A novel strategy for highly selective ethanol synthesis from methane driven by light-driven transformation without reliability for reactive oxygen species

(Press-News.org) Professor Zhongkui Zhao of Dalian University of Technology, in collaboration with Professor Riguang Zhang of Taiyuan University of Technology, Researcher Yuefeng Liu of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Professor Ting Zhang of Qingdao University, and Professor Chunshan Song  of the Chinese University of Hong Kong, constructed a single-atom Cu-N2O1 site with axial oxygen coordination on C3N4. Through the polar activation of the CH bond by the polar Cu-O bond, they successfully pioneered a new photocatalytic methane upgrading strategy independent of reactive oxygen species. This strategy not only significantly increased the rate of photocatalytic methane conversion to ethanol by 226  μmol/g/h under mild conditions, but also achieved an ethanol product selectivity as high as 98%. This achievement not only greatly advances the basic understanding of photocatalytic methane conversion to ethanol, but also creates a new paradigm for photocatalytic methane upgrading, successfully solving the seesaw dilemma between the liquid fuel generation rate and its selectivity in the photocatalytic methane conversion process, and providing new ideas and methods for the innovative development of future photocatalytic methane conversion. The article was published as an open access research article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background information:

Methane is an abundant but low-value fuel and a significant greenhouse gas. Photocatalytic conversion of methane into transportable liquid oxygenated compounds (MTLOs), such as methanol and ethanol, especially ethanol, has long been a sustainable and attractive approach. Methane's stable tetrahedral structure, high CH bond energy (434 kJ/mol), and unique molecular orbital characteristics result in minimal chemical reactivity, making its selective conversion into other substances the "holy grail" of catalysis. Therefore, photo-driven MTLO processes typically require reactive oxygen species (ROS), such as •OH, •OOH , and •O2⁻, to activate the CH bonds of methane. However, because all methane-derived oxygenated compounds are more susceptible to further oxidation, over-oxidation of the products is inevitable. To improve product selectivity, it is necessary to reduce the concentration of ROS in the reaction system,  but this leads to decreased reactivity. This persistent "seesaw effect" makes the development of an innovative pathway independent of reactive oxygen species a pressing need. Furthermore, the photocatalytic conversion of methane to ethanol involves a complex multi-electron CC-coupling process, which also faces challenges such as high energy barriers, slow kinetics, and low efficiency. Therefore, this work presents a novel methane upgrade pathway independent of reactive oxygen species, achieving high activity, high selectivity, and efficient CC-coupling simultaneously, thereby promoting the large-scale application of methane upgrade technology.

Highlights of this article:

This paper explores a novel photocatalytic strategy for the conversion of methane to ethanol that is independent of reactive oxygen species. By polarizing and activating the CH bond of methane at the Cu-O site , selectively attacking methyl/hydroxymethyl radicals by methane and water molecules, and suppressing hydroxyl radicals in the reaction system, a highly efficient and selective photocatalytic conversion of methane to ethanol was achieved, yielding an ethanol production rate of 226 μmol/g/h (higher than the conversion number 13.9 h⁻¹) and a selectivity as high as 98% (only methanol and formic acid were formed as byproducts; CO, CO₂, HCHO, or other substances were not detected). Using this catalytic system, under natural sunlight (Dalian, 121°44' E, 39°04' N), the ethanol production rate can reach 123 μmol/g/h, with a selectivity exceeding 96%. This work provides a new approach for the high-value-added clean conversion of methane, an abundant C1 resource.

Through controlled experiments, reactive oxygen species capture, online infrared spectroscopy, and density functional theory calculations, a novel polarization activation pathway for single-atom copper coupled to carbon nitride with axial oxygen coordination was revealed. This unexpectedly superior catalytic performance can be attributed to the promoting effect of the direct polarization activation of the CH bond by the Cu-O polar bond, the suppression of reactive oxygen formation in the reaction with water as the oxidant under anaerobic conditions, and the kinetically guided selective attack (water to •CH3 and CH4 to •CH2OH). This novel reactive oxygen-independent pathway breaks the activity-selectivity seesaw effect in existing processes. This work opens a new route for the upgrading of methane to transportable liquid oxygenated compounds.

Summary and Outlook:

In summary, this study reveals a novel and highly attractive reactive oxygen species (ROS)-free pathway for the efficient photo-driven conversion of methane in water to ethanol. Under visible light irradiation, we achieved an ethanol production rate as high as 226 μmol/g/h (conversion rate, 13.9 h⁻¹) with unprecedented selectivity (98%) and good cycling stability. Experimental studies and density functional theory calculations demonstrate that photocatalytic methane-to-ethanol conversion without relying on reactive oxygen species can be achieved by combining direct CH bond  polarization activation with effective suppression of ROS formation and guiding reactant attack via H2O to •CH3 and CH4 to •CH2OH processes. This method breaks the inherent seesaw effect between activity and selectivity  in traditional reactive oxygen species-based photocatalytic methane conversion and eliminates the possibility of over-oxidation, thus it can be applied to other oxygen-containing compounds in photo-driven methane upgrading processes. Future work will focus on optimizing catalyst design to further increase ethanol production while maintaining extremely high selectivity, thereby driving the development of sustainable methane value-added technologies.

The research findings were published as a Research Article in CCS Chemistry, the flagship journal of the Chinese Chemical Society. Professor Zhongkui Zhao of Dalian University of Technology, Professor Riguang Zhang of Taiyuan University of Technology, Researcher Yuefeng Liu of the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Professor Ting Zhang of Qingdao University, and Professor Chunshan Song of the Chinese University of Hong Kong are the co-corresponding authors of the paper. Yu Zhang, a graduate student at Dalian University of Technology, and Zun Guan, a graduate student at Taiyuan University of Technology, are the co-first authors of the paper.

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About the journal: CCS Chemistry is the Chinese Chemical Society’s flagship publication, established to serve as the preeminent international chemistry journal published in China. It is an English language journal that covers all areas of chemistry and the chemical sciences, including groundbreaking concepts, mechanisms, methods, materials, reactions, and applications. All articles are diamond open access, with no fees for authors or readers. More information can be found at https://www.chinesechemsoc.org/journal/ccschem.

About the Chinese Chemical Society: The Chinese Chemical Society (CCS) is an academic organization formed by Chinese chemists of their own accord with the purpose of uniting Chinese chemists at home and abroad to promote the development of chemistry in China. The CCS was founded during a meeting of preeminent chemists in Nanjing on August 4, 1932. It currently has more than 120,000 individual members and 184 organizational members. There are 7 Divisions covering the major areas of chemistry: physical, inorganic, organic, polymer, analytical, applied and chemical education, as well as 31 Commissions, including catalysis, computational chemistry, photochemistry, electrochemistry, organic solid chemistry, environmental chemistry, and many other sub-fields of the chemical sciences. The CCS also has 10 committees, including the Woman’s Chemists Committee and Young Chemists Committee. More information can be found at https://www.chinesechemsoc.org/. 

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