NH3 Solubilities and Physical Properties of Ethylamine Hydrochloride

Article pubs.acs.org/jced

Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

NH3 Solubilities and Physical Properties of Ethylamine Hydrochloride Plus Urea Deep Eutectic Solvents Jia-Yin Zhang,† Li-Yun Kong,*,‡ 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, Nanchang, Jiangxi 330031, China ‡ School of Public Health, Xi’an Jiaotong University, Xi’an, Shaanxi 710061, China

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S Supporting Information *

ABSTRACT: In the present work, the solubilities of NH3 in deep eutectic solvents formulated by ethylamine chloride (EaCl) and urea were determined at (313.2−353.2) K and (0−300.0) kPa. It is found that the isotherms for NH3 absorption display slightly nonideal profiles, and EaCl contributes slightly more than urea to the absorption of NH3 in EaCl + urea mixtures. Overall, EaCl + urea mixtures possess quite promising absorption capacities for NH3, with the values of (4.179−4.573) mol·kg−1 at 313.2 K and ∼100.0 kPa. The Krichevsky−Kasarnovsky (K−K) equation was used for the correlation of NH3 solubilities, and Henry’s constants and infinite-dilution partial molar volumes of NH3 in EaCl + urea mixtures were obtained. The thermodynamic functions of the NH3 absorption process were also calculated based on the relationship between Henry’s constants and temperatures. Considering that the physical properties are basic data for liquid solvents, the densities, viscosities, glass transition temperatures, melting temperatures, and freezing temperatures of EaCl + urea mixtures at different temperatures were also determined.

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INTRODUCTION Ammonia (NH3) is a gaseous pollutant with a foul smell. It has been widely concerned for years because it may induce many serious diseases if breathed in by human bodies and is the main cause of haze formation.1,2 One of the most abundant sources of NH3 emission is the tail gas of industrial processes, such as the synthesis of ammonia and urea.3,4 The large-scale use of nitrogen fertilizers in agricultural activities also releases a considerable amount of NH3 into the atmosphere.5,6 Though it has disadvantages, NH3 is also an essential feedstock for the synthesis of many fine chemicals, such as fertilizers, medicines, rubbers, fibers, explosives, and so forth.7,8 Therefore, the elimination and recovery of NH3 from industrial gas has become an important area of chemical engineering research. Currently, the most widely used method for the elimination of NH3 from industrial gas is scrubbing with water or acids.9−12 However, this method is accompanied by some shortcomings. Water is highly volatile and with relatively low efficiency for NH3 absorption. For example, the solubility of NH3 in water is 0.4 g·g−1 at 303.2 K and 95.8 kPa.9 Acids exhibit too strong an interaction with NH3, resulting in the difficult desorption of NH3 and significant loss of NH3 resources. For example, the equilibrium constant for the reaction of NH3 with phosphoric acid in aqueous solution is 1.16 × 107 at 298.2 K.12 To address these shortcomings, it is of great importance to develop low-volatile solvents with not only high efficiency but also good reversibility for NH3 absorption. © XXXX American Chemical Society

Ionic liquids (ILs) have been proposed as alternative solvents for NH3 absorption13−15 owing to their negligible volatility and adjustable structure.16,17 The use of ILs for NH3 absorption has some obvious advantages: first, the evaporative loss of solvents can be significantly restrained; second, there are numerous choices for the structure of ILs to optimize the NH3 absorption performance. Unfortunately, ILs are often criticized for their high cost and viscosity,18−20 which may ultimately hinder their practical application in the NH3 absorption process. Recently, the growing research interests on deep eutectic solvents (DESs) offer another solution to overcome the shortcomings associated with traditional water or acid scrubbing methods for NH3 elimination. DESs are highlighted as a class of green solvents, with similar properties to ILs such as negligible volatility and adjustable structure.21,22 They are normally constructed by simply mixing a hydrogen-bonding acceptor (HBA) and a hydrogen-bonding donor (HBD). The hydrogen-bonding network formed in DESs changes the charge distribution of molecules, thus leading to the reduced melting point for the mixture relative to the single component.23,24 Within this respect, ammonium halide salts are often used as the HBA, while amides, alcohols, or Received: March 19, 2019 Accepted: August 6, 2019

A

DOI: 10.1021/acs.jced.9b00246 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

and melting temperatures of EaCl + urea mixtures were also determined.

