Carbon Dioxide Fixation via the Synthesis of Sodium Ethyl Carbonate

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Carbon Dioxide Fixation via Synthesis of Sodium Ethyl Carbonate in NaOH-dissolved Ethanol Sang-Jun Han, and Jung-Ho Wee Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03250 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 5, 2016

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Industrial & Engineering Chemistry Research

1 2

Title: Carbon Dioxide Fixation via Synthesis of Sodium Ethyl

3

Carbonate in NaOH Dissolved Ethanol

4 5 6

Sang-Jun Han, and Jung-Ho Wee*

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Department of Environmental Engineering, The Catholic University of Korea

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43 Jibong-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do 420-743, Republic of Korea

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* Corresponding author. Tel.: +82-2-2164-4866; fax: +82-2-2164-4765

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* E-mail address: [email protected] or [email protected]

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[email protected] or [email protected]

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-1-

ACS Paragon Plus Environment


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ABSTRACT

16 17

A CO2-fixing material, sodium ethyl carbonate (SEC), is systematically synthesized via

18

carbonation of NaOH dissolved ethanol. The synthesis is conducted by injecting 33.3 vol%

19

CO2 gas into the solution with a concentration of 1-4g NaOH/0.5L ethanol. The SEC

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composition is 97.3 wt%, and NaHCO3 is present in only minor amounts. When the

21

synthesized SEC dissolves in the excess water, it is converted to NaHCO3 precipitate, which

22

can still fix CO2, and ethanol is reproduced in aqueous solution. Therefore, the SEC synthesis

23

and ethanol regeneration process might be an effective means of carbon capture

24

storage/utilization. In addition, SEC thermally decomposes to Na2CO3, CO2, and diethyl ether

25

at 137℃ in an N2 atmosphere and slowly decomposes to Na2CO3·3NaHCO3 with a relatively

26

small release of ethanol and 20% of CO2 fixed in SEC under atmospheric conditions.

27

Furthermore, the main XRD peaks of SEC are herein reported.

28 29

Keywords

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Carbon capture and storage/utilization; Carbon dioxide; Sodium ethyl carbonate; Ethyl ester

32

sodium salt; Monoethyl ester sodium salt; Carbonic acid

33

-2-


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1. INTRODUCTION

35 36

Many studies on the development of renewable energy sources, increases in energy

37

efficiency, and carbon capture, have been conducted to contribute to the efforts to reduce

38

carbon emissions.1-4 Carbon capture and storage/utilization (CCSU) is considered to be one

39

of the most practical technologies because the CO2 that is generated can be directly reduced.5-

40

10

41

Mineral carbonation is also an option for CCSU. This process has various

42

advantageous features such as simplicity of process (integrated capture and storage),

43

favorable thermodynamics, and permanence of CO2 storage. Furthermore, the abundant raw

44

materials such as mineral and industrial residue, which include alkaline component, can be

45

used as source materials for CO2 fixation in the process.11-16 However, there are some hurdles

46

to overcome for practical use. For example, when the process is carried out in dry-based

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conditions, its efficiency is relatively low.17-20 In addition, although a wet process, in which

48

an aqueous solution is used as the solvent to dissolve the source materials such as alkali and

49

alkaline earth metals and to produce carbonated materials, gives high CO2 fixation efficiency,

50

the traditional wet method is not appropriate for CCSU for carbonated materials two

51

significant reasons. The first concerns the solubility of the source materials and carbonated

52

materials. Although the solubility of some alkali metals, especially NaOH, which is used in

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the present paper, is very high, it is difficult to use them as source materials because the

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solubility of the corresponding carbonated alkali metals (Na2CO3 or NaHCO3) in aqueous

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solution is also very high. In addition, although carbonated alkaline earth metals such as

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CaCO3 are almost insoluble in aqueous solution, the same is true for their corresponding

57

alkaline earth metals, such as Ca(OH)2. The second reason is the great amount of energy -3-



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needed to regenerate or treat the solution that remains after carbonation.20,21

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If these issues can be adequately addressed in a system where an alkali metal (NaOH)

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and an ethanol solution are used as the source material and the solvent, respectively, a wet-

61

based process could be developed as another option for CCSU to synergize absorption and

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mineral carbonation.

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In the present system, while the solubility of NaOH in ethanol is very high, the

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solubility of the carbonated alkali metal generated from the carbonation process, presumed to

65

be sodium ethyl carbonate (SEC), is very low and, thus, it can be easily precipitated from the

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solution, which is very important in terms of CCSU. In addition, once SEC has been removed

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from the solution, the remaining solution can immediately be reused for the next carbonation

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process. Furthermore, the energy consumed by the current system may be lower than that of a

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water-based system because the specific heat of ethanol is lower than that of water. Therefore,

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the carbonation process using NaOH dissolved in ethanol may have potential as a CCSU

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technology and would appear to be a niche application comparatively of small scale.

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There have been no published reports that aim to reduce CO2 emissions via formation

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of SEC, and only a few related studies, designed for other purposes, have been presented. In

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1859, Bellstein first reported a means of synthesizing SEC by injecting CO2 into a sodium

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ethoxide-absolute alcohol solution.22 At that time, there was little need for further research on

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SEC because of its limited number of applications; it was useful mainly as a carboxylation

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agent for ethoxide.

