Ionic Liquids for CO2 Capture and Emission Reduction - American

PERSPECTIVE pubs.acs.org/JPCL

Ionic Liquids for CO2 Capture and Emission Reduction Joan F. Brennecke* and Burcu E. Gurkan Department of Chemical and Biomolecular Engineering University of Notre Dame, Notre Dame, Indiana 46556, United States

ABSTRACT Ionic liquids, especially those functionalized with amine groups, show significant potential for a wide variety of CO2 separations, including postcombustion CO2 capture. By tethering the amine to the anion, the stoichiometry of the reaction can be doubled from one CO2 for every two amines (as is the case with aqueous monoethanolamine) to one CO2 for each amine. Moreover, the reaction enthalpy can be actively tuned by the design of the anion, adjusting capacity and regeneration energy. In addition, ILs can be used without added water, further reducing the parasitic energy required for CO2 removal from flue gas.

onic liquids (ILs) are low-melting salts, and they are attractive for a wide variety of applications because they are relatively nonvolatile. Moreover, there are numerous combinations of cations and anions that yield ILs, and this flexibility can be used to tune chemical and physical properties. This is exactly what is needed to design an energyefficient liquid absorbent for CO2 capture. This is shown schematically in Figure 1. Just a few of these possible combinations of anions and cations are shown in the schematic. Candidate ions include ones that would physically dissolve CO2, as well as those that react with CO2, like the amino acids shown in the figure. Once the CO2 has been removed from the gas mixture, it can be released from the IL (which would be reused) by either a decrease in pressure or an increase in temperature, as suggested in the figure. On the basis of this idea, in this Perspective, we will attempt to demonstrate the potential of ILs for a variety of CO2 separations.

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stationary sources of CO2 like power plants rather than mobile sources (transportation vehicles) or distributed sources (buildings). In the U.S., approximately 75% of the electricity is generated from the burning of fossil fuels, with about half coming from conventional pulverized coal power plants. In India, over 70% of the electricity is produced from burning coal; in China, the number is over 80%. CO2 emissions from coal-fired power plants in the U.S. is 2.1 Gt/year.1 Even with aggressive deployment of renewable energy resources like wind and solar, it is likely that it will be several decades before nonfossil fuel resources represent the majority of our electricity production. Therefore, capture of CO2 from the flue gas of conventional coal- and natural-gas-fired power plants is likely to be an important component of our energy portfolio in the coming decades. McKinsey and company published a highly cited study of the most economically attractive and least expensive ways to cut the CO2 emissions of the U.S. in half.4 They concluded that reducing the U.S. CO2 emissions to that extent was not possible without carbon capture from coal-fired power plants. Thus, removal of CO2 from the flue gas of existing power generation facilities is vital. The flue gas from a typical coal-fired power plant is 13% CO2, 68% N2, 16% water, 3% O2, and lower concentrations of other components, as shown in Figure 2.3 It exits the power generation portion of the plant at high temperature and 1 bar of pressure. However, after removal of particulate, mercury, chloride, and SO2 in the flue gas desulfurization unit, it is at roughly 50 °C and 1 bar and has the concentrations shown in Figure 2.3 The technology that is available commercially (e.g., Fluor's Econamine FG or FG Plus technology) would use aqueous amine solutions to remove the CO2 from the flue gas. A typical amine developed for this purpose is monoethanolamine

There are numerous combinations of cations and anions that yield ILs, and this flexibility can be used to tune chemical and physical properties for CO2 capture. Worldwide CO2 emissions from fuel combustion were roughly 29.4 gigatons (Gt) in 2008.1 In the U.S., IEA reported it as 5.6 Gt for 2008; roughly a third of this is from the burning of transportation fuels, a third is from power generation, and a third is from heating and cooling.1 Due largely to rapid growth in the developing world, this is expected to increase to 40.2 Gt by 2030.1 Growing evidence of the link between anthropogenic CO2 emissions and global climate change2 has encouraged the development of a wide range of new CO2 capture technologies.3 Efforts for development and implementation of CO2 capture technology are generally focused on large

r 2010 American Chemical Society

Received Date: November 1, 2010 Accepted Date: November 26, 2010 Published on Web Date: December 01, 2010

