Sifting Ionic Liquids as Additives for Separation of Acetonitrile and

Jun 1, 2012 - The phase behavior of the ternary acetonitrile + water + ionic liquid system was predicted using the COSMO-RS method. The effects of dif...
0 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/IECR

Sifting Ionic Liquids as Additives for Separation of Acetonitrile and Water Azeotropic Mixture Using the COSMO-RS Method Jinlong Li,† Xiaoqian Yang,‡ Kexia Chen,‡ Yeling Zheng,‡ Changjun Peng,*,‡ and Honglai Liu‡ †

Key Laboratory of Advanced Control and Optimization for Chemical Processes, Ministry of Education, and ‡State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, China S Supporting Information *

ABSTRACT: The phase behavior of the ternary acetonitrile + water + ionic liquid system was predicted using the COSMO-RS method. The effects of different ionic liquids on the separation of acetonitrile + water were discussed. It was found that the influence of anions [OAc]− and [Cl]− on the acetonitrile + water mixture is strong, while the cations have fewer effects on the mixture. The interaction energies and mixing enthalpies of binary acetonitrile + ionic liquid and water + ionic liquid systems were predicted indicating that the interaction energy between molecules is stronger in the water + ionic liquid mixture than in the acetonitrile + ionic liquid system. The excess enthalpies of binary mixtures mainly depend on the hydrogen bonds formed between water (or acetonitrile) and ionic liquids. Ionic liquids [EMIM][OAc] and [EMIM]Cl are expected to be favorable solvents and supposed to have practical applications in the separation of acetonitrile from aqueous solution.

1. INTRODUCTION Acetonitrile is an important organic solvent in the chemical industry and is always used in the purification of butadiene and in the manufacture of pharmaceuticals and photographic film.1 Due to its relatively high permittivity, it is also widely used in battery fields2 and in cyclic voltammetry.3 Another popular application is in the field of liquid chromatography4 based on the special properties of low viscosity and low chemical reactivity. Large amounts of acetonitrile of high purity are required to be produced from mixtures, in which the acetonitrile + water5,6 mixture is always encountered. However, the separation of the acetonitrile + water mixture7 is challenging since the two components have similar boiling points and often form azeotropes at a given concentration. Typical solvents used for the separation of the acetonitrile + water mixture at present are those polar substances such as butyl acetate8 and ethylene glycol,9,10 which suffer from high energy consumption and a pollution disadvantage. Thus, it is necessary to look for a suitable environmentally friendly solvent to replace the widely used organic solvents. In the past few years, room temperature ionic liquids11−18 (ILs) that are liquids, entirely consist of ions and are benign solvents have been paid considerable attention for their potential application as entrainers for the separation of azeotropic mixtures, such as ethanol + water mixtures,19,20 because of their low vapor pressure, chemical stability at high temperatures, and excellent solubility for organic and inorganic compounds.21 It has been found through experimental measurement that both [EMIM][BF4] and [EMIM][NO3] can break the azeotrope and enhance the relative volatility of acetonitrile in the acetonitrile + water system.6 Similarly, [N3333][Br] has also been used to improve the vapor−liquid equilibrium (VLE) of the acetonitrile + water system.5,22 Recently, Pereiro et al.23 reviewed the methods using ILs as azeotrope breakers in three typical separation processes, liquid−liquid extraction, extractive distillation, and supported © 2012 American Chemical Society

