Lipophilic alkyl caffeate synthesis using a novel green binuclear ionic liquid 1,1-bis(2-pyrrolidinone) sulfate ([C3(Hnhp)2][HSO4]2) catalyst

Caffeic acid (CA), as a potential green antioxidant, plays an important role in food processing. However, the low liposolubility of CA limits its applications. To overcome this issue, CA is normally modified by introducing a lipophilic group, such as alkyl alcohols, resulting in the formation of alkyl caffeate, which can significantly enhance the liposolubility of CA. In this study, a binuclear ionic liquid, 1,1-bis(2-pyrrolidinone) sulfate ([C3(Hnhp)2][HSO4]2), is successfully synthesized and characterized by FT-IR and 1H NMR. The physicochemical properties of [C3(Hnhp)2][HSO4]2, including the density, viscosity, thermal stability and Bronsted acidity, were analyzed. As a novel catalyst for the esterification of CA with model dodecanol, its catalytic performance was investigated and optimized by response surface methodology. Under the optimal conditions, a 95.42 +/- 1.01% yield of dodecanol caffeate was achieved. Moreover, the [C3(Hnhp)2][HSO4]2 exhibits excellent stability and reusability, making it a highly promising catalyst for the synthesis of various lipophilic alkyl caffeates.