Carbon dioxide/methane mixed-gas adsorption, permeation and diffusion in a carbon molecular sieve film: Experimental observation and modeling

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The CO 2 –CH 4 pure- and mixed-gas transport properties (at 35 °C) of a carbon molecular sieve (CMS) film obtained by pyrolysis of a 6FDA-mPDA polyimide precursor at 900 °C are reported. The competitive mixed-gas adsorption of CO 2 and CH 4 was predicted by the Ideal Adsorbed Solution Theory (IAST) model. The CO 2 /CH 4 mixed-gas solubility selectivity of the CMS film was lower than that of relevant glassy polymer films of various nature and increased with pressure. Mixed-gas adsorption data were coupled with gas permeation tests on the same film sample batch used for barometric adsorption measurements to derive concentration-averaged effective diffusion coefficients. Because of its large fraction of ultramicroporous bottlenecks, the diffusion coefficients of the CMS were of the same order of magnitude as those of glassy polymer films of low free volume (e.g., 6FDA-mPDA and CTA). In the range of pressures explored, the pure-gas and multicomponent CO 2 diffusion coefficients overlapped; most importantly, the methane diffusion coefficient was enhanced by the presence of CO 2 . This result suggests that carbon dioxide dilated the sieving domains of the CMS matrix under mixed-gas environment containing highly sorbing gases such as CO 2 . Consequently, the CMS film lost some of its size-sieving properties as indicated by a drop in mixed-gas CO 2 /CH 4 permselectivity relative to the values obtained under pure-gas conditions. • The first case of CO 2 –CH 4 pure- and mixed-gas adsorption in a CMS film. • Competitive effects are more deleterious for CH 4 than for CO 2 . • Competitive effects are beneficial for the CO 2 /CH 4 adsorption selectivity. • The IAST (Toth) model satisfactory predicts the CMS gas adsorption behavior. • CO 2 -induced alteration of the diffusion domains of the CMS film.