Enhanced H2 Adsorption in Isostructural Metal-Organic Frameworks with Open Metal Sites: Strong Dependence of the Binding Strength on Metal Ions

Metal-organic frameworks (MOFs) with open metal sites exhibit a much stronger H-2 binding strength than classical MOFs, due to the direct interaction between H-2 and the coordinately unsaturated metal ions. Here we report a systematic study of the H-2 adsorption on a series of isostructural. MOFs, M-2(dhtp) (M = Mg, Mn, Co, Ni, Zn). The experimental, initial isosteric heats of adsorption for H-2 (Q(st)) of these MOFs range from 8.5 to 12.9 kJ/mol, with increasing Q(st) in the following order: Zn, Mn, Mg, Co, and Ni. The H-2 binding energies derived from first-principles calculation follow the same trend as the experimental observation on Q(st), confirming the electrostatic Coulomb attraction between the H-2 and the open metals being the major interaction. We also found a strong correlation between the metal ion radius, the M-H-2 distance, and the H-2 binding strength, which provides a viable, empirical method to predict the relative H-2 binding strength of different open metals.