Phenol-Based Ternary Deep Eutectic Solvents for Highly Efficient and

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Phenol-Based Ternary Deep Eutectic Solvents for Highly Efficient and Reversible Absorption of NH3 Fu-Yu Zhong, Hailong Peng, Duan-Jian Tao, Pingkeng Wu, Jie-Ping Fan, and Kuan Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05221 • Publication Date (Web): 09 Jan 2019 Downloaded from http://pubs.acs.org on January 10, 2019

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Phenol-Based Ternary Deep Eutectic Solvents for Highly Efficient and Reversible Absorption of NH3 Fu-Yu Zhong,† Hai-Long Peng,*† Duan-Jian Tao,ǂ Ping-Keng 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. ǂCollege

of Chemistry and Chemical Engineering, Jiangxi Normal University, 99

Ziyang Ave, Nanchang, Jiangxi 330022, China. ‡Department

of Chemical Engineering, Illinois Institute of Technology, 10 West 35th

Street, Chicago, Illinois 60616, United States. *E-mails: [email protected] (K. H.), [email protected] (H. L. P.). ABSTRACT In this work, a class of deep eutectic solvents (DESs) formulated by choline chloride (ChCl), phenol (PhOH) and ethylene glycol (EG) were designed and synthesized for NH3 capture. The effects of temperature, pressure and DES composition on NH3 capacities were investigated systematically. By utilizing the weak acidity of PhOH, highly efficient and reversible absorption of NH3 was realized in PhOH-based ternary DESs. The absorption capacities of NH3 in prepared DESs can reach as high as 9.619 mol/kg (0.162 g/g) at 298.2 K and 101.3 kPa, ranking one of the best reported to date. The captured NH3 could be easily stripped out at elevated temperature and reduced pressure, with negligible loss in NH3 capacities after 10 adsorption-desorption cycles.

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The thermodynamic properties of NH3 absorption process, such as reaction equilibrium constants, Henry’s constants, absorption enthalpies were also calculated with the assistance of thermodynamic modeling. It is found that the NH3 absorption process exhibits a moderate enthalpy change of -36.91 kJ/mol, indicating the potentially energyefficient feature of subsequent desorption process. The results obtained herein suggest that PhOH-based ternary DESs are promising media for the capture of NH3 from industrial gases. KEYWORDS NH3 absorption; deep eutectic solvents; acid-base interaction; thermodynamic modeling; energy efficiency


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INTRODUCTION NH3 is an alkaline gas with odd smell and strong corrosivity. It is mainly present in the exhaust gas of ammonia synthesis process and tail gas of urea synthesis process. Liquid NH3 can also be used as a kind of relatively low-toxic cryogen to substitute Freon, which is destructive to atmospheric ozone layer.1,2 However, the release of NH3 is a big threat to environment, because it may lead to the formation of fine particulate matter (PM) and eutrophication of ecosystem. On the other hand, NH3 is the starting material for a wide range of chemical productions, such as ammonium salts, sulfonamides, polyurethanes, polyamides (Nylon) and nitrile rubbers.3-5 Therefore, it is highly demanded to capture and recycle NH3 from industrial gases. Scrubbing with water or acids is the most widely used method for the capture of NH3 in the industry.6,7 However, this method suffers from high volatility of absorbents, and produces large amount of waste solutions which are difficult to dispose of. Furthermore, water exhibits high heat capacity, and acids exhibit strong reactivity to NH3, both of which contributes to the intensive energy consumption of desorption process. In past years, many efforts have been devoted to developing advanced materials to address the significant issues incurred in traditional NH3 capture process. Within this respect, the utilization of ionic liquids (ILs) as new absorbents for NH3 absorption represents a pioneering example.7-9 ILs are organic salts with melting points below or near ambient temperature. They have many unique properties including wide liquid range, extremely low volatility and structural designability.10 Unfortunately, the high cost and viscosity of ILs limit their practical application in NH3 capture process.



