Selective Collection of Xe under Low Pressure at Room Temperature Driven by the Formation of Ag(I)–Xe Compound Observed in MFI Zeolite Pores

Rare gas elements are chemically stable because of their closed-shell electron configuration, which makes the collection and purification of rare gases very difficult. Here, we present a new concept, orbital trap of Xe, for these applications. This concept was discovered in the Xe adsorption/separation process induced by Ag(I) sites in MFI zeolite pores under low pressure at room temperature. Experiments and DFT calculations showed that Ag(I) ions coordinated to zeolite lattice oxygen have excellent electron accepting property, which induces Xe-5p→Ag(I)-5s electron-donation interactions and forms a stable σ-bond with Xe even in the low-pressure region and at room temperature. On the other hand, in the case of Kr, the relatively high energy of the Kr-4p orbital makes the Kr-4p→Ag(I)-5s electron-donating interaction unstable and stable bonds are not formed. Thus, a driving force to discriminate between Xe and Kr occurs in the Ag(I)-MFI pore. Separation of Xe from Xe/Kr/O2 mixtures at ambient temperature, simulating the practical conditions, was indeed achieved using Ag(I)-MFI with high concentrations of Ag(I) sites, indicating that the orbital trap of Xe is a promising and efficient approach for selective Xe collection.