13C-NMR Study of Acid Dissociation Constant (Pka) Effects on the CO2 Absorption and Regeneration of Aqueous Tertiary Alkanolamine−Piperazine Blends

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Energy Procedia 63 ( 2014 ) 1863 – 1868

GHGT-12 13C-NMR study of acid dissociation constant (pKa) effects on the CO2 absorption and regeneration of aqueous tertiary alkanolamine−piperazine blends

Miho Nittaa, Kogoro Hayashia, Yukio Furukawaa,*, Hiroshi Satob, Yasuro Yamanakac

aDepartment of Chemistry and Biochemistry, Graduate School of Advanced Science and Engineering, Waseda University, Shinjuku-ku, Tokyo 169-8555, Japan bResearch Laboratory, IHI Corporation, 1, Shin-nakahara-cho, Isogo-ku, Yokohama 235-8501, Japan cEnergy & Plant Operations, IHI Corporation, 3-1-1, Toyosu, Koto-ku, Tokyo 135-8710, Japan

Abstract

2– – We have studied the concentration changes of chemical species such as CO3 /HCO3 , amine/protonated species, carbonate/protonated species, carbamates/protonated species, etc. in the course of CO2 absorption and those after CO2 release upon heating at 93 qC for 30 min for aqueous blends of piperazine (PZ) and each of 11 tertiary alkanolamines using 13C-NMR spectroscopy. The initial rates of CO2 capture of the blends ranged between 0.125 and 0.167 mol/L min, which were contributed by the rapid formation of PZ monocarbamate. A positive linear correlation was found between the CO2 release amounts of the blends upon heating and the pKa values of the tertiary alkanolamines. The CO2 content captured as PZ mono- and bis- carbamates decreased upon heating in the pKa range smaller than 9.34, whereas it increased upon heating in the pKa range higher than 9.55. A tertiary alkanolamine having the pKa smaller than 9.34 is promising as a blend amine with PZ.

© 20142013 TheThe Authors. Authors. Published Published by byElsevier Elsevier Ltd. LtdThis. is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Selection and peer-review under responsibility of GHGT. Peer-review under responsibility of the Organizing Committee of GHGT-12

Keywords: 13C-NMR; piperazine; tertiary amine; acid dissociation constant; carbamate; CCS.

1. Introduction

Carbon dioxide (CO2) is a greenhouse gas contributing to global warming and climate change problems.

*Corresponding author: Tel: +81-3-5286-3244; fax: +81-3-3208-7022 E-mail address: [email protected]

1876-6102 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the Organizing Committee of GHGT-12 doi: 10.1016/j.egypro.2014.11.194 1864 Miho Nitta et al. / Energy Procedia 63 ( 2014 ) 1863 – 1868

Aqueous solutions of amines are used as absorbents for removing CO2 from flue gas of fossil fueled power plants [1]. An aqueous amine absorbs CO2 at a low temperature such as 40 qC and releases CO2 at a high temperature such as 120 qC; the separated CO2 will be buried underground. This is called carbon dioxide capture and storage (CCS). A blend of MDEA and PZ is one of high performance absorbents [2]. The kinetics of the reaction of CO2 with aqueous solutions of alkanolamines was reviewed [3]. The CO2 absorption rate of an aqueous solution of a tertiary alkanolamine N-methyldiethanolamine (MDEA) (see Fig.1) is slow, whereas the CO2 release amount is large. The addition of piperazine (PZ) (see Fig. 1) into the MDEA solution enhances the CO2 absorption rate while the CO2 release amount is maintained to be large. Thus, PZ is called "promotor". 13C-NMR spectroscopy is a powerful tool for qualitative and quantitative analyses of chemical species formed in CO2-absorbed aqueous amine solutions [4−11]. It has been elucidated [8, 11] that MDEA and PZ react with CO2, forming ionic species such as bicarbonate – 2– ion (HCO3 ), carbonate ion (CO3 ), protonated MDEA and PZ, PZ mono- and bis-carbamates, and MDEA carbonate, etc. A tertiary alkanolamine does not make its carbamate. It should be noted that it is not possible to − 2− 13 distinguish the signals originating from an amine and its protonated species, HCO3 and CO3 , etc. using C-NMR spectroscopy, because of rapid proton exchanges. However, a systematic 13C- NMR study on the blends of a tertiary alkanolamine and PZ have not been performed yet. In this paper, we will present 13C-NMR study on concentration changes of chemical species generated in the 2– – course of CO2 absorption (CO3 /HCO3 , protonated amines, carbamates, carbonates, etc.) and those after heating at 93 qC for 30 min for blends of PZ and a tertiary alkanolamine such as 2-dimethylamino-1-ethanol (DMAE), 3- dimethylamino-1-propanol (DMAP), 1-dimethylamino-2-propanol (DMA2P), 4-dimethylamino-1-butanol (DMAB), 2-diethylamino-1-ethanol (DEAE), 3-diethylamino-1-propanol (DEAP), 1-diethylamino-2-propanol (DEA2P), 1-(2- hydroxyethyl)piperidine (1-HE-PP), 1-(2-hydroxyethyl)pyrrolidine (1-HE-PRL) or N-ethyldiethanolamine (EDEA), which are shown in Fig. 1. The pKa value of an amine is important in molecular design of high performance amine absorbents [12, 13]. Thus, we will investigate the pKa effects on the rate and the loading of the CO2 absorption 13 process and the CO2 release upon heating using C-NMR spectroscopy.