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Alternatively, solid adsorbents such as zeolites,11 carbons,12,13 metal-organic frameworks (MOFs)14,15 and covalent-organic frameworks (COFs)16 have also been investigated for the capture of NH3. Overall, it still remains a great challenge to achieve both efficient and reversible capture of NH3, together with green and energy-efficient process. Recently, deep eutectic solvents (DESs) have emerged as a class of green solvents with potential application in a variety of fields.17-19 DESs are simple mixtures of hydrogen-bond acceptors (HBAs, such as halide salts) and hydrogen-bond donors (HBDs, such as alcohols, amides and carboxylic acids). They share many features with ILs, including extremely low volatility and structure designability. Therefore, DESs are considered as IL analogues. However, DESs are normally much lower in cost than ILs, because they can be easily prepared from readily available reagents. Han and coworkers first determined the solubilities of CO2 in choline chloride (ChCl) plus urea mixtures, implying that DESs have good ability for dissolving gases.20 Since then, considerable attentions have been paid to the development of DES-based absorbents for gas separation, especially CO2 capture21,22 and SO2 capture.23-

25

There are also

several examples of investigating DESs for NH3 capture: Yang and co-workers found that hybrid DESs with flexible hydrogen-bonded supramolecular networks enables efficient and reversible absorption of NH3;26 Deng and co-workers showed that protic NH4SCN-based DESs possess exceptional NH3 absorption performance and high NH3/CO2 selectivity;27 Vorotyntsev and co-workers evaluated NH3 absorption in


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methanesulfonate-based DESs;28 our group determined the solubilities of NH3 in ChCl plus urea mixtures.29 Though with these progresses, the development of DES-based absorbents for NH3 capture is still very limited. It is of great significance to design DESs with improved performance for NH3 capture. To this end, we proposed to use phenol (PhOH)-based DESs for NH3 capture. In order to form DESs, ChCl was selected as the HBA, and ethylene glycol (EG) was selected as the secondary HBD besides the primary HBD PhOH. Since PhOH exhibit weak acidity, the interaction between PhOH and NH3 should be stronger than ordinary hydrogen-bonding interaction, which is expected to result in efficient absorption of NH3 in proposed PhOH-based ternary DESs. However, the acidity of PhOH is not that strong as carboxylic acids, which is expected to result in reversible absorption of NH3 in proposed PhOH-based ternary DESs. The introduce of EG can not only provide additional hydrogen-bonding sites for complexing with NH3, but also help increase the fluidity of DESs by decreasing the viscosity. The performance of PhOH-based ternary DESs for NH3 capture was thus systematically investigated in this work. EXPERIMENTAL Materials NH3 (99.99 v/v%) and CO2 (99.99 v/v%) were supplied by Jiangxi Huasheng Special Gas Co. Ltd., China. ChCl (99 wt.%), PhOH (99 wt.%) and EG (99 wt.%) were purchased from Adamas Chemicals Co. Ltd., China. All reagents were used as received without further purification. DESs were synthesized by stirring the mixtures of ChCl,



PhOH and EG at 333.2 K until homogeneous liquids were obtained. The prepared DESs were denoted as ChCl+PhOH+EG (a:b:c), where a:b:c is the molar ratio of reagents. Characterizations Densities were measured by an AntonPaar DMA 4500M automatic densimeter with an uncertainty of 0.00001 g/cm3. Viscosities were measured by a Brookfield RVDV2PCP230 viscometer with an uncertainty of 1% in relation to the full scale. Thermal decomposition temperatures were determined by thermogravimetric analysis (TGA) on a PerkinElmer TGA 4000 system under N2 atmosphere with a heating rate of 10 K/min. Glass transition temperatures were determined by differential scanning calorimeter (DSC) on a PerkinElmer DSC 8000 system under N2 atmosphere with a scanning rate of 5 K/min. 1H NMR spectra were collected on a Bruker Avance 600 spectrometer using d6-DMSO containing TMS as the external reference. FTIR spectra were collected on a Thermo Nicolet 5700 spectrometer. Gas absorption The apparatus for measuring gas solubilities has ever been introduced in our previous work.30 It is mainly composed of two chambers made by 316 L stainless steel. One is used as gas reservoir, while the other one is equipped with a magnetic stirrer and used as equilibrium cell. The volumes of chambers are V1 and V2, respectively. The temperature of whole apparatus is controlled at T by a water bath with an uncertainty of 0.1 K. The pressures in two chambers are recorded by Wideplus-8 transducers with an uncertainty of 0.1 kPa. In a typical run, a certain amount of liquid sample (m) was loaded into the equilibrium cell, and the air in whole apparatus was evacuated. The gas


