Efficient and Reversible Absorption of CO2 by Functional Deep

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Environmental and Carbon Dioxide Issues

Efficient and reversible absorption of CO2 by functional deep eutectic solvents Kai Zhang, Yucui Hou, Yiming Wang, Kun Wang, Shuhang Ren, and Weize Wu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01129 • Publication Date (Web): 08 Jun 2018 Downloaded from http://pubs.acs.org on June 8, 2018

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Efficient and reversible absorption of CO2 by functional deep eutectic solvents Kai Zhang a, Yucui Hou b, Yiming Wang a, Kun Wang a, Shuhang Ren a,*, Weize Wu a,* a

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing, 100029, China. b

Department of Chemistry, Taiyuan Normal University, Jinzhong, Shanxi, 030619, China.

*

Corresponding authors. E-mail address: [email protected] (S. Ren) and [email protected] (W. Wu). 1

ACS Paragon Plus Environment

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ABSTRACT Extremely-low-volatility functional deep eutectic solvents (DESs), based on ethylene glycol (EG) and diethylene glycol (DG) as hydrogen bond donor (HBD), and the ammonium salts obtained from triethylenetetramine (TETA) and HCl at different mole ratios as hydrogen bond acceptor (HBA), were designed and used to capture CO2. All the designed DESs can efficiently capture CO2 even at low partial pressures. CO2 absorption capacity of [TETA]Cl-EG DES with n[TETA]Cl : nEG = 1:3 is high up to 17.5 wt% (1.456 mol CO2/mol [TETA]Cl) at 40 oC and 1 atm. CO2 absorption capacity decreases with increasing temperature and decreasing CO2 partial pressure. Regeneration experiments show that CO2 absorption capacities in [TETA]Cl-EG DES and [TETA]Cl-DG DES do not vary after 5 absorption/desorption cycles. It is found that EG or DG can increase the absorption capacity via activating −NH− or −NH2 on [TETA]Cl and enhance the basicity of DESs. In addition, CO2 absorption mechanism in [TETA]Cl-EG DES based on the change of its viscosity during absorption and FTIR analysis indicates that there is a chemical interaction between CO2 and [TETA]Cl, and the stoichiometry for the reversible absorption is 1.5 molecules of CO2 per [TETA]Cl-EG DES molecule.

Keywords: Absorption; CO2; functional deep eutectic solvent; mechanism

2


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1. INTRODUCTION At present, the concentration of CO2 in the atmosphere is increasing significantly due to the use of fossil fuels.1, 2 As one of the most abundant greenhouses gases, CO2 with a high concentration results in many serious environmental problems, such as increasing earth surface temperature and causing frequent and severe climatic disturbance.3 In addition, CO2 is also a cheap, renewable carbon resource. As a result, the capture and utilization of CO2 is significant to the world.4 Up to now, many technologies such as absorption, adsorption and cryogenic capture processes are used to control CO2 emission. Among these processes, selective absorption of CO2 from a gas mixture into a liquid or solvent is an attractive approach. Up to now, the most widely used absorbents for CO2 capture are amine-based solvents, such as monoethanolamine (MEA), piperazine (PIPA), diglycolamine (DGA), and diethanolamine (DEA), in the last century.5 However, there are some drawbacks about the absorbents, such as high energy consumption in the regeneration process and the loss of amine due to its high volatility. In order to solve the drawbacks mentioned above, ionic liquids (ILs), especially functional ILs, have been widely studied for CO2 absorption due to their extremely low vapor pressures, nonflammable nature, and tunable chemical structures.6-8 Functional ILs, such as single amino-based IL,9 amino-free based IL

10

and multiple-site based IL,11 exhibit benign

CO2 capacity under ambient pressure. However, the CO2 uptake capacities by these ILs are usually around at 0.5 mol or 1 mol CO2 per mole IL through the chemical reaction between CO2 and basic IL. It is also noted that the high molecular weight of most functional ILs results in low gravimetric absorption capacity. It was reported that functional IL, ethyltributylphosphonium succinimido ([P4442][Suc]), has the highest gravimetric absorption capacity of 25 wt% at 20 oC, and of 16.2 wt% at 40 oC and 1 atm.12 In addition, the drawbacks of these functional ILs, such as complicated synthesis and high costs of functional 3


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ILs, may limit their large-scale application. Recently, a new type of solvents, deep eutectic solvents (DESs), which are obtained by mixing hydrogen bond acceptor (HBA) and hydrogen bond donor (HBD), have also been studied for CO2 capture due to DESs’ benign characteristics such as easy preparation, no by-product generation and low materials cost.13, 14 Various DESs based on urea,13 glycerol,15 lactic acid

16

and monoethanolamine (MEA) as HBD,17 and choline chloride as HBA were

studied. However, the condition for CO2 capture mostly needs high pressures. The DESs introduced above all exhibit physical CO2 absorption process and hence the CO2 solubility conforms to Henry’s law, which makes these DESs unable to efficiently capture CO2 from flue

gas.

