Reversible Hydrogen Storage with Diol/Lactone Pairs: Strain-Induced Improvement of Reaction Thermodynamics

Abstract

Abstract Liquid organic hydrogen carriers (LOHCs) are an attractive option for long-term hydrogen storage. Among different concepts currently discussed, systems based on α,ω-alkanediols and their corresponding lactones stand out because hydrogen release is significantly less endothermic than the conventional hydrogenated aromatic carriers. This study investigates how the chain length and branching of alkyl substituents on the alkyl-substituted 1,5-pentanediols influence the thermodynamic parameter that determines hydrogen uptake and release. While hydrogen release is favored by a low absolute enthalpy of reaction, hydrogen uptake works better in systems with a high absolute enthalpy of reaction. To address this question, the reaction enthalpies, reaction entropies, Gibbs free energies of reaction, and equilibrium constants were evaluated for both liquid and gas phase processes. These values were derived from the enthalpies of formation and absolute entropies determined by a combined methodology that integrates experimental thermochemical data with empirical and quantum-chemical calculations. The results show that increasing alkyl chain length and branching significantly reduce the enthalpy of the lactonization reactions and lead to significantly more favorable conditions for hydrogen release. Overall, the results show that the size and branching of the substituents in the alkanediol/lactone systems have a decisive influence on the thermodynamic parameters of hydrogen release and offer considerable potential for the development of hydrogen carriers with tailor-made properties.