Selective oxidation of benzene to phenol with nitrous oxide over MFI zeolites: 1. On the role of iron and aluminum

Three MFI zeolites with iron ([Fe]MFI), aluminum ([Al]MFI), or a combination thereof ([Fe,Al]MFI) as framework substituents were hydrothermally synthesized. The evolution of the iron and aluminum species during template removal and further hydrothermal activation was carefully studied by UV–vis, ESR, 27Al NMR, infrared spectroscopy, and high-resolution TEM. Directly after synthesis, Fe and Al appear to be incorporated to a large extent into the zeolite framework. Iron is clearly less stable at framework positions than aluminum. The migration of iron to extraframework positions readily proceeds during template removal. Although a large fraction of framework iron is removed upon calcination in [Fe]MFI and [Fe,Al]MFI, the extent of Fe3+ clustering is more pronounced in the latter material. A more severe steam activation step is required to extract a substantial amount of aluminum from framework positions. In contrast to [Al]MFI and [Fe]MFI, [Fe,Al]MFI shows good performance in selective oxidation of benzene to phenol. A comparison of IR spectra of adsorbed NO2 groups points to the presence of various extraframework species. After the activation of [Fe,Al]MFI an extraframework mixed metal oxide species is formed, which is argued to be essential for selective oxidation of benzene to phenol with nitrous oxide.