Solvent-Free Hydrogenation and Dehydrogenation of Quinoline and Quinaldine for the LOHC Concept

As presented herein, N-heterocyclic quinoline (Q) and quinaldine (MeQ) represent promising liquid organic hydrogen carriers (LOHCs). They can be quantitatively hydrogenated to tetrahydro (TH) forms under mild reaction conditions (100 °C and 10 bar of H2) using Pt/C and Pd/C catalysts. It is noteworthy that the hydrogenation occurs without a solvent, yielding hydrogen storage capacities of up to 2.9 wt·%. Further hydrogenation of Q and MeQ to their decahydro (DH) forms requires the presence of a solvent. In addition to the hydrogenation of Q and MeQ, we succeeded in the dehydrogenation of the TH forms under solvent-free reaction conditions. This suggests that a pressure-controlled system with a single catalyst for hydrogenation and dehydrogenation of the MeQ/Q system could enable hydrogen storage under solvent-free reaction conditions. We also examined various substituted pyridine structures to comprehend the role of protective alkyl groups in hydrogenation. However, solvent-free quinaldine hydrogenation reactions are sensitive to steric changes surrounding pyridine nitrogen. Among the structural variations studied, MeQ with one methyl group has proven to be the most favorable.