New materials can be prepared as membranes that may allow their performance to beat long-standing limits.
Synthetic membranes are used in many separation processes, from industrial-scale ones—such as ...separating atmospheric gases for medical and industrial use, and removing salt from seawater—to smaller-scale processes in chemical synthesis and purification. Membranes are commonly solid materials, such as polymers, that have good mechanical stability and can be readily processed into high–surface area, defect-free, thin films. These features are critical for obtaining not only good chemical separation but also high throughput. Membrane-based chemical separations can have advantages over other methods—they can take less energy than distillation or liquefaction, use less space than absorbent materials, and operate in a continuous mode. In some cases, such as CO2 separations for CO
2
capture, their performance must be improved. We discuss how membranes work, and some notable new approaches for improving their performance.
This paper presents details of recent research progress on CO2 separation membranes and membrane processes using ionic liquids (ILs) over the past few years, including supported ionic liquid ...membranes (SILMs), poly(ionic liquid) membranes (PILMs), poly(ionic liquid)–ionic liquid (PIL–IL) composite membranes, polymer-ionic liquid composite membranes, ion-gel membranes, and membrane absorption processes based on ILs. Descriptions of different approaches to membrane preparation, use of gas transport mechanisms, and state-of-the-art separation results are discussed in the context of breakthroughs and challenges. Furthermore, comprehensive assessment of recently improved membranes and possible future R&D prospective are also discussed.
•An broad overview of recent advances in IL-membrane combinations in CO2 separation.•A clear outline of IL-based CO2 separation membranes and membrane processes.•Comprehensive summary of IL-membrane performance data.•Discussion on issues like trade-off between stability and IL-membrane performance.•Strengths, challenges and future research directions in IL-membrane combinations.
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▶ Applications of ionic liquids in membrane systems are increasing. ▶ Use as supported liquid membranes, composite structures, membrane contactors, and mixed matrix membranes. ▶ ...Platform provides unique opportunities to tune physical/chemical properties. ▶ New opportunities in CO
2 separation from flue gas.
Ionic liquids can be used in various morphologies and configurations as membrane systems including supported liquid membranes, membrane contactors, and mixed matrix membranes. In each case, the negligible vapor pressure can lead to a highly stable structure since the ionic liquid is non-volatile. This perspective is meant to provide some background information on the use of ionic liquids in membrane systems and a discussion of future opportunities for this technology. Ionic liquids with different physical properties can be synthesized in a wide range of structures. In addition, this platform provides an opportunity to “tune” the physical/chemical properties such as density, viscosity, hydrophobicity, and chemical affinity for specific applications. The use of ionic liquids in membrane systems should see continued growth in the future.
The recycling or sequestration of carbon dioxide (CO2) from the waste gas of fossil-fuel power plants is widely acknowledged as one of the most realistic strategies for delaying or avoiding the ...severest environmental, economic, political, and social consequences that will result from global climate change and ocean acidification. For context, in 2013 coal and natural gas power plants accounted for roughly 31% of total U.S. CO2 emissions. Recycling or sequestering this CO2 would reduce U.S. emissions by ca. 1800 million metric tonseasily meeting the U.S.’s currently stated CO2 reduction targets of ca. 17% relative to 2005 levels by 2020. This situation is similar for many developed and developing nations, many of which officially target a 20% reduction relative to 1990 baseline levels by 2020. To make CO2 recycling or sequestration processes technologically and economically viable, the CO2 must first be separated from the rest of the waste gas mixturewhich is comprised mostly of nitrogen gas and water (ca. 85%). Of the many potential separation technologies available, membrane technology is particularly attractive due to its low energy operating cost, low maintenance, smaller equipment footprint, and relatively facile retrofit integration with existing power plant designs. From a techno-economic standpoint, the separation of CO2 from flue gas requires membranes that can process extremely high amounts of CO2 over a short time period, a property defined as the membrane “permeance”. In contrast, the membrane’s CO2/N2 selectivity has only a minor effect on the overall cost of some separation processes once a threshold permeability selectivity of ca. 20 is reached. Given the above criteria, the critical properties when developing membrane materials for postcombustion CO2 separation are CO2 permeability (i.e., the rate of CO2 transport normalized to the material thickness), a reasonable CO2/N2 selectivity (≥20), and the ability to be processed into defect-free thin-films (ca. 100-nm-thick active layer). Traditional polymeric membrane materials are limited by a trade-off between permeability and selectivity empirically described by the “Robeson upper bound”placing the desired membrane properties beyond reach. Therefore, the investigation of advanced and composite materials that can overcome the limitations of traditional polymeric materials is the focus of significant academic and industrial research. In particular, there has been substantial work on ionic-liquid (IL)-based materials due to their gas transport properties. This review provides an overview of our collaborative work on developing poly(ionic liquid)/ionic liquid (PIL/IL) ion-gel membrane technology. We detail developmental work on the preparation of PIL/IL composites and describe how this chemical technology was adapted to allow the roll-to-roll processing and preparation of membranes with defect-free active layers ca. 100 nm thick, CO2 permeances of over 6000 GPU, and CO2/N2 selectivity of ≥20properties with the potential to reduce the cost of CO2 removal from coal-fired power plant flue gas to ca. $15 per ton of CO2 captured. Additionally, we examine the materials developments that have produced advanced PIL/IL composite membranes. These advancements include cross-linked PIL/IL blends, step-growth PIL/IL networks with facilitated transport groups, and PIL/IL composites with microporous additives for CO2/CH4 separations.
