Ind. Eng. Chem. Res.
Triphaee Catalysis in a Batch Reactor. Znd. Eng. Chem. Res. 1991a, 30,2384-2390. Wang, M. L.; Yang, H. M. Dynamica of Phase Transfer Catalyzed Reaction 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 Organic Synthesis; Springer Verlag: New York, 1977.
1992,31,187&1880
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Yang,H. M. Study on the Synthesis of Allyl Polybromophenyl Ether by Phase Transfer Catalytic Reaction. Ph.D. Thesis,Department of Chemical Engineering, National Taing Hue University, Hsinchu, Taiwan, 1990. Received for review November 20,1991 Accepted May 8,1992
Magnesium and Silicon as ZSM-5Modifier Agents for Selective Toluene Disproportionation Maria A. Uguina,* Jose L. Sotelo, David P. Serrano, and Rafael Van Grieken Chemical Engineering Department, School of Chemistry, Compluteme University of Madrid, 28040 Madrid, Spain
Magnesium and silicon have been studied and compared as ZSM-5 modifier agents for selective toluene disproportionation. For both agents, para-selectivity corresponding to the primary product was loo%, p-xylene being the unique isomer able to leave the modified pore structure due to the coupled effects of the steric constraints, enhanced by the modifier agents, and the fast internal isomerization. The further decline of para-selectivity with conversion is caused by the primary product isomerization on the external acid sites. The best para-selectivity/activity relationship obtained with the silicon modification has been assigned to the deposition of this agent on the external zeolite surface, an assumption that showed to be in good agreement with kinetic and adsorption-diffusion measurements. The lower conversion exhibited by the magnesium-modified samples has been related to higher diffusional limitations and a decrease in the number of acid sites due to ion exchange with magnesium species.
Introduction Toluene disproportionation in the vapor phase over a variety of acid zeolites is a commercial way to yield the different xylene isomers and benzene (Schriesheim, 1961; Oliver and Ione, 1970;Bhavikatti and Patwardhan, 1981; Beltrame et al., 1985;Meshram et al., 1986;Chang et al., 1987). When the reaction is carried out over ZSM-5 zeolite, the product distribution can be directed to the selective formation of p-xylene, the moet valuable isomer, modifying the shape-selectivityof the zeolite. It is known that ZSM-5 modification by treatment with different agents and by means of several procedures leads to an enhancement of the selectivity to p-xylene (Kaeding and Butter, 1975; Kaeding and Young, 1977;Kaeding et al., 198la,b;Young et al., 1982), obtaining a proportion of p-xylene rather higher than the thermodynamic equilibrium value (24mol % p-xylene). Olson and Haag (1984)have proposed a reaction scheme for selective toluene disproportionation over modified ZSM-5 zeolites. According with them, toluene disproportionation inside the zeolite structure yields benzene and a xylene mixture known as initial product. Then, this product diffuses out of the channel system but simultaneously it undergoes isomerization reactions which take place within the zeolite crystal,giving the primary product, the first observable outside the zeolite pore and capable of being determined at conversions approaching zero. A p-xylene proportion in the primary product higher than the value initially obtained is expected since ita least minimum diameter allows a faster diffusion than the other two isomers. Finally, the primary product undergoes a second isomerization by reentry into the channel network or over the acid sites located on the external zeolite surface, yielding the secondary product observed in the effluent of each experiment. *To whom correspondence should be addressed.
