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 series of four imidazolium salts containing increasing lengths of fluoroalkyl substituents was synthesized. The three that exist as molten salts at 296
K were tested for their gas separation ...properties relating to CO
2, O
2, N
2 and CH
4 using a supported ionic liquid membrane (SILM) configuration. These fluoroalkyl-functionalized room-temperature ionic liquids (RTILs) were found to exhibit ideal selectivities for CO
2/N
2 separation that were lower than their alkyl-functionalized analogues, but higher ideal selectivity for CO
2/CH
4 separation. The differences in performance of fluoroalkyl-functionalized RTILs relative to their alkyl-functionalized counterparts are explained through the use of solubility parameters, group contributions and in context of the classically observed deviations of fluorocarbons from “regular” solution behavior.
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 (CO(2)), figures prominently in the discussions on the future of global energy policy. Billions of tons of annual CO(2) 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 CO(2) emissions. Current technologies for CO(2) removal from other gases, such as those used in natural gas sweetening, are also capable of capturing CO(2) 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 CO(2) 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 CO(2) 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 CO(2) 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 CO(2) capture. RTILs present a highly versatile and tunable platform for the development of new processes and materials aimed at the capture of CO(2) 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 CO(2) 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 CO(2)-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.
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.
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.
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.
This study focuses on the solubility behaviors of CO2, CH4, and N2 gases in binary mixtures of imidazolium-based room-temperature ionic liquids (RTILs) using 1-ethyl-3-methylimidazolium ...bis(trifluoromethylsulfonyl)imide (C2mimTf2N) and 1-ethyl-3-methylimidazolium tetrafluoroborate (C2mimBF4) at 40 °C and low pressures (∼1 atm). The mixtures tested were 0, 25, 50, 75, 90, 95, and 100 mol % C2mimBF4 in C2mimTf2N. Results show that regular solution theory (RST) can be used to describe the gas solubility and selectivity behaviors in RTIL mixtures using an average mixture solubility parameter or an average measured mixture molar volume. Interestingly, the solubility selectivity, defined as the ratio of gas mole fractions in the RTIL mixture, of CO2 with N2 or CH4 in pure C2mimBF4 can be enhanced by adding 5 mol % C2mimTf2N.
