Catalyst-free upcycling of polycarbonate waste to phenol enabled by temperature–pressure-decoupled subcritical steam flow

Abstract

Polycarbonate (PC) is a widely used engineering plastic, yet its chemical recycling is hindered by poor phenol selectivity under conventional hydrothermal conditions. Here we report a catalyst-free temperature–pressure decoupling strategy that enables selective PC depolymerization in subcritical steam. By separating the roles of temperature and pressure, this approach creates a low-ionic-product steam-phase environment that suppresses proton-driven side reactions while maintaining sufficient reactivity for carbonate bond cleavage. Under optimized conditions (330 °C, water feed rate of 0.05 mL min−1, 4 h), the process achieves a phenol selectivity of 69.1%. Using bisphenol A (BPA) as a molecular probe, together with kinetic, thermodynamic and reaction-network analyses, we show that subcritical steam reprograms the hydrolysis pathway by suppressing alkylphenol-forming branches and favoring rapid conversion of 4-isopropenylphenol (IPP) to phenol and acetone. Kinetic fitting identifies carbonate bond cleavage as the rate-determining step in the overall depolymerization sequence. Extension to post-consumer PC products shows that the chemistry remains effective for real wastes, although morphology and additives can limit downstream conversion. Process-level assessment indicates promising practical potential, with an internal rate of return of 34.8% and a global warming potential as low as 187 kg CO2e per tonne of PC under renewable electricity. These results establish steam-phase reaction-environment engineering as an effective strategy for reprogramming selectivity in polymer hydrolysis and advancing low-carbon recycling of engineering plastics