusion ra te an d th e presence of nonselective acid s i teson th e external zeolite surface. Th is model describes adequately th e selective formation of p-xyleneover modified ZSM-5, reproducing the exper imental produc t dis t r ibut ion with a n average re la tiveerror of 2.8%.

IntroductionToluene disprop ortionation is an ind ustrial process for

the prod uction of benzene and xylenes. Th e reaction canbe carried out over a variety of acid zeolites, such asmordenite, faujasite, and ZSM-5 (Aneke et al., 1979;Bhavikatti and P atwa rdhan , 1981; Meshram e tal., 1983;Beltrame etal., 1985). T he selective formatio n of p-xylene,the most valuable xylene isomer, is obtained w hen mod-ified Z SM-5 zeolites are usedas catalysts (Kaeding etal.,1981; Young et al., 1982; Me shram , 1987). Moreo ver,toluene disproportionation may also be a useful ca talytictest for characterizing the shape selectivity of ZSM-5samples since the kinetic diam eters of th e aromatic hy-drocarbons involved in the reaction are very close to theZSM-5 pore size.

Th e mechanism and kinetics of toluene disproportion-ation over unm odified and modified ZSM -5 zeolitesis stilla matter of disagreeme nt. T hus, three different mecha-nisms have been postulated in the literature (Gnep andGui snet, 1981; Dooley et al., 1990; Frae nkel, 1990): alkyltransfer, dissociative and diphenylalkane mechanisms,which are shown in Figure1. Likewise, with rega rdto th ekinetics of th e reaction over unmodified ZSM-5, Beltramee t al. (1985) proposed a second-order kinetic model as-suming the bimolecular surface reaction between two ad-sorbed toluene moleculesas rate-limiting step. However,Nayak and Riekert (1986) and Bhaskar and Do (1990)found a first-order depend ence of th e reaction rate on th etolue ne par tial pressure. Recen tly, Dooley et al. (1990),collecting da ta from d ifferent autho rs and zeolites, con-cluded that most of the results canbe adequa tely describedby a second-order rate expression obtained w ith the as-sumption of toluene inhibition.

Otherwise, it is known that ZSM-5 modification bytrea tme nt with a variety of agen ts allows enhanc eme nt ofits para selectivity in different reactions with aromatichydrocarbons as products: toluene disproportionation,xylene isomerization, toluene-methanol and toluene-eth-an01 alkylation s, etc. (K aedin g, 1977; Young e t al., 1982;Olson and Haag, 1984; Ashton e tal., 1986; Wang et al.,1989). Th e effect of th e m odifer agent was initially ex-plained by an increase of the diffusional con straints dueto its deposition within the zeolite channel system. Th us,the reaction over the modified ZSM-5 sam plesis controlledby the intracrystalline diffusion which leads to th e for-

*To whom correspondenceshould be addressed.

mation of a primary product (the product just outside th ezeolite channels) with a high proportion of th e pa ra isomerbecause of it s higher diffusivity with regard t o th e othertwo isomers. Th e observed decline in para selectivity with

the increase of th e space time is related to th e primaryproduct isomerization by reentry into the zeolite porestructure . According to this approa ch, several kineticmodels were developed, coupling th e diffusion and intrinsicreaction rates in orderto explain the relationship betweencatalytic activity and para selectivity over modified ZSM-5zeolites (We i, 1982; The odo rou a nd Wei, 1983; Do, 1985;Sundaresan an d H all, 1986).

