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Carbon dioxide capture and utilisation by alkanolamines in deep eutectic solvent medium Anga Muthu Uma Maheswari, and Kandasamy Palanivelu Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01818 • Publication Date (Web): 23 Oct 2015 Downloaded from http://pubs.acs.org on October 31, 2015

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Industrial & Engineering Chemistry Research

Graphical abstract

ACS Paragon Plus Environment


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1

Carbon dioxide capture and utilisation by alkanolamines in deep eutectic solvent

2

medium A. Uma Maheswaria and K. Palanivelua, b*

3 a

4

Centre for Environmental Studies, bCentre for Climate Change and Adaptation

5

Research, Anna University, Chennai-600 025, India

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ABSTRACT: The major drawback of aqueous alkanolamine-based carbon dioxide capture

7

process is the high energy penalty for the regeneration of the absorbent. To overcome this

8

weakness, we studied the absorption of CO2 in alkanolamines dissolved in greener and non-

9

toxic deep eutectic solvents. Among the alkanolamines in various deep eutectic solvent

10

media, 2-amino-2-methyl-1-propanol in choline chloride: urea (1:2) medium was found to

11

exhibit the highest absorption capacity for CO2 gas. In addition to that, the value-added

12

product, 2-amino-2-methyl-1-propanol carbamate, was obtained from all deep eutectic

13

solvent medium, which was analyzed by Fourier transform infrared and 1H and

14

magnetic resonance spectroscopic techniques. Under optimized conditions, the maximum

15

yield of 82 %, 2-amino-2-methyl-1-propanol carbamate was obtained. The deep eutectic

16

solvent used for the process has been recovered and reused for 4 cycles. Thus, the 2-amino-2-

17

methyl-1-propanol in deep eutectic solvent medium emerges to be a novel promising

18

candidate for capture as well as for the utilization of the CO2 gas to obtain the value-added

19

product.

20

*Corresponding author. Tel: + 91 4422359014; Fax: + 91 4422354717

21

E-mail address: [email protected]


13

C nuclear

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Graphic for manuscript 22

1. Introduction

23

The control of anthropogenic carbon dioxide (CO2) emission is one of the most challenging

24

environmental issues faced by industrialized countries, as CO2 is the largest contributor

25

accounting for 60 % of the global warming effect1. Intergovernmental Panel on Climate

26

Change (IPCC)2 predicts that by the year 2100, the atmosphere may contain up to 570 ppmv

27

CO2 causing a rise of mean global temperature of around 1.9 ᴼC and an increase in the mean

28

sea level of 3.8 m. Hence, it is very important from both the environmental and economical

29

point of views to find an efficient way for capturing CO2 from flue gases to minimize its

30

emission into the atmosphere as well as to convert it into value-added products.

31

Among the different capturing techniques developed for the removal of CO2 gas,

32

solvent absorption is the most widely employed method and aqueous alkanolamines are the

33

most commonly used chemical absorbents for the removal of CO2 gas for over 60 years3.

34

Several studies have been reported for the absorption of CO2 in alkanolamines, such as

35

monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), 2-amino-2-

36

methyl-1-propanol (AMP), and 2-methylaminoethanol (MAE) in aqueous medium4, 5.

37

However, the problems associated with the aqueous-based absorbents, such as equipment



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corrosion and high energy consumption for regeneration of the absorbents, make the process

39

complicated and costly.

40

In order to overcome this issue, recently, special attention has been paid to the use of

41

alkanolamines in non-aqueous solvents for the removal of CO2 gas. Because, non-aqueous

42

absorbents have high CO2 absorption capacity and they are low corrosive in nature6. The most

43

important advantage is their low energy consumption during the regeneration of used liquor.

44

In conventional aqueous amine scrubbing methods, relatively low temperatures, less than

45

50 ᴼC, are required for CO2 uptake and high temperatures in the range of 120 ᴼC-140 ᴼC are

46

required for the desorption. The heat required to maintain the thermal differential in aqueous

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amine system is the major factor that increases the total cost and energy consumption of the

48

process and also makes the equipment corrosive. However, in the case of non-aqueous amine

49

absorbents, both the absorption and desorption processes can be carried out at relatively low

50

temperatures (not more than 60 ᴼC) and the equipment corrosion could also be minimized7, 8.

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Park et al. have measured the absorption of CO2 into aqueous and non-aqueous (ethanol,

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methanol, and propylene carbonate) solutions of triethanolamine and methyl diethanolamine

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(MDEA) and reported that the disadvantages witnessed in the case of aqueous absorbents

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could be circumvented by the use of non-aqueous absorbents9.

