Absorption of carbon dioxide using ... - American Chemical Society

Zhuo Li,﹟a Lili Wang,﹟b Changping Li,*a Yingna Cui,b Shenmin Li,b Guang Yang,b Yongming Shen*c a Research Center for Eco-Environmental Engineering...
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Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10403−10414

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Absorption of Carbon Dioxide Using Ethanolamine-Based Deep Eutectic Solvents Zhuo Li,†,∥ Lili Wang,‡,∥ Changping Li,*,† Yingna Cui,‡ Shenmin Li,‡ Guang Yang,‡ and Yongming Shen*,§

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†

Research Center for Eco-Environmental Engineering, Dongguan University of Technology, No. 1 University Road Songshanhu Zone, Dongguan City, Guangdong Province 523106, P. R. China ‡ College of Chemical Engineering and the Environment, Dalian University, No. 10 Xuefu Street Development Zone, Dalian City, Liaoning Province 116622, P. R. China § Institute of Environmental and Ecological Engineering, Guangdong University of Technology, No. 100 Waihuan West Road University Town Panyu District, Guangzhou City, Guangdong Province 510006, P. R. China S Supporting Information *

ABSTRACT: The development of a new, green CO2 absorbent with high energy utilization and low solvent loss can replace CO2 capture by ethanolamine solution, which is currently a necessary and important issue. This work is devoted to the design and synthesis of DESs based on ethanolamine and the absorption of CO2, focusing on the systematic study of the effects of CO2 absorption and the effect of water on the deep eutectic solvents (DESs) of ChCl/MEA, TMAC/MEA, and TEAC/MEA (MEA = ethanolamine, TMAC = tetramethylammonium chloride, and TEAC = tetraethylammonium chloride). A series of DESs comprise various hydrogenbonding donor−acceptor pairs as CO2-capturing solvents. The main factors that influence the absorption process, such as rotational speed, flow rate, temperature, absorption time, molar ratio, and water content on the absorption of CO2, are examined. Simultaneously, the influence on the DESs after absorption was analyzed, and the absorption mechanism was systematically studied. Furthermore, the use of three-component alkaline DESs (TMAC/MEA/MDEA) (MDEA = methyl diethanolamine) and the three components of TMAC/MEA/LiCl, TMAC/MEA/ZnCl2, and TMAC/MEA/NH4Cl were investigated by adding pure water to remove CO2 performance. The TMAC/MEA/LiCl solvent reaches a maximum absorption of 36.81 wt % at 50 °C, and for TMAC/MEA/LiCl + 10% H2O system, the absorption of CO2 decreases with increasing temperature. The TMAC/MEA/LiCl + 10% H2O system has the highest absorption at 30 °C and reaches 37.31 wt %. NMR was applied to investigate the absorption mechanisms. The hydrogen-bonding and electrostatic interactions were the main driving force for this specifically high CO2 absorption process. KEYWORDS: Deep eutectic solvents, CO2 absorption, Ethanolamine, Mechanisms, Green process

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INTRODUCTION Resources and energy have been considerably consumed because of frequent human activities. Particularly, the burning of fuels such as petroleum, coal, and natural gas has intensified, which leads to a dramatic increase in global CO2 emission.1 In recent years, CO2 capture and storage (CCS) technology has been comprehensively investigated. Therefore, developing a new, efficient, economical, and recyclable absorbent is crucial to capture CO2 in flue gas.2 Presently, the most widely used technology for the capture of CO2 in industrial applications is solvent absorption.3 Domestic and foreign CO2 absorption methods are mainly used, such as membrane absorption,4 solvent absorption,5 and chemical absorption.6 Nowadays, the alkanolamine absorption is the most commonly used. The alcohol amine content in the solution is usually controlled at 25−30 wt % to weaken the corrosion of the equipment. The use of alcohol amine © 2019 American Chemical Society

solutions to capture CO2 has the advantages of high absorption, high selectivity, and low cost.7 However, this approach is also faced with many problems, which include decomposition of toxic byproducts at high temperatures, loss of alkanolamines evaporation during regeneration, and corrosion of alkaline aqueous solutions. The most important advantage of ethanolamine is its strong alkalinity, and its heat reaction with CO2 is high (∼80.0 kJ/mol).8,9 Moreover, a large amount of water must be heated during the regeneration process; therefore, the regeneration energy consumption is high. At present, absorption separation is one of the most common technologies available for CO2.10 Absorption can be Received: January 28, 2019 Revised: May 17, 2019 Published: May 28, 2019 10403

