Study of Ion Speciation of CO2 Absorption into Aqueous 1

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Study of ion speciation in CO2 absorption into aqueous solution of 1-dimethylamino-2-propanol using the NMR technique Helei Liu, Raphael O. Idem, Paitoon Tontiwachwuthikul, and Zhiwu Liang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b01540 • Publication Date (Web): 09 Jul 2017 Downloaded from http://pubs.acs.org on July 11, 2017

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Industrial & Engineering Chemistry Research

1

Study of ion speciation of CO2 absorption into aqueous 1-Dimethylamino-2-

2

propanol solution using the NMR technique

3

Helei Liu, Raphael Idem*, Paitoon Tontiwachwuthikul, Zhiwu Liang

4

Clean Energy Technologies Research Institute (CETRI), Faculty of Engineering and Applied

5

Science, University of Regina, Regina, Saskatchewan, S4S 0A2, Canada

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38

*Authors for correspondence: Dr Raphael Idem; Phone: +1-306-585-4470; Fax: +1306-585-4855; Email: [email protected] (R.I.)

1

ACS Paragon Plus Environment


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Abstract

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In this work, the speciation (i.e. 1DMA2P molecule as well as 1DMA2PH+, HCO3-, and

41

CO32- ions) for the CO2 reactive absorption of CO2 in aqueous 1-Dimethylamino-2-propanol

42

solution (i.e. into 1DMA2P-H2O-CO2 system) was studied using the 13C NMR technique at the

43

temperature of 301 K over the 1DMA2P concentration range of 0.5-2.0 mol/L, and CO2 loading

44

range of 0-1.0 mol CO2/mol amine. Also, in addition to other material conservation laws, a new

45

criterion for selection of the protonation calibration curves, the charge balance of the 1DMA2P-

46

H2O-CO2 system, was added in order to generate results with better accuracy. In addition, the

47

equilibrium constants, K1 and K5, were also obtained using the NMR technique, which had good

48

agreement with those from references with AADs of 2.2% and 3.5%, respectively.

49

Key words: Ions speciation plots, 1DMA2P, CO2, NMR, equilibrium constant.

50

51 52 53 54 55 56 57 58

2


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1. Introduction

60

The extensive use of fossil fuels worldwide for power generation is resulting in increased

61

levels of CO2 in the atmosphere, which causes the greenhouse effect leading to global warming

62

and climate change, as manifested in rising sea levels, melting of polar ice, drought, etc. In order

63

to decrease the emissions of CO2 due to human activities, it is essential that a suitable CO2

64

capture technology and reliable resource utilization technology be rapidly developed. The

65

absorption of CO2 into aqueous amine solutions is regarded to be one of the most promising

66

technologies for CO2 capture due to its maturity, cost effectiveness, and capability of handling

67

large amounts of exhaust streams1.

68

Recently, a new tertiary amine, 1-dimethylamino-2-propanol (1DMA2P) has drawn growing

69

attention for its good performance on CO2 capture. As shown in the work of Kadiwala et al.2,

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1DMA2P has faster kinetics than the benchmark commercial tertiary amine (MDEA). Also,

71

Chowdhury et al.3 presented the CO2 cyclic capacities and the absorption rates of 1DMA2P,

72

which were shown to be much higher than those of MDEA. In addition, Liang et al.4 reported

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that 1DMA2P exhibited better mass transfer characteristics than MDEA. Furthermore, it was

74

also observed that the heat of CO2 absorption in 1DMA2P was much lower than other amines

75

(e.g. MEA, DEA, PZ, and MDEA)5. However, a single tertiary amine (e.g. 1DMA2P) cannot be

76

used on its own for commercial application because of its low CO2 absorption rates. It has to be

77

mixed with primary amines or diamines, like MEA and piperazine (PZ), respectively. Also, other

78

performance criteria of 1DMA2P such as vapor-liquid equilibrium (VLE) data, CO2 absorption

79

rate, and mass transfer characteristics must also be investigated before its commercial

80

application. The vapor−liquid equilibrium (VLE) of amine-H2O-CO2 system could be used to

81

test and develop theoretical models and correlations that have vital significance for process

3



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simulation and design of a CO2 treating plant.6 In order to obtain a detailed VLE model of the

83

1DMA2P-H2O-CO2 system, ion speciation and the exact concentrations of cations and anions at

84

different CO2 loadings (VLE plots) are required.7 In addition, ion speciation plots play a

85

significant role in the understanding of the reaction process of 1DMA2P with CO2. All the

86

information is required for the proper design of a CO2 capture plant. The NMR technique is a

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very useful tool to employ for the development of ion speciation plots due to its accuracy and

88

reliability.6,8 The chemical equilibrium constants of the reactions express the relationship

89

between products and reactants of a reaction, which is essential for the development of

90

theoretical models and correlations. These are of great value for building the VLE models that

91

are applied to both process design and simulation for a gas treating plant.9

92

In our previous work10, bicarbonate/carbonate ions of 1.0 M 1DMA2P-H2O-CO2 system

93

were evaluated at a temperature of 293K and used to study their effect on the heat of

