Mining the Carbon Intermediates in Plastic Waste Upcycling for Constructing C–S Bond

Abstract

Postconsumer plastics are generally perceived as valueless with only a small portion of plastic waste being closedloop recycled into similar products while most of them are discarded in landfills. Depositing plastic waste in landfills not only harms the environment but also signifies a substantial economic loss. Alternatively, constructing value-added chemical feedstocks via mining the waste-derived intermediate species as a carbon (C) source under mild electrochemical conditions is a sustainable strategy to realize the circular economy. This proof-of-concept work provides an attractive "turning trash to treasure" strategy by integrating electrocatalytic polyethylene terephthalate (PET) plastic upcycling with a chemical C−S coupling reaction to synthesize organosulfur compounds, hydroxymethanesulfonate (HMS). HMS can be produced efficiently (Faradaic efficiency, FE of ∼70%) via deliberately capturing electrophilic intermediates generated in the PET monomer (ethylene glycol, EG) upcycling process, followed by coupling them with nucleophilic sulfur (S) species (i.e., SO 3 2− and HSO 3 − ). Unlike many previous studies conducted under alkaline conditions, PET upcycling was performed over an amorphous MnO 2 catalyst under near-neutral conditions, allowing for the stabilization of electrophilic intermediates. The compatibility of this strategy was further investigated by employing biomass-derived compounds as substrates. Moreover, comparable HMS yields can be achieved with real-world PET plastics, showing its enormous potential in practical application. Lastly, Density function theory (DFT) calculation reveals that the C−C cleavage step of EG is the rate-determining step (RDS), and amorphous MnO 2 significantly decreases the energy barriers for both RDS and C−S coupling when compared to the crystalline counterpart.