Abstract:Caprolactam-based ionic liquids show many advantages, such as the lower toxicity, lower cost, and a simple preparation process. In this work, caprolactam-based ionic liquids were prepared and adopted as catalysts for the transesterification of Jatropha oil with methanol. The results demonstrated that the SO 3 H-functional caprolactam-based ionic liquids have higher catalytic activity than those of the caprolactam-based ionic liquids without sulfonic group or the SO 3 H-functional pyridine -based ionic liquids, attributed to their stronger Brønsted acidity. By optimizing the reaction parameter, the biodiesel yield catalyzed by 1-(4-sulfonic group) butylcaprolactamium hydrogen sulfate ([HSO 3 -bCPL][HSO 4 ]) could reach above 95% at 140 • C for 3 h. Furthermore, the ionic liquid had a good reusability.