Scholarly article on topic '13C-NMR Spectroscopic Study on Chemical Species in Piperazine−Amine−CO2−H2O System before and after Heating'

13C-NMR Spectroscopic Study on Chemical Species in Piperazine−Amine−CO2−H2O System before and after Heating Academic research paper on "Chemical sciences"

CC BY-NC-ND
0
0
Share paper
Academic journal
Energy Procedia
OECD Field of science
Keywords
{" 13C-NMR" / amine / piperazine / carbamate / "carbon dioxide"}

Abstract of research paper on Chemical sciences, author of scientific article — Miho Nitta, Masaki Hirose, Toru Abe, Yukio Furukawa, Hiroshi Sato, et al.

Abstract Chemical reactions associated with the absorption of CO2 into aqueous solutions of blends of piperazine (PZ) with N- methyldiethanolamine (MDEA), etc. were studied by 13C-NMR spectroscopy. The coexistence of PZ and MDEA enhanced the initial apparent rate of HCO3 −/CO2− 3 formation. This result can be explained by considering that PZ−monocarbamate rapidly formed works as an organocatalyst in the formation reaction of HCO− 3. Concentration changes of chemical species in CO2-absorbed aqueous amine solutions upon heating (80°C, 30min) were studied by 13C-NMR spectroscopy. Carbon dioxide regeneration originates mainly from HCO3 −/CO3, and not form carbamate and carbonate.

Academic research paper on topic "13C-NMR Spectroscopic Study on Chemical Species in Piperazine−Amine−CO2−H2O System before and after Heating"

Available online at www.sciencedirect.com

SciVerse ScienceDirect

Energy Procedia 37 (2013) 869 - 876

GHGT-11

C-NMR Spectroscopic Study on Chemical Species in Piperazine-Amine-CC)2-H2O System before and after

Heating

Miho Nitta a, Masaki Hirose a, Toru Abe a, Yukio Furukawa a*, Hiroshi Sato b,

Yasuro Yamanaka c

a Department of Chemistry and Biochemistry, Graduate School of Advanced Science and Engineering, Waseda University, Shinjuku-

ku, Tokyo 169-8555, Japan b Research Laboratory, IHI Corporation, 1, Shin-nakahama-cho, Isogo-ku, Yokohama 235-8501, Japan _c Energy Operations, IHI Corporation, 1-1, Toyosu 3-chome, Koto-ku, Tokyo 135-8710, Japan_

Abstract

Chemical reactions associated with the absorption of CO2 into aqueous solutions of blends of piperazine (PZ) with N-methyldiethanolamine (MDEA), etc. were studied by 13C-NMR spectroscopy. The coexistence of PZ and MDEA enhanced the initial apparent rate of HCO37CO32- formation. This result can be explained by considering that PZ monocarbamate rapidly formed works as an organocatalyst in the formation reaction of HCO3~. Concentration changes of chemical species in CO2-absorbed aqueous amine solutions upon heating (80 °C, 30 min) were studied by 13C-NMR spectroscopy. Carbon dioxide regeneration originates mainly from HCO37CO32-, and not from carbamate and carbonate.

© 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT

Keywords: 13C-NMR; amine; piperazine; carbamate; carbon dioxide.

1. Introduction

Aqueous solutions of alkanolamines are widely used as absorbers for removing CO2 from flue gas of fossil-fueled power plants [1]. An aqueous solution of an alkanolamine absorbs CO2 chemically at room temperature and releases CO2 at high temperature. Primary amines such as 2-aminoethanol (MEA), 2-amino-2-methyl-1-propanol (AMP), etc., secondary amines such as 2-(isopropylamino)ethanol (IPAE), etc. and tertiary amines such as N-methyldiethanolamine (MDEA), 1-dimethylamino-2-propanol

* Corresponding author. Tel.: +81-3-5286-3244; fax: +81-3-3208-7022. E-mail address: furukawa@waseda.jp.

1876-6102 © 2013 The Authors. Published by Elsevier Ltd. Selection and/or peer-review under responsibility of GHGT doi:10.1016/j.egypro.2013.05.179

(DMA2P), etc. were used as potential candidate absorbers. Their chemical structures are shown in Fig. 1. Since a single amine solution did not show high performance, a blend of two or three amines was used as an absorber. In particular, piperazine (PZ, see Fig. 1f) was used as a so-called "activator" together with an alkanolamine such as MDEA and IPAE. However, the role of PZ remains unclear from a standpoint of molecular chemistry.

