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

## TL;DR

This paper explores quinoline and quinaldine as potential hydrogen storage materials, showing they can be hydrogenated and dehydrogenated efficiently without solvents.

## Contribution

The study demonstrates solvent-free hydrogenation and dehydrogenation of quinoline and quinaldine under mild conditions using single catalysts.

## Key findings

- Quinoline and quinaldine can be hydrogenated to tetrahydro forms without solvents, achieving up to 2.9 wt·% hydrogen storage.
- Dehydrogenation of tetrahydro forms can also occur under solvent-free conditions.
- Quinaldine with one methyl group is most favorable for solvent-free hydrogenation due to steric effects.

## Abstract

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.