Evolution of Iron States and Formation of α-Sites upon Activation of FeZSM-5 Zeolites

Mössbauer spectroscopy in situ was used to study the effect of high-temperature activation on the evolution of iron introduced into a ZSM-5 zeolite matrix by various methods. The activation process (calcination in air, in vacuum, or in the presence of water vapor) was shown to cause an intensive reduction of iron, yielding two types of dinuclear Fe2+ complexes, which may comprise more than 60% of the total metal content. Reduced Fe2+ ions are stable in the presence of O2 but are reversibly oxidized to Fe3+ by nitrous oxide, generating active α-oxygen species, which bring unique oxidation properties to the zeolite. After coordinative saturation by adsorbed water molecules, both iron complexes show identical Mössbauer spectra, which are close to the spectra of dinuclear iron sites in the MMO enzyme. A thorough quantitative comparison between the number of α-oxygen atoms and the number of Fe atoms involved in redox transition shows that α-sites are dinuclear complexes in which both Fe atoms are capable of α-oxygen generation.