Novel Re-Cluster/HZSM-5 Catalyst for Highly Selective Phenol Synthesis from Benzene and O2: Performance and Reaction Mechanism

Abstract

This paper reports the catalytically active structure, its structural transformation and dynamics, and the reaction mechanism for direct phenol synthesis from benzene and molecular oxygen on a novel N-interstitial Re10-cluster/HZSM-5 catalyst, which exhibited remarkable phenol selectivities of 91.6−93.9% at 1.7−9.9% conversions in pulse reactions and 82.4−87.7% at 0.8−5.8% conversions in steady-state reactions. The active N-interstitial Re10 cluster for the direct phenol synthesis with O2 as an oxidant was produced by NH3 at 553 K and the phenol synthesis proceeded with the low activation energy of 24 kJ mol-1. The active Re10 cluster was converted to inactive Re monomers, while NH3 reproduced the active Re10 cluster under the reaction conditions. It was found by means of in situ time-resolved XAFS and DFT calculations that the catalytic phenol synthesis proceeded in conjunction with the structural transformation between the Re10 clusters and the Re monomers. The isosbestic points in energy-dispersive XANES spectra indicated a direct conversion of the Re10 cluster to the Re monomers without any unfavorable metastable intermediates. The benzene oxidation is proposed to proceed via oxygen addition to a C atom of benzene to form an oxygen-added intermediate with a sp3 carbon atom in the benzene ring and subsequent insertion of the oxygen to the C−H bond on the Re10 cluster.