Sulfur Vacancy Boosts Light-Driven C–N Coupling in Upcycling of Polylactic Acid to Alanine

Photocatalytic C-N coupling of lactic acid (LA) and ammonia for alanine synthesis provides an attractive route for the upcycling of poly(lactic acid) (PLA) plastic waste. Although several advancements have been made in enhancing the photocatalytic alanine synthesis, current research lacks a clear understanding of how the photocatalyst structure influences the C-N coupling reaction. It is highly desirable to design a rational photocatalyst system to boost the C-N coupling, both for advancing mechanistic understanding and for optimizing catalysis. In this work, we have successfully developed sulfur-vacancy CdS (S_v-CdS) as a photocatalyst, achieving a record-breaking alanine formation rate of 72.5 mmol g_cat^-1 h^-1, which surpasses the highest value reported in the literature to date. More importantly, for the first time, we have demonstrated that the photocatalytic amination of LA to alanine occurs exclusively through a radical pathway, where S vacancies weaken the adsorption of C_α• and NH₂• radicals on the S_v-CdS surface, thereby promoting the C-N coupling. In contrast, Cd vacancies in cadmium-vacancy CdS are more favorable for activation of the C_α-H bond in LA, but they are less effective for the C-N coupling. This work offers further insights into the mechanism of the photocatalytic C-N coupling reaction and opens a practical route for upcycling PLA plastic waste into value-added chemicals.