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Highly Efficient CO2 Capture by Polyethyleneimine Plus 1-Ethyl-3-methylimidazolium Acetate Mixed Absorbents Jian-Bo Zhang, Hailong Peng, Yong Liu, Duan-Jian Tao, Pingkeng Wu, Jie-Ping Fan, and Kuan Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b00530 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

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ACS Sustainable Chemistry & Engineering

Highly Efficient CO2 Capture by Polyethyleneimine Plus 1Ethyl-3-methylimidazolium Acetate Mixed Absorbents Jian-Bo Zhang,†a Hailong Peng,†a Yong Liu,‡* Duan-Jian Tao,§ Pingkeng Wu,¶ Jie-Ping Fan† and Kuan Huang†* †Key

Laboratory of Poyang Lake Environment and Resource Utilization of Ministry of

Education, School of Resources Environmental and Chemical Engineering, Nanchang University, 999 Xuefu Ave, Nanchang, Jiangxi 330031, China. ‡Henan

Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and

Chemical

Engineering, Henan

University, 85

Minglun

St,

Kaifeng, Henan

475004, China. §College

of Chemistry and Chemical Engineering, Jiangxi Normal University, 99

Ziyang Ave, Nanchang, Jiangxi 330022, China. ¶The

Mcketta Department of Chemical Engineering, The University of Texas at Austin,

200 E Dean Keeton St, Austin, Texas 78712, United States. *Corresponding authors: [email protected] (K. H.); [email protected] (Y. L.).

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ABSTRACT In traditional amine-scrubbing CO2 capture technology, alkanolamines are diluted in physical solvents to retain the good fluidity of CO2-loaded absorbents, which suffers from high volatility of mixed absorbents and sacrifice of absolute CO2 capacities. Herein, we designed a novel class of mixed absorbents comprising of polyethyleneimine (PEI) and 1-ethyl-3-methylimidazolium acetate ([emim][AcO]) for CO2 capture. PEI is a kind of polymeric amine compounds with high amine density and negligible volatility, acting as the key component for CO2 absorption. [emim][AcO] is a kind of functionalized ionic liquids (ILs) with chemical affinity to CO2, but its viscosity is low and does not increase drastically after CO2 absorption, acting as the diluent and supplementary component for CO2 absorption. With such formulation, all the components in designed absorbents are extremely low volatile and exhibit chemical affinity to CO2. And the designed absorbents display fast CO2 absorption, high CO2 capacities and good recyclability. It is believed that PEI+[emim][AcO] mixtures have promising application as alternatives to aqueous alkanolamines for CO2 capture. KEYWORDS CO2 capture; chemical solvents; amine compounds; ionic liquids; mixed absorbents


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INTRODUCTION Global warming has become an increasingly serious issue confronted by human beings. It may lead to various disasters, such as melting of polar glaciers, rising of sea levels, and desertification of green lands. All of these would impose destructive impact on the environment and ecosystem on the earth, and bring great threaten to the survival of human beings. It is believed that the occurrence of global warming is directly related to the accumulation of CO2 in the atmosphere.1 CO2 is a major greenhouse gas, and mainly produced from the energy harvesting processes, especially the combustion of fossil fuels. However, the development of human civilization heavily relies on energy sources, and it is expected that fossil fuels will still be a kind of most consumed energy source in the near future.2 Therefore, it is of great significance reduce the emission of CO2 from anthropic activities. CO2 capture and storage (CCS) has been proposed as a necessary solution to CO2 emission, through which CO2 is separated from gas streams, compressed and finally perfusion to underground.3 At present, the most widely used technology for CO2 capture in the industry is amine scrubbing, which utilizes the reversible chemical reaction between alkanolamines (e.g., MEA, MDEA, etc.) and CO2.4 The reaction of alkanolamines with CO2 produces carbamates (in the absence of water) or bicarbonates (in the presence of water), causing the drastic increase in viscosity of liquids due to the formation of complex hydrogen-bonding networks.5 To address this issue, alkanolamines are normally diluted in low-viscous physical solvents (e.g., water, methanol, ethylene glycol, sulfolane, etc.) for CO2 capture.6 However, there are several 3 ACS Paragon Plus Environment


