Investigation of structural iron species in Fe-mesoporous silicas by spectroscopic techniques

A series of iron-containing mesoporous silicas (Fe-HMS) with Fe contents up to 5.7 wt.% have been prepared at room temperature using the neutral hexadecylamine as surfactant molecule. Characterization of these materials by a variety of spectroscopic techniques, including EXAFS, EPR, Mossbauer and UV-Vis spectroscopies, suggests that Fe species are tetrahedrally coordinated in as-made materials. Tetrahedra are highly distorted with two long and two short Fe-O bond distances due to hydrogen-bonding type interactions with the organic molecules. Removal of the amines by solvent extraction does not modify the chemical environment around the Fe species. Subsequent calcination leads to mainly 3-coordinated Fe species, readily converting to 4-coordinated species in the presence of water. The solvent extraction appears to be a key step to prevent the formation of extraframework iron oxide species, which are formed in large amount when the surfactant is directly removed from as-made materials by calcination in air at high temperature.