Toward Efficient CO2 Capture Solvent Design by Analyzing the Effect

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Toward Efficient CO2 Capture Solvent Design by Analyzing the Effect of Chain Lengths and Amino Types to the Absorption Capacity, Bicarbonate/Carbamate and Cyclic Capacity Rui Zhang, Qi Yang, ZhiWu Liang, Graeme Puxty, Roger J. Mulder, Joanna E. Cosgriff, Hai Yu, Xin Yang, and Ying Xue Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01951 • Publication Date (Web): 14 Sep 2017 Downloaded from http://pubs.acs.org on September 18, 2017

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Toward Efficient CO2 Capture Solvent Design by Analyzing

2

the Effect of Chain Lengths and Amino Types to the

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Absorption Capacity, Bicarbonate/Carbamate and Cyclic

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Capacity

5 6

Rui Zhang1, Qi Yang2*, Zhiwu Liang1*, Graeme Puxty3, Roger J. Mulder2, Joanna E.

7

Cosgriff2, Hai Yu3, Xin Yang4, Ying Xue4

8 9 1

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Joint International Center for CO2 Capture and Storage (iCCS), Provincial Key

11

Laboratory for Cost-effective Utilization of Fossil Fuel Aimed at Reducing

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Carbon-dioxide Emissions, College of Chemistry and Chemical Engineering, Hunan

13

University, Changsha, Hunan, 410082, P.R. China 2

14

3

15 16 17

CSIRO Manufacturing, Clayton Victoria 3168, Australia

4

CSIRO Energy, Newcastle NSW 2300, Australia

College of Chemistry, Key Lab of Green Chemistry and Technology in Ministry of Education, Sichuan University, Chengdu, Sichuan, 610064, P.R. China

18 19

*CORRESPONDING AUTHORS:

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1. Dr. Zhiwu Liang

21

Tel.: +86-13618481627; fax: +86-731-88573033;

22

E-mail address: [email protected]

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2. Dr. Qi Yang

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Tel.: +61-395452574

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Email address: [email protected]

26 1

ACS Paragon Plus Environment

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Highlights:

28

•

This work provided a guide for designing novel efficient CO2 absorbent.

29

•

The relationship of structure-efficiency for CO2 capture was developed.

30

•

The carbamate stability of carbamate were studied using quantum chemistry.

31

2


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ABSTRACT

32 33

Chemical absorption using aqueous amine-based solutions is the leading method

34

for large-scale CO2 capture in industrial plants. This technology, however, still faces

35

many challenges, in particular the high energy requirements for solvent regeneration,

36

which limit the economic viability of the technology. To guide the development of

37

more energy efficient amine solvents, this work studied the effect of molecular

38

characteristics of diamines, including carbon chain length and type of amino

39

functional group, on CO2 absorption and desorption performance. Six linear terminal

40

diamines (NH2CH2CH2-R, R = NH2, NHCH3, N(CH3)2, CH2NH2, CH2NHCH3 and

41

CH2N(NH3)2)

42

(NH2CH2CH2OH, MEA) and 3-aminopropanol (NH2CH2CH2CH2OH, 3AP), were

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also tested as benchmarks. The CO2 absorption capacity in each amine was measured

44

at 40 °C under atmospheric pressure using different CO2 gas partial pressures. 13C and

45

1

were

investigated

and

two

monoamines,

monoethanolamine

H NMR spectroscopy were used to identify and quantify species present in the

46

CO2-amine-H2O system. Computational modelling was also carried out using

47

Gaussian software to explain the effect of chain length change on the stability of the

48

monocarbamate. The experimental results showed that the chain length extension

49

from C2 to C3 led to a higher CO2 absorption capacity and more bicarbonate formation

50

during the CO2 absorption process, and the computational study results supported this

51

conclusion. In addition, the experimental results also demonstrated that increasing the

52

substitution on one N atom in the tested diamines is favorable for a higher CO2

53

absorption capacity and more bicarbonate formation under a CO2 partial pressure of 3


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101 kPa. Both chain length extension from C2 to C3 and an increase in the number of

55

substituents on one N atom yield better performance in the CO2 desorption with

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regards to the CO2 higher cyclic capacity and faster initial CO2 release rate for the

57

tested amines.

58

Keywords: Carbon dioxide, amine, CO2 capture, diamine

59

4


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1. INTRODUCTION

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Climate change has caused strong concern worldwide and excessive CO2

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emission is considered to be the main contributor to global warming, therefore CO2

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capture and storage has become an important research subject1-3. Normally, there are

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three major technologies to capture CO2 are post-combustion, pre-combustion, and

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oxy-fuel combustion CO2 capture4. The post-combustion technology for CO2

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separation is performed from the flue gas after the fuels completely burned5. The

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integrated gasification combined cycle (IGCC) system is used for the pre-combustion

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technology for separating CO2 from mixture gas, this process can be briefly described

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as follows; the coal gasification generates a mixture of CO and H2 which is then

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reformed as CO2 and H2 by steam, the CO2 is removed before H2 is combusted in the

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combustion chamber of the gas turbine5. In oxy-fuel combustion, coal is burned at the

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high concentration of oxygen condition to produce flue gas with high CO2

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concentration which is almost ready for utilization and/or storage5. Huang et al.

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briefly reviewed the existing method and developing technology of CO2 utilization6.

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The conversion of CO2 into chemical products by using the metal-organic frameworks

76

(MOFs) still faces many challenges such as rigorous reaction conditions, high energy

77

consumption and materials costs7.

