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Nov 1, 2016 - Department of Environmental Engineering, The Catholic University of Korea 43 Jibong-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do. 420-743 ...
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Carbon Dioxide Fixation via the Synthesis of Sodium Ethyl Carbonate in NaOH-Dissolved Ethanol Sang-Jun Han† and Jung-Ho Wee* Department of Environmental Engineering, The Catholic University of Korea 43 Jibong-ro, Wonmi-gu, Bucheon-si, Gyeonggi-do 420-743, Republic of Korea

Ind. Eng. Chem. Res. 2016.55:12111-12118. Downloaded from pubs.acs.org by LUND UNIV on 01/02/19. For personal use only.

S Supporting Information *

ABSTRACT: A CO2-fixing material, sodium ethyl carbonate (SEC), is systematically synthesized via the carbonation of NaOH-dissolved ethanol. The synthesis is conducted by injecting 33.3 vol % CO2 gas into the solution with a concentration of 1−4 g of NaOH/0.5 L of ethanol. The SEC composition is 97.3 wt %, and NaHCO3 is present in only minor amounts. When the synthesized SEC is dissolved in excess water, it is converted to a NaHCO3 precipitate, which can still fix CO2, and ethanol is reproduced in aqueous solution. Therefore, the SEC synthesis and ethanol regeneration process might be an effective means of carbon capture storage/utilization. In addition, SEC thermally decomposes to Na2CO3, CO2, and diethyl ether at 137 °C in an N2 atmosphere and slowly decomposes to Na2CO3·3NaHCO3 with a relatively small release of ethanol and 20% of CO2 fixed in SEC under atmospheric conditions. Furthermore, the main X-ray diffraction peaks of SEC are herein reported.

1. INTRODUCTION Many studies on the development of renewable energy sources, increases in energy efficiency, and carbon capture have been conducted to contribute to the efforts to reduce carbon emissions.1−4 Carbon capture storage/utilization (CCSU) is considered to be one of the most practical technologies because carbon dioxide (CO2) that is generated can be directly reduced.5−10 Mineral carbonation is also an option for CCSU. This process has various advantageous features such as simplicity of the process (integrated capture and storage), favorable thermodynamics, and permanence of CO2 storage. Furthermore, abundant raw materials such as mineral and industrial residues, which include an alkaline component, can be used as source materials for CO2 fixation in the process.11−16 However, there are some hurdles to overcome for practical use. For example, when the process is carried out in dry-based conditions, its efficiency is relatively low.17−20 In addition, although a wet process, in which an aqueous solution is used as the solvent to dissolve the source materials such as alkali and alkaline-earth metals and to produce carbonated materials, gives high CO2 fixation efficiency, the traditional wet method is not appropriate for CCSU for carbonated materials for two significant reasons. The first concerns the solubility of the source and carbonated materials. Although the solubility of some alkali metals, especially NaOH, which is used in the present paper, is very high, it is difficult to use them as source materials because the solubility of the corresponding carbonated alkali metals (Na2CO3 or NaHCO3) in aqueous © 2016 American Chemical Society

solution is also very high. In addition, although carbonated alkaline-earth metals such as CaCO3 are almost insoluble in aqueous solution, the same is true for their corresponding alkaline-earth metals, such as Ca(OH)2. The second reason is the great amount of energy needed to regenerate or treat the solution that remains after carbonation.20,21 If these issues can be adequately addressed in a system where an alkali metal (NaOH) and an ethanol solution are used as the source material and the solvent, respectively, a wet-based process could be developed as another option for CCSU to synergize absorption and mineral carbonation. In the present system, while the solubility of NaOH in ethanol is very high, the solubility of the carbonated alkali metal generated from the carbonation process, presumed to be sodium ethyl carbonate (SEC), is very low and, thus, it can be easily precipitated from the solution, which is very important in terms of CCSU. In addition, once SEC has been removed from the solution, the remaining solution can immediately be reused for the next carbonation process. Furthermore, the energy consumed by the current system may be lower than that of a water-based system because the specific heat of ethanol is lower than that of water. Therefore, the carbonation process using NaOH dissolved in ethanol may have potential as a CCSU Received: Revised: Accepted: Published: 12111

