13C NMR Spectroscopy of a Novel Amine Species in the DEAB–CO2

Article pubs.acs.org/IECR

C NMR Spectroscopy of a Novel Amine Species in the DEAB−CO2− H2O system: VLE Model

13

Huancong Shi,†,‡ Teerawat Sema,*,†,‡ Abdulaziz Naami,‡ Zhiwu Liang,*,†,‡ Raphael Idem,*,†,‡ and Paitoon Tontiwachwuthikul*,†,‡ †

Joint International Center for CO2 Capture and Storage (iCCS), Department of Chemical Engineering, Hunan University, Changsha 410082, P.R. China ‡ International Test Centre for CO2 Capture (ITC), Faculty of Engineering and Applied Science, University of Regina, Regina, Saskatchewan S4S 0A2, Canada S Supporting Information *

ABSTRACT: In the present work, ion speciation studies in solutions of the novel amine 4-(diethylamine)-2-butanol (DEAB), at various CO2 loadings (0−0.8 mol of CO2/mol of amine) and amine concentrations (0.52−1.97 M), were determined by 13C nuclear magnetic resonance (NMR) spectroscopy. In addition, the dissociation constant K of DEABH+ was determined at 24.5, 35, and 45 °C using a pH meter. The ion speciation plot, which contains various sets of concentrations of DEAB, protonated DEAB, bicarbonate, and carbonate, was successfully generated. Because DEAB is a novel solvent, this is the first time that the ion speciation plots of the DEAB−CO2−H2O system have been developed. It is also the first time that the 13C NMR calibration technique was applied to develop the vapor−liquid equilibrium (VLE) model for an amine−CO2−H2O system. The results obtained from the present work can be a great help for the further analysis of the DEAB VLE model, as well as CO2 absorption and kinetics studies. Furthermore, it was found that the novel 13C NMR calibration technique developed in this work provides higher accuracy than the conventional technique.

1. INTRODUCTION At present, carbon dioxide (CO2) capture has become a major option in efforts to mitigate the global warming and climate change problems caused by CO2 emissions. The technology of using a proper amine solution as an absorbent has been studied for decades. Recently, the novel amine 4-(diethylamine)-2butanol (DEAB) was developed and synthesized at the International Test Centre for CO2 Capture (ITC) for use in CO2 capture processes.1−3 It was found that DEAB, which is considered as a tertiary amine because it has three carbon atoms attached to the nitrogen atom, has a very high CO2 absorption capacity. The absorption capacity of DEAB is competitive with that of piperazine (PZ) and higher than those of 2-amino-2-methyl-1-propanol (AMP), methyldiethanolamine (MDEA), monoethanolamine (MEA), and diethanolamine (DEA).2,3 Also, it was found that the regeneration energy of DEAB is lower than those of MDEA, DEA, and MEA.2 Because of these outstanding performance characteristics, DEAB is now being considered as a promising alternative solvent for capturing CO2. However, the exact ion speciation of the DEAB−CO2−H2O system is required to comprehensively understand the CO2 absorption and kinetics behavior. The ion concentrations of major cations and anions in the DEAB− CO2−H2O system, including free DEAB, protonated DEAB (DEABH+), bicarbonate (HCO3−), and carbonate (CO32−), need to be determined accurately. Numerous NMR analyses have been performed on the vapor−liquid equilibrium (VLE) model in CO2 absorption process studies within amine−CO2−H2O systems,4−7 as well as H2O−CO2 systems.8 Reliable estimation of the liquid-phase composition plays a key role in the VLE model; however, such © 2012 American Chemical Society

useful information on particular species concentrations is difficult to detect by traditional phase equilibrium measurements for activity coefficient models.4 NMR experiments can provide detailed information about the liquid-phase composition for this purpose. In the conventional method of combining pH measurements with 13C NMR analysis, the concentrations of amine and protonated amine are calculated using the measured pH value (from a pH meter) and the dissociation constant of protonated amine, whereas the concentrations of HCO3− and CO32− are determined by 13C NMR analysis. On the other hand, in the novel 13C NMR calibration technique developed in the present work, the concentrations of all major species (i.e., amine, protonated amine, HCO3−, and CO32−) are determined based on 13C NMR analysis alone. In the present work, the dissociation constant of protonated DEAB (DEABH+) was determined by a titration technique. In addition, the ion speciation results obtained from the newly developed methodology were then compared with those obtained from the method of combining pH measurements and NMR analysis, which is the conventional method reported in the literature.4−7

2. EXPERIMENTAL SECTION 2.1. Chemicals. The novel tertiary amine DEAB was synthesized in the solvent synthesis laboratory at ITC with the Received: Revised: Accepted: Published: 8608

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method described by Tontiwachwuthikul et al.1 The structure of DEAB is shown in Figure 1. Aqueous solutions of DEAB

