Ruthenium-Catalyzed Hydrogen Generation from Alcohols and Formic Acid, Including Ru-Pincer-Type Complexes

The current feedstock for global energy demands is fossil fuels, which are not renewable and therefore have a limited lifetime as an energy supply. Renewable feedstocks such as biologically derived substrates, formic acid, and alcohols have been proposed as alternative energy sources, which can be used to produce hydrogen gas as one of the most simple chemical energy carriers. The dehydrogenation reaction is thus a necessary key step to establish a potential "hydrogen economy." The following review chapter highlights recent advances in the areas of alcohol and formic acid dehydrogenation focusing on ruthenium-catalyzed processes. Although alcohol dehydrogenation has been studied extensively for its organic synthetic aspects, significantly fewer systems have directed efforts towards efficient hydrogen generation; those examples detailing TON and TOF values for gas evolution are described. Not only are ruthenium complexes bearing simple monodentate ligands successful as catalysts for conversion of challenging alcohols, but also those featuring pincer-type ligands. In addition, various ruthenium-catalyzed formic acid dehydrogenation methods have been developed. These protocols are performed mainly in the presence of amine or base to generate hydrogen but also include the absence of base, use of ionic liquids, continuous flow systems as well as hydrogen storage processes. In all of the abovementioned examples, ruthenium catalysts demonstrate high activity at relatively low loadings as well as long-term stability.