Development of Ion Speciation Plots for Three Promising Tertiary

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Development of Ion Speciation Plots for Three Promising Tertiary Amine−CO2−H2O Systems Using the pH Method and the 13C NMR Method Moxia Li,†,§ Helei Liu,*,†,‡,§ Xiao Luo,† Paitoon Tontiwachwuthikul,†,‡ and Zhiwu Liang*,†,‡ †

Joint International Center for CO2 Capture and Storage (iCCS), Provincial Hunan Key Laboratory for Cost-Effective Utilization of Fossil Fuel Aimed at Reducing Carbon-Dioxide Emissions, College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, People’s Republic of China ‡ Clean Energy Technologies Research Institute (CETRI), University of Regina, Regina, Saskatchewan S4S 0A2, Canada ABSTRACT: In this work, three promising tertiary amines1-(2-hydroxyethyl)piperidine (1-(2-HE)PP), 3-(diethylamino)1,2-propanediol (DEA-1,2-PD), and N-(2-hydroxyethyl)pyrolidine (1-(2-HE)PRLD)were experimentally studied and the results presented in terms of pKa, the protonation calibration curves, and ion speciation plots. The pKa of these amines were determined in the temperature range of 294−320 K. The protonation calibration curves for the three amines were also developed based on 13C NMR detection. In addition, ion (amine, amineH+, HCO3−, and CO32−) speciation plots for the three tertiary amines were developed at the temperature of 298 K, at the amine concentration of 1.0 M, over the CO2 loading ranges of 0− 0.897 mol CO2/mol amine for 1-(2-HE)PP, 0−0.915 mol CO2/mol amine for DEA-1,2-PD, and 0−0.927 mol CO2/mol amine for 1-(2-HE)PRLD by using the pH method, the 13C NMR method, and the pH + 13C NMR method. By comparing the ion (amine, amineH+, HCO3−, and CO32−) concentrations of the three tertiary amines obtained from these three methods, it can be concluded that each of these three methods could be used to develop the ion speciation plots of amine−CO2−H2O systems, but there are some significant differences between them.

1. INTRODUCTION As a serious climate problem, global warming is mainly caused by the excessive emissions of anthropogenic greenhouse gases. CO2, which is one of the main products of the burning of fossil fuels (coal, petroleum, and natural gas) accounts for the prime part of anthropogenic greenhouse gases. Because of the exploitation of fossil fuels, global warming and its evolution of polar melting glaciers, rising sea levels, and other consequences has become a threat in our time.1 For this reason, CO2 capture is extremely important, because it provides one avenue to address this challenge. Chemical absorption by aqueous amine solution is one of the most widely used methods of CO2 capture in industry, because of its advantages of high absorption capacity and cost effectiveness.2,3 However, the most important factors that must be considered when selecting a solvent in this technology is that the absorption solvent should possess the advantages of high absorption capacity, fast reaction kinetics, low energy requirement for regeneration, low degradation rate, and low corrosiveness.4 Based on the different numbers of substituents attached to the nitrogen atom of the amino group, amine solvents can be classified into three categories: primary amines, secondary amines, and tertiary amines. Primary and secondary amines react with CO2 according to the zwitterionic mechanism,5 which is the most prominent mechanism and involves the formation of a zwitterionic form of the carbamate, with faster reaction rate, lower CO2 absorption capacity, and higher energy requirements for regeneration than those of the reaction between tertiary amines and CO2, which follows the basecatalyzed hydration mechanism.6,7 The disadvantages of high © 2017 American Chemical Society

molecular weight and high viscosity lead to the poor mass transfer performance of tertiary amine solvents. Three promising amines1-(2-hydroxyethyl)piperidine (1(2-HE)PP), 3-(diethylamino)-1,2-propanediol (DEA-1,2-PD), and N-(2-hydroxyethyl)pyrolidine (1-(2-HE)PRLD), all of which are tertiary aminesas Chowdhury et al. reported,8 have a higher CO2 absorption rate, CO2 absorption capacity, and cyclic capacity than MDEA, which is one of the most widely used tertiary amines. However, some essential information about the performance of these amines, such as vapor−liquid equilibrium (VLE) data and mass-transfer performance, must be taken into account before the commercial usage of these tertiary amines can be considered. The VLE data of amine−CO2−H2O systems are fundamental to the development of theoretical models, which is very important for the design and simulation of CO2-treating plants with high energy utilization efficiency.9 To get the comprehensive VLE data of 1-(2-HE)PP-CO2−H2O, DEA-1,2-PDCO2−H2O and 1-(2-HE)PRLD-CO2−H2O systems, the ion concentrations of free amine, protonated amines, HCO3− and CO32−, and the ion speciation plots of these systems at different CO2 loadings are required. The structures of these three promising amines are shown in Figure 1. In this work, 1-(2-HE)PP), DEA-1,2-PD, and 1-(2-HE)PRLD were experimentally studied and the results are presented in terms of pKa, the protonation calibration curves, and ion speciation plots. The pKa of these amines were Received: December 14, 2016 Revised: February 10, 2017 Published: February 13, 2017 3069

DOI: 10.1021/acs.energyfuels.6b03320 Energy Fuels 2017, 31, 3069−3080

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Energy & Fuels

where amine represents one of three tertiary amines (1-(2HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD), and Ki represents the equilibrium constant of reaction i. There are several equilibrium constants that exist in this system. Among the equilibrium constants, the equilibrium constants of reactions 1 and 5 can be expressed as follows:10,11 K1 =

[amineH+] [amine]·[H+]

(7)

K5 =

[CO32 −]·[H+] [HCO3−]

(8)

The values of K1 for three different tertiary amines systems were obtained in this work. The value of K5 can be obtained from the literature.6,12 In addition, the mass balances of amine and CO2 existing in the tertiary amine−CO2−H2O systems can be expressed as follows: Amine mass balance: [amine]0 = [amineH+] + [amine] Figure 1. Chemical structures of three promising amines in this work: 1-(2-hydroxyethyl)piperidine (1-(2-HE)PP), 3-(diethylamino)-1,2propanediol (DEA-1,2-PD), and N-(2-hydroxyethyl)pyrolidine (1-(2HE)PRLD).

