Improving the Separation of n-Heptane + Ethanol Azeotropic Mixtures

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Improving the separation of n-heptane + ethanol azeotropic mixtures combining ionic liquid 1-ethyl-3methylimidazolium acetate with different inorganic salts Filipe S. Oliveira, Ralf Dohrn, Luis Paulo N. Rebelo, and Isabel M. Marrucho Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b00810 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 29, 2016

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Industrial & Engineering Chemistry Research

Improving the separation of n-heptane + ethanol azeotropic mixtures combining ionic liquid 1-ethyl-3-methylimidazolium acetate with different inorganic salts

Filipe S. Oliveiraa, Ralf Dohrnb, Luís P. N. Rebeloa and Isabel M. Marruchoa,c* a

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de

Lisboa, Apartado 127, 2780-157 Oeiras, Portugal. b

c

Bayer Technology Services GmbH, 51368 Leverkusen, Germany. Centro de Química Estrutural, Instituto Superior Tecnico, Universidade de Lisboa,

Avenida Rovisco Pais, 1049-001 Lisboa, Portugal.

KEYWORDS: ionic liquids; inorganic salts; azeotrope mixtures; liquid-liquid equilibria

ACS Paragon Plus Environment


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Abstract In this work, the ionic liquid (IL) 1-ethyl-3-methylimidazolium acetate, [C2MIM][Ac], was combined with the inorganic salts (ISs) ammonium acetate, [NH4][Ac], ammonium chloride, [NH4]Cl, and ammonium thiocyanate, [NH4][SCN], and used as extraction solvent for the separation of the azeotropic mixture of n-heptane + ethanol. The liquid-liquid equilibria (LLE) of the ternary mixtures of n-heptane + ethanol + IL or IL-IS was experimentally measured at 298.15 K and 0.1 MPa. The feasibility of the extraction solvents was assessed by the distribution coefficient and the selectivity. The results showed that the extraction solvents studied can be suitable candidates for the separation of ethanol from n-heptane. Additionally, we show that the solubilisation of ISs in the IL can greatly increase the selectivity of the latter, while no significant impact on the distribution coefficient is verified. Moreover, the distribution coefficient values obtained for the [C2MIM][Ac] and its mixtures with ISs are the highest among neat ILs tested so far for the azeotropic mixture in study. The experimental liquid-liquid equilibrium data were correlated using the nonrandom two-liquid (NRTL) excess Gibbs free energy model.

1. Introduction In the last decades, the volume of oxygenated additives used in gasolines has been increasing from year to year. Lower alkyl chain alcohols, such as methanol, ethanol or butanol, and ethers such as, methyl-tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE) and ter-amyl ethyl ether (TAEE) are typical examples of oxygenated additives. These compounds are known for their higher octane number, which allows a boost in combustibility and the reduction of CO emissions, when added to gasolines.


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Due to the discontinuation of lead-based additives, the production of oxygenated compounds as gradually become higher, and with it a growing number of processes in which alkanes and alcohols co-exist has emerged 1-3. The presence of an azeotropic point in mixtures of n-hexane or n-heptane + ethanol makes it impossible to separate these mixtures by a simple distillation, hence other techniques must be used. In the literature, the separation of azeotropic mixtures of alkanes + ethanol as already been studied by several group, using liquid-liquid extraction and ionic liquids (ILs) has extraction solvents. Letcher et al. 4 was the first to test an IL as extraction solvent for the separation of mixtures of alkanes + alcohols. Using 1-methyl-3-octylimidazolium chloride, Letcher and co-workers separated alkanes such as n-heptane, dodecane and hexadecane from alcohols methanol and ethanol. The highest selectivity values were obtained for the mixtures of hexadecane + methanol. Pereiro et al.

5, 6

started testing hexafluorophosphate-based ILs in the separation

of n-hexane or n-heptane + ethanol mixtures. The ILs selected were 1-hexyl-3methylimidazolium

([C6MIM][PF6])

and

1-octyl-3-methylimidazolium

hexaflurophosphate ([C8MIM][PF6]) and both proved to break the azeotropic point of the tested mixtures, with the [C6MIM][PF6] showing very high selectivities in the case of n-heptane + ethanol mixtures. However, the low distribution coefficients and its solutropic behaviour makes the use of this IL inadvisable. Afterwards, Pereiro et al focused on alkyl sulfate-based ILs to break the azeotrope mixtures of n-hexane or n-heptane + ethanol. The ILs tested included the 1butyl-3-methylimidazolium dimethylimidazolium

methyl

methyl sulfate

sulfate

[C4MIM][C1SO4]

