Cooperative Reformable Channel System with Unique Recognition of Gas Molecules in a Zeolitic Imidazolate Framework with Multilevel Flexible Ligands

We report a cooperative reformable channel system in a ZIF-L crystal arising from coexistence of three types of local flexible ligands. The reformable channel is able to regulate permeation of a nonspherical guest molecule, such as N₂ or CO₂, based on its longer molecular dimension, which is in a striking contrast to conventional molecular sieves that regulate the shorter cross-sectional dimension of the guest molecules. Our density functional theory (DFT) calculations reveal that the guest molecule induces dynamic motion of the flexible ligands, leading to the channel reformation, and then the guest molecule reorientates itself to fit in the reformed channel. Such a unique “induced fit-in” mechanism causes the gas molecule to pass through six-membered-ring windows in the c- crystal direction of ZIF-L with its longer axis parallel to the window plane. Our experimental permeance of N₂ through the ZIF-L membranes is about three times greater than that of CO₂, supporting the DFT simulation predictions.