Thermodynamics of reversible hydrogen storage with LOHC: a real game changer with diol/lactone pairs

• Release of hydrogen through the cyclization of diols is energetically more favourable. • Thermochemical properties of LOHC pairs diols/lactone were evaluated. • Thermodynamics of system 1,4-butanediol/γ-butyrolactone was studied. • Thermodynamics of system 1,5-pentanediol/δ-valerolactone was studied. • Thermodynamics of system 1,6-hexanediol/ε-caprolactone was studied. To make a hydrogen economy feasible, it is crucial to identify efficient hydrogen storage technologies. While storing hydrogen by hydrogenation of aromatic and dehydrogenation of alicyclic hydrocarbons is technically feasible, it is associated with a high energy demand for hydrogen release that limits the energetic efficiency of the storage cycle if no suitable source of waste heat is available. This study investigates whether the release of hydrogen through the cyclization of diols forming lactones is energetically more favorable. To this end, equilibrium constants for the dehydrogenation reactions at 298.15 K, 400 K, and 500 K were determined using a combination of combustion calorimetry, vapor pressure measurements, and quantum chemical calculations. The results reveal that, compared to aromatic hydrocarbons, the equilibrium position for the dehydrogenation of diols is significantly more favorable for hydrogen release, and the reaction enthalpies are markedly lower. The most advantageous properties were observed for the 1,4-butanediol/γ-butyrolactone system. The reaction enthalpy per mole of hydrogen for the dehydrogenation at 298.15 K is exceptionally low, at 42.5 kJ·mol⁻ 1 . High hydrogen yields can already be expected at temperatures below 400 K at ambient pressure. Under such conditions, hydrogen release from 1,4-butanediol can be carried out with low energy input for heating the hydrogen carrier and for providing the enthalpy of reaction. This reduced thermal demand contributes to a higher prospective energy efficiency of the respective hydrogen storage cycle, which may represent in some application scenarios a significant economic advantage. Thermodynamics of reversible hydrogen storage with LOHC systems: 1,4-butanediol/γ-butyrolactone, 1,5-pentanediol/δ-valerolactone and 1,6-hexanediol/ε-caprolactone was studied in this work.