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I n d . Eng. C h e m . R e s . 1992,31,187&1880 1875Yang, . M. Study on the Synthesis of Allyl PolybromophenylEtherby Phase Transfer Catalytic Reaction. Ph.D. Thesis,Departmentof Chemical Engineering, National Taing Hue University, Hsin-chu, Taiwan, 1990.

Received fo r review November 20,1991Accepted May 8,1992

Triphaee Catalysis in a Batch Reactor. Znd. Eng. Chem. Res.1991a, 30,2384-2390.Wang, M. L.; Yang, H. M. Dynamica of Phase Transfer CatalyzedReaction for the Allylation of 2,4,6-TribromophenoL Chem. Eng.Sci. 1991b,46,619-627.Weber, W. P.; Gokel, G. W. Phase Transfer Catalysis in OrganicSynthesis; Springer Verlag: New York, 1977.

Magnesium and Silicon as ZSM-5Modifier Agents for Selective TolueneDisproportionation

Maria A. Uguina,* Jose L. Sotelo, David P. Serrano, and Rafael Van GriekenChemical Engineering Department, School of Ch emi s t ry , Co mp l u t em e Un i vers it y of Madrid , 28040 Madrid ,SpainMagnesium and silicon have been studied and compared as ZSM-5 modifier ag ents for selectivetoluene disproportionation. For both agents, para-selectivity corresponding to the primary productwas loo%,p-xylene being th e unique isomer able to leave the modified pore structure d ue to th ecoupled effects of the steric constraints, enhanced by the modifier agents, and the fast internalisomerization. T he fu rthe r decline of para-selectivity with conversion iscaused by the primary productisomerization on th e external acid sites. T he best para-selectivity/activity relationship obtainedwith th e silicon modification has been assigned to th e deposition of this agent on t he ex ternal zeolitesurface, an assumption that showed to be in good agreement with kinetic and adsorption-diffusionmeasurem ents. T he lower conversion exhibited by the m agnesium-modified samples has been relatedto higher diffusional limitations and a decrease in th e num ber of acid sites due to ion exchange withmagnesium species.

IntroductionToluene disproportionation in the vapor phase over avariet y of acid zeolites is a com mercial way to yield thediffere nt xylene isomers and benzene (Schriesheim ,1961;Oliver and Ion e, 1970;Bhavikatti and Patwardhan, 1981;Beltrame et al., 1985;Meshram et al., 1986;Chang et al.,1987). When the reaction iscarr ied ou t over ZSM-5 zeolite,the product distribution can be directed to the selective

forma tion of p-xylene, the moet valuable isomer, modifyingthe shape-selectivityof the zeolite. It isknown ha t ZSM-5modification by treatmen t w ith different agents and byme ans of several procedures leads to an enhan cement ofthe selectivity to p-xylene (Kaeding and B utter, 1975;Kaeding and Young, 1977;Kaeding e t al., 198la,b;Younge t al., 1982), obtaining a proportion of p-xylene ratherhigher tha n the thermodynamic equilibrium value (24mol% p-xylene).Olson and Haag (1984) ave proposed a reaction schemefor selective toluene disproportionation over modifiedZSM-5 zeolites. According with them , toluene dispro-portionation inside the zeolite s tructu re yields benzene anda xylene mixture known as initial product. The n, thisproduct diffuses out of the channel system but simulta-neously it undergoes isomerization reactions which takeplace within t he zeolite crystal,giving the primary product,the first observable outside the zeolite pore an d capableof being d etermined a t conversions approaching zero. Ap-xylene proportion in the prim ary product higher thanthe value initially obtained is expected since ita leastminimumdiameter allows a faster diffusion tha n th e othertwo isomers. Finally, the prim ary product undergoes asecond isomerization by reen try into th e cha nnel networkor over the acid sites located on the exte rnal zeolite surface,yielding the secondary product observed in the effluentof each experiment.*Towhom correspondence should be addressed.

