26
Ind. Eng. Chem. Res. 1994,33, 26-31
Deactivation Kinetics of Para-Selective Toluene Disproportionation over Modified ZSM-5 M a r i a A. Uguina, Jose L. Sotelo, David P. Serrano,* and Jose L. Valverde Chemical Engineering Department, Faculty of Chemistry, Complutense University, 28040 Madrid, Spain The deactivation kinetics of toluene disproportionation to yield a mixture of benzene and xylenes with a high para-selectivity has been studied using a Si-Mg modified ZSM-5 catalyst. The kinetic model developed includes the deactivation of the main and the secondary reactions and takes into account the influence of the intracrystalline diffusional steps. The best fit of the experimental data was obtained when toluene is assumed to be the coke precursor. The order of deactivation of the main reaction is close to 3, which agrees with the control of the overall reaction rate by the intracrystalline diffusion. The secondary reaction of p-xylene dealkylation is deactivated faster than the main reaction. However, the activity of the secondary reaction of p-xylene isomerization, which takes place only over the external zeolite surface, remains constant and close to unity. These differences among the deactivation rates have been related to the mechanism of deactivation and to the strength of the acid sites involved in each reaction. Introduction Toluene disproportionation is a commercial method used for the production of benzene and xylenes, usually carried out over acid zeolites (Bhavikatti and Patwardhan, 1981; Beltrame et al., 1985; Nayak and Riekert, 1986; Meshram et al., 1986). The use of the ZSM-5 zeolite as a catalyst is especially interesting due to its high resistance to deactivation by coking and to the possibility of directing the reaction toward the selective formation of the most valuable xylene isomer @-xylene) when the zeolite is modified with different agents (Kaeding et al., 1981;Olson and Haag, 1984; Meshram, 1987). In previous works (Wei, 1982;Do, 1985;Fraenkel, 1990; Uguina et al., 1992), the enhancement of para-selectivity observed in several reactions (toluene disproportionation, toluene alkylation with methanol and ethanol, etc.) carried out over modified ZSM-5 samples has been related to the coupling of the results of the two different roles played by the modifier agent: a decrease of the intracrystalline diffusion rate, which favors the presence of p-xylene in the primary product (the product just outside the zeolite channels), and a deactivation or blockage of the nonselective acid sites located on the external zeolite surface, which prevents the isomerization of the primary product toward m- and o-xylene. Zeolite deactivation during the conversion of organic compounds is mainly produced by coke deposition, two major modes of deactivation having been proposed: site coverageand pore blockage. In contrast with other zeolites also used as industrial catalysts, ZSM-5 undergoes a relatively slow deactivation which has been assigned to an effect of shape-selectivity: the absence of large cavities in the ZSM-5 pore structure avoids the formation of voluminous polyaromatic species which are precursors of the carbonaceous deposits (Guisnet and Magnoux, 1989).The low concentration of acid sites in the ZSM-5 zeolite has been also proposed as a secondary reason for its resistance to deactivation (Magnoux et al., 1987). In spite of the industrial use of zeolites as catalysts for toluene disproportionation to produce benzene and xylenes, only a few works can be found in the literature dealing with the kinetics of its deactivation. Meshram et al. (1983), assuming that toluene disproportionation over
* To whom correspondence should be addressed.
