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Shape selective conversion of 1,2,4-trimethylbenzene over different zeolite frameworks
Studies in Surface Science and Catalysis, volume 154 E. van Steen, L.H. Callanan and M. Claeys (Editors) 9 2004 Elsevier B.V. All fights reserved.
2281
SHAPE SELECTIVE C O N V E R S I O N OF 1,2,4-TRIMETHYLBENZENE OVER DIFFERENT ZEOLITE F R A M E W O R K S Fois, G.A. 1, Bordiga~ Sol, Ricchiardi, G. 1 Dalloro, L. 2, Buzzoni, R. 2, Rivetti, F. 2 and Zecchina, A. 1 1Universit/l di Torino, Dipartimento di Chimica I.F.M. Via Giuria 7, 1-10125 Torino, Italy. 2Polimeri Europa, Istituto Guido Donegani, V. Fauser 4, 1-28100, Novara, Italy.
ABSTRACT The upgrading of Pseudocumene (1,2,4-TMB) to more valuable methylbenzenes over acidic zeolites with variable acidic and diffusive properties is studied, using reactivity tests and IR spectroscopy. The concurrent reactions investigated include isomerization and transalkylation. At low conversion levels the isomer distribution in C8, C9 and C~0 products is not thermodynamically controlled, being different over different zeolite frameworks, indicating shape selectivity. Variable temperature experiments allow to identify the location of active sites for the different reactions on different catalysts, and the nature of shape selectivity.
INTRODUCTION 1,2,4-Trimethylbenzene (TMB) is a relatively abundant low-value petrochemical compound, which can be advantageously upgraded to 1,3,5-Trimethylbenzene (TMB) and/or 1,2,4,5-Tetramethylbenzene (TeMB) by isomerization, transalkylation and eventually alkylation with methanol on solid acid catalysts like zeolites[ 1,2]. In this work we report a study of the isomerization and transalkylation of pure 1,2,4-TMB over Beta, Y and ZSM-5 zeolites. Thermodynamic equilibrium among benzene, toluene, xylenes and polymethylbenzenes (C6-C12 stream) can be calculated using the Stull thermodynamic equilibrium [3]. At 136 ~ the equilibrium composition is: 2.5% toluene, 17% xylenes, 43% C9 and 33% C10. Moreover the equilibrium composition of the C9 fraction at 136~ is: 62.5% pseudocumene, 33% mesitylene and 4.5% hemimellitene. At the same time the composition of Ca0 isomers is approximately: 52% isodurene, 37% durene and 11% prehnitene. The yield of mesitylene is maximized at low temperature, while the durene yield changes very little with temperature. Acid catalysed isomerization of alkylaromatics was shown to occurs by three pathways [4,5,6]: one intramolecular, one intermolecular involving transalkylation and one dissociative by reversible dealkylation-alkylation. Three transalkylation mechanisms have also been proposed [7]: a) dealkylation and alkylation steps via a stable carbenium-ion intermediate;b) a carbocation chain mechanism involving benzylic carbocations and diaryl methane intermediates [8]. Considering the stability of benzenium ions, the formation of diphenylmethane intermediate to give 1,2,3,4-TeMB and m-xylene (from (A)) is preferred [9]. However transition states leading to 1,2,4,5-TeMB and o-xylene are smaller than all others. These considerations can be important on zeolite catalysts, where transition state shape selectivity can play a fundamental role. In addition to this, the diffusivity of the reactants and products in the restricted pore space has to be taken into account. For these reasons, a detailed study of the accessibility of the framework acid sites to different methylbenzenes has been undertaken. The use of 1,2,4-TMB (reactant), 1,3,5-TMB (bulkiest product), CO and Pyridine (basic probes) as probe molecules permits to study the ability of the reactant and the most bulky product to diffuse within the microporous system, and to characterize acid site strength and type.
EXPERIMENTAL The conversion of 1,2,4-Trimethylbenzene has been carried out in a fixed-bed continuous flow reactor. In all experiments we have used temperature varying from 200~ to 400~ at 50 bar of pressure. The WHSV varies from 4 to 16h -1. In these conditions 1,2,4-TMB is liquid.