Clean energy production has become one of the most prominent global issues of the early 21st century, prompting social, economic, and scientific debates regarding energy usage, energy sources, and ...sustainable energy strategies. The reduction of greenhouse gas emissions, specifically carbon dioxide (CO2), figures prominently in the discussions on the future of global energy policy. Billions of tons of annual CO2 emissions are the direct result of fossil fuel combustion to generate electricity. Producing clean energy from abundant sources such as coal will require a massive infrastructure and highly efficient capture technologies to curb CO2 emissions. Current technologies for CO2 removal from other gases, such as those used in natural gas sweetening, are also capable of capturing CO2 from power plant emissions. Aqueous amine processes are found in the vast majority of natural gas sweetening operations in the United States. However, conventional aqueous amine processes are highly energy intensive; their implementation for postcombustion CO2 capture from power plant emissions would drastically cut plant output and efficiency. Membranes, another technology used in natural gas sweetening, have been proposed as an alternative mechanism for CO2 capture from flue gas. Although membranes offer a potentially less energy-intensive approach, their development and industrial implementation lags far behind that of amine processes. Thus, to minimize the impact of postcombustion CO2 capture on the economics of energy production, advances are needed in both of these areas. In this Account, we review our recent research devoted to absorptive processes and membranes. Specifically, we have explored the use of room-temperature ionic liquids (RTILs) in absorptive and membrane technologies for CO2 capture. RTILs present a highly versatile and tunable platform for the development of new processes and materials aimed at the capture of CO2 from power plant flue gas and in natural gas sweetening. The desirable properties of RTIL solvents, such as negligible vapor pressures, thermal stability, and a large liquid range, make them interesting candidates as new materials in well-known CO2 capture processes. Here, we focus on the use of RTILs (1) as absorbents, including in combination with amines, and (2) in the design of polymer membranes. RTIL amine solvents have many potential advantages over aqueous amines, and the versatile chemistry of imidazolium-based RTILs also allows for the generation of new types of CO2-selective polymer membranes. RTIL and RTIL-based composites can compete with, or improve upon, current technologies. Moreover, owing to our experience in this area, we are developing new imidazolium-based polymer architectures and thermotropic and lyotropic liquid crystals as highly tailorable materials based on and capable of interacting with RTILs.
A three-component (cross-linked poly(ionic liquid) (PIL)–ionic liquid (IL)–zeolite), mixed-matrix membrane (MMM) platform based on curable IL prepolymers of controlled length has been developed for ...separating CO2 from CH4. Solutions of these curable prepolymers demonstrate increased resistance to support penetration compared to comparable solutions of analogous cross-linkable IL monomers. By adjusting the curable IL prepolymer chain length, it is possible to manipulate polymer susceptibility to support penetration, polymer solution gelation time, and gas separation performance in MMMs based on these materials. When a 50 wt % solution of the curable IL prepolymer with a degree of polymerization (x) of 87 was cast on an ultrafiltration support membrane, only 3.7 wt % of the polymer penetrated into the support. As the degree of polymerization of the curable IL prepolymer increases, the CO2/CH4 gas separation performance of the resulting MMM performance also improves. For example, an MMM synthesized using 64 wt % curable IL prepolymer (x = 87), 16 wt % EMIMTf2N as the IL, and 20 wt % SAPO-34 zeolite exhibited a CO2/CH4 selectivity of (42 ± 5) and a CO2 permeability of (47 ± 1) barrers. This CO2/CH4 separation performance is comparable to the previous generation of MMMs based on curable small-molecule IL monomers with the same IL and zeolite. However, this new MMM system also exhibits faster curing gelation times and the ability to be solution-cast onto a porous support for formation of thin-film composite membranes without significant selective layer soak-in.
Room-temperature ionic liquids (RTILs) are nonvolatile, tunable solvents that have generated significant interest across a wide variety of engineering applications. The use of RTILs as media for CO2 ...separations appears especially promising, with imidazolium-based salts at the center of this research effort. The solubilities of gases, particularly CO2, N2, and CH4, have been studied in a number of RTILs. Process temperature and the chemical structures of the cation and anion have significant impacts on gas solubility and gas pair selectivity. Models based on regular solution theory and group contributions are useful to predict and explain CO2 solubility and selectivity in imidazolium-based RTILs. In addition to their role as a physical solvent, RTILs might also be used in supported ionic liquid membranes (SILMs) as a highly permeable and selective transport medium. Performance data for SILMs indicates that they exhibit large permeabilities as well as CO2/N2 selectivities that outperform many polymer membranes. Furthermore, the greatest potential of RTILs for CO2 separations might lie in their ability to chemically capture CO2 when used in combination with amines. Amines can be tethered to the cation or the anion, or dissolved in RTILs, providing a wide range of chemical solvents for CO2 capture. However, despite all of their promising features, RTILs do have drawbacks to use in CO2 separations, which have been overlooked as appropriate comparisons of RTILs to common organic solvents and polymers have not been reported. A thorough summary of the capabilitiesand limitationsof imidazolium-based RTILs in CO2-based separations with respect to a variety of materials is thus provided.
A nanoporous, bicontinuous cubic, lyotropic liquid crystal polymer resin with sulfonic acid groups is presented that exhibits high catalytic activity and is capable of molecular-size-selective ...heterogeneous acid catalysis.
A nanoporous, bicontinuous cubic, lyotropic liquid crystal polymer resin with sulfonic acid groups is presented that exhibits high catalytic activity and is capable of molecular-size-selective heterogeneous acid catalysis.
A single-head/single-tail surfactant with a polymerizable group at each end is presented as a new simplified motif for intrinsically cross-linkable, gyroid-phase lyotropic mesogens. The resulting ...nanoporous polymer networks exhibit excellent structural stability in various solvents and are capable of molecular size discrimination.
A simplified design for intrinsically cross-linkable gyroid-forming mesogens is introduced for the fabrication of solvent-resistant nanoporous membranes.
