Regeneration of caprolactam-based Brønsted acidic ionic liquid during transesterification of Jatropha oil

[Display omitted] •The major cause of IL deactivation was the loss of acidity.•There was a negative linear correlation between acidity decline rate and biodiesel yield.•Deactivated IL was regenerated successfully by supplementing acidity with H2SO4. As catalysts for biodiesel preparation, functional acidic ionic liquids (ILs) exhibit excellent catalytic property and good application prospect. However, regeneration and reuse of ILs are crucial issues that limit their industrial application. In this work, the regeneration of caprolactam-based Brønsted acidic IL [HSO3-bCPL][HSO4] during the transesterification of Jatropha oil was investigated. The results indicated that [HSO3-bCPL][HSO4] started to deactivate after repeating use for 9 times, and was deactivated after 12 times. The major cause of the deactivation was that the acidity decreased during the recycling process, and there was a negative linear correlation between acidity decline rate and the biodiesel yield. The deactivated [HSO3-bCPL][HSO4] could be regenerated by supplementing acidity with concentrated sulfuric acid and the dosage of H2SO4 was equal to the acidity decline rate approximately. The catalytic activity and reusability of regenerated [HSO3-bCPL][HSO4] were approached to those of the fresh one.