Controlling Selectivity in Reactions for Sustainable Hydrogen Storage in Liquid Carriers

In order to fully integrate intermittent renewable energy sources into our energy infrastructure, new technologies for long-duration storage and transport of renewable energy are urgently needed. Hydrogen (H₂), produced from renewable electricity via water electrolysis, is an energy carrier that can aid in decarbonizing numerous industries, but storing pure hydrogen requires high pressures (>200 bar) and/or cryogenic temperatures (-250 C). One possible solution is liquid organic hydrogen carriers (LOHCs), molecules which can be reacted with hydrogen to produce hydrogen-rich counterparts that can be stored at ambient conditions, then later dehydrogenated to recover H₂. Viable LOHCs require moderate reaction energies and effective catalysts for the hydrogenation and dehydrogenation reactions to enable reversible H₂ storage. Past research indicates that nitrogen-containing heterocyclic aromatic molecules such as ‘indoles’ (Fig. 1A) can both store and release H₂ at mild conditions (< 200°C).^1. One catalyst that has shown promise for nitrogen containing LOHCs are Pd nanoparticles anchored to high-surface-area, thermally stable metal oxide supports.¹ Reaction selectivity is of key importance because the LOHC must be essentially fully recyclable for subsequent storage cycles. However, the effects of LOHC molecular structure and catalyst surface structure on reaction selectivity remain poorly understood.

To address this, dehydrogenation reactions of 8H-Indole and 8H-N-Methylindole (8H-NMID) to indole and N-methylindole (NMID) were investigated with catalysts of Pd on alumina and silica supports. The reaction pathway for 8H-indole is shown in Figure 1A. Reactions were performed in a high-pressure batch reactor system for 3 hours at 180°C with a feed of 1% 8H-indole or 8H-NMID in dodecane. Products were identified using gas chromatography-mass spectrometry (GCMS).

Fig 1B shows that for two different LOHCs (indole, NMID), catalysts of Pd supported on SiO₂ or Al₂O₃ are active for dehydrogenation, forming a combination of partially and completely dehydrogenated products. However, the sum of product yields did not equal the observed reactant conversion, suggesting the presence of side-reactions (up to 40% losses for indole conversion over Pd/SiO₂). Subsequent control experiments mixing catalyst and reactant solution at ambient conditions revealed that silica and alumina supports were adsorbing indoles, possibly due to hydrogen bonding with OH groups on support surfaces. This accounted for a significant fraction of missing carbon, as displayed in the predicted carbon absorption bar of Figure 1B. A carbon balance is the percent carbon in a reaction feed also detected in the output. Since carbon atoms are not destroyed, values under 100% indicate undetected carbon products.

The extent of adsorption differs between 8H-indole and 8H-NMID (95 mg 8H-NMID vs 130 mg 8H-indole per gram SiO₂), suggesting steric hinderance from the 8H-NMID methyl group suppresses adsorption. These results show that improving selectivity in catalytic LOHC reactions requires tailoring LOHC structure and support properties to avoid parasitic processes such as support adsorption, motivating further research into the molecular factors governing LOHC-support interactions.

(1) Li, Y.; Guo, X.; Zhang, S.; He, Y. A Perspective Review on N-Heterocycles as Liquid Organic Hydrogen Carriers and Their Hydrogenation/Dehydrogenation Catalysts. Energy Fuels 2024, 38 (14), 12447–12471. https://doi.org/10.1021/acs.energyfuels.4c01633.