Transesterification of Waste Cooking Oil Using Calcium Oxide Derived from Eggshells: Response Surface Methodology

The growing need for sustainable energy has elevated biodiesel as a promising substitute for conventional fossil diesel. This study optimized biodiesel production from waste cooking oil (WCO) through transesterification using calcium oxide (CaO) derived from waste chicken eggshells as a low-cost heterogeneous base catalyst. The catalyst was prepared by calcining eggshells at 900°C for 3 hours and characterized by Fourier Transform Infrared Spectroscopy (FTIR) and Scanning Electron Microscopy (SEM). Response Surface Methodology (RSM) combined with Central Composite Design (CCD) was utilized to optimize two critical variables: methanol-to-oil molar ratio (6:1–10:1) and catalyst loading (1.1–1.4 wt%), while keeping reaction temperature at 60°C and time at 60 min. A quadratic model was developed and found statistically significant (F-value = 6.05, p = 0.0176) with R² = 0.8121. Catalyst loading showed the strongest positive linear effect, followed by the methanol-to-oil ratio. The optimal conditions predicted by the model (methanol-to-oil ratio of 9.5:1 and catalyst loading of 1.2 wt%) yielded 89.8% biodiesel, which was experimentally validated at 88.9 ± 0.8%. The resulting biodiesel satisfied key ASTM specifications. This research highlights the viability of eggshell-derived CaO as an eco-friendly catalyst and demonstrates the effectiveness of CCD-RSM for process optimization in waste-to-energy conversion.