Choi

18

reported

that

the

CO2

gravimetric

absorption

capacity

in

[MEA]Cl-ethylenediamine (EDA) DES (n[MEA]Cl : nEDA = 1:4) was high up to 30 wt% at 30 o

C and 1 atm. However, the high volatility of EDA of this DES is still a problem that cannot

be ignored. Thus, it is expected to design functional DESs with non-volatile property that can efficiently capture CO2 with high gravimetric absorption capacity at ambient pressure. In order to design functional DESs, it is necessary to analyze the structures of existing absorbents for CO2 absorption. As we know, amine-based solvents and functional ILs can absorb low-partial pressure CO2 efficiently, which is believed to mainly rely on the chemical reaction between CO2 and basic amino 10. Chloride ion can enhance the thermal stability of amine due to the hydrogen bond between chloride ion and amine.19 Hence, if a polyamine is reacted with hydrochloric acid (HCl) at a certain mole ratio, the volatility of polyamine can decrease significantly. In addition, it has been reported that the polyhydric alcohols can improve the basicity of protic functional ILs to increase the absorption capacity.20 Based on the above information, a new class of functional DESs using ethylene glycol (EG) and diethylene glycol (DG) as HBD, and ammonium salts obtained from triethylenetetramine (TETA) and HCl at different mole ratios as HBA was prepared, as shown in Figure 1. CO2 4


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absorption capacities in these prepared DESs were investigated at different mole ratios of HBA to HBD, various temperatures and CO2 partial pressures, and the volatility of prepared DES was also tested. It was found that the prepared functional DESs exhibited extremely-low volatility and a high CO2 gravimetric absorption capacity (17.5 wt% at n[TETA]Cl : nEG = 1:3, 40 oC, and 1 atm).

2. EXPERIMENTAL SECTION 2.1. Materials. CO2 (99.999 %) and N2 (99.999 %) were obtained from Beijing Haipu Gases Co., Ltd., Beijing, China. Triethylenetetramine (70 %, aqueous solution) was purchased from Aladdin Co., Ltd., Shanghai, China. Ethylene glycol (99 %) was obtained from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Diethylene glycol (99 %) was purchased from Alfa Aesar Chemicals Co., Ltd., Shanghai China. N-Methyl pyrrolidone (99 %) was purchased from Aladdin Co., Ltd., Shanghai, China. Analytical reagent hydrochloric acid was obtained from Beijing Chemical Works, Beijing, China.

2.2. Preparation of DESs. [TETA]Cl used in this work was synthesized via the reaction between TETA aqueous solution and equimolar HCl. After the reaction between TETA and HCl, the water was removed by a rotary evaporator at 80 oC, and then the product was dried under vacuum at 80 o

C for 48 h. [TETA]Cl2 and [TETA]Cl3 were synthesized in a similar way by changing the

mole ratio of TETA to HCl. The DESs were obtained by mixing [TETA]Cln with different mole ratios of [TETA]Cln to EG or DG at 100 oC, and then N2 was bubbled into the mixtures until a homogeneous phase formed and simultaneously impurities in the DESs were removed.21 The DESs were then kept in a desiccator. 5


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2.3. CO2 absorption and desorption experiments. The apparatus in this work is the same as that in our previous work.22 In a typical absorption experiment, CO2 gas with a flow of 100 cm3/min at ambient pressure was bubbled into DES in a glass tube (100 mm in length, 15 mm in inner diameter), which was partly immersed in a water bath with a constant temperature (uncertainty of ± 0.5 oC). An analytical balance with an uncertainty of 0.1 mg was used to determine the absorption capacity of CO2 by weight difference of the glass tube. For the absorption of CO2 less than 1.0 bar, pure CO2 was diluted by N2 in order to reduce the partial pressure of CO2. CO2 desorption experiment was conducted using the same absorption device and only the bubbled gas was changed to pure N2. The desorption efficiency (DE) of the [TETA]Cl-EG DES was determined using equation (1).

DE =

Ra − Rd × 100% Ra

(1)

where Ra is the mole ratio of CO2 to [TETA]Cl when the absorbent was saturated, Rd is the mole ratio of CO2 to [TETA]Cl after desorption. The determination of moist CO2 absorption capacity was measured by a total organic carbon analyzer (TOC, Shimadzu TOC-L CPN, Japan).

2.4. Viscosity and FTIR analyses. The absorption mechanism was analyzed by monitoring the viscosity of DESs during CO2 absorption and FTIR spectroscopy before and after absorption. The viscosity of DESs with different CO2 contents was measured by a viscometer (Lovis 2000M, Austria). FTIR spectra were recorded using a Fourier transform spectrometer (Nicolet 6700, USA) in a wavenumber range of 400 to 4000 cm−1.

