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

Single-crystal structures were obtained of several vanadium-substitute d forms (with coexistent Cu ions) of heteropolyacid catalysts prepared via hydrothermal synthesis. All V-containing species are of the inver se Keggin type; excessive addition of Cu reverts the structure back to the normal Keggin form. Structural details of the two poly-anions ind icate qualitatively different bonding of the bridging oxygen atoms act ive in catalysis. From the single-crystal data reference powder patter ns of the two base structures were simulated and experimentally verifi ed. Failure to observe these patterns in clear form in conventional di ffraction experiments is traced back to rapid changes in the state of hydration of the materials, with time constants comparable to acquisit ion times of powder diffraction data. Thermal dehydration leads to int ermediate structures different from those subjected to dehydration in air. The anhydride of PVMo11 is stable up to 670 K.