Enhancing Alkaline Biodiesel Transesterification Using Protic Ionic Liquid [DETA][Hx] as a Low-Concentration Additive

Abstract Protic ionic liquids (PILs) have emerged as cost-effective alternatives to aprotic ionic liquids in biodiesel production. In biphasic transesterification systems, the intrinsic immiscibility between methanol and oil phases represents an important limitation affecting reaction performance. In this context, the present study investigates the influence of diethylenetriammonium hexanoate ([DETA][Hx]) as a low-concentration physicochemical additive in conventional KOH-catalyzed soybean oil transesterification. A factorial experimental design was employed to evaluate the effects of ionic liquid concentration (0–3% w/w) and reaction time (30–60 min) on biodiesel yield. The results showed that low concentrations of [DETA][Hx] significantly affected process performance, increasing biodiesel yield by 5.1% and reaching a gravimetric biodiesel yield of 97.1%. Physicochemical analyses revealed changes in surface tension behavior in systems containing [DETA][Hx], which, together with the observed yield response, are consistent with a possible influence of the ionic liquid on physicochemical and interfacial interactions within the reaction medium. FT-IR analyses suggested that the ionic liquid structure remained preserved under the evaluated conditions, supporting the interpretation that [DETA][Hx] primarily influences the reaction medium through physicochemical interactions rather than through direct chemical participation in the transesterification reaction. Although direct transport-related measurements were beyond the scope of the present study, the observed behavior suggests that low concentrations of protic ionic liquids may influence phase interactions and reaction performance in alkaline biodiesel transesterification systems. Overall, the present work provides experimental evidence supporting the investigation of protic ionic liquids as low-concentration physicochemical additives for biodiesel production.