3. RESULTS AND DISCUSSION 6


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3.1. Effect of n[HBA]/n[HBD] on CO2 absorption in [TETA]Cl based DES. The mole absorption capacities of pure CO2 at ambient pressure in [TETA]Cl-EG DESs and [TETA]Cl-DG DESs are shown in Figure 2 and Figure 3, respectively, and the gravimetric absorption capacities of CO2 are shown in Figure 4. It can be found that these DESs can efficiently absorb CO2 with high absorption capacity. For [TETA]Cl-EG DES, the mole absorption capacity increases from 0.748 mol CO2/mol [TETA]Cl at n[TETA]Cl : nEG = 1:1 to 1.456 mol CO2/mol [TETA]Cl at n[TETA]Cl : nEG = 1:3. Interestingly, the gravimetric absorption capacities of CO2 reaches the maximum value of 0.175 g CO2/g DES at n[TETA]Cl : nEG = 1:3. The CO2 absorption performance in [TETA]Cl-DG DES is similar to that in [TETA]Cl-EG DES, and the maximum gravimetric absorption capacity of CO2 in [TETA]Cl-DG DES is 0.159 g CO2/g DES. The optimum mole ratios of HBA to HBD are 1:3 and 1:2 for [TETA]Cl-EG DES and [TETA]Cl-DG DES, respectively. However, the mole absorption capacity of CO2 does not increase with decreasing n[TETA]Cl : nEG at n[TETA]Cl : nEG not more than 1:3, and the capacities keep at about 1.45 mol CO2/mol [TETA]Cl. A similar result appears in [TETA]Cl-DG DES. As a contrast, N-methyl pyrrolidone (NMP), instead of EG or DG, was added into [TETA]Cl to form a solution, and the absorption capacity of CO2 in the solution was measured, as shown in Figure 5. The absorption capacity of CO2 in the solution is just about 0.8 mol CO2/mol [TETA]Cl. Compared with EG and DG, NMP cannot enhance the absorption capacity. In addition, the CO2 absorption capacities in [TETA]Cl and EG were also measured separately at 40 oC at ambient pressure. The CO2 absorption capacities in [TETA]Cl and EG are 0.625 mol CO2/mol [TETA]Cl and 0.0132 mol CO2/mol EG, respectively. These results demonstrate that three aminos in [TETA]Cl can be activated by EG or DG, which can improve the basicity of [TETA]Cl to increase the CO2 absorption capacity to 0.5 mol CO2 per mol amino. 7


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3.2. Effect of temperature on CO2 absorption in [TETA]Cl-EG DES. Figure 6 shows the effect of temperature on the absorption of pure CO2 in [TETA]Cl-EG DES with n[TETA]Cl : nEG = 1:3. It can be found that the CO2 absorption capacity decreases with increasing temperature. For example, CO2 absorption capacity decreases from 1.521 mol CO2/mol [TETA]Cl at 30 oC to 1.391 mol CO2/mol [TETA]Cl at 50 oC. The results suggest that the absorbent can be regenerated by increasing the temperature. The gravimetric absorption capacity of CO2 at 50 oC is 0.165 g CO2/g DES, which is very high and demonstrates that the DES is beneficial for CO2 absorption.

3.3. Effect of temperature on CO2 desorption in [TETA]Cl-EG DES. The effect of temperature on CO2 desorption in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) is shown in Figure 7. As expected, the DE of CO2 increases with increasing temperature. For instance, the DE increases from 70.4% to 98.2% when desorption temperature is increased from 60 oC to 100 oC, and the absorbed CO2 can be easily released with a rapid desorption rate at 100 oC. The high DE of CO2 in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at 100 oC leads to high CO2 available absorption capacity (1.427 mol CO2/mol [TETA]Cl, 0.172 g CO2/g DES ), which is beneficial to the absorption process.

3.4. Effect of CO2 partial pressure on CO2 absorption in [TETA]Cl-EG DES. Due to the low partial-pressure of CO2 in flue gas,23 it is important to test CO2 absorption capacity at different CO2 partial pressures. CO2 absorption capacity in [TETA]Cl-EG DES with n[TETA]Cl : nEG = 1:3 as a function of CO2 partial pressure is shown in Figure 8. It can be seen that CO2 absorption capacity decreases with decreasing CO2 partial pressure. For example, CO2 absorption capacity decreases from 1.456 mol CO2/mol [TETA]Cl at 1 atm to 8


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1.256 mol CO2/mol [TETA]Cl at 0.12 atm, close to that in flue gas. Even at the low partial pressure of 0.12 atm, the gravimetric absorption capacity of CO2 is still as high as 0.149 g CO2/g DES. The result also suggests that the absorbed CO2 can be released by decreasing CO2 partial pressure or reduced pressure treatment.

