Ionic Liquid Ion-Gels with High “Free” Ionic Liquid

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Poly(ionic liquid)/Ionic Liquid Ion-Gels with High “Free” Ionic Liquid Content: Platform Membrane Materials for CO2/Light Gas Separations Matthew G. Cowan,†,‡ Douglas L. Gin,*,‡,† and Richard D. Noble*,† †

Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309, United States Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado 80309, United States

‡

CONSPECTUS: 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 tonseasily 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 mixturewhich 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 ≥20properties 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.

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INTRODUCTION As the consequences of anthropogenic climate change become more severe and the underlying causes remain largely unaddressed by humanity as a whole, more scientific attention is being focused on developing short-, mid-, and long-term technological solutions to mitigate or delay the worst economic, political, and social effects of climate change.1 Of the greenhouse gases, CO2 in particular has received significant attention for its role in both climate change and ocean acidification.2 Preventing anthropogenic CO2 from entering the atmosphere requires a three-step process of separation, © XXXX American Chemical Society

compression (for transport), and sequestration or recycling (Figure 1). Previous studies have suggested that the separation of CO2 will be the most energy-intensive step in the overall process.3 Our work has focused on developing membranes for postcombustion CO2 capture because fossil-fuel power plants (i.e., coal- or natural gas-fired) are currently the largest point sources of anthropogenic CO2, and the existing designs would Received: December 21, 2015

A

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Figure 2. Representation of a commercial membrane material made up of an active layer (top), gutter layer (middle), and porous support (bottom). Adapted with permission from ref 8. Copyright 2014 American Chemical Society.

material and the solubility of the gas in the material (eq 1), (units of barrers; 1 barrer = 0.33 × 10−15 mol·m/(m2·s·Pa)); and ‘selectivity’, the rate at which gas i moves across a material in comparison to gas j, which is the product of differences in the diffusivity selectivity and the solubility selectivity of the material for the different gases (eq 2). Gases are transported through dense polymer membranes via the solution-diffusion mechanism and are subject to a permeability/selectivity trade-of f where the permeability decreases as the selectivity of the material increases.10 This is commonly visualized in a “Robeson Plot” where the trade-off is defined by the empirical upper bound line (Figure 3).11 The ‘permeance’ of a membrane represents the net rate of gas transport across it i.e., equivalent to the permeability divided by the thickness (eq 3) (units of gas permeation units (GPU); 1 GPU = 1 barrer/1 μm = 1 × 10−6 cm3gas (STP)/ (cm2material·s·cmHg = 3.35 × 10−7 molgas/(m2material·s·kPa). The permeance of a membrane is dependent on the permeability of the membrane material and the thickness of the active layer (which has the highest resistance to gas transport). These membrane performance metrics are commonly initially evaluated by performing experiments using dry single gases at ambient pressure with 1−2 atm pressure differentials across the membrane and comparing the measurements, a process referred to as “ideal” testing. The majority of the work presented in this review (with the exception of facilitated transport membrane materials) is the result of this “ideal” gas testing. Although it is important to note that mixed gas testing often results in increases in permeability and decreases in selectivity, IL-based materials (the focus of this review) achieve CO2/light gas selectivity largely through solubility selectivity and have been shown to retain their selectivity under mixed gas conditions over significant periods of time (≥100 days)12 and under humidified (95% RH) gas streams.13 In addition to the evaluation of ideal performance metrics, for real-world applications it is also necessary to consider factors that include the ability to prepare extremely thin active layers of the material, performance in a mixed gas feed, performance in a real-world feed, performance stability over time, and production costs.8

Figure 1. Generalized representation of the CO2 sequestration process showing the separation of CO2 from the flue gas stream, compression, and transport to facilities for either sequestration or recycling.

