Study of Ionic Liquids as Entrainers for the Separation of Methyl Acetate Methanol and Ethyl Acetate Ethanol Systems Using the COSMO-RS Model

Transcription

1 pubs.acs.org/iecr Study of Ionic Liquids as Entrainers for the Separation of Methyl Acetate Methanol and Ethyl Acetate Ethanol Systems Using the COSMO-RS Model J. Dhanalakshmi, P. S. T. Sai, and A. R. Balakrishnan* Department of Chemical Engineering, Indian Institute of Technology Madras, Chennai , India *S Supporting Information ABSTRACT: The separation of the nonaqueous azeotropic systems methyl acetate methanol and ethyl acetate ethanol was investigated with 13 cations and 27 anions as entrainers to identify suitable ionic liquids using the COSMO-RS model. It is observed that the cations best suited are imidazolium > pyridinium > methyl pyrrolidinium > octyl quinolinium. The lengthening of the alkyl chain of cations was also studied as well as whether the shorter the alkyl chain, the better is the separation. It was also noticed that the efficacy of the anions is in the order [OAc] > [Cl] > [DHP]. It was seen that the influence of anions is more significant on the behavior of ionic liquid solvation in alcohols. The effects of polarity, excess free energy, and excess enthalpy are addressed on nonaqueous systems containing ionic liquids. It is suggested that [EMIM][OAc], [EMIM][Cl], and [EMIM][DHP] can be used as promising entrainers for practical applications of ester alcohol separation. 1. INTRODUCTION Separation of azeotropic or close boiling mixtures by extractive distillation is a widely used technique. In extractive distillation, the addition of a solvent alters the relative volatility of the components in the mixture by increasing their activity coefficients of the mixture. 1,2 Therefore, the selection of the solvent plays a major role in extractive distillation. 3 Conventional solvents require high energy consumption for regeneration and reuse. Corrosion and handling problems are difficult to handle. Additionally, they lead to severe environmental pollution. Thus, the replacement of conventional solvents is achieved by using environmentally friendly green solvents, called ionic liquids (IL), which are a combination of organic/ inorganic cation and inorganic anion. 4 Extractive distillation with ionic liquid as a separating agent integrates the advantages of liquid solvent (easy operation) and solid salt (high separation ability). It can prevent the limitations of solid salts such as insufficient dissolution and handling in the extractive distillation process. Ionic liquids are molten salts or molten oxides with melting points below 100 C. 5 They are often in the liquid state at ambient or even lower temperatures and are thus referred to as room-temperature ILs (RTILs). The increasing attention paid to ILs from both an industrial and academic viewpoint is due to their unique properties such as negligible vapor pressures and thus being nonflammable, high thermal (up to 200 C) and chemical stability, high electrical conductivity, a large electrochemical window and wide range of liquid state, high dissolving ability in polar and nonpolar, organic, inorganic, and organo-metallic compounds. The applications of ionic liquids in clean synthesis and catalysis, 6 analytical techniques, 7 electrochemistry, and thermodynamics of nonaqueous mixtures 8 have been reported in the literature. In addition to this, potential uses of ILs as lubricants in hightemperature and/or low-pressure applications, as heat transfer fluids, and as cleaning solvents in batch processing equipment have also been observed. 9 A review of applications of ILs on separation processes such as liquid liquid extraction, 10 aromatic and aliphatic hydrocarbon removal, 11 protein extraction, 12 and extractive distillation 13 have also been reported. In recent years, ionic liquids (ILs) have attracted considerable attention for their potential use as entrainers for the separation of azeotropic or close boiling mixtures 14 such as alcohol water 15,16 and tetrahydrofuran (THF) water 17 etc. In the present work, ester alcoholic systems of methyl acetate methanol and ethyl acetate ethanol containing ionic liquids were studied. Many investigators have reported experimental measurements of ternary VLE of ester alcoholic systems using ionic liquids as promising candidate entrainers. Orchilles et al. 18,19 measured isobaric vapor liquid equilibria for methyl acetate methanol and ethyl acetate ethanol systems using 1- ethyl-3-methylimidazolium