Tandem Chemical Depolymerization and Photoreforming of Waste PET Plastic to High-Value-Added Chemicals

Photoreforming of poly(ethylene terephthalate) (PET) wastes to high-value-added chemicals is an emerging and promising approach. Nonetheless, a major obstacle is the harsh alkaline pretreatment (COH - = 5-10 M) to depolymerize PET, resulting in a surge in processing costs and also posing great challenges to subsequent photocatalytic devices, catalysts, and photocatalytic efficiency. Herein, we introduce a tandem process of chemical depolymerization and photoreforming of waste PET plastics. Depolymerization of PET to monomers is achieved through an intramolecular hydrolysis mechanism on a binuclear zinc catalyst under mild conditions (COH- = 0.1 M and T = 60 degrees C). Compared with the traditional harsh alkali pretreatment, the depolymerization rate can be increased by nearly an order of magnitude due to the proximity effect of the bimetallic sites. Technoeconomic analysis shows that processing 50,000 tons of plastic annually can save 15.2 million USD. The photoreforming of PET to formic acid and H2 with an impressive production rate of 2000 mu mol gcat-1 h-1 was achieved on an ultrasmall carbon nitride nanosphere photocatalyst, which is nearly 5-fold higher than the corresponding strong alkali pretreatment system. Mechanism research reveals high photocatalytic activity thanks to the mild PET pretreatment and the efficient electron-hole separation caused by the ultrasmall carbon nitride nanosphere size structure and the electron capture effect of metal Pt. We also demonstrate a gram-scale integrated process for real-world PET plastic wastes including water bottles, clothing fibers, towels, carpets, and mixed plastics containing PET. Our study establishes a new concept of tandem catalysis to reduce the harsh pretreatment of PET by using a synthetic catalyst in polyester plastic photoreforming technology.