Optimization of biodiesel production from Croton Macrostachyus seed oil with calcium oxide (CaO) catalyst and Characterization: Potential assessment of seed and kernel

• Croton macrostachyus seed (CMS) oil was extracted. Oil yield was obtained 40.8% from the seed and 50.3% from the kernel. • Eggshell-derived CaO was prepared as heterogeneous catalyst. • The optimization of the transesterification process was performed using response surface methodology (RSM) based on a Box-Behnken design (BBD). • Analysis of variance (ANOVA) showed that the coefficients of determination (R 2 and R 2 adj) were 99.42% and 98.38%, respectively, indicating a strong correlation between experimental and predicted values. • The optimum conditions identified for maximum yield of CMS biodiesel (98.314 %) were a methanol to oil molar ratio of 8.44:1, a catalyst concentration of 2.78 %, and a reaction time of 108 min while the reaction temperature (60 °C) and stirring speed (450 rpm) were kept constant.. • The oxidation stability of the CMS biodiesel was obtained an induction time of 2.3 h but was enhanced to 16.17 h due to the addition of TBHQ antioxidant. This study investigates the use of Croton macrostachyus (CMS), a widely available, fast-growing, non-edible plant common in Ethiopia’s secondary forests and less productive lands, as a biodiesel feedstock. The research compared oil yields from the seed and kernel, assessed the oxidation stability of the resulting biodiesel, and conducted transesterification using a novel combination of a CaO catalyst with methanol, which had not been explored in previous studies. Oil yield was obtained 40.8 % from the seed and 50.3 % from the kernel. Calcium oxide (CaO) was derived from eggshells through a calcination process and characterized using BET, SEM-EDX, and FTIR techniques to assess its suitability as a heterogeneous catalyst for biodiesel production. The Box-Behnken design (BBD) based on the response surface methodology (RSM) was used to optimize the reaction parameters, including reaction time (60–120 min), methanol to oil molar ratio (4:1 – 12:1), and catalyst concentration (1 % – 5 %), while the reaction temperature (60 °C) and stirring speed (450 rpm) were kept constant. Analysis of variance (ANOVA) showed that the coefficients of determination (R 2 and R 2 adj) were 99.42 % and 98.38 %, respectively, indicating a strong correlation between experimental and predicted values. The optimum conditions identified for maximum yield of CMS biodiesel (98.314 %) were a methanol to oil molar ratio of 8.44:1, a catalyst concentration of 2.78 %, and a reaction time of 108 min. The oxidation stability of the CMS biodiesel was obtained an induction time of 2.3 h but was enhanced to 16.17 h due to the addition of TBHQ antioxidant. All physiochemical properties were compared with ASTM D6751 and EN 14214 biodiesel standards, demonstrating that the biodiesel produced is suitable for internal combustion engine applications.