ON THE ROLE OF PRODUCT ISOMERIZATION FOR SHAPE-SELECTIVE TOLUENE METHYLATION OVER HZSM5

The dependence of the reaction rates and selectivities in toluene meth ylation over zeolite HZSM5 upon the concentration and type of adsorbed surface species are reported. Rather low selectivity to p-xylene was found at 473 K, while high selectivity was observed at 573 K. Under re action conditions which lead to low selectivity to p-xylene (e.g., at 473 K) the reaction can be described best as a network of parallel alk ylation reactions. Under reaction conditions which lead to high select ivity top-xylene the steady state concentration of the builder isomers in the adsorbed phase was so high that the secondary reaction, the is omerization of the formed xylenes, in the HZSM5 pores was faster than the rate of alkylation. Thus, we conclude that for shape selective met hylation of toluene to p-xylene the rate of isomerization has to excee d the rate of alkylation. High concentrations of adsorbed molecules or coke precursors in the zeolite which certainly increase the spatial c onstraints of the reacting molecules do not suffice to achieve high p- selectivity. This indicates that transition state selectivity does not play a major role for the product selectivities in the methylation re actions studied. (C) 1994 Academic Press, Inc.