Turning Waste Cooking Oil into Sustainable Biodiesel: A Review of Waste-Derived Heterogeneous Catalysts, Life-Cycle Performance, and Circular Bioeconomy

Waste cooking oil (WCO) represents both a growing environmental burden and a promising low-cost feedstock for sustainable biodiesel production. This review critically examines recent advances in the conversion of WCO into biodiesel, with particular emphasis on heterogeneous catalysts derived from waste resources and their integration within a circular bioeconomy. Waste-derived catalysts obtained from eggshells, snail shells, spent coffee grounds, fish and animal bones, chicken waste, and mineral residues such as marble are comparatively assessed in terms of biodiesel yield, reaction severity, recyclability, feedstock tolerance, and sustainability. Several Ca-rich waste-derived catalysts achieve biodiesel yields approaching 90–98%, demonstrating their potential to replace conventional homogeneous catalysts while simultaneously valorizing secondary waste streams. However, the analysis shows that maximum biodiesel yield alone is insufficient for identifying the most sustainable catalyst, because high-temperature calcination, alcohol demand, catalyst deactivation, and limited recyclability can offset apparent performance advantages. Particular attention is therefore given to catalyst recovery and reuse, free-fatty-acid-dependent process selection, and the integration of esterification and transesterification routes for variable WCO feedstocks. The review further evaluates biodiesel performance and emissions and critically examines life-cycle assessment (LCA) and techno-economic assessment (TEA). Available LCA evidence indicates that WCO-derived biodiesel can exhibit substantially lower carbon and cumulative energy burdens than first-generation biodiesel, although outcomes remain strongly dependent on system boundaries, allocation procedures, energy sources, catalyst preparation, and avoided-waste credits. By integrating feedstock pretreatment, waste-derived catalyst selection, biodiesel conversion, engine performance, LCA, and TEA within a unified framework, this review identifies the major research gaps and provides a pathway toward scalable, low-carbon, and economically viable WCO valorization.