Hydration-dehydration modified chicken eggshell and sardina pilchardus scale catalyst for high-yield biodiesel...

The rising global need for sustainable energy has driven the exploration of efficient and cost-effective pathways for producing biodiesel from renewable resources. This work presents the development of an enhanced solid catalyst (NES9-3) derived from a synergistic blend of eggshells and sardine scales (ES). Unlike conventional eggshell-derived calcium oxide (CaO) catalysts, which often suffer from rapid deactivation due to leaching, the incorporation of sardine scale-derived hydroxyapatite (HAP) introduces a synergistic effect, providing improved structural stability and partial resistance to catalyst deactivation. The 1:1 ES mixture was first thermally treated at 900 °C for 3 h to obtain the ES9-3 catalyst, which served as a reference catalyst. A subsequent hydration-dehydration-recalcination process was employed to produce the NES9-3 catalyst, aiming to generate a more porous structure with increased surface area and enhanced catalytic activity. Structural characterization using thermogravimetric analysis (TGA), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), and Brunauer-Emmett-Teller (BET) analysis confirmed significant phase transformations, enhanced textural properties, and reduced crystallite size following the modification treatment. The transesterification of waste frying oil (WFO) was optimized using response surface methodology. Under the optimized conditions (2.97 wt% catalyst, 12.35:1 methanol-to-oil molar ratio, and 3.02 h reaction time), the NES9-3 catalyst achieved a biodiesel yield of 93.72% with a conversion efficiency of 97.29%, as confirmed by ¹H nuclear magnetic resonance (¹H NMR). In contrast, ES9-3 exhibited a lower yield of 85.8% under identical conditions and required a longer reaction time and higher methanol consumption to achieve an 89% yield. Gas chromatography-mass spectrometry (GC-MS) analysis confirmed the formation of fatty acid methyl esters (FAMEs), and the resulting biodiesel complied with ASTM D6751 and EN 14214 fuel standards. This study demonstrates that coupling CaO with hydroxyapatite, followed by hydration-dehydration modification, provides a strategy to partially mitigate catalyst deactivation and improve catalytic efficiency, offering a sustainable and economically viable approach for biodiesel production within a circular bioeconomy framework.