## 2 Materials and Methods
### Thermodynamic Analysis of the Hydrogenation/Dehydrogenation Reactions
#### 3.5.3 Liquid-Phase Enthalpies of the Hydrogenation/Dehydrogenation Reactions  
The enthalpy of hydrogenation/dehydrogenation reaction is an important quantity for the hydrogen storage. The \(\Delta_{r}H_{m}^{o}\)(liq)-values calculated according to Hess's Law are given in Table 11, column 6. However, it seems unlikely that the enthalpies of hydrogenation in the gaseous state differ from the enthalpies in the liquid phase, since the enthalpies of vaporization of the reactants do not differ significantly. All reactions with substituted pyridines and quinolines are highly exothermic from \(-180\) to \(-477\) kJ\(\cdot\)mol\({}^{-1}\). This means that extremely accurate enthalpies of reaction must be used for thermal management of hydrogen storage reactors with LOHC. Admittedly, in order to compare different LOHC, it is more useful to relate the enthalpies of reaction to the number of moles of hydrogen released during the dehydrogenation. These relative values per mole of H\({}_{2}\) are given in Table 11, column 7. It has turned out that the enthalpies of reaction for pyridine and quinoline derivatives are not much different and are generally between \(-60\) and \(-65\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\). These results are generally comparable with commercially available thermofluids benzyl-toluenes (Marlothem LH\({}^{\oplus}\), \(-63.5\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\)) [59], dibenzyltoluenes (Marlothem SH\({}^{\oplus}\), \(-65.4\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\)) [59] and biphenyl (\(-65.4\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\)) [40] reported in our previous studies. It is obvious that new results for pyridines and quinolines are not better than those of \(N\)-ethylcarbazole (\(-50.5\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\) [60]), indole (\(-56.6\) kJ mol\({}^{-1}\)/H\({}_{2}\) [61]) and for 2-methyl indole (\(-55.2\) kJ mol\({}^{-1}\)/H\({}_{2}\) [61]). It was found that the reaction enthalpy for 2-phenyl-quinoline (\(-59.7\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\), see Table 11) is one of the lowest among the substituted pyridines and quinolines investigated in this work. For comparison: the reaction enthalpy of \(N\)-phenyl-carbazole (\(-57.7\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\)) [44] is very close to that of 2-phenyl-quinoline, but significantly higher than that of N-alkyl-carbazoles (\(-50.5\) for ethyl-, \(-49.4\) for _iso_-propyl-, \(-50.4\) for n-propyl-, and for n-butyl \(-50.4\) kJ\(\cdot\)mol\({}^{-1}\)/H\({}_{2}\), respectively) [62]. Even if the reaction enthalpies of the hydrogenation/dehydrogenation enthalpies of phenyl-substituted LOHC do not differ significantly from those with aliphatic chains, the presence of the phenyl ring in the structure increases the H\({}_{2}\) storage capacity and makes such compounds more interesting for practical applications. The following are available online at [[https://www.mdpi.com/article/10.3390/app112411758/s1](https://www.mdpi.com/article/10.3390/app112411758/s1)]([https://www.mdpi.com/article/10.3390/app112411758/s1](https://www.mdpi.com/article/10.3390/app112411758/s1)), Figure S1: Temperature dependence of vapor pressures over the 2-methyl-quinoline, Figure S2: Temperature dependence of vapor pressures over the 3-methyl-quinoline, Figure S3: Temperature dependence of vapor pressures over the 4-methyl-quinoline, Figure S4: Evaluation of increments responsible for the exchange of H-atom in piperidine ring with the methyl group. Table S1: Provenance and purity of samples used in this work; Table S2: Compilation of data on molar heat capacities of pyridines and quinolines; Table S3: Coefficients of the Clarke and Glew equation; Table S4: Vapour pressures at different temperatures collected from SciFinder, standard molar vaporization enthalpies and standard molar vaporization entropies; Table S5: Compilation of enthalpies of fusion of quinoline derivatives; Table S6: Thermochemical data for reference compounds; Table S7: Calculation of the liquid phase enthalpies of the decahydroquinolines. Conceptualization, S.P.V. and A.A.S.