be easier to retrofit with postcombustion capture processes than pre- or oxy-combustion equivalents.4 Additionally, postcombustion technology would be suitable for other large point sources such as cement plants, steel plants, chemical refineries, and glass production factories.5 The scientific problem faced by postcombustion CO2 capture is the separation of a low-pressure (ca. 1 atm) gas stream that contains ca. 13% CO2 by volume, along with mainly N2 and ca. 4−5% H2O by volume.6 Furthermore, the scale of the problem is enormous: a 600-MW coal-fired power plant emits 500 m3/s of CO2,6 the equivalent volume to filling one Olympic-size swimming pool every 5 seconds. The current technology standard for postcombustion CO2 capture is amine-based scrubbing. Advantages of amine-based scrubbing include a high-purity product (useful for compression/transport and sequestration/recycling) and technological maturity (making returns on investment predictable),7 while its disadvantages include high energy regeneration and maintenance costs (significantly increasing separation costs) and large equipment footprints. In contrast, membrane-based separations have the ability to process a large amount of gas relatively at relatively low operating energy and maintenance costs with a smaller equipment footprint.6 Current disadvantages include the relative immaturity of membrane-based CO2/ N2 separation technology and the need for additional processing to produce high-purity CO2 as a product. Composite membrane materials (Figure 2) consist of a selective “active layer” that does the bulk of the separation process.8 The active layer typically sits on a highly permeable “gutter layer” which prevents the active layer material from migrating into the “porous support” and helps overcome transport rate limitations that occur with ultrathin active layers,9 the porous support provides the necessary mechanical strength to the membrane. A “protective layer” can also be included to preserve the active layer. The key parameters for evaluating membrane materials that will be used throughout this review include: ‘permeability’, a material property that describes the rate of gas transport, normalized to the active layer thickness and trans-membrane pressure, is related to the diffusivity of a gas through the

Permeabilityi = Diffusivityi ·Solubilityi Selectivityi / j =

Permeancei =

(1)

Diffusivityi · Solubilityi Diffusivityj · Solubilityj

(2)

Diffusivityi · Solubilityi Membrane thickness

(3)

Several model studies have defined the materials targets for economically viable membrane-based postcombustion CO2 capture.6,15 Recent models that propose multistage membrane B

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Figure 3. Left: Robeson plot showing the performance of the membrane materials discussed in this work compared against the 2008 Robeson upper bound for the CO2/N2 gas pair. Right: Robeson plot showing the performance of the membrane materials discussed in this work relative to the 2008 Robeson upper bound for the CO2/CH4 gas pair. Note that all data points are the result of dry single gas studies with the exception of the facilitated transport membrane data points (humidified mixed gas) and the IL data points obtained from ref 14.

capture target ($10 per ton of CO2 captured by 2030−2035),16 the scientific challenge is to develop a membrane with a CO2/ N2 selectivity of ≥20 and a CO2 permeance as high as possible. Previously, we explored the potential of supported ionic liquid membranes (SILMs) and polymerized ionic liquid (PIL) membranes for CO2/light gas separations.17 Ionic liquids (ILs) are organic-based “molten salts” that exhibit liquid-like properties at ambient temperature and pressure. ILs have low vapor pressures, high temperature stability, and synthetic control over their structures and properties. In terms of gas transport, ILs have high CO2 permeability, due to liquid-like diffusivity, and high CO2/N2 selectivity, due to solubility selectivity. For example, the commonly used IL [EMIM][TFSI] has CO2 permeability of 1000 barrers with a CO2/N2 selectivity of 22.14,18 At that CO2 permeability, the membrane active layer would only need to be ca. 100−1000 nm thick to achieve the desired CO2 permeance (1000s of GPU). However, membranes prepared from ILs soaked into a porous support are mechanically unstable and the IL is easily “blown out” of most support materials under several atm of trans-membrane pressure. To overcome the mechanical and processing issues associated with ILs, we (and others) have prepared PILs through direct polymerization of IL monomers.19 PIL materials have excellent mechanical properties, but they also display the gas transport properties expected from a polymeric material.19 Although the gas transport properties could be tuned by synthetically adjusting the IL monomer, the best properties we

processes combined with cooling and compression stages show that the separated CO2 can be produced as a high-purity supercritical fluid.6 The cost of CO2 capture was plotted as a factor of membrane permeance and CO2/N2 selectivity (Figure 4). Those results clearly show that to achieve the DOE CO2

Figure 4. Effect of CO2 permeance and CO2/N2 selectivity on the CO2 capture cost using the model process described by the Membrane Technology and Research, Inc. Reproduced with permission from ref 6. Copyright 2010 Elsevier.

