Study On The Base-catalyzed Glycerolysis Reaction In Organic Solvents

Partial glycerides (monoacylglycerol, MAG and diacylglycerol, DAG) are widely used in food, pharmaceutical and cosmetic industries. Presently, partial glycerides are commonly manufactured by chemcial glycerolysis at high temperatures. The high temperature used caused energy-consuming, low-produnction and it is not suitable for heat-sensitive fatty acid-rich glycerides production. Therefore, low-temperature chemical glycerolysis to obtain high content of MAG or DAG seems attractive.In this study, based on the researches of kinetics of NaOH catalyzed glycerolysis (of triolein) reaction in organic solvent system at low temperatures, and from the theory of chemical potential and activity coefficient point of view, a method based on the solubility of desired product in the selected solvents was proposed to select solvents in order to improve the desired product content by altering the composition at equilibrium state. The mechanism of the increase of monoacylglycerol (MAG) content induced by solvation of tert-butanol was revealed. It was also found that the reaction rate in solvent system was not controlled by the mass transfer between glycerol and triolein, and the rate-limited step of the reaction was deduced and discussed. The main aspects and results are as follows:1. Effects of solvent on the glycerolysis reaction progressChemically or enzymatically (lipozyme TL IM), the glycerolysis reaction was not proceeded at 45 oC in solvent-free system. The reaction was proceeded by introducing tert-butanol solvent into the reaction system, and the reaction rate was faster for chemically than enzymatically. The glycerides composition at equilibrium state, however, was the same for both chemically and enzymatically. No variation was also observed on the fatty acid composition of the products.2. Study on the rate-limited step of the glycerolysis reaction in solvent systemsThe ability of the selected different solvents to improve miscibility of the reactants was studied. Tert-pentanol was found to be the best solvent to improve miscibility of TAG and glycerol followed by tert-butanol and isopropanol. However, the highest initial reaction rate was found at isopropanol solvent system, followed by at tert-butanol and tert-pentanol solvent system. Thus, the glycerolysis reaction was not determined by mass transfer of glycerol and TAG. Based on the fact that the solubility of NaOH in solvents was the higest at isopropanol, followed by at tert-butanol and tert-pentanol solvent system, which was consistent with the sequence of the reaction rate, the rate-limited step of the glycerolysis reaction was therefore deduced to the alkoxide formation step.3. Mechanism of the composition variation at equilibrium state caused by solvationA method based on the solubility of the desired product in the selected solvents was proposed to select solvents for the reaction specificity for partial glycerides. And the feasibility of the proposed method was verified by the present experiment results, which indicated that tert-butanol, tert-pentanol and isopropanol are all MAG-selective solvents, with isopropanol as the most prominent solvents, followed by tert-butanol and tert-pentanol. Glycerolysis of TAG is a two-step consecutive reaction, the rate constant for the reversed reaction of the second step, namely, MAG to DAG and glycerol, is closed to zero in the present study, and the corresponding activation entropy is -270.0 J·K-1·mol-1. The large minus value of the activation entropy indicated that it was because the increased order of the system that the reaction was difficult to proceed. According to the reaction mechanism, the increased order of the system can be attributed to the solvation effect caused by the intense interaction between tert-butanol and MAG anion.4. Production of MAG and DAG through low-temperature chemical glycerolysisHigh content of MAG up to 82.1±1.5% can be obtained with over 97% TAG conversion through the following reaction conditions: tert-butanol/TAG 3:1 (wt/wt), glycerol/TAG 5:1 (mol/mol), NaOH amount, 0.4 wt% of TAG, agitation speed 600 rpm, temperature 50 oC and reaction time for 1-h.Operation modes were critical in the process of DAG production through low-temperature chemical glycerolysis. With acetone dosage (acetone:TAG, wt:wt) 3:1, glycerol/TAG 1:2 (mol/mol), NaOH amount, 0.4 wt% of TAG, a total of 52.0±2.2% DAG can be obtained under the ultrasonic irradiation at 50 oC after 1-h reaction or under magnetic stirring at the same temperature after 4-h reaction.The present results can help for the solvent selection for altering the composition at equilibrium state in organic reactions, it is also beneficial for the low-temperature chemical glycerolysis reaction enhancement. The present reaction system for MAG preparation is potential for industry production.