Recent advances in waste-derived CaO/SiO2 heterogeneous catalysts for sustainable biodiesel production: A critical review

Homogeneous catalysts in biodiesel production face persistent limitations, including inefficient recovery, hazardous by-product generation, and poor reusability. Although heterogeneous catalysts offer a more sustainable alternative, Ca 2+ leaching, progressive loss of activity, and product contamination remain critical constraints. This review critically evaluates waste-derived CaO/SiO 2 heterogeneous catalysts for biodiesel synthesis, encompassing feedstock selection, synthesis methodologies, physicochemical characterization, catalytic performance, leaching behavior, and recyclability. Silica is highlighted as an effective support matrix that enhances CaO dispersion, suppresses Ca 2+ leaching, and sustains catalytic activity over repeated cycles. The dual valorization of waste as precursors for both CaO and SiO 2 is further discussed as a viable circular-economy strategy for cost-effective and sustainable biodiesel production. Reviewed literature demonstrates that waste-derived CaO/SiO 2 catalysts achieve biodiesel yields of up to 100% under mild conditions: temperatures of 55-70 °C, methanol-to-oil molar ratios of 6:1-25:1, and catalyst loadings of 1-10 wt%, while sustaining yields above 90% across eight consecutive cycles. Silica incorporation substantially reduced Ca 2+ leaching from 25.3 ppm in unsupported CaO to 3.4 ppm, declining further to 0.5-0.6 ppm by the eighth cycle. Among the synthesis routes evaluated, wet impregnation consistently delivered superior biodiesel conversion, active-site distribution, and long-term stability. Strong Ca–O–Si interfacial interactions, particularly the formation of CaSiO 3 and Ca 2 SiO 4 phases, are identified as key mechanisms governing leaching resistance and thermal stability. Future research directions are outlined to advance the industrial scalability of waste-derived CaO/SiO 2 catalysts for sustainable biodiesel production.