Ultrasonic-assisted synthesis of mono- and diacylglycerols and purification of crude glycerol derived from biodiesel production

Abstract

Production of monoacylglycerols (MAGs) and diacylglycerols (DAGs) via glycerolysis of fatty acid methyl esters (FAME) with glycerol (G) using an ultrasonic bath and probe was investigated. The glycerolysis using an ultrasonic bath at the optimum condition: KOH 3 wt% of glycerol, ethanol 5.5% v/v solution, and ultrasonic frequency 35 kHz yielded only 3.30% MAGs and 11.47% DAGs because the temperature was limited at 80 °C. The effect of four variables including the molar ratio of G:FAME (2.5:1–3.5:1), KOH (2.5–3.5 wt% of G), time (45–75 min), and temperature (140–160 °C) for glycerolysis utilizing an ultrasonic probe was studied and optimized by response surface methodology (RSM) and box-Behnken design (BBD). The condition: 2.82:1 G:FAME mol:mol, KOH 3.18 wt% of glycerol, 46.33 min, and 155.66 °C was recommended to achieve the highest MAGs (53.32%). To produce the most DAGs, a lower molar ratio, higher temperature, and longer time were suggested. Purification of crude glycerol by a simple multi-step process was proposed and a yield of 45 ± 0.34% was obtained with glycerol purity up to 95%. Glycerolysis of the purified and pure glycerol gave identical contents of MAGs and DAGs. Crystallization was operated to purify MAGs by investigating crystallization temperatures and solvents. Two-step crystallization obtained up to 96% MAG content and the major product was mono-palmitin. The synthesis of MAGs and DAGs was successful through the utilization of an ultrasonic probe, and crystallization was an effective method for MAG purification.