Isomerization of Xylene and Methylation of Toluene on Zeolite H-ZSM-5. Compound Kinetics and Selectivity

Abstract

The alkylation of toluene with methanol and the isomerization of xylene were investigated separately and in detail on zeolite H-ZSM-5 as catalyst. The influence of crystal size alone on rates and product distributions was studied for two Si/Al ratios in the zeolite at 573 and 723 K in a gradientless batch-reactor. The isomer distribution in the xylene resulting from toluene methylation is constant from zero up to at least 50% conversion of the methanol in a stoichiometric mixture, H-ZSM-5 being always para-selective. para-Xylene prevails increasingly in the products when either crystal size, temperature, or Al content in the zeolite increase; essentially pure p-xylene can be obtained in the limit with unmodified H-ZSM-5 as catalyst. The rate of isomerization of xylene alone to an equilibrium mixture of isomers was found to be roughly proportional to the extent of the (external) surface of the zeolite crystals, the ratios of the rates of different reactions in the triangular scheme of isomerization being nearly independent of crystal size. The observations can be understood as resulting from the interaction of reaction and diffusion in the volume of the crystals.