The increasing demand for sustainable and cost-effective fuels has spurred research into biodiesel production using waste resources and heterogeneous catalysts. This study explores the synthesis of activated carbon (AC) from almond shells via phosphoric acid activation, optimizing the process to achieve a high surface area. The use of agricultural waste as a precursor provides a low-cost and sustainable support material for catalyst development. The resulting AC was then used as a support for calcium oxide (CaO), magnesium oxide (MgO), and kettle limescale (KL), forming efficient heterogeneous catalysts for the transesterification of waste cooking oil into biodiesel. The catalysts were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray fluorescence (XRF), and Brunauer-Emmett-Teller (BET) surface area analysis. A Box-Behnken design (BBD) was employed to optimize the effects of reaction time, catalyst concentration, and oil-to-methanol ratio on biodiesel purity, expressed as fatty acid methyl ester (FAME) content. The prepared catalysts exhibited high catalytic activity under the optimized conditions. Optimal conditions for CaO/AC, MgO/AC, and KL/AC catalysts yielded FAME contents of 99.4%, 94.3%, and 98.9%, respectively. Reusability studies demonstrated that the catalysts maintained significant activity after five cycles, with FAME contents of 83.4%, 80.8%, and 83.5% for CaO/AC, MgO/AC, and KL/AC, respectively, indicating good catalytic stability and practical potential for sustainable biodiesel production.
