Low-order polyimide-based COFs incorporated thermally rearranged mixed matrix membranes with enhanced H2/N2 and CO2/N2 separation performance

Interfacial defects between fillers and polymer matrix restrict the separation performance of mixed matrix membranes (MMMs). Herein, low-order polyimide (PI)-based covalent organic frameworks (COFs) fabricated from 2,4,6-tris(4-aminophenyl)-1,3,5-triazine (TAPT) and pyromellitic dianhydride (PMDA) were firstly adopted as fillers to modify thermally rearranged (TR) membranes. Distinct from highly crystalline COFs, low-order PI-COFs contain more exposed terminal reactive groups. These functional groups can form covalent linkages with poly(amic acid) precursors, which effectively suppresses the generation of non-selective interfacial voids and improves the interfacial compatibility between filler and polymer matrix. The hierarchical pores of PI-COFs construct low-resistance gas transport pathways, and triazine/imide polar sites deliver strong affinity for CO 2 . After thermal rearrangement at 450 °C, TR/COFs-2.3 achieves drastically boosted permeabilities: 4632 Barrer (CO 2 ), 3867 Barrer (H 2 ), 795 Barrer (O 2 ) and 215 Barrer (N 2 ). Its CO 2 /N 2 selectivity slightly increases to 21.54, while O 2 /N 2 and H 2 /N 2 selectivities moderately decline to 3.70 and 17.99. This membrane exceeds the 2008 Robeson upper bound for H 2 /N 2 , and nearly reaches the limit for CO 2 /N 2 and O 2 /N 2 . It also displays improved long-term aging stability (66–74% permeability retention after 180 days, versus 52% for neat TR membrane) and stable performance in a 6-day continuous mixed-gas test. This work proposes a low-order COF interfacial modification strategy to alleviate interfacial defects, providing a practical route to prepare high-performance TR-based MMMs.