Effective and Reversible Capture of NH3 by Ethylamine Hydrochloride

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2019, 7, 10552−10560

Effective and Reversible Capture of NH3 by Ethylamine Hydrochloride Plus Glycerol Deep Eutectic Solvents Wen-Jing Jiang,† Fu-Yu Zhong,† Yong Liu,*,‡ and Kuan Huang*,† †

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Key Laboratory of Poyang Lake Environment and Resource Utilization of Ministry of Education, School of Resources Environmental and Chemical Engineering, Nanchang University, 999 Xuefu Avenue, Nanchang, Jiangxi 330031, P. R. China ‡ Henan Key Laboratory of Polyoxometalate Chemistry, College of Chemistry and Chemical Engineering, Henan University, 85 Minglun Street, Kaifeng, Henan 475004, P. R. China S Supporting Information *

ABSTRACT: Achieving effective and reversible capture of ammonia (NH3) is an important task in the chemical industry, to avoid the air pollution potentially induced by NH3 emission, and recycle the NH3 resource for value-added productions. In this work, deep eutectic solvents (DESs) comprising ethylamine hydrochloride (EaCl) and glycerol (Gly) were designed as the media for NH3 absorption, by making use of the protic ionic nature of EaCl and multiple hydroxyl groups in Gly, which enable strong hydrogen-bonding interaction with NH3. The absorption amounts of NH3 in prepared EaCl+Gly mixtures at various temperatures and pressures were experimentally measured. It is found that the NH3 capacities of EaCl+Gly mixtures are quite impressive, with the highest value of 9.631 mol/kg at 298.2 K and 106.7 kPa, surpassing those of most absorbents/adsorbents previously reported. The absorption of NH3 in EaCl+Gly mixtures is also highly reversible, with almost negligible decrease in NH3 capacities during absorption−desorption cycles. The mechanism for interaction between EaCl+Gly and NH3 was validated by spectroscopic characterizations. Furthermore, the NH3 solubility data were fitted by the Krichevsky−Kasarnovsky equation to obtain the Henry’s constants of NH3 in EaCl+Gly mixtures, and estimate the enthalpy changes, Gibbs free energy changes and entropy changes, to evaluate the thermodynamic parameters of NH3 absorption process. KEYWORDS: NH3 capture, Deep eutectic solvents, Hydrogen-bonding interaction, Reversible absorption, Thermodynamic properties

■

INTRODUCTION NH3 is a gaseous pollutant with pungent odor and strong corrosivity. Most NH3 is emitted from the purge gas of ammonia synthesis process. The exhaust gas of urea, chemical fertilizer, and nitric acid synthesis processes, and even the escape gas of ammonia refrigerator also contain large amount of NH3.1,2 The release of NH3 has serious impacts on human health and the environment. For example, the inhalation of NH3 by the human body may cause chronic poisoning. The accumulation of NH3 in atmosphere may also result in the formation of PM2.5, and the oxidation of NH3 may produce NOx which contributes to the formation of acid rain. However, NH3 can be used for the productions of fiber, paint, plastic, resin, etc.3−5 Therefore, achieving effective and reversible capture of NH3 is an important task in the chemical industry, to avoid the air pollution potentially induced by NH3 emission, and recycle the NH3 resource for value-added productions. At present, the most widely used method for NH3 capture is chemical absorption, which utilizes the acid−base interaction of liquid solvents with NH3 to selectively eliminate NH3 from industrial gas. In this respect, water and acids (mostly inorganic acids such as sulfuric acid and hydrochloric acid) are commonly used liquid solvents for chemical absorption of NH3.6−8 However, there are many defects associated with these liquid © 2019 American Chemical Society

solvents. For example, water is highly volatile, which suffers from considerable loss during capture process. The high heat capacity of water also requires a considerable amount of energy input to regenerate the water. Inorganic acids are highly corrosive to equipment, and exhibit extremely strong interaction with NH3, which results in the quite difficult regeneration of inorganic acids. To address these challenges, developing new materials with low volatility, high efficiency, and excellent recyclability for NH3 capture is highly demanded. In the past years, ionic liquids (ILs) have been regarded as promising alternatives to traditional liquid solvents for gas separation.9,10 ILs are a class of room-temperature molten salts possessing many intriguing properties such as wide liquid range, negligible volatility, good thermal stability, and tunable structure.11−13 Using ILs for gas separation can effectively suppress the volatile escape of liquid solvents during separation process. Particularly, the tunable structure of ILs enables the task-specific design of liquid solvents with improved performance for gas separation. In terms of using ILs for NH3 capture, Yokozeki et al. pioneeringly determined the solubilities of NH3 Received: February 24, 2019 Revised: May 24, 2019 Published: May 28, 2019 10552

