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 α-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 α-centers leads to the transformation of inactive Fe3+ to the active, coordinatively unsaturated Fe2+ states. After α-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 α-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.