Shape selectivity of mesoporous aluminosilicates in the transformation of benzenic hydrocarbons

MESOPOROUS MOLECULAR SIEVES 1998 Studies in Surface Science and Catalysis, Vol. 117 L. Bonneviot, F. B~land, C. Danumah, S. Giasson and S. Kaliaguine (Editors) ~ 1998 Elsevier Science B.V. All rights reserved.

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Shape selectivity of mesoporous aluminosilicates in the transformation of benzenic hydrocarbons M. Guisnet ~ N.S. Gnep ~ S. Morin ~ J. Patarin b, F. Loggiac, V. Solinasc ~ Laboratoire de Catalyse en Chimie Organique, UMR 6503, UFR Sciences, 40, ave. du recteur Pineau, 86022 Poitiers cedex, France b Laboratoire des Mat~riaux Min6raux, Mulhouse, France c Laboratorio Chimica Industriale, Cagliari, Italy

The activity and selectivity of three mesoporous aluminosilicates (MCM-41) with SilAI ratios of 10, 30 and 100 and of an amorphous silica alumina (SA) were compared in toluene alkylation with methanol and in xylene transformation at 350~ For both reactions, the order of activity is the one expected from the density and strength of the protonic sites. For toluene alkylation, the selectivity is practically independent of the catalyst: successive formation of xylenes, trimethylbenzenes, tetramethylbenzenes etc., distribution of xylenes in agreement with a FriedeI-Crafts methylation of toluene, high ratio between the rates of methanol conversion into C1Cs hydrocarbons and into xylenes. On the other hand, the selectivity of xylene transformation is completely different on SA and on MCM-41. With SA, the selectivity of m-xylene isomerization (plo - 1.4) is that expected from the classical intramolecular mechanism. In agreement with this mechanism, the addition of methylcyclohexane to the reactant has no effect on the isomerization rate. With MCM-41, m-xylene isomerizes preferentially into o-xylene (p/o = 0.2-0.4) and methylcyclohexane causes an identical decrease in the rates of isomerization and disproportionation. All this can be explained by a bimolecular isomerization mechanism involving successive m-xylene disproportionation and transalkylation with trimethylbenzenes. This particular behaviour of MCM-41 samples is clearly due to the presence of regular non-interconnected long channels in which xylene molecules can undergo, before desorption, successive reactions of disproportionation and transalkylation. This new type of shape selectivity was designated as Tunnel Shape Selectivity. 1. INTRODUCTION Zeolites are used as acid catalysts in various processes of refining and of production of petrochemicals and even of fine chemicals. One of their main

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advantages over the conventional acid catalysts is their shape selective properties caused by the small size of their cages and of their pore apertures [1]. However, this small size becomes a drawback in the treatment of heavy feeds or in the synthesis of large molecules. The need for acid solids with larger pores has initiated an active research with as a consequence the recent discovery of a new family of mesoporous aluminosilicates, designated as M41S, with pore diameters in the 1.510 nm range [2]. The potentialities of MCM-41 (one of the members of this family) as acid catalysts were explored for various reactions. Their activity in the transformation of small molecules is generally similar to that of amorphous silica alumina as it is furthermore expected from the similar acidities of MCM-41 and of silica alumina [3]. However, for the transformation of bulky molecules, e.g. cracking and hydrocracking of gas oil [3,4], much better results are obtained with MCM-41. In this work, the catalytic properties of MCM-41 samples with Si/AI ratios of 10, 30 and 100 are compared to those of an amorphous silica alumina. The reactions which were chosen for this comparison are toluene alkylation with methanol and mxylene transformation. It is shown that, despite the relatively small size of the reactant and product molecules, the unidirectional mesopore system of MCM-41 has a very significant effect on the selectivity of m-xylene transformation. However, this shape selectivity does not exist for toluene alkylation. 2. EXPERIMENTAL The MCM-41 samples with Si/AI ratios of 10, 30 and 100 were synthesized following the procedure given in ref. 2 using cetyltrimethylammonium bromide (Fluka) as the template, sodium silicate (Crossfield) and sodium aluminate (Carlo Erba) as silicon and aluminium sources. In order to eliminate the templating agent, MCM-41 samples were calcined under dry air at 873K. Silica alumina (14 wt% alumina) was supplied by Ketjen. Before use, all the samples were calcined in situ under dry air flow at 500~ for 10h. Alkylation of toluene with methanol (a) and xylene transformation (b) were carried out in a flow reactor at 350~ under the following conditions : a) p~,,, = 0.1 bar; P,=e,== = 0.1 bar, Pm = 0.8 bar; VWVH (weight of toluene injected per hour and per weight of catalyst) = 1.5 - 40 h"1 b) P~,n= = 0.0625 bar; 13,2= 0.9375 bar ; VWVH = 1 - 11 h-~. Analysis of reaction products was performed on line by FID gas chromatography using a 30m fused silica J&W DB Wax capillary column.

