Abstract
Carbon molecular sieve (CMS) membranes were prepared from polyimides with sulfonic acid triethylammonium salt (SO3NH(C2H5)3) and/or hexafluoroisopropylidene (–C(CF3)2–) groups by pyrolyzing precursor composite membranes at the temperatures ranging from 773 to 973 K in an N2 flow for 1 h. The precursor membranes were prepared by coating the polyimides on ceramic porous support tubes. The sulfonic acid salt and the –C(CF3)2– group decomposed in the ranges of 573 to 673 K and 723 to 973 K, respectively. The composite CMS membranes prepared here had a defect-free top layer of 1.5 to 5 μm thick. The CMS membrane derived from the polyimide with both the thermally decomposable groups displayed the highest gas permeances. The decomposition of sulfonic acid groups followed by the decomposition of the –C(CF3)2– group just before the substantial decomposition of the polyimide backbone seemed to significantly enhance the micropore structure of the resultant pyrolytic layer. Compared to the CMS composite or hollow-fiber membranes reported in the literature, the membranes prepared in this study displayed higher O2 and CO2 gas permeances with reasonably high ideal separation factors for O2/N2 and CO2/N2 separations.
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