trifluoromethanesulfonate [EMIM]- [Triflate] at 100 kpa. Isobaric vapor liquid equilibrium data for ethyl acetate ethanol containing ionic liquid 1-ethyl-3- methylimdazolium tetrafluoroborate [EMIM][BF 4 ] at atmospheric pressure ( kpa) were measured with a modified Othmer still by Li et al. 20 They also studied the vapor liquid equilibrium of ethyl acetate ethanol using 1-alkyl-3-methylimdazolium tetrafluoroborate at kpa. 21 Cai et al. 22 measured vapor liquid equilibrium for the ternary system of methyl acetate methanol 1-ethyl-3-methylimdazolium acetate [EMIM][OAc]. They 23 further reported vapor liquid equilibrium data for a methanol methyl acetate system using 1- octyl-3-methylimdazoilum hexafluorophosphate [OMIM][PF 6 ] at kpa. Furthermore, experimental vapor liquid equilibrium studies of the ionic liquid 1-ethyl-3-methylimdazolium acetate [EMIM][OAc] for the separation of an ethyl Received: August 30, 2013 Revised: October 23, 2013 Accepted: October 24, 2013 Published: October 24, American Chemical Society 16396

2 acetate ethanol system was reported by Li et al. 24 In the present work, methyl acetate methanol and ethyl acetate ethanol systems were chosen since these are important solvents in industry. These nonaqueous azeotropic mixtures are used in the poly vinyl alcohol manufacturing process 22,23 and in the process of esertification, 24 among others. Furthermore, ethyl acetate is a basic chemical used in industrial processes such as organic intermediates in pharmaceutical processes; in the essence industry as a solvent; and rayon, paint, and printing ink manufacture. It is most important to choose the best suitable entrainer from the numerous combinations of cations and anions of ionic liquids for the specific application for the separation of azeotropic mixtures. The structural variation of the cations and anions influences the phase behavior of azeotropic mixtures and can be predicted by using conventional activity coefficient models such as NRTL, UNIQUAC, or UNIFAC. 25 These conventional models require the experimental ternary vapor liquid equilibrium data of azeotropic mixtures containing ionic liquids, to estimate the interaction parameters of the models. 26 The UNIFAC model can be used to predict the activity coefficient at infinite dilution for polar systems. 26 The deviation of UNIFAC predictions of relative volatility with experimental data was 9.85% (average relative deviation) for the ethanol water system. Lei et al. 26 further showed that the UNIFAC model overestimated and the COSMO-RS model underestimated the experimental data for the 1-hexene n-hexane system. Lei et al. 26 further stated that both the COSMO-RS model and UNIFAC model agree with the experimental data for nonpolar systems, but for polar nonaqueous systems, the COSMO-RS model may be worse than the UNIFAC model quantitatively. However, the effect of structural variations of ionic liquids on polar systems can be studied qualitatively for rapid screening of potential ionic liquids using the COSMO-RS model. The conductor-like models for real solvents, such as the COSMO-RS model originally developed by Klamt, 27 provide an a priori predictive model to calculate liquid liquid and vapor liquid equilibria based solely on quantum chemical calculations of the involved chemical compounds. 28 It is an efficient method for the prediction of thermophysical data of liquids and an alternative to group contribution methods (UNIQUAC and UNIFAC) and does not make extensive use of experimental data as the GCMs. 29 It deals with the description of the interactions of the molecules in solution and makes it relatively easy to tune the cations and anions for the chemical properties required of the ionic liquids. COSMO-RS is a two step procedure. 30 In the first step, quantum chemical calculations are performed for all the compounds of interest. In the second step of COSMO-RS, the statistical thermodynamics of the molecular interactions, i.e., electrostatics and hydrogen bonding, are considered. COSMO-RS models have been used to predict vapor liquid equilibria for binary and ternary systems. 31,32 The prediction of thermophysical and thermodynamic properties such as mutual solubilities, melting points, densities, viscosities, activity coefficients, and excess enthalpies of ionic liquid mixtures with water and hydrocarbons and pure compound vapor pressures have been performed using the COSMO-RS method. 