A Tale of Two Separation Properties: Bulk and Thin Films of Mixed Matrix Materials

Abstract Mixed matrix materials (MMMs) integrating excellent processability from polymers and distinct separation properties from nanofillers are of interest for membrane gas separations, and they are often made into freestanding films (>100 µm) to demonstrate superior gas separation properties. However, they are difficult to fabricate into thin‐film nanocomposite (TFN) membranes due to interfacial incompatibility between polymers and nanofillers. Here TFN membranes based on MMMs (as thin as 200 nm) are successfully developed comprising amorphous poly(ethylene oxide) (aPEO) and UiO‐66‐NH₂ enabling strong hydrogen bonds between the two matrices. Increasing the UiO‐66‐NH₂ loading unexpectedly decreases CO₂ permeability in freestanding films, but it surprisingly leads to the best CO₂/N₂ separation properties in the membranes at a loading of 10 mass% (CO₂ permeance of 2900 GPU and CO₂/N₂ selectivity of 48). Nanoconfinement significantly influences the morphological and gas separation properties of the MMM layer. The membrane with 10 mass% UiO‐66‐NH₂ demonstrates mixed‐gas CO₂ permeance of 1400 GPU and CO₂/N₂ selectivity of 76 in the presence of 1.2 mol% water vapor at ≈23 °C, surpassing Robeson's upper bound. The membrane also demonstrates stable CO₂/N₂ separation performance when challenged with real flue gas for 700 h continuously.