Acylation of m-cresol with acetic acid supported by in-situ ester formation on H-ZSM-5 zeolites

Abstract

Friedel-Crafts acylation of phenolic compounds with carboxylic acids catalyzed by Brønsted-acid zeolites is an interesting reaction that could find application in the production of fuels and chemicals from renewable feedstocks. The mechanism of direct acylation of m-cresol with acetic acid over protonic zeolites and the role that the confining environment may have on reaction rate and product selectivity are topics of current interest. Here, we evaluate this reaction at varying conditions over H-ZSM-5 catalysts with different acid site densities. By decoupling the effect of parasitic phenomena – diffusion and catalyst deactivation – we have been able to evaluate the true kinetics of this reaction. We have found that, in the absence of a solvent, the reaction over H-ZSM-5 zeolites under commonly studied conditions (Si/Al = 40) at 250 °C is often influenced by the diffusion of reactants to active sites. By further decreasing active site density and operating under diffusion-free conditions, the true activation barrier for acetophenone (2-hydroxyl-4-methylacetophenone) formation obtained on H-ZSM-5 under surface-saturation conditions is relatively low (41 kJ/mol). This value is significantly lower than the barrier associated with the dehydration of the carboxylic acid to form surface acyl species when not assisted by the formation of intermediate ester species. Based on reaction kinetic analysis, it is proposed that the formation of acylium ion from adsorbed m-tolyl acetate is the rate-limiting step. However, feeding the intermediate ester – i.e., m-tolyl acetate – to act as acylating agent, results in reduced rates compared to those in which the acylating agent is acetic acid. This result reveals that diffusion of m-tolyl acetate is slow and that the kinetically relevant ester is formed within the confines of the zeolite pores. By co-feeding water with m-tolyl acetate, we reveal that the bulky ester is hydrolyzed to form smaller molecules that diffuse faster inside zeolite pores and subsequently enhance the reaction rate. Vapor phase acylation of m-cresol with acetic acid over low acid site density H-Beta zeolite confirms the role of the active intermediate ester in accelerating the rate of acylated product formation. Catalyst deactivation during reaction increases with the partial pressure of acetic acid, even under the zero-order kinetic regime, during which the partial pressure of produced gaseous ketene remains constant. This result would indicate that this usual coke precursor is not the main cause of deactivation, but rather bulkier compounds formed by subsequent esterification of acetic acid with the acylated product (e.g., 2-hydroxyl-4-methylacetophenone).