Intensified synthesis of glycerol monocaprylin using ionic liquids by temperature-responsive esterification

Abstract

• [N 1,1,1,1 ][H 2 PO 4 ] is screened by COSMO-RS for selective synthesis of glycerol monocaprylin. • Dual effects of IL as solvent and catalyst are verified by analysis and experiments. • The esterification process with [N 1,1,1,1 ][H 2 PO 4 ] shows a temperature-responsive behavior. • The esterification conditions are optimized to obtain high GMC yield. • The esterification kinetics are correlated with the pseudo-homogeneous model. The selective conversion of glycerol (GL), a surplus by-product in biodiesel production, is deemed as a promising way to produce high value products. Glycerol monocaprylin (GMC), one of the widely utilized industrial chemicals, can be synthesized by esterification of GL with caprylic acid (CA). In this work, the intensified synthesis of GMC is proposed using ionic liquid (IL) as dual solvent and catalyst. The IL namely tetramethylammonium dihydrogen phosphate ([N 1,1,1,1 ][H 2 PO 4 ]) is screened out from 414 acidic IL candidates by calculating the infinite dilution capacity of GL and CA in ILs based on COSMO-RS model. The dual effects of [N 1,1,1,1 ][H 2 PO 4 ] are verified by σ -profile/ σ -potential analysis and experiments. The good solubility of [N 1,1,1,1 ][H 2 PO 4 ] for GL and CA makes the esterification system homogenous during reaction at high temperature, whereas phase splitting occurs after cooling down. Such a temperature-responsive behavior can greatly improve the GMC selectivity and benefit product separation and IL recycling. The reaction conditions are optimized to achieve 87.7% CA conversion and 78.4% GMC yield. The pseudo-homogeneous model is adopted to successfully describe the esterification kinetics with RMSD of 0.0349. Moreover, the recycling tests of [N 1,1,1,1 ][H 2 PO 4 ] are also performed, and the IL can keep its initial catalytic performance after four successive cycles.