Active and selective nature of supported CrOx for the oxidative dehydrogenation of propane with carbon dioxide

Although the supported chromium oxides are important industrial catalysts for a number of reactions including the propane dehydrogenation, the rational catalyst design is still challenged by the deficient understanding on the active and selective nature of the catalysts. In this work, CrOx/silicalite-1 with a Cr loading of 0.5–5 wt.% prepared by the impregnation method is probed for the oxidative dehydrogenation of propane with carbon dioxide (CO2-ODP) under industrially relevant conditions. Characterization results reveal that CrOx is highly dispersed over silicalite-1, and crystal Cr2O3 is still very limited even over the catalyst with the highest Cr loading. Moreover, the isolated and polymeric Cr6+ oxides coexist over all of CrOx/silicalite-1 catalysts, and the amount and the extent of the polymerization of the Cr6+ oxides are continuously increased with increasing the Cr loading. Reaction results indicate that the CrOx/silicalite-1 is superior for CO2-ODP, and the highest space-time yield of 16.7 kg(C3H6) h−1 kgCr)−1 is obtained over the catalyst with a Cr loading of 2 wt.%. By correlating with the reaction results, the polymeric Cr6+ oxides with a high degree of the polymerization are determined to be the most active but least selective sites for CO2-ODP, which is originated from the varied adsorption strengths of C3H8 and the different desorption behaviors of C3H6 on the specifically structured Cr6+ oxides over CrOx/silicalite-1. As a result of the strong adsorption of the C3H8 reactant and the produced C3H6 over the most active polymeric Cr6+ oxides, the deposited coke is found to play dual roles for both decreasing the C3H8 conversion and increasing the C3H6 selectivity. The catalytic results of CrOx/silicalite-1 for CO2-ODP are well explained with these understandings, which are important for guiding the development of the high-performance CrOx-based catalysts for dehydrogenation reactions, especially CO2-ODP.