DBU-derived ionic liquids as tunable basic catalysts for biodiesel transesterification: structure–performance relationships and process optimization

This study evaluates the catalytic performance of DBU-based ionic liquid systems as tunable basic catalyst for biodiesel production from linseed oil. Three ionic liquids [DBUH][IM], [DBUH][1MIM], and [DBUH][2MIM] were synthesized and comprehensively characterized by proton nuclear magnetic resonance ( 1 H NMR), carbon−13 nuclear magnetic resonance ( 13 C NMR, including DEPT), Fourier-transform infrared (FT-IR) spectroscopy, density measurements, Karl–Fischer titration, and pH-metric analysis. The thermogravimetric analysis revealed good thermal stability for all DBU-derived ionic liquids, with decomposition profiles strongly influenced by anion structure. Among the evaluated samples, [DBUH][2MIM] exhibited the highest thermal resistance, indicating that methyl substitution contributes positively to the thermal stability of the ionic liquid system. The process optimization was performed using a Box–Behnken experimental design considering temperature, ethanol-to-oil molar ratio, and catalyst loading. Under the optimized conditions (65 °C, ethanol-to-oil molar ratio of 12.31 M ratio, and 10 wt% catalyst), the predicted conversion of fatty acid ethyl ester (FAEE) was 65.41% after 6 h, while experimental validation reached 70.28 ± 3.23%. Response surface analysis revealed a saddle point behavior, suggesting that the global optimum may lie at the boundary of the investigated experimental domain. The extended kinetic evaluation over 24 h showed equilibrium conversions above 95% for the methyl-substituted ionic liquids, reaching 97.15%, 95.92%, and 88.27% for [DBUH][2MIM], [DBUH][1MIM], and [DBUH][IM], respectively. Notably, catalytic performance did not correlate directly with basicity (pKa), indicating that anion structure and electronic effects play a dominant role in determining catalytic activity. The superior performance of the methyl-substituted imidazolate-derived systems highlights the importance of structural tuning in enhancing catalytic efficiency.