Following the evolution of iron from framework to extra-framework positions in isomorphously substituted [Fe,Al]MFI with 57Fe Mössbauer spectroscopy

Isomorphously substituted [Fe,Al]MFI zeolites enriched with 57Fe isotope were synthesized with varying iron (0.075-0.6 wt%) and aluminum (0-1.1 wt%) concentrations. The as-prepared zeolites underwent sequential post-treatment steps (i.e., calcination, transformation to H-form, and steam treatment), making them active in the one-step oxidation of benzene to phenol with N2O as oxidant. Through 57Fe Mössbauer spectroscopy, and transmission electron microscopy (TEM) as a complementary technique, we followed the evolution of the physico-chemical states of iron during the different post-treatment steps. In general, the extent to which iron is removed from framework to extra-framework positions, during different post-treatments, is higher for [Fe,Al]MFI zeolites containing high iron and aluminum concentrations. For all steamed [Fe,Al]MFI zeolites with an aluminum content of ∼1.1 wt%, iron was predominantly present in the high-spin Fe2+ state (ca. 90% based on spectral contribution). This extraordinarily high concentration of Fe2+ species is significant, since the presence of Fe2+ was correlated to the formation of α-sites, which were reported to be responsible for the direct oxidation of benzene to phenol [1]. For [Fe,Al]MFI with less aluminum (∼0.6 wt%), a mixture of Fe2+ (at least 30%) and Fe3+ ions was observed, whereas for the aluminum-free sample, only iron in the Fe3+ state was obtained.