([C1MIM][C1SO4])

9,

10

7,

and

8

,

1,3-

1-ethyl-3-

methylimidazolium ethyl sulfate ([C2MIM][C2SO4]) 11. All of the ILs used proved to be suitable candidates for the extraction of ethanol, particularly in mixtures of n-heptane +



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ethanol, where particularly high efficiencies were obtained. In addition, it was shown that a shorter alkyl chain on the imidazolium cation led to better n-heptane/ethanol efficiencies. More recently, a lot of ILs based on the bis(trifluoromethylsulfonyl)imide anion have been used in the separation of n-hexane or n-heptane + ethanol azeotropic mixtures 12-16

. Seoane et al.

cation,

16

studied the effect of increasing the alkyl chain of the imidazolium

using

four

different

ILs,

1-ethyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide

([C2MIM][NTf2]),

1-propyl-3-methylimidazolium


([C3MIM][NTf2]),

1-butyl-3-methylimidazolium

bis(trifluoromethylsulfonyl)imide ([C4MIM][NTf2]) and 1-hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C6MIM][NTf2]); González et al.

15

tested two

pyridinium-based ILs, 1-ethyl-3-methylpyridinium bis(trifluoromethylsulfonyl)imide ([C2-3-C1Py][NTf2])

and

1-propyl-3-methylpyridinium

bis(trifluoromethylsulfonyl)imide ([C3-3-C1Py][NTf2]); and Aranda et al.

12

used two

tetraalkyl ammoniums, butyltrimethylammonium bis(trifluoromethylsulfonyl)imide ([BTMA][NTf2])

and

tributylmethylammonium


([TBMA][NTf2]). The results obtained from these three studies, showed similar ranges of the distribution coefficient and selectivity values, with the imidazolium-based ILs showing slightly higher values and the ammonium-based ILs the lowest. On another related work, Corderí and González showed that pyridinium-based ILs could yield better results than imidazolium-based ILs when combined with the trifluoromethanesulfonate anion ([OTf]-) instead of the [NTf2]- anion in the separation of n-hexane + ethanol azeotropic mixtures 13. The same authors, used three different ILs all with the same cation, 1-hexyl-3-methylimidazolium, and observed that the extraction of ethanol in n-hexane + ethanol mixtures was more effective when the dicyanamide


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anion ([DCA]-) was used, followed by the [OTf]- anion and lastly the [NTf2]- anion 14. In addition, these authors also observed high distribution coefficient and selectivity values in the separation of n-hexane or n-heptane + ethanol mixtures, using 1-butyl-1methylpyrrolidinium cation ([C4-3-C1pyr]+) combined with [DCA]- and [OTf]- anions 17

. Cai et al.

18, 19

used phosphate-based ILs in the separation of n-hexane or n-

heptane + ethanol azeotropic mixtures. The results obtained showed that smaller alkyl chain in both cation and anion are preferential, with the IL 1,3-dimethylimidazolium dimethylphosphate ([C1MIM][DMP]) showing higher efficiency in both n-hexane and n-heptane systems than the other two ILs tested, 1-ethyl-3-methylimidazolium diethylphosphate ([C2MIM][DEP]) and 1-butyl-3-methylimidazolium dibutylphosphate ([C4MIM][DBP]). Moreover, the used of low transition temperature mixtures, based on combinations of cholinium chloride with different organic acids or alcohols, has also been attempted by some of us 20 and Rodriguez et al. 21. These compounds proved to be highly efficient extraction solvents for the separation of n-heptane + ethanol mixtures, due to their ability to form H-bonds with the ethanol. However, despite the higher selectivities presented by these compounds, their distribution coefficients fall in the same range of neat ILs. In this work, we explore the use of combinations of one IL and several ISs as extraction solvents for azeotropic mixtures. It is our hope, to combine the advantages of the ILs, easy operation / liquid state and non-volatility, with the advantages of ISs, their high separation ability. The use of mixtures of IL-IS as separating agents for azeotropic mixtures has already been attempted by Lei et al. 22 in the separation of water + ethanol mixtures by extractive distillation, where it was shown that the addition of potassium



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acetate to the IL effectively increased the volatility of ethanol to water, allowing a better separation of the azeotropic mixture. Furthermore, in our previous work

