Theoretical Investigation of Methane Hydroxylation over Isoelectronic [FeO]²⁺- and [MnO]⁺-Exchanged Zeolites Activated by N₂O

While the most likely structure of the active site in iron-containing zeolites has been recently identified as [FeO]²⁺ (Snyder et al. Nature 2016, 536, 317-321), the mechanism for the direct conversion of methane to methanol over this active species is still debatable between the direct-radical-rebound or nonradical (concerted) mechanism. Using density functional theory on periodic systems, we calculated the two reaction mechanisms over two d⁴ isoelectronic systems, [FeO]²⁺ and [MnO]⁺ zeolites. We found that [FeO]²⁺ zeolites favor the direct-radical-rebound mechanism with low CH₄ activation energies, while [MnO]⁺ zeolites prefer the nonradical mechanism with higher CH₄ activation energies. These contrasts, despite their isoelectronic structures, are mainly due to the differences in the metal coordination number and O_α (oxo) spin density. Moreover, molecular orbital analyses suggest that the zeolite steric hindrance further degrades the reactivity of [MnO]⁺ zeolites toward methane. Two types of zeolite frameworks, i.e., medium-pore ZSM-5 (MFI framework) and small-pore SSZ-39 (AEI framework) zeolites, were evaluated, but no significant differences in the reactivity were found. The rate-determining reaction step is found to be methanol desorption instead of methane activation. Careful examination of the most stable sites hosting the active species and calculation for N₂O decomposition over [Fe]²⁺-MFI and -AEI zeolites were also performed.