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) ( a PEO) and UiO‐66‐NH 2 enabling strong hydrogen bonds between the two matrices. Increasing the UiO‐66‐NH 2 loading unexpectedly decreases CO 2 permeability in freestanding films, but it surprisingly leads to the best CO 2 /N 2 separation properties in the membranes at a loading of 10 mass% (CO 2 permeance of 2900 GPU and CO 2 /N 2 selectivity of 48). Nanoconfinement significantly influences the morphological and gas separation properties of the MMM layer. The membrane with 10 mass% UiO‐66‐NH 2 demonstrates mixed‐gas CO 2 permeance of 1400 GPU and CO 2 /N 2 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 CO 2 /N 2 separation performance when challenged with real flue gas for 700 h continuously.