94

regeneration of blended systems by using the NMR technique. In comparison with the previous

95

work, the objective of this present work is to study the speciation of all species (i.e. 1DMA2PH+,

96

1DMA2P, HCO3-, CO32-) using an improved NMR method in order to understand the reaction

97

process of CO2 with 1DMA2P at different conditions. Thus, in this work, CO2 absorption into

98

aqueous 1-Dimethylamino-2-propanol solution was studied in terms of the species produced as a

99

function of CO2 loading. In the 1DMA2P-H2O-CO2 system, 1DMA2P molecule as well as

100

1DMA2PH+, HCO3-, and HCO3- ions were measured by employing the

13

101

which their concentrations were measured at 301K, over the 1DMA2P concentration range of

102

0.5-2.0 mol/L, and the range of CO2 loading 0-1.0 mol CO2/ mol amine. In order to obtain results

103

with better accuracy, a new criterion for selecting the protonation calibration curves was

104

developed by checking the charge balance of the ionic species in the 1DMA2P-H2O-CO2 system.

C NMR technique of

4


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In addition, the equilibrium constants for K1 and K5 were obtained also by using the NMR

106

technique. A comparison of the calculated values with values from the literature for K1 and K5 in

107

terms of their absolute average deviations (ADDs) was also performed.

108

2. Calculation method

109

As the base-catalyzed hydration mechanism shows,11 a tertiary amine does not react directly

110

with CO2 but it acts as a base that catalyzes the hydration of CO2. Since 1DMA2P is a tertiary

111

amine, the reaction between 1DMA2P and CO2 can be described using this mechanism as shown

112

in the following equations:

113

K1 1DMA2 P + H + ←→ 1DMA2 PH +

(1)

114

K 2 , k2 ,k−2 CO2 + 1DMA2P ← →1DMA2PH + + HCO3−

(2)

115

K3 H 2 O + CO2 ←→ H + + HCO3−

(3)

116

K4 CO2 + OH − ←→ HCO3−

(4)

117

K5 HCO3− ←→ H + + CO32−

(5)

118

K6 H 2 O ←→ H + + OH −

(6)

119

where Ki represent the chemical equilibrium constants of reaction i.

120

As shown from those equations, the main ions in the 1DMA2P-H2O-CO2 system are

121

1DMA2PH+, HCO3-, and CO32-, which are related to the reaction of 1DMA2P with CO2 while

122

the molecular species is 1DMA2P.

123

In addition, the process of 1DMA2P reaction with CO2 could be explained by using Figure

124

1. From the figure, CO2 exists in three forms as: CO2, HCO3-, and CO32- after reacting with

125

1DMA2P in the liquid phase. Correspondingly, 1DMA2P exists in two forms as 1DMA2P and

5



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1DMA2PH+. However, free CO2 concentration in the liquid phase could be negligible due to its

127

low concentration.12 Thus, both the molecular and ionic species (1DMA2P, 1DMA2PH+, HCO3-,

128

CO32-) in the system of 1DMA2P-CO2-H2O are considered as important compounds, because

129

they indicate the reaction process of 1DMA2P with CO2. The change of their concentrations as a

130

function of CO2 loading plays a vital role in understanding the reaction process. The

131

technique was employed to determine their concentrations.

132

2.1. Determination of ions concentrations of 1DMA2P, 1DMA2PH+, HCO3- and CO32- in

133

the 1DMA2P-H2O-CO2 system by using 13C NMR technique

134

In this work, the

13

13

C NMR

C NMR technique was employed to determine the concentrations of

135

1DMA2P, 1DMA2PH+, HCO3- and CO32-. The concentrations of 1DMA2P and 1DMA2PH+ can

136

be determined by using the protonation calibration curve equations of 1DMA2P and the total

137

1DMA2P mass balance. The mass balance of 1DMA2P is shown as follows:

138

[1DMA 2 P ] 0 = [1DMA 2 P ] + [1DMA 2 PH + ]

139

(7)

Where [1DMA2P]0 is the initial concentration of 1DMA2P.

140

The protonation calibration curve equations of 1DMA2P solution are obtained as described

141

below. Essentially, the concentrations of HCO3- and CO32- can be represented as in equations (8)

142

and (9)8, 13,14:

143

.84 − δ ) [HCO ] = (168(168 [CO ] .84 − 161.23)

(8)

144

.23) [CO ] = (168(δ.84− 161 [CO ] − 161.23)

(9)

145

where δ is the chemical shift of HCO3-/CO32-; [CO2]0 is the total concentration of CO2 in the

−

3

2 0

2−

3

2 0

6


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1DMA2P-H2O-CO2 system; 168.84 and 161.23 are the chemical shift of solely Na2CO3 and

147

NaHCO3 aqueous solution, respectively.

