High-Capacity Methane Storage in Metal−Organic Frameworks M2(dhtp): The Important Role of Open Metal Sites

We found that metal-organic framework (MOF) compounds M(2)(dhtp) (open metal M = Mg, Mn, Co, Ni, Zn; dhtp = 2,5-dihydroxyterephthalate) possess exceptionally large densities of open metal sites. By adsorbing one CH(4) molecule per open metal, these sites alone can generate very large methane storage capacities, 160-174 cm(3)(STP)/cm(3), approaching the DOE target of 180 cm(3)(STP)/cm(3) for material-based methane storage at room temperature. Our adsorption isotherm measurements at 298 K and 35 bar for the five M(2)(dhtp) compounds yield excess methane adsorption capacities ranging from 149 to 190 cm(3)(STP)/cm(3) (derived using their crystal densities), indeed roughly equal to the predicted, maximal adsorption capacities of the open metals (within +/-10%) in these MOFs. Among the five isostructural MOFs studied, Ni(2)(dhtp) exhibits the highest methane storage capacity, approximately 200 cm(3)(STP)/cm(3) in terms of absolute adsorption, potentially surpassing the DOE target by approximately 10%. Our neutron diffraction experiments clearly reveal that the primary CH(4) adsorption occurs directly on the open metal sites. Initial first-principles calculations show that the binding energies of CH(4) on the open metal sites are significantly higher than those on typical adsorption sites in classical MOFs, consistent with the measured large heats of methane adsorption for these materials. We attribute the enhancement of the binding strength to the unscreened electrostatic interaction between CH(4) and the coordinatively unsaturated metal ions.