In agreement with this mechanism, the para-selectivityenhancem ent and the decrease on toluene conversion ob-served in the modified ZSM-5 zeolites can be related withth e following roles of th e m odifier agent:(a) Pore blockage in t he ZSM-5 zeolite by depo sition ofthe m odifier agent. Tortuousity of the channel system isincreased, which delays the diffusion of the differentmolecules involved in th e reaction. This effect favors therelative p-xylene diffusion, para-selectivity in the primaryproduct being enhanced. Likewise, these steric constraintaprevent the isomerization of the primary product byreentry but may cause a decline on catalytic activity iftoluene diffusion is delayed.(b) Deposition or linkage of th e mod ifier agent to theunselective acid sites located on the external surface of th ezeolite crystals, an effect which avoids the extern al isom-erization of the primary product. Although the percentageof e xterna l acid sites is suppose to be very low, their c at-alytic effecthas o be taken into account since the intrinsicrate constant for isomerization is much higher than theone for disproportionation ( k I / k D 1 7000 according toOlson and Haag (1984)).Among the high numb er of elem ents and compoundswhich have been used asZSM-5 modifier agents (K aedinget al., 1981a,b;Young et al., 1982;Olson and Haag, 1984)in order to enhance ita para-selectivity in different reac-tions (xylene isomerization, toluene disproportionation,and toluene alkylation with methanol, ethanol, etc.), in th epresent work, magnesium and silicon have been selectedwith the aim of studying and comparing their effects oncatalytic and diffusional properties of ZSM-5 zeolite.Whereas magnesium is well-known as a conventionalmodifier agent of ZSM-5 zeolite (Chen e t al., 1979;Kaedinget al., 1981a,b; lson and Haag, 1984;Derewinski et al.,1984;Meahram, 1987), here is not much information aboutthe use of silicon polymers in orde r to enhance the zeoliteshape-selectivity (R odewald,1983). Recently, Wang et al.(1989) toluene alkylation with ethylene) and Handreck

088&5885/92/2631-1875$03.00/0 0 1992American Chemical Society

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1876 Ind . Eng. Chem . Res., Vol. 31, No. 8, 1992and Smith (1990) (n-hexane cracking) have studied theeffects of differe nt silicon compound s asZSM-5 modifieragents. Likewise, Ratn asam y and Pokh riyal (1989) forxylene isomerization have modified the ZSM-48 zeolite bysilylating with tetraeth yl orthosilicate. In all cases, thesilicon m odification is performed usinga silicon compoundwhich is deposited asa layer on the ex terna l surface of thezeolite crystals due to its large molecular size, leading tocatalysts with a sharp shape-selectivity enhancement.According to what has been stated above, a direct re-lationship between the catalytic behavior of the ZSM-5zeolite and its diffusion properties can be established.Therefore, the knowledge of the intracrystalline diffusivityof different molecules in the modified ZSM-5 samples mayprovide essential information to explain their catalyticproperties in selective toluene disproportionation.In the literature, several methods have been used todetermine adsorption-diffusion parameters in zeolites (M aand Ho, 1974; Shah and Ru thve n, 1977; Biilow et al., 1980;Kiirger et al., 1980; Marshall and Weisz, 1988). Thechromatographic method h as shown to be qu ite useful forcalculating equilibrium ad sorption constants, axial dis-persion coefficients, and macropore and micropore diffu-sivities. In this me thod , th e response of a packed columncontaining the adsorbent to a pulse of adsorbate is ob-tained as a chromatographic peak which is commonlyanalyzed by the m oment method: the first and secondmomenta of the response peak can be related with theadsorption-diffusion param eters using different mathe-matic m odels (Schneider and Sm ith, 1968, 1969; Suzuk iand S mith , 1971; Hayn es and Sarma , 1973; Chiang et al.,1984a; Boniface and Ruthv en, 1985).In the present work, in order to determine the diffu-sional limitations present in the magnesium- and silicon-modified ZSM-5 samples, the intracrystalliie diffusivitiesof two hydrocarbons (toluene and n-butane) have beenmeasured by applying the chromatographic method. Theadsorption-diffusion expe rimental data have been fittedto the mathematic model initially developed by Haynesand Sarm a (1973) and later discussed by R uthven (1984)for a packed bed containing a bidisperse porous ad sor ben t

p = & [ l + ( ? ) K p ]

whereK p =tp +(1- p)Kc (3)

Experimental SectionCatalyst Preparation. ZSM-5 eolite was preparedfrom a reaction mixture containing ethanol as templateaccording to the procedure described elsewhere (Costa etal., 1987). T he acid form was obtained by ion exchangeusing a 0.6 M HC1 solution at 25 OC for 4 h and thencalcined in static air a t 550 OC for 14 h. Before modifi-cation, HZSM-5 was bindered with 35 w t % Na-mont-morillonite. Th e product obtained was crushed and sievedto 16-32-mesh pellets an d finally calcined a t 550 OC for5 h. Th e effect of the d ifferent variables involved in theactivation and pelletization steps was studied in an earlierwork (Uguina et al., 1991).