(a) (b) (c)

№ (e) (f)

Fig. 1. Chemical structures of amines: (a) MEA; (b) AMP; (c) IPAE; (d) MDEA; (e) DMA2P; (f) PZ.

It has been demonstrated [2-10] that 13C-NMR spectroscopy is a powerful tool for qualitative and quantitative studies of chemical species formed in aqueous solutions containing amines and CO2. It was elucidated that amines react with CO2, forming ionic species such as bicarbonate ion (HCO3~), carbonate ion (CO32_), protonated amines, amine carbamates, and amine carbonates, etc. It is believed that a carbamate molecule is formed through the deprotonation of the zwitterion of amine and CO2 in the presence of a proton acceptor such as an amine or a water molecule. In the aqueous solutions, CO2 reacts with alkanolamines either directly or via the acid-base buffer mechanism to form HCO3". Recently, a density functional theory study [11] suggests that amine, CO2, and H2O molecules react directly to form HCO3". The reaction mechanism has not been fully clarified yet.

In this paper we focus on quantitative 13C-NMR studies on concentration changes of chemical species in an aqueous solution of a blend of PZ with an amine (MDEA, DMA2P, or IPAE) in the course of CO2 absorptions and on those upon heating as a simple model of CO2 regeneration.

2. Experimental

2.1. Sample preparation

Samples of MEA (Tokyo Chemical Industry), AMP (Acros Organics), IPAE (Tokyo Chemical Industry), MDEA (Tokyo Chemical Industry), DMA2P (Tokyo Chemical Industry), and PZ (Acros Organics) were used without further purification. In the experiments of CO2 absorption kinetics, CO2 gas was bubbled through an aqueous solution of an amine or a blend at a rate of 150 mL/min. The concentration of PZ was 5 or 10 wt%, and that of an amine was 20 or 30 wt%. In the absorptionregeneration experiments, CO2 gas was bubbled through an aqueous solution of an amine or a blend at a rate of 50 mL/min at 50 °C for 60 min; the solution was heated at 80 °C for 30 min as the CO2 regeneration process. The concentrations of MEA, AMP, and MDEA were 20 wt%, and that of PZ was 10 wt%. A blend of 10 wt% PZ and 20 wt% amine (MDEA, DMA2P or IPAE) was used.

2.2. 13C-NMR measurements

The 13C-NMR spectra of a D2O solution containing an amine or a blend were measured at room temperature on a JEOL JNM-500ECX 500 MHz NMR spectrometer by using the inverse gated proton

decoupling method. Because of a long spin-lattice relaxation time, the holding time between scans was set to be 1 min. 128 scans were accumulated for each spectrum. As the standard of chemical shifts, 3-trimethylsilyl-1-propanesulfonic acid-d6 sodium salt was added into the sample solution.

3. Results and discussion

3.1. CO2-absorption kinetics

The 13C-NMR spectrum of an aqueous solution of a blend of PZ (10 wt%) and MDEA (20 wt%) and that after 50-min bubbling of the CO2 gas are shown in Figs. 2a and 2b, respectively. The observed bands have been assigned on the basis of the data in the literature [3, 8]. The assignments of the bands are listed in Table 1. The number of each carbon atom is shown in the chemical structures below. It should be noted that it is not possible to distinguish the signals originating from MDEA and protonated MDEA, because of the fast exchange of a proton between them. Thus a single band is assigned to both the species MDEA/MDEAH+. The chemical shift of this band shifts with increasing CO2-bubbling time because the contents of these species are changed. Similar assignments have been made to another species and its protonated species such as HCO3" and CO32".

Fig. 2. 13C-NMR spectra of (a) an aqueous solution of a blend of PZ (10 wt%) and MDEA (20 wt%) and (b) after 50-min CO2 bubbling.

The following species were observed: (i) MDEA and its protonated species; (ii) MDEA carbonate and its protonated species; (iii) PZ and its protonated species; (iv) PZ monocarbamate and its protonated species; (v) PZ biscarbamate; (vi) HCO3~ and CO32~. This result is consistent with the previous report [8].

The intensity of a band attributed to each species was converted to the concentration of the species. The concentration of each species is plotted as a function of CO2-bubbling time for an aqueous solution of

MDEA (20 wt%), PZ (10 wt%), PZ (10 wt%)-MDEA (20 wt%) blend, and PZ (10 wt%)-MDEA (30 wt%) blend in Figs. 3a-3d, respectively.

