Formation of an iron-oxo species upon decomposition of dinitrogen oxide on a model of Fe-ZSM-5 zeolite

The so-called 'a-oxygen' on Fe-ZSM-5 zeolite, a surface oxygen species responsible for high reactivity in oxidation of methane and of benzene, has been investigated. We present from density-functional-theory (DFT) calculations how such a reactive surface species is generated upon decomposition of dinitrogen oxide (N₂O) and propose a possible form of 'a- oxygen' on Fe-ZSM-5 zeolite. In the initial stages of the reactions, a complex involving an Fe(ON₂) moiety is formed, followed by dissociation into an iron-oxo species and N₂. The activation energy for the decomposition of N₂O on a possible iron active site model of Fe-ZSM-5 zeolite is predicted to be 2.4 kcal mol^-1 at the B3LYP level of theory. Therefore the decomposition of N₂O is expected to take place easily at a coordinatively unsaturated iron active center supported on zeolite. The iron-oxo species thus formed should play an essential role in the direct hydroxylation of methane, benzene, and other hydrocarbons if it involves a coordinatively unsaturated iron. The oxygen exchange on the iron-oxo complex was also investigated. The activation energy for the oxygen exchange on the 'a-oxygen' is predicted to be 26.8 kcal mol^-1. We propose that the bare FeO⁺ complex and the so-called 'a-oxygen' on Fe-ZSM-5 zeolite involve similar catalytic active centers responsible for similar catalytic functions for methane and benzene.