DOI: 10.1021/acssuschemeng.9b01102 ACS Sustainable Chem. Eng. 2019, 7, 10552−10560

Research Article

ACS Sustainable Chemistry & Engineering in various normal ILs.14,15 The solubilities of NH3 in normal ILs are not satisfying owing to the weak interaction between normal ILs and NH3. To this end, Shang et al. designed a series of protic ionic liquids with strong hydrogen-bond donating ability for highly efficient absorption of NH3.16 Subsequently, Zeng et al. and Wang et al. found the effective and reversible dissolution of NH3 in metal-based ILs through Lewis acid−base interaction.17,18 However, the use of ILs for NH3 capture is not preferred from the viewpoints of practical application, because ILs are normally expensive due to the complex synthetic routes, and highly viscous because of the strong electrostatic interaction between ions. Recently, deep eutectic solvents (DESs) have received extensive attentions as IL analogues, because their physical and chemical properties are very similar to ILs.19−23 DESs are composed of hydrogen-bond acceptors (HBAs, such as quarternary ammonium salts) and hydrogen-bond donors (HBDs, such as carboxylic acids, amides, and alcohols), which were simply mixed to form eutectic mixtures. Therefore, DESs are normally advantageous over ILs for practical application. To date, a lot of work have been conducted to investigate the use of DESs for the capture of acid gases such as CO2,24 SO2,25 and H2S.26 Some researchers have started exploring the potential use of DESs for NH3 capture. Li et al. and our group designed phenol-based ternary DESs for highly efficient and reversible absorption of NH3 through weak Lewis acid−base interaction.27,28 Deng et al. achieved highly efficient and reversible absorption of NH3 in protic NH4SCN-based DESs through strong hydrogen-bonding interaction.29 Akhmetshina et al. measured the absorption capacities of NH3 in methanesulfonate-based DESs,30 while Duan et al. and our group measured the absorption capacities of NH3 in choline chloride (ChCl)based DESs.31,32 Although some progresses have been achieved in this field, the application of DESs in NH3 capture is still underexplored. The development of DESs with not only simple formulation but also high performance for NH3 capture is of particular interest. In this work, we designed a new class of DESs composed of ethylamine hydrochloride (EaCl) and glycerol (Gly) as the media for NH3 absorption. The chemical structures of EaCl and Gly are shown in Scheme 1. EaCl is a protic ionic salt with strong