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from cylinder was fed into the reservoir with a pressure of P1. The needle valve between chambers was then turned on to introduce a certain amount of gas into the equilibrium cell. The pressure in gas reservoir thus decreased to P1′. The absorption of gas in liquid sample resulted in the decrease of pressure in equilibrium cell. When the pressure in equilibrium cell remained constant for at least 1 h, the absorption process was considered to reach equilibrium, and the final pressure was recorded as P2. The amount of gas dissolved in liquid sample (n) was thus calculated by the following equation: n  1V1  1 V1   2 (V2 

m

0

)

(1)

where ρ1 is the density of gas at P1 and T, ρ1′ is the density of gas at P1′ and T, ρ2 is the density of gas at P2 and T, and ρ0 is the density of liquid sample at T in g/cm3. The densities of gas were acquired from NIST Chemistry WebBook.31 Gas solubility was calculated as the molality of gas in liquid sample. Continuous measurement of gas solubilities at elevated pressures was performed by introducing more gas into the equilibrium cell to reach new equilibrium. RESULTS AND DISCUSSION Physical properties



1.11 1.10 Density (g/cm3)

1.09 1.08 1.07 1.06 1.05 290

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Tempurature (K) Figure 1. Densities of PhOH-based ternary DESs [■: ChCl+PhOH+EG (1:2:4), ●: ChCl+PhOH+EG (1:3:4), ▲: ChCl+PhOH+EG (1:5:4), ▼: ChCl+PhOH+EG (1:7:4), lines: fitting results].

35 30

Viscosity (cP)

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Temperature (K) Figure 2. Viscosities of PhOH-based ternary DESs [■: ChCl+PhOH+EG (1:2:4), ●: ChCl+PhOH+EG(1:3:4), ▲: ChCl+PhOH+EG (1:5:4), ▼: ChCl+PhOH+EG (1:7:4), lines: fitting results].


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Parameters Equation (2) A B×104 C×107 R2 Equation (3) η0 D T0 R2

Table 1. Fitted parameters for equations (2) and (3) ChCl+PhOH+ ChCl+PhOH+ ChCl+PhOH+ ChCl+PhOH+ EG (1:2:4) EG (1:3:4) EG (1:5:4) EG (1:7:4) 1.3014 -6.7296 0.15 0.9999

1.2915 -6.1195 -1.12 1

1.2883 -6.0402 -1.64 1

1.2894 -6.0970 -1.88 1

0.1764 532.8664 191.2102 0.9999

0.3280 398.2434 206.4176 0.9988

0.3785 341.8674 214.3155 0.9999

0.2468 402.9950 207.4039 0.9998

Four PhOH-based ternary DESs were synthesized in this work: ChCl+PhOH+EG (1:2:4), ChCl+PhOH+EG (1:3:4), ChCl+PhOH+EG (1:5:4) and ChCl+PhOH+EG (1:7:4). ChCl+PhOH+EG mixtures with higher contents of PhOH can not form homogeneous liquids. We first determined the densities and viscosities of prepared DESs, because they are basic data for liquid absorbents. Figures 1 illustrates the densities of prepared DESs at different temperatures. As expected, the densities decrease almost linearly with the increase of temperatures. Moreover, the densities decrease with the increase of PhOH contents if compared at a given temperature. The density data can be fitted by a second-order polynomial equation:32

 =A  BT  CT 2

(2)

Fitted results are summarized in Table 1. Figure 2 illustrates the viscosities of prepared DESs at different temperatures. The viscosities decrease non-linearly with the increase of temperatures. Moreover, the viscosities decrease with the increase of PhOH contents if compared at a given temperature. The viscosity data can be fitted by a VogelTamman-Fulcher (VTF) equation:33