Table 1. Assignments of observed bands in PZ-MDEA-CO2-H2O system._

chemical shift / ppm Species Numbering a)

44.2-43.0 MDEA and its protonated species 1

43.4-43.2 MDEA carbonate and its protonated species 4

44.2-43.5 PZ monocarbamate and its protonated species 11

47.2-44.9 PZ and its protonated species 9

45.9-45.7 PZ monocarbamate and its protonated species 10

46.7 PZ biscarbamate 12

60.9-57.9 MDEA and its protonated species 3

58.6-58.1 MDEA carbonate and its protonated species 6

60.7-59.9 MDEA and its protonated species 2

60.1-60.0 MDEA carbonate and its protonated species 5

63.0-62.5 MDEA carbonate and its protonated species 8

161.1-160.6 MDEA carbonate and its protonated species 13

163.9-163.0 HCO7 and COs2" —

164.6-164.5 PZ monocarbamate and its protonated species 14

165.4 PZ biscarbamate 15

a) The numbering of carbon atoms is shown in the chemical structures below.

In the MDEA-CO2-H2O system (Fig. 3a), the concentration of HCO37CO32" increases with increasing time. The initial apparent rate of HCO37CO32- formation from 0 to 10 min was 0.049 mol/L-min. Probably, this rate depends on the rate of CO2 absorption into the solution. The concentration of HCO37CO32" becomes equilibrium at 50 min. The formation of a small amount of carbonate indicates that carbonate does not play an important role in the CO2 absorption-regeneration process. In the acid-base buffer mechanism, equilibrium reactions can be written as follows:

C02 + H2 O ^ H+ + HCO3 ( 1 )

H2O^H++OH" (2)

HC03^H++C032" (3)

NCH3 (CH2CH2OH)2 + H+ ^ N+HCH3 (CH2CH2OH)2 (4)

In the termolecular mechanism [11], the following reaction will proceed. NCH3 (CH2CH2OH)2 + CO2 + H2O N+HCH3 (CH2CH2OH)2 + HCO"

Fig. 3. Concentrations (mol/L) of HCO37CO3 (♦), MDEA (■), carbonate (Y), PZ (A), PZ monocarbamate PZ biscarbamate (►), total CO2 equivalent content (•), and pH (o) in aqueous solutions of (a) MDEA (20 wt%), (b) PZ (10 wt%), (c) PZ (10 wt%)-MDEA (20 wt%), and (d) PZ (10 wt%)-MDEA (30 wt%) as a function of CO2-bubbling time.

In the PZ-H2O-CO2 system (Fig. 3b), the concentrations of monocarbamate and HCO3 /CO3 increase. The concentration of monocarbamate is higher than that of HCO3 /CO32 . It is believed that a carbamate molecule is formed from the zwitterion of PZ and CO2. Thus, the following reactions are proposed:

The initial apparent rate of PZ monocarbamate formation from 0 to 10 min was 0.087 mol/Lmin. The initial rate of the HCO37CO32~ formation from 0 to 10 min was 0.040 mol/Lmin.

In the PZ (10 wt%)-MDEA (20 wt%)-CO2-H2O system (Fig. 3 c), the initial rate of HCO37CO32" formation from 0 to 10 min was 0.033 mol/Lmin; this is smaller than that (0.049 mol/Lmin) in MDEA (20 wt%)-CO2-H2O system. This decrease is due to the rapid formation of PZ monocarbamate. In this system, the rate of HCO3 /CO32 formation from 10

MDEA (20 wt%)-CO2-H2O system the corresponding rate was 0.058 mol/Lmin. This result indicates that the coexistence of PZ and MDEA enhances the rate. Similar rate enhancements were also observed for PZ (10 wt%)-MDEA (30 wt%)-CO2-H2O, PZ (5 wt%)-MDEA (20 wt%)-CO2-H2O, and PZ (10 wt%)-DMA2P (20 wt%)-CO2-H2O systems. This enhancement of the apparent rate was observed for tertiary amines MDEA and DMA2P. Since the rate of CO2 absorption is low for tertiary amines, this rate enhancement with PZ carbamate can be observed.

