Evolution of Extraframework Iron Species in Fe Silicalite: 2. Effect of Steaming

In the first part of this investigation, we studied in detail the structure, the oxidation state, and the mutual interconversion of extraframework iron species formed in Fe silicalite samples upon migration of iron from framework to extraframework positions induced by thermal activation and redox treatments. In this second part, we used temperature-dependent EPR, UV–vis, and IR spectroscopies to investigate the nature of the extraframework iron species present in Fe silicalite after steaming. Relatively large (≅30 nm) particles of iron oxide were detected. These large clusters are located outside the zeolite pores and are believed not to participate in partial oxidation reactions. It is shown that the Fe species which contribute to the formation of Fe2O3 extralattice particles are preferentially clustered grafted Fe2+ species. In contrast, isolated Fe2+ species are less affected by the steaming treatment. This suggests that the oxygen species active in the selective partial oxidation of benzene are adsorbed on isolated extraframework grafted Fe2+ centers. This result is of primary importance for the understanding of the structure of the oxygen species active in partial selective oxidation using N2O as oxidant.