78

In 1974, DOE researchers described the generation of SEC in order to explain the

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development of a process by which ethanol could be regenerated from sodium ion (or other

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compound) dissolved in waste ethanol solution.23 In that study, a pure Na metal-dissolved

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ethanol solution was prepared and a material presumed to be SEC was synthesized by -4-


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injecting CO2 into the solution. The authors dissolved the synthesized materials in water,

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which removed the Na from the solution in the form of precipitated NaHCO3. However, the

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identity of the synthesized materials was solely confirmed by elemental analysis (EA), and

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only their characteristics when dissolved in water were described. The lack of attention paid

86

to SEC may be because SEC was only an intermediate compound, not the desired end

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product, and because its chemical and physical properties had never been detailed in any

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official data source.

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Two years later, Hirao et al.24 described a new, high-yield method of synthesizing

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alkali alkyl carbonates. First, weakly acidic mixtures of sodium phenoxide (PhONa) and

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sodium alkoxide (p-RC6H4ONa (R; CH3, CH3O, or Br) were dissolved as alkali salts in

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organic solvents such as alcohol, acetone, and tetrahydrofuran. Subsequently, SEC was

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putatively generated with a yield of 63-99% by injecting CO2 into the solution; however, their

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starting material was alkoxide and the synthesis process was relatively complicated. In

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addition, those authors highlighted the kinds of solvent used and the production yield without

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presenting a detailed description of the SEC synthesis process.

97

Except for the three aforementioned papers, no direct study on SEC synthesis and

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carbon fixation has yet been reported. However, some papers have been published and

99

patents have been filed regarding the synthesis of sodium ethoxide, which can act as a

100

precursor for SEC.25-27

101

When synthesizing SEC, CO2 acts as the reactant; therefore, CO2 is fixed during the

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synthesis process. SEC, when dissolved in water, can be easily chemically converted to

103

insoluble NaHCO3 in ethanol solution, so CO2 is immobilized in the form of insoluble

104

NaHCO3. In other words, SEC can be used as a solid material that incorporates the ethanol.

105

Therefore, SEC has promising potential for applications as a raw material in related fields, -5-



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such as the formation of solid fuels, disinfectant agents, and alcoholic beverages.

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Furthermore, SEC can be used as a precursor for diethyl ether (DEE) because it produces

108

DEE and Na2CO3 via thermal decomposition. Therefore, a method for SEC synthesis,

109

disposal, and utilization has a very high potential as an effective CCSU,28-30 and intensive

110

study of SEC may be significant in reducing CO2 emissions.

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As previously mentioned, past studies did not focus on CCSU, and SEC was merely

112

an intermediary in the processes being studied, rather than the target substance. Furthermore,

113

SEC was not systematically synthesized and clearly characterized in the studies. Therefore,

114

the chemical and physical properties of SEC, including its X-ray diffraction (XRD)

115

characterization, and thermal stability, have not yet been reported.

116

In the present study, SEC was directly synthesized injecting gaseous CO2 into a

117

NaOH dissolved ethanol solution. SEC purity was confirmed utilizing the following

118

analytical techniques, elemental analysis (EA), thermogravimetric analysis (TGA), powder x-

119

ray diffraction (XRD), and scanning electron microscopy (SEM). Additional experiments

120

were conducted to examine the chemical and physical features of SEC. These experimental

121

results might provide important data regarding development of wet-based CCSU process.

122

Finally, the paper also discussed the potential uses for the SEC and ethanol regeneration in

123

CCSU technology.

124 125

2. THEORY: Carbonation process for the synthesis of sodium ethyl carbonate (SEC)

126 127

SEC was produced as a precipitate at ambient temperature by the absorption and

128

reaction of CO2 in ethanol solution with dissolved NaOH as the limiting reactant. The overall

129

scheme of SEC synthesis, which is a three-phase reaction, has been previously reported24 and -6-


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can be expressed by eq 1.

131 132

C H OH l + NaOHaq + CO g → C H OCOONas + H Ol

(1)

133 134 135

According to previous authors,24 the reaction consists of two consecutive steps, as represented in eqs 2 and 3.

136 137

C H OH l + NaOHaq → C H O Na aq + H Ol

(2)

138

C H O Na aq + H Ol → C H OCOONas

(3)

139 140

First, NaOH is dissolved in ethanol solution to generate sodium ethoxide and water

141

according to eq 2. Second, as CO2 is injected into the solution, SEC is generated as listed in

142

eq 3. Although eq 1 is the main reaction that takes place under these circumstances and the

143

equilibrium of eq 2 is inclined to the right, other reactions do simultaneously occur. A slight

144

amount of NaOH dissociates to Na+ and OH- in ethanol solution, as shown in eq 4. Thereafter,

145

OH- reacts with the absorbed CO2 to generate HCO3- according to eq 5 and, therefore,

146

NaHCO3 is generated as a precipitate, as shown in eq 6. These overall side reactions are

147

summarized in eq 7.

148 149

C H OH l + NaOHaq → C H OH l + Na aq + OH aq

(4)

150

Na aq + OH aq + CO g → Na aq + HCO aq

(5)

151

Na aq + HCO aq → NaHCO s

(6)

152

C H OH l + NaOHaq + CO g → C H OHl + NaHCO s

(7)

153 -7-



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154

Therefore, the main precipitate generated from the carbonation of NaOH dissolved in an

155

ethanol solution is SEC, and NaHCO3 may be present in trace amounts. Their relative

156

compositions were investigated via various methods in the present work and are detailed in

157

the Results section.

