Highly Efficient and Reversible CO2 Capture by Task-Specific Deep

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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Highly Efficient and Reversible CO2 Capture by Task-Specific Deep Eutectic Solvents Na Zhang,§ Zhaohe Huang,§ Haiming Zhang, Jingwen Ma, Bin Jiang, and Luhong Zhang* School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P. R. China

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S Supporting Information *

ABSTRACT: Herein, a series of imidazole derived task-specific deep eutectic solvents (DESs) based on a protic ionic liquid were synthesized and used for CO2 capture. These DESs exhibited excellent CO2 absorption capacity up to 1.00 mol CO2/mol DES, and the absorption process could be adjusted by the mole ratio of hydrogen-bond acceptor and hydrogen-bond donor. The effect of temperature on the absorption process was also investigated. To deeply investigate the absorption mechanism of these DESs, the corresponding protic ionic liquid was studied and in situ IR analysis was applied for recording the absorption process. With the help of experimental results, NMR spectra analysis, and theoretical calculations, it was found that these DESs captured CO2 via a synergism of cation and anion, and the protonated superbase played a key role in the absorption process. In addition, the DESs studied in this work showed high thermal stability and excellent recyclability.

1. INTRODUCTION As industry rapidly develops, a large amount of CO2 is emitted into the atmosphere, triggering a series of environmental problems, such as the formation of the greenhouse effect.1−5 To reduce CO2 emissions, it is urgent to develop strategies for carbon dioxide capture and storage (CCS), and the development of new absorbents is the core of CCS.6,7 Ionic liquids (ILs), green solvents with high thermal stability and negligible volatility, have been extensively studied as alternatives to aqueous solutions of alkanolamines, which are the common absorbents in the industrial process.8,9 In particular, much effort has been devoted to synthesizing task-specific ionic liquids in recent years, since they show excellent absorption capacity compared with conventional ionic liquids.10−12 However, the synthetic process of a task-specific ionic liquid is complicated.13−17 Moreover, the high cost on large scale production and the generation of solvent byproducts during synthesis and purification further limit its application in industrial application. Hence, it is of urgent necessity to develop new CO2 absorbents. Recently, deep eutectic solvents (DESs), an advanced generation of ILs, have gained the attention of researchers throughout the world.16−19 DESs inherit the advantages of ionic liquids, such as high stability, inherent volatility, etc., but beyond that, the synthesis cost of DESs is lower and the synthesis method is simpler. DESs are typically obtained by means of mixing a hydrogen-bond acceptor (HBA), such as choline chloride, and a hydrogen-bond donor (HBD), such as urea. In 2008, Han and co-workers20 studied the solubility of CO2 in DESs composed of choline chloride (ChCl) and urea © XXXX American Chemical Society

and found that pressure, temperature, and composition ratio had influence on the solubility of CO2 in DESs. Also, ChCl− urea (1:2) exhibited the highest solubility with 0.309 mol CO2/mol DES at 40 °C, 112.5 bar. Subsequently, Leron et al.21 reported that the solubility of CO2 in ChCl−glycerol (1:2) could reach 1.20 mol CO2/mol DES at 30 °C, 58.6 bar. However, ChCl−glycerol (1:2) exhibited only 0.047 mol CO2/ mol DES at low CO2 pressure (1.87 bar). It can be seen that the conventional DESs cannot possess adequate CO 2 absorption capacity at ambient pressure. One of the main reasons is that conventional DESs capture CO2 via physical interaction, such as hydrogen bonds, van der Waals forces, etc.17 Obviously, the CO2 solubility in these DESs at ambient pressure did not meet the requirement for an industrial process. Therefore, it is necessary to develop new strategies to improve CO2 absorption capacity of DES. In the effort to improve the absorption capacity of DES, an effective strategy is to change the CO2 absorption mode of DESs from physical absorption to chemical absorption. In this regard, much research was dedicated to using HBA or HBD with an amino group to obtain task-specific DESs. In 2015, Zhang and co-workers22 reported several DESs composed of 1butyl-3-methylimidazolium chloride (BmimCl) and monoethanolamine (MEA). The CO2 uptake of BmimCl−MEA (1:1) at room temperature and atmospheric pressure is up to Received: Revised: Accepted: Published: A

