Impact of ionic liquid properties on selective enrichment of glycerides in direct lipase-catalyzed esterification

Abstract

The direct lipase-catalyzed esterification of oleic acid and glycerol was studied in 23 different ionic liquids (ILs) in order to deduce the effects of solvent properties such as viscosity, polarity (E^T_N), hydrophobicity (log P) and Kamlet–Taft parameters (α, β and π*) on selective enrichment of glycerides. E^T_N and Kamlet–Taft parameters are most likely the determining factors for glyceride selectivity. ILs with high E^T_N and Kamlet–Taft values might form a hydrogen-bond with monoacylglycerols (MAG) and resulted in high MAG content. ILs with higher log P and lower β resulted in higher FFA conversion and ILs with longer alkyl chains lead to higher contents of DAG and TAG. Viscosity may reduce the reaction rate which could decrease the FFA conversion and change glyceride selectivity. Pearson's correlation analysis showed that the MAG content had a significantly positive correlation with E^T_N value (R = 0.4860, P < 0.05) and Kamlet–Taft parameters (R > 0). log P and hydrogen bond basicity (β) have relatively high positive correlation coefficient (R = 0.6065) and negative coefficient (R = −0.3878) with FFA conversion respectively. [NTF₂]⁻ based ILs were more suitable for the rules of polarity. In summary, selective enrichment of glycerides was not simply influenced by any single property, but by multiple properties collaboratively. However, higher polar ILs could always result in higher polar reaction products.