Synthesis of Lipophilic Caffeoyl Alkyl Ester Using a Novel Natural Deep Eutectic Solvent

In this work, a novel method for lipophilic caffeoyl alkyl ester production was developed using a natural deep eutectic solvent (DES) consisting of choline chloride and caffeic acid (CA) as the caffeoyl donor. Cation-exchange resins were used as the catalyst to catalyze the esterification of fatty alcohols with the DES. Effects of the caffeoyl donor and reaction variables were investigated. Reaction thermodynamics were also analyzed. The results showed that the lipophilic caffeoyl alkyl ester production can be enhanced using the DES as the caffeoyl donor, and cation-exchange resin A-35 showed the best catalytic activity for the reaction. Under the optimized conditions (85 °C, stearyl alcohol/CA 8:1 (mol/mol), A-35 load 5% and 24 h), the maximum octodecyl caffeate (OC) yield (90.69 ± 2.71%) and CA conversion (95.17 ± 2.76%) were obtained with the DES as the caffeoyl donor, which were much higher than those obtained with solid CA as the caffeoyl donor (OC yield 40.97 ± 2.37% and CA conversion 44.26 ± 1.69%). The activation energy of CA conversion (67.57 kJ/mol) with the DES was lower than that with solid CA (90.19 kJ/mol). In addition, the mass transfer limitation can be decreased with the DES. Compared with solid CA as the caffeoyl donor, a fast reaction rate and low mass transfer limitation were obtained using the DES as the caffeoyl donor.