2282 The reactor is tubular (AISI 316L) with internal diameter of 11 mm. The available length of catalytic bed is about 100 mm. Reaction products are collected and analysed by a HP gas chromatograph equipped with FID and HP-PONA Methyl Siloxane capillary column (50m). Materials: 9 Zeolite Beta.The material constituted by 50% of Beta zeolite active phase, extruded with alumina. This sample has a commercial name PBE-1 and it is produced by Enichem. The Si/Al ratio is 14.5. 9 Ultra-stable Y zeolite. Commercial Y zeolite (Zeolyst CBV 600). It is an extruded material with 80% of active phase. The Si/A1 ratio is 2.55. 9 H-Mordenite The catalyst investigated was a commercial zeolite (Si/Al=l 0), supplied by PQ company. 9 H-ZSM-5 This commercial zeolite is supplied by SI]D CHEMIE. IR experiments were done on the same materials in form of pellets, using cells allowing outgassing, activation and probe molecule dosage by gas diffusion and sublimation. RESULTS AND DISCUSSION
Pseudocumene reactivity on 12-membered ring systems: Beta and Y zeolites Reaction results of 1,2,4-Trimethylbenzene over PBE-1 catalyst (50% Beta zeolite) at different temperatures are presented in Fig. 1. We present the conversion of 1,2,4-TMB and the yield of desired products (molar % mesitylene and durene). Moreover, the C6-C12thermodynamic equilibrium (solid lines) and thermodynamic distribution among C9 isomers (dotted lines) are shown. All data are obtained using a fresh catalyst and at WHSV 8 hr -~. 80
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Temperature Figure 1. Conversion of 1,2,4-TMB(m) and yield of 1,3,5-TMB() and 1,2,4,5-TeMB(e) on PBE-1 catalyst, their distribution in C6-C~2thermodynamic equilibrium (solid lines) and distribution in C9 thermodynamic equilibrium (dotted lines). We observe that at high temperature the PBE-1 system approaches the C6-C12 thermodynamic equilibrium. The 1,2,4,5-TeMB yield is always lower than that corresponding to its thermodynamic equilibrium and only at high temperature (350~ equilibrium is reached. Interestingly, above 240~ mesitylene yield exceeds significantly its C6-C12 thermodynamic equilibrium. At 275~ mesitylene yield reaches a maximum of about 20% molar (11% in C6-C12 equilibrium), and only at high temperature approaches the thermodynamic equilibrium. Clearly the mesitylene excess cannot be explained by product shape-selectivity because 1,3,5-TMB is the most bulky product. A plausible explanation for this trend is a
2283 different tendency to make isomerization and transalkylation products. In particular, isomerization reactions occur at lower temperature than transalkylation reactions. The transalkylation mechanism involves a bulky bimolecular diaryl methane intermediate. This bimolecular reaction is more space constrained than the intramolecular isomerization. In Fig. 2 we report the 1,2,4-TMB conversion over USY zeolite and the yields of desirable products versus temperature. The catalyst is more active than PBE-l. This is due both to the lower steric hindrance and to the larger Si/Al ratio of USY zeolite. At high temperature the USY system, like PBE-I, approaches the C6-C12 thermodynamic equilibrium. In particular, on USY the 1,3,5-TMB and the 1,2,4,5-TeMB yields are always lower than their thermodynamic equilibrium. Contrary to what observed on Beta zeolite, the characteristic bell curve for 1,3,5-TMB yield is not observed. This can be interpreted by observing that in Beta zeolite, the bimolecular mechanism for transalkylation takes place only at high temperatures (see previous section) while it is possible at all temperatures on USY zeolite.
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Temperature (~ Figure 2. Conversion of 1,2,4-TMB(w) and yield of 1,3,5-TMB(: ) and 1,2,4,5-TeMB(| on USY catalyst and their distribution in C6-C12thermodynamic equilibrium (solid lines). However, also on USY a certain degree of shape selectivity is observed. In fact, even if the super-cage permits to accommodate the transition state of the transalkylation reaction, nevertheless among the tetramethylbenzenes formed in the super-cage only 1,2,4,5-TeMB can diffuse easily out of the channels. Furthermore, at low conversion, like on PBE-1 catalyst, the 1,3,5-TMB to 1,2,3-TMB ratio is greater than thermodynamic equilibrium. These results indicate that 1,3,5-TMB diffuses out of the channels only slowly. Among the C~0 isomers, the 1,2,4,5-TeMB selectivity is much higher than in PBE-1 catalyst. A possible reason is the following: among C10 only 1,2,4,5-TeMB can diffuse out of the channels, and the 1,2,3,5-TeMB and 1,2,3,4-TeMB isomers can form only on the external surface. The crystallite size of USY zeolite is about 30 times larger than that of theBeta zeolite, that is over USY the Br6nsted acid sites located on external surface are less abundant than over Beta zeolite, so the subsequent isomerization of 1,2,4,5-TeMB to 1,2,3,5-TeMB on external surface is less probable. In fact, also the thermodynamic equilibrium among all C9 isomers occurs only at high temperature on USY, while in PBE-1 it was reached already at 275~ In conclusion, the results of our tests on the two zeolites with 12-membered ring apertures (FAU and BEA) can be interpreted by invoking the following steric effects: 9 BEA framework: transition state shape-selectivity 9 FAU framework: product shape-selectivity
2284
P s e u d o c u m e n e reactivity on Z S M 5 It is important to note that the channels size of ZSM-5 is smaller than the reactants Van der Waals diameter. For this reason the transformation of 1,2,4-Trimethylbenzene was proposed as probe reaction to monitor the catalytic effects of inertization of the external surface of H-ZSM-5 [10]. In particular, several authors [11,12] have suggested that the activity of the external surface is the cause of non-selective transformations. In this reaction the reactant and the products are critically sized for the ZSM-5 pore system. Therefore it may be expected that 1,2,4-TMB transformation is a sensitive test for shape-selective properties and the external surface activity. However molecules with a minimum Van der Waals diameter larger than the pore diameter are not necessarily excluded from the pore system. For example 1,3,5-TMB is not excluded from penetrating the intracrystalline pore space, and a small but not negligible diffusion coefficient (D = 10]~ cm~/s at 588 K) has been determined [13,14]. However, if intracrystalline diffusion limitations are extremely high, the activity of the intracrystalline pore space may be negligible compared to the activity of the external surface. In the following, we will discuss the reactivity of H-ZSM5 in comparison with the other zeolites investigated. Figure 3a) shows the isomerization to transalkylation ratio over H-ZSM-5 (green lines) compared with PBE-1 (blue lines) and USY (orange lines) catalysts. A clear difference between 10 and 12-MR zeolites is observed. Therefore transalkylation of 1,2,4-TMB is strongly hindered in medium pore H-ZSM-5. The 1,3,5-TMB maximum yield, about 22.5%, occurs at a 1,2,4-TMB conversion of about 40%. Clearly, the strong hindrance posed to bimolecular mechanism permits to reach the C9 thermodynamic equilibrium at lower 1,2,4-TMB conversion. Above the temperature at which C9 isomers reach their thermodynamic equilibrium, only transalkylation products may be formed at higher 1,2,4-TMB conversion. It is very interesting to observe that over H-ZSM-5 the xylenes to tetramethylbenzenes ratio is much different than over the three 12-MR zeolites. In the transalkylation reaction, starting from two 1,2,4-TMB molecules, one xylene and one tetramethylbenzene molecule are obtained. Secondary reactions, mainly xylene transalkylation, can lower the xylenes to tetramethylbenzenes ratio. However, without diffusion limitation, the Cs/C10 ratio is comprised in the range 0.7-1 and it is independent from time on stream. This occurs over the three 12-MR zeolites. On the contrary, over H-ZSM-5 this ratio is very high at the beginning and tend to lower at high time on stream being about 1.4 at 50 hours.
