Density functional theory calculation and X-ray absorption spectroscopy studies of structure of vanadium-containing aluminophosphate VAPO-5

Theoretical calculation using density functional B3LYP correlation-exchange is employed to study the structure of different sizes of the VAPO-5 cluste rs. The results indicate that the substitution of either the phosphorus sit e or the aluminum site of the aluminophosphate-5 framework with vanadium is generally not feasible. The calculated structures are comparable with thos e from the in-situ X-ray absorption (XANES and EXAFS) of the V K-edge. The isolated vanadium was found to exist as a mono-ore (V4+ = O)(O-f)(4) specie s for reduced, dehydrated VAPO-5 and as a di-oxo V5+O4 species for calcined , dehydrated VAPO-5. The coordination geometry of the mono-ore V4+ species is sensitive to the presence of water molecules, and the water molecules ma y exchange with the framework oxygens that are coordinated to the V4+ cente r.