Pore Size Control via Multiple-Site Alkylation to Homogenize Sub-Nanoporous Covalent Organic Frameworks for Efficient Sieving of Xenon/Krypton

Among various fission products generated in nuclear reactors, xenon and krypton are two important fission gases with high flow, diffusivity, and radioactivity. Moreover, xenon isolated from these products is an expensive industrial resource with wide applications in medicine and lighting, which makes the development of efficient methods for separation of xenon/krypton significant. However, it is usually difficult for xenon/krypton to be adsorbed by chemical adsorbents due to their inert gas properties, and sub-nanoporous adsorbents proven to be workable for the separation of xenon/krypton are still hard to prepare and regulate the pore size. Herein, we report two novel sub-nanoporous covalent organic frameworks (COFs), which were applied to the sieving of xenon/krypton for the first time. The sub-nanoporous COFs were synthesized via aldehyde-amine polycondensation reactions and the subsequent pore size regulation and homogenization process by using a facile, operational, and efficient multiple-site alkylation strategy. Impressively, the as-prepared sub-nanoporous COFs realized the efficient adsorption and sieving of xenon/krypton owing to their slightly larger pore sizes (∼7 Å) than the dynamic diameters of xenon/krypton and their larger pore volumes. The maximum adsorption capacity for xenon is up to 85.6 cm³/g, and the xenon/krypton selectivity can reach to 9.7. Moreover, the as-prepared COFs possess good γ-ray irradiation stability, which endows them with great potentials for the sieving of radioactive xenon/krypton in the practical application. The multiple-site alkylation strategy proposed in this study provides a valuable approach for the pore construction and control of the porous materials, especially the sub-nanoporous adsorption materials.