158 159

3. ANALYSIS FOR MATERIAL CONFIRMATION AND CHARACTERIZATION

160 161

3.1. Elemental analysis (EA)

162 163

The first method used to confirm the identity of the synthesized materials was

164

qualitative EA of its constituents, C and H. The mass fractions of C and H were analyzed

165

with an EA instrument. If the synthesized material were made up of pure SEC, the mass

166

fractions of C and H would be 32.14 wt% and 4.46 wt%, respectively. On the other hand, if

167

the material were pure NaHCO3, the mass fractions of C and H would be 14.29 wt% and 1.19

168

wt%, respectively. If SEC and NaHCO3 are mixed in the synthesized material, the mass

169

fractions of C and H will be intermediate values.

170 171

3.2. Reaction with excess water

172 173 174

The second approach used to identify the synthesized material was to check whether eq 8 occurred stoichiometrically.

175 176

C H OCOONas + H Ol, excess → NaHCO s + C H OHl + H Ol

177

-8-


(8)

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178

Although this reaction was briefly mentioned in a previous report,23 no supporting

179

evidence was presented to validate the proposed reaction steps. According to eq 8, when SEC

180

is dissolved in excess H2O, NaHCO3 is generated and ethanol is reproduced in the solution.

181

The final state of eq 8 is the same as when a constant amount of NaHCO3 is present in an

182

ethanol-diluted aqueous solution at constant concentration. Therefore, the concentration of

183

ethanol reproduced in the solution is theoretically determined by the amount of SEC and H2O

184

that participate in eq 8. In addition, the proportions of SEC and NaHCO3 in the synthesized

185

material can be similarly estimated as in EA analysis. If the synthesized material contains

186

NaHCO3, the concentration of ethanol in an ethanol-diluted aqueous solution may be lower

187

than the stoichiometric value. NaHCO3 is very slightly soluble in ethanol, and can be

188

precipitated and separated from the solution as a CO2 fixation material.31

189 190

3.3. Reaction with ambient humidity

191 192

In addition to the two aforementioned methods, the present study proposes two other,

193

new, reactions which confirm the synthesized material to be SEC and verifies those reactions.

194

The first new reaction is denoted as eq 9.

195 196

5C H OCOONas + 4H Og → Na CO ∙ 3NaHCO s + 5C H OHl + CO g

(9)

197 198

When SEC reacts with water at trace concentrations, such as the humidity naturally

199

present in the atmosphere, the result is as shown in eq 9. Therefore, when SEC is exposed to

200

the atmosphere, it very slowly decomposes to Na2CO3·3NaHCO3 and releases ethanol and

201

CO2 to the air. However, the ratio of SEC to NaHCO3 in the synthesized material is difficult -9-



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202

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to estimate with this knowledge.

203 204

3.4. Thermal decomposition

205 206 207

The second proposed reaction involves the thermal decomposition of SEC. The standardized specific thermal decomposition of SEC is expressed in eq 10.

208 209

2C H OCOONas → Na CO s + CO g + C H OC H g

(10)

210 211

Pure SEC seems to thermally decompose to Na2CO3, CO2, and DEE in a high-N2

212

atmosphere. As a result, CO2 and gaseous DEE are released, leaving solid Na2CO3 with a

213

weight loss of 52.7 wt%. We confirmed this reaction by thermogravimetric analysis (TGA).

214

In addition, NaHCO3 thermally decomposes to solid Na2CO3, releasing CO2 and H2O, as

215

shown in eq 11.

216 217

NaHCO s → Na CO s + CO g + H Og

(11)

218 219

Because the total weight loss of eq 11 is 36.9 wt%, measuring the weight loss in the

220

synthesized material enables determination of the compositions of SEC and NaHCO3 via

221

thermal decomposition.

222 223

3.5. Characterization of the synthesized material via X-ray diffraction (XRD) and scanning

224

electron microscopy (SEM) imaging

225

- 10 -


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226

Based on the results of the four above-mentioned reactions, the main component of the

227

synthesized material was identified as SEC. As stated above, no significant, qualitative

228

analysis of SEC, such as its XRD pattern, has yet been reported. Therefore, the XRD pattern

229

of SEC is reported herein.

230 231

4. EXPERIMENTAL METHOD

232 233

4.1. Carbonation as a means of synthesizing sodium ethyl carbonate (SEC)

234 235

Four solutions were prepared by dissolving 1, 2, 3, and 4 g of NaOH (OCI Company

236

Ltd., 99.99%) in 0.5 L of highly purified absolute ethanol (OCI Company Ltd., 99.9%). These

237

solutions were used as the chemical absorbents for CO2 fixation as well as the reactants for

238

SEC synthesis. None of the solutions were saturated with NaOH and all were sufficiently

239

stirred to ensure complete NaOH dissolution in the closed vessel. The four solutions are

240

designated 1g-S, 2g-S, 3g-S, and 4g-S, respectively. The procedure for synthesizing SEC is

241

shown schematically in Figure 1.

242

------------------ Figure 1 ------------------

243

A Pyrex cylindrical batch reactor (D, 110mm; h, 80mm) was filled with the sample

244

solutions, which were stirred with a magnetic stirrer bar rotating at 200 rpm in the reactor.

245

The temperature of the reactor and gas mixer was maintained at 25℃. Before injecting CO2

246

into the reactor, every gas line in the apparatus and the absorbent were cleaned by N2 purging.

