CO₂-promoted Cr redox cycle driven ethane catalytic oxidative dehydrogenation to ethylene and its comparative study to ethane direct catalytic dehydrogenation

In the field of ethane conversion, Cr/ZSM-5 catalysts have received considerable attention for application in direct dehydrogenation (DH) and CO₂-assisted oxidative dehydrogenation (CO₂-ODH) reactions, which can be attributed to their excellent catalytic performance. Nevertheless, the speciation of Cr on the catalyst surface within these two reaction systems, as well as the mechanisms accounting for the observed differences in catalytic performance between them, have been scarcely reported. Here, we present a comparative study on the catalytic performance and reaction mechanism of chromium oxide-loaded high-silica ZSM-5 catalysts in two ethane transformation systems: DH and CO₂-ODH. DFT calculations reveal that the adsorption energy of ethane in the CO₂-ODH system is lower than that in the DH system, indicating that the introduction of CO₂ can facilitate ethane adsorption on the catalyst surface. Experimental and computational results demonstrate that the catalysts exhibit high activity and stability in the CO₂-ODH process, achieving an ethane conversion of 51.35 mol% and an ethylene yield of 44.05 mol%. No catalyst deactivation was observed over a 48-h reaction period. Catalysts were characterized before and after the reaction using XRD, N₂ adsorption-desorption, SEM, H₂-TPR, XPS, laser Raman, and UV-Vis. It is revealed that the superior catalytic performance of the catalysts in CO₂-ODH compared to DH is primarily attributed to the introduction of CO₂. Specifically, CO₂ promotes the redox cycle of Cr species during the catalytic reaction, maintains a higher proportion of Cr⁶⁺ in the catalysts, and enhances the dispersion of Cr on the support surface. This work is expected to provide new insights into the effects of CO₂ on Cr species in CO₂-ODH reaction system.