Mechanistic aspects of the oxidative dehydrogenation of propane over an alumina-supported VCrMnWOx mixed oxide catalyst

Mechanistic and kinetic aspects of the catalytic oxidative dehydrogenation of propane (ODP) were studied within a wide range of temperatures (673-773 K), partial pressures of oxygen (0-20 kPa), propane (0-40 kPa) and propene (0-4 kPa) under both steady-state ambient-pressure and transient, vacuum conditions in the temporal analysis of products (TAP) reactor. A Mn0.18V0.3Cr0.23W0.26Ox-Al2O3 catalyst was identified as a selective catalyst for ODP by high-throughput experiments. For comprehensive catalyst characterization, XRD, BET, and in situ UV-visible techniques were applied. The results from transient experiments in combination with UV-visible tests reveal that selective and non-selective propane oxidation occurs on the same active surface sites, i.e., lattice oxygen. COx formation takes place almost exclusively via consecutive propene oxidation, which involves both lattice and adsorbed oxygen species, with the latter being active in CO formation. However, the adsorbed species play a minor role. CO2 formation was found to increase in the presence of propene in the reaction feed. Optimized operating conditions for selective propane oxidation were derived and discussed based on the experimental observations with respect to the influence of temperature and partial pressures of O2, C3H6 and C3H8 on the reaction. In co-feed mode with a propane to oxygen ratio of 2, optimal catalytic performance is achieved at low partial pressures of oxygen and high temperature. Propene selectivity can be also improved by carrying out the ODP reaction in a periodic mode; that is an alternate feed of propane and air.