Theoretical and experimental analysis of ${\text{H}}_2$ binding in a prototypical metal-organic framework material

Hydrogen adsorption by the metal-organic framework (MOF) structure ${\text{Zn}}_2{\left(\text{BDC}\right)}_2\left(\text{TED}\right)$ is investigated using a combination of experimental and theoretical methods. By using the nonempirical van der Waals density-functional approach, it is found that the locus of deepest ${\text{H}}_2$ binding positions lies within two types of narrow channel. The energies of the most stable binding sites, as well as the number of such binding sites, are consistent with the values obtained from experimental adsorption isotherms and heat of adsorption data. Calculations of the shift of the H-H stretch frequency when adsorbed in the MOF give a value of approximately $-30{\text{cm}}^{-1}$ at the strongest binding point in each of the two channels. Ambient temperature infrared-absorption spectroscopy measurements give a hydrogen peak centered at $4120{\text{cm}}^{-1}$, implying a shift consistent with the theoretical calculations.

Figure 1

(Color online) A perspective top view of the unit cell of the MOF. The BDC linkers and the metal atoms are evident in this view, while the TED pillars are more clearly seen in the two subsequent figures. Atomic color scheme: black C, green (black) N, red (black) O, and blue-green (gray) Zn; H atoms are not shown. The locations of channels A (thin blue) and B (thick brown) are schematically indicated. The dashed circles indicate places where the respective channels continue into the adjoining unit cell.

Figure 2

(Color online) A diagonal side view of half the unit cell, showing a contour energy map of one of the four instances of channel A. The ${\text{H}}_2$ molecule was aligned in the $z$ direction. The units of the color scale are kJ/mol.

Figure 3

(Color online) A side view of half the unit cell, showing one of the four instances of channel B, each of which extend an equal amount into the respective neighboring unit cell. The contour map color scale and the ${\text{H}}_2$ orientation are the same as for Fig. 2.

Figure 4

(Color online) Infrared absorbance of the ${\text{Zn}}_2{\left(\text{BDC}\right)}_2\left(\text{TED}\right)$ MOF at room temperature. Top panel: clean MOF at ambient pressure; middle panel: MOF at 800 psi of ${\text{H}}_2$ (upper curve) or ${\text{D}}_2$ (lower curve); bottom panel: difference between the ${\text{H}}_2$ and ${\text{D}}_2$ spectra at several pressures.