Aqueous Biphasic Systems of Pyrrolidinium Ionic ... - ACS Publications

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Aqueous Biphasic Systems of Pyrrolidinium Ionic Liquids with Organic Acid-Derived Anions and K3PO4 Karen G. Joaõ ,†,§ Liliana C. Tomé,†,§ Andreia S. L. Gouveia,† and Isabel M. Marrucho*,†,‡ †

Instituto de Tecnologia Química e Biológica António Xavier, Universidade Nova de Lisboa, Av. da República, 2780-157 Oeiras, Portugal ‡ Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Avenida Rovisco Pais, 1049-001 Lisboa, Portugal S Supporting Information *

ABSTRACT: Aqueous biphasic systems based on ionic liquids (ILs) have been researched as promising extraction and purification routes for a huge diversity of compounds. The inherent tunability offered by ILs combined with the large variety of salts available underlines the reliable phase equilibrium data. In this vein, this work presents novel aqueous biphasic systems based on the 1-butyl-1-methylpyrrolidinium cation combined with anions derived from organic acids, such as acetate, trifluoroacetate, hexanoate, adipate, and one halogenated anion, bromide, in the presence of a powerful salting out species, the inorganic salt K3PO4. The capacity of these ILs to undergo phase separation is discussed in regard to the chemical structure of the IL anion. The results here obtained were compared with those determined for poly(ionic liquid) analogues, and it was observed that while in ILs the hydrophobicity of the anion has the major role in phase splitting, in poly(ionic liquid)s that role is played by the polymer molecular weight. The effect of temperature on the phase equilibria is addressed.

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INTRODUCTION Because of their well-known high versatility, ionic liquids (ILs) have been at the crossroad of several fields for the last two decades. In particular, solvency-related processes, such as dissolution of poorly soluble compounds and complex macromolecules1−4 and engineered extraction and purification technologies with high selective performance and solvent recyclability,5−9 are nowadays major topics of research in the IL field because of the constant advances in the understanding of the fundamental properties of these compounds. Furthermore, the negligible vapor pressure of ILs10 has allowed their recognition as green solvents, while more benign members of this family have been synthesized and tested.11−15 In terms of the development of green separation processes, aqueous biphasic systems (ABSs) based on ILs have been receiving a large amount of attention from academia.16−19 ILbased ABSs have been proposed as an expansion of profiles of “classical” ABSs, which typically consist of two immiscible aqueous-rich phases composed by salt/salt, polymer/salt, or polymer/polymer.20,21 The main advantage of IL-based ABSs is that their phase polarities and affinities can be tailored by a suitable combination of cations and anions or by adjusting their chemical structures, overcoming the limited polarity range of polymer-based ABSs.22−24 Another important advantage of ILbased ABSs is that they are less viscous (∼1−10 mPa·s) than polymer-based ABSs (∼40 mPa·s),25,26 displaying faster phase separation.18 Taking into account the fact that ABSs are © XXXX American Chemical Society

essentially composed of water, they have been recognized as alternative biocompatible media for biologically active substances20,21 and tested for their recovery and purification.18,27,28 A wide number of ILs can be used to prepare ABSs with inorganic salts (ISs). Initially, most IL + IS ABS studies focused on the use of imidazolium ILs, mostly combining halogenated (chloride, bromide), sulfate, or fluorinated anions.29−33 As the portfolio of available ILs expanded, ABSs using a vast range of ILs combined with diverse salting-out agents, such as inorganic salts,26,34−40 polymers (polyethylene glycol,41−43 polypropylene glycol,44−48 and pyrrolidinium-based poly(ionic liquid)s49), amino acids,50 and carbohydrates,51,52 have been proposed, and their use has been successfully illustrated in the extraction of different solutes. However, despite the immense versatility inherent to ILs, the majority of the ABSs composed of ILs and inorganic salts have made use of ILs based on the imidazolium cation, while rather few studies have explored ILs containing other cations.18 In particular, the use of ILs bearing the pyrrolidinium cation, a five-atom heterocyclic compound that is less toxic than imidazolium,53 has been fairly overlooked. Only three studies have reported the ability of 1-butyl-1-methylpyrrolidinium chloride ([Pyr14][Cl]) to endure phase demixing in the presence of ISs such as potassium citrate (C6H5K3O7),54 Received: June 13, 2016 Accepted: February 19, 2017

