Synergistic Effect on the Photoactivation of the Methane CH Bond over Ga³⁺‐Modified ETS‐10

Abstract

During the past decades, many efforts have been devoted to activation of the strong C À H bond in methane under mild conditions [1][2][3][4] 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 SiO 2 -Al 2 O 3 -TiO 2 , [5] for methane conversion. Recently, we used zinc to modify the medium-pore ZSM-5 zeolite and found that the resulting material exhibits substantial photocatalytic activity for the selective conversion of methane to ethane and hydrogen under ultraviolet (UV) irradiation. [6] 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 H 3 CÀH bond cleavage process is a key step in the dehydrogenation and dimerization of methane. [7,8] 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 H 3 C À H bond occurring on substrate surfaces has been revealed so far, [9] and the nature of the photoactive species responsible for the methane C À H activation has yet to be elucidated in more detail. Previously, two models for the photoactivation of the methane CÀH bond were proposed. One considers that the presence of oxygen-centered radicals brings about homolytic CÀH bond cleavage, [10] whereas in the other highly dispersed metal species are believed to be the active sites of methane dehydrogenation. [6,11] Herein, we describe a Ga 3+ -modified ETS-10 zeolite material (ETS-10 = titanosilicate) in which the photogenerated hydroxyl radical and the extraframework metal ion interact with the methane molecule to split the H 3 CÀH bond in a synergistic way under UV irradiation (l < 350 nm). It is demonstrated that the combination of both oxygen-centered and metal-centered active sites in the material significantly enhances its photoactivity for methane CÀH bond activation, leading to efficient non-oxidative coupling of methane at room temperature. An average methane conversion rate of around 29.8 mmol h À1 g À1 was achieved after UV irradiation for 5 h, which is 3 and 20 times faster than those of Zn +modified ZSM-5 zeolite and SiO 2 -Al 2 O 3 -TiO 2 material (the two best photocatalysts for methane conversion developed so far), respectively.ETS-10 is a microporous titanosilicate with a framework containing one-dimensional O-Ti-O-Ti-O semiconducting nanowires (diameter of 0.67 nm) insulated from one another by the surrounding SiO 2 matrix ( Figure 1). [12] The combination of quantum-confined titanate...

Simple and Efficient System for Combined Solar Energy Harvesting and Reversible Hydrogen Storage

“…Based on our recent work on the photoinduced methane conversion, [25][26][27] we reasoned that photo-energy should be effective to overcome the intrinsic thermodynamic constraint of activating C-H bonds of cyclic alkanes and access easy hydrogen release under ambient conditions. Herein, we designed a reversible hydrogen storage/release cycle based on the metal-catalyzed hydrogenation and photoinduced dehydrogenation of organic cyclic hydrocarbons reactions at room temperature.…”

Thermal Non‐Oxidative Aromatization of Light Alkanes Catalyzed by Gallium Nitride

“…[1][2][3][4][5][6][7] Tremendous attempts have been made for such conversions and several promising catalyst systems have been developed, such as the metal-modified zeolites, [8][9][10][11] the pincer-ligated iridium homogeneous complexes, [12] and the lattice-confined single iron sites embedded within a silica matrix. [13] Notably, gallium species in diverse forms turned out to bear remarkable potential for the activation of C À H bonds of light alkanes [14][15][16][17][18] and the selective catalytic reduction (SCR) of nitric oxide by methane. [19] Moreover, the catalytic efficiency for non-oxidative alkane aromatization can be increased significantly by the incorporation of gallium into zeolites.…”

Solar- versus Thermal-Driven Catalysis for Energy Conversion

“…Chen's sunlight-driven CH 4 conversion was the first example of solar-catalyzed CH 4 conversion under environmental conditions. Further, Ga 3+ -modified zeolites 37 and GaNbased UV-responsive photocatalysts 38 also permit the direct conversion of CH 4 to hydrocarbons and benzene at room temperature. Very recently, a new PEC approach for CH 4 oxidation to CO over Ti 3+ doped atomic-layer-deposition-grown TiO 2 has been reported for the CO production with over 80% selectivity in all products (CO, O 2 , and carbonate, etc.).…”