Hierarchical Amine-Functionalized ZIF‑8 Mixed-Matrix Membranes with an Engineered Interface and Transport Pathway for Efficient Gas Separation

Herein, we report a facile approach to engineer a metal–organic framework (MOF) structure and polymer–MOF interface in a mixed-matrix membrane (MMM) to achieve high gas separation performance and plasticization resistance. Hierarchical ZIF-8-NH2 nanoparticles with a relatively small concentration of amine functionality (∼5 mol %) were prepared. The hierarchical MOF structure provides fast molecular transport pathways because of the MOF–MOF percolated network and high MOF packing density. Moreover, the interfacial interaction between the 6FDA-DAM:DABA(3:2) polyimide and the amine-functionalized MOF enhances the chemical stability and polymer chain rigidity. The hierarchical ZIF-8-NH2 MMMs exhibit a significantly improved gas permeability because of the accelerated molecular diffusion through the direction-oriented MOF pathway. For example, the ZIF-8-NH2 30 wt % MMM showed ∼6- and ∼4-fold enhanced H2 (761.7 Barrer) and CO2 (552.4 Barrer) permeabilities when compared to those of a pure polyimide film, with an H2/CH4 selectivity of 35.7 and CO2/CH4 selectivity of 25.9. Additionally, the MMM exhibits an improved plasticization resistance due to the connected MOF structure and MOF–polymer interaction. This strategy provides remarkable insight into the rational design of the polymer and MOF constituents in the MMM system

Ionic Cross-Linked MOF-Polymer Mixed-Matrix Membranes for Suppressing Interfacial Defects and Plasticization Behavior

To address the plasticization phenomenon and MOF-polymer interfacial defects, we report the synthesis of ionic cross-linked MOF MMMs from a dual brominated polymer and MOF components by using N,N′-dimethylpiperazine as the cross-linker. We synthesized brominated MIL-101­(Cr) nanoparticles by using mixed linkers and prepared brominated polyimide (6FDA-DAM-Br) to form ionic cross-linked MMMs. The gas permeation properties of the polyimide, ionic cross-linked MOF-polymer MMMs, and non-cross-linked MOF-polymer MMMs with various MOF weight loadings were investigated systematically to effectively understand the effects of MOF weight loading and ionic cross-linking. The ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly enhanced gas permeability with an H2 permeability of 1640 Barrer and CO2 permeability of 1981 Barrer and slightly decreased H2/CH4, H2/N2, CO2/CH4 and CO2/N2 selectivities of 16.9, 15.4, 20.5, and 18.6, respectively. The H2 and CO2 permeabilities are approximately 2–3 fold higher than those of the pure polyimide (6FDA-DAM) membrane. Moreover, the ionic cross-linked 40 wt % MOF-polymer MMM exhibited significantly increased resistance to plasticization. This is because the brominated MOF incorporation boosted molecular transport and polymer chain rigidity, and ionic cross-linking further reduced the number of interfacial defects and polymer chain mobility

Hollow Heteropoly Acid-Functionalized ZIF Composite Membrane for Proton Exchange Membrane Fuel Cells

Heteropoly acids (HPAs) have been used in perfluorinated sulfonic acid polymers such as Nafion or Aquivion to form organic/inorganic composite membranes with improved proton conductivity and water management ability. However, the HPA has a low BET surface area with water-soluble characteristics, which prevents enhancement in the number of proton-transferable sites and accelerates HPA leaching while operating the proton exchange membrane fuel cells (PEMFCs). The HPA was functionalized on zeolite imidazolate framework-67 (ZIF-67) nanoparticles to address these drawbacks. Incorporating it into the MOF made it water insoluble and enhanced the internal surface area, leading to a good proton conductor. Using a synthetic approach, we were able to form HPA-functionalized ZIF-67 (HZF), which can be optimized with simple compositional modifications and whose HPA content is controllable. The HZF nanoparticles exhibited a hollow structure that formed an HPA–ZIF shell layer because the dissociated cobalt ion and 2-methylimidazole diffused from the core side to the surface layer to interact with the HPA. The HZF/Aquivion composite membranes exhibited excellent mechanical properties and good resistance to the polymer chain swelling phenomenon. The electrochemical properties of the HZF/Aquivion composite membranes with various HZFs were characterized to determine the optimal HPA content in the HZF nanoparticles. The 3 wt % hollow HZF/Aquivion composite membrane with the appropriate HPA content exhibited higher proton conductivities than the pure Aquivion membrane, measuring 0.14 S/cm at 25 °C and 100% RH and 0.09 S/cm at 80 °C and 30% RH. This result indicates that the hollow HZF/Aquivion composite membrane can provide efficient proton transfer and water management ability, suggesting a good strategy for the PEMFC operation