ZSM-5 follows first order kinetics, have described the variation of the corresponding kinetic constant with the time on stream by a simple exponential expression. Bharati and Bhatia (1987) have found that the toluene disproportionation deactivation over a mordenite type zeolite is well described by a heterogeneous model in which the coke formation takes place in parallel with the main reaction. However, no works have been found in the literature about the deactivation kinetics of toluene disproportionation over modified ZSM-5 zeolites. In this case, the deactivation of the secondary reaction of p-xylene isomerization should be included to explain the evolution of para-selectivity with the time on stream and the influence of the intracrystalline diffusion process should be taken into account. In an earlier work (Uguina et al., 19931,we have studied the kinetics of toluene disproportionation at zero time over both unmodified and Si-Mg modified ZSM-5 catalysts. The kinetic modei developed for the modified sample includes the presence of secondary reactions, the effect of the diffusional steps, and the influence of the nonselective acid sites located on the external surface of the zeolite crystals. Using this work as a reference, the main goal of the present paper is to complete the kinetic study taking into account the deactivation of the SiMg/ ZSM-5 catalyst by coke deposition. The main features of this deactivation kinetic model are (i) The main reaction of toluene disproportionation, as well as the deactivation of the secondary reactions of p-xylene dealkylation and isomerization, will be included in the kinetic analysis to describe not only the evolution of toluene conversion but also the evolution of paraselectivity and selectivity toward disproportionation with the time on stream. (ii) The effect of the diffusional steps on the deactivation kinetics will be determined taking into account the conclusions previously obtained by Khang and Levenspiel (1973) about the effect of the pore diffusion regime on the deactivation order for different deactivation mechanisms (parallel, series, side-by-side, and independent). Experimental Section Catalyst Preparation. ZSM-5 zeolite was prepared from a reaction mixture containing ethanol as a template
0888-5885/94/2633-0026$04.50/00 1994 American Chemical Society
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 27 according to the procedure previously reported (Costa et al., 1987). The crystallinity of this material (100%) was determined by X-ray diffraction (XRD) whereas its Si021 A1203 molar ratio (58) was measured by atomic absorption spectroscopy. The acid form of the zeolite was obtained by ion exchange with an HC1 aqueous solution followed by calcination at 550 "C for 14h. Sodium montmorillonite (35 wt %) was used as a binder to get 16-32-mesh pellets. The conditions used in both the activation and pelletization steps were selected in an earlier work (Uguina et al., 1991). The modification of the ZSM-5 zeolite was first performed by impregnation with an organic solution of a dimethylsilicone polymer (Merck, GE SE-30) and subsequently with an aqueous solution of tetrahydrate magnesium acetate accordingto the procedures earlier reported (Uguina et al., 1992). The modifier contents in the final catalysts were 2.5 and 1.08 w t % silicon and magnesium, respectively. Deactivation Kinetic Measurements. Toluene disproportionation was carried out in a downflow fixed bed reactor at atmospheric pressure. Before feeding toluene, the catalyst was treated in a nitrogen stream at 500 "C for 1 h. Then, the reactant was introduced using a syringe pump. Once the flow through the reactor had reached steady state, samples of the gaseous and liquid products were taken and analyzed at different times on stream. The composition of each sample was determined by GC using 5%/5% SP-1200lbentone 34 on a Supelcoport column for liquids and a Porapak Q column for gases. The details of the experimental apparatus have been reported in an earlier work (Uguina et al., 1991). The deactivation kinetic runs were carried out at three temperatures (470,510, and 550 "C) and different space times. For each temperature and space time, time on stream was varied in the range 1-10 h. Results and Discussion The catalyst used in this work is a ZSM-5 zeolite modified by impregnation with silicon and magnesium compounds (SiMgIZSM-5). The contribution of both modifier agents is necessary to decrease the diffusion rate and to deactivate the acid sites located on the external zeolite surface which allow one to obtain an optimum relationship between para-selectivity and catalytic activity. Their contents in the catalyst were selected in a previous work (Serrano, 1990). Mass-Transfer Limitations. The effects on the overall process rate of the external mass-transfer limitations and of the diffusion through the binder matrix were determined to be negligible in previous experiments (Uguina et al., 1993). In regard to the intracrystalline diffusion rate, its influence not only cannot be avoided but it is essential to get a high para-selectivity in the primary product of the reaction. Therefore, the deactivation kinetic model must include the effect of the intracrystalline diffusional steps. Effect of the Time on Stream. The following parameters have been defined and calculated from the product distribution of each experiment: toluene conversion (C),selectivity toward disproportionation (SD, determined as 2 X (mol xylene producedlmol toluene converted)) and para-selectivity (Ps, p-xylene proportion in the xylenes). In the previous kinetic study (Uguina et al., 19931, it was concluded that the system is adequately described by the following reactions.
t
t
Figure 1. Variation of the reaction parameters with the time on stream (T= 510 "C, W/FT, = 255 g h/mol): 0,toluene conversion (C); A,selectivity toward disproportionation (SD); 0,para-selectivity
(Pd.
