Solar-Biohybrid Upcycling of Polylactic Acid Plastics to Alanine

Upcycling of waste plastics into commodity chemicals such as amino acids represents a promising route toward achieving negative carbon emissions and enabling a circular carbon economy. However, existing thermocatalytic methods typically require harsh conditions. While photocatalytic upcycling operates under mild conditions, its dependence on a high-temperature hydrolysis step for plastic pretreatment undermines its overall practicality. Here we report a bio-photocatalytic hybrid system that converts polylactic acid (PLA) plastics into alanine under ambient conditions. The integrated system begins with enzymatic depolymerization of PLA to lactic acid (LA) with 92% conversion at 55°C and pH 10, catalyzed by an engineered peptidase derived from the Micromonospora sp. strain. Without any purification, the resulting hydrolysate is fed directly to the photosynthesis system, where the LA monomers are converted to alanine over a Ni/ZnIn₂S₄ catalyst using ammonia as a nitrogen source. This photocatalytic process, which proceeds via an oxygen-centered radical intermediate pathway, achieves an alanine production rate of 61.91 mmol g^-1 h^-1 and an overall yield of 60%. Life cycle assessment demonstrates that our tandem bio-photoconversion system substantially reduces the carbon footprint compared to single-step thermocatalytic and photocatalytic systems. This work thus establishes a sustainable route for valued chemicals production from low-cost feedstocks.