3.5. Effect of chloride ion and moisture on the absorption of CO2. Figure 9 shows the effect of chloride ion on the absorption of CO2 in [TETA]Cln-EG DES. It is noted that homogenous phase DESs can be formed at nHBA : nHBD not more than 1:4 for [TETA]Cl2-EG DES, and not more than 1:10 for [TETA]Cl3-EG DES. For each additional chloride ion, CO2 absorption capacity is decreased by about 0.5 mol CO2/mol HBD. For example, CO2 absorption capacity is 1.456 mol CO2/mol [TETA]Cl for [TETA]Cl-EG DES, 0.986 mol CO2/mol [TETA]Cl2 for [TETA]Cl2-EG DES, and 0.6 mol CO2/mol [TETA]Cl3 for [TETA]Cl3-EG DES. In addition, CO2 absorption capacities in mole ratio are almost identical with varying nHBA : nHBD for [TETA]Cl2-EG DES and [TETA]Cl3-EG DES. The results demonstrate that free aminos in [TETA]Cl2-EG DES and [TETA]Cl3-EG DES have been completely activated by EG, and one mole free amino in [TETA]Cln can capture about 0.5 mole CO2. As we know, there is always an amount of water in flue gas. Hence, the effect of moisture in CO2 gas was also investigated. The absorption capacity of 92.6% CO2 and 7.4% H2O (g) in [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:3) at 40 oC was measured at ambient pressure. The result indicates that [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:3) can still efficiently capture moist CO2 with an absorption capacity of 1.278 mol CO2/mol [TETA]Cl (0.134 CO2/g DES).

3.6. Volatility and regeneration of DES. When absorbents are applied to CO2 capture, the volatility and regeneration of absorbents are important for the practical application because they have a great impact on the operating 9


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cost. Figure 10 shows the weight losses of [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:3) and [TETA]Cl-DG DES (n[TETA]Cl:nDG = 1:2) as a function of time with bubbling N2 of 100 cm3/min at 100 oC. It can be found that the DES weight loss can be ignored. As mentioned in the literature 19, chloride ion can enhance the thermal stability of amine due to the hydrogen bond between chloride ion and amine. In addition, the high thermal stability can also be attributed to the formation of hydrogen bonding in DES. As we know, the stretching vibration of isolated X−H always appears at high wavenumbers with strong intensity. When hydrogen bond is formed, the peak of X−H is changed to be wide with slight intensity. Take [TETA]Cl-EG DES as an example, Figure 11 shows the FTIR spectra of neat [TETA]Cl, neat EG, and [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:1). It can be found that the stretching vibration of N−H and O−H appears around 3340 cm−1 in neat [TETA]Cl and EG. When [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:1) is formed, the peak width of N−H stretching vibration in [TETA]Cl-EG DES (3705 cm−1 – 3000 cm−1) becomes wider than that in [TETA]Cl (3705 cm−1 – 3140 cm−1), and the peak intensity of O−H in [TETA]Cl-EG DES is obviously weaker than that of in EG. The changes demonstrate the formation of hydrogen bonding in DES. Figure 12 and Figure 13 show five CO2 absorption/desorption cycles in [TETA]Cl-EG DES and [TETA]Cl-DG DES, respectively. [TETA]Cl-EG DES and [TETA]Cl-DG DES absorbed pure CO2 at 40 oC up to saturation and then the absorbed CO2 was desorbed with 100 mL/min N2 at 100 oC. The both two DESs exhibit good absorption/desorption behaviour. There is no notable decrease of the CO2 absorption capacity in the DESs after five cycles. The results demonstrate that the absorption process is reversible and [TETA]Cl-EG DES and [TETA]Cl-DG DES can be regenerated for the absorption of CO2, with high capacities and stability.

3.7. CO2 absorption capacities by different ILs and other DESs for comparison. 10


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CO2 absorption capacities of representative ILs and other DESs with those of the DESs in this work are summarized in Table 1 for comparison. It can be found that the absorption capacities of CO2 in the DESs in this work are comparable with that reported in the literatures.