Figure 5. Range of task-specific materials that we are developing from the PIL/IL ion-gel platform: (a) Linear PIL/IL ion-gels, this system has been prepared as 100-nm-thick active layers in a composite membrane; (b) Cross-linked PIL/IL ion-gel materials that can incorporate a higher wt % of free IL to improve CO2 permeability; (c) PIL/IL ion-gels that incorporate facilitated transport groups to improve CO2 permeability and CO2/N2 selectivity at low CO2 partial pressures; and (d) Mixed-matrix membranes that incorporate a microporous solid incorporated into the PIL/IL ion-gel platform to improve CO2 permeability and CO2/light gas selectivity. C

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Accounts of Chemical Research achieved with a purely PIL material were an ideal CO2 permeability of 16 barrers with an ideal CO2/N2 selectivity of ca. 41.19,20 At these CO2 permeabilities, however, the active layer thickness of this material would need to be ca. 1 nm to achieve the desired CO2 permeance. This review summarizes our recent advances regarding the development of a PIL/IL ion-gel materials platform that combines the best properties of ILs and PILs to produce iongel materials with high CO2 permeability, CO2/N2 selectivity, and the ability to be formed into thin active layers to produce composite membranes with high CO2 permeance. Specifically, this review also includes our work on the diversification and specialization of the PIL/IL ion-gel materials platform to produce materials such as linear PIL/IL blends, cross-linked PIL/IL blends, step-growth PIL/IL networks that enable facilitated transport of CO2, and PIL/IL composites with solid microporous additives (Figure 5).

is largely attributed to the effect of relatively strong electrostatic charge-charge interactions between the PIL and IL components.17 As would be expected, the increasing free IL content led to improvements in gas transport properties compared to the neat PIL materials (e.g., an ideal CO2 permeability of ca. 190 barrers with an ideal CO2/N2 selectivity of 19).23 At these CO2 permeabilities, the active layer of this material would only need to be ca. 20 nm thick to achieve the desired CO2 permeance (1000s of GPU) required for application to postcombustion CO2 capture. Having developed a promising PIL/IL ion-gel materials platform and set a target thickness of ≤100 nm for a membrane active layer, we then worked with industrial collaborators to develop a method to prepare thin films of the linear PIL/IL ion-gel material using a scalable process.24 The preparation of defect-free thin films of the linear PIL/IL materials required developing new methods for the casting of nonvolatile liquids. As a first step toward preparing thin films, we identified a gutter layer that would prevent IL penetration into the porous support without providing significant resistance to gas transport (i.e., a CO2 permeance of 45000 ± 1000 GPU).24 Initial experiments casting linear PIL/IL blends onto the gutter-layer material made it clear that surface tension issues (we found that gutter layers that could prevent the IL from soaking through the layer would also prevent wetting of the surface, causing the IL to bead up and resulting in the formation of defective or inhomogeneous layers) would prevent IL/IL monomer or PIL/IL mixtures being cast directly onto the gutter layer as thin films. To solve the surface tension issue, we developed a two-step casting strategy where a thin (ca. 82 nm thick) defect-free layer of the prepolymerized linear PIL was first coated onto the gutter layer as a methanol solution.24 After drying, a solution of the free IL [EMIM][TFSI] in isopropanol was then soaked into the existing PIL layer to produce a composite PIL/IL material that contained ca. 58 wt % of the free IL. After considerable effort, this strategy was successfully applied to roll-to-roll production of a composite membrane with a defect-free ca. 100-nm-thick active layer that had a CO2 permeance of ≥6100 ± 400 GPU with an ideal CO2/N2 selectivity of ca. 22 (Figure 7).24 Assuming that these materials would retain their performance under real-world flue gas conditions, the gas transport properties of these thin-film composite linear PIL/IL membranes could reduce the cost of CO2 capture to ca. $15 per ton of CO2 captured, according to the model process developed by Membrane Technology and Research, Inc. (Figure 4).6 These properties make these linear PIL/IL iongel composite membranes among the best membrane options for use in a postcombustion CO2 capture process.