148

As the concentration of physically absorbed (free) CO2 is very small, the free CO2 was

149

considered negligible, and was not added in the mass balance equation of carbon. Thus, based on

150

the mass balance of carbon, [CO2]0 can be expressed as in equation (10):

151

[CO2 ]0 = [ HCO3 − ] + [CO3 2− ]

152

3. Chemicals and experiment

153

3.1. Chemical

(10)

154

Reagent grade 1-dimethylamino-2-propanol (1DMA2P) with purity of ≥ 99 wt% was

155

obtained from Sigma-Aldrich. The structure of 1DMA2P is shown in Figure 2. The amine

156

solutions were prepared to the desired concentrations using deionized water. The CO2 gas was

157

obtained from Praxair Inc., Regina, Canada.

158

3.2. The protonation calibration curves of 1DMA2P solution by using 13C NMR

159

Six samples with different protonation ratios ( n a min eH / n a min e = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0)

160

were prepared by adding 1.0 mol/L HCl solution into 1.0 mol/L amine solution. Then, 10wt% of

161

D2O and 5wt% of 1, 4-Dioxane were added into every sample. D2O and 1, 4-Dioxane were

162

added to provide a field-frequency lock and chemical shift reference, respectively. The sample

163

was sent to measure the chemical shifts (δ) of the different carbon atoms of amine by using 13C

164

NMR spectroscopy (Bruker AVANCE 500MHz NMR Spectrometer). The chemical shifts (δ) of

165

different carbon atoms at different protonation ratios were obtained, which were used to develop

166

protonation calibration curves.

+

7



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3.3. CO2 loading measurement and 13C NMR detection.

168

The CO2 absorption apparatus used was the same as in our previous work4 as shown in Figure

169

3. As shown in the figure, this apparatus is made up of 5 parts, namely, a CO2 cylinder, mass

170

flow meter, dryer, saturator, and a three-necked flask reactor. Initially, the CO2 gas from cylinder

171

with the desired flow rate was monitored by using the mass flow meter. Then, the gas was passed

172

through the dryer before passing through the saturator. The dryer was used to dry the saturated

173

backflow gas in order to protect the flow meter. A 100 mL of the aqueous amine solution was

174

introduced into a 150 mL three-necked flask, which was immersed into water bath with the

175

magnetic stirrer at the desired temperature. The CO2 gas flow was then introduced into the

176

reactor to react with CO2. Every five minutes, a 2 mL sample was taken to analyse for CO2

177

loading. The CO2 loading (α) was measured by titration with a Chittick CO2 analyser15. The pH

178

of each of the amine samples was measured with a pH meter. In addition, another 600 µL

179

solution was taken into the NMR sample tube. Drops of D2O (10wt %) as the signal lock and

180

1,4-Dioxane (5wt%, δ=67.19 ppm) as the internal reference were added into the NMR sample

181

tube. Then, the NMR sample tube was sent for

182

500MHz NMR Spectrometer) measurement.

183

4. Results and discussion

184

4.1. The development protonation calibration curves of 1DMA2P solution

13

C NMR spectroscopy (Varian Mercury Inova

185

In this work, the chemical shifts (δ) of four different carbon atoms of 1DMA2P amines were

186

measured at different protonation ratios ( n a min eH / n a min e = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0) using the

187 188

+

13

C NMR technique. As shown in Figure. 1, 1DMA2P has 4 different carbon atoms with 4

different chemical shifts. The chemical shifts for the different carbon atoms are shown in Table 1. 8


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189

As shown in the table, the chemical shift (δ) for the four different carbons changed as function of

190

protonation ratio. The change for different carbons were found to be: C1 for 2.49, C2 for 3.33, C3

191

for 1.71, C4 for 0.98, respectively. The chemical shifts of these carbon atoms decreased as the

192

protonation ratio increased. This is because the chemical shifts are affected by the addition of H+.

193

In order to correlate the results, a linear regression was used to fit the experimental data which

194

fitted the results very well. Thus, the linear relationships between the protonation ratios and the

195

chemical shift can be considered as the protonation calibration curves which can be used to

196

determine the protonation ratio of the amine. However, there are four curves, which could be

197

used as the protonation calibration curve. One of them should be selected as the protonation

198

calibration curve to determine the ratio of amine and protonated amine. According to the work of

199

Shi et al6, the protonation calibration curves should be selected on the basic of a wide range of

200

chemical shifts and good linear fitting.

201

In the 1DAM2P-H2O-CO2 system the charge balance of all ions needs to be considered. In this

202

work, the selection of the protonation calibration curves was developed by also checking the

203

charge balance of the 1DMA2P-H2O-CO2 system. Consequently, the concentrations of all ions

204

(H+, OH-, 1DMA2PH+, HCO3-, CO32-) should be known. Firstly, the chemical shifts of all

205

carbons in the system of 1DMA2P-CO2-H2O were obtained by using the NMR technique as

206

functions of CO2 loading. All the four protonation calibration curves were used to determine the

207

ratio of protonation 1DMA2P. By combing the four protonation calibration curves with the

208

1DMA2P balance, the concentration of 1DMA2PH+ could be measured as a function of CO2

209

loading. The concentration of CO32- and HCO3- were calculated by using equations 8 and 9. The

210

concentration of H+ could be calculated by using equation (11)：

211

[ H + ] = − log( pH )

(11)

9



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The pH values of the 1DMA2P-CO2-H2O system are shown in Table 2. The concentration of

213

OH- was obtained using equation (12):

214

K 6 = [ H + ][OH −]

(12)

215

Where K6 is the equilibrium constant of water.