April 15, 2019 June 27, 2019 July 1, 2019 July 1, 2019 DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 1. Experimental diagram of CO2 absorption: (1) CO2 gas cylinder, (2) N2 gas cylinder, (3) digital thermostat water bath, (4) residual gas absorption bottle (off-gas was absorbed by NaOH solution), (5) Y-shaped glass sampling tube, (6) gas rotor flow meter, and (7) pressure relief valve.

single flask with stirring at 40 °C for 24 h under dry N2 atmosphere. Subsequently, the resultant protic ionic liquid was further freeze-dried to remove excess moisture. The product [DBNH][Im] was clear liquid at room temperature and atmospheric pressure. DBN−BmimCl (1:1) was prepared by a similar method as that used for synthesizing [DBNH][Im], and it was solid at room temperature and atmospheric pressure. 2.2.2. Synthesis of Imidazole Derived Deep Eutectic Solvents. At first, BmimCl and imidazole were freeze-dried for 24 h. Subsequently, BmimCl and imidazole were added into a 50 mL flask at different molar ratios and stirred at 60 °C under dry N2 atmosphere for 24 h to obtain BmimCl−Im (1:1), BmimCl−Im (2:1), and BmimCl−Im (1:2). The resultant deep eutectic solvents were further freeze-dried for preparing task-specific deep eutectic solvents. 2.2.3. Synthesis of Task-Specific Deep Eutectic Solvents. DBN−BmimCl−Im (1:1:1) was obtained by stirring DBN (0.05 mol) and BmimCl−Im (1:1) (0.05 mol) at 50 °C for 24 h. DBN−BmimCl−Im (1:2:1) and DBN−BmimCl−Im (1:1:2) were prepared by a similar method. 2.3. Characterization. 1H NMR and 13C NMR spectra were recorded on a Mercury 300 MHz spectrometer, and the peak frequencies were referenced versus internal standard (TMS) shifts at 0 ppm. CD3OD was selected as solvent. In-situ IR spectra were measured by a Mettler Toledo iC10 ReactIR. The measurement apparatus and steps were shown in Figure 1 and section 2.5. Water contents of the studied absorbents were determined by Karl Fischer coulometer and below 100 ppm for all systems. The decomposition temperatures of the studied absorbents were determined by thermogravimetric analysis (TGA, Netzsch). 2.4. CO2 Capture and Release Experiment. Typically, 1.0 g of task-specific deep eutectic solvent was added into a self-made absorption tube. Then, it was immerged into a water bath for 30 min to reach the desired temperature. After that, CO2 (1 bar) was bubbled through the system at a flow rate of about 60 mL min−1. The weight of the captured CO2 was recorded in real time until the absorption equilibrium was established. For CO2 desorption, the absorption system was put into an oil bath at the appropriate temperature with vigorous stirring. The weight of the system was also recorded in real time until the total weight remained unchanged. All the measurements were repeated for three times and the uncertainties of mass determination were well within ±1%.

0.45 mol CO2/mol DES. Afterward, Choi’s group23 prepared task-specific DESs using monoethanolamine hydrochloride (MEA·Cl) and ethylenediamine (EDA), and [MEA·Cl][EDA] (1:3) exhibited a high absorption capacity of 0.502 mol CO2/ mol DES. Recently, several functional DESs based on ethylene glycol (EG) or diethylene glycol (DG) as HBD, and triethylenetetramine hydrochloride (TETA·Cl) as HBA were reported by Wu and co-workers.24 The CO2 uptake of [MEA· Cl][EDA] (1:3) was up to 1.456 mol CO2/mol [TETA·Cl]. Compared with conventional DESs, these task-specific DESs increased the absorption capacity via adding -NH- or -NH2 group into the absorption system. However, the stability of these DESs was poor. For instance, the decomposition temperature (Td) of BmimCl−MEA (1:1) was just 86 °C. As known to all, if the decomposition temperature of the absorbent is too low, there will be absorbent loss in the recycling process leading to the increase of the operating costs. To overcome this, novel task-specific DESs containing an aprotic heterocyclic anion with high thermal stability was prepared and used for CO2 capture. In this work, a series of imidazole derived DESs were obtained by mixing BmimCl and imidazole at different molar ratios. Then the superbase 1,5-diazabicyclo [4.3.0] non-5-ene (DBN) was added into the DESs to form task-specific DESs. The effects of temperature and mole ratio of HBA and HBD on the absorption capacity were investigated. In order to analyze the CO2 absorption process of task-specific DESs deeply, in situ IR analysis was applied to record the absorption process. Combined with the experimental results, along with NMR spectra analysis and DFT calculations, the corresponding absorption mechanism was proposed.

