Thermodynamics of Reversible Hydrogen Storage: Are Methoxy‐Substituted Aromatics better through Oxygen Functionality?

Abstract

An attractive option for the storage of hydrogen is the reversible hydrogenation of an aromatic substance, a so‐called liquid organic hydrogen carrier. For hydrogen release, it is desirable to find substances with low enthalpies of reaction for dehydrogenation. It has been demonstrated that substitution, e.g., with methyl groups, on the ring can have a positive effect in this regard. In this study, thermochemical properties of methoxy‐substituted pyridines and quinolines and their respective hydrogenated counterparts have been analyzed from a thermodynamic point of view. Combustion calorimetry has been utilized to measure the enthalpy of formation and the transpiration method for determining the vapor pressure. Additionally, high‐level quantum‐chemical approaches have been applied to obtain caloric data in the gas phase, which provide a basis for comparison and evaluation of the experimental data. The results reveal two main effects of methoxy substitution. First, oxygenated substituents like methoxy groups have stronger effect on the Gibbs energy of reaction and thus the temperature needed thermodynamically for hydrogen release than methyl groups. Second, the effect is highly depending on the position of the substitution on the ring. Particularly methoxy groups close to a N heteroatom can have a significant positive effect, which might enable hydrogen release at equilibrium temperatures more than 50 K lower than for their unsubstituted analogue.