Thermolytic Molecular Precursor Route to Active and Selective Vanadia–Zirconia Catalysts for the Oxidative Dehydrogenation of Propane

The thermolytic molecular precursor route was employed in attempts to obtain highly dispersed and structurally well-defined vanadia–zirconia catalysts for the oxidative dehydrogenation of propane. The vanadia–zirconia materials were prepared by cothermolysis of OV(OtBu)3 and Zr(OCMe2Et)4 in a nonpolar solvent, at relatively low temperatures. Prior to calcination, these materials have relatively high surface areas, are amorphous, and appear to be highly dispersed. After calcination to 773 K, nanocrystalline zirconia and various VOx species, which appear to be dispersed on the zirconia surface, can be observed by PXRD and Raman, DR-UV-vis, and 51V NMR spectroscopies. Higher vanadia loadings and/or an increased calcination temperature (823 K) resulted in formation of ZrV2O7 (as demonstrated by Raman, DR-UV-vis, and 51V NMR spectroscopies). In general, the VOx/ZrO2 materials obtained from the molecular precursor method possess a greater surface area and exhibit a higher dispersion of VOx species than materials of the same composition prepared by conventional impregnation methods. For catalysts derived from the alkoxides, the maximum rate for propane oxidative dehydrogenation is more than double that observed for VOx/ZrO2 materials prepared by impregnation, under similar conditions. Likewise, higher selectivities to propene (by more than 15%) were observed with materials derived from the alkoxides. However, analysis of the rate data for the new catalysts revealed that the active sites were also more active in catalyzing the secondary oxidation of propene. As a result of this, the ratio of secondary combustion to oxidative dehydrogenation rate constants (k3/k1) was similar for materials prepared by either wet impregnation or thermolysis of molecular precursors.