Waste-derived nanocatalysts for high-performance and sustainable biodiesel production: Advances, challenges, and future directions

Biodiesel production from low-cost feedstocks remains constrained by catalyst costs, sensitivity to free fatty acids, soap formation, separation difficulties, and inconsistent catalyst reusability. This review critically evaluates waste-derived nanocatalysts for converting waste cooking oil (WCO) and non-edible seed oils into biodiesel, with emphasis on calcium-rich wastes, biomass ash, biochar-supported catalysts, green-synthesized metal oxides, magnetic composites, and bifunctional acid-base systems. Reported catalytic activity generally follows the order: engineered CaO-rich shell/eggshell catalysts and mixed CaO composites > green-synthesized transition-metal oxides > biomass ash and biochar-supported systems, although performance strongly depends on feedstock acidity, catalyst basicity, crystallite size, pore accessibility, leaching resistance, and reactor mode. Recent studies on non-edible oils, including Cannabis sativa, Diospyros malabarica, Carthamus lanatus, Ipomoea carnea, Quercus incana, Mallotus philippensis, Caesalpinia crista, and Phyllanthus maderaspatensis, show biodiesel yields of approximately 94–98% under optimized conditions using WO₃, CdO₂, Bi₂O₃, Al₂O₃, Cr₂O₃, and related green nanocatalysts. The review further integrates bibliometric trends, machine-learning-assisted optimization, kinetic descriptors, activation-energy analysis, cost estimation, catalyst deactivation, barriers to continuous-flow scale-up, and end-of-life catalyst management. The analysis shows that future progress depends less on reporting the maximum single-cycle yield and more on standardized benchmarking of catalytic activity, free fatty acid (FFA) tolerance, reusability, regeneration, pressure-drop behavior, production cost, and life-cycle sustainability.