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physicochemical properties, are considered “green solvents” that have the same importance as ionic liquids and can replace volatile organic solvents. Han and co-workers26 also successfully synthesized a series of choline-based DESs and applied these solvents to CO2 capture and separation studies. Given that ionic liquids are continuously used in the study of absorption CO2, DESs are regarded as an alternative to ionic liquids; the study of CO2 absorption has gradually increased. Li and co-workers27 examined the solubility of DESs (ChCl/urea 1:2) for CO2 at different temperatures and pressures and investigated the influence on the molar ratio of salt and HBD on the solubility of CO2. Li et al.28 also investigated the absorption and desorption of CO2 in [Choline][Pro]/PEG200 solvent. Su et al.29 explored the solubility of CO2 in threecomponent ChCl/urea/H2O. The results showed that the solubility of CO2 decreased with the increase of water content in the solvent, which may be due to the decrease in the concentration of effective reactants. He and co-workers30 explored the solubility of CO2 in ChCl/EG, as well as the conductivity of the mixture. For the first time, Aparicio and coworkers31 studied the mechanism of CO2 absorption by choline chlorides at the molecular level using density functional theory (DFT) and optimized the structure and related parameters of DESs−CO2 system. Stevens and coworkers32 explored CO2 in three different DESs under experimental conditions of 309−329 K and 160 kPa. A thermodynamic model of solubility in three different DESs (ChCl/urea 1:2, ChCl/EG 1:2, ChCl/MA/EG 1.3:1:2.2) was developed. Henry’s constant of the CO2−DESs system was measured in the pressure range from 3.7 to 6.1 MPa. The results indicated that CO2 absorption is an exothermic reaction and entropy decreases with gas absorption. Huang et al.33 studied the absorption of CO2 by MEA in hydroxyl imidazolium-based ionic liquids. The CO2 absorption of MEA−[C3OHmim]BF4 can reach 26.9 wt %, and the CO2 absorption of MEA−[C3OHmim]Cl can reach up to 28.6 wt %. Cl− can increase the absorption of CO2 in MEA, and hydroxyimidazole ionic liquid mixtures can increase the thermal stability of CO2 capture. However, the CO2−HX interaction is confirmed to be different from conventional hydrogen bonding.34−36Notably, valuable information regarding the interaction effects of hydrogen bonds in DESs on CO2 absorption is limited. Choi and co-workers37 synthesized a series of new DESs by using MEA to react with equimolar amounts of HCl (37 wt %), and then maintaining the mixture at 60 °C for 3 h. The purpose of the process is to obtain the MEA-based hydrogen acceptor [MEA·Cl]. Four kinds of DESs, namely, [MEA·Cl][EDA], [TEA·Cl][EDA], [UE·Cl][EDA], and [TAE·Cl][EDA], were synthesized for CO2 absorption. Experiments were also conducted on the effect of water content on CO2 uptake for [MEA·CI][EDA], which was found to have CO2 uptake of 31.5 wt % at 30 °C; the molar ratio is 1:3 after 3 h absorption. The CO2 uptake reached 33.7 wt % after 24 h absorption. Alnashef and co-workers38 synthesized three types of DESs (ChCl/MEA, ChCl/DEA, and ChCl/MDEA), at P = 15 KPa, T = 40 °C, and with the gases CO2 and N2 (15 and 85 vol %, respectively). The absorptions of CO2 at different molar ratios (1:6, 1:8, and 1:10) of three DESs were measured at a flow rate of 15 L/h. Experimental studies have shown that the amino-based DESs CO2 absorption capacity was higher than 30 wt % alkanolamine solution with traditional DESs; ChCl−MEA has the highest absorption of CO2, and ChCl−MDEA has the lowest