CO2 mass balance: α·[amine]0 = [HCO3−] + [CO32 −] + [CO2 (aq)]

⎛ [amine]·[H+] ⎞ pK a = −log10⎜ ⎟ ⎝ [amineH+] ⎠

K2

CO2 + amine + H 2O ↔ amineH+ + HCO3− K3

CO2 + H 2O ↔ H+ + HCO3−

pK a = log10(K1)

K4

(2) (3) (4)

K5 HCO3− ↔

(5)

K6

+

H + CO3

H 2O ↔ H+ + OH−

2−

(12)

2.2. Determination of Ion Concentrations of Amine, AmineH+, HCO3−, and CO32− in the Amine−H2O−CO2 System with the pH Method. Ion concentrations in the amine−H2O−CO2 system are vital to understanding the reaction process of amine and CO2. In the amine−H2O− CO2 system, amineH+, HCO3−, and CO32− were formed based on reactions 1−6. The concentrations of amine, amineH+, HCO3−, and CO32− can be calculated with the following procedures. The concentrations of amine and amineH+ can be calculated using eqs 7, 9, 12, and 16, and the pKa value can be determined as shown in section 3.2.1. The concentrations of HCO3− and CO32− can be determined from eqs 8 and 10, which can be rewritten as follows:

(1)

CO2 + OH− ↔ HCO3−

(11)

Combining eqs 7 and 11, pKa can also be expressed as described in eq 12:

2. CALCULATION METHODS Since 1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD are all tertiary amines, their reactions with CO2 can be explained by the base-catalyzed hydration mechanism. According to the description of the base-catalyzed hydration mechanism, the zwitterionic form of the carbamate in the reaction of CO2 with tertiary amines cannot be generated, and the tertiary amines do not react with CO2 directly, but catalyze the dissociation of H2O molecules.6,7 The reactions in the tertiary amine−CO2− H2O system can be described as follows: K1

(10)

where [amine]0 is the initial concentration of amine, α the CO2 loading in the amine−H2O−CO2 system, and [CO2(aq)] the physical solubility of CO2 in the amine−H2O−CO2 system. Since the physical solubility of CO2 in the amine solution is small, it is ignored in this work.6 2.1. Determination of pKa in Tertiary Amine Solutions. In the tertiary amine solution, the pKa can be expressed as described in eq 11:

determined in the temperature range of 294−320 K. The protonation calibration curves for these amines were also developed based on the 13C NMR detection. In addition, amine and ion (amineH+, HCO3−, and CO32−) speciation plots for these three tertiary amines were developed at the temperature of 298 K, at the concentration of 1.0 M for amine, over the CO2 loading range of 0−0.897 mol CO2/mol amine for 1-(2HE)PP, 0−0.915 mol CO2/mol amine for DEA-1,2-PD, and 0−0.927 mol CO2/mol amine for 1-(2-HE)PRLD by using the pH method, the 13C NMR method, and the pH + 13C NMR method.

amine + H+ ↔ amineH+

(9)

[HCO3−] =

[CO32 −] =

(6) 3070

α ·[amine]0 K5 [H+]

+1

α ·[amine]0 K5 [H+]

+1

(13)

·

K5 [H+]

(14) DOI: 10.1021/acs.energyfuels.6b03320 Energy Fuels 2017, 31, 3069−3080

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Figure 2. Illustration of the CO2 absorption apparatus in this work.

amineH+ can be calculated by eqs 7, 9, 12, and (16 using the pH + 13C NMR method, which is the same calculation as when using the pH method. Also, the concentrations of HCO3− and CO32− can be calculated by eqs 17, 18, and 19, using the pH + 13 C NMR method, which is the same calculation as when using the 13C NMR method.

The value of K5 is obtained from the literature, using the expression shown in eq 15:6,12 36.4385 × 104 1.84157 × 108 − T T2 11 13 0.41579 × 10 0.354291 × 10 ] + − 3 T4 T

K 5 = exp[−294.74 +

= f (T )

3. EXPERIMENTS

(15)

3.1. Chemicals. 1-(2-Hydroxyethyl)piperidine (1-(2-HE)PP) with a purity of 99.9% was obtained from Aladdin Industrial Corporation (Shanghai, China). 3-(Diethylamino)-1,2-propanediol (DEA-1,2-PD) with a purity of 98% was purchased from Adamas Reagent Co., Ltd., China. N-(2-Hydroxyethyl)pyrrolidine (1-(2-HE)PRLD) with a purity of 98% was acquired from J&K Scientific, Ltd., China. The amine solutions were prepared to the desired concentrations using deionized (DI) water. Commercial-grade CO2 with a purity of 99.9% was supplied from Changsha Jingxiang GAS Co., Ltd., China. Deuterium oxide (D2O, 99.9%) and 1,4-dioxane (99.5%) were provided by Sinopharm Chemical Reagent Co., Ltd., China. Guaranteed-reagentgrade sodium hydrogen carbonate (NaHCO3) and analytical-reagentgrade sodium carbonate (Na2CO3) were purchased from Tianjin Kemiou Chemical Reagent Co., Ltd., China. 3.2. Experimental Procedure. 3.2.1. Measurement of Dissociation Constants (K1) of Amines. The procedure for the measurement of dissociation constants in amine solutions is similar to the work of Shi et al. and Kamps et al.,16,17 which show the dissociation equilibrium constants of DEAB and MDEA, respectively. The detailed experimental procedure is described as follows. One hundred milliliters (100 mL) of 1.0 mol/L amine solution was prepared and titrated by using a 1.0 mol/L HCl solution. Then, 5 mL of 1.0 mol/L amine solution was diluted to 100 mL. The 100 mL of 0.05 mol/L amine solution was kept at experimental temperature in a 150 mL three-necked flask, and slowly titrated by using 5 mL of 1.0 mol/L HCl standard solution. The pH value of the solution was recorded when it was stable after the addition of every 0.5 mL of 1.0 mol/L HCl solution into the amine solution. The concentration of H+ can be calculated from the pH value on the basis of eq 16, using the pH meter (standard model Rex PHS-3C, INESA Scientific Instruments Co., China., pH range from 0 to 14 with a ±0.01 accuracy), the details about the application of the pH meter can be found in our published work.3,6,18 The concentrations of amine and amineH+ can be calculated by the mass balance of HCl and amine, as shown in eqs 20 and 21. The dissociation equilibrium constant (K1) was calculated using eq 7. The same procedure was also implemented for the determination of the dissociation constants of these three amines at temperatures in the range of 294−320 K.

