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 COx 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.