Styrene Separation by

ARTICLE pubs.acs.org/IECR

Ionic Liquid Screening for Ethylbenzene/Styrene Separation by Extractive Distillation Mark T.G. Jongmans,* Boelo Schuur, and Andr
e B. de Haan Process Systems Engineering Group, Chemical Engineering and Chemistry Department, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands ABSTRACT: The separation of ethylbenzene from styrene by distillation is very energy-intensive, because of the low relative volatility (1.3
1.4). Extractive distillation is a promising alternative to separate the close boiling mixture, in which the solvent selection is crucial for the process feasibility. In this work, an ionic liquid screening study by liquid
liquid equilibrium (LLE) experiments has been performed to investigate whether ionic liquids (ILs) show potential to separate ethylbenzene from styrene by extractive distillation. The screening method by LLE experiments was validated by VLE experiments for several ILs. The performance of the ILs was compared with the benchmark solvent sulfolane, which displays a selectivity of ∼1.6. Several ILs outperform sulfolane with selectivities up to 2.6. From the results of the LLE experiments, it was concluded that there is a clear tradeoff between capacity and selectivity. ILs with a high capacity usually have a low selectivity, whereas ILs with high selectivity exhibit low capacity. Both cation and anion structure strongly influence the performance. The highest selectivities (2.4
2.6) were obtained with ILs containing aromatic cations, and anions with localized electrons. The largest capacities (0.45
0.6 for styrene) were obtained for ionic liquids with delocalized electrons in the anion and large alkyl chain length in both cation and anion.

1. INTRODUCTION Styrene is most commonly produced via the catalytic dehydrogenation of ethylbenzene.1 Deep vacuum distillation in the pressure range of 5
20 kPa is generally used in this process to separate the unreacted ethylbenzene from styrene. Because of the low relative volatility of 1.3
1.4,2 the distillation of ethylbenzene from styrene accounts for 75%
80% of the total energy use in the distillation section of a typical styrene production plant.3 A dramatic reduction in both capital and energy expenses may be obtained when the traditional distillation is replaced by extractive distillation.2 In extractive distillation, the relative volatility is enhanced by feeding a solvent to the column that modifies the activity coefficients of the components to be separated.4 Typical organic solvents used in extractive distillation are sulfolane, n-methyl pyrrolidone, and n-formyl morpholine.4
8 Sulfolane is known to have a large effect on the relative volatility of an ethylbenzene/styrene mixture.2,9 The selectivity of sulfolane toward styrene in this mixture is ∼1.6.9 However, the disadvantage of sulfolane is its low decomposition temperature.10 An alternative with large potential is to use ionic liquids (ILs), which have recently been reported as promising solvents for extractive distillation processes,4,11,12 as well as in other separation technologies.11,13 In separation technology, ILs are mainly used to separate aromatics from aliphatics,14
17 olefins from aliphatics,18,19 alkynes from olefins,20,21 sulfur- and nitrogen-containing compounds from aromatics/aliphatics,15,22
24 and alcohols/aldehydes/ ketones/ethers from aliphatics/aromatics.16,25 ILs are low melting salts (80 °C),46 which makes them unsuitable for the separation of ethylbenzene from styrene via deep vacuum extractive distillation.

ILs 1
7, 9
19, 21
29, and 31
37 were obtained commercially; ILs 8, 20, 29, and 30 were not available commercially and, therefore, were synthesized in our laboratory. All ILs were liquid at room temperature, except for ILs 8 and 12.

3. EXPERIMENTAL SECTION 3.1. Chemicals. Acetone (g99.5%) was obtained from VWR, cyclohexane (g99%) and styrene (g99.5%) from Merck, ethylbenzene (g99%) and 4-tert-butylcatechol (g98%) from Fluka, acetone-d6 (99.96 at. % D), sodium dicyanamide (g96%), and potassium methyltrifluoroborate (g98%) from Aldrich. The purities in weight percentage were provided by the suppliers. The ILs and some key intermediates used in this study, along 10802