During the last few years, several authors h ave pointedout t he role of th e exte rnal surface of th e ZSM-5 zeoliteas he cause of nonselective transformation s (Farcasiu andDegnan, 1988; Pa pa ratt o e t al., 1988; Wang e t al., 1989).In agreem ent with them, in sp ite of th e low percentage ofacid sites located on the externa l surface of t he zeolitecrystals, their catalytic effect hasto be taken into account

when com paring reactions with very different rates.In th ecase of tolu ene disprop ortionation over ZSM-5, Olson andHaag (1984) have reported th at the ratio between theintrinsic rate consta nts of the seco ndary xylene isomeri-zation an d the m ain reaction is over 7000. The n, the rateof both reactions is comparable if xylene isomerizationtakes place only on the external zeolite surface. Therefore,the high para selectivity of th e unmodified ZSM-5 samplesshould be rela ted with a seco nd role of th e mod ifer agent:the remova l, deactivation, or blockage of the nonselectiveexternal acid sites which avoids the primary productisomerization on the e xtern al zeolite surface. In this way,Fraenkel(1990 ) has developed a kinetic model for toluenedisproportionation over modified ZSM -5 which includesthe effects of th e diffusional limitations and th e externalacid sites, the math ema tic treatment being simplified byassuming tha t the reac tion is irreversible with a pseudo-fmt -ord er rate. However, since toluene disproportionationis known to be reversible, such a kinetic m odel can onlybe applied to a differential reactor operatingat low toluen econversions.

Th e main goal of this work is to stud y the kinetics oftoluene dispropo rtionation over Z SM-5 zeolites, developinga kinetic model which include s the reversibility of th ereaction; hence it canbe used a t high toluene conversions.The reby, the work has been divided into two sections:(i)stud y of th e kinetics of toluene disproportionation overunmo dified Z SM-5 in absence of diffusional effects; (ii)development of a kinetic model for toluene dispropor-

0~88-5885/93/2632-0049$04.o0/00

1993 American C hemical Society


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50 Ind. Eng. Chem. Res., Vol. 32,No. 1, 1993

Figure

+6 Z HC H36 Z H

M3CH $'acH2Q-I 4:;CH 3

1. Mechanism of toluene disproportionation over zeolites: alkyl-transfer ( M l) ,dissociative (M2 ), and diphenylalkane (M3)mechanisms.

tionation over a ZSM-5 sample modified with Si and Mgcompounds, coupling the expression of th e intrinsic reac-tion rate, obtained in the former section, with the r ate ofthe intracrystalline diffusional process and taking intoaccount the role of th e extern al acid sites.

Experimental SectionCatalyst Preparation. ZSM -5 zeolite was synthesized

using ethanol as a template according to the proceduredescribed elsewhere (Costa et al., 1987) with a Si02/A 1203molar ratio of 58. T he natur e and c rystallinity (100% ) ofthe product was determined by X-ray diffraction ( XR D) .Th e acid form was obtained from th e as-synthesized so-dium zeolite by ion exchange with a n HC1 aqueous solutionfollowed by calcination at 550 "C for 14 h. Th e zeolite waspelletized using 35 w t % sodium montmorillonite asbinder. Th e effect of the activation and pelletization stepswere studied in an earlier work (Uguina et al., 1991).

The zeolite modification was carried out first by im-pregnation with an organic solution of a dim ethylsiliconepolymer (M ERCK , GE SE-30) and subsequently with a naqueous solution of tetrahydrate magnesium acetate,yielding a catalyst with 2.5 and 1.08wt 90 silicon andmagnesium, respectively. Th e details of th e ZSM -5 mod-ification with both agents have been earlier reported(Uguina et al., 1992).

Kinetic Measurements. Catalytic experiments werecarried ou t in a fixed bed, continuous down flow reactora t atmospheric pressure. Before toluene or p-xylene (pu-

rity > 99.5%) were fed, the catalysts were treated in anitrogen stream a t 500"C for 1 h. Th en, the reactant wasintroduced using a syringe pump, and the gaseous andliquid products were collected for1 h after the system hadreached steady state. In previous experiments, carried outby varying the time on stream, it was checked tha t th ecatalyst deactivation during the time of the kinetic mea-surem ents could be ruled out. Th e reaction product wasanalyzed by GC using 5%159'0 SP- 1200fb entone 34 on aSupelcoport column for liquids and a Porap akQ columnfor gases.

