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

Herein, we report a systematic approach to engineer a metal-organic framework (MOF) structure and polymer-MOF interface in a mixed-matrix membrane (MMM) to achieve high molecular separation performance and plasticization resistance. Compared to traditional zeolite imidazolate framework-8 (ZIF-8) nanoparticles, hierarchical ZIF-8-NH 2 nanoparticles with a relatively small concentration of amine-functionality (~5 mol%) were prepared based on the mixed organic ligands used in the microemulsion synthetic procedure. The hierarchical MOF structure provides fast molecular transport pathways due to the MOF-MOF percolated network and high MOF packing density. Moreover, the interfacial interaction between the carboxylated polyimide and amine-functionalized MOF exhibits significant characteristic modification in the MMMs, such as peak shifts in the FT-IR spectra and glass transition temperature. In addition, they show tuned surface hydrophobic/hydrophilic properties and high chemical stability toward conventional solvents. The hierarchical ZIF-8-NH 2 MMMs exhibit a significantly improved gas permeability because of the accelerated molecular diffusion through the direction-oriented porous nature of the percolated MOF. For example, the ZIF-8-NH 2 30 wt% MMM showed ~6- and ~4-fold enhanced H 2 (761.7 Barrer) and CO 2 (552.4 Barrer) permeabilities when compared to those of a pure polyimide film, respectively. In particular, the hierarchical MOF structure and strong polymer-MOF interfacial interaction enhances the rigidity of the polymer chain by physically preventing polymer chain swelling and interfacial-chemically hindering the mobile polymer chains at high gas feed pressure. Thus, the hierarchical ZIF-8-NH 2 MMM approach simultaneously improves the molecular transport and plasticization resistance in the separation process. This novel strategy provides remarkable insight for the rational design of the polymer and MOF constituents in the MMM system.