Co-ordination and oxidation changes undergone by iron species in Fe-silicalite upon template removal, activation and interaction with N2O:: an in situ X-ray absorption study

We report on the characterization of a Fe-silicalite sample having a sufficient iron dilution (Si/Fe=90) to be considered a 'true' catalysts in partial oxidation reactions (e.g. benzene-->phenol). Fe3+ species are incorporated during zeolite synthesis in framework Si sites, where they exhibit a tetrahedral symmetry being co-ordinated to four oxygen atoms of the lattice. Template burning in air and sample activation in vacuo causes a progressive migration of ordered iron from framework sites into disordered extra-framework positions. This phenomenon is associated to a reduction from Fe3+ to Fe2+. The fraction of iron species involved in this phenomenon depends upon the activation temperature, and can reach the value of one when severe treatments are adopted. The structural disorder of extra-framework Fe species is monitored by the striking decrease of the EXAFS signal, while the changes of the oxidation state has been indicated in the XANES part of the X-ray absorption spectra. So obtained extra-framework Fe2+ species are characterized by very high co-ordinative unsaturation, being able to form Fe2+(NO)(3) complexes upon interaction with NO at RT (as evidenced by IR spectra). Interaction with N2O at 523 K causes the overall reoxidation of iron species resulting in a catalyst containing extra-framework species characterized by a less pronounced co-ordinative unsaturation. Infrared (IR) spectroscopy shows the dissociation of N2O, while EXAFS spectroscopy shows the increase of the co-ordination sphere of iron. This fact has been interpreted in terms of an additional oxygen atom (coming from the decomposition of N2O) co-ordinated to Fe3+ species. The described Fe3+ --> Fe2+ --> Fe3+ redox cycle has also been qualitatively followed by EPR spectroscopy. (C) 2002 Elsevier Science B.V. All rights reserved.