Novel re-cluster/HZSM-5 catalyst for highly selective phenol synthesis from benzene and O2:: Performance and reaction mechanism

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 Re-10-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 Re-10 cluster for the direct phenol synthesis with O-2 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 Re-10 cluster was converted to inactive Re monomers, while NH3 reproduced the active Re-10 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 Re-10 clusters and the Re monomers. The isosbestic points in energy-dispersive XANES spectra indicated a direct conversion of the Re-10 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 sp(3) carbon atom in the benzene ring and subsequent insertion of the oxygen to the C-H bond on the Re-10 cluster.