Mechanistic Proposals for Direct Benzene Hydroxylation over Fe−ZSM-5 Zeolite

A mechanism of direct benzene hydroxylation over Fe−ZSM-5 zeolite is discussed from B3LYP density functional theory computations. We demonstrate using a mononuclear iron model supported at the AlO 4 - site of zeolite that Fe−ZSM-5 zeolite reasonably catalyzes the conversion of benzene to phenol with N 2 O. A key in this catalytic reaction is that benzene hydroxylation and N 2 O decomposition occur at the same active site in a well-balanced manner. In the initial stages of the reaction, a reactive oxygen species referred to as “α-oxygen” is formed upon decomposition of N 2 O at the iron active site. The first reaction step is the formation of a complex between benzene and the α-oxygen species. The C−H activation of benzene leads to an intermediate that involves OH and C 6 H 5 groups as ligands. After forming this intermediate, the reaction has a junction that leads to two possible pathways, depending on the concentration of N 2 O. The recombination between the OH and C 6 H 5 ligands, which leads to a complex of phenol, can occur when the concentration of N 2 O is low, whereas the decomposition of nitrous oxide can occur on the intermediate involving OH and C 6 H 5 groups when the N 2 O concentration is sufficiently high. Calculated potential energy diagrams and reaction kinetics show that the latter pathway is energetically more favorable than the former. We propose that benzene hydroxylation over Fe−ZSM-5 zeolite should proceed in the following way: (1) the formation of the benzene complex at the iron active site, (2) the hydrogen abstraction from benzene, (3) the decomposition of N 2 O at the active site, (4) the recombination between the OH and C 6 H 5 ligands, and (5) the release of product phenol.