divided into variable-temperature absorption (TSA), pressureswing absorption (PSA), and variable-temperature pressureswing absorption (PTSA). In recent years, metal−organic framework materials (MOFs) with ultrahigh specific surface area have been synthesized for CO2 capture. Because of the high absorption and high selectivity (such as bio-MOF-11 at 25 °C), atmospheric CO2 absorption reached 4.1 mmol/g, and CO2/N2 absorption selectivity was 75:1.11 Mg-MOF-74 has a CO2 uptake of 6.2 mmol/g at 25 °C and atmospheric pressure but has CO2/N2 absorption selectivity of 30:1.12 The absorption process is simple and has strong adaptability to gas. At the same time, it has no corrosion to equipment and is environmentally friendly. However, due to the limitation of absorption capacity, absorption and desorption are relatively frequent and automation is highly demanded. In 2005, the research group reported on the inherent solubility of CO2 in [Bmim][PF6].13 The influence of anions on the solubility of CO2 in ionic liquids was found to be the main factor. Task-specific ionic liquids14 can be designed for specific requirements. According to the actual situation, some ionic liquids with high absorption and high selectivity can be designed. Bates et al.15 synthesized a new type of ionic liquid containing −NH2 functional group [C3H7NH2−Bmim][BF4] for the first time. Under normal temperature and pressure, the functionalized ionic liquid has a CO2 absorption of 7.4 wt %, which belongs to chemical absorption. Although the aminofunctionalized ionic liquids have high absorption of CO2, the viscosity of the ionic liquid increases after amino-functionalization. Imidazole ionic liquids have been reported to have good CO2 solubility and CO2 capture selectivity.16−18 Baj et al.19 showed that the best absorbents of CO2 were [Bbim][Piv] and [Bmim][OAc] with corresponding CO2 absorption capacities of 0.40 and 0.37 mol/mol ionic liquid (IL). Wang et al.20 found the highest uptake value for a superbase protic IL [MTBDH][TFE] (16.4 wt %). A dual amino-functionalized IL (DAIL) can reach up to 18.5 wt %.21 By mixing [Bmim][Cl] and MEA under an argon atmosphere, a colorless, transparent liquid was obtained.22 Furthermore, they also explored the physicochemical properties of the above solvent. The heat of CO2 absorption and their mechanism were also studied. CO2 absorption of MEA/ILs (4:1) has reached 21.4 wt % at room temperature and atmospheric pressure. The absorption of CO2 is higher than that of ILs and DESs of choline chloride. Nguyen and Zondervan23 proposed a new conceptual design for absorption processes, by applying [Bmim][Ac] for separation of CO2 from high CO2 content with the efficiency more than 90%. Pan et al.24 developed a series of chiral amino acid ionic liquids for efficient and reversible capture of CO2. The formation of the intramolecular hydrogen bonding between the carbamate and the protonated OH was found. The viscosity change during absorption provides an important foundation for building absorption systems based on aminofunctionalization. Encapsulated ionic liquids are proposed for overcoming the mass-transfer limitations observed in neat ILs present in the physical absorption of CO2.25 [Emim][TCM], [Bmim][TCM], [Bmim][DCN], and [Bmim][OcSO4] were selected as typical absorption solvents for CO2 solubilities and diffusivities. The results showed that the above-mentioned ILs have the same CO2 solubility capability (3 mg/g at 301 K). CO2 diffusivity also can be of great difference. Deep eutectic solvents (DESs) are generally obtained by mixing a quaternary ammonium salt with metal salt or hydrogen-bond donor (HBD). DESs, with their good 10404

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Figure 1. Structural formula of some typical DESs: (a) ChCl/MEA; (b) ChCl/MEA/MDEA; (c) TEAC/MEA; (d) TMAC/MEA; (e) TMAC/ MEA/NH4Cl; and (f) TMAC/MEA/LiCl.

MDEA and ChCl/MEA/ZnCl2, and tetramethylammonium chloride as hydrogen acceptors in three-component DESs, TMAC/MEA/MDEA and TMAC/MEA/LiCl. Furthermore, the above-mentioned DESs were applied in the CO 2 absorption process. Finally, the absorption mechanisms were also investigated systematically.

absorption of CO2. The solubility of solvent in CO2 increases with the proportion of alkanolamine in DESs. Synthesized DESs are based on ChCl and monobasic acids.39 Under ultraviolet light with air as an oxidant, dibenzothiophene (DBT) sulfur removal rate was 98.6%. When BT > DBT > 4,6Dimethyldibenzothiophene (4,6-DMDBT), the sulfur removal efficiency of different sulfur compounds is lowered. Through choosing ethylene glycol (EG) and diethylene glycol (DG) as HBD, with the ammonium salts obtained from triethylenetetramine (TETA) and HCl as hydrogen acceptor (HBA), a series of functional DESs with extremely low volatility were synthesized.40 This was applied for the absorption of CO2. CO2 absorption capacity of DESs with n[TETA·Cl]/n[EG] = 1:3 is high up to 17.5 wt %. The absorption of CO2 decreases with increasing temperature and decreasing CO2 pressure. It is found that EG or DG can increase absorption by activating −NH− or −NH2 on [TETA·Cl] and enhance the basicity of DESs. The stoichiometry for the reversible absorption is 1.5 molecules of CO2 per [TETA·Cl][EG] DESs molecule. The TETA/PEG200 (polyethylene glycol) solution exhibits a phase-changing feature after absorbing CO2.41 The results showed that PEG200 not only serves as solvent but is also involved in the reaction between TETA and CO2, which further enhances CO2 absorption. Furthermore, the TETA/ PEG200 solution exhibits a high CO2 capacity of 1.63 mol/ mol TETA, which is comparable to that of TETA−water solution. For regeneration processes, microwave heating was identified to be the best choice versus classic heating. After four absorption−desorption cycles, the regeneration efficiency reached 96%. The absorption of CO2 in DESs was investigated at different temperatures, and molar ratios of L-Arg to Gly varied from 1:4 to 1:8, respectively. The L-Arg/Gly DESs (1:6) showed a higher CO2 absorption efficiency (0.511 mol CO2/ mol DESs).42 In this work, the design and synthesis of DESs based on ethanolamine and the absorption of CO2 were investigated. A series of DESs were designed and synthesized as follows: choline chloride/ethanolamine (ChCl/MEA), choline chloride/triethanolamine (ChCl/TEA), tetramethylammonium chloride/ethanolamine (TMAC/MEA), tetrabutylammonium bromide/ethanolamine (TBAB/MEA), and choline chloride as hydrogen acceptors in three-component DESs, ChCl/MEA/