significant defects for traditional amine scrubbing technology: (1) the alkanolamines and physical solvents used are often with high volatility, which does not comply to the principles of green chemistry; (2) the physical solvents themselves enable very weak interaction with CO2, which sacrifices the absolute capacities of mixed absorbents for CO2 capture. Can we construct absorbents with negligible volatility, and all the components of absorbents exhibit chemical affinity to CO2? Such absorbents will be very fascinating for the green and efficient capture of CO2. Polyethyleneimine (PEI) is a kind of polymeric amine compounds with high amine density and negligible volatility. It is a promising candidate to substitute alkanolamines for CO2 capture. However, its viscosity is very high, and increases drastically after CO2 absorption, just as alkanolamines do.7 As a result, PEI can not be used solely for CO2 capture. Actually, PEI is often immobilized in porous supports such as oxides8-9, carbons10-11 and polymers12-14, to prepare solid adsorbents for CO2 capture. However, the sacrifice in absolute capacities of solid adsorbents is also a defect because of the physical interaction of those porous supports with CO2. The absolute CO2 capacity of solid adsorbents is normally defined as the amount of CO2 adsorbed by unit weight of adsorbent (including PEI and supports), for example mol CO2/kg adsorbent. Although PEI itself has large capacity for CO2, the supports have low capacity for CO2 due to the physical interaction of supports with CO2. As a result, the absolute CO2 capacity of whole solid adsorbents should be lower than that of pure PEI (mol CO2/kg PEI).


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Exploring a diluent exhibiting negligible volatility and chemical affinity to CO2 for PEI is believed to be an alternative solution. In the past decades, the emerging of ionic liquids (ILs) provides opportunities for addressing the challenges faced in chemical engineering processes. ILs are organic salts with melting points below or near room temperature. They have many unique properties such as wide liquid range, high ion conductivity and extremely low volatility. Particularly, ILs with task-specific functions can be designed by the rational choice of cations and anions, or tailoring the moieties in structural frameworks.15 As a result, ILs exhibit potential applications in many fields, including gas separation.16 Using ILs as the media for gas separation can effectively avoid the volatile loss of solvents, endowing the separation process with reduced environmental risk and low energy consumption.17 As mentioned above, the major source of CO2 emission is the combustion of fossil fuels. The CO2 content in the product of fossil fuel combustion, i.e., flue gas, is quite low (5~20 v/v%).18 Therefore, chemical solvents with high CO2 capacities at low pressure are often required for the capture of CO2 from flue gas. To this end, a large number of functionalized ILs enabling reversible chemical reaction with CO2 have been developed until now, for example carboxylate-based ILs,19 amine-functionalized ILs,20 azolate-based ILs21 and phenolic ILs.22 1-Ethyl-3-methylimidazolium acetate ([emim][AcO]) is a kind of carboxylatebased ILs with chemical affinity to CO2, but its viscosity is low and does not increase drastically after CO2 absorption.23 Herein, we proposed to use [emim][AcO] as the 5 ACS Paragon Plus Environment


“chemical diluent” for PEI to construct a novel class of mixed absorbents for CO2 capture. In the mixed absorbents, PEI acts as the key component for CO2 absorption, while [emim][AcO] acts as the supplementary component for CO2 absorption. It is believed that PEI+[emim][AcO] mixtures are able to capture CO2 from flue gas very efficiently. Based on this idea, we systematically investigated the performance of PEI+[emim][AcO] mixtures for CO2 capture in this work. EXPERIMENTAL Chemicals

Scheme 1. Chemical structures of PEI, PEG200 and [emim][AcO]. Table 1. Basic information about PEI and [emim][AcO]a Chemicals Purities (wt.%) CAS Nos. Densities (g/cm3) Viscosities (cP) PEI 99 9002-98-6 1.04091 623.4 PEG200 99 25322-68-3 1.10926 10.3 [emim][AcO] 99 143314-17-4 1.10512 24.4 aDensities and viscosities were determined at 313.2 K. CO2 (99.99 mol%) was purchased from Huasheng Special Gas Co. Ltd., China. Polyethyleneimine (PEI, 99 wt.%, branched type, average Mw=600) and polyethylene glycol 200 (PEG200, 99wt.%, average Mw=200) were purchased from Adamas Co. Ltd., 6 ACS Paragon Plus Environment

