Plastic, as one of the greatest scientific discoveries of the 20th century, is celebrated as a symbol of modern industry. Due to its excellent chemical stability and durability, plastic has found widespread application in various fields. However, because of its long degradation cycle and high stability, plastic pollution has become a major source of global environmental issues.
Mechanical recycling is one way to address plastic recycling. Waste plastic is sorted, crushed into polymer particles, and then reused. However, while this method is simple and feasible, the thermal degradation causing polymer chain scission or crosslinking eventually reduces the quality of plastic. Pyrolysis is an alternative approach, chemically converting polymers into monomers or other small molecules, usually requiring high-temperature treatment in an inert atmosphere (>400°C), consuming a lot of energy. Therefore, the development of an environmentally friendly and efficient plastic degradation technology is urgent. Traditional methods such as landfilling and incineration are not only inefficient but may also bring serious secondary pollution.
In this context, photocatalytic degradation technology is gradually emerging as a potential solution for plastic pollution due to its advantages of utilizing solar energy, low energy consumption, and environmental friendliness. In the photocatalytic degradation of plastic and plastic-derived chemicals, photogenerated holes can directly oxidize plastics, and photogenerated electrons and holes react with O₂ or H₂O to form radicals, such as hydroxyl and superoxide radicals, which can also degrade plastics and plastic-derived chemicals.
Polystyrene (PS), a polymer synthesized by free radical polymerization of styrene monomer with a glass transition temperature above 100°C, is often used to make various disposable containers that need to withstand hot water temperatures. In 2018, the annual production capacity of the polystyrene industry was 3.39 million tons. Due to its low weight (especially in foamed types) and low residual value, polystyrene is difficult to recycle. Typically, polystyrene cannot be recycled. Upgrading polystyrene by chemical methods to target small molecules is an ideal way to reduce plastic pollution. Sewon Oh and Erin E. Stache at Cornell University[1] used a catalyst-controlled photo-oxidative degradation method to upgrade polystyrene to benzoyl products, mainly benzoic acid. The authors mixed polystyrene PS (Mn = 89 kg/mol) with 10 mol% ferric chloride in acetone and exposed the mixture to a broad-spectrum LED light source for 20 hours in an air stream, finding that the polystyrene had degraded to oligomers with a number-average molecular weight Mn = 0.8 kg/mol and 11 mol% small molecule products including benzoic acid, benzaldehyde, benzoyl chloride, and acetophenone. Under white light irradiation, FeCl₃ homolytically cleaves to generate chlorine radicals, which abstract an electron-rich hydrogen atom from the polymer backbone. In an oxygen-rich environment, high molecular weight polystyrene (>90 kg/mol) reduces to <1 kg/mol, producing up to 23 mol% benzoyl products. Mechanistic studies show that chlorine radicals promote degradation by dissociating hydrogen atoms.
Photocatalytic Degradation Mechanism of PS
Polyethylene (PE) is a polymer and synthetic material known for its corrosion resistance, UV resistance, durability, ease of processing, and lightweight properties. Consequently, polyethylene is widely used in plastic products, packaging materials, and construction materials. Although polyethylene has many advantages, it also has negative impacts, such as marine debris, land pollution, and air pollution. Photocatalysis is a mild and effective method to address polyethylene plastic pollution. For example, Robert R. Knowles at Princeton University and others[2] reported a photocatalytic method that enables depolymerization and degradation of hydroxylated high molecular weight polyethylene derivatives under mild reaction conditions near room temperature, producing structurally defined products and developing a recycling method for polymers.
Photocatalytic Degradation Mechanism of PE
The Proton-Coupled Electron Transfer (PCET) method activates hydroxyl groups on the polymer backbone, generating reactive alkoxy radicals that then promote carbon chain cleavage via the β-scission process, breaking C-C chemical bonds. This depolymerization process generates structurally defined mixtures, which can be derivatized to yield monomers of certain degrees of polymerization.
Polyvinyl Chloride (PVC) is the third most-produced synthetic polymer plastic globally (after polyethylene and polypropylene), with approximately 40 million tons produced annually and about 5 million tons discarded each year. PVC incineration produces large amounts of HCl gas and highly carcinogenic dioxin gas, greatly impacting the environment and health. The team led by Professor Martin and Researcher Wang Meng at Peking University[3] achieved co-upcycling of PVC and polyester through a catalytic process. By using chlorine-containing ionic liquid as a catalyst/solvent and ZnCl₂ as a catalyst, they successfully converted polyester into terephthalic acid and 1,2-dichloroethane, solving the issue of chlorine interference from PVC on the catalyst, achieving high-yield plastic upcycling. This study used ionic liquid as a medium for PVC dechlorination and HCl storage. The authors screened different ionic liquids, finding Bu₄PCl performed best for chlorine extraction from PVC and HCl absorption. Compared to the situation without ionic liquid, ionic liquid catalyzed the low-temperature dechlorination reaction.
Photocatalytic Degradation Mechanism of PVC
In addition to photocatalytic degradation of PS, PE, and PVC, other plastic waste can be degraded by photocatalysis, such as Polypropylene (PP), Polyester (PET), and Polyurethane (PU), which have received significant research attention. Photocatalytic plastic degradation is a promising environmental technology that decomposes plastic waste into harmless or useful small molecule products through the synergy of light energy and catalysts. However, the technology currently faces challenges in efficiency, product selectivity, and catalyst design. Reactor design needs to optimize light transmission paths and contact area with catalysts to improve light energy utilization; in terms of waste plastic sorting and pretreatment, mechanical crushing, surface oxidation, and chemical pretreatment can increase plastic surface activity and enhance its contact with catalysts. Untreated plastics are often difficult to degrade effectively due to their hydrophobicity and chemical inertness. In terms of product selectivity, future studies should focus on controlling reaction conditions and catalysts to reduce harmful by-products and increase the yield of high-value chemicals. Catalyst design is key in this field, and developing new catalysts with broad-spectrum absorption is expected to improve efficiency. Combining with other technologies, such as electrocatalysis, thermocatalysis, and biodegradation, may further enhance photocatalytic effects. Overall, photocatalytic plastic degradation has broad prospects in addressing global plastic pollution but requires in-depth research and improvement in technical details and large-scale applications.
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