Preparation of Bio-Based Long-Chain Dicarboxylic Acid Ester Precursors Catalyzed by Protic Ionic Liquids

Graphical In this study, we developed a new strategy for the synthesis of long-chain dicarboxylic acid ester precursors via the aldol condensation of 2,5-diformylfuran (DFF) and methyl levulinate (MLA) in the presence of protic ionic liquids. The reaction mechanism was further investigated using nuclear magnetic resonance (NMR) and Fourier transform infrared spectroscopy (FTIR). In this study, a series of long-chain dicarboxylic acid ester precursors were synthesized via aldol condensation reactions catalyzed by protic ionic liquids (PILs). When 2,5-diformylfuran (DFF) and methyl levulinate (MLA) were employed as substrates, the yield of long-chain dicarboxylic acid ester precursors reached 91.0% in the presence of pyrrolidine lactate. This preparation process eliminated the need for additional acids and bases, thereby minimizing the generation of waste salts, which was a common issue in base-catalyzed systems and biosynthesis methods. Nuclear magnetic resonance (NMR) characterization revealed that a strong interaction between the hydroxyl group of the catalyst and the carbonyl group of MLA facilitated the activation of the substrate for the aldol condensation reaction. Notably, the yield of dicarboxylic acid ester precursors remained stable even after five catalytic cycles, demonstrating the excellent reusability of the catalyst. Furthermore, pyrrolidine lactate exhibited broad applicability in both aldol condensation and Knoevenagel condensation reactions. This work presented a novel and sustainable strategy for the synthesis of long-chain dicarboxylic acid ester precursors.