(602i) Post-Synthetic Modification of Hierarchical Branched ZIF Materials

Post-synthetic ligand exchange of metal-organic frameworks (MOFs) has drawn increasing interest over the past few years due to the method's ability to produce porous material structures that otherwise could not be achieved. Recently, our group has developed a method of synthesizing hierarchical branched MOF nanostructures with one-dimensional branches linked into three-dimensional branch and node structures through a modulated synthesis.^1,2 These connected MOF crystals with high aspect ratios show interesting and sometimes exceptional membrane separations performance due to the connectedness of the MOF crystals. In this work, the post-synthetic exchange of hierarchical branched ZIF-8 to hybrid ZIF-8:7 and ZIF-8:90 is developed. ZIF-7 and ZIF-90, exhibiting desirable characteristics for mixed matrix membrane applications, have been challenging to form into hierarchical branched structures with the same success as ZIF-8. This work's first focus is to better understand the role of different synthetic variables on the resulting branched hierarchical structure through ligand exchange. The exchange time, precursor concentrations, and additional modulators during ligand exchange are all considered for their role in retaining the branched structure while exchanging ligands. The change in the branched structure and composition of the ligand-exchanged MOF is studied through PXRD, NMR, and TEM. The new mixed-ligand and branched MOFs will then be discussed, focusing on morphology, MOF chemistry, and gas separation performance of nascent mixed-matrix membranes at variable loadings.

(1) Lee, H.; Chi, W. S.; Lee, M. J.; Zhang, K.; Edhaim, F.; Dewitt, S. J. A.; Smith, Z. P. Branched MOF Percolation Network Suppressing Gate-Opening for Fast and Selective Gas Transport. (In Prep.) 2022.

(2) Chi, W. S.; Sundell, B. J.; Zhang, K.; Harrigan, D. J.; Hayden, S. C.; Smith, Z. P. Mixed-Matrix Membranes Formed from Multi-Dimensional Metal–Organic Frameworks for Enhanced Gas Transport and Plasticization Resistance. ChemSusChem 2019, 12 (11), 2355–2360. https://doi.org/https://doi.org/10.1002/cssc.201900623.