Carbon molecular sieve membrane structure-property relationships for four novel 6FDA based polyimide precursors

This study considers separation performance of carbon molecular sieve (CMS) membranes formed by pyrolysis under argon at 550 °C for four novel polyimide precursors referred to as 6FDA/DETDA, 6FDA:BPDA(1:1)/DETDA, 6FDA/DETDA:DABA(3:2) and 6FDA/1,5-ND:ODA(1:1). The CMS precursors were characterized using Thermogravimetric Analysis (TGA), Differential Scanning Calorimetry (DSC), and Wide-angle X-ray Diffraction (WAXD). Separation performance of polymer precursor films formed from these polymers was examined using pure gases CO2, CH4, O2 and N2; a relationship between gas permeability and polymer fractional free volume (FFV) is reported. The pyrolyzed polymer precursor films to create dense CMS membranes produced separation performance that significantly exceeded the polymer precursor performance in all cases. The 6FDA/DETDA:DABA(3:2) derived CMS membranes showed the highest permeability (above 20,000 barrer for CO2 and above 4000 barrer for O2 at 35 °C) and offered the greatest practical potential among the various precursors. Separation performance of the CMS membranes was studied as a function of time, with storage under vacuum between tests to assess physical aging. As expected, permeability showed a decreasing trend with time, and CO2/CH4, O2/N2 selectivity showed an increasing trend. Sorption isotherms also provided insight into the microstructure evolution over time. For practical applications, continuous active feed of mixed gas 50% CO2/50% CH4 was shown to effectively suppress physical aging in 6FDA/DETDA:DABA(3:2) CMS membrane.