V2O5-SiO2 xerogels for methane oxidation to oxygenates: preparation, characterization, and catalytic properties Original Research Article

V2O5-SiO2 xerogels with a wide range of vanadium contents (1–25 wt.% V2O5) and high surface areas (267–509 m2/g) were synthesized by a two-step sol–gel process as new catalytic materials for selective methane conversion. Three types of V species in the V2O5-SiO2 xerogels were identified by powder X-ray diffraction (XRD), solid-state 51V nuclear magnetic resonance (NMR), and Raman spectroscopy, i.e. (1) accessible surface tetrahedral V5+ species, (2) inaccessible surface tetrahedral V5+ species, and (3) crystalline V2O5. Only the accessible surface tetrahedral V5+ species were the active sites for methane conversion to oxygenates (e.g. methanol and formaldehyde). On the basis of the accessible surface tetrahedral V5+ species, the turnover frequencies of both methane conversion and oxygenates formation substantially decreased with increase in vanadium content and the 1 wt.% V2O5-SiO2 xerogel catalyst exhibited the highest activity as well as the highest selectivity for methane conversion to oxygenates. This result appears to suggest that the accessible surface tetrahedral V5+ species tend to form surface polymeric tetrahedral V5+ species at high vanadium content, which is not active for methane conversion to oxygenates.