2.3. DEABH+ Dissociation Constant Determination. Because the dissociation constant of DEABH+ in aqueous solution is not available in the literature, the K value should be measured carefully in preparation for the further calculation of DEAB and DEABH+ concentrations using the conventional method. The experimental and calculation procedures for the dissociation constant can be found in the work of Kamps and Maurer,10 who studied the dissocoation constant of MDEAH+. The titration technique was validated using MEA. In the present work, ln KMEAH+ at 22 °C was found to be −22.3, whereas the literature value11 is −23.2 (3.8% deviation). In addition, the titration was repeated several times under similar conditions, and the average value was taken. The results showed that the deviation percentage of the DEABH + dissociation constant measurement was 0.4%. Based on the assumption that the solution is ideal (activity coefficient equal to 1),10,11 the DEABH+ dissociation constant K can be calculated using the equation KDEABH+ = ([DEAB][H+])/[DEABH+]

(5)

+

The concentration of H in the solution can be measured by pH meter. Bearing in mind that the disappearance of H+ during titration is a result of its reaction with DEAB to form DEABH+ as shown in reaction 2, the concentration of DEABH+ can then be calculated using the mass balance of protons as in the equation

Figure 1. Molecular structure of 4-(diethylamine)-2-butanol (DEAB , C8H18ON).

were prepared by adding distilled water to the desired concentration. CO2 with a purity of 99.9% was supplied from Praxair Inc., Mississauga, Ontario, Canada. D2O was used for signal locking for 13C NMR spectroscopy (Varian Mercury 500 MHz NMR spectrometer). The pH value was recorded using a pH meter (Accumet AB 15 m) standardized with pH 4, 7, and 10 buffer solutions for both K constant detection and NMR analysis. Standard 1.0 M hydrochloric acid (HCl) solution and NaHCO3 and Na2CO3 (Sigma-Aldrich Co., Oakville, Ontario, Canada) with purities of 99.9% were used for titration and NMR reference analysis. 2.2. Chemical Reaction Scheme. Because DEAB is a tertiary amine, it does not react directly with CO2 but rather acts as a base that catalyzes the hydration of CO2. The major chemical reactions within the DEAB−CO2−H2O systems are listed in eqs 1−4. All of the cations and anions, as well as the neutral compound free DEAB and the CO2 loading, can be determined with proper experiments. The concentrations of free DEAB and DEABH+ can be determined from (i) the DEABH+ dissociation constant K together with pH analysis (conventional method) and (ii) 13C NMR peak calibration (novel method developed in this work). Meanwhile, the CO2 loading can be measured by titration with a Chittick CO2 analyzer through the reverse of reaction 3.5,9 The construction of the apparatus and the procedure for measuring the CO2 loading can be found in the literature.9 Also, the experimental equipment and procedure were validated with conventional amines (i.e., MEA, DEA, MDEA, and AMP) in our previous work.2 The concentration of bicarbonate and carbonate were measured by 13C NMR analysis from reactions 3 and 4.6 The basic reaction scheme in the DEAB−CO2−H2O system can be expressed as CO2 + H 2O + DEAB ⇄ HCO3− + DEABH+

(1)

DEAB + H+ ⇄ DEABH+

(2) +

CO2 + H 2O ⇄ H 2CO3 ⇄ H + HCO3− ⇄ CO32 − + H+

HCO3−

nHCl − [H+]Vtotal = [DEABH+]Vtotal

(6)

Finally, the concentration of free DEAB can be calculated using the DEAB balance equation ([DEAB] + [DEABH+])Vtotal = n0,DEAB

(7)

where nHCl is number of moles of HCl added during the titration; Vtotal is the total liquid volume after titration; and n0,DEAB is the initial number of moles of DEAB, which can be determined by titration with 1.0 M HCl until the methyl orange end point.9 For example, from Table S1 (Supporting Information), 100 mL of 0.91 M DEAB solution was prepared and titrated carefully until the methyl orange end point was reached for n0,DEAB. Then, 5 mL of solution was pipetted and diluted to 100 mL and kept at room temperature. This dilute DEAB solution (0.0455 M) was then slowly titrated with 5 mL of 1.0 M HCl standard solution to bring about reaction 2. The pH meter was placed in the solution containing a magnetic stirrer for measuring the pH value. The pH of the solution was recorded after the addition of every 0.5 mL of 1.0 M HCl solution. As mentioned earlier, the H+ concentration was measured by pH meter, whereas the DEABH+ and DEAB concentrations were calculated using mass balance equations (eqs 6 and 7). Finally, the dissociation constant K was then calculated using eq 5. The same procedure was also applied for determining the dissociation constant K of DEAB at 35 and 45 °C. 2.4. 13C NMR Analysis. The DEAB−CO2−H2O system was prepared at various CO2 loadings (0−0.8 mol of CO2/mol of amine) and DEAB concentrations (0.52−1.97 M). 13C NMR spectroscopy was recorded with sample mixed with drops of D2O (10%) as a signal lock, using a Varian Mercury 500 MHz NMR spectrometer to test at 24.5 °C. By comparison of the 13C NMR peaks of MDEA and DEAB, it was found that the results for DEAB were different. This is because of the difference in the chemical structures of MDEA