carboxylic acids are often used as the HBD. These chemicals are widely available commercially and are inexpensive. Therefore, DESs are usually more suitable for practical application than ILs because of their facile preparation and low cost. Moreover, through the rational choice of HBA and HBD, DESs with a specific function can be fabricated. There has been some progress in the use of DESs for NH3 absorption. Yang et al.25 and our group26 fabricated phenolbased DESs, while Deng et al.27 fabricated protic NH4SCNbased DESs to achieve efficient and reversible absorption of NH3. It is elucidated that the absorption of NH3 in phenolbased DESs is governed by the weak acid−base interaction between phenol and NH3, while the absorption of NH3 in protic NH4SCN-based DESs is governed by the strong hydrogen-bonding interaction between NH4SCN and NH3. Deng et al.28 and our group29 reported the solubilities and related thermodynamic properties of NH3 in some classic choline chloride (ChCl)-based DESs. Meanwhile, Vorotyntsev et al.30 evaluated methanesulfonate-based DESs for NH3 absorption. Even though the investigation of DESs for NH3 absorption is still quite limited, there are many DESs remaining to be explored for potential application in NH3 absorption. In the present work, we proposed the use of ethylamine chloride (EaCl) plus urea mixtures for NH3 absorption. The chemical structures of EaCl and urea are displayed in Scheme 1. Although, it has been reported that EaCl can form DESs

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EXPERIMENTAL SECTION Chemicals. Ammonia (NH3, 99.99 mol %) and carbon dioxide (CO2, 99.99 mol %) were purchased from Huasheng Special Gas Co. Ltd., China. EaCl (98 wt %), urea (99 wt %), and PEG200 (99 wt %) were supplied by Adamas Chemicals Co. Ltd., China. All the chemicals were used as received. The basic information for chemicals used in this work is summarized in Table 1. Preparation of DESs. DESs were prepared by stirring the mixtures of EaCl and urea at 353.2 K vigorously until clear liquids are formed. Because EaCl is very hygroscopic, the prepared DESs were dried at 323.2 K and 0.1 kPa for 24 h before use to remove any water absorbed from the atmosphere. The residual water contents in DESs were measured by a Titan TKF-1B analyzer based on Karl-Fischer titration with a relative uncertainty of ±3%. Table 2 summarizes the abbreviations, liquid compositions, and residual water contents of DESs prepared in this work. Table 2. Chemical Compositions and Water Contents of DESs Prepared in This Worka DESs

nurea/nEaCl

wH2O/%

EaCl + urea (1:0.5) EaCl + urea (1:1) EaCl + urea (1:2)

0.50 ± 0.02 1.00 ± 0.03 1.94 ± 0.06

0.061 ± 0.002 0.048 ± 0.001 0.054 ± 0.002

Scheme 1. Chemical Structures of DESs Investigated in This Work

a

with urea,31 EaCl + urea mixtures have not yet been investigated for NH3 absorption to the best of our knowledge. EaCl is a protic ammonium salt, and urea has dual amide groups. Both components exhibit hydrogen-bond-donating ability. Considering that NH3 has a lone electron pair, thus exhibiting strong hydrogen-bond-accepting ability, we expected that EaCl + urea mixtures may show good performance for NH3 absorption. To verify this assumption, the solubilities of NH3 in EaCl + urea mixtures at different temperatures and pressures were determined. Henry’s constants and thermodynamic functions for NH3 absorption were calculated. The physical properties are basic data for liquid solvents. However, the detailed physical properties of EaCl + urea mixtures are also not available in the literature. Therefore, the densities, viscosities, freezing temperatures, glass transition temperatures,

Measurements of NH3 Solubilities. The measurements of NH3 solubilities were conducted in a dual-vessel apparatus, which has been reported in our previous work.32 Scheme 2 shows the configuration of the whole apparatus. Its reliability for gas solubility measurements was confirmed by measuring the solubilities of CO2 in PEG200 and comparing the results with literature (see Table S1 and Figure S1 in the Supporting Information). The apparatus mainly has two stainless steel vessels, which are used for gas storage and absorption and warmed by an oil bath with a standard uncertainty of ±0.1 K. The absorption vessel is equipped with a magnetic stirrer to facilitate the diffusion of gas in liquids. Two Wideplus-8 transducers with a working range of (0−600) kPa and a standard uncertainty of ±1.2 kPa were used to record the pressures in vessels online. The transducers were calibrated by a Mensor CPC6000 precision pressure controller in the full working range of (0−600) kPa, and the maximum relative deviation is 0.1369%.

nurea/nEaCl is the molar ratio of urea to EaCl in DESs; wH2O is the weight percentage of water in DESs; standard uncertainties u(nurea/ nEaCl) and u(wH2O) are reported following the ±sign.