Overall toluene disproportionation to benzene and p-xylene which includes in one step the toluene conversion into the primary product: (%)ob
2T s B + P X
p-Xylene dealkylation to yield toluene and gaseous hydrocarbons:
-
(Pd)Ob
PX
T+GH
p-Xylene isomerization. This reaction takes place only over the external zeolite surface, not being affected by the diffusional steps: n P X s MX
The variation of the reaction parameters with the time on stream for one of the deactivation experiments is shown in Figure 1. It is observed that the increase of the time on stream leads to a decrease of toluene conversionwhereas para-selectivity and especially selectivity toward disproportionation are enhanced. These results suggest that the catalyst deactivation by coke deposition does not influence the three reactions in the same manner, although the intrinsic relationship between toluene conversion and selectivities observed over the fresh catalyst should be taken into account (the higher the toluene conversion the lower the para-selectivity and selectivity toward disproportionation). Therefore, the selectivities obtained over the fresh and deactivated catalysts at 510 "C have been compared and depicted versus toluene conversion in parts a and b of Figure 2, using the time on stream as curve parameter. Whereas the relationship between paraselectivity and toluene conversion is very similar for the fresh and deactivated catalysts, selectivity toward disproportionation clearly increases with the time on stream when the catalysts are compared at the same level of conversion. Then, the secondary reaction of p-xylene dealkylation seems to be more affected by coke deposition than the main reaction. Deactivation Kinetic Model. The model is concerned with the deactivation kinetics of the main and the secondary reactions. For each reaction, the activity is defined as
with rj(t)and rj(0) being the rate of the reaction j a t any time t and at zero time, respectively. Both rates must be measured at the same composition of the reaction mixture.
28 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994
function has been related to the partial pressure of the species considered as the coke precursor (Pc) by the expression
a6 90
90
1
a)
J o
a
4
12
16
20
24
c (%) Figure 2. Relationship between selectivity and toluene conversion for the fresh and deactivated catalysts: selectivity toward dispro= portionation (a), and para-selectivity (b). 5" = 510 OC, W/FT, 85-425 g h/mol, t = 0 h, 0;4 h, A;and 10 h, 0.
r---
+
MX
OX
I
2 T
~
?
B
+
PX-COKE
COKE
T
c
+
GH
I
Figure 3. Coupling between the coke formation and the different reactions present in the system: coke from toluene (Cl) and coke from p-xylene (C2).