2. EXPERIMENTAL SECTION 2.1. Materials. 1,5-Diazabicyclo [4.3.0] non-5-ene (DBN, 98%) and imidazole (98%) were purchased from Tianjin Heowns Biochemical Technology Co., Ltd. 1-Butyl-3-methylimidazolium chloride (BmimCl, 99%) was supplied by Tokyo Chemical Industry Co., Ltd. CO2(99.999%) and N2 (99.999%) were obtained from Tianjin Shengtang Gas Co., Ltd. 2.2. Preparation of Task-Specific Deep Eutectic Solvents. 2.2.1. Synthesis of Protic Ionic Liquid. Protic ionic liquid [DBNH][Im] was synthesized following a similar procedure reported in the literature.25,26 First, imidazole was freeze-dried for 24 h to remove moisture. After lyophilization, DBN (0.05 mol) and imidazole (0.05) were mixed in a 50 mL B

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 2.5. In-Situ IR CO2 Absorption Measurement. At first, 5.0 g of task-specific DES was added into Y shaped sampling tube. Subsequently, the probe of the in-situ IR was immersed into the task-specific DES. Then the Y shaped sampling tube was put into water bath at 25 °C for 30 min. The IR spectra of the task-specific DES were obtained before bubbling CO2. After that, CO2 (1 bar) was bubbled through the system, and the IR spectra were recorded at the same time. Measurements continued until the IR spectra did not change. The experimental apparatus was shown in Figure 1.

achieve absorption saturation within 30 min with an increase in the mole ratio of imidazole but there was a slight decrease in absorption capacity. According to Shan’s report,27 the Nbonded hydrogen of imidazole could form a hydrogen bond with chloride anions. Therefore, it can be inferred that the complex may provide a medium for the absorption process to improve the absorption efficiency. 3.2. The Effect of Temperature on the CO 2 Absorption Process by Task-Specific DESs. As shown in Figure 3, the effect of temperature on CO2 absorption by a

3. RESULTS AND DISCUSSION 3.1. CO2 Capture. The synthesis of task-specific DESs studied in this work was based on imidazole derived protic ionic liquids. Thus, the corresponding protic ionic liquid [DBNH][Im] was studied first. The CO2 uptake of [DBNH][Im] over time at 25 °C and 1 bar under CO2 flow rate of 60 mL/min was shown in Figure 2. [DBNH][Im] could reach

Figure 3. Effect of temperature on CO2 absorption by DBN− BmimCl−Im (1:1:1) at 1 bar under CO2 flow rate of 60 mL/min.

task-specific DES was investigated, and DBN−BmimCl−Im (1:1:1) was taken as an example. Like that for most of the absorbents studied in the literature,5,22,23,28 the absorption process was exothermic and the uptake of DBN−BmimCl−Im (1:1:1) decreased from 1.02 to 0.81 mol CO2/mol DES with the increase of temperature from 25 to 55 °C. However, these results were not consistent with Zhu’s work.26 They found that the absorption capacity of protic ionic liquid [DBUH][Im] increased when the temperature increased from 25 to 40 °C and thought that the action of mass transfer, reaction dynamics, and reaction thermodynamics had synergistic effects on the absorption process. Obviously, DBN−BmimCl−Im (1:1:1) had better absorption dynamics and thermodynamics at low temperature. Even at 40 °C, DBN−BmimCl−Im (1:1:1) still possessed better absorption capacity than [DBUH][Im]. In addition, the temperature dependence of DBN−BmimCl−Im (1:2:1) and DBN−BmimCl−Im (1:1:2) was also studied. As summarized in Table S1, these two taskspecific DESs showed similar feedback to changes in temperature. Of note, for those three task-specific DESs, the changes in saturated absorption capacity were different with the increase of temperature, implying that the composition of DESs might have an effect on CO2 absorption enthalpy. On the basis of the absorption data at different temperatures listed in Table 1, the CO2 absorption enthalpies were calculated using the Van’t Hoff equation.23,28 The absorption enthalpies of DBN−BmimCl−Im (1:1:1), DBN−BmimCl−Im (1:2:1), and DBN−BmimCl−Im (1:1:2) were −74, −130, and −40 kJ/ mol, respectively. The results demonstrated that the composition of DESs had an effect on CO2 absorption enthalpy, which was in accordance with the work of Choi.23

