Carbon Dioxide Capture by Diethylenetriamine Hydrobromide in

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Carbon Dioxide Capture by Diethylenetriamine Hydrobromide in Nonaqueous Systems and Phase-Change Formation Yang Chen, and Hui Hu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00268 • Publication Date (Web): 03 Apr 2017 Downloaded from http://pubs.acs.org on April 9, 2017

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Carbon Dioxide Capture by Diethylenetriamine Hydrobromide in Nonaqueous Systems and Phase-Change Formation

Yang Chen, Hui Hu* School of Environmental Science and Engineering, Huazhong University of Science & Technology, 1037 Luoyu Road, Wuhan 430074, PR China Email: [email protected]

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ABSTRACT: Diethylenetriamine hydrobromide ([DETAH]Br) was used for capturing CO2 in polyethylene glycol 200 (PEG200) solution, in which phase change was observed. PEG200 could not only act as a cosolvent, but also be involved in CO2 absorption, contributing to biphasic separation and increased CO2 capacity. The potential applications of the mixtures would take advantage of high CO2 capacity (1.184mol/mol [DETAH]Br), and also less energy consumed when considering the mild regeneration condition and low specific heat of PEG200. This system could be recycled at least for five continuous absorption-desorption cycles without significant loss of CO2 capturing and releasing capability. Spectroscopic analysis revealed O–H···:N type of H-bonding and the stabilizing effects of bromide ion would play important roles in stabilizing the system and preventing the biphasic separation by overcoming the electrostatic attractions in the early stage of CO2 absorption. It is estimated that such simple and inexpensive solution provides an excellent alternative to current CO2 capture technologies.

1. Introduction The increasing emissions and accumulation of carbon dioxide (CO2) in the atmosphere, which potentially contribute to global warming, have aroused immediate need and considerable interest in developing efficient, reversible and economic technologies for capturing and sequestration of large quantities of CO2.1,2 Currently, several technologies have being widely investigated for CO2 capture on the laboratory and industrial scales.3,4 Among them, the state of the art in the use of aqueous alkanolamine solutions seems to be the established leading candidate in industry for CO2 capture due to their high absorption reactivity, high selectivity, scale-up feasibility and relatively low cost.5 Although these aqueous amine solutions are effective, they are subjected to severe economic and environmental problems such as equipment corrosion, volatilization and degradation of amine absorbents.6 Unaffordably high energy cost for solvent regeneration and solvent loss due to evaporation

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is the major obstacle in large scale application of aqueous alkanolamine based CO2 capture processes.7,8 It is estimated that the energy consumption for solvent regeneration accounts for 25–40% of the power output loss, and the cost of absorbent regeneration accounts for 70–80% of the overall operation costs, which hinder the wide application of aqueous amines in CO2 capture.9,10 It has been reported that energy more than 90 kJ is required to produce enough water evaporation for removing 1 mol CO2 from the monoethanolamine (MEA) solution, and the regeneration temperature is generally operated at as high as 110–125℃.9,11 Consequently, it is urgent to develop efficient, reversible and less energy intensive alternatives to overcome the above drawbacks. Ionic liquid (IL) has been proposed as a prospective CO2 absorbent due to the appealing and diverse characteristics, such as high CO2 absorption capacity, high thermal stability, negligible vapor pressure, and tunable properties.12,13 As compared to alkanolamines, IL for CO2 capture has no contamination of gas stream, negligible losses, and lower energy consumption. Many research groups have focused on the development of IL for CO2 capture. Hu et al. reported that the CO2 capacity of poly-amino-based ionic liquid could be up to 2.04mo1 CO2/mol IL.14 Shiflett et al. reported that CO2 capture from postcombustion flue gas by 60% IL aqueous solution gave possible energy savings between 12% and 16%.15 Lv et al. found that the regeneration efficiency of 30% [Apmim][Gly]) aqueous solution was higher than 97.5%, much better than MEA aqueous solution.16 In spite of this evolution, ILs as CO2 absorbent have not yet be widely applied in industry on a large scale. One reason is that several synthetic and purification steps are required when preparing functionalized ionic liquids. Together with the expensive and not readily available starting materials, the prices of ionic liquids are probably not cost-competitive when compared with commodity chemicals such as MEA.17,18 Another drawback that impedes their further application is the high viscosity.

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Particularly, task-specific ionic liquids (TSILs) with amino groups may suffer from higher viscosity, with their viscosities further increase after absorbing CO2.19 One strategy in addressing these problems while maintaining tunability of IL is the use of additives or co-solvents, such as the mixtures of low viscosity ionic liquids, water and amines. Mirarab et al. employed ethanol–[EMIM][Tf2N] mixtures for CO2 capture, but this option would partially counterweigh the associated advantages of the use of ILs, such as high thermal stability and low vapor pressure.20 Based on the purpose of keeping the significantly attractive property prevailing, while at the same time allowing an improved tunability, it is beneficial that the capture reaction is performed in nonvolatile medium. In view of this, researchers resort to nonvolatile solvents such as ionic liquids, polymeric solvents, and deep eutectic solvents.3 Given the drawbacks of state of the art, polyethylene glycol (PEG) was quickly identified as potential alternative solvent for reaction and separation processes. Compared to water, PEGs have competitively distinctive properties such as high thermal stability, almost negligible vapor pressure, being nontoxic, relatively inexpensive and readily available, which enable PEGs to be excellent solvent for gas separation even at high temperature.21 In addition, the characteristics of PEG could be tuned to some extent by varying the length of its polymeric chains. As for application in CO2 capture, PEGs are promising in improving the efficiency of CO2 capture and reducing the energy consumption of regeneration.22 Li et al. detailedly investigated CO2 capture by [Choline][Pro]/PEG200 mixture, and found that the addition of PEG200 significantly enhanced the kinetics of absorption and desorption.1 During CO2 capture process, the emergency of phase changing solvent system has drawn much focus in the past few years. Precipitation or biphasic carbonated solution is formed in this CO2 capturing system with completion of CO2 absorption.2 Hu et al. had investigated CO2 capture into precipitate with