August 24, 2016 October 31, 2016 November 1, 2016 November 1, 2016 DOI: 10.1021/acs.iecr.6b03250 Ind. Eng. Chem. Res. 2016, 55, 12111−12118

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characterization and thermal stability, have not yet been reported. In the present study, SEC was directly synthesized by injecting gaseous CO2 into a NaOH-dissolved ethanol solution. The SEC purity was confirmed by utilizing the following analytical techniques: EA, thermogravimetric analysis (TGA), powder XRD, and scanning electron microscopy (SEM). Additional experiments were conducted to examine the chemical and physical features of SEC. These experimental results might provide important data regarding the development of a wet-based CCSU process. Finally, the paper also discussed the potential uses for SEC and ethanol regeneration in CCSU technology.

technology and would appear to be a niche application of comparatively small scale. There have been no published reports that aim to reduce CO2 emissions via the formation of SEC, and only a few related studies, designed for other purposes, have been presented. In 1859, Bellstein first reported a means of synthesizing SEC by injecting CO2 into a sodium ethoxide−absolute alcohol solution.22 At that time, there was little need for further research on SEC because of its limited number of applications; it was useful mainly as a carboxylation agent for ethoxide. In 1974, U.S. Department of Energy researchers described the generation of SEC in order to explain the development of a process by which ethanol could be regenerated from sodium ion (or another compound) dissolved in a waste ethanol solution.23 In that study, a pure sodium-metal-dissolved ethanol solution was prepared and a material presumed to be SEC was synthesized by injecting CO2 into the solution. The authors dissolved the synthesized materials in water, which removed the sodium from the solution in the form of precipitated NaHCO3. However, the identity of the synthesized materials was solely confirmed by elemental analysis (EA), and only their characteristics when dissolved in water were described. The lack of attention paid to SEC may be because SEC was only an intermediate compound, not the desired end product, and because its chemical and physical properties had never been detailed in any official data source. Two years later, Hirao et al.24 described a new high-yield method of synthesizing alkali alkylcarbonates. First, weakly acidic mixtures of sodium phenoxide (PhONa) and sodium alkoxide (p-RC6H4ONa, where R = CH3, CH3O, or Br) were dissolved as alkali salts in organic solvents such as alcohol, acetone, and tetrahydrofuran. Subsequently, SEC was putatively generated with a yield of 63−99% by injecting CO2 into the solution; however, their starting material was alkoxide, and the synthesis process was relatively complicated. In addition, those authors highlighted the kinds of solvents used and the production yield without presenting a detailed description of the SEC synthesis process. Except for the three aforementioned papers, no direct study on the SEC synthesis and carbon fixation has yet been reported. However, some papers have been published, and patents have been filed regarding the synthesis of sodium ethoxide, which can act as a precursor for SEC.25−27 When synthesizing SEC, CO2 acts as the reactant; therefore, CO2 is fixed during the synthesis process. SEC, when dissolved in water, can be easily chemically converted to insoluble NaHCO3 in an ethanol solution, so CO2 is immobilized in the form of insoluble NaHCO3. In other words, SEC can be used as a solid material that incorporates ethanol. Therefore, SEC has promising potential for applications as a raw material in related fields, such as the formation of solid fuels, disinfectant agents, and alcoholic beverages. Furthermore, SEC can be used as a precursor for diethyl ether (DEE) because it produces DEE and Na2CO3 via thermal decomposition. Therefore, a method for SEC synthesis, disposal, and utilization has a very high potential as an effective CCSU,28−30 and intensive study of SEC may be significant in reducing CO2 emissions. As previously mentioned, past studies did not focus on CCSU, and SEC was merely an intermediary in the processes being studied rather than the target substance. Furthermore, SEC was not systematically synthesized and clearly characterized in the studies. Therefore, the chemical and physical properties of SEC, including its X-ray diffraction (XRD)