247

CO2 and N2 were mixed in the gas mixer equipped with a temperature controller at a constant

248

proportion. The flow rates were controlled by a mass flow controller at 1 L/min for CO2 and

249

2 L/min for N2. Before the reaction, the gas mixture by-passed the reactor and was directly - 11 -



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250

fed to a non-dispersive infrared (NDIR)-based gas analyzer (maMos-200, Madur Electronics)

251

to confirm that the CO2 gas composition had stabilized at 33.3%. Thereafter, the gas mixture

252

was injected into the reactor via a three-way valve in the apparatus and dispersed in the

253

reactor via a sparger made by glass filter. After passing through the reactor, the gases were

254

directed to a condenser maintained at 4℃ by a circulator to eliminate the volatilized ethanol.

255

The CO2 concentration in the gas leaving the condenser was measured every second by a gas

256

analyzer in order to determine the final gas composition. The reaction was considered

257

complete when the outgoing CO2 composition reached the level of the initial CO2 feed

258

composition, 33.3%. The amount of CO2 absorbed (or reacted) could be calculated based on

259

the variation of the outgoing CO2 composition according to reaction time. The method used

260

to calculate the amount of CO2 absorbed in the reaction was detailed in our previous paper.32

261

When the absorption of gas was terminated, a fine white powder (shown in Figure S1 in

262

Supporting Information), which was presumed to be SEC and which had been precipitating

263

continuously throughout the reaction, was enriched in the solution.

264

Finally, the solution was filtered and then the cake was dried in a vacuum oven at 50℃

265

for more than 1 hour to obtain the precipitate (presumed to be SEC) for confirmation and

266

characterization.

267 268

4.2. Confirmation and characterization of the synthesized material

269 270

EA (Perkin Elmer 2400 series II, Perkin Elmer) was used to positively identify the

271

synthesized material as SEC. The C and H compositions in the material were analyzed by EA

272

in CHN mode.

273

The occurrence of the reaction in eq 8 was confirmed by analyzing the ethanol - 12 -


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274

concentration in an aqueous solution containing the synthesized material. First, a 2.50 wt%

275

ethanol aqueous solution was prepared by dissolving stoichiometric amounts of the

276

synthesized material (presumed to be SEC) calculated based on eq 8 in a given amount of

277

distilled water. The material dissolved readily and completely disappeared in the solution

278

within a short time, releasing a certain amount of heat. Subsequently, a small amount of white

279

powder, presumed to be NaHCO3, was very slowly generated in the solution. Thereafter, the

280

transparent solution that had been obtained by filtration was pre-treated for ethanol analysis.

281

The ethanol composition in the transparent solution was measured by gas chromatography-

282

mass

283

(30m×250µm×0.25µm)). Meanwhile, the filter cakes separated from the solution were dried

284

for more than 3 hours at 50℃ in a vacuum oven and qualitatively analyzed by XRD.

285

spectrometry

(GC-MS,

Agilent

7890,

Agilent

Tech,

DB-WAX

column

To verify eq 9, 3 g of synthesized material in a vial (D; 30 mm, h; 60 mm) was exposed

286

to ambient air (25±3℃, 30-60% relative humidity) for 2 months, and then analyzed by XRD.

287

TGA (TGA N-1000, SCINCO) was used to validate eqs 10 and 11. After the sample was

288

loaded, high-purity N2 gas (99.99%) was fed into the TGA instrument at a flow rate of 25.5

289

mL/min. The weight loss and temperature derivative were measured as the sample was raised

290

from room temperature to 400℃ at a ramp rate of 2℃/min. In addition, to qualitatively

291

measure the final solid product resulting from eq 10, a substantial amount of the synthesized

292

material was thermally decomposed in a high-temperature vacuum tube furnace (GSL-1100,

293

MTI Corp.) under the same conditions as those used for TGA analysis. Thereafter, the final

294

product of eqs 10 and 11 was sampled and analyzed by XRD. All XRD patterns were

295

obtained with a Siemens Bruker AXS D-5000 instrument using Cu Ka1 radiation in Bragg-

296

Brentano reflecting and Debye-Scherrer transmission geometry. The samples were scanned in

297

a 2θ range of 10-90˚ with a step size of 0.2˚ and a scan rate of 1 min per step. In addition, the - 13 -



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298

particle shape of the synthesized material was measured by scanning electron microscopy

299

(SEM; S-4800, Hitachi).

300 301

5. RESULTS AND DISCUSSION

302 303

5.1. Amount of CO2 absorbed during the synthesis of sodium ethyl carbonate (SEC)

304 305 306 307

Figure 2 shows the theoretical and experimentally observed amounts of CO2 absorbed during the production of SEC in the four NaOH-dissolved ethanol solutions. ------------------ Figure 2 ------------------

308

As CO2 was injected into the solution, a constant amount of CO2 was absorbed and

309

reacted with the components of the solution. The saturation level of CO2 in 500 mL of pure

310

ethanol was 1.88 g, as assessed by conducting an experiment prior to the reaction. The total

311

amount of theoretically-absorbed CO2 in the solution, which is the summation of 1.88 g and

312

the theoretical amount of CO2 reacted based on eq 1, is shown as a dotted line in Figure 2.