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Figure 3. a) Percentage distribution between Trimethylbenzenes (I,3,5-TMB+ 1,2,3-TMB) ( 9 and TeMB (-) products and b) 1,3,5-TMB(e), 1,2,4,5-TeMB(-) yields on PBE-1 (blue lines), in USY (orange lines) and at ZSM-5 (green lines). This means that the transalkylation reaction occurs over ZSM-5 mainly within the pore system. In fact, if the bimolecular mechanism should occur mainly on the external surface, the absence of steric hindrance would lead to a Cs/C10 ratio of about one. On the contrary, the C10 product obtained (1,2,4,5-TeMB), diffuses
2285 slowly within the 10-MR channels, while xylene isomers can isomerize within channels and diffuse out of the micropores easily. However, 1,2,4,5-TeMB formed within the pore system is consecutively isomerised on the external surface (Fig. 5.28b). In fact, for pentasil materials such as ZSM-5, the percentage of the external surface tetrahedral sites, T~(%), either Si or A1, can be estimated to be given approximately by Ts(%) = 181 (1/D) where D is the mean crystallite size (in nm). This estimate will give a non negligible amount of external surface aluminium for zeolite crystallites with size of 100 nm or less. Therefore at medium-high 1,2,4-TMB conversion, the external surface isomerizes significantly the products formed within the pores. An other important point is the distribution of isomerization products. Over ZSM-5 the bulky 1,3,5-TMB is formed with a higher rate than 1,2,3-TMB at every 1,2,4-TMB conversion. Contrary to that observed on 12-MR zeolites, the distribution of C9 products is thermodynamically controlled. Therefore it may be concluded that the activity of the intracrystalline pore space in the isomerization reaction is negligible. Thus the isomerization of 1,2,4-TMB to 1,3,5-TMB and 1,2,3-TMB occurs primarily on the external surface of the H-ZSM-5 crystallites. The formation of 1,2,4,5-TeMB inside the micropores of ZSM-5 is consistent with observations made by Yashima et al. [15]. We conclude that only xylenes and 1,2,4,5-TeMB are formed in the micropores, while 1,2,3,4-TeMB, 1,2,3,5-TeMB, 1,2,3-TMB and 1,3,5-TMB are formed on the external surface.
Interaction of the Beta zeolite catalyst with 1,2,4-TMB In order to understand if the beta channel system is accessible to the reactant molecule, we have studied the spectroscopic perturbation of the OH stretching groups of the catalyst caused by the interaction with pseudocumene. The results of pseudocumene adsorption at room temperature are reported in figure 4 for the beta zeolite pre-treated in vacuum at 673K. Upon 1,2,4-TMB dosage the two silanol bands at 3784 and 3748 cm -1 disappear and a clear peak at 3571 cm -1 is formed. The strength of the interaction (as deduced from the OH shift Av = -165 cm -]) induced by pseudocumene interaction is the one expected upon the formation of the H-bond between free silanols and the It electron density of the benzene ring of 1,2,4-TMB [16]. It is interesting to note that the BrOnsted site band at 3610 cm -] is completely eroded upon pseudocumene dosage. This clearly indicates that the reactant can diffuse within the channel system. As far as the strong and broad band centred at 3247 cm -~, induced by the formation of the H-bond between Br6nsted acidic sites and the rt electron density of the benzene ring of 1,2,4-TMB, no substantial modification is observed upon pseudocumene dosages. This result means that all the Br6nsted acidic sites are engaged in stronger H-bonds with 1,2,4-TMB and are not ready to further interaction.
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Interaction of the Beta zeolite catalyst with 1,3,5-TMB Using the more bulky 1,3,5-TMB as probe molecule, the results of the absorption experiment over H-Beta are very similar to that obtained dosing 1,2,4-TMB. Also in this case, Fig. 5, the silanol groups located at 3784 and 3748 cm 1 and the BrOnsted acid sites at 3610 cm -~ are completely eroded upon mesitylene dosage. We conclude that also the bulky mesitylene molecule is capable to diffuse into the micropore system of the
2286 H-Beta zeolite. This fact is particularly important for the mechanistic consideration discussed for the 1,2,4-TMB isomerization reaction. From these IR results we can affirm that the two isomerization reactions of 1,2,4-TMB to 1,2,3-TMB and 1,3,5-TMB can occur inside and outside the channel system.
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Figure 5. IR spectra at RT for increasing dosages of 1,3,5-TMB on H-Beta outgassed at 673K (background spectra red line). Only at low temperature the slow diffusion of mesitylene inside the pores permits to obtain a 1,3,5-TMB to 1,2,3-TMB ratio lower than the thermodynamic equilibrium as discussed above. The bands formed upon mesitylene dosage due to the H-bond between hydroxyl groups and the n electron density of the benzene ring of 1,3,5-TMB are similar to those found using 1,2,4-TMB as probe molecule: 3571 cm ] for silanols and 3226 cm ~ for Br6nsted sites. Also in this case, the H-bond formed between BrOnsted sites and mesitylene resists at prolonged outgassing treatment at room temperature.