2. THEORY: CARBONATION PROCESS FOR THE SYNTHESIS OF SEC SEC was produced as a precipitate at ambient temperature by the absorption and reaction of CO2 in an ethanol solution with dissolved NaOH as the limiting reactant. The overall scheme of the SEC synthesis, which is a three-phase reaction, has been previously reported24 and can be expressed by eq 1. C2H5OH(l) + NaOH(aq) + CO2 (g) → C2H5OCOONa(s) + H 2O(l)

(1)

24

According to previous authors, the reaction consists of two consecutive steps, as represented in eqs 2 and 3. C2H5OH(l) + NaOH(aq) → C2H5O−Na +(aq) + H 2O(l) (2) −

+

C2H5O Na (aq) + CO2 (g) → C2H5OCOONa(s)

(3)

First, NaOH is dissolved in an ethanol solution to generate sodium ethoxide and water according to eq 2. Second, as CO2 is injected into the solution, SEC is generated as listed in eq 3. Although eq 1 is the main reaction that takes place under these circumstances and the equilibrium of eq 2 is inclined to the right, other reactions do simultaneously occur. A slight amount of NaOH dissociates to Na+ and OH− in an ethanol solution, as shown in eq 4. Thereafter, OH− reacts with the absorbed CO2 to generate HCO3− according to eq 5 and, therefore, NaHCO3 is generated as a precipitate, as shown in eq 6. These overall side reactions are summarized in eq 7. C2H5OH(l) + NaOH(aq) → C2H5OH(l) + Na +(aq) + OH−(aq)

(4)

Na +(aq) + OH−(aq) + CO2 (g) → Na +(aq) + HCO3−(aq)

(5)

Na +(aq) + HCO3−(aq) → NaHCO3(s)

(6)

C2H5OH(l) + NaOH(aq) + CO2 (g) → C2H5OH(l) + NaHCO3(s)

(7)

Therefore, the main precipitate generated from the carbonation of NaOH dissolved in an ethanol solution is SEC, and NaHCO3 may be present in trace amounts. Their relative compositions were investigated via various methods in the present work and are detailed in the Results and Discussion section. 12112

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Figure 1. Schematic diagram for SEC synthesis via the absorption of CO2 in a NaOH-dissolved ethanol solution: (1) CO2 cylinder; (2) N2 cylinder; (3) mass flow controller; (4) gas mixer; (5) temperature controller; (6) sparger; (7) magnetic stirrer; (8) pH meter; (9) EC meter; (10) pH/EC recorder; (11) condenser; (12) gas analyzer; (13) computer used for data acquisition.

SEC and verifies those reactions. The first new reaction is denoted as eq 9.

3. ANALYSIS FOR MATERIAL CONFIRMATION AND CHARACTERIZATION 3.1. EA. The first method used to confirm the identity of the synthesized materials was qualitative EA of its constituents, carbon and hydrogen. The mass fractions of carbon and hydrogen were analyzed with an EA instrument. If the synthesized material were made up of pure SEC, the mass fractions of carbon and hydrogen would be 32.14 and 4.46 wt %, respectively. On the other hand, if the material were pure NaHCO3, the mass fractions of carbon and hydrogen would be 14.29 and 1.19 wt %, respectively. If SEC and NaHCO3 are mixed in the synthesized material, the mass fractions of carbon and hydrogen will be intermediate values. 3.2. Reaction with Excess Water. The second approach used to identify the synthesized material was to check whether eq 8 occurred stoichiometrically.

5C2H5OCOONa(s) + 4H 2O(g) → Na 2CO3 · 3NaHCO3(s) + 5C2H5OH(l) + CO2 (g) (9)

When SEC reacts with water at trace concentrations, such as the humidity naturally present in the atmosphere, the result is as shown in eq 9. Therefore, when SEC is exposed to the atmosphere, it very slowly decomposes to Na2CO3·3NaHCO3 and releases ethanol and CO2 to the air. However, the ratio of SEC to NaHCO3 in the synthesized material is difficult to estimate with this knowledge. 3.4. Thermal Decomposition. The second proposed reaction involves thermal decomposition of SEC. The standardized specific thermal decomposition of SEC is expressed in eq 10.

