**2.2 Quaternary systems with IL mixtures as extraction agents**

Among all the ILs that have been studied as separation agents, only a small number of them have shown simultaneously solute distribution coefficients and selectivity higher than sulfolane since, generally, high selectivities are related to low solute distribution coefficients and vice versa. On the other hand, most ILs present quite high viscosity, which make their use difficult in extraction processes. The proper selection of two ILs could lead to a separation agent, whose physical properties and extraction performance overcome those of sulfolane. Therefore, some pairs of ILs have been tested to extract aromatics, mainly benzene or toluene, from their mixtures with alkanes.

*Solvents, Ionic Liquids and Solvent Effects*

generally used when aromatic content is high. After the extraction process, further separation units are needed to recover sulfolane from raffinate and extract phases. The increasing concern for the environment has induced the search of new solvents that reduce pollution and energy costs, making the extraction process more environmentally friendly. One of the alternatives proposed is the use of ionic liquids (ILs) as solvents for the extraction of aromatic compounds, and consequently a large number of papers reporting liquid-liquid equilibria (LLE) data of ternary system aliphatic + aromatic + IL can be found in the literature. From experimental data, selectivity and distribution coefficients (β) can be calculated and used to evaluate the capability of the solvent since high selectivity together with high distribution coefficients are suitable. Canales and Brennecke [1] compared ILs and conventional solvents for the extraction of aromatic from aliphatic compounds,

Even though the information obtained from ternary systems is crucial, we cannot be certain that selectivity and distribution coefficients obtained from ternary systems will remain unchanged when more components, aromatics or aliphatic are present in the mixture. Because of this, selectivity and solute distribution coefficients should be determined for complex mixtures, in order to obtain more actual data. Some papers related to quaternary or higher systems using sulfolane to extract aromatics from aliphatic compounds can be found in the literature [2–8], while similar information using ILs as extractant agents is scarce, taking into account the huge number of ILs that can be used [9–22]. It also should be noted that mixtures of ILs or organic solvent + IL can be selected as separation agents [23], greatly increas-

In this chapter, a literature review on the results obtained using ILs for the extraction of aromatic compounds in systems with more than three components is performed. Furthermore new liquid-liquid equilibrium experimental data for the quaternary systems heptane + cyclohexane + toluene + 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [EMim][NTf2], and heptane + cyclohexane + toluene + 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide,

Up to now, a quite wide range of papers dealing with liquid-liquid equilibrium data of hydrocarbon + aromatic + IL ternary systems can be found in literature. These studies are essential to determine the capability of ILs to extract aromatic compounds; however, many other compounds are present in refinery streams, and

With the aim of analysing the influence of more than one aliphatic compound on the extraction of aromatics, liquid-liquid equilibrium data of several quaternary

Requejo et al. [9–11] studied the extraction of benzene from its mixtures with octane and decane using tributylmethylammonium bis(trifluoromethylsulfonyl) imide, [N4441][NTf2]; 1-butyl-1-methylpyrrolidinium dicyanamide, [BMpyr] [DCA]; and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [BMpyr][NTf2], and the results of the quaternary system were compared with those obtained for the ternary systems octane + benzene + IL and decane + benzene + IL. Using [N4441] [NTf2], the highest values of β were obtained for the ternary system octane + benzene + [N4441] [NTf2], while the β values calculated for the

finding that ILs are potentially alternatives to currently used solvents.

ing the number of systems that should be studied.

**2.1 Quaternary systems with two aliphatic compounds**

these compounds can modify the extraction process.

systems have been published [9–14].

[HMim][NTf2], are presented.

**2. Literature review**

**68**

Sakal et al. [15] carried out the extraction of benzene from benzene + cyclohexane mixtures using 1,3-dimethylimidazolium dimethylphosphate, [MMim][DMP], and 1-methylimidazolium tetrafluoroborate, [Mim][BF4], and the mixtures [Mim] [BF4] wt. 50% + [MMim][DMP] wt. 50% and [Mim][BF4] wt. 25% + [MMim] [DMP] wt. 75%. Both selectivity and solute distribution coefficient follow the same trend: [MMim][DMP] > [Mim][BF4] wt. 25% + [MMim][DMP] wt. 75% > [Mim] [BF4] wt. 50% + [MMim][DMP] wt. 50%. Additionally, IL mixtures of 1-methylimidazolium perchlorate, [Mim][ClO4] and [MMim][DMP], at the same mass ratios were also checked, finding that the addition of [Mim][BF4] or [Mim][ClO4] does not improve the extraction capacity of [MMim][DMP].

