Amphiphilic heteropolyacid-based sulfonic acid-functionalised ionic liquids as efficient catalysts for biodiesel production

Aiming to address the mass transfer limitation in biphasic transesterification reactions for biodiesel production, as well as the poor affinity between conventional catalysts and feedstock oils, a series of amphiphilic heteropolyacid-based sulfonic acid-functionalised ionic liquids (AHPA-SFILs) were synthesised. Impressively, the solid-state AHPA-SFILs as catalysts would dissolve in the reaction medium during the transesterification of palm oil with methanol, enabling efficient homogeneous catalysis. Subsequently, the AHPA-SFILs could be easily precipitated and recovered by evaporating the methanol. Under the reaction temperature of 140 °C, methanol/oil molar ratio of 20, catalyst dosage of 7 wt%, and reaction time of 1.8 h determined by response surface methodology, the optimum AHPA-SFIL, namely [C16N-‍PS]-HPMo-1, exhibited a 95.86% yield of biodiesel. As revealed by experiments and quantum chemical calculations, the C16 chain of [C16N-PS]-HPMo-1 on one hand enhanced the mass transfer by promoting the dissolution of methanol in oil phase, and on the other hand facilitated the enrichment of triglycerides through intensified van der Waals interaction, thereby triggering the remarkable catalytic performance. This reaction intensification effect derived from long alkyl groups provided a theoretical basis for the design of future ILs. Furthermore, [C16N-PS]-HPMo-‍1 AHPA-SFIL showed satisfactory generality in the biodiesel production from different oils, signifying its promising application potential.