78

Among different CO2 capture methods, post-combustion capture using chemical

79

absorbents, especially amine absorbents, is regarded as a feasible method for

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mitigating the CO2 emissions from many industrial sources8-11. The most commonly

81

studied amine absorbent is monoethanolamine (MEA)12-16. Aqueous solutions of other 5


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primary, secondary and tertiary amines and amino acid salts have also been studied to

83

investigate their mass transfer performance, reaction kinetics and energy consumption

84

in

85

demonstrated some challenges for use in CO2 capture processes. For example,

86

primary amines such as MEA have a fast reaction rate with CO2 during absorption but

87

require large energy consumption during CO2 desorption; a tertiary amine such as

88

N-methyldiethanolamine (MDEA) requires less energy for CO2 desorption but has a

89

slow reaction rate with CO2 during absorption. Other challenges for CO2 capture

90

processes include amine degradation, facility corrosion, and the escape of amine or

91

other substances to the environment14, 22-24. These challenges, in particular high energy

92

consumption, result in high CO2 capture costs

93

important to develop better solvents and better processes.

absorption-desorption

processes17-21.

All

of

25

these

absorbents,

however,

. To address these challenges, it is

94

An ideal amine absorbent for CO2 capture should have fast reaction kinetics

95

during CO2 absorption, large CO2 cyclic capacity (the difference between rich amine

96

CO2 loading and lean amine CO2 loading), and low energy consumption during

97

desorption. The performance of amines during CO2 absorption and stripping is

98

strongly related to amine molecular structure. To design and develop better amine

99

absorbents for CO2 capture, it is necessary to understand the effect of amine

100

molecular structure on amine performance in CO2 capture.

101

A molecule with multiple amino groups results in a higher CO2 absorption

102

capacity as more sites for reaction with CO2 are available 26. Yang et al. and Conway

103

et al., have studied a number of specifically designed amine absorbents for CO2 6


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capture

. These designer amines contained two or three amino groups of different

105

types which were spaced by two or three carbons in the same molecules. Their results

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showed that the designer amines demonstrated a significant improvement in CO2

107

cyclic capacity. Despite these studies, no systematic investigation has been carried out

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to investigate the effects of molecular structure changes of diamines on CO2

109

absorption-desorption performance.

110

In this work, six linear diamines with the structure NH2(CH2)n-R were

111

investigated for their absorption-desorption performance (n = 2 or 3; and R = NH2,

112

NHCH3, N(CH3)2). One terminal nitrogen was kept as a primary amino group and the

113

other varied among primary, secondary and tertiary amino groups. In addition to

114

chemical names and abbreviations, the diamines presented in this paper are also given

115

structural codes of mNCxNn (m, n = 1, 2 or 3, which represent the primary, secondary

116

and tertiary amino group respectively; and x = 2 or 3, which represents the chain

117

length with two or three carbons respectively) to emphasize their chemical structure

118

variations. In the current study, CO2 absorption capacity in each amine at the

119

concentration of 2 M for a diamine or 4 M for a monoamine was first measured at

120

40.0 °C under atmospheric pressure using different CO2 gas partial pressures. The

121

proportions of carbamate and bicarbonate/carbonate were determined by the peak

122

areas of their 13C and 1H NMR spectra. The total CO2 loading of each amine solution

123

was determined from the NMR spectroscopic results. The stripping experiments were

124

carried out to evaluate the CO2 removal performance of the amines tested with regards

125

to the CO2 cyclic capacity and initial CO2 release rate. The results were evaluated in 7


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terms of the effect of varying the carbon chain length and the types of amino groups.

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Computational modelling was also carried out for the pair of 1NC2N1 and 1NC3N1 and

128

the pair of 1NC2N2 and 1NC3N2 to improve understanding of the influence of the

129

structural changes on diamine speciation behaviour.

130 131

2. EXPERIMENTAL SECTION

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2.1 Materials and apparatus

133

MEA used was purchased from Merck, and six diamines (Table 1) and 3AP were

134

all from Sigma Aldrich. All the chemicals were used as purchased without further

135

purification. The details of these amines including their structure codes are shown in

136

Table 1. CO2 (>99.9%) and N2 gases (>99.999%) were purchased from BOC.

137

Hydrogen (1H) and carbon (13C) NMR spectra were acquired at 25.0 °C on a

138

Bruker Avance III 400 NMR spectrometer operating at frequencies of 400.13 MHz

139

(1H) and 100.62 MHz (13C). The external chemical shift reference used in this study

140

consisted

141

3-(trimethysilyl)propinonate-2,2,3,3-d4 (TMSPd) (1%, w/v) in a mixture of

142

dioxane-D2O (50% v/v). The trimethylsilyl resonance of TMSPd was defined as 0

143

ppm for 1H NMR and the dioxane resonance as 66.83 ppm for

144

spectra were obtained as the sum of 32 scans with an interscan delay of 3.73 seconds.

145

Inverse-gated 1H-decoupled

146

(zgig30 pulse program, Bruker) as the sum of 32 scans with a minimum pulse delay

147

time (D1) of 70 to 90 seconds which corresponded to a value of (AQ + D1) ≥ 5 ×

of

a

capillary

13

containing

a

solution

of

sodium

13

C NMR. 1H NMR

C NMR spectra were obtained at a pulse angle of 30°

8


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T1max of the slowest relaxing carbon for each sample. The NMR data were processed

149

using Bruker TopSpin 3.5 software.

150

Bronkhorst flow-controllers were used to control gas flow rates in experiments.

151

Table 1. The details of each amine used in this work.