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EXPERIMENTAL SECTION

Choline chloride (ChCl), tetramethylammonium chloride (TMAC), tetraethylammonium chloride (TEAC), monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) were purchased with purities more than 98%; zinc chloride (ZnCl2), lithium chloride (LiCl), ammonium chloride (NH4Cl), sodium hydroxide (NaOH), and DMSO were purchased with analytical purity. CO2 gas (≥99.9996%) was supplied by Dongguan Special Gas, China. Quaternary ammonium salt as HBA and different types of alcohol amines as HBD were utilized to synthesize a series of new DESs. In this experiment, a series of solvents were synthesized by means of an oil bath thermostatic heating. The HBA and HBD are mixed at a certain molar ratio (TMAC/MEA 1:5, TEAC/MEA 1:5, TMAC/ MEA/MDEA 1:5:0.5, and TMAC/MEA/LiCl 1:5:0.1). The synthesis process was conducted in a round-bottomed flask at a temperature range of 60−130 °C, and the mixture was heated and stirred until both became homogeneous liquids. The system would be ∼3−5 h. The synthetic process of ChCl/MEA was taken as an example. The molar ratio (1:5) of ChCl and MEA was mixed in a 50 mL roundbottomed flask equipped with a magnetic stirrer. After the two compositions became liquid with heating, the system was stirred at 80 °C for 4 h to form DESs. The structural formulas of the six typical DESs are shown in Figure 1. 1H NMR and 13C NMR spectra were used to characterize the synthesized DESs (DRX-500 MHz NMR). DMSO was used as the solvent. 1H NMR and 13C NMR spectra of some typical DESs are listed in Figures S1−S6. For CO2 absorption, small magnetite was added to a gas absorption tube with 10 mL. Then, 2 g of DESs was added, and the gas absorption tube was placed in a water bath. The temperature of the heated magnetic stirrer (30 °C) and the rotational speed (800 rpm) were set. CO2 gas was bubbled into the DESs at atmospheric pressure, the CO2 gas flow rate was controlled at 50 mL/min, and the electronic analytical balance with an accuracy of ±0.1 mg was used at equal intervals (for example, 10 min) to determine the CO2 absorption. The absorption equilibrium of CO2 is reached when the mass of the DESs no longer increases or the simultaneous mass increase is TEAC > TEAB > TBAC > TBAB. Meanwhile, the order for the HBD is MEA > DEA > MDEA > TEA. The better HBD candidate is MEA, which shows much higher absorption efficiencies than the other ones. MEA is a moderate compound from all aspects among its analogous ones, so MEA was chosen as typical content. The amount of hydrogen atoms on the nitrogen atoms of MEA is one more than that of DEA, and no hydrogen is observed on the nitrogen atoms of MDEA and TEA, which do not directly interact with CO2 and are slowly absorbed. As the amount of hydroxyl groups in the alkanolamine increases, the intramolecular and intermolecular hydrogen-bonding forces in the DESs, the viscosity, and the absorption of CO2 increase. For HBAs, as the volume of the attached group on the nitrogen atom increases, the distance between the Cl− and Br− and the N+ center is farther. Moreover, forming a hydrogen bond with the alkanolamine is easy, and the hydrogen-bonding force of DESs and the solvent viscosity both increase. Chemical structures of ChCl and TMAC are similar, and chloride ions are similarly affected by adjacent groups. Therefore, the absorption of CO2 is similar between two DESs. For example, the best absorption for CO2 is ChCl/MEA (0.2523 g CO2/g DESs, 25.23 wt %). Thus, ChCl/MEA, TMAC/MEA, and TEAC/MEA are chosen to study the effect of reaction conditions on CO2 absorption and the use of DESs. Some factors, including the molar ratio of DESs, system temperature, absorption time, and rotational speed, were

Figure 4. Three DESs with different times on absorption.

However, the CO2 slowly increases from 20 to 40 min. After 40 min, the CO2 absorption remained stable. The reaction reached an equilibrium at 40 min. Given that the viscosity of DESs is low at the first 10 min, CO2 absorption sharply increased. The absorption efficiencies of the unsaturated DESs became large as time increased before 10 min. With time lengthened, the CO2 absorption of the semisaturated DESs will increase slowly until equilibrium is reached. 10406

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is >800 rpm, the rotation speed has a slight effect on the absorption, which indicates that the CO2 gas can be fully mixed with the DESs. Therefore, 800 rpm was chosen as the experiment speed to absorb CO2. After CO2 absorption by DESs, absorption and desorption experiments of DESs were conducted to study the absorption properties of CO2 by solvents. The result is shown in Figure 7.