Fig. 1. Chemical structures of amines: (a) DMAE; (b) DMAP; (c) DMA2P; (d) DMAB; (e) DEAE; (f) DEAP; (g) DEA2P; (h) 1-HE-PP; (i) 1- HE-PRL; (j) MDEA; (k) EDEA; (l) PZ

2. Experimental

The samples of DMAP and 1-HE-PP were purchased from Sigma−Aldrich. Other tertiary alkanolamines were purchased from Tokyo Chemical Industry. PZ was purchased from Kanto Chemical. In the experiments of CO2 absorption kinetics, gaseous CO2 was bubbled through a D2O solution of an amine blend at a rate of 150 mL/min at room temperature in a flask. The concentrations of PZ and a tertiary alkanolamine were 10 and 20 wt%, respectively. The volume of the solution was about 50 mL. A small amount of the solution was taken from the 13 solution and used for C-NMR measurements. In the absorption−regeneration experiments, a D2O solution (50 mL) of the amine blend bubbled with gaseous CO2 at a rate of 150 mL/min for 60 min at room temperature was heated at 93 °C for 30 min. 13C-NMR spectra 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 at 1 min. The data of 64 scans were accumulated for each spectrum. A small amount of 1,4-dioxane-d6 (D, 99%) was added into the sample; its standard chemical shift 67.4 ppm [14] was used for the calculation of chemical shifts. The pKa value of each amine was measured at room Miho Nitta et al. / Energy Procedia 63 ( 2014 ) 1863 – 1868 1865 temperature using the potentiometric titration method [15]. The pH values were measured on a Horiba F-51 pH meter. A 2.0u10−4 mol/L amine aqueous solution was titrated with the 0.5 mol/L HCl aqueous solution. The average of three measurements was used as pKa for each amine.

3. Results and discussion

3.1. Absorption of CO2

13 The C-NMR spectrum of an aqueous solution of the EDEA−PZ blend and that after 50-min CO2 bubbling are shown in Fig. 2a and 2b, respectively. The observed 13C-NMR spectra were assigned on the basis of the data in the literature [4−12]. The assignments of the observed bands are listed in Table 1. No signals of EDEA carbamate, PZ

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

Table 1. Assignments of observed bands chemical shift / ppm Species Numbering a) 12.0−10.1 EDEA carbonate and its protonated species 5 12.2−9.8 EDEA and its protonated species 1 45.3−42.8 PZ monocarbamate and its protonated species 13 46.7−44.2 PZ and its protonated species 11 46.1−45.0 PZ monocarbamate and its protonated species 12 46.0 PZ biscarbamate 14 50.6−49.8 EDEA and its protonated species 2 54.1−53.7 EDEA carbonate and its protonated species 6 56.1−55.8 EDEA and its protonated species 4 59.2−57.2 EDEA carbonate and its protonated species 8 59.4−56.1 EDEA and its protonated species 3 60.2−57.8 EDEA carbonate and its protonated species 7 63.8−61.7 EDEA carbonate and its protonated species 10 160.0−159.9 EDEA carbonate and its protonated species 15 − 2− 164.8−162.0 HCO3 and CO3 ũ 164.5−163.8 PZ monocarbamate and its protonated species 16 164.7 PZ biscarbamate 17 a) The numbering of carbon atoms is shown in the chemical structures in Fig. 3. 1866 Miho Nitta et al. / Energy Procedia 63 ( 2014 ) 1863 – 1868