Results and Discussion(i ) Toluene Disproportionation over Unmodified

ZSM-5. Mass-Transfer Limitations. T he effect of the

mass-transfer limitations was tested in two series of pre-vious experiments, carried out by varying the toluene feedrate and t he m acroparticle size of th e pellets, respectively.It was found that the mass transfer between the bulkgaseous phase and the external surface of the catalystparticles does not affect the ove rall process rate. Likewise,it was found th at the resistanceto the m acropore diffusionthrough the binder m atrixis negligible in the macroparticlesize range used i n this work (0.39-1.55 mm ).

Otherwise, the effect of t he intracrystalline diffusionallimitations on the toluene disproportionation rate has beenestimated by calculating the observable Weisz mo dulus(a)and the effectiveness factor ( q ) in the way shown in aprevious work (Uguina et al., 1991). From the value of7N 0.95 obtained, it is concluded tha t th e intracrystallinediffusion slightly influences th e ra te of the main reactionand can be neglected in the study of the kinetics andmechanism over the unmodified ZSM-5. However, in theexperim ents carried out a t low toluene conversions, a paraselectivity (p-xylene percentage in t he total of xylenes)higher tha n th e thermodynamic equilibrium value (24%)has been obtained. This result shows the influence of theintracrystalline diffusion on the secondary reaction ofxylene isomerization even in the unmodified zeolite. Inorder to avoid including the effect of th e diffusiona l processin the first section of th is work, the three xylene isomershave been grouped and consideredas a single component(xylene).

Reactions and Mechanism. Th e toluene conversionand benzenefxylene molar ratio obtained in a series ofexperiments conducted over th e unmodified ZSM-5 at 475"C and different space times are depicted in Figure 2.Asexpected, toluene conversion increases with the space time,but the benzenefxylene molar ratio deviates from1 an da m aximum is observed in the xylene yield (not shown inFigure 2). This excess of benzene with regard to thestoichiometry of the rea ction and th e presence of differentgaseous hydrocarbons in th e effluent (see product distri-bution in T able I ) reveal tha t secondary reactions of aro-matic dealkylation tak e place on the catalyst in additionto toluene disproportionation. T he maximum in the xyleneyield could be due to xylene disproportionation, but it m ustbe ruled out since trimethylbenzenes are prese nt a t verylow concentrations. Th e production of gaseous hydro-


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Ind. Eng. Chem. Res.,Vol. 32, No. 1, 1993 51

c ( V O )

20

10

0

0 20 4 0 60

W/ 5, (4. h /mol)

01 X

1. 2

10

0.8

Figure 2. Toluene disproportionation over unmodified ZSM-5(T= 475 "C): ( 0) oluene conversion (C); (0 ) enzene/xylene molarratio ( B I X ) .

Table I . Toluene and p-Xylen e Conversion over theUnmodified ZSM-5

feed toluene p-xyleneOperation Conditions

tem perat ure ("C) 475 47 5space time (gh/ mo l) 70.7 8.1

methane 0.5 0.6ethane 0.2 0.5

propane 0.2 1.3propene 0.3benzene 14.2 0.3toluene 72.9 10.6

p-xylene 3.7 72.1m-xylene 5.5 9.2

Product Distribution (molW )

ethyle ne 0.5 2.0

ethylbenzene 0.1

o-xylene 2.1 2.5ethyltoluene 0.1 0.2trimethylbenzene 0.1 0.3

carbons has been also observed by other authors, being

usually assignedto toluene dealkylation, coupledin parallelwith the m ain reaction (Aneke e t al., 1979; Bhaska r a ndDo, 1990). However, in our experiments, the benzene/xylene ratio approach es1 a t low toluen e conversions; i.e.,selectivity toward toluene disproportionation tends toloo% , which suggests a relationship between the presenceof xylene and th e extension of dealk ylation.

