Indole
and methylindole are heterocyclic aromatics, which can be
hydrogenated and used for hydrogen storage. A huge advantage of heterocyclic
components compared to homocyclic aromatics is the lower enthalpy
of reaction for hydrogen release by dehydrogenation. In this study,
thermochemical properties of indole and 2-methylindole and its partially
and fully hydrogenated derivatives have been determined. Hydrogenation
of indoles is a two-step reaction, which is highly influenced by reaction
thermodynamics. High precision combustion calorimetry was used to
measure enthalpies of formation of indole derivatives. The gas-phase
enthalpies of formation were derived with the help of vapor pressure
measurements. The high-level quantum-chemical methods were used to
establish consistency of the experimental data. The standard molar
thermodynamic functions of formation (enthalpy, entropy, and Gibbs
energy) of indole derivatives were derived. The results showed that
the partially hydrogenated species, 2-methylindoline, tends to dehydrogenate
easily under hydrogen release conditions. Thus, indoline is only expected
in trace amounts in the respective reaction mixture.
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