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 H-2 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/cm(3) by mechanical compaction. We find that optimal hydrogen storage properties are achieved for rho similar to 0.5 g/cm(3), yielding a 350% increase in volumetric H-2 density with only a modest 15% reduction in gravimetric H-2 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/cm(3), 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/cm(3) and 0.6 g/cm(3). Thermal conductivity increases slightly with tablet density, but remains below the single crystal value. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.