Interaction of the H-ZSM5 zeolite catalyst with 1,2,4-TMB In order to understand if the 10-MR three-dimensional channels of H-ZSM-5 are accessible to the reactant molecule, we have studied the spectroscopic perturbation of the OH stretching groups of the catalyst caused by the interaction with pseudocumene. The results of pseudocumene adsorption at room temperature are reported in Fig. 6 for the sample pre-treated in vacuum at 673K. Upon 1,2,4-TMB dosage the silanol band at 3748 cm -] disappears and a clear peak at 3571 cm -~ is formed. The strength of the interaction (as deduced from the OH shift Av= -165 cm -~) induced by pseudocumene interaction is the one expected upon the formation of the H-bond between free silanols and the n electron density of the benzene ring of 1,2,4-TMB. On the contrary the A1-OH and Br6nsted sites located at 3665 and 3610 cm -~ are completely unperturbed upon 1,2,4-TMB dosages at room temperature. This means that in these conditions (room temperature and very low pressure) 1,2,4-TMB interacts with the external surface, but it does not diffuse within the channel system (blue line Fig. 6). In order to observe 1,2,4-TMB diffusion into the channels, we have heated the sample containing the probe molecule at 623K for 15 minutes. IR spectra where then collected after cooling to RT, without further dosages. Upon heating, the probe molecule gradually reaches the internal channels. In fact, it is possible to note the formation of a little and broader band between 3400 and 3050 cm -~. At the same time, the silanol peak at 3748 cm -] increases, while the broader band centred at 3575 cm -~, assigned to silanoi hydroxy groups interacting with 1,2,4-TMB, decreases. Clearly in this phase a slow diffusion of the probe molecule from the external surface to the internal channels takes place. From this consideration we can conclude that over H-ZSM-5, diffusion limitations are present for the reactant molecule. However, in reaction conditions (350~ and 50 bar), 1,2,4-TMB can probably penetrate more easily within the micropores. It is evident that the reactant molecule are critically sized for the ZSM-5 pore system. It may therefore be expected that the 1,2,4-TMB reaction is a sensitive test for shape selective properties and external surface activity.
2287
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Figure 6. IR spectra at RT for increasing dosages of 1,2,4-TMB on H-ZSM-5 outgassed at 673K (background spectra red line). CONCLUSIONS In this work we have studied the 1,2,4-TMB (pseudocumene) transformation over three different 12-MR framework zeolites (Beta, Y) and a 10-MR structure (ZSM-5). At low conversion of 1,2,4-TMB, it is possible to identify primary products since secondary reactions are negligible. At very low conversion, among the xylene isomers and TeMB isomers, o-xylene and 1,2,4,5-TeMB are formed in excess of the thermodynamic equilibrium. The selective formation of o-xylene and 1,2,4,5-TeMB can be explained by transition state and product shape selectivity. In fact, among the nine possible TS of transalkylation reaction, the one producing o-xylene and 1,2,4,5-TeMB is the most slender. The isomerization of 1,2,4-TMB yields 1,3,5-TMB and 1,2,3-TMB. Whereas in Beta, USY and Mordenite at low conversion the 1,3,5-TMB/1,2,3-TMB is lower than thermodynamic equilibrium, in ZSM-5 the thermodynamic equilibrium is maintained at all conversions. These results indicate that 1,2,3-TMB is produced in the channel system of 12-MR zeolites, while 1,3,5-TMB diffuse out of the pores rather slowly. On the contrary over ZSM-5 all C9 isomers are formed mainly on the external surface. An other interesting data is the ratio TeMB/TMB: in Beta this ratio approaches the thermodynamic equilibrium at high conversion only, in ZSM-5 this equilibrium is never achieved, while in USY this ratio is always equal to the equilibrium value. We may explain these results with transition-state shape selectivity: the presence of super-cages in USY promotes the bulky TS of transalkylation reaction. This TS is disfavoured in ZSM-5 channel system. However, in USY too, among the TeMB formed in super-cage only 1,2,4,5-TeMB desorbs out of the channels (product shape selectivity). As the conversion increases the distribution of xylenes, TMB and TeMB approach their respective equilibrium composition (secondary reaction on external surface). These conclusions are supported by the results of probe-molecule experiments where the reactant and the bulkiest product are contacted with the catalyst. REFERENCES 1. H.G. Franck, J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer-Verlag, Berlin 1998 2. Chemical Society, Washington DC, 1976, p.680. 3. D.R. Stull, E.F. Westrum, G.C. Sinke, "The Chemical Thermodynamics of Organic Compounds", Wiley, New York, 1969. 4. M. Guisnet, N.S. Gnep and S.Morin, Microporous and Mesoporous Materials 35-36 (2000) 47-59. 5. G.A. Olah, M.W. Meyer, N.A. Overchuck, J. Org. Chem 29 (1964) 2313-2315. 6. R.H. Allen, L.D.Yats, D.S. Erley, J. Am. Chem. Soc. 82 (1960) 4853. 7. M.L. Poutsma, in: J.A. Rabo(Ed.), Zeolite Chemistry and Catalysis, ACS Monographs vol.171, American Chemical Society, Washington, DC, 1976, p.491. 8. A. Streitwieser Jr., L. Reif, J. Am. Chem. Soc. 82 (1960) 5003.
2288 9. 10. 11. 12. 13. 14. 15. 16.