liquid membranes, indicating that ILs can be considered as good alternatives to conventional organic solvents to separate azeotropic mixtures. However, experiments to explore the influence of different ILs on different azeotropic mixtures are retarded because of too many choices of ILs based on the different combinations of cations and anions. A suitable prediction method through computer calculation to save experimental efforts, time, and cost of materials is then highly expected. A recent, refreshing approach called COSMORS24−27 (COnduct-like Screening Model for Real Solvent) proposed by Klamt and co-workers based on ab initio calculation might serve as an effective candidate. In the COSMO-RS method, the molecule is considered to be a collection of separated surface segments with different surface charge densities. The chemical potential of any solute in solvent as a function of temperature and concentration can be obtained via integrating the potential of the solvent segment over the surface of the solute, which gives access to the prediction of many bulk thermodynamic properties such as phase behaviors, free energies, and heats. The COSMO-RS theory has been embedded in the commercial computer package COSMOthermX,27 which can give predictions of activity coefficients, phase equilibria, vapor pressures, heats of vaporization, Henry constants, partition coefficients, heats of mixing (excess enthalpies and free enthalpies), reaction thermodynamics, chemical potentials, densities, viscosities of pure compounds, and so on for organic solutions, ionic solutions, polymers, drug molecules, and other more types of solutions. So far, the COSMO-RS approach has been applied to the molecular design of ILs based on their specific properties involving densities,28 vapor pressures,29 activity coefficients,30−33 excess Received: Revised: Accepted: Published: 9376

January 11, 2012 May 8, 2012 June 1, 2012 June 1, 2012 dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

properties,34,35 and phase equilibria (VLEs,36−41 liquid−liquid equilibria (LLEs),37,40 solid−liquid equilibria (SLEs)42). Many VLEs containing ILs predicted by COSMO-RS method has been reported in the literature, but they are mostly limited to binary mixtures. For instance, Banerjee et al.36 predicted VLEs for 116 non-IL binary sets in which 33 mixtures are azeotropic, with the parametrizations using 10 associated and 22 binary nonassociated non-IL systems, and with the effective contact surface areas and the hydrogen-bonding coefficients estimated using a sequential scheme. Diedenhofen et al.30 calculated the activity coefficients at infinite dilutions for 38 compounds in the ILs [BMPY][BF4], [EMIM][NTf2], and [EM2IM][NTf2] solvents, respectively. Freire et al.37 evaluated binary LLEs and VLEs for mixtures of alcohols with several imidazoliumand pyridinium-based ILs. In addition, the LLEs and VLEs of various binary mixtures of water and ILs have been evaluated in Freire’s subsequent published work.40 Recently, Li and Paricaud43 extended COSMO models to predicte partition coefficients, VLEs, and LLEs for biofuel-related mixtures. For more applications of COSMO-RS, the reader is directed to some detailed reviews.44−46 However, so far, few predictions for the phase behaviors of ternary mixtures containing ILs have been made in the past few years. In this work, the COSMO-RS model was used to predict the VLEs of ternary acetonitrile + water + IL mixtures, which gives a guide for the selection of environmentally friendly solvent, ILs, for the separation of the azeotropic mixture composed of acetonitrile and water. The effects of different types of ILs with different anions and cations, the length of the branched chain in cations, and the interaction energy between different functional groups on acetonitrile and water mixing properties were tested and checked. Also, the excess enthalpies of binary mixtures of acetonitrile + ILs and water + ILs have been calculated and discussed.

3. RESULTS AND DISCUSSION In this work, we mainly aim at sifting ILs as additives for the separation of the acetonitrile + water azeotropic mixture based on the a priori COSMO-RS method. First, it is necessary to check the feasibility of the COSMO-RS model for the prediction of the phase behavior of mixtures consisting of water, acetonitrile, and ILs, or both. For the phase equilibrium, only the VLE and relative volatility properties were considered here. As is well-known, VLE needs the temperature, pressure, and chemical potential of component i in both equilibrium phases to be equal. Based on the activity coefficient method, the equilibrium condition for VLE should satisfy pyi = pi s xiγi

(i = 1, ..., N )