and glycerol (Gly, 99 wt %) were supplied by Adamas Co. Ltd., China. All the chemicals were directly used as-received. The mixtures of EaCl and Gly were stirred at 333.2 K, and DESs were obtained when the mixtures turned to transparent liquids. Prepared DESs are abbreviated as EaCl+Gly (1:x), where x represents the molar ratio of Gly to EaCl. Characterizations. Water contents were measured by a Titan TKF-1B analyzer based on Karl Fischer titration with relative uncertainty of 0.3%. Densities were collected on an Anton Paar DMA 4500 M densiometer with standard uncertainty of 0.000 05 g/cm3. Viscosities were collected on a Brookfield RVDV2PCP230 viscometer with relative uncertainty of 1%. Thermogravimetric analysis (TGA) was conducted on a PerkinElmer TGA4000 analyzer at heating rate of 5 K/ min under N2 atmosphere. Glass transition temperatures and melting points were determined by a PerkinElmer DSC 8000 analyzer at a scanning rate of 10 K/min under N2 atmosphere. 1H NMR spectra was collected on a Bruker Avance 600 spectrometer using d6-DMSO as the solvent and TMS as the internal standard. FTIR spectra was collected on a Thermo Nicolet 5700 spectrometer. NH3 Solubility Measurements. The apparatus used for collecting the absorption capacities of NH3 has frequently been reported in previous work, and its reliability has also been validated.33−35 The apparatus has two stainless steel chambers, which are used as the gas reservoir and equilibrium cell, respectively. The equilibrium cell is placed on a magnetic stirrer. The temperature of the whole apparatus is adjusted by an oil bath with a standard uncertainty of 0.1 K. The pressures of the two chambers are monitored by Wideplus-8 transducers with a standard uncertainty of 1.2 kPa. As an example, ∼1.0 g of DES which was accurately weighted by an analytical balance with a standard uncertainty of 0.0001 g, was placed in the equilibrium cell. The whole apparatus was then totally evacuated, and the pressure of equilibrium cell (P0) was measured to be 0−0.2 kPa, being close to the detection limit of pressure transducers. The increase of P0 was found to be less than 0.1 kPa after 24 h, proving the excellent tightness of whole apparatus. The gas from tank was then introduced into the gas reservoir to a pressure of P1. The valve between two chambers was switched on to feed the gas into equilibrium cell. Equilibrium was thought to be reached for gas absorption if the pressures kept constant for 2 h. The pressures were expressed as P2 for equilibrium cell and P′1 for gas reservoir after absorption equilibrium being reached. Thus, the gas partial pressure in equilibrium cell was PNH3 = P2 − P0. The gas solubility could be calculated using eq 1.

Scheme 1. Chemical Structures of EaCl and Gly

1,T 1′,T is the gas density at P1 and T in mol/cm3, ρPNH is temperature in K, ρPNH 3 3 NH3,T the gas density at P1′ and T in mol/cm3, ρPNH is gas density at P and NH 3 3 T in mol/cm3, V1 is the gas reservoir volume including connecting parts in cm3, V2 is the equilibrium cell volume including connecting parts in cm3, wDES is the DES mass in g, and ρDES is the DES density at T in g/

P

3

3

(1)

where mNH3 is the gas solubility in mol/kg, the meanings of P1, P′1, and PNH3 have been introduced in above section (in kPa), T is the

1,T 1′,T NH3,T cm3. ρPNH , ρPNH and ρPNH were acquired from the NIST Chemistry 3 3 3 36 WebBook. V1 and V2 were measured by helium with standard uncertainty of 0.01 cm3. The solubility of gas at elevated pressure was continuously measured by feeding more gas into the equilibrium cell to achieve a new equilibrium for gas absorption. After the completion of measurements, the gas was purged into a glass tank with diluted H2SO4. Since the contribution of the uncertainties of temperature, volume, mass, and density to the uncertainties of gas solubility data can be ignored, the standard uncertainties of gas solubility data were estimated from the standard uncertainty of pressure through error propagation.

hydrogen-bond donating ability, and Gly is an alcohol compound with multiple hydroxyl groups. Both components are expected to enable strong hydrogen-bonding interaction with NH3, which have lone pair electrons. In addition, EaCl and Gly are commercially available, and with lower cost than most ILs. It is believed that EaCl+Gly mixtures have promising application in NH3 capture. Holding these in mind, the NH3 capture performance of EaCl+Gly mixtures and related absorption mechanism were systematically examined in the present work.

■

,T

P1, T P1′ , T 3 m NH3 = [ρNH V1 − ρNH V1 − ρNHNH (V2 − wDES/ρDES )] /wDES 3

■

RESULTS AND DISCUSSION Physiochemical Properties. In this work, four DESs were synthesized: EaCl+Gly (1:2), EaCl+Gly (1:3), EaCl+Gly (1:4), and EaCl+Gly (1:5). The water contents of EaCl+Gly mixtures are found to be in the range of 0.047 to 0.059 wt % (see Table S1

EXPERIMENTAL SECTION

Materials and Synthesis. NH3 (99.99 mol %) was purchased from Huasheng Co. Ltd., China. Ethylamine hydrochloride (EaCl, 98 wt %) 10553


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ACS Sustainable Chemistry & Engineering

