(609a) Synthesis and Characterization of Multi-Dimensional Metal–Organic Frameworks for Efficient Gas Separations with Percolation Networks

Metal–organic frameworks (MOFs) are porous crystals formed from organic ligands coordinated with metal ions or metal clusters. These materials are of great interest for various applications because of their wide range of accessible structures and chemistries. However, for gas separation membranes, MOF crystals are difficult to process into continuous films or fibers. Instead, MOFs are often incorporated into polymer matrices to form mixed-matrix membranes (MMMs). Even with this approach, particles tend to aggregate, resulting in non-selective defects. Thus, it is challenging to form MMMs where MOFs are percolated, precluding the possibility of interconnected channels for enhanced separation performance even at near theoretical maximum loading.. To overcome these challenges, we have developed a method to synthesize multi-dimensional MOFs (MDMs) that form a hierarchical network in MMMs without the need for particle alignment. These MDM structures have 1D thread-like branches of ~10 nm diameter at the length of tens of nanometers interconnected at random branch points to form 3D hierarchical structures. Thus, built-in percolation networks can be formed. For this project, MDMs were dispersed in polymer matrices at different loadings. To demonstrate the networks’ benefits, we compared gas transport properties of the MMMs formed with MDMs and traditional MOFs that are roughly spherical. Changes in transport properties with different MOF loadings suggest percolation behavior, a phenomena that is further supported by selective MOF digestion experiments. The interconnected network of these MDMs provides a secondary benefit of enhanced plasticization resistance by confining polymer chain mobility in the intersticial spaces between MDM particles. Results will be presented for several separations of interest with a particular emphasis on H₂-based separations, where we have used this percolation strategy to surpass upper bound performance limits. Furthermore, we will present evidence that polymer confinement can be used to restrict MOF ligand rotation, and thus shift size sieving to smaller molecules. The approach, implications, and future directions of applying MDM materials to membrane-based gas separations will be outlined in this presentation.