An important factor in CH₄ activation by Zn ion in comparison with Mg ion in MFI: The superior electron-accepting nature of Zn²⁺

We present clear IR and density functional theory (DFT) evidence demonstrating that the electron-accepting nature of Zn²⁺ ion exchanged in MFI-type zeolite (ZnMFI) plays a dominant role in CH₄ activation. The IR study revealed that the heterolytic dissociation of CH ₄ takes place on the Zn²⁺ ion exchanged in MFI under a CH₄ atmosphere even near room temperature, whereas a similar reaction scarcely occurred on Mg²⁺ ion exchanged in MFI, although the ionic radius and charge of Mg²⁺ are almost the same as those of Zn ²⁺. These data indicate that the dissociation reaction of CH ₄ on Zn²⁺ in MFI is facilitated not only by the electrostatic interaction but also by the electron-transfer interaction. This interpretation was clearly evidenced by the observed v₁ mode of the C-H symmetric stretching vibration, i.e., a larger band shift toward lower wavenumbers, for the molecular CH₄ adsorbed on ZnMFI, compared with those for a gaseous CH₄ molecule. Additional experiments were also performed by the IR method utilizing CO as a probe molecule that has an electron-donating nature. All experimental data presented were successfully explained in terms of the superior electron-accepting nature of Zn²⁺ exchanged in MFI. Furthermore, the DFT calculation method completely explained all experimental data by adopting the M7S2 model, which was truncated from the ZnMFI structure; the electron-accepting nature is dominant in the heterolytic activation of CH₄ in the Zn²⁺ ion in MFI in comparison with that of Mg²⁺ exchanged at the same site. We have thus shown that the electron-transfer interaction between Zn²⁺ and CH₄ plays a key role in the heterolytic CH₄ activation process: the σ donation from the σ(C-H) orbital of CH₄ toward the Zn 4s orbital through overlapping with each orbital.