Factors Determining the Selectivity in the Oxidative Dehydrogenation of Propane over Boria—Alumina Catalysts

Boria-alumina catalysts are active in the selective oxidative dehydrogenation of propane. For undoped Al2O3, carbon oxides were the main products while the addition of 30 wt% B2O3 led to a significant increase in propene selectivity; propene yields of 18 % were achieved at 823 K. Pulse experiments with C3D8 in the temporal-analysis-of-products reactor and spectroscopic studies showed that an addition of boria in the amount of less than a monolayer eliminates hydroxyl groups of γ-alumina involved in H-D exchange with propane. The suppression of the dissociative adsorption of propane resulted in a decrease of catalyst activity but did not improve propene selectivity. Oxygen isotopic exchange between gas-phase 18O2 and surface 16O was used to study the role of oxygen activation in the overall process; only for high boria loading no oxygen exchange products were detected. The suppression of both, dissociative adsorption of propane and oxygen, was found to be necessary for the improvement of propene selectivity in the oxidative dehydrogenation of propane over boria-alumina catalysts. It was assumed that higher propene selectivity can be achieved by secondary reactions of propyl radicals formed on the surface after their release into the gas phase due to diminished surface oxidation.