In agreement with this mechanism, the para-selectivity enhancement and the decrease on toluene conversion observed in the modified ZSM-5 zeolites can be related with the following roles of the modifier agent: (a) Pore blockage in the ZSM-5 zeolite by deposition of the modifier agent. Tortuousity of the channel system is increased, which delays the diffusion of the different molecules involved in the reaction. This effect favors the relative p-xylene diffusion, para-selectivity in the primary product being enhanced. Likewise, these steric constrainta prevent the isomerization of the primary product by reentry but may cause a decline on catalytic activity if toluene diffusion is delayed. (b) Deposition or linkage of the modifier agent to the unselective acid sites located on the external surface of the zeolite crystals, an effect which avoids the external isomerization of the primary product. Although the percentage of external acid sites is suppose to be very low, their catalytic effect has to be taken into account since the intrinsic rate constant for isomerization is much higher than the one for disproportionation ( k I / k D31 7000 according to Olson and Haag (1984)). Among the high number of elements and compounds which have been used as ZSM-5 modifier agents (Kaeding et al., 1981a,b;Young et al., 1982;Olson and Haag, 1984) in order to enhance ita para-selectivity in different reactions (xylene isomerization, toluene disproportionation, and toluene alkylation with methanol, ethanol, etc.), in the present work, magnesium and silicon have been selected with the aim of studying and comparing their effects on catalytic and diffusional properties of ZSM-5 zeolite. Whereas magnesium is well-known as a conventional modifier agent of ZSM-5 zeolite (Chen et al., 1979;Kaeding et al., 1981a,b;Olson and Haag, 1984;Derewinski et al., 1984;Meahram, 1987),there is not much information about the use of silicon polymers in order to enhance the zeolite shape-selectivity (Rodewald, 1983). Recently, Wang et al. (1989)(toluene alkylation with ethylene) and Handreck
088&5885/92/2631-1875$03.00/0 0 1992 American Chemical Society
1876 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992
and Smith (1990) (n-hexane cracking) have studied the effects of different silicon compounds as ZSM-5 modifier agents. Likewise, Ratnasamy and Pokhriyal (1989) for xylene isomerization have modified the ZSM-48 zeolite by silylating with tetraethyl orthosilicate. In all cases, the silicon modification is performed using a silicon compound which is deposited as a layer on the external surface of the zeolite crystals due to its large molecular size, leading to catalysts with a sharp shape-selectivity enhancement. According to what has been stated above, a direct relationship between the catalytic behavior of the ZSM-5 zeolite and its diffusion properties can be established. Therefore, the knowledge of the intracrystalline diffusivity of different molecules in the modified ZSM-5 samples may provide essential information to explain their catalytic properties in selective toluene disproportionation. In the literature, several methods have been used to determine adsorption-diffusion parameters in zeolites (Ma and Ho, 1974; Shah and Ruthven, 1977; Biilow et al., 1980; Kiirger et al., 1980; Marshall and Weisz, 1988). The chromatographic method has shown to be quite useful for calculating equilibrium adsorption constants, axial dispersion coefficients, and macropore and micropore diffusivities. In this method, the response of a packed column containing the adsorbent to a pulse of adsorbate is obtained as a chromatographic peak which is commonly analyzed by the moment method: the first and second momenta of the response peak can be related with the adsorption-diffusion parameters using different mathematic models (Schneider and Smith, 1968, 1969; Suzuki and Smith, 1971; Haynes and Sarma, 1973; Chiang et al., 1984a; Boniface and Ruthven, 1985). In the present work, in order to determine the diffusional limitations present in the magnesium- and siliconmodified ZSM-5 samples, the intracrystalliie diffusivities of two hydrocarbons (toluene and n-butane) have been measured by applying the chromatographic method. The adsorption-diffusion experimental data have been fitted to the mathematic model initially developed by Haynes and Sarma (1973) and later discussed by Ruthven (1984) for a packed bed containing a bidisperse porous adsorbent p = &U[ l + ( ? ) K p ]
where Kp = tp + (1- tp)Kc
(3)
Experimental Section Catalyst Preparation. ZSM-5zeolite was prepared from a reaction mixture containing ethanol as template according to the procedure described elsewhere (Costa et al., 1987). The acid form was obtained by ion exchange using a 0.6 M HC1 solution at 25 OC for 4 h and then calcined in static air at 550 OC for 14 h. Before modification, HZSM-5 was bindered with 35 w t % Na-montmorillonite. The product obtained was crushed and sieved to 16-32-mesh pellets and finally calcined at 550 OC for 5 h. The effect of the different variables involved in the activation and pelletization steps was studied in an earlier work (Uguina et al., 1991).
