Highly Selective Separation of CO₂, CH₄, and C₂-C₄ Hydrocarbons in Ultramicroporous Semicycloaliphatic Polyimides

Ultramicroporous semicycloaliphatic polyimides with major pore sizes less than 0.5 nm are synthesized through imidization reaction between different aromatic tetraamines and cycloaliphatic dianhydrides. The synergistic role of abundant CO₂-philic imide rings and the molecular sieving effect of ultrasmall pores in the polyimide network bring about high adsorption selectivity of CO₂/CH₄ (37.2) and CO₂/N₂ (136.7). In addition, it is interesting to observe that, under ambient condition (298 K/1 bar), n-butane exhibits the highest uptake (3.15 mmol/g) among the C₁-C₄ alkanes, and the adsorbed amount significantly drops with the reduction of the number of carbon atoms. As a result, the mixed light alkanes can be effectively separated according to the carbon numbers. The separation factors of n-butane/propane and propane/ethane reach 3.1 and 6.5, whereas those of n-butane, propane, and ethane over methane are as high as 414.5, 217.4, and 19.6, respectively. Moreover, the polyimides display large adsorption capacities for 1,3-butadiene (4.64 mmol/g) and propene (2.68 mmol/g) with good selectivity over 1-butene and propane of 3.2 and 3.0, respectively. Together with the excellent thermal and physicochemical stabilities, the ultramicroporous polyimides obtained in this work show promising applications in adsorption/separation for CO₂, CH₄, and C₂-C₄ hydrocarbons.