TRANSFORMATION OF VOHPO4-CENTER-DOT-1/2H(2)O TO (VO)(2)P2O7 - CRYSTALLOGRAPHIC, MICROSTRUCTURAL, AND MECHANISTIC ASPECTS

A detailed study of the transformation of the butane oxidation catalyst precursor VOHPO4 . 1/2H(2)O to gamma-(VO)(2)P2O7 is reported. The vanadyl hydrogen phosphate precursors used in this study were in the form of single crystals and powders, each prepared from aqueous media. Electron microscopy and X-ray diffraction clearly show the transformation to be topotactic: single crystals are converted to pseudomorphs which are unchanged in size or shape with respect to the starting crystals, The pseudomorphs are made up of highly-oriented vanadyl pyrophosphate microcrystals. A large contraction of the atomic structure in the direction perpendicular to the basal plane induces cracks and voids within the crystal. A transformation mechanism involving phosphorus-atom inversion, along with supporting MNDO SCF-MO calculations, is presented. This mechanism accounts for at least some of the interlayer, crystallographic disorder that is often attributed to the sterically hindering effects of residual organics. The generation of cracks to accommodate unit cell shrinkage along the thin dimension of the crystal plates creates additional basal plane surface area that may be beneficial in terms of catalytic performance. (C) 1995 Academic Press, Inc.