3. RESULTS AND DISCUSSION 3.1. Acidity and porosity of the samples XRD spectra of the three MCM-41 samples are typical of these materials as described by the researchers from Mobil [2]. The BET surface area is equal to 1350, 1020 and 1050 m= g-1 for MCM-41(10, (30) and (100) respectively. For all the samples, there is, in the isotherm for nitrogen adsorption at-196~ a sharp

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inflection for P/Po = 0.35 (Figure 1), which is characteristic of MCM-41 samples. Moreover, the t-plot of data in figure 1 indicates that no micropores are present. However, large differences are found between the adsorption-desorption isotherms. With MCM-41(100), sorption curves are typical of type IV isotherms without any hysteresis, while with MCM-41(30) and especially with MCM-41(10), hysteresis are observed (Figure 1). The pore size distribution is very sharp with MCM-41(100), sharp with MCM-41(30) and relatively large with MCM-41(10). The average pore diameter is equal to 28~ for MCM-41(100), 30.5A for MCM-41(30) and 34A for MCM41(10). Furthermore, TEM of the samples shows that with the latter samples, the elimination of the surfactant by oxidative treatment creates large pores owing to a partial degradation of the framework, whereas it is not the case for MCM-41(100). The number of these large pores is more significant for MCM-41(10) than for MCM41(30). It can therefore be concluded that the differences in pore distribution of the MCM-41 samples shown by nitrogen sorption are due to differences in the extent of the framework degradation. Silica alumina has a BET surface area of 365 m= g-l. Its isotherm for nitrogen sorption presents a hysteresis. The size of the pores is between 30 and 80~., the average pore diameter being of approximatively 40A. 1.2

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Figure 1. Nitrogen adsorption (~k) -desorption (El) isotherms of the MCM-41(100) (a), (30) (b), (10) (c) samples. Table 1 Concentrations (mol g-l) of Br0nsted (Ca) and Lewis (C,) acid sites, activities (mmol h.+ g-Z) for toluene alkylation and for m-xylene transformation of silica alumina and of the MCM-41 samples and selectivity of m-xylene transformation (DII and plo extrapolated at zero conversion) Sample " . CB CL A tew,, A m-m~,,, DII

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The acidity of all the samples was characterized by pyridine sorption followed by IR spectroscopy. The densities of Br0nsted and Lewis acid sites estimated for a desorption temperature of 150~ are given in table 1. With all the samples, the density of Lewis acid sites is higher than that of the Br0nsted sites. Moreover, pyridine thermodesorption shows that the Lewis acid sites are stronger [5]. The concentrations of Br0nsted acid sites of silica alumina and of MCM-41 (10) are similar and approximatively 2.5 times greater than those of MCM-41(30) and MCM-41(100). However, silica alumina has stronger protonic sites than the MCM-41 samples: some pyridine remains adsorbed at 350~ over silica alumina but not over the MCM41 samples [5]. It should be remarked that the ratio between the number of protonic sites and the total number of aluminium atoms increases with the Si/AI ratio (from 0.035 with MCM-41(10) to 0.11 with MCM-41(100)). This suggests that, as is the case with zeolites, the difficulty in the dealumination of the mesoporous aluminosilicate framework increases with their Si/AI ratio. 3.2. A l k y l a t i o n o f t o l u e n e with m e t h a n o l

With all the samples, the reaction products are xylenes and other methylbenzenes resulting from toluene alkylation and C1-Cs hydrocarbons resulting from methanol transformation. For toluene alkylation, silica alumina is initially 1.5 times more active than MCM-41(10) which is 2.5 times more active than MCM41(30) and 4 times active than MCM-41(100) (Table 1). This order in activity is the one expected from the acidity measurements: decrease in the concentration of protonic sites with the increase in the Si/AI ratio of the MCM-41 samples, quasi identical concentration of the protonic sites of MCM-41(10) and of silica alumina, but higher strength of the acid sites of silica alumina [5]. Figure 2 shows that the ratio between the amounts of methanol transformed into light hydrocarbons and into alkylation products are almost identical with silica alumina and with the MCM-41 samples. This ratio, very high (>10) at low conversion of toluene decreases with increasing this conversion. 12 X O