(3) (4) 8609

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and DEAB. MDEA exhibits three 13C NMR peaks at δ = 58.7− 59.8 ppm (−CH2−OH), δ = 57.3−59.6 ppm (−CH2−N< ), and δ = 41.7−42.7 ppm (CH3−N < ).4 However, DEAB exhibits six 13C NMR peaks for C1−C6 (as marked in Figure 2) at 10.2, 22.3, 33.9, 46.3, 48.56, and 67.2 ppm.

NaHCO3 solutions at 24.5 °C were observed in this work at 160.716 and 168.467 ppm, respectively, which are very close to the results observed by Holmes et al.6 (i.e., 160.33 and 168.09 ppm, respectively, at 25.0 °C). From multiple sets of reference results for HCO3−/CO32− in Table 1, the trends are clear and matched well with the conclusion of Holmes et al.6 that the NMR peaks of pure HCO3− and CO32− shift to the shielded region (chemical shift decreases) as temperature increases. Conversely, the NMR peaks shift to larger chemical shift as temperature decreases.6 The chemical shift of HCO3−/CO32− observed in this work (at 24.5 °C) is 0.23% higher than that from the work of Holmes et al.6 (at 25.0 °C). This matches well with the temperature deviation of 0.17%. Even though, there is no reference point for the size of the deviations and the magnitude of the temperature correction, the deviation of 0.23% different from the work of Holmes et al.6 is considered to be very small (the deviation caused by the difference in temperature of 0.17%). Therefore, the sets of NMR data collected in this work are considered to be very accurate. 2.5. Determination of Ion Concentration of HCO3−/ CO32− and DEABH+/DEAB. As suggested by Holmes et al.,6 the concentration of CO32− and HCO3− can be calculated using eqs 8 and 9. The chemical shift of the HCO3− and CO32− reflects the exact ratio of these two ions. The initial concentration of CO2 ([CO2]0) can be determined using a Chittick CO2 analyzer.9 However, the 13C NMR peak of HCO3−/CO32− is hardly detected at very low CO2 loadings. In this case, the concentration of HCO3−/CO32− can then be calculated using the simplified charge balance equation (eq 10), which is simplified from eq 11 based on the assumption that the trace amount of H+ and OH− can be neglected.

Figure 2. Protonated DEAB with its six specific carbon groups. These carbons are labeled in specific 13C chemical shift order from the shielded region to the deshielded region (low to high).

Because of the fast proton transfer reactions of eqs 2 and 4, it is impossible to distinguish between the pairs HCO3−/CO32− and DEABH+/DEAB in the NMR spectra.4 The 13C chemical shifts of these proton-exchanging species are represented by one common peak for HCO3−/CO32− and six common peaks for DEABH+/DEAB. The chemical shifts of Na2CO3 and

[CO32 −] = [(δ − 160.716)/(168.467 − 160.716)][CO2 ]0 (8)

Table 1. 13C NMR Chemical Shifts (δ, ppm) of HCO3−/CO32− and pH Values of DEAB−CO2−H2O Solutions at Various CO2 Loadings and DEAB Concentrations at 24.5 °C sample 0

1

2

loading δ (ppm)

0.000

0.052 N/Aa

0.119 166.402

loading δ (ppm)

0.000

0.046 167.035

0.094 166.581

loading pH δ (ppm)

0.000 12.32

0.113 10.67 166.006

0.169 10.57 165.578

loading pH δ (ppm)

0.000 12.61

0.126 10.72 165.699

0.161 10.56 165.183

δ (ppm)

160.716

ref

δ (ppm)

168.467

ref

3

4

CDEAB = 0.52 mol/L 0.241 0.359 165.293 164.044 CDEAB = 0.98 mol/L 0.179 0.257 166.054 165.468 CDEAB = 1.5 mol/L 0.253 0.364 10.37 10.14 164.926 163.996 CDEAB = 1.97 mol/L 0.204 0.251 10.47 10.38 164.857 164.542 NaHCO3 160.27b Na2CO3 167.74 −

a

5

6

7

8

0.479 162.722

0.610 161.693

0.720 161.170

0.807 160.646

0.327 164.751

0.409 163.938

0.545 162.737

0.702 161.675

0.450 10.00 163.337

0.578 9.65 162.466

0.712 9.40 161.660

0.815 9.00 160.957

0.313 10.24 163.960

0.394 10.04 163.385

0.553 9.84 162.568

0.615 9.58 161.876

160.33c

161.4d

161.3e

168.09

168.9

169.5

2−

Concentration of CO2 too low to detect by NMR analysis. Concentration of HCO3 /CO3 calculated using the charge balance equation (eq 11). Measured at 27 °C.14 cMeasured at 25 °C.6 dMeasured at 20 °C.4 eMeasured at 32 °C.15 Sample of bicarbonate mixed with acid and base, and NMR instrument not that accurate (10−100 MHz). This can be recognized as an exception. b