3.8. CO2 absorption mechanism in [TETA]Cl-EG DES. The result mentioned above shows that CO2 in [TETA]Cl-EG DES can react reversibly in 1.5:1 stoichiometric ratio with [TETA]Cl at 40 oC and 1 atm. The high absorption capacity of CO2 in DES demonstrates that the absorption process is mainly attributed to chemical reaction. It was reported that the viscosity of absorbents increased dramatically when the absorbents absorb CO2 via chemical interaction.24 The viscosities of [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:4) during CO2 absorption at different temperatures are shown in Figure 14. The viscosity increases from 110.4 mPa•s for virgin DES to 1326.9 mPa•s for DES with absorbed CO2 (1.258 mol CO2 per mol [TETA]Cl), which indicates that there are chemical reaction between CO2 and DES.24 Furthermore, 1.45:1 stoichiometric ratio of the reaction of CO2 with [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:3) can be confirmed by FTIR spectroscopy, and the FTIR spectra are shown in Figure 15. For virgin DES, a wide peak with weak intensity around 3340 cm−1 can be attributed to the strong hydrogen bond in DES. However, after absorption of CO2, the peak was changed to be narrow with high intensity, probably due to the formation of −NH2+, where the hydrogen bond of O−H···NH2+ or N−H···NH2+ is weakened. In addition, there are strong peaks at 1327 cm−1 and 1571 cm−1 which are associated with the symmetric and asymmetric stretching frequencies of carboxylate −COO−,25 respectively. The results demonstrated that CO2 was captured by the DES to form carboxylate. Based on the stoichiometry of the reaction of CO2 with [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:3), viscosity during CO2 absorption and the FTIR analysis, the reaction mechanism between CO2 and free amino in DESs was proposed and showed in Scheme 1. 11


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R

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R Absorption

2 NH + CO 2 R'

N Desorption

R'

R

O + O

NH 2

R'

Scheme 1. Proposed reaction mechanism between CO2 and free amino in DESs.

4. CONCLUSION All prepared functional DESs can efficiently capture CO2 at ambient pressure. The absorption capacity of CO2 is increased with decreasing chloride ion, decreasing temperature, and increasing CO2 partial pressure. It is found that EG or DG can enhance the absorption capacity by activating the −NH− and −NH2 in [TETA]Cl and improving the basicity of DESs. Among the synthesized DESs, [TETA]Cl-EG DES and [TETA]Cl-DG DES are promising absorbents for CO2 capture. [TETA]Cl-EG and [TETA]Cl-DG DES can absorb CO2 in a 1:1.5 stoichiometry with a high 17.5 wt% and 15.9 wt% CO2 gravimetric absorption capacity for [TETA]Cl-EG DES and [TETA]Cl-DG DES, respectively, with nHBA : nHBD = 1:3 at 40 oC and 1 atm. In addition, the CO2 absorption capacities in [TETA]Cl-EG DES and [TETA]Cl-DG DES do not vary after 5 absorption/desorption cycles. The DESs’ viscosity increases during CO2 absorption for [TETA]Cl-EG, indicating that there is a chemical reaction between CO2 and [TETA]Cl. The FTIR spectra of [TETA]Cl-EG and [TETA]Cl-EG + CO2 indicate that CO2 is absorbed chemically by [TETA]Cl with forming a carboxylate.

ACKNOWLEDGMENTS The authors thank Professors Zhenyu Liu and Qingya Liu for their help and the financial support of the National Natural Science Foundation of China (No. 21306007), the Research Fund for the Doctoral Program of Higher Education of China (20130010120005) and the long-term subsidy mechanism from the Ministry of Finance and the Ministry of Education of PRC (BUCT). 12


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reversible capture of

low-concentration CO2 by ionic liquids. Angew. Chem. Int. Ed. 2017, 56, 13293-13297. (13) Li, X. Y.; Hou, M. Q.; Han, B. X.; Wang, X. L.; Zou, L. Z. Solubility of CO2 in a choline chloride + urea eutectic mixture. J. Chem. Eng. Data 2014, 53, (2), 548-550. (14) Abbott, A. P.; Capper, G.; Davies, D. L.; Rasheed, R. K.; Tambyrajah, V. Novel solvent properties of choline chloride/urea mixtures. Chem. Commun. 2003, 9, (1), 70-71. (15) Sze, L. L.; Pandey, S.; Ravula, S.; Pandey, S.; Zhao, H.; Baker, G. A.; Baker, S. N. Ternary deep eutectic solvents tasked for carbon dioxide capture. ACS Sustainable Chem. Eng. 2014, 2, (9), 2117-2123. (16) Francisco, M.; Bruinhorst, A.; Zubeir, L. F.; Peters, C. J.; Kroon, M. C. A new low transition temperature mixture (lttm) formed by choline chloride + lactic acid: Characterization as solvent for CO2 capture. Fluid Phase Equilib. 2013, 340, 77-84. (17) Hsu, Y. H.; Leron, R. B.; Li, M. H. Solubility of carbon dioxide in aqueous mixtures of (reline + monoethanolamine) at T = (313.2 to 353.2) K. J. Chem. Thermodyn. 2014, 72, 94-99. (18) Trivedi, T. J.; Ji, H. L.; Lee, H. J.; You, K. J.; Choi, J. W. Deep eutectic solvents as attractive media for CO2 capture. Green Chem 2016, 18, (9), 2834-2842. (19) Huang, Q.; Li, Y.; Jin, X. B.; Zhao, D.; Chen, G. Z. Chloride ion enhanced thermal stability of carbon dioxide captured by monoethanolamine in hydroxyl imidazolium based ionic liquids. Energ. Environ. Sci. 2011, 4, (6), 2125-2133. 15


