The CO2-assisted oxidative dehydrogenation of propane (CO2-ODHP) over Cr-based catalysts represents a promising approach to produce propylene, while the nature of active Cr species and associated reaction mechanisms remains debatable. In this work, a series of Cr-containing MFI-structured zeolite catalysts (i.e., S-1, ZSM-5-100, and ZSM-5-13) were synthesized and evaluated for CO2-ODHP to elucidate active Cr sites and reaction pathways. Through reaction rate analysis combined with multiple characterizations, we quantitatively correlated the distribution of Cr species with their intrinsic activities. It is found that the isolated Cr6+ cations and their reduced form, coordinatively unsaturated Cr3+ cations (denoted as isolated Cr3+), give turnover frequencies (TOFs) of 1.9 × 10−3 and 2.5 × 10−3 s−1 at 550 °C, respectively, which are much higher than those of crystalline Cr2O3 (0.24 × 10−3 s−1), polymeric Cr6+ (0.023 × 10−3 s−1), and its reduced form Cr3+ (0.016× 10−3 s−1). Density functional theory calculations further illustrate the feasible dehydrogenation of propane on both isolated Cr6+ species and isolated Cr3+ species. The Cr6+/Cr4+ redox pair highlights a Mars−van Krevelen mechanism with the involvement of CO2 as a soft oxidant. Both direct propane dehydrogenation and oxidative dehydrogenation with activated CO2 can readily occur on isolated Cr3+ sites, with notably lower energy barriers for the former. Therefore, our findings unequivocally clarify the active sites and the main reaction network, in which the role of CO2 is also highlighted.
Zhu et al. (Fri,) studied this question.