Oxidative dehydrogenation of propane on VAPO-5, V2O5/ALPO4-5 and V2O5/MgO catalysts. Nature of selective sites

Abstract

The catalytic properties for the oxidative dehydrogenation of propane of selective VAPO-5, V2O5/ALPO-5 (V/AP) and V2O5/MgO (V/MgO) catalysts have been compared in the following reaction conditions: atmospheric pressure, C3H8/O2/He molar ratio of 4/8/88, in the 475–550°C temperature range. The catalysts have also been characterized by several techniques (SBET, XRD, FTIR, H2-temperature programmed reduction, UV-Vis). It was found that isolated tetrahedral V5+ species are mainly present on VAPO-5 (0.5 wt% of V atoms) and on V/MgO (8.8 wt% of V atoms) catalysts, while isolated and associated V5+ species are present on V/AP samples with vanadium contents between 0.8–2.2 wt% of V atoms. The better catalytic properties for the oxidative dehydrogenation of propane of VAPO-5 are related to the presence of isolated VO4 tetrahedron in the framework which, in absence of vanadium, exhibits a very low activity. However, the existence of Mg2+ sites free of vanadium on the surface of the V/MgO sample favours the direct formation of carbon oxides from propane. In the case of V/AP samples the presence of polymeric vanadium species favours the degradation of the formed propene to carbon oxides.