Thermally rearranged polyimide membranes incorporated with polyimide-type flaky COF toward high-efficiency gas separation

While conventional polymeric gas separation membranes are limited by the inherent permeability-selectivity trade-off, mixed matrix membranes (MMMs) often suffer from poor interfacial compatibility and non-selective void defects. Herein, rigid flaky polyimide (PI)-type covalent organic frameworks (COFs) were solvothermally synthesized using 2,4,6-triaminopyrimidine (TAP) and pyromellitic dianhydride (PMDA). The obtained COFs were adopted as functional fillers and blended into hydroxyl-containing polyimide (HPI) precursor derived from 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (6FAP) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) to prepare thermally rearranged (TR) MMMs. The as-synthesized COFs possess excellent thermal stability, distinctive porous structure and moderate interfacial affinity with the polymer matrix, which effectively enhances the gas separation performance of the obtained membranes. At a PI-type COF loading of 2.0 wt%, the MMM treated at 450 °C for 1 h achieves superior separation performance. The permeabilities toward H 2 , CO 2 , O 2 and N 2 reach 2446, 2920, 495 and 131 Barrer, and the corresponding ideal selectivities of H 2 /N 2 , CO 2 /N 2 and O 2 /N 2 are 18.67, 22.29 and 3.77, respectively. Remarkably, the H 2 /N 2 separation behavior exceeds the 2008 Robeson upper bound, and CO 2 /N 2 as well as O 2 /N 2 separation performances are close to this limit. Elevated treatment temperature and extended heating time remarkably improve gas permeability but sacrifice membrane selectivity. Moreover, the fabricated MMMs display satisfactory anti-aging stability after 180-day ambient storage. This study develops a valid modification strategy via flaky PI-type COF incorporation to elevate the gas separation performance of TR polymers, providing a promising route for constructing high-efficiency gas separation membranes.