Microwave assisted process intensification of lipase catalyzed transesterification of 1,2 propanediol with dimethyl carbonate for the green synthesis of propylene carbonate: novelties of kinetics and mechanism of consecutive reactions

Abstract

Green chemistry inherently hinges on process intensification through catalysis and waste minimization. The synergism of enzyme catalysis with microwave irradiation was studied systematically in this work for a consecutive reaction about which no literature report exists. Transesterification reaction of renewable starting materials (1,2 propanediol and dimethyl carbonate) was studied as a new strategy for environmentally benign route of cyclic carbonate synthesis in non aqueous media. A variety of supported enzymes were considered including Lipozyme RM IM, Lipozyme TL IM, Amano and Candida antarctica (Novozyme 435) among which Novozym 435 was the best. Both reactions occur under the influence of enzyme and also are enhanced by microwave irradiation. Thus, the example reported here serves as the first of its kind. The studies cover influence of different process parameters on reaction rate and selectivity of propylene carbonate and include agitation speed, solvent, loading of catalyst, reactant concentration (mole ratio) and temperature. A novel kinetic model was developed by finding forward and backward reaction rate constants for each step of the reaction. Microwave irradiation not only increases reaction rate but also improves the thermal stability of enzyme. Maximum conversion of 93% was obtained in 6 h using 250 mg of enzyme loading with 1 and 3 mmol of 1,2 propanediol and dimethyl carbonate in 15 mL solution made up with 1,4-dioxane as solvent at 70 °C as compared to 54.4% conversion under conventional heating, thus proving the synergism between microwave irradiation and lipase. The enzyme is reusable and the results are novel.