Exploring the Feasibility of 2,5-Dimethylindole as a Liquid Organic Hydrogen Carrier: A Combined Theoretical and Experimental Investigation

Abstract

We present 2,5-dimethylindole (2,5-DMID) as a novel and promising liquid organic carrier (LOHC) for hydrogen storage. A systematic study combined experiments with calculations was conducted on the hydrogen storage properties of 2,5-DMID. Hydrogenation reactions catalyzed by 5 wt % Ru/Al2O3 catalyst under 120–160 °C at 5 MPa H2 pressure and 120 °C at 3–6 MPa H2 pressure indicate that the reaction rate is temperature-dependent and weakly impacted by hydrogen pressure. Full hydrogenation was achieved within 80 min at 150 °C and 5 MPa. Two intermediates, 2H-2,5-DMID and 4H-2,5-DMID, were detected. Conversion from 2,5-DMID to 2H-2,5-DMID was readily achieved at all temperatures with an apparent activation energy of 81.58 kJ/mol H2. Complete dehydrogenation of 8H-2,5-DMID was achieved at 170–210 °C for 180 min on a 5 wt % Pd/Al2O3 catalyst, generating high purity hydrogen gas up to 99.99%. Five cycles of hydrogenation and dehydrogenation processes confirmed the full achievement of both processes with no significant side reactions observed. Density functional theory calculations of the optimal adsorption sites and corresponding reaction energies of 2,5-dimethylindole and intermediates on the catalyst surface perfectly aligned with experimental reactions. This study expands the LOHC family of nitrogen heterocyclic molecules.

A Perspective Review on N-Heterocycles as Liquid Organic Hydrogen Carriers and Their Hydrogenation/Dehydrogenation Catalysts

“…NEC was initially proposed as a carbazole molecule, and later indole derivatives were suggested as LOHCs due to their structural similarity to carbazole. Quinoline compounds were also proposed as LOHCs due to their aqueous solubility …”