313

Overall, the experimentally observed CO2 values were similar to the theoretical values in the

314

four solutions. This offered direct confirmation that the reaction in eq 1 (alone or in

315

combination with that in eq 7) had occurred and proceeded stoichiometrically. However, the

316

slope of the experimental value according to NaOH concentration in the solution was slightly

317

higher than that of the theoretical values. For example, the amount of CO2 absorbed in 1g-S

318

was 2.78g, which was 6.7% smaller than the theoretical amount. However, the experimental

319

value increased and became almost identical to the theoretical value in 2g-S and 3g-S. Finally,

320

the experimental value of CO2 absorbed in 4g-S was slightly larger than the theoretical value.

321

This phenomenon was attributed to the following two side reactions: - 14 -


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322 323

H Ol + CO g → H aq + HCO aq

(12)

324

C H OH l + HCO aq → C H OCOO aq + H Ol

(13)

325 326

In other words, water, the product of eq 1, reacts with the continuously supplied CO2 to

327

generate HCO3- in the solution. After that, HCO3- reacts with ethanol to produce C2H5OCOO-,

328

according to eq 13, until equilibrium is attained. Finally, the water produced from eq 1

329

triggers eqs 12 and 13 and the reactions are intensified according to the concentration of

330

NaOH in the solution. Therefore, the relative amount of CO2 absorbed in a highly

331

concentrated NaOH solution is larger than that in a weakly concentrated solution. However,

332

because this additional CO2 is not directly used for SEC synthesis and cannot be

333

quantitatively included in the theoretical amount of CO2 absorbed, the experimental value of

334

CO2 absorbed in the highly concentrated NaOH solution is larger than the theoretical value.

335

Therefore, the SEC manufacturing process denoted by eq 1 has potential as a new CCSU

336

technology.

337

The process, however, as with other traditional CCSU technologies, may involve

338

economic hurdles. Ethanol price might be one of the important factors influencing the

339

economy. Therefore, reusability of the ethanol seems to be the most critical point in

340

improving the economy of this process. Although the reusability of the solutions was not

341

discussed in the paper, SEC mixed with a small amount of NaHCO3 powder was also

342

produced via direct carbonation in ethanol solution that had been reused six or even seven

343

times without any additional treatment. In addition, the net amount of ethanol solely

344

consumed for SEC synthesis is very small and the ethanol was not difficult to recover leaving

345

NaHCO3 and water according to eq 8. Furthermore, there are a variety of means to increase - 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

346

the economy of the process as being one of the CCSU processes. Theoretically, SEC can be

347

the economically available materials such as the feedstock of solid fuels, disinfectant agents,

348

powdered alcoholic beverages and a precursor for diethyl ether (DEE). Including these SEC

349

application potentials, many of fundamental issues regarding the economy should be

350

intensively reviewed whether it can meet an economically acceptable cost value of CCSU

351

process, 50~150 $/tCO2 avoided. However, it is not easy to be conducted at current stage

352

because the process is suggested as an alternative conceptual methodology in the paper. They

353

will be achieved in near future.

354 355

5.2. Confirmation of the synthesized material

356 357

5.2.1. Element analysis (EA) and reaction with excess water

358 359 360

The compositions of C and H in the synthesized material as measured by EA were compared to the theoretical values in pure SEC and NaHCO3, as listed in Table 1.

361

------------------ Table 1 ------------------

362

The compositions of C and H in the synthesized material were intermediate between

363

those of pure SEC and NaHCO3. Therefore, the synthesized material was physically

364

confirmed to be a mixture of SEC and NaHCO3, and the amounts of each could be quantified.

365

The portion of SEC in the synthesized material based on the composition of C and H in the

366

synthesized material was 98.23 wt% and 96.19 wt%, respectively. Therefore, the main

367

component of the synthesized material is SEC, and NaHCO3 is present in only minor

368

amounts.

369

A constant amount of the synthesized material was dissolved in excess water to verify - 16 -


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370

eq 8. 2 g of the synthesized material was dissolved in 32.36 g of water to prepare a 2.50 wt%

371

ethanol aqueous solution according to eq 8. However, the real concentration of ethanol in the

372

solution was measured to be 2.42 wt%; this result supports the notion that a slight amount of

373

NaHCO3 is present in the synthesized material. The composition of SEC in the synthesized

374

material can be calculated by the difference between the theoretical and measured ethanol

375

concentration. It was thusly estimated to be 97.60 wt%.

376 377 378

The XRD peaks of the precipitated substance generated from eq 8 are shown in Figure 3. ------------------ Figure 3 ------------------

379

The XRD patterns perfectly matched those of the NaHCO3 standard. These results offer

380

further evidence confirming SEC as the synthesized material. Therefore, when SEC reacted

381

with excess water, CO2 was fixed in the form of NaHCO3 and ethanol was reproduced at a

382

constant concentration in the aqueous ethanol solution.

383 384

5.2.2. Thermogravimetric analysis (TGA)

385 386

The results of TGA analysis of the synthesized material are shown in Figure 4.

387

------------------ Figure 4 ------------------

388

As the temperature increased from 100 to 150℃, the sample weight rapidly

389

decreased to a minimum, at which it remained. SEC, therefore, thermally decomposed in this

390

temperature range. The thermal decomposition point and total weight loss of the sample were

391

137℃ and 52.1%, respectively, while the theoretical weight loss of pure SEC and NaHCO3

392

are 52.7% and 36.9%, respectively. Based on these findings, the SEC composition in the

393

synthesized material was calculated to be 97.21 wt%. - 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

394 395

The XRD patterns of the synthesized materials decomposed in an electrical furnace under the same condition as that of the TGA analysis are shown in Figure 5.