The concentration of H+ can be obtained through pH measurement of the solution on the basis of eq 16. pH = −log10([H+])

(16)

2.3. Determination of Concentrations of Amine, AmineH+, HCO3−, and CO32− in the Amine−H2O−CO2 System with the 13C NMR Method. In this work, the 13C NMR technique was also employed to determine the concentrations of amine, amineH+, HCO3−, and CO32−; this is a technique that was well applied in the studies of Holmes et al.,9 Ahn et al.,13 and Perinu et al.14 The concentrations of amine and amineH+ can be determined by using the protonation calibration curve equations of amines and eq 9. The protonation calibration curve equations of these three new tertiary amines can be obtained as shown in section 3.2.2. The concentrations of HCO3− and CO32− can be represented as shown in the following equations: [HCO3−] = [CO32 −] =

168.831 − δ ·[CO2 ]0 168.831 − 161.089 δ − 161.089 ·[CO2 ]0 168.831 − 161.089

[CO2 ] = α · [amine]0

(17)

(18) (19)

HCO3−/CO32−;

where δ is the chemical shift of [CO2]0 is the total concentration of CO2 in the amine−H2O−CO2 system; 168.831 and 161.089 are the chemical shifts of solely Na2CO3 and NaHCO3 aqueous solutions in this work, respectively. 2.4. Determination of Concentrations of Amine, AmineH+, HCO3−, and CO32− in the Amine−H2O−CO2 System by Using the pH + 13C NMR Method. To determine the concentrations of amine, amineH+, HCO3−, and CO32− in the amine−CO2−H2O system, Jakobsen et al.15 reported that the pH values, as a function of the chemical shift and calibration curves, were used to calculate the ion speciation, whereas, in this work, the concentrations of amine and 3071

nHCl − [H+]·Vtotal = [amineH+]·Vtotal

(20)

([amine] + [amineH+])·Vtotal = namine

(21)


Article

Energy & Fuels where nHCl is the total number of moles of HCl added into the solution, Vtotal the total volume of the solution, and namine the primary number of moles of amine. 3.2.2. Validation of pH and 13C NMR Protonation Calibration Tests. Based on the research of Shi et al.,19 validation tests were performed to determine the accuracy of the measured protonation ratios of 1-(2-HE)PP by the pH method and the 13C NMR method, and the results were compared to the protonation ratios obtained from titration with HCl, which was taken to be the actual value. For the pH method, 10 mL of 1.0 M 1-(2-HE)PP solution was prepared for titration with 1.0 M HCl. A magnetic stirrer was used in the beaker to establish equilibrium quickly after each addition of HCl, and stirred continuously for the entire titration duration. The pH value was recorded for every addition of 2 mL of 1.0 M HCl solution of standard solution until a total of 10 mL of HCl solution had been added. The measured protonation ratios were then calculated based on the pH value and equilibrium constant Ka for 1-(2-HE)PP generated in section 3.2.1. For the NMR calibration method, it is well-known that the range of protonation ratios of amines can be confirmed by the chemical shift (δ).20 Six samples with different protonation ratios (namineH+/namine,0 = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0) were prepared by adding the 1.0 M HCl solution into a 1.0 M 1-(2-HE)PP solution. Then, 10 wt % of D2O and 5 wt % of 1,4-dioxane were added into every sample. D2O and 1,4dioxane were added to provide a field-frequency lock and chemical shift reference, respectively. The samples were measured using 13C NMR spectroscopy to determine the chemical shifts (δ) of the different carbon atoms of each amine. The chemical shifts (δ) of different carbon atoms at different protonation ratios were obtained and were then used to develop the protonation calibration curves. Moreover, the experimental procedures of the protonation calibration tests on the DEA-1,2-PD and 1-(2-HE)PRLD solutions were the same as that of 1-(2-HE)PP solution. 3.2.3. The CO2 Loading Measurement and 13C NMR Detection. The CO2 absorption apparatus is illustrated in Figure 2; it consists of a CO2 cylinder, a gas mass flow meter, a dryer (which prevents the gas mass flow meter from being damaged by water), a saturator (for avoiding a loss of amine solution), a heating magnetic stirrer with a temperature controller, a three-necked flask (as an absorption reactor), an iron stand fixing the three-necked flask, a thermometer displaying the actual temperature in the reactor, and a pH meter (to record the pH value in the amine solution). The CO2 loading (α) was measured by titration with a Chittick CO2 analyzer.21 A standard 100 mL of 1.0 mol/L amine aqueous solution was transferred into a 150 mL three-necked flask, which was stirred with a magnetic stirrer. CO2 was passed through the mass flow meter to get a stable CO2 flow of 0.3 L/min. The gas flow was introduced into the reactor immersed in the water bath at the desired temperature after passing through the dryer and saturator. Every few minutes, a 2 mL sample was taken to analyze CO2 loading. Meanwhile, the pH value of the solution was also recorded.21 In addition, another 600 μL solution was placed into an NMR sample tube. Drops of D2O (10 wt %) as the signal lock and 1,4-dioxane (5 wt %, δ = 67.19 ppm) as the internal reference were added into the NMR sample tube and the sample was then measured by 13C NMR spectroscopy (Varian Mercury Inova 400 MHz NMR spectrometer).22

promotes the hydration of CO2 in tertiary amine absorption of CO2.18,24 In this work, the pKa values of 1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD were experimentally generated over the temperature range of 294−320 K. The pKa values of these tertiary amines are shown in Figure 3. As Figure 3 shows, the

4. RESULTS AND DISCUSSION 4.1. pKa Values for the Three Tertiary Amines. It is known that pKa (namely, log10(K1)) is an important parameter for the screening of amine solvents for acid gas absorption and dynamic process analysis.23 Generally speaking, pKa represents the basicity of the amine solvent. Based on the reports of Liu et al.,3 Hamborg et al.,24 Brønsted et al.,25 Littel et al.,26 and Li et al.,18 the higher the pKa value is, the faster the absorption kinetics will be in tertiary amine−CO2−H2O. Based on the base-catalyzed hydration mechanism, the stronger basicity

1‐(2‐HE)PRLD > DEA‐1,2‐PD > 1‐(2‐HE)PP > MDEA

Figure 3. pKa values of tertiary amines within a temperature range of 294−320 K.

pKa values decreased as the temperature increased. The empirical correlations were employed to fit the experimental result and are expressed in the following equations. pK a(1‐(2‐HE)PP) = 4.08 +

1767.8 T

(AAD = 0.6%) (22)

1878.2 pK a(DEA‐1,2‐PD) = 3.75 + T

(AAD = 0.8%) (23)

pK a(1‐(2‐HE)PRLD) = 4.63 +

1652.1 T

(AAD = 0.8%) (24)

The predicted pKa values for the three tertiary amines have excellent agreement with the experimental results, with the AAD (average absolute deviation) being 0.6% for 1-(2-HE)PP, 0.8% for DEA-1,2-PD, and 0.8% for 1-(2-HE)PRLD. In addition, the pKa values of the three tested tertiary amines were compared with that of tertiary amine MDEA, which was published by Kamps et al.17 It can be seen from Figure 3 that the pKa values for these three promising tertiary amines are much higher than that of MDEA, which means that these three promising tertiary amines may show faster absorption kinetics than that of MDEA. 1-(2-HE)PRLD shows the highest pKa values and the order of pKa at the experimental temperature can be ranked as Thus, the absorption kinetics for those tertiary amines at the experimental temperature can be ranked as 1‐(2‐HE)PRLD > DEA‐1,2‐PD > 1‐(2‐HE)PP > MDEA