Figure 2. CO2 absorption by protic ionic liquid and task-specific deep eutectic solvents as a function of time at 25 °C and 1 bar under CO2 flow rate of 60 mL/min.

absorption equilibrium within 160 min, and the absorption capacity was 0.82 mol of CO2/mol IL. Of note, the viscosity of [DBNH][Im] increased rapidly after bubbling CO2 into the absorption system and became solid after absorption. A similar phenomenon was found in Dai’s work.25 In their work, protic ionic liquids [MTBDH][PhO] and [(P2-Et)H][PhO] became gel after absorbing CO2. In addition to [DBNH][Im], DBN− BmimCl (1:1) was synthesized in this work. However, it was solid at 25 °C which limited its use for CO2 capture at low temperature. Subsequently, the absorption processes of taskspecific DESs were investigated. As shown in Figure 2, as compared with [DBNH][Im], DBN−BmimCl−Im (1:1:1), DBN−BmimCl−Im (1:2:1), and DBN−BmimCl−Im (1:1:2) exhibited much higher absorption capacities with 1.02, 1.07, and 0.97 mol CO2/mol DES, respectively. Moreover, their time consumption of reaching the adsorption balance was much shorter than that of [DBNH][Im]. In addition, they were all clear liquids after absorbing CO2. When the mole ratio of BmimCl in DES was increased from DBN−BmimCl−Im (1:1:1) to DBN−BmimCl−Im (1:2:1), the absorption capacity increased slightly, but the time to reach equilibrium was almost unchanged. Obviously, DBN−BmimCl−Im (1:1:2) could C

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research Table 1. Comparison of CO2 Capture by Different Functionalized Ionic Liquids and Deep Eutectic Solvents tempa

pressureb

absorption capacityc

absorption enthalpy

Tdd

(kJ/mol)

°C

absorbents

°C

bar

(mol/mol)

[ApBmim][BF4] [Emim][Im] [Bis(mim)C2][Im]2 [P66614][Im] coline chloride−urea(1:4) coline chloride−glycerol(1:3) [MEA·Cl][EDA](1:3) [bmim][MeSO3]−urea(1:1) MEA−[C3OHmim]BF4(1:1) MEA−BmimCl(1:1) DBN−BmimCl−Im (1:1:1) DBN−BmimCl−Im (1:1:2) DBN−BmimCl−Im (1:2:1)

22 40 40 23 25 25 30 30 35 40 25 25 25

1 ∼0.7 ∼0.7 1 10 10 1 ∼7 1 1 1 1 1

0.5 0.55 0.75 1 0.024 0.045 0.502 ∼0.1 0.373 ∼0.3 1.02 0.97 1.07

−89.9

−61.18 −3.75 −64.3 −95∼−113 −74 −40 −130

148.2e 142.3e 252e

115

86 134.2 146.5 148.6

ref 10 33 33 12 36 36 23 37 38 22 this work this work this work

a e

Absorption temperature. bAbsorption equilibrium pressure. cmol CO2/mol solvent. dTd is defined as the temperature with the weight loss of 5%. According to the reference, the decomposition temperature corresponding to a 10% mass loss.

Figure 4. (a) The absorption IR spectra of [DBNH][Im] at 1 bar under CO2 flow rate of 60 mL/min; (b) typical changes of the absorption peaks over time.

Figure 5. NMR spectra of [DBNH][Im] collected before and after absorption: (a) 1H NMR; (b) 13C NMR.

3.3. Investigation on the Reaction Mechanism between Task-Specific DESs and CO2. The experimental

results of DBN−BmimCl−Im (1:1:1) and [DBNH][Im] indicated that these two absorbents exhibited different D

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 6. Scheme for the absorption mechanism of [DBNH][Im].

Figure 7. (a) The absorption IR spectra of DBN−BmimCl−Im (1:1:1) at 1 bar under CO2 flow rate of 60 mL/min; (b) the change of typical absorption peaks over time.