Table I. Details of the Packed Bed and Column n theAdsorption Systemcolumnbed diameter 0.533 cmbed length 5.5 cmbed voidage 0.46adsorbentpellet voidage 0.38average pellet radiu s 0.375 mmaverage crystal radiu s 3.7 pmadsorbate toluene and n-butane

Magnesium Modification (Mg/ZSM-6). "he ZSM-6zeolites modified with magnesium were prepared by im-pregnation of the bindered samples with aqueous solutionsof magnesium acetate tetrahydrate a t 40 "C for 4 h. Afterfdtxation, the zeolite was dried a t 110 "C and the n calcineda t 550 "C for 16 h.Silicon Modification (Si/ZSM-6). ZSM-5 zeolitepellets were suspended in a boiling solution of a di-methylsilicon polymer (M erck, GE SE -30) in a toluene-xylene mixtur e (molar ratio =l / l) .After 1h, the pelletswere filtered, the y were dried a t 250 "C or 1h, and thenthe modification degree was increased by repeated inci-pient w etness im pregnation with a fresh silicon polymersolution. Finally, the sam ple was calcined a t 550 O C or16 h.Catalyst Characterization. The crystallinity of theZSM-5 zeolite (100%)was measured by X-ray diffraction(XRD ), using a ZSM-5 standa rd pattern. Th e averagecrystal size of thi s zeolite (7.4 pm) was evaluated from t hecrystal size distribution o btain ed by means of laser gran-ulome try using a CILAS 715 granulometer. Th e Si/Alratio (29) of th e ZSM-5 zeolite and th e magnesium con tentin the samples modified with this agent were determ inedby atom ic absorption spectroscopy whereas in the samplesmodified with silicon the m odifier content was measuredby therm ogravim etric analysis in air (Serrano, 1990).Activity Measurements. Toluene disproportionationwas performed in a downflow fixed bed reactor at at-mosp heric pressure. Toluene flow was delivered andcontrolled by a syringe pum p while the effluent from t hereactor wa s cooled and separated in gaseous and liquidstreams. Th e compositions of reactantsand products weredetermine d by gas chroma tography (GC ), using a 5%SP-1200/5% bentone 34 on Supelcopo rt column (3 m) anda Porapak Q column (3 m) for the liquid and gaseousproducts, respectively.The reaction was carried out at 475 "C. For the un-modified ZSM-5 zeolite the space velocity was varied be-tween 2.17 and 27.06 h-l. Th e magnesium- and silicon-modified catalysts were tested a t two different space ve-locities (WHSV = 2.17 an d 4.34 h-l). Previously, th enegligible catalyst aging at these operation conditions, evenafter several hours of time on stream, wa s checked.Adsorption-Diffusion Measurements. Th e adsorp-tion column packed with the ZSM-5 sample was kept atconstant tem perature, and flowing helium was used ascarrier gas. Th e injection of the adsorbate (toluene orn-butane) in the column inlet was done manually, and thecomposition in the column ou tlet was determined with atherma l conductivity detector, the response being recordedas a chromato graphic peak. Details of the column andadsorbent bed are given in Table I.Prior to each exp eriment the zeolite sam ple was evacu-ated a t 300 "C for 16 h in an helium flow and then cooledto the desired temperature for the adsorption-diffusionmeasu remen t. Several injections of adsorbate in thecolumn inlet were performed, varying the carrier flow, foreach adsorbe nt and obtaining the corresponding chroma-

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Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1877

2o120 40 60 80W / F ( g h/rnol)Figure 1. Toluene disproportionation over unmodified ZSM-5zeolite. Reaction param eters versus space time (T=475 "C).tographic peak in the column outlet. Th e first and thesecond mom ents of th e response were evaluated from thepeaks by numerical integration.The mathematic model used in this work is based indifferent assumptions which have been checked for eachadsorbate-adsorbent system. Thus , pressure drop acrossthe column was found to be negligible, adsorption waschecked to be reversible by repeating injections of theadsorbate, the linearity of the adsorption isotherm wa stested by decreasing the size of the pulse, and th e retentiontime of the pulse was corrected taking into account th epresence of dead space. Likewise, diffusion in microporeswas considered as he controlling rate step since in previousexperi men ts carried ou t with a column packed only withth e binder, witho ut zeolite, very sh ort retention times forboth adsorbates were found. Thisfinding ed us to simplifyeq 2 by assuming that the contributions of externaltransport and macropore diffusion to the overall resistanceare negligible. Th e fitting of the first an d the secondmoments obtained from the chromatographic peak atdifferent carrier flows by means of eqs 1-3 allows calcu-lation of th e adsorption equilibrium constant (K,)nd theintracrystalline diffusivity (D,) for each adsorbate-ad-sorbent system.Results and Discussion