216

The charge balance of 1DMA2P-H2O-CO2 system was verified with equation (13):

217

[ H + ] + [1DMA2 PH + ] = [OH − ] + [ HCO3 ] + [CO3 ]

218

All the results of charge balances extracted from the four protonation calibration curves are

219

shown in Figure 4. As shown in the figure, the charge of this system did not show a zero balance.

220

This is because of the errors from the experiments. As shown in Figure 4, the charge balance

221

extracted from Carbon 4 shows a better result than other three carbons. It means that the

222

protonation calibration curves calibration curve based on carbon 4 should be considered to

223

determine the protonation calibration curves ratio of 1DMA2P. In order to produce accurate

224

results, the protonation calibration curve of carbon 4 was used to determine the concentrations of

225

1DMA2P and 1DMA2PH+, as given in equation (14):

−

2-

226

C4: δ = 45.279 - 0.01607 x × 100

227

where x is the ratio of 1DMA2P concentration to 1DMA2PH+ concentration

228

(13)

(14)

4.2. Validation of NMR technique used in this work

229

Before using the NMR technique for this work, a validation was performed in terms of CO2

230

loading. As is well known, the speciation quantification could be calculated on the basis of the

231

area of its peak. The CO2 loading could be represented as the ratio of the sums of the integral

232

areas for HCO3- and CO32- and the sums of the integral areas for 1DMA2P and 1DMA2PH+. In

233

this work, the CO2 loading was calculated by using equation (15)16, 17:

10


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( AHCO − + ACO 2 − ) × 4 3

3

234

CO2 loading =

235

where A is the area of the peak for each species.

A1DMA2 P + A1DMA2 P +

(15)

236

In the analysis of NMR, the peaks of HCO3- and CO32- as well as the peaks of 1DMA2P and

237

1DMA2PH+ are respectively indistinguishable, because the proton exchange is dramatically

238

fast18-20. Thus, the proton-exchanges are given as sums represented by using the integral areas.

239

Thus, CO2 loading could be calculated as in equation (16):

240

CO2 loading =

AHCO − / CO 2 − × 4 3

3

(16)

A1DMA2 P / 1DMA2 PH +

241

The CO2 loading was calculated using the NMR technique. The calculated values for CO2

242

loading extracted from the NMR technique were compared with the experimental values of CO2

243

loading from the titration technique. All the results are presented in Figure 5. From the figure, it

244

could be seen that the calculated values from NMR technique have good agreement with the

245

values from the titration technique with an AAD of 4.0%. Thus, the NMR technique used in this

246

work is reliable and accurate.

247

4.3. The ion speciation plots of 1.0 mol/L 1DMA2P-CO2-H2O system at 301K by using 13C

248

NMR technique

249

The chemical shifts and pH values of 1.0 mol/L1DMA2P-CO2-H2O system were determined

250

at the temperature of 301K, the CO2 loading range of 0-0.83 mol CO2/ mol amine. All the results

251

are shown in Figure 6 and Table 2. From Table 2, it can be seen that the pH values of 1DMA2P

252

solutions decreased as a function of CO2 loading over the concentration of 0.5-2.0 mol/L. This is

253

mainly because the more CO2 (an acid gas) that is introduced in the amine solution, the more

254

acidic (i.e. less basic) the solution becomes. From Figure 6, it can be seen that the chemical shift 11



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Page 12 of 34

255

of the different carbons of 1DMA2P change as a function of CO2 loading. This is mainly because

256

1DMA2P receives H+ to be 1DMA2PH+ which affects the chemical shift. As the CO2 loading

257

increases, more 1DMA2PH+ is produced, which leads to the decrease in chemical shift. As

258

shown in Figure 6, the chemical shift of CO32-/HCO3- appears once CO2 is introduced in the

259

1DMA2P solution. As 1DMA2P is a tertiary amine, the reaction of 1DMA2P with CO2 produces

260

carbonate and bicarbonate ions but not the carbomate. The ion concentrations of 1DMA2P,

261

1DMA2PH+, HCO3- and CO32- were calculated using the 13C NMR method over the CO2 loading

262

range of 0-0.83 mol CO2/mol amine, at 301K, at the concentration of 1.0 M. All the results are

263

plotted in Figure 7. All results are used to establish the ion speciation plots of 1DMA2P-CO2-

264

H2O system. From the figure, it is clearly seen that the concentration of free 1DMA2P decreased

265

with increasing CO2 loading, which is mainly caused by the reaction of 1DMA2P solution with

266

CO2 and the appearance of the protonated 1DMA2P. As a result, the concentration of

267

1DMA2PH+ (protonated 1DMA2P) increased gradually with an increase in CO2 loading. As one

268

of the principal products, HCO3− increased as the CO2 loading increased. However, the

269

concentration of CO32− (another product) was not straightforward, as it increased with an

270

increased CO2 loading at low CO2 loading but then after reaching a maximum, it decreased with

271

further increase in CO2 loading. This is mainly attributed to the existence of an excess amount of

272

1DMA2P at the low CO2 loading, which led to the increase in CO32- concentration. At higher

273

CO2 loading, the decrease of free 1DMA2P resulted in a weaker basic solution so that the CO32-

274

concentration decreased by converting to the HCO3- through the reverse of reaction (5).

