Selective adsorption of trace CO2 by immobilized amino acid ionic liquids with ultra-micropores based on amino MOFs

Great attention has been paid to effectively capture trace CO₂ in air or a confined space for ensuring human safety, but it remains challenging to enhance CO₂ capacity and selectivity concurrently. In this study, metal–organic frameworks (MOFs) were combined with amino groups (NH₂-MIL-125(Ti) and NH₂-UIO-66(Zr)) and amino acid anion-functionalized ionic liquids (ILs), namely 1-Butyl-3-methylimidazolium glycinate ([BMIm]Gly) and 1-Butyl-3-methylimidazolium arginine ([BMIm]Arg) to prepare various ionic liquid (IL) composites containing various IL contents. Relative to pristine supports, incorporating ILs significantly enhanced CO₂ capacity together with CO₂/N₂ selectivity, in particular in the confined spaces (<5000 ppm) or in air (415 ppm). At a 50 wt% IL loading, new ultra-micropores (<0.65 nm) could be obtained in IL-NH₂-UIO-66(Zr) composite, but they did not occur within pristine supports or the rest IL-NH₂-UIO-66(Zr) (10 wt%, 30 wt% and 70 wt%). Among them, CO₂ uptake of 50 wt% [BMIm]Arg-NH₂-UIO-66(Zr) peak at 0.0005 and 0.005 bar (2.93 and 3.87 mmolCO₂/g-adsorbent, respectively) at 313 K; besides, recyclability significantly increased compared with the latest values reported. Additionally, the excellent optimal CO₂/N₂ selectivity was determined to be 8916 at 0.005 bar and 288 K, which increased by 254.7 folds relative to NH₂-UIO-66(Zr). In the meantime, as revealed by mixed gas breakthrough experimental results, 50 wt% [BMIm]Arg-NH₂-UIO-66(Zr) showed superb CO₂ separation effect in air and simulated confined spaces. Such ultra-high CO₂ separation efficacy is associated with the synergistic effect of chemical interaction of IL anion with amino groups in MOFs and CO₂ and the novel ultra-micropore effect. Findings in the present work shed more lights on designing hierarchically porous IL composites that possess ultra-micropores to efficiently remove trace CO₂.