CHEMICALLY COMPLEXING IONIC LIQUIDS FOR POST-COMBUSTION CO 2 CAPTURE

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1 CHEMICALLY COMPLEXING IONIC LIQUIDS FOR POST-COMBUSTION CO 2 CAPTURE Burcu E. Gurkan, Juan C. de la Fuente, Elaine M. Mindrup, Lindsay E. Ficke, Brett F. Goodrich, Erica A. Price, William F. Schneider*, Edward Maginn*, Joan F. Brennecke* Department of Chemical and Biomolecular Engineering, University of Notre Dame, IN, USA Departamento de Ingeniería Química y Ambiental, Universidad Técnica Federico Santa María, Valparíso, Chile * wschneider@nd.edu, ed@nd.edu, jfb@nd.edu ABSTRACT New materials are vital to the development of more energy efficient processes for CO 2 capture from post-combustion flue gas. Here we show that amino-acid based ionic liquids are excellent candidates for this technology. Using a combination of theory and experimental CO 2 uptake, calorimetry and infrared spectroscopy measurements, we show that capacities of one CO 2 per mole of ionic liquid are possible. INTRODUCTION The utility sector produces roughly a third of the CO 2 emissions in the United States. Conventional coal fired power plants currently produce over 50% of the electricity in the United States. Even with dramatic growth of power generation from renewable sources, this percentage is not expected to drop significantly in the next two decades. Therefore, capturing CO 2 from post-combustion flue gas is a major target in efforts to avoid serious consequences for the Earth s ecosystem. The most mature technology for CO 2 capture is aqueous amine scrubbing, with monoethanolamine (MEA) most commonly used as the baseline. Unfortunately, the MEA process would result in a 28% parasitic energy loss and roughly double the cost of electricity. The heat required to regenerate the aqueous MEA solution is a major contributor to this parasitic energy loss. Developing new materials is the key to selectively and efficiently removing CO 2 from post-combustion flue gas. Ionic liquids (ILs) are attractive candidates because they have very low vapor pressures and high thermal stability. CO 2 is highly soluble in many ILs even when the mechanism of solubilization is just physical dissolution 1-6. Most importantly, ILs can be tuned limitlessly by the selection of anion, cation and the subsitutents. For post-combustion CO 2 capture applications, very high capacities are required at extremely low partial pressures (i.e., 0.1 to 0.15 bar) of CO 2. Therefore, here we investigate the functionalization of the IL with amines that can react with CO 2, as first suggested by Bates et al. 7. In that case the amine was tethered to an imidazolium cation and the suggested reaction chemistry was similar to that which occurs in aqueous amines. It was believed that a carbamate was formed by reaction of one CO 2 with two amines, as shown in the equations below:

2 All the groups that have investigated ionic liquids that incorporate amine functionality have assume this 2:1 stoichiometry. Systems investigated include functionalized sultones 8, and amino acids anions with imidazolium 9 and phosphonium 10 cations. The ability to terminate the reaction of an amine with CO 2 at eqn. 1 would allow 1:1 stoichiometry; i.e., one would need only one amine for each CO 2 captured. This would greatly increase the CO 2 capacity but would require eliminating the deprotonation of the carbamic acid intermediate. Using ab initio calculations we predicted that 1:1 stoichiometry could be achieved when tethering the amine to the anion 11. Here we confirm that high CO 2 uptake, up to 1 mole of CO 2 per mole of IL, can be achieved with ILs by using a combination of theory, experimental uptake measurements, calorimetry and Fourier Transform Infrared Spectroscopy. EXPERIMENTAL Isotherms of CO 2 + trihexyl(tetradecyl)phosphonium methioninate, [P ][Met], and CO 2 + trihexyl(tetradecyl)phosphonium prolinate, [P ][Pro], systems at 22 ⁰C were measured using a CO 2 Uptake Apparatus. In a separate system that incorporates an IR probe, physical and chemical portions of the capacities were determined. This also enabled the analysis of the reaction spectrum in-situ and identification of the end products. The IR spectrum of trihexyl(tetradecyl)phosphonium glycinate, [P ][Gly], was also analyzed. The reaction enthalpies for [P ][Met] and [P ][Pro] were measured by calorimetry and compared with theoretical calculations based on the [Met] and [Pro] anions following 1:1 versus 2:1 reaction stoichiometry. Figure-1. Structures of trihexyl(tetradecyl)phosphonium [P ] cation and glycine [Gly], methionine [Met] and proline [Pro] anions CO 2 Uptake Apparatus: Neat IL samples of 2.5 ml were allowed to react with CO 2 at 22 ⁰C in a closed glass vessel with a volume of 129 ml. Before adding CO 2, vacuum was applied to remove any gases in the vessel. CO 2 was allowed to react with the IL while the mixture was magnetically stirred. Equilibrium was verified by maintenance of constant pressure for several hours. After equilibrium was reached, the pressure was recorded and the amount of CO 2 remaining in the gas space was calculated using the ideal gas law. The difference in the amount of CO 2 in the gas space at the beginning and end of the experiment was calculated as the amount absorbed by the IL. All experiments are performed at constant temperature. Reaction enthalpy measurements by Calorimeter: Reaction enthalpies of CO 2 + IL systems were directly measured using a calorimeter (Setaram MicroDSCIII). One drop of IL (approximately 40 mg) was used. The entire experimental setup was degassed at elevated

