Photocatalytic upcycling of polylactic acid to alanine by sulfur vacancy-rich cadmium sulfide
Yue Wu, Phuc T.T. Nguyen, Sie Shing Wong, Minjun Feng, Peijie Han, Bingqing Yao, Qian He, Tze Chien Sum, Tianyong Zhang, Ning Yan
Abstract
Photocatalytic conversion has emerged as a promising strategy for harnessing renewable solar energy in the valorization of plastic waste. However, research on the photocatalytic transformation of plastics into valuable nitrogen-containing chemicals remains limited. In this study, we present a visible-light-driven pathway for the conversion of polylactic acid (PLA) into alanine under mild conditions. This process is catalyzed by defect-engineered CdS nanocrystals synthesized at room temperature. We observe a distinctive volcano-shaped relationship between sulfur vacancy content in CdS and the corresponding alanine production rate reaching up to 4.95 mmol/g catalyst/h at 70 oC. Ultraviolet-visible, photocurrent, electrochemical impedance, transient absorption, photoluminescence, and Fourier-transform infrared spectroscopy collectively highlight the crucial role of sulfur vacancies. The surface vacancies serve as adsorption sites for lactic acid; however, an excessive number of vacancies can hinder charge transfer efficiency. Sulfur vacancy-rich CdS exhibits high stability with maintained performance and morphology over several runs, effectively converts real-life PLA products and shows potential in the amination of other polyesters. This work not only highlights a facile approach for fabricating defect-engineered catalysts but also presents a sustainable method for upcycling plastic waste into valuable chemicals. Photocatalysis offers a promising pathway to upcycle plastic waste. Here the authors demonstrate how defective cadmium sulfide photocatalysts can convert real-world polylactic acid into the amino acid alanine under visible light.