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Ind. Eng. Chem. Res. 2008, 47, 8496–8498
Room-Temperature Ionic Liquid-Amine Solutions: Tunable Solvents for Efficient and Reversible Capture of CO2 Dean Camper, Jason E. Bara,* Douglas L. Gin, and Richard D. Noble* Department of Chemical and Biological Engineering, UniVersity of Colorado, Boulder, Colorado 80309
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 waterbased counterparts but may offer many advantages, including increased energy efficiency, compared to current aqueous amine technologies. The immediate need to reduce greenhouse gas emissions has brought great attention to CO2 capture and sequestration.1 The most viable near-term approach to post-combustion CO2 capture is chemical absorption.1 Acid gases (CO2, H2S, etc.) must also be removed from natural gas.2 While CO2 removal from natural gas increases the fuel value, H2S removal is also crucial, because it is extremely toxic.2 Amine-based “scrubbing” is used in 95% of U.S. natural gas “sweetening” operations.2 In this process, CO2 reacts with 2 equiv of monoethanolamine (MEA) to form an aqueous carbamate. CO2 can be released if the solution is heated and/or the partial pressure is reduced (see Scheme 1). This process is energy-intensive, yet effective for separating CO2 from other gases on large and small scales.1 Substantial improvements in energy efficiency are desirable to the economics of CO2 capture.1 The capture of acid gases from natural gas is performed at higher pressures than from post-combustion processes.1,2 The type of amine most effective in a given application is related to the partial pressure of the acid gas in the stream, with primary (1°) alkanolamines (i.e., MEA), secondary (2°) alkanolamines (e.g., diethanolamine (DEA)), and tertiary (3°) alkanolamines (e.g., triethanolamine (TEA)) being best-suited for low, moderate, and high pressures, respectively.1b The 3° amines can also selectively separate H2S from CO2.3 Room-temperature ionic liquids (RTILs) are nonvolatile solvents that have recently garnered considerable attention for several applications (see Figure 1).4 Imidazolium-based RTILs (1a) possess good CO2 solubility and selectivity for CO2, relative to N2 and CH4.4 These properties make RTILs interesting media for gas separation4 and capture.5 Gas dissolution in RTILs is primarily a physical phenomenon, with no chemical reaction. A “task-specific” ionic liquid (TSIL) (denoted here as 1b) was recently reported to be capable of CO2 capture, in an effort to combine the desirable properties of RTILs for gas separations4 with the reactivity of amines.5 However, 1b and its derivatives
do not appear to be viable for industrial processes, because there are many drawbacks associated with their use for CO2 capture: These functionalized RTILs are highly viscous in their unreacted states,5 and their corresponding CO2 adducts are intractable tars,5 thus limiting the utility of TSILs as neat solvents for CO2 capture. Furthermore, the production of amine-functionalized imidazolium salts requires several synthetic and purification steps5 and is not cost-competitive with commodity chemicals such as MEA. While TSILs can be dissolved in normal solvents or other RTILs to somewhat offset their inherently high viscosities, there are simpler ways to achieve efficient CO2 capture in RTIL solvents. Herein, we present an industrially attractive and unprecedented method to realize the CO2-capture performance of amines while utilizing the desirable properties of RTILs without the need for functionalized TSILs. This method is the use of organic amine/RTIL solutions, which affords many of the desired properties of TSILs for CO2 capture, but without many of their inherent drawbacks. A fact that has been seemingly overlooked thus far in RTIL gas separations research is that commercially available and inexpensive organic amines, such as MEA, can be readily dissolved in imidazoliumbased RTILs with Tf2N anions. We have found that RTIL-MEA solutions can be used for CO2 capture in a manner similar to their aqueous counterparts. RTIL-MEA solutions exhibit rapid and reversible CO2 uptake, and they are capable of capturing 1 mol of CO2 per 2 moles of dissolved amine. In addition, DEA can be utilized for CO2 capture in RTILs that contain Tf2N anions, but requires that the RTIL be tailored to contain a tethered 1° alcohol. RTIL-amine solutions may also offer significant advantages over conventional aqueous amine solutions, especially in regard to the energy required to process CO2. Imidazolium-based RTILs Scheme 1. Reversible Reaction of CO2 with 2 equiv of MEA To Form a Carbamate Salt
* To whom correspondence should be addressed. E-mail addresses:
[email protected],
[email protected]. 10.1021/ie801002m CCC: $40.75 2008 American Chemical Society Published on Web 10/08/2008
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Figure 1. General structures of (a) imidazolium-based RTILs and (b) aminesubstituted “task-specific” ionic liquids (TSILs).
