Analysis of factors influencing the performance of CMS membranes for gas separation
Williams, Paul Jason
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Carbon molecular sieve (CMS) membranes represent the most attractive pure component materials to compete against polymer membranes for high performance gas separations. CMS membranes are formed from the thermal decomposition of polymer precursors and can therefore be formed into continuous defect free membranes with excellent gas separation performance. Over the last 20 years, CMS membranes have been produced in a variety of geometries and have a wide range of separation performance applicable to several important gas separations. Though research into CMS membrane formation is quite extensive, the relationship between synthesis factors and separation performance is still not well understood. The goal of this study was to elucidate the effect of two different synthesis factors on the separation performance of CMS membranes to allow more control over separation performance. The foci of this study were to clarify (1) the effect of pyrolysis atmosphere and (2) the effect of polymer precursor composition. Dense flat CMS membranes were synthesized from 6FDA:BPDA-DAM precursor at 550 oC using several pyrolysis atmospheres including vacuum pyrolysis (<0.05 torr), helium and argon flowing at atmospheric pressure, and helium and argon flowing at reduced pressures. The separation performance of CMS membranes produced under different pyrolysis atmospheres suggests that the amount of oxygen available during pyrolysis has a significant affect on the microstructure of membrane. CMS membranes were produced from 6FDA:BPDA(1:1)-DAM and 6FDA:BPDA(1:1)-DAM under identical pyrolysis conditions to determine the utility of polymer precursor composition as an engineering tool to fine-tune the performance of CMS membranes. In a second study utilizing 6FDA-6FpDA and 6FDA-6FmDA precursors, the separation performance of CMS membranes was shown to be dependent on the intrinsic precursor free volume. These studies have shown that two factors to be considered when choosing a polymer precursor are the intrinsic free volume of the polymer and the composition of the by-products evolved during pyrolysis.