A

DOI: 10.1021/acs.jced.6b00487 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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citrate buffer (C6H5K3O7/C6H8O7),55 sodium carbonate (Na2CO3),56 and phosphate buffer (K2HPO4/KH2PO4).57 In our previous work,49 we have shown that it is possible to implement poly(ionic liquid)-based ABSs in the presence of a powerful salting out species, the inorganic salt K3PO4. In particular, pyrrolidinium-based poly(ionic liquid)s bearing counteranions derived from different organic acids were synthesized and used for this purpose. We observed that the polymer molecular weight is the major parameter controlling the aptitude of the polymer for phase splitting, irrespective of the counteranion. In other words, the higher the polymer molecular weight, the higher its capacity to promote phase splitting. This work presents the first evaluation of the ability of pyrrolidinium-based ILs containing anions derived from organic acids, the same as those used to prepare the pyrrolidiniumbased poly(ionic liquid)s, to promote ABSs with the same inorganic salt, K3PO4. These systems were chosen to provide a direct comparison between poly(ionic liquid)s and ionic liquids and to draw conclusions about the phase behavior of the pyrrolidinium-based ILs. In this context, ILs containing a fixed cation ([Pyr12]+) and different organic acid-derived anions (acetate [Ac]−, trifluoroacetate [TFAc]−, hexanoate [Hex]−, adipate [Adi]−) and one halogenated anion (bromide [Br]−) were synthesized, and the respective cloud point curves were measured. Figure 1

mentioned before, only a few works have used pyrrolidiniumbased ILs,54−57 and these typically used the cation [Pyr14]+.

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EXPERIMENTAL SECTION Materials. Trifluoroacetic acid (≥99%), acetic acid (≥99%), hexanoic acid (≥99.5%), adipic acid (≥99%), SUPELCO Amberlite IRN-78, diethyl ether (99.8%), and potassium phosphate tribasic (K3PO4, ≥98%) were purchased from Sigma-Aldrich. IoLiTec GmbH provided the 1-ethyl-1-methylpyrrolidinium bromide ([Pyr12][Br]) (99 wt % purity). The starting materials utilized in the synthesis of ILs were used as received, and the water was double-distilled. The sources and purities of the materials used are listed in Table 1. Pyrrolidinium-Based Ionic Liquids. The pyrrolidinium carboxylate ILs, namely, 1-ethyl-1-methylpyrrolidinium acetate ([Pyr12][Ac]), 1-ethyl-1-methylpyrrolidinium trifluoroacetate ([Pyr12][TFAc]), 1-ethyl-1-methylpyrrolidinium hexanoate ([Pyr12][Hex]), and 1-ethyl-1-methylpyrrolidinium adipate ([Pyr12][Adi]) were synthesized by a two-step anion exchange reaction according to a synthetic route previously described elsewhere for cholinium carboxylate ILs.11 Briefly, an aqueous solution of 1-ethyl-1-methylpyrrolidinium hydroxide, [Pyr12][OH], was prepared by passing an aqueous solution of [Pyr12][Br] through a column packed with SUPELCO Amberlite IRN-78. Subsequently, the prepared [Pyr12][OH] was neutralized with a slight excess of the desired carboxylic acid using an ice bath. After 12 h of stirring at room temperature, the excess water was removed from the mixtures by rotary evaporation. In order to remove the unreacted acid, the resulting products were then washed with diethyl ether. The chemical structures of the synthesized pyrrolidinium carboxylate ILs and their purities were determined by 1H NMR analysis (spectra in the Supporting Information) and are listed in Table 1. All of the pyrrolidinium-based IL samples were dried (∼1 Pa and 318.15 K) for 4 days before their use. Phase Diagrams. The binodal curves were determined in a glass vessel by the cloud point titration method at the desired temperature (288.15, 298.15, or 308.15 K) and pressure (0.1 MPa) as described elsewhere.49 The experimental procedure adopted follows a method previously validated for ABSs formed by ILs and inorganic salts.18 In particular, aqueous solutions of K3PO4 at ca. 40 wt % and aqueous solutions of the different ILs with various concentrations (from 42 to 65 wt %) were prepared. The binodal curves were correlated according to the following expression:58

Figure 1. Chemical structures of the cation and anions studied: (a) 1ethyl-1-methylpyrrolidinium ([Pyr12]+), (b) bromide ([Br]−), (c) acetate ([Ac]−), (d) trifluoroacetate ([TFAc]−), (e) hexanoate ([Hex]−), and (f) adipate ([Adi]−).