System temperature influences CO2 absorption, and the result is shown in Figure 5. The viscosity of DESs is reduced

Figure 5. Three DESs with different temperatures on absorption.

due to the increase in absorption temperature. The CO2 absorption considerably changed when the temperature ranged from 20 to 30 °C. Meanwhile, the CO2 absorption remained relatively stable and reached 25 wt % when the temperature ranged from 30 to 60 °C. The above results indicated that this absorption process is an endothermic reaction. Higher system temperature is beneficial for the CO2 absorption process. However, in most industrial applications, the absorption process is always carried out at room temperature. Excessive lower or higher temperature will increase the cost and consumption of energy. Considering all the factors, 30 °C was selected as the optimal absorption temperature. The effect of rotation speed on CO2 absorption was also studied. The experiments were performed under the condition of 200, 400, 600, 800, and 1000 rpm, and the result is shown in Figure 6. When the rotational speed ranges from 200 to 800 rpm, the absorption of CO2 increases with the rotational speed. The absorption of CO2 is slightly reduced when the rotational speed reaches 1000 rpm. The viscosity of the solvent increases as the CO2 is continuously absorbed. When the rotation speed

Figure 7. Effect of repeated times on absorption.

The DES starts to be resolved at 75 °C to speed up the resolution of CO2, and the temperature of the CO2 desorption experiment is 100−140 °C. Absorption for the first time to resolve CO2 needs ∼5−6 h, and extended resolution from 2 to 5 times needs ∼2 h. With the increase in the repeated times, the absorption of CO2 by the DESs is reduced, and the analytical residue of CO2 gradually increases, which is listed in Figure S7. The released CO2 can be recovered by NaOH solution during the desorption process. After 7 times, the amount of absorbed CO2 still tends to decline. The CO2 desorption should be performed so that the absorption capability can be recovered. Water content is another important factor that influence the absorption process. ChCl/MEA, TMAC/MEA, and TEAC/ MEA were selected as typical two-component DESs to investigate the influence of water content. The absorption experiment of CO2 was conducted under the condition of water content of 5−25%. As shown in Figure 8, the absorption of ChCl/MEA CO2 in the DESs is basically unchanged with the increase in water content, which shows that the water has a slight effect on the ChCl/MEA. At room temperature, the viscosity of ChCl/MEA is 36 cP. After adding 20% water, the viscosity of ChCl/MEA decreased to 30 cP. The viscosity of ChCl/MEA with absorbed CO2 was 988 cP. When the water content ranged from 5% to 10%, TMAC/MEA and TEAC/ MEA increased the CO2 absorption; when the water content ranged from 10% to 25%, TMAC/MEA and TEAC/MEA gradually reduced CO2 absorption. The probable reason is that the free volume of the solvent is increased after adding water in DESs, and the viscosity of DESs is reduced. The reaction kinetics between the CO2 and DESs is increased, the reaction becomes sufficient, and the absorption is increased. The CO2 absorption was conducted under the condition of TMAC/MEA + 10% H2O system. As shown in Figure S8, the absorption of CO2 increases when the temperature rises from

Figure 6. Three DESs with different speeds on absorption. 10407

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Figure 8. Different water contents on absorption of three DESs.

25 to 30 °C; when the temperature rises from 30 to 60 °C, the amount of CO2 is gradually reduced. The absorption amount of CO2 is similar within the first 10 min. At the temperature of 30 °C, the CO2 absorption gradually increases as the absorption time increases. Therefore, 30 °C was selected as the best absorption temperature. Due to its flexible design, different types of basic HBD can be selected to design and synthesize a series of threecomponent alkaline DESs for CO2 absorption. Ethanolamine (MEA), N-methyldiethanolamine (MDEA), and triethanolamine (TEA) were selected as HBDs, while ChCl, TMAC, and TEAC were used as HBAs. MDEA and TEA are tertiary amines and cannot react directly with CO2. Two alcohol amines were selected as hydrogen donors to increase the basicity of DESs. The absorption rate is slow. The amino group is protonated to better combine with CO2 after the addition of water. As shown in Figure 9a, the quaternary ammonium salt as HBA and the alkanolamine as HBD have an effect on CO2 absorption. The system temperature is 30 °C, the rotation speed is 800 rpm, the flow rate is 50 mL/min, and the molar ratio of DESs is 1:5:0.5 because it has the best CO2 absorption capability. The TMAC/MEA/MDEA absorption of CO2 reached 30 wt %. The results show the change in CO2 uptake by the three-component DESs after the addition of 10% water. After adding water, the amino group is protonated to improve the capability of the mentioned DESs to combine with CO2. The results indicate that the addition of water slightly affects the CO2 absorption, which increases the absorption of CO2 by 2 wt %. Owing to the flexible design of the DESs, a series of threecomponent DESs were designed to explore CO2 absorption. ChCl, TMAC, and TEAC were selected as HBAs, and MEA was used as HBD. Ferric chloride (FeCl3), cobalt chloride (CoCl2), nickel chloride (NiCl2), cupric chloride (CuCl2), zinc chloride (ZnCl2), lithium chloride (LiCl), and ammonium chloride (NH4Cl) were selected to synthesize three components of DESs at a certain molar ratio. A variety of DESs were synthesized to perform absorption of CO2. The results are shown in Figure 9b. Three-component DESs containing FeCl3, CoCl2, NiCl2, and CuCl2 have the same amount of CO2 absorption. The three-component DESs containing zinc chloride (ZnCl2), lithium chloride (LiCl), and ammonium chloride (NH4Cl) have similar CO2 absorption and higher