313.2 K. Overall, EaCl+Gly mixtures are less viscous than most ILs reported in the literature.37−41 As is known, liquid solvents with lower viscosities are more easily to transport in pipelines, and usually have fast absorption rate for gases. Therefore, EaCl +Gly mixtures are more appropriate for application in gas absorption process than most ILs. Furthermore, it is observed that the densities decrease linearly with temperature, while the viscosities decrease nonlinearly with temperature. These are common trends for liquid solvents.42−45 The density and viscosity data were correlated with linear eq 2 and VFT eq 3 respectively:

of the Supporting Information, SI), which are quite low and negligible. We then measured the physiochemical properties of prepared EaCl+Gly mixtures, including densities, viscosities, and decomposition temperatures, because these are fundamental data for gas absorption process design. Figures 1 and 2 show

ρDES = a + bT

(2)

ji D zyz ηDES = η0expjjj z j T − T0 zz k {

(3)

where ρDES is the density in cm /g, ηDES is the viscosity in cP, T is the temperature in K, and a (in g/cm3), b (in g/cm3·K), η0 (in cp), D (in K), and T0 (in K) are empirical parameters. Fitted lines are also shown in Figures 1 and 2, and fitted parameters are summarized in Table 1. 3

Table 1. Fitted Parameters for Eqs 2 and 3 Figure 1. Densities of EaCl+Gly mixtures [■: EaCl+Gly (1:2), ●: EaCl +Gly (1:3), ▲: EaCl+Gly (1:4), and ▼: EaCl+Gly (1:5), lines: fitting results].

parameters eq 2 a (g/cm3) b × 10−4 (g/cm3·K) R2 eq 3 η0 × 10−5 (cp) D (K) T0 (K) R2

EaCl+Gly (1:2)

EaCl+Gly (1:3)

EaCl+Gly (1:4)

EaCl+Gly (1:5)

1.353 −5.391 1

1.374 −5.543 1

1.387 −5.619 1

1.396 −5.776 1

889 1283 156.81 0.9999

3539 907 187.03 1

3200 934 186.27 1

923 1210 170.73 0.9999

Figure 3 shows the TGA curves for EaCl+Gly mixtures. It is found that EaCl+Gly mixtures start to decompose at ∼425 K. The decomposition temperatures decrease slightly with the increase of Gly contents in mixtures, because the ionic

Figure 2. Viscosities of EaCl+Gly mixtures [■: EaCl+Gly (1:2), ●: EaCl+Gly (1:3), ▲: EaCl+Gly (1:4), and ▼: EaCl+Gly (1:5), lines: fitting results].

the densities and viscosities of EaCl+Gly mixtures, respectively (see Table S2 for density and viscosity data). It is found that the densities increase with the increase of Gly contents in mixtures. For example, the density is 1.18377 cm3/g at 313.2 K for EaCl +Gly (1:2), while the value is 1.22115 cm3/g for EaCl+Gly (1:5) at the same temperature. However, the viscosities first increase but then decrease with the increase of Gly contents in mixtures. When the contents of Gly are not high, the addition of Gly results in the formation of more intermolecular hydrogen bonds, and leads to the increase of viscosities for mixtures. When the contents of Gly are high enough, the addition of Gly results in the dilution of EaCl, and leads to the decrease of viscosities for mixtures. As a result, EaCl+Gly (1:4) has the highest viscosities among the four EaCl+Gly mixtures, with the value of 50.5 cP at

Figure 3. TGA curves for EaCl+Gly mixtures [black line: EaCl+Gly (1:2), red line: EaCl+Gly (1:3), blue line: EaCl+Gly (1:4), and green line: EaCl+Gly (1:5)]. 10554


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ACS Sustainable Chemistry & Engineering compound EaCl is more stable than neutral compound Gly. We also attempted to determine the glass transition temperatures and melting points of EaCl+Gly mixtures. However, they were found to be not available in the experimental temperature range of 200−300 K (see Figure S1). Therefore, the melting points of EaCl+Gly mixtures are much lower than those of pure EaCl (383 K)46 and pure Gly (291 K),47 validating that EaCl+Gly mixtures can be classified as DESs. NH3 Absorption Rates. To examine the rates of NH3 absorption in EaCl+Gly mixtures, ∼1 g of DES was placed in the equilibrium cell, followed by feeding gas to a pressure of ∼200 kPa, monitoring the pressure change online, and calculating the amount of gas absorbed in relation to absorption time. Results are shown in Figure 4. It is found that the

Figure 5. Solubilities of NH3 in EaCl+Gly (1:2) (■: 298.2 K, ●: 313.2 K, ▲: 333.2 K, ▼: 353.2 K, and lines: fitting results).

