Dual Active Sites of Metal-Free N,S-Doped Lignin-Derived Carbon Catalysts for Oxidative Esterification Depolymerization of Polyethylene Terephthalate

The methanolysis of polyethylene terephthalate (PET) to produce dimethyl terephthalate (DMT) and ethylene glycol (EG) is a promising route for sustainable PET valorization. Conventional metal-based catalysts often suffer from high cost, large loading, low processing capacity, and environmental concerns. Here, we report a nitrogen–sulfur codoped lignin-derived carbon catalyst (S–N@C-800 °C) for oxidative alcoholysis of PET. Under 180 °C and 30 min with a PET-to-catalyst ratio of 250:1, PET conversion reached 99.58%, with EG and DMT yields of 97.67 and 95.37%, respectively. Even at a 600:1 ratio (0.16% catalyst by mass), the system maintained high depolymerization efficiency, highlighting its performance under low catalyst usage and high solid loading. Characterization revealed strong correlations among activity, acid–base sites, nitrogen species, and vacancies. Mechanistic studies using electron paramagnetic resonance (EPR) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that basic sites activate O 2 to generate superoxide (·O 2 – ), which reacts with methanol to form OOH· and CH 3 O-, while acidic sites activate carbonyl groups, forming carbocations that lower ester bond dissociation energy and facilitate nucleophilic attack. This metal-free, efficient, and environmentally benign catalytic system provides a practical strategy for PET recycling.