Water Microdroplets-Initiated Methane Oxidation

The special redox reactivity of water microdroplets causes “mild ignition” of methane gas to form methane oxygenates. The C(sp³)-H bond of methane can be activated by the hydroxyl radical (OH·) or the hydrogen radical (H·) across the air-water interface (AWI) of microdroplets to generate the methyl radical (CH₃·). Once CH₃· is formed, it undergoes free-radical reactions with O₂ in the air, excessive OH· and H· across the AWI, and H₂O₂ present at the AWI and generated CH₃· itself to produce methanol and other species. Production of the methanol and other oxygenates was confirmed by gas chromatography, mass spectrometry, and ¹H- and ¹³C-nuclear magnetic resonance. Formic acid, acetic acid, ethanol, carbon dioxide, and methyl peroxide were also detected as methane oxidation byproducts. This water microdroplet-initiated oxidation process can be further enhanced under ultrasonication to yield 2.66 ± 0.77 mM methanol conversion from the methane gas in a single spray run for 30 min, with a selectivity of 19.2% compared with all other oxygenated species.