Potdar et al. [16] determined LLE data for the quaternary system hexane + benzene + 1-ethyl-3-methylimidazolium ethylsulphate, [EMim][ESO4], + 1-ethyl-3-methylimidazolium methylsulphate, [EMim][MSO4]. The comparison between β and S obtained using the mixture of ILs and each of them separately shows that the mixture of ILs has a lower extraction capacity. These worse results can be due to the fact that there is less free volume in a combination of these two ILs, hindering the solution of benzene in the ILs mixture.

The mixtures of N-butylpyridinium tetrafluoroborate, [Bpy][BF4], with N-butylpyridinium bis(trifluoromethylsulfonyl)imide, [Bpy][NTF2], and with 1-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl) imide, [4BMpy][NTf2], were tested for the extraction of toluene from the mixture heptane-toluene. These ILs were selected because of the higher selectivity and the lower solute distribution coefficient showed by [Bpy][BF4] and the opposite behaviour showed by [Bpy] [NTf2] and [4BMpy][NTf2] [17–18]. In both cases, β and S values higher than those obtained using sulfolane were achieved with a [Bpy][BF4] mole fraction of 0.7 in the mixture of ILs.

On the basis of the results obtained using 1-ethyl-3-methylimidazolium tricyanomethanide, [EMim][TCM], as well as 1-ethyl-3-methylimidazolium dicyanamide, [EMim][DCA], for the extraction of toluene from heptane, a mixture of [EMim][TCM] + [EMim][DCA] with a [EMim][TCM] mole fraction of 0.8 was selected to carry out the same extraction process [19]. Both solute distribution coefficient and selectivity are higher using this IL mixture than using sulfolane. Since selectivity values using the IL mixture were almost double than those using sulfolane, toluene extracted by the IL mixture would be significantly purer.

The capability of the binary mixture of 1-ethyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide, [4EMpy][NTf2], and 1-ethyl-3-methylimidazolium dicyanamide, [EMim][DCA], as solvent for the extraction of toluene from its mixtures with heptane or 2,3-dimethylpentane or cyclohexane was evaluated by Larriba et al. [20]. The IL [4EMpy][NTf2] was selected because it showed solute distribution coefficients for toluene higher than sulfolane. On the other hand, toluene selectivity of [EMim][DCA] is substantially higher than that of sulfolane. The extraction of toluene from other alkanes (hexane, octane or nonane) was also performed [21]. According to the authors, selectivity values increase when n-alkane chain length increases, while toluene distribution coefficients follow the opposite trend. Taking into account the results, the IL mixture can be an alternative to sulfolane in the extraction of toluene from mixtures toluene-n-alkane with low concentration of toluene.

The same mixture of ILs was selected for the extraction of benzene or ethylbenzene or xylenes from heptane [22]. The mixture with a [4EMpy][NTf2] molar fraction of 0.3 was chosen in all cases because of its density and viscosity similar to sulfolane and good extraction performance obtained in preliminary tests. As well as using a single IL, best results were obtained in the separation of benzene, whereas the lowest values of β and S were obtained in the separation of ethylbenzene. Compared to sulfolane, the mixture of ILs showed higher extractive properties in

**71**

*Extraction of Aromatic Compounds from Their Mixtures with Alkanes: From Ternary…*

the extraction of benzene and higher selectivity and slightly lower solute distribu-

As mentioned above, one of the main drawbacks of the use of ILs as extraction agents is their high viscosity that is difficult for the separation process. The selection of an appropriate solvent to mix with the IL could break the hydrogen bonds between the cation and the anion, lowering the viscosity but retaining the intrinsic properties of the IL. With the aim of lowering viscosity and decreasing costs, the mixtures of [EMim][OAc] + acetonitrile and [EMim][ESO4] + acetonitrile were used to extract benzene from a benzene + hexane mixture [23]. It is worth noting that solute distribution coefficients are very low compared to the values obtained using a single IL; however, both hexane composition in extract phase and solvent composition in raffinate phase are practically zero. This fact would considerably simplify the solvent recovery.

LLE data for systems with more than four components are very scarce. As a continuation of the extraction studies using a mixture of ILs, Larriba et al. [24] analysed the separation of BTEX fraction from a reformer gasoline model (n-hexane, n-heptane, n-octane). In that case, aromatic distribution coefficients using the IL mixture were significantly lower than using sulfolane, whereas selectivity values were higher. It is interesting to point out that in all pseudoternary system alkane + aromatic + IL mixture previously studied by the authors, they achieved better results using the mixture [4EMpy][NTf2] + [EMim][DCA] than using sulfolane. In order to improve these results, [EMim][DCA] was replaced by [EMim][TCM], and a mixture of [4EMpy][NTf2] + [EMim][TCM] with a [EMim][TCM] mole fraction of 0.6 was selected on the bases of extraction yield and thermophysical properties of the mixture [25]. The extraction results were used to perform the simulation of the extraction process including the extraction column and the recovery section of the solvent.