275

However, CO32− was observed to be a major component rather than HCO3− at the lower

276

CO2 loading (CO2 loading < 0.20 mol CO2/mol amine). This is mainly attributed to the change of

277

the pH of the solution. At the low CO2 loading, the solution pH and the concentration of free

12


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278

1DMA2P were so high that reactions (2), (3), (4) and (5) were greatly promoted. Hence, the

279

concentration of CO32- increased significantly. At higher CO2 loading, the decrease of free

280

1DMA2P resulted in a lower pH value and weaker basic solution. Thus, the concentration of

281

CO32- decreased by the conversion of this ion to HCO3- through the reverse reaction of reaction

282

(5). This leads CO32− to accept a proton and convert to HCO3−, which results in a decrease in the

283

concentration of CO32−, and HCO3− becoming the major component rather than CO32−.

284

4.4 Comparison of ions speciation of 1.0 M 1DMA2P-H2O-CO2 system obtained from two

285

different methods.

286

There are different methods (such as calculation method based on equilibrium constants4,

287

NMR technique6, pH+NMR method21), which could be employed to evaluate the speciation of

288

the 1DMA2P-H2O-CO2 system. In the previous work, the speciation plots of 1DMA2P,

289

1DMA2PH+, HCO3-, and CO32- were developed by using the calculation method with the

290

concentration 1DMA2P of 2M and the temperature of 298K. By using the calculation method,

291

concentrations of all species were obtained from the equilibrium constants in the equations.

292

However, ions speciation of 1DMA2P in this work was obtained by using the experimental

293

method. The concentrations of all species (ions and molecular) for different conditions were

294

determined based on the NMR results. In order to give a clearer picture, the ions speciation of

295

1DMA2P-H2O-CO2 under the same conditions (301K, 1.0M) are shown as a comparison

296

between the NMR method and the calculation method. All the results are shown in Figures 7 and

297

8. By comparing those two figures, it can be seen that the concentrations of species obtained

298

from the two different methods at the same CO2 loading are different. The experimental results

299

obtained from NMR method gives better accuracy, because the NMR gives lower systemic error.

300

13



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

301 302

Page 14 of 34

4.5 The ion speciation plots of 1DMA2P-CO2-H2O system at 301K by using 13C NMR technique

303

The species concentrations of 1DMA2P, 1DMA2PH+, HCO3- and CO32- in 1DMA2P-CO2-

304

H2O system over the concentration range of 0.5-2.0 mol/L and CO2 loading range of 0-1.0 mol

305

CO2/mol amine were calculated using the 13C NMR method, at temperature of 301K are plotted

306

in Figures 9-11 to establish the ion speciation plots of 1DMA2P-CO2-H2O system. From Figure

307

9-11, it can be seen that the concentration of free 1DMA2P decreased with increasing CO2

308

loading for the three systems. This could be explained by the reaction of 1DMA2P solution with

309

CO2 and the appearance of the protonation of 1DMA2P, which leads to a decrease of free

310

1DMA2P as was discussed previously.

311

4.6 Equilibrium constant extricated from NMR.

312

The chemical equilibrium constants of each reaction expresses the relationships between the

313

products and reactants of the reaction. In this work, the equilibrium constants, K1 and K5, are

314

presented based on the results extracted from NMR.

315

Based on reactions (1) and (5), K1 and K5 could be expressed as in equations (17) and (18),

316

respectively:

317

K1 =

[1DMA2 PH + ] [1DMA2P][ H + ]

(17)

318

K5 =

[ H + ][CO32 − ] [ HCO 3− ]

(18)

319

The values of K1 and K5 at different CO2 loadings and 1DMA2P concentrations can be

320

calculated by using the results extracted from the NMR technique. K1 and K5 for 301K with

321

different CO2 loadings in 1.0 mol/L 1DMA2P solution have been calculated and are presented in

14


Page 15 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


322

Figure 12. From the figure, it can be seen that both K1 and K5 could be considered to be

323

constants with a small change as a function of CO2 loading. Because the chemical equilibrium

324

constants represent the chemical equilibrium status of the reaction no matter the amount of

325

reactants and products. Thus, the values of K1 and K5 of 1.0 mol/L 1DMA2P were obtained by

326

averaging all values of K1 and K5 at different CO2 loading in order to obtain better accuracy. The

327

procedures for the calculation for K1 and K5 for 1.0 mol/L 1DMA2P, K1 and K5 for 0.5 mol/L,

328

1.5mol/L, and 2.0mol/L are presented in Table 3. From the Table 3, it can be seen that the value

329

of K1 and K5 for different concentrations are almost same. The absolute average deviations

330

(ADD) between the value for different concentrations and the average value for K1 and K5 were

331

0.5% and 1.5%, respectively. These are acceptable. Thus, K1 and K5 at different concentration

332

are considered as constants. In this case, the equilibrium constants, K1 and K5, are not related to

333

concentration, and are only related to temperature, which is confirmed by Kent and Eisenberg22.