23

, we also

showed that by combination the IL 1-ethyl-3-methylimidazolium ethyl sulfate ([C2MIM][C2SO4]) with different amounts of [NH4][SCN] (HIIL17, HIIL33 and HIIL45), the selectivity values could be increased due to the higher ionicity of the mixtures, leading to a higher efficiency in the extraction of ethanol from n-heptane + ethanol azeotropic mixtures. Therefore, as a continuation of our previous work, in this study we used mixtures of the IL 1-ethyl-3-methylimidazolium acetate ([C2MIM][Ac]) with the same concentration of three different inorganic salts (ISs), ammonium acetate ([NH4][Ac]), ammonium chloride ([NH4]Cl) and ammonium thiocyanate ([NH4][SCN]) as extraction solvents for n-heptane + ethanol azeotropic mixtures. The liquid-liquid equilibria for the ternary or pseudoternary systems n-heptane + ethanol + IL or IL-IS was measured at 298.15 K and 0.1 MPa and the distribution coefficient and selectivity parameters were determined in order to assess the feasibility of the IL and IL-IS mixtures as extraction solvents.

2. Experimental 2.1. Materials N-heptane and ethanol were supplied by Riedel-de-Haën with 99 % purity and by Scharlau with 99.9 % purity, respectively. The inorganic salts, ammonium acetate ([NH4][Ac]), ammonium chloride ([NH4]Cl) and ammonium thiocyanate ([NH4][SCN]), were all provided by Sigma-Aldrich with a purity content superior to 98.0 %, 99.5 % and 99.0 %, respectively and were used as received.


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The ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2MIM][Ac]) was purchased from Iolitec with a mass fraction purity of ≥ 95 %. 1H NMR was used for purity check and the water content was measured by Karl Fischer coulometric titration (Metrohm 831 KF Coulometer). The ionic liquid was used as received and the final water mass fraction ranged between 5000 − 8000 ppm of water.

2.2. Mixtures of ionic liquids with inorganic salts Three different inorganic salts, [NH4][Ac], [NH4]Cl and [NH4][SCN] were combined with the same ionic liquid [C2MIM][Ac]. All of the IL-IS mixtures were prepared at the same IS concentration, 0.33 molar fraction, i.e., 1 mol of IS per 2 mol of IL. This concentration was chosen because it allows for a direct comparison of the effects caused by the different ISs, but mainly due to the higher ionicity of the system at this molar fraction. Table 1, presents the ionicity of the IL-IS mixtures tested, calculated by the Walden Plot method24. In this method, the determination of the ionicity is made by considering the deviation of the substance to the ideal Walden line (ideal electrolyte), meaning that a smaller deviation leads to a higher ionicity.

Table 1 Ionicity (∆W) calculated from the Walden Plot deviation for the neat IL and the mixtures IL + IS tested in this work. Solvent

∆Wa

[C2MIM][Ac]

0.1238

0.67 [C2MIM][Ac] + 0.33 [NH4][Ac] (IL-33AC)

0.0881

0.67 [C2MIM][Ac] + 0.33 [NH4]Cl (IL-33CL)

0.0078

0.67 [C2MIM][Ac] + 0.33 [NH4][SCN] (IL-33SCN)

-0.0098



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a

the ionicity data is taken from reference 25.

2.3. LLE measurements The ternary LLE experiments were performed at 298.15 K in a glass cell thermostatically regulated by a water jacket connected to an external water bath controlled to ± 0.01 K. The temperature in the cell was measured with a platinum resistance thermometer coupled to a Keithley 199 System DMM/Scanner that was calibrated with high–accuracy mercury thermometers (0.01 K). The mixing was assured by a magnetic stirrer. The binodal curve of the ternary system was determined by preparing several binary mixtures of IL or (IL+IS) + n-heptane in the immiscible region and then ethanol was added until the ternary mixture became miscible. Afterwards, the density and the refractive index of those ternary mixtures was determined in duplicate. The measurements of density and refractive index were both performed at 303.15 K and atmospheric pressure, in order to assure that no phase splitting happened during the measurements. The density measurements were performed using an automated Anton Paar Densimiter DMA 5000, while the refractive index measurements were performed using an automated Anton Paar Refractometer Abbemat 500, where the relative standard deviation for the measurements was of 0.007 % and 0.002 % for the density and the refractive index, respectively. The determination of the tie-lines, was performed by preparing ternary mixtures of known composition that were vigorously stirred for at least 1 h and left to settle for at least 12 h in a thermostatic bath at 298.15 K. Then, samples from both phases were taken with a syringe and their densities and refractive indexes were determined in duplicate. The composition of both phases in equilibrium was calculated by


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MATLAB23, using the fitting of the densities and of the refractive indexes with the composition along the binodal curve by the following equations:

