Mesopore engineering of ZIF-8 by [Bmim][Tf2N] positioning into nanocage for enhanced CO2 capture

The regulation on cage size and CO2-philic environment of metal-organic frameworks (MOFs) is essential to improve CO2 capture. In this work, [Bmim][Tf2N] was proposed to position into cages of ZIF-8 to achieve the selective permeation of CO2 by pore screening and CO2 adsorption. [Bmim][Tf2N] was introduced into cages of ZIF-8 by "adsorption-infiltration" strategy. Due to the strong interaction between [Tf2N]- and ZIF-8, the cages present a CO2-philic environment, which preferentially attracts CO2 to enter these cages, ensuring the dissolution selectivity of CO2. Besides, the proper cage size tuned by [Bmim][Tf2N] provides CO2 with a faster transport channel due to its smaller average kinetic diameter than N2, guaranteeing the diffusion selectivity of CO2. Consequently, the CO2 separation in [Bmim][Tf2N]@ZIF-8/Pebax MMMs is intensified. With increasing the [Bmim][Tf2N] content in cages of ZIF-8, the pore volume by BET results decreases and the CO2 adsorption from molecular simulation increases. In response, the CO2/N2 selectivity of [Bmim][Tf2N]@ZIF-8/Pebax MMMs is enhanced constantly. Meanwhile, the CO2 permeability is declined due to the narrowed cage size. All of these results demonstrate the significance of pore screening and CO2 adsorption induced by [Bmim][Tf2N]. Specifically, [Bmim][Tf2N]8@ZIF-8/Pebax MMM at filler loading of 15 wt% exhibits the CO2 permeability of 108.3 Barrer and the CO2/N2 selectivity of 86.7, exceeding the 2008 Robeson upper bound. Especially the CO2/N2 selectivity locates at a higher level than those of most reported works, indicating that appropriate tuning of the physicochemical properties within the nanocages of MOFs is significative to improve CO2 capture.