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CO 2 -CH 4 mixed-gas sorption and permeation properties of a ladder polymer (PIM-Trip-TB) were measured experimentally at 35 °C to interpret nonideal transport behavior of polymers of intrinsic microporosity (PIMs). Both CH 4 and CO 2 mixed-gas solubilities were lower than those in the pure-gas environment mainly due to competitive sorption. In the range of pressures tested, the CO 2 /CH 4 mixed-gas solubility selectivity of PIM-Trip-TB coincided on average with the value at infinite dilution, and at all pressures, it was higher than the pure-gas solubility selectivity. Because CO 2 diffusion coefficient was found insensitive to mixture effects, we inferred that the increased diffusion coefficient of CH 4 and the consequent loss of CO 2 /CH 4 permselectivity in mixture environment were correlated to CO 2 -induced alteration of the selective diffusion domains of PIM-Trip-TB. Similar effects were also found for PIM-1 by an analysis of pure- and mixed-gas experimental permeation and sorption data. The increase of CH 4 mixed-gas diffusion coefficients from the pure-gas values was more pronounced for both PIMs (PIM-Trip-TB and PIM-1) than for a conventional low-free volume polymer 6FDA-mPDA polyimide reported previously; this indicates that the high intrachain rigidity in PIMs cannot restrain unfavorable mixture effects on CO 2 /CH 4 diffusion and permeability selectivity. Image 1 • CO 2 -CH 4 mixed-gas solubility in PIM-Trip-TB deviates from pure-gas values. • CO 2 vs. CH 4 mixed-gas solubilities can be fitted linearly regardless of concentration. • Equimolar CO 2 /CH 4 solubility selectivity in mixture is higher than pure-gas one. • CO 2 alters polymer size sieving domains thus depressing mixed-gas permselectivity. • Intrachain rigidity does not limit CO 2 -induced plasticization during CO 2 -CH 4 separations. [ABSTRACT FROM AUTHOR]

Abstract Mixed-gas sorption of CO 2 -CH 4 mixtures in rubbery polydimethylsiloxane (PDMS) at 35 °C demonstrated that the presence of CH 4 changed the behavior of CO 2 sorption and vice versa. This mutual interaction indicated that gases in mixtures do not sorb independently in rubbery membranes. Moreover, we observed that at increasing pressures the interaction between PDMS and CO 2 -CH 4 mixtures enhanced the solubility selectivity of PDMS. Mixed-gas solubility coefficients of CH 4 in PDMS were lower than 0.5 cm3(STP) cm−3 atm−1. To accurately measure these values, a new sorption system was designed, constructed, and optimized for low solubility coefficients; an operator-friendly approach to mixed-gas sorption experiments is also discussed in this work. CO 2 -CH 4 mixed-gas diffusivity trends were evaluated from Maxwell-Stefan model fitting of mixed-gas permeation and sorption data. The analysis of both mixed-gas diffusion and sorption data demonstrated that CO 2 /CH 4 mixed-gas permselectivity of PDMS was mainly influenced by CO 2 sorption. In mixtures, CH 4 diffusion coefficients increased with higher volumetric CO 2 concentration, whereas the CO 2 diffusion coefficient was essentially concentration independent in both pure- and mixed-gas environments. Graphical abstract fx1 Highlights • A new mixed-gas sorption system with optimized volumes for low-solubility detection. • CO 2 -CH 4 mixed-gas sorption in rubbery PDMS deviates from pure-gas trends. • Enhanced CO 2 /CH 4 solubility selectivity was caused by mixture effects and pressure. • CO 2 induced phenomena increased CH 4 diffusion coefficient. [ABSTRACT FROM AUTHOR]

Membrane technology is employed in large-scale removal of acid gases from natural gas, and cellulose acetate is by far the most adopted material for this application. Because of its utmost industrial relevance, we analyzed the gas sorption behavior of CO 2 –CH 4 mixtures in cellulose triacetate (CTA) at 35 °C. CO 2 solubility in CTA was only slightly affected by the presence of methane, whereas competition effects sharply reduced CH 4 uptake. Regardless of mixture concentration, CO 2 vs. CH 4 solubility coefficients regressed linearly, which translated in solubility selectivities that increased as equilibrium pressures increased. Specifically, compared to other relevant glassy polymer membrane materials, CTA positioned very close to the solubility selectivity upper bound at infinite dilution and demonstrated the highest affinity to CO 2 at all investigated pressures. The experimental solubility and permeability data were used in the framework of the solution-diffusion theory to determine pure- and mixed-gas concentration averaged diffusion coefficients of CTA. CO 2 diffusion was essentially unaffected by mixture effects, whereas methane diffusivity was boosted by the CO 2 -induced plasticization of CTA. The ratio between the pure- and mixed-gas concentration averaged diffusion coefficients of methane was used to quantify the effect of plasticization on the mixed-gas performance of CTA and other relevant membrane materials previously analyzed in similar experimental studies. When we further extended this comparison in a mixed-gas diffusion analysis (at 10 atm partial pressure), we observed that CTA had lower diffusion selectivity due to an inferior size-sieving capability than a reference material, 6FDA-mPDA polyimide, but displayed superior solubility selectivity. Image 1 • Mixed-gas solubility of CH 4 is reduced in CO 2 /CH 4 mixture environment. • Regardless of concentration, CO 2 vs. CH 4 mixture solubility coefficients regress linearly. • Mixed-gas equimolar CO 2 /CH 4 solubility selectivities are higher than pure-gas values. • CTA mixed-gas diffusion- and permselectivity is depressed by CO 2 -induced polymer plasticization. [ABSTRACT FROM AUTHOR]

The nonideal behavior of polymeric membranes during separation of gas mixtures can be quantified via the solution-diffusion theory from experimental mixed-gas solubility and permeability coefficients. In this study, CO2-CH4 mixtures were sorbed at 35 °C in 4,4′-(hexafluoroisopropylidene)diphthalic dianhydride (6FDA)-m-phenylenediamine (mPDA)—a polyimide of remarkable performance. The existence of a linear trend for all data of mixed-gas CO2 versus CH4 solubility coefficients—regardless of mixture concentration—was observed for 6FDA-mPDA and other polymeric films; the slope of this trend was identified as the ratio of gas solubilities at infinite dilution. The CO2/CH4 mixed-gas solubility selectivity of 6FDA-mPDA and previously reported polymers was higher than the equimolar pure-gas value and increased with pressure from the infinite dilution value. The analysis of CO2-CH4 mixed-gas concentration-averaged effective diffusion coefficients of equimolar feeds showed that CO2 diffusivity was not affected by CH4. Our data indicate that the decrease of CO2/CH4 mixed-gas diffusion, and permeability selectivity from the pure-gas values, resulted from an increase in the methane diffusion coefficient in mixtures. This effect was the result of an alteration of the size sieving properties of 6FDA-mPDA as a consequence of CO2 presence in the 6FDA-mPDA film matrix. [ABSTRACT FROM AUTHOR]