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Microporous two-dimensional covalent organic framework (2D COF) membranes offer promise for gas separation applications, but their gas transport mechanism remains unclear. In this study, a TpHz 2D COF membrane supported on a macroporous nylon substrate is prepared by substrate-assisted interfacial polymerization under mild conditions. The formation of a continuous and dense thin (∼300 nm thick) TpHz layer is confirmed by scanning electron microscopy and Fourier transform infrared spectroscopy. Characterization by X-ray diffraction, grazing incidence wide-angle X-ray scattering, and N2 porosimetry qualitatively reveals the microstructures of the supported TpHz membranes, i.e., they comprise partially oriented 2D COF lamellar crystallites with moderate crystallinity in an eclipsed (AA) stacking geometry, centering the effective membrane pore size distribution at ∼1.1 nm. Single gas permeation data show that the transport of common molecular gases, including H2, He, CH4, N2, and CO2, through the synthesized TpHz membranes follows the Knudsen transport mechanism, where single gas permeance decreases with an increasing molecular weight and permeation temperature. Binary gas separation results show that in the equimolar CO2/N2 mixture, the presence of the CO2 surface flow slightly hinders the N2 flow at room temperature due to the reduced membrane channel size by the adsorbed CO2 gas layer on TpHz’s pore wall. In contrast, permeation of the equimolar CH4/N2 binary mixture does not exhibit a discernible surface flow of both gases due to their much lower gas uptake on TpHz, and their transport mechanism follows Knudsen-like behavior.
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Effect of the annealing temperature of polybenzimidazole membranes in high pressure and high temperature H2/CO2 gas separations
The effect of the annealing temperature of polybenzimidazole (PBI) membranes on H2/CO2 gas separations was investigated. Membranes annealed from 250 ◦C to 400 ◦C were tested for gas permeation with pure H2, CO2, and N2 gases and a H2:CO2 (1:1) mixture at 35 ◦C, 100 ◦C, 200 ◦C, and 300 ◦C and at pressures up to 45 bar. Gas permeation data show that permeability and selectivity of the membranes is significantly impacted by the annealing temperature, the presence of adsorbed water, and remaining casting solvent (DMAc). At a testing temperature of 35 ◦C, ideal H2/CO2 selectivities of 50, 49, and 66 with pure H2 permeabilities of 1.5, 0.8, and 1.5 Barrer were obtained for membranes annealed at 250 ◦C, 300 ◦C, and 400 ◦C, respectively. At this temperature, high gas mixture H2/CO2 selectivities of 41, 73, and 47 with H2 permeabilities of 1.03, 0.26, and 0.50 Barrer were also obtained for these membranes. At testing temperatures of 300 ◦C, both the ideal and gas mixture H2/ CO2 selectivities dropped to 44, 28, and 30 (ideal, H2 = 45, 45, 44 Barrer) and to 19, 22, and 23 (mixture, H2 = 41, 43, and 44 Barrer) for membranes annealed at 250 ◦C, 300 ◦C, and 400 ◦C, respectively. As water was removed from the membranes at temperatures greater than 100 ◦C during permeation cycles, where the testing temperature was increased from 35 ◦C to 300 ◦C, the permselectivity properties of the membranes annealed at 400 ◦C became more reproducible. Permeabilities at 35 ◦C from a second permeability cycle increased, but H2/ CO2 selectivities decreased to 21 for gas mixtures (H2 = 1.4 Barrer) and to 34 for pure gases (H2 = 2.2 Barrer). The results suggest that high annealing temperatures may induce changes in the configuration and conformation of the polymer chains, imparting distinctive permselectivity properties to the membranes. Activation energies of permeability for H2, CO2, and N2 from pure gases and H2:CO2 mixtures correlated with these changes as well.
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- Award ID(s):
- 1917747
- PAR ID:
- 10472980
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Journal of Membrane Science
- Volume:
- 677
- Issue:
- C
- ISSN:
- 0376-7388
- Page Range / eLocation ID:
- 121619
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.memsci.2023.121619
