Transfer hydrogenation of phenol over Co-CoO/N-doped carbon: boosted catalyst performance enabled by synergistic catalysis between Co⁰ and Co

Selective hydrogenation of biomass-derived phenols into cyclohexanones or cyclohexanols is an industrially important fundamental reaction. Traditional processes commonly used noble metal catalysts and high-pressure H₂ as a donor, which are not cost-saving and selectivity-controllable. Herein, we fabricated highly dispersed cobalt nanoparticles (<5 nm) supported on mesoporous N-doped carbon spheres (Co-CoO/NCS) via an ion exchange-pyrolysis strategy, which showed excellent activity and good selectivity in one-pot transfer hydrogenation of phenol to cyclohexanol with 2-PrOH as a hydrogen donor. It was found that the surface cobalt species of Co-CoO/NCS could be tuned by simply adjusting the pyrolysis temperature, thus resulting in a boosted catalytic performance of Co-CoO/NCS-600 (obtained at 600 °C), which was more active than other counterparts as well as Co/NCS-600 and Co₃O₄/NCS-600. Controlled experiments revealed that Co⁰ was mainly responsible for dehydrogenation of 2-PrOH, while phenol hydrogenation could be promoted by Lewis acidic Co (especially by Co²⁺), and the coexistence of Co⁰ and Co was indispensable for boosting the CTH activity of Co-CoO/NCS. This work provides an economical and environmentally-friendly method for the selective hydrogenation of phenols into value-added chemicals.