On the topotactic dehydration of VOHPO4 · 0.5 H2O into vanadyl pyrophosphate

The thermal analysis (DTA, TGA) of the dehydration under inert atmosphere of the vanadyl hydrogen phosphate hydrate VOHPO₄ · 0.5 H₂O recently studied, together with the characterization of both starting and final compounds (XRD, ir, and uv-visible spectroscopies) show that pseudomorphic vanadyl pyrophosphate is obtained in these conditions. This form γ-(VO)₂P₂O₇ is the active and selective catalyst in butane oxidation to maleic anhydride whereas β-(VO)₂P₂O₇ is selective in butene oxidation only. Pseudomorphic relations between VOHPO₄ · 0.5 H₂O and γ-(VO)₂P₂O₇ are evidenced by the comparison of XRD patterns, SEM and TEM experiments, and justified with the help of both structures. The low values of the activation energies calculated from kinetic data, E₁ = 21.4 and E₂ = 24.7 kcal · mole^?1 show that the dehydration of the hydrate and the formation of γ are topotactic, this hypothesis is confirmed and the mechanism stated precisely in view of the preceding results.