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DOI: 10.1021/jz1014828 |J. Phys. Chem. Lett. 2010, 1, 3459–3464


Figure 2. Schematic of an aqueous amine scrubbing operation.

corrosivity, volatilization of the amine, and oxidative degradation of the amine. The theoretical minimum energy requirement for the separation of CO2 from flue gas and compression up to pipeline pressure is about 10% of the energy produced by the coal-fired power plant.7 Thus, there is tremendous room for improvement in the technology for CO2 separation. As a result, there has been significant activity in the development of a wide variety of different techniques for postcombustion CO2 capture. These include solid adsorbents, membranes, new liquid absorbents, as well as processes to directly convert the CO2 into carbonates or other manageable species.8 Solid sorbents like metal organic frameworks (e.g., see ref 9) represent innovative materials. However, any industrial process using solid adsorbents would likely involve temperature swing. Unfortunately, heat transfer to and from a solid support is very challenging and is likely to provide some serious limits on solid adsorbent technology. Membranes are attractive, but scaling up to power plant size would be challenging. Given these considerations, we believe that new liquid absorbents represent the best alternative to aqueous amine technology.

Figure 1. Schematic of a few possible combinations of anions and cations that could be used to make ionic liquids to preferentially dissolve CO2. One example, trihexyltetradecylphosphonium prolinate, which reacts with CO2, is shown in detail.

(MEA). Two amines react with the CO2 to form a carbamate salt, as shown in eqs 1 and 2.

If CO2 capture from coal-fired power plants had to be deployed today, this is the technology that would be used. Aqueous amines have been used for many decades in the removal of CO2 from natural gas and are even used to clean the air in submarines. However, aqueous amines have never been deployed at the scale that would be necessary for a typical power plant. CO2 emissions from a typical 500 MW coal-fired power plant are >7 tons/min, and the volume of gas that has to be treated is roughly 30 000 m3/min. A simple schematic of an aqueous amine scrubbing operation is shown in Figure 2. The flue gas is contacted countercurrently with the amine solution, which reacts with the CO2. The cleaned flue gas exits the top of the absorber, and the CO2-rich amine solution is taken to a stripper. The CO2 is removed from the amine solution by adding heat and raising the temperature to reverse the reaction and desorb the concentrated CO2. This CO2 can be compressed for geological storage in oil and gas reservoirs, unminable coal seams, or deep saline aquifers.2 The aqueous amine solution can then be reused in the absorber. The main problem with aqueous amine solutions (like MEA) is that they require large amounts of energy to operate the separation process. In fact, estimates are that almost 30% of the energy of the power plant would have to be diverted to run the CO2 capture process.5 This could result in a doubling of the cost of electricity.5 The large energy load with MEA can be attributed to much extent to the large heat of reaction, which can be as high as -85 kJ/mol of CO2 at 40 °C,6 and heat lost to vaporization of water in the stripper. Other problems include


Capture of CO2 from the flue gas of conventional coal- and naturalgas-fired power plants is likely to be an important component of our energy portfolio in the coming decades. New liquid absorbents include chilled ammonia10 and lowvolatility aminosilicones.11 However, here, we will focus on ionic liquids because of the ability to design and tune them for specific CO2 separations. CO2 Solubilities in ILs. In using ILs as sorbents for postcombustion flue gas or any of a wide variety of other CO2 separation processes, the most important property is the selectivity of CO2 over other components in the gas mixture. Figure 3 shows the solubility of various gases in 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide.

