Hydrogen adsorption in the metal–organic frameworks Fe2(dobdc) and Fe2(O2)(dobdc)

Abstract

The hydrogen storage properties of Fe₂(dobdc) (dobdc⁴⁻ = 2,5-dioxido-1,4-benzenedicarboxylate) and an oxidized analog, Fe₂(O₂)(dobdc), have been examined using several complementary techniques, including low-pressure gas adsorption, neutron powder diffraction, and inelastic neutron scattering. These two metal–organic frameworks, which possess one-dimensional hexagonal channels decorated with unsaturated iron coordination sites, exhibit high initial isosteric heats of adsorption of −9.7(1) and −10.0(1) kJ mol⁻¹, respectively. Neutron powder diffraction has allowed the identification of three D₂ binding sites within the two frameworks, with the closest contacts corresponding to Fe–D₂ separations of 2.47(3) and 2.53(5) Å, respectively. Inelastic neutron scattering spectra, obtained from p-H₂ (para-H₂) and D₂–p-H₂ mixtures adsorbed in Fe₂(dobdc), reveal weak interactions between two neighboring adsorption sites, a finding that is in opposition to a previous report of possible ‘pairing’ between neighboring H₂ molecules.