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

Abstract

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 CO2, 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 CO2/CH4 selectivities of near 50 with CO2 and H2S permeabilities over an order of magnitude higher than Torlon. CO2 and CH4 sorption in the DAM- and TmPDA-based materials was reduced, whereas H2S sorption was enhanced relative to membranes containing fluorinated 6FpDA. Consequently, DAM- and TmPDA-based membranes showed increased stability toward high pressure CO2 but lower plasticization resistance toward pure H2S. These results highlight the differences between CO2 and H2S that challenge the rational design of materials targeting simultaneous separation of both contaminants.