Recent Advances in Liquid Organic Hydrogen Carriers: An Alcohol-Based Hydrogen Economy

Abstract

Energy storage and the use of abundantly available feedstock without contributing to the carbon footprint are two significant global challenges. In this regard, the development of high-performance, low-cost, sustainable, and environmentally friendly energy storage and production systems is crucial to fulfill the growing energy demands of the current society. The use of hydrogen will diversify energy sources as it significantly reduces greenhouse gas emissions and environmental pollution during energy conversion. Although the hydrogen economy is quite beneficial, hydrogen storage is still very challenging, and the existing methods suffer from a lot of problems and drawbacks. The conventional liquefaction and compression hydrogen storage technologies are associated with several challenges, including low storage density, boil-off losses, relatively high costs, and safety and transportation concerns. In recent years, liquid organic hydrogen carrier (LOHC) systems have attained a lot of importance as a substitute for the traditional storage methods. Hydrogen storage and transport using LOHCs are based on two-step cycles, such as (i) loading/storage of hydrogen by catalytic hydrogenation of H2-lean compounds and (ii) unloading/releasing hydrogen by dehydrogenating the resulting H2-rich liquids. Since alcohols are widely accessible from various industrial processes or even from biomass-derived precursors, the catalytic acceptorless dehydrogenation of alcohols is an attractive approach for future hydrogen storage applications. Hence, the catalytic dehydrogenation-hydrogenation of alcohols can be used for the development of alcohol-based LOHC systems which are economical, safe, and easy to handle. Further, they are similar to crude oils under ambient conditions and thus are suitable for use in the current energy infrastructure. This Review covers several essential aspects of these developing efficient and abundantly available LOHC systems for efficient hydrogen storage and transport applications. Additionally, reversible LOHC systems based on the catalytic dehydrogenation-hydrogenation of alcohols and their corresponding carbonyl compounds have been discussed.

Homogeneous catalysis with polyhydride complexes

“…62–64 As a consequence, the metal-promoted acceptorless dehydrogenation of cyclic amines are receiving noticeable attention. 65 Other liquid organic hydrogen carriers of interest are alcohols 66 and formic acid. 67…”

Visible-Light-Promoted Iridium(III)-Catalyzed Acceptorless Dehydrogenation of N-Heterocycles at Room Temperature

“…The catalytic (de)­hydrogenation of N-heterocycles involving H 2 is a fundamental organic transformation being present in the manufacturing of multistep synthesis of drugs and biologically relevant molecules. − Notably, the use of N-heterocycles has recently gained considerable attention for storage of hydrogen as liquid organic hydrogen carrier (LOHC) which is based on catalytic (de)­hydrogenations of organic substrates. − Still, development of efficient catalysts for hydrogen-storage has proven challenging, being subjected to fulfill a series of technical and practical prerequisites to become a disruptive technology. − One of the most constraining factors is the temperature at which the hydrogen is recovered from the organic carrier (90–300 °C). , Thermodynamically, dehydrogenation of organic molecules to release H 2 is an uphill process requiring high temperatures. , To overcome this energetic barrier, the use of visible light (400–700 nm) has emerged as a valuable and cleaner alternative. − Specially, because photon energies exhibit values superior or within the range of those energy barriers found for many catalyzed reactions thermally activated. Therefore, many efforts have been directed to provide viable alternatives.…”

Water as a solvent: transition metal catalyzed dehydrogenation of alcohols going green

“…Over the past few decades, considerable progress has been realized in transition-metal catalyzed dehydrogenation of alcohols and related transformations. Several review articles have also emerged in the literature based on C–C and C–N bond formation, 59,60 acceptorless dehydrogenative coupling, 61 and N-heterocycle synthesis, 62,63 and the use of 3d metal-based catalysts, 57,64–66 cooperative ligands, 67 heterogeneous catalysts, 68 tandem catalysis, 69 liquid organic hydrogen carriers, 70 etc . However, most of the alcohol dehydrogenation related transformations have been extensively explored in organic solvents or using a mixture of water and organic solvents.…”