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Similar distributions of the aromatic products and of the light products are found with silica alumina and MCM-41. The scheme of alkylation is successive, only

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xylenes appearing at low conversion. A very small difference in xylene distribution is observed between silica alumina and MCM-41 samples, the latter samples being slightly more selective for o-xylene as shown below by the distributions extrapolated at zero conversion : Silica alumina : ortho 55%, meta 20%, para 25% MCM-41 : ortho 58%, meta 19%, para 23% These product distributions are very close to that generally obtained in FriedeI-Crafts methylation of toluene : ortho : 60%, meta : 14%, para : 26% [6]. The distribution of the light products is also practically independent of the catalyst, e.g. at a toluene conversion of 4%, the distribution obtained with silica alumina and MCM-41 samples (% mol) is the following: C1: 2.5-4%, C2: 25-26%, Ca: 34-35%, C4: 28-33%, Cs: 4-7%. The only appreciable difference concerns the alkane/alkene ratio which is lower with MCM-41 than with silica alumina: e.g. 0.015 with MCM-41(30) and 0.15 with silica alumina. This indicates that hydrogen transfer is more significant with silica alumina than with MCM-41.

3.3. Xylene transformation With all the samples, xylenes are transformed by isomerization and disproportionation, this latter reaction leading to a quasi equimolar mixture of trimethylbenzenes (in proportion close to that of thermodynamic equilibrium) and of toluene. A detailed comparison of the catalytic properties of MCM-41 samples and of silica alumina was carried out in the case of m-xylene transformation. Initially, silica alumina was found to be 4 times more active than MCM-41(10) which is 2 times more active than MCM-41(30) and 4 times more active than MCM-41(100) (Table 1). This order in activity which is similar to the one obtained from toluene (and methanol conversions) can also be explained by the changes in acidity. However, contrary to what was obtained for the transformation of the toluene-methanol mixture, large differences in selectivity can be observed between silica alumina and MCM-41 samples. The main difference concerns the (p/o) ratio which, at low conversions, is equal to 1.4 with silica alumina while it is lower than 1 with MCM-41 samples : from 0.4 with MCM-41(10) to 0.15 with MCM-41(100) (Table 1). Furthermore, with silica alumina the disproportionationlisomerization ratio (D/I) is lower than with the MCM41 samples. With these samples, the greater the Si/AI ratio, the lower DII (Table 1). The large difference in isomerization selectivity suggests that the isomerization mechanisms are different over silica alumina and over MCM-41. Indeed, the plo ratio found with silica alumina is close to that attributed to a purely intramolecular mechanism (1.2). On the other hand, the one found with MCM-41 is close to that of a bimolecular pathway (Figure 3) approximately 0.275 [7]. In agreement with this proposal, a direct isomerization of o-xylene into p-xylene (and vice versa) is observed with MCM-41(30), as could be expected from the bimolecular mechanism : e.g., m/p ratio in the products of o-xylene isomerization over MCM-41 equal to 12 at conversion close to zero (Figure 4). On the other hand, as is expected from a monomolecular mechanism, no direct interisomerization of o- and p-xylene is observed : e.g., very high initial m/p ratio over silica alumina (Figure 4).

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The effect that branched alkanes such as methylcyclohexane have on the rate of xylene isomerization allowed us to discriminate between the mono and the bimolecular isomerization mechanisms. Indeed, it has been shown on FAU zeolites

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Figure 3. Bimolecular mechanism of m-xylene isomerization on MCM-41 50 40 0,,

Figure 4. Isomerization (I) of o-xylene on silica alumina (~) and on MCM41(30) (:~). m-xylene / p-xylene (m/p) = f (%1)

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that branched alkanes inhibit xylene disproportionation without affecting their monomolecular isomerization [8]. This inhibiting effect was attributed to the following reaction 9

This reaction limits the concentration of benzylic carbocations which are involved as intermediates in disproportionation (and transalkylation) reactions. Figure 5 shows that with silica alumina, methylcyclohexane decreases the rate of m-xylene disproportionation but has no effect on isomerization as it is expected from an unimolecular mechanism. On the other hand, methylcyclohexane decreases both isomerization and disproportionation and exactly to the same extent. This identical effect of methylcyclohexane on the rates of these reactions is in favour of a pure bimolecular isomerization pathway.

