Extensive efforts have been devoted towards the development of methods for the direct conversion of methane (CH₄), ethane (CH₃CH₃), or other abundant natural gasses into useful products, such as the corresponding alcohols, aldehydes, ketones, and carboxylic acids, liquid fuels, and precursors of chemical and pharmaceutical products. Selective aerobic oxygenation of CH₄ into liquid products without the concomitant formation of CO₂ and CO has served as an elusive target reaction. The selective oxygenation of CH₄ to CH₃OH with molecular oxygen (O₂) has been unknown because the oxidation of oxygenated products, CH₃OH and formic acid (HCOOH) is much easier than that of CH₄, leading to over-oxidation products such as CO and CO₂. Here, we report that oxygenation of methane photochemically occurred in the presence of ClO₂^•. The yields of methanol and formic acid as products were 17% and 82%, respectively, with a methane conversion of 99% in a two-phase system comprising perfluorohexane and water under ambient conditions. The reaction occurred by the efficient radical chain process.^[1–3]

UV light irradiation of chlorine dioxide radical results in the excited state of one-electron reduction potential to be E_red^* = +3.22 V vs. SCE. The highly oxidative power of chlorine dioxide at the excited state allowed electron-transfer oxidation of benzene and cyclohexane as hydrocarbon substrates to yield the corresponding oxygenated products. The reaction is initiated by the formation of the photoexcited state of ClO₂^•. Electron transfer from benzene (Eox = 2.48 V vs SCE) to the excited state of ClO₂^• occurs to form a radical ion pair composed of a benzene radical cation and ClO₂^–. Phenol, as a final product, is formed from the reaction between benzene radical cation and H₂O.

[1] K. Ohkubo, K. Hirose, T. Shibata, T. Takamori, S. Fukuzumi, J. Phys. Org. Chem. 2017, 30, e3619.

[2] K. Ohkubo, K. Hirose, Angew. Chem. Int. Ed. 2018, 57, 2126.

[3] Y. Itabashi, H, Asahara, K, Ohkubo, Chem. Commun. 2023, 59, 7506.