Novel proline trifluoromethanesulfonate ionic liquid as an effective and renewable catalyst for oleic acid esterification: Process optimization, kinetic studies, and thermodynamic studies

Eight novel amino acid (AA)–based trifluoromethanesulfonate ionic liquids (ILs; named [AAH][CF 3 SO 3 ]-ILs) were synthesized via neutralization reactions from natural AAs and trifluoromethanesulfonate (CF 3 SO 3 H). The catalytic performance of [AAH][CF 3 SO 3 ]-ILs in biodiesel preparation was evaluated with oleic acid and methanol as substrates. Among the eight [AAH][CF 3 SO 3 ]-ILs, the proline trifluoromethanesulfonate ([ProH][CF 3 SO 3 ]) catalyst displayed the best catalytic activity. The optimal process conditions for the response surface methodology (RSM) were as follows: [ProH][CF 3 SO 3 ] loading = 14.99 wt%, MeOH-to-oleic acid molar ratio = 17.94:1, reaction time = 1.95 h, and temperature = 71°C, and an oleic acid conversion rate of 98.97% was obtained. A first-order reaction kinetics of [ProH][CF 3 SO 3 ]–catalyzed oleic acid esterification showed an activation energy of 18.25 kJ·mol −1 and a frequency factor of 14.04 min −1 . ΔH, ΔS, and ΔG of oleic acid esterification reaction were 15.42 kJ·mol −1 , −0.232 kJ·mol −1 ·K −1 , and 97.32 kJ·mol −1 , respectively. The catalytic activity of [ProH][CF 3 SO 3 ] did not considerably change after eight reuse cycles, demonstrating its excellent stability and reusability. When applied as a catalyst, [ProH][CF 3 SO 3 ] dissolved in the reaction medium during the esterification reaction, enabling efficient homogeneous catalysis . After the reaction, phase separation automatically occurred between biodiesel (methyl oleate) and the catalyst, minimizing the product separation and purification process. The application of [ProH][CF 3 SO 3 ] in this study provides an environmentally friendly technique for biodiesel production . [ProH][CF 3 SO 3 ] is a promising renewable catalyst that it can be used to catalyze fatty acid esterification for biodiesel preparation instead of traditional catalysts.