Tailoring shell-derived CaO catalysts via Ni/Fe oxide incorporation and controlled calcination for efficient biodiesel production from jatropha oil

In this work, shell-derived CaO catalysts were engineered through Ni and Fe oxide incorporation and controlled calcination to enhance biodiesel production from Jatropha curcas oil (JCO). The study systematically compares CaO catalysts derived from four biogenic CaCO₃ sources (eggshell, oyster shell, cockle clam shell, and waste dead coral) under consistent preparation and reaction conditions. The CaO-series catalysts were characterized using comprehensive physicochemical analysis. To reduce the free fatty acid (FFA) content, a methanol extraction pretreatment was applied as a safer alternative to conventional acid esterification. Metal modification improved catalytic performance across all supports compared with pristine CaO. Among the investigated materials, Fe/CaO derived from cockle clam shell achieved the highest FAME yield of 92% under mild conditions (65 °C, methanol-to-oil molar ratio of 20:1, 10 wt% catalyst). Calcination temperature significantly influenced activity: moderate calcination (500 °C) provided an ideal catalytic performance, while higher temperatures (≥750 °C) led to reduced activity, consistent with sintering and structural changes. Reusability tests showed relatively stable performance for Ni/CaO-OS over three cycles, with deactivation mainly associated with surface carbonation and partial agglomeration rather than metal loss. Overall, this work demonstrates that combining biogenic CaO source selection, transition metal modification, controlled calcination, and a simplified pretreatment approach provides an effective and more sustainable strategy for heterogeneous biodiesel production.