Selective methane oxidation by molecular iron catalysts in aqueous medium

• Published in: Nature (London) 2023-04, Vol.616 (7957), p.476-481
• Main Authors: Fujisaki, Hiroto, Ishizuka, Tomoya, Kotani, Hiroaki, Shiota, Yoshihito, Yoshizawa, Kazunari, Kojima, Takahiko
• Format: Article
• Language: English
• Subjects: 119/118; 140/131; 140/133; 639/638/77/888; 639/638/911/406/939; Alkanes; Aqueous solutions; Catalysis; Catalysts; Catalytic oxidation; Ethanol; Gas mixtures; High temperature; Holes; Humanities and Social Sciences; Hydrophobicity; Iron constituents; Ligands; Mass spectrometry; Methane; Methanol; multidisciplinary; Natural gas; Oxidation; Propane; Reforming; Science; Science (multidisciplinary); Scientific imaging; Selectivity; Substrates
• ISSN: 0028-0836; 1476-4687
• DOI: 10.1038/s41586-023-05821-2

Using natural gas as chemical feedstock requires efficient oxidation of the constituent alkanes—and primarily methane 1 , 2 . The current industrial process uses steam reforming at high temperatures and pressures 3 , 4 to generate a gas mixture that is then further converted into products such as methanol. Molecular Pt catalysts 5 – 7 have also been used to convert methane to methanol 8 , but their selectivity is generally low owing to overoxidation—the initial oxidation products tend to be easier to oxidize than methane itself. Here we show that N -heterocyclic carbene-ligated Fe II complexes with a hydrophobic cavity capture hydrophobic methane substrate from an aqueous solution and, after oxidation by the Fe centre, release a hydrophilic methanol product back into the solution. We find that increasing the size of the hydrophobic cavities enhances this effect, giving a turnover number of 5.0 × 10 2 and a methanol selectivity of 83% during a 3-h methane oxidation reaction. If the transport limitations arising from the processing of methane in an aqueous medium can be overcome, this catch-and-release strategy provides an efficient and selective approach to using naturally abundant alkane resources. Methane can be oxidized to methanol using N -heterocyclic carbene-ligated Fe II complexes, in which the hydrophobic cavity captures the methane substrate from an aqueous solution and releases the hydrophilic methanol product back into the solution.