In order to confirm this estimation of the kinetics trend, it is necessary to investigate the CO2 absorption kinetics in those tertiary amines by experiment. 4.2. Protonation Calibration Curves of Amines at 298 K Obtained with the 13C NMR Method and the 3072


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Table 1. 13C NMR Protonation Calibration Table of 1-(2-HE)PP at 298 K, Based on the Molar Ratio of H+ and 1-(2-HE)PP (nH+:n1‑(2‑HE)PP) δ(1-(2-HE)PP) (ppm) carbon atom

nH+:n1‑(2‑HE)PP = 0:1.0

nH+:n1‑(2‑HE)PP = 0.2:1.0

nH+:n1‑(2‑HE)PP = 0.4:1.0

nH+:n1‑(2‑HE)PP = 0.6:1.0

nH+:n1‑(2‑HE)PP = 0.8:1.0

nH+:n1‑(2‑HE)PP = 1.0:1.0

Δδ (ppm)

C1 C2 C3 C4 C5

60.250 59.039 54.610 25.413 24.044

59.931 58.403 54.467 24.981 23.585

59.626 57.744 54.339 24.543 23.128

59.330 57.085 54.219 24.107 22.676

59.038 56.413 54.105 23.667 22.221

58.751 55.751 53.997 23.230 21.769

−1.499 −3.288 −0.613 −2.183 −2.275

Table 2. 13C NMR Protonation Calibration Table of DEA-1,2-PD at 298 K, Based on the Molar Ratio of H+ and DEA-1,2-PD (nH+:nDEA‑1,2‑PD) δDEA‑1,2‑PD (ppm) carbon atom

nH+:nDEA‑1,2‑PD = 0:1.0

nH+:nDEA‑1,2‑PD = 0.2:1.0

nH+:nDEA‑1,2‑PD = 0.4:1.0

nH+:nDEA‑1,2‑PD = 0.6:1.0

nH+:nDEA‑1,2‑PD = 0.8:1.0

nH+:nDEA‑1,2‑PD = 1.0:1.0

Δδ (ppm)

C4 C5 C3 C2 C1

69.969 65.327 55.574 47.72 10.771

69.322 65.065 55.362 47.905 10.348

68.646 64.8 55.133 48.104 9.922

67.965 64.543 54.933 48.307 9.511

67.294 64.29 54.735 48.511 9.108

66.616 64.037 54.537 48.715 8.707

−3.353 −1.290 −1.037 0.995 −2.064

Table 3. 13C NMR Protonation Calibration Table of 1-(2-HE)PRLD at 298 K, Based on the Molar Ratio of H+ and 1-(2HE)PRLD (nH+:n1‑(2‑HE)PRLD) δ1‑(2‑HE)PRLD (ppm) carbon atom

nH :n1‑(2‑HE)PRLD = 0:1.0

nH :n1‑(2‑HE)PRLD = 0.2:1.0

nH :n1‑(2‑HE)PRLD = 0.4:1.0

nH+:n1‑(2‑HE)PRLD = 0.6:1.0

nH+:n1‑(2‑HE)PRLD = 0.8:1.0

nH+:n1‑(2‑HE)PRLD = 1.0:1.0

Δδ (ppm)

C1 C2 C3 C4

60.587 57.396 54.202 23.339

59.939 57.300 54.291 23.299

59.261 57.211 54.398 23.259

58.535 57.119 54.520 23.219

57.871 57.045 54.641 23.187

57.208 56.968 54.764 23.153

−3.379 −0.428 0.562 −0.186

+

+

+

Validation of pH and 13C NMR Protonation Calibration Tests. In this work, the chemical shifts (δ) of different carbon atoms of these three promising tertiary amines were presented at the different protonation ratios (namineH+/namine,0 = 0.0, 0.2, 0.4, 0.6, 0.8, and 1.0). As shown in Tables 1−3 and Figures 4−6, 1-(2-HE)PP has five different C atoms and five different carbon 13C NMR peaks, DEA-1,2-PD has five different C atoms and five different carbon 13C NMR peaks, and 1-(2-HE)PRLD has four different C atoms and four different carbon 13C NMR peaks, respectively. These carbon 13C NMR peaks can be categorized into two groups: the ones in the shielded region

Figure 5. Protonation calibration curve of DEA-1,2-PD at 298 K.

and the ones in the deshielded region. For 1-(2-HE)PP, the chemical shifts of C1, C2, C3, C4, and C5, which are considered to be in the shielded region, decreased as the protonation ratio increased at −1.499, −3.288, −0.613, −2.183, and −2.275 ppm, respectively. For DEA-1,2-PD, the chemical shifts of C1, C3, C4, and C5, which are considered to be in the shielded region, decreased as the protonation ratio increased at −2.064, −1.037, −3.353, and −1.290 ppm, respectively, while the chemical shift of C2, which is considered to be in the deshielded region, increased as the protonation ratio increased at 0.995 ppm. For 1-(2-HE)PRLD, the chemical shifts of C1, C2, and C4, which are considered to be in the shielded region, decreased as the

Figure 4. Protonation calibration curve of 1-(2-HE)PP at 298 K. 3073


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(2-HE)PPH+, DEA-1,2-PD and DEA-1,2-PDH+, and 1-(2HE)PRLD and 1-(2-HE)PRLDH+, respectively. Moreover, the chemical shifts of these reference C atoms changed with the increasing protonation ratio. This is because the chemical shifts were affected by the addition of H+. In order to correlate the results, linear regression was used to fit the experimental data. From Figures 4−6, it was found that linear regression can fit the results very well. Thus, the linear relationship between the protonation ratios and the chemical shift can be considered as the protonation calibration curves to determine the protonation ratio of amine as shown in eqs 25−30. The deviations of both the pH method and 13C NMR method for 1-(2-HE)PP solution are demonstrated in Table 4. Table 4. Validation of Deviations of pH and NMR Protonation Calibration Tests of 1-(2-HE)PP Solution

Figure 6. Protonation calibration curve of 1-(2-HE)PRLD at 298 K.

pH Method

protonation ratio increased at −3.379, −0.428, and −0.186 ppm, while the chemical shift of C3, which is considered to be in the deshielded region, increased as the protonation ratio increased at 0.562 ppm. Based on the comparison of the ranges of peak shifting, there were two C atoms with the widest ranges of peak shifting in the shielded regions for each amine (C2 and C5 for 1-(2-HE)PP, C1 and C4 for DEA-1,2-PD, C1 and C2 for 1-(2-HE)PRLD) and these were selected as the protonation calibration sets. Since the chemical shifts of C2 of DEA-1,2-PD and C3 of 1-(2-HE)PRLD, which were in the deshielded regions, were in different chemical surroundings from those of the relative C atoms in the shielded regions, the chemical shifts of C2 of DEA-1,2-PD and C3 of 1-(2-HE)PRLD were not included in the calibration curves, in order to prevent systematic errors. The protonation calibration curve equations of 1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD were established as eqs 25−30. For 1-(2-HE)PPH+/1-(2-HE)PP: (AAD = 0.26%)