3 temperatures (up to 70 ⁰C) for several hours prior to introducing CO 2 to the calorimeter cell with the IL. in-situ IR measurements: An Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) manufactured by Mettler Toledo (React-IR ic10) was used to monitor the reaction spectrum of the CO 2 + IL systems. The IR probe was mounted at the bottom of the micro reactor with a volume of 58 ml (Parr Instrument Company). The reactor is jacketed and the contents are constantly stirred. Approximately 6 ml of IL sample was used in each experiment. Samples were dried prior to experiments and loaded into the reactor under a nitrogen atmosphere. After the reactor was sealed, the gas space was evacuated by a turbo vacuum pump (Pfeifer). The reactor was charged with CO 2 and the equilibrium pressure was recorded after a stable IR spectrum was achieved. The total amount of CO 2 absorbed by the IL was calculated using the initial and equilibrium pressures at constant experimental temperature. The calculated overall absorption capacities for all ILs were verified with isotherm measurements obtained by the CO 2 uptake apparatus. The physical portion of the capacity was calculated using the IR calibrations for the CO 2 peak. Calibrations were developed through the use of Beer-Lambert s Law, using IL standards with known gas solubilities. THEORY Density Functional Theory Calculations: Energies at 25 ⁰C were calculated for the anions of the ILs before and after CO 2 attachment and the CO 2 itself in the gas phase. DFT calculations were performed at the B3LYP level of theory with G(d,p) basis set using Gaussian software. For each species, minimum energy conformers were determined through a systematic search. Calculated reaction enthalpies include the zero-point and thermal corrections to 25 ⁰C. RESULTS AND DISCUSSION CO 2 capacity measurements show that for each mole of IL, almost 1 mole of CO 2 is absorbed, suggesting a 1:1 overall reaction stoichiometry. The isotherms obtained for [P ][Met] and [P ][Pro] from both the Uptake Apparatus and the IR setup agree well as seen in Fig 2. While the mole ratio of absorbed CO 2 vs IL does not reach exactly to 1, it is significantly greater than 0.5, which is the theoretical mole ratio for 2:1 reaction stoichiometry 12. a b Figure-2. Isotherms of CO 2 with a) [P ][Met] and b) [P ][Pro], measured with Uptake Apparatus at 22 ⁰C and the IR setup at 25 ⁰C.

4 At low pressures, the isotherms display a steep slope where CO 2 is chemically absorbed. At higher pressures, the chemical absorption capacity is reached and additional uptake is due to physical absorption. From the IR calibrations, the physical uptake is quantified to be not more than 3% of the overall capacity at atmospheric pressure. Accordingly, the Henry s constants for the physical solubility are calculated as in Table 1. For this reason, the observed capacity can only be explained by chemical absorption that has a reaction stoichiometry of 1:1, as suggested in Fig. 3. Table-1. Experimental and calculated enthalpies for the CO 2 -IL reaction and the measured physical solubility Henry s constant at 25 ⁰C for the studied ILs Ionic Liquid H (bar) H rxn (kj/mol of CO 2 ) experimental experimental calculated [P ][Met] [P ][Pro] DFT calculations performed for the anions reacting with CO 2 following both 1:1 and 2:1 reaction stoichiometry show that the product of the 1:1 reaction has a lower relative energy; therefore, it is more favorable. This is consistent with the observations in the capacity measurements. The relative energies calculated for methioninate and prolinate (optimized structures are as shown in Fig 3) reacting with CO 2 by a 1:1 mechanism are in good agreement with the direct calorimetric measurements (see Table 1). The uncertainty in measured enthalpies is ±5 kj/mol. Therefore there is a clear difference between the attraction of [Met] and [Pro] for CO 2. This can also be seen in the isotherms, Fig. 2, where [P ][Pro] has a steeper slope compared to [P ][Met] in the chemical absorption region. Figure-3. Suggested reaction mechanism of CO 2 absorption by [P ][Met] and [P ][Pro] with optimized minimum energy conformers of the reacted anions in gas phase. The IR spectra of [P ][Pro], [P ][Met] and [P ][Gly] before and after reaction with CO 2 are shown in Fig 4. The reaction spectra shown here are at low CO 2 pressures. Therefore, the physically dissolved CO 2 peak that is generally seen between 2370 and 2310 cm -1

5 is not observed. In order to calculate the Henry s constants, higher pressures were applied, which is not shown here. All three ILs show asymmetric and symmetric stretches of COO - around 1600 and 1470 cm -1. Overlapping with the v = 1600 cm -1 asymmetric stretch of carbonyl group is the NH 2 bending for [P ][Gly] and [P ][Met] or NH bending for [P ][Pro]. For ILs incorporating a primary amine, N-H stretch is weakly observed as a doublet centered at approximately 3300 cm -1. a b c Figure-4. IR spectrum of a) [P ][Pro], b) [P ][Met] + 40 w% tetraglyme and c) [P ][Gly] + 35 w% tetraglyme at 25 ⁰C initially and at 1 st absorption point of CO 2 at pressures of 27, 12 and 15 kpa, respectively. Upon exposure to CO 2 three main features are observed: 1) A new band appears for all ILs around 1700 cm -1 which is due to COOH formation as CO 2 bonds to the nitrogen of the amine. 2) Amine transformation takes place for [P ][Met] and [P ][Gly], where a singlet is observed instead of a doublet at 3300 cm -1. For [P ][Pro], NH stretching and bending peaks disappear. 3) There is no evidence of ammonium formation except for [P ][Gly]. Upon repeating the absorption-desorption cycles it is observed that this IL loses its CO 2 capacity. Precipitation was observed in the sample. It is evident in Fig. 5 that there is secondary reaction taking place and that it can be attributed to a 2:1 reaction stoichiometry.

6 Figure-5. Reaction spectrum of diluted (with 25w% tetraglyme) [P ][Gly] a) before b) after reaction with CO 2 at 25 ⁰C and c) after heating (b) up to 80 ⁰C and cooling back down to 25 ⁰C It is important to note that [P ][Met] is observed to decompose during desorption at elevated temperatures (80 ⁰C), therefore multiple absorption-desorption cycles were not performed. CONCLUSIONS Using a combination of theoretical calculations and experimental uptake, calorimetry and IR measurements, we have shown that CO 2 binds with ILs with anion-based amine functionalization with a 1:1 mechanism. This high capacity and the controllable heat of reaction make ILs excellent candidates of CO 2 capture processes. ACKNOWLEDGEMENTS This material is based upon work supported by the Department of Energy under Award Number DE-FC26-07NT REFERENCES 1. J. L. Anderson, J. K. Dixon and J. F. Brennecke, Accounts of Chemical Research, 2007, 40, J. L. Anthony, J. L. Anderson, E. J. Maginn and J. F. Brennecke, Journal of Physical Chemistry B, 2005, 109, R. E. Baltus, B. H. Culbertson, S. Dai, H. M. Luo and D. W. DePaoli, Journal of Physical Chemistry B, 2004, 108, Y. H. Chen, S. J. Zhang, X. L. Yuan, Y. Q. Zhang, X. P. Zhang, W. B. Dai and R. Mori, Thermochimica Acta, 2006, 441,

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