Rational Creation of the Second Xe-Matched Adsorption Site in a Covalent Organic Framework via a Digging-Patching...

It is highly desirable to achieve both high Xe uptake and big Xe/Kr selectivity for Xe/Kr separation but it remains a long-term challenging issue due to an inherent trade-off between Xe uptake and Xe/Kr selectivity. We herein show a general and convenient digging-patching (DP) strategy in a covalent organic framework for simultaneously boosting Xe uptake and Xe/Kr selectivity. Notably, this DP method plays dual functions, creating dual micropores to match the kinetic diameter of Xe and increasing the porosity with surface areas from 573 m²/g to 1134 m²/g. As a result, we observed a large enhancement in both Xe uptake and selectivity from 1.37 to 2.29 mmol/g and from 14.6 to 22.0. It was found that structural design by the digging (D) operation in the DP strategy functions to create dual adsorption sites for Xe, which helps to improve Xe uptake, while the patching (P) operation functions to optimize the second adsorption site with its size close to the kinetic diameters of Xe, which helps to improve Xe/Kr selectivity, finally breaking the inherent trade-off between Xe uptake and Xe/Kr selectivity. The actual Xe-Kr separation capability was further confirmed by breakthrough experiments. This work highlights a fundamental structural design of two-dimensional COFs for size-based gas separation.