Synthesis of Polyimides Containing Benzimidazole Groups in Side Chains for Natural Gas Separation

Polyimides for natural gas separation are synthesized from copolycondensation of 5-(1H-benzimidazol-2-yl)benzene-1,3-diamine (BIB) with 2,3,5,6-tetramethyl-1,4-phenylenediamine (Durene) and 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) with different 6FDA/BIB/Durene ratios. All polyimides containing benzimidazole groups in side chains display good processability, outstanding thermal stability, and excellent mechanical properties. The CO2 permeability and CO2/CH4 selectivity of the synthesized polyimides change with the 6FDA/BIB/Durene ratio or the BIB content. At a total pressure of 2 bar and a CO2/CH4 feed ratio of 50:50, the polyimide synthesized with 30% BIB content (F10B3D7) exhibits a high CO2 permeability of 197.2 Barrer coupled with a high CO2/CH4 selectivity of 38.6, which is close to the 2008 Robeson’s upper bound. Besides, CO2-induced plasticization and physical aging of polyimide membranes can also be regulated by tuning the 6FDA/BIB/Durene ratio. Specifically, F10B3D7 also exhibits good plasticization resistance and excellent stability against physical aging, and it is demonstrated to be a promising candidate for the purification of biogas and the removal of CO2 molecules from natural gas.