Table 1. Information for Chemicals Used in This Worka chemicals

CAS numbers

M.W./(g·mol−1)

suppliers

purities

analysis methods

purification methods

ammonia carbon dioxide ethylamine hydrochloride urea PEG200

7664-41-7 124-38-9 557-66-4 57-13-6 25322-68-3

17.04 44.01 81.54 60.06 180−220

Huasheng Huasheng Adamas Adamas Adamas

99.99 mol % 99.99 mol % 98 wt % 99 wt % 99 wt %

GC GC HPLC HPLC HPLC

none none none none none

a

M.W. is the molecular weight; GC is the gas chromatograph; HPLC is the high-performance liquid chromatograph. B



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NH3 in DESs at equilibrium pressure of P2 could be calculated using eq 1

Scheme 2. Apparatus for NH3 Solubility Measurements (1: NH3 Cylinder; 2: N2 Cylinder; 3−13: Needle Valves; 14: Digital Displayer; 15: Oil Bath; 16: Gas Storage Vessel; 17: Gas Absorption Vessel; 18: Glass Tank; 19: Dilute H2SO4; 20 and 21: Pressure Transducers)

mNH3 = [ρΝΗ P0 , T V1 − ρΝΗ P1, T V1 − ρΝΗ P2 , T 3

3

3

(V2 − wDESs/ρDESs )] /wDESs

(1)

where mNH3 is the solubility of NH3 in DESs (mol·kg−1); Pm (m = 0−2) is the pressure in vessels (kPa); T is the temperature of the whole apparatus (K); ρNH3Pm,T is the density of NH3 at Pm and T (mol·cm−3); Vn (n = 1−2) is the volume of vessels including connecting parts (cm3); wDESs is the mass of DESs (g); and ρDESs is the density of DESs at T (g·cm−3). ρNH3Pm,T was acquired from the NIST Chemistry WebBook.33 Vn was measured by using He as the probing gas. The measurements of NH3 solubilities at higher equilibrium pressures were conducted by intruding more NH3 to the absorption vessel. After the completion of measurements, the residual NH3 in the whole apparatus was purged to a glass tank filled with dilute H2SO4 by flowing N2. The standard uncertainties of NH3 solubilities were estimated from the standard uncertainties of pressures by error propagation because the contribution of the uncertainties of temperatures, volumes, mass, and densities to the uncertainties of NH3 solubilities are negligible.34 Measurements of Physical Properties. The densities were measured at (313.2−353.2) K and 101.3 kPa by an Anton Paar DMA 4500M densimeter with a standard uncertainty of ±0.00005 g·cm−3. The densimeter was calibrated by water, and the density of water at 298.2 K and 101.3 kPa was measured to be 0.99688 g·cm−3, being comparable to the value of 0.99705 g·cm−3 taken from NIST Chemistry WebBook.33 Considering the purities of EaCl and urea used for preparation of DESs (0.98 and 0.99 in mass fraction, see Table 1), the standard uncertainties of density data were estimated to be ±0.002 g· cm−3 according to the literature.35 The viscosities were measured at (313.2−353.2) K and 101.3 kPa by a Brookfield RVDV2PCP230 viscometer with a relative uncertainty of ±1%.

In a typical run, a specific amount of DESs was loaded in the absorption vessel. The mass of DESs was measured by an analytical balance with a standard uncertainty of ±0.0001 g. The whole apparatus was then completely evacuated, and the residual pressure was measured to be (0−0.2) kPa, approaching the detecting limit of transducers. The residual pressure remained unchanged for 12 h, proving the excellent tightness of the whole apparatus. NH3 was loaded in the storage vessel from the cylinder to a pressure of P0. The needle valve connecting two vessels was turned on to introduce partial NH3 to the absorption vessel. The absorption of NH3 in DESs caused the decrease of pressure in the absorption vessel. If the pressure in the absorption vessel remained unchanged for 1 h, it is considered that equilibrium was reached for NH3 absorption. The pressures were recorded as P1 for the storage vessel and P2 for the absorption vessel. Thus, the solubility of Table 3. Solubilities of NH3 in EaCl + Urea (1:0.5)a T/K

PNH3/kPa

313.2 313.2 313.2 313.2 313.2 313.2 313.2 323.2 323.2 323.2 323.2 323.2 323.2 323.2 333.2 333.2 333.2 333.2

8.5 44.5 98.5 146.8 201.0 248.9 301.7 13.0 57.0 107.5 152.2 199.9 248.9 299.2 20.8 49.9 100.3 155.0

mNH3/(mol·kg−1)

T/K

PNH3/kPa

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

333.2 333.2 333.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2

201.5 249.2 299.3 16.3 50.5 100.6 148.0 204.7 250.6 301.5 22.3 51.3 101.2 151.0 204.0 249.3 301.2

0.518 2.520 4.396 5.939 7.674 9.401 11.022 0.660 2.247 3.570 4.613 5.683 6.815 8.049 0.448 1.205 2.257 3.238