The variation of the activity with the time on stream has been described by the deactivation equation proposed by Corella and Asua (1981):
where dj is the order of deactivation and \kj(Pj,l') the deactivation function which includes the influence of the operation conditions. Khang and Levenspiel (1973) have performed a theoretical study about the influence of the diffusional steps on the deactivation of catalyst pellets. Accordingto them, the order of deactivation may change due to the diffusional control of the reaction rate. This effect depends on the mechanism of coupling between the poisoning and the main reaction: (i) Parallel deactivation. When coke is formed from one of the reactants, the order of deactivation depends on the Thiele modulus: as the diffusional resistance increases, the value of dj changes from 1 to 3. (ii) Series deactivation. The order of deactivation is insensitive to the magnitude of the Thiele modulus and close to unity if coke is produced from one of the reaction products. Taking into account these conclusions, we have developed two series of deactivation models, assuming that the coke precursor is toluene in one and p-xylene in the other (Figure 3). The other major species present (benzene) was not considered as a coke precursor since in experiments carried out feeding pure benzene to the reactor coke deposition on the catalyst was negligible. The deactivation
kjd being the rate constant of the deactivation reaction. In each series of deactivation models, the influence of the diffusional control has been included in the values used for the corresponding deactivation orders: (i) Toluene as coke precursor (reaction Cl). The reaction of coke formation is coupled in parallel with the main reaction of toluene disproportionation hence the corresponding order of deactivation (dD) must depend on the diffusional control, varying in the range 1-3. At the same time, coking is coupled in series with the secondary reaction of p-xylene dealkylation which should lead to an order of deactivation for this reaction almost constant and close to unity (dd = 1). (ii) p-Xylene as coke precursor (reaction C2). In this case, the coke formation takes place in series with the main reaction (dD is assumed to be 1)and in parallel with the secondary reaction of p-xylene dealkylation (dd depends on the diffusional control with values between 1 and 3). Since the secondary reaction of p-xylene isomerization takes place only over the external zeolite surface, ita deactivation must not be affected by the rate of the intracrystalline diffusion process. Therefore, a constant value of 1has been used for its order of deactivation ( d ~ ) in both series of deactivation models. It must be noted that most of the kinetics studies of deactivation reported in the literature have been performed applying the differential method; the activity at every time on stream, space time, and temperature being determined from the reaction rates by means of eq 1. This method has the disadvantage of the errors introduced in the calculated activities when the product distribution curves are differentiated. In contrast, we have used an integral method to compare the equations of the deactivation models with the experimental data. Thereby, two numerical integrations are carried out: one with respect to the time on stream to obtain the activities and other with respect to the space time to determine the molar fraction of each component. The fit between the experimental data and the deactivation models has been performed using a computer program which combines a fourth-order Runge-Kutta method to carry out the numerical integrations with a nonlinear regression method based on Marquardt's algorithm to estimate the different parameters of the model (Marquardt, 1963). These parameters were calculated by minimizing the objective function: N
F = C
- (xi)o)J2
(4)
(Xi)ob2 The equations which form the two series of deactivation kinetic models proposed, as well as the block diagram of the computer program used for the calculations, have been summarized in the Appendix. The comparison between the experimental and the predicted data has been performed using simultaneously all the results obtained at the different temperatures and space times investigated. The influence of temperature on the deactivation kinetic constants has been described using the Arrhenius equation. The discrimination among the models has been based on the values of the objective function, the average relative error in the reproduction of the experimental product P I
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 29 Table 1. Fitting of the Deactivation Kinetic Models model
coke Precursor
dn
dd
dr
T1 T2 T3 PX1 PX2 PX3
toluene toluene toluene p-xylene p-xylene p-xylene
1 2 3 1 1 1
1 1 1 1 2 3
1 1 1 1 1 1
B
(%)
F
5.3 3.7 3.6 11.6 11.7 11.7
2.39 1.30 1.22 11.07 11.07 11.07
a i 0.8 '
o'6 0.4
Table 2. Kinetic Parameters of the T3 Model parameter
k u kdd
preexp factor (h-1) 2.145 2.178
,
O
1\
E 00
T-51OoC
activ energy (kJ/mol) 22.4 9.9
