Methylation of toluene

Effect of physico-chemical properties of ZSM-5 on shape selective methylation of toluene. 215 National Meeting, American Chemical Society at Dallas, Texas, March 1998. 

Xie, Y., Zhao, B., Long, X., and Tang, Y. (2000) Dispersion of oxides on H-ZSM-5 and tlireshold effect on shape selective methylation of toluene. ACS Symp. Sen, 738 (Shape Selective Catalysis), 188-200. 

Because BaO/NaX zeolite catalysts exhibited the best performance, further investigations have been carried out recently to characterize the oxidative methylation of toluene catalyzed by BaO-modified X- and Y-zeolites, mordenite, ZSM-5, sil-icalite, and ALPO4-5 (230). The authors found that activity and basicity of BaO-modified zeolites and zeolite-like catalysts depend on both the structural type and composition. Thus, for samples of the same structural type (BaO/NaX zeolite. 

Monsanto disclosed the manufacture of ethylbenzene through a different approach by the methylation of toluene in the side chain.318 A cesium-exchanged faujasite promoted by boron or phosphorus is used as the catalyst. Toluene and methanol (5 1) reacting at 400-475°C produce an ethylbenzene-styrene mixture at very high toluene conversion. About 50% of the methanol is converted to carbon monoxide and hydrogen, which is a disadvantage since such a plant should operate in conjunction with a methanol synthesis plant. 

Figure 34 shows how the para selectivity of the methylation of toluene depends on the content of Sb203 in the catalyst. The para selectivity reaches the maximum as the content of Sb203 in the catalyst approaches... 

In general the selectivity in toluene methylation found with MFI molecular sieve catalysts is proposed to be caused either by a restricted transition state to form m- and o-xylene [10,11,12] and/or diffusional constraints of the bulkier isomers, o- and m-xylene, in the pores of zeolite ZSM5 [3,4], Recent results on the methylation of toluene based on in situ analysis of the working catalyst showed that all three isomers were primary products in toluene methylation [13]. The high p-selectivity was explained to be due to transport constraints of the bulkier... 

The catalytic activity and selectivity for the methylation of toluene was studied over 1 hour on stream at 570 K. The kinetic results are summarized in Figures 1 and 2 as well as in Table 2... 

Catalytic activities and selectivities of ZSM5 for the methylation of toluene at 570 K... 

The activity and selectivity of HZSM5 zeolites in the catalytic methylation of toluene at 570 K is found to be constant for small particle samples. While p-xylene can easily be transported in the pores of ZSM5, the bulkier isomers, m- and o-xylene (which are also primary products in toluene methylation) are partly retained in the pore system. Adsorbed at the catalytically acitve sites, these xylene molecules isomerize to the faster diffusing p-xylene and increase in this way the p-selectivity. After reaching a steady state in the surface coverage of the xylenes, the rate of formation of m- and o-xylene equals the rate of isomerization to p-xylene plus the rate of transport out of the pores. 

R Mantha.S.Bathia.M.S.Rao Kinetics of Deactivation In Methylation of Toluene over H-ZSM-5 and Hydrogen Mordenite Catalysts itKtEng.Chem.Res. 30 (1991) 281... 

The Dependency of Shape Selectivity on Diffusivitv. The methylation of toluene to produce xylene isomers using ZSM-5 (MFl type) zeolite catalyst is a typical reaction, in which the shape selectivity is observed. In this reaction, xylene isomers once produced inside the zeolite crystal diffuse to the outer surface of the crystal, and the shape selectivity is ascribed to the difference of the diffusivity among the isomers. 

Figure 6. Reaction rate of methylation of toluene over HZSM-5 is directly proportional to the concentration of chemisorbed methanol [108].

Product selectivity results from differences in the size of the products produced from a given reaction. In a homogeneous reactions methylation of toluene gives a mixture of ortho, meta and para xylenes but when H-ZSM-5 is used as the acid, p-xylene is the almost exclusive product (Eqn. 10.21) because the passage of this less bulky isomer through the pores of the zeolite is not restricted while the more bulky o- and m-isomers are too large to easily go through them. 2... 

The chemical selectivity obtained in the alkylation of aromatic molecules over zeolitic catalysts critically depends upon their acid base properties. While xylenes are the primary products in the methylation of toluene over acidic zeolites like HZSM5 [e.g. 1,2,3], ethylbenzene and styrene are formed over basic zeolites such as Rb-X and Cs-X [e g. 4,5,6]. Previous reports suggested the surface chemistry of chemisorbed methanol to be the most decisive parameter to determine the selectivity [7,8]. Recent experiments on toluene methylation to xylenes indicated, however, that various bimolecular precursors to the transition state in the rate determining step exist and may be important for the catalytic properties of zeolites [9,10],... 

Upon heating, the loaded HZSM5 catalyst in a gas stream containing both reactants, the coadsorption complex was observed up to temperatures of 453 K. The decrease in its concentration occurred in parallel with the appearance of the first reaction products (xylenes)(see Figure 3). Thus, we concluded that it is likely to be a possible precursor to the transition state in the methylation reaction. This is supported by the fact that under reaction conditions the rate of methylation of toluene was found to be directly proportional to the surface concentration of the activated methanol species [23,10]. We think that during the reaction only a small concentration of the bimolecular complex exists which can not be monitored by IR spectroscopy. Its abundance should, however, depend upon the concentration of chemisorbed methanol. 

