Recent advances in plastic recycling and upgrading under mild conditions

Abstract

Plastic depolymerization usually requires rigorous conditions such as high temperature, high pressure, and caustic bases or acids, resulting in high energy costs and environmental issues. Recently, new technologies that operate under mild conditions are emerging. This review summarizes the progress made in the past decade in the conversion of waste plastics into high-purity monomers or value-added products under mild conditions by bio-, photo-, electro-, and low-temperature thermocatalysis. First, the enzymatic depolymerization of plastics is discussed, with emphasis on the progress in enzyme engineering to improve the thermal stability of enzymes. Next, electro- and photocatalytic approaches for the conversion of waste plastics into high value-added fuels and chemicals at room temperature are presented. Specifically, the status and problems of upgrading plastics via electrocatalysis and photoreforming are described, and the photocatalytic oxidation of plastics to chemical feedstocks using oxygen or air as the oxidant is also highlighted. Then, the recent progress in low-temperature thermocatalysis for plastic recycling and upgrading is surveyed. Finally, the challenges and opportunities of catalytic recycling and upgrading of waste plastics under mild conditions are discussed in terms of the catalyst rational design, catalytic system optimization and scalability, and economic feasibility.