Methanol toluene alkylation, zeolite

Since their development in 1974 ZSM-5 zeolites have had considerable commercial success. ZSM-5 has a 10-membered ring-pore aperture of 0.55 nm (hence the 5 in ZSM-5), which is an ideal dimension for carrying out selective transformations on small aromatic substrates. Being the feedstock for PET, / -xylene is the most useful of the xylene isomers. The Bronsted acid form of ZSM-5, H-ZSM-5, is used to produce p-xylene selectively through toluene alkylation with methanol, xylene isomerization and toluene disproportionation (Figure 4.4). This is an example of a product selective reaction in which the reactant (toluene) is small enough to enter the pore but some of the initial products formed (o and w-xylene) are too large to diffuse rapidly out of the pore. /7-Xylene can, however. 

The acidic/basic properties of zeolites can be changed by introdnction of B, In, Ga elements into the crystal framework. For example, a coincorporation of alnminnm and boron in the zeolite lattice has revealed weak acidity for boron-associated sites [246] in boron-snbstitnted ZSM5 and ZSMll zeolites. Ammonia adsorption microcalorimetry gave initial heats of adsorption of abont 65 kJ/mol for H-B-ZSMll and showed that B-substituted pentasils have only very weak acidity [247]. Calcination at 800°C increased the heats of NH3 adsorption to about 170 kJ/mol by creation of strong Lewis acid sites as it can be seen in Figure 13.13. The lack of strong Brpnsted acid sites in H-B-ZSMll was confirmed by poor catalytic activity in methanol conversion and in toluene alkylation with methanol. 

Fig. 25. Proposed mechanism of the side-chain alkylation of toluene by methanol on basic zeolites (a) and the surface species formed during the decomposition of methanol on basic zeolites (b).

A comparative study with various types of zeolite showed that Cs-exchanged X and Y zeolites were active for toluene alkylation but primarily catalyzed the decomposition of methanol to CO.431 L and Beta zeolites, in turn, were less active and required higher reaction temperature but were much more selective, providing only very little CO. Adding boron to Cs-exchanged zeolites promotes the alkylation reaction 432 It appears that boron reduces the decomposition of methanol to CO without inhibiting active sites for side-chain alkylation. 

Catalytic Reactions. As the techniques for solid-state n.m.r. continue to improve with the simultaneous improvement in sensitivity and hence speed, there will be a growing trend to look at chemical reactions occurring on or in catalysts. There have already been a number of instances where catalytically stimulated reactions have been studied by 13C n.m.r. - the alkylation of toluene by methanol on X zeolite, for example,138 in which the influence of the cation, Na+ or Cs+, on selectivity was deduced. The adsorption binding and decomposition of various metal carbonyls on A1203 or in zeolites has been studied,139 likewise, the nature and sites of interaction of CO and C02 on X and Y zeolites.140... 

Scheme 16. Side chain alkylation of toluene with methanol over basic zeolites.

Figure 3. Para-selectivity as a function of temperatixre for the alkylation of toluene with methanol catalyzed by zeolite ZSM-5.

These effects have been used to explain the case of toluene alkylation by methanol in large zeolites, but other reactions are also being studied, and the results obtained so far are in agreement with the theoretical development. 

The effect of the acidity of polyfunctional zeolite catalysts on their activity in benzene alkylation by propene has been examined.Crystalline zeolites exist with a variety of characteristic pore and channel sizes. In the case of ZSM-5 zeolite, the interconnected channels formed by 10-membered rings of oxygen atoms allows certain benzene derivatives to fit rather closely, diffuse into the pores, and, after undergoing a reaction, diffuse out. This is the origin of the high para selectivity in reactions such as alkylation of toluene by methanol catalyzed by zeolites like ZSM-5 and related modifications. ... 

When refering to shape selectivity properties related to diffusivity, it seems obvious that the larger the zeolite grain, the higher will be the volume/sur f ace ratios and the shape selectivity, since the reaction will be more diffusion controlled. The external surface area represents different percents of the total zeolite area depending on the size of the grains which could be important if the active sites at the external surface also play a role in the selectivity. For instance in the case of toluene alkylation by methanol, the external surface acid sites will favor the thermodynamical equilibrium due to isomerization reactions (o m p-xylene - 25 50 25 at 400 C) while diffusivity resistance will favor the less bulky isomer namely the para-xylene. It may therefore be useful to neutralize the external surface acidity either by some bulky basic molecules or by terminating the synthesis with some Al free layers of siliceous zeolite. 

The only aromatic components that appeared at reaction temperatures below 300 °C were toluene and p-xylene. In the case of the small-crystalline H-ZSM-5(M), however, some m- and o-xylene were present in the product mixture even at 245 °C (WHSV = 6 h- ). This can be explained by xylene isomerization at the outer zeolite surface. At conditions where the para-selectivity was high (more than 90% para), the amount of p-ethyl-toluene (PET) in the product were one order of magnitude greater than that of any other Cg-component, but when it was low, the ratio 1,2,4-trimethyl-benzene (124TMB) PET was found to be about 10 1. These experimental facts indicate that 124TMB is mainly formed by secondary xylene alkylation with methanol. Toluene, p-xylene, PET and perhaps ethyl-benzene are more likely to be the primary aromatic products formed in the MTG-reaction. To confirm this suggestion the molar product ratios EB/PX,... 

