Hierarchical mesoporous Fe/ZSM-5 with tunable porosity for selective hydroxylation of benzene to phenol

Abstract

Hierarchical mesoporous Fe–ZSM-5 zeolite with catalytically active surface Fe species is prepared by introduction of Fe³⁺ in the synthesis gel of mesoporous ZSM-5 and the material synthesised with the optimum Fe content and optimum porosity is highly active and selective for the hydroxylation of benzene to phenol with nitrous oxide. The porosity of the material is controlled by the synthesis gel composition (organosilane template concentration). Samples prepared at high template/Si ratio (0.11) show lower zeolitic crystallinity and lower micropore volume but higher strong acidity than the samples prepared with low template/Si ratio (0.036). Samples with very high surface area, high strong acidity, low zeolitic wall crystallinity and 0.93 wt% Fe content show 1.3 times higher phenol formation rate than a steamed H-ZSM-5 sample. Silylation of the mesoporous catalyst resulted in 2-fold increase in initial phenol formation rate compared to steamed microporous H-ZSM-5 which is attributed to improved hydrophobicity and reduced Lewis acid sites, resulting in better desorption of the phenol product from catalyst.