In order to clarify this point, additional catalytic runswere carried out at the same tem perature (475"C) ndseveral space times feeding pure p-xylene. A typicalproduct composition has been summarized in Table I,whereas Figure 3 shows p-xylene conversion and theproduct distribution versus the space time. It can be ob-served th at p-xylene undergoes isomerization to given-and o-xylene but, a t the same time, a high proportion of

toluene and gaseous hydrocarbons are produced whichshows that p-xylene is dealkylated to a great extent.Likewise, it is remarkab le the low benzene c oncentrationeven at high conversions. Since some benzene has to beproduced by toluene disproportionation, itcan be proposedtha t toluene dealkylationis negligible. Th is conclusion hasalso been supported by furth er kinetic analysis: if toluenedealkylation is included, a negative and low valueis ob-tained for the corresponding kinetic constant. Therefore,two reactions have been co nsidered in the kin etic studyof toluene disproportionation over th e unmo difiedZSM-5toluene disproportionation 2T ?r? B + Xxylene dealkylation X -+ T + GH

i /-(%) 20 t / Ilo

0 2 4 6 a

WIFpx, (g .h /md)

Figure 3. p-Xylene conversion over unmodified ZSM-5(T = 475OC): ( 0 ) -xylene conversion;( 0 ) m- + o-xylenes; (A) oluene; (0 )benzene.

where T, B, X, and GH d enote toluene, benzene, xylene,and gaseous hydroc arbons, respectively.

Figure 1 hows the three different m echanism s proposedfor toluene disproportionation over zeolites. Th e disso-ciative mechanism (M2) involves in the first step thetoluene adsorption on an acid site which leads to the

formation of a methyl carbocation(CH,+). his speciesis known by its high instability an d hence in man y casesshould lead to t he formation of gaseous hydrocarbons in-stead of the su bsequent alkylation step. Then , toluenedealkylation should be also observed. Since we haveconcluded above tha t this reaction does not happen , thedissociative mechanism ha s to be discarded. Otherwise,in the diphenylalkane m echanism, the form ation of thisvoluminous intermed iate may be hindered by a restrictedtrans ition- state selectivity because of th e ZSM-5 struct urewithout large cavities an d with narrow 10-ring channels.Furtherm ore, the different kinetic equations based on thismodel which have been proposed earlier (Gnep andGuisnet, 1981; Dooley e t al., 1990) include th e hydrogenpartia l pressure , species involved in two steps of th eM3

mechanism. Taking into account th at our experimentshave been carried o ut in th e absen ce of hyd rogen, we haveconsidered the alkyl-transfer mechanism (M l) more likelyfor toluene dispropor tionation over ZSM-5 under our op-eratin g conditions.

Kinetics and Models. Since the experimental da tahave been obtained in an integral reactor, th e differentkinetic models tested include the ra te of the reverse re-action in agre emen t with the reversible natu re of toluenedisproportionation. Likewise, in all cases, it has beensupposed th at the secondary xylene dealkylationis anirreversible reaction with ps eudo-first-or der kinetics.

Taking into accoun t the alkyl-transfer mechanism, threeLangmuir-Hinshelwood rate expressions have been derivedassuming th at each one of th e mechanism steps is rate-controlling:toluene adsorption on an acid site

surface reaction between two toluene molecules, one ad-sorbed an d the o ther in th e gaseous phase, to give xylene(adsorbed) and benzene (gaseous phase)


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52 Ind. Eng. Chem. Res., Vol.32, No. 1, 1993

xylene desor ption of th e acid site

Likewise, different pseudohomogeneous models havealso been tested correspond ing to first- and second-orderrates:

(4 )

The fit between the ex perimental data and t he kineticmodels was performed using a computer program whichcombines a fourth-order Runge-Kutta methodto integratethe rate expressions with a nonlinear regression methodbased on Marquar dt's algorithm to estim ate the kineticand adsorption constants. These parameters were calcu-lated by minimizing the objective function

TD = k D ( P T n l - P B ~ ~ P x ~ ~ / K D )

(5 )[(Xi)calc - ( x ~ ) o b s ~ ~F = C

l = l (XI)&'

with (XJcalcnd (Xl)obs eing the molar fraction of thedifferen t species, calculated and observed, respectively.