illustrates the chemical structures of the ILs used. To the best of our knowledge, there have been no studies using ILs with this particular cation ([Pyr12]+) to promote ABSs. As

Y = A exp[(BX 0.5) − (CX3)]

(1)

Table 1. Sources, Purification Methods, and Final Purities of All Compounds Used chemical name

abbreviation

potassium phosphate tribasic

K3PO4

1-ethyl-1-methylpyrrolidinium bromide 1-ethyl-1-methylpyrrolidinium acetate 1-ethyl-1-methylpyrrolidinium trifluoroacetate 1-ethyl-1-methylpyrrolidinium hexanoate 1-ethyl-1-methylpyrrolidinium adipate

[Pyr12][Br]

source

purification method

final mass fraction purity

final mole fraction purity

analysis method

none

0.98

−

−

none

0.99

−

−

extraction with diethyl ether and drying

−

0.95

1

[Pyr12][Ac]

SigmaAldrich IoLiTec GmbH synthesis

[Pyr12][TFAc]

synthesis

−

0.984

[Pyr12][Hex]

synthesis

−

0.973

[Pyr12][Adi]

synthesis

−

0.973

B

H and 13C NMR



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Table 2. Correlation Parameters of Equation 1 (A, B, and C) Adjusted to the Binodal Experimental Data of Each System, with the Respective Standard Deviations (σ) and Correlation Coefficients (R2) IL

T (K)

[Pyr12][Br] [Pyr12][Ac] [Pyr12][TFAc] [Pyr12][Hex]

298.15 298.15 298.15 288.15 298.15 308.15 298.15

[Pyr12][Adi]

A±σ 77 65 79 77 64 70 77

± ± ± ± ± ± ±

B±σ −0.320 −0.312 −0.312 −0.329 −0.265 −0.289 −0.221

1 1 1 2 2 2 28

± ± ± ± ± ± ±

0.005 0.006 0.003 0.009 0.009 0.011 0.083

105·(C ± σ)

R2

± ± ± ± ± ± ±

0.9994 0.9994 0.9999 0.9992 0.9997 0.9995 0.9999

5.61 3.21 6.60 6.65 7.34 6.76 5.91

0.20 0.12 0.79 0.23 0.16 0.19 0.35

Figure 2. Phase diagrams of the ternary systems composed of pyrrolidium-based ILs + K3PO4 + H2O at T = 298.15 K and p = 0.1 MPa: blue ■, [Pyr12][Br]; red ◆, [Pyr12][Ac]; green ▲, [Pyr12][TFAc]; orange ○, [Pyr12][Hex]; purple bars, [Pyr12][Adi].

Influence of IL Chemical Structure. In order to understand the influence of the chemical structure of the IL, in particular the character of the anion, on ABS formation, the ternary phase diagrams measured for all of the systems at 298.15 K are graphically presented in Figure 2. It is well-known that the area of the biphasic region is proportional to the ability of each IL to undergo phase demixing with the IS solution. The larger the biphasic region is, the lower is the IS concentration needed for ABS formation, and consequently, the stronger is the IL’s ability to phase split. From Figure 2 it can be noticed that the binodal curves of the different ILs are very close, some of them even crossing each other, meaning that the anion has little effect in the ABS formation of these pyrrolidinium-based ILs. However, at an arbitrary concentration between 1.2 and 2.4 mol·kg−1 of K3PO4, the aptitudes of these pyrrolidinium-based ILs to form ABSs with K3PO4 occur in the order [Hex]− > [TFAc] − > [Br]− > [Adi]− > [Ac]−, in accordance with the anion hydrophilic character.26,30,41,42,59 This is in agreement with the usually observed mechanism behind the formation of ABSs by ILs and ISs, which is characterized by the occurrence of a contest involving the IL and IS ions for the molecules of water to form hydration complexes. The competition is typically won by the salt, owing to the favored hydration of their high-charge-density ions compared with the IL ions, leading to migration of water from the solvation shell of the IL to that of IS. Consequently, salting-out of the IL occurs, and an IL-rich phase separates from the inorganic salt solution. Hence,

where Y and X are the IL and K3PO4 mass fraction percentages, respectively, and A, B, and C are constants obtained by regression of the experimental binodal data.