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alkanolamine–alcohol mixture and found that the regeneration energy consumption of the phase transitional absorption process is approximately 15% of that of the MEA-based process.23 Kassim et al. proposed that biphasic phenomenon could be due to the formation of insoluble MEA–carbamate in the mixture.2 Hasib-ur-Rahman investigated diethanolamine carbamate and 2-amino-2-methyl-1-propanol carbamate formation in alkanolamine-RTIL (room-temperature ionic liquid) mixtures. The results showed readily and high efficient regeneration.4 It is worthy noting that the greater mass transfer capacity could partially offset the downside posed by higher viscosity of the ionic liquids. As a result, it is helpful to overcome the equilibrium limitation thus not only allowing maximum CO2 loading but also enabling easy separation of the solid product and less regeneration energy.18,24 Besides, the phase change system could partially reduce corrosion to equipment.17 Prior to conducting this study, Ilioudis et al. have focused on conformations, binding sites, geometries and interaction types of protonated polyamine matrix by the analysis of crystal structures, the effects of crystal structures on basicity as well as solid-state and solution phase properties provide important reference for CO2 capture research.25-27 Hu et al. have used a series of polyamine-based ionic liquid prepared from DETA and TETA for CO2 capture in aqueous solution.28 The high energy consumption for regeneration and corrosion phenomenon in gas processing would be tricky problems. As for non-aqueous absorbent, Doyle et al. have been concentrated on removal of low levels of CO2 from ambient air with [DETAH]NO3 in PEG solution.29 However, inexpensive and readily available ionic liquid/PEG systems are still worthy of being exploited. Particularly, phase-change formation was extensively observed, but there is a lack of detailed reports about the mechanism of biphase formation in CO2 capture system.

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In this research, diethylenetriamine hydrobromide ([DETAH]Br) was chosen as CO2 absorbent in PEG solution. As illustrated previously, one of the reasons that limits the application of ionic liquids and other exotic trapping species for CO2 capture is the unaffordable cost of the required materials and synthesis process. Compared to some other task-specific ionic liquids that require several reaction steps for synthesis with high cost materials, [DETAH]Br can be easily prepared from less expensive and readily available commercial sources with a relatively simple synthesis by one step neutralization reaction. On the other hand, one molecule of [DETAH]Br could bind one molecule of CO2, which enables [DETAH]Br to have a higher molar capacity than MEA. In comparison with many other TSILs, the simple bromine salt is usually solid at room temperature, which facilitates transportation and storage. In this research, different molecular weights of PEG were taken into account as cosolvent, CO2 capture performance of [DETAH]Br was compared with a series of ionic liquid of different anions prepared from DETA, and effects of water content, temperature as well as viscosity on CO2 capture and biphase formation

were

scrutinized.

Corrosion

and

degradation

behaviors

were

also

evaluated.

Absorption-desorption cycles were undertaken at relatively low temperature with bubbling nitrogen gas through the solution. Detailed thermogravimetric-differential scanning calorimetry (TG-DSC), Fourier Transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) spectroscopic characterization were performed to understand the CO2 absorption and biphase formation mechanism of [DETAH]Br-PEG system. The changes of hydrogen bonding in CO2 capture process, including the stabilizing effects of bromide ion on biphase formation were examined in detail. We propose that such simple and inexpensive solution would provide an excellent alternative to current CO2 capture technologies.

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Table 1. Chemical information Name

CAS number

Molecule weight (g/mol)

Purity (wt %)

Diethylenetriamine

111-40-0

103.17

≥99.5%

Hydrobromic acid

10035-10-6

80.91

≥40.0%

Hydrochloric acid

7647-01-0

36.46

36.0%-38.0%

Fluoroboric acid

16872-11-0

87.81

≥40.0%

Nitric acid

7697-37-2

63.01

65.0%-68.0%

Formic acid

64-18-6

46.03

≥88.0%

Acetic acid

64-19-7

60.05

≥99.7%

Ethylene glycol

107-21-1

62.07

≥99.5%

Polyethylene glycol 200

25322-68-3

190-210

≥99.5%


25322-68-3

280-320

≥99.5%


25322-68-3

380-430

≥99.5%

2. Experimental apparatus and procedures 2.1. Materials The ionic liquid diethylenetriamine hydrobromide ([DETAH]Br) was prepared by reacting diethylenetriamine slowly (over ice) with 1 equiv of aqueous hydrobromic acid (Scheme 1). Water was removed by rotary evaporation, and the product was dried under vacuum to obtain a transparent orange solid. The detailed characterization and structural information were included in Supporting Information. Other ionic liquid could be prepared with similar method. The chemicals were supplied by Sinopharm Chemical Reagent Co., LTD (Beijing, China) and the chemical information was shown in Table 1. Nitrogen (N2) and carbon dioxide (CO2) with purity of 99.999% were supplied by Wuhan Specialty Gases Co., LTD (Wuhan, China).

Scheme 1. Schematic illustration to synthesize [DETAH]Br.

2.2. Absorption and desorption processes

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The absorption/desorption experiments were carried out in a laboratory bubbling reactor under ambient pressure (1atm), which had been used in our previous work.14 The apparatus mainly consisted of a constant temperature water bath, a test tube (10 mm×150 mm), a CO2 gas cylinder and a rotameter. The temperature of the water bath was maintained within 20±0.5℃. 20.0 g absorbent consisted of 20 wt% [DETAH]Br and 80wt% PEG was loaded in the test tube, and the flow rate of CO2 was controlled at 250 mL/ min (PCO2 = 1atm) by a rotameter. An electronic balance (Sartorius AG, TE124S) with a precision of 0.1mg was used to measure the weight of the test tube every 5 minutes. The solubility of CO2 in the solution could be calculated based on the weight increase. The CO2 release was performed after the absorption. The CO2 saturated [DETAH]Br-PEG200 mixture was heated to 80 ℃ in a preheated oil bath, refluxed with N2 bubbled through (250mL/min), PCO2 = 0, PN2=1 atm, and then cooled for another absorption cycle. Both repeated experiments on CO2 absorption and repeated experiments on CO2 desorption gave an estimate of experimental error at a level of ±5%. The rich phase was filtered and washed thoroughly with ethanol, dried under vacuum at 60℃ for 8h, and then stored at room temperature.