Figure 9. (a) Different kinds of DESs on absorption before and after the addition of water. (b) Effect of DESs on absorption.

absorption than that of the former. For the three HBAs, the TMAC class has higher CO2 uptake than that of ChCl and TEAC. The metal chloride effect observed on the CO2 absorption is due to the following: the extranuclear electron arrangement of Fe3+ in FeCl3 is 1s22s22p63s23p63d54s0, of Co2+ in CoCl2 is 1s22s22p63s23p63d74s0, of Ni2+ in NiCl2 is 1s22s22p63s23p63d84s0, of Cu2+ in CuCl2 is 1s 2 2s 2 2p 6 3s 2 3p 6 3d 9 4s 0 , and of Zn 2 + in ZnCl 2 is 1s22s22p63s23p63d104s0. Moreover, the extranuclear electron arrangement of Li+ in LiCl is 1s2. No unpaired fully charged electrons are observed outside the nuclear. The former has a single electron outside the core, which facilitates easy bonding with carbonyl oxygen, improving the stability of the absorption product. Consequently, the electron cannot continue to react with CO2. So, CO2 absorption efficiencies are lower, while for Li+ and Zn2+, they accept electron pairs and complex with carbonyl oxygen in CO2. The complexation of lithium or zinc ions with the carbonyl oxygen of CO2 is the main reason that accounts for increasing the formation of the carbonyl group, thereby causing the amino group to react with the carbonyl group of CO2 to form carbamate. Three types of DESs, namely, TMAC/MEA/LiCl, TMAC/ MEA/ZnCl2, and TMAC/MEA/NH4Cl, were chosen to optimize the absorption process. As shown in Figure S9, a three-component DES with better CO 2 absorption is preferably subjected to conduct the absorption under different 10408

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Figure 10. 1H NMR characterization of ChCl/MEA (1:5) before and after CO2 absorption (DMSO): (a) ChCl/MEA before CO2 absorption, (b) ChCl/MEA after CO2 absorption, and (c) ChCl/MEA + 10% H2O after CO2 absorption.

Figure 11. 1H NMR characterization of TMAC/MEA/LiCl (1:5:0.1) before and after CO2 absorption (DMSO): (a) TMAC/MEA/LiCl before CO2 absorption, (b) TMAC/MEA/LiCl after CO2 absorption, and (c) TMAC/MEA/LiCl + 10% H2O after CO2 absorption.

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DOI: 10.1021/acssuschemeng.9b00555 ACS Sustainable Chem. Eng. 2019, 7, 10403−10414

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Figure 12. 13C NMR characterization of CO2 absorption (DMSO): (a) ChCl/MEA before CO2 absorption, (b) ChCl/MEA after CO2 absorption, and (c) ChCl/MEA + 10% H2O after CO2 absorption.

the chemical shift of the original δ = 2.735 ppm absorption peak increases to δ = 2.940 ppm (ChCl/MEA + 10% H2O). After CO2 absorption, the chemical shift is increased to δ = 2.941 ppm. The electron absorption capacity of H3N+−CH2− CH2−OH in the H3N+−CH2−CH2−OH is reduced due to the ammonium-bonded protons in the MEA, and the chemical shift is increased. The degree of amino-bonded protons is also increased. However, the absorption peak of the original δ = 3.286 ppm has a chemical shift decrease in the next two absorption cases. The chemical shift of the original δ = 3.286 ppm absorption peak decreases to δ = 3.105 ppm (ChCl/MEA + 10% H2O). After CO2 absorption, the chemical shift decreases to δ = 3.121 ppm. This result is also consistent with the aforementioned observation because the electron absorption capacity of the hydroxyl group is larger than that of the amino group, and the electron protonation ability is improved after the protonation of the amino group. The effect of the methylene group is equivalent to lowering the electronwithdrawing ability of the hydroxyl group. Therefore, the methylene-hydrogen electron cloud density connected to the hydroxyl group is increased, and the chemical shift is lowered. As shown in parts b and c of Figure 11, the direct absorption of TMAC/MEA/LiCl is consistent with the 1H NMR characterization of the absorption after the addition of 10% water, indicating that the absorption product is the same. The difference is that the amino group is protonated after the addition of water. When CO2 is bubbled into the absorbent to form weak acid, the protonated amino group is easily combined with CO2, and the mass of the absorbed CO2 is increased. As shown in Figure 12, according to the 13C NMR of the ChCl/MEA solvent system, δ = 44.661 ppm represents the absorption peak of C in the methylene group near the amino group in NH2−CH2−CH2−OH, δ = 63.787 ppm represents