C2H5OCOONa(s) + H 2O(l, excess) → NaHCO3(s) + C2H5OH(l) + H 2O(l)

(8)

2C2H5OCOONa(s)

Although this reaction was briefly mentioned in a previous report,23 no supporting evidence was presented to validate the proposed reaction steps. According to eq 8, when SEC is dissolved in excess water, NaHCO3 is generated and ethanol is reproduced in the solution. The final state of eq 8 is the same as that when a constant amount of NaHCO3 is present in an ethanol-diluted aqueous solution at constant concentration. Therefore, the concentration of ethanol reproduced in the solution is theoretically determined by the amount of SEC and water that participate in eq 8. In addition, the proportions of SEC and NaHCO3 in the synthesized material can be similarly estimated as in EA. If the synthesized material contains NaHCO3, the concentration of ethanol in an ethanol-diluted aqueous solution may be lower than the stoichiometric value. NaHCO3 is very slightly soluble in ethanol and can be precipitated and separated from the solution as a CO2 fixation material.31 3.3. Reaction with Ambient Humidity. In addition to the two aforementioned methods, the present study proposes two other new reactions that confirm the synthesized material to be

→ Na 2CO3(s) + CO2 (g) + C2H5OC2H5(g)

(10)

Pure SEC seems to thermally decompose to Na2CO3, CO2, and DEE in a pure N2 atmosphere. As a result, CO2 and gaseous DEE are released, leaving solid Na2CO3 with a weight loss of 52.7 wt %. We confirmed this reaction by TGA. In addition, NaHCO3 thermally decomposes to solid Na2CO3, releasing CO2 and water, as shown in eq 11. 2NaHCO3(s) → Na 2CO3(s) + CO2 (g) + H 2O(g)

(11)

Because the total weight loss of eq 11 is 36.9 wt %, measuring the weight loss in the synthesized material enables determination of the compositions of SEC and NaHCO3 via thermal decomposition. 3.5. Characterization of the Synthesized Material via XRD and SEM Imaging. On the basis of the results of the four above-mentioned reactions, the main component of the synthesized material was identified as SEC. As stated above, no significant, qualitative analysis of SEC, such as its XRD pattern, 12113

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The material dissolved readily and completely disappeared in the solution within a short time, releasing a certain amount of heat. Subsequently, a small amount of white powder, presumed to be NaHCO3, was very slowly generated in the solution. Thereafter, the transparent solution that had been obtained by filtration was pretreated for ethanol analysis. The ethanol composition in the transparent solution was measured by gas chromatography−mass spectrometry [GC−MS; Agilent 7890, Agilent Tech, DB-WAX column (30 m × 250 μm × 0.25 μm)]. Meanwhile, the filter cakes separated from the solution were dried for more than 3 h at 50 °C in a vacuum oven and qualitatively analyzed by XRD. To verify eq 9, 3 g of synthesized material in a vial (D, 30 mm; h, 60 mm) was exposed to ambient air (25 ± 3 °C and 30−60% relative humidity) for 2 months and then analyzed by XRD. TGA (TGA N-1000, SCINCO) was used to validate eqs 10 and 11. After the sample was loaded, high-purity N2 gas (99.99%) was fed into the TGA instrument at a flow rate of 25.5 mL/min. The weight loss and temperature derivative were measured as the sample was raised from room temperature to 400 °C at a ramp rate of 2 °C/min. In addition, to qualitatively measure the final solid product resulting from eq 10, a substantial amount of the synthesized material was thermally decomposed in a high-temperature vacuum tube furnace (GSL1100, MTI Corp.) under the same conditions as those used for TGA. Thereafter, the final product of eqs 10 and 11 was sampled and analyzed by XRD. All XRD patterns were obtained with a Siemens Bruker AXS D-5000 instrument using Cu Ka1 radiation in Bragg−Brentano reflecting and Debye−Scherrer transmission geometry. The samples were scanned in a 2θ range of 10−90° with a step size of 0.2° and a scan rate of 1 min/step. In addition, the particle shape of the synthesized material was measured by SEM (S-4800, Hitachi).

has yet been reported. Therefore, the XRD pattern of SEC is reported herein.