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Industrial & Engineering Chemistry Research with their abbreviation, supplier, and purity are listed in Table 1. The synthesized ILs are described in the following section. All ILs were dried in a rotary evaporator (B€uchi Model Rotavapor R-200) and stored under vacuum in a desiccator filled with molecular sieves. All chemicals were used without further purification. 3.2. Ionic Liquid Syntheses. Anion exchange (metathesis) is a common procedure to prepare ILs.49
51 The ILs 3-cyano-Nbutylpyridinium dicyanamide, 1-ethyl-3-methylimidazolium methyltrifluoroborate, 1-allyl-3-methylimidazolium dicyanamide, and 1-benzyl-3-methylimidazolium dicyanamide were all prepared using the anion-exchange procedure, stirring a mixture of a sodium or potassium salt with the desired anion together with a chloride of the desired cation in 500 mL of acetone for 60 h at room temperature. During the reaction, the formed NaCl or KCl precipitated, and was filtered off. After evaporation of the acetone, the completeness of the reactions was verified with 1H NMR (Varian Oxford NMR, 200 MHz, using acetone-d6 as a solvent). 3-Cyano-N-butylpyridinium Dicyanamide. The reaction of 3-cyanobutylpyridinium chloride (34.6 g, 0.176 mol) and sodium dicyanamide (16.2 g, 0.182 mol) yielded 38.4 g (0.169 mol, 96% yield) of 3-cyano-N-butylpyridinium dicyanamide, appearing as a dark reddish brown solid. 1H NMR (200 MHz, acetone-d6): 9.91(s, 1H), 9.57(m, 1H), 9.17(m, 1H), 8.54(m, 1H), 4.97(m, 2H), 2.16(m, 2H), 1.5(m, 2H), 0.98(m, 3H). 1-Ethyl-3-methylimidazolium Methyltrifluoroborate. The reaction of 1-ethyl-3-methylimidazolium chloride (11.4 g, 0.078 mol) and potassium methyltrifluoroborate (10.3 g, 0.084 mol) yielded 15.6 g (0.080 mol, 95% yield) 1-ethyl-3-methylimidazolium methyltrifluoroborate, a viscous yellow liquid. 1H NMR (200 MHz, acetone-d6): 9.28(s, 1H), 7.80(s, 1H), 7.72(s, 1H), 4.4(m, 2H), 4.01(s, 3H), 1.52(m, 3H),
0.41(s, 3H). 1-Allyl-3-methylimidazolium Dicyanamide. The reaction between 1-allyl-3-methylimidazolium chloride (22.4 g, 0.141 mol) and sodium dicyanamide (13.8 g, 0.155 mol) yielded 23.65 g (0.135 mol, 96% yield) 1-allyl-3-methylimidazolium dicyanamide, a viscous liquid of light yellow color. 1H NMR (200 MHz, acetone-d6): 9.1(s, 1H), 7.73(m, 2H), 6.25
6.02(m, 1H), 5.5
5.36(m, 2H), 5.03
4.95(m, 2H), 4.05(s, 3H). 1-Benzyl-3-methylimidazolium Dicyanamide. The reaction between 1-benzyl-3-methylimidazolium chloride (22.0 g, 0.105 mol) and sodium dicyanamide (10.73 g, 0.121 mol) yielded 23.7 g (0.099 mol, 94% yield) 1-benzyl-3-methylimidazolium dicyanamide, a viscous yellow liquid. 1H NMR (200 MHz, acetone-d6): 9.21(s, 1H), 7.77(m, 2H), 7.47(m, 5H), 5.59(s, 2H), 4.06(s, 3H). 3.3. Experimental Procedure of Phase Equilibria. Liquid
Liquid Extraction. The liquid
liquid equilibrium experiments were performed by mixing 20 mL of the feed mixture with 10 mL of IL in 70-mL jacketed glass vessels. The vessels were equipped with a double impeller on a single axis, powered by an electronic stirrer (IKA Model Eurostar powercontrol-visc P1). The impellers were placed at the interface between the solvent and the feed, to provide good mixing. The double-walled vessels were thermostatted using a thermostatic bath (Julabo F25). The extraction experiments were performed at 313.2 and 348.2 K ( 0.1 K. The ethylbenzene/styrene mass ratio in the feed was 40/60, which is representative for the feed to the current distillation column. Since the behavior between ethylbenzene and styrene is almost ideal,9 it is expected that other ethylbenzene/styrene ratios will give similar results. 4-Tert-butylcatechol (0.005 wt %) was added as a polymerization inhibitor. The time to equilibration was typically 5 min at 1300 rpm. In order to ensure equilibrium,

