Possible active sites in Fe/ZSM-5 zeolite for the direct benzene hydroxylation to phenol: 1. μ-Oxo[Fe, M] species (M = Fe, Al)

Abstract

Density functional theory was employed to obtain the configurations of oxo-bridged [Fe, M] exchanged ZSM-5 zeolite clusters, possible active sites for direct benzene hydroxylation. For oxo-bridged [Fe, Fe] species, the Fe–Fe and Fe–O distances in Configurations a, b, c, d, e 1 and f were more consistent with experimental (ca. 2.53 or 3.06 Å) than in the cluster models designed by Yakovlev et al. with two framework Al atoms nearby. For oxo-bridged [Fe, Al] species, the geometric parameters of Configurations g, i and j agreed well with experimental. Configuration i was found to be of higher stability by 64.2 kJ mol −1 compared to its isomer Configuration k 1. +II and +III constituted the dominant valence states for Fe in Fe/ZSM-5 zeolite, and the state of mixed +II and +III valences did exist in ZSM-5 zeolite as in Configuration b 1, b 2 or c. Fe of +I state was absolutely non-existent in ZSM-5 zeolite even if predefined so. The calculated reaction heats of “α-oxygen” formation clearly showed that the oxidation processes were always facilitated by N 2O than O 2. The reaction heats of ZSM-5 zeolites exchanged with mono-Fe and binuclear Fe species ( n → o, a 1,2 → b 1,2 and e 1 → f) were −114.9, −65.3, −67.4 and −48.8 kJ mol −1, respectively, in excellent agreement with the results obtained by Ryder, Yakovlev et al. Moreover, “α-oxygen” generation was favored by introduction of extra-framework Al ( i → j: −73.9 kJ mol −1) compared to the binuclear Fe species. All factors considered, the best candidates for “α-oxygen” generation were Configuration a 1,2 for oxo-bridged [Fe, Fe] species and Configuration i for oxo-bridged [Fe, Al] species, respectively.