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.