396

------------------ Figure 5 ------------------

397

These XRD patterns perfectly corresponded with those of a standard Na2CO3 sample

398

and, thus, eqs 10 and 11 could be confirmed. However, diethyl ether generated during the

399

decomposition could not be identified.

400 401

5.3. Characterization of the synthesized material

402 403

5.3.1. X-ray diffraction (XRD)

404 405

The combination of the aforementioned results confirmed that the synthesized

406

material was mainly SEC, with a very small amount of NaHCO3. The XRD patterns of the

407

synthesized material were scanned and the result in the maximum intensity range is shown in

408

Figure 6a.

409

------------------ Figure 6 ------------------

410

In Figure 6a, the main peaks do not correspond with those of any previously reported

411

Na, C, O, and H-based substances. In addition, the standard peaks of NaHCO3 are not clearly

412

identifiable. This may be because NaHCO3 is present merely at trace levels and its

413

crystallinity may be very small compared to that of SEC. Nevertheless, to confirm the

414

presence of NaHCO3 as an impurity, the small peaks in the low-intensity zone were

415

magnified, as shown in Figure 6b. Some of the small peaks were identified as those of

416

NaHCO3. Finally, the main peaks shown in Figure 6a are believed to be the standard XRD

417

patterns of SEC; this is the first report of its standard XRD patterns in the published literature. - 18 -


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418

The d value, intensity, and relative intensity of the synthesized materials are summarized in

419

Table S1 in Supporting Information and the expected main peaks of SEC are highlighted.

420 421

5.3.2. Reaction with ambient humidity

422 423 424

The XRD pattern of the synthesized material following exposure to ambient air for two months is shown in Figure 7.

425

------------------ Figure 7 ------------------

426

The exposed material was confirmed to be Na2CO3·3NaHCO3, which supports the

427

validity of eq 9 and the role of water as the limiting reactant in this reaction. Theoretically,

428

when 1 g of SEC is exposed to ambient air, it reacts with 0.13 g of H2O to generate 0.41 g of

429

ethanol and 0.08 g of CO2. Finally, 0.40 g of CO2 was reacted to synthesize 1 g of SEC

430

according to eq 1, and 20% of the CO2 reacted was released, as shown in eq 9, with the

431

balance of CO2 fixed in the form of Na2CO3·3NaHCO3.

432 433

5.3.3. SEM photographs

434 435 436

SEM images of the synthesized SEC are shown in Figure 8. ------------------ Figure 8 ------------------

437

The SEC grain exhibits a rhombic- and plate-type structure. The maximum grain size

438

is about 14.2×19.5 µm, as shown in Figure 8a, with a thickness of approximately 0.5 µm, as

439

shown in Figure 8b. The synthesized SEC was physically very weak and brittle, as

440

demonstrated when the crystal was easily cracked during the SEM analysis.

441

- 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

442

6. CONCLUSION

443 444

In the present paper, a CO2-fixing material was synthesized by absorbing gaseous CO2

445

in NaOH-dissolved ethanol solution. Sodium ethyl carbonate (SEC) was the main component

446

of the synthesized material, along with a very slight amount of NaHCO3, at an average

447

composition of approximately 2.7 wt%. This was confirmed via various analytical methods,

448

such as gas analysis, EA, TGA, GC-MS, XRD, and in a furnace.

449

Although the synthesized SEC is itself a CO2-fixing material, when it dissolves excess

450

water, it is converted to insoluble NaHCO3, which can also fix CO2, and ethanol is

451

reproduced at a constant concentration in the solution. Although the recycling of SEC-

452

separated carbonated solutions was not detailed in the paper, we confirmed that the solution

453

can be reused for the next carbonation cycle by adding NaOH to the solution, without any

454

other treatment, which will be reported in the near future. Therefore, the SEC synthesis and

455

ethanol regeneration process proposed in this paper has the potential to be an effective CCSU

456

technology. However, the CO2 fixing properties described in the paper leave many challenges

457

to be solved for its practical use because the study primarily focused on the SEC synthesis

458

and it characterization. The challenges include the optimum condition determination of the

459

various factors which influence the performance of the process such as the ratio of solids-to-

460

liquid (solution concentration), temperature, pressure (or composition) of feeding gas and etc.

461

In addition, the number of reusable cycles of the ethanol and make-up amount of NaOH for

462

regeneration in the process should be intensively analyzed for feasibility study of the process.

463

However, as aforementioned, the process is suggested briefly as an alternative conceptual

464

methodology with very limited results and thus they will be conducted in near future.

465

Some of the chemical and physical characteristics of the synthesized materials were - 20 -


Page 20 of 40

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466

presented. Chemically, as the synthesized material was exposed to water, which acted as the

467

limiting reactant, in the form of atmospheric humidity, it slowly decomposed to

468

Na2CO3·3NaHCO3 with a relatively small release of ethanol and 20% of CO2 fixed in SEC.

469

Furthermore, SEC thermally decomposed to Na2CO3, CO2, and diethyl ether at 137℃ in an

470

N2 atmosphere. The main XRD pattern of SEC was herein reported, which can be regarded as

471

the standard XRD pattern. These chemical and physical characteristics might offer the

472

constructive information to find the SEC application and to develop the conceptual process of

473

CCSU technology. However, these characteristics should be required a more in-depth

474

analysis and these will be also conducted in near the future.

