Aerobic oxidation of fluorene to fluorenone over Co–Cu bimetal oxides

Abstract

Oxidation of sp³ C–H bonds has attracted increasing attention, and the aim of our work is to prepare catalysts for oxidation of sp³ C–H bonds using O₂ without an initiator. In this paper, a series of Co–Cu bimetal oxides with different Co/Cu ratios were synthesized by a sol–gel method and tested for catalytic oxidation of fluorene to fluorenone using molecular oxygen as an oxidant in the absence of a radical initiator. The best catalytic performance was achieved over the Co_0.7Cu_0.3 catalyst and the catalysts could be reused without significant loss of the catalytic activity. The characterization results indicated that some Cu entered the Co₃O₄ lattices, leading to more high-valence metal ion sites (Co³⁺ and Cu²⁺) and surface oxygen species (O₂⁻, O₂²⁻, and O⁻) as well as promoted redox ability, which all enhanced the catalytic activity. In addition, the catalysts were also efficient for the oxidation of other benzylic C–H containing aromatic hydrocarbons such as tetralin, indan, diphenylmethane and ethylbenzene.