temperature conditions. For the TMAC/MEA/MDEA + 10% H2O system, the CO2 absorption decreases with increasing temperature. This specific CO2 absorption is found to be an exothermic reaction. It is exothermically different from twocomponent DESs (no water added). Therefore, the reaction proceeds in the direction of the reverse reaction, and the amount of absorbed CO2 decreases. Thus, while temperature rises from 30 to 60 °C, the CO2 absorption gradually decreases. Hence, 30 °C is selected as the optimum absorption temperature of the system. Investigation of the CO2 uptake at different molar ratios is shown in Figure S10. The changes in CO2 absorption were measured for the three aforementioned types of DESs with the addition of 10% H2O. The molar ratio of DESs of 1:5:0.1 is chosen as the best absorption condition. The absorption of TMAC/MEA/LiCl + 10% H2O system at 30 °C is much higher and reaches 37.31 wt %. Compared with the previous research showing 26.9 and 33.7 wt % from the literature 33 and 37, respectively, our result is higher than the other DESs-based absorption process. As for effect of time on CO2 absorption, it is shown in Figure S11. Repeated absorptions were also conducted and are shown in Figure S12. DESs have high CO2 absorption efficiency. However, these solvents are repeatedly absorbed for 7 times, and the CO2 absorption capability of DESs is rapidly reduced. The mechanism is very important to the CO2 absorption process. So, NMR is used to investigate the absorption mechanisms systematically. As shown in Figure 10, the structure of the DESs (ChCl/MEA) changes after CO2 absorption. According to the nuclear magnetic resonance spectrum of the ChCl/MEA, δ = 2.735 ppm is determined to represent H on C near N in N−CH2−CH2−OH, while δ = 3.286 ppm represents H on methylene C near OH in H2N− CH2−CH2−OH, as shown in parts b and c of Figure 9, respectively. After absorption of CO2 in ChCl/MEA solvent, 10410

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Figure 13. 13C NMR characterization of TMAC/MEA/LiCl (1:5:0.1) (DMSO): (a) TMAC/MEA/LiCl before CO2 absorption, (b) TMAC/ MEA/LiCl after CO2 absorption, and (c) TMAC/MEA/LiCl + 10% H2O after CO2 absorption.

Figure 14. Presumed absorption mechanism of CO2 and ChCl/MEA.

near the amino group in NH2−CH2−CH2−OH, δ = 63.463 ppm represents the absorption peak of C in the methylene group near the hydroxyl group in NH2−CH2−CH2−OH, and δ = 54.244 ppm represents methyl carbon in tetramethylammonium chloride. After metal chloride addition and CO2 absorption, new peaks are observed in both solvents at 61, 42, and 161.863 ppm because the carbamate can affect the chemical environment of carbon. The product obtained by the absorption of CO2 by the two solvents is carbamate, and no hydrolysis occurs after the addition of water. The complexation of lithium ions with carbonyl oxygen in CO2 increases the activity of the carbonyl group, thereby causing the amino group to react with the carbonyl group of CO2 to form carbamate, which increases the amount of CO2 absorbed. Chloride ion can form a hydrogen bond with the amino group to improve the stability of the ethanolamine. After CO2 absorption, the protonated amino group is easily combined with CO2 to form stable carbamate. The viscosity of the solvent is lowered when the DESs are added to 10% water. By contrast, the amino group is protonated, the protonated amino group is easily combined with CO2 to form carbamate, and the absorption of CO2 is increased, as shown in Figures 14 and 15.