Th e average relative error(4 in the reproduction of theprodu ct distribution and the value of th e objective functionobtain ed with the different kinetic models tested have beensummarized in Table 11. It can be seen tha t the hetero-geneous models provide a much b ette r fit of the experi-mental data than the pseudohomogeneous models. Th ebest fit is achieved with t he expression rate deduced byassuming toluene adsorption as the rate-limiting step,although from a statis tical point of view it is difficult toestablish differences between this m odel and t ha t whichconsiders the rate controlled by the xylene desorption.Nevertheless, in the th ree heterogeneous models, a strongcorrelation between the toluene and xylene equilibriumadsorp tion co nstan ts were observed; i.e., infinite com bi-nations of them led to the same value of the objectivefunction. In order to obtain reliable values of th e param -

eters , the heterogen eous models were simplified by con-sidering tha t both constants are the same. This simpli-fication almost did no t change the fit to the experimentaldata, th e best correlation being again obtained with th emodel corresponding to toluene adsorptionas the ra te-limiting step. The values calculated for the parametersincluded in this model are

k D = 0.0047 mol/(g.h.atm )

k d = 0.0070 mol/(g h-atm )

KT = Kx = 9.72 atm-'

(ii) Para-Selective Toluene Disproportionation overSi-Mg Modified ZSM-5. Catalyst. T he catalyst used

in this section was a ZSM -5 zeolite modified by im preg-nation with silicon and magnesium compounds (SiM g/ZSM-5). The silicon modification was carried out usinga dim ethylsilicone polymeras precursor, which, dueto it slarge molecular size, cannot prenetrate into the zeolitechannel system, being deposited on th e external surfaceof the cry stah a nd d eactivating the nonselective acid sites.In the other hand , thesize and na ture of the precursorusedin the magnesium m odification allow this agen t to be lo-cated inside th e zeolite pore, leading to an enhancementof the diffusional limitations. The contributionsof bothmodifier agents are necessary to o btain an optimum re-lationship between p ara selectivity and catalytic activity,their contents in the catalyst having been chosen in aprevious work (Serrano,1990) by means of three factorial

Ta b l e 11. Toluene Disp ropor t iona t ion ove r Unmodi fi edZSM-5. Fi t t ing of the K ine t i c Mode ls

homogeneous models

nl n7 n, F (90)1 0 1 1.540 18.81 1 0 1.360 17.61 1 1 0.383 9.32 1 1 0.217 7.12 0 1 0.204 6.82 1 0 0.197 6.7

heterogeneous models F f (% )toluene adsorption 0.065 4.0surface reaction 0.075 4.3xylene desorption 0.072 4.2

t 0 - 1t

4 0

G o I 11 .20 100 200 300 4 00

W/F,, (g h/rnol)

Figure 4. Toluene disproportionation over modified SiMg/ZSM -5(2' = 510 OC): (0 ) oluene conversion(C) ;(0 ) enzene/xylene molarratio ( B I X ) ;(A ) ara selectivity (Ps ) .

designs of experiments which included the effect of theoperation conditions (space time, tem peratu re, and toluenepartial pressure).

Reactions and Mechanism. Th e kinetic runs over theSiMgf ZSM-5catalyst were carried ou t at three tempera-tures (470, 510, and 550"C) nd different space times.Figure 4 shows toluene conversion, benzenelxylene molarratio, and para selectivity versus space time for the ex-periments at 510"C. It can also be observed th at in th emodified catalyst th e ben zenelxylene molar ratio deviatesfrom 1 as the space time increases: the dealkylation re-action takes place only if there a re xylenes in th e reactionmixture. Th e high para selectivity exhibited by this cat-alyst due to the shape-selectivity enhan cem ent caused bythe S i and M g modifications can be noted. Likewise, thepara selectivity is increased a t lower space times, beingremarkablethat a value of 100% is obtaine d by space timeapproaching zero. This finding shows that p-xyleneis th eonly xylene isomer which can be detected just outside thechann el system of the modified zeolite. Th e inter nalxylene isomerization is completely controlled by th e in-

