Extremely low barrier activation of methane on spin-polarized ferryl ion [FeO]2+ at the four-membered ring of zeolite

Abstract

The Panov's α-O center in ZSM-5 zeolite revealing extraordinary activity in the oxidation of methane to methanol has been a subject of intense experimental and theoretical studies. Consensus on the structure and the origin of α-O superactivity seems to be achieved only in that α-O represents the ferryl group [FeO]2+ (bound by four lattice oxo sites) in cation-exchange position at some zeolite ring containing two framework Al atoms. Among rings with various sizes only three ones (four-, five-, and six-membered) can provide the ferryl ion with the necessary four number of oxo ligands (in the “plane” of ring) at reasonable Fe-O distance of about 2.0 Å. The six-membered ring is considered as the most probable one due to the 29Si MAS NMR data on Al pairs distribution on zeolite rings. However, the six-membered model in ZSM-5 has not been checked on behalf of the possibility of axial Fe-O coordination along Fe and terminal oxygen axis. By means of plane-wave DFT modeling, the present work shows that this coordination in fact takes place for six-membered ring unlike the four- and five-membered ones in which the Fe cation appears well above the plane of ring making the axial coupling impossible. Therefore, by known strong negative effect of axial oxo ligand on the ferryl species oxidizing activity, the six-membered model has to be discarded. Instead, a model of [FeO]2+ at four- or five-membered ring is to be put forward. In this work the four-membered model is shown to form the oxyl ground state Fe(III)–O• with the radical-like β-spin polarized oxygen. Calculated barrier of hydrogen abstraction from methane on this center is as low as 1.7 kcal/mol. The obtained prediction opens a way for other reactions to be probed on this center, particularly, the condensation of methane and oxidative dehydrogenation of hydrocarbons at low temperatures.