37 A review of the applications of the COSMO-RS model in ionic liquid property predictions for vapor liquid equilibria, liquid liquid equilibria, gas solubilities, and Henry s law constants have been documented. 38 The COSMO-RS model is an excellent method for the evaluation of liquid liquid and vapor liquid equilibria of binary mixtures of water and ionic liquids 39 and of alcohols and ionic liquids. 40 Banerjee et al. 41 have used the COSMO-RS model to predict vapor liquid equilibria for 116 non-il binary sets, out of which 33 were azeotropic systems. COSMO-RS is widely used for solubility and solvent evaluation of ionic liquids in vapor liquid equilibria, 42,43 liquid liquid equilibria, 44 and hydrodesulfurization and denitrification of diesel oil. 45 Recently, the conceptual design of ionic liquids and the selection of a solvent and its optimization criteria have been documented by Jork et al., 46 especially for the aqueous azeotropic (THF+water) separation by varying the structure of the ionic liquids on the basis of the COSMO-RS predictions. Banerjee and Verma 47 performed the solvent evaluation to identify the potential ionic liquids for the separation of azeotropic mixtures of ethanol water, 2-propanol water, and THF water with the combinations of 10 cations and 24 anions using the COSMO-RS approach. The effect of different ionic liquids on the phase behavior of acetonitrile water with the help of the COSMO-RS model was predicted by Li et al. 48 Gutierrez et al. 49 examined a solvent selection methodology for extractive distillation processes of methylcyclohexane toluene, 1-hexane n-hexane, and ethanol water. The solvent preselection is done with the COSMO-RS model to study the variations of ionic liquid (cation anion) structure and their effect on solubility and selectivity. Ferro et al. 50 used the COSMO-RS methodology to determine criteria for the design and selection of ionic liquids to specific applications. The criteria are related to operational and energy consumption in the separation processes. However, the prediction of phase behavior of potential ionic liquids for nonaqueous systems is limited. In the present work, COSMOtherm software was used to obtain the vapor liquid equilibria of the ternary nonaqueous systems of methyl acetate methanol and ethyl acetate ethanol involving ionic liquids. The software was used to study the different cations and anions in ionic liquids which are suited as entrainers. The influences of structural variations and the length of the alkyl chain of cations and anions in ionic liquids on the phase behavior of nonaqueous mixtures were studied. A combination of 13 cations and 27 anions was investigated (Table 1). 2. COMPUTATIONAL DETAILS COSMOtherm (version C2.1, release 01.11) software was used in this work to compute the VLE for ternary systems with ionic liquids, which is based on the COSMO-RS model. The standard procedure for COSMO-RS calculations involves two steps. The first step involves quantum chemical calculations of molecules and ions which are computed by TURBOMOLE. The calculation of molecular energies was accomplished using the triple-ζ valence polarized (TZVP) basis set and Becke Perdew (BP) function. As the result of COSMO calculation, a COSMO file is generated, and it is stored in a COSMO database, which is predefined and created by COSMOlogic. 51 The COSMO files were used in the COSMO-RS statistical thermodynamic calculations to calculate the properties of ionic liquid mixtures, and the ionic liquids are treated as two individual ions in an equimolar mixture. In the present work, several organic compounds and ionic liquids with cations and anions of theoretically possible combinations were selected from the COSMO database

3 Table 1. Ionic Liquids Investigated in the Present Work: Combinations of Cations and Anions no abbreviation cations imidazolium type (a) 1-methyl-3-methylimdazolium [MMIM] (b) 1-ethyl-3-methylimdazolium [EMIM] (c) 1-butyl-3-methylimdazolium [BMIM] (d) 1-hexyl-3-methylimdazolium [HMIM] (e) 1-octyl-3-methylimdazolium [OMIM] pyridinium type (a) 1-ethylpyridinium [Epy] (b) 1-butylpyridinium [Bpy] (c) 1-hexylpyridinium [Hpy] (d) 1-octylpyridinium [Opy] methyl pyrrolidinium type (a) 1-ethyl-1-methyl pyrrolidinium [EMPYR] (b) 1-butyl-1-methyl pyrrolidinium [BMPYR] (c) 1-hexyl-1-methyl pyrrolidinium [HMPYR] quinolinium type (a) 1-octyl quinolinium OQY no abbreviation anions 1 tetrafluoro borate [BF 4 ] 2 bis(trifluoro methyl)imide [BTI] 3 bromide [Br] 4 butyl sulfate [ButSO 4 ] 5 chloro(trifluoro)borate [BClF 3 ] 6 chloride [Cl] 7 dibutylphosphate [DBP] 8 dicyanamide [DCA] 9 diethylphosphate [DEP] 10 dihydrogen phosphate [DHP] 11 dimethylphosphate [DMP] 12 ethoxyethylsulfate [EEtSO 4 ] 13 ethylsulfate [EtSO 4 ] 14 heptafluorobutanoate [HFB] 15 hydrogen sulfate [HSO 4 ] 16 methanesulfonate [CH 3 SO 3 ] 17 methoxymethylsulafte [MMeSO 4 ] 18 methylsulfate [MeSO 4 ] 19 nitrate [NO 3 ] 20 bis(trifluoromethanesulfonyl)imide [NTf 2 ] 21 acetate [OAc] 22 octylsulfate [OctSO 4 ] 23 hexafluorophosphate [PF 6 ] 24 thiocyanate [SCN] 25 tricyanomethane [C 4 N 3 ] 26 trifluoroacetate [CF 3 COO] 27 trifluoromethanesulfonate [CF 3 SO 3 ] 3. RESULTS AND DISCUSSION The feasibility of the COSMO-RS model for solvent evaluation is examined by estimating vapor liquid equilibria of methyl acetate methanol and ethyl acetate ethanol systems. In VLE calculations, the vapor phase was assumed to be ideal. Isobaric VLE calculations were performed for the ternary system containing ionic liquid using the following equations. P tot = p = γxp i tot p i o i i i o o o o P = γx p + γx p + γx p + γx p where P tot is the total pressure of the ternary system and p i o is the vapor pressure of the pure component i. x i is the liquid phase mole fraction of component i, and the third and fourth terms may be neglected, as they are very small compared to the other two terms. o o γxp γxp y = and y = 1 tot 2 tot P P where y i is the vapor phase mole fraction of component i, and γ i is the liquid phase activity coefficient of component i. The optimization of the solvent screening was performed for these nonaqueous azeotropic mixture separations by considering relative volatility as a parameter. The relative volatility is obtained using vapor phase mole fractions of y i and y j, and the liquid phase mole fraction on IL free basis of x i and x j and is given by α = ij (/) y y i ( x / x ) i j j The deviations of vapor phase mole fraction and temperature between the COSMO-RS model predictions and literature experimental data were estimated by using RAAD (Relative Absolute Average Deviation) as defined by Banerjee et al. 47 and is given by 1 exp cal RAADy = [ y y ] j j M 1 RAADT = [ Texp Tcal] M where M denotes number of data points Benchmarking of Isobaric VLE of Ternary Systems. The benchmarking was carried out for two nonaqueous systems, namely methyl acetate methanol and ethyl acetate ethanol. For these nonaqueous systems, ionic liquids as an entrainer for their separation was investigated Methyl Acetate Methanol Ionic liquid. The vapor liquid equilibrium of methyl acetate (1) methanol (2) ionic liquids (3) was predicted for two different ionic liquids, namely, [EMIM][Triflate] and [EMIM][OAc], which was studied experimentally earlier by Orchilles et al. 18 and Cai et al. 22 by varying the concentrations under isobaric conditions. The T x y data were obtained and compared with literature data. The errors of absolute average deviation in temperature and vapor phase mole fraction between experimental and predicted data are given in Table 2. The deviation is higher for increasing the mole fraction of ionic liquids. However, the COSMO-RS model predicts the trend of ionic liquid behavior on azeotropic systems qualitatively as reported earlier by Li et al Ethyl Acetate Ethanol Ionic Liquid. The effect of ionic liquids on the ethyl acetate (1) ethanol (2) system was measured experimentally earlier using ionic liquids (3) such as [EMIM][OAc], [EMIM[BF 4 ], [EMIM][Triflate], [BMIM]- [BF 4 ], and [OMIM][BF 4 ]. All these ionic liquids showed a salting out effect on the ethyl acetate ethanol system. The predictions of temperature and mole fraction were made using the COSMO-RS model for comparison with the experimental results reported and are shown in Table 3. It is observed that the deviation increases with increasing ionic liquid content in the nonaqueous azeotropic system. Thus, the capability of the COSMO-RS model can be used for the qualitative measurement of influence of ionic liquid on the azeotropic mixture and 16398