2.3. Characterization A nuclear magnetic resonance spectrometer (AV600MHz, Bruker Co. LTD.) was used to detect the structural changes of absorbents before and after CO2 absorption. 0.1mL liquid sample or 30mg solid sample was dissolved in 0.5mL D2O for

13

C nuclear magnetic resonance (13C NMR) (1024 scans)

analysis. 0.1mL liquid sample was dissolved in 0.5mL CD3OD for 1H nuclear magnetic resonance (1H NMR) (32 scans) analysis. The thermal stability of the reaction products was measured by TG and DSC, using a thermogravimetric analyzer (Diamond TG/DTA6300, PerkinElmer Instruments Co. LTD., USA) in a nitrogen atmosphere, with a heating rate of 10℃/min from room temperature to 800℃. The FTIR

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spectra were carried out using a VERTEX 70 FTIR spectrometer (Germany) with a resolution of 4 cm-1 and 64 scans by attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) from 450 cm-1 to 4000 cm-1. The water content was measured by coulometric titration using a Karl Fischer method (756 KF, Metrohm). The viscosity of the solution was measured by using a digital rotational viscometer (NDJ-5S, Shanghai Bangxi Instrument Technology Co., Ltd., China). The temperature was controlled by water bath.

3. Results and discussion 3.1. The effect of cosolvent on CO2 capture and phase-change formation To investigate the CO2 absorption performance of [DETAH]Br-PEG system, different molecular weights of PEG were taken into account, and the absorption in EG and water solution were used as comparisons.

Figure 1. (a) Real-time change of CO2 absorption in [DETAH]Br-PEG200 system; (b) The effects of anions on phase-change formation in PEG200 solution; (c) Biphasic separation of the saturated solution after reaction; (d) The effect of concentration on phase-change formation. Figure 1(a) showed the visible changes of [DETAH]Br-PEG200 system as a function of time with 9


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the continuous addition of CO2. The initial state of the mixture was viscous clear liquid. And the time of obvious opaque gel (not stable) formation was 60min. After absorption, biphasic separation was observed when the saturated emulsion was stored at 15℃ for 48h (Figure 1(c)). The creation of the emulsion and biphase formation could be due to the changes in the chemical structure of the [DETAH]Br-PEG200 complex as CO2 contacted with [DETAH]Br and PEG molecular. However, in case of PEG400 as cosolvent, the blended absorbent immediately became cloudy and gradually resembled an emulsion with the addition of CO2. As for PEG300, the system visibly changed from clear liquid to cloudy liquid when continuously bubbling CO2 for 10min. While systems of [DETAH]Br-EG and [DETAH]Br-water kept limpid all the time.

Figure 2. (a) The effect of cosolvent on CO2 absorption by [DETAH]Br (mass ratio 1:4, 20℃, PCO2=1atm, 250mL/min); (b)The effect of anion on CO2 absorption in PEG200 solution; (c) The effect of water content on CO2 absorption in [DETAH]Br-PEG system; (d) Five consecutive cycles of CO2

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absorption (20℃, PCO2=1atm, 250mL/min.) and CO2 release (80℃, N2:250mL/min, PCO2=0, PN2=1atm) by [DETAH]Br-PEG200 system. The corresponding CO2 absorption performance was shown in Figure 2(a). It can be observed that the CO2 capacity showed an decrease as the molecular weight of PEG increased from 200 to 400 (1.184mol vs 0.868mol vs 0.324mol for [DETAH]Br-PEG with a mass ratio of 1:4). On the basis of physical absorption, PEGs with higher molecular weights had higher CO2 solubility.30 However, [DETAH]Br played the dominant role in absorption, and the CO2 capacity was mainly determined by [DETAH]Br concentration (Figure 1(d)). As a result, the observed decrease of CO2 capacity may be attributed to the relatively high viscosity of PEGs that increased from 63.01 to 115.00 mPa·s as molecular weight of PEG increased from 200 to 400 at 20℃.31 Compared to CO2 absorption in [DETAH]Br-water, [DETAH]Br-PEG200 system could not only show advantage on CO2 capacity (1.184mol vs 0.868mol), but also less energy consumed than the aqueous solution when considering the same amount of water in aqueous solution required higher energy to heat the solution as the heat capacity of water was twice as that of PEG200 (2.11 kJ/(kg·K)).9 As for CO2 capture by [DETAH]Br-EG system, no precipitate was formed. Therefore, The observed higher CO2 capacity of [DETAH]Br-PEG200 system than that of [DETAH]Br-EG system(1.184mol vs 0.997mol) could be attributed to phase-change formation, which allowed maximum CO2 loading by eliminating the products out of liquid solution.

3.2. The effect of anion on CO2 capture and phase-change formation We also compared the effect of anion (cation: [DETAH]+) on phase-change formation in PEG200 solution (Figure 1(b)). It can be observed that [DETAH]Cl-PEG200 system became cloudy after 30min and gradually resembled an emulsion with the addition of CO2, while CO2 saturated [DETAH]NO3-PEG200 system remained as viscous clear liquid after completing absorption and formed

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an emulsion in a few days, indicating the instability of the mixture. Similarly, CO2 absorption in [DETAH]BF4-PEG200 system partially formed white tiny precipitate, which seemed to be stable and viscous, adhering to the bottle wall. But in case of ionic liquid with formate ion and acetate ion, the CO2 saturated solution remained clear, barely precipitate and biphasic separation were observed. This could be due to the formation of carbamic acid structures but not carbamate. The different CO2 capture behaviors may be attributed to different CO2 loading, structure of products, hydrogen bonding network and stabilizing effect of the anion. Figure 2(b) showed that CO2 capture in [DETAH]Br-PEG200 system was much better than other ionic liquid with different anions in PEG200. It was noteworthy that [DETAH]OAc-PEG200 system showed faster capture rate at the beginning but less CO2 loading in the end, indirectly indicating the advantage of biphasic separation, which facilitated CO2 absorption. The least CO2 loading (0.651mol) for [DETAH]BF4-PEG200 system may be attributed to the strong bonding effect of BF4-, which decreased the reactivity of amino-groups. The less formation of precipitate could reveal that the biphasic separation may not only be related to CO2 loading, but also be involved in hydrogen bonding network and stabilizing effects of the anion in ionic liquid.

