Polyoxometalates catalyst materials: X-ray thermal stability study of phosphorus-containing heteropolyacids H3 +xPM12 –xVxO40·13–14H2O (M = Mo,W; x= 0–1)

Abstract

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.