152 Amine

Acronym

Code

Mol Wt

Purity (%)

MEA

1

NC2OH

61.08

99.0

3-Aminopropanol

3AP

1

NC3OH

75.11

99.0

Ethylenediamine

EDA

1

NC2N1

60.10

99.5

MEDA

1

NC2N2

74.13

95.0

2-Dimethylaminoethylamine

DMAEA

1

NC2N3

88.15

99.0

1,3-Propanediamine

1,3-PDA

1

NC3N1

74.13

99.0

N'-Methylpropane-1,3-diamine

MAPA

1

NC3N2

88.15

97.0

3-Dimethylaminopropylamine

DMAPA

1

NC3N3

102.18

99.0

Monoethanolamine

N-Methylethylenediamine

Molecular structure

153 154

2.2 Absorption and desorption experiments

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2.2.1 CO2 absorption capacity measurement

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The CO2 absorption capacity is the maximum CO2 loading at a specific

157

temperature and pressure. The absorption capacity of CO2 in aqueous amine solutions

158

under atmospheric pressure at two CO2 partial pressures (101 kPa and 10 kPa) were

159

measured using the CO2 absorption experimental apparatus (Fig. 1).

160

Amine solutions (2 M for all diamines and 4 M for all monoamines) were

161

prepared by dissolving the appropriate quantity of amine in deionized water in a 9


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volumetric flask. 50 mL of the freshly prepared amine solution was then placed in a

163

jacketed 100 mL pear-shaped flask topped with a cooling condenser (Fig. 1) and

164

thermometer. The solution was stirred with an egg-shaped magnetic stirring bar (900

165

rpm) and heated by the circulation of thermostatted water (40.0 °C) through the flask

166

jacket. The internal temperature of the absorption was maintained at 39.7 ± 0.1 °C.

167

Pure CO2 gas (101 kPa) was humidified by passage through a flask of water at room

168

temperature (25 ± 0.1 °C) and was then bubbled into the sample at a flow rate of 20

169

mL/min for 18 hours to ensure that the amine solutions were fully saturated with CO2.

170

The authors repeated the experiments 6 times using 2 different amines (one

171

monoamine and one diamine). The deviations of the volume change for the 6 runs are

172

-0.11, -0.10, -0.12, -0.14, -0.14 and -0.13 ml, respectively, and the AAD (absolute

173

average deviation) is 1.23% relative to the initial total volume, this is an acceptable

174

errors and that means the volume changes during the absorption process for 18 hours

175

can be neglected.

176

The fully CO2 loaded amine samples under a CO2 partial pressure of 10 kPa were

177

prepared as described above except that the gas composition was changed to 10% CO2

178

in N2 with a total flow rate of 50 mL/min (5 mL CO2/min, 45 mL N2/min) and the

179

amine sample volume reduced to 10 mL.

10


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Figure 1. The CO2 absorption-desorption experiment diagram. 2.2.2 Preparation of samples at different CO2 loadings

183

The amine samples with different CO2 loadings were prepared by mixing the

184

above CO2-rich solution prepared using pure CO2 gas with the corresponding fresh

185

amine solution in a volume ratio of CO2-rich amine/total mixed amine (sum of

186

CO2-rich amine and fresh amine) of 0, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0

187

respectively. These samples were equilibrated for 60 minutes, and then 0.5 mL of each

188

sample was placed in a 5 mm NMR tube for NMR analysis.

189

2.2.3 Desorption process

190

The stripping experiments were also carried out in the apparatus shown in Fig. 1.

191

The experiments started with the preparation of the CO2-rich amine solution using a

192

gas mixture of 10 kPa of CO2 partial pressure in N2 under an atmosphere pressure as

193

described above. After the solutions reached equilibrium, the water flow through the

194

water jacket was isolated and the gas inlet was replaced by a stopper. The amine

195

solution was sealed in the reaction flask and kept at room temperature. A sample was

196

taken for NMR analysis. The temperature of the water bath was increased to 90 °C.

197

Once this temperature was reached the reaction flask was re-equipped with the 11


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198

condenser and the solution was stirred, followed by recommencing the circulation of

199

the thermostatted water reservoir (now 90 °C) through the water jacket; desorption

200

time was recorded from this point. The desorption samples (0.5 mL) were taken from

201

the solution at 2, 5, 10, 20, 30, 45 and 60 min after the start of desorption and

202

transferred to NMR tubes. The samples were analyzed by

203

spectroscopy at 25 °C. The fully loaded amine samples (10 kPa CO2) and the

204

corresponding 60 minute desorption samples were used for the CO2 cyclic capacity

205

calculation.

206

2.3 Computational study of C2 and C3 diamines

13

C and 1H NMR

207

Geometry optimizations were performed at the M06-GD329/6-31+G(d,p) level in

208

water (the solvent used experimentally) solution using the SMD30, 31 solvation model

209

with the keyword “int=ultrafine”. Grimme’s recent studies32, 33 have suggested that it

210

is important to include empirical dispersion corrections with these regularly used

211

functionals. The vibrational frequency outcomes were examined at the same level of

212

theory to confirm stationary points as minima (no imaginary frequencies).

213

Single-point

214

M06-GD3/6-31+G(d,p)-optimized structures at the M06-GD3/6-311++G(2d,2p) level

215

with solvation effects modelled by SMD in water. The thermal corrections evaluated

216

from the unscaled vibrational frequencies at the M06-GD3/6-31+G(d,p) level on the

217

optimized geometries were then added to the (SMD)M06-GD3/6-311++G(2d,2p)

218

electronic energies to obtain the free energies. To obtain further insight into the

219

electronic properties of the present system, natural bond orbital (NBO)34 analyses

energy

calculations

were

carried

12


out

for

all

the

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were performed at the M06-GD3/6-31+G(d,p) level in water solution with the SMD

221

model on selected systems. All calculations were performed with Gaussian 09.

