Salt-Promoted Water Removal from Reflux Toluene for Efficient One-Step Lactide Synthesis

One-step lactide (LD) synthesis from lactic acid (LA) is a promising method for an economically viable polylactic acid (PLA) industry. However, this synthesis is based on a highly water-sensitive esterification reaction, and few studies focus on developing an efficient method to improve its activity by water removal from the reaction environment. Herein, we report a new strategy coupling salt-promoted toluene–water phase separation with heteroazeotropic distillation to enhance water removal to realize highly efficient LD synthesis. Different kinds of chloride salts (LiCl, NaCl, CsCl, MgCl2, and CaCl2) were selected to study their influence on the reaction. The highest yield of LD (84.7%) was obtained when CaCl2 was added, much higher than that of the salt-free reaction (72.3%). Hydrated Ca2+ cations with relatively large charge number and small radius can efficiently reduce the solubility of water in toluene due to their strong electrostatic force to attract more water molecules. Mechanism studies demonstrate that the increased yield of LD after adding CaCl2 was attributed to its significant inhibition of LD hydrolysis resulting from the deep water removal from the toluene solvent.