Oxidative Dehydrogenation of Propane on Vanadium Catalysts: Role of V Site Isolation

The aim of the study was to test the hypothesis that an isolation of the vanadium active sites is pivotal for high activity in oxidative dehydrogenation (ODH) of propane. A series of materials in which isolated vanadium ions were introduced into faujasite was synthetized. The nature of the VO_x species was confirmed by different physicochemical techniques showing that they constitute highly dispersed VO_x species of tetrahedral/square pyramid coordination. The samples were tested in ODH in air in the temperature range 400–500 °C under atmospheric pressure. Their activity was compared with analogous systems in which nonisolated, polymeric vanadium species were introduced into the same zeolite by wet impregnation. The resulting catalysts with isolated vanadium ions exhibited higher propylene selectivity than those containing polymeric species in temperature range 425–500 °C (23.6%–31.7% vs. 3.6%–10.5%), confirming the hypothesis. Additionally, the reaction mechanism was proposed based on density functional theory calculations, indicating that the first C─H bond breaking in propane is the rate determining step (the energy barrier equals to 27.5 kcal/mol). The formed propyl radical diffuses then to reach the new vanadium site, where the second C─H bond breaking occurs (the energy barrier equals to 10.5 kcal/mol).