334

The calculated values of K1 and K5 at the temperature of 301K are also compared with values

335

from the literature2,22,23 and presented in terms of AAD. It was found that the calculated values of

336

K1 and K5 have an excellent agreement with literature values with AADs of 2.2% for K1 and 3.6%

337

for K5, respectively. K1 and K5 obtained from this work are therefore reliable. Therefore, the

338

method of obtaining equilibrium constant by using the NMR method could be used to obtain the

339

equilibrium constants of CO2 absorption in amine systems.

340

341

5. Conclusions

342

In this present work, species (1DMA2P, 1DMA2PH+, HCO3-, CO32-) and the speciation plots for

343

the 1DMA2P-H2O-CO2 system were developed at the temperature of 301K, over 1DMA2P

344

concentration of 0.5-2.0mol/L, and over the CO2 loading range of 0-0.83 mol CO2/mol amine.

15



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 34

345

The species (1DMA2P, 1DMA2PH+, HCO3-, CO32-) concentrations for the 1DMA2P-CO2-H2O

346

system were determined based on the

347

protonation calibration curve was also developed by considering the charge balance in the

348

1DMA2P-H2O-CO2 system in order to give better accuracy. In addition, the equilibrium

349

constants, K1 and K5, were also obtained by using the NMR technique. The results from this

350

new method for estimating K1 and K5 gave good agreement with results from the literature with

351

AADs of 2.2% and 3.5%, respectively for the 1DMA2P-H2O-CO2 system. Therefore, the NMR

352

technique is a suitable and reliable method to obtain chemical equilibrium constants.

353

Acknowledgements

354

The financial support from the National Natural Science Foundation of China (NSFC-Nos.

355

21536003, 21476064, U1362112, 21376067 and 51521006), National Key Technology R&D

356

Program (MOST-No.2014BAC18B04), Innovative Research Team Development Plan(MOE-

357

No.IRT1238), Specialized Research Fund for the Doctoral Program of Higher Education (MOE-

358

No. 20130161110025), Key Project of International &Regional Cooperation of Hunan Provincial

359

Science and Technology plan (2014WK2037), the China Outstanding Engineer Training Plan for

360

Students of Chemical Engineering & Technology in Hunan University (MOE-No.2011-40), the

361

China Scholarship Council (CSC), and the Natural Science and Engineering Research Council

362

of Canada (NSERC) is gratefully acknowledged.

13

C NMR technique. A new method for selecting the

363

364

References

365 366 367 368

1. D'Alessandro, D. M.; Smit, B.; Long, J. R., Carbon dioxide capture: prospects for new materials. Angewandte Chemie International Edition 2010, 49, (35), 6058-6082. 2. Kadiwala, S.; Rayer, A. V.; Henni, A., Kinetics of carbon dioxide (CO2) with ethylenediamine, 3-amino-1-propanol in methanol and ethanol, and with 1-dimethylamino-216


