Qualitative Determination of Species in DETAâH2OâCO2 System

Ind. Eng. Chem. Res. 2007, 46, 249-254

249

Qualitative Determination of Species in DETA-H2O-CO2 System Using NMR Spectra

13C

Ardi Hartono,*,†,| Eirik F. da Silva,‡ Hans Grasdalen,§ and Hallvard F. Svendsen† Department of Chemical Engineering and Department of Biotechnology, Norwegian UniVersity of Science and Technology, N-7491 Trondheim, Norway, and SINTEF Materials and Chemistry, N-7465 Trondheim, Norway 13 C NMR spectroscopic investigations were performed on aqueous solutions of diethylenetriamine (DETA) and carbon dioxide at 298.0 K. Systems with loadings ranging from 0 to 1.69 were studied. Results suggest that carbamate is one of the main species formed in the system at loadings below 1.0. At higher loadings (>1.0), it appears that dicarbamate is dominating and HCO3-/CO32- are also formed. No clear evidence was found of any tricarbamate species in the system.

Introduction There is at present great interest in improving CO2 absorption technology, to make it a more attractive technology for largescale capture of CO2 from exhaust gases. Finding better solvents is one way of improving the absorption process. A number of different amines have been considered in the carbon dioxide absorption process. Absorption with diamines has been the subject of some studies in recent years;1-4 such solvents are expected to have a higher capacity to bind CO2 than solvents with a single amine functionality. A molecule with three amine functionalities can potentially have an even higher capacity to bind CO2. The amount of CO2 in the solution is usually expressed in terms of the loading; this is defined as the number of moles of CO2 dissolved per mole of amine in the solution. In this paper we present initial results on the use of the triamine diethylenetriamine (DETA) as a solvent for CO2 absorption. DETA has two primary and one secondary amine groups. The presence of three amine groups suggests that a DETAH2O-CO2 system is potentially more complex than systems with a single amine group. There is a large number of species that may be formed and reactions that may take place. Nuclear magnetic resonance (NMR) techniques are widely used in qualitative and quantitative identification of chemical species. In the context of carbon dioxide absorption NMR has been used to determine carbamate stability and liquid-phase composition/speciation.3-9 The objective of this work is to use NMR techniques to determine which species are formed in a DETA-H2O-CO2 system. Determination of the species formed is a first step to understanding the chemistry of the system. Such insight is needed to develop thermodynamic models of the system. Experimental Section

mol % of amine) was added as internal reference solvent. CO2 (purity >99.9 mol % from AGA Gas GmbH) was added by bubbling the gas into the solution. The loading was determined from the weight change of the solution after bubbling with CO2. Loaded and unloaded solutions were filled into Norrel 507-HP tubes and weighed. A small amount of D2O (5-10 mass % of solution) was added to get a signal lock. All spectra at different loadings were acquired on a Bruker Avance DPX 400 MHz spectrometer with a 5 mm DUAL 1H/ 13C probe head. 13C NMR spectra were recorded at 298.0 K, by applying a one-dimensional sequence with decoupling, a 3.63 s pulse repetition time, and a 90° excitation pulse. At these conditions, all signals originating from CO2 will show a lower intensity compared to those from DETA carbons. All acquired spectra are plotted with Mestre-C NMR Data Processing Software from MestreLab Research. Chemical System Each amine group in DETA can potentially act as a base or to form carbamate. Considering all possible combinations of CO2 and protons (H+) binding to the amine groups, we are left with 18 different, i.e., symmetrically nonequivalent, DETA species. These potential species that may be formed are shown in Figure 1. In addition, the following species are known to be present: H2O, H3O+, OH-, CO2, HCO3-, and CO32- . In total this leaves us with 24 potential species. For these species the following chemical equilibrium equations may be set up: Dissociation of water:

2H2O a H3O+ + OH-

(1)

Dissociation of carbon dioxide:

A batch solution was prepared by weighing DETA (purity >98.5 mass %, from Acros Organics, without further purification) into distilled water (unloaded solution). 1,4-Dioxane (∼5

CO2 + 2H2O a HCO3- + H3O+

(2)

Dissociation of bicarbonate ion: * To whom correspondence should be addressed. Tel.: +4773594125. Fax: +47-73594080. E-mail: [email protected]. † Department of Chemical Engineering, Norwegian University of Science and Technology. ‡ SINTEF Materials and Chemistry. | Permanent address: Department of Chemical Engineering, Lambung Mangkurat University, Jl. A. Yani Km 35 Banjarbaru, Kalimantan Selatan, Indonesia. § Department of Biotechnology, Norwegian University of Science and Technology.

