Beyond waste: sardine scales and eggshell catalyst for biodiesel production from waste frying oil

The urgent need for sustainable fuel alternatives is highlighted by the current global energy crisis, driven by increasing demand, climate change, and resource depletion. Biodiesel represents a promising option that combines effectiveness, sustainability, economic feasibility, and environmental benefits. This study explores an innovative waste-to-material approach for biodiesel production by developing catalysts from waste materials, specifically sardine scales and eggshells, while reusing waste frying oil (WFO) as a feedstock. The structural and functional characteristics of the catalyst were thoroughly investigated. X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FTIR) analyses identified three distinct active phases: hydroxyapatite, β-tricalcium phosphate, and CaO. Thermal stability was assessed via thermogravimetric and differential thermal analysis (TGA-DTA), while surface area and porosity were determined using the Brunauer-Emmett-Teller (BET) method, and morphology was examined by scanning electron microscopy (SEM). The calcination of a 1:1 mixture of sardine scales and eggshells at 900 °C for 3 h resulted in the highest catalyst activity. The transesterification process was optimized, achieving a maximum biodiesel yield of 89% under the following conditions: a 15:1 methanol-to-WFO ratio, a 2.5 wt% catalyst loading, and a reaction time of 4.5 h at 65 °C. The composition and structure of the resulting biodiesel were confirmed by gas chromatography-mass spectrometry (GC-MS), as well as ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. ¹H NMR analysis confirmed the successful conversion of triglycerides into fatty acid methyl esters (FAMEs), with a conversion of 92.5%. Moreover, the catalyst exhibited sustained activity even after 60 days of moisture exposure, highlighting its stability and durability. Notably, the catalyst maintained its performance over five consecutive reaction cycles after reactivation and exhibited an activation energy of 51.59 kJ mol^-1, with a pre-exponential factor of 6.4 × 10⁵ min^-1. Compliance with ASTM D6751, EN 14214, and ASTM D7467 standards confirms that the synthesized biodiesel and its blends meet industry requirements. This study demonstrates the potential of waste-derived materials as effective heterogeneous catalysts, thereby promoting sustainability and waste valorization in energy production.