Solid state, surface and catalytic studies of oxides

The properties of oxide catalysts that determine selectivity in oxidation of alkanes were investigated. Upon examining the product distribution in the oxidation of ethane, propane, butane, pentane, and cyclohexane over Mg orthovanadate, Mg pyrovanadate, and vanadyl pyrophosphate, it was found that whether dehydrogenation, formation of oxygen-containing organic products, or combustion was the predominant reaction depended on the catalyst and alkane. The product distribution could be explained by considering whether the active sites contained easily removable lattice oxygen ions, and whether the size of the alkane molecule was sufficiently large to be bonded to two adjacent vanadium ions in the active site to enhance the probability of reaction between the adsorbed hydrocarbon intermediate and reactive lattice oxygen. Selectivity for oxidative dehydrogenation of butane on silica-supported vanadium oxide depended on the vanadia loading. The selectivity was much higher on a 1 wt.% than on a 10 Wt-% V[sub 2]O[sub 5]/SiO[sub 2] sample. It was also found that on the 10 wt.% sample, agglomeration of vanadia into V[sub 2]O[sub 5] crystallites occurred during reaction. Results suggested that isolated VO[sub 4] units were more selective for dehydrogenation of butane, consistent with the idea that the selectivity depended on the availability of reactive oxygen in the active site