Page 17 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414


propanol and 3-dimethylamino-1-propanol in water using stopped-flow technique. Chemical Engineering Journal 2012, 179, 262-271. 3. Chowdhury, F. A.; Yamada, H.; Higashii, T.; Goto, K.; Onoda, M., CO2 capture by tertiary amine absorbents: a performance comparison study. Industrial & Engineering Chemistry Research 2013, 52, (24), 8323-8331. 4. Liang, Y.; Liu, H.; Rongwong, W.; Liang, Z.; Idem, R.; Tontiwachwuthikul, P., Solubility, absorption heat and mass transfer studies of CO2 absorption into aqueous solution of 1-dimethylamino-2-propanol. Fuel 2015, 144, 121-129. 5. Liu, H.; Gao, H.; Idem, R.; Tontiwachwuthikul, P.; Liang, Z., Analysis of CO2 solubility and absorption heat into 1-dimethylamino-2-propanol solution. Chemical Engineering Science, http://dx.doi.org/10.1016/j.ces.2017.02.032., 2017. 6. 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. Industrial & Engineering Chemistry Research 2012, 51, (25), 8608-8615. 7. Liu, H.; Luo, X.; Liang, Z.; Tontiwachwuthikul, P., Determination of Vapor–Liquid Equilibrium (VLE) Plots of 1-Dimethylamino-2-propanol Solutions Using the pH Method. Industrial & Engineering Chemistry Research 2015, 54, (17), 4709-4716. 8. 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. Industrial & Engineering Chemistry Research 2014, 53, (20), 8577-8591. 9. Shen, K. P.; Li, M. H., Solubility of carbon dioxide in aqueous mixtures of monoethanolamine with methyldiethanolamine. Journal of chemical and Engineering Data 1992, 37, (1), 96-100. 10. Zhang, R.; Liang, Z.; Liu, H.; Rongwong, W.; Luo, X.; Idem, R.; Yang, Q., Study of Formation of Bicarbonate Ions in CO2-Loaded Aqueous Single 1DMA2P and MDEA Tertiary Amines and Blended MEA–1DMA2P and MEA–MDEA Amines for Low Heat of Regeneration. Industrial & Engineering Chemistry Research 2016, 55, (12), 3710-3717. 11. Donaldson, T. L.; Nguyen, Y. N., Carbon dioxide reaction kinetics and transport in aqueous amine membranes. Industrial & Engineering Chemistry Fundamentals 1980, 19, (3), 260-266. 12. Aroonwilas, A.; Veawab, A.; Tontiwachwuthikul, P., Behavior of the mass-transfer coefficient of structured packings in CO2 absorbers with chemical reactions. Industrial & engineering chemistry research 1999, 38, (5), 2044-2050. 13. Holmes, P. E.; Naaz, M.; Poling, B. E., Ion concentrations in the CO2−NH3−H2O system 13 from C NMR spectroscopy. Industrial & engineering chemistry research 1998, 37, (8), 32813287. 14. Suda, T.; Iwaki, T.; Mimura, T., Facile determination of dissolved species in CO2-amineH2O system by NMR spectroscopy. Chemistry letters 1996, 25, (9), 777-778. 15. Horwitz, W., Association of official analytical chemists (AOAC) methods. George Banta Company, Menasha, WI 1975. 16. Ciftja, A. F.; Hartono, A.; Svendsen, H. F., Experimental study on carbamate formation in the AMP–CO2–H2O system at different temperatures. Chemical Engineering Science 2014, 107, 317-327. 17. Ciftja, A. F.; Hartono, A.; Svendsen, H. F., 13C NMR as a method species determination in CO2 absorbent systems. International Journal of Greenhouse Gas Control 2013, 16, 224-232. 17



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432

Page 18 of 34

18. Abbott, T. M.; Buchanan, G. W.; Kruus, P.; Lee, K. C., 13C nuclear magnetic resonance and Raman investigations of aqueous carbon dioxide systems. Canadian Journal of Chemistry 1982, 60, (8), 1000-1006. 19. Hartono, A.; da Silva, E. F.; Grasdalen, H.; Svendsen, H. F., Qualitative determination of species in DETA−H2O−CO2 system using 13C NMR spectra. Industrial & engineering chemistry research 2007, 46, (1), 249-254. 20. Jakobsen, J. P.; Krane, J.; Svendsen, H. F., Liquid-phase composition determination in CO2−H2O−alkanolamine systems: An NMR study. Industrial & engineering chemistry research 2005, 44, (26), 9894-9903. 21. Jakobsen, J. P.; Krane, J.; Svendsen, H. F., Liquid-phase composition determination in CO2-H2O-alkanolamine systems: An NMR study. Industrial & engineering chemistry research 2005, 44, (26), 9894-9903. 22. Kent, R. L.; Eisenberg, B., Better data for amine treating. Hydrocarbon process 1976, 55, (2), 87-90. 23. Chang, Y.-C.; Leron, R. B.; Li, M.-H., Equilibrium solubility of carbon dioxide in aqueous solutions of (diethylenetriamine+ piperazine). The Journal of Chemical Thermodynamics 2013, 64, 106-113.

433

18


Page 19 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


434

Figures Captions

435

Figure 1. The reaction process of CO2 reaction with 1DAM2P solution.

436

Figure 2 The molecular structure of 1DMA2P with specific carbon groups (C1-C4)

437

Figure.3. The CO2 absorption apparatus in this work.

438

Figure 4. The charge balance of the system of 1DMA2P-H2O-CO2 for different 4 carbons

439

Figure 5. Parity plot for CO2 loading in 1.0 mol/L aqueous 1DMA2P solution obtained by using

440

titration and NMR.

441

Figure 6. Chemical shift of 1DMA2P-H2O-CO2 system as function of CO2 loading.

442

Figure 7. Ion speciation (concentration) plots in the system of 1DMA2P-H2O-CO2 at the

443

temperature of 298K and 1DMA2P concentration of 1.0 mol/L by using NMR technique.

444


445

temperature of 301K and 1DMA2P concentration of 1.0 mol/L by using the calculated method.

446


447

temperature of 301K and 1DMA2P concentration of 0.5 mol/L.

448


449


450


451


452

Figure 12. K1 and K5 as function of CO2 loading in 1.0 mol/L 1DMA2P-CO2-H2O system.

453 454 455 456

19



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 34

457

458 459

Figure 1. The reaction process of CO2 reaction with 1DAM2P solution.

460 461 462 463 464 465 466 467 468 469

20


Page 21 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


470 471

Figure 2 The molecular structure of 1DMA2P with specific carbon groups (C1-C4).

472 473

Figure.3. The CO2 absorption apparatus in this work.