0.023 0.023 0.023 0.023 0.024 0.024 0.024 0.022 0.022 0.023 0.023 0.023 0.023 0.023 0.020 0.020 0.020 0.020

mNH3/(mol·kg−1) 3.987 4.727 5.542 0.345 0.914 1.585 2.441 3.176 3.714 4.348 0.394 0.822 1.470 2.068 2.610 3.075 3.554

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.020 0.021 0.021 0.019 0.019 0.019 0.019 0.019 0.019 0.019 0.019 0.019 0.019 0.019 0.019 0.020 0.020

a

T is the temperature; PNH3 is the pressure of NH3; mNH3 is the solubility of NH3 in DESs; standard uncertainties u are u(T) = 0.1 K and u(PNH3) = 1.2 kPa, u(mNH3) are reported following the ±sign. C



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Table 4. Solubilities of NH3 in EaCl + Urea (1:1)a T/K

PNH3/kPa

313.2 313.2 313.2 313.2 313.2 313.2 313.2 323.2 323.2 323.2 323.2 323.2 323.2 323.2 333.2 333.2 333.2 333.2

9.5 56.8 99.1 151 200.5 248.1 300.6 9.2 47.1 99.1 151.5 202.0 250.9 298.9 23.1 53.0 101.7 151.8

mNH3/(mol·kg−1)

T/K

PNH3/kPa

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

333.2 333.2 333.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2

202.9 249.8 298.9 17.7 53.2 103.6 150.3 201.0 249.5 299.5 15.7 52.1 100.8 150.4 200.6 250.2 301.5

0.647 2.958 4.573 6.322 7.798 9.116 10.511 0.400 1.737 3.186 4.488 5.599 6.613 7.559 0.271 1.210 2.326 3.283

0.023 0.023 0.023 0.024 0.024 0.024 0.024 0.022 0.022 0.023 0.023 0.023 0.023 0.023 0.023 0.021 0.021 0.021

mNH3/(mol·kg−1) 4.268 5.210 5.948 0.402 1.090 1.954 2.788 3.529 4.222 4.955 0.252 0.752 1.362 1.964 2.515 3.034 3.555

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.022 0.022 0.022 0.021 0.021 0.021 0.021 0.021 0.021 0.022 0.020 0.020 0.020 0.020 0.020 0.020 0.020

a

T is the temperature; PNH3 is the pressure of NH3; mNH3 is the solubility of NH3 in DESs; standard uncertainties u are u(T) = 0.1 K and u(PNH3) = 1.2 kPa, u(mNH3) are reported following the ±sign.

Table 5. Solubilities of NH3 in EaCl + Urea (1:2)a T/K

PNH3/kPa

313.2 313.2 313.2 313.2 313.2 313.2 313.2 323.2 323.2 323.2 323.2 323.2 323.2 323.2 333.2 333.2 333.2 333.2

8.4 48.6 96.3 150.0 199.4 247.7 296.2 8.5 47.6 97.8 146.5 192.6 246.3 298.8 10.0 46.6 109.8 152.7

mNH3/(mol·kg−1)

T/K

PNH3/kPa

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

333.2 333.2 333.2 343.2 343.2 343.2 343.2 343.2 343.2 343.2 353.2 353.2 353.2 353.2 353.2 353.2 353.2

203.5 252.4 303.4 11.7 54.4 104.9 153.4 202.6 249.2 299.6 20.0 51.4 97.6 149.3 199.9 249.5 300.7

0.527 2.395 4.179 5.939 7.371 8.733 10.061 0.305 1.465 2.719 3.797 4.717 5.736 6.724 0.242 1.050 2.255 3.000

0.023 0.023 0.023 0.023 0.023 0.024 0.024 0.022 0.022 0.022 0.022 0.022 0.022 0.023 0.021 0.021 0.021 0.022

mNH3/(mol·kg−1) 3.823 4.564 5.311 0.209 0.879 1.603 2.349 2.881 3.525 4.126 0.274 0.635 1.147 1.707 2.226 2.712 3.204

± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.022 0.022 0.022 0.021 0.021 0.021 0.021 0.021 0.021 0.022 0.018 0.018 0.019 0.019 0.019 0.019 0.019

a

T is the temperature; PNH3 is the pressure of NH3; mNH3 is the solubility of NH3 in DESs; standard uncertainties u are u(T) = 0.1 K and u(PNH3) = 1.2 kPa, u(mNH3) are reported following the ±sign.

