Efficient Epoxidation over Faujasite Zeolites: Unprecedented O₂ Cooperative Activation by Co(II)–Ba(II) Cation Pair Sites Confined in the Supercage

Styrene oxide is one of the most important epoxides for manufacturing fine chemicals and pharmaceuticals, but its current synthesis process is environmentally unfriendly. As an eco-friendly and safe system, Co²⁺(Ba²⁺)-exchanged faujasite zeolites were found to efficiently catalyze styrene epoxidation with molecular oxygen. Despite having been discovered for nearly two decades, the nature of the active site and the mechanism of the reaction remain unclear. Herein, we carried out detailed experimental characterizations and density functional theory calculations. We concluded that the active sites are binuclear, rather than single-site Co²⁺ ions as generally believed, where Co²⁺···Co²⁺ or Ba²⁺···Co²⁺ pairs cooperatively activate O₂ to O₂^–. As compared to that on the symmetric Co²⁺···Co²⁺ pair site, the ethenyl CC in styrene tends to be exclusively attacked by the terminal oxygen closer to Ba²⁺ on the asymmetric Ba²⁺···Co²⁺ pair site, thus enhancing the epoxidation selectivity. This new insight will enable the optimization of oxidation catalysts.