Thermodynamics of hydrogen storage: Equilibrium study of the LOHC system indole/octahydroindole

Reversible hydrogenation of indole or its derivatives could be an attractive hydrogen storage technology. Particularly for hydrogen release, the reaction equilibrium is a crucial factor determining the conditions in the respective process. This study is supposed to provide inside into this reaction equilibrium. An experimental study of the chemical equilibrium of hydrogenation/dehydrogenation reactions was performed in a stirred autoclave with n-hexane as a solvent by varying the temperature, pressure, and hydrogen:feedstock ratio. Based on the compositions in stationary state, equilibrium constants have been derived. Furthermore, thermodynamic prop-erties of chemical reactions were derived from the equilibrium constant temperature dependence. This tem-perature dependence reveals that the enthalpy of reaction for the dehydrogenation of the six-membered ring is significantly larger than in the nitrogen containing five-membered ring. To get deeper insides, the molar en-thalpies of formation of intermediates have been calculated using high-level quantum chemical methods. This analysis supports the observations from the equilibrium reactions. Additionally, it show that the heat of reaction for the endothermal dehydrogenation is about 2 kJ mol (H2)-1 smaller in the liquid phase than in case of a gas phase reaction.

Automated summary

This study investigates the reversible hydrogenation of indole and its derivatives. The experimental results provide equilibrium constants and thermodynamic properties, showing that the dehydrogenation of the six-membered ring has a larger enthalpy change than the nitrogen-containing five-membered ring. It also reveals that the heat of reaction for the endothermal dehydrogenation is smaller in the liquid phase than in the gas phase, by about 2 kJ mol (H2)-1.