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

Abstract

We found that metal−organic framework (MOF) compounds M₂(dhtp) (open metal M = Mg, Mn, Co, Ni, Zn; dhtp = 2,5-dihydroxyterephthalate) possess exceptionally large densities of open metal sites. By adsorbing one CH₄ molecule per open metal, these sites alone can generate very large methane storage capacities, 160−174 cm³(STP)/cm³, approaching the DOE target of 180 cm³(STP)/cm³ for material-based methane storage at room temperature. Our adsorption isotherm measurements at 298 K and 35 bar for the five M₂(dhtp) compounds yield excess methane adsorption capacities ranging from 149 to 190 cm³(STP)/cm³ (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, Ni₂(dhtp) exhibits the highest methane storage capacity, ∼200 cm³(STP)/cm³ in terms of absolute adsorption, potentially surpassing the DOE target by ∼10%. Our neutron diffraction experiments clearly reveal that the primary CH₄ adsorption occurs directly on the open metal sites. Initial first-principles calculations show that the binding energies of CH₄ 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 CH₄ and the coordinatively unsaturated metal ions.