3.3. The effect of water content on CO2 capture It has been assumed that water in absorbents could act as a diffusive intermediate to transport CO2 or increase the flexibility of the amine chains, thus have a positive effect on the CO2 capture through reduction of kinetic restrictions.32 Besides, It was believed that the solvating water molecules could reduce the electrostatic attractions, thus achieve lower cohesive energy of the system.33 Goodrich et al. had reported that the CO2 capacity was reduced with the presence of water in trihexyl (tetradecyl)phosphonium prolinate ([P66614][Pro]), presumably due to reprotonation of the anion.34 In

12


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contrast, the solubility of CO2 in [P66614][2-CNPyr] increased slightly and the slope of the isotherm was altered significantly in the presence of water. In this research, as shown in Figure 2(c), CO2 absorption capacity of [DETAH]Br in PEG200 increased from 1.046mol to 1.184mol as the water content rose from 1.3wt% to 4.7wt%, similar trend was shown in the result of CO2 absorption in [DETAH]Br-PEG300 system. However, when the water content increased to 6.5wt%, the CO2 capacity showed a slightly decrease. It seemed that water disrupted the anion–cation interaction in which an enhancement was observed allowing increased interaction of the cation with CO2, water also seemed to compete effectively with CO2.35 The captured species may change with more addition of water. More importantly, the variation of water content in the mixture could change the hydrogen bond network and affect the stabilizing effect of bromide ion.

Figure 3. (a) The effect of temperature on CO2 capture in [DETAH]Br-PEG200 system (mass ratio 1:4,

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20℃, PCO2=1atm, 250mL/min); (b) Viscosity at different CO2 loading (20℃).

3.4. The effect of temperature and viscosity on CO2 capture The temperature dependence of CO2 capture into [DETAH]Br-PEG200 system was evaluated in Figure 3. It seemed that the effect of temperature on CO2 loading was slight between 30-50℃, and CO2 loadings of the saturated solution in this range were all kept at around 0.85 mol CO2/mol [DETAH]Br. In most IL-CO2 capture system, the increased temperature may result in improved CO2 capacity due the decreased viscosity which contributed to enhanced kinetics. In this case, however, when the temperature decreased to 20℃, the CO2 capacity increased to 1.18mol/mol [DETAH]Br, which could be attributed to the favorable CO2–amine interactions at lower temperature originating from the exothermic acid–base interaction between [DETAH]Br and CO2.36 The saturated uptake was 0.84mol at 50℃ though the viscosity after CO2 absorption was only 83.2mPa·s. As for the significant decrease of CO2 capacity when temperature increased to 60℃, it could be attributed to the instability of CO2 adduct, which led to CO2 release from the system. It is generally observed that the viscosity of absorbent increased after CO2 absorption, which would impede CO2 diffusion and decrease the absorption rate. In this research, the viscosity of [DETAH]Br-PEG200 (mass ratio 1:4) system at 20℃ increased from 71.7 to 305.2 mPa·s after saturated absorption. This increase of viscosity was limited when compared to IL-based absorbents reported in the literatures shown in Table 2. In spite of a viscosity higher than MEA aqueous solution, but the data was much lower than that of most CO2-saturated TSILs. It could be attributed to the phase-change formation, which eliminated amine species out of liquid phase after CO2 absorption and reduced the hydrogen bonding interaction between amine species and PEG200. Otherwise, phase-change formation may also contribute to CO2 loading, which can be concluded from the increased phase-change formation time (65min) at 60℃but obviously dropped saturated uptake.

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Table 2. Comparisons of CO2 loading and viscosity of absorbents in literatures. Conditions

mol

g CO2/g

Viscosity/mPa·s

Absorbents

Ref. T/℃

P/kPa

CO2/mol IL

absorbent

unloading

loading

[DETAH]Br+PEG200 a

20

101

1.18 b

0.0585 c

71.7 d

305.2 d

This work

[P66614][4NH2-PhCOO]

30

101

0.8

0.0568

18460 e

＞99220 e

Luo37

[P66614][6NH2-PyCOO]

30

101

0.78

0.0553

10300 e

40130 e

Luo37

[P66614][2NH2-PyCOO]

30

101

0.56

0.0397

840 e

895 e

Luo37

[P66614][Me-Gly]

30

101

0.90

0.0692

312 e

41280 e

Luo37

[P66614][Ac-Gly]

30

101

0.49

0.0359

1169 e

964 e

Luo37

[P66614][Ac-PhO]

30

101

1.20

0.0798

2690 e

2565 e

Luo37

[P66614][Ala]

25

101

0.66

0.0508

450 f

53000 f

Goodrich38

[P66614][Sar]

25

101

0.91

0.0700

440 f

83000 f

Goodrich38

[P66614][3-CF3Pyr]

22

97.8

0.89

0.0633

76 g

83 g

Gurkan39

[P66614][2-CNPyr]

22

93.1

0.89

0.0681

98 g

96 g

Gurkan39

[C3OHmin]Cl+MEA h

60

101

0.298 i

0.0552 c

48 j

271 j

Huang40

30wt% MEA in H2O

40.6

101

0.339 i

0.0733c

EDA+PEG300 k

20

101

0.8807 l

0.1076c

a

Mass ratio of [DETAH]Br to PEG200 is 1:4, phase change was observed

b

mol CO2/mol [DETAH]Br.

c

Calculated from the total weight of absorbent.

d

Viscosity measured at 20℃ and around 101 kPa.

e


Jassim41 53.5 e

15


2000 e

Zhao42

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f


g

Viscosity measured at 50℃ around 100 kPa.

h

mole ratio of MEA to [C3OHmim]Cl is 1:1, phase change was observed.

i

mol CO2/mol MEA.

j


k

mole ratio of EDA to PEG300 is 1:1, phase change was observed.

l

mol CO2/mol EDA.

3.5. Absorption-desorption Cycles In every CO2 capture and release cycle, the [DETAH]Br-PEG200 system was exposed to a CO2 atmosphere until the system remained stable, then the CO2 capturing system was plunged into a hot oil bath (80 ℃) with N2 bubbled through (250mL/min) to measure the CO2 release. The experiment of regeneration without N2 bubbling was also considered. The corresponding regeneration rate was 88.25% at 90℃ and 0.1 kPa (see details in Supporting Information).