distribution (e), and the values and significance (determined by means of a t Student's test) of the parameters. The results obtained with the different deactivation models tested have been summarized in Table 1. It is interesting to point out that, with all the deactivation kinetic models, the activity obtained for the secondary reaction of p-xylene isomerization was around unity regardless of the temperature, space time, and time on stream. Furthermore, the values estimated for the corresponding deactivation kinetic constant were shown to be not significant. These results clearly show that this reaction is not affected by the coke deposition, at least in the range of time on stream studied. Since the reaction of p-xylene isomerization takes place only over the acid sites located on the external zeolite surface, its resistance to deactivation may be related to the mechanism of coking. For the external acid sites, deactivation is produced only by site coverage, not being affected by the pore blockage which is the mechanism responsible of a faster decay in the activity of the catalyst. A second reason to explain the negligible deactivation of p-xylene isomerization may be the nature of the acid sites. The ZSM-5 sample used in this work has been modified by impregnation with a Si polymer and a Mg salt to increase the diffusional constraints and to deactivate the external acidity. Then, a low strength of the residual acid sites, still remaining on the external surface of the SiMg/ZSM-5 after both modifications, may be argued to explain their resistance to deactivation by coke formation. This hypothesis is in agreement with the relatively weak strength of the acidity required to catalyze the reactions of xylene isomerization as proposed by Kim et al. (1992). If the deactivation models are compared on the basis of the coke precursor, a much better fit of the experimental data is obtained with those that assume toluene to be the coke precursor than with those that consider the coke formation is from p-xylene. In the last case, the fit seems to be unaffected by the order of deactivation, similar results being obtained with the PX1, PX2, and PX3 models. Therefore, the system is more adequately described by a deactivation in parallel with the main reaction of toluene disproportionation. In regard to the order of deactivation of the main reaction, it can be seen in Table 1 how the fit is improved when d D has values between 2 and 3, which shows a strong control of the reaction rate by the diffusional process, in good agreement with the conclusions previously obtained with the kinetic model at zero time (Uguina et al., 1993) and the postulates of Khang and Levenspiel (1973). When the deactivation order d D is introduced as a parameter in the computer program, the best fit of the experimental data is obtained for a value of 2.87, hence the T3 model has been selected. With this model an average error of 3.6% is obtained in the reproduction of the experimental product distribution.
1.0 a!J 0.9
o.a 0.7 0
2
4
6
8
1
0 t(h)
Figure 4. Deactivation of the main and the secondary reactions with the time on stream = 255 g h/mol).
The values of the kinetic parameters corresponding to the T3 model are shown in Table 2. For both reactions, toluene disproportionation and p-xylene dealkylation, the activation energies of the deactivation constants are much lower than those previously calculated for the kinetic constants at zero time (99 and 176 kJ/mol, respectively). The evolution of the activities with the time on stream a t the three temperatures investigated has been depicted in Figure 4 for a space time of 255 g h/mol. (Similar trends are observed in the rest of the experiments.) The fast deactivation undergone by the secondary reaction of p-xylene dealkylation is remarkable in contrast with the main reaction, which agrees with the above mentioned increase of selectivity toward disproportionation with the time on stream. Both reactions take place on the acid sites of the ZSM-5 zeolite, but it is known that the dealkylation reaction requires a stronger acidity than toluene disproportionation (Meshran, 19861, Le., not all the acid sites present in the catalyst are active sites in p-xylene dealkylation. In a previous work (Uguina et al., 1993),we have found by means of NH3 TPD measurements that the coking of ZSM-5 with toluene leads to a decrease in the strength of the acid sites. Then, coke is preferentially deposited on the stronger acid sites, which explains the faster deactivation of the p-xylene dealkylation.
Conclusions A kinetic model of the toluene disproportionation deactivation over a SiMg/ZSM-5 catalyst has been developed including the presence of secondary reactions @xylene isomerization and dealkylation) and the effect of the diffusional steps. The best fit of the experimental data is obtained when toluene is assumed to be the coke precursor with a deactivation order for the main reaction of 3, which shows the strong influence of the intracrystalline diffusion on the overall reaction rate.
-
30 Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994
i
INITIALVALUE OF THE PARAMETERS
1
INTEGRATION OF ( S ) , ( 6 1 , ( 7 1
I
KINETIC MODEL AT ZERO TIME
STOICHIOMETRY
i
INTEGRATION OF (111, ( 1 2 ) , (1311
kDd, kdd, kId = deactivation kinetic constants of toluene disproportionation, p-xylene dealkylation, and p-xylene isomerization, respectively, h-1 PC = partial pressure of the coke precursor, atm rD, rd, rI = reaction rates of toluene disproportionation, p-xylene dealkylation, and p-xylene isomerization, respectively, mol/(g h) RB,RT, RPX = formation rates of benzene, toluene, and p-xylene, respectively, mol/(g h) t = time on stream, h w/FTo = space time, g h/mol XB,XT, Xpx = mole fraction of benzene, toluene, and p-xylene, respectively Greek Symbols t = average relative error 9 = deactivation function, eq 3 Appendix Figure 5 shows the block diagram of the computer program used to calculate the evolution with the time on stream of the product distribution predicted by each deactivation model. The activity of each reaction is calculated by the integration of eq 2 and 3:
NEW VALUE OF THE P W T E R S
Figure 5. Block diagram of the computer program used to fit the experimental data and the deactivation models.