HCO3- + H2O a CO32- + H3O+

(3)

Dissociation of protonated DETA:

DETAH+(p) + H2O a DETA + H3O+

(4a)

DETAH+(s) + H2O a DETA + H3O+

(4b)

10.1021/ie0603868 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006

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Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007

Figure 1. Potential DETA species in DETA-H2O-CO2 system: p, primary amine; s, secondary amine; pp, two primary amines; ps, primary and secondary amines.

Dissociation of diprotonated DETA:

DETAH22+(pp) + H2O a DETAH+(p) + H3O+

(5a)

DETAH22+(ps) + H2O a DETAH+(s) + H3O+

(5b)

Dissociation of triprotonated DETA:

DETAH33+ + H2O a DETAH22+(pp) + H3O+

(6a)

DETAH33+ + H2O a DETAH22+(ps) + H3O+

(6b)

DETA + CO2 + H2O a DETACO2

(p)

+ H3O

[DETAH2(ps)CO2(p)]+ + H2O a DETAH(s)CO2(p) + H3O+ (13b) [DETAH2(pp)CO2(s)]+ + H2O a DETAH(p)CO2(s) + H3O+ (14) Dissociation of protonated dicarbamates:

Formation of carbamates on primary or secondary amine groups: -

[DETAH2(ps)CO2(p)]+ + H2O a DETAH(p)CO2(p) + H3O+ (13a)

+

DETA + CO2 + H2O a DETACO2-(s) + H3O+

(7) (8)

Formation of dicarbamates on primary and/or secondary amine groups:

DETACO2-(p) + CO2 + H2O a DETA(CO2)22-(pp) + H3O+ (9) DETACO2-(s) + CO2 + H2O a

[DETAH(s)(CO2)2(pp)]- + H2O a DETA(CO2)22-(pp) + H3O+ (15) [DETAH(p)(CO2)2(ps)]- + H2O a DETA(CO2)22-(ps) + H3O+ (16) Formation of tricarbamate:

DETA(CO2)22-(pp) + CO2 + H2O a DETA(CO2)33- + H3O+ (17) DETA(CO2)22-(ps) + CO2 + H2O a DETA(CO2)33- + H3O+ (18)

DETAH(s)CO2(p) + H2O a DETACO2-(p) + H3O+ (11b)

Due to the fast exchanging of protons, it is not possible to distinguish signals of protonated and unprotonated species.4,5 In Figure 2 the DETA species are separated into different groups. In the interpretation of experimental NMR data we can identify which groups are present, but assignment of species within the group cannot be done directly from the experimental NMR data. The observed signals represent the average of the species in a group. If the concentration ratios between the species in a group change, the signal may also shift. Such a shift is for example expected for protonating species with changes in pH.

DETAH(p)CO2(s) + H2O a DETACO2-(s) + H3O+

Result and Discussion

DETA(CO2)22-(ps) + H3O+ (10) Dissociation of protonated carbamates:

DETAH(p)CO2(p) + H2O a DETACO2-(p) + H3O+ (11a)

Dissociation of diprotonated carbamates:

(12)

13C NMR was performed on DETA-H O-CO systems at 2 2 different loadings. The chemical shifts for the system are in

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 251

Figure 2. Molecular structures and types of carbon nuclei in DETA species.

Figure 3.

13C

NMR spectrum for unloaded DETA.