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RESULTS AND DISCUSSION Phase Equilibria. The binodal curves of K3PO4, water, and the pyrrolidinium-based ILs 1-ethyl-1-methylpyrrolidinium bromide ([Pyr12][Br]), 1-ethyl-1-methylpyrrolidinium acetate ([Pyr12][Ac]), 1-ethyl-1-methylpyrrolidinium trifluoroacetate ([Pyr12][TFAc]), 1-ethyl-1-methylpyrrolidinium hexanoate ([Pyr12][Hex]), and 1-ethyl-1-methylpyrrolidinium adipate ([Pyr12][Adi]) (Figure 1) were determined by the cloud point titration method. It should be mentioned that 1-ethyl-1methylpyrrolidinium citrate was also tested, but this IL did not undergo phase splitting when combined with K3PO4. The experimental weight fraction data for each phase diagram are provided in Tables S1−S4. Nevertheless, all of the binodal curves are presented and discussed in molality units (mol of solute per kg of solvent) in order to remove the influence that can result from the ILs’ molecular weight. Table 2 shows the correlation parameters, along with their standard deviations and correlation coefficients, obtained by regression of the experimental binodal curve data (in weight fraction) using eq 1. Considering the correlation coefficients and standard deviations obtained (Table 2), it is possible to presume that eq 1 provides a good description of the experimental data. C



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Figure 3. Influence of the nature of the cation on the ternary phase diagrams composed of ILs + K3PO4 + H2O at at T = 298.15 K and p = 0.1 MPa: blue ■, [Pyr12][Br]; green ▲, [Pyr12][TFAc]; red ◆, [Pyr12][Ac]; gray bars, [C2mim][Br];30 orange ●, [C2mim][Ac];30 cyan ×, [Ch][TFAc];59 red +, [Ch][Ac].26

Figure 4. Influence of the temperature on the phase diagram of the ternary system composed of [Pyr12][Hex] + K3PO4 + H2O: green ▲, 288.15 K; orange ●, 298.15 K; red ■, 308.15 K.

the salting-out order of the studied ILs in K3PO4 aqueous solution follows the order of the ILs’ hydrophobicities. Figure 2 shows that the IL that most easily splits into two phases is [Pyr12][Hex]. The larger biphasic region of [Pyr12][Hex] compared with [Pyr12][Ac] can be explained by the larger hydrophobicity of the former IL due to its longer alkyl chain (weaker affinity for water). The same argument is valid for the IL with the fluorinated anion, [Pyr12][TFAc], compared with the phase diagram of [Pyr12][Ac]. The presence of the trifluoromethyl group increases the hydrophobicity of [Pyr12][TFAc], making this IL more prone to phase demixing, and thus, [Pyr12][TFAc] is more easily salted-out than the [Pyr12][Ac]. On the other hand, when evaluating both [Pyr12][Hex] and [Pyr12][Adi], which anions have the same alkyl chain length (six carbons), the presence of an extra carboxylic group in the adipate anion boosts the affinity of

[Pyr 12 ][Adi] for water, and for that reason, high IS concentrations are required for the phase splitting to occur. In the case of [Pyr12][Cit], with its three carboxylic groups and also an extra hydroxyl group, which greatly increase the hydrophylicity of this IL, no phase behavior was observed, as previously mentioned. In general, it can be stated that the most hydrophobic pyrrolidinium-based ILs, those that are less prone to hydration, are easily salted-out by K3PO4 and thus more able to promote ABSs. These findings are in accordance with what has been usually obtained for ABSs of other ILs and K3PO4.26,30,31,33 Despite the differences between the binodal curves depending on either the cation or its alkyl side chain length,18 it is important to point out that the demixing ability of the [Pyr12]based ILs from K3PO4 aqueous solutions follows the same anion trend observed for 1-butyl-3-methylimidazolium D