The base-catalyzed reactions were carried out over Na, K, Cs modified Y zeolites for example the conversion of toluene to ethylbenzene and styrene in presence of methanol over CsY zeolite. Also the side chain alkylation of picolines with methanol was carried out over CsY. CsY is also used in the synthesis of 4-methylthiazole from acetone, methylamine and SO2... 

The coke deposited during the isobutane alkylation with C4 olefins on zeolites was extracted with a mixture of methanol/toluene, and with CI2CH2. None of these solvents were effective to remove the coke. Only a very small fraction was removed, which was attributed to the dissolution of external coke. The coke molecules formed inside the channels have a large size and therefore cannot diffuse out of the pores . 

However, another type of reactions of benzene derivatives was studied by in-situ IR spectroscopy as well, viz. the side-chain alkylation of alkylbenzenes, for instance of toluene, over basic zeolite catalysts such as M -X zeolites (M=Na, K, Rb, Cs) [901,902]. The intermediate conversion of methanol to formaldehyde turned out to be crucial for the side-chain alkylation as well as a strong polarization of the methyl group of toluene, the preferential adsorption of toluene, and a sufficient basicity, i.e., base strength of the catalyst. Related to these IR studies of side-chain alkylation of toluene were in-situ IR spectroscopic investigations of the decomposition of methanol over basic zeolites (M+-X, M =Na+, K+, Rb, Cs+ Na-ZSM-5 and Cs-ZSM-5 [903]). It was shown that over weakly basic zeolites (Na-ZSM-5, Cs-ZSM-5) dimethyl ether was formed from methanol, whereas over more strongly basic X-type zeolites formaldehyde was produced, which is an indispensable intermediate for the side-chain reaction (vide supra). 

Figure 4.17. Zeolite transition-state selectivity. Toluene alkylation with methanol catalyzed by H-MOR showing the energies of the key reaction intermediates . Reaction energy diagram for ortho-, meta- and para-xylene are compared.

Vasihev and Gahnsky [224] and Palomares et al. [225] studied X-type zeolites exchanged with K, Rb, and Cs and found that the Cs-exchanged X zeolite was the most active for side chain alkylation, producing nearly no xylene isomers. On the other hand, toluene alkylation over Na-X mainly led to the formation of xylenes. An IR-spectroscopic study in [225] revealed that on Na-X, methanol was by far the most abimdant species on the catalyst smface, while on the more basic Cs,Na-X it was toluene. Miyamoto et aL [226] visualized the acid-base cooperative catalysis in the side chain alkylation of toluene by computer graphics and also used it to explore geometrical factors. 

Toluene alkylation with methanol using Friedel-Crafts catalysts results in mixed products since isomerization reactions and fiirther methylation of the desired products readily occur under these reaction conditions [49,50]. Recent work has therefore been aimed at the development of processes with high selectivity, and zeolite catalysts appear to have the most promise in this regard. 

Among the wide variety of organic reactions in which zeolites have been employed as catalysts, may be emphasized the transformations of aromatic hydrocarbons of importance in petrochemistry, and in the synthesis of intermediates for pharmaceutical or fragrance products.5 In particular, Friede 1-Crafts acylation and alkylation over zeolites have been widely used for the synthesis of fine chemicals.6 Insights into the mechanism of aromatic acylation over zeolites have been disclosed.7 The production of ethylbenzene from benzene and ethylene, catalyzed by HZSM-5 zeolite and developed by the Mobil-Badger Company, was the first commercialized industrial process for aromatic alkylation over zeolites.8 Other typical examples of zeolite-mediated Friedel-Crafts reactions are the regioselective formation of p-xylene by alkylation of toluene with methanol over HZSM-5,9 or the regioselective p-acylation of toluene with acetic anhydride over HBEA zeolites.10 In both transformations, the p-isomers are obtained in nearly quantitative yield. 

On selectivity aspects of alkylation of toluene with methanol over zeolites, DGMK tagungsbericht, 9903. Proceedings of DGMK-Confer-ence The Future Role of Aromatics in Refining and Petrochemistry, 1999, pp. 279-286. 

Yashima, T., Ahmad, H., Yamazaki, K Katsuta, M and Hara, N. (1970) Alkylation on synthetic zeolites, 1. alkylation of toluene with methanol. 

Lacroix, C., Deluzarche, A., and Kiennemann, A. (1984) Promotion role of some metals (Cu, Ag) in the side chain alkylation of toluene with methanol. Zeolites, 4,109-111. 

Uu, H.-C. (1985) Modified zeolite catalyst composition and process for alkylating toluene with methanol to form styrene. U.S. Patent 4, 499,318. 

Borgna, A., Magni, S., Speulveda, )., Padro, C.L., and Apesteguia, A.R. (2005) side chain alkylation of toluene with methanol on Cs-exchanged Na-Y zeolite effect of Cs loading. Catal. 

ZSM-5 is a Mobil-proprietary, shape-selective zeolite which is used commercially in synthetic fuels (methanol-to-gasoline), petrochemicals (xylene isomerization, toluene disproportionation, benzene alkylation) and in petroleum refining (lube and... 

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