(347g) A Supramolecular Metal-Organic Framework with Permanent Porosity for Xe/Kr Separation

Xenon and krypton have been widely used in various fields including medical imaging, anesthesia, lighting, laser, and ion propulsion. Typically, a gas mixture of Xe/Kr (20/80, v/v) can be obtained as a by-product of air separation, and the pure Xe and Kr can be collected after distillation processes. Another promising Xe source is the reprocessing off-gases of UNF (used nuclear fuel), but it remains a challenge to separate radioactive noble gas isotopes due to their chemical inertness. The cryogenic distillation based on the difference in boiling points to separate noble gases from air and UNF reprocessing off-gases is energy and capital-intensive, which finally leads to the high prices of pure products. The adsorption methods using porous materials have been regarded as a promising method to achieve noble gases separation. Numerous porous materials such as activated charcoal, zeolite, metal-organic frameworks (MOFs) and porous molecular solids have been reported to demonstrate excellent performance for Xe/Kr separation. Hydrogen-bonded organic frameworks (HOFs) with advantageous features have presented potential in various aspects, however, there is limited number of reports on HOFs performing well in noble gas separation.

Herein, we report a supramolecular metal-organic framework, SMOF-SIFSIX-2, self-assembled from cation metal complexes and inorganic anions through hydrogen-bonding interactions combined the advantages of MOFs and HOFs. SMOF-SIFSIX-2 not only possesses permanent porosity, but also realizes highly Xe/Kr separation. (Scheme 1). Single-component adsorption isotherms of Xe and Kr on SMOF-SIFSIX-2 revealed that the uptake of Xe can reach 2.73 mmol/g at 298 K and 1.0 bar, while the Kr uptake was 1.64 mmol/g (Figure 1). The IAST selectivity for Xe/Kr (20/80, v/v) at 298 K and 100 kPa were calculated to be 16.8, and Henry’s constant were calculated to be 18.4. The column breakthrough experiments using a binary gas mixture of Xe/Kr (20/80, v/v) and a quinary gas mixture (Xe 400 ppm and Kr 40 ppm balanced with dry air without CO₂) were conducted to simulate the practical industrial separation at 298 K and 1.0 bar on SMOF-SIFSIX-2. For the binary gas mixture, Kr broke through immediately while Xe was not eluted until 35 min (Figure 2). For the gas mixture containing a low concentration of noble gases, the Xe broke through at around 48 min after the feed gas in with a total Xe capacity of 25.8 mmol/g (Figure 3). This work demonstrated that SMOF-SIFSIX-2 was a promising adsorbent for Xe/Kr separation.

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