Visualizing MOF mixed matrix membranes at the nanoscale: Towards structure-performance relationships in CO₂/CH₄ separation over NH₂-MIL-53(Al)@PI

Mixed matrix membranes (MMMs) composed of metal organic framework (MOF) fillers embedded in a polymeric matrix represent a promising alternative for CO₂ removal from natural gas and biogas. Here, MMMs based on NH ₂-MIL-53(Al) MOF and polyimide are successfully synthesized with MOF loadings up to 25 wt% and different thicknesses. At 308 K and ΔP = 3 bar, the incorporation of the MOF filler enhances CO₂ permeability with respect to membranes based on the neat polymer, while preserving the relatively high separation factor. The rate of solvent evaporation after membrane casting proves key for the final configuration and dispersion of the MOF in the membrane. Fast solvent removal favours the contraction of the MOF structure to its narrow pore framework configuration, resulting in enhanced separation factor and, particularly, CO₂ permeability. The study reveals an excellent filler-polymer contact, with ca. 0.11% void volume fraction, for membranes based on the amino-functionalized MOF, even at high filler loadings (25 wt%). By providing precise and quantitative insight into key structural features at the nanoscale range, the approach provides feedback to the membrane casting process and therefore it represents an important advancement towards the rational design of mixed matrix membranes with enhanced structural features and separation performance. Mixed-matrix membranes composed of the flexible NH ₂-MIL-53(Al) metal-organic framework (MOF) embedded in polyimide represent a promising alternative for CO₂ removal from natural gas and biogas. Quantitative focused ion beam scanning electron microscopy (FIB-SEM) tomography evidences an excellent filler-polymer contact. The loading of MOF crystals and its framework configuration, which can be adjusted during membrane casting, are key parameters for the gas separation performance.