Restricting Linker Rotation in Nanocages of ZIF-8 Membranes using Crown Ether "Molecular Locks" for Enhanced Propylene/Propane Separation.

ZIF-8 membranes have long been prized for their exceptional CH/CH separation performance. On the other hand, ZIF-8 has structural flexibility, where the external pressure triggers channel expansion, potentially deteriorating the molecular sieving ability. Here, we demonstrate a reliable strategy to fine-tune the flexible pore structure of ZIF-8 by embedding crown ether within a ZIF-8 membrane. Benzo-15-crown-5 (15C5) was selected as the cavity occupant and perfectly confined in the sodalite (SOD) cage of ZIF-8. The 15C5 molecules, which have a size comparable to the nanocage, impose a spatial constraint on linker rotation, enabling the phase transition to a rigid structure in the flexible ZIF-8. The corresponding 15C5@ZIF-8 membranes achieve an ultrahigh CH/CH selectivity of 220, outperforming that of most membranes. Unlike their flexible counterparts, the resulting membranes manifest a positive increase in the CH/CH separation factor with elevated pressure, securing a record-high CH/CH separation factor of 331 under 7 bar. More importantly, extraordinary separation stability was demonstrated with continuous measurement, which is highly desirable for practical applications.