From microscopic insights of H-2 adsorption to uptake estimations in MOFs

The adsorption of hydrogen in MOFs takes place mainly close to the inorganic secondary building unit (IBU). The adsorption capacities on MIL-88, UiO-66, MIL-47 and MFU-1 were investigated. Quantum chemical calculations at the ab initio HF/MP2 theoretical level were employed to estimate the maximum uptake of H-2 molecules per metallic centre. Extrapolating the results on small clusters to the unit cell of each particular MOF, the H-2 uptakes (gravimetric and volumetric) were estimated. The loading of hydrogen per metal atom (H-2 molecules/M-atom) and the density of metal atoms (M-atoms angstrom(-3)) were defined as useful parameters to assess hydrogen storage properties and to estimate the optimum density that the material should have to be a good H-2 adsorbent. It was found that values above 3 H-2 molecules/M-atom and around 0.004 M-atoms angstrom(-3) for MOFs with densities around 0.7-1.0 g cm(-3) are required to reach the 2015 storage targets.