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experiments were run for 20 min, after which the two phases were allowed to settle for 2 h. Samples for analysis (0.2 mL) were taken from both extract and raffinate phase using a 2-mL syringe. Since water has an influence on the extraction performance, the water content in the ILs was measured before and after the experiments with Karl Fischer titration and was in all cases determined to be 0.3 are observed) and a relative low selectivity (typically, 2). This behavior is also often observed in aromatic/aliphatic separations.39 Anions with localized electrons have a higher dipole moment than anions with delocalized electrons.53 Charge localization facilitates stronger Coulombic interactions between the ions, thereby creating more cation
anion affinity.53 Nonpolar molecules, such as ethylbenzene and styrene, also tend to reside in the nonpolar domains of an IL.53 Since ILs that contain anions with many localized electrons have more polar regions, there are less domains for the aromatics available. Because of these two reasons, the capacity decreases and selectivity increases. The capacity and selectivity are also dependent on the alkyl chain length of the anion. The capacity increases and selectivity decreases when the alkyl chain length of the anion increases. This conclusion can be drawn by comparing the ILs with the anions [MeSO4]
and [EtSO4]
. This trend for anion alkyl chain length was also observed by Garcia et al.55 4.4. Effect of Cation Structure. In this section, the effect of cation structure on the capacity and selectivity is discussed. The anions [BF4]
, [Tf2N]
, and [N(CN)2]
were selected as

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counterions, because many different ILs are commercially available with these types of anions. Some additional ILs with the [N(CN)2]
anion were custom-synthesized (see section 3.2). The results are displayed in Figure 4. Figure 4a illustrates that pyridinium-based ILs have a higher capacity and only a slightly lower selectivity for styrene, compared to imidazolium ILs. This conclusion was drawn by comparing the results for [4-mebupy][BF4] (Dstyrene = 0.414, S = 1.77) and [BMIM][BF4] (Dstyrene = 0.243, S = 1.82) and may be explained by the pyridinium cation having a slightly larger π-system and, therefore, a stronger π
π interaction with aromatics.56 This trend is also known for aromatics/ aliphatics separations.45 The attachment of an extra methyl group to an imidazolium cation does not have a positive influence on the extraction performance, which can be derived from the results for [BMMIM][BF4] (Dstyrene = 0.239, S = 1.92) and [BMIM][BF4]. An extra methyl group being attached to an imidazolium cation should enhance the interaction of the cation methyl groups with the π-system of the aromatics.36 However, the extra methyl group makes the [BMMIM]+-cation more ionic, because the positive charge is more localized than in a [BMIM]+ cation.57 Charge localization facilitates stronger Coulombic interactions, and thereby more cation
anion interaction. This explains the lower distribution coefficient for aromatics if an extra methyl group is added to an imidazolium cation. Moreover, an extra methyl group also causes more steric hindrance for aromatics.58 The attachment of a benzyl group to an imidazolium cation also does not positively influence the aromatic capacity. In general a more extended π-electron ring system should have stronger π
π interaction with aromatics.56 However, in the case of ILs, most probably only one aromatic moiety will take part in the π
π interaction.56 Moreover, the benzyl group, just like an extra methyl group, can cause steric hindrance. A polar group in the side chain of a cation negatively affects the capacity of ILs toward aromatics. This conclusion can be drawn by comparing [HEMIM]+ with [EMIM]+ (Figure 4b) and [3-cybupy]+ with [3-mebupy]+ (Figure 4c). It was expected that the nitrile group in [3-cybupy]+ should enhance the distribution coefficient, because an electron-withdrawing group in the side chain enhances the π-acceptor capability of an aromatic IL cation,49 thereby enhancing the affinity toward aromatics. However, the polar groups in the side chain of the cations cause an increase in cation
anion affinity and, therefore, the capacity for aromatics is lower. This, most probably, can also explain the lower capacity for the [AMIM]+ cation, compared to the [EMIM]+ cation. The allyl group in the side chain of the imidazolium cation should increase the π
π interaction; however, the capacity of the [AMIM]+ cation (Dstyrene = 0.24) is lower than that for the [EMIM]+ cation (Dstyrene = 0.30). The pyrrolidinium cation has a much higher capacity, but much lower selectivity, compared to the pyridinium and imidazolium cations, as can be seen in Figure 4b. The large capacity of the pyrrolidinium cation originates from the low cation
anion affinity with the [N(CN)2]
anion. The lower selectivity of the pyrrolidinium cation can be explained by the fact that pyrrolidinium cations are more aliphatic in nature, compared to pyridinium and imidazolium ILs, and therefore do not have the specific π
π interaction.45 Since the selectivity is much lower, compared to the benchmark solvent sulfolane, pyrrolidinium cations are not interesting to use for ethylbenzene/styrene separation. The same holds for ILs with noncyclic cations such as sulfonium, phosphonium, and ammonium, which is illustrated in Figure 4c. 10806