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RESULTS AND DISCUSSION NH3 Solubilities. The solubilities of NH3 in EaCl + urea mixtures were measured at (313.2−353.2) K and (0−300.0) kPa in this work. The detailed solubility data are summarized in Tables 3−5 and graphically shown in Figures 1−3. It is obvious that the solubilities of NH3 increase with the increase in pressures but decrease with the increase in temperatures. This is a common trend for gas absorption in liquid solvents.36 However, the NH3 absorption isotherms display profiles that are slightly deviated from a linear trend. It is well known that the ideal absorption of gas in liquid solvents follows Henry’s law, and the gas solubilities are directly proportional to pressures.37 The nonlinear trend for NH3 absorption isotherms suggests that the absorption of NH3 in EaCl + urea mixtures is

The viscometer was calibrated also by water, and the viscosity of water at 298.2 K and 101.3 kPa was measured to be 0.87 mPa·s, being comparable to the value of 0.89 mPa·s taken from NIST Chemistry WebBook.33 The glass transition temperatures, melting temperatures, and freezing temperatures were determined on a PerkinElmer DSC 8000 system by heating from (223.2 to 373.2) K and then cooling from (373.2 to 273.2) K at a rate of 5 K/min under flowing atmospheric N2. The determination was performed three times to give the average values of freezing temperatures, glass transition temperatures, and melting temperatures, and the standard uncertainties were estimated to be ±1 K. D



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exhibits both hydrogen-bond-accepting ability and hydrogenbond-donating ability, and it is actually the chloride anion in EaCl that acts the HBA to form DESs with urea. Figure 4 shows the solubilities of NH3 in EaCl + urea mixtures with different molar ratios of urea to EaCl at 323.2 K.

Figure 1. Solubilities of NH3 in EaCl + urea (1:0.5).

Figure 4. Solubilities of NH3 in EaCl + urea mixtures with different molar ratios of urea to EaCl at 323.2 K.

It is found that solubilities of NH3 decrease slightly with the increase of the urea/EaCl molar ratio, suggesting that EaCl contributes more than urea to the absorption of NH3 in EaCl + urea mixtures. On the other hand, the interaction of NH3 with EaCl is stronger than that of NH3 with urea. The interaction between NH3 and EaCl + urea mixtures was then examined by 1 H NMR spectra (see Figure S2 in the Supporting Information). Before NH3 absorption, the peaks at 5.54 and 7.93 ppm are attributed to the protons of amide groups in urea (−NH2) and ammonium cation in EaCl (−NH3+), respectively. After NH3 absorption, the peak of −NH3+ shifts significantly upfield and overlaps with the peak of −NH2 at (5.29−6.19) ppm. The significant shift for the peak of −NH3+ agrees well with the stronger interaction of NH3 with EaCl. The EaCl + urea mixtures were then compared with other DESs25−30 and ILs38−44 reported in the literature to evaluate their performance for NH3 absorption. Table 6 summarizes the solubilities of NH3 in different solvents. It can be seen that the solubilities of NH3 in EaCl + urea mixtures are quite promising, with the values of (4.179−4.573) mol·kg−1 at 313.2 K and ∼100.0 kPa (entries 1−3). Although these values are lower than the solubilities of NH3 in phenol-based DESs (entries 7−8), protic NH4SCN-based DESs (entry 9) and metal-based ILs (entries 18−19), they are much higher than the solubilities of NH3 in normal DESs (entries 4−6), normal ILs (entries 10−13) and hydroxyl-functionalized ILs (entry 14). The solubilities of NH3 in EaCl + urea mixtures are also comparable to those in protic ILs (entries 15−17). This comparison highlights the importance of multiple hydrogenbond-donating sites in DESs for efficient absorption of NH3. Henry’s Constants and Infinite-Dilute Partial Molar Volumes. Because of the nonlinear trend for NH3 absorption isotherms, the Krichevsky−Kasarnovsky eq 2 was used for the correlation of NH3 solubilities in EaCl + urea mixtures45

Figure 2. Solubilities of NH3 in EaCl + urea (1:1).

Figure 3. Solubilities of NH3 in EaCl + urea (1:2).

not an absolutely ideal type, which should arise from the strong interaction between NH3 and EaCl + urea mixtures. This is within our expectation considering the protic ammonium cation in EaCl and dual amide groups in urea. Both protic ammonium cation and amide groups exhibit hydrogen-bonddonating ability. As a result, EaCl and urea can interact strongly with NH3, which has a lone electron pair and exhibits hydrogen-bond-accepting ability. It should be noted that EaCl

ln E

fNH

3

mNH3

≈ ln

PNH3 mNH3

= ln Hm +

∞ V NH P 3 NH3

RT

(2)



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Table 6. Comparison of NH3 Solubilities in Different DESs and ILsa entries

solvents

T/K

PNH3/kPa

mNH3/(mol·kg−1)

refs

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

EaCl + urea (1:0.5) EaCl + urea (1:1) EaCl + urea (1:2) [bmim][MeSO3] + urea (1:1) ChCl + urea (1:2) ChCl + EG (1:2) ChCl + PhOH + EG (1:7:4) ChCl + Res + Gly (1:3:5) NH4SCN + G (2:3) [Bmim][BF4] [Bmim][PF6] [Bmim][Tf2N] [Hmim][Cl] [EtOHmim][BF4] [TMGH][BF4] [DMEA][Ac] [Bim][NTf2] [Bmim]2[CuCl4] [Emim]2[Co(NCS)4]