(1)

pis

where p is the total pressure of the system and is the saturation vapor pressure of pure component i. γi is the activity coefficient of component i in the liquid phase; xi and yi are mole fractions of component i in the liquid and gas phases, respectively. Note that the vapor phase is considered to be an ideal mixture and the changing of liquid volume with pressure is neglected as the investigated systems here are at low pressure. The relative volatility is defined by the following expression:

a12 =

y1 /y2 x1′/x 2′

(2)

where x1′ and x2′ are the mole fractions of acetonitrile and water based on IL-free solution in the liquid phase and y1 and y2 are the mole fractions of acetonitrile and water in the vapor phase at equilibrium, respectively. The VLE of the acetonitrile (1) and water (2) mixture without ILs over a full concentration range was first predicted based on COSMO-RS, as shown in Figure 1, where the top and bottom panels are x−y and x−T diagrams, respectively. Acetonitrile forms an azeotrope with water, and the minimum azeotropic point reaches 76 °C when the mole fraction of acetonitrile is 0.713.50 The predicted results including the azeotropic point are in good agreement with experiments, suggesting that the COSMO-RS can well be used for the azeotropic mixture of acetonitrile and water. A typical VLE for a ternary mixture consisting of acetonitrile (1), water (2), and [EMIM][NO3] (3) at x3 = 0.2 and p = 100 kPa was predicted and illustrated in Figure 2, in which x1′ is the mole fraction of acetonitrile on an IL-free basis. The x−y curve for the mixture without IL is also given in Figure 2. As shown in Figure 2, the prediction for the ternary mixture is not accurate enough: the predicted azeotropic point does move toward a favorable direction, but the experiment indicates an elimination of azeotrope. However, it is supposed that the COSMO-RS model can be used to find out the right tendency of ILs on the improvement of the azeotropic behavior of the acetonitrile + water system, as Freire et al.40 gave a right tendency for the prediction of the phase behavior of the water + IL system. Sections 3.1−3.4 will give a detailed discussion about the effects of different types of ILs, interaction energies between molecules, and mixing properties on the phase behavior of the acetonitrile and water mixture. As is well-known, ILs can be freely tailed by different cations and anions for many special requirements in practice. Therefore, we supposed that 182 different ILs could be combined from 14 kinds of cations and 13 types of anions and were used for this investigation, although not all of them are ILs. The detailed structures of the

2. COMPUTATION DETAILS The main working equations of the COSMO-RS model are not repeated, and only some calculation principles are provided in this work. The reader is directed to some related literatures24−27 for the detailed description of this model. The standard calculation procedure of the COSMO-RS method essentially consists of two steps: one is the quantum chemical COSMO calculations for the molecules and the other is COSMO statistical calculations performed with the COSMOtherm package. In the former, the geometry of the molecular structure has to be optimized based on ab initio calculations with the Gaussian 03,47 Turbomole,48 or DMol349 software package. In this work, the optimized geometry was obtained using the Gaussian 03 package with the BP86/TZVP quantum chemical level, which is the same as the one used in COSMORS calcualtions. In calculations, an ionic liquid was considered to be a mixture consisting of equimolar cations and anions, and only the geometry of one ion (cation or anion) can be optimized once. In the latter step, the COSMO file produced from ab initio calculation was considered as the only inputs and put into the COSMOtherm package (COSMOtherm-C210110); the many other generalized parameters included in COSMO-based models have been determined in advance by adjusting a large set of experimental VLE and/or LLE data.24,26 9377

dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

cations and anions including imidazole ions, pyridinium ions, methyl sulfate, tetrafluoroborate, etc. are listed in Table S1 in the Supporting Information. To check the effect of a solute on the VLE of a mixture, the solvent capacity between the solute and the solvent is always considered as a useful method and it can usually be represented by the reciprocal of the activity coefficient at infinite dilution.51 The solvent capacity SF of water in an IL in this work is written as SF =

1 γ2∞

(3)

Here γ2∞ is the activity coefficient at infinite dilution of water in an IL. The theoretical SF values of water in 182 different ILs at atmospheric temperature were obtained and are listed in Table S2 in the Supporting Information. As shown in Table S2 in the Supporting Information, the theoretical SF values for systems where ILs are formed by [PF6]− and [NTf2]− anions and a variety of cations are far less than 1, indicating that ILs containing [PF6]− and [NTf2]− ions have worse solubility capacities, while the others have comparatively better solubility capacities with water at the given conditions. It is assumed that ILs having worse mutual solubilities with water cannot be well applied to improve the VLE of the acetonitrile and water mixture.52 Therefore, all attention is paid to those ILs having good solubilities with water in the following discussion. 3.1. Influence of Anion Class. Figure 3 compares the relative volatilities at x3 = 0.1 and x1′ = 0.7 in acetonitrile (1) +

Figure 1. Comparison of VLE for acetonitrile (1) + water (2) at 101.32 kPa between experimental (symbols)25 and COSMO-RS predicted (lines) results.

