SCALABLE FABRICATION OF MIXED-MATRIX MEMBRANES FOR GAS SEPARATIONS: POLYMER-MODIFICATION-ENABLED IN-SITU METAL-ORGANIC FRAMEWORK FORMATION

Mixed-matrix membranes (MMMs) combining the advantages of polymer and inorganic membranes have been intensively studied for gas separations by incorporating molecular sieves such as zeolite, silica, and metal-organic framework (MOF) to a polymer matrix. Despite the potential, there have been no commercial applications of MMMs due to the several challenging MMM processing issues. One of the major challenges is the difficulty of controlling defects of a thin selective layer and microstructure of polymer/filler upon the single-step MMM spinning at the same time. We investigated a new paradigm of scalable MMM fabrication, named polymer-modification-enabled in-situ metal-organic framework formation (PMMOF) by decoupling polymer membrane fabrication and filler incorporation. PMMOF involves four steps, hydrolysis, ion-exchange, ligand treatment, and imidization, enabling in-situ formation of metal-organic framework (MOF) fillers inside polymers. The first MMMs by PMMOF were demonstrated by in-situ forming zeolitic-imidazole framework-8 (ZIF-8) fillers up to 32.9 vol% in the 6FDA-DAM polymer. The binary C3H6/C3H8 separation performance of the MMMs showed much higher separation factors than conventionally-prepared 6FDA-DAM/ZIF-8 MMMs at similar filler loadings, satisfying the commercial C3H6/C3H8 separation performance criteria. For the more in-depth study of PMMOF, the actual reaction conditions synthesizing MOF crystals in a polymer free volume was investigated using the phase transformation of ZIF-7. In addition, the C3H6/C3H8 separation performance of MMMs by PMMOF was further improved by combining PMMOF with linker-doping strategy. Despite these successes, there were several remaining issues for PMMOF. Among them, most importantly, as the filler contents increased, the permeability of MMM decreased continuously despite the higher separation factors compared with those of MMMs prepared by conventional blending methods. To address this issue, cross-linked polyimides (i.e., 6FDA-DAM:DABA (3:2)) were used with different cross-linking degrees. It was found that the inherent rigidity of polymer as well as the swelling of the polymer followed by chain rearrangement were critical to prevent the severe permeation reduction. Finally, the first MMM module containing multi-stranded mixed-matrix hollow fiber membranes with submicron-thick selective skin layers was demonstrated by transforming a preformed module with PI-coated polyethersulfone hollow fibers using the PMMOF