Chemical Upcycling of Polylactic Acid Plastic Waste into Alanine over Ru/CeO2 with Decoration of Indium

Alanine as a versatile chemical can be synthesized by catalytic amination of biomass-derived polylactic acid (PLA) in ammonia. However, accelerating the production rate of alanine with high selectivity through the chemical upcycling of PLA waste remains a great challenge. Herein, we proposed Ru/Ce–In-mixed metal oxide (MMO) catalysts with decoration of indium and found that they could efficiently facilitate the amination of PLA into alanine with a high selectivity toward alanine. Among the Ru/Ce–In-MMO catalysts with different molar ratios of Ce/In, Ru/Ce3In-MMO exhibited outstanding catalytic performance and achieved an impressive alanine yield up to 76.1% with an excellent alanine production rate of 3.01 molAla·molRu–1·h–1 at 180 °C for 18 h under 0.1 MPa of N2, which was remarkably higher than those of individual Ru/CeO2 and Ru/In2O3 as catalysts. These results revealed that the incorporation of In into Ru/CeO2 not only created an electron-rich coordination environment for stabilizing Ru particles but also reinforced the interaction between Ce and In species, which was beneficial to the dehydrogenation, amination, and hydrogenation steps of PLA to alanine by interacting with intermediates formed during the reaction process. Most importantly, Ru/CeO2 with decoration of In resulted in a higher ratio of Ce3+/(Ce3+ + Ce4+) (28.6% vs 16.2%), plentiful oxygen vacancies (48.5% vs 12.7%), larger specific surface areas, and stronger total surface basicity, which collectively/synergistically improved the catalytic activity and the yield of alanine. Furthermore, the systemic studies confirmed that the as-prepared catalysts exhibited excellent recyclability without significant deactivation over three consecutive cycles. This study provides new vistas to rationally design efficient catalysts toward sustainable production of alanine in upcycling of PLA waste.