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The first measurement of high physical CO2 solubility in an IL was reported in 1999.13 Subsequently, there have been numerous studies of the solubility of CO2 in a wide variety of ILs (e.g., see refs 12 and 14-22). The general observations are that the nature of the anion plays a very important role in determining the solubility. For instance, CO2 solubility is ILs with the bis(trifluoromethylsulfonyl)imide anion is higher than that in ILs with a nitrate anion.23 In general, fluorination on the anion or, to a lesser extent, on the cation increases CO2 solubility.23,24 To a somewhat lesser degree, increasing the length of alkyl chains on the cation can increase CO2 solubility. Recently, Carvalho and Coutinho25 suggested that the physical solubility of CO2 in ILs is dominated by entropic effects, so that the solubilities of CO2 in all ILs at pressures to 5 MPa and temperatures between 298 and 363 K fall on a common curve when plotted as molality versus pressure. Unfortunately, many ILs and data that do not follow this trend were left out of the analysis, even from the references cited (e.g., ref 23). While entropic effects are likely important, the model suggested by Carvalho and Coutinho25 is, unfortunately, an oversimplification. In general, the physical solubility of N2, O2, H2, and CO in ILs is quite low,12 as shown in Figure 3. Therefore, selectivities for removing CO2 from both postcombustion and precombustion gases is quite high. However, for postcombustion gas separation, there is the capacity issue, as mentioned above. Hydrocarbon solubilities, including methane, are higher.12,13 For instance, in 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide, the CO2/CH4 selectivity would be on the order of 10 at room temperature.12 By contrast, the solubility of sulfur-containing compounds from postcombustion (SO2) and precombustion (H2S) gases tends to be relatively high in ILs; 26-29 therefore, one could envision either sequential or simultaneous removal of both CO2 and SO2 or H2S. One item of serious concern in using ILs for CO2 separations from flue gases is water solubility. All ILs, even the so-called “hydrophobic” ILs, are hygroscopic. This means that the IL in the absorber will pick up water from the flue gas. Even if the water does not decrease the capacity, this is a concern in an actual process because some of the water would be evaporated in the stripper, adding to the energy required for regenerating the IL. It is envisioned that most postcombustion CO2 capture processes would be placed after the flue gas desulfurization (FGD) unit. The most effective of these uses a wet process, so that the flue gas will be saturated with water at the temperature exiting the FGD, which is likely to be around 40-50 °C. At this temperature, the flue gas would contain 0.07-0.12 bar of water vapor. Even for ILs that are not completely miscible with water (i.e., the hydrophobic ILs), this could result in 1-15 wt % water in the liquid phase.30 However, this is significantly less than aqueous amine solutions, which usually contain 50-70 wt % water. Thus, for similar processing conditions, the ILs have a significant advantage over aqueous amine solutions because less water would be evaporated in the stripper. This means a lower energy requirement for the ILs. In general, CO2 solubilities in ILs are quite high compared to other gases in postcombustion flue gas; therefore, the

Figure 3. Solubility of various gases in 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide.12

Even for this IL, which simply absorbs CO2 physically, the selectivity of CO2 relative to N2 and O2 at flue gas compositions and 40 °C would be roughly 20 and 50, respectively. Another important property is capacity. Using the IL shown in Figure 3 would require 33 mol of IL for every mole of CO2 captured if the CO2 pressure were 1 bar. Given the high molecular weight of the IL, this would require an inordinately large mass of IL in the system for separation of CO2 from the postcombustion flue gas, where the partial pressure is about 0.13 bar. Thus, capacity is as important as selectivity for practical postcombustion CO2 separation processes. Capacity is not as much of a problem for a variety of other CO2 separation processes. For instance, in the removal of CO2 from natural gas, the partial pressure of CO2 at the wellhead could be tens of bars, depending on the CO2 content of the natural gas mixture. Thus, ILs that physically dissolve CO2 and have a high selectivity over CH4 (like that shown in Figure 3) are attractive for natural gas sweetening. Another emerging CO2 separation of importance is precombustion gases. The primary products from the gasification of coal, biomass, coke, or other fuel sources are H2 and CO. This syngas can be used for hydrocarbon synthesis, such as in hydroformylation. However, use of syngas as a decarbonized fuel requires the addition of steam to run the water gas shift (WGS) reaction (eq 3), which converts the CO to CO2 and produces more H2. The precombustion gases then consist primarily of CO2 and H2. The key then becomes separation of the CO2 from the H2 prior to combustion of the H2 or its use in fuel cells and so forth. Here again, the partial pressure of CO2 exiting the WGS reactor can be tens of bars. Therefore, ionic liquids that physically dissolve CO2 may have sufficient selectivity and capacity for use in precombustion CO2 capture. COðgÞ þ H2 OðvÞ f CO2ðgÞ þ H2ðgÞ

ð3Þ

ILs that physically dissolve CO2 are attractive for natural gas sweetening and precombustion gas separation.