 =0 exp

D T  T0

(3)



Fitted results are also summarized in Table 1. Overall, the viscosities of PhOH-based ternary DESs (20~30 cP at 298.2 K) are considerably lower than those of most ILs and other DESs.18,34 This feature endows them with good fluidity, which is very beneficial for their practical application in NH3 capture process. We also determined the thermal decomposition temperatures and glass transition temperatures of prepared DESs (see Figures S1 and S2 in the Supporting Information). It is found that the thermal decomposition temperatures are above 373.2 K, while the glass transition temperatures are not available from 223.2 K to room temperature. NH3 capture performance NH3 capacities

12

Solubility of NH3 (mol/kg)

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Pressure (kPa) Figure 3. Solubilities of NH3 in PhOH-based ternary DESs at 313.2 K [■: ChCl+PhOH+EG (1:2:4), ●: ChCl+PhOH+EG (1:3:4), ▲: ChCl+PhOH+EG (1:5:4), ▼: ChCl+PhOH+EG (1:7:4)].


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7.8 7.6


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7.4

y=5.47578x+0.30842 R2=0.9987

7.2 7.0 6.8 6.6 6.4 6.2 6.0 2

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Molar ratio of PhOH to ChCl in DESs Figure 4. Variation of NH3 solubilities at 313.2 K and 101.3 kPa with molar ratios of PhOH to ChCl in prepared DESs (lines: fitting results). Figure 3 shows the NH3 absorption isotherms of prepared DESs at 313.2 K. It is found that the solubilities of NH3 increase with the increase of pressure, which is a common phenomenon for gas absorption process. However, the variation of NH3 solubilities with pressures is obviously non-linear, suggesting that the absorption of NH3 is a non-ideal type. Such non-ideal behavior should be caused by the acid-base interaction between PhOH and NH3. Moreover, the solubilities of NH3 increase with the increase of PhOH contents, because DESs with higher PhOH contents exhibit more acidic sites. Figure 4 shows the variation of NH3 solubilities at 313.2 K and 101.3 kPa with molar ratios of PhOH to ChCl in prepared DESs. It is observed that there is a linear relationship between NH3 solubilities and PhOH contents.



14


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Pressure (kPa) Figure 5. Comparison of NH3 solubilities in different absorbents at 313.2 K [■: ChCl+PhOH+EG (1:7:4), ●: ChCl+EG (1:4), ▲: EG]. In order to demonstrate the important role of PhOH in NH3 capture by prepared DESs, we compared the NH3 absorption isotherms of ChCl+PhOH+EG (1:7:4), ChCl+EG (1:4) and pure EG, as shown in Figure 5. It is observed that the solubilities of NH3 in ChCl+PhOH+EG (1:7:4) are higher than those in ChCl+EG (1:4) and pure EG, especially at low pressures. Therefore, PhOH is the key component in prepared DESs for NH3 capture. In addition, the solubilities of NH3 in ChCl+EG (1:4) and pure EG increase almost linearly with the increase of pressures, suggesting the weak interaction between solvent and solute. Such weak interaction should mainly come from the hydrogen-bonding interaction between EG and NH3, because the solubilities of NH3 in pure EG are even higher than those in ChCl+EG (1:4). However, it should be pointed out that pure EG is somewhat volatile, and the presence of non-volatile ChCl is helpful to reduce the volatility of whole DES-based absorbents.


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Solubility of NH3 (mol/kg）

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Pressure (kPa) Figure 6. Solubilities of NH3 in ChCl+PhOH+EG (1:5:4) at different temperatures (■: 298.2 K, ●: 313.2 K, ▲: 333.2 K, ▼: 353.2 K, lines: fitting results). Figure 6 shows the NH3 absorption isotherms of ChCl+PhOH+EG (1:5:4) at different temperatures. It is within expectation that solubilities of NH3 decrease with the increase of temperature, because gas absorption process is normally exothermic. Recycling of DESs


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NH3 absorption capacity (mol/kg)


