Oxidative Dehydrogenation of Propane over Vanadia–Magnesia Catalysts Prepared by Thermolysis of OV(OtBu)3 in the Presence of Nanocrystalline MgO

Abstract

The influence of vanadium content on the performance of V–Mg–O catalysts for the oxidative dehydrogenation (ODH) of propane was investigated. High-surface-area (380 m2/g) MgO was prepared by hydrolysis of Mg(OCH3)2 followed by hypercritical drying. Vanadia was deposited on this support by thermolysis of OV(OtBu)3. Catalysts prepared by this means have BET surface areas of 187–299 m2/g and apparent surface densities of V2O5 of 1.1–10.3 VOx/nm2. All of the catalysts were characterized by X-ray diffraction, temperature-programmed reduction, and Raman, UV–visible, and nuclear magnetic resonance spectroscopy. The environment of the V atoms depends strongly on the apparent surface density of vanadia. Isolated VO42− units are present at very low apparent surface densities (∼1 VOx/nm2). As the vanadia density increases, magnesium vanadate structures are formed and above a surface density of 3.5 VOx/nm2 well-dispersed magnesium orthovanadate domains become evident. The rate of ODH per V atom increases with increasing VOx surface density and reaches a maximum value at 3.5 VOx/nm2. Above this surface density, the rate of ODH per V atom decreases because an increasing fraction of the V atoms lie below the catalyst surface and, hence, are inaccessible. Consistent with this interpretation, the ODH activity per unit surface area reaches a plateau at a VOx surface density of about 4 VOx/nm2. The propane ODH selectivity of the catalysts increases with increasing VOx surface density and reaches a plateau of 80% for an apparent surface density of about 4 VOx/nm2. Rate coefficients for propane ODH (k1), propane combustion (k2), and propene combustion (k3) were calculated for each catalyst. The value of k1 increases with increasing VOx surface density, reaching a maximum at about 4 VOx/nm2. By contrast, the ratios (k2/k1) and (k3/k1) decrease monotonically with increasing VOx surface density. The observed trends in k1, (k2/k1), and (k3/k1) are discussed in terms of the surface structure of the catalyst.