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, NH₂-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 CO₂ separation. Unlike conventional MMMs containing single fillers, the porous crystalline COF@MOF nanosheets (TpPa-1@NH₂-ZIF-8) designed in this study significantly enhance performance through a multimechanism synergistic strategy: TpPa-1 not only exhibits high CO₂ adsorption capacity but also serves as a two-dimensional transport channel that facilitates rapid CO₂ diffusion. Concurrently, the NH₂-ZIF-8 coating enables precise molecular sieving, effectively rejecting larger molecules such as N₂ 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 CO₂, 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@NH₂-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/NH₂-ZIF-8@TpPa-1 MMMs containing 0.4 wt% filler exhibit outstanding CO₂ separation performance: the CO₂ permeability reaches 760.85 Barrer, corresponding to a 292% increase relative to the pristine PEO membrane, while the CO₂/N₂ 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.