Figure 4. TG-DSC data for [DETAH]Br and rich phase of [DETAH]Br-PEG200-CO2 in a nitrogen atmosphere at a heating rate of 10℃/min. 16


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Before conducting this experiment, thermogravimetric analysis was performed. As shown in Figure 4, the CO2-adduct had an onset of decomposition temperature at 98℃ (corresponding to 5% mass loss, weight loss began at 48.2℃). The DSC curve showed an endothermic peak at 128℃for [DETAH]Br, which can be attributed to the melting point of [DETAH]Br. The endothermic peak at 108℃ for rich phase demonstrated the release of CO2 was completed. Figure 2(d) showed that the [DETAH]Br-PEG200 system could be recycled at least for five continuous absorption-desorption cycles without significant loss of CO2 capturing and releasing capability. The regeneration temperature, lower than that of the traditional MEA system (100-140℃), also lower than DETA-tetradecyl(trihexyl)phosphonium chloride

solution(75-145℃) and DETA-RevIL solution

(131.69℃),3,43,44 was probably due to the much lower thermostability of the formed species. The maximum

CO2

loading

capacity

could

reach

1.18

mol

CO2/mol

[DETAH]Br

in

the

absorption-desorption cycles. However, since [DETAH]Br possessed a relatively low molecular weight than many other ionic liqiuids such as [P66614]+, the CO2 loading per kg of [DETAH]Br (6.41mol CO2/kg [DETAH]Br) was also higher than that of most functionalized ILs, but was slightly lower than that of MEA (7.5mol CO2/kg MEA) reported in literature.45 Nevertheless, the recycling performance and corrosion resistance of [DETAH]Br-PEG200 system were better than that of the MEA process, which was confirmed in the following investigation. Therefore, the [DETAH]Br-PEG200 system could provide high loading capacity, stable absorption-desorption cycles, limited energy loss and low energy requirements for CO2 capture.

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Figure 5. Corrosion penetration rates of [DETAH]Br-PEG200 (mass ratio 1:4), 20% MEA aqueous solution, and 20% DETA aqueous solution with CO2 loading of 0.3 mol, data of [DETAH]Br-PEG200 system loaded with 0.9mol CO2 was used as a comparison.

3.6. Corrosion and degradation behavior discussion Since DETA as well as the active species contained DETA and Br- were supposed to be corrosive, it is necessary to test the corrosiveness of [DETAH]Br-PEG200 system and make a comparison with MEA and DETA aqueous solution. Weight-loss method was employed to determine the corrosion rate of steel in absorbents at 50℃ (see details in the Supporting Information). After immersing in absorbents with CO2 loading of 0.3mol for 15 days, the corrosion rate of the [DETAH]Br-PEG200 system, MEA, and DETA solutions were 0.09, 1.30, and 2.20 mm per year, respectively (Figure. 5), and decreased to 0.05 mm per year for [DETAH]Br-PEG200 when CO2 loading was 0.9mol. It is indicated that the [DETAH]Br-PEG200 system would be more corrosion-resistant if practically used. The diminished corrosiveness was ascribed to several reasons. First, the relatively higher viscosity resulted in a stronger adhesive contact to the steel surface and lower oxygen/moisture permeability.36 Second, the pervading hydrogen bond network made the amine groups less available for protonation, thus less actively

18


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Energy & Fuels

involved in corrosion redox cycle. Third, the water content was relatively low. Last but not the least, in this phase-change system, it was favorable for reducing the participation of amine species in corrosion redox cycle by eliminating them out of the liquid phase as insoluble solid.8,46 That was also the reason why higher CO2 loading promised a more suppressed corrosiveness in [DETAH]Br-PEG200 system. As for degradation, the Fe2+, which catalyzed amine oxidation would be in low concentration in the light of low corrosion rate, together with the hydrogen bonded amino groups and lower oxygen/moisture permeability, the oxidative degradation could be limited. Additionally, the formation of a protective layer by PEG and precipitate on the metal surface blocked the active sites against approaching oxidizing species.46

3.7. Discussion on changes of reaction products and hydrogen bonding in CO2 capture 3.7.1. ATR-IR analysis In order to analyze the changes of carbonate species and hydrogen bonds, ATR-IR spectra of [DETAH]Br-PEG system before and after CO2 bubbling were compared. As shown in Figure 6(a), in the high frequency region, the -NH2 and -OH stretching bands observed in the range of 3000–3250 cm−1 showed significant broadening, and a shoulder at around 3160 cm−1 appeared. The emergence of this band was resulted from hydrogen bonding between -NH2 of [DETAH]Br and -OH groups of PEG.47 In the absence of CO2, the OH-stretching vibration of PEG200 appeared at 3340 cm-1, and gradually shifted to higher frequency with the dissolution of CO2. It is indicated that the hydrogen bond interaction between the terminal hydroxyl of PEG200 and the amine groups of [DETAH]Br or terminal hydroxyl groups of other PEG200 molecules was weakened.48

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Figure 6. ATR-IR spectra of A—[DETAH]Br-PEG200, B—[DETAH]Br-PEG200-CO2-50min, C— [DETAH]Br-PEG200-CO2-50min-24h,

D

—

[DETAH]Br-PEG200-CO2-95min

and

E

—

[DETAH]Br-PEG200-CO2 (equilibrium). (a) 3700-2500cm-1; (b) 1750-800cm-1.

Figure 7. ATR-IR spectra of A — PEG400, B — PEG300, C — PEG200, D — lean phase of [DETAH]Br-PEG200-CO2(equilibrium)

and

E — [DETAH]Br-PEG200-CO2 (equilibrium).