474 475 476

21



0.06

0.04

0.02

Charge

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 34

0 0

0.1

0.2

0.3

0.4

0.5

0.6

-0.02

0.7

0.8

0.9

C1 C2 C3 C4

-0.04

-0.06

-0.08

CO2 loading (mol CO2/mol amine)

477 478

Figure 4. The charge balance of the system of 1DMA2P-H2O-CO2 for different 4 carbons

22


Page 23 of 34

1.00

AAD=4.0%

0.80 CO2 loading by Titration

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.60

0.40

0.20

0.00 0

0.2

0.4

0.6

0.8

1

CO2 loading by NMR

479 480

Figure 5. Parity plot for CO2 loading in 1.0 mol/L aqueous 1DMA2P solution obtained by using

481

titration and NMR.

23



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 34

482 483

Figure 6. Chemical shift of 1DMA2P-H2O-CO2 system as function of CO2 loading.

484 485 486 487 488

24


Page 25 of 34

1.2

Concentration (mol/L)

1 0.8 0.6 0.4 0.2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

CO2 loading (mol CO2/mol amine) 1DMA2P

1DMA2PH+

HCO3-

CO32-

489 490

Figure 7. Ion speciation (concentration) plots in the 1DMA2P-H2O-CO2 system at the

491

temperature of 301K and 1DMA2P concentration of 1.0 mol/L by using the calculated method

492

by using NMR method.

493 1.2 1

Concentration (mol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4 0.2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9


494 495

1DMA2PH+

HCO3-

CO32-

Figure 8. Ion speciation (concentration) plots in the system of 1DMA2P-H2O-CO2 at the 25



496

temperature of 301K and 1DMA2P concentration of 1.0 mol/L by using the calculated method.

497 498 499 500 501 502 0.6

Concentration(mol/L)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 34

0.5 0.4 0.3 0.2 0.1 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


1DMA2PH+

HCO3-

CO32-

503 504


505


506 507 508 509 510 511

26


Page 27 of 34

1.8 1.6


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1.4 1.2 1 0.8 0.6 0.4 0.2 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


1DMA2PH+

HCO3-

CO32-

512 513


514


515 516 517 518 519 520

27



2.5

2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 34

1.5

1

0.5

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1


1DMA2PH+

HCO3-

CO32-

521 522


523


524 525 526 527 528 529

28


Page 29 of 34

12 10 8

-log(K)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6 4 2 0 0

0.2

0.4

0.6

0.8

1

1.2

CO2 loading (mol CO2/mol amine) -log(K1)

-log(K5)

530 531

Figure 12. K1 and K5 as function of CO2 loading in 1.0 mol/L 1DMA2P-CO2-H2O system.

532 533 534 535 536 537 538 539 540 541 542 543 544 545

29



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

546

Table Captions

547

Table 1. Chemical shift of different carbons as function of protonation

548

Table 2. The pH values of 1DMA2P solution as function of CO2 loading over the 1DMA2P

549

concentration of 0.5-2.0mol/L.

550

Table 3. K1 and K5 value of different 1DMA2P-CO2-H2O systems.

Page 30 of 34

551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568

30


Page 31 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

569


Table 1. Chemical shift of different carbons as function of protonation Chemical shift Protonation (%) C1

C2

C3

C4

0

66.45

65.55

21.53

45.27

20

65.92

64.87

21.33

44.93

40

65.39

64.15

21.11

44.59

60

64.85

63.44

20.9

44.25

80

64.35

62.78

20.71

43.95

100

63.96

62.22

20.55

43.56

Change

2.49

3.33

1.71

0.98

570 571 572 573 574 575 576 577 578 579

31



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 34

580

Table 2. The pH values of 1DMA2P solution as function of CO2 loading over the 1DMA2P

581

concentration of 0.5-2.0mol/L. 0.5mol/L

1.0mol/L

1.5mol/L

2.0mol/L

loading

pH

loading

pH

loading

pH

loading

pH

0.00

11.63

0.000

11.61

0

11.8

0.000

11.81

0.11

10.28

0.06

10.48

0.12

10.23

0.12

10.26

0.15

10.16

0.15

10.09

0.24

9.94

0.23

9.94

0.32

9.86

0.27

9.80

0.42

9.59

0.33

9.78

0.42

9.69

0.35

9.66

0.53

9.40

0.42

9.60

0.57

9.47

0.46

9.46

0.67

9.03

0.52

9.41

0.65

9.31

0.54

9.31

0.78

8.76

0.65

9.15

0.72

9.07

0.65

9.10

0.91

8.06

0.77

8.84

0.85

8.83

0.74

8.85

0.96

7.78

0.86

8.56

0.96

8.33

0.83

8.26

0.91

8.20

0.94

7.94

582 583 584 585 586 587 588 589 590 591 592 593 32


Page 33 of 34

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

594


Table 3. K1 and K5 value of different 1DMA2P-CO2-H2O systems. Concentration

-log(K1)

-log(K5)

0.5mol/L

9.73

10.21

1.0mol/L

9.65

9.95

1.5mol/L

9.70

9.70

2.0mol/L

9.60

9.87

Average

9.67

9.93

595

33



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Study of Ion Speciation of CO2 Absorption into Aqueous 1

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