Effect of the Amide Bond Diamine Structure on the CO₂, H₂S, and CH₄ Transport Properties of a Series of Novel 6FDA-Based Polyamide–Imides for Natural Gas Purification

A series of higher permeability polyamide–imides based on 2,2′-bis(3,4-dicarboxyphenyl) hexafluoropropane dianhydride with comparable plasticization resistance to Torlon were synthesized and formed into dense film membranes. Polymers possessing 2,4-diamino mesitylene (DAM) were stable up to 56 atm of pure CO 2, which is due to enhanced charge transfer complex formation compared to polymers containing 4,4′-(hexafluoroisopropylidene) dianiline (6FpDA) and 2,3,5,6-tetramethyl-1,4-phenylenediamine (TmPDA). The new polymers containing DAM and TmPDA showed ideal CO 2 /CH 4 selectivities of near 50 with CO 2 and H 2 S permeabilities over an order of magnitude higher than Torlon. CO 2 and CH 4 sorption in the DAM- and TmPDA-based materials was reduced, whereas H 2 S sorption was enhanced relative to membranes containing fluorinated 6FpDA. Consequently, DAM- and TmPDA-based membranes showed increased stability toward high pressure CO 2 but lower plasticization resistance toward pure H 2 S. These results highlight the differences between CO 2 and H 2 S that challenge the rational design of materials targeting simultaneous separation of both contaminants.