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.
Bibliographical noteKAUST Repository Item: Exported on 2020-10-01
Acknowledgements: P.S, R.S and C.S wish to thankfully acknowledge the Council of Scientific and Industrial Research (CSIR), New Delhi, for their respective junior and senior research fellowships. J.M. thanks the Department of Science and Technology, India, for DST-INSPIRE Faculty Research project grant (GAP-0522) at CSIR-IICT, Hyderabad. We are also grateful with Dr. Nakka Lingaiah, CSIR-IICT for his kind help in evaluation of the N2-sorption analysis data.