222 223

3. THEORY

224

3.1 Chemistry

225

In terms of the expected chemical species at equilibrium the following chemical

226

reactions were assumed to occur in the diamine-CO2-H2O system studied in this work.

227

The symbol dAm is used in the following equations for both diamines and amino

228

groups to illustrate reactions occurring in the system.

229

Formation of monocarbamate by primary or secondary amino groups:

230

+ + ↔ +

231

Formation of dicarbamate:

232

+ + ↔ + ()

233

Hydrolysis of carbamate:

234

+ ↔ +

(3)

235

() + ↔ +

(4)

236

() + 2 ↔ 2 +

(5)

237

Formation of protonated amine:

238

+ ↔ +

239

+ ↔ + (for diamine)

(1)

(2)

(6)

13


(7)

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240

The monocarbamate or dicarbamate in eqs. (1) - (7) represent all possible

241

carbamate species in the formation and hydrolysis regardless of the amino-group type.

242

The amino-group type details are discussed with the relevant results.

243

3.2 Identification and quantification of species by 13C NMR spectroscopy

244

Quantitative

13

C NMR spectroscopy has been widely used to measure species

245

formed in the mixture of CO2 with aqueous amines35-37. This method is able to

246

identify and quantify carbon-containing species formed during absorption and

247

desorption processes, including bicarbonate/carbonate, and determine the total CO2

248

loading in the sample. It is not possible to distinguish the bicarbonate and carbonate

249

species separately by 13C NMR spectroscopy due to the fast proton transfer reaction 36,

250

hence only one peak appears in the

251

amount of bicarbonate and carbonate. Also, according to the work of Jakobsen, et al.,

252

the bicarbonate concentration is much higher than the carbonate concentration in the

253

highly CO2 loaded butyl-ethanolamine (BEA), methyl-di-ethanolamine (MDEA) and

254

MEA solutions

255

amount of bicarbonate is significantly higher than carbonate at the high CO2 loading

256

in the single or blended amine systems and carbonate can therefore be neglected38, 39.

13

C NMR spectrum which represents the total

36

. Our previous work and that of Shi et al. also suggested that the

257

The ratio of the amount of each species to the total amine is used to quantify

258

speciation of each amine solution under different CO2 loadings. In the example of

259

MEA displayed in Fig. 2, the ratio of carbamate species can be calculated by the ratio

260

of signal a’ or b’ or c versus the sum of signals a and a’ or the sum of signals b and b’.

261

Different calculation methods should give same results, however in practice, the 14


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integral of each carbamate signal varied slightly. To avoid this possible small variation

263

by selection of different signals, the average integral of carbamate was used in the

264

speciation calculation as shown in eqs. (8) - (10). The 13C NMR results for carbamate

265

(Fig. 2(B)) were validated by the 1H NMR results (Fig. 2(C)) and the average absolute

266

deviation (AAD) between these two methods is 0.4% as shown in Fig. 2(D).

267

In addition, the total CO2 loading in the CO2 loaded MEA solution discussed 13

268

above was also determined by HCl titration to compare with that obtained by

269

NMR spectroscopy. The CO2 loading measurement with acid-basic titration method

270

used in this work is proposed by Horwitz et al.40 This method has been used in our

271

previous work and reported by Xiao et al.21 and Zhang et al.41, the details can be

272

found in S1 in the supporting information. The AAD of the CO2 loading obtained

273

from these two methods is presented in Fig. 2(E). The results showed that the total

274

CO2 loading values obtained from NMR spectroscopy were in good agreement with

275

those obtained from acid titration when the CO2 loading was below 0.4 mole

276

CO2/mole amine, but the difference between the results from these two methods

277

appeared when the CO2 loading was higher than 0.4 mole CO2/mole amine. Fig. 2(E)

278

shows that the AAD of CO2 loading between the NMR method (eq. (12)) and the

279

titration method is 1.98% which is an acceptable deviation. In addition, the AAD

280

between these two methods for 3AP, EDA, 1,3-PDA, MEDA, MAPA, DMAEA and

281

DMAPA are 1.10%, 2.55%, 1.52%, 2.45%, 1.9%, 1.59% and 1.90%, respectively.

282

This indicates that the CO2 loading obtained by NMR method is reliable.

283

The molecular structures of each species and the stacked 15


13

C

C NMR spectra for

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each amine used in this work can be found in the S2 in the supporting information

285 286

Figure 2. (A) The molecular structure of possible species in CO2 loaded MEA

287

solution; (B) The stacked 13C NMR spectrum of CO2-MEA-H2O system; (C) The

288

stacked 1H NMR spectrum of CO2-MEA-H2O system; (D) The validation of speciation

289

methods by calculating carbamate; (E) The validation of the NMR method for

290

determining the CO2 loading

291

/

292

293

(8)

( )/ ( )/

(9)

( )/ ( )/

( )/

+

( )/

(10)

294 295

4. RESULTS AND DISCUSSION

296

4.1 Evaluation of CO2 absorption performance

297

In this work, the CO2 absorption capacity of each amine was investigated at 40 °C

298

with 101 kPa (100% CO2) and 10 kPa (10% CO2 in N2) CO2 partial pressure at 16


Page 17 of 35

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Energy & Fuels

13

299

atmospheric pressure. The details of the

C NMR spectra of CO2 loaded-amine

300

solution obtained at 40 °C with different CO2 partial pressure for each amine are

301

shown in S3 in supporting information. The molar CO2 capacity is defined as moles of

302

CO2 per mole of nitrogen in an amine to account for the different numbers of nitrogen

303

atoms in the molecule. The mass basis CO2 absorption capacity is defined as the

304

grams of CO2 per gram of amine (grams CO2/gram amine). Fig. 3 presents the CO2

305

absorption capacity in each amine on both molar and mass basis under the CO2 partial

306

pressures of 101 kPa and 10 kPa. The amine pairs which contain the same amino

307

groups but different chain lengths (C2 or C3) are presented next to each other in Fig. 3.

