Zeolitic Imidazolate Framework-Mediated Synthesis of Co3O4 Nanoparticles Encapsulated in N-doped Graphitic Carbon as an Efficient Catalyst for Selective Oxidation of Hydrocarbons

The aerobic solvent-free selective oxidation of the C-H bonds of hydrocarbons to the corresponding ketones by earth-abundant catalysts would provide high added value both from economy and environment points of view. In this respect herein, Co3O4 nanocrystals embedded into N-doped graphitic carbon nanohybrids (Co3O4@GNC) are prepared by uniform seed-mediated growth and deposition of Co-based zeolitic imidazolate framework-9 (ZIF-9) nanocrystals on graphene oxide (GO) nanosheets followed by facile carbonization of the ZIF-GO composite under inert atmosphere at high temperature. The specific textural and chemical characteristics of as-synthesized nanohybrids at different pyrolysis temperatures were comprehensively investigated by performing various spectroscopic tools. N-doped graphitic carbon wrapped Co3O4 nanocrystals is demonstrated to be an efficient catalyst for the selective oxidation of arylalkanes under aerobic and solvent-free conditions. Under the optimized reaction conditions, Co3O4@GNC-B catalyst (as-synthesized at 700ºC carbonization temperature) was found to exhibit superior catalytic performance with improved stability (reproducible conversion values upon sixth cycles) in ethyl benzene oxidation, providing 65.8% conversion of ethyl benzene with an exclusive selectivity of 72.6% for acetophenone. Combination of the catalytic results of the control group and the different characterization methods including XPS and XAFS, the superior catalytic activity and stability of Co3O4@GNC is attributed to the presence of Co-Nx active site and the synergistic effect between Co3O4 nanocrystals and unique N-containing interconnected carbonaceous framework. Moreover, the Co3O4@GNC catalyst could also be explored for a variety of arylalkane substrates with high catalytic activity for the aerobic and solvent-free oxidation. The design concept of precious-metal-free robust catalysts from ZIFs can be further extended to fabricate other novel and stable catalytic systems for advanced applications in fine-chemical production.