Figure 4. NH3 absorption rates in EaCl+Gly mixtures at 298.2 K [■: EaCl+Gly (1:2), ●: EaCl+Gly (1:3), ▲: EaCl+Gly (1:4), and ▼: EaCl +Gly (1:5)].

absorption of NH3 in EaCl+Gly mixtures is quite fast, with the equilibrium time being shorter than 60 s. The fast absorption of NH3 should be attributed to the low viscosity of EaCl+Gly mixtures. There is no obvious difference in NH3 absorption rates for the four EaCl+Gly mixtures, implying the minimized diffusion barrier at such low level of viscosities. NH3 Solubilities. The solubilities of NH3 in EaCl+Gly mixtures were measured at 298.2−353.2 K and 0−250 kPa, as shown in Figures 5, 6, 7, and 8 (see Tables S3∼S6 for NH3 solubility data). The results show that the solubilities of NH3 increase with the increase of pressures, but decrease with the increase of temperatures. These are common trends for gas absorption in liquid solvents.48,49 However, the absorption of NH3 in EaCl+Gly mixtures is slightly deviated from idea type, which should be attributed to the strong hydrogen-bonding interaction between EaCl+Gly and NH3. Since the viscosities of DESs during NH3 absorption are very important, we collected the viscosities of EaCl+Gly (1:2) with different NH3 loadings at 298.2 K (see Figure S3). Although the experimental data are somewhat scattered, because the absorbed NH3 may escape during the measurements of viscosities, it can still be found that the dissolution of NH3 has a minor effect on the fluidity of EaCl +Gly mixtures. Figure 9 compares the absorption capacities of NH3 in EaCl+Gly mixtures at 298.2 K. It is found that the compositions of mixtures have little effect on the solubilities of NH3, and the four EaCl+Gly mixtures have almost identical



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range and pressure range (see Figure S4). It is found that the solubilities of CO2 in EaCl+Gly (1:2) are in the range of 0.0025−0.0017 mol/kg at 100 kPa, which are extremely low relative to the solubilities of NH3. The ideal NH3/CO2 selectivities (defined as the ratio of NH3 solubility to CO2 solubility at 100 kPa) are thus calculated to be 818−5567, suggesting the excellent ability of EaCl+Gly mixtures for NH3/ CO2 separation. Absorption Mechanism. In order to validate the strong hydrogen-bonding interaction of EaCl+Gly with NH3, the 1H NMR and FTIR spectra of EaCl+Gly (1:2) before and after NH3 absorption were measured, as shown in Figures 10 and 11. In the 1 H NMR spectra, the proton signals for −NH3+ in EaCl (7.91 ppm) and −OH in Gly (4.44 and 4.51 ppm) shift together to 4.79 ppm, and become indistinguishable after NH3 absorption. However, the proton signals for other groups in EaCl and Gly do not show obvious shift after NH3 absorption. This observation suggests that the protons of −NH3+ and −OH are active sites for NH3 absorption in EaCl+Gly mixtures, and the interaction with NH3 changes the electronic environment of −NH3+ and −OH. In the FTIR spectra, there is no obvious change after NH3 absorption, indicating that the interaction between EaCl+Gly and NH3 is not strong enough to transport the proton from −NH3+ and −OH to NH3. Therefore, the interaction between EaCl+Gly and NH3 is classified as strong hydrogen-bonding interaction. Thermodynamic Properties. Considering the strong hydrogen-bonding interaction between EaCl+Gly and NH3, and the slight deviation of NH3 absorption from ideal type, the NH3 solubility data were correlated with the Krichevsky− Kasarnovsky (K−K) eq 4:


ln

PNH3 m NH3

= ln Hm +

∞ V NH P 3 NH3

RT

(4)

