Innovative and sustainable approaches to aerobic oxidation reactions for organics upgrading

Recently, the research team of Tierui Zhang from the Institute of Chemistry and Physics, Chinese Academy of Sciences, published a Mini-Review titled "Innovative and Sustainable Approaches to Aerobic Oxidation Reactions for Organics Upgrading" in the flagship journal of the Chinese Chemical Society, CCS Chemistry. The article summarizes innovative and sustainable methods for the aerobic oxidation upgrading of organic compounds based on new photocatalysis, electrocatalysis, and photoelectrocatalysis technologies, offering new insights for the green chemistry and sustainable development of chemical synthesis.

Background:

Aerobic oxidation upgrading of organic compounds is crucial for the synthesis of value-added chemicals supporting various industries, including pharmaceuticals, agrochemicals, and materials science. With the growing global demand for sustainable chemistry, traditional high-temperature and high-pressure organic oxidation processes are facing significant challenges. How to use green energy sources, such as solar and electrical energy, to drive molecular oxygen (O₂) for efficient, low-energy organic oxidation upgrading has become a hot topic in the field of chemistry. In response, researchers, including Guangbo Chen and Tierui Zhang, have comprehensively summarized the innovative applications of photocatalysis, electrocatalysis, and photoelectrocatalysis in the aerobic oxidation of organic compounds, providing new ideas for green chemical synthesis. 

Highlights:

The article systematically reviews recent examples of aerobic oxidation upgrading of organic substrates using molecular oxygen (O₂) or air as green oxidants, utilizing the three novel technologies: photocatalysis, electrocatalysis, and photoelectrocatalysis. It compares and summarizes their representative catalytic mechanisms, catalytic system designs, and catalytic reactor setups. 

Photocatalysis: Semiconductor photocatalysts have become a research hotspot due to their easy separation, recyclability, and tunable redox behavior. Photocatalytic aerobic oxidation reactions rely on the electron-hole pairs generated by semiconductor materials (such as TiO₂, g-C₃N₄) upon light absorption. These charge carriers further react with O₂ and H₂O to generate reactive oxygen species (ROS), such as singlet oxygen (¹O₂), superoxide radicals (·O₂⁻), hydroxyl radicals (·OH), and hydrogen peroxide (H₂O₂), thereby driving organic oxidation reactions. This article summarizes key reactions in organic synthesis, such as selective oxidation of C(sp³)-H bonds and oxidation of alcohols, amines, and sulfides, and provides an overview of catalyst design (e.g., doping, oxygen vacancies, heterojunctions), reaction mechanisms (e.g., EPR, FTIR-MS revealing ROS dynamic behavior), and reactor design optimization, highlighting the advantages and challenges of photocatalytic organic oxidation. 

Electrocatalysis: Electric energy can be generated by various renewable energy sources (such as solar, wind, hydro, and biomass), and the rapid development of photovoltaic technology in recent years has enabled efficient conversion of solar energy into electricity, providing a sustainable energy source for electrocatalytic reactions. Similar to photocatalysis, O₂ does not directly react with organic substrates but first undergoes a 2e⁻ transfer oxygen reduction reaction (ORR) at the cathode to generate intermediate H₂O₂, which then oxidizes the organic substrate. This article compares non-interfacial and interfacial (e.g., ion membranes seamlessly integrated with thermal catalysts) systems for this cascade reaction, analyzing the advantages of each in suppressing side reactions and utilizing in situ generated high-concentration H₂O₂. Additionally, the article highlights an electrocatalytic synthesis strategy based on "linear pairing," which cleverly couples the cathode and anode reactions through system design, reducing the complexity of reactor construction and improving atomic utilization efficiency. 

Photoelectrocatalysis: Photoelectrocatalysis (PEC) combines the advantages of both photocatalysis and electrocatalysis and is considered a more sustainable method for organic compound oxidation upgrading. In a PEC system, the semiconductor photoelectrode serves as the core component, capturing and converting solar energy, while the introduction of electrical energy helps reduce reaction energy consumption and improve catalytic efficiency. The principle is that the photoelectrode first absorbs photons to generate electron-hole pairs, which separate under the influence of an electric field and participate in redox reactions. O₂ is reduced to H₂O₂ at the cathode, which acts as the oxidant for the oxidation reaction of organic compounds. This article also summarizes current PEC aerobic oxidation systems and introduces homogeneous cascade catalysts that have been applied, such as TS-1 and AaeUPO. 

Summary and Outlook:

This article summarizes recent progress in the use of molecular oxygen (O₂) or air as green oxidants, utilizing photocatalysis, electrocatalysis, and photoelectrocatalysis for organic substrate oxidation upgrading. These advances have significantly promoted the development of organic green oxidation upgrading. However, there are still several issues that need to be addressed in the field of photocatalytic/electrocatalytic aerobic oxidation of organic compounds. Future directions to focus on include: 1) continuing to explore new strategies and methods to improve the intrinsic performance of catalysts; 2) developing advanced in situ detection technologies to capture the dynamic behavior of key ROS species in photocatalytic systems and accurately clarify reaction mechanisms; 3) continuing to optimize reactor designs in electrocatalysis and photoelectrocatalysis to achieve seamless integration of (photo)electrochemical and thermal chemical processes, reducing system complexity and further improving catalytic efficiency; 4) considering economic factors and life cycle analysis alongside catalyst development and reactor design, evaluating energy costs, overall process efficiency, environmental impact, and catalyst lifespan to ensure the feasibility of these processes at an industrial scale; and 5) exploring the potential of water as a sustainable oxygen source for organic oxidation reactions. Future designs should focus on optimizing system structures and regulating catalyst properties to use water as the main ROS source, reducing reliance on traditional oxidants to align with green chemistry principles. The authors hope this review will stimulate researchers' interest in photocatalytic/electrocatalytic aerobic oxidation of organic compounds and contribute to further advancements in this field.

Article Details:

Innovative and Sustainable Approaches to Aerobic Oxidation Reactions for Organics Upgrading

Guangbo Chen, Pengfei Shao, Xiaoxiao Niu, Li-Zhu Wu & Tierui Zhang

CCS Chem. 2025, Just Published. DOI: 10.31635/ccschem.024.202405369