ρ = A⋅ w1 + B⋅ w12 +C ⋅ w13 + D⋅ w14 + E ⋅ w2 + F ⋅ w22 + G⋅ w23 + H ⋅ w24 + I ⋅ w3 + J ⋅ w32 + K ⋅ w33 + L⋅ w34

(1)

nD = M ⋅ w1 + N ⋅ w12 + O⋅ w13 + P⋅ w14 + Q⋅ w2 + R⋅ w22 + S ⋅ w23 +T ⋅ w24 +U ⋅ w3 +V ⋅ w32 +W ⋅ w33 + X ⋅ w34

(2)

w 3 = 1 − (w1 + w 2 )

(3)

where, ρ and nD correspond to the density and refractive index, respectively, w1, w2 and w3 correspond to the mass fraction compositions of heptane, ethanol and IL or IL-IS, respectively, and A to X are adjustable parameters which are given in Table S1 (Supporting Information). The root-mean-square deviations obtained for the fittings were always lower than 0.02 % and 0.03 % for the refractive indexes and the density, respectively in all systems studied. In order to determine the uncertainty of the composition of the tie lines, duplicate samples with the same initial composition were prepared and their respective tie-lines were calculated, showing a standard deviation lower than 0.003 in the mass fraction.

3. Modelling 3.1. The NRTL model A flexible way to correlate liquid-liquid equilibria is the use of excess Gibbs free energy models, like the non-random two-liquid (NRTL) model 26 or the universal quasichemical (UNIQUAC) model

27

. Although these widely used models were not

originally intended for systems containing ILs, they have been successfully applied to



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calculate LLE of systems containing ILs; there are several examples in the literature, both for NRTL 28-30 and for UNIQUAC 29-32. Due to its slightly higher flexibility and its simplicity concerning IL + IS systems, we used the NRTL model in this study. Calculations were performed using the Aspen Plus 8.4 process simulator. Since NRTL is well known, only the essential equations are given here. If NRTL is used, the activity coefficient equation for component i in a multicomponent mixture is given by:

∑τ G x G x = +∑ ∑G x ∑G x ji

ln γ i

ji

j

j

ij

ki k

k

j

j

kj

k

 ∑n τ nj Gnj xn   τ ij −  Gkj xk  ∑ k k  

Gij = exp(− α ij ⋅ τ ij )

with

(4)

(5)

For the interaction parameters, τij, Aspen Plus uses the following temperature dependence:

τ ij = aij +

bij

(6)

T

where aij and bij are adjustable parameters. Each IL-IS mixture was treated like a pseudo-component, of which the molar mass was calculated from the molar mass of the IL and of the salt according to the total composition of the IL-IS mixture.


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3.2. Strategy of Parameter Fitting Since all experimental data have been measured at 298.15 K, we used only the aij parameter in Eq. (6), except for the ethanol – n-heptane system, for which b12 and b21 are given in the Aspen Plus parameter data bank APV84 VLE-LIT. In the Aspen Plus parameter data bank NISTV84 NIST-IG there are NRTL parameters available for nheptane and [C2MIM][Ac]. We used these data bank parameters without further adjustments and fitted the interaction parameters of the remaining binary systems. For a large number of binary systems, non-randomness factor αij varies between 0.20 and 0.47 33

. For the interactions between n-heptane (1) and IL-IS (3) we set α13 to 0.3, being the

standard value in Aspen Plus and in agreement with the value used for n-heptane – [C2MIM][Ac] in the NISTV84 NIST-IG databank. Interaction parameters a13 and a31 were fitted to the experimental data by using the root mean square deviation (rmsd) as an objective function:

rmsd =

∑ ∑ ∑ (x i

p

t

exp i, p ,t

− xicalc , p ,t

)

2

6k

(7)

where superscripts i, p, and t refer to the component, phase and tie line, x is the mole fraction, and k is the number of tie lines. For the interaction between ethanol (2) and IL (3) or IL-IS (3), a value of

α23 = - 0.1 lead to significantly better results concerning the slope of the tie lines and the shape of the binodal curve in the ternary systems than using the standard value α23=0.3. All binary interaction parameters used for the calculations are summarized in Table 2.



