Tailored pore-confined single-site iron(III) catalyst for selective CH4 oxidation to CH3OH or CH3CO2H using O2

Abstract

Direct oxidation of methane to valuable oxygenates like alcohols and acetic acid under mild conditions poses a significant challenge due to high C‒H bond dissociation energy, facile overoxidation to CO and CO₂ and the intricacy of C−H activation/C−C coupling. In this work, we develop a multifunctional iron(III) dihydroxyl catalytic species immobilized within a metal-organic framework (MOF) for selective methane oxidation into methanol or acetic acid at different reaction conditions using O₂. The active-site isolation of monomeric Fe^III(OH)₂ species at the MOF nodes, their confinement within the porous framework, and their electron-deficient nature facilitate chemoselective C‒H oxidation, yielding methanol or acetic acid with high productivities of \(38,592\,\upmu {{{\rm{mol}}}}_{{{{\rm{CH}}}}_{3}{{\rm{OH}}}}{{{{\rm{g}}}}_{{{\rm{Fe}}}}}^{-1}{{{\rm{h}}}}^{-1}\) and \(81,043\,\upmu {{{\rm{mol}}}}_{{{{\rm{CH}}}}_{3}{{{\rm{CO}}}}_{2}{{\rm{H}}}}{{{{\rm{g}}}}_{{{\rm{Fe}}}}}^{-1}{{{\rm{h}}}}^{-1}\), respectively. Experiments and theoretical calculations suggest that methanol formation occurs via a Fe^III-Fe^I-Fe^III catalytic cycle, whereas CH₃CO₂H is produced via hydrocarboxylation of in-situ generated CH₃OH with CO₂ and H₂, and direct CH₄ carboxylation with CO₂.