Figure 8. NMR spectra of DBN−BmimCl−Im (1:1:1) collected before and after absorption: (a) 1H NMR; (b) 13C NMR.

absorption behavior. DBN−BmimCl−Im (1:1:1) was prepared by adding DBN into DES BmimCl−Im (1:1). This can also be obtained by mixing protic ionic liquid [DBNH][Im] and BmimCl. Thus, in order to investigate the absorption process of DBN−BmimCl−Im (1:1:1) systematically, [DBNH][Im] was studied first by employing in situ infrared spectroscopy utilizing a ReactIR iC10 spectrometer. As shown in Figure 4, there was a new peak at 1678 cm−1, which was ascribed to the stretching vibration of CO in OC−O after absorbing CO2.26,29 Moreover, due to the C−O stretching vibration in OC−O, the corresponding absorption peak intensity at 1282 cm−1 increased. The new peak at 1174 cm−1 was nominated as the bending modes of imidazole anion combined

CO2, which was also found in other IR spectra of the imidazole anion functionalized ionic liquid after capturing CO2.12,29 The absorption peak at 1044 cm−1 was assigned to the stretching vibration of the newly formed C−N bond. To investigate the CO2 absorption mechanism of [DBNH][Im] further, 1H NMR and 13C NMR analyses were conducted. Superbase DBN was a strong proton acceptor, the protonated conjugate acid if which cannot react with a hydroxide ion; thereby the stable and reactive protic ionic liquid was obtained.12 As shown in Figure 5a, the peak of H on the N of the imidazole ring was not found in the spectrum of fresh [DBNH][Im], indicating the formation of [DBNH][Im]. After absorbing CO2, the peak related to −COOH was not E

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research found, verifying that [DBNH+] did not react with CO2 directly. Furthermore, the signals of the hydrogen in the [DBNH+] shifted to downfield. The result suggested a strong hydrogen bond formed between [DBNH+] and [Im-CO2−], which also can be the reason that the viscosity of [DBNH][Im] increased rapidly after bubbling CO2 and finally became a solid. The 13C NMR spectrum of the saturated [DBNH][Im] displayed a resonance peak at 163.8 ppm, which was consistent with the 13C NMR spectra of [DBUH][Im]26 and [P66614][Im].12 On the basis of the experimental results and spectra analysis, the CO2 absorption mechanism of [DBNH][Im] was proposed and shown in Figure 6. The absorption process of DBN−BmimCl−Im (1:1:1) was also studied employing in situ infrared spectroscopy as depicted in Figure 7. The stretching vibration peak for C O in OC−O was also found and enhanced quickly at 1678 cm−1 after bubbling CO2 into the absorbent, and the absorption peak at 1320 cm−1 was assigned to the C−O stretching vibration in OC−O. Of note, for the fresh DBN− BmimCl−Im (1:1:1), there was absorption peak at 1174 cm−1, which was also a characteristic peak of the [Bmim+] cation according to a previous report.22,30 Furthermore, unlike the absorbance IR spectra shown in Figure 4, the peaks at 1174 and 1044 cm−1 increased slightly in the absorption process. In addition, a new peak appeared at 1506 cm−1, which was attributed to the stretching mode of −NH− on the imidazole ring,31 suggesting that the imidazole anion might combine with a proton to form imidazole in the absorption process. On the basis of the above analysis, the absorption mechanism of DBN−BmimCl−Im (1:1:1) might be different from that of [DBNH][Im]. This again suggested that the absorption process was changed after adding BmimCl into [DBNH][Im]. The 1H NMR and 13C NMR spectra of fresh and CO2 saturated DBN−BmimCl−Im (1:1:1) were investigated to verify the aforementioned deduction. As shown in Figure 8a, a new peak appeared at 9.08 in the 1H NMR spectrum of CO2saturated DBN−BmimCl−Im (1:1:1), while there was no peak in the 1H NMR spectrum of fresh absorbent, suggesting that this peak was not assigned to the C2 hydrogen in BmimCl. So it can be inferred that the signal at 9.08 ppm might correspond to the −NH− group on the imidazole ring.32 That is to say, the result again indicated that the imidazole molecule was generated after absorbing CO2. According to Zhang’s work,33 [Im−] combined with the C2 hydrogen on the imidazolium ring to form the imidazole molecule and then [Emim] reacted with CO2 leading to the formation of [Emim+−COO−]. For DBN−BmimCl−Im (1:1:1), [Bmim+] and [Im−] existed simultaneous in the absorption system, so a similar reaction process might occur in DBN−BmimCl−Im (1:1:1). Brennecke and co-workers34 found that C4 and C5 hydrogen in the imidazolium cation shifted to downfield when the cation combined with CO2, which agreed well with the 1H NMR spectrum studied in this work. The double peaks at 7.70 ppm might be attributed to C4 and C5 hydrogen in [Bmim+]. A similar phenomenon was found using DBN−BmimCl−Im (1:2:1) as absorbent presented in Figure S1. Furthermore, as depicted in Figure 8b, a new peak at 164.5 ppm was ascribed to the newly formed COO− group in [Bmim+−COO−]. On the basis of the above analysis, at the early stage of the absorption process, the absorption mechanism of DBN− BmimCl−Im (1:1:1) was similar to that of imidazolate-based ionic liquids studied in Zhang’s work.33 It is of interest to note that the CO2 capacity of [Emim][Im] was just 0.5 mol CO2/

