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, [Fe^III(μ-O)₂Fe^III]²⁺ and [Fe^II(μ-O)Fe^II]²⁺. The [Fe ^II(μ-O)Fe^II]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 [Fe^III(μ- O)₂Fe^III]²⁺ is favored upon the high-temperature calcination in an O₂-rich environment.