4. EXPERIMENTAL METHOD 4.1. Carbonation as a Means of Synthesizing Sodium Ethyl Carbonate (SEC). Four solutions were prepared by dissolving 1, 2, 3, and 4 g of NaOH (OCI Company Ltd., 99.99%) in 0.5 L of highly purified absolute ethanol (OCI Company Ltd., 99.9%). These solutions were used as chemical absorbents for CO2 fixation as well as reactants for SEC synthesis. None of the solutions were saturated with NaOH, and all were sufficiently stirred to ensure complete NaOH dissolution in the closed vessel. The four solutions are designated as 1g-S, 2g-S, 3g-S, and 4g-S, respectively. The procedure for synthesizing SEC is shown schematically in Figure 1. A Pyrex cylindrical batch reactor (D, 110 mm; h, 80 mm) was filled with the sample solutions, which were stirred with a magnetic stirrer rotating at 200 rpm in the reactor. The temperature of the reactor and gas mixer was maintained at 25 °C. Before CO2 was injected into the reactor, every gas line in the apparatus and absorbent was cleaned by N2 purging. CO2 and N2 were mixed in a gas mixer equipped with a temperature controller at a constant proportion. The flow rates were controlled by a mass flow controller at 1 L/min for CO2 and 2 L/min for N2. Before the reaction, the gas mixture bypassed the reactor and was directly fed to a nondispersive-infrared-based gas analyzer (maMos-200, Madur Electronics) to confirm that the CO2 gas composition had stabilized at 33.3%. Thereafter, the gas mixture was injected into the reactor via a three-way valve in the apparatus and dispersed in the reactor via a sparger made by a glass filter. After passing through the reactor, the gases were directed to a condenser maintained at 4 °C by a circulator to eliminate the volatilized ethanol. The CO2 concentration in the gas leaving the condenser was measured every 1 s by a gas analyzer in order to determine the final gas composition. The reaction was considered complete when the outgoing CO2 composition reached the level of the initial CO2 feed composition, 33.3%. The amount of CO2 absorbed (or reacted) could be calculated based on the variation of the outgoing CO2 composition according to the reaction time. The method used to calculate the amount of CO2 absorbed in the reaction was detailed in our previous paper.32 When the absorption of gas was terminated, a fine white powder (shown in Figure S1 in the Supporting Information), which was presumed to be SEC and which had been precipitating continuously throughout the reaction, was enriched in the solution. Finally, the solution was filtered, and then the cake was dried in a vacuum oven at 50 °C for more than 3 h to obtain the precipitate (presumed to be SEC) for confirmation and characterization. 4.2. Confirmation and Characterization of the Synthesized Material. EA (PerkinElmer 2400 series II) was used to positively identify the synthesized material as SEC. The carbon and hydrogen compositions in the material were analyzed by EA in CHN mode. The occurrence of the reaction in eq 8 was confirmed by analyzing the ethanol concentration in an aqueous solution containing the synthesized material. First, a 2.50 wt % ethanol aqueous solution was prepared by dissolving stoichiometric amounts of the synthesized material (presumed to be SEC) calculated based on eq 8 in a given amount of distilled water.

5. RESULTS AND DISCUSSION 5.1. Amount of CO2 Absorbed during the Synthesis of SEC. 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. Theoretical and experimental amounts of CO2 absorbed to create the synthesized material, SEC, in each solution. 12114

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However, it can not easily be conducted at the current stage because the process is suggested as an alternative conceptual methodology in the paper. They will be achieved in the near future. 5.2. Confirmation of the Synthesized Material. 5.2.1. EA and Reaction with Excess Water. The compositions of carbon and hydrogen in the synthesized material as measured by EA were compared to the theoretical values in pure SEC and NaHCO3, as listed in Table 1.

