Synergistic Effect on the Photoactivation of the Methane C-H Bond over Ga~(3+)-Modified ETS-10

摘要

During the past decades, many efforts have been devoted to activation of the strong C-H bond in methane under mild conditions because through suitable conversion methane may be used to substitute for the dwindling petroleum resources as a chemical feedstock. Among the various strategies is a compelling approach that uses photoenergy to drive the conversion of methane to more valuable molecules through dehydrogenation at room temperature. Yoshida et al. developed several effective photocatalysts, such as the ternary oxide SiO2-Al2O3-TiO2, for methane conversion. Recently, we used zinc to modify the medium-pore ZSM-5 zeolite and found that the resulting material exhibits substantial photo-catalytic activity for the selective conversion of methane to ethane and hydrogen under ultraviolet (UV) irradiation. However, there are still huge challenges in finding more powerful systems to achieve a practical product yield and to exploit solar energy more effectively for the photodriven conversion of methane. It is now a consensus that the H3C-H bond cleavage process is a key step in the dehydrogenation and dimerization of methane. Investigations into this process provide insights into the essence of methane conversion at a molecular level, and thus enable us to design better performing systems. Nevertheless, only limited information about the mechanism for the photoinduced cleavage of the H3C-H bond occurring on substrate surfaces has been revealed so far, and the nature of the photoactive species responsible for the methane C-H activation has yet to be elucidated in more detail. Previously, two models for the photoactivation of the methane C-H bond were proposed. One considers that the presence of oxygen-centered radicals brings about homolytic C-H bond cleavage, whereas in the other highly dispersed metal species are believed to be the active sites of methane dehydrogenation.