Fig. 3. Numbering of carbon atoms.

carbonate, and CO2 (125.3 ppm [7]) were observed. A small amount of the carbonate of EDEA was observed. The concentrations of PZ/protonated species, EDEA/protonated species, EDEA carbonate/protonated species, 2− − CO3 /HCO3 , PZ monocarbamate/protonated species, and PZ biscarbamate were calculated using the band at 44.2−46.7 ppm, the band at 56.1−59.4 ppm due to the N-bonded carbon atom of the hydroxyalkyl chains, the band − 2− − at approximately 160 ppm due to CO2 of the amine carbonate, and the band at 162−167 ppm due to CO3 /HCO3 , the band at approximately 164 ppm, and the band at approximately 165 ppm, respectively. 2− − Temporal concentration changes of CO3 /HCO3 , amine/protonated amine, EDEA carbonate/protonated carbonate, PZ mono- and bis-carbamates for the aqueous solution of the EDEA−PZ blend in the course of CO2 2− − absorption are shown in Fig. 4. The sum of the concentrations of CO3 /HCO3 , EDEA carbonate/protonated carbonate, and PZ mono- and bis-carbamates is equal to the amount of captured CO2; the sum is also shown in Fig. 2− − 4. The CO2 capture originates mainly from the formation of CO3 /HCO3 and PZ monocarbamate. The 2− − concentration of PZ monocarbamate is higher than that of CO3 /HCO3 at 5 and 10 min. Thus, the formation of PZ monocarbamate contributes mainly to the initial rate of CO2 capture. The concentration of PZ monocarbamate is 2− − 2− − almost the same as that of CO3 /HCO3 at 15 min. After 20 min, the concentration of CO3 /HCO3 is higher than that of PZ monocarbamate. The concentration of PZ biscarbamate shows the maximum at 15 min. The contribution of PZ biscarbamate to CO2 capture is small. These reaction features are essentially the same as those of an MDEA−PZ aqueous solution [11].

2− − Fig. 4. Concentration changes of CO3 /HCO3 (●), EDEA/protonated EDTA (■), PZ/protonated PZ (□), EDEA carbonate/protonated carbonate

(● ), PZ monocarbamate/protonated monocarbamate (●), PZ biscarbamate/protonated biscarbamate (●), CO2 capture (●), and pH(○) in the

aqueous solution of the EDEA−PZ blend as a function of CO2 bubbling time.

The initial rate (vi) of CO2 capture was calculated from the data between 0 and 5 or 10 min for each amine. The obtained initial rates are plotted against pKa values in Fig. 5; the measured pKa values of DMAE, DMAP, DMA2P, DMAB, DEAE, DEAP, DEA2P, 1-HE-PP, 1-HE-PRL, MDEA, and EDEA were 9.34, 9.57, 9.55, 9.79, 9.94, 10.17, 10.14, 9.56. 9.73, 8.60, and 9.08, respectively. The initial CO2 absorption rates ranged between 0.125 and 0.167 mol/L min. The blend of EDEA and PZ exhibited the highest rate. The rate of an aqueous solution of PZ (10 wt%) was 0.150 mol/L min. It is well known that the rate of PZ is fast. Thus, the initial rates of the blends of a tertiary 13 alkanolamine and PZ are fast. C-NMR results indicated that the initial CO2 capture rates of all the amine blends Miho Nitta et al. / Energy Procedia 63 ( 2014 ) 1863 – 1868 1867 are mainly contributed by the formation of PZ monocarbamates. The initial rates of PZ monocarbamates were not sensitive to pKa in the range between 8.60 and 10.25. On the other hand, carbamates were not formed in aqueous solutions of tertiary alkanolamines, and the initial rate increased with increasing pKa [13].

Fig. 5. Initial rates of CO2 capture versus pKa values for aqueous blends of a tertiary alkanolamine (20 wt%) and PZ (10 wt%).