From the hydrocarbon distribution obtained in eachexperiment t he following parameter s have been definedand calculated:toluene conversion (CT)

x 100ol of toluene convertedmol of toluene fedT =selectivity toward disproportionation (SD)

2 X mol of xylene obtaineds = x 100mol of toluene convertedpara-selectivity (Ps)

mol of p-xylene obtainedPs = x 100mol of xylene obtainedUnmodified ZSM-5 Zeolite. A series of toluene dis-proportionation experiments has been carried out over theunmodified ZS Md zeolite at 47 5 "C arying the spacetime. Figure 1 shows the reaction param eters obtainedversus the space time. It can be seen that, besides theexpected increase on to luene conversion, selectivity towarddisproportionation slowly falls with th e space time sinceseveral secondary reactions such as toluene and xylene

l o o

t6o t4o t

Si/ZSM- 5 I

I 1 I I t I I I I I I I I I0 10 20

c (%Figure 2. Relationship between para-selectivity and toluene con-version over the silicon- and magnesium-modified ZSM-5 samples(T=475 O C , WHSV =2.17h-l).dealkylation and xylene disproportionation take place,being responsible of th e presence of gaseous hydrocarbonsand trimethylbenzenes in th e effluent.The para-selectivity corresponding to the primaryproduct can be obtained a t space time approaching zero,which leads to a value close to 80%. Therefore, as t wa sassumed, p-xylene proportion in the p rimary prod uct israther higher than the thermodynam ic equilibrium value(24%), even for the unm odified ZSM-5 zeolite. Th e fur-ther isomerization of the product by reentry into the zeolitepore structu re or over th e acid sites located on the ex ternalsurface of the crystals explains the observed decrease ofpara-selectivity with the space time.Magnesium- and Silicon-ModifiedZSM-5 eolites.In order to determ ine and compare th e effects of magne-sium and silicon as modifier agents of th e ZSM -5 zeolite,several catalysts (modified in different degrees with theseelements) have been prepared. For Mg/ZSM-5 samplesthe magnesium content in the catalyst (0.4-0.9 wt W ) asfixed varying th e salt concentration in the im pregnationstep. Likewise, for Si/ZSM-5 samples th e modifier content(1.1-14 wt 96)was fixed varying the amount of polymeradded du ring the incipient w etness impregnation.Figure 2 shows the relationship between toluene con-version and para-selectivity obtained a t the same operationconditions with these catalysts. In both cases,the modifieragent has a remarkable effect on the para-selectivity whichundergoes an enhanceme nt to reach values close to 807'0,but their effects on catalytic activity are very different. Inthe catalystsmodified with m agnesium, toluene conversionsharply drops with the m odifier conten t whereas in thesilicon-modified zeolites, the modification has a weakereffect. Therefo re, for the same conversion, silicon-modifiedzeolites show a much higher para-selectivity than themagnesium-modified catalysts.The se results can be initially related with the differentroles playe d by the se modifier agents. In the siliconmodification, the agent must be deposited only on theexternal surface of th e ZSM -5 zeolite since the large m0-lecular size of the polymer used as precursor avoids itsaccess into the channel system. Therefore, the effect ofthe silicon modification is the deactivation of th e externalacid sites by deposition of a silica layer on th e zeolitecrystal, which explains why, at low m odification degrees,this agent affects mainly the para-selectivity whereas thecatalytic activity remains constant. However, for highersilicon con tent s, th e polymer depos ition likely leads to adecrease in the effective size of the entrances to thechannel system, which may limit or even prevent the accessof toluene to the internal acid sites, toluene conversionbeing decreased. By contrast, the size of the magnesium

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1878 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992Table XI. Kinetic Measurements (T=475 " C)

