Ionic Covalent Organic Frameworks with Krypton-Specific Molecular Recognition

Radioisotopes of xenon and krypton (85Kr and 133Xe) are by-products released from nuclear fuel reprocessing plants. The release of 85Kr and 133Xe is environmentally concerning due to their hazardous radioactive nature and long half-lives of 10.8 years and 36.3 days, respectively. Furthermore, the high concentration of xenon in exhaust gases marks it as a valuable secondary source of pure xenon with various industrial uses. However, capturing and separating these gases is extremely challenging because of their similar dynamic diameters (4.047 Å vs. 3.655 Å), spherical shapes, and very low polarizabilities. In this project, we develop novel solid adsorption materials with krypton-specific size-based molecular recognition. These materials feature thermoelastic apertures that selectively block larger molecules over marginally smaller ones with 0.2 Å resolution. We aim to achieve low-cost and high selectivity Kr/Xe separation near ambient conditions. Our molecular sieving approach offers the potential to effectively remove trace amounts of radioactive Kr in off-gas streams as well as in the Xe-rich phase. This strategy is significantly more cost-effective and energy-efficient than the current cryogenic distillation methods. In phase I, we will synthesize various Ionic Covalent Frameworks (ICOFs) with temperature-regulated dynamic pores from readily available alcohols and organoboron reagents in a cost-effective and scalable manner. Both static and dynamic gas adsorption experiments will be performed to select the best performing ICOF materials that offer high Kr/Xe separation selectivity. Lastly, we will evaluate recyclability of end-of-life ICOF materials to further improve the sustainability and cost-effectiveness of the proposed approach. During phase II and beyond, we will explore scaling up the technology with potential commercial partners for further development and commercialization. The concentrations of Xe and Kr in the exhaust gases from nuclear fission are 4,500 times and 40 times higher than in air, respectively, with xenon at 400 ppm and krypton at 40 ppm. Therefore, exhaust gases can be an important secondary source for harvesting pure xenon. Efficient recovery and separation of krypton and xenon from nuclear fuel waste thus not only mitigates environmental pollution but also brings considerable economic benefits. These gases hold high commercial values, with xenon priced at approximately $1,800 per kilogram and krypton at around $300 per kilogram. Therefore, our project not only contributes to environmental stewardship but also stimulates technological innovation, economic growth, and the expansion of markets for these high-value gases.