where PNH3 is the NH3 partial pressure in kPa, mNH3 is the NH3 solubility in mol/kg, Hm is the Henry’s constant of NH3 in DES in kPa·kg/mol, V∞ NH3 is the partial molar volume of NH3 in DES at infinite dilution in L/mol, R is the universal gas constant (8.314 J/mol·K), and T is the temperature in K. Correlated lines are also shown in Figures 5∼8. It is found that the K−K equation can correlate the NH3 solubility data very well. Through the correlation, the Henry’s constants and partial molar volumes at infinite dilution were obtained, as presented in Tables 3 and 4. It is found that the Henry’s constants of NH3 in EaCl+Gly mixtures increase with the increase of temperatures, which is in accordance with the negative dependence of NH3 solubilities on temperatures. The partial molar volumes of NH3 in EaCl+Gly mixtures at infinite dilution are with positive values, implying that the absorbed NH3 may penetrate into the compact phase of mixtures, and expands the distance between solvent molecules. This normally suggests a relatively stable thermodynamic status, which should be a result of the strong hydrogen-bonding interaction between EaCl+Gly and NH3. Furthermore, the four EaCl+Gly mixtures have almost identical Henry’s constants of NH3 and partial molar volumes of NH3 at infinite dilution if compared at the same temperature, which is in accordance with the comparable ability of four mixtures for NH3 absorption. With the Henry’s constants of NH3 at different temperatures, the enthalpy changes for NH3 absorption process were estimated by the van’t Hoff eq 5:

Figure 9. Comparison of NH3 solubilities in EaCl+Gly mixtures at 298.2 K [■: EaCl+Gly (1:2), ●: EaCl+Gly (1:3), ▲: EaCl+Gly (1:4), and ▼: EaCl+Gly (1:5)].

solubilities of NH3. The solubilities of NH3 in pure Gly were also measured at the same temperature range and pressure range (see Figure S2). It is found that the solubilities of NH3 in pure Gly are similar as those in EaCl+Gly mixtures. For example, the solubility of NH3 is 9.519 mol/kg at 102.3 kPa and 298.2 K in pure Gly, while the value is 9.631 mol/kg at 106.7 kPa and 298.2 K in EaCl+Gly (1:2). These findings suggest that EaCl and Gly contribute the same to the absorption of NH3 in mixtures. It should be noted that pure EaCl is solid in the experimental temperature range of 298.2−353.2 K, thus the solubilities of NH3 in pure EaCl cannot be measured. Table 2 gives a summary of the NH3 capacities for different absorbents/adsorbents reported in the literature, including DESs, 2 7 − 3 0 , 3 2 ILs,14,15,17,18,50−52 porous carbons,53 and covalent organic frameworks (COFs).54,55 Overall, the solubilities of NH3 in EaCl+Gly are quite impressive, with the highest value of 9.631 mol/kg at 298.2 K and 106.7 kPa, surpassing those of most absorbents/adsorbents. Since the selective separation of NH3 from CO2 is an important task in chemical industry, we also measured the solubilities of CO2 in EaCl+Gly (1:2) at the same temperature 10556


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ACS Sustainable Chemistry & Engineering Table 2. Comparison of NH3 Capacities of Different Absorbents and Adsorbents solvents

T (K)

P (kPa)

mNH3 (mol/kg)

refs.

EaCl+Gly (1:2) [bmim][MeSO3]+urea (1:1) ChCl+urea (1:2) ChCl+PhOH+EG (1:5:4) ChCl+Res+Gly (1:7:5) NH4SCN+Gly (2:3) [Bmim][BF4] [Bmim][PF6] [Bmim][Tf2N] [Emim][Ac] [Hmim][Cl] [EtOHmim][BF4] [TMGH][BF4] [DMEA][Ac] [Bim][Tf2N] [Bmim]2[CuCl4] [Emim]2[Co(NCS)4] C-NiCl2-EM [HOOC]17-COFs BBP-5

298.2 313.2 298.2 298.2 313.0 313.2 298.2 298.2 299.4 298.3 297.8 313.2 293.2 298.1 313.2 303.2 303.2 298.2 298.0 298.0