55

Now-a-days, a growing attention has been paid to the use of the greener and non-toxic

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deep eutectic solvents (DESs) for CO2 absorption10-13. DESs have many desirable solvent

57

properties compared with molecular solvents. In fact, they share many unusual characteristics

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with room-temperature ionic liquids (RTILs) such as negligible vapour pressure, wide liquid

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range, high thermal and chemical stabilities, non-flammability, and high solvation

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capacity14, 15. However, unlike the latter, DESs are easy to prepare at high purity; thus, they

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can be manufactured at a considerably lower cost than RTILs16. Furthermore, they can be

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made from biodegradable components, and their toxicities are well characterized.


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Li et al. determined the solubility of CO2 in a choline chloride (ChCl)/urea (U) DES at

64

different temperatures, pressures, and for different ChCl/U molar ratios17. The study revealed

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that the ChCl:U of 1:2 composition showed high absorption capacity for CO2 gas. Leron et al.

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measured the solubility of CO2 in ChCl:glycerol (1:2) DES and stated that the solubility of

67

CO2 in the DES was found to be comparable with the typical solubility of CO2 in ionic

68

liquids18. Recently, Li et al. studied the solubility of CO2 in ChCl and glycol-based DES and

69

reported that the ChCl:triethylene glycol (1:4) DES shows higher absorption capacity for CO2

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gas19. However, the reported CO2 solubility in the studied DESs was very low compared with

71

the amine-based absorption used in industry. This logically suggests us to improve the

72

solubility of CO2 by incorporating alkanolamines into the DES medium. By applying DES as

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a non-aqueous medium, the CO2 solubility could be improved and the benefits of DES

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compared with aqueous and other non-aqueous solvents could also be employed. Ali et al.

75

have prepared the new type ammonium and phosphonium based DES with alkanolamines as

76

hydrogen bond donor (HBD)20. The study reported that at atmospheric temperature and at a

77

pressure of 10 bar, the absorption capacity of ChCl:MEA (1:6) is 62 % higher than the

78

aqueous MEA.

79

Moreover, DES as a non-aqueous medium for alkanolamines would be a viable

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candidate to reduce the corrosion problem occurring in aqueous alkanolamine-based gas

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absorption plants. It was reported that about 9 million dollars are annually spent to mitigate

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the corrosion from the plant systems. Two major types of corrosion are reported to be

83

encountered in the aqueous alkanolamine-based power plants, namely wet acid gas (or CO2)

84

corrosion and alkanolamine solution corrosion21. In both the corrosion, CO2 will react with

85

water and iron and forms FeCO3 (Equation 1) which is only slightly soluble and does not

86

form a very protective film.

87

Fe + H 2 O + CO 2 → FeCO3 + H 2


(1)


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It is well known that the alkanolamines are not responsible for the corrosion. It is

89

corrosive in nature only in the presence of aqueous medium. So, the exclusion of water

90

prevents the conversion of CO2 into carbonic acid and restricts its further reaction with iron.

91

Hence, by replacing the aqueous part with DES, it would be possible to reduce corrosion

92

virtually to negligible levels. Hasib ur rahman et al. reported22 that by replacing the aqueous

93

amine with RTIL-amine blend the corrosion could be reduced to 72 % at 60 ᴼC. Since, DES

94

is analogue to RTIL, the application of DES-amine blend would also reduce the corrosion

95

and avoids the addition of costly and toxic corrosion inhibitors, and consequently reduces the

96

operational cost of the process.

97

In addition to the separation process, the utilization of CO2 as a resource is the

98

strategic idea in the mitigation of global warming effect. CO2 can be converted into an

99

assortment of value-added products such as bicarbonates, carbonates and carbamates. Among

100

them, carbamate is one of the most substantial value-added products obtained by the reaction

101

of CO2 with amines. It is being used as an insecticide, human medicine and as a

102

preservative23. In our earlier study, we have obtained 52 % of AMP-carbamate by the

103

reaction of CO2 with AMP in coconut oil medium 24.

104

In this study, an attempt has been made to measure the absorption of CO2 in

105

alkanolamines in DES (non-toxic and greener) medium. As well as, the value-added product

106

AMP-carbamate was isolated as a stable compound from AMP in DES medium and

107

characterized by FT-IR, 1H NMR, and 13C NMR spectroscopic studies. The various operating

108

parameters such as amine concentration, reaction duration, temperature, and pressure of CO2

109

gas were optimized to obtain the maximum yield of AMP-carbamate. To our knowledge, this

110

is the first study that reports the employment of AMP in DES medium for the capture as well

111

as for the utilization of CO2 gas to get the value-added product.

