Phase equilibrium in binary systems of ionic liquids, or deep eutectic solvents with 2-phenylethanol (PEA), or water
Graphical abstract
Introduction
The 2-phenylethanol (PEA) can be obtained by chemical synthesis or biosynthesis, but its separation from fermentation broths is very difficult [1], [2], [3]. In an industrial process, the cost of separation and concentration of PEA from aqueous media is very high. The accurate choose of the proper entrainer is an essential for the design of equilibrium-based separation processes. This study presents experimental solubility data of sustainable solvents such as ionic liquids (ILs) and deep eutectic solvents (DES) in PEA and water, accompanied by the interpretation of interactions occurring in such binary systems. The ILs are highly solvating solvents, with non-, or very low vapor presure, non-flammable materials for a variety of innovative applications [4], [5], [6], [7]. In particular, ILs can offer favorable alternatives to common organic solvents in new technologies [5]. The physico-chemical properties of ILs may be tailored by changing the length and branching of the alkyl chain, or by the addition the polar groups at the cataion or anion of the IL. Nowadays, they are viewed as a new class of solvents for various applications in chemical engineering [5], biotechnology [7], [8], pharmaceutical systems [9], as herbicides, wood preservatives [10] and in electrochemistry [11]. They are seriously considered as new class of solvents for liquid–liquid extraction for gasoline and diesel desulfurization [12], [13], [14]. In this work we give attention to ILs with maximum four carbon chain on the cation, or anion, which were found very attractive in that extraction processes [14].
Last decade also DES [15] have been proposed as new solvents in several chemical reactions, metal-organic frameworks, as extraction media, for electrodeposition of metals and many others [15], [16], [17], [18], [19], [20], [21], [22], [23]. DESs have properties comparable to ILs. DESs are potential candidates to be used as non-volatile ionic liquids-based. The advantages compared to traditional ILs are: nonreactive with water, simple to synthesize, the materials easily mixed and converted to ILs without further purification. DESs are mostly biodegradable and very cheap, because of the low cost of raw materials. They are binary hydrogen bonded mixtures of quaternary ammonium salts, or others with donors such as acids, amines, and alcohols. The eutectic mixtures reveal much lower the melting point of the mixture in comparison to the melting points of the individual components. Herein, we describe the use of DES based on choline chloride, or choline acetylchloride with different acids, or diol. The DES of choice were based on the assumption that they will not be toxic to the yeast. The use of DES was inspired by those innovative works, where DES has given good results as entrainers [23], [24], [25], [26], [27], [28], [29]. They were used with success in the extraction of DNA and as a media in the enzymatic reactions [25], [26]. Mixtures of DES with water were used in extraction of phenol-based compounds with high selectivities [27]. Mixtures of choline chloride, or tetramethyl ammonium chloride, or tetrabutyl ammonium chloride with malonic acid, or glycerol, or tetraethylene glycerol, or ethylene glycol, or polyethylene glycol were used as new entrainers for desulfurization of gasoline and diesel with selectivity of 82.8% for one cycle [28]. Similar good results were obtained in the separation of aromatics from aliphatics with DESs containing ammonium salts and ethylene glycol, or lactic acid and sulfolane as a hydrogen bonding donors [29].
PEA is an important flavor with rose-like aroma. It is used in perfume, cosmetics flavor compositions for food, soft drinks, candy and cookies. In chemical processes the product purification is a major problem. For mentioned applications natural PEA is preferred from biotechnological production [30], [31]. Different entrainers were used in bioconversion by the yeast from l-phenylalanine (L-Phe) as oleic acid, or long chain hydrocarbons and also ionic liquids [32]. However, production of PEA from L-Phe by yeast is limited by a synergistic inhibitory impact of PEA toward the cell. Thus, it is necessary to extract PEA by a separate organic phase in the fermenter as “in situ product recovery” to enhance productivity [2], [30], [31]. ILs have been already studied in the bioproduction of PEA from L-Phe in a biphasic system with various yeast strain cultures [2], [30], [31], [32]. The IL used in such process has to reveal high extractive capacity and biocompatibility to the yeast. The ILs proposed for this technology must exhibit appreciable large solubility of PEA and immiscibility with water.
