(424c) Modeling Hydrogen Adsorption in Microporous Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are one kind of the potential materials for hydrogen storage because of their high pore volume and surface area. However, none of the current known MOFs meet DOE's 2010 target of 6 wt % at room temperature. We have carried out a series of calculations on hypothetical MOF structures in order to identify adsorption potentials needed to facilitate uptake of large uptake of H₂ in MOFs. We have focused on using two known structures, IRMOF-1 and IRMOF-14, and exploring the effect of changing the interaction potential between the adsorbate (H₂) and the adsorbent framework atoms. Our approach involves hypothetical changes to the chemical makeup of the framework, without changing the geometry of the framework. We have computed the volumetric density of states (VDOS) as a way to estimate the storage capacity of a material. An integral over the VDOS gives the amount of volume available for adsorption in a given range of adsorption energies. A simple approximation states that the amount of H₂ adsorbed at room temperature and moderate pressures is largely determined by the amount of volume available to the adsorbate where the potential is lower than about -1000 K.

Our model calculations suggest that the adsorption potential must be changed quite dramatically (by a factor of about 20) in order to achieve a very high uptake of hydrogen at room temperature. Mechanisms for increasing the adsorption potential by this amount are probably limited to weak chemisorption interactions that result, for example, from H₂ molecules interacting with under-coordinated transition metal atoms, such as Ti and Sc. We have investigated placement of Sc within the linker groups of MOFs through ab initio density functional theory.