112


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1.1. Reaction fundamentals

114

1.1.1. Alkanolamines in aqueous medium

115

Zwitterion mechanism is a well-established mechanism for the description of the reaction

116

between CO2 and primary or secondary amines. It was originally proposed by Caplow, and

117

then reintroduced by Danckwerts 25, 26. The electrophilic nature of the carbon atom of CO2

118

makes it susceptible to nucleophilic attack. The nucleophilic addition of primary or secondary

119

amine with CO2 can create a zwitterionic transition state, which can undergo an

120

intramolecular proton transfer to form a neutral carbamic acid. The subsequent reaction of

121

this carbamic acid with a Bronsted base amine can lead to the formation of carbamate (0.5:1

122

CO2: amine ratio). Therefore, primary and secondary amines can exhibit both Lewis and

123

Bronsted basicity in an aqueous medium27.

124

CO2 + RR’NH

RR’NH+COO-

(2)

125

RR’NH+COO-

RR’NCOOH

(3)

126

RR’NCOOH + RR’NH

127

RR’NCOO- + RR’NH2+

(4)

RR’NCOO- + RR’NH2+

(5)

The overall reaction is CO2 + 2RR’NH

128 129

In the case of sterically hindered amine, since the carbamate formed is unstable, further

130

reaction of the carbamate with water may lead to a bicarbonate formation with a 1:1 CO2:

131

amine ratio,

132

RR’NCOO- + H2O

RR’NH

+ HCO3−

(6)

133

In tertiary amines, the reaction pathway depends on the nucleophilicity of water rather

134

than amine, leading to the formation of carbonic acid, which on further reaction with a

135

Bronsted base amine produces bicarbonate (1:1 amine: CO2). Due to the lack of proton, they

136

cannot undergo an intramolecular proton transfer to form a neutral carbamic acid; therefore,



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they act as a chemical sink for CO2 in aqueous solution simply by providing only Bronsted

138

basicity.

139

RR’R”N + CO2 + H2O

RR’R”NH + HCO3−

(7)

140

1.1.2. Alkanolamines in non-aqueous medium

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In the case of primary and secondary alkanolamines in non-aqueous system, they follow the

142

same mechanism as in the case of aqueous medium. However, in the case of sterically

143

hindered amine in non-aqueous system, the possibility of further reaction of carbamate with

144

water is less. Thus, the carbamate formation is more favourable than bicarbonate formation

145

and the loading capacity can exceed the predicted theoretical value due to the equilibrium

146

established between zwitterions and carbamic acid. Calabro et al. achieved the molar ratio of

147

0.91:1 (CO2: amine) for sterically hindered primary amines in non-aqueous DMSO medium

148

which is higher than the theoretical value (0.5:1 CO2: amine) 28. In the case of tertiary amines,

149

due to the absence of water nucleophile, they are unable to form the carbamic acid/carbamate

150

species and the reaction of tertiary amines with CO2 in non-aqueous system is less

151

pronounced 29.

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2. Experimental

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2.1. Chemicals

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Various chemicals employed in this study are monoethanolamine (MEA), diethanolamine

155

(DEA), triethanolamine (TEA), 2-methylaminoethanol (MAE), 2-amino-2-methyl-1-propanol

156

(AMP), choline chloride (ChCl), urea (U), glycerol (Gly), ethylene glycol (EG), diethylene

157

glycol (DEG), triethylene glycol (TEG), diethyl ether, and hexane. All chemicals were

158

purchased from the Merck company with 96-98% purity. The chemicals were used without

159

further purification. Research-grade CO2 gas of different concentrations (99% CO2 gas, 15%

160

CO2 gas in 85 % N2 gas, and 15% CO2 gas in 1000 ppm of SO2 gas balance N2 gas) was

161

purchased from Supreme Engineering Services, India.


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2.2. Preparation of absorbents

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Alkanolamines in aqueous medium were prepared using distilled water. ChCl:U (1:2),

164

ChCl:Gly (1:2), ChCl:EG (1:2), ChCl:DEG (1:4), and ChCl:TEG (1:4) DES were prepared

165

by mixing the hydrogen bond donor (ChCl) with the acceptor (U, Gly, EG, DEG, and TEG)

166

in the required ratio and heating them at 80 ᴼC with continuous stirring (300 rpm) for 2

167

hours30. Then, the alkanolamines in non-aqueous (DES) media were prepared by dissolving

168

the alkanolamines in DESs. The chemical structure of the DESs used in this study is

169

presented in Figure 1.