(25)

(AAD = 0.08%)

(26)

(AAD = 0.4%)

(27)

(AAD = 0.2%)

(28)

C5: δ = 24.041 − 0.02274x × 100

For DEA-1,2-PDH+/DEA-1,2-PD: C1: δ = 10.760 − 0.02064x × 100

C NMR Method

titration ratio (%)

by pH test (%)

deviation (%)

by NMR test (%)

deviation (%)

0 20 40 60 80 100 average

1.71 18.64 40.34 64.54 84.00 99.95

1.71 1.36 0.34 4.54 4.00 0.05 2.00

0.16 19.90 39.95 59.88 80.08 100.06

0.16 0.10 0.05 0.12 0.08 0.06 0.10

The accurate protonation ratios are listed in the first column, the measured values from the pH method are listed in the second column, for which the dissociation equilibrium constant of protonated amines are generated in section 3.2.2, and the concentration of H+ is based on the pH value. The measured values from the 13C NMR calibration method are listed in the fourth column, for which the protonation ratio was the average of the protonation ratios calculated from the protonation calibration curve equations of C2 and C5 for 1-(2-HE)PP. The absolute deviation of value of the measured protonation rate of both methods from the accurate value is marked as “deviation (%)”. As shown in Table 4 and Figure 7, the absolute average deviations (AAD%) of the six tested points are 2.00% for the pH method and 0.10% for the 13C NMR method, respectively. Thus, it can be concluded that 13C NMR method is more reliable than the pH method in this work.

C2: δ = 59.054 − 0.03296x × 100

13

C4: δ = 69.983 − 0.03361x × 100 +

For 1-(2-HE)PRLDH /1-(2-HE)PRLD: C1: δ = 60.604 − 0.03404x × 100

(AAD = 0.5%)

(29)

(AAD = 1.4%)

(30)

C2: δ = 57.387 − 0.00428x × 100

where δ is the chemical shift of reference carbon atom and x is the protonation ratio of amines (1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD). The average values of the protonation ratios calculated from C2 and C5 for 1-(2-HE)PP, C1 and C4 for DEA-1,2-PD, C1 and C2 for 1-(2-HE)PRLD were generated to be used to calculate the concentrations of 1-(2-HE)PP and 1-

Figure 7. Validation of pH and NMR protonation calibration tests of 1-(2-HE)PP solution. 3074


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Energy & Fuels Table 5. CO2 Loading and pH Value As Functions of Time in 1.0 M Amine−CO2−H2O at 298 K 1-(2-HE)PP

DEA-1,2-PD

1-(2-HE)PRLD

time, t (s)

CO2 loading

pH

time, t (s)

CO2 loading

pH

time, t (s)

CO2 loading

pH

0 175 229 320 430 498 588 1550 1946 2186 2329 2930

0 0.061 0.130 0.230 0.329 0.398 0.473 0.577 0.677 0.784 0.826 0.897

11.77 10.24 10.06 9.88 9.68 9.58 9.48 9.27 9.10 8.86 8.68 7.90

0 61 109 203 224 295 412 741 860 1032 1160 2060

0.000 0.041 0.101 0.154 0.206 0.277 0.462 0.623 0.704 0.783 0.874 0.915

11.80 10.96 10.67 10.38 10.26 10.05 9.73 9.29 9.07 8.74 8.52 7.65

0 249 803 1179 1739 2047 2624 3114 3472 4372

0.000 0.063 0.157 0.261 0.368 0.449 0.556 0.736 0.855 0.927

11.92 10.79 10.42 10.21 9.99 9.77 9.39 8.99 8.58 7.64

4.3. CO2 Absorption Rate and Capacity in Three Promising Tertiary Amine Solutions. The experimental data for pH value and CO2 loading, each as a function of time (t) in 1.0 M solutions of the three tertiary amines (1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD) at 298 K are shown in Table 5. The plots of CO2 loading, as a function of time, and the change in pH value, as a function of CO2 loading, are shown in Figures 8 and 9, respectively. As shown in Figure 8,

CO2 loading in DEA-1,2-PD solution changed faster than in 1(2-HE)PP and 1-(2-HE)PRLD solutions at 298 K. This means that the CO2 absorption rate in DEA-1,2-PD solution is faster than those of 1-(2-HE)PP and 1-(2-HE)PRLD solutions. The rank of the three tested amines in terms of CO2 absorption rate at 298 K can be shown as 1‐(2‐HE)PRLD < 1‐(2‐HE)PP < DEA‐1,2‐PD

Compared to the result in section 4.1, it is evident that the rank of CO2 absorption rate for three promising solvents shows a trend that is contradictory with the rank of pKa, while the ranking of the three tested amines, in terms of CO2 absorption kinetics, should be consistent with the ranking of the pKa values. This is because the absorption rate obtained in this work does not equate to the absorption kinetics. The possible reason is that the absorption rates might have been influenced by the experimental environment, such as the viscosity of the solvent, the room temperature of the laboratory, the CO2 partial pressure, and so on, which would affect the mass-transfer performance of CO2 absorption. To confirm this result, further study on the CO2 absorption rate should be performed. From Figure 9, it can be seen that the pH value change increased (the pH value decreased) with the increase of CO2 loading. Because CO2 is an acidic gas, when it was bubbled into the amine solution, the concentration of H+ in the solution was increased. Furthermore, the last pH value change of 1-(2HE)PRLD is higher than those of 1-(2-HE)PP and DEA-1,2PD, and the last CO2 loading of 1-(2-HE)RLD is higher than those of 1-(2-HE)PP and DEA-1,2-PD. Thus, it can be concluded that the ranking of 1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD, in terms of CO2 absorption capacity, can be shown as

Figure 8. Plot of CO2 loading with time (t) in 1.0 M amine−CO2− H2O at 298 K.

1‐(2‐HE)PRLD > DEA‐1,2‐PD > 1‐(2‐HE)PP

which is consistent with the ranking of pKa values in section 4.1. However, the pH value changes of 1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD were 3.87, 4.15, and 4.28, respectively, and the CO2 loadings were 0.897, 0.915, and 0.927 mol CO2/mol amine, respectively. In terms of the pH value change, the CO2 absorption capacity of 1-(2-HE)PRLD and DEA-1,2-PD were similar and significantly better than that of 1-(2-HE)PP, whereas for the last CO2 loading, the CO2 absorption capacity of these promising tertiary amines were similar to each other. 4.4. Ion Speciation Plots of Amine−CO2−H2O Systems at 298 K Using the pH Method. In this work, the ion concentrations of 1-(2-HE)PP, 1-(2-HE)PPH+, HCO3−, and

Figure 9. Plot of pH value change with the CO2 loading in 1.0 M amine−CO2−H2O at 298 K.