Table I. Details of the Packed Bed and Column in the Adsorption System column bed diameter 0.533 cm bed length 5.5 cm bed voidage 0.46 adsorbent pellet voidage 0.38 average pellet radius 0.375 mm average crystal radius 3.7 pm adsorbate toluene and n-butane
Magnesium Modification (Mg/ZSM-6). "he ZSM-6 zeolites modified with magnesium were prepared by impregnation of the bindered samples with aqueous solutions of magnesium acetate tetrahydrate at 40 "C for 4 h. After fdtxation, the zeolite was dried at 110 "C and then calcined at 550 "C for 16 h. Silicon Modification (Si/ZSM-6). ZSM-5 zeolite pellets were suspended in a boiling solution of a dimethylsilicon polymer (Merck, GE SE-30) in a toluenexylene mixture (molar ratio = l/l). After 1h, the pellets were filtered, they were dried at 250 "C for 1h, and then the modification degree was increased by repeated incipient wetness impregnation with a fresh silicon polymer solution. Finally, the sample was calcined at 550 O C for 16 h. Catalyst Characterization. The crystallinity of the ZSM-5 zeolite (100%) was measured by X-ray diffraction (XRD), using a ZSM-5 standard pattern. The average crystal size of this zeolite (7.4 pm) was evaluated from the crystal size distribution obtained by means of laser granulometry using a CILAS 715 granulometer. The Si/Al ratio (29) of the ZSM-5 zeolite and the magnesium content in the samples modified with this agent were determined by atomic absorption spectroscopy whereas in the samples modified with silicon the modifier content was measured by thermogravimetric analysis in air (Serrano, 1990). Activity Measurements. Toluene disproportionation was performed in a downflow fixed bed reactor at atmospheric pressure. Toluene flow was delivered and controlled by a syringe pump while the effluent from the reactor was cooled and separated in gaseous and liquid streams. The compositions of reactants and products were determined by gas chromatography (GC), using a 5% SP-1200/5% bentone 34 on Supelcoport column (3 m) and a Porapak Q column (3 m) for the liquid and gaseous products, respectively. The reaction was carried out at 475 "C. For the unmodified ZSM-5 zeolite the space velocity was varied between 2.17 and 27.06 h-l. The magnesium- and siliconmodified catalysts were tested at two different space velocities (WHSV = 2.17 and 4.34 h-l). Previously, the negligible catalyst aging at these operation conditions, even after several hours of time on stream, was checked. Adsorption-Diffusion Measurements. The adsorption column packed with the ZSM-5 sample was kept at constant temperature, and flowing helium was used as carrier gas. The injection of the adsorbate (toluene or n-butane) in the column inlet was done manually, and the composition in the column outlet was determined with a thermal conductivity detector, the response being recorded as a chromatographic peak. Details of the column and adsorbent bed are given in Table I. Prior to each experiment the zeolite sample was evacuated at 300 "C for 16 h in an helium flow and then cooled to the desired temperature for the adsorption-diffusion measurement. Several injections of adsorbate in the column inlet were performed, varying the carrier flow, for each adsorbent and obtaining the corresponding chroma-
Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 1877 loo
I
Si/ZSM- 5
t
6o
2o0
1t 4o
20
0
40
60
W/F
80
( g h/rnol)
Figure 1. Toluene disproportionation over unmodified ZSM-5 zeolite. Reaction parameters versus space time (T= 475 "C).
tographic peak in the column outlet. The first and the second moments of the response were evaluated from the peaks by numerical integration. The mathematic model used in this work is based in different assumptions which have been checked for each adsorbate-adsorbent system. Thus, pressure drop across the column was found to be negligible, adsorption was checked to be reversible by repeating injections of the adsorbate, the linearity of the adsorption isotherm was tested by decreasing the size of the pulse, and the retention time of the pulse was corrected taking into account the presence of dead space. Likewise, diffusion in micropores was considered as the controlling rate step since in previous experiments carried out with a column packed only with the binder, without zeolite, very short retention times for both adsorbates were found. This finding led us to simplify eq 2 by assuming that the contributions of external transport and macropore diffusion to the overall resistance are negligible. The fitting of the first and the second moments obtained from the chromatographic peak at different carrier flows by means of eqs 1-3 allows calcuand the lation of the adsorption equilibrium constant (K,) intracrystalline diffusivity (D,) for each adsorbate-adsorbent system.