In order to explain this rate enhancement, we consider a novel mechanism of HCO3" formation-decomposition reactions as follows. The formation reaction of PZ monocarbamate can be expressed as follows:

A proton generated from the reaction of PZ and CO2 is accepted by an MDEA molecule. In addition, we assume the decomposition reaction of PZ monocarbamate as follows:

The combination of Eqs (8) and (9) leads to Eq (5). In Eq (5) PZ monocarbamate does not appear. Thus, the PZ monocarbamate molecule helps the formation of HCO3" like an organocatalyst. Since similar reactions associated with PZ biscarbamate are possible, the PZ biscarbamate molecule also may help the formation of HCO3" as an organocatalyst.

3.2. CO2 regeneration upon heating

Molar concentrations of the chemical species in amine-CO2-H2O system before and after heat-treatment for MEA, AMP, MDEA, and PZ are shown in Fig. 4. In an MEA aqueous solution bubbled with CO2, the concentrations of carbamate and HCO37CO32" were 1.34 and 0.79 mol/L, respectively. After heat-treatment, the concentration of carbamate increased to 1.55 mol/L. This suggests that carbamate is not decomposed to CO2 and MEA upon heating. The concentration of HCO37CO32" species decreased to 0.37 mol/L; 0.42 mol/L of CO2 was regenerated from the HCO37CO32" species. In an AMP aqueous solution bubbled with CO2, the concentrations of HCO37CO32" and carbonate were 2.26 and

0.05 mol/L, respectively. After heat-treatment, the

concentrations of HCO3

7CO32"

and carbonate

decreased to 1.84 and 0.04 mol/L, respectively. In an MDEA aqueous solution bubbled with CO2, the concentration of HCO37CO32" was 1.32 mol/L; upon heating, the concentration decreased to 0.76 mol/L. The 0.56 mol/L of CO2 was regenerated from the HCO37CO32" species. In an aqueous solution of PZ bubbled with CO2, there exist HCO37CO32" (0.31 mol/L), PZ monocarbamate (0.75 mol/L), and PZ biscarbamate (0.03 mol/L). The concentrations of HCO37CO32", monocarbamate, and biscarbamate were changed to 0.24, 0.63, and 0.10 mol/L, respectively. This result indicates that mono- and bis-carbamates are not decomposed to PZ and CO2.

Carbon dioxide is mainly saved in an aqueous amine solution as the chemical species of HCO37CO32", monocarbamate, and biscarbamate. Carbonate may be ignored because the concentration of carbonate is low. The observed data suggests that upon heating, the regeneration of CO2 mainly originates from the HCO37CO32" species. This is probably attributed to the temperature dependence of chemical equilibrium in reactions (1)-(4).

MEA AMP MDEA 1 1 1 1 Pz

1.34 I 1 1 1 1 1 1

0.79 1 L 1 1.55 0.37 2.26 1.84 1 1.32 : 1 1 0.75 ■ 1 0.03 I 0.76 1 N 1 : o.3i 1 0.63 0.10 ■ 0.24

Fig. 4. Concentrations of HCO3 /CO32 (black), monocarbamate (pink), biscarbamate (yellow), and carbonate (blue). Bars in the left- and right-hand sides for each amine are before and after heat-treatment, respectively.

The concentrations of HCO37CO32", PZ monocarbamate, PZ biscarbamate, and amine carbamate in the aqueous solution of a PZ blend with an amine such as MDEA, DMA2P, or IPAE before and after heat-treatment are listed in Table 2.

Table 2. Concentrations (mol/L) of HCO37CO32", PZ monocarbamate, PZ biscarbamate, and amine carbamate

before and after heating._

species_heating equivalent CO2 change MDEA PZ PZ-MDEA PZ-DMA2P PZ-IPAE

HCOb'/COb2" before 1.32 0.31 1.00 1.42 1.25

after 0.76 0.24 0.40 0.64 0.78

_A1_-0.56 -0.07 -0.60_-0.78_-0.47

PZ monocarbamate before — 0.75 0.74 0.68 0.67

after — 0.63 0.59 0.55 0.50

A2 — -0.12 -0.15 -0.13 -0.17

PZ biscarbamate before — 0.03 0.18 0.27 0.29

after — 0.10 0.22 0.40 0.44

A3 — 0.07 0.04 0.13 0.15

A2 + 2A3 — 0.02 -0.07 0.13 0.13

carbamate before — — — — 0.0

after — — — — 0.0

A, — — — — 0.0

A = A1 + A2 + 2A3 + At -0.56 -0.05 -0.67 -0.65 -0.34

Table 2 shows the concentration changes upon heating in HCO3 /CO32 (Ai), PZ monocarbamate (A2),PZ biscarbamate (A3), amine carbamate (A4) and total equivalent CO2 content (A = A1 + A2 + 2A3 + A4). In