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Energy & Fuels CO32− were calculated by using the pH method over the CO2 loading range of 0−0.897 mol CO2/mol amine, at a temperature of 298 K and a concentration of 1.0 M. The ion speciation results of the 1-(2-HE)PP-CO2−H2O system are plotted in Figure 10. The ion speciation plots of the DEA-1,2-

Figure 12. Ion speciation plot of the 1.0 M 1-(2-HE)PRLD-CO2− H2O system at 298 K via the pH method.

decrease of free 1-(2-HE)PP resulted in the lower pH value and weaker basic solution. Hence, the concentration of CO32− decreased because of the conversion of this ion to HCO3− through the reverse reaction of reaction 5. For the DEA-1,2-PD-CO2−H2O system, the ion speciation curves of DEA-1,2-PD, DEA-1,2-PDH+, HCO3−, and CO32− show the same trend as those in the 1-(2-HE)PP-CO2−H2O system. However, CO32− was observed to be a major component, rather than HCO3−, at lower CO2 loadings (CO2 loading < 0.17 mol CO2/mol amine). The reason is that the pKa value of DEA-1,2-PD is higher than that of 1-(2-HE)PP at the same temperature. The amine with higher pKa value is a stronger base and its solution is more basic than that of the amine with lower pKa values. Hence, reactions 2, 3, 4, and 5 are more greatly promoted, and the concentration of CO32− was higher than that of HCO3− in DEA-1,2-PD-CO2−H2O system at lower CO2 loadings. As the CO2 loading increased, the basicity of the solution became weaker, because of the decrease of free DEA-1,2-PD, which led the CO32− ion to accept a proton and convert to HCO3− through the reverse reaction of reaction 5, which resulted in a phenomenon of a decrease in the concentration of CO32−, so that HCO3− became the major product of the reaction with CO2, rather than CO32−. In the 1-(2-HE)PRLD-CO2−H2O system, the concentrations of 1-(2-HE)PRLD, 1-(2-HE)PRLDH+, HCO3−, and CO32− showed the same trends as those in the DEA-1,2-PDCO2−H2O system. The reasons can be found in the description of the DEA-1,2-PD-CO2−H2O system. The only significant difference is the critical CO2 loadings of the DEA-1,2-PDCO2−H2O system and the 1-(2-HE)PRLD-CO2−H2O system, which were ∼0.17 and 0.19 mol CO2/mol amine, respectively. They were similar, but there was still a difference of 0.02 mol CO2/mol amine; this is caused by the difference in the molecular structures of DEA-1,2-PD and 1-(2-HE)PRLD. In the comparison of the three promising solvents, in terms of the concentrations of CO32− and HCO3−, it can be concluded that the concentration of HCO3− was always higher than that of CO32− for 1-(2-HE)PP-CO2−H2O system, the concentration of HCO3− was higher than that of CO32− with the CO2 loading being >0.17 mol CO2/mol amine for the DEA-1,2-PD-CO2−H2O system, and the concentration of HCO3− was higher than that of CO32− when the CO2 loading was >0.19 mol CO2/mol amine for the 1-(2-HE)PRLD-CO2− H2O system. This means that the 1-(2-HE)PRLD solution

Figure 10. Ion speciation plot of the 1.0 M 1-(2-HE)PP-CO2−H2O system at 298 K via the pH method.

PD-CO2−H2O and 1-(2-HE)PRLD-CO2−H2O systems were also developed using the same procedure; these are shown in Figures 11 and 12, respectively. From Figure 10, it is clearly

Figure 11. Ion speciation plot of the 1.0 M DEA-1,2-PD-CO2−H2O system at 298 K via the pH method.

seen that the concentration of free 1-(2-HE)PP decreased as the CO2 loading increased, which is mainly caused by the reaction of 1-(2-HE)PP solution with CO2 and the appearance of the protonation of free 1-(2-HE)PP. As a result, the concentration of 1-(2-HE)PPH+ (protonated 1-(2-HE)PP) increased gradually with the increase of CO2 loading. As one of the principal products, HCO3− increased as the CO2 loading increased. However, the plot of the increasing CO 3 2− concentration is curved, showing that the concentration of CO32− increased with CO2 loading at low CO2 loading, but after reaching a maximum, it then decreased as the CO2 loading increased. This is mainly attributed to the change in solution pH. At the low CO2 loading, the solution pH value and the concentration of free 1-(2-HE)PP were so high that reactions 2, 3, 4, and 5 were greatly promoted. Hence, the concentration of CO32− increased significantly. At higher CO2 loading, the 3076


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Energy & Fuels requires more CO2 to make the concentration of HCO3− higher than that of CO32−. This is because the pKa value of 1(2-HE)PRLD solution is higher than those of DEA-1,2-PD and 1-(2-HE)PP. This leads to more CO2 being necessary to lower the basicity of the aqueous 1-(2-HE)PRLD solution. As shown in Table 1, the primary pH values of three amines at 298 K can be ranked as 1‐(2‐HE)PRLD > DEA‐1,2‐PD > 1‐(2‐HE)PP

while the last pH values of those three amines at 298 K can be ranked as 1‐(2‐HE)PP > DEA‐1,2‐PD > 1‐(2‐HE)PRLD

Thus, the amount of CO2 needed to lower the basicity of the solution of those three amines would have the same ranking as the pKa value and change in pH value. In tertiary amine solutions, CO2 was converted to HCO3− and CO32− once it was introduced into the solutions, while the reduced free amines(1-(2-HE)PP, DEA-1,2-PD, and 1-(2HE)PRLD) were converted to the protonated amines (1-(2HE)PPH+, DEA-1,2-PDH+, and 1-(2-HE)PRLDH+). According to the charge conservation, the equation

Figure 13. Ion speciation plot of the 1.0 M 1-(2-HE)PP-CO2−H2O system at 298 K via the 13C NMR method.

