Selective mild oxidation of methane to methanol or formic acid on Fe–MOR catalysts

Abstract

Controllable methane oxidation directly into value-added products under mild conditions remains a challenge. Herein, an active Fe/MOR catalyst was synthesized via simple solid-state ion exchange, and its activity in the selective oxidation of methane with H₂O₂ in the aqueous phase was intensively investigated. The octahedral dimeric Fe³⁺ species [Fe₂(μ-O)₂] in the extra framework was confirmed as the initial active site by X-ray photoelectron spectroscopy, X-ray absorption near-edge structure and extended X-ray absorption fine structure, UV-vis diffuse-reflectance spectroscopy, and high-angle annular dark-field scanning transmission electron microscopy in combination with DFT calculations. The DFT calculations indicated that methanol formation via methyl peroxide (CH₃OOH*) on [Fe₂(μ-OH)₂O₂] is the most favorable pathway compared to the direct formation of methanol via CH₃O*. The formed CH₃OH is easily further oxidized by hydroxyl radicals (˙OH) resulting in non-selective methane oxidation. In contrast, the Fe/MOR catalyst could lead to a high methanol selectivity of 71.3% in the presence of homogeneous Cu²⁺ precursor, which efficiently suppressed the over-oxidation of methanol, and a high formic acid selectivity up to 81–82% at a slightly higher reaction temperature by mildly shifting the oxidation of methanol and formaldehyde to the target product.