From microscopic insights of H2adsorption to uptake estimations in MOFs

Abstract

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 initioHF/MP2 theoretical level were employed to estimate the maximum uptake of H₂ molecules per metallic centre. Extrapolating the results on small clusters to the unit cell of each particular MOF, the H₂ uptakes (gravimetric and volumetric) were estimated. The loading of hydrogen per metal atom (H₂ molecules/M-atom) and the density of metal atoms (M-atoms Å⁻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 H₂ adsorbent. It was found that values above 3 H₂ molecules/M-atom and around 0.004 M-atoms Å⁻³ for MOFs with densities around 0.7–1.0 g cm⁻³ are required to reach the 2015 storage targets.