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China. 1-Ethyl-3-methylimidazolium acetate ([emim][AcO], 99 wt.%) was purchased from Chengjie Co. Ltd., China. The chemical structures of PEI, PEG200 and [emim][AcO] used in this work are shown in Scheme 1, and Table 1 summarizes the basic information about them. [emim][AcO] was dried at 353.2 K and 0.1 kPa for 48 h before use. PEI+[emim][AcO] and PEI+PEG200 mixtures were prepared by directly mixing PEI with [emim][AcO] or PEG200. The prepared mixtures were denoted as PEI+[emim][AcO] (a:b) or PEI+PEG200 (a:b), where a:b is the mass ratio of PEI to [emim][AcO] or PEG200 in mixtures. Characterizations Densities were measured by an AntonPaar DMA 4500M automatic densimeter with an uncertainty of 0.00005 g/cm3. Viscosities were measured by a Brookfield RVDV2PCP230 viscometer with a relative uncertainty of 1%. Thermal stability was examined on a Netzsch STA 2500 thermogravimetric analyzer. Gas absorption

Scheme 2. Apparatus for CO2 absorption measurements (1: CO2 cylinder; 2: pump; 3: water bath; 4, 7, 12, and 13: needle valves; 5: gas reservoir; 6: equilibrium cell; 8 and 9: pressure transducers; 10: computer; 11: temperature probe; 14: magnetic stirrer). The apparatus for CO2 absorption measurements is shown in Scheme 2, which has ever been introduced in our previous work.24 It has two chambers made by 316 L 7 ACS Paragon Plus Environment


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stainless steel. The big one is used as gas reservoir, while the small one is used as equilibrium cell. The volumes are V1 for gas reservoir and V2 for equilibrium cell, respectively. A water bath with an uncertainty of 0.1 K is used to control the temperature of whole apparatus. Two transducers with an uncertainty of 1.2 kPa are used to record the pressures in two chambers. To measure the rates of CO2 absorption, a certain amount of liquid sample was loaded into the equilibrium cell, and the air in whole apparatus was evacuated. CO2 was fed into the gas reservoir from CO2 cylinder, and the pressure was recorded as P1. The needle valve between two chambers was then turned on to introduce a certain amount of CO2 into the equilibrium cell. The pressure in gas reservoir thus decreased to P1′, while the pressure in equilibrium cell increased to P2. The absorption of CO2 in liquid sample resulted in the continuous decrease of P2, which was recorded online. Assuming that the mass of liquid sample is m, and the experimental temperature is T, the amount of CO2 dissolved in liquid sample (n) at any time was thus calculated by the following equation:

n  1V1  1 V1   2 (V2 

m

0

)

(1)

where ρ1 is the density of CO2 at P1 and T, ρ1′ is the density of CO2 at P1′ and T, ρ2 is the density of CO2 at P2 and T, and ρ0 is the density of liquid sample at T. The densities of CO2 were acquired from NIST Chemistry WebBook.25 The procedure for CO2 solubility measurements is similar to that for CO2 absorption rate measurements. When P2 remained constant for at least 1 h, the absorption process was considered to reach


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equilibrium, and the final value of P2 was used to calculate the amount of CO2 dissolved in liquid sample by equation (1). Continuous measurement of CO2 solubilities at elevated pressures was performed by introducing more CO2 into the equilibrium cell to reach new equilibrium. The CO2 solubilities were finally transformed to the molality of CO2 in liquid sample. The uncertainties of gas solubilities were estimated from the uncertainties of pressures by error propagation. RESULTS AND DISCUSSION Physical properties

Figure 1. Densities of PEI+[emim][AcO] mixed solvents {■: PEI+[emim][AcO] (0.05:0.95); ●: PEI+[emim][AcO] (0.1:0.9); ▲: PEI+[emim][AcO] (0.2:0.8); ▼: PEI+[emim][AcO] (0.3:0.7); lines: fitted results}.

