Propane-CO₂ Coupling Reaction over Isolated Cr Active Centers in the Beta Zeolite

The propane-CO₂ coupling reaction (CO₂-PDH) enables propylene production with simultaneous CO₂ utilization. Elucidating the nature of active sites and the role of CO₂ remains challenging due to catalyst heterogeneity and parallel pathways. Herein, we construct a well-defined catalytic platform comprising isolated metal sites embedded in the Beta zeolite framework. Among the metals examined (V, Nb, Ta, Cr, Mo, and W), Cr-Beta exhibits the best performance, achieving 66.9% propane conversion and 51.2% propylene yield with syngas coproduction (H₂/CO = 0.82). Characterizations identify the active site as a pseudotetrahedral framework Cr species, {(≡SiO)₃Cr···(HO-Si≡)}. Framework Cr═O species, generated during air pretreatment, serve as the active centers for initial oxidative dehydrogenation and are subsequently reduced to tetrahedral Cr sites. These reduced Cr sites catalyze CO₂-PDH by coupling propane dehydrogenation with the reverse water-gas shift reaction, while CO₂ simultaneously promotes propane conversion through a formate-mediated surface hydrogen consumption pathway that shifts thermodynamic equilibrium and suppresses coke formation via the reverse Boudouard reaction. This work provides mechanistic insight into active-site evolution and CO₂ participation in alkane-CO₂ co-conversion, offering guidance for the rational design of efficient catalysts for simultaneous fossil-resource valorization and CO₂ utilization.