Solubility of Ionic Liquid [Bmim] Ac in Supercritical CO2 Containing

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

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Solubility of Ionic Liquid [Bmim]Ac in Supercritical CO2 Containing Different Cosolvents Gen Li, Dan Zhou, Qin-Qin Xu, Guo-Yue Qiao, and Jian-Zhong Yin* State Key Laboratory of Fine Chemicals, School of Chemical Machinery, Dalian University of Technology, Dalian 116024, PR China S Supporting Information *

ABSTRACT: The solubility of the ionic liquid (IL) 1-butyl-3methylimidazolium acetate ([Bmim]Ac) in supercritical carbon dioxide (scCO2) with cosolvents, including ethanol, acetone, dimethyl sulfoxide (DMSO), and acetonitrile, was determined at 40, 50, and 60 °C and with a pressure up to 15.0 MPa. The results showed that the addition of cosolvents has a significant effect on the solubility of [Bmim]Ac in scCO2. The ability of different cosolvents to enhance the solubility of [Bmim]Ac in scCO2 is in the following order: ethanol > DMSO > acetone > acetonitrile. The solubility of [Bmim]Ac in the scCO2/cosolvent mixture increased dramatically as the cosolvent concentration exceeded 20.0 mol %. The effect of the temperature on the solubility is more complicated. The solubility of [Bmim]Ac in scCO2/cosolvent increased slowly when using ethanol as the cosolvent, decreased slowly when using acetone or acetonitrile as the cosolvent, and increased first and then decreased when using DMSO as the cosolvent as the temperature increased from 40 to 60 °C. Moreover, the values of solubility increased as the pressure increased from 8 to 15 MPa. The increased tendency in the high pressure area is more obvious. The maximum solubility is 3.66 × 10−2 mol %, which can be obtained when using 26.0 mol % ethanol at 60 °C, 14.55 MPa, and the minimum solubility is 1.89 × 10−4 mol %, which can be obtained when using 10.5 mol % DMSO at 40 °C, 9.93 MPa. The modified Christal equation was used to correlate the solubility data, and the average absolute relative deviations are in the range of 4.36−14.35%. The maximum correlation accuracy is obtained when using ethanol as the cosolvent, and the minimum value for the system is obtained when using DMSO as the cosolvent.

1. INTRODUCTION The prevention of pollution has been a focus around the world in recent years as the chemical industry develops. The utilization of green solvents is an effective and alternative solution, ILs and scCO2 are excellent choices among all green solvents.1−5 Ionic liquids have attracted much attention in the recent years because of their extremely low vapor pressures, high thermal and chemical stability, and their structure can be designed according to requirement.6−8 Therefore, ILs are considered to be a new generation of green solvents, which have a wide range of applications in extraction, separation, material preparation, catalyzing reactions, and they can be used as environmentally benign media for organic synthesis reactions.9−11 In order to prepare a high-purity product and recycle the ionic liquids efficiently, a separation without crosscontamination is urgently needed. Usually, evaporation or liquid−liquid separation is employed to separate IL from organics, but these traditional methods are invalid for highboiling point or thermally labile compounds. Supercritical CO2 has been widely used as a green solvent because of its nontoxic, gas-like viscosity and liquid-like solubility for several decades.12 During the recent years, several studies have been made about the application of scCO2 for separation of ionic liquids from organic compounds.13−17 However, it is well-known that cosovlent can enhance the solubility of solute in scCO2. In © XXXX American Chemical Society

