Density functional theory study on biodiesel production from yeast lipid catalyzed by imidazolium ionic liquid

Imidazolium ionic liquid is a novel and efficient catalyst for biodiesel production from oleaginous yeast. However, the catalytic mechanism of how imidazolium ionic liquid works in the reaction is not clear. Herein, Density functional theory (DFT) was employed to investigate the catalytic mechanism of three imidazolium ionic liquids in esterification and transesterification reactions. Pathways containing three intermediates and two transition states were proposed for the two reactions. In the subsequent DFT calculation, PBE0-D3BJ functional and 6-31G* basis set were used to optimize the intermediates and transition states. Then the Gibbs free energy of the intermediates and transition states in each reaction step was calculated to obtain the energy barrier for each reaction. The calculation results showed that 1-butyl-3-methylimidazolium hydrogen sulfate ([Bmim][HSO4]) had the minimum energy barrier of 37.77 kcal/mol in esterification, and 1-sulfobutyl-3-methylimidazolium hydrosulfate ([HSO3-Bmim][HSO4]) had the minimum energy barrier of 20.80 kcal/mol in transesterification, which were consistent with previous experimental results. The catalytic mechanism validated in this study provides a new idea for subsequent optimization of imidazolium ionic liquid structure to improve their catalytic efficiency in biodiesel production from oleaginous yeast.