Coconut-Shell Activated Carbon-Supported CaO for Biodiesel Production from Waste Cooking Oil: Catalyst Preparation and Process-Parameter Screening

Valorising waste cooking oil and agricultural residues provides a practical route to integrate low-cost biodiesel production with the circular use of biomass resources. In this study, we produced activated carbon (AC) from waste coconut shell and used it as a support for a calcium-based heterogeneous catalyst prepared by wet impregnation with calcium nitrate, followed by thermal treatment. We evaluated the resulting CaO/AC material for transesterification of pretreated waste cooking oil with methanol at 60 °C. A one-factor-at-a-time screening examined nominal oil:methanol ratios of 1:4, 1:8, and 1:16; catalyst loadings of 0.05, 0.10, and 0.20 g; and reaction times of 30, 60, and 120 min. We used FTIR and atomic absorption spectroscopy (AAS) to characterise the prepared catalyst. FTIR bands were consistent with oxygen-containing surface groups and calcium-containing species, while AAS confirmed calcium in the supported material. The highest gravimetric yield of recovered biodiesel-rich product was approximately 90%, obtained at an oil: methanol ratio of 1:16, a catalyst loading of 0.10 g, and a reaction time of 60 min. Under the selected conditions, CaO/AC substantially outperformed activated carbon alone (approximately 90% versus 32%). These findings support the feasibility of coconut-shell-derived activated carbon as a low-cost support for calcium-based transesterification catalysts. Because the source dataset lacked replicate variability, XRD/SEM-EDX confirmation, FAME quantification, and standardised fuel-property data, the results are interpreted as process screening rather than definitive statistical optimisation or proof of fuel-standard compliance.