Alkylation reactions over ion-exchanged molecular sieve zeolite catalysts. Part 4.—Competitive alkylation of toluene and benzene with ethanol, n-propyl alcohol, isopropyl alcohol and t-butyl alcohol: consideration of the reactivity and selectivity of the alkylating species

Abstract

Results for the competitive alkylation of toluene and benzene with ethanol, n-propyl alcohol, isopropyl alcohol and t-butyl alcohol over NiY and Ni, SnY zeolite catalysts are reported and analysed. The effects of time on stream, reaction temperature (473–543 K) and space velocity on the positional and substrate selectivity in ethylation under non-isomerizing conditions are examined in detail for catalysts which had already reached their steady-state activity in methylation, a high coking reaction. In all cases the substrate selectivity and ethyltoluene isomer composition (27%para, 22%meta, 51%ortho) remained virtually constant.

In contrast to alkylation with ethanol, isomerization of the product alkyltoluenes occurred when n- and iso-propyl alcohol were the alkylating reagents; however, as in ethylation no di- or poly-substituted products were detected.

A thorough investigation of the substrate selectivity (reactivity) and para/meta selectivity of the alcohols as alkylating agents is reported, with reference to both the literature on homogeneous Friedel–Crafts alkylation and heterogeneous catalysis and also linear free energy relationships. The high ortho/para ratios and the high quantities of meta isomer obtained indicates that in all cases the alkylating agent is a highly reactive species; the electrophilic character or reactivity is very similar for methylation, ethylation, n-propylation and isopropylation and the isomer distributions obtained in the present work are remarkably similar to those reported in the literature for homogeneous catalysis. Since the alkylating agents had approximately the same reactivity the enormous increase in rates on changing from methanol to isopropyl alcohol as alkylating agent is attributed to the relative quantities of the intermediate carbonium ions on the catalyst surface.