mol IL. For ionic liquid [Emim][Im], zwitterion [Emim+COO−] formed a dimer with another cation. At the same time, the imidazole molecule conbined with the imidazole anion to form a complex, which explained why [Emim][Im] yielded 0.5 mol CO2/mol IL. However, DBN−BmimCl−Im (1:1:1) exhibited much higher absorption uptake with 1.02 mol CO2/mol DES, suggesting that it experienced a different reaction process in the next stage. Of note, in addition to [Bmim+] and [Im−], [DBNH+] and [Cl−] existed in DBN− BmimCl−Im (1:1:1). Considering the fact that [DBNH][Im] became a solid after absorbing CO2, it was inferred that there was strong interaction between [DBNH+] and [Im-CO2+]. Moreover, imidazole can form a strong hydrogen bond with [Cl−]. Therefore, [DBNH+] and [Cl−] might have an effect on the absorption process. To investigate the effect of [DBNH+] on the absorption process further, density-functional theory (DFT) calculations using the Gaussian 09 program at the B3LYP/6-311++G (d,p) level of theory,35 were carried out to calculate the interaction energy ΔE between [DBNH+] or [Bmim+] and [Bmim+− COO−] with the BSSE correction. The optimized structures of dimers were depicted in Figure 9. For [Bmim+]−[Bmim+−

Figure 9. Possible existing dimer after absorbing CO2: (a) [Bmin]+[Bmim+−COO−]; (b) [DBNH]+[Bmim+−COO−].

COO−], the distance between O in the zwitterion [Bmim+COO−] and C2 hydrogen in [Bmim+] was calculated to be 1.94 Å, and it reduced by approximately 28.7% compared with the sum of van der Waals radii of the two interacting atoms. Also for [DBNH+]−[Bmim+−COO−], the distance between O and H was 1.69 Å, which corresponded to a reduction of approximately 37.9%. In addition, the interaction energy between [Bmim+] and [Bmim+−COO−] was −121.3 kJ/mol, while the interaction energy between [DBNH+] and the zwitterion was −133.4 kJ/mol. Obviously, [DBNH+] had a stronger interaction with the zwitterion [Bmim+−COO−] than [Bmim+], demonstrating that the dimer [DBNH+]−[Bmim+− COO−] was more stable in the CO2 saturated absorption system. A plausible mechanism between CO2 and DBN−BmimCl− Im (1:1:1) was depicted in Figure 10. At first, [Im−] combined with C2 hydrogen on the [Bmim+] and then [Bmim] reacted with CO2 to form [Bmim+-COO−]. Subsequently, [Bmim+COO−] formed a dimer with [DBNH+], and a strong hydrogen bond was formed between imidazole and [Cl−]. Therefore, DBN−BmimCl−Im (1:1:1) can yield as high as 1.02 mol CO2/mol DES absorption capacity. 3.4. The Stability and Recycling Property of TaskSpecific DESs. The thermal stability of protic ionic liquid [DBNH][Im] and task-specific DESs was determined by TGA analysis. As depicted in Figure 11, the decomposition temperature (Td) of [DBNH][Im] was about 119.5 °C, and F

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 10. Plausible mechanism between DBN−BmimCl−Im (1:1:1) and CO2.

