Thiophene-doped fully conjugated covalent organic frameworks for efficient photocatalytic hydrogen peroxide production

The research teams of Professors Yu Tang and Fengjuan Chen from Lanzhou University proposed a new mixed ligand strategy. By introducing complementary building units into covalent organic frameworks (COFs) and systematically regulating their ratios, they achieved synergistic optimization of the four key steps in the photocatalytic hydrogen peroxide synthesis, effectively breaking through the constraints between various performance indicators and significantly improving the overall catalytic efficiency. Studies have shown that the introduction of the DTTA unit significantly broadens the light absorption range and enhances the charge carrier separation ability; while the TA component improves the crystallinity and hydrophilicity of the material, thereby promoting the transport and mass transfer of photogenerated charges. At the optimal ratio, TA/DTTA-2-TMT achieved an H₂O₂ generation rate of 3451 μmol·g^-1·h^-1 in pure water, air and 100 mW·cm^-2 light. This work provides new ideas for the development of high-performance H₂O₂ photocatalysts. The article was published as an open access research article in CCS Chemistry, the flagship journal of the Chinese Chemical Society.

Background:
Hydrogen peroxide (H₂O₂) is an important oxidant used widely in fields such as papermaking and disinfection. However, the traditional anthraquinone method to produce H₂O₂ suffers from high energy consumption and environmental pollution concerns. In recent years, photocatalytic synthesis technology using water and oxygen as raw materials and driven by solar energy has become a research hotspot for the green production of H₂O_2. This process includes four key steps: light absorption, charge separation, migration, and surface reaction. However, there are often mutual constraints between these steps, which affect the overall efficiency improvement. Among the many photocatalytic materials, covalent organic frameworks (COFs) have become the star materials in this field since 2020 due to their tunable chemical structure, ordered pores, and excellent photoelectric properties. Currently, researchers have improved the photocatalytic performance of COFs through strategies such as constructing donor-acceptor structures, introducing functional groups, and adjusting bonding methods. However, how to synergistically optimize the entire photocatalytic process and balance conflicting factors such as narrow band gap and carrier recombination, hydrophilicity and crystallinity remains a key challenge. Therefore, achieving precise control of COFs structure at the molecular level and promoting multi-step synergistic efficiency have become important research directions for promoting the green synthesis of H₂O₂.

Highlights of this article:
Inspired by the "barrel effect", the team proposed a new mixed ligand strategy, namely, by precisely regulating the ratio of two aldehyde monomers, terephthalaldehyde (TA) and 2,5-di(thiophen-2-yl)terephthalaldehyde (DTTA), to react with 2,4,6-trimethyl-1,3,5-triazine (TMT), and successfully developed a high-performance TA/DTTA-2-TMT photocatalyst. This material synergistically optimizes the four key steps of photocatalytic H₂O₂ production, thereby breaking through the constraints between various key performance indicators and significantly improving the overall catalytic efficiency (Figure 1).

Research has shown that the introduction of the DTTA unit significantly broadens the material's light absorption range and enhances charge carrier separation, while the TA component effectively improves the material's crystallinity and hydrophilicity, creating favorable conditions for charge transport and plasmid transfer during the reaction (Figures 2 and 3). Ultimately, the optimally formulated TA/DTTA-2-TMT photocatalyst achieved an exceptional H₂O₂ yield of 3451 μmol·g⁻¹·h⁻¹ in pure water, air, and under 100 mW·cm⁻² illumination.

Summary and Outlook:
In summary, the authors successfully designed and synthesized a series of COFs with similar backbone structures by combining two functionally complementary building blocks, TA and DTTA, with TMT in varying ratios. Experimental data and theoretical calculations demonstrate a clear synergistic effect between the two linking units: the introduction of DTTA broadens the light absorption boundary and enhances charge carrier separation, while TA improves the crystallinity and surface hydrophilicity of the framework. The optimally matched TA/DTTA-2-TMT photocatalyst exhibits exceptional H₂O₂ generation activity, outperforming not only the control material composed of a single linking unit, but also most reported COF-based catalytic systems. This synergistic enhancement effect stems from the balanced optimization of key processes such as light absorption, charge separation and transport, and surface redox reactions, thereby establishing a rational design strategy for developing high-performance photocatalytic materials through component complementarity.

This research was published as a research article in CCS Chemistry. Professors Yu Tang and Fengjuan Chen of Lanzhou University are the corresponding authors, and doctoral students Hailong Lin and Guoying Tan are co-first authors. This research was funded by the National Natural Science Foundation of China's Innovative Research Group and Key Project, the Gansu Provincial Science and Technology Plan, and the Fundamental Research Funds for Central Universities.

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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/.