Mechanistic Interpretations and Insights for the Oxidative Dehydrogenation of Propane via CO₂over Cr₂O₃/Al₂O₃Catalysts

Oxidative dehydrogenation (ODH) of alkanes using carbon dioxide as a soft oxidant has recently emerged as a potentially attractive alternative to steam cracking for the production of light olefins. To elucidate reaction pathways and their dependence on the operating conditions, CO₂-assisted propane dehydrogenation over a redox-active Cr₂O₃/Al₂O₃catalyst was examined in a packed bed reactor as a function of temperature, Cr₂O₃/CO₂feed ratio, and residence time. Previous ODH studies have largely focused on CO₂-rich conditions with the aim of preventing coke formation. However, at T = 600 °C the present study finds that the use of propane-rich conditions (1 ≤ C₃H₈/CO₂≤ 2.5) maximizes propylene production and selectivity while maintaining catalyst stability. It is postulated that the selective Mars van Krevelen dehydrogenation process is optimized at these ratios. Excess CO₂apparently promotes nonselective dehydrogenation and dry reforming pathways that generate additional CO, adversely impacting catalyst stability via the Bouduard reaction. This hypothesis is supported by complementary investigations of the reverse water gas shift reaction and thermodynamic analysis. The findings and methodology presented here are likely applicable to related ODH processes with other alkanes and redox-active catalysts.