E. Kikuchi, T. Matsuda, H. Fujiki, Y. Morita, Applied Catalysis, 11, 1984, 331. H.P. R/3ger, K.P. M/311er, C.T. O'Connor, Microporous Materials 8 (1997) 151-157. M. Farcasiu, D.F. Degnan, Ind. Eng. Chem. Res. 27, 1988, 45. L. Wang, C.L. Ay, Appl. Catal. 54 (1989) 257. D.H. Olson, G.T. Kerr, S.L. Lawton, W.M. Meier, J. Phys. Chem., 85 (1981) 2238. P.B. Weisz, Pure Appl. Chem., 52 (1980) 2091. T. Yashima, A. Inaka, S. Namba, Sekiyu Gakkaishi (J. Jpn. Petrol. Inst.) 28(1), 13 (1985). C. Pazb, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina, G. Bellussi, J. Phys. Chem, 101 (1997) 4741.
2281
SHAPE SELECTIVE C O N V E R S I O N OF 1,2,4-TRIMETHYLBENZENE OVER DIFFERENT ZEOLITE F R A M E W O R K S Fois, G.A. 1, Bordiga~ Sol, Ricchiardi, G. 1 Dalloro, L. 2, Buzzoni, R. 2, Rivetti, F. 2 and Zecchina, A. 1 1Universit/l di Torino, Dipartimento di Chimica I.F.M. Via Giuria 7, 1-10125 Torino, Italy. 2Polimeri Europa, Istituto Guido Donegani, V. Fauser 4, 1-28100, Novara, Italy.
ABSTRACT The upgrading of Pseudocumene (1,2,4-TMB) to more valuable methylbenzenes over acidic zeolites with variable acidic and diffusive properties is studied, using reactivity tests and IR spectroscopy. The concurrent reactions investigated include isomerization and transalkylation. At low conversion levels the isomer distribution in C8, C9 and C~0 products is not thermodynamically controlled, being different over different zeolite frameworks, indicating shape selectivity. Variable temperature experiments allow to identify the location of active sites for the different reactions on different catalysts, and the nature of shape selectivity.
INTRODUCTION 1,2,4-Trimethylbenzene (TMB) is a relatively abundant low-value petrochemical compound, which can be advantageously upgraded to 1,3,5-Trimethylbenzene (TMB) and/or 1,2,4,5-Tetramethylbenzene (TeMB) by isomerization, transalkylation and eventually alkylation with methanol on solid acid catalysts like zeolites[ 1,2]. In this work we report a study of the isomerization and transalkylation of pure 1,2,4-TMB over Beta, Y and ZSM-5 zeolites. Thermodynamic equilibrium among benzene, toluene, xylenes and polymethylbenzenes (C6-C12 stream) can be calculated using the Stull thermodynamic equilibrium [3]. At 136 ~ the equilibrium composition is: 2.5% toluene, 17% xylenes, 43% C9 and 33% C10. Moreover the equilibrium composition of the C9 fraction at 136~ is: 62.5% pseudocumene, 33% mesitylene and 4.5% hemimellitene. At the same time the composition of Ca0 isomers is approximately: 52% isodurene, 37% durene and 11% prehnitene. The yield of mesitylene is maximized at low temperature, while the durene yield changes very little with temperature. Acid catalysed isomerization of alkylaromatics was shown to occurs by three pathways [4,5,6]: one intramolecular, one intermolecular involving transalkylation and one dissociative by reversible dealkylation-alkylation. Three transalkylation mechanisms have also been proposed [7]: a) dealkylation and alkylation steps via a stable carbenium-ion intermediate;b) a carbocation chain mechanism involving benzylic carbocations and diaryl methane intermediates [8]. Considering the stability of benzenium ions, the formation of diphenylmethane intermediate to give 1,2,3,4-TeMB and m-xylene (from (A)) is preferred [9]. However transition states leading to 1,2,4,5-TeMB and o-xylene are smaller than all others. These considerations can be important on zeolite catalysts, where transition state shape selectivity can play a fundamental role. In addition to this, the diffusivity of the reactants and products in the restricted pore space has to be taken into account. For these reasons, a detailed study of the accessibility of the framework acid sites to different methylbenzenes has been undertaken. The use of 1,2,4-TMB (reactant), 1,3,5-TMB (bulkiest product), CO and Pyridine (basic probes) as probe molecules permits to study the ability of the reactant and the most bulky product to diffuse within the microporous system, and to characterize acid site strength and type.
EXPERIMENTAL The conversion of 1,2,4-Trimethylbenzene has been carried out in a fixed-bed continuous flow reactor. In all experiments we have used temperature varying from 200~ to 400~ at 50 bar of pressure. The WHSV varies from 4 to 16h -1. In these conditions 1,2,4-TMB is liquid.