In order to study the extraction of BTEX fraction from naphtha reformate using 1-butyl-3-methylimidazolium hexafluorophosphate, [BMim][PF6], Al-Rashed et al. [26] determined liquid-liquid equilibrium data for the system hexane + heptane + octane + benzene + toluene + o-xylene + [BMim][PF6]. Comparing the aromatic extraction between this system and those systems with only one aromatic, they concluded that a combination of the three aromatic compounds affected negatively on the extractive capability of the IL. According to the authors, this can be because the aromatic compounds in one phase can strongly associate between them through π-π forces. Furthermore, the electrostatic association between the aromatic alkyl chains and alkanes present in the mixture also influences the extractive capability of the IL. The gasoline or naphtha model was constituted by equal amounts of alkanes in

In order to analyse the influence of the alkyl chain length of the IL cation on the extraction of toluene from its mixture with heptane and cyclohexane, liquidliquid equilibrium data of the quaternary systems heptane + cyclohexane + toluene + 1-ethyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [EMim] [NTf2], and heptane + cyclohexane + toluene + 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, [HMim][NTf2], were determined.

Experimental data were compared to the results obtained for the ternary systems heptane + toluene + [EMim][NTf2], cyclohexane + toluene + [EMim][NTf2], heptane + toluene + [HMim][NTf2] and cyclohexane + toluene + [HMim][NTf2] [27, 28].

*DOI: http://dx.doi.org/10.5772/intechopen.86229*

tion coefficients for p-xylene/heptane separation.

**2.3 Systems with more than four components**

all the above-mentioned studies.

**3. Experimental part**

#### *Extraction of Aromatic Compounds from Their Mixtures with Alkanes: From Ternary… DOI: http://dx.doi.org/10.5772/intechopen.86229*

the extraction of benzene and higher selectivity and slightly lower solute distribution coefficients for p-xylene/heptane separation.

As mentioned above, one of the main drawbacks of the use of ILs as extraction agents is their high viscosity that is difficult for the separation process. The selection of an appropriate solvent to mix with the IL could break the hydrogen bonds between the cation and the anion, lowering the viscosity but retaining the intrinsic properties of the IL. With the aim of lowering viscosity and decreasing costs, the mixtures of [EMim][OAc] + acetonitrile and [EMim][ESO4] + acetonitrile were used to extract benzene from a benzene + hexane mixture [23]. It is worth noting that solute distribution coefficients are very low compared to the values obtained using a single IL; however, both hexane composition in extract phase and solvent composition in raffinate phase are practically zero. This fact would considerably simplify the solvent recovery.

### **2.3 Systems with more than four components**

*Solvents, Ionic Liquids and Solvent Effects*

Sakal et al. [15] carried out the extraction of benzene from benzene + cyclohexane mixtures using 1,3-dimethylimidazolium dimethylphosphate, [MMim][DMP], and 1-methylimidazolium tetrafluoroborate, [Mim][BF4], and the mixtures [Mim] [BF4] wt. 50% + [MMim][DMP] wt. 50% and [Mim][BF4] wt. 25% + [MMim] [DMP] wt. 75%. Both selectivity and solute distribution coefficient follow the same trend: [MMim][DMP] > [Mim][BF4] wt. 25% + [MMim][DMP] wt. 75% > [Mim] [BF4] wt. 50% + [MMim][DMP] wt. 50%. Additionally, IL mixtures of 1-methylimidazolium perchlorate, [Mim][ClO4] and [MMim][DMP], at the same mass ratios were also checked, finding that the addition of [Mim][BF4] or [Mim][ClO4]

Potdar et al. [16] determined LLE data for the quaternary system hexane + benzene + 1-ethyl-3-methylimidazolium ethylsulphate, [EMim][ESO4], + 1-ethyl-3-methylimidazolium methylsulphate, [EMim][MSO4]. The comparison between β and S obtained using the mixture of ILs and each of them separately shows that the mixture of ILs has a lower extraction capacity. These worse results can be due to the fact that there is less free volume in a combination of these two ILs, hindering

