Iron Site Evolution in Fe-MFI Zeolite under Thermal Treatment: Spectroscopic Insights and Acetone Vapor Interactions

Iron-containing MFI zeolites (Fe-MFI) and purely siliceous MFI were compared to assess the structural and electronic effects of iron incorporation. Powder X-ray diffraction confirms that the MFI framework is preserved after Fe introduction during synthesis. UV-vis and 57Fe Mössbauer spectroscopy show that iron is mainly present as Fe3+ in distorted [5]/[6]-coordinated and octahedral environments. In the as-synthesized material, Fe3+ accounts for ∼88% of the total iron, accompanied by a minor Fe2+ fraction (13%). After calcination, Fe2+ is fully oxidized to Fe3+ and redistributed nearly equally between distorted and octahedral sites. Nitrogen physisorption indicates a moderate loss of microporosity due to the partial pore occupation by FeOx species. Overall, Fe incorporation generates dispersed Fe3+ centers and small extra-framework FeOx species that subtly modify the structural and electronic properties of the MFI zeolite. These powders were incorporated into a membrane for acetone vapor detection, the Fe-MFI membrane showing a stronger and more distinct response to acetone exposure than the pure MFI membrane. In summary, this study establishes Fe-MFI membranes as promising active materials for acetone vapor interaction, providing a robust proof-of-concept for future gas-sensing applications. The key novelty of this work is the direct correlation between the calcination-induced redistribution of iron sites and the resulting vapor-sensing performance of the membrane.