Results and Discussion From the hydrocarbon distribution obtained in each experiment the following parameters have been defined and calculated: toluene conversion (CT)
CT =
mol of toluene converted x 100 mol of toluene fed
selectivity toward disproportionation (SD)
s,
t
=
2 X mol of xylene obtained
mol of toluene converted para-selectivity (Ps)
Ps =
x 100
mol of p-xylene obtained x 100 mol of xylene obtained
Unmodified ZSM-5 Zeolite. A series of toluene disproportionation experiments has been carried out over the unmodified ZSMd zeolite at 475 "C varying the space time. Figure 1 shows the reaction parameters obtained versus the space time. It can be seen that, besides the expected increase on toluene conversion, selectivity toward disproportionation slowly falls with the space time since several secondary reactions such as toluene and xylene
I
0
1
I
I
t
I
10
I
I
I
I
I
I
I I
20
c,
( %)
Figure 2. Relationship between para-selectivity and toluene conversion over the silicon- and magnesium-modified ZSM-5 samples (T= 475 O C , WHSV = 2.17 h-l).
dealkylation and xylene disproportionation take place, being responsible of the presence of gaseous hydrocarbons and trimethylbenzenes in the effluent. The para-selectivity corresponding to the primary product can be obtained at space time approaching zero, which leads to a value close to 80%. Therefore, as it was assumed, p-xylene proportion in the primary product is rather higher than the thermodynamic equilibrium value (24%), even for the unmodified ZSM-5 zeolite. The further isomerization of the product by reentry into the zeolite pore structure or over the acid sites located on the external surface of the crystals explains the observed decrease of para-selectivity with the space time. Magnesium- and Silicon-ModifiedZSM-5Zeolites. In order to determine and compare the effects of magnesium and silicon as modifier agents of the ZSM-5 zeolite, several catalysts (modified in different degrees with these elements) have been prepared. For Mg/ZSM-5 samples the magnesium content in the catalyst (0.4-0.9 wt W )was fixed varying the salt concentration in the impregnation step. Likewise, for Si/ZSM-5 samples the modifier content (1.1-14 wt 96)was fixed varying the amount of polymer added during the incipient wetness impregnation. Figure 2 shows the relationship between toluene conversion and para-selectivity obtained at the same operation conditions with these catalysts. In both cases,the modifier agent has a remarkable effect on the para-selectivity which undergoes an enhancement to reach values close to 807'0, but their effects on catalytic activity are very different. In the catalysts modified with magnesium, toluene conversion sharply drops with the modifier content whereas in the silicon-modified zeolites, the modification has a weaker effect. Therefore, for the same conversion, silicon-modified zeolites show a much higher para-selectivity than the magnesium-modified catalysts. These results can be initially related with the different roles played by these modifier agents. In the silicon modification, the agent must be deposited only on the external surface of the ZSM-5 zeolite since the large m0lecular size of the polymer used as precursor avoids its access into the channel system. Therefore, the effect of the silicon modification is the deactivation of the external acid sites by deposition of a silica layer on the zeolite crystal, which explains why, at low modification degrees, this agent affects mainly the para-selectivity whereas the catalytic activity remains constant. However, for higher silicon contents, the polymer deposition likely leads to a decrease in the effective size of the entrances to the channel system, which may limit or even prevent the access of toluene to the internal acid sites, toluene conversion being decreased. By contrast, the size of the magnesium
1878 Ind. Eng. Chem. Res., Vol. 31, No. 8, 1992 Table XI. Kinetic Measurements (T= 475 "C) Darameter ZSM-5 Mg/ZSM-5 80 100 Pso (%) 0.0046 O.OOO9 &Jobs (mol g-' h-' atm-') 0.0143 0.1039 (KI)ob.(mol g-' h-' atm-') 22.6 15.9 (KI/ KD)ob
Si/ZSM-5 100 0.0019 0.0118 6.2