Figure 3. Relative volatilities predicted for acetonitrile (1) + water (2) + ionic liquid (3) (cations fixed with [EMIM]+, [BUPY]+, or [N3333]+) when x1′ = 0.7 and xIL = 0.1 at 100 kPa.

water (2) + IL (3) systems, in which different types of ILs consist of [EMIM]+, [BUPY]+, or [N3333]+ cations and various anions, respectively. One can see that anions have a significant influence on the relative volatilities of acetonitrile and water in mixtures when the mole fraction of acetonitrile is 0.7, very close to the azeotropic composition. A significant increase of the relative volatility of acetonitrile near the azeotropic point illuminates that ILs with anions [OAc]− and Cl− could effectively improve the VLE of the acetonitrile + water system, while the improvement of the VLE by anions like [BF4]− are weak since the corresponding relative volatility is not obviously

Figure 2. Comparison of VLE for acetonitrile (1) + water (2) + [EMIM][NO3] (3) at 100 kPa and xIL = 0 or 0.2 between experimental (symbols)22 and the COSMO-RS predicted (lines) results.

9378

dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

improved. On the other hand, one can see from Figure 3 that the predicted relative volatilities for some systems from the theoretical model are less than 1 since the COSMO-RS method underestimated the experimental results in this work, which is verified in Figure 4. As illustrated in Figure 4, the experimental

Figure 5. σ-Profiles for acetonitrile, water, and different anions.

water has peaks at σ < −0.0082 e/Å2 and the range of the polarization charge density is up to about −0.02 e/Å2, owing to very strong hydrogen bond donator abilities, so that it can combine well with those anions with the hydrogen bond acceptor abilities through hydrogen bonding. Acetonitrile is characterized by hydrogen acceptor abilities due to its nitrile group and cannot form a stable complex with anions. Thus, the fact that the ILs containing anions such as [OAc]− and Cl− can well improve the VLE of the water and acetonitrile mixture lies in their abilities to form hydrogen bonds with water and break the interactions between acetonitrile and water. Furthermore, the improvement of the VLE for the investigated case through ILs containing [OAc]− and Cl− anions have been verified through experimental measurement53 as well, as shown in Figure 6. In Figure 6, the experimental data for the two ternary mixtures acetonitrile (1) + water (2) + [Bmim][OAc] (3) and acetonitrile (1) + water (2) + [Bmim][Cl](3) were measured at 101.3 kPa and xIL = 0.05 (mole fraction) with the isobaric method (Ellis still). The COSMO predicted results for the two mixture are also depicted in Figure 6. One can see that both

Figure 4. Comparison of theoretical (lines) and experimental (symbols) relative volativities for acetonitrile (1) + water (2) + ionic liquid (3) systems at 100 kPa and xIL = 0.2.

