Ionic liquid tuning nanocage size of MOFs through a two-step adsorption/infiltration strategy for enhanced gas screening of mixed-matrix membranes

Cage-type MOFs based Mixed-matrix membranes (MMMs) hardly exhibit screening effect of gas molecules due to the flexible aperture. Regulation of nanocages of MOFs by Ionic Liquid (IL) has been proved to be effective. Therefore, in this work, an IL adsorption/infiltration two-step strategy is proposed to confine IL into the nanocages of MOFs, achieving fine tuning of effective cage size for screening gas pair with similar size. [Bmim] [PF6] are first adsorbed on the outer-surface of ZIF-8, then through heat treatment, the aperture of ZIF-8 is dilated, and the bigger [Bmim][PF6] molecules infiltrate through the dilated aperture into the nanocage driven by thermodynamics and differential concentration. When the temperature is down to room temperature, the aperture is narrowed to 3.4 angstrom, thus [Bmim][PF6] are confined into nanocages, which are confirmed by PXRD, TGA and FT-IR characteristics. By controlling IL content in the cage, the effective cage size of ZIF-8 is tuned to specific slope among dynamic diameters of CO2 over N-2, which is perfectly reflected in the CO2/N-2 separation. Compared with ZIF-8/Pebax MMMs, the IL@ZIF-8/Pebax MMMs exhibit improved CO2/N-2 selectivity but decreased CO2 permeability, demonstrating effectiveness of cage size adjustment. Furthermore, with the IL molecules inside ZIF-8 cage increasing, the CO2/N-2 selectivity of the MMMs continues to rise, indicating cage size screening effect of ZIF-8 by IL infiltration. Especially, the IL8@ZIF-8/Pebax MMMs exhibit the best gas separation performances with CO2 permeability of 117 Barrer and CO2/N-2 selectivity of 84.5, beyond the 2008 Robinson upper bound. The proposed strategy provides an effective but facile way to confine IL into the cages of MOFs to precisely screen gas pairs with similar size and can be easily expanded to other IL and MOFs system.