| Heteropolyacids (HPAs) are good materials for both acid and oxidation catalysis. In this work, Keggin-type heteropolyacids have been examined to elucidate the effects of primary structure, composition and secondary structure on catalytic activity. The specific rates of double bond isomerization of both 1-butene and cis-2-butene were orders of magnitude greater on the tungsten heteropolyacids than on molybdenum heteropolyacids. Results from ammonia sorption microcalorimetry showed DeltaHsorp on tungsten-based heteropolyacids was approximately 40 kJ mol--1 higher than the corresponding enthalpy obtained on molybdenum-based heteropolyacids. Quantum chemical calculations revealed the most energetically favorable site of the acidic proton to be a bridging oxygen atom in the anhydrous heteropolyacid. The proton affinity of the tungsten Keggin unit was lower than that of an analogous molybdenum HPA by 38 kJ mol--1. Ammonia binding energies were also computed on the Keggin units.; The investigation of oxidation chemistries over heteropolyacids focused on catalysts composed primarily of molybdenum but containing some vanadium and copper. The activity of Cu,V-containing heteropolyacid for oxidative dehydrogenation of propane to propene at 573 K was greater than that of H3PMo 12O40 and H4PMo11VO40, but was similar to that of a physical mixture of CuO and H4PMo 11VO40.; Finally, the catalytic activity of silica supported tungsten based heteropolyacid for double bond isomerization of butene and skeletal isomerization of butane and pentane was evaluated. The catalysts were characterized with a variety of techniques including X-ray diffraction, X-ray absorption spectroscopy, UV-vis spectroscopy and ammonia sorption microcalorimetry. Well dispersed HPAs were not active for reactions that required strong acid sites, whereas catalysts with small crystallites of HPA were active. The catalysts were easily regenerated by water at low temperatures, presumably by rehydration of partially decomposed Keggin units.; In these studies, the HPA has been examined on the scale of individual molecules, bulk systems and supported systems with both well dispersed molecules of HPA and crystallites of HPA. |
