Stable Fe/ZSM‑5 Nanosheet Zeolite Catalysts\nfor the Oxidation of Benzene to Phenol

Abstract

Fe/ZSM-5 nanosheet\nzeolites of varying thickness were synthesized\nwith di- and tetraquaternary ammonium structure directing agents and\nextensively characterized for their textural, structural, and catalytic\nproperties. Introduction of Fe³⁺ ions in the framework\nof nanosheet zeolites was slightly less effective than in bulk ZSM-5\nzeolite. Steaming was necessary to activate all catalysts for N₂O decomposition and benzene oxidation. The higher the Fe content,\nthe higher the degree of Fe aggregation was after catalyst activation.\nThe degree of Fe aggregation was lower when the crystal domain size\nof the zeolite or the Fe content was decreased. These two parameters\nhad a substantial influence on the catalytic performance. Decreasing\nthe number of Fe sites along the <i>b</i>-direction strongly\nsuppressed secondary reactions of phenol and, accordingly, catalyst\ndeactivation. This together with the absence of diffusional limitations\nin nanosheet zeolites explains the much higher phenol productivity\nobtainable with nanostructured Fe/ZSM-5. Steamed Fe/ZSM-5 zeolite\nnanosheet synthesized using C_22‑6‑3·Br₂ (domain size in <i>b</i>-direction ∼3 nm)\nand containing 0.24 wt % Fe exhibited the highest catalytic performance.\nDuring the first 24 h on stream, this catalyst produced 185 mmol_phenol g^–1. Calcination to remove the coke\ndeposits completely restored the initial activity.