Mixed matrix and thin-film nanocomposite membranes of PIM-1 and hydrolyzed PIM-1 with Ni- and Co-MOF-74 nanoparticles for CO₂ separation: Comparison of blending, grafting and crosslinking fabrication methods

The prototypical polymer of intrinsic microporosity, PIM-1, was synthesized with
branched topology, then acid-hydrolyzed to introduce carboxylic acid functional groups (cPIM-1). Self-standing membranes and thin film nanocomposite (TFN) membranes were fabricated from PIM-1, cPIM-1, and Ni- or Co-MOF-74 nanoparticles (NPs), with a particular focus on TFNs for industrial viability. TFN membranes on polyacrylonitrile support were fabricated utilizing the following methods: (i) conventional blending; (ii) a grafting reaction between the hydroxyl group in the MOF-74 NPs and fluoro- groups of the chain-ends during PIM-1 synthesis; and (iii) an in-situ cross-linking reaction between the carboxylic acid groups in cPIM-1 and the metal ions of MOF-74 synthesized in-situ. Both conventional blending and grafting of PIM-1 with MOF-74 showed significant increase in permeance in TFN membranes. A grafted PIM-1 TFN membrane produced a CO₂ permeance of 9600 GPU, which is roughly 170% higher than a pristine PIM-1 thin film composite (TFC) membrane, while also improving the selectivity. After 28 days of aging, a blended TFN membrane based on PIM-1 showed a CO₂ permeance of 1000 GPU and selectivities of 20.0 and 20.5 for CO₂/N₂ and CO₂/CH₄, respectively. Crosslinked cPIM-1 TFNs showed significant improvement in ideal selectivity from 65 in cPIM-1 to over 90 for CO₂/N₂, with permeance almost unchanged after crosslinking with Ni-MOF-74. However, small differences in the polyacrylonitrile support can result in significant changes in gas separation behavior. In mixed 3 gas conditions, crosslinked cPIM-1/Co-MOF-74 produced an impressive 1842 GPU and CO₂/N₂ selectivity of 33. These results fall within the performance range that is suitable for post combustion carbon capture.