Abstract

Abstract Molecular sieving is an ideal separation mechanism, but controlling pore size, restricting framework flexibility, and avoiding strong adsorption are all very challenging. Here, we report a flexible adsorbent showing molecular sieving at ambient temperature and high pressure, even under high humidity. While typical guest‐induced transformations are observed, a high transition pressure of 16.6 atm is observed for C 2 H 4 at 298 K because of very weak C 2 H 4 adsorption (~16 kJ mol −1 ). Also, C 2 H 6 is completely excluded below the pore‐opening pressure of 7.7 atm, giving single‐component selectivity of ca. 300. Quantitative high‐pressure column breakthrough experiments using 1 : 1 C 2 H 4 /C 2 H 6 mixtures at 10 atm as input confirm molecular sieving with C 2 H 4 adsorption of 0.73 mmol g −1 or 32 cm 3 (STP) cm −3 and negligible C 2 H 6 adsorption of 0.001(2) mmol g −1 , and the adsorbent can be completely regenerated by inert gas purging. Furthermore, it is highly hydrophobic with negligible water adsorption, and the C 2 H 4 /C 2 H 6 separation performance is unaffected at high humidity.