measurements show that both [EMIM][BF4] and [EMIM][NO3] can improve the relative volatility of the acetonitrile + water system over the entire concentration range, while the theoretical predictions can only provide the right trends. For the systems investigated in this work, the intermolecular interactions between molecules mainly consist of electrostatic interaction, hydrogen bonding, and van der Waals forces, which will be proved via computer simulation in Section 3.4. Being the only description in COSMO-RS, the σ-profile used to characterize the local polarity of the molecular surface determines the attractive interaction energy. Thus, from the σ-profile of different molecules, one can know that the interactions between different components are strong or weak. From the above discussion, we know that the anions [OAc]− and Cl− can effectively improve the relative volatilities of water and acetonitrile and their σ-profiles are illustrated in Figure 5, in which σ-profiles of the anions [BF4]− and [NO3]−, water, and acetonitrile are also given to show a clear comparison. Note that the two vertical dashed lines in Figure 5 are the cutoff values for the hydrogen bond donor (σ < −0.0082 e/Å2) and acceptor (σ > 0.0082 e/Å2).36 If the profile lies in the left side of −0.0082 e/Å2, it is suggested that the surface segment will have hydrogen bond donator ability, while it will have acceptor ability if it is in the right side of 0.0082 e/ Å2. Also, the further the peak of the profile is from the absolute value 0.0082 e/Å2 on the left or the right, the stronger the hydrogen bond donator or acceptor ability is, respectively. From Figure 5, one can see that both selected anions [OAc]− and Cl− of ILs have peaks in the region of σ > 0.0082 e/Å2 and far from 0.0082 e/Å2, indicating that these anions have strong hydrogen bond acceptor abilities, while the anions [BF4]− and [NO3]− have weak hydrogen bond acceptor ability compared to the former. In addition, the hydrogen bond acceptor ability of the [NO3]− ion is stronger than the one of [BF4]− since the peak of the profile for the former is on the right of the latter and the polarization charge density of the former ranges more widely (up to about 0.022 e/Å2) (see Figure 5). Meanwhile,

Figure 6. Comparison of VLEs between experiments53 and COSMO predictions for acetonitrile (1) + water (2) + [Bmim][OAc] (3) and acetonitrile (1) + water (2) + [Bmim][Cl] (3) at 101.3 kPa and xIL = 0.05. All experimental data were measured with the Ellis method. 9379

dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

the length of the alkyl chain in cations decreases the relative volatilities of acetonitrile and water, and an imidazolium-based IL with a short carbon chain has a more active role in the separation of the acetonitrile + water mixture. In Figure 9, the

[Bmim][OAc] and [Bmim][Cl] can well eliminate the azeotropic point of the acetonitrile + water mixture, and the former is better than the latter due to their different hydrogen bond acceptor abilities as shown in Figure 5. However, the COSMO-RS model cannot yet provide accurate predictions for the two systems and can only give the right trends in improving their VLEs. 3.2. Influence of Cation Class. Compared with the remarkable effects of anions on the acetonitrile + water system, the cations have far less influence on the investigated mixture, as shown in Figure 7, in which the relative volatilities of

Figure 9. σ-Profiles for acetonitrile, water, and cations ([BMIM]+, [HMIM]+, and [BEIM]+).

σ-profiles of different imidazole ILs with various alkyl chain lengths in the cations are depicted and one can see that these types of cations have small peaks in the region of σ < −0.0082 e/Å2, indicating their weak capability in donating hydrogen bonds. Meanwhile, those cations with shorter alkyl chains show stronger donation of hydrogen bonding compared to those with longer lengths of alkyl groups, as observed by Freire et al.37 Note that [BMIM]1+ and [BMIM]2+ in Figure 9 are the isomers of cation [BMIM]+ and the small differences between them in stable structure and lowest energy lead to slightly different σ-profiles.37,54 The differences in isomers for other cations was neglected in this work. From the above discussions, one can see that the anions [OAc]− and Cl− and the cation [EMIM]+ have good performances in the separation of acetonitrile−water, respectively. Naturally, one would be concerned with their real performances in ternary VLEs. Figure 10 demonstrates the VLEs of acetonitrile + water + [EMIM][OAc] and acetonitrile + water + [EMIM]Cl ternaries in the whole concentration range when the mole fractions of ILs in the mixture are 0.1, 0.2, and 0.3, respectively. One can see that the azeotropic point of the acetonitrile and water system can well be eliminated when the mole fraction of [EMIM][OAc] is 0.1. Similarly, when the concentration of [EMIM]Cl in the mixture is 0.2 or 0.3, the azeotropic point can also be eliminated. The obtained results suggest that the ILs [EMIM][OAc] and [EMIM]Cl could be expected to have practical application in the separation of acetonitrile−water. 3.3. Effect of Interaction Energy. The intermolecular energy can be defined as the energy difference between a real system and a pseudosystem with all the molecules isolated which gives a description of the interaction between molecules. Similarly, the interaction energy of ILs usually referring to the energy difference between an ion pair and isolated ions reflects to some extent the strength of the interaction between ions. On the other hand, the interaction energy could provide information on selectivity. López-Pastor et al.55 studied water and methanol associations in ILs and showed that most of the