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DEVELOPMENT OF PIL/IL ION-GEL MATERIALS An initial strategy we explored to form solid-like materials with high IL content was the use of low-molecular-weight organic gelators to form IL-gel materials.21 While this initial PIL-free strategy developed materials that contained extremely high amounts of IL (98 wt %) and had correspondingly good gas transport properties (an ideal CO2 permeability of 650 barrers with an ideal CO2/N2 selectivity of 22), the gel materials lacked the mechanical strength to form stable thin films and the iongels would phase-separate into gel and liquid domains over time. Consequently, we developed other strategies for preparing IL-containing materials that had better, more solidlike, mechanical properties. a. Linear PIL/IL Ion-Gels

In order to prepare hybrid materials with the desired mechanical properties, we prepared composite materials by polymerizing IL monomers in “free” ILs to form PIL/IL ion-gel materials (Figure 6).22 Qualitatively, these linear PIL/IL ion-gel materials have solid-like mechanical properties at a free IL loading of ca. 15 wt %. When supported membranes of these materials are produced, there is the potential to increase the IL loading and use materials that have more gel-like mechanical properties for a thin active layer. The relatively good mechanical integrity of these linear PIL/IL ion-gel materials

b. Cross-linked PIL/IL Materials for Increased Free IL Content and Improved CO2 Permeability

To further improve the CO2 permeability of the PIL/IL ion-gel membranes, we explored cross-linking strategies that allow increased free IL loadings while retaining the mechanical stability of the membrane material. While cross-linking of the neat PIL materials reduced the CO2 permeability (mainly due to reduced CO2 diffusion) relative to the neat linear PIL,25 it enabled inclusion of higher free IL loadings (≥75 wt %) in the resulting PIL/IL ion-gels, which offset the relatively minor permeability loses due to cross-linking. The best cross-linked materials that we produced had an ideal CO2 permeability of ≥500 barrers with an ideal CO2/N2 selectivity of ≥35.26

Figure 6. Generalized structures of PIL and IL cations and anions that we have used to prepare PIL/IL ion-gels for CO2/light gas separations. D

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PIL/IL ion-gels had ideal CO2 permeabilities of 500 ± 60 barrers and ideal CO2/N2 selectivites of 24 ± 4. However, the mechanical properties of the ion-gel materials prepared from the initial curable PIL were less than ideal and the resulting ion-gel would crumble under physical manipulations.28 We attributed the fragility of those materials to excessive cross-linking producing a heterogeneous material with loosely connected cross-linked PIL and IL domains. By reducing the number of cross-linking groups (i.e., randomly incorporating nonpolymerizable N-methylimidazolium groups to dilute the cross-linkable N-vinylimidazolium groups in the PIL structure), we were able to produce materials with dramatically improved mechanical propertieswhile retaining the high free IL content. Most importantly, the curable PIL strategy is compatible with the two-step casting method for preparing thin f ilms described in section a.

Figure 7. SEM cross-sectional image of the composite membrane with a ca. 100-nm-thick active layer of linear PIL/IL material that has a CO2 permeance of ≥6100 ± 400 GPU with an ideal CO2/N2 selectivity of ca. 20. Reproduced with permission from ref 24. Copyright 2014 American Chemical Society.

Furthermore, we showed that both the free IL and cross-linked PIL structures could be modified in these materials to optimize the gas transport properties.20b,23,27 Although these cross-linked PIL/IL ion-gel materials performed better than the linear PIL ion-gels, they were not initially compatible with the two-step casting strategy described above in section a. The small-molecule IL monomers and crosslinkers could not be cast directly, nor could we dissolve and cast the cross-linked PILs, which are insoluble.24 To overcome these issues, we developed a “curable PIL” platform.28 The postpolymerization functionalization of poly(chloromethylstyrene) allowed the inclusion of polymerizable pendant-groups onto the PIL polymer backbone (Figure 8), meaning that it could act as a IL-based macro-cross-linker. By swelling this curable PIL with free IL, then using free-radical polymerization to cross-link the linear PIL chains, we were able to prepare cross-linked ion-gels that contained ≥80 wt % of the free IL [EMIM][TFSI] (Figure 9). The resulting cross-linked

