Zhejiang University Li Zhenglong Team: Photocatalytic Oxidative Depolymerization of Polyethylene Plastic Waste into Aliphatic Long-Chain Dicarboxylic Acids

Recently, the global annual production of polyethylene (PE) has reached 83 million tons, making it one of the most widely used plastics. The long-term accumulation of polyethylene waste poses a serious threat to the environment. This study presents a method for catalytically oxidizing polyethylene (PE) using BiOI/BiVO4 p-n heterojunction materials under visible light irradiation to generate valuable aliphatic dicarboxylic acids (C4-C30). The method achieves a carbon yield of up to 83% under mild conditions, and by optimizing the composition of the heterojunction and reaction conditions, the yield of long-chain dicarboxylic acids (C10-C30) can be enhanced to 75%. The authors also investigate the effects of reaction temperature, time, air pressure, and the stability of the catalyst after multiple cycles on the photocatalytic oxidation of PE. Furthermore, the reaction mechanism is elucidated using various technical approaches: superoxide radicals (•O2-) are responsible for activating the C-H bonds in the PE backbone, while hydroxyl radicals (•OH) cleave the C=C bonds. Additionally, the photocatalytic upgrading process shows significant efficiency for various commercial PE types used in daily life. This photocatalytic system offers a promising method for the efficient recycling of PE waste.

Figure 1. Schematic illustration of the photocatalytic oxidation depolymerization of polyethylene waste plastics into dicarboxylic acids proposed in this study.

Background introduction:

As the amount of plastic waste continues to increase, the efficient recycling of plastics has become a severe environmental issue. Especially for plastics like polyethylene (PE), due to their stability and durability, the degradation rate in the environment is very slow, posing a long-term threat to ecosystems. Currently, there are some limitations to the chemical recycling methods for plastics, particularly polyethylene, such as high energy consumption processes and the issue of secondary pollution generated. To address these problems, scientists are seeking more environmentally friendly and energy-efficient methods for plastic recycling. Photocatalytic systems have emerged as an attractive option due to their use of renewable solar energy and relatively low operational conditions. In recent years, progress has been made in the degradation and recycling of plastic waste using photocatalytic systems, with researchers aiming to simulate the natural degradation mechanisms through reactive species such as free radicals, thereby achieving green recycling of polyolefin waste. BiOI/BiVO4 p-n heterojunction materials have been proposed as photocatalysts due to their stability and non-toxicity. In this study, researchers developed this material and successfully achieved photocatalytic oxidation of polyethylene under mild conditions using visible light irradiation. This research contributes to the search for environmentally friendly and low-energy plastic recycling technologies, utilizing the photocatalytic performance of BiOI/BiVO4 p-n heterojunction materials to efficiently convert polyethylene waste into higher value chemicals.

Figure 2. Comparison of this catalytic system with existing methods.

Highlights of this article:

This work successfully achieved the photo-catalytic oxidation of polyethylene (PE) to valuable aliphatic dicarboxylic acids (C4-C30) under mild conditions using BiOI/BiVO4 p-n heterojunction photocatalyst, with a carbon yield up to 83%. Notably, the carbon yield for long-chain dicarboxylic acids (C10-C30) reached 75%.

Figure 3. (a) Reaction results of PE on different catalysts, (b) molecular weight of PE, (c) dicarboxylic acids, (d) monocarboxylic acid product distribution over time.

In the photocatalytic oxidation process, superoxide radicals (•O₂⁻) and hydroxyl radicals (•OH) play a crucial role. The superoxide radicals activate the C-H bonds in the PE main chain, while the hydroxyl radicals oxidize and cleave the C=C bonds, ultimately converting PE into dicarboxylic acid products.

Figure 4. Proposed reaction mechanism

This catalytic system can oxidatively depolymerize most commercial PE into dicarboxylic acids, with carbon yields of the dicarboxylic acids ranging from 59% to 64%. Small amounts of branched alkanes and cycloalkanes were also detected in the products. The photocatalytic system exhibits excellent stability when processing commercial PE waste, indicating its potential for practical applications.

Figure 5. Photocatalytic oxidation depolymerization results of pure PE particles and common PE waste plastics in daily life

Summary and Prospects:

Developing green chemical methods for the efficient recycling of waste polyolefins is essential for reducing carbon emissions and supporting sustainable development. By converting polyethylene (PE) into valuable chemicals (long-chain dicarboxylic acids), the recycling of waste plastics can be enhanced, providing raw materials for the production of biodegradable polymers, lubricants, and surfactants. The photocatalytic oxidation method proposed in this study operates under mild conditions, significantly reducing the energy and resource inputs required by traditional methods, and is expected to advance the development of recycling technologies for polyolefin waste plastics.