Darameter ZSM-5 Mg/ZSM -5 Si/ZSM-5Pso (%) 80 100 100&Jobs (mo l g-' h-' atm-') 0.0046 O.OOO9 0.0019(KI)ob.mol g-' h -' atm-') 0.1039 0.0143 0.0118(KI KD) b 22.6 15.9 6.2precursor allows its location either inside or outside thezeolite pore structure, which leads to an enhancem ent ofthe diffusive hindrance. Nevertheless, the natu re of themagnesium linkage to the zeolite is not still clear, andbesides a physical deposition, a pa rtial ion exchange be-tween MgZt in the solution an d Ht from th e zeolite mayhappen during the impregnation. Therefore, the sha rpdecrease of toluene conversion observed in th e magnesiummodification can be related with the enhanced diffusionallimitations a nd a decrease in the num ber of active sitesfor toluene disproportionation.Kinetic Measurements. From kinetic measurementscarried out a t the same temperature and two differentspace times for every catalyst, the following parametersrelated with th e mechanism of th e selective toluene dis-proportionation have been determined:(a) Para-selectivity corresponding to the primary prod-uct, Pso, calculated at space time ap proaching zero.(b) Observed toluene disproportionation rate, (rD),,bs,determined from the va riation of th e toluene mole fractionwith the space time:

(4)(c) Observed p-xylene isomerization ra te, (rI)obs,valu-ated from the variation of p-xylene mole fraction with th espace time, takinginto accou nt the p-xylene formation rateby disproportionation:

1 dXT( rD)obs =--2 d(W/FTJ

Likewise, th e kinetic con stants (KD),,~nd (K1Iobs anbe calculated from the rate values assuming first-orderkinetics for th e disproportionation and isomerization re-actions.The kinetic parameters have been determined from thereaction data obtained in experiments carried out at 475"C and two space times, usingunmodifiedZSM-5 (3.40 an d7.28 g h/mol) a nd Mg/ZSM-5 with wMg=0.9 wt % an dSi/ZSM-5 with wsi =7.3 w t % (21.23 an d 42.46 g h/mol).Th e da ta have been used to calculate Ps by space timeapproaching zero whereas the kinetic consta nts have beenevaluated from t he results obtained a t th e lowest spacetime with toluene conversions clearly below l o % ,whichallows assuming differential conditions and irreversiblereactions. Th e values of these parameters are summ arizedin Table 11.Bo th mo dified catalysts yield PS values close to 100%,hence p-xylene must be th e only xylene isomer present inthe primary product, just outside the zeolite channelsystem. Therefore, th e steric constraint enhancem entproduced by these two modifier agents drastically affectsthe diffusion of m- nd o-xylene and with th e fast internalisomerization is the cau se of th e para-selectivity ach ievedin the primary product. Furthermore, if p-xylene is theonly isomer able to leave the modified zeolite structure,the primary product isomerization by reentry must benegligible, the observed isomerization rat e being a mea-surement of p-xylene isomerization over the acid siteslocated on the external surface of the zeolite crystals.Likewise, if the steric constraints in these catalysts are still

Table 1x1. Adsorption-Diffusion Measurementsadsorbate adsorbent T " C) K, D, cm2/s)toluene ZSM-5 290 830 3.0 Xtoluene Mg/ZSM-5 290 2600 1.9 Xtoluene Si/ZSM-5 290 492 2.7 X271 3.9 X lo*-butane ZSM-5 180n-butane Mg/ZSM-5 180 304 2.1 X lo*n-butane Si/ZSM-5 180 236 2.8 X lo*