313.2 313.2 313.2 313.2 313.2 313.2 313.2 313.2 313.2 298.4 298.0 313.0 297.8 313.2 293.2 298.1 313.0 303.2 303.2

98.5 99.1 96.3 172.6 108.2 100.5 101.3 101.3 101.3 128.0 174.0 100.0 133.0 115.6 100.0 163.0 100.0 100.0 100.0

4.396 4.573 4.179 1.049 1.599 2.729 7.652 7.647 10.353 0.911 1.843 0.668 1.445 2.928 4.874 5.497 6.414 10.118 11.647

this work this work this work 30 29 28 26 25 27 38 38 39 38 40 41 42 39 43 44

a

T is the temperature; PNH3 is the pressure of NH3; mNH3 is the solubility of NH3 in DESs.

where f NH3 is the fugacity of NH3 (kPa); mNH3 is the solubility

ΔH ° =

−1

of NH3 in DESs (mol·kg ); PNH3 is the pressure of NH3 (kPa); Hm is Henry’s constant of NH3 in DESs (kPa·kg· ∞ mol−1); V NH is the infinite-dilute partial molar volume of NH3 3 in DESs (L·mol−1); R is the universal gas constant (8.314 J· mol−1·K−1); and T is the temperature (K). Because the NH3 solubility measurements were conducted at a relatively moderate pressure range of (0−300.0) kPa, the fugacity of NH3 is approximately equal to the pressure of NH3. Thus, Henry’s constants of NH3 in EaCl + urea mixtures can be directly obtained by fitting the solubilities of NH3 to the pressures of NH3. The fitting lines are also shown in Figures 1−3, and it is found that the Krichevsky−Kasarnovsky equation can correlate the solubilities of NH3 very well. Henry’s constants of NH3 in EaCl + urea mixtures are presented in Table 7. Henry’s constants of NH3 increase with temperatures increasing, being consistent with the negative impact of temperatures on NH3 solubilities. Thermodynamic Functions. Based on the relationship between Henry’s constants and temperatures, the thermodynamic functions for the NH3 absorption process can be calculated using eqs 3−545

R ∂(ln(Hm/P°)) ∂(1/T )

ΔG° = RT ln(Hm/P°)

(3) (4)

ΔH ° − ΔG° (5) T where ΔH° is the enthalpy change; ΔG° is the Gibbs free energy change; ΔS° is the entropy change; and P° is the standard pressure (100 kPa). According to eq 3, the enthalpy changes can be obtained by drawing a linear fit to ln(Hm/P°) and 1/T, as demonstrated in Figure 5. However, the change of ΔS° =

Table 7. Henry’s Constants of NH3 in EaCl + Urea Mixturesa Hm/(kPa·kg·mol−1) T/K 313.2 323.2 333.2 343.2 353.2

EaCl + urea (1:0.5) 21.0 27.2 41.5 55.6 62.0

± ± ± ± ±

1.1 1.4 0.9 1.6 1.2

EaCl + urea (1:1) 19.1 27.9 40.3 48.6 68.8

± ± ± ± ±

0.6 0.7 0.8 0.8 0.8

EaCl + urea (1:2) 20.8 33.0 44.7 60.9 80.8

± ± ± ± ±

Figure 5. Linear fit of ln(Hm/P°) and 1/T for EaCl + urea mixtures.

0.6 0.7 0.5 1.3 0.9

slope and hence crossover is observed as shown in Figure 5. There are two reasons for this phenomenon. First, the relationship between ln(Hm/P°) and 1/T is not exactly linear. As can be seen, eq 3 is a differential equation. However, a linear fit can be drawn to ln(Hm/P°) and 1/T when the temperature range is not large, and this is a widely used method for the estimation of enthalpy changes in gas

a

T is the temperature; Hm is the Henry’s constant of NH3 in DESs; standard uncertainties u are u(T) = 0.1 K, u(Hm) are reported following the ±sign. F



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Table 8. Thermodynamic Functions for NH3 Absorption in EaCl + Urea Mixturesa DESs

T/K

ΔH°/(kJ·mol−1)

EaCl + urea (1:0.5)

313.2 323.2 333.2 343.2 353.2 313.2 323.2 333.2 343.2 353.2 313.2 323.2 333.2 343.2 353.2

−23.6 ± 2.8

EaCl + urea (1:1)

EaCl + urea (1:2)

