Mechanism of CH₄ activation on a monomeric Zn²⁺-ion exchanged in MFI-Type zeolite with a specific Al arrangement: Similarity to the activation site for H₂

In this work, we used both experimental and density functional theory (DFT) calculation methods to examine the mechanism of CH₄ activation taking place on the Zn²⁺ ion exchanged MFI-type zeolite (ZnMFI). The heterolytic dissociation of CH₄ on ZnMFI around 300 K was observed experimentally, causing the appearance of IR bands at 3615, 2930, and 2892 cm^-1. The first band can be assigned to the OH stretching vibration associated with the formation of the Brønsted acid site and the latter to the C-H stretching modes ascribable to the -[ZnCH₃]⁺ species. Combining the IR spectroscopy with a DFT calculation, it is apparent that the heterolytic C-H bond dissociation of CH₄ has an activation energy of 15 kJ mol^-1 and takes place on a monomeric Zn²⁺ at the M7S2 site. The M7S2 site has a specific Al arrangement in MFI and exhibits a pronounced reactivity for the H-H bond cleavage of H₂, even at room temperature. In addition, to our knowledge, we are the first to succeed in explaining the dissociation process of CH₄ by applying natural bond orbital (NBO) and interaction localized orbital (ILO) analyses to the present system; the donation interaction from the CH₄-σ(C- H) orbital to the Zn-4s orbital triggers the cleavage of the C-H bond of CH ₄ under mild conditions.