Metal-Free Photoredox Upcycling of Chemically Diverse Plastics and Mixed Plastics into High-Value Chemicals under Visible Light

Abstract

The growing accumulation of plastic waste poses a major global environmental challenge, yet existing chemical recycling technologies remain constrained by high energy demand, narrow substrate scope, intolerance to poly(vinyl chloride) (PVC) and mixed plastic waste, and heavy reliance on precious, toxic, or base-metal catalysts. Here, we report a metal-free, defect-engineered polymeric carbon nitride photocatalyst containing tri-coordinated nitrogen vacancies and oxygen-linked heptazine units (O-VN-CN) that confronts this challenge under visible light and ambient conditions. Our developed O-VN-CN catalyst converts chemically distinct commodity plastics, including aliphatic polypropylene (PP), aromatic polystyrene (PS), and halogenated PVC, both individually and as a ternary mixture, into value-added chemicals under a unified set of reaction conditions without substrate-specific modification. Notably, the co-upcycling of ternary PP/PS/PVC mixture represents, to the best of our knowledge, the first demonstration of photocatalytic upcycling of mixed plastics using a recyclable, metal-free heterogeneous photocatalyst under ambient conditions. Mechanistic and computational investigations reveal that the co-existence of tri-coordinated nitrogen vacancies and oxygen-linked heptazine units creates shallow electron-trapping states that enhance visible light absorption, promote charge separation and interfacial transfer, and facilitate the activation of O2 to •O2⁻ that actively participates in the C–C bond cleavage and breaks down the polymer chains into value-added small molecules. Synthesized from cheap, abundant urea and ammonium acetate by simple calcination, the catalyst offers a potentially scalable, green and economically compelling route to closing the plastics carbon loop.