Complementary use of IR and EPR spectroscopies for characterization of iron species in thermally treated MFI-type zeolites

Iron species formed by thermal treatment of iron- and aluminium-based ion exchanged MFI-type zeolites were investigated by IR and EPR spectroscopies and by the adsorption of NO as a probe molecule. The parallel investigation of the residual acidity by adsorption of pyridine allowed to calculate the number of acidic sites and the number of Lewis centres both in iron- and aluminium-based MFI zeolites. It was confirmed that the activation process could be divided into two regions, depending on temperature. At lower temperatures the main process is hydrolysis and thermal decomposition of precursors such as iron aquacomplexes and/or hydroxides, but at higher temperatures dehydroxylation and subsequent redox reaction, balancing the amounts of Fe(II) and Fe(III) ions, takes place. Combined EPR and IR studies showed that the process of thermal activation can be tracked over wide temperature range (from ambient temperature to 1020 K) by following the properties of both ferrous and ferric sites and implied that at least the part of Fe cations cannot be observed by neither method, suggesting they adopt a form of bulk oxide-like species.