this equation A3 is times by 2, because a biscarbamate molecule is made from two CO2 molecules. In all the solutions, A1 decreases upon heating, indicating that HCO37CO32" species contribute to CO2 regeneration. On the other hand, the sum of the equivalent CO2 contents in PZ monocarbamate and biscarbamate A2 + 2A3 increases for PZ-DMA2P and PZ-IPAE and decreases for PZ-MDEA. The total equivalent CO2 change A = A1 + A2 + 2A3 + A4 is large for PZ-MDEA and PZ-DMA2P. The observed data suggests that the regeneration of CO2 upon heating mainly originates from the HCO37CO32" species. Carbamate has a negative effect in CO2 regeneration upon heating.

4. Conclusions

We have studied the concentration changes of chemical species in an aqueous solution of a blend of PZ and an amine such as MDEA, DMA2P, or IPAE as a function of CO2-bubbling time by 13C-NMR spectroscopy. At the beginning of the reaction, PZ carbamate was formed. The coexistence of PZ and MDEA or DMA2P enhanced the initial rate of HCO37CO32" formation. This result probably indicates that PZ monocarbamate rapidly formed works as an organocatalyst in the formation reaction of HCO3". We have studied the concentrations of chemical species in CO2-absorbed aqueous amine (MEA, AMP, MDEA, PZ, PZ-MDEA, PZ-DMA2P, and PZ-IPAE) solutions upon heating (80 °C, 30 min) by 13C-NMR spectroscopy. The results indicate that CO2 regeneration originates from the HCO37CO32" species, and not from mono- and bis-carbamates and carbonate. 13C-NMR spectroscopy is a powerful tool for qualitative and quantitative analyses of chemical species in amine absorbers of CO2.

Acknowledgements

The authors are grateful to Professors H. Nakai and N. Kanomata, and Dr. H. Yamamoto of Waseda University for valuable discussion and suggestions.

References

[1] Aarou D, Tsouris C. Separation of CO2 from flue gas: review. Sep Sci Technol 2005; 40: 321-48.

[2] Suda T, Iwaki T, Mimura T. Facile determination of dissolved species in CO2-amine-H2O system by NMR spectroscopy. Chem Lett 1996; 777-8.

[3] Bishnoi S, Rochelle GT. Absorption of carbon dioxide in aqueous piperazine: reaction kinetics, mass transfer and solubility. Chem Eng Sci 2000; 55: 5531-43.

[4] Bishnoi S, Rochelle GT. Thermodynamics of piperazine/methyldiethanolamine/water/carbon dioxide. Ind Eng Chem Res 2002; 41: 606-12.

[5] Jakobsen JP, Krane J, Svendsen HF. Liquid-phase composition determination in CO2-H2^alkanolamine systems: an NMR study. Ind Eng Chem Res 2005; 44: 9894-903.

[6] Hartono A. da Silva EF, Grasdalen H, Svendsen F. Qualitative determination of species in DETA-H2^CO2 system using 13C NMR spectra. Ind Eng Chem Res 2007; 46: 249-54.

[7] Böttinger W, Maiwald M, Hasse H. Online NMR spectroscopic study of species distribution in MEA-H2^CO2 and DEA-H2^CO2. Fluid Phase Equilibria 2008; 263: 131-43.

[8] Böttinger W, Maiwald M, Hasse H. Online NMR spectroscopic study of species distribution in MDEA-H2^CO2 and MDEA-PIP-H2O-CO2. Ind Eng Chem Res 2008; 47: 7917-26.

[9] Yamada H, Shimizu S, Okabe H, Matsuzaki Y, Chowdhury FA, Fujioka Y. Prediction of basicity of aqueous amine solutions and the species distribution in the amine-H2^CO2 system using the COSMO-RS method. Ind Eng Chem Res 2010; 49: 2449-55.

[10] Ciftja AF, Hartono A, da Silva EF, Svendsen HF. Study on carbamate stability in the AMP/CO2/H2O system from 13C-NMR spectroscopy. Energy Procedia 2011; 4:614-20.

[11] Yamada H, Matsuzaki Y, Higashii T, Kazama S. Density functional theory study on carbon dioxide absorption into aqueous solutions of 2-amino-2-methyl-1-propanol using a continuum solvation model. J Phys Chem A 2011; 115: 3076-86.