Structure–Transport Relationships of Water–Organic Solvent Co-transport in Carbon Molecular Sieve (CMS) Membranes

Abstract

We explore the effects of the carbon molecular sieve (CMS) microstructure on the separation performance and transport mechanism of water–organic mixtures. Specifically, we utilize PIM-1 dense films and integrally skinned asymmetric hollow fiber membranes as polymer precursors for the CMS materials. The PIM-1 membranes were pyrolyzed under several different pyrolysis atmospheres (argon, carbon dioxide, and diluted hydrogen gas) and at multiple pyrolysis temperatures. Detailed gas physisorption measurements reveal that membranes pyrolyzed under 4% H2 and CO2 had broadened ultramicropore distributions (pore diameter <7 Å) compared to Ar pyrolysis, and pyrolysis under CO2 increased ultramicropore volume and broadened micropore distributions at increased pyrolysis temperatures. Gravimetric water and p-xylene sorption and diffusion measurements reveal that the PIM-1-derived CMS materials are more hydrophilic than other CMS materials that have been previously studied, which leads to sorption-diffusion estimations showing water-selective permeation. Water permeation in the vapor phase, pervaporation, and liquid-phase hydraulic permeation reveal that the isobaric permeation modes (vapor permeation and pervaporation) are reasonably well predicted by the sorption-diffusion model, whereas the hydraulic permeation mode is significantly underpredicted (>250×). Conversely, the permeation of p-xylene is well predicted by the sorption-diffusion model in all cases. The collection of pore size analysis, vapor sorption and diffusion, and permeation in different modalities creates a picture of a combined transport mechanism in which waterunder high transmembrane pressurespermeates via a Poiseuille-style mechanism, whereas p-xylene solutes in the mixture permeate via sorption-diffusion.

Separation of Liquid Xylene Isomers Using Thin-Film Composite Carbon Molecular Sieve Hollow Fiber Membranes

“…In addition, asymmetric PIM-1 hollow fibers were pyrolyzed to obtain CMS membranes to separate xylene isomers 9 and p-X/ water. 10 Although the asymmetric structures enable a thin selective layer, porous structures in the asymmetric precursors would partially collapse to be dense during a pyrolytic conversion process, resulting in a thick CMS selective layer. Similarly, polyimides were spun into asymmetric hollow fibers and pyrolyzed to produce CMS hollow fiber membranes, and they were tested for organic solvent forward osmosis of hexane isomers.…”

“…Consequently, the overall flux remained low, compromising the separation efficacy. In addition, asymmetric PIM-1 hollow fibers were pyrolyzed to obtain CMS membranes to separate xylene isomers and p -X/water . Although the asymmetric structures enable a thin selective layer, porous structures in the asymmetric precursors would partially collapse to be dense during a pyrolytic conversion process, resulting in a thick CMS selective layer.…”