Formation of an Iron-Oxo Species upon Decomposition of Dinitrogen Oxide on a Model of Fe-ZSM-5 Zeolite

The so-called ``α-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 ``α-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 ``α-oxygen" is predicted to be 26.8 kcal mol-1. We propose that the bare FeO+ complex and the so-called ``α-oxygen" on Fe-ZSM-5 zeolite involve similar catalytic active centers responsible for similar catalytic functions for methane and benzene.