Ultrasound-assisted interesterification of palm oil for biodiesel production using biogenic calcium-based catalyst from cuttlefish bone waste

Interesterification is the latest approach to producing glycerol-free biodiesel. However, conventional heating makes this process ineffective. This study utilized ultrasound-assisted interesterification of palm oil with ethyl acetate, catalyzed by a biogenic calcium-based catalyst derived from cuttlefish bone waste calcined at 800 °C for 4 h. X-ray diffraction (XRD) with Rietveld refinement revealed that the catalyst predominantly consisted of portlandite (77.8 wt%) and calcite (13.2 wt%) due to atmospheric hydration and carbonation, achieving a crystallinity index of 95.78% and a crystallite size of 17.50 nm. Using palm oil and ethyl acetate, the transesterification process was conducted at an ultrasonic frequency of 40 kHz with modifications in reaction duration (3–15 min), temperature (55–65 °C), and molar ratio (1:8–1:12). The optimum biodiesel yield of 94.35% ± 0.51 was obtained at 65 °C, 12 min, and a 1:10 molar ratio, with apparent reaction rate constants of 0.2156 and 0.9298 L mol⁻¹ min⁻¹ at 55 and 65 °C, respectively. The produced biodiesel exhibited excellent fuel properties: density (0.88 g/mL), kinematic viscosity (3.90–4.20 cSt), cetane number (54.70), flash point (145 °C), cloud point (13 °C), pour point (7 °C), and sulfur content (0.0178%), all complying with ASTM D6751 standards. GC–MS analysis confirmed a total fatty acid ethyl ester (FAEE) content of 79.04%, dominated by ethyl palmitate (25.54%) and ethyl oleate (25.87%). These findings demonstrate that combining the nano-crystallite biogenic calcium-based catalyst with ultrasound intensification is a highly efficient and sustainable approach for producing high-quality, glycerol-free biodiesel.