Active and spectator iron species in Fe/MFI catalysts for benzene selective hydroxylation with N2O

The presence and nature of active and spectator iron species in Fe/MFI catalysts is analyzed by comparing reactivity and physico-chemical properties (by ESR, Mössbauer and 29Si-NMR) of a series of samples pretreated in different conditions and in which part of the iron is selectively removed by extraction. A large part of the extra-framework (EFW) iron present inside zeolite channels could be removed without affecting significantly the productivity or selectivity to phenol. This shows that the most abundant species which may be identified in these samples are not those responsible for activity. During the catalytic reaction further modification occurs with the migration to EFW positions of the largest part of framework (FW) iron due to Fe3+ to Fe2+ reduction forming first disordered clustered iron oxide/hydroxide species (ferrihydrite) which then nucleate forming magnetite nanodomains. This initial dislodgement of FeFW determines an initial increase in phenol productivity. After about 3h the productivity to phenol reaches a nearly constant value, but this residual activity does not correlates with the amount of FeEFW further evidencing that the largest part of EFW iron species are only spectator in benzene selective hydroxylation, while the active species are very limited and possibly associated to sites interacting with defect sites suggested to be (-Si-O)2Fe2+ species which reacts with N2O to give rise to an (-Si-O)2Fe4+=O oxo-type species.