An official website of the United States government
The implementation of synthetic polymer membranes in gas separations, ranging from natural gas sweetening, hydrogen separation, helium recovery, carbon capture, oxygen/nitrogen enrichment, etc. , has stimulated the vigorous development of high-performance membrane materials. However, size-sieving types of synthetic polymer membranes are frequently subject to a trade-off between permeability and selectivity, primarily due to the lack of ability to boost fractional free volume while simultaneously controlling the micropore size distribution. Herein, we review recent research progress on microporosity manipulation in high-free-volume polymeric gas separation membranes and their gas separation performance, with an emphasis on membranes with hourglass-shaped or bimodally distributed microcavities. State-of-the-art strategies to construct tailorable and hierarchically microporous structures, microporosity characterization, and microcavity architecture that govern gas separation performance are systematically summarized.
more »
« less
Permeabilities and selectivities in anisotropic planar membranes for gas separations
The use of membrane technologies for separation processes is an alternative approach to reduce the environmental impact and energy demand of separations. The development of new membrane materials plays a central role to overcome the limitations of membranes in terms of selectivity, permeability, and stability. Most membrane materials in the past have been engineered to control the relative magnitude of the flux of the species diffusing through the membrane. However, mass flux is a vector and controlling its direction can open new opportunities to design separation processes. In this paper we characterize the separation capabilities of metamaterial-inspired anisotropic planar membranes by studying the development of spatially dependent permeabilities and selectivities as a consequence of manipulating the flux direction within the membrane. Specifically, we show how the performance of anisotropic planar membranes for separations can be characterized in terms of permeability, selectivity, and the collected permeate proportion. In contrast to isotropic membrane materials, we show how the selectivity under single stage operation can be increased beyond the selectivities of the constituent materials by reducing the permeate proportion that is collected. Our work provides new opportunities for the design of alternative separation processes that take advantage of flux directional control within membrane materials.
more »
« less
- Award ID(s):
- 1744212
- PAR ID:
- 10200518
- Date Published:
- Journal Name:
- Separation and purification technology
- Volume:
- 228
- ISSN:
- 1383-5866
- Page Range / eLocation ID:
- 115762
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Biological membranes can achieve remarkably high permeabilities, while maintaining ideal selectivities, by relying on well-defined internal nanoscale structures in the form of membrane proteins. Here, we apply such design strategies to desalination membranes. A series of polyamide desalination membranes—which were synthesized in an industrial-scale manufacturing line and varied in processing conditions but retained similar chemical compositions—show increasing water permeability and active layer thickness with constant sodium chloride selectivity. Transmission electron microscopy measurements enabled us to determine nanoscale three-dimensional polyamide density maps and predict water permeability with zero adjustable parameters. Density fluctuations are detrimental to water transport, which makes systematic control over nanoscale polyamide inhomogeneity a key route to maximizing water permeability without sacrificing salt selectivity in desalination membranes.more » « less
-
Abstract Porous graphene and other atomically thin 2D materials are regarded as highly promising membrane materials for high‐performance gas separations due to their atomic thickness, large‐scale synthesizability, excellent mechanical strength, and chemical stability. When these atomically thin materials contain a high areal density of gas‐sieving nanoscale pores, they can exhibit both high gas permeances and high selectivities, which is beneficial for reducing the cost of gas‐separation processes. Here, recent modeling and experimental advances in nanoporous atomically thin membranes for gas separations is discussed. The major challenges involved, including controlling pore size distributions, scaling up the membrane area, and matching theory with experimental results, are also highlighted. Finally, important future directions are proposed for real gas‐separation applications of nanoporous atomically thin membranes.more » « less
-
Gas separation membranes incorporating two-dimensional (2D) materials have received considerable attention in recent years, as these membranes have shown outstanding physical, structural, and thermal properties and high permeability- selectivity. The reduced thickness and diversity of the gas transport mechanisms through in-plane pores (intrinsic defects), in-plane slitlike pores, or plane-to-plane interlayer galleries provide the membranes with a significant sieving ability for energy-efficient gas separation. The discovery of 2D transition metal carbides/nitrides materials, MXenes, has provided new opportunities in the gas separation membrane area because of their hydrophilicity, rich chemistry, high flexibility, and mechanical strength. This Review puts into perspective recent advances in 2D-material-based gas separation membranes. It discusses research opportunities mainly in MXene-based gas membranes, highlights modification approaches for tuning the in-plane and plane-to-plane nanoslits, explains governing mechanisms of transport through these membranes, and compares their advantages and disadvantages with those of other 2D materials. It also discusses current challenges and provides prospects in this area.more » « less
-
Abstract Membrane-based separation technologies are attractive to remediating unconventional water sources, including brackish, industrial, and municipal wastewater, due to their versatility and relatively high energy efficiency. However, membrane fouling by dissolved or suspended organic substances remains a primary challenge which can result in an irreversible decline of the permeate flux. To overcome this, membranes have been incorporated with photocatalytic materials that can degrade these organic substances deposited on the surface upon light illumination. While such photocatalytic membranes have demonstrated that they can recover their inherent permeability, less information is known about the effect of photocatalysis on the kinetics of the permeate flux. In this work, a photocatalytic mesh that can selectively permeate water while repelling oil was fabricated by coating a mixture of nitrogen-doped TiO2(N-TiO2) and perfluorosilane-grafted SiO2(F-SiO2) nanoparticles on a stainless steel mesh. Utilizing the photocatalytic mesh, the time-dependent evolution of the water-rich permeate flux as a result of photocatalytic degradation of the oil was studied under the visible light illumination. A mathematical model was developed that can relate the photocatalytic degradation of the organic substances deposited on a mesh surface to the evolution of the permeate flux. This model was established by integrating the Langmuir–Hinshelwood kinetics for photocatalysis and the Cassie–Baxter wettability analysis on a chemically heterogeneous mesh surface into a permeate flux relation. Consequently, the time-dependent water-rich permeate flux values are compared with those predicted by using the model. It is found that the model can predict the evolution of the water-rich permeate flux with a goodness of fit of 0.92.more » « less
- Free Publicly Accessible Full Text
-
Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.seppur.2019.115762