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Table 2 Binary interaction parameters for the NRTL equation. Component. i Component j

Source

aIJ

aJI

bIJ

bJI

αIJ

n-heptane

ethanol

APV84

0

0

558,28

411,88

0,3

[C2MIM][Ac]

NISTV84

3,846

4,766

0

0

0,3

ethanol

IL+33% [NH4][Ac]

fitted

4

5

0

0

0,3

IL+33% [NH4]Cl

fitted

5

7

0

0

0,3

IL+33% [NH4][SCN]

fitted

4,8

6,8

0

0

0,3

[C2MIM][Ac]

fitted

3

-6,5

0

0

-0,1

IL+33% [NH4][Ac]

fitted

2,2

-7

0

0

-0,1

IL+33% [NH4]Cl

fitted

-0,4

-10,1

0

0

-0,1

IL+33% [NH4][SCN]

fitted

0,2

-9,8

0

0

-0,1

4. Results and discussion 4.1. Ternary diagrams The ternary diagrams for the systems n-heptane (1) + ethanol (2) + IL or IL-IS (3) obtained at 298.15 K and 0.1 MPa are presented in Figures 1-4. From the inspection of the latter Figures it can be seen that in all systems studied, with the neat IL and with the IL-IS mixtures, big immiscibility regions were obtained independently of the extraction solvent used, where the n-heptane is always immiscible in the IL or IL-IS mixtures and the ethanol is always miscible in the hole composition range in both nheptane and extraction solvent. In addition, Figure S1 (Supporting Information) depicts a comparison between the binodal curves of all studied extraction solvents in this work, where it is shown that the addition of an IS to the IL increases the immiscibility region. This behaviour is consistent with our previous work where different amounts of the same IS ([NH4][SCN]) were added to the IL [C2MIM][C2SO4], and it was observed that the immiscibility region increased as the [NH4][SCN] content increased as well

23

. In

this study, the IS [NH4][Ac] showed the smallest increase of the immiscibility region,


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possibly due to the effect of the common ion, whereas the two other ISs, [NH4]Cl and [NH4][SCN], showed similar increments on the immiscibility region. Moreover, Figures 1-4 also show the experimental tie-lines for each extraction solvent, where it can be seen that positive slopes were obtained in all systems. This behaviour indicates that the ethanol is more soluble than the n-heptane in the extraction solvents studied, meaning that the extraction of ethanol from n-heptane is always favourable. Indeed, in Tables S2-S5 (Supporting Information), the compositions for the tie-lines are presented and it is clear that in the n-heptane rich phase of every system studied, the compositions of ethanol and IL or IL-IS are always very low, to the point that, in some cases they fall under the error of our determination method and the distribution coefficient and selectivity values are not possible to calculate. In Figures 1-4 the results of the modelling are compared with the experimental data. The agreement between the calculated slopes of the tie lines and the experimental values is very good. The binodal curves are well represented by the NRTL model, except for the region close to the critical point were the model predicts a too large liquid-liquid region.



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Ethanol 0.0 0.1

1.0 0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Heptane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

[C2MIM][Ac]

Figure 1. Ternary diagram for the system n-heptane (1) + ethanol (2) + [C2MIM][Ac] (3) at 298.15 K and 0.1 MPa. The gray dots represent the binodal curve; the gray squares the composition of the initial mixture; the black dots and lines represent the experimental tie-lines, while the dotted lines the calculated tie-lines by the NRTL model; the compositions are given in mass fraction.


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Ethanol 0.0 0.1

1.0 0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Heptane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

IL-33AC

Figure 2. Ternary diagram for the system n-heptane (1) + ethanol (2) + (0.67[C2MIM][Ac] + 0.33[NH4][Ac]) (3) at 298.15 K and 0.1 MPa. The blue dots represent the binodal curve; the blue squares the composition of the initial mixture; the black dots and lines represent the experimental tie-lines, while the dotted lines the calculated tie-lines by the NRTL model; the compositions are given in mass fraction.



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Ethanol 0.0 0.1

1.0 0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Heptane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

IL-33CL

Figure 3. Ternary diagram for the system n-heptane (1) + ethanol (2) + (0.67[C2MIM][Ac] + 0.33[NH4]Cl) (3) at 298.15 K and 0.1 MPa. The green dots represent the binodal curve; the green squares the composition of the initial mixture; the black dots and lines represent the experimental tie-lines, while the dotted lines the calculated tie-lines by the NRTL model; the compositions are given in mass fraction.


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Ethanol 0.0 0.1

1.0 0.9

0.2

0.8

0.3

0.7

0.4

0.6

0.5

0.5

0.6

0.4

0.7

0.3

0.8

0.2

0.9

0.1

1.0

Heptane

0.0

0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

IL-33SCN

Figure 4. Ternary diagram for the system n-heptane (1) + ethanol (2) + (0.67[C2MIM][Ac] + 0.33[NH4][SCN]) (3) at 298.15 K and 0.1 MPa. The orange dots represent the binodal curve; the orange squares the composition of the initial mixture; the black dots and lines represent the experimental tie-lines, while the dotted lines the calculated tie-lines by the NRTL model; the compositions are given in mass fraction.