Figure 7. Relative volatilities predicted for acetonitrile (1) + water (2) + ionic liquid (3) (anions fixed with [OAc]−, Cl−, or [BF4]−) when x1′ = 0.7 and x3 = 0.1 at 100 kPa.

different ILs composed of the anions [OAc]−, Cl−, and [BF4]− and various cations are illustrated. The difference in the relative volatilities between different types of cations generally becomes smaller. For imidazole group based ILs, the alkyl chain length has an effect on relative volatilities as illustrated in Figure 8. Increasing

Figure 8. Relative volatilities predicted for acetonitrile (1) + water (2) + ionic liquid (3) ([EMIM][OAc], [BMIM][OAc], or [HMIM][OAc]) at 100 kPa and xIL = 0.1. 9380

dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

where IE is the interaction energy in kilojoules per mole and E is the total energy of mixture in kilojoules per mole. The interactions between water molecules and ILs containing the imidazolium cation and the anions Cl−, Br−, [BF4]−, and [PF6]− were investigated by Wang61 with quantum chemical calculations. The predicted geometries and interaction energies imply that the water molecules can interact with the anions Cl−, Br−, and [BF4]− and form complexes. In addition, the cations could form strong interactions with the water molecules as well. In this work, the interaction energies of acetonitrile−water and ILs are further discussed. The optimized geometry with the minimum energy of [EMIM]Cl is taken as an example here due to its good capability of eliminating azeotropy. Its detailed structure is drawn in Figure 11a. Hydrogen atoms located around the anion

Figure 10. VLEs predicted for acetonitrile (1) + water (2) + ionic liquid (3) at 100 kPa. (a) [EMIM][OAc]; (b) [EMIM]Cl. Figure 11. (a) Optimized geometries of [EMIM]Cl, (b) [EMIM]Cl− water, and (c) [EMIM]Cl−acetonitrile.

water and methanol molecules tend to be isolated from each other and to interact with the anion of the IL via H-bonding. Hong et al.56 found that the strong interactions between [EMIM][NTf2] and CH3CN probably belong to the ion-dipole type. Asaki et al.57 obtained the frequency dependent complex dielectric functions for pure acetonitrile, pure [EMIM][TfO], and mixtures of the two liquids, and found that the interaction of [EMIM]+ is stronger than that of the anion with acetonitrile. To calculate the interaction energies between acetonitrile− water and ILs, the stable structures of ILs with minimum energy were first obtained. The minimum energy was found through the Gaussian 03 package based on the BP86 functional and the TZVP basis set; then vibrational analysis58 (frequency calculations) was used to determine the nature of stationary points searched by geometry optimization. The interaction energies of acetonitrile + IL and water + IL can be calculated by the following expressions:59,60 IE H2O − IL = E H2O + IL − E H2O − E IL

(4)

IECH3CN − IL = ECH3CN + IL − ECH3CN − E IL

(5)

and imidazole ring hold the highest charges of [EMIM]+ to obtain high polarity and provide the possibility of forming stable hydrogen bonds as mentioned earlier. Chlorine anions are likely to form a hydrogen bond with acetonitrile−water; as a result, acetonitrile or water molecules would be close to the chlorine anion and imidazole ring in the favorable conformation, which can be illustrated by Figure 11b,c. Typically, the maximum distance of hydrogen bonds between atoms should be lower than the van der Waals radii of two atoms, and the values of the van der Waals radii of atoms are listed in Table 1.62 For example, the van der Waals radii of H atoms and Cl atoms are respectively 1.20 and 1.52 Å; the maximum value of the H···Cl bond length is then 2.72 Å. Table 1. van der Waals Radii of Atoms atom vdW radius /Å 9381

