Thermochemical properties of benzyltoluenes and their hydro- and perhydro-derivatives as key components of a liquid organic hydrogen carrier system

Benzyltoluenes can be utilized for hydrogen storage through reversible hydrogenation in form of pure regioisomers or as isomeric mixture. For the design of the process and in-depth understanding of the reaction, knowledge about the thermochemical properties of the different chemical species is mandatory. This work aims at providing data on relevant thermochemical properties such as vapor pressure, enthalpy of vaporization and enthalpy of formation for benzyltoluenes and their hydro-and perhydro-derivatives forming the respective liquid organic hydrogen carrier (LOHC) system. Combustion calorimetry has been utilized to determine enthalpy of reaction and high-level quantum chemical methods have been applied for validation. It has been found that the different steps of the reaction and reactions of different isomers are similar as expected, but there are still clear differences. For instance, enthalpy of reaction is higher for the hydrogenation of the meta isomer than for the other two isomers. Hydrogenation of the first ring is more exothermal than hydrogenation of the second ring and hydrogenation of the less substituted ring is more exothermal than hydrogenation of the higher substituted ring. These findings shed new light on kinetic observations, which so far have only been explained by steric effects.