8610

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3.2. Conventional 13C NMR Results and pH Value of the DEAB−CO2−H2O System. Table 1 demonstrates the exact chemical shifts of HCO3−/CO32− for the DEAB−CO2− H2O system at various CO2 loadings (0−0.8 mol of CO2/mol of amine) and DEAB concentrations. In total, 36 samples with four different DEAB concentrations (i.e., 0.52, 0.98, 1.5, and 1.97 M) were tested. The pH test was also performed at DEAB concentrations of 1.50 and 1.97 M to generate an ion speciation plot with conventional pH measurements. It can be clearly observed from Table 1 that, for each concentration, along the samples from lean to rich amine solutions, the pH value and the chemical shift of the HCO3−/CO32− ratio decreased as the CO2 loading increased. This is plausible chemically, because, when acidic CO2 gas is introduced into basic amine solution, the pH of the solution decreases. After CO2 is dissolved in water, it generates carbonic acid (H2CO3) and then releases protons. At very low loadings, the excess DEAB accepts the protons and generates CO32−, which becomes one of the major components in the solution. When the CO2 loading is increased, more protons are released, and CO32− starts to accept protons and converts to HCO3−, resulting in an increase in the HCO3− concentration. Therefore, the chemical shift of HCO3−/CO32− decreased and finally reached 100% DEABH+ and HCO3−. The data in Table 1 show that pure HCO3− and CO32− have similar chemical shifts under certain conditions. However, the chemical shifts of HCO3− and CO32− were found to be slightly different from those reported in the literature.4,6,14,15 After a careful analysis, it was found that the main reason for the difference was the operating temperature. The chemical shifts of HCO3− and CO32− are slightly different at different operating temperatures. This is because the chemical shifts of HCO3− and CO32− are directly related to the equilibrium constants, which are very sensitive to temperature. Higher operating temperatures result in smaller chemical shifts.6 For example, the chemical shifts of HCO3− and CO32− obtained in this work at 24.5 °C were 160.712 and 168.467 ppm, respectively. In contrast, those observed by Usubharatana14 at 27.0 °C were 160.27 and 167.74 ppm, respectively. Note that the NMR instruments used in the work of Usubharatana14 and this work were the same. Therefore, to avoid the deviation of chemical shifts with temperature, the chemical shifts of pure HCO3− and CO32− should be tested before performing the NMR analysis of amine−CO2−H2O systems rather than quoting the reference data directly. 3.3. Novel 13C NMR Calibration Curve of the DEAB/ BEABH+ System. The 13C NMR spectrum of DEABH+/ DEAB exhibits six peaks, which represent six types of carbons based on the chemical structure. Even though the DEAB molecule contains eight carbon atoms, six types of carbons can be observed because of structural symmetry, as shown in Figures 1 and 2. These six carbons were labeled C1−C6 in numerical order of their 13C chemical shifts from smallest to largest. The entire chemical shifts at various protonation ratios (ratio of concentrations of added H+ and initial DEAB) of 0:1, 0.2:1, 0.4:1, 0.6:1, 0.8:1, 1:1, and 1.2:1 were recorded and are plotted as calibration curves in Table S2 and Figure S1 (Supporting Information). The ratio of 1.2:1 was prepared (nH+/nDEAB = 1.2:1) to ensure that the sample was fully protonated. From Table S2 and Figure S1 (Supporting Information), six carbon 13C NMR peaks can be categorized into two groups: those in the shielded region and those in the deshielded region. The chemical shifts of C1, C3, and C6, which are considered to

[HCO3−] = [(168.467 − δ)/(168.467 − 160.716)] [CO2 ]0

[DEABH+] = [HCO3−] + 2[CO32 −]

(9) (10)

[DEABH+] + [H+] = [HCO3−] + 2[CO32 −] + [OH−] (11)