As CO2 was injected into the solution, a constant amount of CO2 was absorbed and reacted with the components of the solution. The saturation level of CO2 in 500 mL of pure ethanol was 1.88 g, as assessed by conducting an experiment prior to the reaction. The total amount of theoretically absorbed CO2 in the solution, which is the summation of 1.88 g and the theoretical amount of CO2 reacted based on eq 1, is shown as a dotted line in Figure 2. Overall, the experimentally observed CO2 values were similar to the theoretical values in the four solutions. This offered direct confirmation that the reaction in eq 1 (alone or in combination with that in eq 7) had occurred and proceeded stoichiometrically. However, the slope of the experimental value according to the NaOH concentration in the solution was slightly higher than that of the theoretical value. For example, the amount of CO2 absorbed in 1g-S was 2.78 g, which was 6.7% smaller than the theoretical amount. However, the experimental value increased and became almost identical with the theoretical values in 2g-S and 3g-S. Finally, the experimental value of CO2 absorbed in 4g-S was slightly larger than the theoretical value. This phenomenon was attributed to the following two side reactions: H 2O(l) + CO2 (g) → H+(aq) + HCO3−(aq)

Table 1. Composition of Carbon and Hydrogen in the Synthesized Material, in Pure SEC, and in Pure NaHCO3 elemental composition (wt %) C

H

pure SEC pure NaHCO3 synthesized material

32.14 14.29 31.73

4.46 1.19 4.30

The compositions of carbon and hydrogen in the synthesized material were intermediate between those of pure SEC and NaHCO3. Therefore, the synthesized material was physically confirmed to be a mixture of SEC and NaHCO3, and the amounts of each could be quantified. The portion of SEC in the synthesized material based on the composition of carbon and hydrogen in the synthesized material was 98.23 and 96.19 wt %, respectively. Therefore, the main component of the synthesized material is SEC, and NaHCO3 is present in only minor amounts. A constant amount of the synthesized material was dissolved in excess water to verify eq 8. A total of 2 g of the synthesized material was dissolved in 32.36 g of water to prepare a 2.50 wt % ethanol aqueous solution according to eq 8. However, the real concentration of ethanol in the solution was measured to be 2.42 wt %; this result supports the notion that a slight amount of NaHCO3 is present in the synthesized material. The composition of SEC in the synthesized material can be calculated by the difference between the theoretical and measured ethanol concentrations. It was thus estimated to be 97.60 wt %. The XRD peaks of the precipitated substance generated from eq 8 are shown in Figure 3.

(12)

C2H5OH(l) + HCO3−(aq) → C2H5OCOO−(aq) + H 2O(l)

material

(13)

In other words, water, the product of eq 1, reacts with the continuously supplied CO2 to generate HCO3− in the solution. After that, HCO 3 − reacts with ethanol to produce C2H5OCOO−, according to eq 13, until equilibrium is attained. Finally, the water produced from eq 1 triggers eqs 12 and 13, and the reactions are intensified according to the concentration of NaOH in the solution. Therefore, the relative amount of CO2 absorbed in a highly concentrated NaOH solution is larger than that in a weakly concentrated solution. However, because this additional CO2 is not directly used for the SEC synthesis and cannot be quantitatively included in the theoretical amount of CO2 absorbed, the experimental value of CO2 absorbed in the highly concentrated NaOH solution is larger than the theoretical value. Therefore, the SEC manufacturing process denoted by eq 1 has potential as a new CCSU technology. The process, however, as with other traditional CCSU technologies, may involve economic hurdles. The ethanol price might be one of the important factors influencing the economy. Therefore, reusability of ethanol seems to be the most critical point in improving the economy of this process. Although the reusability of the solutions was not discussed in the paper, SEC mixed with a small amount of NaHCO3 powder was also produced via direct carbonation in an ethanol solution that had been reused six or even seven times without any additional treatment. In addition, the net amount of ethanol solely consumed for SEC synthesis is very small and ethanol was not difficult to recover, leaving NaHCO3 and water according to eq 8. Furthermore, there are a variety of means to increase the economy of the process as being one of the CCSU processes. Theoretically, SEC can be the economically available materials such as the feedstock of solid fuels, disinfectant agents, powdered alcoholic beverages, and a precursor for DEE. Including these SEC application potentials, many of fundamental issues regarding the economy should be intensively reviewed on whether it can meet an economically acceptable cost value of the CCSU process, $50−150/tCO2 avoided.

Figure 3. XRD patterns of the precipitated substance generated by dissolving the synthesized material, SEC, in excess water. 12115

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5.3. Characterization of the Synthesized Material. 5.3.1. XRD. The combination of the aforementioned results confirmed that the synthesized material was mainly SEC, with a very small amount of NaHCO3. The XRD patterns of the synthesized material were scanned, and the result in the maximum intensity range is shown in Figure 6a.

The XRD patterns perfectly matched those of the NaHCO3 standard. These results offer further evidence that SEC is the synthesized material. Therefore, when SEC was reacted with excess water, CO2 was fixed in the form of NaHCO3 and ethanol was reproduced at a constant concentration in the aqueous ethanol solution. 5.2.2. TGA. The results of TGA of the synthesized material are shown in Figure 4.

Figure 4. TGA results of the synthesized material SEC.

As the temperature increased from 100 to 150 °C, the sample weight rapidly decreased to a minimum, at which it remained. SEC, therefore, thermally decomposed in this temperature range. The thermal decomposition point and total weight loss of the sample were 137 °C and 52.1%, respectively, while the theoretical weight loss of pure SEC and NaHCO3 are 52.7% and 36.9%, respectively. On the basis of these findings, the SEC composition in the synthesized material was calculated to be 97.21 wt %. The XRD patterns of the synthesized materials decomposed in an electrical furnace under the same conditions as those of TGA are shown in Figure 5. These XRD patterns perfectly corresponded with those of a standard Na2CO3 sample and, thus, eqs 10 and 11 could be confirmed. However, DEE generated during the decomposition could not be identified.

Figure 6. XRD patterns of the synthesized material SEC in the high(a) and low-intensity (b) zones.

In Figure 6a, the main peaks do not correspond with those of any previously reported sodium, carbon, oxygen, and hydrogenbased substances. In addition, the standard peaks of NaHCO3 are not clearly identifiable. This may be because NaHCO3 is present merely at trace levels and its crystallinity may be very small compared to that of SEC. Nevertheless, to confirm the presence of NaHCO3 as an impurity, the small peaks in the low-intensity zone were magnified, as shown in Figure 6b. Some of the small peaks were identified as those of NaHCO3. Finally, the main peaks shown in Figure 6a are believed to be the standard XRD patterns of SEC; this is the first report of its standard XRD patterns in the published literature. The d value, intensity, and relative intensity of the synthesized materials are summarized in Table S1 in the Supporting Information, and the expected main peaks of SEC are highlighted. 5.3.2. Reaction with Ambient Humidity. The XRD pattern of the synthesized material following exposure to ambient air for 2 months is shown in Figure 7. The exposed material was confirmed to be Na2CO3· 3NaHCO3, which supports the validity of eq 9 and the role of water as the limiting reactant in this reaction. Theoretically, when 1 g of SEC is exposed to ambient air, it reacts with 0.13 g of water to generate 0.41 g of ethanol and 0.08 g of CO2. Finally, 0.40 g of CO2 was reacted to synthesize 1 g of SEC according to eq 1, and 20% of the CO2 reacted was released, as shown in eq 9, with the balance of CO2 fixed in the form of Na2CO3·3NaHCO3. 5.3.3. SEM Photographs. SEM images of the synthesized SEC are shown in Figure 8.