4 Table 2. Benchmarking of Isobaric Vapor Liquid Equilibrium of Methyl Acetate Methano -ILs (Ionic Liquids) a ionic liquid [EMIM] [OAc] [EMIM] [Triflate] reference mole fraction of ionic liquids temperature range/k RAADy RAADT a RAAD, Relative Absolute Average Deviation; -y, mole fraction of component i in gas phase, -T, temperature. Figure 1. Relative volatility of imidazolium based cations and anions of ionic liquid of a methyl acetate methanol system at x 1 = 0.7 and x 3 = 0.1 (as listed in Table 1). Table 3. Benchmarking of Isobaric Vapor Liquid Equilibrium of Ethyl Acetate-Ethanol-ILs (Ionic Liquids) a ionic liquid [EMIM] [OAc] [EMIM] [Triflate] [EMIM] [BF 4 ] [BMIM] [BF 4 ] [OMIM] [BF 4 ] reference mole fraction of ionic liquids temperature range/k RAADy RAADT a RAAD, Relative Absolute Average Deviation; -y, mole fraction of component i in gas phase; -T, temperature. Figure 2. Relative volatility of pyridinium based cations and anions of ionic liquid of a methyl acetate methanol system at x 1 = 0.7 and x 3 = 0.1 (as listed in Table 1). can be used for further solvent evaluation of a specific combination of cations and anions for separation Influence of Anion type Methyl Acetate Methanol Ionic Liquid. The isobaric VLEs for different anions were compared using relative volatility at an azeotropic composition of a methyl acetate methanol system with various cations such as imidazolium, pyridinium, pyrrolidinium, and quinolinium. Figures 1 3, show that the relative volatility of a methyl acetate methanol system with [OAc], [Cl], and [DHP] is superior to those of other anions. The higher the relative volatility of the low volatile component at x 1 = 0.7 and x 3 = 0.1, the easier is the separation. It is clearly seen that the influence of the anion plays a major role on the solubility of ionic liquids in methanol. 40 The solubility of the ionic liquid has a great impact on the relative volatility of the nonaqueous mixture and that the interactions between methanol and ionic liquid are very strong due to hydrogen bonding of solvation. 52 Cai et al. 22 reported that the [OAc] anion gives good separation of the methyl acetate methanol system. The Figure 3. Relative volatility of methyl pyrrolidinium based cations and anions of ionic liquid of methyl acetate methanol system at x 1 = 0.7 and x 3 = 0.1 (as listed in Table 1). COSMO-RS model gave similar predictions. Hence, it may be concluded by extension that [Cl] and [DHP] anions can also be selected as entrainers for the methyl acetate methanol system Ethyl Acetate Ethanol Ionic Liquid. The same 27 anions were used to predict the relative volatility of the ethyl acetate ethanol system. A similar trend was found on the 16399

5 phase behavior of the ethyl acetate ethanol system, which is shown in Figures 4 6. Similar solubility behavior is seen for Figure 4. Relative volatility of imidazolium based cations and anions of ionic liquid of the ethyl acetate ethanol system at x 1 = 0.6 and x 3 = 0.1 (as listed in Table 1). acetate ethanol system. This is consistent with the experimental data of the [OAc] anion which leads to higher ethyl acetate compositions as reported by Li et al. 24 The anions [PF 6 ], [NTf 2 ], and bis(trifluoromethyl)imide have a relative volatility of less than one, showing that the interactions with methanol and ethanol are very weak, resulting in a salting in effect on the nonaqueous azeotropic systems. These anions are generally named hydrophobic anions and make the separation impossible for ester alcohol systems. However, the effect of the anions [Triflate] and [BF 4 ] on the VLEs of methyl acetate methanol and ethyl acetate ethanol systems, respectively, were measured experimentally. 18,20 It shows that a salting out effect was found with these anions. The COSMO-RS model predicts similar behavior for these anions but not as satisfactorily as that of [OAc], [Cl], and [DHP]. This is discussed later in section Influence of Cation Type and Its Alkyl Chain Length. The anions [OAc], [Cl], and [DHP], which showed a strong influence on the VLE of ester alcohol systems, were further studied using 13 cations and are shown in Figures 7 and 8. The cations of the imidazolium group have a Figure 5. Relative volatility of pyridinium based cations and anions of ionic liquid of the ethyl acetate ethanol system at x 1 = 0.6 and x 3 = 0.1 (as listed in Table 1). Figure 7. Comparison of relative volatility of different cations on the methyl acetate methanol system at x 1 = 0.7 and x 3 = 0.1 (as listed in Table 1). greater effect than pyridinium, pyrrolidinium, and quinolinium types of cations. 56 The influence of cations is small when compared to anions. 57 Figure 6. Relative volatility of methyl pyrrolidinium based cations and anions of ionic liquid of the ethyl acetate ethanol system at x 1 = 0.6 and x 3 = 0.1 (as listed in Table 1). alcohols (methanol, ethanol) and water with ionic liquids since methanol and ethanol polarity are close to each other and are far from that of water. 8 The three anions [OAc], [Cl], and [DHP] showed higher relative volatility for the ethyl Figure 8. Comparison of relative volatility of different cations on the ethyl acetate ethanol system at x 1 = 0.6 and x 3 = 0.1 (as listed in Table 1)