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([C4mim])-based30 and cholinium ([Ch])-based ILs.26,59 In regard to the influence of the nature of the cation, Figure 3 compares the ternary phase diagrams obtained in this work for [Pyr12]-based ILs with those reported in the literature for ILs comprising different cations, namely, 1-ethyl-3-methylimidazolium ([C2mim]+)30 and cholinium ([Ch]+).26,59 It can be seen that, independently of the anion, the cation has a major effect on the ABS formation since the cation order is pyrrolidinium > imidazolium > cholinium. This cation trend is governed by the hydrophilicity of the cation, since the well-known high hydrophilicity of the cholinium cation is translated into a smaller capacity to phase-split and thus a smaller two-phase region. This is in agreement with what was observed by Marques et al.56 for pyrrolidinium- and imidazolium-based ILs and by Sintra et al.60 for imidazolium- and cholinium-based ILs. It can also be observed that within each of the cation families the anion order is maintained ([TFAc]− > [Br]− > [Ac]−), and thus, the ILs considered in Figure 3 can be ordered according to their ABS formation ability in the following way: [Pyr12][TFAc] > [Pyr12][Br] > [Pyr12][Ac] > [C2mim][Br] > [C2mim][Ac] > [Ch][TFAc] > [Ch][Ac]. As mentioned before, a direct comparison between the results obtained in the present work for pyrrolidinium ILs and those obtained in our previous work49 for the pyrrolidiniumbased poly(ionic liquid)s bearing the corresponding counteranions is not straightforward. The ability of the studied poly(ionic liquid)s to promote ABSs with K3PO4 follows the next trend of the counteranions: [Cl]− > [TFAc]− > [Hex]− > [Ac]− > [Adi]− > [Cit]−.49 However, for the pyrrolidiniumbased ILs, a different trend of the anions was found: [Hex]− > [TFAc]− > [Br]− > [Adi]− > [Ac]−, and the IL 1-ethyl-1methylpyrrolidinium citrate did not undergo phase demixing with K3PO4. Thus, it can be concluded that for these ILs the hydrophobicity of the anion rules the phase splitting, while for the respective polymers the molecular weight is the most important factor affecting the phase equilibria. Consequently, when working with poly(ionic liquid)s, the molecular weight of the polymer is an extra parameter that can be used to tune the phase equilibria. Influence of Temperature. Figure 4 illustrates the influence of temperature on the phase diagram behavior of the system [Pyr12][Hex] + K3PO4 + H2O. The phase diagram displays a slight decrease in the immiscibility region as the temperature increases, which is certainly due to the more favorable interactions of [Pyr12][Hex] with water and their enhanced mutual solubilities at higher temperatures. This means that higher K3PO4 and [Pyr12][Hex] concentrations are required for phase separation, and consequently, lower temperatures seem to be more favorable for the creation of the [Pyr12][Hex]−K3PO4-based ABS. Even though the effect of temperature on the phase diagram is dependent on the saltingout agent used, the weak influence of temperature on the ABSs presented in this work is in accordance with what has been reported in other studies for ABSs formed by distinct ILs and ISs.61−64

larger biphasic regions than those of their analogues based on the imidazolium and cholinium cations, enabling the conclusion that the IL cation is more important to distinguish between ABS formation than the anions. The phase-splitting order of the IL anions observed for imidazolium and cholinium cations is maintained for pyrrolidinium-based cations, indicating that the mechanism of phase formation, preferential hydration of the inorganic salt, is maintained. Nevertheless, in a comparison of the results obtained for pyrrolidinium ILs with those of poly(ionic liquid) analogues and K3PO4, a different salting-out order was found, confirming that the molecular weight of the polymer has a major role in the phase-splitting ability of these polymers. Moreover, temperature does not have much influence on the phase-splitting ability of the ABSs contemplated in the present work.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jced.6b00487. 1 H and 13 C NMR spectra of the synthesized pyrrolidinium carboxylate ILs and weight fraction data determined for the studied ternary systems (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Tel: +351-21-8413385. Fax: +351-21-8499242. E-mail: isabel. [email protected]. ORCID

Andreia S. L. Gouveia: 0000-0003-2809-7863 Isabel M. Marrucho: 0000-0002-8733-1958 Author Contributions §

L.C.T. and K.G.J. contributed equally to this work.

Funding

L.C.T. is grateful to Fundaçaõ para a Ciência e a Tecnologia (FCT) for her postdoctoral research grant (SFRH/BPD/ 101793/2014). I.M.M. acknowledges FCT/MCTES (Portugal) for a contract under Investigador FCT (IF/363/2012). This work was supported by FCT through Project PTDC/QUIQUI/121520/2010 and R&D Unit UID/Multi/04551/2013 (GreenIT). The NMR spectrometer used in this work is part of the National NMR Facility supported by FCT (RECI/BBBBQB/0230/2012). Notes

The authors declare no competing financial interest.

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REFERENCES

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CONCLUSIONS In this work, ILs containing a fixed cation (1-ethyl-1methylpyrrolidinium) and different organic acid-derived anions (acetate, trifluoroacetate, hexanoate, adipate) and one halogenated anion (bromide) were used to determine phase diagrams in the presence of potassium phosphate. The pyrrolidinium-based ILs were shown to provide ABSs with E



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