tracrystalline diffusion dueto the co ntraints caused by themodifier agents. Althoughm- nd o-x ylenes may b e ini-tially f orm ed on th e active sites, they diffuse very slowlythrough t he modified channel system, p-xylene being theunique isom er able to leave the zeolite. Ther efore, thefurther decline on para selectivity with the space timecanonly be explained by th e p-xylene isomerization over theacid sites still remaining on th e ex ternal surface of thezeolite even a fter th e Si an d Mg modifications, whereasthe isomerization by reentry into the channel systemcannot be considered. According to this, th e system isdescribed by t he following reactions:

overall toluene disproportionation to benzene andp-xylene, including in an unique ste p th e toluene conversioninto the initial product and the subsequen t transformation


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Ind. Eng. Chem. Res., Vol. 32,No. 1, 1993 53

of this into the prim ary product a s a consequence of thediffusion-isomerization coupling

( P D k2T e + P X

p-xylene dealkylation( r d ) c b

P X - T T G H

p-xylene isomerization

PX MX

The two first reactions take place mainly within thechannel system of th e zeolite; hence their kinetics havetobe described by taking into account th at th e intrinsic re-action rates can be influenced by the hindered toluene andp-xylene diffusion through the modified channel of theSiMg/ZSM-5. However,as t has been concluded, p-xyleneisomerization is carried ou t on the externa l zeolite surfacewhich implies th at its rate is not altered by the diffusionalsteps.

Kinetic Model. In agreement with the mechanism andreactions proposed, the rates of tolue ne disappear ance andbenzene and p-xylene formation are given by

(7)

where the observed reaction rates are related with th eintrinsic rates by mea ns of the corr espon dingeffectivenessfactors:

(rD)obs = ?DrD (9 )

(rd)oba= vdrd (10)Th e intrinsic toluene disproportionation rate has been

described using kinetic equation1 obtained in th e firstsection of this work, whereas p-xylene dealky lation andisomerization hav e been co nsider ed irreversible first-orderreactions. Although xylene isomerization isknown to bereversible, the form er assum ption is valid for our experi-men ts over SiMg/ZS M-5 because inall experiments verylow conce ntrations ofm- nd o-xylenes were obtained ,asa consequence of t he high par a selectivity observed forthiscatalyst.

The effectiveness factors can be related with the dif-ferent kinetic parame ters and reaction conditions usingthe generalized expression proposed by Froment andBischoff (1979):

where co may be assumed to be zero (irreversible reactions)or the equilibrium concentration (reversible reactions) inthe presence of strong diffusional limitations. If eq11 isapplied to the intrinsic rate expressions of toluene dis-proportionation and p-xylene dealkylation, a mathem aticmodel is obtained which allows one to calculate the cor-responding effectiveness factors. Th is model has beensummarized in the Appendix.

In order to avoid including an excessive num ber of pa-rameters, th e kinetic m odel has been simplified assuming

Table 111. Para S elective Toluene Disproportionation overSiMg/ZSM-5. Parameters of the Kinetic Model

activationenergy

parameter preexponential factor (kJ/m ol)k D 4280 mol/(g.h-atm) 99

K T 1.45 X IO-* atm-' -54Deif 2.44 X IO-' m z / h 57kl 74.9 mol/ (gheatm ) 69

tha t th e diffusion coefficients of toluen e and p- xylene haveapproximately th e sam e value (DT,eff- DpX,eff), supp o-sition based in th e same minimu m m olecular diame ter ofboth compounds.

The fit between the model and the experimental datahave been performed using simultaneously the resultsobtained at the three different tempe ratures, the regressionand integration m ethods being the same as in section i.The influence of temperature was described using theArrhenius equation for the kinetic constants and thediffusion coefficient, whereas the van't Hoff equ ationwasused for the equilibrium adsorption constants. Th e valuesobtained for the different param eters of th e model aresummarized in Table 111.

