Synthesis of Cu Single Atoms Supported on Mesoporous Graphitic Carbon Nitride and Their Application in Liquid-Phase Aerobic Oxidation of Cyclohexene

Different loadings of Cu single atoms were anchored on a graphitic carbon nitride (g-C3N4) matrix using a two-step thermal synthesis method and applied in liquid-phase cyclohexene oxidation under mild conditions using molecular O-2 as the oxidizing agent. The oxidation state of Cu was determined to be Cu+, which is in linear coordination with two neighboring nitrogen atoms at a distance of 1.9 angstrom. The catalyst with 0.9 wt % Cu pyrolyzed at 380 degrees C was found to exhibit the best catalytic performance with the highest conversion up to 82% with an allylic selectivity of 55%. It also showed high reusability over four catalytic runs without any detectable Cu leaching. Cyclohexene oxidation followed first-order kinetics with an apparent activation energy of 66.2 kJ mol(-1). The addition of hydroquinone as a radical scavenger confirmed that cyclohexene oxidation proceeds via a radical mechanism. Time-resolved in situ attenuated total reflection infrared (ATR-IR) spectroscopy was carried out to qualitatively monitor the cyclohexene oxidation pathways. The comparison with the homogeneous analogue Cu(I) iodide indirectly verified the linearly N-coordinated single Cu(I) species to be the active sites for cyclohexene oxidation.