The Fe Active Sites in FeZSM-5 Catalyst for Selective Oxidation of CH4 to CH3OH at Room Temperature

It was shown recently that iron complexes formed during the thermal treatment of FeZSM-5 zeolite perform single-turnover cycles of methane oxidation to methanol at ambient conditions when nitrous oxide is used as a source of oxygen. The long-living active intermediate is capable of transferring accepted O atom (called -oxygen) into C-H bond of methane to produce methanol at 100% selectivity. The present work is aimed to the identification of iron active sites through a comparison of in situ ⁵⁷Fe Mössbauer spectra of FeZSM-5 after various thermal treatments and reaction stages. It is established that vacuum activation at 900 °C accompanied by a manifold increase of -centers leads to the transformation of inactive Fe³⁺ to the active, coordinatively unsaturated Fe²⁺ states. After -oxygen loading, active Fe²⁺ states transform to a new Fe³⁺ states responsible for further methane oxidation. The latter reaction, as well as reaction with ²H₂, is not fully reversible: part of active Fe³⁺ transforms to other inactive Fe³⁺ form. On the contrary, reaction of -oxygen with CO leads to a complete restoration of the initial, vacuum activated Fe²⁺ states. On the base of joint Mössbauer and catalytic data, the structure and composition of iron active centers are suggested.