Figure 1. Chemical structures and respective acronyms of the DESs studied in this work 170

2.3. Apparatus and procedure

171

A schematic diagram of the experimental setup for the absorption of CO2 gas is shown in

172

Figure 2. The reactor consists of 150 mL capacity stainless steel cylindrical tank equipped



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with pressure gauge, two baffles to endure intense mixing, and a stainless steel diffuser to

174

generate fine gas bubbles. The tank was kept on the magnetic stirrer with hot plate for

175

stirring. The alkanolamine sample (10 ml) was taken in the gas absorption tank, and before

176

each run, any air or gas in the loaded reactor was removed by applying vacuum for 1h and

177

then desired quantity of CO2 gas was passed from the cylinder to the tank. Then, the samples

178

were withdrawn from the absorption tank and analyzed to measure the CO2 content by a

179

titrimetric method31. Each experimental run was conducted in duplicates and the experimental

180

error was found to be within ±5 % in terms of reproducibility.

Figure 2. Experimental setup for absorption of CO2 gas 181 182

The CO2 loading31 is calculated using the following equation:

α=

Wt CO2 × M. Wt amine mole of CO 2 = mole of amine Wt amine × M. Wt CO2


(8)

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The precipitated AMP-carbamate has been filtered and isolated in pure form with hexane and

184

then dried. The yield32 of the AMP carbamate was calculated using Equation 9. The CO2

185

content in the precipitate was calculated from the weight of AMP carbamate.

186

Yield =

Actual mass of the product

× 100

(9)

Theoritical mass of the prdouct 187

After the recovery of the AMP-carbamate, 10 ml of diethyl ether was added to the

188

filtrate to remove any unreacted amine and the remaining DES was heated at 60 ᴼC for 1 h,

189

and reused for further absorption study33.

190

The yield of AMP carbamate obtained by the reaction of AMP with CO2 diluted with

191

N2 and SO2 was measured by CO2 gas analyzer (VAISALA M170). Viscosity of the DES

192

was measured by Modular Compact Rheometer (Anton Paar, MCR 102). FTIR spectra were

193

recorded using a Perkin Elmer spectrometer using KBr disk in the range from 4000 to

194

400 cm-1. 1H NMR and 13C NMR spectra were recorded in DMSO using a Bruker Avance III

195

500 MHz spectrometer.

196

3. Results and Discussion

197

The absorption of CO2 in various DES medium is presented in Table 1. The results reported

198

in the table were in good agreement with the literature values, nearly under the same

199

experimental conditions. Among all DES medium, the CO2 solubility is higher in ChCl:U

200

medium compared with others. This is due to the unique properties of ChCl:U mixture

201

compared with other DES. The freezing point of the ChCl:U eutectic mixture is 12 ᴼC, which

202

is considerably lower than that of either of the constituents (m.p. choline chloride = 302 ᴼC

203

and urea = 33 ᴼC). Furthermore, there is a strong hydrogen bonding between urea and choline

204

chloride which contributes to low freezing point34. Due to their low freezing point, they are

205

being a homogeneous liquid at an ambient temperature, which contributes towards higher

206

solvation capacity to dissolve the CO2 gas. Li et al. measured17,


19

the absorption of CO2 in


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ChCl:U and ChCl:glycol-based solvent and reported that ChCl:U showed maximum

208

absorption of CO2 gas compared with other DES.

209

Table 1. Absorption of CO2 in DES media (Temperature=25 ᴼC; Pressure=2 bar) DES

CO2 loading

CO2 loading

Ref

g of CO2/g of solvent

g of CO2/g of solvent

(This work)

(Literature value)

ChCl:U (1:2)

0.0120

0.0122

17

ChCl:Gly (1:2)

0.0024

0.0026

18

ChCl:EG (1:2)

0.0028

0.0030

16

ChCl:DEG (1:4)

0.0043

0.0047

19

ChCl:TEG (1:4)

0.0082

0.0064

19

210

The absorption of CO2 in various alkanolamines in aqueous medium and in DES

211

media is presented in Table 2. As expected logically, the alkanolamine incorporated DES

212

media showed higher absorption capacity for CO2 gas than the DES media. This is well

213

supported by the data provided by Ali et al20. They reported that the solubility of the CO2 in

214

ethylene glycol and glycerol based DESs were much smaller than that in the amine

215

containing DES. Moreover, it was found that using alkanolamine as HBD increased the CO2

216

solubility substantially.

217

In addition to that, it could be observed from the table that the CO2 capture by

218

alkanolamines in DES medium is higher than in aqueous medium. Due to the high polar

219

nature of DESs, they are capable of solvating the hypothetical zwitterions and carbamic acid

220

pairs easily. This could additionally or alternatively increase the absorption of CO2 gas35. In

221

the case of AMP in DES media, the value-added product AMP-carbamate was obtained

222

which is not possible in the case of aqueous medium, because, in the case of AMP in aqueous

223

medium the carbamate will hydrolyze to bicarbonate as in equation 6, but in the case of non-


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224

aqueous medium, only equation 5 is applicable which facilitates the formation of AMP-

225

carbamate36. The mechanism of reaction of CO2 with AMP in non aqueous medium is

226

presented in Figure 3.

