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.
This study focuses on bulk fluid solubility of carbon dioxide (CO2), methane (CH4), hydrogen (H2), and nitrogen (N2) gases in the imidazolium-based RTILs: 1-ethyl-3-methylimidazolium ...bis(trifluoromethylsulfonyl)imide (emimTf2N), 1-ethyl-3-methylimidazolium tetrafluoroborate (emimBF4), 1-n-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (hmimTf2N), and 1,3-dimethylimidazolium methyl sulfate (mmimMeSO4) as a function of temperature (25, 40, 55, and 70 °C) at near-atmospheric pressures. The experimental behaviors are explained in terms of thermodynamic relationships that account for the negligible vapor pressure of the RTIL as well as the low solubilities of the gases. Results show that, as temperature increases, the solubility of CO2 decreases in all RTILs, the solubility of CH4 remains constant in emimTf2N and hmimTf2N but increases in mmimMeSO4 and emimBF4, and the solubility of N2 and H2 increases. Also, the ideal solubility selectivity (ratio of pure-component solubilities) increases as temperature decreases for CO2/N2, CO2/CH4, and CO2/H2 systems. Experimental values for the enthalpy and entropy of solvation are reported.
Imidazolate salts are powerful and versatile starting materials that can be used to generate many useful compounds. To this end, a method for producing sodium imidazolate (NaIm) from only commodity ...starting materials has been developed. NaIm was directly produced at a scale of ∼400 g via the neutralization of molten imidazole by NaOH, followed by dehydration. The NaIm product was then utilized as a starting material to produce 20 different N-functionalized imidazole and bis(imidazole) compounds, with the application of a common procedure involving minimal solvent volumes and straightforward purification via flash chromatography and solvent evaporation. Generating N-functionalized imidazoles “on demand” from NaIm and alkyl halides (or similar compounds) can eliminate the need for hazardous starting materials such as NaH and anhydrous solvents that have typically been employed in their synthesis. This method may ultimately enhance the availability of N-functionalized imidazoles for a variety of research and commercial applications.
While molar volume-based models for gas solubility in ionic liquids (ILs) have been proposed, free volume within the IL can be shown to be the underlying property driving gas solubility and ...selecitivity. Previously published observations as to the distinct differences in solubility trends for gases such as CH4 and N2 relative to CO2 in systematically varied ILs can be attributed to positive and negative effects arising from increasing free volume with increasing alkyl chain length. Through the use of COSMOtherm as a powerful and rapid tool to calculate free volumes in 165 existing and theoretical 1-n-alkyl-3-methylimidazolium (CnmimX) ILs, a previously unreported, yet speculated, critical underlying relationship between gas solubility in ILs is herein described. These results build upon previous assertions that Regular Solution Theory is applicable to imidazolium-based ILs, which appeared to indicate that a global maximum had already been observed for CO2 solubility in imidazolium-based ILs. However, the findings of this computational study suggest that the perceived maximum in CO2 solubility might be exceeded through rational design of ILs. We observe that although Henry’s constants in ILs are dependent on the inverse of molar volume and free volume, the volume-normalized solubility of CH4 and N2 are proportional to free volume, while CO2 is inversely proportional to the square root of free volume. Our free volume model is compared to experimental data for CO2/CH4 and CO2/N2 selectivity, and a nearly identical plot of selectivity relative to IL molar volume can be generated from the computational method alone. The overall implication is that large, highly delocalized anions paired with imidazolium cations that have minimally sized alkyl chains may hold the key to achieving greater CO2 solubility and selectivity in ILs.
Composite poly(ionic liquid)-ionic liquid membranes containing copper (I) chloride (CuCl) have been successfully fabricated via photopolymerization of an IL monomer, 1-vinyl-3-butylimidazolium ...bistriflimide (C4vimTf2N), in the presence of CuCl and a non-polymerizable IL, 1-butyl-3-methylimidazolium chloride (C4mimCl), forming the chlorocuprate anion (CuCl2−) in situ. The influence of the metal salt content on the gas separation performance of the composite membranes was assessed. Results showed that increasing the content of non-polymerizable IL enhanced the permeabilities of CO2, H2, N2 and CO relative to those obtained in the pristine poly(C4vimTf2N); whereas the addition of CuCl induced a general reduction of gas diffusivity. On the whole, an enhancement of both gas permeability and ideal gas pair selectivity were observed for CO2/N2 and H2/N2 separations in the Cu-containing composite membranes with respect to the neat poly(C4vimTf2N).
•Poly(IL)-IL composites containing Cu+ (as CuCl2−) fabricated via photopolymerization.•Presence of Cu+ did not promote facilitated transport of carbon monoxide.•Design approach could enable new poly(IL) materials with other transition metals.
Ionenes, condensation polymers wherein the charge (typically cationic) lies directly within the polymer backbone, have been known for over 85 years. Historically, ionenes have been synthesized from ...3o diamines and α,ω‐dihaloalkanes, forming chains of ammonium cations tethered by flexible hydrocarbon linkages with “free” halide anions. However, the requisite building blocks of ionenes are by no means limited only to such molecules. In recent years, ionenes with more sophisticated backbone chemistries have been produced, with a trend toward the use of functionalities associated with classical “high‐performance” condensation polymers such as polyimides and polyarylamides. The expansion of ionenes is also catalyzed by the rapid growth of research in imidazolium‐based ionic liquids (ILs), wherein the combination of ionenes with ILs can yield unexpected behaviors. Furthermore, when considering the largely unexplored experimental space in anionic ionenes, the opportunities for new materials are virtually endless. This review primarily focuses on developments in ionenes published in the scientific literature since 2010, but also includes some older examples that may not have received sufficient attention at the time of their original publication yet can provide some key lessons for the future of ionene design.
