Alcohol-Based Liquid Organic Hydrogen Carriers

Hydrogen gas (H2) is a promising clean energy source that does not emit CO2 during combustion. In establishing the H2 supply chain (including its production, storage, transportation, and utilization), the safe, high-density storage and transportation of H2 are bottlenecks. In this regard, liquid organic hydrogen carriers (LOHCs, e.g., methylcyclohexane/toluene) have attracted considerable attention as they can safely and reversibly store H2 through covalent bonding via hydrogenation and dehydrogenation reactions and are highly compatible with the existing gasoline infrastructure. Among LOHCs, this review focuses on alcohols/carbonyl compounds (ketones or esters) as environmentally friendly LOHCs, which can be produced from biomass and can release H2 under mild conditions (e.g., <200 °C) due to their low dehydrogenation enthalpies, and first summarizes alcohols/carbonyl compounds proposed as candidates for LOHCs. Subsequently, dehydrogenation methods for releasing H2 from the alcohols are also described. In addition, electrolytic hydrogenation and alcoholic fermentation with baker’s yeast are described as methods for the hydrogenation of carbonyl compounds, which do not require a supply of H2. These hydrogenation methods, which can store hydrogen directly from sustainable hydrogen sources (water or coenzymes contained in baker's yeast), enable the conventional multi-step process for storing H2 in LOHCs (including the production of pure H2 and hydrogenation) to be accomplished efficiently in a single step. This review outlines methods for the hydrogenation of carbonyl compounds and the dehydrogenation of alcohols and highlights the potential of alcohols/carbonyl compounds as LOHCs for realizing a carbon-neutral and sustainable society.