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 CO 2 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 CO 2 separation performance of the membranes. By optimizing the functionality and loading of the porous fillers, the CO 2 /CH 4 /N 2 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 CO 2 permeability of ∼1299 Barrers, a CO 2 /CH 4 selectivity of ∼41.3, and a CO 2 /N 2 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 CO 2 and Lewis basic bipyridine, ultimately delivering outstanding performance that surpasses the Robeson curves for CO 2 /CH 4 /N 2 separations.