Formyl Radical Generation from Methane Oxidation Promoted by the Superoxide Radical in Tungsten Oxide Cations WO₄

The formyl radical (HCO^•) is a critical intermediate species involved in atmospheric, combustion, interstellar, and catalysis chemistry. The complicated reaction networks of HCO^• formation from a variety of substrates such as CO_x molecules as well as unsaturated hydrocarbons and their derivatives have been widely proposed. By contrast, the contribution of saturated hydrocarbons (e.g., methane) to HCO^• generation remains largely elusive. Herein, we successfully identify that the metal oxide cluster cation, WO₄⁺, that features a superoxide radical (O₂^-•) can activate three C-H bonds of methane and then directly oxidize methane to the HCO^• radical at room temperature. This finding represents a sharp improvement in the field of direct methane oxidation to oxygenated compounds by metal oxide systems wherein selective activation of one, two, or four C-H bonds was generally involved. We propose that the presence of a superoxide radical is very effective to prevent the high-valent metal center from being reduced, which is thermodynamically and kinetically advantageous for direct oxidation of methane to a free HCO^• radical. This work not only reports an unprecedented route for production of the HCO^• radical but also provides convincing evidence to consider methane as an important source in constructing the reaction networks relevant with HCO^• generation.