227 228 229

Figure 3. Mechanism of reaction of CO2 with AMP in non aqueous medium

230

The CO2 loading capacity of AMP in all DES medium (sum of CO2 content in both

231

precipitate and in filtrate) is higher than the AMP in an aqueous medium. It has been reported

232

in literature that AMP has higher loading capacity (1.0 mol of CO2/ mol of amine) than other

233

amines37, 38. This may be due to the bulkiness of the group attached to the tertiary carbon

234

atom and higher reaction rate constant of AMP with CO2. Among all alkanolamines in DES

235

media, AMP in ChCl:U medium shows the maximum absorption of CO2. Therefore, the high



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236

absorption of CO2 in AMP in DES medium is due to the combination of high reactivity of

237

AMP and high solvation capacity of DES. Table 2. Absorption of CO2 in alkanolamines in aqueous medium and DES media (Temperature=25 ᴼC; Pressure=2 bar) CO2 loading α = (g of CO2/g of solvent) Medium

MEA

DEA

TEA

MAE

AMP In

In

filtrate

precipitate

Aqueous

0.3333

0.0988

0.0573

0.3814

0.4242

-

ChCl:U (1:2)

0.4331

0.1705

0.0848

0.4385

0.4822

0.0016

ChCl:Gly (1:2)

0.3335

0.1224

0.0575

0.3822

0.4313

0.0009

ChCl:EG (1:2)

0.3546

0.1260

0.0586

0.3950

0.4412

0.0010

ChCl:DEG (1:4)

0.3697

0.1567

0.0633

0.4057

0.4548

0.0011

ChCl:TEG (1:4)

0.4218

0.1665

0.0766

0.4313

0.4612

0.0012

238

Following AMP, the absorption of CO2 is higher in MAE, which is higher than that in

239

the most commonly used conventional amine MEA. According to the study of Mimura et al.

240

even though, MAE is not a hindered amine as by the definition of sterically hindered amine,

241

it is more hindered than MEA39. Therefore, in such a comparison, the hindrance effect can

242

make some differences. Next, the absorption of CO2 is higher in MEA compared with DEA,

243

which is higher than in TEA. This trend could be reasonably explained based on the rate of

244

absorption. Even though TEA has higher CO2 loading capacity (1.0 mol of CO2/ mol of

245

amine), due to the very low reaction rate the absorption of CO2 is low in TEA among various

246

alkanolamines40.


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Since, the highest AMP carbamate formation was occurred in ChCl:U (1:2) DES

248

medium, the relevant parameters have been optimized to obtain the maximum yield of AMP

249

carbamate and the results are presented in the following sections:

250

3.1. Effect of amine concentration on the yield of AMP-carbamate

251

The concentration of AMP was varied from 1M to 5M at reaction temperature of 25 ᴼC,

252

pressure of 2 bar, and for reaction duration of 60 min. The results depicted in Figure 4a

253

indicate that the yield of AMP-carbamate increased with increasing amine concentration and

254

reached the maximum yield of 61 % at 3M and beyond this concentration there was a

255

decrease in the yield. This might be due to the excess reactant, which might have produced

256

the decrease in the yield. Pahlavanzaden et al. reported that at higher concentration, the

257

reactivity of AMP with CO2 decreased41.

258

3.2. Effect of reaction duration on the yield of AMP-carbamate

259

The effect of reaction duration on the yield of AMP-carbamate was studied by varying the

260

reaction duration from 30 to 150 min at reaction temperature of 25 ᴼC, pressure of 2 bar, and

261

with amine concentration of 3M. The results illustrated in Figure 4b indicate that the yield of

262

AMP-carbamate increases with increase in time; after 60 min, the curve turns to saturation.

263

This indicates that the reaction reached its maximum yield, and the further extension in

264

reaction duration does not result in further enhancement of the yield. On the basis of this,

265

further experiments were carried out for reaction duration of 60 min.

266

3.3. Effect of temperature on the yield of AMP-carbamate

267

The effect of temperature on the yield of the AMP-carbamate has been investigated by

268

varying the temperature from 25 ᴼC to 65 ᴼC at a pressure of 2 bar, with the amine

269

concentration of 3M, and for the reaction duration of 60 min. Figure 4c shows that the

270

maximum yield was obtained at 35 ᴼC. It has been reported that when CO2 is used as a

271

solvent or reactant, the reaction rate is maximum near the critical temperature42. With further



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

272

increase in temperature the yield of AMP-carbamate was found to be decreased. Ion et al.

273

reported that at high temperature the reaction of the amine with CO2 will become

274

exothermic43. Hence, the equilibrium shifts to the left with the increase in temperature and the

275

yield decreases. Alessandro et al. have also reported that the stability of carbamates will

276

decrease with increase in temperature44.

