Chemical recycling of Polylactic acid to methyl lactate catalyzed by choline chloride/ZnCl2 deep eutectic solvents

Abstract

Methyl lactate (ML) is a green and biodegradable chemical product, which can be used as a solvent for cellulose, paints and dyes. Although several kinds of catalysts have been reported for the methanolysis of Polylactic acid (PLA) into ML, low catalytic activity and high cost and complicated operation are still problems. Renewable, environmentally friendly, and non-toxic choline chloride-based Deep eutectic solvents (DESs) are the newest alternatives to conventional catalysts for the methanolysis of PLA to obtain the valuable product ML. In this study, the methanolysis of PLA into ML using DESs composed of choline chloride (ChCl) and Zinc chloride (ZnCl2) was investigated by experiments and calculations. Experimental results reveal that ChCl-2ZnCl2 can catalyze the methanolysis of PLA to produce ML, more importantly, ML is easily obtained by vacuum distillation, and the catalyst can be reused for 5 times without significant loss of activity. Moreover, the possible reaction mechanism was derived from experimental data and density functional theory (DFT) calculations. And the kinetics of the methanolysis of PLA catalyzed by ChCl-2ZnCl2 was studied and the activation energy was obtained. This work reveals that ChCl/ZnCl2 DESs are conducive to the methanolysis of PLA to ML under moderate conditions, which can also provide strategies for other polymer degradation.