Transition State and Diffusion Controlled Shape Selectivity in the Formation and Reaction of Xylenes

Abstract

Abstract The influence of the nature and concentration of the adsorbed species on the reaction rates and selectivities in toluene methylation and xylene isomerization over the zeolite HZSM5 is reported. In the both reactions, the reaction rates were found to be of first order with respect to the surface concentration of the reactants, i.e. methanol in the toluene methylation and xylene in the isomerization. The selectivity in m-xylene isomerization is explained by restrictions to the transition state to form o-xylene. In the case of p- and o-xylene isomerization, the selectivity is controlled by restrictions of the transport of the primary product m-xylene out of the pores. In toluene methylation, all three isomers were found to be primary products but due to the different rates of transport, the bulkier isomers are accumulated in the pores. High selectivity to p-xylene was achieved at elevated temperatures (i.e. at 573 K and above), when the surface concentration of the bulkier xylene isomers was so high, that the rate of the isomerization of the formed xylenes was higher than the rate of alkylation. At low temperatures (i.e, 473 K), low selectivity to p-xylene was found. The accumulated xylene molecules blocked the catalytically active sites and decreased the overall reaction rate. Thus, we conclude that for shape selective methylation of toluene, the rate of isomerization has to exceed the rate of alkylation. Transition state selectivity does not play a major role for the product selectivites in the methylation.