4.2. Distribution coefficient and selectivity In order to evaluate the feasibility of the neat IL and the mixtures of IL-IS in the separation of n-heptane + ethanol mixtures, two widely used parameters were calculated, the distribution coefficient and the selectivity. Since we are evaluating the ability of the extraction solvent (IL or IL-IS) to extract the ethanol from the n-heptane, the distribution coefficient, β, was calculated in regards to the ethanol as shown in the following equation:

β2 =

w2II w2I

(4)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

where the w2I and w2II represent the mass fraction of ethanol in the n-heptane rich phase (upper phase) and in the IL or IL-IS rich phase (lower phase), respectively. The distribution coefficient represents the ratio between the amount of ethanol that is extracted to the IL or IL-IS and the amount that remains in the n-heptane phase, thus accounting for the solute-carrying capacity of the extraction solvent. Figure 5 depicts the distribution coefficients obtained for all systems studied as a function of the mass fraction of ethanol in the n-heptane rich phase, these values are also presented in Tables S2-S5 (Supporting Information). As expected, the distribution coefficient increases as the concentration of ethanol decreases in the n-heptane rich phase. In addition, the values obtained are very similar for all the system studied, meaning that the addition of the IS or the increase in the ionicity did not promote a significant effect on this parameter. Nevertheless, the distribution coefficients presented are higher than those obtained with other neat ILs or other IL-IS mixtures as shown in Figure S2 (Supporting Information), consistently showing values superior to 10 for w2I < 0.05. The high distribution coefficient obtained were somewhat expected due to the presence of the acetate anion on the IL, which will certainly establish stronger H-bond interactions with the ethanol, owing to its carboxylate group -COO-. A higher distribution coefficient allows the use of lower amounts of the extraction solvent and usually leads to a lower solvent flow rate, a smaller-diameter column and lower operating costs.


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160 [C2MIM][Ac]

140

[C2MIM][Ac] + [NH4][Ac] [C2MIM][Ac] + [NH4][Cl]

80

β2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[C2MIM][Ac] + [NH4][SCN]

60 40 20 0 0.00

0.02

0.04

0.06

0.08

0.10

wEthanol Upper Phase Figure 5. Distribution coefficient values, β2, for the ternary systems n-heptane (1) + ethanol (2) + IL or IL-IS (3), as a function of ethanol mass fraction in n-heptane-rich phase at 298.15 K and 0.1 MPa.

The selectivity, S, is the parameter that evaluates the efficiency of the extraction solvent, indicating the ease of extraction of the solute, ethanol, from its diluent, nheptane, and was determined by the following equation:

S=

w1I × β2 w1II

(5)

where, β2 is the distribution coefficient of ethanol and w1I and w1II are the mass fractions of n-heptane in the n-heptane rich phase (upper phase) and IL or IL-IS rich phase (lower phase), respectively. Figure 6 and Tables S2-S5 (Supporting Information) present the selectivity values obtained for all the systems studied. In agreement with the β2 behaviour, the selectivity also increases as the concentration of ethanol decreases in the n-heptane rich



phase. As shown in Figure 6, the addition of IS to the IL positively affects this parameter and a relation with the ionicity of the system is possible to establish. It can be seen that the selectivity increases in the following trend [C2MIM][Ac] < IL-33AC < IL33CL < IL-33SCN, which follows the same order as the ionicity of these extraction solvents, presented in Table 1. In our previous work

23

, we had already shown that by

increasing the ionicity of the IL, the selectivity of the extraction could also be increased, thus showing that the addition of an IS to an IL could improve the extraction of ethanol from n-heptane + ethanol azeotropic mixtures. A high selectivity usually means that we have a decrease of the amount of inert, n-heptane in the studied systems, in the extract which will be the IL-IS rich phase to where the ethanol is extracted. As shown in the ternary diagrams, the addition of IS increases in the immiscibility region of the system, making the n-heptane more insoluble in the IL-IS mixture than in the neat IL, consequently leading to a reduction of its amount in the IL-IS rich phase. In addition, a higher selectivity also allows for fewer stages in the process of extraction.