For the conventional method of determining the ion speciation, the pH method is used to calculate the exact AmH+/Am ratio (where Am = amine).4−7 Then, the ion concentration of free DEAB and DEABH+ can be calculated using the DEABH+ dissociation constant K and pH value at a certain temperature. In addition to the conventional method, a novel 13C NMR analysis methodology was developed based on the fact that the 13 C NMR peaks will shift either upfield or downfield upon protonation. Several researchers have applied 13C NMR analysis to amine−CO2−H2O systems as well, but not for VLE models. They have tested the carbamate stability12 and performed qualitative determinations.13 As for HCO3−/CO32−, the chemical shifts (positions) of the 13C NMR peaks of DEAB/ DEABH+ also represent the relative amounts of initial DEAB and DEABH+. The only difference between the pairs DEABH+/ DEAB and HCO3−/CO32− is that the 13C NMR spectrum of DEABH+/DEAB exhibits six peaks whereas that of HCO3−/ CO32− exhibits only one peak. The calibration curves of 13C NMR spectra for DEABH+/DEAB can be established at various protonation ratios (ratio of the initial DEAB concentration to the DEABH+ concentration). The 13C NMR spectra of the liquid samples were then examined. Thus, the concentrations of DEAB and DEABH+ can be calculated based on the developed NMR calibration curves. Therefore, in the case of the novel 13C NMR calibration method, the pH test and acid dissociation constant Ka are not needed.

3. RESULTS AND DISCUSSION 3.1. DEABH+ Dissociation Constant Analysis Using a pH Meter. The pH values (each of which is an average from four pH tests) of DEAB solution titrated with HCl solution at 24.5 °C are listed in Table S1 (Supporting Information). The values of the DEABH+ dissociation constant K were calculated from eqs 5−7. For example, at 24.5 °C, the DEABH+ dissociation constants K for 0.5−4.0 mL were taken into consideration, with an average value of 3.94 × 10 −11 mol/L (ln KDEABH+ = −23.96). (The subscript 4 in −3.94 × 10−11 mol/L is included to indicate two significant digits obtained from pH measurements using the pH meter. This notation was used to ensure that all of the data were considered to be accurate and reliable.) This value is higher than that of MEAH+ (ln KMEAH+ = −25.62 at 24.5 °C),11 but lower than that of MDEAH+ (ln KMDEAH+ = −23.73 at 24.5 °C).4,10 This result indicates that the binding energy of DEAB with a proton is smaller than that of MEA, but higher than that of MDEA. The same method was applied for DEAB at 35 and 45 °C. The values of the DEABH+ dissociation constant K were found to be 4.50 × 10−11 mol/L (ln K = −23.82) and 7.80 × 10−11 mol/L (ln K = −23.27), respectively. Then, a predictive correlation for the DEABH+ dissociation constant K was established as ln KDEABH+ = −13.46 − 3147.5/ T(K) with R2 = 0.87, valid in the temperature range of 24.5−45 °C. 8611

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Figure 3. Ion speciation (mole fraction) plot of the DEAB−CO2−H2O system at 24.5 °C and an initial DEAB concentration of 1.50 M. Solid lines were obtained from the conventional method of combining pH testing and NMR analysis; dashed lines were obtained from the novel 13C NMR calibration method propsed herein.

figures, and the accuracy is down to 0.001 ppm with our powerful 500 MHz NMR instrument or 0.01 ppm with common NMR instruments. The 0.01 unit error from a pH meter could end up with a significant error in the DEABH+/ DEAB ratio. On the other hand, the 0.01 ppm error is considered to be negligible because three calibration curves were used (the system will minimize error itself). Finally, the calibration method requires some effort in the preparation, but it is much easier in terms of sample analysis, and the novel NMR method generates the results for the speciation of DEABH+/DEAB and HCO3−/CO32− simultaneously, which minimizes the systematic error of stepwise performing pH tests before NMR analysis. This novel method has not been recognized because of two major reasons: (i) pH testing is convenient because acid dissociation constants K are available from literature, and it can provide the AmH+/Am ratio right away. (ii) Only one chemical shift can be used as a calibration curve for several amines. For instance, MEA has a simple structure, with two 13C peaks at 63.7 ppm (CH2OH) and 42.9 ppm (CH2NH2), but only one of them can be used for calibration (Δδ = −4.2 ppm).5 The other 13 C peak hardly changes with protonation (Δδ = −0.8 ppm).5 Under these conditions, one reference curve would be less accurate for analysis. For other amines with more complicated structures (e.g., AMP, DEA, and MDEA), their 13C spectra contain multiple peaks, which can be used as calibration curves. Jakobsen et al.4 also published the 13C chemical shift range of MDEA. For MDEA, the molecule contains three types of carbon peaks, −CH2−OH (δ = 58.7−59.8 ppm), −CH2−N< (δ = 57.3−59.6 ppm), and CH3−N< (δ = 41.7−42.7 ppm). Jakobsen et al.4 already acknowledged the change of δ 13C with protonation (increase CO2 loading and decrease pH of the system). However, they did not perform further analysis of the different types of shifts (upfield and downfield) or determine the relationship of the change in 13C chemical shift upon protonation. 3.4. Ion Speciation Plot of the DEAB−CO2−H2O System Using a Combination of 13C NMR Analysis and pH Testing (Conventional Method). For the conventional

