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In advancing sustainable energy solutions, an international collaborative team of scientists has achieved a significant milestone in low-carbon chemical conversion. In their in Nature, the team has discovered a photocatalytic approach to converting methane to ethanol with high selectivity of about 80% and a methane conversion rate of 2.3% in a single run using a packed-bed flow reactor.

The system achieves an impressive apparent quantum efficiency (AQE) of 9.4%, which measures how effectively it converts incident photons into electrons that participate in the reaction under specific wavelength conditions.

The group was led by Professors Zhengxiao Guo of the Department of Chemistry at The University of Hong Kong (HKU), Weixin Huang of the University of Science and Technology of China, Richard Catlow of the University College London and Junwang Tang of Tsinghua University.

Ethanol is well known to spirit up many celebratory occasions, but, more significantly, it serves as an ideal liquid hydrogen carrier and a chemical feedstock for a wide range of applications towards carbon neutrality.

The global market for ethanol exceeds USD 100 billion, with a current compound annual growth rate (CAGR) of approximately 7%. Methane, the primary constituent of natural and shale gas, is often burned for heating. Despite its potential as a carbon source for chemical synthesis, its inherent chemical inertness poses substantial hurdles to its efficient conversion.

Traditional industrial methane conversion is typically conducted via syngas under high temperatures and pressures, a process that is energy-intensive and exhibits poor product selectivity. Efforts to directly convert methane into ethanol often encounter challenges in controlling highly selective carbon–carbon (C–C) coupling to produce a specific C₂₊ chemical, such as ethanol.

The efficient conversion is achieved through a unique intra-molecular junction formed between alternate benzene and triazine units within a covalent triazine framework (CTF-1) polymer. The intra-molecular junction enhances the lifetime and the efficient separation of photo-generated charges while enabling preferential adsorption of O₂ and H₂O to the benzene and triazine units, respectively, to facilitate C–C coupling.

Moreover, this intrinsically asymmetric dual-site feature effectively delineates the C–C coupling sites from the hydroxyl radical formation sites, thereby mitigating the risk of overoxidation of the intermediate into CO₂ and water. When further enhanced by the addition of Pt, the intramolecular junction photocatalyst demonstrates a very promising ethanol production rate, as stated above.

"This is a step-change advancement in the photocatalytic conversion of methane into value-added green chemicals—not only in terms of a newly identified metal-free 'intramolecular junction' for effective C–C coupling; but also by turning methane into a much more desirable liquid chemical, relatively efficiently at ambient conditions," Professor Guo, one of the corresponding authors of the paper, remarked.

Conventionally, as in the Fischer−Tropsch synthesis, methane conversion to liquid chemicals requires high temperature (> 700°C) and pressure (∼ 20 bar) to activate its C–H bond, involving high energy input and multiple steps. Previous attempts in the photocatalytic conversion of methane to a C₂₊ product often encountered either low selectivity and/or low efficiency, due to the limited capabilities of the specific catalysts. The newly developed CTF-1 catalyst demonstrates more than 20 times higher quantum efficiency along with a very high selectivity.

Methane is an abundant yet climate-potent gas. Its one-step photocatalytic conversion represents a highly desirable approach to decarbonizing the chemical and fuel industries. Particularly in liquid form, ethanol is much easier to store, transport and distribute, compared to gaseous hydrogen. It can be directly reformed onboard of low-carbon vehicles—on land, at sea or in the air, offering great potential for applications in urban transport, shipping and the upcoming low-altitude economy, thereby paving the way towards carbon neutrality.