Selective mild oxidation of methane to methanol or formic acid on Fe-MOR catalysts
The controllable methane oxidation directly into value-added products under mild conditions is still the remaining challenge. Herein, an active Fe/MOR catalyst has been synthesized by the simple solid-state ion exchange, which was intensively investigated for selective oxidation of methane with H2O2 in aqueous phase. The octahedral dimer Fe3+ species [Fe2(μ-O)2] in 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 spectra, and high-angle annular dark field and scanning transmission electron microscopy with the combination of DFT calculations. DFT calculations indicate that the methanol formation via methyl peroxide (CH3OOH*) on [Fe2(μ-OH)2O2] is most favorable pathway compared to direct formation of methanol via CH3O*. The formed CH3OH was easily further oxidized by hydroxyl radical (●OH) to result in non-selective methane oxidation. In contrast, The Fe/MOR catalyst can produce a high methanol selectivity of 71.3% by the presence of homogeneous Cu2+ precursor that can efficiently suppress the over-oxidation of methanol, and a high formic acid selectivity up to 81-82% by just slightly higher reaction temperature that can mildly shift the oxidation of methanol and formaldehyde to the target product.