2282 The reactor is tubular (AISI 316L) with internal diameter of 11 mm. The available length of catalytic bed is about 100 mm. Reaction products are collected and analysed by a HP gas chromatograph equipped with FID and HP-PONA Methyl Siloxane capillary column (50m). Materials: 9 Zeolite Beta.The material constituted by 50% of Beta zeolite active phase, extruded with alumina. This sample has a commercial name PBE-1 and it is produced by Enichem. The Si/Al ratio is 14.5. 9 Ultra-stable Y zeolite. Commercial Y zeolite (Zeolyst CBV 600). It is an extruded material with 80% of active phase. The Si/A1 ratio is 2.55. 9 H-Mordenite The catalyst investigated was a commercial zeolite (Si/Al=l 0), supplied by PQ company. 9 H-ZSM-5 This commercial zeolite is supplied by SI]D CHEMIE. IR experiments were done on the same materials in form of pellets, using cells allowing outgassing, activation and probe molecule dosage by gas diffusion and sublimation. RESULTS AND DISCUSSION
Pseudocumene reactivity on 12-membered ring systems: Beta and Y zeolites Reaction results of 1,2,4-Trimethylbenzene over PBE-1 catalyst (50% Beta zeolite) at different temperatures are presented in Fig. 1. We present the conversion of 1,2,4-TMB and the yield of desired products (molar % mesitylene and durene). Moreover, the C6-C12thermodynamic equilibrium (solid lines) and thermodynamic distribution among C9 isomers (dotted lines) are shown. All data are obtained using a fresh catalyst and at WHSV 8 hr -~. 80
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Temperature Figure 1. Conversion of 1,2,4-TMB(m) and yield of 1,3,5-TMB() and 1,2,4,5-TeMB(e) on PBE-1 catalyst, their distribution in C6-C~2thermodynamic equilibrium (solid lines) and distribution in C9 thermodynamic equilibrium (dotted lines). We observe that at high temperature the PBE-1 system approaches the C6-C12 thermodynamic equilibrium. The 1,2,4,5-TeMB yield is always lower than that corresponding to its thermodynamic equilibrium and only at high temperature (350~ equilibrium is reached. Interestingly, above 240~ mesitylene yield exceeds significantly its C6-C12 thermodynamic equilibrium. At 275~ mesitylene yield reaches a maximum of about 20% molar (11% in C6-C12 equilibrium), and only at high temperature approaches the thermodynamic equilibrium. Clearly the mesitylene excess cannot be explained by product shape-selectivity because 1,3,5-TMB is the most bulky product. A plausible explanation for this trend is a
2283 different tendency to make isomerization and transalkylation products. In particular, isomerization reactions occur at lower temperature than transalkylation reactions. The transalkylation mechanism involves a bulky bimolecular diaryl methane intermediate. This bimolecular reaction is more space constrained than the intramolecular isomerization. In Fig. 2 we report the 1,2,4-TMB conversion over USY zeolite and the yields of desirable products versus temperature. The catalyst is more active than PBE-l. This is due both to the lower steric hindrance and to the larger Si/Al ratio of USY zeolite. At high temperature the USY system, like PBE-I, approaches the C6-C12 thermodynamic equilibrium. In particular, on USY the 1,3,5-TMB and the 1,2,4,5-TeMB yields are always lower than their thermodynamic equilibrium. Contrary to what observed on Beta zeolite, the characteristic bell curve for 1,3,5-TMB yield is not observed. This can be interpreted by observing that in Beta zeolite, the bimolecular mechanism for transalkylation takes place only at high temperatures (see previous section) while it is possible at all temperatures on USY zeolite.
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Temperature (~ Figure 2. Conversion of 1,2,4-TMB(w) and yield of 1,3,5-TMB(: ) and 1,2,4,5-TeMB(| on USY catalyst and their distribution in C6-C12thermodynamic equilibrium (solid lines). However, also on USY a certain degree of shape selectivity is observed. In fact, even if the super-cage permits to accommodate the transition state of the transalkylation reaction, nevertheless among the tetramethylbenzenes formed in the super-cage only 1,2,4,5-TeMB can diffuse easily out of the channels. Furthermore, at low conversion, like on PBE-1 catalyst, the 1,3,5-TMB to 1,2,3-TMB ratio is greater than thermodynamic equilibrium. These results indicate that 1,3,5-TMB diffuses out of the channels only slowly. Among the C~0 isomers, the 1,2,4,5-TeMB selectivity is much higher than in PBE-1 catalyst. A possible reason is the following: among C10 only 1,2,4,5-TeMB can diffuse out of the channels, and the 1,2,3,5-TeMB and 1,2,3,4-TeMB isomers can form only on the external surface. The crystallite size of USY zeolite is about 30 times larger than that of theBeta zeolite, that is over USY the Br6nsted acid sites located on external surface are less abundant than over Beta zeolite, so the subsequent isomerization of 1,2,4,5-TeMB to 1,2,3,5-TeMB on external surface is less probable. In fact, also the thermodynamic equilibrium among all C9 isomers occurs only at high temperature on USY, while in PBE-1 it was reached already at 275~ In conclusion, the results of our tests on the two zeolites with 12-membered ring apertures (FAU and BEA) can be interpreted by invoking the following steric effects: 9 BEA framework: transition state shape-selectivity 9 FAU framework: product shape-selectivity
2284
P s e u d o c u m e n e reactivity on Z S M 5 It is important to note that the channels size of ZSM-5 is smaller than the reactants Van der Waals diameter. For this reason the transformation of 1,2,4-Trimethylbenzene was proposed as probe reaction to monitor the catalytic effects of inertization of the external surface of H-ZSM-5 [10]. In particular, several authors [11,12] have suggested that the activity of the external surface is the cause of non-selective transformations. In this reaction the reactant and the products are critically sized for the ZSM-5 pore system. Therefore it may be expected that 1,2,4-TMB transformation is a sensitive test for shape-selective properties and the external surface activity. However molecules with a minimum Van der Waals diameter larger than the pore diameter are not necessarily excluded from the pore system. For example 1,3,5-TMB is not excluded from penetrating the intracrystalline pore space, and a small but not negligible diffusion coefficient (D = 10]~ cm~/s at 588 K) has been determined [13,14]. However, if intracrystalline diffusion limitations are extremely high, the activity of the intracrystalline pore space may be negligible compared to the activity of the external surface. In the following, we will discuss the reactivity of H-ZSM5 in comparison with the other zeolites investigated. Figure 3a) shows the isomerization to transalkylation ratio over H-ZSM-5 (green lines) compared with PBE-1 (blue lines) and USY (orange lines) catalysts. A clear difference between 10 and 12-MR zeolites is observed. Therefore transalkylation of 1,2,4-TMB is strongly hindered in medium pore H-ZSM-5. The 1,3,5-TMB maximum yield, about 22.5%, occurs at a 1,2,4-TMB conversion of about 40%. Clearly, the strong hindrance posed to bimolecular mechanism permits to reach the C9 thermodynamic equilibrium at lower 1,2,4-TMB conversion. Above the temperature at which C9 isomers reach their thermodynamic equilibrium, only transalkylation products may be formed at higher 1,2,4-TMB conversion. It is very interesting to observe that over H-ZSM-5 the xylenes to tetramethylbenzenes ratio is much different than over the three 12-MR zeolites. In the transalkylation reaction, starting from two 1,2,4-TMB molecules, one xylene and one tetramethylbenzene molecule are obtained. Secondary reactions, mainly xylene transalkylation, can lower the xylenes to tetramethylbenzenes ratio. However, without diffusion limitation, the Cs/C10 ratio is comprised in the range 0.7-1 and it is independent from time on stream. This occurs over the three 12-MR zeolites. On the contrary, over H-ZSM-5 this ratio is very high at the beginning and tend to lower at high time on stream being about 1.4 at 50 hours.