The deactivation by coke deposition affects in a different way the three reactions considered, which is probably due to differences in the mechanism of deactivation and in the strength of the acid sites involved: The activity of p-xylene isomerization remains close to 1in the range of time on stream investigated. This reaction takes place on the acid sites located over the external zeolite surface and hence is not affected by the pore blockage with coke, the major deactivating effect of the coke deposition. Furthermore, it requires the presence of relatively weak acid sites, with high resistance to deactivation by site coverage with coke. The decay of the catalytic activity of toluene disproportionation is much slower than that ofp-xylene dealkylation. Both reactions take place within the zeolite pore structure, being affected by both site coverage and pore blockage. However, p-xylene dealkylation requires stronger acid sites, which are supposed to be first deactivated by the coke deposition.
(7) According to eq 1, the reaction rates at any time t can be calculated from the activities by means of [(rD)ob(t)l
[(rD)&(O)l [('D)&(t)l
[r,(t)l = [rI(0)l[aI(t)l
(8)
(10)
where (rD)obs(O),(rd)ob(O) and rI(0) are given by the kinetic model at zero time (Uguina et al., 1993). The formation rate of each species at any time t is related to the reaction rates through the stoichiometry:
Acknowledgment We wish to thank CICYT of Spain for the support of this work.
Nomenclature aD, ad, a1 = activities of toluene disproportionation,p-xylene dealkylation, and p-xylene isomerization, respectively dD, dd, dI = orders of deactivation of toluene disproportionation, p-xylene dealkylation, and p-xylene isomerization, respectively F = objective function, eq 4 kD, kd, kI = rate constants of toluene disproportionation, p-xylene dealkylation, and p-xylene isomerization,respectively, mol/(g h atm)
Finally, the proportion of toluene, benzene, and p-xylene in the system a t any time, temperature, and space time is obtained by integration of these equations. Literature Cited Beltrame, P.;Beltrame, P. L.; Carniti, P.; Forni, L.; Zuretti, G.Toluene Disproportionation Catalysed by Various Zeolites. Zeolites 1986, 5 , 400-405.
Ind. Eng. Chem. Res., Vol. 33, No. 1, 1994 31 Bharati, S. P.; Bhatia, S. Deactivation Kinetics of Toluene Disproportionation over Hydrogen Mordenite Catalyst. Znd.Eng. Chem. Res. 1987,26,1854-1860. Bhavikatti, S. S.; Patwardhan, S. R. Toluene Disproportionation over Nickel-Loaded Aluminum-DeficientMordenite. 2. Kinetics. Znd. Eng. Chem. Prod. Res. Dev. 1981,20, 106-109. Corella, J.; Asua, J. M. Kinetic Equations of Mechanistic Type with Non-separable Variables for Catalyst Deactivation. Ind. Eng. Chem. Process Des. Dev. 1982,21, 55-61. Costa, E.; Uguina, M. A.; Lucas, A.; Blanes, J. Synthesis of ZSM-5 Zeolites in the C2H~OH-Na20-Al20~-SiO~H~O System. J. Catal. 1987,107,317-324. Do, D. D. Enhanced Para-xylene Selectivity in a Fixed-Bed Reactor. AZChE J. 1985,31, 547-580. Fraenkel, D. Role of External Surface Sites in Shape-Selectivity Catalysis over Zeolites. Znd. Eng. Chem. Res. 1990, 29, 18141821. Guisnet, M.; Magnoux, P. Coking and Deactivation of Zeolites. Influence of the Pore Structure. Appl. Catal. 