Hydrogen carrier with ring dehydrogenation and lactonization in the same molecule: A thermodynamic perspective

Organic hydrogen carrier molecules can operate based on different chemical principles. A rather conventional approach is the reversible hydrogenation of an aromatic ring. A more recent alternative, with promising thermodynamic characteristics, is the lactonization of diols. These lactone systems can release hydrogen at much lower temperatures, while at the same time requiring more effort for hydrogen uptake. In this work, the combination of both reaction mechanisms within one molecule is evaluated from a thermodynamic point of view. For this task, calorimetric measurements have been applied and validated with quantum-chemical calculations to assess enthalpies and entropies of reaction. These data form the basis for a subsequent calculation of the composition of the reaction mixture as a function of pressure. The results show that dehydrogenation will proceed stepwise. Hydrogen release can start under mild conditions through lactonization (releasing up to 40% of the overall stored H 2 ) and, after the carrier is further heated, full hydrogen release can proceed via aromatization.