Optimization of microwave-assisted etherification of glycerol to polyglycerols by sodium carbonate as catalyst

• Process optimizations on the etherification of glycerol to polyglycerols are reported.
• Reaction temperature is the most significant variable for the optimized process.
• Microwave radiation shortens the reaction time for the etherification of glycerol to oligoglycerols.
• HPLC was used to determine the quantity of polyglycerol products without any product modifications.

The process optimization of etherification of glycerol to polyglycerols by sodium carbonate as a catalyst using microwaves as a heat source in solvent free conditions has been investigated in this study. The regression models describing the linear correlations between reaction parameters and reaction outcomes were developed. The three reaction parameters studied are: reaction temperature (220 °C and 270 °C), catalyst concentration (1 wt% and 3 wt%), and reaction time (0.5, 1.0, 1.5, 2.0, and 3.0 h). The reaction temperature was found to have the most significant effect on the percent conversion of glycerol (X); combined yields of diglycerols, triglycerols, and tetraglycerols products (YDG+TG+TtG); selectivity toward pentaglycerols (SPG); and selectivity toward cyclic diglycerols (Sc-DG). Undesirable cyclic diglycerols formed at high temperature and the amount increased along with the longer reaction time. The higher catalyst concentration (3 wt%) provides higher amounts of the desired products. Moreover, the longer reaction time also resulted in higher conversions and yields. For the optimized conditions, X, YDG+TG+TtG, and Sc-DG were predicted to be at 84%, 63%, and 9%, respectively, at the reaction temperature of 270 °C, catalyst concentration of 3 wt%, and reaction time of 1.0 h. Experimental verification of the predicted optimum conditions gave the actual responses of 93%, 70%, and 7% for X, YDG+TG+TtG, and Sc-DG, respectively, with small deviations from the predicted responses. The results indicated that the developed models were valid and accurate in describing the actual experimental data at any conditions within the range studied. The observation suggests that the sodium carbonate catalyst with microwave heat source have the potential to be used in glycerol conversions to polyglycerols.