308

As expected, the absorption capacity of CO2 increased with an increase in CO2 gas

309

partial pressure. The extension of the chain length from C2 to C3 corresponded to a

310

higher CO2 absorption capacity for each pair of the comparable amines (Fig. 3),

311

therefore it can be inferred that the C3 chain improved the CO2 absorption capacity.

312

This conclusion is consistent with the results published by Singh et al.42. The results

313

in Fig. 3 show that diamines which contain the same carbon chain lengths increased in

314

CO2 absorption capacity at high CO2 partial pressure if one of their amino groups

315

changed from primary to secondary or tertiary (an increase of substituent on one N

316

atom). Interestingly, however, diamines of the same chain length containing either one

317

secondary or tertiary amino group, e.g. MEDA (1NC2N2) - DMAEA (1NC2N3) and

318

MAPA (1NC3N2) - DMAPA (1NC3N3), showed a different trend depending upon the

319

CO2 partial pressure used. For example, MEDA (1NC2N2) and DMAEA (1NC2N3) had

320

similar CO2 absorption capacity when pure CO2 was used in the experiments but 17


Energy & Fuels

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

321

DMAEA (1NC2N3) had lower CO2 absorption capacity than MEDA (1NC2N2) when

322

10% CO2 was used. A similar response to CO2 partial pressure change was also

323

displayed by MAPA (1NC3N2) and DMAPA (1NC3N3). This is likely caused by

324

differences in the overall carbamate stability and the pKa values of the amino groups.

325

As the molecular structure of each paired amines are similar so it can be inferred that

326

the pKa of each paired amine are almost same. In addition, it is well known that the

327

secondary carbamate and the dicarbamate are less stable than the monocarbamate.

328

Both of MEDA and MAPA can produce the secondary carbamate and dicarbamate but

329

the DMAEA and DMAPA only can produce the monocarbamate. The secondary

330

carbamate and dicarbamate from MEDA and MAPA are easy to decompose to be free

331

amine and monocarbamate due to H+ even at a low CO2 loading stage. In contrast, the

332

monocarbamate cannot decompose at the low CO2 loading stage due to low level of

333

H+ in the solution. The released free MEDA/MAPA and monocarbamate can

334

continues to absorb CO2 then lead to a high CO2 absorption capacity (at low CO2

335

partial pressure). For DMAEA and DMAPA, because no free amine can be released

336

from their monocarbamate, so a lower CO2 absorption capacity was obtained

337

compared to the MEDA and MAPA at a low CO2 partial pressure. As a result, the

338

different trend of the CO2 absorption capacity obtained in MEDA (1NC2N2) -

339

DMAEA (1NC2N3) and MAPA (1NC3N2) - DMAPA (1NC3N3) at the low CO2 partial

340

pressure (10% CO2). The mass basis results in Fig. 3 also indicated that the tested

341

diamines showed higher CO2 absorption capacity than the corresponding monoamines.

342

EDA (1NC2N1) demonstrated the best mass basis CO2 absorption capacity among the 18


Page 18 of 35

Page 19 of 35

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Energy & Fuels

343

tested diamines because EDA (1NC2N1) has the smallest molecular weight (Table 1)

344

which results in a largest ratio of grams CO2/gram amine.

345 346

Figure 3. The molar/mass based CO2 absorption capacity in each aqueous amine

347

solution

348

Besides the CO2 absorption capacity in aqueous amine solutions, the formation of

349

O

350

bicarbonate (

351

-O

) and carbamate including monocarbamate (

NH2

-

,

O

O

N H

NH2

,

H 2N

N

-

,

O

O O

352

-O

N H

O

N H

OH

-

O

N

) and dicarbamate (

,

-O

OH

N H

,

O

O

N H

-

O-

O

O

O

O

O-

HO

N H

O

H N

N H

-O

N H

,

-O

O

N H

O

OO

O

-

, H N

O

-

,HN 2

O

O N H

N H

N

,

O

O-

O-

-

,

O

N H

N H

N

,

O

N

,

O

N H

N O

O-

)

353

formed in corresponding amines are also critical, not only for total CO2 loading but

354

also for CO2 desorption.