more enhanced by a more severe modification, tolueneand/or p-xylene diffusion may be affected leading to adecrease of cataly tic activity witho ut improving further thepara-selectivity.Otherwise, for both modifications th e disproportionationand isomerization kinetic constants are lower than thatCorresponding to the un modified ZSM-5 zeolite. It can benoted that th e silicon modification strongly affects (KI)obscompared with th e effect on (KD)o b , a finding which is ingood agreement with the location of the modifier agent onthe external surface of ZSM-5 zeolite deactivating theunselective acid sites. Th e decrease of the observed dis-proportionation rate m ay be explained by blocking of th eentrances to the channel system when a large am ount ofpolymer is deposited on the zeolite. In the magnesiummodification, although th e isomerization ra te is also veryreduced, the rate of the main reaction is rather lower tha nin the unmodified catalyst, a fact which agrees well withthe above-mentioned sharp decrease of toluene conversionwith the magnesium agent.T h e ( K I / K ~ I o batio, even for the unmodified ZSM-5zeolite, is by fa r lower than the value of 7000 reported byOlson and Haag (1984). Nevertheless, i t must be takeninto account that the value obtained by these authors isreferred to the intrinsic kinetic constants, without anyeffect due to the diffusionalsteps. Th e diffusive hindrancepresen t even in th e unm&ied zeolite has a stronger effecton the internal xylene isomerization rate than on thetoluene disproportionation ra te. Gilson and Derouane(1984) have proposed an equation to calculate the per-centage of external surface tetrahe dral sites asa functionof the average crystal size for the unmodified ZSM-5zeolite. Applying this equation to the ZSM-5 sample usedin thiiwork, th e estimate d percentage of external surfaceacid sites is about 0.02%,a value that m ust be drasticallyreduced when th e zeolite is modified with magnesium a ndparticularly with silicon. In spi te of th is low propo rtionof external acid sites, the observed isomerization kineticconstant for th e Si/ZSM-5 sam ple is still 6 times highertha n the kinetic constant of the m ain reaction. Th is factpoints out that the selective deactivation of external sur-face sites is a very impo rtant condition in order t o get ahigh para-selectivity in the secondary product.Adsorption-Diffusion Measurements. The chro-matographic method has been applied in th e present workin order to determine and compare the diffusional limi-tations present in the unmodified ZSM-5, Mg/ZSM-5 ( w ~=0.9 w t %), and Si/ZSM-5 (wsi =7.3 w t %) samples. Themeasurements have been carried out at different tem-peratures for each adsorbate, selected in order to achievesuitable retention times with a defined shape of thechromatographic peak. Th us, when toluene was used asadsorbate, a temperature of 290 "C had to be selected dueth e highly skewed peak obtained a t lower temperatures.In this way, Shah and Oey (1988) have reported th at themethod of momenta can be successfully applied even tohighly skewed chromatographic responses.Th e results obtained from these chromatographic mea-surements for the different adso rbatea dsorb ent systemsare summarized in Table 111.

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Ind. Eng. Chem. Res., Vol. 31,No. 8,1992 1879the presence of magnesium increases ita value. This effectismore remarkable when toluene is used as adsorbate andmay be related to he earlier mentioned dependen ce of theequilibrium adsoption constant on the ion form of theZSM-5 zeolite; i.e., th e presence of m agnesium seems toincrease the adsorption capacity of the ZSM-5 zeolite,which may be explained by the generation of new ad-sorption sites related to magnesium species.Conclusions

Th e main conclusions obtained from th e results of thiswork can be summarized as follows:Modifications of ZSM -5 zeolite with magn esium or sil-icon compounds leads to a high para-selectivity enhance-ment in toluene disproportionation, although catalyticactivity is decreased. Th e dro p in toluene conversion ismore remarkable for the Mg/ZSM-5 samples.Para-selectivity corresponding t o the primary p roductin Mg/ZSM-5 and Si/ZSM-5 catalysts was found to beclose to 100%. The n, p-xylene is the only xylene isomerable to leave the zeolite pore stru cture due to the stericconstraint enhancement caused by the modifier agentbesides th e high rate of the intern al xylene isomerization.Silicon mo dification, using a large polymer as precursor,deactivates selectively the ex ternal acid site s of ZSM -5zeolite by formation of a silica layer on th e external surfaceof the zeolite crystals.From adsorption-diffusion m easurements obtained bymeans of th e chromatographic method, it isobserved th attoluene and n-butan e diffusivities are drastically decreasedwith the magnesium modification, supporting that thediffusive hindrance enhancem ent an d th e decrease in th enumber of acid sites by ion exchange with M g2+are thecauses of th e observed toluene conversion dro p with themagnesium content in these samples.In th e silicon- and magnesium-modified ZSM-5 samples,th e key point t o obtain highly para-selective ca talysts isth e deactivation of th e unselective acid sites located on theexternal surface of the zeolite crystals.NomenclatureD,: zeolite (cry stal) diffusivityDL: xial dispersion coefficientD : macropore diffusivity(gD)ob:bserved kinetic constan t of toluene disp roportion-(KJObs:bserved kinetic constan t of p-xylene isomerizationk f : gas-to-pellet mass-transfer coefficientK,: adsorption equilibrium constant for the crystalKp: adsorption equilibrium constant for the pelletL: length of the chromato graph ic columnr,: zeolite crystal radiusR,: pellet radiusT: temperatureu : interstitial fluid velocityG r e e k Symbols6: void fraction of bedtp: void fraction of pelletp : first moment of the response peaku: second moment of the response peakp-xylene, 106-42-3.Literature CitedBeltrame, P.; Beltrame, P. L.; Carniti, P.; Forni, L. ; Zuretti, G.Toluene Disproportionation Catalysed by Various Zeolites.Zeolites 1986, 5, 400-405.Bhavikatti, S. S.;Patwardhm, S.R. Toluene Disproportionation overAluminum-Deficient and Metal-Loaded Mordenites. 1. Catalytic

