CARBON MOLECULAR SIEVE MEMBRANES FOR XYLENE ISOMER SEPARATIONS
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The purification of benzene derivatives, particularly xylene isomers, is one of the most important organic mixture separations practiced in industry. The separation of xylene isomers is especially challenging due to the similarity of their physical properties. Carbon molecular sieve (CMS) membranes are promising materials for such challenging solvent separations due to their stability, ability to be scaled at practical form factors, and the avoidance of expensive supports or complex multi-step fabrication processes, but these materials have not been studied in detail in the case of large organic molecules. Xylene isomer transport and sorption properties in a CMS membrane derived from a prototypical polymer of intrinsic microporosity (PIM-1) reveal that diffusion selectivity is the dominant factor in contributing to the preferential permeation of p-xylene over o-xylene. Moreover, the contributions of “enthalpic” and “entropic” selectivity to the diffusion selectivity are studied in detail and reveal that entropic factors dominate the xylene selection mechanism. Another critical challenge is the creation of “mid-range” (e.g., 5-9 Å) microstructures that allow for facile permeation of organic solvents and selection between similarly-sized guest molecules. Here, we create these microstructures via the pyrolysis of the polymer of intrinsic microporosity under low concentrations of hydrogen gas. The introduction of H2 inhibits the aromatization of the decomposing polymer and ultimately results in the creation of a well-defined bimodal pore network that exhibits an ultramicropore size of 5.1 Å. The H2-assisted CMS dense membranes show a dramatic increase in p-xylene ideal permeability (~15 times), with little loss in p-xylene/o-xylene selectivity (18.8 vs. 25.0) when compared to CMS membranes pyrolyzed under a pure argon atmosphere. This approach is successfully extended to hollow fiber membranes operating in organic solvent reverse osmosis mode, highlighting the potential of this approach to be translated from the laboratory to the field. Moreover, this thesis demonstrates that the gradient of the fractional occupancy of penetrant molecules within the micropores of the membrane is the driving force for permeation without requiring assumptions about pressure within the CMS membrane. Flux equations are derived using both Fickian and Maxwell-Stefan approaches, and different behavior in the permeate flux versus upstream hydraulic pressure relationship is shown to arise as a result of differences in the loading dependence of the single component Maxwell-Stefan diffusivity. Molecular modeling results available in the literature and experimental data obtained from CMS membranes showcase that these loading-dependent changes in the Maxwell-Stefan diffusivity are possible. This loading dependence is separated into three regimes: so-called “weak confinement” diffusion and “strong confinement” diffusion, both of which have been discussed at length in the literature, and a new “hybrid confinement” diffusion, which is introduced here. Overall, this thesis opens up new opportunities for the membrane-based applications of CMS materials and provides fundamental insight and guidance into the osmotically-moderated sorption-diffusion transport of solvent molecules through CMS membranes.