the range δ ) 37-165 ppm (where δ is the chemical shift in parts per million). In the figures only regions of the spectra of interest are shown. The upfield region (37-52 ppm) corresponds to DETA carbons, while the downfield region (160-165 ppm) corresponds to the CO2 carbon of carbamate species. To show close signals in the downfield region, this part of the spectra is shown with greater resolution on the x-axis and the intensity of the signals is amplified. Results for unloaded DETA are shown in Figure 3. The two different signals (δ ) 39.98 ppm; δ ) 50.65 ppm) in the figure represent the average peaks of aC1, aC4 and aC2, aC3, respectively. The values of the chemical shift and the difference of both chemical shifts are in good agreement with reported data in the literature.10,11 These are the signals of DETA itself; the overlap of the signals is partly due to the symmetry of the molecule (see Figure 2A). While the molecule may take on conformers that are not symmetrical, the shift between conformers is likely to be rapid on the NMR time scale and the NMR signal only shows the average of signals of different conformers. Figure 4 shows the spectrum obtained at a loading of 0.22. In this spectrum five new peaks appear; we believe these to be the signals of carbamate species. The bC5 peak position is in

the range for a carbamate carbon, and bC1-bC4 are likely to be the signals of the four DETA carbons (see Figure 2B). The assignment of these peaks is based on two-dimensional NMR spectra of both COSY and HSQC techniques in combination with quantum mechanical calculations (given in the Supporting Information).12 Whereas DETA itself is symmetrical, the primary carbamate (DETACO2-(p)) is not symmetrical, and four different peaks for the DETA carbons are to be expected. Figure 5 shows the spectrum obtained at a loading of 0.43. It can be seen that the peaks for DETA and the carbamate species are shifted somewhat. This is most likely the effect of protonated forms of DETA and carbamate increasing in concentration. Six new peaks appear in this spectrum at cC1, cC4 (δ ) 40.05 ppm); cC2, cC3 (δ ) 48.12 ppm); cC7 (δ ) 163.95 ppm) and dC1, dC4 (δ ) 39.40 ppm); dC2, dC3 (δ ) 48.39 ppm); dC5, dC6 (δ ) 164.44 ppm). We believe these to be the signals of species of the secondary carbamate and of the dicarbamate attached at the two primary amine groups, respectively. Primary-primary dicarbamate or secondary carbamate both have symmetrical structure and thus it may be expected that three signals occur in the spectrum for each species (Figure 2, groups C and D).

252


Figure 4.

13C

NMR spectrum for DETA at loading of 0.22.

Figure 5.

13C


Figure 6.

13C


Figure 7.

13C


Figure 6 shows the spectrum obtained at a loading of 0.66. In this spectrum, six new peaks appear; we assign these to be the signals of a dicarbamate species. We believe that the signal at eC5 (δ ) 164.40 ppm) and eC7 (δ ) 164.01 ppm) correspond to the primary-secondary dicarbamate groups and that the four peaks at δ ) 40.02, 40.39, 47.17, and 47.74 ppm are the signals of the four DETA carbons (Figure 2E).

Figure 7 shows the spectrum acquired at a loading of 0.80. The species seen in this spectrum are likely to be the same as those at a loading of 0.66 (Figure 6). The signals due to the primary carbamate at bC2and bC3 are shifted; this suggests the presence of a protonated primary carbamate species. The signals due to the DETA at aC2, aC3, the primary carbamate at bC2 and bC3, and the primary-secondary dicarbamate are also

Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 253

Figure 8.

13C


Figure 9.

13C


Figure 10.

13C


shifted; these suggest the presence of a protonated species. The dC2 and dC3 overlap with bC3, because of shift, and eC3 overlaps with aC2 and aC3. Figure 8 shows the spectrum acquired at loading of 1.0. No new signals are found in this spectrum, but some signals are shifted as a result of the presence of protonated species. The shifted signals are bC2 at the primary carbamate; cC1, cC4 at the secondary carbamate; eC2, eC3 at the primary-secondary dicarbamate; and dC1, dC4 at the primary-primary dicarbamate, which overlaps with bC3. Also, the two peaks of free DETA (aC1, aC4 and aC2, aC3) have shifted. Figure 9 shows the spectrum at a loading of 1.38. A new signal appears at δ ) 160.71 ppm; we believe that this signal is due to the presence of HCO3-/CO32-. We see a reduction in the intensities of the signals corresponding to the primary carbamate DETA carbons (bC1, bC2, bC3, and bC4), suggesting that the concentrations of this species is decreasing. The intensities of the primary-primary dicarbamate (dC1, dC4 and dC2, dC3) are increasing. Both are expected, because of the increased loading. Figure 10 shows the spectrum at a loading of 1.69. The intensity of signals due to the primary carbamate (e.g., bC2 and