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Recycling time Figure 7. Variation of NH3 solubilities in ChCl+PhOH+EG (1:5:4) with recycling times (absorption condition: 313.2 K and 101.3 kPa, desorption condition: 333.2 K and 0.1 kPa for 2 h). Based on the dependence of NH3 solubilities on temperature and pressure, it is inferred that the captured NH3 can be stripped out by increasing temperature and reducing pressure. In order to evaluate the recyclability of prepared DESs for NH3 absorption, the NH3-saturated ChCl+PhOH+EG (1:5:4) was treated at elevated temperature and reduced pressure to strip out the captured NH3. The recycled ChCl+PhOH+EG (1:5:4) was then used for NH3 absorption again. The absorptiondesorption cycle was performed for 10 times, and results are shown in Figure 7. It is found that there is no obvious loss in NH3 capacities, suggesting the good reversibility of NH3 absorption in PhOH-based ternary DESs. Although the prepared DESs enable acid-based interaction with NH3, such interaction should be not very strong because the acidity of PhOH is weak (pKa=9.94).


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In order to examine the stability of ChCl+PhOH+EG (1:5:4) during the 10 absorption-desorption cycles in Figure 7, ChCl+PhOH+EG (1:5:4) was purged with N2 at 333.2 K for 20 h. It is found that the sample does not show obvious loss in weight (see Figure S3), proving the good stability of ChCl+PhOH+EG (1:5:4). Comparison with other absorbents/adsorbents

Table 2. Comparison of NH3 capacities for different absorbents/adsorbents Entries Solvents T (K) P (kPa) mNH3 (mol/kg) Refs. 1 ChCl+PhOH+EG(1:5:4) 298.2 101.3 9.619 This work 2 ChCl+PhOH+EG(1:5:4) 313.2 101.3 6.988 This work 3 ChCl+PhOH+EG(1:7:4) 313.2 101.3 7.652 This work 4 ChCl+urea (1:2) 298.2 95.0 2.213 [29] 5 ChCl+Res+Gly (1:3:5) 298.2 101.3 9.982 [26] 6 ChCl+Res+Gly (1:3:5) 313.0 101.3 7.647 [26] 7 ChCl+D-fructose+Gly (1:3:5) 313.0 101.3 6.471 [26] 8 NH4SCN+Gly (2:3) 313.2 101.3 10.353 [27] 9 [bmim][MeSO3]+urea (1:1) 313.2 172.6 1.049 [28] 10 [Bmim][BF4] 298.2 101.3 0.998 [40] 11 [Bmim][PF6] 298.2 101.3 1.233 [40] 12 [Bmim][Tf2N] 299.4 101.3 0.311 [40] 13 [Emim][Ac] 298.3 101.3 1.879 [38] 14 [Emim][SCN] 298.1 101.3 2.642 [38] 15 [Hmim][Cl] 297.8 101.3 1.409 [40] 16 [Bmmim][ Tf2N] 313.0 100.5 0.460 [7] 17 [Bmmim][DCA] 313.0 103.4 0.622 [7] 18 [EtOHmim][BF4] 313.2 101.3 2.642 [41] 19 [TMGH][BF4] 293.2 101.3 5.285 [39] 20 [DMEA][Ac] 298.1 101.3 5.872 [38] 21 [Bim][Tf2N] 313.2 101.3 6.459 [8] 22 [Bim][SCN] 313.0 96.6 10.658 [7] 23 [Bim][NO3] 303.0 100.1 7.937 [7] 24 C-NiCl2-EM 298.2 0.1 4.800 [37] 25 C-CuCl2-EPM 298.2 0.1 4.200 [37] 26 C-ZnCl2-EPM 298.2 0.1 3.900 [37] 27 [HOOC]17 COFs 298.0 100.0 9.340 [36] 28 [HOOC]33 COFs 298.0 100.0 8.210 [36] 29 [HOOC]0 COFs 283.0 100.0 9.230 [36] 30 [HOOC]0 COFs 298.0 100.0 6.850 [36] 31 BPP-7 298.0

Phenol-Based Ternary Deep Eutectic Solvents for Highly Efficient and

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