practical applications, the organic compounds may act as cosolvents to increase the solubility of ILs. Several attempts have been made concerning this concept. Brennecke performed studies on the phase behavior of some scCO2/IL systems. They found that scCO2 was very soluble in five imidazolium-based ILs ([Emim][HSO4], [Emim][MeSO4], [Emim][MeSO3], and [Emim][SCN], [Emim][DEP]), and the expansion of ILs was not obvious, while the solubility of these ILs in scCO2 was extremely low with an order of magnitude of 10−5 mol %.18,19 Wu Wei-ze and co-workers reported the solubility of two commonly used ILs ([Bmim][PF6] and [Bmim][BF4]) in scCO2 without any cosolvent, the results showed that the solubility of the ILs in scCO2 was very low with an order of magnitude below 10−4 mol % at 40 °C, 15 MPa. However, the addition of cosolvents can significantly enhance the dissolution.20−22 [Bmim][PF6], [Bmim][BF4], and [Bmim]Ac are the most frequently used ILs, but there is no report about the solubility of [Bmim]Ac in scCO2 with cosolvents. It is of great significance to replenish the solubility database. So in this work, we measured the solubility of the IL 1-butyl-3Received: December 25, 2017 Accepted: March 9, 2018

A

DOI: 10.1021/acs.jced.7b01108 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

±0.01 MPa. The maximum pressure which the apparatus could bear was 20 MPa. Pressure was measured by a pressure sensor with an experimental error of ±0.0069 MPa. Temperature in the visible reactor was controlled using a constant temperature water bath, and was recorded by a temperature sensor with an experimental error of ±0.01 °C. The resolution of the camera wass 1920*1080 dpi, and the image capture speed was 60 fps. In a typical experiment, a certain amount of IL and cosolvent were put into the visible reactor, and then the air in the system was replace with CO2. Related needle valves were opened slowly when the temperature reached the set value. The pressure increased as CO2 was compressed into the visible reactor. The experimental process phenomena are presented in Figure 2. There were two phases at the beginning, one was a liquid phase, which mainly contained cosolvent, and the other was avapor phase, which mainly contained CO2. Then, highpressure CO2 was continually compressed into the system and solubilized in the liquid phase until a second liquid phase appeared. In this course, phase transition of LV to LLV could be observed. The first liquid phase was an organic-rich phase at the bottom of the reactor, the second liquid phase was a CO2rich phase in the middle of the reactor. At this state, IL ([Bmim]Ac) mainly existed in the organic-rich phase. Then the magnetic stirrer was turned on, and the pressure was adjusted by controlling the high-pressure chromatography syringe pump until the mixture of IL/cosolvent/CO2 achieved a state of single phase. The adjustable volume plunger was then adjusted repeatedly to obtain the cloud point. The determination of the cloud point was also evidenced by the Tyndall effect. The Tyndall effect could be clearly observed with the incomplete dissolution of the IL ([Bmim]Ac) in the scCO2/cosolvent mixture but could not be observed when the dissolution process was completed. The Tyndall effect phenomena are shown in Figure 3. There was a “light patch” across the visible

methylimidazolium acetate ([Bmim]Ac) in scCO2 with four different cosolvents, namely ethanol, acetone, DMSO, and acetonitrile, at 40, 50, and 60 °C and a pressure up to 15.0 MPa. The trend of the solubility with the different cosolvents, the amount of cosolvent, the temperatures, and the pressures were analyzed, and then the four-parameter Christal equation was employed to correlate the solubility data.

2. EXPERIMENTAL SECTION 2.1. Materials. CO2 (99.9%) was purchased at Dalian Gas Co., Ltd. 1-Butyl-3-methylimidazolium acetate ([Bmim]Ac, 98%) and 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6], 98%) were purchased at Lanzhou Institute of Physics, Chinese Academy of Sciences. The ionic liquids were dried under vacuum at 120 °C for 12 h before use. Ethanol, acetone, dimethyl sulfoxide (DMSO), and acetonitrile were purchased at Hua Hao Chemical Co., Ltd., with a purity of 99.7% mass fraction. CO2, ethanol, acetone, DMSO, and acetonitrile were used with no more purification. Further details are presented in the Supporting Information. 2.2. Equipment and Procedures. A static method was employed to determine the solubility of the ILs in the scCO2/ cosolvent mixture, the experimental apparatus is shown in Figure 1. The reactor had a visual window to observe the

