Microporous polyimides with functional groups for the adsorption of carbon dioxide and organic vapors

Abstract

Highly cross-linked microporous polyimide networks using (hexafluoroisopropylidene)diphenyl, benzophenone and biphenyl as linking struts and tetraphenylmethane and tetraphenyladamantane as net nodes were synthesized under well controlled polymerization conditions. The resultant polyimides show initial decomposition temperatures over 500 °C and BET surface areas up to 781 m² g⁻¹ with quite uniform pore sizes at around 0.5 nm. It is found that the presence of a high density of trifluoromethyls gives rise to a simultaneously larger CO₂ uptake (13.5 wt%, 273 K/1 bar) and CO₂/N₂ selectivity (53.5, IAST method), while the introduction of benzophenone groups enhances the interaction between the CO₂ molecules and polymer skeleton, thereby leading to a high heat of adsorption (33.3 kJ mol⁻¹) and selectivities of CO₂/N₂ (59.2) and CO₂/CH₄ (15.8). The polyimides exhibit large adsorption capacities for organic vapors such as benzene (104.9 wt%) and cyclohexane (60.2 wt%) at 298 K and P/P_o = 0.8. In addition, the low-surface-energy trifluoromethyls significantly decrease the water uptake in the fluorinated polyimide compared to the non-fluorinated samples. The improved hydrophobicity is advantageous for the practical application of porous adsorbents in CO₂ capture from flue gas and natural gas.