Thermodynamics of Chemical Hydrogen Storage: Are Sterically Hindered and Overcrowded Molecules More Effective?

Homocyclic aromatics with different degrees of alkylation have been investigated so far as Liquid Organic Hydrogen Carriers (LOHC). A low enthalpy of reaction for the dehydrogenation reaction is generally considered beneficial. Values available for crowded, multi-alkylated aromatics, such as hexamethyl benzene, indicate that these substances could be utilized efficiently as LOHCs. However, no clear trend can be identified in the existing data. The aim of this study is to provide a consistent and comprehensive data set on this substance class to evaluate if multi-alkylation is indeed beneficial. For this purpose, own and literature results from experimental methods such as combustion calorimetry and the transpiration method for measuring the enthalpy of vaporisation were combined with quantum chemical approaches to obtain a validated, consistent data set. This comprehensive study reveals that the positive effect on enthalpy of reaction for dehydrogenation is comparatively weak. A slightly lower enthalpy of reaction is actually observed for crowded alkylbenzenes, but it is most likely not sufficient to reach a significant decrease in temperature for hydrogen release. Nevertheless, the results are of high importance for the further development of LOHC systems with optimal structural motifs.