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Figure 3. a) Percentage distribution between Trimethylbenzenes (I,3,5-TMB+ 1,2,3-TMB) ( 9 and TeMB (-) products and b) 1,3,5-TMB(e), 1,2,4,5-TeMB(-) yields on PBE-1 (blue lines), in USY (orange lines) and at ZSM-5 (green lines). This means that the transalkylation reaction occurs over ZSM-5 mainly within the pore system. In fact, if the bimolecular mechanism should occur mainly on the external surface, the absence of steric hindrance would lead to a Cs/C10 ratio of about one. On the contrary, the C10 product obtained (1,2,4,5-TeMB), diffuses
2285 slowly within the 10-MR channels, while xylene isomers can isomerize within channels and diffuse out of the micropores easily. However, 1,2,4,5-TeMB formed within the pore system is consecutively isomerised on the external surface (Fig. 5.28b). In fact, for pentasil materials such as ZSM-5, the percentage of the external surface tetrahedral sites, T~(%), either Si or A1, can be estimated to be given approximately by Ts(%) = 181 (1/D) where D is the mean crystallite size (in nm). This estimate will give a non negligible amount of external surface aluminium for zeolite crystallites with size of 100 nm or less. Therefore at medium-high 1,2,4-TMB conversion, the external surface isomerizes significantly the products formed within the pores. An other important point is the distribution of isomerization products. Over ZSM-5 the bulky 1,3,5-TMB is formed with a higher rate than 1,2,3-TMB at every 1,2,4-TMB conversion. Contrary to that observed on 12-MR zeolites, the distribution of C9 products is thermodynamically controlled. Therefore it may be concluded that the activity of the intracrystalline pore space in the isomerization reaction is negligible. Thus the isomerization of 1,2,4-TMB to 1,3,5-TMB and 1,2,3-TMB occurs primarily on the external surface of the H-ZSM-5 crystallites. The formation of 1,2,4,5-TeMB inside the micropores of ZSM-5 is consistent with observations made by Yashima et al. [15]. We conclude that only xylenes and 1,2,4,5-TeMB are formed in the micropores, while 1,2,3,4-TeMB, 1,2,3,5-TeMB, 1,2,3-TMB and 1,3,5-TMB are formed on the external surface.
Interaction of the Beta zeolite catalyst with 1,2,4-TMB In order to understand if the beta channel system is accessible to the reactant molecule, we have studied the spectroscopic perturbation of the OH stretching groups of the catalyst caused by the interaction with pseudocumene. The results of pseudocumene adsorption at room temperature are reported in figure 4 for the beta zeolite pre-treated in vacuum at 673K. Upon 1,2,4-TMB dosage the two silanol bands at 3784 and 3748 cm -1 disappear and a clear peak at 3571 cm -1 is formed. The strength of the interaction (as deduced from the OH shift Av = -165 cm -]) induced by pseudocumene interaction is the one expected upon the formation of the H-bond between free silanols and the It electron density of the benzene ring of 1,2,4-TMB [16]. It is interesting to note that the BrOnsted site band at 3610 cm -] is completely eroded upon pseudocumene dosage. This clearly indicates that the reactant can diffuse within the channel system. As far as the strong and broad band centred at 3247 cm -~, induced by the formation of the H-bond between Br6nsted acidic sites and the rt electron density of the benzene ring of 1,2,4-TMB, no substantial modification is observed upon pseudocumene dosages. This result means that all the Br6nsted acidic sites are engaged in stronger H-bonds with 1,2,4-TMB and are not ready to further interaction.
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Interaction of the Beta zeolite catalyst with 1,3,5-TMB Using the more bulky 1,3,5-TMB as probe molecule, the results of the absorption experiment over H-Beta are very similar to that obtained dosing 1,2,4-TMB. Also in this case, Fig. 5, the silanol groups located at 3784 and 3748 cm 1 and the BrOnsted acid sites at 3610 cm -~ are completely eroded upon mesitylene dosage. We conclude that also the bulky mesitylene molecule is capable to diffuse into the micropore system of the
2286 H-Beta zeolite. This fact is particularly important for the mechanistic consideration discussed for the 1,2,4-TMB isomerization reaction. From these IR results we can affirm that the two isomerization reactions of 1,2,4-TMB to 1,2,3-TMB and 1,3,5-TMB can occur inside and outside the channel system.