6 Increasing alkyl chain length of the cation decreases the relative volatility of methyl acetate methanol and ethyl acetate ethanol systems. This results in a decrease in solubility of ionic liquid alcohols as can be seen in Figures 1 6. This is due to increase of the hydrophobic nature of the cations by lengthening the alkyl chain. 58 The shorter the alkyl chain of the cation, for example, MMIM and EMIM, the better is the separation of azeotropic mixtures among all the cations considered in this work. A similar trend has been reported for ionic liquid with alcohols. 40 Therefore, the cations and anions of ionic liquids selected from the above discussion on the basis of higher relative volatility of ester alcohol systems are the [EMIM] cation and [OAc], [Cl], and [DHP] anions of ionic liquids and are promising entrainers for extractive distillation of ester alcohol systems. Ionic liquid combinations of these cations and anions can be tailored for practical applications. It has been shown experimentally 22 that [EMIM]- [OAc] with nonaqueous mixtures are suitable ionic liquids for the separation of ester alcohol systems. The VLE data obtained from the COSMO-RS model with the ionic liquids of [EMIM][Cl] and [EMIM][DHP] at mole fractions of 0.1 and 0.2 are shown in Figures 9 and 10 for methyl acetate methanol and ethyl acetate ethanol system, respectively. Hence, these ionic liquids are promising candidate solvents for the removal of methyl acetate and ethyl acetate from their alcohol mixtures. It is also observed that increasing the ionic Figure 10. Effect of selected ionic liquid concentrations on the isobaric vapor liquid equilibrium of the ethyl acetate ethanol system at (a) x 3 = 0.1 and (b) x 3 = 0.2., ionic liquid free;, [EMIM][OAc];, [EMIM][Cl];, [EMIM][DHP]. Figure 9. Effect of selected ionic liquid concentrations on the isobaric vapor liquid equilibrium of the methyl acetate methanol system at (a) x 3 = 0.1 and (b) x 3 = 0.2., ionic liquid free;, [EMIM][OAc];, [EMIM][Cl];, [EMIM][DHP]. liquid concentrations moves the azeotropic point into higher ester concentrations as expected. The [EMIM][Cl] and [EMIM][DHP] remove azeotrope at x 3 = 0.1 in ester alcohol systems Influence of Polarity. In the COSMO-RS model, 51 local pairwise molecular interactions are described as σprofiles for the compounds involved. σ-profiles give a polarity of molecules by which interactions among different molecules being strong or weak can be identified. The polarity effect was analyzed by Li et al. 48 for the acetonitrile water system. The interactions of [OAc] and [Cl] anions with the acetonitrile water system were investigated, and the cutoff value of σ was given in the range to A similar procedure was followed in the present work to investigate the methyl acetate methanol and ethyl acetate ethanol systems with ionic liquids in order to ascertain the interactions among the molecules in the solution. It is known that [OAc], [Cl], and [DHP] provide promising candidates for the elimination of the azeotrope of nonaqueous mixtures. These σ-profiles for the methyl acetate methanol and ethyl acetate ethanol systems with [OAc], [Cl], and [DHP] anions are shown in Figures 11 and 12. Also, anions [Triflate] (in Figure 11) and [BF 4 ] (in Figure 12) are included for ease in comparison to clarify the polarity effect. In Figures 11 14, the value of σ lies in the range 0.01 to e/å 2. Negative σ (σ < 0.01) indicates hydrogen bond donor ability, whereas a positive σ (σ > 0.01) shows hydrogen bond acceptor ability. In Figures 11 and 12, it is observed that 16401

7 Figure 11. σ-profiles for polarity of molecules of methyl acetate methanol with different anions. Figure 14. σ-profiles for polarity of molecules of ethyl acetate ethanol cations of different types of interactions in solution. Figure 12. σ-profiles for polarity of molecules of ethyl acetate ethanol with different anions. Figure 13. σ-profiles for polarity of molecules of methyl acetate methanol cations of different types of interactions in solution. the peaks of the anions of [OAc], [Cl], and [DHP] lie on the right side of the positive σ region (σ > 0.01), showing a strong hydrogen bond acceptor. The farther the profile peak on the left side of the absolute value of σ 0.01 e/å 2, the stronger is the hydrogen bond donor, and the farther to the right, the