•MMMs were formed by blending up to 50 wt% pillar [5]arene-SOF in Matrimid-5218™.•Among the first SOF-containing MMMs with high (light gas/CH4) selectivity.•(Light gas/CH4) selectivity increased with SOF loading and annealing time.•MMMs performed at the upper bound line for CO2/CH4, N2/CH4, and H2/CH4 separations.Supramolecular organic frameworks (SOFs) represent a class of microporous solids with excellent light-gas selectivity, as well as good solubility/dispersibility in common organic solvents. We created new mixed-matrix membrane (MMM) materials by blending the microporous, solid supramolecular organic framework material, pillar[5]arene (P5-SOF) with the commercial polymer, Matrimid-5218™, and tested their light gas separation performance. These purely organic MMM materials were found to have very high (light gas/CH4) selectivity values (CO2/CH4: 180, N2/CH4: 6.5, and H2/CH4: 600), illustrating that they have promise as membranes for high-purity CH4 separations.Download high-res image (222KB)Download full-size image

•A lyotropic liquid crystal polymer (TFC QI) membrane to treat flowback water.•Permeance of the TFC QI was comparable to that of commercial RO and NF membranes.•Selectivity of the TFC QI was unique compared to commercial RO and NF membranes.•The TFC QI allowed for the recovery of labile carbon while rejecting salt.•Pretreatment enabled the TFC QI to recover 50% of DOC and still reject 75% of TDS.A thin-film composite, bicontinuous cubic lyotropic liquid crystal polymer (TFC QI) membrane with uniform-size, ionic nanopores was studied for the treatment of hydraulic fracturing flowback water. The TFC QI membrane performance was compared to those of a commercial nanofiltration (NF) membrane (NF270) and a commercial reverse osmosis (RO) membrane (SW30HR) for the filtration of flowback water from the Denver-Julesburg Basin. The permeability, salt rejection, and organic solute rejection for each membrane was evaluated. The results illustrate that the TFC QI membrane maintained its performance to a similar degree as the commercial NF and RO membranes while demonstrating a unique selectivity not observed in the commercial membranes. Specifically, the TFC QI membrane rejected 75% of the salt while recovering 9.6% of the dissolved organic carbon (DOC) and 50% of the water. Of particular interest was the recovery of labile DOC, which was assessed through biodegradation experiments. Analysis following biodegradation of the TFC QI membrane permeate demonstrates the membrane's ability to recover labile DOC in a reduced-saline permeate. Improved recovery of labile DOC (increased to 22%) was demonstrated by reducing the pH of the flowback water. Therefore, the selectivity of the TFC QI membrane provides an opportunity to recover resources from hydraulic fracturing flowback.Download high-res image (341KB)Download full-size image

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.

We demonstrate that an ether-based n-alkoxy-2,4-hexadiene polymerizable tail system is an effective and modular alternative to traditional ester-based polymerizable tail groups (i.e., acrylate, methacrylate, sorbate) and alkyl-1,3-diene tails for the design of radically polymerized ionic liquid crystal (ILC) monomers. Several series of nonsymmetric 1-vinylimidazolium-bromide-based ILC monomers containing these different polymerizable tail systems were synthesized and compared for their ability to form thermotropic liquid crystal (TLC) phases and to be photo-cross-linked with TLC phase retention. The n-alkoxy-2,4-hexadiene tail system was found to be more favorable/conducive to TLC phase formation than acrylate, methacrylate, and sorbate tails. It was more similar to the alkyl-1,3-diene tail system in terms of its more favorable effect on TLC behavior; however, it is more modular/easier to synthesize, more resistant to thermal Diels–Alder side reaction, and more isomerically pure, making it better for ILC monomer design. Also, the n-alkoxy-2,4-hexadiene tail system was found to be very amenable to radical photo-cross-linking with TLC phase retention. To demonstrate this feature, an example cross-linkable ILC monomer with this tail system was synthesized and polymerized in the smectic A TLC phase, and the monomer and polymerized material were characterized for their ionic conductivity behavior.

