(697d) Enhanced Gas Separation Performance and Plasticization Resistance through Mixed Matrix Membrane Approach Incorporating Multi-Dimensional Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are an emerging class of crystalline materials that are formed by coordination reactions between metal cations and organic ligands. MOFs exhibit high internal surface area, tunable pore size, and tunable chemical functionality, thus making these materials interesting for gas storage and separation applications. In regard to gas separations, mixed-matrix membranes (MMMs) are formed by combining MOFs into polymers to achieve high separation performance while maintaining processability. This study reports a new type of multi-dimensional MOF structure that has 1D branches that are interconnected into 3D hierarchical structures. The multi-dimensional MOFs were incorporated into a polyimide matrix to form MMMs with uniform MOF dispersion. At relatively low MOF loadings, the high effective aspect ratio of interconnected branches creates a percolation network, which results in an increase in gas permeability far above the theoretical permeability predicted by the Maxwell Model. Additionally, the hierarchical architecture leads to a suppression of polymer chain relaxation, thereby reducing effects from plasticization. This study focuses on two multi-dimensional MOFs with distinct mechanisms for molecular transport performance: HKUST-1, which has open-metal sites, has beneficial sorption selectivity, and ZIF-8, which has an appropriately sized molecular-sieving window, has beneficial size-sieving selectivity. Both structures are formed into multi-dimensional MOFs, and a comparison is made between their separation performance and plasticization resistance relative to their bulk counterparts in MMMs. The multi-dimensional structure can improve separation performance for certain separations and simultaneously improve resistance to plasticization.