277

Figure 4. (a) Effect of amine concentration on the yield of AMP-carbamate (b) Effect of

278

reaction duration on the yield of AMP-carbamate (c) Effect of temperature on the yield of

279

AMP-carbamate (d) Effect of pressure on the yield of AMP-carbamate

280

3.4. Effect of pressure on the yield of AMP-carbamate

281

The effect of CO2 pressure on the yield of the AMP-carbamate was studied by varying the

282

pressure from 1 to 10 bar at a temperature of 35 ᴼC, for the reaction duration of 60 min, and

283

with amine concentration of 3M. The results obtained by varying the pressure of 99% CO2


Page 16 of 28

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284

and 15 % CO2 in 85 % N2 gas are presented in Figure 4d. The yield of AMP carbamate

285

increases with the increase in pressure of both 15% and 99 % CO2 gas and reached a

286

maximum yield at 6 bar. The results show that the maximum yield of 82 % and 30 % was

287

obtained with pure CO2 and 15 % CO2. The uncertainty in the analysis was 82 ± 0.5 %. The

288

increase in pressure produces the interfacial mass transfer by dominating the gas phase mass

289

transfer as a result of the increase in the driving force from the bulk of the gas phase to the

290

gas-liquid interface. Beyond the pressure of 6 bar, with further increase in pressure there is a

291

decrease in the yield. This might be due to the dilution effect. In other words, further increase

292

in CO2 pressure may retard the interaction between the AMP and CO2 and may cause a low

293

concentration of amine in the vicinity of CO2 gas, thus resulting in a decrease in yield45.

294

Under optimized conditions, the same experiment has been conducted with AMP in

295

aqueous medium (Amine concentration = 3 M, pressure = 6 bar, absorption temperature = 35

296

ᴼC, and desorption temperature = 120 ᴼC). The total energy consumption for this process is

297

71.2 KJ/g CO2 gas. This energy consumption is only for the absorption and desorption

298

process. However, in the case of AMP in DES (ChCl:U) medium, the CO2 has been captured

299

as well as utilized with the energy consumption of 22.46 KJ/g CO2 gas, which is 68 % lower

300

than the aqueous AMP.

301

Barbarossa et al.46 studied the CO2 capture and utilization capacity of AMP in various

302

alcohol medium. Hasib-ur-Rahman et al.22 reported the capture and utilization of CO2 by

303

DEA and AMP in ionic liquid medium. The comparison of the present findings with their

304

results is presented in Table 3. Nevertheless, the studies have proposed the alternative

305

absorbents for the aqueous medium, the properties of alcohol such as high volatility and high

306

vapour pressure and high cost of ionic liquid restricts their application in industries. But,

307

these problems could be overcome by the employment of greener and non-toxic DES as a

308

non-aqueous medium.



Page 18 of 28

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309

Table 3. Comparison of the CO2 capture and utilization capacity of AMP in non aqueous

310

medium

Non aqueous medium

Concentration of AMP 3M

Alcohol medium

Temperature 20 ᴼC

atm gas flow (14 dm3h-1)

Ethylene glycolethanol (1:2) ionic liquid medium

Pressure

CO2 concentration

Yield of AMPcarbamate

Ref.

50 % saturated with pure CO2 and remaining with 12 % (v/v) CO2 in air

94.3 %

46

15 wt%

35 ᴼC

atm

99 % CO2

50 mol %

22

3M

35 ᴼC

10 bar

99 % CO2

52 %

24

15 % CO2 in 85 % N2 gas

11 %

24

99 % CO2

82 %

15 % CO2 in 85 % N2 gas

30 %

This work

[Hmin][Tf2N] Vegetable medium

oil

(coconut oil) DES medium ChCl: U (1:2)

3M

35 ᴼC

6 bar

311

Generally, the ChCl:U (1:2) eutectic mixture has higher viscosity compared with

312

other organic solvents, which represents a real drawback on its application in a continuous

313

process. This is due to the strong hydrogen bonding between the ChCl and U. However, at 25

314

ᴼC, when AMP is added to the ChCl:U (1:2) mixture, the viscosity of the resulting mixture is

315

55 % lower than the pure DES. The viscosity of the absorbents is presented in Table 4. This

316

is due to the partial rupture of the hydrogen bonding network by AMP47. Similarly, when the

317

temperature of the AMP-ChCl:U (1:2) mixture was increased to 35 ᴼC, the viscosity of the

318

amine-DES mixture is 82 % lower than the pure DES. This behaviour of DES could be

319

explained based on Arrhenius equation48, in which the viscosity is inversely proportional to

320

temperature. So, albeit, the ChCl:U is highly viscous, the addition of AMP would decrease

321

the mass transfer resistance from gas phase to gas-liquid interface, resulting in the high yield

322

of AMP carbamate. Thus, the incorporation of alkanolamines into the DES medium would

323

make the DES as a feasible candidate for CO2 capture application.


