Local environment of iron in the mesoporous hexagonal aluminophosphate catalyst

Iron-containing hexagonal mesoporous aluminophosphate (FeHMA) was synthesised hydrothermally in a microwave oven in the presence of cetyltrimethylammonium chloride (CTACl) as a template. The local environment of aluminium and phosphorus forming the aluminophosphate walls was studied by MAS NMR. The local environment of iron incorporated in HMA was investigated by X-ray absorption spectroscopy (XAS). X-ray absorption near edge structure (XANES) studies showed that iron in the as-synthesised and template-free FeHMA was present predominantly in the form of octahedrally coordinated Fe3+ ions. X-ray absorption fine structure (EXAFS) analyses confirmed that iron cations in the as-synthesised and template-free FeHMA were in octahedral coordination with the Fe-O distances of 1.97 Å. Mössbauer spectroscopy revealed the presence of octahedrally coordinated Fe3+ and also tetrahedrally coordinated Fe2+ in the template-free FeHMA in the ratio of 4:1, which could not be detected by XANES. After the thermal treatment of template-free FeHMA at 427 °C in vacuum the Mössbauer spectra showed that almost half of the iron cations present in the sample were tetrahedrally coordinated. The framework positions of tetrahedrally coordinated Fe2+ and octahedrally coordinated Fe3+ are suggested.