3.2. Regeneration of CO2

2− − The concentrations of PZ/protonated species, tertiary alkanolamine/protonated species, CO3 /HCO3 , PZ monocarbamate/protonated species, PZ biscarbamate, and tertiary alkanolamine carbonate/protonated species in the 13 CO2-saturated amine blend and after heating at 93 qC for 30 min were obtained from the C-NMR measurements. 2− − CO2 was captured as CO3 /HCO3 , PZ monocarbamate, PZ biscarbamate, and tertiary alkanolamine carbonate/protonated species in an tertiary alkanolamine−PZ blends. The loading of CO2 can be expressed by dividing the molar concentration equivalent to absorbed CO2 by the total molar concentration of PZ and the tertiary amine, mol-CO2/mol-amine. The maximum CO2 loadings ranged between 0.927 and 1.05 for all the blends. The CO2 loadings decreased upon heating at 93 qC for 30 min. The change in mol-CO2/mol-amine upon heating is expressed by '(mol-CO2/mol-amine). The decrease in '(mol-CO2/mol-amine) upon heating means the release of CO2 from the solution. In Fig. 5, '(mol-CO2/mol-amine) values are shown in red closed circles. Among these values, a positive linear correlation is found, as the dashed red line in Fig. 6.

2− − Fig. 6. Loading changes of CO3 /HCO3 (●), PZ monocarbamate/protonated species (● ), PZ biscarbamate ( ●), and CO2 contained as PZ mono-

and bis-carbamates (○), and total CO2 capture (●) upon heating at 93 qC for 30 min as a function of pKa for aqueous blends of a tertiary alkanolamine (20 wt%) and PZ (10 wt%). 1868 Miho Nitta et al. / Energy Procedia 63 ( 2014 ) 1863 – 1868

The concentration of a tertiary alkanolamine carbonate/protonated species was small. Thus, '(mol-CO2/mol- 2− − amine) mainly originates from the concentration changes of CO3 /HCO3 , PZ monocarbamate/protonated species 2− − (abbreviated as MC), and PZ biscarbamate (abbreviated as BC) upon heating. The CO3 /HCO3 , MC, and BC concentration changes divided by the total molar concentration of PZ and the tertiary alkanolamine are called 2− − 2− − loading of CO3 /HCO3 , MC, and BC, respectively. The changes in CO3 /HCO3 , MC, and BC loadings are 2− − expressed as '(mol-X/mol-amine) (X = CO3 /HCO3 , MC, and BC), and also shown in Fig. 6. The MC loading changes, '(mol-MC/mol-amine)s, show values between −0.148 and −0.056; minus sign means decrease in concentration upon heating. On the other hand, the BC loading changes, '(mol-BC/mol-amine)s, show similar plus values in the pKa range below 9.34; they increase with increasing pKa in the range above 9.34. The plus sign means increase in concentration. The sum of '(mol-MC/mol-amine) and 2'(mol-BC/mol-amine) indicates the CO2 capture changes as carbamates upon heating. The sum increases with increasing pKa as the black dashed line in Fig. 6. In the present experimental conditions, one can conclude that a tertiary alkanolamine having pKa lower than 9.34 will show high performance as a blend amine with PZ.

2. Conclusions

2– – We have studied the concentration changes of CO3 /HCO3 , PZ/protonated species, tertiary alkanolamine/protonated species, carbonate/protonated species, PZ monocarbamate/protonated species, and biscarbamates in the course of CO2 absorption and those after CO2 release upon heating at 93 qC for 30 min for aqueous blends of PZ and each of 11 kinds of tertiary alkanolamines using 13C-NMR spectroscopy. The observed initial rates of CO2 capture ranged between 0.125 and 0.167 mol/L min, which originates from the rapid formation of PZ monocarbamate; the capture rates were fast. The CO2 release amount of each blend upon heating increased with decreasing pKa of the tertiary alkanolamine. The concentrations of PZ monocarbamate decreased upon heating. On the other hand, the concentrations of PZ biscarbamates increased upon heating. The CO2 loading captured as PZ carbamates decreased upon heating in the pKa range smaller than 9.34, whereas it increased in the range higher than 9.34. PZ biscarbamate plays a negative role in CO2 release. A blend of a PZ derivative that does not form its biscarbamate and a tertiary amine having a small pKa value is promising as a CO2 absorbent.

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