Thermodynamics of reversible hydrogen storage with diol/lactone pairs: what effect does the branching of molecules have?

Liquid organic hydrogen carriers (LOHCs) based on α,ω-alkanediol/lactone systems offer significantly lower dehydrogenation enthalpies than conventional aromatic carriers. This study investigates how the chain length and branching of alkyl substituents on the lactone ring influence the thermodynamic equilibrium of reversible hydrogen storage. Using a hybrid approach combining experimental thermochemistry and high-level quantum-chemical calculations (G3MP2), the reaction enthalpies, entropies, and Gibbs energies for both liquid and gas phases have been determined. Results reveal that extending the chain and increasing the branching of the alkyl group substantially reduce the Gibbs reaction energy. For instance, 2-tert-butyl-γ-butyrolactone exhibits a negative Gibbs reaction energy (−5.3 kJ·mol −1 at 298.15 K), indicating that thermodynamically spontaneous hydrogen release is possible at room temperature, provided suitable catalysts are available. However, while highly branched substituents greatly facilitate hydrogen release, they reduce volumetric hydrogen storage density and make the reverse hydrogenation step more challenging. Ultimately, a structure-property analysis demonstrates that tuning the size and branching of alkyl substituents provides a versatile strategy to balance the competing thermodynamic demands of hydrogen storage and release, enabling the rational design of tailored diol/lactone LOHCs.