An Important Factor in CH₄ Activation by Zn Ion in Comparison with Mg Ion in MFI: The Superior Electron-Accepting Nature of Zn²⁺

Abstract

We present clear IR and density functional theory (DFT) evidence demonstrating that the electron-accepting nature of Zn 2+ ion exchanged in MFI-type zeolite (ZnMFI) plays a dominant role in CH 4 activation. The IR study revealed that the heterolytic dissociation of CH 4 takes place on the Zn 2+ ion exchanged in MFI under a CH 4 atmosphere even near room temperature, whereas a similar reaction scarcely occurred on Mg 2+ ion exchanged in MFI, although the ionic radius and charge of Mg 2+ are almost the same as those of Zn 2+ . These data indicate that the dissociation reaction of CH 4 on Zn 2+ 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 v 1 mode of the C–H symmetric stretching vibration, i.e., a larger band shift toward lower wavenumbers, for the molecular CH 4 adsorbed on ZnMFI, compared with those for a gaseous CH 4 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 Zn 2+ 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 CH 4 in the Zn 2+ ion in MFI in comparison with that of Mg 2+ exchanged at the same site. We have thus shown that the electron-transfer interaction between Zn 2+ and CH 4 plays a key role in the heterolytic CH 4 activation process: the σ donation from the σ(C–H) orbital of CH 4 toward the Zn 4s orbital through overlapping with each orbital.