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

(20) Zhao, J. H.; Ren, S. H.; Hou, Y. C.; Zhang, K.; Wu, W. Z. SO2 absorption by carboxylate anion-based task-specific ionic liquids: Effect of solvents and mechanism. Ind. Eng. Chem. Res. 2016, 55, 12919-12928. (21) Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Liu, W. N. Purification of ionic liquids: Sweeping solvents by nitrogen. J. Chem. Eng. Data 2010, 55, (11), 5074-5077. (22) Ren, S. H.; Hou, Y. C.; Wu, W. Z.; Tian, S. D.; Liu, W. N. CO2 capture from flue gas at high temperatures by new ionic liquids with high capacity. Rsc Adv. 2012, 2, (6), 2504-2507. (23) Cui, G. K.; Wang, J. J.; Zhang, S. J. Active chemisorption sites in functionalized ionic liquids for carbon capture. Chem. Soc. Rev. 2016, 45, (15), 4307-4339. (24) Gutowski, K. E.; Maginn, E. J. Amine-functionalized task-specific ionic liquids: A mechanistic explanation for the dramatic increase in viscosity upon complexation with CO2 from molecular simulation. J. Am. Chem. Soc. 2008, 130, (44), 14690-14704. (25) Lee, K. Y.; Kim, H. S.; Chang, S. K.; Jung, K. D. Behaviors of SO2 absorption in [Bmim][oAc] as an absorbent to recover SO2 in thermochemical processes to produce hydrogen. Int. J. Hydrogen Energy 2010, 35, (19), 10173-10178. (26) Yokozeki, A.; Shiflett, M. B.; Junk, C. P.; Grieco, L. M.; Foo, T. Physical and chemical absorptions of carbon dioxide in room-temperature ionic liquids. J. Phys. Chem. B 2008, 112, (51), 16654-16663. (27) Zhang, Y. Q.; Wu, Z. K.; Chen, S. L.; Yu, P.; Luo, Y. B. CO2 capture by imidazolate-based ionic liquids: Effect of functionalized cation and dication. Ind. Eng. Chem. Res. 2013, 52, (18), 6069-6075. (28) Luo, X.; Guo, Y.; Ding, F.; Zhao, H.; Cui, G.; Li, H.; Wang, C. Significant improvements in CO2 capture by pyridine-containing anion-functionalized ionic liquids through multiple-site cooperative interactions. Angew. Chem. Int. Ed. 2014, 53, (27), 7053-7057. (29) Leron, R. B.; Li, M. H. Solubility of carbon dioxide in a choline chloride–ethylene 16


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Page 17 of 35 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

Energy & Fuels

glycol based deep eutectic solvent. Thermochim. Acta 2013, 551, (1), 14-19.

17


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

Table captions Table 1. CO2 absorption capacity in different ILs and DESs.

Figure captions Figure 1. Chemical structures of HBAs ([TETA]Cl, [TETA]Cl2, and [TETA]Cl3) and HBDs (EG and DG). Figure 2. CO2 absorption as a function of time by [TETA]Cl-EG DESs with different mole ratios of EG to [TETA]Cl at 40 oC and ambient pressure. Figure 3. CO2 absorption as a function of time by [TETA]Cl-DG DESs with different mole ratios of DG to [TETA]Cl at 40 oC and ambient pressure. Figure 4. Gravimetric absorption capacity of CO2 as a function of mole ratio of HBD to HBA at 40 oC and ambient pressure. Figure 5. CO2 absorption in three absorbents: ■, [TETA]Cl-NMP (n[TETA]Cl : nNMP = 1:7); ●, [TETA]Cl-EG (n[TETA]Cl : nEG = 1:7); ▲, [TETA]Cl-DG (n[TETA]Cl : nDG = 1:7). Figure 6. Effect of temperature on CO2 absorption in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at ambient pressure. Figure 7. Effect of temperature on the desorption of CO2 in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at ambient pressure. Figure 8. Effect of CO2 partial pressure on CO2 absorption in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at 40 oC.