the absorption peak of C in the methylene group near the hydroxyl group in NH2−CH2−CH2−OH, δ = 67.431 ppm represents the absorption peak of methylene carbon adjacent to ammonium ion in ChCl, δ = 55.391 ppm represents the absorption peak of methylene carbon adjacent to the hydroxyl group in ChCl, and δ = 53.568 ppm represents the absorption peak of methyl carbon on the ammonium ion in ChCl. Owing to hydrogen bonding and electrostatic interactions, the chemical environment of the three methyl carbons has changed, causing several absorption peaks at that point to provide a basis for the interaction of hydrogen bonds. After absorption of CO2, two additional absorption peaks at 60.750 and 43.088 ppm are observed, indicting after CO2 absorption will change the original chemical environment. New peaks also appeared at 162.242 ppm, indicating the formation of carbamate after CO2 absorption. Moreover, after adding 10% H2O water, 13C NMR was the same as for direct absorption. That is, water did not hydrolyze the carbamate. As shown in Figure 13a, in the carbon spectrum before absorption in the TMAC/MEA/LiCl system, three carbon atoms with different chemical environments are observed. δ = 44.214 ppm represents the absorption peak of C in the methylene group 10411

DOI: 10.1021/acssuschemeng.9b00555 ACS Sustainable Chem. Eng. 2019, 7, 10403−10414

Research Article

ACS Sustainable Chemistry & Engineering

Figure 15. Presumed absorption mechanism of CO2 and TMAC/MEA/LiCl.

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We speculate on the absorption mechanism of CO2 with DESs (ChCl/MEA and TMAC/MEA/LiCl), which indicates changes of the final product during absorption.

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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/acssuschemeng.9b00555. 1 H NMR spectrum of TMAC/MEA (1:5), 1H NMR spectrum of TEAC/MEA (1:5), 1H NMR spectrum of ChCl/MEA (1:5), 1H NMR spectrum of TMAC/MEA/ LiCl (1:5:0.1), 13C NMR spectrum of ChCl/MEA (1:5), 13 C NMR spectrum of TMAC/MEA/LiCl (1:5:0.1), effect of desorption times on desorption residue, effect of temperature on CO2 absorption (TMAC/MEA + 10% H2O), effect of temperature on CO2 absorption (TMAC/MEA/MDEA + 10% H2O), effect of molar ratio on CO2 absorption, effect of time on CO2 absorption, and effect of repeated times on CO2 absorption (PDF)

CONCLUSIONS

The design, synthesis, and absorption of CO2 based on ethanolamine-based DESs is discussed, focusing on the effects of CO2 absorption and the influence of water on the CO2 absorption of DESs. The higher the content of ethanolamine in the DESs, the higher is the CO2 absorption. When the molar ratio of the DESs is 1:5, the absorption of ChCl/MEA reaches 25.23 wt %. The alkalinity of the DESs has a certain influence on the absorption of CO2. The absorption product of CO2 by ChCl/MEA can release CO2 gas under high-temperature conditions, and the DESs can be reused. The TMAC/MEA + 10% H2O system has a molar ratio of 1:5 at 30 °C and reaches a maximum absorption of 34.39 wt %. A series of threecomponent DESs were designed and synthesized to explore the influence of metal chloride on the absorption of CO2 in DESs. The absorption time is 60 min, and the molar ratio is 1:5:0.1 at 50 °C. TMAC/MEA/LiCl as the absorbent reaches the highest, and its absorption was 36.81 wt %. The absorption can reach 37.31 wt % at 30 °C in the TMAC/MEA/LiCl after adding 10% H2O. While increasing the temperature, the CO2 absorption decreases after adding 10% H2O. Although TMAC/MEA/LiCl (1:5:0.1) reaches high CO2 absorption, its recycling is more difficult. These DESs have high stability, high absorption CO2 capability, and short absorption saturation time. Moreover, although the absorption of ChCl/ MEA is lower than that of the three-component ones, the repeated CO2 absorption of ChCl/MEA decreases to 15 wt % after 7 times. NMR was applied to investigate the absorption mechanisms, and the hydrogen-bonding and electrostatic interactions were the main driving force for this specifically high CO2 absorption process.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: 086-769-22861921. *E-mail: [email protected]. ORCID

Changping Li: 0000-0003-1084-4023 Present Address

E-mail: [email protected] (Z.L.). E-mail: wanglili1994@ 126.com (L.W.). E-mail: [email protected] (Y.C.). E-mail: [email protected] (S.L.). E-mail: [email protected] (G.Y.). Author Contributions ∥

Z.L. and L.W. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Sincere thanks are given to National Natural Science Foundation of China (Grant no. 201546007), Dalian Outstanding Scholar Project (Grant no. 2016RJ11), Dalian 10412

DOI: 10.1021/acssuschemeng.9b00555 ACS Sustainable Chem. Eng. 2019, 7, 10403−10414

Research Article

ACS Sustainable Chemistry & Engineering

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Technology Star Project (Grant no. 2016RQ079), and Natural Science Foundation of Liaoning Province of China (Grant no. 20180550078) for financial support of this project.

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DOI: 10.1021/acssuschemeng.9b00555 ACS Sustainable Chem. Eng. 2019, 7, 10403−10414