The cooperative effect of Co and CoO in Co/CoO enabled efficient catalytic hydrogenation and demethoxylation of guaiacol to cyclohexanol

Abstract

The selective catalytic transformations of lignin-derived phenolics into cyclohexanol provide an alternative renewable solution to fine-chemical industries over the fossil-fuel-dependent routes. Herein, low-cost, non-noble metal-based Co/CoO catalysts with oxygen vacancies were prepared by a simple reduction of Co₃O₄ at 300 °C for 20 minutes. The catalyst was utilized for the hydrogenation and demethoxylation of guaiacol, a lignin model compound, into cyclohexanol, an industrially important compound. Under relatively milder reaction conditions of 170 °C, 2 MPa H₂ pressure and 2 h, 94% conversion of guaiacol with 80% cyclohexanol selectivity was obtained. The catalytic activity of Co/CoO towards hydrogenation and demethoxylation of guaiacol relied on the Co and CoO ratio, which in turn was varied through alteration in the reduction conditions of Co₃O₄. Metallic Co nanoparticles growth and its influence on the electronic environment of CoO in Co/CoO catalysts was confirmed using XPS, H₂-TPD, NH₃-TPD, and H₂-TPR techniques. Several phenolics were successfully converted to corresponding cyclohexanols with the developed catalyst. The catalyst was efficiently recycled with no appreciable loss in the activity. The spent catalyst showed similar chemical environments to that of the fresh catalyst. The present work provides useful insights into the simple synthesis of an economical heterogeneous catalyst that can encourage researchers to produce other valuable chemicals and fuels from non-fossil resources under milder conditions required by the petrochemical, fine chemical, and pharmaceutical industries.