18


Page 18 of 35

Page 19 of 35 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

Energy & Fuels

Figure 9. Effect of chloride ion on the absorption of CO2 in three DESs at 40 oC with nHBA : nHBD = 1:3 for [TETA]Cl-EG DES, nHBA : nHBD = 1:4 for [TETA]Cl2-EG DES, and nHBA : nHBD = 1:10 for [TETA]Cl3-EG DES. Figure 10. Weight ratios of [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) and [TETA]Cl-DG DES (n[TETA]Cl : nDG = 1:2) at 100 oC and a N2 flow of 100 cm3/min. Figure 11. FTIR spectra of [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:1), [TETA]Cl, and EG. (a), [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:1); (b), [TETA]Cl; (c), EG. Figure 12. Absorption and desorption cycles of CO2 in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3). In each cycle, CO2 was absorbed at 40 oC and desorbed at 100 oC. Figure 13. Absorption and desorption cycles of CO2 in [TETA]Cl-DG DES (n[TETA]Cl : nDG = 1:2). In each cycle, CO2 was absorbed at 40 oC and desorbed at 100 oC. Figure 14. Viscosity of [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:4) as a function of mole ratio of absorbed CO2 to [TETA]Cl. Figure 15. FTIR spectra of (a) [TETA] Cl-EG DES and (b) CO2-absorbed [TETA]Cl-EG DES.

19


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

Page 20 of 35

Table 1. CO2 absorption capacity in different ILs and DESs Tabs /oC

Absorbent

CO2 absorption capacity

Tdes /oC PCO2 /atm

Ref. mol CO2/ mol abs

g CO2/ g abs

[Bmim][Ac]a

25

-

1

0.38

0.084

26

[Bmim][Im]a

40

90

1

0.54

0.115

27

[N66614][Lys]a

22

80

1

2.10

0.151

11

[N66614][Asn]a

22

80

1

2.00

0.147

11

[N66614][Gln]a

22

80

1

1.90

0.136

11

[N66614][His]a

22

80

1

1.90

0.136

11

[P66614][Lys]a

22

80

1

1.60

0.112

11

[P66614][2-Op]a

20

80

1

1.58

0.120

28

[P4442][Suc]a

40

60

1

1.21

0.162

12

[ChCl]-Ureab

30

-

60

0.92

0.156

13

[ChCl]-EGb

30

-

58

0.84

0.137

29

[MEA Cl]-EDAc

30

100

1

0.54

0.337

18

[TEA Cl]-EDAc

30

100

1

0.45

0.242

18

[UE Cl]-EDAc

30

100

1

0.26

0.178

18

[TAE Cl]-EDAc

30

100

1

0.25

0.146

18

[TETA]·Cl-EGd

40

100

1

1.46

0.175

This work

[TETA]·Cl-DGe

40

100

1

1.42

0.159

This work

[TETA]·Cl-EG

40

-

0.92f

1.28

0.134

This work

a

, IL; b, DES (nHBA : nHBD = 1:2); c, DES (nHBA : nHBD = 1:3); d, DES (nHBA : nHBD = 1:3); e,

DES, (nHBA : nHBD = 1:2). [Bmin][Ac], 1-butyl-3-methylimidazolium acetate; [Bmin][Im], 1-butyl-3-methylimidazolium lysinate;

[N66614][Asn],

imidazolate;

[N66614][Lys],

trihexyl(tetradecyl)ammonium

trihexyl(tetradecyl)ammonium asparaginate;

[N66614][Gln],

Trihexyl(tetradecyl)ammonium glutaminate; [N66614][His], trihexyl(tetradecyl)ammonium histidinate;

[P66614][Lys],

trihexyl(tetradecyl)phosphonium

lysinate;

[P66614][2-Op],

Trihexyl(tetradecyl)phosphonium 2-hydroxypyridine; [ChCl]-Urea, choline chloride-urea; f, other fraction is water.

20


Page 21 of 35 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

Energy & Fuels

H N H 2N

H N

NH3 Cl

N H

Cl H3N

[TETA]Cl

[TETA]Cl2

H2 N

NH3 Cl N H

Cl H3N

NH3 Cl

N H

Cl

O HO

HO

OH

[TETA]Cl3

DG

OH

EG

Figure 1. Chemical structures of HBAs ([TETA]Cl, [TETA]Cl2, and [TETA]Cl3) and HBDs (EG and DG).

21


Energy & Fuels

1.8

Mole ratio of CO2 to [TETA]Cl

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

Page 22 of 35

1:1 1:2 1:3 1:4 1:5 1:6 1:7

1.5 1.2 0.9 0.6 0.3 0.0 0

100

200

300

400

500

Time /min

Figure 2. CO2 absorption as a function of time by [TETA]Cl-EG DESs with different mole ratios of EG to [TETA]Cl at 40 oC and ambient pressure.

22


Page 23 of 35

1.8


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

Energy & Fuels

1.5 1:1 1:2 1:3 1:4 1:5 1:6 1:7

1.2 0.9 0.6 0.3 0.0 0

50

100 150 200 250 300 350 400

Time /min

Figure 3. CO2 absorption as a function of time by [TETA]Cl-DG DESs with different mole ratios of DG to [TETA]Cl at 40 oC and ambient pressure.

