Thermally Tunable Adsorption of Xenon in Crystalline Molecular Sorbent

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 [ConFe4–nL6]4– (n = 1, 2, 3, and 4),
where L2– 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) Å3 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 oversized molecular cavities suggests a new strategy for
separation and storage of xenon, through introduction of kinetic barriers,
such as H-bond networks.