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Figure 4. Comparison of CO2 solubility in terms of the mole ratio of the reactive IL, trihexyltetradecyl phosphonium prolinate, [P66614][Pro], with that of the unreactive IL, 1-hexyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide, [hmim][Tf2N], at 25 °C.

Figure 5. CO2 absorption capacities of the amino-acid-based ILs, trihexyltetradecyl phosphonium methionate, [P66614][Met], and trihexyltetradecyl phosphonium prolinate, [P66614][Pro], at 25 °C.

selectivity for removing CO2 is quite good. As a result, ILs that physically dissolve CO2 are attractive for natural gas sweetening and precombustion gas separation. However, the capacity is much too low at the low partial pressures present in postcombustion flue gas. Taking the lead from the conventional aqueous amine technology, one solution to this problem is to include functional groups in the IL that can react with CO2. This should maintain or increase selectivity while dramatically increasing capacity. Chemical Complexation with ILs. Bates et al.31 were the first to incorporate a free amine functional group in an IL. They attached it to the alkyl chain on an imidazolium cation and showed that the resulting IL could be used to take up significant CO2. Including chemical complexation in the IL can dramatically increase the CO2 capacity at low partial pressures, as shown in Figure 4. Bates et al.31 noted that the IL that they synthesized was initially quite viscous. As a result, they may not have noticed even further increases in viscosity upon reaction of the IL with CO2. Subsequent research, which has focused on amino-acidbased ILs,32-34 clearly demonstrates this phenomenon. In fact, Zhang and co-workers34 were not able to measure the CO2 uptake in the neat IL because of high viscosities. Instead, they coated the IL on a solid support to measure the capacity. For a practical process, we do not believe that a solid-supported IL is an attractive option. The solid would add significant mass so that the sensible heat required for the regeneration would increase and it would necessitate the incorporation of solid/gas heat exchange, which is quite challenging in practice. The increase in viscosity, which also occurs with molecular amines like MEA, is likely due to the formation of a hydrogenbonded network.35 However, for MEA, the problem is ameliorated by the large dilution with water. With ILs, we do not want to dilute with water because the subsequent evaporation of water during regeneration of the IL would add significant energy requirements. Fortunately, we have discovered how to eliminate the viscosity increase upon reaction of ILs with CO2.36 The key is the frustrating formation of a hydrogenbonding network by enlisting the use of aprotic heterocyclic anions.

As shown in eqs 1 and 2, the reaction chemistry of conventional amines with CO2 involves one CO2 molecule reacting with two amines (1:2 reaction stoichiometry). Because even efficiently designed ILs will inevitably have higher molecular weights than their molecular counterparts, IL CO2 capacity is lower than aqueous amines (e.g., MEA) on a mass or volumetric basis. Therefore, it would be desirable to improve the stoichiometry of the reaction. On the basis of theoretical calculations,33,37 it has been shown that reaction of CO2 with an amine tethered to the anion of an IL should terminate at eq 1 and exhibit 1:1 stoichiometry. This would double the CO2 capacity of ILs on a molar basis and bring mass and volumetric capacities in line with aqueous amine solutions. As shown in Figure 5,33 amino-acid-based ILs do absorb close to 1 mol of CO2 per mole of IL. Zhang and co-workers34 also studied anion-functionalized amino acid ILs that should reach 1:1 capacity. However, they did not recognize this change in reaction chemistry because the ILs were on a solid support, and they erroneously attributed any unexpectedly high uptakes to the support. Another approach to using ILs for postcombustion CO2 capture is simply to dissolve conventional amines in nonfunctionalized ILs.38 In this case, some CO2 would be absorbed physically by the IL, in addition to that chemically complexing with the dissolved molecular amine. Depending on the choice of IL and the hydrophilicity of the amine, this would likely eliminate some of the energy requirements associated with water evaporation. However, the heat of reaction (and subsequent heat required for regeneration) would be fixed by the choice of conventional amine added to the IL and could not be tuned to minimize the parasitic energy load. Process Modeling. Alternatives to aqueous amines for postcombustion CO2 capture applications must have comparable or improved performance and must be less expensive. Cost involves both operating cost and capital investment. For postcombustion CO2 capture processes, a significant part of the operating cost is the energy required. Thus, a lower parasitic energy requirement than that for aqueous MEA processes is a must for any new technology. Energy needs


