Fe³⁺-substituted silicate gels: Precursors for zeolite-based catalysts

In order to investigate the possibility of isomorphous substitution of Fe³⁺ for Si⁴⁺ in the tetrahedral silicate network, two Fe-substituted silicate gel samples with Si/Fe atom ratios of 50 and 25 were prepared by the solution-sol-gel processing technique. The samples were characterized by magnetization and Mössbauer spectroscopic measurements over the temperature range 293-4.2 K. The magnetic moment of the Fe ions and Mössbauer isomer shifts were used to determine the valence state and coordination symmetry of the Fe species present in these gel samples. It has been found that iron enters substitutionally into the tetrahedral silicate structure, especially at low concentrations. This result is of importance, since Fe substituted silicates can be used as catalyst precursors in several technologically relevant chemical reactions.