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 obtai n highly dispersed and structurally well-defined vanadia-zirconia catalysts for the oxidative dehydrogenation of propane. The vanadia-zirconia materia ls were prepared by cothermolysis of OV((OBu)-Bu-t)(3) and Zr(OCMe2Et)(4) i n a nonpolar solvent, at relatively low temperatures. Prior to calcination, these materials have relatively high surface areas, are amorphous, and app ear to be highly dispersed. After calcination to 773 K, nanocrystalline zir conia and various VOx species, which appear to be dispersed on the zirconia surface, can be observed by PXRD and Raman, DR-UV-vis, and V-51 NMR spectr oscopies. Higher vanadia loadings and/or an increased calcination temperatu re (823 K) resulted in formation of ZrV2O7 (as demonstrated by Raman, DR-UV -vis, and 51V NMR spectroscopies). In general, the VOx/ZrO2 materials obtai ned from the molecular precursor method possess a greater surface area and exhibit a higher dispersion of VOx species than materials of the same compo sition 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 impre gnation, under similar conditions. Likewise, higher selectivities to propen e (by more than 15%) were observed with materials derived from the alkoxide s. However, analysis of the rate data for the new catalysts revealed that t he active sites were also more active in catalyzing the secondary oxidation of propene. As a result of this, the ratio of secondary combustion to oxid ative dehydrogenation rate constants (k(3)/k(1)) was similar for materials prepared by either wet impregnation or thermolysis of molecular precursors. (C) 2000 Academic Press.