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emanate from three main sources, (1) heat that has to be added to drive the CO2 off of the absorbent so that the absorbent can be reused, (2) pumping requirements, and (3) energy needed to compress the purified CO2 up to pipeline pressure, which is generally considered to be 2000 psi. Compressing the entire flue gas stream is prohibitively expensive. A low-capacity solvent means that excessively large amounts would be needed; this would increase pumping costs and capital costs because much larger equipment would be required. An absorbent with a large heat of reaction means that the energy required to uncomplex it in the regenerator would be large. However, the heat of reaction (the enthalpy of reaction) is related to the CO2 capacity because ln K = -ΔG/RT, where K is the equilibrium constant for the complexation reaction and ΔG is the Gibbs free energy change. ΔG = ΔH - T ΔS, where ΔH and ΔS are the enthalpy and entropy of reaction, respectively. Thus, there is a trade-off between wanting higher CO2 capacity at a particular temperature and wanting to keep the heat needed to reverse the reaction low. Being able to tune this reaction enthalpy is one of the attractive features of ILs. This can be done by incorporating electron-withdrawing or electron-donating groups in the anion that weaken or strengthen the complex between the anion and the CO2. In general, it is beneficial to desorb the CO2 at as high of a temperature as possible so that the partial pressure exiting the regenerator (i.e., stripper) is as high as possible. This is because the work required for compression is roughly proportional to the logarithm of the ratio of the final and initial pressures. Thus, the higher the pressure of CO2 being produced, the lower the compression costs. This is where the fact that ILs do not require added water is really beneficial. The only water present with ILs will be that absorbed by the IL in the absorber due to the water vapor in the flue gas. The heat of evaporation of water is very large; thus, additional heat is required in the generator for every mole of water that is evaporated. This limits the regeneration temperature (and CO2 pressure coming off of the stripper) for aqueous amines. This is not a limitation for ILs. Our work with process modeling of IL systems for postcombustion flue gas capture indicates significantly lower parasitic energy losses than aqueous MEA. How much this can be reduced will depend on further optimization of the ILs and their chemical and physical properties. In conclusion, energy- and cost-efficient separation of CO2 from postcombustion flue gas is a significant technical challenge. Ionic liquids present a promising new technology with the potential to achieve this goal. They show inherently high selectivity for CO2, and their capacity can be dramatically enhanced by incorporation of amine functional groups, especially on the anion where they react in a 1:1 stoichiometry with CO2. Chemical properties, such as the heat of reaction, can be adjusted with the choice of functional groups near the amine, and physical properties, such as heat capacity, density, and viscosity, can be tuned as well. This design capacity, along with the ability to work in the absence of significant amounts of water translates to decreased parasitic energy requirements.


Challenges remain in optimizing all of the chemical and physical properties.

Ionic liquids present a promising new technology for energy and costefficient separation of CO2 from postcombustion flue gas. AUTHOR INFORMATION Corresponding Author: *To whom correspondence should be addressed. E-mail: jfb@ nd.edu.

Biographies Joan F. Brennecke is the Keating-Crawford Professor of the Chemical and Biomolecular Engineering Department at the University of Notre Dame and the director of the Notre Dame Energy Center. She received her B.S. in chemical engineering from the University of Texas and a M.S. and Ph.D. from the University of Illinois at Urbana-Champaign. Burcu Gurkan is a graduate student at the University of Notre Dame in the Department of Chemical and Biomolecular Engineering. She currently has a CEST/Bayer Fellowship and plans to complete her Ph.D. in 2011. Her current work focuses on characterization of CO2 absorption reaction of ionic liquids.

ACKNOWLEDGMENT This material is based upon work supported by the Department of Energy under Award Number DE-FC-07NT43091.

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Ionic Liquids for CO2 Capture and Emission Reduction - American

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