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][CF3SO3]-ILs) were synthesized via neutralization reactions from natural AAs and trifluoromethanesulfonate (CF3SO3H). The catalytic performance of [AAH][CF3SO3]-ILs in biodiesel preparation was evaluated with oleic acid and methanol as substrates. Among the eight [AAH][CF3SO3]-ILs, the proline trifluoromethanesulfonate ([ProH][CF3SO3]) catalyst displayed the best catalytic activity. The optimal process conditions for the response surface methodology (RSM) were as follows: [ProH][CF3SO3] 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][CF3SO3]–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][CF3SO3] did not considerably change after eight reuse cycles, demonstrating its excellent stability and reusability. When applied as a catalyst, [ProH][CF3SO3] 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][CF3SO3] in this study provides an environmentally friendly technique for biodiesel production. [ProH][CF3SO3] is a promising renewable catalyst that it can be used to catalyze fatty acid esterification for biodiesel preparation instead of traditional catalysts.