This work is a continuation of our study on possible extractin of PEA “in situ” from the water biomass [33], [34], [35], [36]. We have measured the phase equilibria for binary or ternary systems of PEA with IL and/or water.
The (solid–liquid) phase equilibrium (SLE) of pirydynium, or imidazolium-based IL, was measured in our laboratory with PEA [33]. The complete miscibility in the liquid phase was observed for 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate [EMIM][FAP] with PEA at T = 298.15 K [33]. The N-octylisoquinolinium bis{(trifluoromethyl)sulfonyl}imide, [OiQin][NTf2] has been found by us as extrimly good entrainer for the extraction of PEA from water in ternary (liquid–liquid) phase equilibrium measurements [34]. Recently, the SLE of binary systems composed of the piperidinium-based ILs with PEA were measured [35]. The majority of the systems containing piperidinium-based ILs have showed complete miscibility with PEA and immiscibility with water [35].
During the last few years, the ILs based on pyrrolidinium cation have received much of our attention in different extraction processes [37], [38], [39], [40]. In literature, the pyrrolidinium-based ILs are described mainly in context of their physical and electrochemical properties as well as surfactants [41], [42], [43], [44]. The mutual miscibility of 1-butyl-1-methylpyrrolidinium-based ILs with water were reported as well [45], [46], [47], [48].
The goal of this work is to assess the suitability of chosen pyrrolidynium, or imidazolium, or phosphonium-based ILs and DESs for use in PEA-enhanced separation process from an aqueous phase. The SLE or LLE data of new entrainers in binary systems with PEA and water are presented. The results from binary systems will focused our attention to the biosynthesis of PEA.
Section snippets
Chemicals and materials
The substances, names, CAS numbers, abreviations, molar masses, structure, producer, purity and water content are listed in Table 1, Table 2 for IL and DES, respectively [45], [46], [47], [48], [49]. The samples of ILs were dried for 24 h at T = 320 K under the reduced pressure to remove volatile impurities and trace water. The water content was of below 780 ppm, as determined by Karl Fisher titration. The PEA used in phase equilibrium measurements was purchased from Sigma–Aldrich Chemie GmbH
Binary systems with ILs
In this work the SLE, or LLE measurements of binary systems of five 1-butyl-1-methylpyrrolidinium, or one 1-butyl-1-methylimidazolium-based ILs and one phosphonium-based IL with different anions are presented. Systematic measurements of the binary mixtures up to the temperature T = 280 K have shown complete liquid miscibility with PEA for four ILs: [BMPYR][FAP], [BMPYR][DCA], [BMIM][TCM] and [P14,6,6,6][TCM] (see Table 1). However, the immiscibility is expected simultaneously for these ILs with
Modeling
Since no solid–solid phase transition was observed in this work (except for [BMPYR][TCM]), and the change of heat capacity at the melting temperature was not measured, a simplified general thermodynamic equation relating temperature, T/K and the mole fraction of the IL, x1 in solvent have been fitted to all the sets of experimental SLE data [51]:where Tfus,1, ΔfusH1, T, x1 and γ1 stand for: melting temperature for the pure IL, enthalpy of fusion for the pure IL,
Conclusions
The new experimental diagrams of (solid, or liquid + liquid) phase equilibrium of eight ILs with PEA and of six DES with PEA and water have been measured. The solubility of ILs in water were taken from the literature. An increase of solubility of ILs in PEA for different anions was in order: [TCM]− > [CF3SO3]− > [TCB]−. All ILs revealed complete miscibility with PEA without three showing the simple eitectic mixtures: [BMPYR][TCM], [BMPYR][TCB] and [BMPYR][CF3SO3]. The acceptable immiscibility
Acknowledgments
Funding for this research was provided by the project of National Science Center in Poland for years 2015–2018, No. 2014/15/B/ST5/00136.
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