be in the shielded region, decreased as the protonation ratio increased at −1.77, −2.00, and −1.49 ppm, respectively. On the other hand, the chemical shifts of C2, C4, and C5, which are considered to be in the deshielded region, increased as the protonation ratio increased at 0.04, 1.35, and 0.66 ppm, respectively. After comparing the range of peak shifting, the shielded region (C1, C3, and C6) was selected as the calibration set. Even though the range of peak shifting of C4 was comparable to those of C1, C3, and C6, the chemical shift of C4 (deshielded region) was in a different chemical surroundings from those of C1, C3, and C6 (shielded region). Therefore, the chemical shift of C4 was not included in the calibration curve to avoid systematic errors. The correlations of these three calibration curves (C1, C3, and C6) were established (as shown in Figure S1 in the Supporting Information). The average value of the three protonation ratios (which were calculated from the correlations for the C1, C3, and C6 calibration curves) was obtained for use in calculating the exact concentrations of both DEAB and DEABH+. In this novel method, multiple calibration curves were used to increase the accuracy and precision of the VLE model by reducing systematic and operational errors. By using one calibration curve (with seven data points at 0:1, 0.2:1, 0.4:1, 0.6:1, 0.8:1, 1:1, and 1.2:1 protonation ratios), there will be systematic and operational error for the curve [expected value, E(x), might deviate from the real value (μ) with a certain value of the standard variance (S1)]. If three calibration curves (21 data points) are used, the standard variance S2 will be reduced (S2 < S1). This is because the standard variance S is reduced as the number of data points increases as S/n0.5 ≈ σ. (For experiments, S is the standard variance; for the real system, σ is the standard variance. σ is real value that is constant, whereas S is variable and depends on the number of data points.) This novel method for determining the protonation ratio has several advantages. First, this method no longer requires a pH test. The NMR analysis can entirely determine the ratios AmH+/Am and HCO3−/CO32− for most amine−CO2−H2O systems independently. Second, the pH meter has only two significant figures, but NMR spectra have four significant 8612

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Figure 4. Ion speciation (mole fraction) plot of the DEAB−CO2−H2O system at 24.5 °C and an initial DEAB concentration of 1.97 M. Solid lines were obtained from the conventional method of combining pH testing and NMR analysis; dashed lines were obtained from the novel 13C NMR calibration method proposed herein.

method of combining pH testing with 13C NMR analysis, the DEABH+/DEAB ratio was obtained using the pH value and DEABH+ dissociation constant K. Based on this method, the ion speciation (concentrations of DEABH+, DEAB, HCO3−, and CO32−) at 24.5 °C and various CO2 loadings (0−0.8 mol of CO2/mol of amine) and initial DEAB concentrations (1.50 and 1.97 M) are reported in Table S3 (Supporting Information). The results are plotted as solid lines in Figures 3 and 4, respectively. 3.5. Ion Speciation Plot of the DEAB−CO2−H2O System Using a Novel 13C NMR Calibration Method. For the novel 13C NMR calibration method, the ion speciation (concentrations of DEABH+, DEAB, HCO3−, and CO32−) was analyzed using 13C NMR spectroscopy. The results at 24.5 °C and various CO2 loadings (0−0.8 mol of CO2/mol of amine) and initial DEAB concentrations (0.52, 0.98, 1.50, 1.97 M) are reported in Table S4 (Supporting Information). The results (at only 1.50 and 1.97 M) are plotted as dashed lines in Figures 3 and 4, together with the results obtained from the conventional method. By comparing the ion speciation results from the conventional (solid lines) and novel (dashed lines) methods in Figures 3 and 4, it was found that the two methods provide similar speciations with a deviation percentage of 8.2%. However, the experimental VLE results obtained from the novel 13C NMR calibration method seem to be more accurate than those of the conventional method because the 13C NMR calibration technique provides more significant figures than the conventional technique combining pH testing and NMR analysis. [Statistically, the pH meter can generate only two significant figures (e.g., 10.56 ppm, where the last two digits are counted). The NMR instrument can generate four significant figures (e.g., 1.487 ppm).] Therefore, it can be concluded that the systematic error in the novel method should be lower than that in the conventional technique. In the conventional method of combining pH testing and 13 C NMR analysis, the concentrations of HCO3− and CO32− are determined using NMR analysis, whereas the concen-