355

In the CO2 stripping process, bicarbonate and carbamate are heated to decompose

356

and release CO2, but the energy required to break the C-N bond of carbamate is more

357

than that required to break the C-O bond of bicarbonate 43. As a consequence of the

358

ease of decomposition, the amines which generate more bicarbonate during absorption 19


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Page 20 of 35

359

should benefit the CO2 stripping process. It is known that, in CO2 absorption, one

360

monoamine such as MEA (1NC2OH) and 3AP (1NC3OH) can form one carbamate

361

species while one diamine is able to form more than one carbamate when it contains

362

suitable amino groups. These carbamate species can be complicated when the two

363

amino groups in the molecule are different. For example, MEDA (1NC2N2) contains

364

one primary and one secondary amino group and both of them are able to form

365

carbamates with CO2. There are two different monocarbamates and one dicarbamate

366

species produced in the reaction of MEDA (1NC2N2) with CO2, as identified by

367

NMR spectroscopy. There were small amounts of dicarbamate formed in all

368

experiments in which primary-primary diamines and primary-secondary diamines

369

were used, however the carbamate formed by the primary amino-groups appeared first

370

and remained the dominant carbamate species throughout the whole reaction for all

371

diamines studied. The dicarbamate and monocarbamate formed by the secondary

372

amino-group are therefore not discussed in detail, but their quantities (converted to

373

CO2) were added to the dominant monocarbamate formed by the primary

374

amino-group to give the sum of all carbamates. Fig.4 plots the ratio of total carbamate

375

species in each amine solution tested. As expected, higher CO2 partial pressure

376

favored bicarbonate formation in all amines and this factor impacted more on the

377

amines which contained tertiary amino groups.

20


13

C

Page 21 of 35

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Energy & Fuels

378 379

Figure 4. The moles of carbamate/bicarbonate per mole N in each aqueous amine

380

solution

381 382

The experimental results also showed that the chain extension from C2 to C3 for

383

every comparable amine pair increased the ratio of bicarbonate to carbamate under

384

both high and low CO2 partial pressures. It is understandable that the molecular

385

structural factors which favour the carbamate stability restrict the conversion of

386

carbamate to bicarbonate in general. To gain clearer knowledge of the impact of chain

387

lengths on the conversion of carbamate to bicarbonate in the systems studied, the

388

stabilities of the carbamates from two C2 diamines and two C3 diamines have been

389

studied by quantum chemistry modelling. From quantum mechanical calculations, it is

390

clear that the carbamate zwitterions containing C2 or C3 chains are able to be

391

stabilized by intramolecular hydrogen bonding effects between the O1 atom in the

392

-COO group and the H2 atom in the -NH2R group (R = H or Me) (Fig. 5). It appears

393

that molecules with shorter carbon chains (C2 vs C3) can form more stable 21


Energy & Fuels

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

hydrogen

bonds,

which

can

Page 22 of 35

394

intramolecular

be

explained

by

a

shorter

395

O1(-COO)-H2(-NH2R) hydrogen bond length. Lengths of the O1(-COO)-H2(-NH2R)

396

hydrogen bonds are 1.58 Å in EDA carbamate, 1.99 Å in 1,3-PDA carbamate (R=H),

397

1.63 Å in MEDA carbamate and 2.12 Å in MAPA carbamate (R=Me). All the

398

carbamate zwitterions containing C2 chains were found to be energetically more

399

favorable than the corresponding ones containing C3 chains (Table 2). The second

400

order perturbation theory analysis of the Fock matrix in natural bond orbital (NBO)

401

basis shows the donations from the lone pair (LP) of the O1 atom in the -COO group

402

to the anti-bonding orbital of H2-N3 (BD* H2-N3) in -NH2Me group are 58.73 and

403

50.97 kcal/mol in EDA carbamate and MEDA carbamate respectively, all stronger

404

than the corresponding ones for molecules containing C3 chains (Table 3). The

405

electron cloud movement from donor to acceptor can make the molecule highly

406

polarized, therefore the stabilization of C2 carbamate should benefit more from such

407

delocalization. Consequently, the decrease of the carbon chain length from C3 to C2

408

supports carbamate formation.

409

The results also clearly show that the induction effect of the methyl substituent on

410

nitrogen produced smaller changes of bond length, relative free energy and energy of

411

hyperconjugative interactions compared to the changes caused by the chain length

412

differences.

413

22


Page 23 of 35

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Energy & Fuels

414 415

Figure 5. The structures of the C2 and C3 mono-carbamate species at the

416

(SMD)M06-GD3/6-31+G(d, p) level. Dashed lines indicate hydrogen bonds.

417

Table 2. Free energies (a.u.) of all the reactants and carbamate products and

418 419

relative free energiesa (kcal/mol) of carbamate products using

420

(SMD)M06-GD3/6-311++G(2d,2p)// (SMD)M06-GD3/6-31+G(d,p). C2 diamines Name

421

C3 diamines

Free

Relative free

energy

energy

(a.u.)

(kcal/mol)

CO2

-188.58390

EDA

-190.42358

EDA-Carbamate

-379.02139

MEDA

-229.69276

MEDA-Carbamate

-418.29097

a

Name

-8.73219 -8.97943

Free

Relative

energy

free energy

(a.u.)

(kcal/mol)

1,3-PDA

-229.70173

1,3-PDA-Carbamate

-418.29519

MAPA

-268.97166

MAPA-Carbamate

-457.56576

-5.99739 -6.4026

Energy values are relative to the corresponding diamine reactant + CO2.

422 423

Table 3. The second order perturbation theory analysis of the Fock matrix in NBO

424

basis for all the carbamate zwitterions under (SMD)M06-GD3/6-31+G(d,p). a 23


Energy & Fuels

425

426

a

E(2)a (kcal/mol)

Items

chain

C3

C2

Chain Donor (i) LP O

BD* H -N

58.73

50.97

chain

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 35

Donor (i)

Acceptor (j)

1,3-PDA Carbamate

MAPA Carbamate

4.32

2.65

1

1

LP O

Acceptor (j) 2

2

EDA Carbamate

MEDA Carbamate

3

3

BD* H -N

E(2) means energy of hyperconjugative interactions.

