PHYSICOCHEMICAL STUDY OF STRUCTURAL DISORDER IN VANADYL PYROPHOSPHATE

Vanadium phosphate specimens have been prepared in aqueous or organic media following procedures typical in the preparation of catalysts for selective hydrocarbon oxidation, and the materials obtained are examined with XRD, TGA, IR, ESR, and EXAFS techniques. The results show a particular ability of the (VO)2P2O7 structure to keep (most of the) vanadium in mixed-valence VIV-VV pairs, probably ordered in the lattice in a specific manner, while the corresponding excess oxygen seems to remain trapped between the layers of the structure. In spite of this nonstoichiometric character, however, XRD continues to show only the reflections of the (VO)2P2O7 lattice. The possibility that the use of both an organic solvent and P: V ratios higher than 1 during catalyst preparation help to avoid the phenomena, possibly detrimental for catalyst operation, of deeper vanadium oxidation (beyond the mentioned mixed-valence state) and generation of faults in the P-O-P links between the layers of the (VO)2P2O7-type structure is discussed. The possible relevance of these results for the operation of these catalysts in practical reactions is also discussed. © 1993 Academic Press, Inc.