The Crude Glycerol Challenge: Purification Technologies and Possible Valorization Pathways in the Biodiesel Industry.

The rapid expansion of the biodiesel industry has led to a significant surplus of crude glycerol, creating both a challenge and an opportunity for its sustainable valorization. The aim of this review is to provide a comprehensive overview of glycerol upgrading strategies that integrate upstream purification methods with downstream catalytic conversion pathways. Conventional and emerging purification technologies are critically discussed, with particular attention to their efficiency, cost, and compatibility with catalytic processes. Major catalytic routes such as acetalization, oligomerization, carbonation, hydrogenolysis, oxidation, etherification, esterification and steam reforming are systematically analyzed in terms of reaction mechanisms, catalyst design, and process conditions. Although high conversions and selectivities are often achieved under optimized conditions, key limitations such as catalyst deactivation, equilibrium constraints, feedstock impurities, and scalability issues remain significant barriers to industrial implementation. The review highlights the strong interdependence between glycerol purity, catalytic performance, and process design, emphasizing the need for integrated approaches. Recent advances in multifunctional catalysts, reactor engineering, and process intensification are discussed as promising strategies to overcome current challenges. Overall, this work provides a critical perspective on the state of the art and identifies future directions toward the development of efficient, scalable, and sustainable glycerol-based biorefinery processes.