Tuning the acidity of pyridinium-based ionic liquid catalysts for enhanced biodiesel production using oleic acid: a DFT study

This work presents the density functional theory (DFT) study on the synthesis of methyl oleate using ionic liquids, specifically pyridinium hydrogen sulfate (PHS) and its derivatives, as catalysts. Herein, we investigated the effect of push and pull group substitution on pristine PHS and its influence on catalytic activity toward biodiesel production. The structure, electronic properties, and reactivity descriptors were reported for five ionic liquid catalysts. It is found that these push and pull group substituents influence the acidity of pristine PHS. Further, we analyzed Gibbs free energy profile for the esterification of oleic acid using pyridinium-based ionic liquids. The key intermediates and transition states involved in the reaction pathway were identified and optimized at the B3LYP/6-31+G(d,p) level. Among the derivatives, PHS-CN showed the lowest energy barrier of 37.89 kcal/mol, indicating better catalytic activity for biodiesel production. Therefore, our findings offer valuable insights into the rational design and structural optimization of other pyridinium-based ionic liquids as efficient catalysts for biodiesel production.