Confinement of Ionic Liquids in Nanocages: Tailoring the Molecular Sieving Properties of ZIF‐8 for Membrane‐Based CO 2 Capture

Fine‐tuning of effective pore size of microporous materials is necessary to achieve precise molecular sieving properties. Herein, we demonstrate that room temperature ionic liquids can be used as cavity occupants for modification of the microenvironment of MOF nanocages. Targeting CO 2 capture applications, we tailored the effective cage size of ZIF‐8 to be between CO 2 and N 2 by confining an imidazolium‐based ionic liquid [bmim][Tf 2 N] into ZIF‐8’s SOD cages by in‐situ ionothermal synthesis. Mixed matrix membranes derived from ionic liquid‐modified ZIF‐8 exhibited remarkable combinations of permeability and selectivity that transcend the upper bound of polymer membranes for CO 2 /N 2 and CO 2 /CH 4 separation. We observed an unusual response of the membranes to varying pressure, that is, an increase in the CO 2 /CH 4 separation factor with pressure, which is highly desirable for practical applications in natural gas upgrading.