Enhanced H$_{2}$ adsorption in metal-organic frameworks with open metal sites: Binding mechanism and strong dependence on metal ions

Metal-organic frameworks (MOFs) with open metal sites exhibit much stronger H$_{2}$ binding strength than classical MOFs, due to the direct interaction between H$_{2}$ and the coordinately unsaturated metal ions. [1] Here we will present a systematic study of the H$_{2}$ adsorption on a series of isostructural MOFs, M$_{2}$(dhtp) with open metals 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.[2] 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. [1] J. Phys. Chem. C, 112, 8132 (2008). [2] J. Am. Chem. Soc., 130, 15268 (2008).