1989,54, 1-27. Kaeding, W. W.; Chu, C.; Young, L. S.; Butter, S. A. Selective Disproportionation of Toluene to Produce Benzene and p-Xylene. J. Catal. 1981,69,392-398. Khang, S. J.; Levenspiel, 0. The Suitability of an nth-Order Rate Form to Represent DeactivatingCatalyst Pellets. Znd.Eng. Chem. Fundam. 1973,12,185-190. Kim, J.; Namba, S.; Yashima, T. Para-selectivity of Zeolites with MFI Structure. Difference between Disproportionation and Alkylation. Appl. Catal. 1992,83, 51-58. Magnoux, P.; Cartraud, P.; Mignard, S.; Guisnet, M. Coking, Aging, and Regenerationof Zeolites. 111. Comparison of the Deactivation Modes of H-Mordenite, ‘HZSM-5, and HY during n-Heptane Cracking. J. Catal. 1987,106, 242-250. Marquardt, D. W. An Algorithm for Least-Squares Estimation of Nonlinear parameters. J. SOC.Znd. Appl. Math. 1963,11,471481. Meshram, N. R. Selective Toluene Disproportionation over ZSM-5 Zeolites. J. Chem. Technol. Biotechnol. 1987,37,111-122. Meshram, N. R.; Hegde, S. G.; Kulkami, S. B. Active Sites on ZSM-5 Zeolites for Toluene Disproportionation. Zeolites 1986, 6, 434438.
Meshram, N. R.; Hegde, S. G.; Kulkami, S. B.; Ratnasamy, P. Disproportionationof Toluene over HZSM-5Zeolites. Appl. Catal. 1983,8, 359-367. Nayak, U. S.; Riekert, L. Catalytic Activity and Product Distribution in the Disproportionation of Toluene on Different Preparations of Pentasil Zeolite Catalysts. Appl. Catal. 1986,23, 403-411. Olson, D. H.; Haag,W. 0. Stmcture-SelectivityRelationshipin Xylene Isomerization and Selective Toluene Disproportionation. In Catalytic Materials: Relationship Between Structure and Reactivity; Whyte, T. E., Jr., Della Betta, R. A., Derouane, E. G., Baker, R. T. K., Eds.; ACS Symposium Series 248; American Chemical Society: Washington, DC, 1984; pp 275-307. Serrano,D. P. Ph.D. Dissertation, ComplutenseUniversityof Madrid, 1990. Uguina, M. A.; Sotelo,J. L.; Serrano,D. P. Toluene Disproportionation over ZSM-5 Zeolite. Effects of Crystal Size,Silicon-to-Aluminum Ratio, Activation Method and Pelletization. Appl. Catal. 1991, 76,183-198. Uguina, M. A,; Sotelo, J. L.; Serrano, D. P.; Van Grieken, R. Magnesium and Silicon as ZSM-5 Modifier Agents for Selective Toluene Disproportionation. Znd. Eng. Chem.Res. 1992,31,18751880. Uguina, M. A.; Sotelo, J. L.; Serrano, D. P. Kinetics of Toluene Disproportionationover Unmodified and ModifiedZSM-5 Zeolites. Znd. Eng. Chem. Res. 1993,32,49-55. Uguina, M. A.; Serrano, D. P.; Van Grieken, R.; VBnes, S. Adsorption, Acid and Catalytic Changes Induced in ZSM-5 by Coking with Different Hydrocarbons. Appl. Catal. A 1993,99,97-113. Wei, J. A. Mathematical Theory of Enhanced para-Xylene Selectivity in Molecular Sieve catalysts. J. Catal. 1982, 76,433-439.
Receiued for review April 23, 1993 Revised manuscript received September 13, 1993 Accepted September 22, 1993.
* Abstract published in Aduance ACS Abstracts, December 1, 1993.
