Thermodynamic study of the adsorbed hydrogen phase in Cu-based metal-organic frameworks at cryogenic temperatures

The hydrogen adsorption properties of two isostructural copper-based microporous metal-organic frameworks (MOFs) materials which differ in the length of the bridging ligands were investigated over the 50-100 K and 0-40 bar ranges. The measured excess adsorption isotherms were analyzed in terms of adsorption enthalpies, adsorbed phase densities and volumes. The characteristic excess maximum was found to be displaced to lower pressures and to vary less as a function of temperature on the material with the shorter ligand. This behaviour could be explained, on the basis of enthalpy calculations, by an enhanced hydrogen affinity in smaller pores. On the other hand, it was also found that the material with the shorter ligand has a reduced storage capacity. This observation could be explained, from measurements near saturation, by a reduction of both adsorbed phase density and volume. Despite their structural difference, both MOFs adsorbed hydrogen near saturation under an incompressible phase reaching bulk liquid densities. These properties suggest that repulsive forces may ultimately limit the packing of supercritical molecules in small pores despite apparently stronger solid-gas interactions.