The mechanism of transmethylation in anisole decomposition over Brønsted acid sites: density functional theory (DFT) study

Abstract

In this work, the mechanism and intrinsic reaction energy barriers of transmethylation, as the initial stage of catalytic and non-catalytic anisole decomposition, were investigated by Density Functional Theory (DFT). Molecular analyses indicated that methyl free radical transfer occurred in the absence of a catalyst, and the catalytic transmethylation over Brønsted acid sites was considered based on a dual electrophilic attack mechanism with protonation and carbocation substitution. Reaction modelling for the formation of methyl-containing compounds in both non-catalytic and catalytic anisole decomposition indicated that the energy barriers were significantly decreased in the presence of a catalyst by 60 kcal mol⁻¹ at the most in the case of o-cresol. The results also revealed that the intrinsic transmethylation orientation preferred the ortho- and para-positions on the acceptor compounds containing oxygen-rich substituents due to their large electronegativity, and the lowest energy barrier was observed in the case of transmethylation towards the para-position of the cresol molecule (54.1 kcal mol⁻¹).