Molecular Engineering of Supported Vanadium Oxide Catalysts Through Support Modification

Abstract

Highly dispersed, multilayered surface metal oxide catalysts (V2O5/MO x /SiO2, M = Ti(IV), Zr(IV) or Al(III)) were successfully synthesized by taking into account various factors that govern the maximum dispersion of metal oxide species on silica. The characterization results revealed that the molecular structures of the surface vanadium oxide species on the modified supports are a strong function of environmental conditions. The surface vanadium oxide species under dehydrated conditions are predominantly isolated VO4 units, similar to the dehydrated V2O5/SiO2 catalysts. Upon hydration, the surface vanadium oxide species on the modified supports consist of polymerized VO5/VO6 units and/or less polymerized (VO3) n species, which depend on the vanadia content and the specific second metal oxide loading. The surface V cations are found to preferentially interact with the surface metal (Ti, Zr or Al) oxide species on silica. The V(V) cations in the dehydrated state appear to possess both oxygenated ligands of Si(IV)–O− and M–O−. Consequently, the reducibility and catalytic properties of the surface vanadium oxide species are significantly altered. The turnover frequencies of the surface VO4 species on these modified supports for methanol oxidation to redox products (predominantly formaldehyde) increase by more than an order of magnitude relative to the unmodified V2O5/SiO2 catalysts. These reactivity enhancements are associated with the substitution of Si(IV)–O− oxygenated ligands by less electronegative M–O− ligands in the O=V(–O–support)3 structure, which strongly suggests that the bridging V–O–support bonds play a key role in determining the reactivity of the surface vanadium oxide species on oxide supports.