Changes in states of substituted iron in microporous (MFI analogue) and mesoporous (MCM-41) hosts exposed to redox conditions

Iron was incorporated into microporous MFI analogue (ZSM-5, Si/Fe ∼ 200 and FER, Si/Fe = 16) and mesoporous MCM-41 hosts of different preparations (Si/Fe ∼ 100 and 20, respectively). Samples were characterized by in situ Mössbauer spectroscopy under various conditions. Framework (FW) and extra-framework (EFW) sites can be distinguished in relation with strict three-dimensional (3D) crystallinity of the structure. In addition, in the microporous ZSM-5 system occurrence of Fe4+ oxidation state is hypothesized and reversible Fe4+ ↔ Fe3+ transformation is presumed for a minor portion of iron. Spectra of Fe-FER are interpreted in terms of dynamic formation and decomposition of FeFW–O–FeEFW dinuclear centres. In the mesoporous systems, the partly amorphous character of the pore wall is correlated with the redox behaviour and coordination of incorporated iron; larger portion of iron may take part in Fe3+ ↔ Fe2+ reversible processes and Fe2+ may be stabilized in distorted coordination (not appearing in microporous structures).