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 $38,592\mathrm{\mu }{\mathrm{mol}}_{{\mathrm{CH}}_3\mathrm{OH}}{{\mathrm{g}}_{\mathrm{Fe}}}^{-1}{\mathrm{h}}^{-1}$ and $81,043\mathrm{\mu }{\mathrm{mol}}_{{\mathrm{CH}}_3{\mathrm{CO}}_2\mathrm{H}}{{\mathrm{g}}_{\mathrm{Fe}}}^{-1}{\mathrm{h}}^{-1}$, 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.

The development of catalytic technology for direct oxidation of methane into value-added products is highly lucrative. Here, a metal-organic framework supported mono iron(III)-dihydroxyl catalyst selectively oxidizes methane into methanol or acetic acid using only oxygen, where acetic acid formation occurs via in-situ methane carboxylation and methanol hydrocarboxylation.