Reversible storage and release of hydrogen with LOHC: Evaluation of thermochemical data for methyl-quinolines with complementary experimental and computational methods

The liquid organic hydrogen carriers (LOHC) are aromatic molecules, which can be considered as an attractive option for the storage and transport of hydrogen. A considerable amount of hydrogen can be loaded and unloaded with a reversible chemical reaction. Methyl-substituted quinolines are available from petroleum, coal processing, wood preservation or they can be synthesized from aniline. Quinolines can be considered as potential LOHC systems, provided they have favorable thermodynamic properties, which were the focus of this current study. The absolute vapor pressures of methyl-quinolines were measured using the transpiration method. The standard molar enthalpies of vaporization of methyl-quinolines were derived from the vapor pressure temperature dependences. Thermodynamic data on vaporization and formation enthalpies available in the literature were collected, evaluated, and combined with own experimental results. The theoretical standard molar gas-phase enthalpies of formation of methyl-quinolines, calculated using the quantum chemical G3MP2 and G4 methods, agreed well with the evaluated experimental data. The hydrogenation/dehydrogenation reaction enthalpies of methyl-quinolines were calculated and compared with the data for other potential liquid organic hydrogen carriers. The comparatively low enthalpies of reaction make these heteroaromatics a seminal LOHC systems.