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).