precursor allows its location either inside or outside the zeolite pore structure, which leads to an enhancement of the diffusive hindrance. Nevertheless, the nature of the magnesium linkage to the zeolite is not still clear, and besides a physical deposition, a partial ion exchange between MgZt in the solution and Ht from the zeolite may happen during the impregnation. Therefore, the sharp decrease of toluene conversion observed in the magnesium modification can be related with the enhanced diffusional limitations and a decrease in the number of active sites for toluene disproportionation. Kinetic Measurements. From kinetic measurements carried out at the same temperature and two different space times for every catalyst, the following parameters related with the mechanism of the selective toluene disproportionation have been determined: (a) Para-selectivity corresponding to the primary product, Pso, calculated at space time approaching zero. (b) Observed toluene disproportionation rate, (rD),,bs, determined from the variation of the toluene mole fraction with the space time: 1 dXT (4) (rD)obs = -2 d(W/FTJ (c) Observed p-xylene isomerization rate, (rI)obs,evaluated from the variation of p-xylene mole fraction with the space time, taking into account the p-xylene formation rate by disproportionation:
Likewise, the kinetic constants (KD),,~and (K1Iobscan be calculated from the rate values assuming first-order kinetics for the disproportionation and isomerization reactions. The kinetic parameters have been determined from the reaction data obtained in experiments carried out a t 475 "C and two space times, using unmodifiedZSM-5 (3.40 and 7.28 g h/mol) and Mg/ZSM-5 with wMg= 0.9 wt % and Si/ZSM-5 with wsi = 7.3 wt % (21.23 and 42.46 g h/mol). The data have been used to calculate Ps by space time approaching zero whereas the kinetic constants have been evaluated from the results obtained at the lowest space time with toluene conversions clearly below lo%, which allows assuming differential conditions and irreversible reactions. The values of these parameters are summarized in Table 11. Both modified catalysts yield PS values close to 100%, hence p-xylene must be the only xylene isomer present in the primary product, just outside the zeolite channel system. Therefore, the steric constraint enhancement produced by these two modifier agents drastically affects the diffusion of m-and o-xylene and with the fast internal isomerization is the cause of the para-selectivity achieved in the primary product. Furthermore, if p-xylene is the only isomer able to leave the modified zeolite structure, the primary product isomerization by reentry must be negligible, the observed isomerization rate being a measurement of p-xylene isomerization over the acid sites located on the external surface of the zeolite crystals. Likewise, if the steric constraints in these catalysts are still
Table 1x1. Adsorption-Diffusion Measurements adsorbate adsorbent T ("C) K, D, (cm2/s) toluene ZSM-5 290 830 3.0 X toluene Mg/ZSM-5 290 2600 1.9 X toluene Si/ZSM-5 290 492 2.7 X ZSM-5 180 271 3.9 X lo* n-butane Mg/ZSM-5 180 304 2.1 X lo* n-butane Si/ZSM-5 180 236 2.8 X lo* n-butane
more enhanced by a more severe modification, toluene and/or p-xylene diffusion may be affected leading to a decrease of catalytic activity without improving further the para-selectivity. Otherwise, for both modifications the disproportionation and isomerization kinetic constants are lower than that Corresponding to the unmodified ZSM-5 zeolite. It can be noted that the silicon modification strongly affects (KI)obs compared with the effect on (KD)ob, a finding which is in good agreement with the location of the modifier agent on the external surface of ZSM-5 zeolite deactivating the unselective acid sites. The decrease of the observed disproportionation rate may be explained by blocking of the entrances to the channel system when a large amount of polymer is deposited on the zeolite. In the magnesium modification, although the isomerization rate is also very reduced, the rate of the main reaction is rather lower than in the unmodified catalyst, a fact which agrees well with the above-mentioned sharp decrease of toluene conversion with the magnesium agent. The (KI/K~Iob ratio, even