bC3), the secondary carbamate (cC1-cC4), and the primarysecondary dicarbamate species (e.g., eC3 and eC7) appear to decrease from a loadings of 1.38 to 1.69. Also, the primaryprimary carbamate decreases, e.g., dC1, dC4. At the same time, the HCO3-/CO32- peak becomes visible in Figures 9 and 10. We cannot see any signals due to the presence of a tricarbamate species. Conclusion In a DETA-H2O-CO2 system 24 species may potentially be formed. In this work we have determined the nature of some of the species formed in this system. Results suggest that carbamate, dicarbamate, and HCO3-/CO32- species are the main species formed in the system. No clear indication was found of a tricarbamate species or of free CO2. Acknowledgment This work was supported financially by the Directorate General of Higher Education (DGHE), Ministry of National Education, Republic of Indonesia, through The Technological

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and Professional Skills Development Sector Project (TPSDP), ADB Loan No. 1792-INO. The work has also been supported by the Norwegian Research Council through the BIGCO2 Project and a grant for computation time. Supporting Information Available: Details of the NMR nuclear shielding calculations, two-dimensional NMR spectra and some of the conformers in the DETA-H2O-CO2 system. This material is available free of charge via the Internet at http:// pubs.acs.org. Literature Cited (1) Ma’mun, S.; Svendsen, H. F.; Hoff, K. A.; Juliussen, O. Selection of New Absorbent for Carbon Dioxide Capture. Energy ConVers. Manage., published online 2006, http://doi.org/10.106/j.encoman.2006.04.007. (2) Ma’mun, S.; Jakobsen, J. P.; Svendsend, H. F.; Juliussen, O. Experimental and Modeling Study of the Solubility of Carbon Dioxide in Aqueous 30 Mass % 2-(2-Aminoethyl-amino)ethanol solution. Ind. Eng. Chem. Res. 2006, 45 (8), 2505-2512. (3) Bishnoi, S.; Rochelle, G. Thermodynamics of Piperazine/Methyldiethanolamine/Water/Carbon Dioxide. Ind. Eng. Chem. Res. 2002, 41, 604612. (4) Cullinane, J. T.; Rochelle, G. T. Thermodynamic of Aqueous potassium carbonate, piperazine and carbon dioxide. Fluid Phase Equilib. 2005, 227, 197-213. (5) Jakobsen, J. P.; Krane, J.; Svendsen, H. F. Liquid-Phase Composition Determination in CO2-H2O-Alkanolamines Systems: An NMR Study. Ind. Eng. Chem. Res. 2005, 44, 9894-9903.

(6) Bishnoi, S.; Rochelle, G. Absorption of Carbon Dioxide into Aqueous Piperazine: Reaction Kinetics, Mass Transfer and Solubility. Chem. Eng. Sci. 2000, 55, 5531-5543. (7) Chakraborty, A. K.; Astarita, G.; Bischoff, K. B. CO2 Absorption in Aqueous Solution of Hindered Amines. Chem. Eng. Sci. 1986, 41, 9971003. (8) Park, J. Y.; Yoon, S. J.; Lee, H. Effect of The Steric Hindrance on Carbon Dioxide Absorption into New Amine Solution: Thermodynamic and Spectroscopic Verification through Solubility and NMR Analysis. EnViron. Sci. Technol. 2003, 37, 1670-1675. (9) Suda, T.; Iwaki, T.; Mimura, T. Facile Determination of Dissolved Species in CO2-Amine-H2O System by NMR Spectroscopy. Chem. Lett. 1996, 9, 777-778. (10) Dagnall, S. P.; Hague, D. N.; McAdam, M. E. 13C Nuclear Magnetic Resonance Study of Some Aliphatic Polyamines. J. Chem. Soc., Perkin Trans. 2 1984, 435-440. (11) Pouchert, C. J.; Behnke, J. The Aldrich Library of 13C and 1H FT NMR Spectra, 1st ed.; Aldrich Chemical Company, Inc., Milwaukee, WI, 1993; Vol. 1, p 499. (12) B3LYP/6-311++G(d,p) gas-phase calculations carried out in Gaussian03. To be published in future work.

ReceiVed for reView March 28, 2006 ReVised manuscript receiVed September 6, 2006 Accepted September 16, 2006 IE0603868

Qualitative Determination of Species in DETAâH2OâCO2 System

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Qualitative Determination of Species in DETAâH2OâCO2 System