Oxidative C−C Cleavage of Ketols over Vanadium–Carbon Catalysts

The oxidation of 2-hydroxycyclohexanone and carbohydrates to adipic acid and formic acid, respectively, was performed with a combination of a vanadium catalyst, carbon as the cocatalyst, and O₂ in water under mild conditions (353 K, 0.1–0.3 MPa). The catalytic activity of aqueous V₂O₅ was increased significantly by the addition of activated carbon (C), whereas the addition of carbon black, graphene oxide, and carbon nanotubes has a much smaller effect. UV/Vis and inductively coupled plasma optical emission spectroscopy were used to show that most V^V species are adsorbed on C at least in a short reaction time. The V^IV species (VO²⁺) was not adsorbed on C. The order of activity of vanadium species was V^V on C>dissolved free V^V≫dissolved free V^IV. To help the oxidation of V^IV, the further addition of phosphomolybdate (PMo₁₂O₄₀ ³⁻; PMo) was also tested, and the activity was improved. The selectivity and yield of adipic acid from 2-hydroxycyclohexanone was also improved slightly by the addition of PMo. However, in the oxidation of glucose, the addition of PMo did not improve the final formic acid yield. The oxidation of glucose with the V+C system gave a 42 % yield of formic acid, which was comparable to the values reported with a more expensive PVMo polyoxometalate catalyst. A reaction mechanism is proposed in which the reversibly formed “biradical” state of the vanadium ketol complex reacts with O₂ and accompanying rearrangement dissociates the C−C bond.