⎡⎣amineH+⎤⎦ + ⎡⎣H+⎤⎦ = ⎡⎣HCO3−⎤⎦ + 2⎡⎣CO32 −⎤⎦

could be established in these three tertiary amine systems, while the concentrations of H+ and OH− were very small, so that they could be ignored in the equation ⎡⎣amineH+⎤⎦ + ⎡⎣H+⎤⎦ = ⎡⎣HCO3−⎤⎦ + 2⎡⎣CO32 −⎤⎦ + ⎡⎣OH−⎤⎦

and the charge conservation equation can be simplified to ⎡⎣amineH+⎤⎦ = ⎡⎣HCO3−⎤⎦ + 2⎡⎣CO32 −⎤⎦

Therefore, the molar concentrations of the reduced free amines should be equal to the concentration of HCO3− plus two times of the concentration of CO32−, which has been well shown in Figures 10−21. 4.5. Ion Speciation Plots of Amine−CO2−H2O Systems at 298 K Using the 13C NMR Method. In this work, the ion concentrations of free amines, protonated amines, HCO3−, and CO32− were also obtained by using the 13C NMR method over the CO2 loading range of 0−0.897 mol CO2/mol amine, at a temperature of 298 K and a concentration of 1.0 M. The speciation results of the 1-(2-HE)PP-CO2−H2O, DEA-1,2-PDCO2−H2O, and 1-(2-HE)PRLD-CO2−H2O systems are plotted in Figures 13, 14, and 15, respectively. From these figures, it can be seen that the ion speciation curves of free amines, protonated amines, HCO3−, and CO32− show the same trends as those in the 1-(2-HE)PP-CO2−H2O system calculated using the pH method described in section 4.4. Compared with the results generated by using the pH method, the results obtained by using the 13C NMR method show that the critical CO2 loadings for the numerical size relationship between the concentrations of HCO3− and CO32− were higher than those of the same systems conducted via the pH method. The critical CO2 loadings were ∼0.10, 0.30, and 0.35 mol CO2/mol amine for the 1-(2-HE)PP-CO2−H2O, DEA-1,2-PD-CO2−H2O, and 1-(2-HE)PRLD-CO2−H2O systems, respectively. The more specific description about the differences between the same systems conducted by different methods is described in section 4.7. 4.6. Ion Speciation Plots of Amine−CO2−H2O Systems at 298 K Using the pH + 13C NMR Method. In this work,

Figure 14. Ion speciation plot of the 1.0 M DEA-1,2-PD −CO2−H2O system at 298 K via the 13C NMR method.

Figure 15. Ion speciation plot of 1.0 M 1-(2-HE)PRLD-CO2−H2O system at 298 K via the 13C NMR method.

ion speciation plots for the three tertiary systems are presented using the pH + 13C NMR method. Actually, the concentrations of amine and amineH+ obtained via the pH + 13C NMR method are the same as those by the pH method, which gave the same results as the pH method, and the concentrations of HCO3− and CO32− calculated by the pH + 13C NMR method are the same as those obtained with the 13C NMR method. All 3077


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Energy & Fuels the results for the 1-(2-HE)PP-CO2−H2O, DEA-1,2-PD-CO2− H2O, and 1-(2-HE)PRLD-CO2−H2O systems are shown in Figures 16, 17 and 18, respectively. The tendency of the corresponding ion speciation curve in the same system and its reason are the same as those mentioned in the two previous methods.

4.7. Comparison of Three Different Methods for the Development of Ion Speciation Plots in 1.0 M Tertiary Amine Solutions. In this work, the ion speciation plots of three promising tertiary amines were generated by the pH method, the 13C NMR method, and the pH + 13C NMR method. As shown in Figure 19, the ion speciation plots for the

Figure 19. Ion speciation plot in the 1.0 M 1-(2-HE)PP-CO2−H2O system at 298 K.

Figure 16. Ion speciation plot of the 1.0 M 1-(2-HE)PP-CO2−H2O system at 298 K via the pH + 13C NMR method.

1-(2-HE)PP-CO2−H2O system were developed using three different methods, and it can be seen that the tendency of ions (1-(2-HE)PP, 1-(2-HE)PPH+, HCO3−, CO32−) generated by using different methods were identical to the others. As shown in Figure 19, the concentration of 1-(2-HE)PP obtained using the pH method is much lower than that obtained by using the 13 C NMR method, and the concentration of 1-(2-HE)PPH+ by the pH method is much higher than that of 1-(2-HE)PPH+ via the 13C NMR method. As shown in section 2 (“Calculation Methods”), the calculation of the concentrations of amine, amineH+, HCO3−, and CO32− involves several equilibrium constants, such as Ka and K5, by using the pH method. The equilibrium concentration described by eqs 7 and 8 are ideal models, so that the influences of the activity coefficients of the components were ignored, which would increase the errors of the measured pKa and the calculated concentrations. However, the calculation of these concentrations does not involve any equilibrium constants when using the 13C NMR method, and the entire computational process was based on the experimental data from the 13C NMR technique. For concentrations of HCO3− and CO32−, the two methods show similar results. Figure 20 shows the ion speciation plots for the DEA-1,2PD-CO2−H2O system, which were developed by using the pH method, the 13C NMR method, and the pH + 13C NMR method. As shown in Figure 20, all three of these methods represent the ion speciation plots for the DEA-1,2-PD-CO2− H2O system very well. In addition, the ion speciation plots for the 1-(2-HE)PRLD-CO2−H2O system were developed from the three methods, as shown in Figure 21. From Figure 21, it can be found that the ion concentrations calculated using the pH method have good agreement with those using the 13C NMR method. Based on the previous discussion for these three tertiary amines, it can be concluded that the pH method, the 13C NMR method and the pH + 13C NMR method can produce the ion speciation plots for the amine−CO2−H2O systems. For the pH

Figure 17. Ion speciation plot of the 1.0 M DEA-1,2-PD-CO2−H2O system at 298 K via the pH + 13C NMR method.

Figure 18. Ion speciation plot of the 1.0 M 1-(2-HE)PRLD −CO2− H2O system at 298 K via the pH + 13C NMR method.

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Energy & Fuels

concentrations of HCO3− and CO32− calculated by using the pH + 13C NMR method are the same as those obtained using the 13C NMR method. Thus, the order of the accuracy of these three methods could be ranked as 13

C NMR method > pH + 13C NMR method > pH method

However, the order of the cost of these three methods could be ranked as pH method < pH + 13C NMR method < 13C NMR method

All the results in this work have been obtained at atmosphere pressure and a temperature of 298 K, which would provide basic research preparation for further study on the typical concentrations of flue gas absorbed by these three promising tertiary amines.

Figure 20. Ion speciation plot in the 1.0 M DEA-1,2-PD-CO2−H2O system at 298 K.

