Melting Points of Potential Liquid Organic Hydrogen Carrier Systems Consisting of N-Alkylcarbazoles

Abstract

Liquid organic hydrogen carriers (LOHCs) represent an attractive concept for storing hydrogen by the hydrogenation of usually aromatic compounds. One of the best investigated LOHCs is N-ethylcarbazole because of its favorable thermodynamic properties. However, its high melting point of 343.1 K could be a major drawback particularly in mobile applications. Therefore, it is desired to decrease the melting point of N-ethylcarbazole without significantly changing favorable properties such as the storage density or the reaction behavior of the carrier compound. To investigate the solid–liquid behavior during hydrogenation, the melting points of pure N-ethylcarbazole derivatives with increasing degree of hydrogenation as well as the liquidus line of the binary mixture of N-ethylcarbazole and N-ethyl-dodecahydro-carbazole were measured. Because of their structural and chemical resemblance binary mixtures consisting of different alkylcarbazole combinations were analyzed regarding their potential for a melting point depression. By the appropriate combination of N-alkylcarbazoles, it is possible to achieve a considerable melting point decrease to 297.1 K.

Large-scale stationary hydrogen storage via liquid organic hydrogen carriers

“…Another drawback for this system is the relatively high melting point, 68°C, of NEC. Stark et al. (2016) attempted to decrease the melting point of NEC via mixing with its hydrogenation derivatives or various N -alkylcarbazoles, but generally, the melting points of the mixtures are above room temperature.…”

2‐(N‐Methylbenzyl)pyridine: A Potential Liquid Organic Hydrogen Carrier with Fast H2 Release and Stable Activity in Consecutive Cycles

“…H 2 ‐lean NEC is converted into the corresponding perhydro product (H 12 ‐NEC) by hydrogenation, where 6 mol of H 2 per mol of NEC can be added to yield an H 2 storage capacity of 5.8 wt % . However, the pair NEC/H 12 ‐NEC has two main drawbacks that complicate implementation for practical applications: a) the fully dehydrogenated, pure NEC is a solid at room temperature with a melting point of 68 °C and even after >10 % hydrogenation NEC can be solid under ambient conditions; b) the thermal stability of NEC is limited by dealkylation to carbazole, which possibly happens above 270 °C in the presence of dehydrogenation catalysts, although the typical temperature for H 12 ‐NEC dehydrogenation is around 230 °C. To overcome these limitations, Wasserscheid and co‐workers suggested alternative LOHC systems incorporating isomeric benzyltoluenes (BT) and dibenzyltoluenes (DBT), which have been widely used as industrial heat‐transfer fluids .…”

Study of catalytic hydrogenation and dehydrogenation of 2,3-dimethylindole for hydrogen storage application

“… 48 Fully dehydrogenated N -propylcarbazole can be obtained at 200 °C in 240 min with applied Pd/Al 2 O 3 catalyst. 49 Arlt et al investigated the thermochemical properties of N -alkylcarbazoles ( N -isopropylcarbazole, N -propylcarbazole, and N -butylcarbazole) 50 and found the melting point of eutectic systems composed of N -alkylcarbazoles can be lowered dramatically to 24 °C, 51 which are suitable for large scale and long distance storage and delivery at ambient conditions with existing infrastructures. In addition, Wasserscheid et al proposed benzyltoluene (BT) and dibenzyltoluene (DBT) as promising LOHCs.…”