Development of a CaO-Rich Catalyst from Three Waste-Shell Sources for Transesterification of Gliricidia sepium Seed Oil

Calcium-rich biological wastes offer potential low-cost precursors for heterogeneous catalysts used in biodiesel processing. This study prepared a CaO-rich catalyst from snail shell, eggshell and periwinkle shell wastes and applied it to the transesterification of Gliricidia sepium seed oil. Shell proportions were evaluated using a constrained three-component mixture design, while the calcined materials were characterised by X-ray fluorescence and qualitative X-ray diffraction. Acid esterification reduced the free fatty acid content of the oil from 10.01% to 2.19%. Transesterification was subsequently investigated using a 25-run experimental matrix comprising methanol-to-oil molar ratio, catalyst loading, reaction temperature and reaction time. The mean oil yield was 34.50 ± 10.85%. The 20:20:60 snail shell:eggshell:periwinkle shell formulation produced the highest XRF-measured CaO content of 89.672%. The highest recovered product yield was 85.00%, obtained at a 12:1 methanol-to-oil molar ratio, 3 wt% catalyst loading, 50 °C and 30 min. GC–MS qualitatively identified methyl linoleate in the recovered product. The findings demonstrate that three waste-shell sources can be combined to produce a CaO-rich material for oil transesterification. However, the gravimetric recovery value does not represent quantitative FAME content or triglyceride conversion, which require validated chromatographic determination.