Microporous polyimides with high surface area and CO₂ selectivity fabricated from cross-linkable linear polyimides

Linear polyimides of intrinsic microporosity have been intensively investigated for gas separation due to their microporous structure and high surface area. The microporous structure in the linear polyimides of intrinsic microporosity comes from their contorted structure. Therefore, most linear polyimides without contorted structure do not have micropores. In this work, the microporous polyimides are constructed through the condensation of a cross-linkable dianhydride monomer with two novel nitrogen-rich diamine monomers and post crosslinking reaction. The linear polyimide precursors without contorted structure have the same main-chain structure. The introduction of crosslinked structure endow the crosslinked polyimides (PI-CLs) with microporous structure. The microporous structure in PI-CLs can be tuned by changing the substituents of the linear polyimide precursors. The PI-CLs have competitive CO₂ uptake capacity (7.3-9.4 wt%) at 273 K and 1 bar. Particularly, the crosslinked polyimide containing trifluoromethyl groups (CF₃-PI-CL) shows high CO₂/N₂ and CO₂/CH₄ selectivity (72 and 22) at 273 K, which are among the best results for reported porous materials. This work reveals that the introduction of crosslinked structure and changing substituents is an efficient method for constructing microporous polyimides with abundant micropores and excellent CO₂ selective adsorption capacity. This method also has great potential for fabricating high-performance microporous polymers based on other linear polymers without rigid contorted structure.