Figure 5. XRD patterns of the thermally decomposed substance of the synthesized material SEC. 12116

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pressure (or composition) of the feeding gas, etc. In addition, the number of reusable cycles of ethanol and the makeup amount of NaOH for regeneration in the process should be intensively analyzed for feasibility studies of the process. However, as aforementioned, the process is suggested briefly as an alternative conceptual methodology with very limited results, and thus they will be conducted in the near future. Some of the chemical and physical characteristics of the synthesized materials were presented. Chemically, as the synthesized material was exposed to water, which acted as the limiting reactant, in the form of atmospheric humidity, it slowly decomposed to Na2CO3·3NaHCO3 with a relatively small release of ethanol and 20% of CO2 fixed in SEC. Furthermore, SEC thermally decomposed to Na2CO3, CO2, and DEE at 137 °C in an N2 atmosphere. The main XRD pattern of SEC was herein reported, which can be regarded as the standard XRD pattern. These chemical and physical characteristics might offer constructive information to find the SEC application and to develop the conceptual process of CCSU technology. However, these characteristics require a more in-depth analysis, and these also will be conducted in the near future.

Figure 7. XRD patterns of the synthesized material SEC exposed to ambient air for 2 months.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03250. Photograph of the synthesized material SEC (Figure S1) and d values and intensities in the XRD pattern of the synthesized material, where bold and underlined values represent the main SEC peaks (Table S1) (PDF)

Figure 8. SEM images of the synthesized material SEC: (a) upper view; (b) side view.

■

The SEC grain exhibits a rhombic- and plate-type structure. The maximum grain size is about 14.2 × 19.5 μm, as shown in Figure 8a, with a thickness of approximately 0.5 μm, as shown in Figure 8b. The synthesized SEC was physically very weak and brittle, as demonstrated when the crystal was easily cracked during SEM analysis.

AUTHOR INFORMATION

Corresponding Author

*Tel.: +82-2-2164-4866. Fax: +82-2-2164-4765. E-mail: [email protected] or [email protected]. Notes

The authors declare no competing financial interest. † E-mail: [email protected] or [email protected].

6. CONCLUSION In the present paper, a CO2-fixing material was synthesized by absorbing gaseous CO2 in a NaOH-dissolved ethanol solution. SEC was the main component of the synthesized material, along with a very slight amount of NaHCO3, at an average composition of approximately 2.7 wt %. This was confirmed via various analytical methods, such as gas analysis, EA, TGA, GC− MS, XRD, and in a furnace. Although the synthesized SEC is itself a CO2-fixing material, when it dissolves excess water, it is converted to insoluble NaHCO3, which can also fix CO2, and ethanol is reproduced at a constant concentration in the solution. Although the recycling of SEC-separated carbonated solutions was not detailed in the paper, we confirmed that the solution can be reused for the next carbonation cycle by adding NaOH to the solution, without any other treatment, which will be reported in the near future. Therefore, the SEC synthesis and ethanol regeneration process proposed in this paper has the potential to be an effective CCSU technology. However, the CO2-fixing properties described in the paper leave many challenges to be solved for its practical use because the study primarily focused on the SEC synthesis and it characterization. The challenges include the optimum condition determination of the various factors that influence the performance of the process such as the ratio of solids-to-liquid (solution concentration), temperature,

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ACKNOWLEDGMENTS This research was supported by the Basic Research Program through the National Research Foundation of Korea funded by the Ministry of Education (Grant 2013R1A1A2A10010414) as well as by the Catholic University of Korea, Research Fund, 2015.

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DOI: 10.1021/acs.iecr.6b03250 Ind. Eng. Chem. Res. 2016, 55, 12111−12118

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NOTE ADDED AFTER ASAP PUBLICATION This paper was published ASAP on November 10, 2016, with an error to equation 11. The corrected version was reposted on November 11, 2016.

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DOI: 10.1021/acs.iecr.6b03250 Ind. Eng. Chem. Res. 2016, 55, 12111−12118