Engineering Multiple Transport Pathways in MMMs Using COF@MOF Hierarchical Nanostructures for Efficient CO ₂ Separation

A novel porous crystalline composite with a hierarchical architecture is constructed by encapsulating a covalent organic framework (COF, TpPa-1) within an amino-functionalized zeolitic imidazolate framework (ZIF, NH 2 -ZIF-8; pore size ∼3.4 Å). This hierarchical structure is employed as an engineered filler for fabricating polyethylene oxide (PEO)-based mixed-matrix membranes (MMMs) to achieve efficient CO 2 separation. Unlike conventional MMMs containing single fillers, the porous crystalline COF@MOF nanosheets (TpPa-1@NH 2 -ZIF-8) designed in this study significantly enhance performance through a multimechanism synergistic strategy: TpPa-1 not only exhibits high CO 2 adsorption capacity but also serves as a two-dimensional transport channel that facilitates rapid CO 2 diffusion. Concurrently, the NH 2 -ZIF-8 coating enables precise molecular sieving, effectively rejecting larger molecules such as N 2 and thereby enhancing sieving selectivity. Moreover, surface amino groups impart hydrophilicity, enabling water adsorption that maintains surface wetting. The adsorbed water molecules subsequently undergo reversible interactions with CO 2, further enhancing affinity selectivity and transport behavior. This synergistic structure, which integrates molecular sieving, masking effects, and affinity interactions, transcends conventional solution–diffusion mechanisms and substantially elevates separation efficiency. TpPa-1@NH 2 -ZIF-8 composite nanosheets are prepared via an in situ growth method. They are uniformly dispersed within a PEO matrix and subjected to UV cross-linking to fabricate MMMs. Characterization shows that PEO/NH 2 -ZIF-8@TpPa-1 MMMs containing 0.4 wt% filler exhibit outstanding CO 2 separation performance: the CO 2 permeability reaches 760.85 Barrer, corresponding to a 292% increase relative to the pristine PEO membrane, while the CO 2 /N 2 selectivity attains 65.09, representing a 146% enhancement. These results conclusively demonstrate the membrane's exceptional efficiency and application potential in gas separation. Consequently, the strategy of constructing a COF@MOF hierarchical pore structure while integrating masking effects and multimechanism transport pathways provides a promising approach for the development of high-performance gas separation membranes.