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

Polyimides for natural gas separation\nare synthesized from copolycondensation\nof 5-(1<i>H</i>-benzimidazol-2-yl)benzene-1,3-diamine (BIB)\nwith 2,3,5,6-tetramethyl-1,4-phenylenediamine (Durene) and 4,4′-(hexafluoroisopropylidene)diphthalic\nanhydride (6FDA) with different 6FDA/BIB/Durene ratios. All polyimides\ncontaining benzimidazole groups in side chains display good processability,\noutstanding thermal stability, and excellent mechanical properties.\nThe CO<sub>2</sub> permeability and CO<sub>2</sub>/CH<sub>4</sub> selectivity\nof the synthesized polyimides change with the 6FDA/BIB/Durene ratio\nor the BIB content. At a total pressure of 2 bar and a CO<sub>2</sub>/CH<sub>4</sub> feed ratio of 50:50, the polyimide synthesized with\n30% BIB content (F<sub>10</sub>B<sub>3</sub>D<sub>7</sub>) exhibits\na high CO<sub>2</sub> permeability of 197.2 Barrer coupled with a\nhigh CO<sub>2</sub>/CH<sub>4</sub> selectivity of 38.6, which is close\nto the 2008 Robeson’s upper bound. Besides, CO<sub>2</sub>-induced\nplasticization and physical aging of polyimide membranes can also\nbe regulated by tuning the 6FDA/BIB/Durene ratio. Specifically, F<sub>10</sub>B<sub>3</sub>D<sub>7</sub> also exhibits good plasticization\nresistance and excellent stability against physical aging, and it\nis demonstrated to be a promising candidate for the purification of\nbiogas and the removal of CO<sub>2</sub> molecules from natural gas.