Stability of solid rhenium catalysts for liquid-phase biomass valorization–various facets of catalyst deactivation and rhenium leaching

Rhenium is a versatile element and increasingly used in solid catalysts for the conversion of biomass, where it can fulfill different roles in providing or improving catalytic activity. On the other hand, this also makes Re-based catalysts susceptible to various types of catalyst deactivation. Deactivation mechanisms, detection methods, and coping strategies are discussed for each type of deactivation using the collected literature on Re-containing catalysts for biomass utilization in liquid phase. Particular focus is placed on the correlation between catalyst deactivation and Re leaching, a commonly observed problem when using Re-containing catalyst in liquid-phase reactions that can lead to severe and irreversible loss of catalytic activity. Material properties, reaction conditions, and other factors influencing Re leaching are systematically assessed and leaching mechanisms discussed, which also opens possibilities how the problem can be mitigated. In particular, the role of the Re oxidation state is identified as a key material property influencing Re leaching and a variety of processes and parameters that alter the Re oxidation state during the lifetime of the catalyst are analyzed. Moreover, insights into the intricate procedure of detecting Re leaching and identifying the correlation between leaching and catalyst deactivation are presented. Finally, strategies to purposefully use Re leaching are shown.