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Abstract
A new method for the preparation of anhydrous ethyl ester of lactic acid was studied. The selected method is based on catalytic transesterification of lactic acid oligomers, which were prepared for this purpose by autocatalytic oligomerization of lactic acid. In this work, a kinetic model for the case of catalytic alcoholysis of oligoesters was derived assuming a first-order reaction and equimolar content of reactants in the reaction mixture. The model makes it possible to obtain the values of the reaction rate and equilibrium constants and the equilibrium alcohol concentration by regression analysis at one time. The model was verified by measuring the rate of consumption of ethanol over the time at various reaction temperatures with anhydrous FeCl3 as the catalyst. The reaction was studied at overpressure under autogenous conditions in the temperature range of 100–180 °C. For the catalyst concentration of 1 mol %, the activation energy value was 64.35 kJ·mol−1. The dependence of equilibrium composition and rate constant on the temperature was obtained. The derived model is generally applicable to all first-order equilibrium reactions. The presumption is that the forward and reverse reactions are of the same order and have the same stoichiometry and equivalent amounts of reactants at the beginning of the reaction.
Highlights
IntroductionEsters of lactic acid are an important group of derivates with increasing use as bio-solvents
Esters of lactic acid are an important group of derivates with increasing use as bio-solvents.The reason for this is that the use of these esters has a small environmental impact owing to their partial or complete renewability of feedstock, miscibility with water and organic phase, and their great dissolving power [1]
As an alternative, based on conventional unit operations, we suggest the preparation of lactic acid esters by the transesterification of oligomeric lactic acid with anhydrous alcohols, while water from the reaction system has already been removed during prior oligomerization of aqueous lactic acid solution
Summary
Esters of lactic acid are an important group of derivates with increasing use as bio-solvents. As an alternative, based on conventional unit operations, we suggest the preparation of lactic acid esters by the transesterification of oligomeric lactic acid with anhydrous alcohols, while water from the reaction system has already been removed during prior oligomerization of aqueous lactic acid solution. Experimental verification is demonstrated by the preparation of anhydrous ethyl lactate by the transesterification of oligomeric lactic acid at an equivalent content of reactants. Anhydrous FeCl3 was used as a catalyst, as previously reported for the alcoholysis of high molecular weight poly(lactic acid) [19] This ecological catalyst belongs to the broad group of Lewis acids that are commonly used in organic synthesis [20,21]. Pre-oligomerization limits the amount of water in the esterification environment prior to the addition of the catalyst This allows the use of these catalysts, which undergo rapid hydrolysis and a change of catalytic effect, in an aqueous medium. Lewis acids can replace the commonly used strong organic and inorganic acids, thereby reducing the environmental impact of a given esterification process [27]
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