20


(a)

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3700-2500cm-1; (b) 1750-800cm-1. In Figure 6(b), characteristic peaks in spectrum of PEG200 were observed at 1455, 1350, 1323, 1296, 1248, 1060 and 935cm-1.49,50 The broad band at 1610cm-1 in spectrum of [DETAH]Br-PEG200 was attributed to N-H deformation of primary amine, but shifted to 1634 cm-1 in the spectrum of [DETAH]Br-PEG200-CO2 due to the formation of RNH3+ with the presence of CO2.51 Additionally, compared to the spectrum of [DETAH]Br-PEG200, new peaks emerged at 1583, 1545, 1504, 1471, 1411, 1402, 1365, 1227 and 814cm-1. Peaks at 1583 and 1545cm-1 were assigned to the asymmetric stretch of –COO- species, 1523 and 1504cm-1 were attributed to NH deformation in –NHCOO-/C-N stretching, indicating the formation of carbamate species.52,53 The C-N stretching vibration/NCOO- skeletal vibration peak was observed at 1411cm-1, while the -OCOO symmetric stretching mode coupled with the CH2 twisting mode, was observed at 1402 cm-1.52,53 Besides, the emergency of peak at 1365 cm-1, which assigned to −COO− symmetric stretching, probably revealed the presence of bicarbonate or alkylcarbonate species.52,53 Interestingly, the -CH2 twisting modes at 1296cm-1 was not observed in the spectrum of [DETAH]Br-PEG200-CO2, but shifted to higher wavenumbers (1305cm-1) with the presence of CO2. Similarly, the –CH2 scissor (gauche) of PEG200 at 1471cm-1 was present after CO2 absorption, which probably indicated the structural change of PEG200 molecules or the emergency of new species with PEG200 moiety.50 As for the peak at 1227cm-1, it was attributed to –C-C-O stretching vibration. Combined with the increased intensity of band at 1114cm-1 corresponding to C-O-C asymmetric stretching vibration, it is reasonably concluded that there was structural change of the PEG200 molecules.54-57 The appearance of peak at 814cm-1 was OCN out of plane bending or -OCOO out-of-plane bending mode.54-57 The analysis of ATR-IR spectrum of the rich phase separated from [DETAH]Br-PEG200-CO2 mixture supported the presence of alkylcarbonate species (see details in

21


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Supporting Information).

Figure 8. ATR-IR spectra of the [DETAH]Br-PEG-CO2 systems when they began to be cloudy. A—In PEG400; B—In PEG300; C—In PEG200. (a) 3700-2500cm-1; (b) 1750-800cm-1. In order to figure out the effects of changes of hydrogen bonds on phase-change formation, we made a comparison with the spectra of lean phase, PEG200 and [DETAH]Br-PEG200-CO2(equilibrium) in Figure 7. In the high frequency region, the position of –OH in spectrum of [DETAH]Br-PEG200-CO2 was close to that in spectrum of PEG200. It is suggested almost no hydrogen bond interaction between the products and PEG200 molecules, which well accounted for the precipitate and phase change formation. Instead, the absorption peak of –OH in spectrum of lean phase shifted to lower wavenumber, attributing to weaker hydrogen bond between –OH in PEG200 and amine groups in residual [DETAH]Br. Nevertheless, the lack of characteristic peaks of the CO2-containing species revealed that they should have been transferred into rich phase. The possibility of changes of hydrogen bonds contributed to phase-change formation drove us to focus on the structural difference of PEGs. In Figure 7, the broader shape and lower wavenumber of –OH peak in spectrum of PEG200 indicated stronger

22


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Energy & Fuels

hydrogen bonding than that in PEG300 and PEG400. Similarly, the different peak intensities at 1094 and 1061cm-1 were attributed to more C-O-C groups in PEG molecule with longer chain length, but less –OH groups with the same mass of cosolvent.49,50 However, the possibility of the reaction product formation contributed to phase-change formation should also be considered. In Figure 8, there were no prominent product peaks in spectra of [DETAH]Br-PEG300-CO2 and [DETAH]Br-PEG400-CO2 when they became cloudy, but the spectrum of [DETAH]Br-PEG200-CO2 mixture showed evident characteristic peaks of carbamate species. We had postulated the precipitate formation was only attributed to bicarbonate or alkylcarbonate species, but carbamate species were included in the rich phase, almost no carbamate species were found in the lean phase. Considering the clear state of [DETAH]Br-PEG200-CO2 mixture lasted for 60min, it is indicated that the formation of precipitate by carbamate species could probably be more related to the strength of hydrogen bonding network in the mixture than the immiscibility of the products and PEG. Though both the polar hydroxyl groups and the ether bonds of the surrounding solvent molecules could hydrogen bond to the amine groups of [DETAH]Br, it should be noted that the polarity of the hydroxyl group was stronger than that of the ether bond. Besides, it has been reported that the N–H···:O═C H-bond (ca. 8 kJ/mol) and N–H···:N(ca. 8-17 kJ/mol) H-bond were weaker than O–H···:N H-bond (29 kJ/mol).58,59 We proposed that O–H···:N type of H-bonding played an important role in stabilizing the system and preventing biphasic separation by overcoming the electrostatic attractions in the early stage of CO2 absorption, which supported the experiment phenomenon. Interestingly, it was observed that sample obtained from the [DETAH]Br-PEG200-CO2-35min mixture maintained as no change clear liquid, but the clear sample obtained from the [DETAH]Br-PEG200-CO2-50min mixture changed into opaque gel in 5min. It seemed that biphasic

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Page 24 of 40

separation was related to CO2 loading in mixture, and [DETAH]Br-PEG200-CO2 mixture with relatively high CO2 loading was not stable. In Figure 6, the spectrum of [DETAH]Br-PEG200-CO2-50min mixture after 24h showed no obviously characteristic peaks of other species except weaker hydrogen bond feature. It is suggested that the biphasic separation could not only be related to new species formation, but also resulted from the instability of the [DETAH]Br-PEG200-CO2 mixture with relatively high CO2 loading, leading to hydrogen bond network changing or rearranging. Due to the relatively low solubility of bicarbonate and alkylcarbonate species in PEG solution, we postulated that there were other reactions contributed to biphasic separation and increased CO2 capacity. The reactions could be shown as Equation (1)-(4),. (1) (2) (3) (4) (ROH denotes to PEG) 3.7.2. 13C NMR analysis In order to further identify the products and confirm the reaction process, [DETAH]Br-PEG200 system loaded with CO2 was analyzed qualitatively by 13C NMR spectroscopy.

24


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Figure 9. 13C NMR spectra of [DETAH]Br-PEG200-CO2 mixture at different time. In Figure 9, it can be observed that new peaks belonged to the primary carbamate appeared at 164.47ppm when a small amount of CO2 was added in the solution (5min). With continuously bubbling CO2 through the absorbent, new peaks attributed to dicarbamate of [DETAH]Br appeared at 164.09 ppm.60 Additional signals at 160.08–158.87 ppm could be ascribed to alkyl carbonate or bicarbonate species.54,61 The chemical shift of carbamate (R1R2NCOO–) decreased with increasing CO2 loading. The peaks at 47.07 ppm showed the similar trend.