for the unmodified ZSM-5 zeolite, is by far lower than the value of 7000 reported by Olson and Haag (1984). Nevertheless, it must be taken into account that the value obtained by these authors is referred to the intrinsic kinetic constants, without any effect due to the diffusional steps. The diffusive hindrance present even in the unm&ied zeolite has a stronger effect on the internal xylene isomerization rate than on the toluene disproportionation rate. Gilson and Derouane (1984) have proposed an equation to calculate the percentage of external surface tetrahedral sites as a function of the average crystal size for the unmodified ZSM-5 zeolite. Applying this equation to the ZSM-5 sample used in thii work, the estimated percentage of external surface acid sites is about 0.02%,a value that must be drastically reduced when the zeolite is modified with magnesium and particularly with silicon. In spite of this low proportion of external acid sites, the observed isomerization kinetic constant for the Si/ZSM-5 sample is still 6 times higher than the kinetic constant of the main reaction. This fact points out that the selective deactivation of external surface sites is a very important condition in order to get a high para-selectivity in the secondary product. Adsorption-Diffusion Measurements. The chromatographic method has been applied in the present work in order to determine and compare the diffusional limitations present in the unmodified ZSM-5, Mg/ZSM-5 ( w ~ = 0.9 wt %), and Si/ZSM-5 (wsi = 7.3 wt %) samples. The measurements have been carried out at different temperatures for each adsorbate, selected in order to achieve suitable retention times with a defined shape of the chromatographic peak. Thus, when toluene was used as adsorbate, a temperature of 290 "C had to be selected due the highly skewed peak obtained at lower temperatures. In this way, Shah and Oey (1988) have reported that the method of momenta can be successfully applied even to highly skewed chromatographic responses. The results obtained from these chromatographic measurements for the different adsorbateadsorbent systems are summarized in Table 111.
Ind. Eng. Chem. Res., Vol. 31,No. 8,1992 1879 Table IV. Adsorption-Diffusion Data from the Literature on Silicalite and ZSM-5 Gravimetric Method adsorbate adsorbent T ("C) D, (cm2/s) research group 40 2.5 X Doelle et al. (1981) benzene HSZM-5 p-xylene HSZM-5 40 0.2-6 X Le Van Mao et al. (1983) 40 0.2-2.6 X lo-'' Ragahi et al. (1984) p-xylene HSZM-5 WU et al. (1983) 200 1.2 X benzene silicalite 200 1.0 X Wu et al. (1983) p-xylene silicalite Chromatographic Method adsorbate benzene toluene n-butane
Dc
adsorbent T ("C) K, (cm2/s) research group Forni et al. (1986) NaZSM-5 290 8900 4.9 X Forni et al. (1986) NaZSM-5 290 9300 7.5 X Chiang et al. silicalite 180 292 3.5 X (1984b)
In order to check the accuracy of these values, in Table IV we have summarized several literature data about adsorption-diffusion of benzene, toluene, p-xylene, and nbutane on silicalite and ZSM-5 zeolite. These measurements have been carried out by both gravimetric and chromatographic methods, and some values have been calculated or extrapolated from the data directly reported by the authors. Comparing the data shown in Tables I11 and IV, it can be noted that for the unmodified ZSM-5 zeolite the intracrystalline diffusivity of toluene obtained in the present work is very close to the values obtained by Forni and Viscardi (1986) for benzene and toluene but their equilibrium adsorption constants are higher by an order of magnitude. Nevertheless, it must be taken into account that the latter values have been obtained with the sodium form of the ZSM-5 zeolite and these own authors found rather lower retention times with the acid form, which leads to a' lower value of the equilibrium adsorption constant for HZSM-5. Likewise, the equilibrium adsorption constant of n-butane in the unmodified ZSM-5 zeolite obtained in this work agrees very well with the value reported by Chiang et al. (1984b) on silicalite,although their intracrystalline diffusivity is lower by 2 orders of magnitude. This divergence between diffusivities on silicalite and ZSM-5 has been already