NEW INSIGHTS INTO VOPO(4) PHASES THROUGH THEIR HYDRATION

Some VOPO4 phases can be involved when the V-P-0 catalysts used to oxidize n-butane to maleic anhydride are activated. Evidence is given that correct characterization can only be achieved if the catalysts are protected from hydration. Hydration Of VOPO4 phases (alpha(I)-, alpha(II)-, beta-, gamma-, delta-) was monitored by Raman spectroscopy. It is shown that, with the exception of beta-VOPO4, all VOPO4 phases can be hydrated, with hydration leading to the same VOPO4 . 2H2O. NH3 was also intercalated within alpha(I)-VOPO4. These experiments give clues to a unified description of all (with the exception of beta-VOPO4) VOPO4 phases as layered arrangements of VO6 distorted octahedra linked by PO4 tetrahedra. Proposals are given for the structure of gamma- and delta-VOPO4. The proposed structure of delta-VOPO4 allows epitaxy of this peculiar phase onto (VO)2P2O7 as well as growth of alpha(II)-VOPO4 onto delta-VOPO4. (C) 1994 Academic Press, Inc.