Figure 1. Layout of experimental apparatus. (1) CO2 cylinder, (2) condenser, (3) plunger pump, (4,7,11,18) needle valve, (5) buffer tank, (6) pressure gauge, (8) high-pressure chromatography syringe pump, (9) filter, (10) buffer tank, (12) micro-adjustment valve, (13) high-pressure visible reactor, (14) magnetic stirrer, (15) camera and accent light, (16) temperature sensor, (17) pressure sensor, (19) adjustable volume plunger, (20) collector for waste gas, (21) magnetic stirrer, and (22) water bath cycle.

Figure 3. IL is not completely dissolved (a). IL is completely dissolved (b).

experimental phenomena and its effective volume was 29.28 mL, including pipelines in this system. The plunger’s volume was adjustable in the range of 0−1.30 mL. Thus, the experimental system’s volume could be adjusted linearly between 29.28−30.58 mL. The measurement uncertainty of volume wass ±0.01 mL. Pressure was controlled using a highpressure chromatography syringe pump with an uncertainty of

window’s glass and the cavity of the reactor when the ionic liquid was not completely dissolved, and the “light patch” across the cavity of the reactor disappeared as the ionic liquid was completely dissolved. In this work, solubility is defined as the mole fraction of IL in the ternary system (except for Figure 10). PR-BM EOS was

Figure 2. Phase behavior of ethanol/[Bmim]Ac/scCO2 at 40 °C. (a) 0 MPa, two phases: LV; (b) 8 MPa, three phases: LLV; (c) 13.56 MPa, cloud point; and (d) 13.56 MPa, complete dissolution. B



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used to calculate the density of the mixture, which is shown in the Supporting Information. Then the total mass of the ternary system could be calculated because the volume was known already. The mass of the IL and cosolvent put in the visible reactor were known, so the mass of CO2 could be calculated and then the mole fraction of the IL in the ternary system. 2.3. Reliability of Experimental Apparatus. In order to testify the reliability of the experimental apparatus, we repeated part of the data in the literature reported by Wu Wei-ze.20 The results are presented in Table 1 and Figure 4, which show that we can reproduce Wu’s data with good accuracy. Table 1. Solubility of [Bmim][PF6] in scCO2/Ethanol at 40 °C, 12 MPa, Reported in the Literature20 and the Values We Determined in This Workab x1

106 × x2

0 0.085 0.113 0.165 0.225 0.254

0.18 6.26 15 50.7 219

P (MPa)

106 × x2*

Figure 5. Effect of different cosolvents on solubility at 40 °C and a cosolvent concentration of 21 mol %.

AARD (%)

Table 2. Hydrogen Bond Donor/Receptor Number of Cosolvents27 12.05 12.1 12.07 12.11

14.5 50 141 203

3.3 1.4

item

ethanol

acetone

DMSO

acetonitrile

7.3

H-bond donor H-bond receptor

1 1

0 1

0 2

0 1

a

x1 is the mole fraction of ethanol, x2 is the mole fraction of [Bmim][PF6] reported in the literature,20 x2* is the mole fraction of [Bmim][PF6] we measured, P is the pressure, and AARD is the relative error. bRelative standard uncertainties for pressure and solubility are ur(P) = 0.02 and ur(x2*) = 0.01

Figure 6. Effect of concentration of solvents on solubility using ethanol as the cosolvent at 40 °C.

Figure 4. Solubility of [Bmim][PF6] in scCO2/ethanol at 40 °C, 12 MPa, reported in the literature20 and the values we determined in this work.