427 428

4.2 Evaluation of CO2 desorption performance

429

As discussed above, the chain length and the amino group can affect the CO2

430

absorption capacity and the formation of carbamate and bicarbonate. DMAPA

431

demonstrate a higher CO2 absorption capacity at high CO2 partial pressure but not at

432

the low CO2 partial pressure compared to other amines. Because the CO2 mainly

433

released from carbamate and bicarbonate in the solvent regeneration process and the

434

DMAPA generate less carbamate and more bicarbonate compared to others amines at

435

high/low CO2 partial pressure, consequently, it is necessary to investigate the effect of

436

the chain length and amino group to the CO2 desorption performance of the diamines

437

in current study. It is well known that the CO2 loading in amine solution decreases as

438

the heating time increases during the CO2 desorption process. To evaluate the CO2

439

desorption performance of the diamines tested, the cyclic capacity, the initial CO2

440

release rate and the CO2 removal efficiency were investigated to assess the desorption

441

performance. CO2 stripping of each diamine on a laboratory scale was performed as

442

described in section 2.2.3. The CO2 desorption of these diamines was investigated

443

under mild conditions for 60 minutes and samples taken at different reaction times

444

were analyzed. The CO2 loading changes with the CO2 desorption time are presented 24


Page 25 of 35

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Energy & Fuels

445

in Fig. 6. The results showed that all the diamines had significant CO2 release in the

446

early stages after which the CO2 loading curves changed slowly with increase of

447

stripping time except for DMAEA (1NC2N3) and DMAPA (1NC3N3).

448

449

Figure 6. CO2 loading versus desorption time

450 451 452

In addition, the CO2 removal performance of the amines studied in this work was

453

evaluated by their CO2 cyclic capacity and CO2 removal efficiency. The CO2 cyclic

454

capacity ( ∆

455

equilibrium CO2 loading obtained at 40 °C and that after heating at 90 °C for one hour.

456

The CO2 removal efficiency (φ) was defined as the ratio of the removed CO2 (CO2

457

cyclic capacity) to the full CO2 loading. Note that the fully loaded CO2-rich amine

458

solution prepared with 10% CO2 partial pressure at atmosphere pressure was used in

459

this study to simulate the industrial CO2 concentration sourced from flue gas. The

!"

) of each amine was defined by the difference between the

25


Energy & Fuels

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Page 26 of 35

460

CO2 cyclic capacity and the CO2 removal efficiency of each amine tested are

461

expressed as eqs. (11) and (12) respectively. & '() +& '() #$. − #$.

462

∆

463

,

464

The results of CO2 cyclic capacity, CO2 removal efficiency and the mole fraction

465

of initial bicarbonate of the amines studied are displayed in Fig. 7. Each of the studied

466

diamines has a better CO2 removal performance than their corresponding monoamine

467

(MEA and 3AP) as determined by the CO2 removal efficiency and cyclic capacity.

468

These results also indicate that the C3 chain amines have better CO2 cyclic capacity

469

and better CO2 removal efficiency than the corresponding C2 amines. This is due to

470

the preference of C2 chain amines to form carbamates as discussed above. C3

471

diamines also form more bicarbonate initially, which is easier to decompose to form

472

free CO2 than carbamate. Furthermore, as predicted in the absorption study above, the

473

results in Fig. 7 also show that increasing the number of substituents on the nitrogen

474

atom increases the amount of bicarbonate formed in the absorption, which gives a

475

better cyclic capacity. Hence, for the CO2 cyclic capacity and the CO2 removal

476

efficiency, the chain length extension from C2 to C3 and an increase in substitution on

477

nitrogen atoms in diamines benefits the CO2 desorption process.

!"

∆-./" 3 456 -012.

(11)

× 100

(12)

478

26


Page 27 of 35

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Energy & Fuels

479 480

Figure 7. Comparison of CO2 removal performance of each studied amine

481 482

Besides the CO2 loading changes and the CO2 removal performance, the overall

483

initial CO2 release rate is also an important factor to evaluate the amine desorption

484

performance. Because the higher CO2 cyclic capacity mainly contributed to higher

485

CO2 absorption capacity which lead to more bicarbonate formed. The more

486

bicarbonate in CO2 rich solution leads to a faster initial CO2 release rate because the

487

bicarbonate is easy to decompose. So, Fig. 8 present the overall CO2 desorption

488

performance of tested amines in terms of the cyclic capacity and initial CO2 release

489

rate. The initial desorption rate was calculated using the data for the sample which

490

was collected at 2 minutes of reaction time (eq. (13)). The real CO2 release rate is

491

faster than the result obtained from eq. (13) as the reaction time was counted from the

492

time thermal heating was introduced, and CO2 release started 30 seconds later when

493

the internal temperature of the reaction solution reached 70 °C. These results, however,

494

are representative of the relative overall CO2 release rates of the diamines tested as all

495

of these desorption experiments were carried out under the same reaction conditions. 27


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Page 28 of 35

496

This method is not suitable for calculation of the CO2 desorption rate over extended

497

reaction time frames, however, as the CO2 desorption rate changes significantly

498

during the reaction process.

499

:;> ?@ABA=> C;>;=D; C=B;

3 " E./ E./ " "

×+&

（13）

500 501

Figure 8. The initial desorption rate vs. cyclic capacity of each amine used in this

502

work

503

Results in Fig. 8 showed that every diamine containing a C3 chain had better

504

cyclic capacity and a faster initial CO2 desorption rate than those of its C2 analogue.

505

There are several possible factors contributing to the better CO2 desorption

506

performance of 1NC3Nn in comparison to the corresponding paired diamine 1NC2Nn.

