Sulfur-Vacancy-Engineered In2S3 Nanoflowers for Visible-Light-Driven One-Pot Upcycling of Polylactic Acid Waste to Alanine

Abstract

The accumulation of polylactic acid (PLA) waste poses a growing environmental challenge, calling for sustainable upcycling strategies that transform waste plastics into value-added chemicals. Here, sulfur-vacancy-engineered In2S3 nanoflowers were synthesized through a facile hydrothermal method and applied as an efficient photocatalyst for visible-light-driven one-pot upcycling of PLA into alanine. The optimized In2S3−S0.2 catalyst exhibited an alanine formation rate of 3.32 mmol·g−1·h−1 at 50 °C under visible-light irradiation. Structural and photoelectrochemical characterizations revealed that the enhanced activity of In2S3−S0.2 was mainly attributed to the sulfur-vacancy-induced bandgap narrowing, suppressed charge-carrier recombination, and strengthened reduction capability toward imine intermediates. Combined with radical-trapping experiments and in situ EPR analysis, PLA was sequentially transformed through ammonolysis and photocatalytic amination pathways involving lactamide, ammonium lactate, carbon-centered radicals, amino radicals, and imine intermediates before alanine formation. Moreover, the catalyst exhibited excellent stability and enabled the direct conversion of post-consumer PLA products into alanine. This work demonstrates an effective defect-engineering strategy for sustainable photocatalytic upcycling of biodegradable plastics into value-added amino acids.