ation

Registry No.Mg, 7439-95-4; i,7440-21-3;oluene, 108-88-3;

Table IV. Adsorption-Diffusion Data from the Litera ture onSilicalite and ZSM-5adsorbate adsorbent T "C) D , (cm2/s) research groupbenzene HSZM-5 40 2.5 X Doelle et al. (1981)p-xylene HSZM-5 40 0.2-6 X Le Van Mao et al.p-xylene HSZM-5 40 0.2-2.6 X lo-' Ragahi et al. (1984)benzene silicalite 200 1.2X W U et al. (1983)p-xylene silicalite 200 1.0 X Wu et al. (1983)

Gravimetric Method

(1983)

Chromatographic MethodDcadsorbate adsorbent T "C) K, (cm2/s) research group

benzene NaZSM-5 290 8900 4.9 X Forni et al. (1986)toluene NaZSM-5 290 9300 7.5 X Forni et al. (1986)n-butane silicalite 180 292 3.5 X Chiang et al.(1984b)In order to check th e accuracy of these va lues, in Tab leIV we have summarized several literature d ata a bout ad -sorption-diffusion of benzene, toluene, p-xylene, and n-bu tan e on silicalite an d ZSM-5 zeolite. These m easure-ments have been carried out by both gravimetric andchromatographic methods, and some values have beencalculated or extrapolated from th e da ta directly reportedby the authors.Comparing the dat a shown in Tables I11and IV, it canbe noted that for the unmodified ZSM-5 zeolite the in-tracrystalline diffusivity of toluene obtained in th e presentwork is very close to the values obtained by Forni andViscardi (1986) for benzene and toluene but their equi-librium adsorption constants are higher by an order ofmagnitude. Nevertheless, it must be take n into accounttha t the la tter values have been obtained with the sodiumform of the ZSM -5 zeolite and the se own auth ors foundrather lower retention times with the acid form, whichleads to a' lower value of the equilibrium adsorption con-sta nt for HZSM-5. Likewise, the equilibrium adsorptionconstant of n-butane in the unmodified ZSM-5 zeoliteobtained in this work agrees very well with t he value re-ported by Chiang et al. (1984b) on silicalite, lthough theirintracrystalline diffusivity is lower by 2 orders of magni-tude . Th is divergence between diffusivities on silicaliteand ZSM-5 has been already reported. Thu s, the diffu-sivity on silicalite app ears to be between 1 and 2 ordersof magnitude lower than that corresponding to ZSM-5zeolite. In spite of the disparity between th e gravimetricand chromatographic data recorded in the available lit-erature, it can be concluded t ha t th e values obtained inthe present work are consistent with those obtained byother auth ors using chromatographic methods and there-fore suitable to compare the effect of magnesium andsilicon modifications on the diffusional limitations throughthe ZSM-5 pore structure.From Table 111, it can be observed th at both modifica-

tions lead to a decrease of toluene an d n-bu tane diffusiv-ities as regards the unmodified zeolite, which is more re-markable for the magnesium-modified ZSM-5 sample.This fact confirms a higher blockage of th e chann el systemby magnesium species compared w ith th e silicon modifi-cation, in good agreement w ith the location of the siliconmodifier on the external surface of the zeolite and themagnesium either outside and within t he zeolite crystal.Nevertheless, the measured decrease of toluene and n-butane diffusivities in the Mg/ZSM-5 sample is not enoughto assign ita low cataly tic activity only to diffusional ca w sbu t again he reduction in the numb er of acid sites becauseof the ion exchange with h@+ has to be taken into account.Other interesting point is tha t, whereas the equilibriumadsorption constant is reduced by ad ding a silicon polymer,

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