Novel proline p-toluenesulfonate ionic liquid as a recyclable catalyst for lauric acid esterification: process optimisation, kinetic and thermodynamic studies

Abstract

In this study, eight amino acid-based p-toluenesulfonic acid ionic liquids ([AAH][C₇H₇O₃S]-ILs) were successfully prepared by the neutralisation reaction of natural amino acids with p-toluenesulfonic acid (p-TSA). The catalytic performance of the newly synthesized [AAH][C₇H₇O₃S]-ILs was systematically investigated for biodiesel preparation using lauric acid and ethanol as model reactants. Experimental results demonstrated that proline p-toluenesulfonic acid ionic liquid ([ProH][C₇H₇O₃S]) exhibited optimal catalytic performance for esterification of lauric acid and ethanol with a lower H₀ (1.6601). Response surface methodology (RSM) optimized the process conditions to achieve 96.1 % lauric acid conversion under the following conditions: 14.4 wt.% catalyst loading, 16.3:1 ethanol/lauric acid molar ratio, 2.46 h reaction time, and 89 °C temperature. According to the ¹H NMR method, the yield of biodiesel (ethyl laurate) was determined to be 96.0 %. The kinetic investigation revealed that the reaction followed first-order kinetic, exhibiting relatively low activation energy (7.04 kJ•mol⁻¹) and a frequency factor reaching 0.208 min⁻¹. Calculated thermodynamic parameters showed ΔH = 3.94 kJ•mol⁻¹, ΔS = -0.268kJ•mol⁻¹•K⁻¹, ΔG = 103.9kJ•mol⁻¹. After undergoing ten reaction cycles, the catalyst maintained 93.9 % of its initial catalytic activity, confirming its excellent structural stability. It was noteworthy that [ProH][C₇H₇O₃S] exhibited homogeneous catalytic properties during the reaction, and the system can achieve spontaneous liquid–liquid partitioning after the reaction, which significantly simplifies the product separation process. The catalyst had the advantages of high catalytic efficiency, green renewability and easy operation, which provided a new catalytic system with the potential of industrialisation for biodiesel production.