Activity–selectivity engineering of waste-derived CaO catalysts via KOH and ZnO surface modification for coconut biodiesel production

Abstract

The development of sustainable heterogeneous catalysts for biodiesel production requires not only high conversion efficiency, but also precise control over catalytic activity and product selectivity. However, the influence of surface modification on the activity–selectivity balance of waste-derived CaO catalysts remains poorly understood. In this study, eggshell-derived CaO catalysts were chemically modified with KOH and ZnO in order to investigate how surface engineering affects catalytic behaviour during coconut oil transesterification. The catalysts were structurally and compositionally characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray fluorescence (XRF), energy-dispersive spectroscopy (EDS), and thermogravimetric analysis (TG/DTG), confirming the successful conversion of CaCO₃ into catalytically active CaO, incorporation of potassium- and zinc-containing phases, and significant changes in surface morphology and thermal stability after modification. Under fixed reaction conditions (methanol-to-oil molar ratio of 19:1, 1.5 wt% catalyst, 64°C, 120 min), a clear activity–selectivity trade-off was identified. The KOH-modified catalyst exhibited the highest biodiesel yield (86.9%) and the highest pseudo-first-order kinetic constant (0.0223 min⁻¹), suggesting enhanced catalytic activity and predominantly reaction-controlled behaviour. In contrast, the ZnO-modified catalyst achieved the highest methyl ester content (98.1%) and showed better agreement with pseudo-second-order kinetics, suggesting a greater contribution of adsorption-mediated surface interactions. Biodiesel characterisation by GC-FID, FTIR, TG/DTG, viscosity, and density analyses confirmed the formation of high-quality fatty acid methyl esters compliant with fuel quality standards. Taken together, these findings indicate that chemical modification of waste-derived CaO catalysts enables the rational tuning of catalytic activity and ester selectivity, providing mechanistic insights for the rational design of waste-derived heterogeneous catalysts for sustainable biofuel production within circular economy systems. Graphical Abstract

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