A critical review of pyrolysis-based conversion and catalytic upgrading of waste oils for resource recovery and valorization

Waste oils are not merely hazardous residues but hydrocarbon-rich secondary resources with considerable potential for fuel production and chemical recovery. However, their resource utilization is constrained by strong feedstock heterogeneity, including variations in water content, metals, additives, oxidation products, gums, asphaltenes, free fatty acids, and other contaminants. These compositional differences complicate pretreatment, affect cracking pathways, accelerate coke formation, and reduce catalyst stability. Although many reviews have discussed waste oil regeneration, biodiesel production, or general recycling technologies, the links among waste oil characteristics, pyrolysis conversion behavior, catalytic upgrading, and practical scalability remain insufficiently integrated. This critical review addresses this gap by examining pyrolysis-based conversion and catalytic upgrading as a connected strategy for waste oil resource recovery and valorization. It emphasizes how feedstock properties determine pretreatment needs, reactor selection, product distribution, and upgrading routes, and further evaluates the roles of process intensification, co-pyrolysis, and catalysts in improving fuel quality and conversion efficiency. Key challenges, including feedstock variability, catalyst deactivation, coke deposition, product standardization, and scale-up uncertainty, are also discussed. This review provides a feedstock-to-process-to-product perspective for advancing waste oil pyrolysis toward cleaner, more efficient, and more sustainable resource utilization.