Structural Transformations of VOHPO_4·1/2H_2O in the Presence of Ammonia

摘要

The structural transformation of vanadium phosphate hemihydrate, VOHPO_4·1/2H_2O, into ammonium vanadyl pyrophosphate, (NH_4)_2[(VO)_3(P_2O_7)_2], was studied by XRD and spectro-scopic 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 α-(NH_4)_2[(VO)_3(P_2O_7)_2], which is isostructural to α-K_2[(VO)_3(P_2O_7)_2]. Both materials are characterized by the orthorhombic space group Pna2_1 and an intersecting tunnel structure. Apart from α-(NH_4)_2[(VO)_3(P_2O_7)_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 VOHPO_4·1/2H_2O carried out only under an ammonia/ oxygen (air) flow led to the generation of α-(NH_4)_2[(VO)_3(P_2O_7)_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 ammoniaitrogen flow generated an XRD-amorphous product and a deep reduction of V~Ⅳ = > V~Ⅲ proceeded.