3. RESULTS AND DISCUSSION 3.1. Effect of Different Cosolvents on Solubility. In order to investigate the influence of different cosolvents on solubility, four organic solvents with different polarities were selected as the cosolvents, including ethanol, acetone, dimethyl sulfoxide (DMSO), and acetonitrile. The integral solubility data measured in the work are presented in the Supporting Information. For example, when the concentration of the cosolvent is 21 mol % at 40 °C, the solubilitiy values are as shown in Figure 5. The solubility is between 1.76 × 10−2 and 2.09 × 10−2 mol % when using ethanol as the cosolvent, followed tightly by DMSO, while the solubility is between 5.00 × 10−3 and 7.00 × 10−3 mol % when using acetone or acetonitrile as the cosolvent. The enhancement of acetone and acetonitrile is basically equal considering the experimental

Figure 7. Densities of corresponding systems using ethanol as the cosolvent at 40 °C.

error. The solubility of the imidazolium-based ionic liquids in scCO2 is lower than 5 × 10−5 mol % without any cosolvent.18 The solubility of [Bmim]Ac in scCO2 without any cosolvent was also determined at 40, 50, and 60 °C and at a pressure up to 14.5 MPa in this work. The results are shown in Table S2 of C



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Figure 8. Solubility isotherm (a) at 25 mol % ethanol, (b) at 21 mol % acetone, (c) at 21 mol % DMSO, and (d) at 29 mol % acetonitrile, respectively.

linearly with the concentration of the cosolvent under the same conditions. The main reason is that more ethanol can form stronger interactions and dissolve more ionic liquid, and ethanol is much more soluble in the scCO2. Another reason is that the density of the scCO2/cosolvent mixture increases with the increasing concentration of the cosolvent, as illustrated in Figure 7. Therefore, the solubility of the ionic liquid in the mixture is obviously increased. The same results can be obtained under other conditions. As shown in Figure 6, the slopes of the solubility curves increased when the concentration of ethanol increased, that is to say, the solubility is more sensitive to pressure when using more ethanol. The densities of the corresponding systems using ethanol as the cosolvent at 40 °C are exhibited in Figure 7. The density of the supercritical mixture reached a relatively high level with little fluctuations when the concentration of ethanol was more than 21.0 mol %, and it was helpful for the dissolution process in this density range, thus higher solubility can be obtained. 3.3. Effect of Temperature on Solubility. Solubilities are determined under three temperatures, 40, 50, and 60 °C, in this article. As was illustrated in Figure 8, the solubility of [Bmim]Ac in the scCO2/cosolvent mixture increased slowly when using ethanol as the cosolvent (Figure 8a), decreased slowly when using acetone or acetonitrile as the cosolvent (Figure 8b,d), and increased first and then decreased when using DMSO as the cosolvent as the temperature increased from 40 to 60 °C (Figure 8c). The influence of the temperature on solubility is more complicated. On one hand, the density of scCO2 decreases with the increasing temperature, as does its ability to dissolve solute.

the Supporting Information, which indicates that the solubility of [Bmim]Ac in scCO2 is lower than 1.16 × 10−4 mol %. The results indicated that the enhancement of the cosolvents on the solubility of the ionic liquid [Bmim]Ac in scCO2 is remarkable, and [Bmim]Ac has the highest solubility using ethanol as the cosolvent. Under the same conditions, the ability of the cosolvents to enhance the solubility of the IL follows the order: ethanol > DMSO > acetone > acetonitrile. Usually, the effect of the cosolvent on the solubility and selectivity of the solute in scCO2 is determined by two aspects: (1) the density of the solvent and (2) the interactions between the solute and solvent. The latter one is the decisive factor under normal conditions, which includes isotropic forces (dispersion force, inducement force, and electrostatic force) and anisotropic forces (hydrogen bond and interaction forces caused by electron transfer). The hydrogen bond donor/ receptor number can reflect the ability of the cosolvents to form hydrogen bonds. As presented in Table 2, the ability of ethanol to form a hydrogen bond is similar to that of DMSO, while the ability of acetone or acetonitrile is much smaller. In this work, the addition of the cosolvent has little effect on the density of the supercritical mixture, so maybe the influence of the cosolvents on the dissolution process is mainly reflected in the interaction between the solvent and solute, namely the hydrogen bond.23−26 The same results can be obtained under other conditions. 3.2. Effect of Concentration of Solvents on Solubility. The concentration of the solvent is defined as the molar fraction of the cosolvent in the ternary system in this work. The solubility data obtained using ethanol as the cosolvent at 40 °C are as shown in Figure 6. The solubility increased almost D



