Structural transformations of VOHPO4 center dot 1/2H(2)O in the presence of ammonia

The structural transformation of vanadium phosphate hemihydrate, VOHPO4 . 1/2H(2)O, into ammonium vanadyl pyrophosphate, (NH4)(2)[(VO)(3)(P2O7)(2)], was studied by XRD and spectroscopic methods such as FTIR and Raman spectroscopy. The reaction was carried out at 673 K in the presence of the ammoxidation feed (toluene, oxygen (air), ammonia, and water vapor), proceeding through an intermediate crystalline phase. The main product of the transformation was alpha-(NH4)(2)[(VO)(3)(P2O7)(2)], which is isostructural to alpha-K-2[(VO)(3)(P2O7)(2)]. Both materials are characterized by the orthorhombic space group Pna2(1) and an intersecting tunnel structure. Apart from alpha-(NH4)(2)[(VO)(3)(P2O7)(2)], a vanadium-rich phase was formed according to the stoichiometry which is probably a mixed valent vanadium oxide. The structural transformation proceeded also in the same direction without participation of the aromatic substrate. The reaction of VOHPO4 . 1/2H(2)O carried out only under an ammonia/oxygen (air) flow led to the generation of alpha-(NH4)(2)[(VO)(3)(P2O7)(2)] as well, but to a minor degree. An XRD-amorphous material was obtained, being the main phase. The treatment of the hemihydrate with an ammonia/nitrogen flow generated an XRD-amorphous product and a deep reduction of V-IV double right arrow V-III proceeded.