Enhancing waste eggshell-derived CaO catalysts for biodiesel production through synergistic oxide modification: a...

Valorizing waste materials for sustainable biodiesel production offers dual benefits of environmental remediation and cost reduction. In this study, calcium oxide (CaO) derived from waste eggshells was modified with metal oxides-SiO₂, TiO₂, and Co₂SiO₄-to overcome limitations of pure CaO catalysts, such as leaching, low surface area, and reduced reusability. The catalysts were synthesized via wet impregnation followed by calcination, and their physicochemical properties were systematically correlated with transesterification performance under identical optimized reaction conditions, enabling direct structure-activity comparison. Among the modified catalysts, TiO₂-modified CaO exhibited the highest FAME yield (89.7%) due to synergistic effects that enhanced basicity (1.103 mmol g^-1) and surface area (121.9 m² g^-1), supported by XRD data of CaTiO₃ phase formation. Detailed kinetic analysis confirmed pseudo-first-order behavior, and equilibrium data for all catalysts collapsed onto a single Hill isotherm, revealing cooperative adsorption effects that appear intrinsic to the transesterification equilibrium rather than catalyst specific. Apparent turnover frequency (TOF) analysis, normalized by basic site concentration, showed broadly comparable per-site activity across the catalyst series, with CaO-SiO₂ giving the highest operational TOF, indicating improved site accessibility rather than intrinsically faster kinetics. The modified catalysts demonstrated minimal deactivation (<2% loss over four cycles), attributed to oxide-induced stabilization of CaO active sites. This work provides new mechanistic insight into cooperative adsorption and site utilization in waste-derived CaO catalysts, advancing the rational design of durable heterogeneous catalysts for biodiesel synthesis.