Understanding Hydrogen Adsorption in MIL-47-M (M = V and Fe) through Density Functional Theory

The present paper aims to investigate the role of open metal site metal-organic frameworks (MOFs) on hydrogen adsorptivity using periodic boundary condition (PBC) density functional theory (DFT). Hence, MIL-47-M (M = V and Fe) were selected and one hydrogen molecule adsorptivity was calculated in different orientations on them. Four different chemical sites were identified in every cluster section of these MOFs, and molecular hydrogen adsorption was studied in these sites. In these MOFs, V has fewer electrons than Fe in its valence layer. Results demonstrated that when dihedral angle of M-O-H-H is 0̊, the binding energy of hydrogen adsorption is higher than that in other orientations in both MILs (-26.16 and -19.73 kJ mol-1 for V and Fe, respectively). In this orientation, hydrogen molecule has a head-to-head interaction with O in M-O-M. Also, hydrogen desorption in various orientations in MIL-Fe was found, and minimum distances between the hydrogen molecule and one atom of MILs were calculated in all the orientations. It was revealed that there are not significant differences in all orientations and the best adsorptivity condition is hydrogen molecule in a head-to-head orientation with O, (M-O-M), which is 2.419 and 2.338 Å for V and Fe, respectively. According to PBC-DFT results, hydrogen adsorption on MIL-V is energitically more stable than that on MIL-Fe. Findings indicate that MOFs with an open metal site are more proper candidates for hydrogen adsorptions.