Figure 2. Viscosities of PEI+[emim][AcO] mixed solvents {■: PEI+[emim][AcO] (0.05:0.95); ●: PEI+[emim][AcO] (0.1:0.9); ▲: PEI+[emim][AcO] (0.2:0.8); ▼: PEI+[emim][AcO] (0.3:0.7); lines: fitted results}. 9 ACS Paragon Plus Environment


Parameter Equation (2) A B×104 C×107 R2 Equation (3) A K T0 R2

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Table 2. Fitted parameters for equations (2) and (3) PEI+[emim][AcO] PEI+[emim][AcO] PEI+[emim][AcO] PEI+[emim][AcO] (0.05:0.95) (0.1:0.9) (0.2:0.8) (0.3:0.7) 1.276 -4.901 -2.143 0.9999

1.263 -4.477 -2.643 1

1.270 -5.512 -0.9404 1

1.266 -5.271 -1.333 1

0.01919 1262 165.1 0.9999

0.01511 1358 162.1 0.9998

0.02472 1261 171.0 0.9999

0.02042 1402 167.0 0.9998

In this work, PEI+[emim][AcO] mixtures with four mass ratios were prepared: PEI+[emim][AcO] (0.05:0.95), PEI+[emim][AcO] (0.1:0.9), PEI+[emim][AcO] (0.2:0.8) and PEI+[emim][AcO] (0.3:0.7). PEI+[emim][AcO] mixtures with higher PEI contents are too viscous to be applied for CO2 capture. Since physical properties such as densities and viscosities are basic data for CO2 absorbents, the densities and viscosities of PEI+[emim][AcO] mixtures at different temperatures were determined first. Figure 1 presents the densities of PEI+[emim][AcO] mixtures. It is observed that the densities decrease almost linearly with the increase of temperature, which is a common trend for liquids. In addition, the densities decrease with the increase of PEI contents, because PEI is the light component in mixtures (see Table 1). The density data can be correlated by the following equation:26

 =A  BT  CT 2

(2)

The fitted values for parameters of equation (2) are summarized in Table 2. Figure 2 presents the viscosities of PEI+[emim][AcO] mixtures. It is observed that the viscosities decrease non-linearly with the increase of temperature, which is also a 10 ACS Paragon Plus Environment

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common trend for liquids. In addition, the viscosities increase with the increase of PEI contents, because PEI is the more-viscous component in mixtures (see Table 1). The viscosity data can be correlated by the following equation:26

 =AT 0.5exp

K T  T0

(3)

The fitted values for parameters of equation (3) are also summarized in Table 2. Overall, PEI+[emim][AcO] mixtures are with moderate viscosities of 98.2~362.8 cP at 313.2 K. CO2 absorption rates

Figure 3. Rates of CO2 absorption in different solvents at 313.2 K and initial pressure of 120 kPa {■: PEI+[emim][AcO] (0.05:0.95); ●: PEI+[emim][AcO] (0.1:0.9); ▲: PEI+[emim][AcO] (0.2:0.8); ▼: PEI+[emim][AcO] (0.3:0.7); ◆: [emim][AcO]; ◄: PEI}.

Scheme 3. Mechanism for reactions of PEI and [emim][AcO] with CO2. Figure 3 compares the rates of CO2 absorption in different solvents, including PEI+[emim][AcO] mixtures, pure [emim][AcO] and pure PEI. It can be seen that the