25


Energy & Fuels

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

Figure 10. (a)

13

Page 26 of 40

C NMR spectra of the rich phase and lean phase separated from

[DETAH]Br-PEG200-CO2. (b) A comparison of

13

C NMR spectra of the rich phase separated from

[DETAH]Br-PEG200-CO2 and [DETAH]Cl-PEG200-CO2. Lean phase and rich phase of IL-PEG200-CO2 mixture were also analyzed qualitatively by

13

C

NMR spectroscopy in Figure 10 (a). The presence of characteristic signals in the range 60.42–71.79 ppm and a shift to upfield indicated the formation of PEG-carbonate. The signals at δ=71.79 and 69.73 ppm were resulted from -CH2O- in PEG.47,61 And the peak observed at δ=60.42 ppm was belonged to the carbon attached to the hydroxyl groups of PEG.47,61 The presence of characteristic peaks of [DETAH]Br at 45.90 and 38.90ppm in the spectrum of lean phase was also consistent with the previous ATR-IR analysis.

In

Figure

10

(b),

the

13

C

NMR

spectrum

of

rich

phase

separated

from

[DETAH]Cl-PEG200-CO2 mixture was used as a comparison. It can be observed that the characteristic peaks of the reaction species almost appeared at the same position, indicating the reaction process and reaction species were similar. The difference in the amount of reaction species, as well as an upfield shift of PEG characteristic peaks for rich phase of [DETAH]Cl-PEG200-CO2 mixture could be attributed to the effect of the different anion. 26


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Energy & Fuels

3.8. The proposed mechanism of precipitate and biphasic change formation In the early stage of CO2 absorption, [DETAH]Br was continuously dissolved in PEG. The solvation of the [DETAH]Br reduced the cohesive force within molecules and allowed for complete contact between PEG200 and CO2. As a result, some of the O–H···:O H-bonds and N–H···:N H-bonds were substituted by O–H···:N H-bonds, and some [DETAH]Br molecules began to be transformed into carbamate with continuously CO2 bubbling. Strong gel formation but not biphasic separation was observed may partly due to the O–H···:N H-bond between the dissolved [DETAH]Br and PEG molecule, which contributed to stable conformation without biphasic separation. Previously ATR-IR analysis has demonstrated that O–H···:N H-bond could play a more important role in stabilizing the product from precipitation, though the R-O-R groups could also form hydrogen bond with [DETAH]Br.

Figure 11. Expanded 1H NMR spectra of [DETAH]X–PEG200 mixture before and after 40 min CO2 bubbling (mass ratio 1:4, 20℃, 1atm). The solvent was CD3OD as indicated by peaks near 3.33 ppm. (a) 4.70-5.00ppm; (b) 2.80-3.55ppm. The mixture of [DETAH]Br–PEG200 remained as a stable colorless liquid at room temperature in

27


Energy & Fuels

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Page 28 of 40

60min, but was transformed from transparent gel to turbid aggregation as continuously bubbling CO2 through the absorbent, which was different from the CO2 absorption of ionic liquid with other different anions in PEG200. This phenomenon may also be explained by Br− ions being capable of pairing with cations through either or both of electrostatic interaction and hydrogen bonding, which could stabilize the RNH3+ ion and prevent the recombination of RNH3+ and RNHCOO-, hence increase the capacity and stability of the captured CO2.36,

40

Similarly, the deprotonated PEG ions may also contribute to the

increased stability of the solution by interaction with RNH3+. The CO2 capture reactions in the [DETAH]Br-PEG system could be depicted as Equation (1)-(5). (1) (2) (3) (4) (ROH denotes to PEG) (5) To understand the [DETAH]Br–CO2 interactions and the influence of Br− ion on CO2 capture in PEG200 solution, 1H NMR spectra were recorded with tetradeuteromethanol (CD3OD) as the solvent. Besides, to further confirm whether the aforementioned influence of Br− ion to be generic on CO2 capture in PEG, four ILs, [DETAH]+ coupled with Br−, Cl−, BF4−, and NO3−, were compared. As shown in Figure 11(b), the expanded range spectrum of [DETAH]Br–PEG200 exhibited the characteristic peaks at the expected chemical shifts, and the solvent peak at around 3.33 ppm. After absorption of CO2, which should react with [DETAH]Br to form RNHCOO− and RNH3+, two small sets of triplets at 3.13 and 3.54 ppm emerged in the NMR spectrum for 40min CO2 bubbling. These can be assigned to the two kinds of CH2N protons in the [DETAH]BrCOO− ion. Due to the electron

28


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Energy & Fuels

withdrawing ability of the carbonyl group, the chemical shifts of the CH2N protons in [DETAH]BrCOO− ion were located downfield, to their counterparts in [DETAH]Br.17,40 H2 N

H2 N HN

HN

NH H H

Br O

O

O

HO O

O

O H

H O

H

OH

O

H

N

HO

O H

H 2N

O

HO

O

HN

O

N H

O

O

N

HO

O

OCOO H2 N

O H2N

O H

Br

H

O N H

H

Br

O H

OH H2 N

O H2N O

O H2N H

HO

H

OH H2 N

N Br

H

N

Br N H2

OCOO H2 N Br

H

O

O H2N H

O H2N H

OCOO

O H2 N

N

HO

O

N H2

H

CO2

Br

H

Br

H 2N

H

H2 N

O H 2N H

OH

O

O

N

HO

O

NH

O Br N H2

O

O

H

N H2

N2 + Heating

H2N

O

Br

N H2

H

H

Br O

HN

NH

HN

H

H H

Br

H

NH

H

H H

O

Br

Figure 12. The proposed mechanism of precipitate and biphasic change formation. It is worthy noting that in the 1H NMR spectra, fast proton exchange caused the merge of –NH2 and –NH3+ groups. As a result, the two kinds of CH2N protons in [DETAH]Br and [DETAH2]+Br shifted their peaks from 2.82 and 2.94 ppm in the absence of CO2, to 2.92 and 3.08 ppm upon 40 min CO2 bubbling, respectively.17,40 These changes after CO2 bubbling could also be found in the 1H NMR spectra of other kinds of IL mixed with PEG200. However, the [DETAH]Cl–PEG200–CO2 mixture exhibited most peaks at lower chemical shifts than those of [DETAH]Br–PEG200–CO2. These shift differences were more remarkable on the peaks attributed to –NH2, –NH3+ and –OH groups in the range of 4.8-5.0ppm (Figure 11(a)), which resulted from the more positively charged protons for hydrogen bonding with Br− after CO2 bubbling. Similar changes of chemical shift were present in the 1H NMR spectrum of [DETAH]NO3–PEG200–CO2. It is indicated that NO3− may have similar hydrogen bonding 29


