Nature of Catalytic Active Sites in Supported MOx/ZSM-5 Catalysts: Anchoring Sites, Electronic Structures, Molecular Structures and Reactivity

Abstract The non-oxidative dehydroaromatization (DHA) of methane to aromatics by ZMS-5 supported group V-VII transition metal oxide (VOx, CrOx, MoOx, WOx and ReOx) catalysts has received much attention in recent years. Despite the intensive catalysis studies, many fundamental issues still remain unclear such as the anchoring sites and molecular structures of the initial supported MOx phases on the ZSM-5 support (Si/Al= 15, 25, 40 and 140), and the nature of the M catalytic active sites during the CH4 DHA reaction. A systematic in situ molecular spectroscopic study under fully dehydrated conditions and operando spectroscopic study during methane DHA were investigated over a series of supported MOx/ZSM-5 catalysts in order to resolve the molecular level details.The in situ Raman spectra identified that the initially supported MOx phases are completely dispersed as surface monoxo (M=O) and dioxo (O=M=O) MOx species on the ZSM-5 supports. The corresponding in situ UV-vis spectra of supported MOx/ZSM-5 catalysts have high edge energy (Eg) values for the Ligand-to-Metal-Charge Transfer (LMCT) transitions and the absence of d-d transitions are consistent with that the supported MOx species are isolated and fully oxidized (V+5, Cr+6, Mo+6, W+6 and Re+7) on the ZSM-5 support, respectively.The reactivity of the supported MOx/ZSM-5 catalysts was studied with temperature-programmed operando Raman-MS spectroscopy. The supported VOx/ZSM-5 and CrOx/ZSM-5 catalysts were not selective to benzene and formed only carbon monoxide and hydrogen. Only supported ReOx/ZSM-5, MoOx/ZSM-5 and WOx/ZSM-5 were found to be active catalysts for methane DHA and the temperature for initial benzene formation decreased in the order of WOx/ZSM-5 (~800 oC) < MoOx/ZSM-5 (~700 oC)