absorption of CO2 in pure PEI is very slow, and still far away from reaching equilibrium after 40 min. In contrast, the absorption of CO2 in pure [emim][AcO] is quite fast, with the equilibrium time of ~5 min. There are two reasons for the significantly faster absorption of CO2 in [emim][AcO] than in PEI. First, [emim][AcO] itself has much lower viscosity than PEI (see Table 1). Second, the reaction of [emim][AcO] with CO2 follows a different route from the reaction of PEI with CO2. The mechanism for reactions of PEI and [emim][AcO] with CO2 has been well established in the literature.27,28 As depicted in Scheme 3, the reaction of PEI with CO2 produces carbamates, which results in the formation of complex hydrogen-bonding networks. Therefore, the viscosity of PEI increases drastically after CO2 absorption. However, the reaction of [emim][AcO] with CO2 produces carbene-CO2 adducts, which has little effect on the viscosity of [emim][AcO]. It is further demonstrated in Figure 3 that the absorption of CO2 in PEI+[emim][AcO] mixtures are much faster than in pure PEI, but slower than in pure [emim][AcO]. Therefore, diluting PEI by [emim][AcO] can significantly accelerate the absorption of CO2 in PEI, and the CO2 absorption rates increase with the increase of [emim][AcO] contents in mixtures. As can be seen, the equilibrium time for CO2 absorption in PEI+[emim][AcO] (0.05:0.95) and PEI+[emim][AcO] (0.1:0.9) are less than 10 min, while the equilibrium time for CO2 absorption in PEI+[emim][AcO] (0.2:0.8) and PEI+[emim][AcO] (0.3:0.7) are more than 30 min.


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Figure 4. Viscosity changes for different solvents before and after CO2 absorption at 313.2 K and 200 kPa. The viscosity changes for different solvents before and after CO2 absorption are shown in Figure 4. It should be noted that, the viscosity of pure PEI after CO2 absorption exceeds the upper detecting limit of densimeter (>38000 cP). Therefore, the viscosity data of pure PEI before and after CO2 absorption are not presented in this figure. Interestingly, the viscosities of pure [emim][AcO] increase only by a factor of 1.7 (from 24.4 to 42.5 cP) after CO2 absorption. The increase of viscosities for PEI+[emim][AcO] mixtures after CO2 absorption is also not that significant in comparison with that for pure PEI. For example, the viscosities of PEI+[emim][AcO] (0.05:0.95), PEI+[emim][AcO] (0.1:0.9), PEI+[emim][AcO] (0.2:0.8) and PEI+[emim][AcO] (0.3:0.7) increase by factors of 1.3 (from 98.2 to 127.2 cP), 1.5 (from 117.5 to 181.5 cP), 8.3 (from 175.5 to 1465 cP) and 25.7 (from 362.8 to 9341 cP) respectively after CO2 absorption (see Figure 4). Obviously, PEI+[emim][AcO] mixtures still have good fluidity after CO2 absorption except for PEI+[emim][AcO] (0.3:0.7). Based on these results, it can be concluded that diluting PEI by [emim][AcO] can not only decrease the viscosities of PEI, but also suppress the increase of viscosities after CO2 absorption.



Both factors contribute to the significantly accelerated absorption of CO2 in PEI+[emim][AcO] mixtures in relative to that in pure PEI. CO2 absorption capacities

Figure 5. Solubilities of CO2 in different solvents at 313.2 K {■: PEI+[emim][AcO] (0.05:0.95); ●: PEI+[emim][AcO] (0.1:0.9); ▲: PEI+[emim][AcO] (0.2:0.8); ▼: PEI+[emim][AcO] (0.3:0.7); ◆: [emim][AcO]}. Figure 5 compares the solubilities of CO2 in different solvents, including PEI+[emim][AcO] mixtures and pure [emim][AcO]. It should be noted that the absorption of CO2 in pure PEI can not reach equilibrium under the experimental conditions. Therefore, the solubilities of CO2 in pure PEI are not presented in this figure. Obviously, the variation of CO2 solubilities with CO2 pressures for PEI+[emim][AcO] mixtures and pure [emim][AcO] displays non-linear profiles, which is caused by the chemical affinity of PEI and [emim][AcO] to CO2. Furthermore, the solubilities of CO2 increase with the increase of PEI contents in mixtures, suggesting that PEI is the key component for CO2 absorption while [emim][AcO] is the supplementary component. As demonstrated in Scheme 3, the reactions of PEI and [emim][AcO] with CO2 both follow the stoichiometry of 2:1. However, the enthalpy changes for reaction of PEI with CO2 are much more negative than the enthalpy changes for reaction of [emim][AcO] (14 ACS Paragon Plus Environment

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60~-80 vs. -30~-40 kJ/mol), according to previous reports.23,29

Therefore, PEI has

much stronger chemical affinity to CO2 than [emim][AcO].