and 148.6 °C, respectively. Apparently, the task-specific DESs studied in this work were more stable than [DBNH][Im]. Moreover, Td increased as the mole ratio of BmimCl and imidazole increased. Hence, these three task-specific DESs can be potentially used under high temperature conditions. Since DBN−BmimCl−Im (1:1:2) had good absorption dynamics and thermodynamics at 25 °C, it achieved equilibrium within 30 min and possessed good mole absorption capacity. Besides, it exhibited excellent thermal stability. So DBN−BmimCl−Im (1:1:2) was selected as a representative to investigate the reversible property of the taskspecific DESs studied in this work. As shown in Figure 12, the CO2 absorption capacity almost remained unchanged after repeating the absorption and desorption cycle for six times, indicating that it exhibited excellent reusability. 3.5. CO2 Uptakes by Different ILs and Other DESs for Comparison. The absorption capacity and CO2 absorption enthalpy of some representative task-specific ionic liquids and DESs are summarized in Table 1. For task-specific ionic liquids, such as [ApBmim][BF4] and [Emim][Im], though they exhibited good absorption capacity and higher decom-

Figure 11. TGA curves of the task-specific DESs.

the Td of DBN−BmimCl−Im (1:1:1), DBN−BmimCl−Im (1:1:2), and DBN−BmimCl−Im (1:2:1) were 134.2, 146.5,

Figure 12. Reusability of DBN−BmimCl−Im (1:1:2) in CO2 absorption. G

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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position temperature, the complicated synthetic process limited their application in the industrial process. The conventional DESs were easy to prepare by mixing HBA and HBD, but they did not have adequate absorption capacity because they captured CO2 via physical interaction. Taskspecific DESs have attracted much attention in the scientific community in recent years and these DESs exhibit excellent CO2 uptake. However, the decomposition temperatures of these DESs were relatively low. As the synthesis of the DESs studied in this work was based on a protic ionic liquid, they showed higher thermal stability as well as good absorption capacity. In addition, the task-specific DESs with higher absorption efficiency and lower absorption enthalpy can be obtained by adjusting the mole ratio of HBA and HBD in DES, which has potential theoretical significance in carbon dioxide capture research.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86 22 27400199. ORCID

Luhong Zhang: 0000-0002-7073-4793 Author Contributions §

These two authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was financially supported by National Key R&D Program of China (No. 2016YFC0400406).

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4. CONCLUSIONS In summary, this work puts forward a novel kind of taskspecific DESs containing aprotic heterocyclic anions and provides a comprehensive study of absorption process based on various analytical methods. As the synthesis of the functionalized DESs was based on protic ionic liquid, they showed higher thermal stability compared with the state-ofthe-art task-specific DESs, which showed even higher thermal stability than the corresponding protic ionic liquid [DBNH][Im]. The uptake of [DBNH][Im] was just 0.82 mol CO2/mol IL and it was a solid after absorbing CO2, while DBN− BmimCl−Im (1:1:1) showed higher absorption capacity with 1.02 mol CO2/mol IL and remained liquid in the absorption process. By adjusting the mole ratio of BmimCl and imidazole, the uptake of DES and the time to reach equilibrium can be tailored. DBN−BmimCl−Im (1:1:2) reached absorption equilibrium within 30 min with just a slight decrease in absorption capacity. In addition, the absorption enthalpies of DESs can also be adjusted by the molar ratio of BmimCl and imidazole. The absorption enthalpies of DBN−BmimCl−Im (1:1:1), DBN−BmimCl−Im (1:2:1), and DBN−BmimCl−Im (1:1:2) were −74, −130, and −40 kJ/mol, respectively. Obviously, DBN−BmimCl−Im (1:1:2) was easier to recirculate than DBN−BmimCl−Im (1:2:1). Moreover, DBN− BmimCl−Im (1:1:2) exhibited excellent thermal stability with a Td value of 146.5 °C. The CO2 absorption capacity of DBN−BmimCl−Im (1:1:2) almost remained unchanged after six absorption and desorption cycles, indicating that it exhibited excellent reusability. Of note, the task-specific DESs studied in this work experienced a different absorption path compared to [DBNH][Im]. [Im−] combined with C2 hydrogen on the [Bmim+] and then [Bmim] reacted with CO2 to form [Bmim+-COO−]. Afterward, [Bmim+-COO−] formed a dimer with [DBNH+], and a strong hydrogen bond was formed between imidazole and [Cl−].

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02041. Saturated mole absorption capacity of DESs, NMR spectra of DBN−BmimCl−Im (1:2:1) before and after absorption of CO2 (PDF) H

DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.iecr.9b02041 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX