Ind. Eng. Chem. Res. 2009, 48, 2717–2720
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NMR Studies of Amine Species in MEA-CO2-H2O System: Modification of the Model of Vapor-Liquid Equilibrium (VLE) Gao-jun Fan,*,†,‡,§ Andrew G. H. Wee,*,‡ Raphael Idem,*,† and Paitoon Tontiwachwuthikul† International Test Centre for CO2 Capture (ITC), Faculty of Engineering, UniVersity of Regina, and Department of Chemistry and Biochemistry, UniVersity of Regina, Regina, Saskatchewan S4S0A2, Canada
Speciation studies in aqueous monoethanolamine (MEA) solution at different CO2 loadings have been conducted via nuclear magnetic resonance (NMR) spectroscopy. The results showed that the diagnostic peaks corresponding to the MEA and the protonated amine (MEAH+) observed by 1H NMR spectroscopy were gradually shifted downfield, whereas the corresponding 13C peaks were shifted upfield as the CO2 loading increased. Quantitatively, the molar fraction of the carbamate was obtained by integration of the relevant peak areas in the 1H NMR spectra while the molar fractions of free MEA and MEAH+ were determined according to the MEAH+ dissociation constant and pH value of the solution. Furthermore, as the CO2 loading increased, the fraction of free MEA dropped steadily, while the ratio of carbamate and MEAH+ increased monotonically. The molar ratio of the carbamate reached its maximum at a CO2 molar loading of 0.5. The data obtained from this study have led us to propose a modification to the current vapor-liquid equilibrium (VLE) model for this system. Introduction Considered as one of the main greenhouse gases, carbon dioxide (CO2) has been associated with climate change and global warming problems. Techniques that use an absorbent to remove CO2 from a variety of source gases (such as natural gas, synthesis gas, flue gas, and various refinery streams) have been known.1-3 Among the absorbents, monoethanolamine (MEA) is one of the most-studied solvents. MEA is commercially available and also the most widely used solvent in industrial processes for the removal of CO2 from flue gases. To enhance the CO2 uptake, even at low CO2 partial pressure, as well as to reduce the energy required in the recovery of alkanolamines, a kinetic study between the amines and CO2 is very important. Nuclear magnetic resonance (NMR) spectroscopy is a valuable tool for studying chemical reactions via the detection of intermediates with sufficient lifetimes. Thus, NMR spectroscopy has been applied to study aqueous CO2 systems,4,5 to determine the carbamate stability constant,6,7 to study ion concentrations in the CO2-NH3-H2O system,8 and to determine the formation of protonated amine:carbamate salts derived from the reaction of CO2 and hydrogen sulfide (H2S) with anhydrous alkanolamine.9 Furthermore, evaluation of the liquid-phase composition in the alkanolamine-CO2-H2O system by NMR has been conducted by Suda et al.10 and Jakobsen et al.11 Suda and co-workers studied the chemical species in the amine solution based on a series of electroneutrality, material balances, and equilibrium constants in combination with NMR; however, only limited data were provided.10 Jakobsen and co-workers have studied the liquidphase composition with the aid of 13C NMR spectroscopy. Unfortunately, the quantitative evaluation of the species through the integration of 13C NMR peaks lacked accuracy.11 * To whom correspondence should be addressed. Tel.: +1-416-6659696. E-mail address:
[email protected]. † International Test Centre for CO2 Capture (ITC). ‡ Department of Chemistry and Biochemistry. § Present address: Toronto Research Chemicals, 2 Brisbane Road, Toronto, Ontario M3J 2J8, Canada.
Herein, we would like to report our studies on the composition of the MEA-CO2-H2O system, using both 1H and 13C NMR spectroscopy, in combination with a pH meter. Experimental Section The loading of CO2 into the 5.0 M aqueous MEA solution was performed in a rotary-type 600-mL stainless steel semibatch autoclave reactor (model Parr 5500, obtained from Parr Instrument Co., Moline, IL), which consisted of a variable-speed impeller, cooling coils, a gas feed port, a liquid sampling port, a thermo well, and a pressure gauge. The reagent-grade MEA (99% purity), obtained from Fisher Scientific, was diluted with deionized water to a concentration of 5.0 M. A 1.0 M aqueous hydrochloric acid (HCl) solution, also obtained from Fisher Scientific, was used to establish the exact concentration of MEA. Analytical-grade CO2 that was supplied by Praxair was used for loading. Various levels of CO2 were loaded into the 5.0 M aqueous MEA solution in the reactor. CO2 loading was achieved by bubbling the CO2 gas into the MEA solution at a pressure of 50-150 kPa for 1-4 h to obtain a loading of ∼0.05-0.6 mol CO2/mol MEA without heating. The CO2 loading in each sample was ascertained by titration, using a Chittick CO2 analyzer. The actual CO2 loadings that resulted from this procedure are provided in Figure 3, shown later in this paper. 1 H and 13C NMR spectra were recorded in D2O, using a Varian Mercury 500 MHz NMR spectrometer at 22.5 °C. The D2O resonance (δ ) 4.80 ppm) was used as an internal reference in the 1H NMR analysis, whereas 1,4-dioxane (δ ) 67.19 ppm) was used as an internal standard in the case of 13C NMR. The pH was recorded using the Accumet AB15 m standardized with pH 4, 7, and 10 buffer solutions. Samples with varying CO2 loading in 5.0 M aqueous MEA solution were then studied qualitatively and quantitatively. Results and Discussion 1
H NMR Spectra Studies. Representative 1H NMR spectra are shown in Figure 1. Before CO2 loading, the free MEA
10.1021/ie8015895 CCC: $40.75 2009 American Chemical Society Published on Web 02/06/2009
2718 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009
Figure 1. 1H NMR spectra for a 5.0 M MEA solution with varying CO2 loading at 22.5 °C. Figure 3. Plots of the amine species concentration against the CO2/MEA molar loading.
Figure 2. 13C NMR spectra for 5.0 M MEA solution with varying CO2 loadings at 22.5 °C. (The carbonyl peaks of MEACO2- and HCO3- are not shown here.)
appears as two triplet peaks, at 3.62 (J ) 5.6 Hz) and 2.75 ppm (J ) 5.6 Hz), respectively. After the CO2 loading, MEA reacted with CO2 to form the carbamate ion (MEACO2-), which resulted in another set of two peaks in the 1H NMR spectra, while the amine was protonated to give the ammonium ion. Because of the fast exchange of the proton, it is not possible to distinguish between the amine form and the protonated amine form in the NMR spectra. Thus, the amine and protonated amine peaks were treated as MEA/ MEAH+ peaks in the NMR spectra. With CO2 loadings at 0.16 mol CO2/mol MEA, the MEA/ MEAH+ resulted in two triplet peaks at 3.63 and 2.78 ppm, and two triplet peaks at 3.61 (J ) 6.0 Hz) and 3.16 ppm (J ) 6.0 Hz) were assigned to the protons of CH2O and CH2N of the carbamate, respectively (refer to Figure 1). Because of the strong electron-withdrawing ability of the carbonyl group in the carbamate, the chemical shift of the CH2N protons of the carbamate was located downfield, in comparison to the chemical shift of the CH2N protons of MEA. In contrast, the chemical shift of the CH2O protons of the carbamate was located upfield, in comparison to the chemical shift of the CH2O protons of MEA. At CO2 loadings of 0.24 mol CO2/mol MEA, the MEA/MEAH+ peaks were shifted to 3.64 and 2.83 ppm, whereas the carbamate peaks were located at 3.60 and 3.16 ppm, respectively. During the entire process of increased CO2 absorption from 0 mol CO2/mol MEA to 0.54 mol CO2/mol MEA, the chemical shifts of the two MEA/MEAH+ peaks were shifted significantly to lower field, from 3.62 and 2.75 ppm to 3.73
and 3.01 ppm, respectively. The shift of the MEA/MEAH+ peaks downfield in the 1H NMR spectra is due to the concomitant increase in the fraction of the protonated amine, in which the strongly electron-withdrawing ammonium group inductively withdraws electron density from the R and β protons. The change in the chemical shift of the CH2N protons (∆δ ) 0.26 ppm) is more pronounced than that of the CH2O protons (∆δ ) 0.11 ppm). During the CO2 absorption process, the relative intensity of the carbamate peaks gradually increased, because of the increased ratio of carbamate to MEA. In contrast to the downfield shift of the MEA/MEAH+ peaks, the two carbamate peaks were slightly shifted upfield. For example, the peaks at 3.61 and 3.16 ppm at a CO2 loading of 0.16 mol CO2/mol MEA were eventually shifted to 3.56 and 3.12 ppm, respectively, when the loading reached 0.54 mol CO2/mol MEA. This upfield shift of the carbamate peaks as the acidity increased upon absorption of CO2 is contrary to the deshielding effects that are caused by the increased acid content that has been described in the literature.12 13 C NMR Spectra Studies. Representative 13C NMR spectra are shown in Figure 2. Before CO2 loading, free MEA appears as two peaks, at 63.7 (CH2OH) and 42.9 ppm (CH2NH2), respectively. After loading with CO2, the results showed two sets of peaks: three peaks that corresponded to the carbamate and two peaks that corresponded to the MEA/MEAH+. At a CO2 loading of 0.16 mol CO2/mol MEA, the two MEA/ MEAH+ peaks were located at 63.0 and 42.8 ppm, and the three carbamate peaks were located at 165.3, 62.0, and 43.8 ppm, respectively. When the CO2 loading was increased, the relative intensity of carbamate peaks gradually increased; meanwhile, the intensity of MEA/MEAH+ peaks decreased. Different from the upfield-shift effects of their respective protons in the 1H NMR spectra, the 13C chemical shifts of the three carbamate peaks did not change, although the chemical environment around the resonating nuclei changed, because of the increased acidity. However, the chemical shifts of the two MEA/MEAH+ peaks were shifted significantly upfield, from 63.7 and 42.9 ppm to 59.5 and 42.1 ppm, respectively, when CO2 absorption was increased from 0 mol CO2/mol MEA to 0.54 mol CO2/mol MEA. Surprisingly, the change of the chemical shifts on the carbon adjacent to the hydroxyl group (∆δ ) 4.2 ppm) were more significant than that on the carbon adjacent to the nitrogen group (∆δ ) 0.8 ppm).
Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 2719 Table 1. NMR Data for the Carbamate (HO-aCH2-bCH2-NHc COO-) and MEA/MEAH+ (HO-dCH2-eCH2NH2) for the 0.32 CO2 Molar Loading Solution NMR Peak, δ (ppm) atom a b c d e
Figure 4. Plot of the vapor-liquid equilibrium (VLE) model reported by Austgen et al.14
However, the HCO3-/CO32- peaks that were present at 162.3 ppm in the 0.54 M CO2/MEA sample were not observed in solutions with lower CO2 loadings. The carbamate and MEA/MEAH+ peaks have been fully characterized by the two-dimensional (2D) NMR spectra (gCOSY, gHMQC, and gHMBC) for the 0.32 CO2 molar loading solution, as shown in Table 1. Quantitative Studies. The molar ratio of the carbamate (MEACO2-) to the sum of MEA and protonated MEA can be readily determined based on the ratio of the integrated peak area in the 1H NMR spectra. The total amine in the aqueous solution is considered to be 100%, namely, [MEA] + [MEAH+] + [MEACO2-] ) 100%. The respective ratio of MEA and protonated MEA can be determined by applying the equation of MEAH+ dissociation constant (eq 1). [MEA][H3O+] [MEAH+]
) Kp
(1)
The concentration of hydrogen ions can be obtained using a pH meter and the equilibrium constants Kp can be calculated from a simple correlation with temperature T (given in Kelvin) that was developed by Kent and Eisenberg13 (eq 2). ln Kp )
-5851.11 - 3.3636 T
(2)
The results are plotted as points in Figure 3. As the CO2 absorption increased, the pH value of the solution decreased and, therefore, the concentration of free MEA decreased. The concentration of MEAH+ gradually increased during the process of increasing CO2 loading and became predominant after half-molar loading. The ratio of the carbamate increased steadily and reached its maximum (ca. 41%) at half-molar loading. This result is in good agreement with that which was previously reported by Austgen et al.14 and Suda et al.;10 however, it is different from that of Jakobsen et al., which showed its maximum at ∼0.6 mol CO2/mol MEA.11 The model of the vapor-liquid equilibrium (VLE) that was developed by Austgen et al. showed that the ratio of MEAH+ is higher than that of the carbamate during the entire CO2 loading process14 (see Figure 4). In our studies, the concentration of MEAH+ is less than that of carbamate before the half-molar loading (see Figure 3), which means that most of the loaded CO2 reacted with MEA to form carbamate and only a small amount of CO2 reacted with water and free MEA, resulting in MEAH+. This observation is consistent
13
C
62.0 43.8 165.3 61.3 42.4
1
H
COSY
HMBC
3.59 3.15
b a
3.68 2.89
e d
b a, c b e d
with the results reported by Sartori and Leder.15 Hence, the formation of carbamate is comparatively faster than the formation of protonated MEA. This is likely one of the reasons why, in the widely used industrial amines, MEA absorbs CO2 faster than does N-methyl-2,2′-iminodiethanol (MDEA), which cannot form carbamates. Conclusions In conclusion, the amine species in the CO2-MEA-H2O system were studied qualitatively and quantitatively by NMR spectroscopy. As the acidity increased upon CO2 absorption, the MEA/MEAH+ peaks were shifted downfield, whereas the carbamate peaks were shifted upfield in the 1H NMR spectra. In contrast, in the 13C NMR spectra, the peaks of MEA/ MEAH+ were shifted upfield, but the carbamate peaks remained unchanged. The ratio of amine species was determined by calculating the 1H NMR peak integration ratio and the pH value of the solution. The ratio of amine species were based only on the 1H NMR peak areas and, therefore, were not affected by the error in the calibration. The vapor-liquid equilibrium (VLE) model has been modified as a consequence of our studies. Acknowledgment We thank the Natural Sciences and Engineering Research Council, Canada for the financial support. Literature Cited (1) Kohl, A. L.; Reisenfeld, F. C. Gas Purification, 4th Edition; Gulf Publishing Co.: Houston, TX, 1997. (2) Astarita, G.; Savage, D. W.; Bisio, A. Gas Treating with Chemical SolVents; John Wiley & Sons: New York, 1983. (3) Hendricks, C. Carbon Dioxide RemoVal from Coal-Fired Power Plants; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1994. (4) Patterson, A.; Ettinger, R. Nuclear Magnetic Resonance Studies of the Carbon Dioxide-Water Equilibrium. Z. Elektrochem. 1960, 98. (5) Abbott, T. M.; Buchanan, G. W.; Kruus, P.; Lee, K. C. 13C Nuclear Magnetic Resonance and Raman Investigations of Aqueous Carbon Dioxide Systems. Can. J. Chem. 1982, 60, 1000. (6) Barth, D.; Rubini, P.; Delpuech, J. J. Determination of the Thermodynamic Equilibrium Parameters for the Formation of Amino-alcohol Carbamates in Aqueous Solution by Carbon-13 Nuclear Magnetic Resonance. Bull. Soc. Chim. Fr. 1984, 7, 227. (7) Bishnoi, S.; Rochelle, G. T. Absorption of Carbon Dioxide into Aqueous Piperazine: Reaction Kinetics, Mass Transfer and Solubility. Chem. Eng. Sci. 2000, 55, 5531. (8) Holmes, P. E.; Naaz, M.; Poling, B. E. Ion Concentrations in the CO2-NH3-H2O System from 13C NMR Spectroscopy. Ind. Eng. Chem. Res. 1998, 37, 3281. (9) Maddox, R. N.; Mains, G. J.; Rahman, M. A. Reactions of Carbon Dioxide and Hydrogen Sulfide with Some Alkanolamines. Ind. Eng. Chem. Res. 1987, 26, 27. (10) Suda, T.; Iwaki, T.; Mimura, T. Facile Determination of Dissolved Species in CO2-Amine-H2O System by NMR Spectroscopy. Chem. Lett. 1996, 777.
2720 Ind. Eng. Chem. Res., Vol. 48, No. 5, 2009 (11) Jakobsen, J. P.; Krane, J.; Svendsen, H. F. Liquid-Phase Composition Determination in CO2-H2O-Alkanolamine Systems: An NMR Study. Ind. Eng. Chem. Res. 2005, 44, 9894. (12) Sorrell, T. N. Organic Chemistry; Daily Grind: Sausalito, CA, 1998; Chapter 15. (13) Kent, R. L.; Eisenberg, B. Better Data for Amine Treating. Hydrocarbon Process. 1976, 87. (14) Austgen, D. M.; Rochelle, G. T.; Peng, X.; Chen, C.-C. Model of Vapor-Liquid Equilibria for Aqueous Acid Gas-Alkanolamine Systems Using the Electrolyte-NRTL Equation. Ind. Eng. Chem. Res. 1989, 28, 1060.
(15) Sartori, G.; Leder, F. Process and Amine-Solvent Absorbent for Removing Acidic Gases from Gaseous Mixtures, U.S. Patent 4,112,051, Sept. 5, 1978.
ReceiVed for reView October 20, 2008 ReVised manuscript receiVed January 5, 2009 Accepted January 28, 2009 IE8015895