The activation energy calculated for toluene dispro-portionation is close to the values reported by other au-thors also over ZSM-5 (Beltram e etal. (1985), 85 kJ /mo l;Bhaskar and Do (1990), 100-121 kJ /m ol) . Thi s fact showsth at th e kinetic model developed here has been able toisolate th e effects of th e chemical and phy sical steps onth e overall reaction rate. In con trast with the value re-ported by Olson and H aag (1984) for the rati okI/kD overunmodified ZS M-5 (-7000), from Tab le I11 values closeto 2 are obtained for th e Si and Mg modified sample. Itis concluded tha t bo th modifications have sharply reducedthe p-xylene isomerization rate, w hereas their effects onthe toluene disproportionation rate have no t beenso im -portant. Also remarkable is the high activation energyobta ined for p-xylene dealkylation, which shows how this

secondary reaction is very favored by th e temp eratureincrease. The relatively high activation energy calculatedfor the effective diffusion coefficient of toluene andp-xylene suggests that the diffusion of these compoundsthrough t he modified chann el system is a process stronglydepende nt on the temperatur e: the increase of this var-iable allows toluene an d p-xylene m olecules to have th enecessary energy to save the constraints imposed by themodifier agents. Finally, th e value obtained for theequilibrium adsorption constant has been checked tosatisfy the thermodynamic requirements postulated byMears and Boudart(1966) and Vannice et al. (1979).Partsa-c of Figure 5 show a comparison betw een the ex peri-mental toluene, benzene, and p-xylene molar fractions andthose calculated with th e kinetic model.A very good f itcan be observed with an average relative erro r of 2.8%.Thu s, the proposed kinetic model can describe the m ax-imum obtained in th e p-xylene molar fraction at 510 and550 "C with the increase of space time.

Conclusions

When toluene disproportionation is carried out overZSM-5 in the absenceof hydrogen, a benze ne/xy lene molarratio higher than 1 s observed in the product due to thesecon dary reaction of xylene dealkylation, toluene deal-kylation being negligible in the t em per atur e range studied.For the unmodifiedZSM-5,heterogeneous models basedon the alkyl-transfer m echanism allowed a much betterfitof th e experimental results than first- and second-order

kd 3.5 X IO7 mol/(gh.atm) 176


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54 Ind. Eng. Chem. Res., Vol. 32, No . 1, 1993

100

1

I c )

00 100 20 0 300 400 m

WIF,, ( g himol)

Figure 5 . Comparison between the observed(0 , A, 0 ) nd calcu-lated (-) prod uct distributions over modifiedSiMg/ZSM-5 toluene(a), benzene (b ), and p-xylene (c);( 0 ) 70 OC, (A ) 10 "C, and ( 0 )550 "C.

pseudohomogeneous m odels, the best concorda nce beingobtain ed with the ra te expression correspo ndingto tolueneadsorption as the rate-limiting step. When a ZSM-5modified with Si and Mg compoundsis used as the cata-lyst, p-xylene is th e only xylene isomer jus t outside t hechannel system; hence the further decline on para selec-tivity with th e space time has been assigned to p-xyleneisomerization over the acid sites still remaining on theexternal zeolite surface after both modifications. T heexpression rate obtained for toluene disproportionationover th e unmodified ZSM-5 has been applied to t he d ataof th e modified ZSM-5, taking into acc ount th e rates ofthe intracrys talline tolueneand p-xylene diffusion a nd t hepresence of nonselective extern al acid sites. T he kinetic

model developedfits

very well the ex perim ental productdistributionand can be applied at high toluene conversionssince the reversibility of toluene disproportionationistaken in to account. Th e relatively high activation energycalculated for toluene and p-xylene effective diffusioncoefficient shows th e great influence of th e tem pera tureon th e arom atic hydrocarbon diffusion through th e mod-ified channel system of ZSM-5.