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factor, as mentioned before. Similar results can be obtained under other conditions. 3.5. Correlation of Solubility. The solubility data obtained are very limited but of great importance. Experiments of solubility measurement in supercritical fluids are generally carried out under high pressure, which are difficult to achieve and take a long time. The correlation of the solubility data is of certain significance and can be used to predicate more solubility data within the experimental range. Calculation and formula association are promising supplements and could be used to verify the solubility data. Equation of states, solubility parameter models, and empirical models are commonly used for solubility calculations. But the first two involve some physical parameters of the ILs, which are difficult to be determined accurately, and as a result, the accuracy of these two methods is very low. The empirical models do not need physical parameters of the solute and usually have good correlation accuracy. The Christal equation,28 which is one of the most important empirical models, was used to associate the solubility of the ionic liquid in the scCO2/cosolvent mixture in this article. The Christal equation is based on the theory of the formation of complex compounds, which are generated by the interactions existing between the solute and solvent molecules. When an equilibrium is reached at an ideal state, 1 mol of solute A and n mol of solvent B generate 1 mol of the complex compound ABn: A + n B → ABn Figure 9. Effect of pressure on solubility using DMSO as the cosolvent (a) at 40 and (b) 60 °C.

k=

(1)

[ABn] [A][B]n

(2)

Take a logarithm on both sides: ln[ABn] = ln k + ln[A] + n ln[B]

On the other hand, the saturated vapor pressure increases with the increasing temperature, which is conducive to the diffusion of solute in the solvent. Therefore, the solubility of the ionic liquids in scCO2 is the result of these two aspects. 3.4. Effect of Pressure on Solubility. The solubility data obtained are exhibited in Figure 9 using DMSO as the cosolvent at 40 and 60 °C. The solubility of [Bmim]Ac in the scCO2/DMSO mixture increased with increasing pressure at all studied temperatures and with all studied cosolvents. But the solubility is more sensitive to the pressure when the concentration of DMSO is larger than 20.0 mol %. The reason for this regular change is that the density and solvent properties of scCO2 get closer to the liquid with increasing pressure, and usually the solubility of solute increases linearly with increasing scCO2 density. But the compressibility of scCO2 reduces when getting much closer to the liquid, and the density of scCO2 increases a little with increasing pressure in high pressure areas. The results suggested that the interactions between the IL and cosolvent were the decisive

(3)

[A], [B], and [ABn] represent the molar concentrations of the solute, solvent, and complex compound, respectively. From the Clausius Clabelon equation and Van’t Hoff equation we can get:

ln[A] = ln k =

Δv H + qv RT

(4)

Δv H + qs RT

(5)

ΔvH, ΔsH, qv, and qs are the enthalpy of vaporization, enthalpy of dissolution, and integration constants, respectively. ρ [B] = MB (6) [ABn] =

C MA + nMB

(7)

Table 3. Correlation of Solubility Using the Four-Parameter Christal Equationa

a

cosolvent

k

r

a

b

AARD (%)

R2

ethanol acetone DMSO acetonitrile

3.4404 6.6570 18.4440 10.1460

2.5842 5.8251 2.9377 4.3711

−2068.4462 −4607.9944 −7705.6801 −8149.7388

−30.4642 −62.8316 −119.0011 −65.4443

4.36 6.50 14.35 13.52

0.9940 0.9822 0.9662 0.9696

k, r, a, and b are model parameters; AARD is the relative error; and R2 is the correlation coefficient. E



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Figure 10. Comparison of the Christal correlation and the experimental data (a) at 40 °C using ethanol as the cosolvent, (b) at 60 °C using acetone as the cosolvent, (c) at 50 °C using DMSO as the cosolvent, and (d) at 40 °C using acetonitrile as the cosolvent.