23


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

Gravimetric ratio of CO2 to DES

Energy & Fuels

Page 24 of 35

0.20

0.16

0.12

0.08

TETA Cl-EG TETA Cl-DG

0.04 1:1

1:2

1:3

1:4

1:5

1:6

1:7

Mole ratio of HBD to HBA

Figure 4. Gravimetric absorption capacity of CO2 as a function of mole ratio of HBD to HBA at 40 oC and ambient pressure.

24


Page 25 of 35

1.6


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

Energy & Fuels

1.2

0.8 [TETA]Cl-NMP [TETA]Cl-EG [TETA]Cl-DG

0.4

0.0 0

20

40

60

80

100

Time/min

Figure 5. CO2 absorption in three absorbents: ■, [TETA]Cl-NMP (n[TETA]Cl : nNMP = 1:7); ●, [TETA]Cl-EG (n[TETA]Cl : nEG = 1:7); ▲, [TETA]Cl-DG (n[TETA]Cl : nDG = 1:7).

25


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


Energy & Fuels

Page 26 of 35

1.8 1.5 1.2 o

30 C o 40 C o 50 C

0.9 0.6 0.3 0.0 0

50

100

150

200

250

300

350

Time /min

Figure 6. Effect of temperature on CO2 absorption in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at ambient pressure.

26


Page 27 of 35

o

100 C o 80 C o 60 C o 40 C

100 80

DE /%

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

Energy & Fuels

60 40 20 0 0

50

100

150

200

250

300

350

Time /min

Figure 7. Effect of temperature on the desorption of CO2 in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at ambient pressure.

27


Energy & Fuels

1.6


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

Page 28 of 35

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

20

40

60

80

100

CO2 partial pressure /kPa

Figure 8. Effect of CO2 partial pressure on CO2 absorption in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) at 40 oC.

28


Page 29 of 35

1.6

Mole ratio of CO2 to HBA

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

Energy & Fuels

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 [TETA]Cl-EG

[TETA]Cl2-EG [TETA]Cl3-EG

DES

Figure 9. Effect of chloride ion on the absorption of CO2 in three DESs at 40 oC with nHBA : nHBD = 1:3 for [TETA]Cl-EG DES, nHBA : nHBD = 1:4 for [TETA]Cl2-EG DES, and nHBA : nHBD = 1:10 for [TETA]Cl3-EG DES.

29


Energy & Fuels

100

Weight ratio/%

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

Page 30 of 35

80 [TETA]Cl-EG [TETA]Cl-DG

60 40 20 0

0

1

2

3

4

5

6

Time /h

Figure 10. Weight ratios of [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3) and [TETA]Cl-DG DES (n[TETA]Cl : nDG = 1:2) at 100 oC and a N2 flow of 100 cm3/min.

30


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Energy & Fuels

3340 cm

3000 cm

-1

3705 cm

-1

3140 cm

-1

-1

(a) (b)

3705 cm

-1

3705 cm

4000

-1

3500

3000 cm

3000

-1

(c)

2500 2000

1500

1000

500

-1

Wavenumber /cm

Figure 11. FTIR spectra of [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:1), [TETA]Cl, and EG. (a), [TETA]Cl-EG DES (n[TETA]Cl:nEG = 1:1); (b), [TETA]Cl; (c), EG.

31


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


Energy & Fuels

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

200 400 600 800 1000 1200 1400

Time /min

Figure 12. Absorption and desorption cycles of CO2 in [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:3). In each cycle, CO2 was absorbed at 40 oC and desorbed at 100 oC.

32


Page 32 of 35

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

Energy & Fuels


Page 33 of 35

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

500

1000

1500

2000

2500

Time/min

Figure 13. Absorption and desorption cycles of CO2 in [TETA]Cl-DG DES (n[TETA]Cl : nDG = 1:2). In each cycle, CO2 was absorbed at 40 oC and desorbed at 100 oC.

33


Energy & Fuels

3000 o

30 C o 35 C o 40 C o 45 C o 50 C

2500

η /mPa⋅s

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

Page 34 of 35

2000 1500 1000 500 0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4


Figure 14. Viscosity of [TETA]Cl-EG DES (n[TETA]Cl : nEG = 1:4) as a function of mole ratio of absorbed CO2 to [TETA]Cl.

34


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Energy & Fuels

3347 cm

-1

1571 cm

-1

1327 cm 3340 cm

-1

-1

(b)

(a)

4000

3500

3000

2500 2000

1500

Wavenumber /cm

-1

1000

500

Figure 15. FTIR spectra of (a) [TETA] Cl-EG DES and (b) CO2-absorbed [TETA]Cl-EG DES.

35


Efficient and Reversible Absorption of CO2 by Functional Deep

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