Alkylation of Toluene over Zeolite Catalysts

Alkylation of toluene has been studied extensively, and as a result, the ortho/para orientation of substit-uents has been established. However, isomerization under its own containing reaction conditions always resulted in a mixture containing xylene isomer in a thermodynamical equilibrium, i.e., 24% para isomer, 55% meta isomer, and 21% ortho isomer. Zeolite catalysts, which possess shape selective characteristics, are expected to produce higher concentration of para-xylene than that in equilibrium by selective toluene alkylation with methanol. Thus, such selective alkylation has been studied extensively.^2),6) One of the objectives of this study is to find a practical method for alkylation of toluene with methanol or ethanol to produce high yields of para-xylene or para-ethylto-luene. The catalysts used in this study are shown in Table 1. Prior to reaction, Na⁺ of the zeolites was exchanged with H⁺, and the catalysts were calcined at 673K for 4h. Activities and selectivities of these catalysts were evaluated in a fixed bed flow reactor of atmospheric pressure. The feed stream was a mixture of N₂, toluene and methanol or ethanol (N₂: toluene + methanol or ethanol=9 : 1). Runs were normally carried out at 523K. W/F was usually 10g-cat.h/mol, where W is the catalyst weight (gram) and F is the total flow rate (moles per hour). Product distribution was analyzed with gas chromatography. Based on the results of alkylation over a variety of zeolites (Fig. 3), pentasil zeolite exhibited both catalytic activity (toluene conversion> 10%) and para-xylene isomer selectivity (≃30%). It was evident that the shape selective characteristics of zeolites were dependent on their effective pore size and acidity (Figs. 4, 5, 6), i.e., zeolites that have smaller pore sizes and lower Al contents exhibited higher selectivities for para-isomer. It was reported¹³⁾ that the shape selective characteristics of zeolites could be enhanced by selective poisoning of their outer surface acid sites. Therefore, such selective poisoning was attempted using 2,4-dimethyl quinoline (Fig. 9), which, because of its effective molecular size, cannot enter the zeolite pores. However, based on the absence of para isomer selectivity observed, the shape-selective characteristics of the zeolites were not affected by mere selective poisoning of their outer surface acid sites. It is well-known^3),5) that modification of pentasil zeolite by phosphorus or boron enhances the shape-selective characteristics because of the decrease in the size of the pores and number of strong acid sites. The modification of pentasil by impregnation with a phosphorus compound or boron compound also enhances para isomer selectivity (Figs. 10 and 11). Para xylene selectivity was more than 40% on these modified pentasil zeolite catalysts, while it was about 30% on untreated pentasil zeolite. However, offretite/eri-onite zeolite, whose pore size is nearly equal to that of pentasil zeolite, was not effective, even when it was modified with phosphorus. Compared with methyla-tion of toluene, ethylation of toluene exhibited greater shape selective characteristics (Fig. 12). In the ethylation of toluene, the untreated pentasil zeolite dispro-portionates toluene to produce dominantly benzene and xylenes, and produces only a small amount of ethyl-toluene nearly in the equilibrium para-isomer composition (26%). On the other hand, the modified pentasil zeolite produces more than 70% para isomer in ethyltoluene but scarcely any ortho isomer.