Simultaneous esterification and transesterification reactions of acidic palm oil catalyzed by zinc stearate

The production of fatty acid ethyl esters (FAEE) from high acid value palm oil (APO) was investigated. Reactions were carried out in a single step (simultaneous esterification and transesterification—SET) using zinc stearate as the catalyst precursor. Zinc stearate was characterized for its structural patterns and functional groups before and after SET using X-ray diffraction, Fourier-transform infrared spectroscopy, thermogravimetric analysis (TG), differential scanning calorimetry (DSC), scanning electron microscopy (SEM), and gas chromatography (GC). The effects of SET parameters such as the ethanol-to-APO molar ratio, the reaction temperature, and the amount of catalyst were investigated and pre-optimized using a Central Composite Design (CCD). The presence of the catalyst proved to be a significant parameter, contributing 90.4% of the FAEE content compared to 65.9% of the FAEE content under the same reaction conditions in its absence. The best APO-to-FAEE yield of 88.6% was achieved using an ethanol-to-APO molar ratio of 12:1 and 10 wt% catalyst at 180 °C for 120 min. Under these conditions, the predicted and experimental FAEE yields were 88.5% and 88.6%, respectively. The R-squared value of the CCD mathematical model was 0.9989, indicating its high predictability and goodness of fit. Zinc stearate maintained its catalytic activity in the SET of APO for five consecutive reuse cycles, but changes in the catalyst chemical composition were observed mainly due to the conversion of zinc stearate to zinc palmitate. This new lamellar structure was maintained after being formed without changes in catalytic performance.