Reactivity of generated oxygen species from nitrous oxide over [Fe,Al]MFI catalysts for the direct oxidation of benzene to phenol

The catalytic performance of various steam-activated [Fe,Al]MFI catalysts in the direct oxidation of benzene to phenol using N2O as oxidant is described. All [Fe,Al]MFI catalysts contain ca. 90% of iron in the high-spin Fe2+ state, independent of the iron concentration (0.075-0.6 wt.% iron). In the presence of N2O at 623 K, most Fe2+ ions (>90%) were oxidized to Fe3+ ions as deduced from Mössbauer spectroscopy. In the presence of benzene, subsequent reduction of Fe3+ to Fe2+ takes place. However, not all of the oxidized Fe2+ to Fe3+ ions were able to selectively oxidize benzene to phenol. This indicates that only a fraction of iron is catalytically active. For [Fe,Al]MFI catalysts with relatively high iron concentration, most of the extra-framework iron species formed are inactive in the direct oxidation of benzene to phenol. Finally, a more detailed in situ Mössbauer study for one sample, i.e. [Fe,Al]MFI (1:8) catalyst, was performed to illustrate the reduction/oxidation properties of the different iron species formed after steam-treatment.