Figure 9. Free-standing cross-linked PIL/IL ion-gel membrane containing 80 wt % of the free IL [EMIM][TFSI]. These cross-linked membranes, prepared from the curable polymer described in section b, have an ideal CO2 permeability of 500 ± 60 barrers and an ideal CO2/N2 selectivity of 24 ± 4.

c. Epoxide/Amine PIL/IL Composites for Facilitated Transport of CO2 and Processing Advantages

The low CO2 partial pressure in flue gas provides an opportunity for facilitated transport membranes. Fixed-site facilitated transport membranes operate via a “hopping” mechanism, depicted in Figure 10, where the targeted gas hops across the membrane by undergoing a reversible chemical reaction with active sites dispersed throughout the membrane (e.g., amines for CO2 transport).29 Facilitated transport is reliant on the concentration gradient of “open” active sites across the membrane, and therefore is strongly dependent on the concentration (i.e., partial pressure) of the target gas making facilitated transport membranes particularly useful for the first separation stage in the CO2/flue gas separation process, where low CO2 partial pressures are available. To include amines into our PIL/IL materials, we used stepgrowth polymerization to develop epoxy-amine PIL/IL iongels, which afforded several benefits including: formation of facilitated transport groups within the PIL matrix, stoichiometric control over amine nature and cross-linking, and polymerization in the presence of O2.13,30 Testing of the epoxy-amine PIL/IL ion-gels with humidified, mixed CO 2 /N 2 gas streams showed increases in CO 2 permeability and CO2/N2 selectivity consistent with facilitated transport (Figure 11).13 We determined that the performance of these materials was largely determined by the nature of the anion (hydrophilic vs hydrophobic) and amine groups (primary

Figure 8. Synthetic route to “curable” PILs with different cross-linking densities (4, 5) via reacting poly(chloromethylstyrene) (1) with variable amounts of N-vinyl- and N-methyl-imidazole (2, 3). Adapted with permission from ref 28. Copyright 2014 American Chemical Society. E

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selective CO2 transport pathway through the PIL/IL ion-gel that is dependent on the size, shape and orientation of the zeolite particles.33 Previous research on the inclusion of porous materials into neutral polymer matrices to improve gas transport properties had highlighted the formation of highpermeance, nonselective free volume defects that impose significant limitations on MMM selectivity (Figure 12).32,34 With the combination of charged PILs as the polymer matrix and the free IL as a nonvolatile interfacial lubricant, the PIL/IL matrix has good adhesion to the zeolite particles, and the IL is also able to fill interfacial defects, making the PIL/IL ion-gel an excellent platform for producing selectivity-optimized membranes.31a Our initial work with these new three-component PIL/IL/ zeolite MMMs used the zeolites SAPO-34 and SSZ-13, which have shown particular promise for CO2/CH4 separations. This work established that even modest zeolite particle loadings of 10 wt % had a beneficial effect on the ideal CO2 permeability (e.g., 527 vs 67 barrers) and the ideal CO2/N2 selectivity (21 vs 15) compared to the neat PIL material.31b Additionally, the CO2/CH4 selectivity of the MMMs also improved (25 vs 11), prompting us to investigate these materials for the removal of CO2 from natural gas (i.e., CO2/CH4 separation). In contrast to the separation of CO2 from flue gas, the purification of natural gas occurs at higher pressure (e.g., 30−60 bar), providing a high CO2 partial pressure driving force that can be taken advantage of by membranes with high CO2 selectivity.35 We systematically optimized the PIL/IL-based materials platform with regard to the void space defects previously identified (Figure 12).36 Controlling the IL:zeolite ratio significantly reduced the contribution of defects at the zeolite:zeolite interfaces (Figure 12a) and PIL:zeolite interfaces (Figure 12b). We noted that controlling the IL:zeolite ratio was particularly important when switching between the zeolites SAPO-34 and SSZ-13, suggesting that the IL component could be tuned to accommodate different porous solidsa point of interest that we are continuing to explore. Using the “optimal” IL:zeolite ratio also improved the mechanical properties of the membrane materials (storage moduli values of ≥2200 MPa). By reducing the cross-linking density of the materials, we reduced the impact of the “densification” (Figure 12c) that occurs around the zeolite particles (which creates a high-resistance diffusion pathway into and out of the zeolite particles). Reducing the densification significantly improved the gas transport properties of the resulting MMMs, in one case increasing the ideal CO2/CH4 selectivity f rom 35 to 93. In all cases, targeting each defect provided a significant boost in the CO2/CH4 selectivity of the membrane material.36 Additionally, we realized that the fractional free volume (FFV) of the polymer component in a MMM can be considered a “defect” due to its relatively high CO2 permeability and relatively low CO2/light gas selectivity. By decreasing the FFV of the PIL component, we substantially increased the ideal CO2/CH4 selectivity (31, 60, and 70) at the cost of decreases to the ideal CO2 permeability (301, 172, and 200 barrers)a trade worth making for the CO2/natural gas separation application. Overall, this work provides a new platform technology and guidelines for preparing high-performance PIL/IL/zeolite iongel MMMs with a current best performance of an ideal CO2/ CH4 selectivity of 93 with an ideal CO2 permeability of 261 barrers. This performance is well-above the 2008 Robeson upper bound and among the best of any current membrane material for CO2/CH4 separations.11

