(293f) Separation of Diethyl Citrate and Phase Equilibria in Mixtures with Ethanol and Water

The potential harmful effects caused by petrochemical plasticizers, specifically phthalates, are a continuous concern for consumers and producers. Plasticizer market is growing mainly in the biobased sector (based on natural resources) and stagnation is observed in the traditional phthalates. Among the variety of biobased plasticizers, the esters from citric acid (with or without acetylation) are preferred as ingredient in the fabrication of polymers and resins used in medical devices, food packaging, cosmetics, and many other applications. This preference is because citrates are generally recognized as safe (GRAS) by different regulatory agencies worldwide.

Current citrates production is carried out in semibatch processes and large excess of alcohol is required to overcome chemical equilibrium limitations of the reaction. The need for the excess alcohol removal and product purification make the process energy intensive, inefficient and costly. In this regard, intensification by using reactive distillation (RD) technologies is envisioned as an alternative to improve process sustainability.   

For a complete RD feasibility study, thermodynamic and kinetic data associated to the reactive system are required. This include properties of intermediate esters that in the case of the esterification with ethanol correspond to monoethyl citrate and diethyl citrate. These are low-volatile, water-soluble acidic esters, and differ only in an ethyl radical, which makes difficult their separation and further purification. Despite molecular distillation and preparative chromatography used in the past were not efficient enough, a simple pH-controlled solvent extraction was successful to accomplish intermediate ester separation.

A mixture of ethyl citrates rich in dyethyl citrate (DEC) was prepared by incomplete esterification of citric acid and anhydrous ethanol. Reaction was carried out in a 2 L stainless steel reactor coupled with a reflux condenser and a mechanical stirrer. An initial citric acid/ethanol molar ratio of (1:5) and 5 wt.% Amberlyst 15 catalyst were used during reaction. The temperature was set at 73 °C and the reaction was carried out for 5 hours. After the reaction, the excess ethanol was removed by vacuum distillation, leaving behind a yellowish liquid mixture of citric acids and citrates.

By performing a pH controlled separation, trietyl citrate (TEC) and DEC were isolated from the citrates mixture, and 97 wt% DEC was obtained with citric acid as major impurity. Even if TEC and citric acid are commercially available to be used as standards in HPLC calibration, intermediate esters are not. In this sense, calibration of HPLC diode array detector was performed to improve quantitative analysis in further experiments.

Once isolated, diethyl citrate was characterized by NMR, DSC, and vapor pressure tests. Binary vapor liquid equilibria at constant temperature in mixtures with ethanol and water were obtained, and parameters for activity-based models were regressed. Good agreement of predicted and experimental phase equilibrium data were observed, and the model can be used in further reactive distillation feasibility studies.