Considerable study has been done on methane oxidation to yield methanol, used in producing valuable substances like formaldehyde, dimethyl ether, and more. Yet, methane remains the most challenging hydrocarbon to oxidize. To our knowledge, no mild process exists currently for efficiently and selectively converting methane into methanol.
Inspired by the architecture and reaction mechanism of naturally existing metalloenzymes engaged in methane oxidation, scientists have devised a mononuclear iron compound possessing a hydrophobic setting near its active iron core. By utilizing this iron compound as a catalyst alongside a cost-effective and secure oxidizing agent, they have successfully achieved the direct transformation of methane into methanol, exhibiting remarkable efficiency and selectivity.
During this reaction, methane oxidation takes place under mild conditions, with a temperature of 50 °C and approximately 10 atm pressure, within an aqueous solution. The catalytic turnover number surpasses 500 within a 3-hour timeframe, demonstrating exceptional efficiency. Moreover, methanol is produced with a notable selectivity of 83%. This remarkable achievement is attributed to the existence of a hydrophobic environment adjacent to the iron center, which serves as the catalyst's active site. The hydrophobic cavity adeptly captures methane, a hydrophobic substance, and releases the resulting methanol, a hydrophilic substance, into the aqueous solution. Simultaneously, it acts as a barrier, preventing excessive oxidation of methanol by hindering its access to the active iron center.
The "catch-and-release" mechanism employed in this study is anticipated to serve as a highly effective model not only for the conversion of methane to methanol but also for the efficient chemical conversion of diverse hydrophobic organic compounds. This mechanism, utilizing a hydrophobic environment to selectively trap and release target molecules, holds great promise in expanding the scope of transformations beyond methane oxidation. By leveraging this approach, researchers may unlock new pathways for the conversion of hydrophobic organic substances into valuable products, leading to advancements in the field of efficient chemical conversions.
This work was supported by JST CREST (grant nos. JPMJCR16P1 and JPMJCR15P5) and by Grants-in-Aid (grant nos. 17H03027, 18K19089 and 21H01947) from the Japan Society for the Promotion of Science (JSPS). K.Y. and Y.S. acknowledge the MEXT projects of Cooperative Research Program of Network Joint Research Centre for Materials and Devices, Integrated Research Consortium on Chemical Sciences and the Elements Strategy Initiative to Form Core Research Centre. H.F. also acknowledges funding from JST SPRING (grant no. JPMJSP2124) and Research Fellowship for Young Scientists provided by JSPS (grant no. 22J10804).