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Interaction of the H-ZSM5 zeolite catalyst with 1,2,4-TMB In order to understand if the 10-MR three-dimensional channels of H-ZSM-5 are accessible to the reactant molecule, we have studied the spectroscopic perturbation of the OH stretching groups of the catalyst caused by the interaction with pseudocumene. The results of pseudocumene adsorption at room temperature are reported in Fig. 6 for the sample pre-treated in vacuum at 673K. Upon 1,2,4-TMB dosage the silanol band at 3748 cm -] disappears and a clear peak at 3571 cm -~ is formed. The strength of the interaction (as deduced from the OH shift Av= -165 cm -~) induced by pseudocumene interaction is the one expected upon the formation of the H-bond between free silanols and the n electron density of the benzene ring of 1,2,4-TMB. On the contrary the A1-OH and Br6nsted sites located at 3665 and 3610 cm -~ are completely unperturbed upon 1,2,4-TMB dosages at room temperature. This means that in these conditions (room temperature and very low pressure) 1,2,4-TMB interacts with the external surface, but it does not diffuse within the channel system (blue line Fig. 6). In order to observe 1,2,4-TMB diffusion into the channels, we have heated the sample containing the probe molecule at 623K for 15 minutes. IR spectra where then collected after cooling to RT, without further dosages. Upon heating, the probe molecule gradually reaches the internal channels. In fact, it is possible to note the formation of a little and broader band between 3400 and 3050 cm -~. At the same time, the silanol peak at 3748 cm -] increases, while the broader band centred at 3575 cm -~, assigned to silanoi hydroxy groups interacting with 1,2,4-TMB, decreases. Clearly in this phase a slow diffusion of the probe molecule from the external surface to the internal channels takes place. From this consideration we can conclude that over H-ZSM-5, diffusion limitations are present for the reactant molecule. However, in reaction conditions (350~ and 50 bar), 1,2,4-TMB can probably penetrate more easily within the micropores. It is evident that the reactant molecule are critically sized for the ZSM-5 pore system. It may therefore be expected that the 1,2,4-TMB reaction is a sensitive test for shape selective properties and external surface activity.
2287
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Figure 6. IR spectra at RT for increasing dosages of 1,2,4-TMB on H-ZSM-5 outgassed at 673K (background spectra red line). CONCLUSIONS In this work we have studied the 1,2,4-TMB (pseudocumene) transformation over three different 12-MR framework zeolites (Beta, Y) and a 10-MR structure (ZSM-5). At low conversion of 1,2,4-TMB, it is possible to identify primary products since secondary reactions are negligible. At very low conversion, among the xylene isomers and TeMB isomers, o-xylene and 1,2,4,5-TeMB are formed in excess of the thermodynamic equilibrium. The selective formation of o-xylene and 1,2,4,5-TeMB can be explained by transition state and product shape selectivity. In fact, among the nine possible TS of transalkylation reaction, the one producing o-xylene and 1,2,4,5-TeMB is the most slender. The isomerization of 1,2,4-TMB yields 1,3,5-TMB and 1,2,3-TMB. Whereas in Beta, USY and Mordenite at low conversion the 1,3,5-TMB/1,2,3-TMB is lower than thermodynamic equilibrium, in ZSM-5 the thermodynamic equilibrium is maintained at all conversions. These results indicate that 1,2,3-TMB is produced in the channel system of 12-MR zeolites, while 1,3,5-TMB diffuse out of the pores rather slowly. On the contrary over ZSM-5 all C9 isomers are formed mainly on the external surface. An other interesting data is the ratio TeMB/TMB: in Beta this ratio approaches the thermodynamic equilibrium at high conversion only, in ZSM-5 this equilibrium is never achieved, while in USY this ratio is always equal to the equilibrium value. We may explain these results with transition-state shape selectivity: the presence of super-cages in USY promotes the bulky TS of transalkylation reaction. This TS is disfavoured in ZSM-5 channel system. However, in USY too, among the TeMB formed in super-cage only 1,2,4,5-TeMB desorbs out of the channels (product shape selectivity). As the conversion increases the distribution of xylenes, TMB and TeMB approach their respective equilibrium composition (secondary reaction on external surface). These conclusions are supported by the results of probe-molecule experiments where the reactant and the bulkiest product are contacted with the catalyst. REFERENCES 1. H.G. Franck, J.W. Stadelhofer, Industrial Aromatic Chemistry, Springer-Verlag, Berlin 1998 2. Chemical Society, Washington DC, 1976, p.680. 3. D.R. Stull, E.F. Westrum, G.C. Sinke, "The Chemical Thermodynamics of Organic Compounds", Wiley, New York, 1969. 4. M. Guisnet, N.S. Gnep and S.Morin, Microporous and Mesoporous Materials 35-36 (2000) 47-59. 5. G.A. Olah, M.W. Meyer, N.A. Overchuck, J. Org. Chem 29 (1964) 2313-2315. 6. R.H. Allen, L.D.Yats, D.S. Erley, J. Am. Chem. Soc. 82 (1960) 4853. 7. M.L. Poutsma, in: J.A. Rabo(Ed.), Zeolite Chemistry and Catalysis, ACS Monographs vol.171, American Chemical Society, Washington, DC, 1976, p.491. 8. A. Streitwieser Jr., L. Reif, J. Am. Chem. Soc. 82 (1960) 5003.
2288 9. 10. 11. 12. 13. 14. 15. 16.
E. Kikuchi, T. Matsuda, H. Fujiki, Y. Morita, Applied Catalysis, 11, 1984, 331. H.P. R/3ger, K.P. M/311er, C.T. O'Connor, Microporous Materials 8 (1997) 151-157. M. Farcasiu, D.F. Degnan, Ind. Eng. Chem. Res. 27, 1988, 45. L. Wang, C.L. Ay, Appl. Catal. 54 (1989) 257. D.H. Olson, G.T. Kerr, S.L. Lawton, W.M. Meier, J. Phys. Chem., 85 (1981) 2238. P.B. Weisz, Pure Appl. Chem., 52 (1980) 2091. T. Yashima, A. Inaka, S. Namba, Sekiyu Gakkaishi (J. Jpn. Petrol. Inst.) 28(1), 13 (1985). C. Pazb, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina, G. Bellussi, J. Phys. Chem, 101 (1997) 4741.