Efficient Sunlight‐Driven Dehydrogenative Coupling of Methane to Ethane over a Zn⁺‐Modified Zeolite

Abstract

Effective conversion of methane to a mixture of more valuable hydrocarbons and hydrogen under mild conditions is a great scientific and practical challenge. [1][2][3][4][5][6][7] Up to date, the thermal routes for activation of the strong CÀH bond (104 kcal mol À1 ) in methane require high temperatures and multistep processes, and therefore they are energy-consuming and inefficient. [8,9] Compared to methods powered by thermal energy, [10][11][12][13][14] techniques that use photonic energy have substantial advantages, such as the capacity to minimize coking at room temperature. A promising approach to methane conversion is the direct non-oxidative coupling of methane (NOCM) to form ethane and hydrogen powered by photons [Equation (1)]. [15,16] 2 CH 4The produced ethane can in turn be conveniently converted to liquid fuels or ethene through metathesis and dehydrogenation, respectively. [17] Furthermore, this NOCM reaction is the best way to produce clean H 2 energy from fossil fuels because methane has the highest H/C ratio among all hydrocarbons. However, the methane conversion using photocatalysts previously reported for the NOCM reaction is very low (less than 4 % upon UV irradiation for 90 hours).[18] More importantly, the wavelength of the light used in the photocatalytic systems previously reported for NOCM needs to be shorter than 270 nm, which is beyond the region of the solar spectrum (wavelength l > 290 nm) reaching the surface of the Earth. To achieve a substantial yield and to exploit solar energy effectively, the development of photocatalytic systems with a distinctly higher activity, higher selectivity, and lower photon energy threshold is desired.Herein, we report a Zn + -modified ZSM-5 zeolite catalyst which exhibits superior photocatalytic activity for selective C À H activation of an alkane molecule and methane conversion both upon high-pressure irradiation of a mercury lamp and sunlight irradiation at room temperature. An optimized catalyst converts 24 % of methane upon irradiation for 8 hours by a high-pressure mercury lamp with a selectivity larger than 99 % for ethane and hydrogen products. Mechanistic studies suggest a two-stage photoexcitation process, which lowers the energy threshold (l < 390 nm) needed to power our photocatalytic system relative to that (l < 270 nm) required by previously reported systems.Interactions of zeolites with metal vapors are an effective approach for the preparation of metal-containing zeolites. [19,20] Using this approach, we have synthesized a zincmodified ZSM-5 catalyst with a Brunauer-Emmett-Teller (BET) surface area of 362 m 2 g À1 through a solid-vapor reaction between a dehydrated HZSM-5 zeolite (protonated ZSM-5 with a Si/Al ratio of the framework of 14.8) and metallic zinc vapor. During the reaction, the protons of the Brønsted acidic sites (OH groups bridging Al and Si atoms of the framework) in the zeolite are reduced by zinc atoms to form H 2 molecules (as detected by gas chromatography, GC), whereas the zinc atoms undergo two different oxidation r...

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

“…As illustrated in Figure S3, upon visible light irradiation, the photoexcited hot electrons of Pt species can be temporarily added to an empty C-H σ*-antibonding orbital of the adsorbed cyclohexane to active and homolytically cleave its C-H bond, followed by the formation of an H atom which then couples with another H atom to produce H 2 . It is worth pointing out that the alkane dehydrogenation process does not consume extra electrons 25 and thus, the photoexcited electrons will finally fall back to Pt. This mechanism is consistent with the experimental and XPS results that the Pt species with high electron density (low electron binding energy), which can offer electrons efficiently, are critical to access a high reactivity to the cyclohexane dehydrogenation reaction under visible light.…”

“…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.…”

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

“…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. [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. 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.…”

Layered Double Hydroxide Nanostructured Photocatalysts for Renewable Energy Production

“…This g-value of observed for ZnAl-LDH nanosheets is in good agreement with ESR data previously reported by Chen et al that the Zn + in Zn-ZSM-5 exhibited an ESR signal around g = 1.998. [ 41 ] In the ZnAl-LDH system, the V o defect is likely located next to Zn cations, leading to the existence of coordinatively unsaturated Zn ions in the form of Zn + -V o complexes.…”