Rhenium - a tuneable player in tailored hydrogenation catalysis
Although rhenium may not be the most common choice of active species in catalysis, it has been reported as a highly active and selective catalyst over a wide range of reactions. Its applications include hydrogenation reactions of great relevance in the field of renewable materials and bio-derived platform molecules, such as valorisation of lignin, CO 2 , and carboxylic acids. Different from several transition metals, rhenium presents oxidation numbers varying from -3 to +7. Such diversity in the coordination chemistry of rhenium is reflected in the variety of known rhenium compounds since this metal can compose stable structures as ligand-bridged multinuclear and organometallic compounds, as well as inorganic oxides, metal-organic frameworks, and clusters. The exceptional flexibility in rhenium speciation yields numerous selective catalysts; however, it also makes the characterization of rhenium catalysts challenging, and its influence on the catalytic activity is not trivial. This review will outline the most established rhenium-based materials used in hydrogenation catalysis and shed some light on the relation of rhenium species to selectivity based on advanced characterization techniques. Finally, our perspectives on the use of rhenium catalysts to produce value-added products will be given.