C (g/L) is the solubility of the solute in solvent and ρ is the density of the solvent. MA and MB are the molar mass of the solute and solvent, respectively. Eqs 3, 4, 5, 6, and 7 were integrated to achieve: ln C = k ln ρ +

Three different cosolvent concentrations at the same temperature with the same cosolvent were used as examples, the experimental values and calculated values are as shown in Figure 10. The regression results showed that the four-parameter Christal equation has a good correlation for the solubility of the IL [Bmim]Ac in CO2 considering the addition of different cosolvents. The average absolute relative deviations are in the range of 4.36−14.35%. The maximum correlation accuracy was obtained when using ethanol as the cosolvent, and the minimum value for the system was obtained using DMSO as the cosolvent.

ΔsH + Δ v H + ln(MA + nMB) + qs + qv − n ln MB RT

(8)

a and b are defined in eqs 9 and 10 as: a=

ΔsH + Δ v H ΔH = RT R

b = ln(MA + nMB) + qs + qv − n ln MB

(9) (10)

4. CONCLUSION The results showed that the solubility of the ionic liquid [Bmim]Ac in scCO2 can be remarkably enhanced by adding cosolvents. The ability of different cosolvents to increase the solubility of [Bmim]Ac in scCO2 is in the following order: ethanol > DMSO > acetone > acetonitrile, which mainly depends on the ability of the cosolvents to form a hydrogen bond. The solubility of [Bmim]Ac in the scCO2/cosolvent mixture increased dramatically as the cosolvent concentration exceeded 20 mol %. The temperature has different effects on the solubility for different cosolvents. The solubility of [Bmim] Ac in the scCO2/cosolvent mixture increased slowly when using ethanol as the cosolvent, decreased slowly when using acetone or acetonitrile as the cosolvent, and increased first and then decreased when using DMSO as the cosolvent as the temperature increased from 40 to 60 °C. The solubility of [Bmim]Ac in the scCO2/cosolvent mixture increased as the pressure increased from 8 to 15 MPa, and the solubility was more sensitive to the pressure when using much more cosolvent. The Christal equation with four parameters can be

The Christal equation is as follows: ln C = k ln ρ +

⎛a ⎞ ⎜ + b⎟ ⎝T ⎠

(11)

Usually, cosolvents are added to the supercritical fluid to increase the solubility and selectivity of the solute. Considering the effect of the cosolvent concentration on the solubility, González29 modified the Christal equation as follows: a ln C = k ln ρ + r ln m + +b (12) T C (g/L) is the solubility of the solute; ρ (g/L) is the density of SCF; m (g/L) is the mass concentration of cosolvent; and k, r, a, and b are model parameters. The four-parameter Christal equation was used to correlate the solubility of the ionic liquid in the scCO2/cosolvent mixture in this article. The parameters in the equation, the average absolute relative deviations (AARD), and the correlation coefficients are obtained by nonlinear regression using Origin 9.0. The results are shown as Table 3. F



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used for the correlation of the solubility of [Bmim]Ac in the scCO2/cosolvent mixture, and the average absolute relative deviations for four cosolvents are in the range of 4.36−14.35%.

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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.7b01108. All solubility data determined in this work and PR-BM equation of state for the calculation of the density and solubility data (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jian-Zhong Yin: 0000-0003-4529-3743 Funding

The work was financially supported by the National Natural Science Foundation of China (U1662130, 21506027), Chinese Postdoctoral Science Foundation (2017T100175, 2015M571307), and the Fundamental Research Funds for the Central Universities (DUT17JC34). Notes

The authors declare no competing financial interest.

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REFERENCES

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Solubility of Ionic Liquid [Bmim] Ac in Supercritical CO2 Containing

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