(446c) Nature of Surface Metal Oxide Sites Present for Supported MOx/ZSM-5 Catalysts

The well-dispersed group V-VII transition metal oxides supported on zeolites have received significant attention in recent years due to their industrial potential as catalysts for numerous chemical reactions. In spite of the extensive characterization studies on these catalyst systems, a general consensus has not been reached about many fundamental details such as the anchoring sites and the molecular and electronic structures of the surface MOx species on ZSM-5 supports. This study aims to resolve the debates on the anchoring sites and electronic and molecular structures of surface MOx species (M=V,Cr, Mo, W and Re) on ZSM-5 supports with the aid of modern in situ spectroscopy (FT-IR, UV-vis, Raman and XANES/EXAFS). In situ Fourier Transform Infrared (FT-IR) Spectroscopy showed that Al-(OH)⁺-Al Brønsted acid sites are the preferred anchoring site of the ZSM-5 support for surface MO_x species, but external silanol and extra framework aluminum hydroxyls also become anchoring sites at high metal oxide loadings. In situ UV-vis diffuse reflectance spectroscopy (DRS) demonstrated that all the supported MOx species are fully oxidized and present as isolated sites. In situ Raman and in situ extended X-ray absorption spectroscopy (XAS) revealed that only one surface MOx structures are present for supported V₂O₅/ZSM-5 and Re₂O₇/ZSM-5 catalysts (mono-oxo O=VO₃ and trioxo (O=)₃ReO, respectively), two surface CrOx structures are present for supported CrO₃/ZSM-5 catalysts (mono-oxo O=CrO₃ and dioxo (O=)₂CrO₂), and four distinct surface MoOx and WOx structures are present on supported MoO₃/ZSM-5 and WO₃/ZSM-5 catalysts dependent of the Si/Al ratio of the ZSM-5 support(dioxo (O=)₂MO₂ species anchored to two Brønsted acid sites at low MOx loading and high Al content, dioxo (O=)₂MO₂ species anchored to one Brønsted acid site at increasing MOx loading, dioxo (O=)₂MO₂ species anchored to two Si-OH sites at low Al content and high MOx loading and mono-oxo O=MO₄ anchored at extra framework Al-OH sites. These new studies demonstrate how systematic, modern in situ spectroscopic studies are able to resolve the fundamental issues of complex metal oxide catalytic systems.