Sustainable transformation of waste cooking oil: A global review of valorization pathways and future directions

The escalating global energy demand and persistent environmental concerns necessitate a paradigm shift towards sustainable resource management, with waste cooking oil (WCO) emerging as a promising and abundant feedstock. This comprehensive review systematically analyzes recent advancements in WCO valorization, focusing on literature published from 2019 to 2025. Significant progress in biodiesel production is highlighted, where innovative bifunctional and nanocatalysts have demonstrated conversion efficiencies typically ranging from 87 % to 96 %, often achieving high yields in significantly reduced reaction times, some within 5 min through process intensification. Beyond traditional fuels, the conversion of WCO into diverse hydrocarbon fuels is examined, noting deoxygenation degrees reaching up to 99 % and high selectivity (approx. 70 %) for C11–C17 fuel fractions suitable for bio-jet applications. The practical viability of WCO-derived products is supported by engine performance assessments, which show smoke opacity reductions of up to 50 % and improved thermal efficiency, despite marginal increases in specific fuel consumption. This review also evaluates a wide array of non-fuel applications, including bio-asphalt modifiers that restore aged bitumen properties to near-virgin levels and the production of biopolymers with 19–30 % polyhydroxybutyrate content. Despite these achievements, the transition from laboratory to industrial scale demands intensified research. Key future directions are identified, including overcoming feedstock variability, developing robust next-generation catalysts, and leveraging machine learning for process optimization. This review underscores WCO's immense potential to significantly contribute to environmental protection, foster economic viability, and accelerate the transition towards a resource-efficient, low-carbon circular bioeconomy.