106.7 172.6 95.0 101.3 101.0 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 101.3 100.0 100.0 0.1 100.0 100.0

9.631 1.049 2.213 9.619 10.600 10.353 0.998 1.233 0.311 1.879 1.409 2.642 5.285 5.872 6.459 10.118 11.647 4.800 9.340 17.700

this work 30 32 28 27 29 15 15 15 14 15 50 51 14 52 18 17 53 54 55

Table 3. Henry’s Constants of NH3 in EaCl+Gly Mixtures Hm (kPa·kg/mol) T (K)

EaCl+Gly (1:2)

EaCl+Gly (1:3)

EaCl+Gly (1:4)

EaCl+Gly (1:5)

298.2 313.2 333.2 353.2

8.2 ± 0.3 11.8 ± 0.3 23.8 ± 0.4 44.6 ± 0.8

8.2 ± 0.3 12.7 ± 0.3 24.6 ± 0.4 44.5 ± 0.4

8.1 ± 0.3 12.7 ± 0.3 25.0 ± 0.4 45.8 ± 0.5

8.1 ± 0.3 12.6 ± 0.3 25.2 ± 0.3 46.1 ± 0.4

Table 4. Partial Molar Volumes of NH3 in EaCl+Gly Mixtures at Infinite Dilution Figure 10. 1H NMR spectra of EaCl+G ly (1:2) before and after NH3 absorption. (green line: before NH3 absorption, red line: after NH3 absorption).

V∞ NH3 (L/mol) T (K)

EaCl+Gly (1:2)

EaCl+Gly (1:3)

EaCl+Gly (1:4)

EaCl+Gly (1:5)

298.2 313.2 333.2 353.2

6.23 ± 0.50 6.03 ± 0.37 4.29 ± 0.25 2.76 ± 0.24

6.18 ± 0.51 5.40 ± 0.30 4.13 ± 0.22 2.69 ± 0.14

6.20 ± 0.56 5.47 ± 0.34 3.87 ± 0.21 2.50 ± 0.16

6.40 ± 0.56 5.39 ± 0.32 3.77 ± 0.18 2.23 ± 0.12

ΔH =

R ∂(ln Hm)

( T1 )

∂

(5)

where ΔH is the enthalpy change in kJ/mol. Figure 12 shows the linear fit of ln Hm and 1/T for NH3 absorption in EaCl+Gly mixtures, and estimated enthalpy changes are presented in Table 5. The Gibbs free energy changes and entropy changes for NH3 absorption process were further estimated by the following eqs 6 and 7: ΔG = RT ln Hm ΔS = Figure 11. FTIR spectra of EaCl+Gly (1:2) before and after NH3 absorption. (black line: before NH3 absorption, red line: after NH3 absorption).

ΔH − ΔG T

(6)

(7)

where ΔG is the Gibbs free energy change in kJ/mol, and ΔS is the entropy change in J/mol·K. Estimated Gibbs free energy change and entropy changes are also presented in Table 5. The enthalpy changes for NH3 absorption in EaCl+Gly mixtures are 10557


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Figure 12. Linear fit of lnHm and 1/T for NH3 absorption in EaCl+Gly mixtures [■: EaCl+Gly (1:2), ●: EaCl+Gly (1:3), ▲: EaCl+Gly (1:4), and ▼: EaCl+Gly (1:5)].

Figure 13. Recycling of EaCl+Gly (1:2) for NH3 absorption (absorption conditions: 313.2 K and 100 kPa, desorption conditions: 353.2 K and 0.1 kPa).

Table 5. Thermodynamic Properties of NH3 Absorption in EaCl+Gly Mixtures

can also be inferred from the magnitude of enthalpy changes for NH3 absorption in EaCl+Gly mixtures (−25.9∼−28.0 kJ/mol). The stability of EaCl+Gly mixtures during the absorption− desorption cycles was further examined by purging EaCl+Gly (1:2) with N2 at the desorption temperature of 353.2 K, and the weight was recorded online (see Figure S5). It is found that there is little change in the weight of EaCl+Gly (1:2) over 20 h, proving the good stability of EaCl+Gly mixture.

solvents

T (K)

