Liquid Organic Hydrogen Carriers: Thermophysical and Thermochemical Studies of Carbazole Partly and Fully Hydrogenated Derivatives

Abstract

Carbazole derivatives are promising liquid organic hydrogen carrier (LOHCs). They can take up and release hydrogen in catalytic hydrogenation/dehydrogenation reactions. The thermophysical properties (heat capacity, density, viscosity, surface tension, and refractive index) of carbazole and N-ethylcarbazole, as well as those of their hydrogenated derivatives, were measured. Furthermore, thermochemical properties (enthalpy of vaporization, sublimation, and fusion) were derived from experiments. Molar enthalpies of formation of the gaseous carbazole derivatives were calculated and validated with high-level quantum chemical calculations. Molar enthalpies of formation data in the liquid phase were derived by combining the high-level quantum chemistry values of the gas-phase enthalpies of formation with experimentally determined enthalpies of vaporization. Thermodynamic analysis of the hydrogenation/dehydrogenation reactions of the carbazole derivatives was performed. This analysis, among others, revealed a rather weak dependence of thermodynamic hydrogen release characteristics on the alkyl chain length. The thermophysical properties of the derivatives show a strong dependence on the degree of hydrogenation. Therefore, the state of hydrogenation should always be considered when dealing with substance properties of LOHCs. Using the data presented in this work, a more reliable process modeling is possible.

Palladium Supported on Carbon Nanotubes as a High-Performance Catalyst for the Dehydrogenation of Dodecahydro-N-ethylcarbazole

“…Therefore, N-ethylcarbazole, with a gravimetric density of 5.8 wt.%, has been identified as the most prospective candidate for hydrogen storage (Scheme 1). Although there are many studies about the dehydrogenation reaction from calculations and experiments [20][21][22][23][24][25][26][27][28], the dehydrogenation reaction is still the key to limit its large-scale application, especially, the development of dehydrogenation catalysts with outstanding activity and stability is the hotspot of current research. A large number of dehydrogenation catalysts have been extensively investigated, including homogeneous catalysts and the heterogeneous catalysts [29][30][31][32][33][34][35].…”

Energietransport über weite Strecken

“…Für das LOHC‐System NEC/PH‐NEC wurden folgende thermodynamische Werte für die Gleichgewichtsreaktion in der Flüssigphase verwendet 14, 15: …”

Energetics of LOHC: Structure-Property Relationships from Network of Thermochemical Experiments and in Silico Methods

“…It was found that the introduction of a nitrogen atom to aromatic organic hydrides is effective for lowering of the dehydrogenation temperature [1]. This idea was realized in N-alkyl-carbazoles as the second generation of nitrogen-containing organic hydrides for hydrogen storage [2][3][4]. So far, however, no single material has been found that can meet numerous criteria expected for the inexpensive but effective hydrogen storage material that must consume and release H 2 with fast kinetics at moderate temperatures of 350-400 K. Thus, the search for new LOHC systems based on abundant and inexpensive organic compounds with a high capacity to store and release hydrogen and ideally use the same catalyst to charge and discharge hydrogen under relatively mild conditions, and compatible with existing infrastructure for transport and refuelling, is a major challenge [5].…”

“…However, these methods generally fail for aromatic, branched, and strained cyclic molecules [9]. In a series of our recent studies, we successfully tested quantum-chemical composite methods of G* family to provide reliable gas phase thermochemical data for N-alkyl-carbazoles [2][3][4]. The hydrogenation/dehydrogenation reactions suitable for hydrogen storage, however, usually take place in the liquid phase.…”