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.
ASJC Scopus subject areas
- Electronic, Optical and Magnetic Materials
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films