trations of amine and protonated amine are calculated using a predictive equation of dissociation constant Ka, concentrations of H+ and OH− through a pH test, and mass balance equations (eqs 10 and 11). In the novel method of using a 13C NMR calibration curve, all concentrations of HCO3−, CO32−, amine, and protonated amine can be determined using the NMR analysis. Thus, the pH test and dissociation constant Ka are not needed. As can be seen in Figure S1 (Supporting Information), the chemical shift of DEAB changed as the protonation reaction proceeded. However, this novel technique is not applicable in the case of an amine that has a simple chemical structure, such as MEA. This is because only two 13C NMR peaks can be detected at 63.7 ppm (CH2OH) and 42.9 (CH2NH2) ppm. Moreover, one peak was found to rarely change as protonation proceeded with Δδ = −0.8 ppm.5 This novel technique is suitable for amines that have more complex structures (e.g., MDEA, DEAB, and AMP) because several calibration curves can be used for the aid of more accurate results. In addition, it can be observed from Figures 3 and 4 that the DEAB concentration decreased as the CO2 loading increased. This is because of the CO2−DEAB−water reaction and the DEAB protonation, as shown in eqs 1 and 2, respectively. As a result, the concentration of protonated DEAB, DEABH+, increased as the CO2 loading increased. One of the major products, HCO3−, was found to increase as the CO2 loading increased. As the CO2 loading increased (which means that more CO2 was introduced into the system), HCO3− was formed by CO2 reacting with DEAB−water and CO2 reacting with water, as shown in eqs 1 and 3, respectively. The last major component, CO32−, was found to increase and reach to a maximum, after which it decreased as the CO2 loading increased further. At the lean loading of CO2, the solution contained excess DEAB where the solution was strongly basic (pH > 10). At this point, CO32− was observed to be a major component rather than HCO3−. However, after the introduction of more CO2 (increased CO2 loading), the solution became more weakly basic (pH < 10). Thus, CO32− was converted to HCO3− through the reverse reaction of eq 4. 8613

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After comparing the ion speciation of DEAB with that of conventional tertiary amine (i.e., MDEA), it was found that almost 100% of the DEAB was converted to DEABH+ at a rich CO2 loading of 0.8 mol of CO2/mol of amine, but only 67% of the MDEA was converted to MDEAH+ at the same rich CO2 loading.4 It can be inferred from this observation that DEAB has a stronger proton affinity than MDEA. Therefore, DEAB was found to be a stronger Brønsted base (higher pKa value) than MDEA. (The pKa values at 25 °C of DEAB and MDEA are 10.4 and 10.2, respectively.) As mentioned by Versteeg et al.16 and da Silva and Svendsen,17 the reaction rate constant for CO2 absorption can be simply predicted using pKa, as the higher pKa, the faster the reaction kinetics. Thus, it can be said that DEAB has faster reaction kinetics that MDEA. This observation is in good agreement with our kinetics analysis of DEAB using a laminar jet absorber.18 Moreover, the ion speciation of DEAB was compared with that of a conventional primary amine (i.e., MEA). The results showed that 100% of the MEA was converted to MEAH+ at a CO2 loading of 0.7 mol of CO2/mol of amine.5 Thus, it can be inferred that MEA has a stronger proton affinity (the pKa of MEA is 11.1) than DEAB and MDEA. This is because MEA (which is primary amine) is more reactive than DEAB (which is tertiary amine).

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NOMENCLATURE AMP = 2-amino-2-methyl-1-propanol DEA = diethanolamine DEAB = 4-(diethylamino)-2-butanol KDEABH+ = DEABH+ dissociation constant, mol/L MDEA = methyldiethanolamine MEA = monoethanolamine n = number of data points n0,DEAB = initial DEAB amount, mol nHCl = number of moles of HCl added during the titration, mol S = standard variance for the experimental system T = absolute temperature, K Vtotal = total liquid volume after titration, L

Greek Letters

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δ = chemical shift, ppm σ = standard variance for the real system μ = real value

REFERENCES

(1) Tontiwachwuthikul, P.; Wee, A. G. H.; Idem, R. O.; Maneeintr, K.; Fan, G. J.; Veawab, A.; Aroonwilas, A.; Chakma. A. Method for capturing carbon dioxide from gas streams. U.S. Patent Application 2008/0050296 A1, 2008. (2) Sema, T.; Naami, A.; Idem, R.; Tontiwachwuthikul, P. Correlations for equilibrium solubility of carbon dioxide in aqueous 4-(diethylamino)-2-butanol solutions. Ind. Eng. Chem. Res. 2011, 50, 14008. (3) Maneeintr, K.; Idem, R. O.; Tontiwachwuthikul, P.; Wee, A. G. H. Synthesis, solubilities, and cyclic capacity of amino alcohols for CO2 capture from flue gas streams. Energy Proc. 2009, 1, 1327. (4) Jakobsen, J. P.; Krane, J.; Svendsen, H. F. Liquid-phase composition determination in CO2−H2O−alkanolamine systems: An NMR study. Ind. Eng. Chem. Res. 2005, 44, 9894. (5) Fan, G. J.; Wee, A. G. H.; 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, 2717. (6) Holmes, P. E.; Naaz, M.; Poling, B. E. Ion concentrations in the CO2−NH3−H2O system from 13C NMR spectroscopy. Ind. Eng. Chem. Res. 1998, 37, 3281. (7) Suda, T.; Iwaki, T.; Mimura, T. Facile determination of dissolved species in CO2−amine−H2O system by NMR spectroscopy. Chem. Lett. 1996, 777. (8) Patterson, A.; Ettinger, R. Nuclear magnetic resonance studies of the carbon dioxide−water equilibrium. Z. Elektrochem. 1960, 98.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

