Polymer/metal-organic framework junctions in polyamide membrane via in-situ amine etching for stable CO₂/N₂ selectivity at elevated pressures

Polyamide thin-film composite (PA-TFC) membranes with ultrathin selective layers, fabricated via facile interfacial polymerization process, are fascinating for CO₂/N₂ separation. But they always suffer from drastic deterioration of CO₂/N₂ selectivity at elevated pressures due to CO₂-induced swelling of polymer chains. Herein, we report an in-situ amine etching strategy to construct robust polyamide/polyethyleneimine (PEI)-functionalized ZIF-8 (PA/PEI-ZIF-8) junctions within PA-TFC membranes. In this strategy, PEI-ZIF-8 colloid was sprayed onto triethylenetetramine (TETA)-preimpregnated polysulfone substrates prior to interfacial polymerization, leading to controllable etching of PEI-ZIF-8 particles into Zn-imidazole clusters and ultra-small PEI-ZIF-8 nanocrystals. These etched species subsequently coordinated with TETA, forming highly integrated PA/PEI-ZIF-8 junctions with enhanced interfacial compatibility. Benefiting from the high porosity of PEI-ZIF-8 nanocrystals and excellent interfacial compatibility, the optimized composite membrane realizes a CO₂ permeance of 478 GPU (GPU = gas permeance unit) and a CO₂/N₂ selectivity of 60.3. Notably, owing to the rigidity of PEI-ZIF-8 nanocrystals and coordination interactions between PEI-ZIF-8 and polyamide chains, the PA/PEI-ZIF-8 junctions effectively improve the anti-swelling property of composite membranes. Contrary to reduced CO₂/N₂ selectivity of pristine PA membrane at elevated pressures, the CO₂/N₂ selectivity of composite membrane remains stable as the feed gas pressure increases from 1 to 4 bar. This work provides a novel strategy for fabricating robust PA-TFC membranes for pressurized CO₂ separation.