Figure 2. RTIL solvents for alkanolamines in CO2 capture.
Figure 3. CO2 uptake in equimolar solutions of 2a-MEA and 2b-DEA.
have less than one-third the heat capacity of water (1.30 J g-1 K-1 vs 4.18 J g-1 K-1), or less than one-half on a volume basis (1.88 J cm-3 K-1 vs 4.18 J cm-3 K-1).6 Decomplexation of CO2 from aqueous carbamates requires heating the solution to elevated temperature, thus vaporizing water and amine, which must be condensed or replenished to the process.7 To investigate the performance and properties of RTIL-amine solutions, we first examined a widely studied RTIL6c,8 (2a) as a solvent for alkanolamines. However, it was quickly found that all amines useful for CO2 capture are not necessarily soluble in every RTIL. DEA was found to be immiscible with RTILs that contain solely alkyl substituents (i.e., 2a). To expand RTIL-amine solutions to 2° alkanolamines, an RTIL that contains a tethered 1° alcohol (2b) was used, which was miscible with MEA and DEA. The ability to tune the solubility and compatibility properties of RTILs is a powerful tool for process optimization,9 and it allows for these solutions to be used for CO2 capture over a range of pressures.7 These broad capabilities of RTIL-amine solutions are not easily obtainable with TSILs. To assess the CO2 capture performance of RTILs with 1° amines, MEA was dissolved in 2a and 2b as 50:50 (mol/mol) solutions (16% v/v MEA), containing one 1° amine per ion pair, analogous to amine-substituted TSILs.5 However, because MEA is miscible with 2a and 2b, the maximum loading of amine is by no means limited to 50 mol %. Solutions were loaded into a sealed vessel of known volume and exposed to CO2 at ∼1 atm and 40 °C while stirring at a moderate speed. The pressure of CO2 was observed to decay rapidly and capture of CO2 was >90% within 15 min, and the reaction was complete after 25 min (see Figure 3). Because RTILs with Tf2N anions have relatively low viscosities, the RTIL-amine solutions can be stirred. As a result, these times are ∼10% of that needed for CO2 capture by a highly viscous TSIL.5 CO2 can be rapidly decomplexed from MEA-carbamate in 2a by heating the resulting mixture to 100 °C and reducing the system pressure (see the Supporting Information). The solubility of CO2 in neat 2a and 2b under these conditions has been well-described in a previous work,4c and the uptake of CO2 by RTIL-amine solutions is at least 20 times greater than that which can be achieved in neat RTILs.4,5 Because MEA-carbamate is not soluble in 2a (see Figure 4) or 2b, the reaction equilibrium favors the formation of more
Figure 4. Photograph of equimolar 2a-MEA solutions: (a) CO2-lean, and (b) CO2-rich with precipitated MEA-carbamate salt.
carbamate as it precipitates (see Scheme 1). By reducing the carbamate concentration in solution, the residual CO2 content in the gas stream can be reduced to very low levels (see the Supporting Information).7 The insolubility of MEA-carbamate in RTILs is in sharp contrast to that in aqueous (or polar organic) solutions, in which the carbamate salt remains soluble, thereby slowing the rate of the CO2 capture reaction.7 The advantages and challenges of processing precipitated CO2 adducts have not yet received consideration. Our initial assessment is that if the carbamate salts can be collected and separated from the bulk RTIL solvent, CO2 may be decomplexed without having to heat the entire process fluid volume. Thermal input could be more directly applied to the carbamate-bound CO2, providing increased energy efficiency. By the same methodology, the use of DEA for CO2 capture in 2b was also examined. DEA and DEA-carbamate are both soluble in 2b, and this behavior is similar to that of aqueous DEA solutions.7 At 40 °C and a final CO2 pressure of 30.4 Torr (0.588 psia), an equimolar 2b-DEA solution contained 0.29 mol of CO2 per mol of DEA (see Figure 3). CO2 and DEA are in equilibrium with DEA-carbamate at low pressures