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Industrial & Engineering Chemistry Research

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Figure 4. Effect of cation structure on the capacity and selectivity in LLE experiments at 348.2 K for a model feed, consisting of 60 wt % styrene and 40 wt % ethylbenzene ((a) anion [BF4]
, (b) anion [N(CN)2]
, and (c) anion [Tf2N]
).

Overall ,the conclusion can be drawn that ILs with aromatic cations have a larger selectivity than ILs with aliphatic cations. Because ILs with anions such as [BF4]
have a high selectivity and low capacity, the alkyl chain length of the imidazolium cation was increased in small steps, from ethyl to hexyl, to increase the capacity. The results of this increase in alkyl chain length of the imidazolium cation with a [BF4]
anion is shown in Figure 5. From this figure, the conclusion can be drawn that the capacity increases from 0.13 to 0.38 and the selectivity decreases from 2.15 to 1.46 with increasing alkyl chain length. This is common behavior for the solubility of aromatics in ILs.14,16,39 If the side chains of a cation or anion are small, the IL will have a fluid structure quite similar to that of common inorganic molten salt and, therefore, will have rather high cation
anion affinity. In that case, the IL mixture will contain mostly polar regions, just as for anions with localized electrons.53 Although aromatics contain a π-bonding surface, these molecules are still quite nonpolar (low dipole moment), in comparison with hydrocarbons that contain functional groups, such as alcohols or ketones.59 Therefore, if the interaction between the cation and the anion of an IL is very strong, it is difficult to get a high capacity for aromatics. However, the polarity of the IL will decrease if the alkyl chain length of the cation increases and more nonpolar domains will be

formed and cation
anion affinity will decrease.53 Therefore, the aromatic capacity will increase with increasing alkyl chain length of the cation. 4.5. Overview of All Measured ILs. In Figure 6, the results from the LLE experiments for the majority of the ILs that form liquid
liquid biphasic systems with the ethylbenzene/styrene feed are illustrated. The selectivity of sulfolane for the model ethylbenzene/styrene mixture containing 60 wt % styrene is indicated with a line. Since sulfolane mixes over the full composition range with aromatics, no capacity for sulfolane is available. Figure 6 shows that, overall, there is a clear tradeoff between the capacity and the selectivity. ILs with a high capacity usually have a low selectivity, whereas ILs with low capacity exhibit high selectivity. This is a general trend in solvent screening studies for extraction and extractive distillation processes.60 Both the IL cation and anion structure have a strong influence on the performance of the IL. The largest selectivities (2.4
2.6) were measured for ILs with aromatic cations, because of the specific aromatic π
π interactions, especially those with short alkyl chain length, such as [EMIM]+, and anions with localized electrons, such as [MeSO4]
and [Mesy]
. The largest capacities were obtained for ILs containing anions with delocalized electrons, such as [B(CN)4]
and [N(CN)2]
, and large alkyl 10807

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Industrial & Engineering Chemistry Research chain length in the cation as well as the anion. From the overall results, one also can reach the conclusion that, although the aromatic
IL interaction is very important, the IL cation
anion affinity also has a large impact on the performance. Figure 6 also demonstrates that several ILs outperform sulfolane, with selectivities up to 2.6. Therefore, ILs are promising solvents, which can perform better than sulfolane in an extractive distillation process to separate ethylbenzene from styrene. This is also depicted in Figure 7, in which the reflux ratio of the extractive distillation column is shown as function of the solvent selectivity. The minimum reflux ratio was calculated

Figure 5. Effect of cation alkyl length on the capacity and selectivity in LLE experiments at 348.2 K with an imidazolium cation with [BF4]
anion for a model feed consisting of 60 wt % styrene and 40 wt % ethylbenzene. (Experimental data are shown as data points ((9) [EMIM][BF4], (b) [PMIM][BF4], (2) [BMIM][BF4], and (1) [HMIM][BF4].)