New imidazolium- and pyrrolidinium-based bis(epoxide)-functionalized ionic liquid (IL) monomers were synthesized and reacted with multifunctional amine monomers to produce cross-linked, epoxy–amine poly(ionic liquid) (PIL) resins and PIL/IL ion-gel membranes. The length and chemical nature (i.e., alkyl versus ether) between the imidazolium group and epoxide groups were studied to determine their effects on CO2 affinity. The CO2 uptake (millimoles per gram) of the epoxy–amine resins (between 0.1 and 1 mmol/g) was found to depend predominately on the epoxide-to-amine ratio and the bis(epoxide) IL molecular weight. The effect of using a primary versus a secondary amine-containing multifunctional monomer was also assessed for the resin synthesis. Secondary amines can increase CO2 permeability but also increase the time required for bis(epoxide) conversion. When either the epoxide or amine monomer structure is changed, the CO2 solubility and permeability of the resulting PIL resins and ion-gel membranes can be tuned.

A metal-containing ionic liquid (MCIL) has been prepared in which the [CoII(salicylate)2]2− anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.

An aspartame-based, low molecular-weight organic gelator (LMOG) was used to form melt-infused and composite membranes with two different imidazolium-based room-temperature ionic liquids (RTILs) for CO2 separation from N2. Previous work demonstrated that LMOGs can gel RTILs at low loading levels, and this aspartame-based LMOG was selected because it has been reported to gel a large number of RTILs. The imidazolium-based RTILs were used because of their inherent good properties for CO2/light gas separations. Analysis of the resulting bulk RTIL/LMOG physical gels showed that these materials have high sol–gel transition temperatures (ca. 135 °C) suitable for flue gas applications. Gas permeabilities and burst pressure measurements of thick, melt-infused membranes revealed a trade-off between high CO2 permeabilities and good mechanical stability as a function of the LMOG loading. Defect-free, composite membranes of the gelled RTILs were successfully fabricated by choosing an appropriate porous membrane support (hydrophobic PTFE) using a suitable coating technique (roller coating). The thicknesses of the applied composite gel layers ranged from 10.3 to 20.7 μm, which represents an order of magnitude decrease in active layer thickness, compared to the original melt-infused gel RTIL membranes.

Imidazolium- and oligo(imidazolium)-based ionic organic compounds are important in the design of room-temperature ionic liquid materials; however, the chromatographic analysis and separation of such compounds are often difficult. A convenient and inexpensive method for effective thin-layer chromatography (TLC) analysis and column chromatography separation of imidazolium-based ionic compounds is presented. Normal-phase ion-pair TLC is used to effectively analyze homologous mixtures of these ionic compounds. Subsequent separation of the mixtures is performed using ion-pair flash chromatography on normal-phase silica gel, yielding high levels of recovery. This method also results in a complete exchange of the counter anion on the imidazolium compounds to the anion of the ion-pair reagent.

The synthesis and characterization of a new class of 1-(2,3-dihydroxypropyl)-3-alkylimidazolium bis(trifluoromethanesulfonimide) room-temperature ionic liquids (RTILs) with tunable water miscibility are reported. The presence of a vicinal diol substituent on these RTILs allows for variable water miscibility, depending on the nature of the alkyl substituent on the cation. Water-miscible, imidazolium-based RTILs with the bis(trifluoromethanesulfonimide) anion are unprecedented.

Room-temperature ionic liquid (RTIL) based monomers were photopolymerized in the presence of nonpolymerizable RTILs with various types of organic functional groups attached to the imidazolium cation. Groups employed include alkyl, ether, nitrile, fluoroalkyl, and siloxane functionalities. This straightforward method allows for a broad range of functional groups to be incorporated into poly(RTIL) matrices without the need to develop new monomers. The resultant poly(RTIL)−RTIL composites were homogeneous, optically transparent solids that exhibited no signs of phase separation, even after many months of storage. As thin films, poly(RTIL)−RTIL composites were utilized as gas separation membranes and tested for their permeabilities to CO2, N2, and CH4. The presence of 20 mol % “free” RTIL within the poly(RTIL) matrix was shown to increase CO2 permeability by ∼100−250% relative to the neat poly(RTIL) membrane with no free RTIL component. The nature of the organic functional group associated with the free RTIL cation can influence both gas permeability and CO2/N2 and CO2/CH4 selectivity.

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 capabilities—and limitations—of imidazolium-based RTILs in CO2-based separations with respect to a variety of materials is thus provided.

A metal-containing ionic liquid (MCIL) has been prepared in which the [CoII(salicylate)2]2− anion is able to selectively coordinate two water molecules with a visible colour change, even in the presence of alcohols. Upon moderate heating or placement in vacuo, the hydrated MCIL undergoes reversible thermochromism by releasing the bound water molecules.