Ultralow-loading single-atom cobalt on graphitic carbon nitrogen with robust Co-N pairs for aerobic cyclohexane oxidation

Selectively aerobic oxidation of cyclohexane using the homogeneous catalysts is an industrially important process, suffering from the difficult catalyst separation, low conversion and selectivity. Herein, a series of single-atom Co catalysts, possessing the ultralow Co loading of below 1.0 wt.‰, supported on mesoporous graphitic carbon nitrogen (Co/g-C₃N₄-w) were prepared by a simple adsorption method and applied into the cyclohexane oxidation. Characterization results demonstrate that the confinement effect of the voids in g-C₃N₄ facilitates the formation of the single-atom Co, which is stabilized by bonding with the pyridinic nitrogen of g-C₃N₄ and accompanied by the electron transfer from Co to g-C₃N₄. The catalytic performance presents an increasing trend with the increment of the Co loading, and the superior value with the conversion of 23.8% and selectivity of 95.6% is obtained over Co/g-C₃N₄-0.9. Kinetic study, density functional theory (DFT) calculations, and characterizations reveal that the decreased activation energy of cyclohexane oxidation over Co/g-C₃N₄-w can be attributed to the favorable dissociation activation of O₂ molecules and decomposition of cyclohexylhydroperoxide (CHHP) intermediate on the coordination unsaturated single-atom Co as well as the enhanced adsorption of cyclohexane on the electron-rich g-C₃N₄ surface, boosting the cyclohexane oxidation following the surface catalytic mechanism. Distinctively, robust Co-N structures and hydrophobic nature of g-C₃N₄ contribute to the high stability of Co/g-C₃N₄-0.9 for cyclohexane oxidation.