Stable copper nanoparticle photocatalysts for selective epoxidation of alkenes with visible light

Selective epoxidation of various alkenes with molecular oxygen (O2) under mild conditions is a longstanding challenge in achieving syntheses of epoxides. Cu-based catalysts have been found to be catalytically active for selective epoxidations. However, the application of copper nanoparticles (CuNPs) for photocatalyzed epoxidations is encumbered by the instability of CuNPs in air. Herein we report that CuNPs supported on titanium nitride (TiN) without additional stabilizers not only are stable in air but also can catalyze selective epoxidation of various alkenes with O2 or even air as a benign oxidant under light irradiation. CuNPs remain in the metallic state due to the significant charge transfer that occurs between CuNPs and TiN. The epoxidation is driven by visible light irradiation at moderate temperatures, achieving good to high yields and excellent selectivity. The photocatalytic process is applicable to the selective epoxidation of various alkenes. In this photocatalytic system, reactant alkenes chemically adsorb on CuNPs, forming Cu–alkene surface complexes, and light irradiation can activate the complexes for reaction. The cyclic ether solvent also plays a key role, reacting with O2 on the surface of CuNPs under light irradiation, yielding oxygen adatoms. The activated surface complexes react with the adatoms, yielding the corresponding epoxides. Analysis of the influence of irradiation wavelength and intensity on the epoxidation suggests that light-excited electrons of CuNPs drive the reaction. The adatoms formed react with alkenes, producing the final product epoxides. We also observed interesting product stereoselectivity, predominantly generating the trans isomers for the epoxidation of stilbene (up to 97%). The findings reported here not only provide an effective and selective reaction system for alkene epoxidations but also are a step toward demonstrating the practical use of CuNPs as photocatalysts for various applications.