Direct synthesis of lactide from lactic acid

Abstract

L-lactide was directly synthesized from L-lactic acid solution through 3 methods. In the first method, L-lactide was synthesized under steam distillation, but could not be isolated via steam distillation. Addition of toluene did not improve the process. The highest yield of L-lactide from this method was 3 %. In the second method, silica gel was used as dehydrating catalyst. The extraction with boiling solvent and baking L-lactic acid with silica gel were investigated. The highest yield of lactide was 9 % when 1:1 ratio of L-lactic acid and silica gel were boiled in toluene at reflux for 12 hours. In the third method, L-lactide was synthesized and isolated via the reduced pressure distillation in the presence of acid or metal catalyst. Magnesium sulfate was found to be the most effective catalyst for L-lactide production, giving pure product in 20 % yield at distillation temperature of 180-200 °C for 1 h. This simple method enables the production of L-lactide without numerous subsequent separation and purification steps.