stronger the acceptor ability. The [Triflate] anion also lies in the same positive σ region, but it has a weaker hydrogen bond acceptor compared to other anions. Methyl acetate has hydrogen bond acceptor ability due to the presence of an acetate group, and a stable complex with anions is not possible. Methanol has a stronger hydrogen bond donor because of its high polarity, and it lies in the negative σ region and extends up to e/å 2 (see Figure 11), which can interact with the anions with hydrogen bond acceptor ability through the formation of hydrogen bonding. Hence, the selected anions of ionic liquids such as [OAc], [Cl], and [DHP] form strong hydrogen bonds with methanol and break the methyl acetate methanol interactions, which leads to improvement of the VLE of the methyl acetate methanol system. For the same anions, a similar trend is also observed for the ethyl acetate ethanol system as shown in Figure 12 in which [BF 4 ] has a peak up to e/ A 2, which is certainly less than that of e/å 2, e/å 2, and e/å 2 of [OAc], [Cl], and [DHP], respectively. The hydrogen bond donor for ethanol has a similar magnitude of e/å 2 to that of methanol, which is strong. Hence, the interaction between ethyl acetate and ethanol becomes weaker, and the separation of ethyl acetate from ethanol containing ionic liquids is facilitated. The choice of cation can be explained by comparing their different polarities as seen in Figures 13 and 14 for both methyl acetate methanol and ethyl acetate systems. From Figures 13 and 14, it is seen that all the cations have hydrogen bond donor capability since negative σ is obtained. [EMIM] cations have a stronger hydrogen bond donor (σ = 1.80 e/å 2 ) than that of other cations. Further, it is seen that strong interactions through the hydrogen bond are possible when the length of the alkyl chain is shorter, and a longer alkyl chain length produces a weak hydrogen bond donor ability of the cation, as observed by Li et al. 48 Hence, it may be concluded that [MMIM] and [EMIM] are better cations than [BMIM] and other cations Influence of Excess Free Energy. The excess free energy gives information on molecular interactions between ester alcohol and ionic liquids. Li et al. 48 calculated the interaction free energy for the acetonitrile water system with [EMIM] cations with anions of [OAc] and [Cl] using the Gaussian 03 package. The interaction free energies of water [EMIM][OAc] and water [EMIM][Cl] are greater than that of acetonitrile [EMIM][OAc] and acetonitrile [EMIM][Cl], respectively, which indicates a stronger interaction of ionic liquids with water than acetonitrile by hydrogen bonding. For the systems investigated in the present work, the free energy of interaction is calculated using the COSMOtherm software package. At mixture azeotropic concentration, the effect of free energy is estimated for ionic liquids (x 3 = 0.1) of [EMIM][OAc], [EMIM][Cl], [EMIM][DHP], [EMIM]- [Triflate], and [EMIM][BF 4 ]. These are selected because of their capability of removing the azeotrope of a nonaqueous mixture at lower concentrations. In Table 4, the interaction free 16402

8 Table 4. Excess Free Energy of Methyl Acetate Methanol and Ethyl Acetate Ethanol Systems with Ionic Liquid (Mole Fraction x 3 = 0.1) at Mixture Azeotropic Concentration a G E /kcal/mol ionic liquids methyl acetate methanol ethyl acetate ethanol [EMIM][OAc] [EMIM][Cl] [EMIM][DHP] [EMIM][BF 4 ] [EMIM][Triflate] a G E, excess free energy of ester alcohol with IL (Ionic Liquids). energies for methyl acetate methanol with various ionic liquids are listed, and it is observed that higher free energy is obtained in the order of [EMIM][OAc] > [EMIM][Cl] > [EMIM]- [DHP] > [EMIM][Triflate]. Similarly, [EMIM][OAc] > [EMIM][Cl] > [EMIM][DHP] > [EMIM][BF 4 ] is observed for the ethyl acetate ethanol system. The results obtained show that the interaction of solvated anions with alcohol molecules forms a complex by means of hydrogen bonding due to their high polarity as well as cation interactions with alcohols. Hence, the alcohol activity is reduced, which leads to improve the relative volatility of ester alcohol systems. This behavior is similar to that of the acetonitrile water system with various ionic liquids. 