Preparation of ZIF-67@PDA/fluorine-containing polyimide mixed matrix membrane and gas separation performance

Metal-organic framework (MOF)/polymer mixed matrix membranes (MMMs) have shown great promising application in gas separation fields by combining the features of MOF with molecular sieving effect and polymer matrix with lower cost, better processing properties, and high mechanical strength. However, their applications are greatly limited due to MOF poor dispersity in polymer matrix. The metal frame material ZIF-67 was synthesized by solvothermal method, and the polydopamine (PDA) layer was modified on the surface of ZIF-67 by solution oxidation to prepare ZIF-67@PDA nano-porous materials. The ZIF-67/FPI composite membrane and ZIF-67@PDA/FPI composite membrane with different mass fraction were prepared by using 4, 4'-oxydianiline-2, 2'-(hexafluoroisopropylidene)diphthalic anhydride (ODA-6FDA) fluorinated polyimide (FPI) as matrix and ZIF-67 and ZIF-67@PDA as fillers. The structure and properties of MMMs were characterized by FTIR, WAXD, TGA, SEM, specific surface and pore size distribution analyzer, gas permeant, and the permeability of four gases including N₂, O₂, CO₂, and He was tested. The results show that the nanomicro porous material ZIF-67 modified with polydopamine can disperse uniformly in the polymer matrix and provides fast channels for the passage of gas molecules, and exhibits good thermal stability. ZIF-67@PDA has a good affinity for CO₂, which is beneficial to improve the CO₂/N₂ selectivity. When the ZIF-67@PDA loading is 10wt%, the CO₂ permeability and CO₂/N₂ selectivity of MMMs increases by 131% and 50% respectively compared with pure FPI membrane, and MMMs show good gas separation performance.