H 1.20

C 1.70

N 1.55

O 1.52

Cl 1.52

Br 1.85

F 1.47

dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

It can be found from Figure 11b,c that both acetonitrile and water molecules can form hydrogen bonds with [EMIM]Cl. If the anion Cl− is replaced by other anions, the hydrogen bonds between acetonitrile−water and anions and, furthermore, the global structure of complex would be modified due to the strong interactions between them. However, no obvious difference in improving the acetonitrile−water selectivity was observed when different ILs were used as entrainers, in which only the cations in the ILs were different (refer to Figure 7), although the different cations in ILs might induce small changes in the global structure of the complex. Meanwhile, the alkyl chain length in cations has little effect on acetonitrile−water selectivity, partly because the hydrogen bond forms mainly in the place closing the imidazole ring and altering the cation alkyl chain length does not work. Therefore, it can be concluded that the anions play an even greater role in acetonitrile−water separation than cations. The interaction energies of acetonitrile−water with three different ILs are shown in Table 2. The interaction energies of

Figure 12. Excess enthalpies predicted for water (1) + ionic liquid (2) and acetonitrile (1) + ionic liquid (2) mixtures at 298.15 K.

Table 2. Interaction Energies of Acetonitrile−IL and Water− IL [EMIM][OAc] [EMIM][Cl] [EMIM][DCA]

acetonitrile/kJ·mol−1

water/kJ·mol−1

−34.08 −43.34 −38.81

−69.17 −65.66 −61.89

[EMIM]Cl−water, [EMIM]Cl−acetonitrile, and water−acetonitrile are −65.66, −43.34, and −23.27 kJ·mol−1, respectively. The interaction energies of [EMIM]Cl with water and acetonitrile are all sufficient for separating acetonitrile from water. The interaction energies of water with ILs are all greater than those of acetonitrile with ILs, indicating strong attraction between water and ILs, which could reduce the water activity coefficient and increase the acetonitrile−water relative volatility. 3.4. Effect of Mixing Enthalpy. An endothermic or exothermic process frequently appears when the pure components are mixed to form a solution at a certain temperature and pressure. Mixing enthalpy is an important thermodynamic property of a solution and a good measurement of nonideal characteristics of a solution. Considerable progress has been made in predicting VLEs with mixing enthalpies, and the mixing enthalpies for binary systems have also been verified with COSMOtherm software.34 The COSMO-RS method was employed to study the mixing enthalpies of water + ILs and other mixtures as well in the current work. Figure 12 shows the mixing enthalpies of water + [EMIM][OAc], water + [EMIM]Cl, acetonitrile + [EMIM][OAc], and acetonitrile + [EMIM]Cl. As is well-known, the mixing is exothermic if the mixing enthalpy is negative, and vice versa. The mixing enthalpies of water + [EMIM][OAc] and water + [EMIM]Cl are negative, indicating strong heat production when water is mixed with ILs and illustrating the strong interaction between them. This is in accordance with water−IL interaction energy calculated above. In addition, the mixing enthalpy curve of water + [EMIM][OAc] is located below the curve of water + [EMIM]Cl, showing stronger interactions between [EMIM][OAc] and water. Figure 13 illustrates an example where the IL [HMIM][DCA] has a poor impact on acetonitrile−water separation. The mixing of water and [HMIM][DCA] is exothermic, while that of acetonitrile and [HMIM][DCA] is endothermic.

Figure 13. Excess enthalpies of water (1) + [HMIM][DCA] (2) and acetonitrile (1) + [HMIM][DCA] (2) systems at 298.15 K.