Energy & Fuels

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ability

as

Br−,

which

was

consistent

with

the

Page 30 of 40

phenomenon

that

the

mixture

of

[DETAH]NO3–PEG200–CO2 could remain as a stable clear liquid at room temperature for a couple of days. As for [DETAH]BF4–PEG200–CO2 mixture, the less precipitate formation but higher chemical shifts revealed that BF4– could strongly interact with both PEG molecule and [DETAH]+ ion within [DETAH]BF4. With more and more amine groups being transformed into charged ammonium and carbamate groups during the chemical absorption, the original H-bond(s) system was broken up, and electrostatic attractions between the charged centers of the CO2 adduct were strengthened (stronger than the hydrogen bonding ones). The decreasing CO2 capture rate and transition of the mixture from a gel to aggregation suggested that the local environment was unable to solvate and stabilize the cationic and anionic centers of isolated salt molecules, and forced them to interact intimately. As shown in Figure 12, electrostatic attractions and intermolecular interaction between -C═O and -NH of carbamate species were prevailing in the mixture. Additionally, intramolecular interaction between -C═O and -NH of carbamate could also be prevasting (see details in Supporting Information). At the same time, some carbamate species were transformed into bicarbonate and alkylcarbonate species. As a result, precipitation and biphasic change formation were observed. Ordered layer packing structures were formed as the salts molecular self-assembly aggregated.

4. Conclusions In conclusion, [DETAH]Br was prepared from less expensive and readily available commercial sources with a relatively simple synthesis by the neutralization reaction, and used for capturing CO2 in PEG solution. In this research, the use of PEG200 showed higher CO2 capacity than in other cosolvent (1.184mol/mol [DETAH]Br), and [DETAH]Br also exhibited advantage over other ionic liquid of

30


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Energy & Fuels

different anions prepared from DETA with respect to CO2 loading. The capture reaction was sensitive to temperature higher than 60℃ due to the instability of CO2 adduct. The viscosity after absorption was only 83.2mPa·s at 50℃, much lower than that of most CO2-saturated TSILs. Phase-change formation could not only increase CO2 capacity, but also contribute to excellent corrosion-resistant performance. This system could be recycled at least for five continuous absorption-desorption cycles without significant loss of CO2 capturing and releasing capability under mild conditions. The regeneration temperature of [DETAH]Br-PEG200 system was lower than that of MEA in aqueous solution and DETA in IL or PEG. The analysis of ATR-IR and 13C NMR revealed that the CO2 adduct could be a mixture of carbamate, bicarbonate and alkylcarbonate species. There were strong hydrogen-bonding interactions between hydroxyl groups in PEG200 and amine groups in [DETAH]Br. PEG200 could not only act as a cosolvent, but also be partially involved in CO2 absorption, contributing to biphasic separation and increased CO2 capacity. Biphase formation was different in solution of different molecule weights PEG and in solution with ionic liquid of different anions, which could not only be related to CO2 loading and new species formation, but also be involved in hydrogen bonding network and stabilizing effects of the anion in [DETAH]Br. It is proposed that O–H···:N type of H-bonding and the stabilizing effects of bromide ion would play important roles in stabilizing the system and preventing the formation of biphasic separation by overcoming the electrostatic attractions in the early stage of CO2 absorption. Corresponding Author: *Telephone/Fax: +86-027-87792141. E-mail: [email protected]. ORCID Yang Chen: 0000-0002-1110-373X Hui Hu: 0000-0002-8417-9671

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Page 32 of 40

Supporting Information: Experiment equipment of CO2 absorption, real-time change of CO2 absorption/desorption, CO2 absorption in neat PEGs, method of corrosion rate test, CO2 desorption without N2 bubbling, ATR-IR spectra of absorbent at different time and the rich phase,

13

C NMR analysis for regeneration,

characterization and structural information of [DETAH]Br, structural analysis by DFT calculation, and scanning electron microscopy analysis of the rich phase.(PDF)

Acknowledgments This work was supported by the Innovative Cross Key Team Project of Self-determined and Innovative Research Funds of HUST (Nos. 20152DTD063).

References [1] Li, X. Y.; Hou, M. Q.; Zhang, Z. F.; Han, B. X.; Yang, G. Y.; Wang, X. L.; Zou, L. Z. Absorption of CO2 by ionic liquid/polyethylene glycol mixture and the thermodynamic parameters. Green Chem. 2008, 10 (8), 879–884. [2] Kassim,

M.

A.; Sairi,

N.

A.; Yusoff, R.;

Alias,

Y.;

Aroua,

M.

A.

Evaluation

of

1-Butyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)imide–Alkanolamine Sulfolane-Based System as Solvent for Absorption of Carbon Dioxide. Ind. Eng. Chem. Res. 2016, 55 (29), 7992–8001. [3] Harper, N．D.; Nizio, K. D.; Hendsbee, A. D.; Masuda, J. D.; Robertson, K. N.; Murphy, L. J.; Johnson, M. B.; Pye, C. C.; Clyburne, J. A. C. Survey of Carbon Dioxide Capture in Phosphonium-Based Ionic Liquids and End-Capped Polyethylene Glycol Using DETA (DETA=Diethylenetriamine) as a Model Absorbent. Ind. Eng. Chem. Res. 2011, 50 (5), 2822–2830. [4] Hasib-ur-Rahman, M.; Larachi, F. CO2 Capture in Alkanolamine-RTIL Blends via Carbamate

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