ΔG°/(kJ·mol−1) −4.1 −3.5 −2.4 −1.7 −1.4 −4.3 −3.4 −2.5 −2.1 −1.1 −4.1 −3.0 −2.2 −1.4 −0.6

−28.0 ± 1.6

−29.6 ± 0.8

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

ΔS°/(J·mol−1·K−1) −62.5 −62.3 −63.6 −64.0 −63.0 −75.5 −75.9 −76.4 −75.5 −76.1 −81.4 −82.3 −82.1 −82.0 −82.0

0.1 0.1 0.1 0.1 0.1 0.2 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

8.6 8.3 8.3 8.0 7.8 4.7 4.6 4.7 4.5 4.5 2.2 2.2 2.3 2.2 2.2

T is the temperature; ΔH° is the enthalpy change; ΔG° is the Gibbs free energy change; ΔS° is the entropy change; standard uncertainties u are u(T) = 0.1 K, u(ΔH°), u(ΔG°) and u(ΔS°) are reported following the ±sign. a

Table 9. Densities and Viscosities of EaCl + Urea Mixturesa

absorption research.28−30 Second, Henry’s constants were obtained by fitting the solubilities of NH3 to the pressures of NH3 using the Krichevsky−Kasarnovsky equation. Because of the error propagation, the uncertainties of Henry’s constants are relatively larger than those of NH3 solubilities, and the uncertainties of enthalpy changes are even larger than those of Henry’s constants. The calculated values for enthalpy changes, Gibbs free energy changes, and entropy changes are summarized in Table 8. It can be seen that the enthalpy changes are with negative values of (−29.6 to −23.6) kJ·mol−1, suggesting an exothermic process for the absorption of NH3 in EaCl + urea mixtures, which is consistent with the negative impact of temperatures on NH3 solubilities. However, the magnitude of enthalpy changes is moderate, agreeing well with the strength of hydrogen-bonding interaction between NH3 and EaCl + urea mixtures. It can also be seen that the Gibbs free energy changes are also with negative values of (−4.3 to −4.1) kJ·mol−1 at 313.2 K, suggesting a thermodynamically favorable process for the absorption of NH3 in EaCl + urea mixtures, which is consistent with the positive infinite-dilute partial molar volumes of NH3. Physical Properties. Because the physical properties are basic data for liquid solvents, the densities and viscosities of EaCl + urea mixtures were measured at (313.2−353.2) K and 101.3 kPa. The detailed density and viscosity data are summarized in Table 9 and graphically shown in Figures 6 and 7. It is obvious that the densities and viscosities decrease with increase in temperatures, which is a common trend for liquid solvents.46 Furthermore, the densities increase with the increase in the molar ratio of urea to EaCl. However, the viscosities first increase and then decrease with the increase in the molar ratio of urea to EaCl. The viscosities of DESs are essentially determined by the hydrogen bonds formed. When the molar ratio of urea to EaCl is low, the addition of urea contributes to the formation of more intermolecular hydrogen bonds between EaCl and urea and results in the increase of viscosities for EaCl + urea mixtures. When the molar ratio of urea to EaCl is high, the addition of urea contributes to the disruption of intermolecular hydrogen bonds formed between EaCl and urea and results in the decrease of viscosities for EaCl + urea mixtures. Therefore, EaCl + urea (1:1) has higher viscosities than EaCl + urea (1:0.5) and EaCl + urea (1:2).

DESs

T/K

ρDES/(g·cm−3)

EaCl + urea (1:0.5)

313.2 323.2 333.2 343.2 353.2 313.2 323.2 333.2 343.2 353.2 313.2 323.2 333.2 343.2 353.2

1.103 1.098 1.093 1.088 1.083 1.142 1.136 1.130 1.125 1.119 1.179 1.173 1.167 1.161 1.155

EaCl + urea (1:1)

EaCl + urea (1:2)

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002 0.002

ηDES/(mPa·s) 97.8 69.6 50.8 40.9 34.8 197.7 110.1 68.3 47.6 34.8 105.5 67.5 49.2 37.5 28.7

± ± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.0 0.7 0.5 0.4 0.3 2.0 1.1 0.7 0.5 0.3 1.0 0.7 0.5 0.4 0.3

T is the temperature; ρDES is the density of DESs; ηDES is the viscosity of DESs; ρDES and ηDES were measured at 101.3 kPa; standard uncertainties u are u(T) = 0.1 K, u(ρDES) and u(ηDES) are reported following the ±sign. a

Figure 6. Densities of EaCl + urea mixtures.