This work

Page 19 of 28


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324

Table 4. Viscosity of the absorbents DES

Viscosity mPa.s

Ref

This work

Literature

ChCl:U (1:2)

650 (25 ᴼC)

750 (25 ᴼC)

14

AMP

140 (25 ᴼC)

147 (25 ᴼC)

37

3M AMP in ChCl:U (1:2) medium

291 (25 ᴼC)

-

-

3M AMP in ChCl:U (1:2) medium

117 (35 ᴼC)

-

-

325

3.5. Recycling of DES and Validation of optimum conditions

326

After the recovery of the value-added product, under optimized conditions, 98 % of DES has

327

been recovered, and reused for further absorption study which is presented in Figure 5. It

328

shows that the yield of AMP-carbamate remains almost constant up to first four recycle runs.

329

Beyond that, there is a decrease in the yield due to loss of DES as a consequence of reuse. So,

330

the ChCl:U (1:2) DES could be effectively used for CO2 capture as well as utilization up to 4

331

cycles without change in their performance.

Figure 5. Recovery and reusability of DES (amine concentration=3M; temperature=35 ᴼC, reaction duration=60 min, and pressure=6 bar)



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

332

For the feasibility of this study in industries, the other components of flue gas should

333

also be considered. In addition to the CO2 gas, the flue gas contains H2O, O2, NOx, SOx, CO

334

and particulates. Among them SO2 is pronounced to have more interference in CO2 capture.

335

In order to identify the interference of SO2 gas the study has been conducted with 15 % CO2

336

in 1000 ppm SO2 balance N2 under optimized conditions. The comparison of the yield

337

obtained for various gas compositions under optimized conditions is presented in Figure 6.

338

The yield of AMP carbamate is lower in the presence of SO2 gas. Bonenfant et al. reported

339

that this is due to the amine degradation produced by the SO2 gas49. Both CO2 and SO2 are

340

acidic gases, and thus, it is expected that solvents that are designed to capture CO2 may

341

capture SO2 as well. In fact, SO2 has a net dipole moment which allows it to engage in

342

dipole-dipole interactions with the amine. In contrast, CO2, which is linear with no net dipole

343

moment, interacts with the amine via Vander Waals forces alone. Therefore, the bonding

344

between SO2 and the amine is stronger resulting in the formation of sulfamates rather than

345

carbamates, which would reduce the CO2 capturing capacity of amines. This phenomenon has

346

been observed by many CO2 capture solvents, including MEA, and it is one of the major

347

pathways for the solvent degradation.

348

The maximum yield of AMP carbamate was obtained with pure CO2 gas compared

349

with CO2 diluted with N2 and SO2. In order to apply this technology in the industry, this

350

aspect has to be considered. The flue gases from fossil fuel power stations typically contain

351

about 7-15 volume per cent of CO2 depending on the fuel. Since, the maximum yield is

352

obtained with 99% CO2 gas; the maximum recovery of value-added product, AMP carbamate

353

could be obtained by utilizing CO2 gas after separating and concentrating it from flue gases.


Page 20 of 28

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20 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

Figure 6. Comparison of yield of AMP carbamate under optimized conditions (amine concentration=3M; temperature=35 ᴼC, reaction duration=60 min, and pressure=6 bar) 354

3.6. Characterization of AMP-Carbamate

355

3.6.1. FTIR Analysis

356

The FTIR spectra of the DES, its parent materials, and precipitated AMP-carbamate are

357

presented in Figure 7. Urea was employed as a hydrogen bond donor. The FTIR spectrum of

358

urea (Figure 7a) exhibited bands with strong intensity at 3439 and 3344 cm-1 for 1ᴼ amide -

359

NH2 bond stretching vibrations (asymmetric and symmetric). The peaks at 1677 cm-1,

360

1625 cm-1, 1464 cm-1, and 789 cm-1 correspond to –NH2 symmetric bending, -NH2

361

asymmetric bending, -CN bond stretching, and –NH bond out of plane bending50. On the

362

other hand, ChCl was used as a hydrogen bond acceptor. The spectrum of ChCl (Figure 7b)

363

indicates the presence of a strong O–H bond stretching vibration at 3372 cm-1(broad) along

364

with C–H bond stretching peak at 2,955 and 2,905 cm-1, CH2 bond bending vibration peak at

365

1478 cm-1, and C–N+ symmetric stretching vibration peak51 at 614 cm-1.