5. CONCLUSION (1) Based on the experimental results of the pKa values of these three promising tertiary amines (1-(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD), it can be concluded as the pKa values of these three promising tertiary amines decreased as the temperature increased. Moreover, the corresponding empirical correlations shown as eqs 22, 23, and 24 could predict pKa very well with AADs 0.6% for 1-(2-HE)PP, 0.8% for DEA-1,2-PD, and 0.8% for 1-(2-HE)PRLD, respectively. Furthermore, the order of pKa value for three amines can be ranked as 1‐(2‐HE)PRLD > DEA‐1,2‐PD > 1‐(2‐HE)PP > MDEA

(2) Based on the experimental results of the protonation calibration curves of these three promising tertiary amines (1(2-HE)PP, DEA-1,2-PD, and 1-(2-HE)PRLD), the corresponding equations of the protonation calibration curves used for the calculation of the protonation ratios of amines were provided as described by eqs 25−30. (3) The ranking of these three promising tertiary amines, in terms of CO2 absorption rate at 298 K, can be shown as

Figure 21. Ion speciation plot in the 1.0 M 1-(2-HE)PRLD-CO2− H2O system at 298 K.

1‐(2‐HE)PRLD > 1‐(2‐HE)PP > DEA‐1,2‐PD

method, with its involvement of some equilibrium constants, there may be more error. However, this method is simple and could be used to develop the ion speciation plots for amines without a lot of experimental data. For the 13C NMR method, all ion concentrations are calculated based on the experimental results and should be more accurate. Obviously, the cost of the 13 C NMR method is higher and this method is more complicated. The reasons for the larger error of the pH method and the greater accuracy for the 13C NMR method have been well shown in the first paragraph of section 4.7. Moreover, as shown in Table 4 and Figure 7, the absolute average deviations (AAD%) of the protonation ratios used for the calculations of the concentrations of free amines and protonated amines by using the 13C NMR method are 2.00% for the pH method and 0.10% for the 13C NMR method, respectively. Thus, it can be concluded that the 13C NMR method is more reliable than the pH method in this work. Furthermore, eqs 13 and 14 were used for the calculation of the concentrations of HCO3− and CO32−, in which the value of K5 was derived from refs 6 and 12. Therefore, some operating and systematic errors might be introduced into the calculation results, while the entire computational process was based on the experimental data from the 13C NMR technique. As described in section 4.6, the concentrations of amine and amineH+ obtained using the pH + 13C NMR method are the same as those obtained using the pH method, and the

which is contradictory with the ranking of pKa values. While the ranking of these three promising tertiary amines, in terms of CO2 absorption capacity, can be shown as 1‐(2‐HE)PRLD > DEA‐1,2‐PD > 1‐(2‐HE)PP > MDEA

which is consistent with the ranking of pKa values, although that the CO2 absorption capacity of these three tertiary amines were similar to each other. (4) Based on the comparison of ion speciation plots of these three promising tertiary amines obtained by using the pH method, the 13C NMR method, and the pH + 13C NMR method, it can be concluded that the pH method is very simple but shows more error, while the NMR method shows more accuracy but is more complicated. Moreover, the order of the accuracy of these three methods can be ranked as 13

C NMR method > pH + 13C NMR method > pH method

The order of the cost of these three methods can be ranked as

■

pH method < pH + 13C NMR method < 13C NMR method

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H. Liu). 3079


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Energy & Fuels *E-mail: [email protected] (Z. Liang).

(14) Perinu, C.; Arstad, B.; Bouzga, A. M.; Svendsen, J. A.; Jens, K. J. NMR-Based Carbamate Decomposition Constants of Linear Primary Alkanolamines for CO2 Capture. Ind. Eng. Chem. Res. 2014, 53 (38), 14571−14578. (15) 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 (26), 9894−9903. (16) Shi, H.; Sema, T.; Naami, A.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P. 13C NMR spectroscopy of a novel amine species in the DEAB−CO2−H2O system: VLE model. Ind. Eng. Chem. Res. 2012, 51 (25), 8608−8615. (17) Pérez-Salado Kamps, Á .; Maurer, G. Dissociation constant of Nmethyldiethanolamine in aqueous solution at temperatures from 278 to 368 K. J. Chem. Eng. Data 1996, 41 (6), 1505−1513. (18) Li, J.; Liu, H.; Liang, Z.; Luo, X.; Liao, H.; Idem, R.; Tontiwachwuthikul, P. Experimental study of the kinetics of the homogenous reaction of CO2 into a novel aqueous 3-diethylamino1,2-propanediol solution using the stopped-flow technique. Chem. Eng. J. 2015, 270, 485−495. (19) Shi, H.; Naami, A.; Idem, R.; Tontiwachwuthikul, P. 1D NMR analysis of a quaternary MEA−DEAB−CO2−H2O amine system: liquid phase speciation and vapor−liquid equilibria at CO2 absorption and solvent regeneration conditions. Ind. Eng. Chem. Res. 2014, 53 (20), 8577−8591. (20) Fan, G.-j.; Wee, A. G.; Idem, R.; Tontiwachwuthikul, P. NMR studies of amine species in MEA−CO2−H2O system: Modification of the model of vapor−liquid equilibrium (VLE). Ind. Eng. Chem. Res. 2009, 48 (5), 2717−2720. (21) Horwitz, W. Association of Official Analytical Chemists (AOAC) Methods, 12th Edition; George Bant: Gaithersburg, MD, 1975. (22) Danckwerts, P. The reaction of CO2 with ethanolamines. Chem. Eng. Sci. 1979, 34 (4), 443−446. (23) Thee, H.; Suryaputradinata, Y. A.; Mumford, K. A.; Smith, K. H.; da Silva, G.; Kentish, S. E.; Stevens, G. W. A kinetic and process modeling study of CO2 capture with MEA-promoted potassium carbonate solutions. Chem. Eng. J. 2012, 210, 271−279. (24) Hamborg, E. S.; Versteeg, G. F. Dissociation constants and thermodynamic properties of alkanolamines. Energy Procedia 2009, 1 (1), 1213−1218. (25) Brønsted, J.; Guggenheim, E. Contribution to the theory of acid and basic catalysis. The mutarotation of glucose. J. Am. Chem. Soc. 1927, 49 (10), 2554−2584. (26) Littel, R.; Versteeg, G.; Van Swaaij, W. P. Kinetics of CO2 with primary and secondary amines in aqueous solutionsI. Zwitterion deprotonation kinetics for DEA and DIPA in aqueous blends of alkanolamines. Chem. Eng. Sci. 1992, 47 (8), 2027−2035.

ORCID

Zhiwu Liang: 0000-0003-1935-0759 Author Contributions §

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the National Natural Science Foundation of China (NSFC Nos. 21536003, 21606078, 21476064, 21376067 and 51521006), National Key Technology R&D Program (MOST No. 2014BAC18B04), Innovative Research Team Development Plan (MOE No. IRT1238), Specialized Research Fund for the Doctoral Program of Higher Education (MOE No. 20130161110025), and China Outstanding Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan University (MOE No. 2011-40) are gratefully acknowledged. We also greatly appreciate Mr. Wilfred Olson for his great contribution to helping us correct any grammar mistakes and providing insightful inputs to our research work.

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REFERENCES

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