ΔH (kJ/mol)

ΔG (kJ/mol)

ΔS (J/mol·K)

EaCl+Gly (1:2)

298.2 313.2 333.2 353.2 298.2 313.2 333.2 353.2 298.2 313.2 333.2 353.2 298.2 313.2 333.2 353.2

−25.9 ± 0.8

−6.2 ± 0.1 −5.6 ± 0.1 −4.0 ± 0.1 −2.4 ± 0.1 −6.2 ± 0.1 −5.4 ± 0.1 −3.9 ± 0.1 −2.4 ± 0.1 −6.2 ± 0.1 −5.4 ± 0.1 −3.8 ± 0.1 −2.3 ± 0.1 −6.2 ± 0.1 −5.4 ± 0.1 −3.8 ± 0.1 −2.3 ± 0.1

−66.1 ± 2.3 −64.9 ± 2.2 −65.8 ± 2.2 −66.6 ± 2.0 −69.5 ± 1.0 −68.8 ± 1.0 −69.2 ± 1.0 −69.5 ± 1.0 −71.5 ± 1.0 −70.9 ± 1.0 −71.2 ± 1.0 −71.6 ± 1.0 −72.9 ± 0.9 −72.1 ± 1.0 −72.5 ± 1.0 −72.8 ± 1.0

EaCl+Gly (1:3)

EaCl+Gly (1:4)

EaCl+Gly (1:5)

−26.9 ± 0.4

−27.6 ± 0.4

−28.0 ± 0.4

■

CONCLUSIONS In summary, a new class of DESs comprising EaCl and Gly were designed and synthesized for NH3 absorption. The physiochemical properties and NH3 capture performance of EaCl +Gly mixtures were examined systematically. On the basis of the experimental results, it is concluded that the strong hydrogenbonding interaction formed between EaCl+Gly and NH3 endowing EaCl+Gly mixtures with high NH3 solubilities and excellent absorption reversibility. Furthermore, the two components in EaCl+Gly mixtures contribute the almost same to the absorption of NH3 in mixtures, resulting in the negligible effect of mixture compositions on NH3 solubilities. Thermodynamic analysis reveals that the absorption of NH3 in EaCl+Gly mixtures is a relatively favorable process. Given that EaCl+Gly mixtures are easy to prepare from commercially available materials, and with relatively low viscosity, they are believed be to promising materials for the capture and recycle of NH3 from industrial gas.

−25.9∼−28.0 kJ/mol, which agree well with the magnitude for thermal effect of hydrogen-bonding interaction. The Gibbs free energy changes are with the values of ∼−6.2 kJ/mol at 298.2 K, suggesting that the absorption of NH3 in EaCl+Gly mixtures is a thermodynamic favorable process. Regeneration of DESs. To examine the reversibility of NH3 absorption in EaCl+Gly mixtures, the NH3-saturated EaCl+Gly (1:2) was regenerated at 353.2 K and 0.1 kPa for 1 h. After the completion of regeneration, EaCl+Gly (1:2) was reused for NH3 absorption. The absorption−desorption cycle was conducted for 5 times. Figure 13 shows the solubilities of NH3 at different recycling times. The results show that the solubilities of NH3 display no obvious decrease after 5 times of recycling, indicating the high reversibility of NH3 absorption in EaCl+Gly mixtures. It should be noted that the NH3 solubilities shown in Figure 13 are working capacities, because the NH3 solubilities were measured from the decrease of gas-phase pressure in equilibrium cell. Although EaCl+Gly mixtures enable strong hydrogenbonding interaction with NH3, such interaction can be easily destroyed at increased temperature and decreased pressure. This

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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.9b01102. Water contents, densities, viscosities, DSC traces, NH3 solubilities, viscosity changes, CO2 solubilities, and weight changes (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.L.). *E-mail: [email protected] (K.H.). 10558


Research Article

ACS Sustainable Chemistry & Engineering ORCID

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Kuan Huang: 0000-0003-1905-3017 Notes

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) and the National Natural Science Foundation of China (21676072). K.H. also appreciates the sponsorship from Nanchang University.

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Effective and Reversible Capture of NH3 by Ethylamine Hydrochloride

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