In situ time-resolved DXAFS for the determination of kinetics of structural changes of H-ZSM-5-supported active Re-cluster catalyst in the direct phenol synthesis from benzene and O2

Abstract

Dynamic structural changes and their kinetics of a Re₁₀-cluster catalyst in the direct phenol synthesis from benzene and O₂ were investigated by in situ time-resolved Re L_III-edge energy-dispersive XAFS (DXAFS). We have successfully monitored the structural transformation of active Re₁₀ clusters to inactive Re monomers in the course of the selective oxidation of benzene with O₂ on the catalyst by the DXAFS technique in a real time. The results obtained suggested that the Re₁₀ cluster transformed directly to the Re monomers, which showed first order kinetics with respect to the quantity of Re₁₀ clusters. The absence of undesirable intermediate structures with low phenol selectivity during the structural transformation may be an advantageous issue for the high phenol selectivity of 93.9% at 9.9% conversion in a pulse reaction and 87.7% at 5.8% conversion in a steady-state reaction on the Re₁₀-cluster catalyst. The reactant benzene inhibited the unfavorable structural transformation of the Re₁₀ cluster to the Re monomers during the selective benzene oxidation to phenol.