We acknowledge the research support over the past many years of the Industrial Research Consortium - Future Cap Phase II of the International Test Centre for CO2 Capture (ITC) at the University of Regina. We also acknowledge the research support from the followings organizations: Natural Sciences and Engineering Research Council of Canada (NSERC), Canada Foundation for Innovation (CFI), Saskatchewan Ministry of Energy & Resources, Western Economic Diversification, Saskatchewan Power Corporation, Alberta Energy Research Institute (AERI), and Research Institute of Innovative and Technology for the Earth (RITE). In addition, we acknowledge the recent research support from the Provincial Government of Hunan, Federal Government of China, as well as Hunan University, to the Joint International Center for CO2 Capture and Storage (iCCS).

4. CONCLUSIONS (1) The DEABH+ dissociation constant was calculated using the pH test technique over a temperature range of 24.5− 45 °C and can be expressed as a function of temperature in Kelvin as ln KDEABH+ = −13.46 − 3147.5/T(K) with R2 = 0.87. (2) A novel calibration method using 13C NMR spetra was successfully developed and applied for the first time to detect AmH+/Am (i.e., DEABH+/DEAB) ratio. This novel method is apparently more accurate than the conventional method of combining pH testing and NMR analysis because the NMR technique provides higher accuracy than the pH testing technique. In addition, the novel method is more convenient because the samples can be subjected to NMR analysis right away and additional pH testing beforehand is not required, which might minimize operational error. (3) Based on the protonation ratio observed from ion speciation, MEA (100% of MEA is protonated at 0.8 CO2 loading) seems to be a better absorbent than DEAB (almost 100% of DEAB is protonated at 0.8 CO2 loading) and MDEA (only 67% of the MDEA is protonated at 0.79 CO2 loading).

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Article

AUTHOR INFORMATION

Corresponding Author

* E-mail: [email protected] (P.T.), [email protected] (Z.L.), [email protected] (R.I.), [email protected] (T.S.). Tel.: +1-306-585-4160 (P.T.), +86-136-184-81627 (Z.L.), +1-306-585-4470 (R.I.), +1-306-502-4989 (T.S.). Notes

The authors declare no competing financial interest. 8614

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(9) Horwitz, W. Association of Official Analytical Chemists (AOAC) Methods, 12th ed.; George Bant: Gaithersburg, MD, 1975. (10) Kamps, Á . P. S.; Maurer, G. Dissociation constant of Nmethyldiethanolamine in aqueous solution at temperatures from 278 to 368 K. J. Chem. Eng. Data 1996, 41, 1505. (11) Kent, R. L.; Eisenberg, B. Better data for amine treating. Hydrocarbon Process. 1976, 87. (12) Ciftja, A. F.; Hartono, A.; da Silva, E. F.; Svendsen, H. F. Study on carbamate stability in the AMP/CO2/H2O system from 13C-NMR spectroscopy. Energy Proc. 2011, 4, 614. (13) Hartono, A.; da Silva, E. F.; Grasdalen, H.; Svendsen, H. F. Qualitative determination of species in DETA−H2O−CO2 system using 13C NMR spectra. Ind. Eng. Chem. Res. 2007, 46, 249. (14) Usubharatana, P. A study of monoethanolamine−methanol hybrid solvents for carbon dioxide capture by absorption. Ph.D. Dissertation, University of Regina, Regina, Saskatchewan, Canada, 2009 (15) Abbott, T. M.; Buchanan, G. W.; Kruus, P.; Lee, K. C. 13C Nuclear magnetic resonance and raman investigation of aqueous carbon dioxide systems. Can. J. Chem. 1981, 60, 1000. (16) Versteeg, G. F.; van Dijck, L. A.; van Swaaij, P. M. On the kinetics between CO2 and alkanolamines both in aqueous and nonaqueous solutions. An overview. Chem. Eng. Commun. 1996, 144, 113. (17) da Silva, E. F.; Svendsen, H. F. Computational chemistry study of reactions, equilibrium and kinetics of chemical CO2 absorption. Int. J. Greenhouse Gas Control 2007, 1, 151−157. (18) Sema, T.; Naami, A.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P.; Shi, H.; Wattanaphan, P.; Henni, A. Analysis of reaction kinetics of a novel reactive 4-diethylamino-2-butanol solvent. Chem. Eng. Sci., revision submitted.

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