Selective oxidation of benzene to phenol with nitrous oxide over MFI zeolites

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.