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Figure 5. m-Xylene transformation on MCM-41(30) (a) and on silica alumina (b). Effect of methylcyclohexane (MCH) on the rate of isomerization (~) and of disproportionation (D). This very high selectivity of MCM-41 samples for bimolecular isomerization cannot be due to the characteristics of their acidity: weak protonic acidity, large concentration of strong Lewis acid sites. Indeed, no bimolecular isomerization can be observed on silica alumina which presents the same acidity characteristics. Therefore, this selectivity is certainly due to the presence of uniform noninterconnected channels with a relatively large diameter (30A). When passing through these channels, the molecules of xylene reactant would be transformed into isomer molecules through successive reactions of diproportionation and transalkylation with trimethylbenzene molecules. We propose to call this particular selectivity of MCM-41 : Tunnel Shape Selectivity. Curiously, this shape selectivity cannot be observed in the narrow pores of unidirectional zeolites such as mordenites. This is probably because steric constraints strongly limit the formation of the bulky intermediates of disproportionation and transalkylation reactions. It should furthermore be emphasized that this tunnel shape selectivity was proposed in order to explain the particular distribution of the products of n-butane transformation (which involves non-bulky bimolecular intermediates) over a non-dealuminated mordenite sample [9]. It is possible through the bimolecular pathway (Figure 3) to explain the differences in D II and p/o ratio obtained in m-xylene transformation over the MCM41 samples (Table 1). Indeed, trimethylbenzene molecules resulting from m-xylene disproportionation can undergo during their passage through the channels, one or several successive reactions of transalkylations (known to be faster than disproportionation [7]) with production of xylene molecules. Therefore, the decrease in DII with increasing the Si/AI ratio of the MCM-41 samples would be related to an increase in the number of transalkylation steps which lead to xylene isomers. This increase in the number of transalkylation steps could also be responsible for the decrease in the plo ratio. Indeed, not only m-xylene but also o- and p-xylenes can undergo transalkylation reactions and the transalkylation of trimethylbenzenes with p-xylene is faster than with o-xylene [10] and moreover leads to a supplementary formation of this isomer, e.g. :

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The increase in the number of transalkylation steps is most likely due to the decrease in the number of large pores created during oxidative removal of the surfactant (as was shown by TEM), hence to an increase with Si/AI of MCM-41 samples in the length of the mesoporous channels. 4. CONCLUSION A new mode of shape selectivity has been discovered in MCM-41 mesoporous aluminosilicates. This new mode called Tunnel Shape Selectivity is responsible for a purely bimolecular pathway of xylene isomerization : in the non- interconnected long channels of MCM-41, reactant molecules undergo, before desorption, various successive disproportionation and transalkylation reactions with formation of xylene isomers. Curiously, no shape selectivity is observed in toluene alkylation with methanol. The selectivity of MCM-41 is identical to that of an amorphous silica alumina presenting a similar acidity : in particular there is no preferential formation of polymethyibenzenes. This could be due to the very low rate of alkylation with consequently the possibility of only one alkylation step during the passage of toluene molecules through the channels and / or to the preferential transformation of methanol into C1-Cs hydrocarbons.

REFERENCES 1. N.Y. Chen, W.E. Garwood, F.G. Dwyer, in Shape Selective Catalysis in Industrial Applications, Chemical Industries, H. Heinemann (ed.), vol. 36, M. Dekker, Inc., New York, 1989. 2. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli and J.S. Beck, Nature, 359 (1992) 710; J.S. Beck, M.C. Vartulli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.O. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Amer. Chem. Soc. 114 (1992) 10834. 3. A. Corma, A. Martinez, V. Martinez-Soria and J.B. Monton, J. Catal. 153 (1995) 25. 4. A. Corma, M.S. Grande, V. Gonzalez-Alfaro and .~V. Orchilles, J. Catal. 159 (1996) 375. 5. S. Morin, P. Ayrault, S. El Mouahid, N.S. Gnep and M. Guisnet, Appl. Catal. A: General 159 (1997) 317. 6. J.S. Beck and W.O. Haag, in Handbook of Heterogeneous Catalysis, G.ErU, H. Kn0zinger, J. Weitkamp (eds.), V.C.H. Weinheim, vol. 5, p. 2123 (). 7. S. Morin, N.S. Gnep and M. Guisnet, J. Catal. 159 (1996) 296. 8. N.S. Gnep and M. Guisnet, React. Kin. Catal. Lett. 22 (1983) 237. 9. M.T. Tran, N.S. Gnep, G. Szabo and M. Guisnet, J. Catal. 174 (1998) 10. S. Morin, unpublished results.