Investigation of Silica-Supported Vanadium Oxide Catalysts by High Field 51V Magic-Angle Spinning NMR

Supported V2O5/SiO2 catalysts were studied using solidstate V-51 magic-angle spinning NMR at a sample spinning rate of 36 kHz and at a magnetic field of 19.975 T to provide a better understanding of the coordination of the vanadium oxide as a function of environmental conditions. Structural transformations of the supported vanadium oxide species between the catalyst in the dehydrated state and hydrated state under an ambient environment were revisited to examine the degree of oligomerization and the effect of water. The experimental results indicate the existence of a single dehydrated surface vanadium oxide species that resonates at-67S ppm and two vanadium oxide species under ambient conditions that resonate at 566 and 610 ppm. No detectable structural difference was found as a function of vanadium oxide loading on SiO2 (3% V2O5/SiO2 and 8% V2O5/SiO2). Quantum chemistry simulations of the 51V NMR chemical shifts on predicted surface structures were used as an aid in understanding potential surface vanadium oxide species on the silica support. The results suggest the formation of isolated surface VO4 units for the dehydrated catalysts with the possibility of dimer and cyclic trimer presence. The absence of bridging V-O-V vibrations (similar to 200-300 cm(-1)) in previous Raman spectra, however, indicates that the isolated surface VO4 sites are the dominant dehydrated surface vanadia species on silica. Upon exposure to water, hydrolysis of the bridging V-O-Si bonds is most likely responsible for the decreased electron shielding experienced by vanadium. No indicators for the presence of hydrated decavanadate clusters or hydrated vanadia gels previously proposed in the literature were detected in this study.