1-Butyl-3-methylimidazolium hydrogen sulfate catalyzed in-situ transesterification of Nannochloropsis to fatty acid methyl esters

[Display omitted] •[Bmim][HSO4] catalyzed in-situ transesterification of wet algae.•[Bmim][HSO4] served as both effective solvent and excellent acid catalyst.•Proposed a mechanism for [Bmim][HSO4] catalyzed in-situ transesterification.•Identified cell walls and lipid droplets in algae using confocal imaging tests.•Obtained crude biodiesel yield about 95% in 30min at 200°C. 1-Butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) is used as a solvent and an acid catalyst for in-situ extractive transesterification of wet Nannochloropsis with methanol. The reaction is supposed to be a five-step process: (1) wet algae cell wall dissolves in ionic liquid at reaction temperatures; (2) hydrogen ions and sulfate ions release from the ionization of HSO4−. The hydrogen ions (H+) act as catalysts for accelerating the reactive extraction of triglyceride from wet Nannochloropsis; (3) hydrogen ions and methanol molecules transfer from bulk to active site of cells without passing through cell wall that is dissolved by ionic liquid; (4) in-situ transesterification of lipid (mainly triglycerides) with methanol; and (5) products transfer from inside of algae cells to outside of cells. The crude biodiesel yield of [Bmim][HSO4] catalyzed in-situ transesterification is about 95.28% at reaction temperature of 200°C, reaction time of 30min, mass ratio of [Bmim][HSO4] to wet Nannochloropsis of 0.9:1, and a mass ratio of methanol to wet algae of 3:1. It decreases to 81.23% after [Bmim][HSO4] is recycled for 4 times, which indicates that [Bmim][HSO4] catalyzed in-situ transesterification is an economic approach for biodiesel production from wet algae.