Increased volumetric hydrogen uptake of MOF-5 by powder densification

The metal-organic framework MOF-5 has attracted significant attention due to its ability to store large quantities of H2 by mass, up to 10 wt.% absolute at 70 bar and 77 K. On the other hand, since MOF-5 is typically obtained as a bulk powder, it exhibits a low volumetric density and poor thermal conductivity—both of which are undesirable characteristics for a hydrogen storage material. Here we explore the extent to which powder densification can overcome these deficiencies, as well as characterize the impact of densification on crystallinity, pore volume, surface area, and crush strength. MOF-5 powder was processed into cylindrical tablets with densities up to 1.6 g/cm3 by mechanical compaction. We find that optimal hydrogen storage properties are achieved for ρ ∼ 0.5 g/cm3, yielding a 350% increase in volumetric H2 density with only a modest 15% reduction in gravimetric H2 excess in comparison to the powder. Higher densities result in larger reductions in gravimetric excess. Total pore volume and surface area decrease commensurately with the gravimetric capacity, and are linked to an incipient amorphization transformation. Nevertheless, a large fraction of MOF-5 crystallinity remains intact in densities up to 0.75 g/cm3, as confirmed from powder XRD. Predictably, the radial crush strength of the pellets is enhanced by densification, increasing by a factor of 4.3 between a density of 0.4 g/cm3 and 0.6 g/cm3. Thermal conductivity increases slightly with tablet density, but remains below the single crystal value.