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

Developing efficient and environmentally friendly catalysts for sustainable biodiesel production is a promising approach to meet global energy demands and address environmental concerns. In this study, four 2-alkyl-1,3-di(3-sulfopropyl)-1H-benzo[d]imidazol-3-ium ionic liquids (ASBILs) were synthesized as catalysts for biodiesel production and characterized using FT-IR, NMR, and MS analyses. Their catalytic activities were evaluated in a model esterification reaction between oleic acid and bioethanol. Proton NMR spectroscopy (400 MHz) was employed to determine the conversion of oleic acid to ethyl oleate. A simple methodology for the quantification of ethyl oleate in unpurified reaction mixtures directly in the 1H NMR spectra through the relationship between the areas of a characteristic signal was proposed and tested. The experimental results identified the efficient catalyst amount of 20 mol% (based on oleic acid mass) for optimal reaction parameters: 16.7:1 molar ratio of ethanol to oleic acid and reaction temperature of 102°C. The reactions reached equilibrium within approximately 75 minutes, achieving conversions of 97.5 – 100%. Notably, 100% conversion was obtained using the ASBIL with a C11 alkyl chain, and its catalytic activity showed no significant decline even after ten cycles of reuse.