Fabrication of single Co sites in graphitic carbon nitride via the ion exchange to boost aerobic cyclohexane oxidation

Cyclohexane oxidation is an industrial process for manufacturing cyclohexanol, cyclohexanone and adipic acid, while it remains a huge challenge to develop highly efficient catalysts for intensifying this process. Herein, a bottom-up synthesis approach, coupling the one-pot and ion exchange strategy, is developed to fabricate atomically dispersed Co on graphitic carbon nitride (g-C3N4) for the aerobic cyclohexane oxidation. The loading of the single-atom Co on Co/g-C3N4 can be facilely tuned by adjusting the ion exchange extent of Na + on Na/g-C3N4 precursor. Comprehensive characterizations demonstrate the atomical location of Co in sixfold cavities of g-C3N4, accompanied by the electron interaction between Co atoms and g-C3N4. The resulting Co/g–C3N4–1 catalyst possesses the superior oxidation performance with the 28.8 % cyclohexane conversion and 93.0 % overall selectivity toward desired products, and it also exhibits an acceptable universality to the catalytic oxidation of various hydrocarbons. The mechanism study revealed that besides the dissociation of O2 molecules on single-atom Co sites, the electron-rich g-C3N4 surface stemming from atomic Co doping facilitated the cyclohexane adsorption, which synergically boosted the subsequent oxidation reaction following the surface catalytic mechanism. We anticipate that such a concept of fabricating atomically dispersed metals via the ion exchange strategy provides a novel insight and feasible approach for the design of highly efficient single-atom catalysts for the cyclohexane oxidation and beyond.