A novel caffeic acid-based deep eutectic solvent as caffeoyl donor to enhance glycerol caffeates synthesis

Glycerol caffeates (GC) is one kind of hydrophilic caffeoyl derivatives, which can be synthesized by the reaction of solid caffeic acid (CA) with different caffeoyl acceptors. The poor solubility of solid CA in the reaction resulted in great mass transfer limitation, low yield and time-consuming. In the work, in order to improve reaction efficiency, a novel liquid CA-based deep eutectic solvent (DES) was prepared and firstly used as caffeoyl donor for GC synthesis. Several solid acids were used as catalysts and the maximum GC yield (~55%) was achieved using NKC-9 as catalyst. Compared with solid CA as caffeoyl donor, CA conversion and initial reaction rate can be enhanced using liquid CA-based DES as caffeoyl donor. Especially at low temperature, initial CA conversion rate of liquid CA-based DES at 70 °C was 7 times that of solid CA. And the maximum CA conversion (~91%) was 2 times that of CA (44.6 ± 3.0%). When liquid CA-based DES was used as caffeoyl donor, the effect of external mass transfer limitation can also be neglected, and the activation energies of CA conversion and glyceryl monocaffeate formation (70.66 and 65.38 kJ/mol) were lower than those (83.31 and 80.41 kJ/mol) of solid CA. These indicated that liquid CA-based DES was favorable for hydrophilic GMC and GDC selective formations.