Thermally Tunable Adsorption of Xenon in Crystalline Molecular Sorbent

Abstract

The thermostability of encapsulated xenon is investigated in a series of isostructural crystalline sorbents. These sorbents consist of metal-organic capsules, with the general formula of [Co_nFe_4-nL₆]^4- (n = 1, 2, 3 and 4), where L^2- is an organic linker with two sulfonate groups. In the crystalline sorbent, guanidinium cations form H-bond networks with the peripheral sulfonate groups in the solid state and trap xenon in the molecular cavities, which are at least 2.7 times the volume of xenon. When heated, the sorbent retains xenon up to 561 K, i.e., 396 K higher than the boiling point of xenon. Furthermore, the thermostability of trapped xenon can be modulated by varying the ratio of Co:Fe in the crystalline sorbent. Elemental analysis on a single crystal by energy dispersive X-ray spectroscopy confirms the homogeneous distribution of Co and Fe in the sorbent. Structural analyses reveal that the expansion of capsule cavity is proportional to the Co:Fe ratio, with increases of 0.049(1) Å and 6.4(8) ų in metal-metal distance and cavity volume, per substitution of Fe by Co center. Steric repulsion between peripheral sulfonate groups is found to render a hypothetical face-centered cubic structure of (C(NH₂)₃)₄[Fe₄L₆] not accessible, which would have trapped xenon with exceptional thermostability. The stable and tunable trapping of xenon in crystalline sorbents by over-sized molecular cavities suggests a new strategy for separation and storage of xenon, through introduction of kinetic barriers, such as H-bond networks.

Figure 1.

(a) Preparations of heterobimetallic capsules and the corresponding Xe-entrapped molecular crystals. (b) A representative demonstration of the six H-bonds formed between (C(NH₂)₃)⁺ and interfacial sulfonate groups on each facet of Co_nFe_4–n tetrahedra, as observed in 1–7. (c) Chemical structure of the (C(NH₂)₃)⁺-capped M^II₄ capsules in the solid state and illustration of the dihedral angle ∠C_sulfonateCCʹCʹ_sulfonate (θ). Gray, blue, red, yellow, dark green, light blue spheres represent C, N, O, S, Fe/Co (M^II), Xe atoms, respectively. H atoms are omitted for clarity, except for those participating in H-bonding, which are represented by light green spheres.

Figure 2.

Summary of intramolecular metal–metal distances (black) and internal volumes (red) with respect to Co content in [Co_nFe_4–nL₆] cages. Stars represent data from reported (Fe(H₂O)₆)₂[(X)Fe₄L₆], where X stands for an undefined guest molecule. Empty diamonds and solid circles represent data from compounds 1–3 (guest-free) and 4–7 (Xe-encapsulated), respectively.

Figure 3.

EDX analyses of a crystal of 2 that was adhered to an Al sample holder. Top left: SEM image of a crystal. EDX mapping for O (top middle), S (top right), Co (bottom left), Fe (bottom middle), and superimposed Fe and Co (bottom right), with each element monitored at its K_α1 wavelength.

Figure 4.

Summary of TGA-MS plots for (a) 4, (b) 5, (c) 6, and (d) 7. Black and red lines represent the thermogravimetric trace and the chromatograph monitored at m/z = 131, respectively.