Development of CaO/Al2O3 bifunctional catalyst for biodiesel production from jatropha oil: optimization and kinetic study
Abstract
Biodiesel production from non-edible Jatropha curcas oil, which have high free fatty acid (FFA) content, presents technical challenges due to soap formation during conventional base-catalyzed transesterification. This often necessitates a costly and time-consuming acid pretreatment. To address this, the study developed a bifunctional heterogeneous calcium-/aluminum oxide (CaO/Al2O3) catalyst synthesized from calcined snail shells (SS) and impregnated with alumina. The catalyst enables direct transesterification of high-FFA jatropha oil, thereby simplifying the process and reducing wastage. Optimization of the transesterification parameters using central composite design (CCD) revealed an optimal biodiesel yield of 93.4% at 64.3°C, 2.65 h, 2.29% catalyst loading and a methanol-to-oil molar ratio of 0.396. Catalyst characterization using X-ray diffraction (XRD) analysis of a raw (uncalcined) sample, showed that the dominant component was aragonite or calcium carbonate (CaCO3) at 86.6%, after 900°C calcination. Based on the XRD trend, a shift from aragonite to CaO (quicklime) as the dominant phase at 85.9% occured. SEM images of CaO before impregnation showed a highly porous structure with fine, loosely aggregated particles. After impregnating the CaO with Al2O3, the structure appeared denser and more compact, with the presence of larger cluster or agglomerates resulting from coating or filling pores by Al2O3. Structural changes in the catalyst due to Al2O3 impregnation are reflected in the Fourier Transform Infrared (FTIR) data, a validation of the formation of a more stable and potentially more active CaO/Al2O3 composite catalyst. Kinetic studies showed the reaction followed second-order kinetics with an activation energy of 37.54 kJ/mol. The catalyst retained over 85% yield after regenerative studies. From the foregone, the potential of CaO/Al2O3 as an efficient, cost-effective, and sustainable catalyst for biodiesel production from high FFA feedstock is described as solid. Graphical Abstract.