DFT Study of the Possible Reaction Path for Radical Promoted Esterification Mechanism of Free Fatty Acids of Soapnut Oil for the Production of Biodiesel

Esterification of higher free fatty acids content by using a photo-catalyst has recently been proved as the most efficient method for the pretreatment of non-edible oil to synthesize biodiesel. Methods: In the current study, mechanistic details of photo-catalyzed esterification reaction for four different fatty acids through density functional theory (DFT) calculations are explored and compared with un-catalyzed esterification reaction. Results: Revealed that the presence of photo-catalyst lowers the activation barrier and the structure of fatty acid has no significant effect on its reactivity. Thermodynamic data also revealed that the presence of photo-catalyst lowered the activation energy from 51.67 kcal/mol to 0.7495 kcal/mol. Furthermore, Gibbs free energy changes (Δ_rG_m Ø) and molar enthalpy changes (Δ_rH_m Ø) of the photo-catalyzed esterification reaction are negative, indicating that it is a spontaneous exothermic reaction. On the other hand, free fatty acids esterification in the absence of a catalyst is a kinetically unfavorable process with positive values of Δ_rG_m Ø and Δ_rH_m Ø. Conclusion: Our findings theoretically clarify the mechanism of the photo-catalyzed esterification reaction of FFA present in non-edible oil, which facilitates the process of biodiesel production.