Efficient epoxidation of styrene over single-atomic copper-doped ordered mesoporous alumina (Cu-OMA) composited with carbon nitride

Abstract

The present study assessed the catalytic performance in epoxidation of styrene using tert‑butyl hydroperoxide (TBHP) as an oxidant over single-atomic copper-doped Ordered mesoporous alumina (Cu-OMA) composited with g-C₃N₄ (CN). The thermal treatment at three temperatures (300, 500, 700 °C) provided a facile mean to modify the texture and electronic structure of the Cu-OMA@CN composite. The X-Ray absorption spectroscopy proved the single-atomic Cu-O-Al and Cu-O-C in Cu-OMA@CN-500 catalyst. Catalytic reaction parameters, namely temperature (60–80 °C), catalyst amount (10–70 mg), substrate/oxidant ratio (1:1–1:3), and solvents (acetone, ethanol, acetonitrile, N, N-dimethylacetamide) were optimized to obtain high conversion of styrene and selectivity to styrene epoxide. Under the optimized reaction conditions (75 °C, 30 mg of catalyst, 1:2 of styrene/TBHP and acetonitrile as solvent), the conversion decreased in the order of Cu-OMA@CN-500 > Cu-OMA@CN-300 > Cu-OMA@CN-700. This sequence was related to the formation of electron-rich single-atom copper sites, high surface area, medium alkaline environment and high content of graphic nitrogen species in Cu-OMA@CN-500. This structure differed from the blocky structure of C₃N₄ in Cu-OMA@CN-300 and the destroyed tri-s-triazine ring in Cu-OMA@CN-700. The most active Cu-OMA@CN-500 catalyst displayed a high styrene conversion (87.2 %) and striking selectivity to styrene oxide (90.0 %). Its catalytic activity was retained after five recyclability tests. The possible catalytic mechanism for epoxidation of styrene over this catalyst was tentatively proposed.