No abstract

Theoretical calculation using density functional B3LYP correlation-exchange is employed to study the structure of different sizes of the VAPO-5 clusters. The results indicate that the substitution of either the phosphorus site or the aluminum site of the aluminophosphate-5 framework with vanadium is generally not feasible. The calculated structures are comparable with those 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-oxo (V 4+ ) O)(O f ) 4 species for reduced, dehydrated VAPO-5 and as a di-oxo V 5+ O 4 species for calcined, dehydrated VAPO-5. The coordination geometry of the mono-oxo V 4+ species is sensitive to the presence of water molecules, and the water molecules may exchange with the framework oxygens that are coordinated to the V 4+ center.

Based on the density functional theory calculation using B3LYP hybrid method, a distorted square pyramidal (V 4+ dO)(O f ) 4 unit in a reduced and dehydrated VAPO-5 is formed by the vanadium (IV) oxide ion anchored to two neighboring 12-membered TO 4 rings of the aluminophosphate-5 lattice. By breaking two V-O-T bonds on one twelve-membered-ring to form another VdO bond, V 4+ can be oxidized to a tetrahedral di-oxo V 5+ O 4 unit. The computed structures of vanadium oxide species have been confirmed by Raman spectroscopy.