Abstract: The direct propylene epoxidation with molecular oxygen offers an atom-economical route to propylene oxide (PO). However, the oxidative reaction conditions required at high temperatures often induce complete propylene oxidation and the reconstruction of the active centers, compromising PO selectivity. Herein, we propose a redispersion-confinement strategy to encapsulate small, low-valence Cu nanoparticles (NPs) within a silica shell, thereby achieving stable dispersion across various supports. We confirm that the formation of Cu-O-Si coordination between Cu and silica, together with cesium promotion, could stabilize the small Cu NPs and enhance the PO formation. The optimum Cs-Cu@Si/SBA-15 catalyst delivers a 3.9% propylene conversion and a 68% PO selectivity, markedly surpassing the 0.6% conversion and 42% selectivity obtained over Cs-Cu/SBA-15 without the silica shell. A significant space and chemical state confinement effect is evidenced between CuO and the silica shell, which retards the reactivity of lattice oxygen and allows the moderate activation of molecular oxygen to electrophilic oxygen species; thus, promoting the selective epoxidation of propylene. This strategy offers a general approach for molecular-level control of active-site structure and electronic states in heterogeneous catalysts.

Key words: Copper catalyst, Confinement, Surface engineering, Propylene epoxidation, Oxidation-reduction reactions