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with the Underwood expression:61 Rmin ¼

ðxD =xF Þ
α½ð1
xD Þ=ð1
xF Þ α
1

ð6Þ

where α is the relative volatility and xD and xF are, respectively, the ethylbenzene mole fractions in the distillate and the feed. Typical values for xF and xD in a common ethylbenzene/styrene

Figure 7. Effect of solvent selectivity on the required reflux ratio (R) of the extractive distillation column for a model feed consisting of 60 wt % styrene and 40 wt % ethylbenzene. The dashed line represents the Underwood equation (R = 1.3Rmin), and the symbols represent results for sulfolane (denoted by the open left-facing triangle) and ionic liquids (the black square represents [P14][N(CN)2], the gray square represents [P4441][MeSO4], the black circle represents [3-mebupy][B(CN)4], the gray circle represents [4-mebupy][BF4], the black upright triangle represents [EMIM][N(CN)2], the gray upright triangle represents [EMIM][SCN)], the black inverted triangle represents [EMIM][Mesy], and the gray inverted triangle represents [EMIM][MeSO4]).

Figure 6. Comparison of capacity (mass base) and selectivity of several ILs in LLE experiments at 348.2 K for a model feed consisting of 60 wt % styrene and 40 wt % ethylbenzene. Symbols represent the experimental results of ILs; the dashed line represents the selectivity of sulfolane.9 10808

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Industrial & Engineering Chemistry Research distillation are 0.4 and 0.98, respectively.3 The relative volatility was calculated using eq 5, in which the selectivities from the LLE experiments at 348.2 K were used. The pure-component vapor pressures of ethylbenzene and styrene were calculated for the same temperature. The selectivity values of 348.2 K were used. The reflux ratio was calculated by multiplying the minimum reflux ratio by 1.3. Figure 7 illustrates that sulfolane can decrease the reflux ratio from 7.4 to 2.4. Several ILs with a larger selectivity than sulfolane, such as [EMIM][SCN] and [EMIM][CH3SO4], can decrease the reflux ratio even more, to values of 1.1
1.5. This can result in a lower energy requirement for the IL extractive distillation process, compared to the sulfolane process. However, the selectivity and thereby the reflux ratio only give an initial indication about possible energy requirements. The solvent capacity also influences the performance of the extractive distillation process, as discussed in the Introduction. Therefore, a detailed process evaluation should confirm whether ILs are indeed more promising than a conventional organic solvent, such as sulfolane.

5. CONCLUSIONS An ionic liquid (IL) screening study, using liquid
liquid extraction (LLE) experiments, has been performed to investigate whether ILs are promising solvents in an extractive distillation to separate ethylbenzene from styrene. It was demonstrated that LLE experiments are a reliable and simple method to obtain estimates for the VLE selectivity. The temperature only slightly influences the performance, with a small decrease in selectivity being observed with increasing temperature. The results of all ILs were compared to the benchmark solvent sulfolane, which has a selectivity of 1.6. For the IL selectivities, a tradeoff is observed between the capacity and the selectivity. ILs with high capacity usually have a low selectivity, whereas ILs with high selectivity exhibit low capacity. Several ILs outperform sulfolane, with selectivities up to 2.6. Therefore, ILs can be promising solvents in an extractive distillation unit to separate ethylbenzene from styrene. A detailed process evaluation should confirm whether ILs are indeed more promising. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +31 40 247 2424. Fax: +31 40 246 3966. E-mail: [email protected].

’ ACKNOWLEDGMENT This is an ISPT project (Institute for Sustainable Process Technology, Netherlands). ’ REFERENCES (1) Ward, A. L.; Robert, W. J. In Kirk
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