Step towards nanoscale fe moieties: Intercalation of simple and Keggin-type iron-containing ions in-between the layers of Na-montmorillonite

The synthesis and characterization of iron-containing Na-montmorillonite prepared by various techniques are reported. Methods include ion exchange by Fe(II) ion and pillaring by partially hydrolysed Fe(II) salt or by Al13O407+ where the substitution of Fe(III) for AI(III) was attempted. Substances were characterized by elemental analysis, thermogravimetry, X-ray diffractometry (XRD), Al-27 and Si-29 MAS NMR, FT-IR spectroscopies, and BET measurements. The coordination and oxidation states of iron were deduced from Fe-57 Mossbauer spectra. Ion exchange resulted in the incorporation of polynuclear hydrous Fe(III) oxides characterized by octahedral coordination in significant amounts. In contradiction to earlier reports, we find no evidence for isomorphous substitution of iron for aluminium in Al13O407+ Keggin ions. Instead, mixed pillars were formed containing Al13O407+ Keggin ions and hydrous iron oxide oligomers. After reduction, large magnetite patches kept the layers apart when the partially hydrolysed pure iron salt was used for pillaring. Heat treatment influenced the water content of the intercalated species. The oxidation state of iron could be significantly influenced by reduction with hydrogen at various temperatures depending on the method of preparation.