An important item related to the hardness and softness of the active sites of the zeolites is the change in selectivity in the alkylation reactions of aromatic compounds. In the methylation of toluene, orbital control tends to give p-xylene [19]. In this context, it has been shovra that H-ZSM-5 and H-ZSM-11 with a Si/Al ratio of 17 is a soft electrophile [20]. Soft electrophile have a low-energy LUMO. A soft-soft reaction is fast because of a large interaction between the HOMO of the nucleophile and the LUMO of the electrophile... 

The same catalysts were used for the disproportionation-hydrode-methylation of toluene with hydrogen (1 5 molar ratio) at 550°. The results obtained are shown in Fig. 23. Figure 23A indicates the reaction results at the total feed of 8 ml of toluene over 8 gm of catalyst. Figure 23B indicates the same at a total feed of 40 ml. The comparison of these... 

The methylation of toluene with CH3OH over ZSM-5 zeolite impregnated H3PO4 led to p-xylcnc selectively(Scheme 5.40). 

Xu, H.-S., Pu, S.-P. and T. Inui. Improvement of para-Selectivity in Methylation of Toluene on Various MFI-Type Metallosilicate Catalysts. Catal. Letter, 1996,41, 83-87. 

The products resulting from replacement of aromatic H by CH3, in the vapor-phase radiolyses of the alkylbenzenes, show a strong preference for meta orientation. That these products are derived from radicals is suggested by our observation that no xylenes are formed when toluene vapor is radiolyzed in the presence of iodine. The distribution of isomers does not, however, correspond to the preferential ortho substitution found (16) in the homolytic methylation of toluene, nor does it resemble the nearly statistical distribution observed in liquid phase radiolysis or in vapor-phase photolysis at 2537 A. (17). The process is obviously complex, and there is little direct evidence on which to base a mechanism. It may, nevertheless, be of interest that the C8H9+ ion is observed (15) in the mass spectrum of toluene at moderate pressures. Neutralization of such an ion could lead to radicals and ultimately to aromatic products. It is also of interest that the high pressure mass spectrum shows (15) abundant formation of dimeric ions, which may well be involved in the enhanced production of dimer and higher polymeric products in the vapor phase radiolysis. 

The catalytic oxidation of toluene over metal oxides to benzaldehyde and benzoic acid are well-known industrial processes and minor amounts of coupling products are detected among the products. It is shown that the oxidative coupling of toluene is favoured under anaerobic conditions and that metal oxides of the groups III to V of the periodic table catalyse methyl-methyl coupling [1]. Pb/Li/MgO is chosen for this study because it is known as a selective catalyst for the oxidative methylation of toluene with methane [2-4] and as good benzyl radical producer [1]. 

The present world rcscr es of natural gas that contains mainly methane are still underutilized due to high cost of transportation. Considerable interest is therefore presently shown in the conversion of methane to transportable liquids and feedstocks in addition to its previous sole use for heating purposes by combustion. One possible new route for the utilization of methane derived from natural gas or other sources for conversion to more valuable higher hydrocarbons is the methylation of aromatic hydrocarbons. This chapter provides a general overview of the work that has been done so far on the use of methane for catalytic methylation of model aromatic compounds and for direct liquefaction of coal for the production of liquid hydrocarbons. The review is especially focused on the use of both acidic and basic zeolites in acid-catalyzed and base-catalyzed methylation reactions, respectively. The base-catalyzed methylation reaction covered in this discussion is mainly the oxidative methylation of toluene to produce ethylbenzene and styrene. This reaction has been found to occur over basic sites incorporated into zeolites by chemical modification or by changing the electronegative charge of the zeolite framework. 

The effect of the content of alkali earth oxides MgO and BaO on the catalytic and basic properties of NaX zeolites have also been investigated by Kovacheva ct al. (32. 46]. For the MgO-modified NaX zeolites, the best catalytic properties were obtained by NaX zeolite impregnated with a maximum of 13 wt. % MgO. The significant improvement in the catalytic properties was attributed to the presence of a moderate amount of strong basic sites. The catalytic activity was found to depend on the temperature and the MgO content. It was demonstrated that the catalysts possess well preserved crystal structure and low amount of MgO crystal phase. It was reported that the amount of basic active sites but not the strength is dependent on the content of MgO in the zeolite. BaO/NaX was also found to be an effective catalyst for the oxidative methylation of toluene with methane. The catalytic properties of this zeolite was reported to be superior to those of the MgO/NaX zeolite. It was demonstrated that an increase in the barium content leads to an increase in the amount of basic sites and a corresponding increase in catalytic activity. The catalysts were found to possess a preserved crystal structure and the existence of differently dispersed BaO and formation of BaC03 were detected in the catalysts. The conversion of toluene, selectivity and the yield of ethylbenzene plus styrene were reported to depend on (i) the content of barium in the zeolite, (ii) the reaction temperature, (iii) the time of catalytic use, (v) the state of the zeolite crystal structure and (vi) the state of barium in the zeolite lattice. 

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