6000 5000

Selectivity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 33

4000

1800

[C2MIM][Ac] 1500

[C2MIM][Ac] + [NH4][Ac] [C2MIM][Ac] + [NH4][SCN]

1200

[C2MIM][Ac] + [NH4][Cl] 900

3000 2000 1000 0 0.00

600 300 0 0.00

0.02

0.01

0.02

0.04

0.03

0.04

0.05

0.06

wEthanol Upper Phase


0.06

0.07

0.08

0.08

0.10

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Figure 6. Selectivity, S, for the ternary systems n-heptane (1) + ethanol (2) + IL or ILIS (3), as a function of ethanol mass fraction in n-heptane-rich phase at 298.15 K and 0.1 MPa. The inset figure represents an enlargement of the data, whereas the lines are just for eye guidance.

4.3. Comparison with the literature In this section, the β and the S values obtained in this work were compared to those of other neat ILs and IL-IS mixtures found in literature for the same azeotropic mixture. The β values found in literature are expressed in molar fractions, hence they were converted in mass fraction because of the big differences in the molecular weights of the ILs, which resulted in lower values of β. In Figure 7 the comparison between the β values of this work and those found in literature is establish for approximately the same concentration of ethanol, while Figure S2 (Supporting Information) depicts the same comparison in the whole ethanol concentration range. As mentioned in the previous section, the presence of ions that can establish Hbonds with the ethanol is crucial to increase the β values. Since ethanol is a compound of small molecular weight and with a -OH group, IL or IL-IS mixtures with smaller ions and with the capacity to establish H-bonds will be preferential to the extraction of ethanol. It can be seen that either the neat [C2MIM][Ac] or its mixtures with ISs are better suitable to extract the ethanol than the other neat ILs found in literature. The increase in the distribution coefficient follow the trend acetated-based ILs > dicyanamide-based ILs > sulfate-based ILs > phosphate-based ILs > [OTf]- based ILs > [NTf2]- based ILs. In addition it can also be seen that the anion of the IL plays a much more important role in the β values, than the cation. This behaviour is better illustrated



in the work of Corderí et al. 14 where several ILs with the same cation where tested and only the anion was changed, and in the works with [NTf2]- based ILs 12, 15, 16, where the different cations tested yielded β values in the same range (2-4), and in the works with phosphate-based ILs

5, 18

, where [C8MIM][PF6], that presents a bigger alkyl chain than

[C1MIM][DMP], [C2MIM][DEP] and [C4MIM][DBP], has a higher distribution coefficient than [C4MIM][DBP] but smaller than the other two.

20

15

β2

10

5

0

[C2MIM][Ac] IL-33AC IL-33CL IL-33SCN [C1MIM][C1SO4] [C4MIM][C1SO4] [C2MIM][C2SO4] HIIL17 HIIL33 HIIL45 [C2MIM][NTf2] [C3MIM][NTf2] [C4MIM][NTf2] [C6MIM][NTf2] [BTMA][NTf2] [TBMA][NTf2] [C2-3-C1Py][NTf2] [C3-3-C1Py][NTf2] [C4-3-C1Py][OTf] [C4-3-C1Py][DCA] [C8MIM][PF6] [C1MIM][DMP] [C2MIM][DEP] [C4MIM][DBP]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 7. Comparison between the distribution coefficient values, β2, of the systems studied in this work, other IL-IS mixtures 23 and neat ILs found in literature 5, 7, 9, 11, 12, 15-18

. The comparison was done at an ethanol mass fraction between 0.02 - 0.03 wt% in

n-heptane-rich phase at 298.15 K and 0.1 MPa.

In Figures 8 and S3 (Supporting Information), the comparison between the S values of this work and those found in literature is establish for approximately the same concentration of ethanol and in the whole ethanol concentration range, respectively. It


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can be seen from panel a of Figure 8, that the ILs and IL-IS mixtures based on sulfate anions are the most efficient extraction solvents for ethanol in n-heptane + ethanol azeotropic mixtures. It was our expectation that the selectivity values for the [C2MIM][Ac] and its mixtures with ISs, to be higher than those obtained since we have an IL with a small alkyl chain in the cation and an anion capable of establishing strong H-bonds with the ethanol. However, the reason for the lower than expected selectivity values of the [C2MIM][Ac] and its mixtures, could be due to its great solvent properties. As shown in other works, [C2MIM][Ac] as proven to be capable of dissolving a wide range of compounds, raging form different ISs proteins and enzymes

34

, to lignocellulosic materials

39, 40

35-38

, and also of having a high performance in capturing CO2

,

41-

43

. As mentioned in the previous section, the immiscibility of the IL in the n-

heptane can play a crucial role in the determination of the selectivity parameter. Hence ILs with lower solubility in n-heptane can produce higher selectivity values. From panel b in Figure 8, it is possible to see that as the alkyl chain of the IL is increase (either in the cation or in the anion), the selectivity decreases. Nevertheless, the results show that with the addition of an IS, the selectivity parameter can be greatly increased reaching a value more than 3 times higher when [NH4][SCN] is added. Indeed, the IL-33SCN displayed selectivity values comparable to some of the better neat ILs, such as [C2MIM][C2SO4], [C4-3-C1Pyr][DCA] and [C1MIM][DMP], and higher than the remain neat ILs, including [C4MIM][C1SO4], [C43-C1Pyr][OTf], some phosphate based ILs and all the [NTf2]- based ILs.



