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Hydrogen Storage in the Expanded Pore Metal-Organic Frameworks M-2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn)

The hydrogen storage properties of a new family of isostructural metal organic frameworks are reported. The frameworks M-2(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc(4-) = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are analogous to the widely studied M-2(dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn; dobdc(4-) = 2,5-dioxido-1,4-benzenedicarboxylate) family of materials, featuring the same weak-field oxo-based ligand environment for the M2+ metal centers, but with a larger pore volume resulting from the extended length of the dobpdc(4-) linker. Hydrogen gas adsorption isotherms measured at 77 and 87 K indicate strong H-2 binding at low pressures, corresponding to the adsorption of one molecule per M2+ site. Isosteric heats of adsorption indicate adsorption enthalpies ranging from -8.8 to -12.0 kJ/mol, with the trend Zn < Mn < Fe < Mg < Co < Ni. Room-temperature high-pressure adsorption isotherms indicate enhanced gravimetric uptakes compared to the M-2(dobdc) analogues, a result of the higher surface areas and pore volumes of the expanded frameworks. Indeed, powder neutron diffraction experiments performed on Fe-2(dobpdc) reveal two additional secondary H2 adsorption sites not observed for the nonexpanded framework. While displaying higher gravimetric capacities than their nonexpanded counterparts, the larger pore volumes result in lower volumetric capacities. Upon comparison with other promising frameworks for hydrogen storage, it becomes evident that in order to design future materials for on-board hydrogen storage, care must be placed in achieving both a high surface area and a high volumetric density of exposed metal cation sites in order to maximize gravimetric and volumetric capacities simultaneously.

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