Ionenes are condensation polymers which contain ionic moieties directly in the polymer backbone, rather than as pendants. Although ionenes have been known for many decades, recent interest in charged polymers for a variety of applications has sparked the evolution of ionenes from ammonium cations tethered by relatively simple functional groups to much more robust architectures found in high‐performance polymers.
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.
•Olefin separation by silver(I) ions in ionic liquids and polymeric ionic liquids.•Silver(I) based polymeric ionic liquid used as gas chromatographic stationary phase.•Chemical structure of ionic ...liquid affects interaction of silver(I) ion and olefin.•Two-dimensional gas chromatography enhanced olefin separation using silver(I) ion.•Polymeric structure in ionic liquid eased silver(I) reduction at high temperatures.
Silver(I) ions have been used in various studies as components within polymer membranes or ionic liquids (ILs) to enable separation of olefins from paraffins. Polymeric ionic liquids (PILs) are a class of polymers synthesized from IL monomers and typically possess higher thermal and chemical stability than the ILs from which they are formed. Until now, very little is known about the difference in strength of silver(I) ion-olefin interactions when they take place in an IL compared to a PIL. In this work, the chromatographic separation of olefins by stationary phases composed of silver(I) bis(trifluoromethyl)sulfonylimide (Ag+NTf2−) incorporated into the 1-hexyl-3-methylimidazolium NTf2 (HMIM+NTf2−) IL and poly(1-hexyl-3-vinylimidazolium NTf2) (poly(HVIM+NTf2−)) PIL at varying concentrations was investigated. Olefins were more highly retained by silver(I) ions in PILs than in ILs as the silver(I) salt concentration in the stationary was increased. The potential separation power of silver(I)-containing IL and PIL stationary phases in comprehensive two-dimensional gas chromatography (GC×GC) was compared to the conventional one-dimensional system. The separation selectivity of alkenes and alkynes from paraffins was significantly increased, while dienes and aromatic compounds showed insignificant changes in retention. The chemical structural features of IL and PIL that enhance silver(I) ion stability and olefin separation were investigated by using silver(I) trifluoromethanesulfonate (Ag+OTf−), 1-decyl-3-methylimidazolium NTf2 (DMIM+NTf2−) IL, poly(1-decyl-3-vinylimidazolium NTf2 (poly(DVIM+NTf2−)) PIL, HMIM+OTf− IL and poly(HVIM+OTf−) PIL. Longer alkyl substituents appended to the IL (and PIL) cation increased the strength of silver(I) olefin interaction, and OTf− anions in the IL (and PIL) tended to preserve silver(I) ion from thermal reduction, while also retaining olefins less than the NTf2−-containing columns. In general, silver(I) ions in PILs possessing analogous chemical structures to ILs exhibited higher silver(I) ion-olefin interaction strength but were less thermally stable.
Understanding the molecular-level solubility of CO2 and its mixtures is essential to the progress of gas-treating technologies. Herein, we use grand canonical Monte Carlo simulations to study the ...single-component gas absorption of SO2, N2, CH4, and H2 and binary mixtures of CO2/SO2, CO2/N2, CO2/CH4, and CO2/H2 of varying mole fractions within multivalent ionic liquids (ILs). Our results highlight the importance of the free volume effect and the anion effect when interpreting the absorption behavior of these mixtures, similar to the behavior of CO2 found in our previous study (Phys. Chem. Chem. Phys. 2020, 22, 20618–20633). The deviation of gas solubility between the pure component absorption versus the binary absorption, as well as the solubility selectivity, highlights the importance of the relative affinity of gas species within a mixture to the different anions. The absorption selectivity within a specific IL system can be predicted based on the relative gas affinity to the anion.
Solutions of room-temperature ionic liquids (RTILs) and commercially available amines were found to be effective for the capture of CO2 as carbamate salts. RTIL solutions containing 50 mol % (16% ...v/v) monoethanolamine (MEA) are capable of rapid and reversible capture of 1 mol of CO2 per 2 moles MEA to give an insoluble MEA−carbamate precipitate that helps to drive the capture reaction (as opposed to aqueous amine systems). Diethanolamine (DEA) can also be used in the same manner for CO2 capture in RTILs containing a pendant hydroxyl group. The captured CO2 in the resulting RTIL−carbamate salt mixtures can be readily released by either heating and/or subjecting them to reduced pressure. Using this unprecedented and industrially attractive mixing approach, the desirable properties of RTILs (i.e., nonvolatility, enhanced CO2 solubility, lower heat capacities) can be combined with the performance of amines for CO2 capture without the use of specially designed, functionalized “task-specific” ionic liquids. By mixing RTILs with commercial amines, reactive solvents with a wide range of amine loading levels can be tailored to capture CO2 in a variety of conditions and processes. These RTIL−amine solutions behave similarly to their water-based counterparts but may offer many advantages, including increased energy efficiency, compared to current aqueous amine technologies.
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