SINGLE-CRYSTAL AND POWDER DIFFRACTION STUDIES OF THE STRUCTURE OF HETEROPOLYMOLYBDOPHOSPHORIC ACID CATALYSTS

Single-crystal structures were obtained of several vanadium-substituted forms (with coexistent Cu ions) of heteropolyacid catalysts prepared via hydrothermal synthesis. All V-containing species are of the inverse Keggin type; excessive addition of Cu reverts the structure back to the normal Keggin form. Structural details of the two poly-anions indicate qualitatively different bonding of the bridging oxygen atoms active in catalysis. From the single-crystal data reference powder patterns of the two base structures were simulated and experimentally verified. Failure to observe these patterns in clear form in conventional diffraction experiments is traced back to rapid changes in the state of hydration of the materials, with time constants comparable to acquisition times of powder diffraction data. Thermal dehydration leads to intermediate structures different from those subjected to dehydration in air. The anhydride of PVMo11 is stable up to 670 K.