Polyoxometalates catalyst materials: X-ray thermal stability study of phosphorus-containing heteropolyacids H 3 +x PM 12 –x V x O ₄₀ ·13–14H ₂ O (M = Mo,W; x= 0–1)

The phase evolution, under thermal treatment, of the 12-molybdo(tungsto)phosphoric acids and their related 1-vanado-substituted compounds has been studied by in situ X-ray diffraction methods. The anhydrous acid temperature range is clearly observed for the 12-molybdo(tungsto)phosphoric acid parents via formation of a tetragonal phase. However, the vanadium monosubstituted acids seem to transform into the 12-metallo parents. In the case of the 1-vanado-11-tungstophosphoric acid a curious evolution, via a very stable cubic lattice, suggests thermal stabilization of the 12-tungstophosphate anion through the formation of an anhydrous VO2+ salt.