Nomenclaturec = concentration, mol/&c , = concentration in the external surface of the particle,

c , = concentration in the center of th e particle, mol/m 3Deff= effective diffusion coefficient, m 2/ h

mol/m3

DT,eff, DyX,eff= toluene and p-xylene effective diffusion

k D = rate constant of toluene disproportionation, mol/(g

kd = rate c onstan t of xylene dealkylation, mo l/(gh.atm )k I = rate co nstant of p-xylene isomerization, m ol/(g h-at m)K D = thermodynamic eq uilibrium constan t of toluene dis-

KT, Kx , PX= equilibrium adsorption constants of toluene,

L = catalyst diffusional half-width, mPB, T, x, pX= partia l pressure of benzene, toluene, xylene,

and p-xylene, respectively, atmP T , ~ ,px:, = partial pressure of toluene and p-xylene corre-

sponding to the equilibrium composition of toluene dis-proportionation, atm

rD , rd , rI = intrinsic reaction rate s of toluene disproportion-ation, p-xylene dealkylation, and p-xylene isomerization,respectively, mol/ (g h)

(rD)obs,rd),bs = observed reaction rates of toluene dispro-portionation and p-xylene dealkylation, respectively,

rv = reaction rate referredto the catalyst volume, mol/ (m3.h)R = constant of the ideal gas law, atm m3 /(m obK )R g , R T , RPX = formation rate of benzene, toluene, and p-

xylene, respectively, mol/ (g h)

XB, T , Xpx = molar fraction of benzene, toluene, an d p-xylene, respectively

Greek Symbolst = average relative error4 = observable Weisz modulusq = effectiveness f actorq D , q d = effectiveness factors for toluene dispropo rtionation

pp = catalyst density, g/m3n = global pressure, a tmAppendix

Th e following assum ptions have been carried o uttoobtain th e expressions of the effectiveness factors from eq11:

(a) Deffdoes not depend on th e concentration.(b) Mole concentration (ci) has been related to the

partial pressure (Pi) sing th e ideal gas law.(c) There a re strong diffusional limitations hence th e

concentration in the center of th e zeolite crystal(c,) isassumed to be the equilibrium concentration.

(d) The shape of the zeolite crystal is consideredspherical.

With these assumptionsand with the introduction of theintrinsic rate expressions, the integration of eq11 leadstoeffectiveness factor for toluene disproportionation

coefficient, respectively, m 2/ h

hsatm)

proportionation

xylene, and p-xylene, respectively, atm-'

mol/ (gh )

and p -xylene dea lkylation, respectively

( 2 ~ T, e f & D / ( RT p p ) ) ' ' 2(I) '2

(12)LrDD =

being

I, =

B p~I, = [- In ( A P T 2 + BP T + C) - I 3 ] (15)

2A 2A Pr..


7/7

Ind. Eng. Chem. Res., Vol. 32, No. 1, 1993 55

Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis andDesign; Wiley: New York, 1979.

Gnep, N. S.; Guisnet, M. Toluene Disproportionation overMordenites-11 Kinetic Stu dy. Appl. Catal. 1981, 1 , 329-342.

Kaedi ng, W. W. DisDroDortionation of Toluen e. U.S. Pa ten t

(19)

effectiveness fa ctor for p-xylene dealkylation

Registry No. PhCH ,, 108-88-3; CH3CsH 4-p-CH3,106-42-3;

CH3CBH4-rn-CH3,08-383; CH3C6H4-o-CH3, 547-6; ethyltoluene,25550-14-5; trimethylbenzene, 25551-13-7; magnesium acetate,

CHI, 74-82-8; CZHB,74-84-0; CH zn CH z, 74-85-1; C3H8, 74-98-6;CHZ=CHCH,, 115-07-1; P h H , 71-43-2; E tP h, 100-41-4;

142-72-3.

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Received fo r review April 20, 1992Revised manuscript received August 28, 1992

Accepted Sep tember 30, 1992

1875-1880.

kinetics of toulene disproportionation over unmodified and modified zsm-5 zeolites

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