Figure 10. Fixed-site facilitated CO2 transport “hopping” mechanism in an epoxide-amine PIL/IL ion-gel membrane. Adapted with permission from ref 13. Copyright 2015 Elsevier.

vs secondary). At the optimal conditions for facilitated CO2 transport (low CO2 partial pressure and high humidity), the best performance exhibited by these membranes was a CO2 permeability of 900 barrers and a CO2/N2 selectivity of 140. While the performance of these ion-gel membrane materials is impressive, the low CO2 partial pressures where the facilitated transport mechanism is most effective limits the application of these materials to the first stage of a CO2 separation process (i.e., where low CO2 partial pressures of ca. 10 kPa are typically present).

Figure 11. Plot of the CO2 permeability and CO2/N2 selectivity of an epoxide-amine ion-gel membrane as a function of CO2 partial pressure under a humidified (95% relative humidity), mixed CO2/N2 gas stream at 25 °C. Partially reproduced with permission from ref 13. Copyright 2015 Elsevier.

d. PIL/IL Ion-Gel Materials Containing Microporous Solid Additives: Mixed-Matrix Membranes (MMMs)

With the aim of improving CO2 permeability, we explored the effect of including zeolite particles (a crystalline microporous solid) into PIL/IL ion-gels to form mixed-matrix membranes (MMMs).31 Zeolite materials selectively transport CO2 over other gases due to a combination of preferential adsorption and molecular sieving32 and essentially provide a faster and more F

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Figure 12. Defects commonly encountered in preparing MMM materials comprised of a porous solid dispersed in a polymer matrix: (a) solid/solid surface defects; (b) polymer/solid surface defects; (c) densification of the matrix around the porous solid particles; and (d) transport through the polymer matrix.

cost savings in other industrially and economically important gas separation processes involving CO2.

The underlying science behind our most recent results suggest that the best results will be obtained using low FFV PIL/IL matrices and that the upper limits of PIL/IL/zeolite ion-gel MMM materials will be determined by the performance of the microporous solid additive and the loading at which it can be incorporated. Therefore, the platform technology we have focused on CO2/CH4 separations can be adjusted to the separation of many other gas pairs by selection of an appropriate microporous solid. In the immediate future, we also aim to adjust these materials for compatibility with the two-step casting method described in section a for preparing thin-films of the PIL/IL/zeolite ion-gels.

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AUTHOR INFORMATION

Corresponding Authors

*(D.L.G.) E-mail: [email protected]. *(R.D.N.) E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies

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Matthew G. Cowan currently works as a postdoctoral research associate in the Department of Chemistry & Biochemistry and the Department of Chemical & Biological Engineering at the University of Colorado at Boulder on a variety of projects related to light gas separations. His current and future research interests focus on combining chemistry and chemical engineering perspectives and technologies to address significant problems related to global energy use. He received his B.Sc. (Hons) (2008) and Ph.D. (2012) degrees in chemistry from the University of Otago in New Zealand.

