Metal-Organic Frameworks for Xe/Kr Separation

Northwestern researchers have developed an improved method to separate, concentrate and purify xenon and krypton from air using metal-organic frameworks (MOFs). Abstract Xe and Kr are important noble gases with lighting, laser, medicine, nuclear, and industrial applications. MOFs are suitable for chemical separation because they are nanoporous materials that are composed of organic linkers and metal corners that self-assemble in solution to stable crystalline structures. Unlike the energy intensive process of current methodology, this improved technology offers an ambient temperature alternative. The investigators determined the optimal characteristics of MOFs for Xe/Kr separation based upon the ideal adsorbed solution theory (IAST), which accurately predicts gas selectivity and mixture behavior from single-component isotherms. More specifically, they identified ideal linkers, topologies, pore sizes and metal atoms that would optimize xenon adsorption over krypton in multi-component mixtures. They tested mixture isotherms that had a fixed 80/20 molar composition of Kr to Xe in the gas phase which is representative of industrial mixtures. Simulations identified a single MOF exhibiting excellent Xe selectivity over a wide pressure range (0.1-1.0 MPa) and practical capacity, compared to other reported MOFs (Figure 1). This pressure range is particularly attractive because gas separation in industrial pressure swing adsorption processes operates between 0.1 and 0.5 MPa. This combination of Xe/Kr selectivity, capacity, and suitability to pressure swing adsorption promises an efficient new separation technology for noble gas separation.