Suppose the molar concentration of [HMIM][DCA] is 0.1, the mixing enthalpy of water + [HMIM][DCA] is −1409.51 kJ·mol−1, and that of acetonitrile + [HMIM][DCA] is 333.98 kJ·mol−1. Comparing with Figure 12, from the energy change, [EMIM][OAc] and [EMIM]Cl may have better separation impacts than [HMIM][DCA]. The estimations of mixing enthalpy in the COSMO-RS model arise from summing the three contributions associated with interactions: polar misfit HE(misfit), hydrogen bonds HE(H-bond), and van der Waals forces HE(vdW). HE = HE(misfit) + H E(H‐bond) + HE(vdW)

(6)

The mixing enthalpies including HE(misfit), HE(H-bond), and HE(vdW) of water + IL and acetonitrile + IL are severally depicted in Figure 14. Hydrogen bond interaction is the main part of the mixing enthalpy in both exothermic and endothermic processes. The second contribution to the mixing enthalpy is electrostatic interaction HE(misfit) between molecules, whereas the van der Waals forces HE(vdW) hardly affect the mixing enthalpy. 9382

dx.doi.org/10.1021/ie3000985 | Ind. Eng. Chem. Res. 2012, 51, 9376−9385

Industrial & Engineering Chemistry Research

Article

would have significant application in separation of the acetonitrile + water system.

■

ASSOCIATED CONTENT

S Supporting Information *

Detailed structures of 14 kinds of cations and 13 types of anions and the theoretical solvent capacity parameters for 182 ion combinations at atmospheric temperature. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: 86-21-6425 2767. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Basic Research Program of China (Nos. 2012CB720500 and 2009CB219902), the National Natural Science Foundation of China (No. 21136004), and the 111 Project (Grant B08021) of China.

■

Figure 14. Excess enthalpies of water (1) + ionic liquid (2) and acetonitrile (1) + ionic liquid (2) systems at x2 = 0.1 and T = 298.15 K. (a) Water + ionic liquids; (b) acetonitrile + ionic liquids.

■

4. CONCLUSIONS The COSMO-RS model embodied in the COSMOtherm package was employed to predict the ternary phase behavior of acetonitrile + water + IL. The effects of anions and cations on the VLE were investigated in detail. The anions [OAc]− and Cl− can effectively improve the VLE of the acetonitrile + water system to eliminate the azeotrope, while the cations have less influence on the vapor−liquid phase diagram. The common imidazolium-based ILs can easily extract acetonitrile from aqueous solution, and both [EMIM][OAc] and [EMIM]Cl are expected to have better development in practical application. The obtained interaction energy between [EMIM][Cl] and one of acetonitrile and water show that ILs can form hydrogen bonds with acetonitrile−water, and the predicted mixing enthalpies of binary water + IL and acetonitrile + IL systems suggest that the enthalpy from hydrogen bond interaction plays a decisive role in the mixing enthalpy. In addition, the hydrogen bond interaction between water and ILs is larger than that of acetonitrile and ILs, indicating that the interaction energies between ILs and water are enough to break the complex of acetonitrile + water. In general, ILs regarded as “green solvents”

NOMENCLATURE p = total pressure of the system, kPa pis = saturation vapor pressure of pure component i, kPa γi = activity coefficient of component i xi = mole fraction of component i in liquid phase yi = mole fraction of component i in gas phase xi′ = mole fraction of component i on basis of IL-free solution α12 = relative volatility SF = solvent capacity of water in IL γi∞ = activity coefficients at infinite dilution σ = segment charge density, e/Å2 IE = interaction energy of water/acetonitrile with IL, kJ·mol−1 E = total energy of mixture, kJ·mol−1 HE = mixing enthalpy, kJ·mol−1 REFERENCES

(1) Srinivasu, M. K.; Raju, A. N.; Reddy, G. O. Determination of lovastatin and simvastatin in pharmaceutical dosage forms by MEKC. J. Pharm. Biomed. Anal. 2002, 29, 715−721. (2) Imanishi, N.; Fujiyoshi, M.; Takeda, Y.; Yamamoto, O.; Tabuchi, M. Preparation and 7Li-NMR study of chemically delithiated Li1−xCoO2 (0