Synthesis of CO ₂ -Selective PIM-1 Composite Membranes via Spin-Coating and Thermal Treatment of Polyimidazolium

Polymer of intrinsic microporosity-1 (PIM-1) is recognized for its exceptional gas permeability, attributed to its high free volume and microporosity. However, it presents several disadvantages, including low selectivity, plasticization, and poor aging properties. To address these challenges, PIM-nPI-Tz, a series of composite membranes containing triazine cross-links, was developed herein by spin-coating nitrile-functionalized polyimidazolium (PI-CN) onto a PIM-1 membrane, followed by heat treatment. The resulting coating layer increased the selectivity of the PIM-1 membrane, given its CO2 affinity, while the formation of triazine increased the backbone-to-backbone spacing, enhancing the diffusivity and permeability of the membrane. Notably, PIM-5PI-Tz (involving PIM-1 coated with a 5% PI-CN solution) exhibited a 73% increase in CO2 permeability (7539 barrer) and a 34% rise in CO2/N2 selectivity (24.4), nearly equivalent to the 2019 Robeson upper bound in terms of overall gas separation. Moreover, the plasticization of the membrane was effectively suppressed, even at a feed gas pressure of 20 bar. Further, its gas separation performance was retained over 300 days under CO2/N2 mixed-gas conditions, demonstrating its superior antiplasticization and antiaging characteristics. The coating material used herein, PI-CN, exhibits a significantly higher CO2-philicity than conventional coating materials, such as Pebax, PEG, and PDMS. Moreover, during the spin-coating and subsequent heat treatment processes, molecular rearrangement within PI-CN facilitates CO2 diffusion, further enhancing the gas transport performance of the PIM membrane.