Figure 6. Solubilities of CO2 in PEI+[emim][AcO] (0.1:0.9) at different temperatures (■: 298.2 K; ●: 313.2 K; ▲: 333.2 K; ▼: 353.2 K).

Figure 7. Solubilities of CO2 in PEI+[emim][AcO] (0.2:0.8) at different temperatures (■: 298.2 K; ●: 313.2 K; ▲: 333.2 K; ▼: 353.2 K).



Figure 8. Solubilities of CO2 in [emim][AcO] at different temperatures (■: 298.2 K; ●: 313.2 K; ▲: 333.2 K; ▼: 353.2 K). PEI+[emim][AcO] (0.1:0.9) and PEI+[emim][AcO] (0.2:0.8) were then selected as two representatives of mixtures for the investigation of temperature effect on CO2 solubilities, considering that they have good fluidity after CO2 absorption, and relatively high CO2 solubilities. Results are shown in Figures 6 and 7. For comparison, the effect of temperature on CO2 solubilities in pure [emim][AcO] was also investigated, as shown in Figure 8. Obviously, the solubilities of CO2 decrease with the increase of temperature, which is a common trend for CO2 absorption process. However, the negative dependence of CO2 solubilities on temperature is less significant for PEI+[emim][AcO] mixtures than for pure [emim][AcO]. For example, the solubilities of CO2 in PEI+[emim][AcO] (0.1:0.9) and PEI+[emim][AcO] (0.2:0.8) at 15 kPa decrease by only 35% (from 2.02 to 1.31 mol/kg) and 34% (from 3.13 to 2.05 mol/kg) respectively, while the solubilities of CO2 in pure [emim][AcO] at 15 kPa decrease by 60% (from 0.80 to 0.32 mol/kg), when the temperatures increase from 298.2 to 353.2 K. This is consistent with the fact that PEI has much stronger chemical affinity to CO2 than [emim][AcO]. Therefore, PEI+[emim][AcO] mixtures can retain their relatively high CO2 solubilities at elevated temperatures. Recycling of mixed absorbents


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Figure 9. Recycling of PEI+[emim][AcO] (0.1:0.9) for CO2 absorption (absorption condition: 313.2 K and 15 kPa; desorption condition: 353.2 K, 0.1 kPa, 1 h).

Figure 10. Recycling of PEI+[emim][AcO] (0.2:0.8) for CO2 absorption (absorption condition: 313.2 K and 15 kPa; desorption condition: 353.2 K, 0.1 kPa, 1 h). PEI+[emim][AcO] mixtures saturated with CO2 were subjected to heating and evacuating, and then reused for CO2 absorption to test their recyclability. Figures 9 and 10 present the CO2 solubilities of PEI+[emim][AcO] (0.1:0.9) and PEI+[emim][AcO] (0.2:0.8) in 8 absorption-desorption cycles. It should be noted that since the CO2 solubilities were determined from the decrease of gas-phase pressure in equilibrium cell, the CO2 solubilities presented are working capacities. There is no obvious loss in CO2 solubilities, implying that CO2 can be completely desorbed from PEI+[emim][AcO] mixtures. To examine whether the thermal stability of PEI+[emim][AcO] mixtures is



high enough for their long-term use in CO2 absorption-desorption cycles, thermal gravimetric analysis (TGA) was performed. It is found that PEI+[emim][AcO] mixtures start to lose weight at ~470 K (see Figure S1), which is much higher than the temperature required for CO2 desorption (353.2 K). CO2/N2 Selectivities As there is large amount of N2 coexisting with CO2 in flue gas, it is important to investigate the ability of PEI+[emim][AcO] mixtures for selectively absorbing CO2 from N2. Figure 11 shows the solubilities of N2 in PEI+[emim][AcO] (0.1:0.9) at different temperatures. It is found that the N2 solubilities are quite low, being less than 0.0350 mol/kg at 100 kPa. The ideal selectivities of CO2/N2 (defined as the ratio of CO2 solubility to N2 solubility at 100 kPa) are thus calculated to be 81~188. The large CO2/N2 selectivities suggest the excellent ability of PEI+[emim][AcO] mixtures for CO2 and N2 separation.

