Stability of Extraframework Iron-Containing Complexes in ZSM-5 Zeolite

The stability of oxygenated and hydroxylated iron complexes in Fe/ZSM-5 is studied by periodic DFT calculations. The reaction paths for the interconversion of various potential iron-containing complexes confined in the zeolite matrix are discussed. It is demonstrated that the distribution of mononuclear [FeO]+ species depends only slightly on the specific local zeolite environment. For all binuclear complexes considered, a notable preference for the location at the larger eight-membered ring γ site in the sinusoidal channel is observed. Nevertheless, the formation of the mononuclear species [FeO]+ in realistic systems is very unlikely. Irrespective of their location inside the zeolite matrix, such species show a strong tendency toward self-organization into binuclear oxygen-bridged [Fe(μ-O)2Fe]2+ complexes. Using ab initio thermodynamic analysis of the stability of different Fe complexes in ZSM-5, it is demonstrated that two distinct extraframework cationic complexes can be present in the Fe/ZSM-5 catalyst, namely, [FeIII(μ-O)2FeIII]2+ and [FeII(μ-O)FeII]2+. The [FeII(μ-O)FeII]2+ complexes containing bivalent iron centers are mainly present in the Fe/ZSM-5 catalyst activated at low oxygen chemical potential and H2O-free conditions, whereas the formation of its Fe3+-containing counterpart [FeIII(μ-O)2FeIII]2+ is favored upon the high-temperature calcination in an O2-rich environment.