Direct and Selective Photocatalytic Oxidation of CH4 to Oxygenates with O2 on Cocatalysts/ZnO at Room Temperature in Water

Abstract

Direct conversion of methane into methanol and other liquid oxygenates still confronts considerable challenges in activating the first C–H bond of methane and inhibiting overoxidation. Here, we report that ZnO loaded with appropriate cocatalysts (Pt, Pd, Au, or Ag) enables direct oxidation of methane to methanol and formaldehyde in water using only molecular oxygen as the oxidant under mild light irradiation at room temperature. Up to 250 micromoles of liquid oxygenates with ∼95% selectivity is achieved for 2 h over 10 mg of ZnO loaded with 0.1 wt % of Au. Experiments with isotopically labeled oxygen and water reveal that molecular O₂, rather than water, is the source of oxygen for direct CH₄ oxidation. We find that ZnO and cocatalyst could concertedly activate CH₄ and O₂ into methyl radical and mildly oxidative intermediate (hydroperoxyl radical) in water, which are two key precursor intermediates for generating oxygenated liquid products in direct CH₄ oxidation. Our study underlines two equally significant aspects for realizing direct and selective photooxidation of CH₄ to liquid oxygenates, i.e., efficient C–H bond activation of CH₄ and controllable activation of O₂.