and the capture does not go to completion, as with MEA.7 Under these equilibrium conditions, the amount of CO2 captured by equimolar 2b-DEA is the same as that achievable in aqueous DEA solutions.7 These properties make RTIL-DEA solutions especially attractive for CO2 scrubbing, because they may be interchangeable with aqueous DEA solutions that are currently used in industry.1,3,7 In summary, we have demonstrated through mixing that RTILs and alkanolamines may be combined in an efficient and effective manner for CO2 gas capture, superior to the use of analogous, amine-functionalized TSILs. The capture of CO2 in RTIL-amine solutions occurs rapidly and is readily reversed. Tuning of the RTIL allows for compatibilization with 2° alkanolamines, enabling these solvent systems to be effective for acid gas capture at a range of pressures. RTIL-amine solutions behave similarly to the aqueous amine solutions currently used, and they may offer significant advantages in the amount of energy consumed to process a given volume of CO2. Certainly, more research is needed before RTIL-amine solutions can be shown to be viable replacements for aqueous amines for CO2 capture in continuous processes. However, RTIL-amine solutions already present a new and promising approach to an established, yet highly energy-intensive, technology. Acknowledgment Primary funding from the U.S. Army Research Office (AB07CBT010 and HDTRA1-08-1-0028) is gratefully acknowledged. Partial funding from the NSF is also gratefully acknowledged (Grant No. DMR-0552399, to D.L.G.). The authors also
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gratefully acknowledge Dr. Richard Shoemaker for his assistance with the NMR section of the Supporting Information. Supporting Information Available: Details of the synthesis of 2a and 2b. Gas uptake experimental procedures and data. (PDFs) This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Recent papers on the issues and challenges of CO2 capture: (a) Hammond, G. P.; Akwe, S. S. O. Int. J. Energy Res. 2007, 31, 1180. (b) Tobiesen, F. A.; Svendsen, H. F.; Mejdell, T. Ind. Eng. Chem. Res. 2007, 46, 7811. (c) Eimer, D. Post-Combustion CO2 Separation Technology Summary. In Proceedings of the Carbon Dioxide Capture for Storage in Deep Geologic Formation; Thomas, D. C., Benson, S. M., Eds.; Elsevier: Oxford, U.K., 2005; p 91. (2) www.naturalgas.org/naturalgas/processing_ng.asp. (3) Li, Y.; Mather, A. E. Ind. Eng. Chem. Res. 1996, 35, 4804. (4) Gas separations using RTILs: (a) Han, X.; Armstrong, D. W. Acc. Chem. Res. 2007, 40, 1079. (b) Anderson, J. L.; Anthony, J. L.; Maginn, E. J.; Brennecke, J. F. In Ionic Liquids in Synthesis, 2nd Edition; Wasserscheid, P., Welton, T., Eds.; Wiley-VCH: Weinheim, Germany,
2007. (c) Camper, D.; Bara, J.; Koval, C.; Noble, R. Ind. Eng. Chem. Res. 2006, 45, 6279. (5) Bates, E. D.; Mayton, R. D.; Ntai, I.; Davis, J. H. J. Am. Chem. Soc. 2002, 124, 926. (6) Heat capacities of RTILs: (a) Valkenburg, M. E. V.; Vaughn, R. L.; Williams, M.; Wilkes, J. S. Thermochim. Acta 2005, 425, 181. (b) Waliszewski, D.; Stepniak, I.; Piekarski, H.; Lewandowski, A. Thermochim. Acta 2005, 433, 149. (c) Shimizu, Y.; Ohte, Y.; Yamamura, Y.; Saito, K.; Atake, T. J. Phys Chem. B. 2006, 110, 13970. (7) Astarita, G.; Savage, D. W.; Bisio, A. Gas Treating with Chemical SolVents; John Wiley & Sons: New York, 1983. (8) Various studies focused on 2a: (a) Shiflett, M. B.; Yokozeki, A. J. Phys. Chem. B 2007, 111, 2070. (b) Widegren, J. A.; Magee, J. W. J. Chem. Eng. Data 2007, 52, 2331. (c) Blokhin, A. V.; Paulechka, Y. U.; Kabo, G. J. J. Chem. Eng. Data 2006, 51, 1377. (9) Reviews on tuning the physical properties of RTILs: (a) Castner, E. W.; Wishart, J. F.; Shirota, H. Acc. Chem. Res. 2007, 40, 1217. (b) Smiglak, M.; Metlen, A.; Rogers, R. D. Acc. Chem. Res. 2007, 40, 1182.
ReceiVed for reView June 27, 2008 ReVised manuscript receiVed September 4, 2008 Accepted September 17, 2008 IE801002M
