Metal-organic frameworks with optimal pore cages for high Xe adsorption and efficient Xe/Kr separation

Abstract

Separation of Xe/Kr mixtures obtained from the by-products of air separation is a paramount industrial process but still a formidable challenge due to the inherent inertness and close physicochemical properties of Xe and Kr. Herein we present two isostructural metal–organic frameworks (MOF), MIL-88B-tpt and MIL-88B-tpt-F, which were found to have high Xe uptake capacities and efficient Xe/Kr separation resulted from the suitable pore caged structures and pore sizes. Remarkably, MIL-88B-tpt exhibits a record-high Xe sorption capacity of 210.2 cm3 g−1 at 273 K and 1 bar. The highest Xe uptake and moderate Xe/Kr separation selectivity endow MIL-88B-tpt with high-performance Xe/Kr separation, ranking it among the best-performing MOF. The comparable Xe/Kr separation performance observed for the fluorinated version of MIL-88B-tpt-F and MIL-88B-tpt indicates the fluorine groups have a negligible effect on the Xe/Kr separation. Thus the efficient Xe/Kr separation for both MOF is mainly resulted from the optimal pore cages for Xe trapping and the well-matched pore sizes for Xe atom. The extensive Xe···framework interactions unveiled by computational simulations facilitate a strong binding affinity to Xe, thus leading to the highest Xe uptakes and efficient separation ability for both MOF.