6FDA-DAM Polyimide-Based Mixed Matrix Membranes with Functionalized UiO-67 Nanoparticles for Improved CO 2 Separation Performance

This work presents a 6FDA-DAM polyimide (PI) ((4,4′-hexafluoroisopropylidene) diphthalic anhydride 1,3,5-trimethyl-2,6-phenylenediamine) as the continuous polymeric phase for preparing mixed matrix membranes. The successful synthesis of the 6FDA-DAM polymer was verified by FTIR spectroscopy. N-heterocyclic linkers were incorporated into the UiO-67 framework, creating highly stable materials with exceptional surface areas. This enhancement improved CO2 affinity and polymer adhesion. Field emission scanning electron microscopy images showed that the resulting films were uniformly coated with the synthesized UiO-67s, reducing the likelihood of nonselective defects in the interphase region. Gas permeation measurements demonstrated that these functionalized porous nanofillers significantly enhanced the CO2 separation performance of the membranes. By optimizing the functionality and loading of the porous fillers, the CO2/CH4/N2 separation performance was dramatically improved. Specifically, the insertion of 20 wt % of bpy25 (a mixture of biphenyl-4,4′-dicarboxylate (bpdc) and 2,2′-bipyridine-5,5′-dicarboxylic acid (bpy) in a 3:1 ratio) resulted in an exceptional CO2 permeability of ∼1299 Barrers, a CO2/CH4 selectivity of ∼41.3, and a CO2/N2 selectivity of ∼50.7. These values are approximately 180%, 170%, and 166% higher than those of the unfilled PI. The improved separation factor is likely attributed to the abundant presence of the bipyridine moiety within the 6FDA-DAM matrix. This presence facilitates the interactions between Lewis acidic CO2 and Lewis basic bipyridine, ultimately delivering outstanding performance that surpasses the Robeson curves for CO2/CH4/N2 separations.