21 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

Figure 7. FTIR spectra of urea (a), ChCl (b), DES (c), and (d) AMP carbamate 366

When comparing the spectra of urea and choline chloride with DES, there are

367

significant changes observed in both regions of the spectrum upon formation of the DES. The

368

band at 1450 cm−1 attributes (Figure 7c) to CH3 rocking associated with ChCl. It can be noted

369

that the absorption bands at 3439 cm−1, 3344 cm−1 in the spectrum of urea has changed to

370

broader bands in the spectrum of DES. This is due to the formation of strong hydrogen bonds

371

between urea and ChCl52. The hydrogen bonds may exist as N H· · ·N H, N H· · ·O H, H O· ·

372

·H O and O H· · ·N H. In addition to that, because of this more hydrogen bonding, the NH2

373

symmetric bending at 1677 cm−1 and NH2 asymmetric bending at 1625 cm−1 in the spectrum

374

of urea shift to 1665 cm−1 and 1620 cm−1. The C=O stretching at 1599 cm−1 in the spectrum


Page 22 of 28

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22 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

375

of urea disappears in the spectrum of DES and appears as C=O bending at 583 cm−1. The

376

C-C=O stretching at 955 cm−1 in the DES spectrum shows that the structure of Ch+ is not

377

destroyed in urea–ChCl system53.

378

The FTIR spectrum of AMP-carbamate obtained by the reaction of CO2 with AMP in

379

DES medium is presented in Figure 7d and the absorption bands of the spectra along with

380

their assignments are presented in Table 5. When CO2 reacted with AMP in aqueous medium

381

AMP-carbamate (RNCOO-), protonated AMP (AMPH+), and bicarbonate (HCO3-) are the

382

expected potential ionic reaction products. But, in the case of non-aqueous medium, AMP

383

being a sterically hindered amine should typically react with CO2 to form a carbamate

384

derivative. Specific assignments of the spectral peaks that emerged are detailed below. Table 5. FTIR band positions (cm-1) and the assignments of the AMP carbamate ion

Assignment

IR frequency (cm-1)

RNHCOO-

NH Stretching

3352

RNH3+

NH3 Symmetric Stretching

2963

RNH3+

NH3 Asymmetric stretching

1658

RNHCOO-

COO- Asymmetric stretching

1629

RNHCOO-

NH bending

1537

RNHCOO-

COO- Symmetric Stretching

1450

RNH3+

NH3 bending

1255

RNHCOO-

CN Stretching

1185

RNH3+

CN stretching

1065

RNHCOO-

CN Out of plane bending

866

385

On comparing with the most typical bands expected for an alkyl carbamate anion, it

386

has been observed that the features located at 3352 cm-1, 1629 cm-1 and 866 cm-1 are assigned



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

387

to the NH stretching, COO- asymmetric stretching and OCN out of plane bending,

388

respectively; corresponding to the vibrational modes of AMP-carboxylate anion. The most

389

typical vibrational bands of the AMPH+ cation is observed at 1658 cm-1 assigned to NH3

390

asymmetric bending54. The lack of weak and broad absorption bands in the region of 1350-

391

1360 cm-1 corresponds to the absence of bicarbonate ion55. The peaks between 3000 cm-1 and

392

2000 cm-1 can be explained as combination bands or overtones of lower mode of alkyl

393

ammonium ion. It is evident from these values that the IR spectrum is in good agreement

394

with the formation of AMP-carbamate.

395

4. Conclusions

396

CO2 absorption of alkanolamines in aqueous and DES medium was investigated. The

397

absorption was observed to be higher in DES medium compared with aqueous medium.

398

Among, the alkanolamines in various DES media, the CO2 absorption was higher in AMP in

399

the ChCl:U (1:2) medium. In addition to that, under optimized conditions, the maximum

400

yield of 82 %, 30 %, and 22 % of AMP-carbamate was obtained for 99 % CO2 gas, 15 % CO2

401

gas in 85 % N2, and 15 % CO2 gas in 1000 ppm of SO2 and balance N2 and it was confirmed

402

by FTIR, 1H and 13C NMR spectroscopic techniques. The maximum yield was found near the

403

critical temperature of CO2 and at a pressure of 6 bar. Thus, this work will provide some

404

technological insight into the identification of economical gas absorbents for the CO2 capture.

405

Hence, it is proper to say, that the application of alkanolamines in DES medium would be a

406

promising viable option for the capture as well as utilization of CO2 gas.

407

Supporting Information

408

1

409

consumption calculation for CO2 capture by AMP in aqueous and DES medium

H NMR (Figure S1),

13

C NMR (Figure S2) analysis of the AMP carbamate, energy

410 411


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412

Acknowledgement

413

A.Uma Maheswari acknowledges the financial support provided by the DST-INSPIRE (IF

414

10580), New Delhi, India for this research work.

415

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416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457

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