Figure 11. Solubilities of N2 in PEI+[emim][AcO] (0.1:0.9) at different temperatures (■: 298.2 K; ●: 313.2 K; ▲: 333.2 K; ▼: 353.2 K). Comparison with other absorbents


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Figure 12. Comparison of CO2 absorption rates in PEI+[emim][AcO] mixed solvents and PEI+PEG200 mixed solvents at 313.2 K and 100 kPa {■: PEI+PEG200 (0.1:0.9); ●: PEI+PEG200 (0.2:0.8); ▲: PEI+[emim][AcO] (0.1:0.9); ▼: PEI+[emim][AcO] (0.2:0.8)}.

Figure 13. Comparison of viscosity changes for PEI+[emim][AcO] mixed solvents and PEI+PEG200 mixed solvents before and after CO2 absorption at 313.2 K and 100 kPa.

Figure 14. Comparison of CO2 solubilities in PEI+[emim][AcO] mixed solvents and PEI+PEG200 mixed solvents at 313.2 K {■: PEI+PEG200 (0.1:0.9); ●: PEI+PEG200 (0.2:0.8); ▲: PEI+[emim][AcO] (0.1:0.9); ▼: PEI+[emim][AcO] (0.2:0.8)}.



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To further justify the advantage of using [emim][AcO] as the “chemical diluent” for PEI, the CO2 capture performance of PEI+PEG200 mixtures was also investigated and compared with that of PEI+[emim][AcO] mixtures. Here, PEG200 is a “physical diluent”, but it has lower viscosity than [emim][AcO] (see Table 1). Figures 12 compares the rates of CO2 absorption in PEI+[emim][AcO] mixtures and PEI+PEG200 mixtures. It can be seen that the absorption of CO2 in PEI+[emim][AcO] mixtures is slightly slower than the that in PEI+PEG200 mixtures, if compared at the same PEI contents. Figures 13 compares the viscosity changes for PEI+[emim][AcO] mixtures and PEI+PEG200 mixtures before and after CO2 absorption. It can be seen that the viscosity of PEI+[emim][AcO] (0.1:0.9) after CO2 absorption is slightly higher than the that of PEI+PEG200 (0.1:0.9) (181.6 vs. 164.7 cP). These are due to the higher viscosity of [emim][AcO] than PEG200. Figures 14 compares the solubilities of CO2 in PEI+[emim][AcO] mixtures and PEI+PEG200 mixtures. It can be seen that PEI+[emim][AcO] mixtures have much higher CO2 solubilities than PEI+PEG200 mixtures. Based on these comparisons, it can be concluded that the “chemical diluent” [emim][AcO] can not only accelerate the absorption of CO2 in PEI as other “physical diluents” do, but also avoid the significantly sacrificed solubilities of CO2 in mixed absorbents. Table 3. Comparison of CO2 capacities in different absorbents Absorbent Temperature (K) Pressure (kPa) CO2 capacity (mol/kg) PEI+[emim][AcO] (0.1:0.9) 298.2 15 2.02 298.2 100 2.72 313.2 15 1.86 313.2 100 2.39 PEI+[emim][AcO] (0.2:0.8) 298.2 15 3.13 298.2 100 3.81 20 ACS Paragon Plus Environment