475 476

Acknowledgements

477 478

This research was supported by Basic Research Program through the National Research

479

Foundation of Korea (NRF) funded by the Ministry of Education (2013R1A1A2A10010414)

480

as well as supported by the Catholic University of Korea, Research Fund, 2015.

481 482

Supporting Information

483 484

Figure S1.Photograph of the synthesized material, sodium ethyl carbonate (SEC).

485 486

Table S1. The d values and intensities in the XRD pattern of the synthesized material; bold an

487

d underlined values are expected to represent the main SEC peaks.

488 489

This information is available free of charge via the Internet at http://pubs.acs.org. - 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

490

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CO2 Capture Process. Int. J. Greenhouse Gas Control 2015, 43, 33.

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from The Basic Ionic Liquids Orientated on Mesoporous Materials. ACS Appl. Mater.

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Interfaces 2014, 6, 12947.

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Kim, Y. H.; Kim, S. O. Selective and Regenerative Carbon Dioxide Capture by Highly

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Polarizing Porous Carbon Nitride. Ind. Eng. Chem. Res. 2015, 9, 9148.

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Microsized and Nanosized Mesoporous Carbons for Carbon Dioxide Capture. Ind. Eng.

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Chem. Res. 2016, 55, 7355.

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in Coal-Fired Power Plants: Energy-Saving Mechanism, Proposed Methods, and Performance

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(8) Kumar, S.; Saxena, S. K.; Drozd, V.; Durygin, A. An Experimental Investigation of

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Mesoporous MgO As a Potential Pre-Combustion CO2 Sorbent. Mater. Renew. Sustain.

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Energy 2015, 4, 1.

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(9) Meng, L. Y.; Park, S. J. MgO-Templated Porous Carbons-Based CO2 Adsorbents

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Produced by KOH Activation. Mater. Chem. Phys. 2012, 137, 91.

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(10) Labus, K.; Tarkowski, R.; Wdowin, M. Modeling Gas–Rock–Water Interactions in

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Carbon Dioxide Storage Capacity Assessment: A Case Study of Jurassic Sandstones in

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Poland. Int. J. Environ. Sci. Technol. 2015, 8, 2493.

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(11) Samanta, A.; Zhao, A.; Shimizu, G. K.; Sarkar, P.; Gupta, R. Post-Combustion CO2

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Capture Using Solid Sorbents: A Review. Ind. Eng. Chem. Res. 2011, 51, 1438.

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(12) Lu, H.; Reddy, E. P.; Smirniotis, P. G.; Calcium Oxide Based Sorbents for Capture of

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Carbon Dioxide at High Temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944.

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535

Brown Coal Fly Ash for CO2 Sequestration: Multiple-Cycle Leaching-Carbonation and

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(14) Guo, Y.; Li, C.; Lu, S.; Zhao, C. K2CO3-Modified Potassium Feldspar for CO2 Capture

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from Post-Combustion Flue Gas. Energy Fuels 2015, 29, 8151.

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(15) Wang, C.; Yue, H.; Li, C.; Liang, B.; Zhu, J.; Xie, H. Mineralization of CO2 Using

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Natural K-Feldspar and Industrial Solid Waste to Produce Soluble Potassium. Ind. Eng. Chem.

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Res. 2014, 53, 7971.

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(16) Sanna, A.; Maroto-Valer, M. M. CO2 Capture at High Temperature Using Fly Ash-

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Derived Sodium Silicates. Ind. Eng. Chem. Res. 2016, 55, 4080.

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(17) Azdarpour, A.; Asadullah, M.; Mohammadian, E.; Junin, R.; Hamidi, H.; Manan, M.;

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Daud, A. R. M. Mineral Carbonation of Red Gypsum via pH-Swing Process: Effect of CO2

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Pressure on The Efficiency And Products Characteristics. Chem. Eng. J. 2015, 264, 425.

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(18) Zingaretti, D.; Costa, G.; Baciocchi, R. Assessment of Accelerated Carbonation

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Processes for CO2 Storage Using Alkaline Industrial Residues. Ind. Eng. Chem. Res. 2013, 53,

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(19) Lee, J. B.; Ryu, C. K.; Baek, J. I.; Lee, J. H.; Eom, T. H.; Kim, S. H. Sodium-Based Dry

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Regenerable Sorbent for Carbon Dioxide Capture from Power Plant Flue Gas. Ind. Eng.

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Chem. Res. 2008, 47, 4465.

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and Pressures in Flue Gas Conditions. Environ. Sci. Technol. 2014, 48, 5163.

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Brown Coal Fly Ash Using Regenerative Ammonium Chloride–Process Simulation and

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Techno-Economic Analysis. Appl. Energy 2016, 175, 54.

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of Components; Atomics International Div, 1974.

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Derivatives. XXII. Formation of Alkali Alkyl Carbonate by the O-Carboxylation of Alcohol

576

in the Presence of an Alkali Salt of a Weak Acid. Bull. Chem. Soc. Jpn. 1976, 49, 2775.

577 578

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579

Characterization of Sodium Alkoxides. Bull. Mater. Sci. 2006, 29, 173.

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Thermal Decomposition of Sodium Methoxide and Ethoxide. J. Nucl. Mater. 2006, 358, 111.

583 584

(27) Vacek, J. Alkali Metal Alcoholates There of. C.S. Patent 213,119, 1984.