The densities and viscosities can be fitted to the linear eq 6 and Vogel−Fulcher−Tammann eq 7, respectively46 G



Article

Table 11. Freezing Temperatures, Glass Transition Temperatures, and Melting Temperatures of EaCl + Urea mixturesa DESs

Tg/K

Tm/K

Tf/K

EaCl + urea (1:0.5) EaCl + urea (1:1) EaCl + urea (1:2)

N.A. N.A. 237

310 321 313

300 313 307

a The experimental pressure is 101.3 kPa; Tf is the freezing temperature of DESs; Tg is the glass transition temperature of DESs; Tm is the melting temperature of DESs; N.A. means not available in the temperature range of (223−373) K; standard uncertainties u are u(Tf) = 1 K, u(Tg) = 1 K and u(Tm) = 1 K.

EaCl + urea (1:1). Furthermore, EaCl + urea (1:0.5) and EaCl + urea (1:1) do not have the glass transition temperatures available in the range of (223−323) K, while EaCl + urea (1:2) displays the glass transition temperature of 237 K.

Figure 7. Viscosities of EaCl + urea mixtures.

ρDES = a + bT

■

CONCLUSIONS In summary, the behavior of NH3 absorption in DESs formulated by EaCl, and urea was investigated systematically. Specifically, the solubilities of NH3 at different temperatures and pressures were determined; Henry’s constants, infinitedilute partial molar volumes, enthalpy changes, Gibbs free energy changes, and entropies changes were also calculated. The multiple hydrogen-bond-donating sites in EaCl + urea mixtures contribute to the efficient absorption of NH3, with the NH3 solubilities being quite competitive in comparison with other DESs and ILs reported in the literature. The present work not only reported important thermodynamic data for NH3 absorption in DESs, but also provided useful insights for the design of DESs with high NH3 absorption performance. Furthermore, the densities, viscosities, freezing temperatures, glass transition temperatures, and melting temperatures of EaCl + urea mixtures were also determined because they are basic data for liquid solvents.

(6)

ij D yz zz ηDES = η0 expjjj j T − T0 zz k {

(7) −3

where ρDES is the density of DESs (g·cm ); ηDES is the viscosity of DESs (mPa·s); and a, b, η0, D, and T0 are the empirical parameters. The fitting lines are also shown in Figures 6 and 7, and calculated parameters are presented in Table 10. The viscosities of liquid solvents have a great impact on their fluidity and mass transfer, and those with low viscosities are very useful for application in the gas absorption process. It is interestingly found that EaCl + urea mixtures have moderate viscosities of (97.8−197.7) mPa·s at 323.2 K. To explore the feasibility of NH3 absorption by DESs, the time-dependent absorption of NH3 in EaCl + urea mixtures was examined at 313.2 K (see Figure S3 in the Supporting Information). It can be seen that the absorption of NH3 in EaCl + urea mixtures is very fast, with the equilibrium time being shorter than 30 s. In addition, the viscosities of EaCl + urea (1:1) after NH3 absorption at ∼100 kPa were also measured (see Figure S4 in the Supporting Information). It can be seen that the viscosities of EaCl + urea (1:1) increase only slightly after NH3 absorption, which should have little effect on the fluidity and mass transfer of EaCl + urea (1:1). The glass transition temperatures, melting temperatures, and freezing temperatures of EaCl + urea mixtures were also determined, and results are summarized in Table 11 [see Figure S5 in the Supporting Information for differential scanning calorimetry (DSC) traces]. It is found that EaCl + urea (1:1) has higher freezing temperature and melting temperature than EaCl + urea (1:0.5) and EaCl + urea (1:2), probably because of the tighter stacking of molecules in

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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.jced.9b00246. Solubilities of CO2 in PEG200, 1H NMR spectra, timedependent absorption curves, viscosities after NH3 absorption, and DSC traces (PDF)

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (L.-Y.K.). *E-mail: [email protected] (K.H). ORCID

Kuan Huang: 0000-0003-1905-3017

Table 10. Calculated Parameters for Eqs 6 and 7a parameters a/(g·cm−3) b × 104/(g·cm−3·K−1) η0/(mPa·s) D/K T0/K

EaCl + urea (1:0.5) 1.2606 −5.028 6.422 177.4 248.2

± ± ± ± ±

EaCl + urea (1:1)

0.0002 0.005 2.192 55.0 11.9

1.3157 −5.558 1.468 361.0 239.6

± ± ± ± ±

0.0008 0.025 0.214 25.9 3.0

EaCl + urea (1:2) 1.3640 −5.917 1.876 347.0 227.0

± ± ± ± ±

0.0002 0.007 1.078 116.2 16.4

Standard uncertainties u(a), u(b), u(η0), u(D) and u(T0) are reported following the ±sign.

a

H



Article

Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Natural Science Foundation of Jiangxi Province (20171BAB203019). K.H. also appreciates the sponsorship from Nanchang University.

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