507

Firstly, the relatively lower stability of 1NC3Nn carbamates makes these species easier

508

to decompose with heating. In addition, the solution of each of the C3 molecules has a

509

lower heat capacity than the C2 molecules due to the larger mass fraction of organic

510

components. This means that the C3 chain length diamines heat up faster than C2

511

chain length diamines. As a result, the CO2 loaded C3 diamines should decompose 28


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Energy & Fuels

512

faster than CO2 loaded C2 diamines. Furthermore, the effects of viscosity on heat

513

transfer also affect the overall CO2 release. The current overall initial desorption rate,

514

however, is a valuable simple parameter for comparison of the CO2 release rate for

515

each diamine.

516

The results in Fig. 8 also showed a parallel response of the increase of cyclic

517

capacity to the increase of methyl substitution on N (x-axis), and a parallel response of

518

relative initial CO2 release rate to the chain length variation (y-axis). Hence, the best

519

performance area is at the right-top corner as summarized in the insert picture located

520

in right-bottom of Fig. 8. This suggests some favorable structural factors of amine

521

absorbents in relation to their CO2 separation performance.

522

5. CONCLUSIONS In this work, six linear diterminal diamines of 2 M concentration containing C2 or

523 524

C3

carbon

chains and

two

monoamines,

monoethanolamine

525

3-aminopropanol (3AP), of 4 M concentration, were evaluated for CO2

526

absorption-desorption performance. The CO2 absorption capacity of each amine was

527

measured at 40 °C at 101 kPa and 10 kPa of CO2 gas partial pressures. In addition, the

528

speciation in each CO2 loaded aqueous amine solution was determined by 13C and 1H

529

NMR spectroscopy at various CO2 loadings. The absorption experimental results

530

show that the chain length extension of an amine from C2 to C3 increases the CO2

531

absorption capacity and generates more bicarbonate and less carbamate. This was

532

further confirmed by the results from quantum chemistry modelling calculations. The

533

experimental results also suggest that an increase of substitution on one terminal 29


(MEA)

and

Energy & Fuels

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

534

nitrogen atom of a diamine benefits the formation of bicarbonate versus carbamate.

535

More substitution on one nitrogen also causes a higher CO2 absorption capacity at 101

536

kPa CO2 partial pressure. On the other hand, the desorption experimental results show

537

that, with consideration of the cyclic capacity and initial CO2 release rate, the diamine

538

chain length extension from C2 to C3 and more substitution on nitrogen also benefits

539

CO2 desorption.

540 541

SUPPORTING INFORMATION

542

The measurement of the CO2 loading by using 1M HCl titration method. The

543

molecular structures of each species and the 13C NMR spectra for each amine used in

544

this work.

545 546 547

ACKNOWLEDGEMENTS The authors are very grateful for the CSIRO PhD student scholarship (R-09237-1)

548

to support this research and would like to acknowledge the research contribution to

549

the CO2 loaded amines test by the CSIRO PCC team in Australia. The authors also

550

would like to express thanks for the financial support from the National Natural

551

Science Foundation of China (NSFC Nos. 21536003, 21476064, 21376067 and

552

51521006), National Key Technology R & D Program (MOST-No. 2014BAC18B04),

553

Graduate Student Innovation Project of Hunan Province (CX2016B121). Specialized

554

Research Fund for the Doctoral Program of Higher Education (MOE-No.

555

20130161110025), China’s State “Project 985” in Hunan University Novel 30


Page 30 of 35

Page 31 of 35

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Energy & Fuels

556

Technology Research & Development for CO2 Capture, and China Outstanding

557

Engineer Training Plan for Students of Chemical Engineering & Technology in Hunan

558

University (MOE-No. 2011-40), Innovative Research Team Development Plan

559

(MOE-No. IRT1238) is also gratefully acknowledged. China Scholarship Council

560

(201606130057) are also greatly appreciated.

31


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561

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Wigley, T. M.; Richels, R.; Edmonds, J. A., Economic and environmental choices in the

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Markewitz, P.; Kuckshinrichs, W.; Leitner, W.; Linssen, J.; Zapp, P.; Bongartz, R.; Schreiber, A.;

Müller, T. E., Worldwide innovations in the development of carbon capture technologies and the utilization of CO2. Energy & Environmental Science 2012, 5, (6), 7281-7305. 4.

Figueroa, J. D.; Fout, T.; Plasynski, S.; McIlvried, H.; Srivastava, R. D., Advances in CO2 capture

technology—the US Department of Energy's Carbon Sequestration Program. International journal of

greenhouse gas control 2008, 2, (1), 9-20. 5.

Wang, Y.; Zhao, L.; Otto, A.; Robinius, M.; Stolten, D., A Review of Post-combustion CO2

Capture Technologies from Coal-fired Power Plants. Energy Procedia 2017, 114, 650-665. 6.

Huang, C.-H.; Tan, C.-S., A review: CO2 utilization. Aerosol Air Qual. Res 2014, 14, 480-499.

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Li, P.-Z.; Wang, X.-J.; Liu, J.; Lim, J. S.; Zou, R.; Zhao, Y., A triazole-containing metal–organic

framework as a highly effective and substrate size-dependent catalyst for CO2 conversion. Journal of

the American Chemical Society 2016, 138, (7), 2142-2145. 8.

Boot-Handford, M. E.; Abanades, J. C.; Anthony, E. J.; Blunt, M. J.; Brandani, S.; Mac Dowell,

N.; Fernández, J. R.; Ferrari, M.-C.; Gross, R.; Hallett, J. P., Carbon capture and storage update. Energy

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Toward Efficient CO2 Capture Solvent Design by Analyzing the Effect

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