Novel Sulfonic Acid-Functionalized Ionic Liquids: Synthesis and Catalytic Performance in Biodiesel Production via Quantitative 1H-NMR Analysis

The development of efficient and environmentally compatible catalysts for sustainable biodiesel production is a promising approach to meet global energy demands and address environmental challenges. This study reports the synthesis and characterization of four 2-alkyl-1,3-di(3-sulfopropyl)-1 H -benzo[d]imidazol-3-ium ionic liquids (ASBILs) and provides new insights into the influence of alkyl chain length (C9–C17) on their catalytic performance. A direct analytical methodology based on quantitative 1 H NMR was established to monitor the catalyzed esterification of oleic acid with ethanol. The experimental results reveal a non-linear structure-activity relationship, identifying the C11 variant as the optimal catalyst. This chain length provides a superior balance between lipophilic affinity and minimized steric hindrance, achieving a 99% ethyl oleate yield in the esterification of oleic acid within 45 min at 102 °C, and its catalytic performance showed no significant degradation over ten consecutive reuse cycles. These findings also establish design rules for optimizing the hydrophobic-hydrophilic balance in multifunctional ionic liquids.