CO2-Assisted Oxidative Dehydrogenation of Propane to Propylene over Modified SiO2 Based Catalysts

The oxidative dehydrogenation of propane with CO2 (CO2-ODP) was investigated over different metal oxides MxOy (M: Ca, Sn, Cr, Ga) supported on a SiO2 surface. Catalysts were characterized employing nitrogen adsorption/desorption, X-ray diffraction (XRD), CO2 temperature programmed desorption (CO2-TPD) and pyridine adsorption/desorption experiments in order to identify their physicochemical properties and correlate them with their activity and selectivity for the CO2-ODP reaction. The effect of operating reaction conditions on catalytic performance was also examined, aiming to improve the propylene yield and suppress side reactions. Surface acidity and basicity were found to be affected by the nature of MxOy, which in turn affected the conversion of propane to propylene, which was in all cases higher compared to that of bare SiO2. Propane conversion, reaction rate and selectivities towards propylene and carbon monoxide were maximized for the Ga- and Cr-containing catalysts characterized by moderate surface basicity, which were also able to limit the undesired reactions leading to ethylene and methane byproducts. High surface acidity was found to be beneficial for the CO2-ODP reaction, which, however, should not be excessive to ensure high catalytic activity. The silica-supported Ga2O3 catalyst exhibited sufficient stability with time and better than that of the most active Cr2O3-SiO2 catalyst. Decreasing the weight gas hourly space velocity resulted in a significant improvement in both propane conversion and propylene yield as well as a suppression of undesired product formation. Increasing CO2 concentration in the feed did not practically affect propane conversion, while led to a decrease in propylene yield. The ratio of propylene to ethylene selectivity was optimized for CO2:C3H8 = 5:1 and space velocity of 6000 mL g?1 h?1, most possibly due to facilitation of the C–H bond cleavage against that of the C–C bond. Results of the present study provided evidence that the efficient conversion of propane to propylene is feasible over silica-based composite metal oxides, provided that catalyst characteristics have been optimized and reaction conditions have been properly selected.