reported. Thus, the diffusivity on silicalite appears to be between 1 and 2 orders of magnitude lower than that corresponding to ZSM-5 zeolite. In spite of the disparity between the gravimetric and chromatographic data recorded in the available literature, it can be concluded that the values obtained in the present work are consistent with those obtained by other authors using chromatographic methods and therefore suitable to compare the effect of magnesium and silicon modifications on the diffusional limitations through the ZSM-5 pore structure. From Table 111, it can be observed that both modifications lead to a decrease of toluene and n-butane diffusivities as regards the unmodified zeolite, which is more remarkable for the magnesium-modified ZSM-5 sample. This fact confirms a higher blockage of the channel system by magnesium species compared with the silicon modification, in good agreement with the location of the silicon modifier on the external surface of the zeolite and the magnesium either outside and within the zeolite crystal. Nevertheless, the measured decrease of toluene and nbutane diffusivities in the Mg/ZSM-5 sample is not enough to assign ita low catalytic activity only to diffusional c a w s but again the reduction in the number of acid sites because of the ion exchange with h@+ has to be taken into account. Other interesting point is that, whereas the equilibrium adsorption constant is reduced by adding a silicon polymer,
the presence of magnesium increases ita value. This effect is more remarkable when toluene is used as adsorbate and may be related to the earlier mentioned dependence of the equilibrium adsoption constant on the ion form of the ZSM-5 zeolite; i.e., the presence of magnesium seems to increase the adsorption capacity of the ZSM-5 zeolite, which may be explained by the generation of new adsorption sites related to magnesium species.
Conclusions The main conclusions obtained from the results of this work can be summarized as follows: Modifications of ZSM-5 zeolite with magnesium or silicon compounds leads to a high para-selectivity enhancement in toluene disproportionation, although catalytic activity is decreased. The drop in toluene conversion is more remarkable for the Mg/ZSM-5 samples. Para-selectivity corresponding to the primary product in Mg/ZSM-5 and Si/ZSM-5 catalysts was found to be close to 100%. Then, p-xylene is the only xylene isomer able to leave the zeolite pore structure due to the steric constraint enhancement caused by the modifier agent besides the high rate of the internal xylene isomerization. Silicon modification, using a large polymer as precursor, deactivates selectively the external acid sites of ZSM-5 zeolite by formation of a silica layer on the external surface of the zeolite crystals. From adsorption-diffusion measurements obtained by means of the chromatographic method, it is observed that toluene and n-butane diffusivities are drastically decreased with the magnesium modification, supporting that the diffusive hindrance enhancement and the decrease in the number of acid sites by ion exchange with Mg2+are the causes of the observed toluene conversion drop with the magnesium content in these samples. In the silicon- and magnesium-modified ZSM-5 samples, the key point to obtain highly para-selective catalysts is the deactivation of the unselective acid sites located on the external surface of the zeolite crystals. Nomenclature D,: zeolite (crystal) diffusivity DL:axial dispersion coefficient D : macropore diffusivity (gD)ob: observed kinetic constant of toluene disproportionation (KJObs:observed kinetic constant of p-xylene isomerization k f : gas-to-pellet mass-transfer coefficient K,: adsorption equilibrium constant for the crystal Kp: adsorption equilibrium constant for the pellet L: length of the chromatographic column r,: zeolite crystal radius R,: pellet radius T: temperature u: interstitial fluid velocity Greek Symbols
void fraction of bed void fraction of pellet p: first moment of the response peak u: second moment of the response peak Registry No. Mg, 7439-95-4;Si, 7440-21-3; toluene, 108-88-3; 6:
tp:
p-xylene, 106-42-3.
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