Immobilized Deep Eutectic Solvent ChCl–2ZnCl2@ZIF-8 Composite as an Efficient and Reusable Catalyst for the Methanolysis of Poly(lactic Acid)

Polylactic acid (PLA), as a biodegradable plastic, is considered a potential alternative to alleviate the environmental problems caused by the accumulation of petroleum-based plastic wastes. However, PLA is relatively expensive and naturally degrades very slowly. Therefore, compared to disposal, recycling PLA is more conducive to sustainable development. In this study, a novel immobilized deep eutectic solvent (DES) ChCl–2ZnCl₂@ZIF-8 was developed and used for the methanolysis of PLA to produce a valuable product methyl lactate (ML). The synthesized heterogeneous catalyst exhibits excellent thermal and chemical stability, as well as excellent catalytic performance. For example, the reaction temperature significantly decreased, the dosage of the catalyst decreased, and the PLA conversion and the ML yield were both improved, reaching 98.2% and 96.3%. After the reaction, the catalyst can be recovered through simple filtration, and reused for five times with negligible loss of catalytic activity. The catalyst was characterized by FT-IR, SEM, BET, TGA and XRD. Based on the experimental results, a possible reaction mechanism of PLA methanolysis was proposed. In addition, the kinetics of the PLA methanolysis catalyzed by ChCl–2ZnCl₂@ZIF-8 was examined and the activation energy was obtained. This work mainly provided a way of recovering catalysts and promoted the development of PLA methanolysis into ML.