1800

a)

1600 1400

Selectivity

1200 1000 800 600 400 200 0

b)

350 300

Selectivity

250 200 150 100 50 0

[C2MIM][Ac] IL-33AC IL-33CL IL-33SCN [C1MIM][C1SO4] [C4MIM][C1SO4] [C2MIM][C2SO4] HIIL17 HIIL33 HIIL45 [C2MIM][NTf2] [C3MIM][NTf2] [C4MIM][NTf2] [C6MIM][NTf2] [BTMA][NTf2] [TBMA][NTf2] [C2-3-C1Py][NTf2] [C3-3-C1Py][NTf2] [C4-3-C1Pyr][OTf] [C4-3-C1Pyr][DCA] [C8MIM][PF6] [C1MIM][DMP] [C2MIM][DEP] [C4MIM][DBP]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 33

Figure 8. Comparison between the selectivity values, S, of the systems studied in this work other IL-IS mixtures

23

and neat ILs found in literature

5, 7, 9, 11, 12, 15-18

. The

comparison was done at an ethanol mass fraction between 0.02 - 0.03 wt% in nheptane-rich phase at 298.15 K and 0.1 MPa. Panel b) is an enlargement of panel a).


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5. Conclusion In this work, the use of mixtures of an IL with different IS was assessed for the separation of n-heptane + ethanol azeotropic mixtures. Three different IS were tested at the same concentration and the LLE for the ternary systems n-heptane + ethanol + IL or IL-IS was measured at 298.15 K and atmospheric pressure. The feasibility of the IL and IL-IS mixtures was determined by the calculation of the distribution coefficient and selectivity values. The results obtained show that the addition of IS to the IL lead to the increase of the immiscibility region of the systems, which was translated in the increase in the selectivity of the extraction solvent. The increase in the selectivity observed followed the trend IL-33SCN > IL-33CL > IL-33AC > [C2MIM][Ac], which could be correlated with the increase in the ionicity of the IL. The distribution coefficient values were not significantly affected by the introduction of the IS, nevertheless the data obtained showed that both ILs as IL-IS mixtures based on acetate anions leads to higher distribution coefficients than all the other neat ILs tested so far. The measured phase equilibria were successfully correlated by using the NRTL model and by treating the mixtures of IL + IS as a pseudo-component. The slope of the tine lines and the binodal curves are well represented by the model, except for the region close to the critical point were the model predicts a too large liquid-liquid region. Thus the introduction of IS in the ILs could provide a boost in the feasibility of the ILs as extraction solvents for the separation of azeotropic mixtures in LLE systems that can be obtained at a low cost. Therefore, the knowledge gained in this study could open the door for the production more promising extraction solvents, due to the wide range of combinations that can be establish between ILs and ISs.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Associated content The Supporting Information presents the tables with the calculated compositions for the experimental tie-lines, ethanol distribution coefficient and selectivity (S) for each of the four ternary systems studied in this work. In addition, a table with all the adjustable parameters used in equations 1 and 2 was also introduced. Figures comparing the selectivity and ethanol distribution coefficient values obtained in the present work with those in literature are also presented as well as a figure mapping binodal curve for systems studied. This information is available free of charge via the Internet at http://acs.pubs.org.

Author information Corresponding author. *E-mail: [email protected]

Acknowledgements Filipe S. Oliveira gratefully acknowledges the financial support of FCT/MCTES (Portugal) through the PhD fellowship SFRH/BD/73761/2010 and Isabel M. Marrucho for a contract under FCT Investigator 2012 Program. The authors also acknowledge the Fundação para a Ciência e Tecnologia (FCT) for the financial support through the project PTDC/EQU-FTT/1686/2012 and Research Unit GREEN-it "Bioresources for Sustainability" (UID/Multi/04551/2013). The NMR spectrometers are part of the National NMR Facility (RECI/BBB-BQB/0230/2012), also supported by FCT.


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Improving the Separation of n-Heptane + Ethanol Azeotropic Mixtures

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