Fivefold increase of hydrogen uptake in MOF74 through linker decorations

We present ab initio results for linker decorations in Mg-MOF74, i.e., attaching various metals $\mathcal{M}=\text{Li}$, Na, K, Sc, Cr, Mn, Fe, Ni, Cu, Zn, Rb, Pd, Ag, and Pt near the ring of the linker, creating new strong adsorption sites and thus maximizing small-molecule uptake. We find that in most cases these decorations influence the overall form and structure of Mg-MOF74 only marginally. After the initial screening, we chose metals that bind favorably to the linker and further investigated adsorption of ${\mathrm{H}}_2,{\mathrm{CO}}_2$, and ${\mathrm{H}}_2\mathrm{O}$ for $\mathcal{M}=\text{Li}$, Na, K, and Sc. For the case of ${\mathrm{H}}_2$ we show that up to 24 additional guest molecules can be adsorbed in the metal-organic framework (MOF) unit cell, with binding energies comparable to the original open-metal sites at the six corners of the channel. This leads to a fivefold increase of the molecule uptake in Mg-MOF74, with tremendous impact on many applications in general and hydrogen storage in particular, where the gravimetric hydrogen density increases from 1.63 to 7.28 mass % and the volumetric density increases from 15.10 to 75.50 g ${\mathrm{H}}_2{\mathrm{L}}^{-1}$.

Figure 1

MOF74 with modified linker 2,5-dihydroxybenzenedicarboxylic acid. (top) Decoration center, where one metal atom is attached on each side of the linker, directly above and below the center of the ring. (bottom) Decoration outside, where one metal atom is attached on each side of the linker, outside the ring of the linker. Carbon atoms are depicted as gray, oxygen is red, hydrogen is white, and the additional metals are blue. The six original open-metal sites are visible as green in the corners of each channel.

Figure 2

Li-H radial distribution functions $g_{\text{Li–H}}\left(r\right)$ for the center and outside decorations at 1 bar and temperatures of 77, 200, and 298 K. The 298 K radial distribution function for the outside decoration is not shown, as this decoration becomes unstable at that temperature.