The mixtures of N-butylpyridinium tetrafluoroborate, [Bpy][BF4], with N-butylpyridinium bis(trifluoromethylsulfonyl)imide, [Bpy][NTF2], and with 1-butyl-4-methylpyridinium bis(trifluoromethylsulfonyl) imide, [4BMpy][NTf2], were tested for the extraction of toluene from the mixture heptane-toluene. These ILs were selected because of the higher selectivity and the lower solute distribution coefficient showed by [Bpy][BF4] and the opposite behaviour showed by [Bpy] [NTf2] and [4BMpy][NTf2] [17–18]. In both cases, β and S values higher than those obtained using sulfolane were achieved with a [Bpy][BF4] mole fraction of 0.7 in

On the basis of the results obtained using 1-ethyl-3-methylimidazolium tricyanomethanide, [EMim][TCM], as well as 1-ethyl-3-methylimidazolium dicyanamide, [EMim][DCA], for the extraction of toluene from heptane, a mixture of [EMim][TCM] + [EMim][DCA] with a [EMim][TCM] mole fraction of 0.8 was selected to carry out the same extraction process [19]. Both solute distribution coefficient and selectivity are higher using this IL mixture than using sulfolane. Since selectivity values using the IL mixture were almost double than those using sulfolane, toluene extracted by the IL mixture would be significantly purer. The capability of the binary mixture of 1-ethyl-4-methylpyridinium bis(trifluoromethylsulfonyl)imide, [4EMpy][NTf2], and 1-ethyl-3-methylimidazolium dicyanamide, [EMim][DCA], as solvent for the extraction of toluene from its mixtures with heptane or 2,3-dimethylpentane or cyclohexane was evaluated by Larriba et al. [20]. The IL [4EMpy][NTf2] was selected because it showed solute distribution coefficients for toluene higher than sulfolane. On the other hand, toluene selectivity of [EMim][DCA] is substantially higher than that of sulfolane. The extraction of toluene from other alkanes (hexane, octane or nonane) was also performed [21]. According to the authors, selectivity values increase when n-alkane chain length increases, while toluene distribution coefficients follow the opposite trend. Taking into account the results, the IL mixture can be an alternative to sulfolane in the extraction of toluene from mixtures toluene-n-alkane with low concentration of toluene. The same mixture of ILs was selected for the extraction of benzene or ethylbenzene or xylenes from heptane [22]. The mixture with a [4EMpy][NTf2] molar fraction of 0.3 was chosen in all cases because of its density and viscosity similar to sulfolane and good extraction performance obtained in preliminary tests. As well as using a single IL, best results were obtained in the separation of benzene, whereas the lowest values of β and S were obtained in the separation of ethylbenzene. Compared to sulfolane, the mixture of ILs showed higher extractive properties in

does not improve the extraction capacity of [MMim][DMP].

the solution of benzene in the ILs mixture.

the mixture of ILs.

**70**

LLE data for systems with more than four components are very scarce. As a continuation of the extraction studies using a mixture of ILs, Larriba et al. [24] analysed the separation of BTEX fraction from a reformer gasoline model (n-hexane, n-heptane, n-octane). In that case, aromatic distribution coefficients using the IL mixture were significantly lower than using sulfolane, whereas selectivity values were higher. It is interesting to point out that in all pseudoternary system alkane + aromatic + IL mixture previously studied by the authors, they achieved better results using the mixture [4EMpy][NTf2] + [EMim][DCA] than using sulfolane. In order to improve these results, [EMim][DCA] was replaced by [EMim][TCM], and a mixture of [4EMpy][NTf2] + [EMim][TCM] with a [EMim][TCM] mole fraction of 0.6 was selected on the bases of extraction yield and thermophysical properties of the mixture [25]. The extraction results were used to perform the simulation of the extraction process including the extraction column and the recovery section of the solvent.

In order to study the extraction of BTEX fraction from naphtha reformate using 1-butyl-3-methylimidazolium hexafluorophosphate, [BMim][PF6], Al-Rashed et al. [26] determined liquid-liquid equilibrium data for the system hexane + heptane + octane + benzene + toluene + o-xylene + [BMim][PF6]. Comparing the aromatic extraction between this system and those systems with only one aromatic, they concluded that a combination of the three aromatic compounds affected negatively on the extractive capability of the IL. According to the authors, this can be because the aromatic compounds in one phase can strongly associate between them through π-π forces. Furthermore, the electrostatic association between the aromatic alkyl chains and alkanes present in the mixture also influences the extractive capability of the IL.

The gasoline or naphtha model was constituted by equal amounts of alkanes in all the above-mentioned studies.