References This work This work This work This work This work This work

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313.2 15 2.81 This work 313.2 100 3.42 This work PEI+PEG200 (0.1:0.9) 313.2 15 0.81 This work 313.2 100 0.96 This work PEI+PEG200 (0.2:0.8) 313.2 15 1.63 This work 313.2 100 1.91 This work [emim][AcO] 313.2 15 0.64 This work 313.2 100 1.26 This work [APbim][BF4] 298.2 10 1.82 30 [emim][Gly] 313.2 15 2.29 28 [emim][Ala] 313.2 15 1.81 28 [P66614][Pro] 295.2 15 1.56 31 [P4442]2[IDA] 313.2 100 2.95 32 [P66614][2-CNpyr] 295.2 15 1.01 33 [P66614][Im] 296.2 100 1.82 34 [MTBDH][TFE] 296.2 100 4.32 35 [P66614][PhO] 303.2 100 1.36 36 [P66614][2-Op] 293.2 15 2.33 37 [emim][Gly]+[emim][Ac]a 298.2 15 2.05 28 b [emim][Ala]+[emim][Ac] 313.2 15 1.41 28 c 313.2 26 1.02 38 [Ch][Pro]+PEG200 d 298.2 100 1.52 39 [C2(N114)2][Gly]2+H2O e [N1111][Gly]+H2O 298.2 100 1.62 39 f 303.2 10 0.23 40 [C2OHmim][Gly]+H2O g [APmim][Gly]+H2O 303.2 100 0.62 41 h DAIL+H2O 303.2 100 2.01 42 [DETA][Cl]+EGi 303.2 100 2.32 43 j MEA+H2O 298.2 10 2.72 44 MDEA+H2Ok 298.2 48 3.29 45 aThe mass fraction of [emim][Gly] is 52%; bthe mass fraction of [emim][Ala] is 54%; cthe mass fraction of [Ch][Pro] is 50%; dthe mass fraction of [C (N 2 114)2][Gly]2 is 40%; ethe mass fraction of [N f 1111][Gly]2 is 40%; the mass fraction of [C2OHmim][Gly] is g 8%; the mass fraction of [APmim][Gly] is 11%; hthe mass fraction of DAIL is 50%; ithe mass fraction of [DETA][Cl] is 36%; jthe mass fraction of MEA is 30%; kthe mass fraction of MDEA is 50%. Table 3 also summarizes the solubilities of CO2 in different chemical absorbents, including pure functionalized ILs,28,30-37 IL-based mixtures,23,38 and aqueous alkanolamines.39-45 As can be seen, PEI+[emim][AcO] mixtures designed in this work have much higher CO2 solubilities than most pure functionalized ILs and all the other IL-based mixtures reported in the literature. The CO2 solubilities of PEI+[emim][AcO] mixtures are even comparable to those of traditional aqueous alkanolamines. 21 ACS Paragon Plus Environment


CONCLUSIONS In summary, [emim][AcO] was used as a “chemical diluent” for PEI to construct mixed absorbents in this work. The physical properties and CO2 capture performance such as absorption rates, capacities and recyclability of PEI+[emim][AcO] mixtures were investigated. It is found that PEI+[emim][AcO] mixtures are with moderate viscosities, and still have good fluidity even after CO2 absorption. As a result, PEI+[emim][AcO] mixtures display significantly improved rates of CO2 absorption in comparison with pure PEI. In addition, PEI+[emim][AcO] mixtures exhibit quite high CO2 capacities, owing to the fact that both components in mixtures exhibit chemical affinity to CO2. The CO2 capacities of PEI+[emim][AcO] mixtures are higher than most other CO2 absorbents reported in the literature. Further considering that the volatility of PEI and [emim][AcO] are extremely low, it is concluded that PEI+[emim][AcO] mixtures are a class of green and efficient absorbents for CO2 capture. AUTHOR INFORMATION Contributions aThese

authors contributed equally to this work.

Corresponding authors *E-mails: [email protected] (K. H.); [email protected] (Y. L.). Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS


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This work was supported by the Natural Science Foundation of Jiangxi Province (20171BAB203019) and the National Natural Science Foundation of China (31660482 and 21676072). K. H. also appreciates the sponsorship from Nanchang University. ASSOCIATED CONTENTs Supporting information Density, viscosity, solubility data and TGA curves. REFERENCES 1.

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Graphic Abstract

SYNOPSIS: [emim][AcO] was proposed as the “chemical diluent” for PEI to construct mixed absorbents with negligible volatility and high capacity for CO2 capture.


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