CONCLUSIONS AND OUTLOOK We have developed chemical, materials, and processing technologies that allow PIL/IL ion-gels to be used as highperformance, ultrathin selective layers in composite membranes for CO2/N2 separations. These include the initial linear PIL/IL ion-gels that have been used to produce 100-nm-thick active layers in high-performance composite membranes (CO2 permeance ≥6000 GPU with an ideal CO2/N2 selectivity of ca. 22). These membranes have the potential to reduce the cost of postcombustion CO2 capture to ca. $15 per ton of CO2 captured. Recent improvements to the original PIL/IL materials platform have included: cross-linked PIL/IL blends that can entrain high (80 wt %) loadings of free IL while retaining mechanical strength and are compatible with the twostage casting process used for the preparation of thin films (an ideal CO2 permeability of 500 barrers with an ideal CO2/N2 selectivity of 24); step-growth epoxide-amine PIL/IL ion-gels that incorporate facilitated transport groups and exhibit extremely good CO2 transport under low-pressure, humidified mixed gas conditions (CO2 permeability of 900 barrers with a CO2/N2 selectivity of 140); and PIL/IL composites with added microporous solids (MMMs) that increase CO2 permeability and CO2/light gas selectivity and can be tailored for CO2/CH4 separationsand presumably other CO2/light gas separations (an ideal CO2 permeability 250 barrers with an ideal CO2/CH4 selectivity of 90). We are also interested in other emerging PIL/ IL ion-gels such as nanostructured materials, such as the block copolymer (BCP)-based materials prepared by Lodge and coworkers37 (an ideal CO2 permeability of ca. 900 barrers with an ideal CO2/N2 selectivity of ca. 40), and adding to our work in the area of nanostructured PIL-based BCPs.38 Moving forward, our aims are to test the PIL/IL membrane technology platform for extended periods under “real-world” conditions, similar to the other leading membrane technologies that are being evaluated for long-term use under realistic operating conditions in postcombustion CO2 capture processes.39 We also plan to diversify and specialize the PIL/IL ion-gel platform for other CO2/light gas separations so that these membrane materials can provide options for energy and

Douglas L. Gin received his B.Sc. in Chemistry from the University of British Columbia (1988) and his Ph.D. in Chemistry from Caltech (1993) working with Robert Grubbs. After postdoctoral work with Alan MacDiarmid at the University of Pennsylvania, he began his independent research career in the Department of Chemistry at the University of California at Berkeley in 1994. In 2001, he moved to the University of Colorado at Boulder, where he is currently a joint Professor in the Department of Chemistry & Biochemistry and the Department of Chemical & Biological Engineering. Richard D. Noble is the Alfred T. and Betty E. Look Professor of Chemical Engineering at the University of Colorado. His research interests involve experimental and theoretical studies of processes that can attain highly selective chemical separations. He is interested in the use of specific interactions that can enhance the selectivity of a separation process. These specific interactions involve unique solvents such as ionic liquids, reversible chemical complexation, use of external fields (light or electrical), or nanoporous structures (such as zeolites or lyotropic liquid crystals) to provide the selectivity. He has received several awards, including the ACS National Award in Separation Science and Technology in 2015.

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ACKNOWLEDGMENTS We greatly appreciate the efforts of the researchers who are listed as coauthors in the references below for their contributions to this work. In particular, we thank Dr. William M. McDanel for his input on this manuscript. We thank Lily A. Robertson for preparing the drawn figures used in this manuscript. The work described in this review was funded by grants from the Advanced Research Projects AgencyEnergy program (grant DE-AR0000098) with matching funds from G

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TOTAL S.A. (France), the National Science Foundation (grant IIP1047356), and the U.S. Defense Threat Reduction Agency (grant HDTRA1-08-1-0028).

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DOI: 10.1021/acs.accounts.5b00547 Acc. Chem. Res. XXXX, XXX, XXX−XXX