Liquid Organic Hydrogen Carriers (LOHCs) represent a promising technology for a hydrogen-based energy economy, particularly when considering the transition to liquid hydrogen storage. According to the Department of Energy, the benchmarks for light-duty on-board hydrogen storage are set at a minimum of 6.5 wt% hydrogen storage capacity and a volumetric energy density threshold of 1.7 kWh/L. The principal challenge in the development of LOHC systems lies in the energy-intensive dehydrogenation process. The high storage capacity of naphthalene, even with solvent, meets the DOE requirements and is inexpensive and abundant. Furthermore, decalin dehydrogenation has few side reactions which eliminate an energy-intensive hydrogen cleaning step (for side-product contamination). Prior studies have demonstrated a preference for cis-decalin when using Pt group metals, especially Ru. Cis-decalin is less thermodynamically stable than trans-decalin and can ring flip, and is dehydrogenated at over three times the rate of trans-decalin, reducing the energetic demand of dehydrogenation. Therefore, non-noble metal catalysts with desirable stereoselectivity enhance LOHC system viability.
Metal hydroxide organic frameworks (MHOFs) comprise edge-sharing metal hydroxide octahedra layers interconnected by carboxylate linkers. MHOFs present unique opportunities as sacrificial templates for thermocatalysis, with adjustable metal centers, structural robustness, and heteroatom incorporation through linker selection. We subjected Ni- and terephthalate-based MHOF nanosheets to low-temperature pyrolysis (<450°C), synthesizing a Ni-C composite featuring uniformly distributed Ni sites. The catalyst achieved a surface area of over 160 m²/g and an elemental composition of 60-65 wt% Ni, 30-35 wt% C, and 5-10 wt% O. Comparative kinetic studies among the MHOF-derived catalysts, a commercial 65 wt% Ni/SiO₂-Al₂O₃, and a 20 wt% Ni/γ-Al₂O₃ were conducted at 5–35 bar H₂ and temperatures between 363–440 K using an upflow liquid-phase reactor. The MHOF-based catalysts performed similarly to other Ni-based catalysts in decalin conversion rates per active site, however with a notable selectivity toward cis-decalin across all conditions compared to the other Ni catalysts. Our findings highlight Ni-MHOFs as precursors for transition metal catalysts that emulate the stereoselectivity of NM catalysts, thereby reducing energy requirements in LOHC dehydrogenation.

Matthew Webber

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