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(28) Lin, P. H.; Wong, D. S. H. Carbon Dioxide Capture and Regeneration with

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Amine/Alcohol/Water Blends. Int. J. Greenhouse Gas Control 2014, 26, 69.

588 589

(29) Alves, D. C.; Silva, R.; Voiry, D.; Asefa, T.; Chhowalla, M. Copper Nanoparticles

590

Stabilized by Reduced Graphene Oxide for CO2 Reduction Reaction. Mater. Renew. Sustain.

591

Energy 2015, 4, 1.

592 593

(30) Zhong, N.; Liu, H.; Luo, X.; Al-Marri, M. J.; Benamor, A.; Idem, R.; Tontiwachwuthikul,

594

P.; Liang, Z. Reaction Kinetics of Carbon dioxide (CO2) with Diethylenetriamine and 1-

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Amino-2-propanol in Nonaqueous Solvents Using Stopped-Flow Technique. Ind. Eng. Chem.

596

Res. 2016, 55, 7307.

597 598

(31) Lide, D. R. CRC Handbook of Chemistry and Physics. 85th ed. CRC Press: Boca Raton,

599

FL, 2004.

600 601

(32) Han, S. J.; Yoo, M.; Kim, D. W.; Wee, J. H. Carbon Dioxide Capture Using Calcium

602

Hydroxide Aqueous Solution as the Absorbent. Energy Fuels 2011, 25, 3825.

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604


List of Tables

605 606

Table 1

607

Composition of C and H in the synthesized material, in pure sodium ethyl carbonate (SEC),

608

and in pure NaHCO3. Materials

Elemental composition (wt%) C

H

Pure SEC

32.14

4.46

Pure NaHCO3

14.29

1.19

Synthesized material

31.73

4.30

609

- 27 -



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

610

List of Figures

611 612

Figure 1. Schematic diagram for sodium ethyl carbonate (SEC) synthesis via absorption of

613

CO2 in NaOH-dissolved ethanol solution. (1) CO2 cylinder, (2) N2 cylinder, (3) Mass flow

614

controller, (4) Gas mixer, (5) Temp controller, (6) Sparger, (7) Magnetic stirrer, (8) pH meter,

615

(9) EC meter, (10) pH/EC recorder, (11) Condenser, (12) Gas analyzer, and (13) Computer

616

used for data acquisition.

617 618

Figure 2. Theoretical and experimental amounts of CO2 absorbed to create the synthesized

619

material, sodium ethyl carbonate (SEC), in each solution.

620 621

Figure 3. XRD patterns of the precipitated substance generated by dissolving the synthesized

622

material, sodium ethyl carbonate (SEC), in excess water.

623 624

Figure 4. TGA results of the synthesized material, sodium ethyl carbonate (SEC).

625 626

Figure 5. XRD patterns of the thermally decomposed substance of the synthesized material,

627

sodium ethyl carbonate (SEC).

628 629

Figure 6. XRD patterns of the synthesized material, sodium ethyl carbonate (SEC), in the

630

high (a) and low (b) intensity zone.

631 632

Figure 7. XRD patterns of the synthesized material, sodium ethyl carbonate (SEC), exposed

633

to ambient air for two months. - 28 -


Page 28 of 40

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634 635

Figure 8. SEM images of the synthesized material, sodium ethyl carbonate SEC: (a) upper

636

view and (b) side view.

637

- 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

638

For Table of Contents Only

639

- 30 -


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Figure 1. Schematic diagram for sodium ethyl carbonate (SEC) synthesis via absorption of CO2 in NaOHdissolved ethanol solution. (1) CO2 cylinder, (2) N2 cylinder, (3) Mass flow controller, (4) Gas mixer, (5) Temp controller, (6) Sparger, (7) Magnetic stirrer, (8) pH meter, (9) EC meter, (10) pH/EC recorder, (11) Condenser, (12) Gas analyzer, and (13) Computer used for data acquisition. 217x142mm (96 x 96 DPI)



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 2. Theoretical and experimental amounts of CO2 absorbed to create the synthesized material, sodium ethyl carbonate (SEC), in each solution. 130x127mm (300 x 300 DPI)


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Figure 3. XRD patterns of the precipitated substance generated by dissolving the synthesized material, sodium ethyl carbonate (SEC), in excess water. 151x114mm (300 x 300 DPI)



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 4. TGA results of the synthesized material, sodium ethyl carbonate (SEC). 163x113mm (300 x 300 DPI)


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Figure 5. XRD patterns of the thermally decomposed substance of the synthesized material, sodium ethyl carbonate (SEC). 151x114mm (300 x 300 DPI)



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. XRD patterns of the synthesized material, sodium ethyl carbonate (SEC), in the high (a) and low (b) intensity zone. 174x189mm (300 x 300 DPI)


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Figure 7. XRD patterns of the synthesized material, sodium ethyl carbonate (SEC), exposed to ambient air for two months. 154x114mm (300 x 300 DPI)



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Figure 8. SEM images of the synthesized material, sodium ethyl carbonate SEC: (a) upper view and (b) side view. 127x95mm (96 x 96 DPI)


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Figure 8. SEM images of the synthesized material, sodium ethyl carbonate SEC: (a) upper view and (b) side view. 127x95mm (96 x 96 DPI)



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139x94mm (96 x 96 DPI)


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Carbon Dioxide Fixation via the Synthesis of Sodium Ethyl Carbonate

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