Bidirectional regulation of counterions in cationic MOFs to enhance CO2 separation efficiency in mixed-matrix membranes

Metal-organic frameworks (MOFs) have emerged as promising filler materials for gas separation applications; however, most conventional MOFs rely predominantly on a single dominant separation mechanism, which constrains their maximum achievable performance. To address this limitation, a series of isoreticular ionic metal-organic frameworks (iMOFs), denoted as Ni-btz-X (where X = Cl, Br, and BF 4 − ), were designed and synthesized through an anion regulation strategy. These iMOFs were further incorporated into a polymer of intrinsic microporosity (PIM-1) to construct mixed-matrix membranes (MMMs) for CO 2 /N 2 separation. Systematic investigations into the effects of anion substitution revealed that the Ni-btz-BF 4 /PIM-1 MMMs exhibited exceptional CO 2 recognition capabilities. This superior performance is attributed to the synergistic interplay between thermodynamic (enhanced CO 2 adsorption) and kinetic (selective molecular diffusion) factors, effectively overcoming the intrinsic limitations based on a single mechanism. The MMM containing 20 wt% Ni-btz-BF 4 achieved a CO 2 permeability of 10,595.2 barrer and a CO 2 /N 2 selectivity of 46.7, exceeding the 2019 Robeson upper bound. Further analyses employing solution-diffusion coefficients and grand canonical Monte Carlo (GCMC) simulations further confirmed that the counterions within the iMOFs precisely regulate the gas adsorption and diffusion behaviors by tuning pore size and enhancing affinity toward CO 2 molecules. Additionally, robust pressure resistance, adaptability to a wide temperature range, and commendable stability of 20% Ni-btz-BF 4 /PIM-1 MMM were also confirmed. This work highlights the potential of anion regulation as a versatile strategy for designing high-performance MOF-based MMMs, offering a promising solution for efficient CO 2 /N 2 separation in industrial applications.