48 From the above, it is seen that [OAc] and [Cl] have higher excess free energy; hence these anions together with the [EMIM] cation may be a good choice as entrainer for extractive distillation. Nevertheless, the behaviors of [BF 4 ] and [Triflate] are weaker when compared to other anions even though they have larger excess energy Influence of Excess Enthalpy. The enthalpy of mixing of a solution is an effective tool to describe the process of nonideality of azeotropic systems. It describes a state of the process whether it is exothermic or endothermic. The exothermicity arises due to a negative value of enthalpy, and a positive value of enthalpy indicates that it is endothermic during ionic liquid mixing with an ester alcohol system. In the present work, enthalpy of mixing of ternary nonaqueous system is estimated at its azeotropic concentration and x 3 = 0.1 using the COSMO-RS method, as described by Li et al. 48 and shown in Table 5. All the promising ionic liquids exhibit negative Table 5. Excess Enthalpy Methyl Acetate Methanol and Ethyl Acetate Ethanol Systems with Ionic Liquid (Mole Fraction x 3 = 0.1) at Mixture Azeotropic Concentration a H E /kcal/mol ionic liquids methyl acetate methanol ethyl acetate ethanol [EMIM][OAc] [EMIM][Cl] [EMIM][DHP] [EMIM][BF 4 ] [EMIM][Triflate] a H E, excess enthalpy of mixing. enthalpy of mixing, indicating that the process is exothermic. The high excess enthalpy describes a strong interaction of ionic liquid between ester and alcohol systems, providing better separation. Thus, the order of potential ionic liquids is the [EMIM] cation with [OAc] > [Cl] > [DHP]. Also, the COSMO-RS model evaluates the excess enthalpy in terms of summation of three different interactions of azeotropic mixtures along with ionic liquids. The three contributions to H E are the misfit excess enthalpy H E (MF), a contribution from hydrogen bonding H E (HB), and a contribution from van der Waals interactions H E (vdw) and are given by E E E E H = H (MF) + H (HB) + H (vdw) The three contributions of interactions of [EMIM][OAc], [EMIM][Cl], and [EMIM][DHP] are evaluated at a mole fraction of ionic liquid (x 3 = 0.1), and the results are given in the Supporting Information, Tables S1 S6, for both ester alcohol systems, respectively. From the analysis, it is seen that the excess enthalpy for [EMIM][OAc] is in the range 2.39 to 3.11 kcal mol 1 and for [EMIM][Cl] is in the range 1.70 to 3.01 kcal mol 1 followed by [EMIM][DHP] ( 1.55 to 2.25 kcal.mol 1 ) for methyl acetate methanol and ethyl acetate ethanol systems. From the results, hydrogen bonding plays a major role in the enthalpy of mixing of the solution than that of the other two interactions as reported for the acetonitrile water system. 48 This is seen in the effect of polarity of various cations and anions on ester alcoholic systems as well. It is also noticed that the effect of interaction due to van der Waals contribution is very small. More importantly, the effect of excess enthalpy of the [EMIM] cation with anions [OAc], [Cl], and [DHP] is valid for nonaqueous systems, which has been shown earlier for the aqueous system of acetonitrile water by Li et al CONCLUSION The COSMO-RS model has been used to select suitable solvents for the separation of methyl acetate methanol and ethyl acetate ethanol systems. From the solvent evaluation analysis, it is observed that the [EMIM] cation and [OAc], [Cl], and [DHP] anions of ionic liquids are capable of eliminating the azeotrope of nonaqueous mixtures. It is also noticed that the shorter alkyl chain of the cation favors the azeotrope separation, and the anion plays an important role and decides the solubility of ionic liquid in alcohols by which methyl acetate and ethyl acetate are removed from the alcohol solutions. The effects of polarity of molecules was examined to study the intermolecular interactions between molecules involved in the solution. Further, the effect of excess free energy and enthalpy were examined on the different contributions of interactions of molecules quantitatively. It is seen that the behavior of the VLE data with ionic liquids for aqueous systems is also seen in nonaqueous systems, particularly the acetate and chloride anions. Hence, it can be concluded that a a priori prediction using the COSMO-RS model provides a route to evaluate solvents for a task-specific application, especially in the separation of azeotropic mixtures. Furthermore, these ionic liquids do not pose any problems with corrosion, which is an issue with ionic liquids containing flourides. 59 ASSOCIATED CONTENT *S Supporting Information Interaction contributions to excess enthalpy of mixing. This material is available free of charge via the Internet at pubs.acs.org

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