Deep eutectic mixture

Many of these problems may be overcome by using ionic liquids based on sugars [35] or deep eutectic mixtures. Deep eutectic mixtures such as that derived from choline chloride and urea (m. pt. 12°C [36]) or carboxylic acids [37] can be liquids and have very low vapour pressure. They have been successfully used as electrochemical solvents, but their use in catalysis remains little explored. Urea is a fertiliser and choline chloride (Vitamin B4) is a component of chicken feed so the mixture is environmentally acceptable. 

Although many ionic liquids and deep eutectic mixtures based on choline have the advantages to be cheap and no-toxic, they are all hydrophilic and miscible with aqueous solvents. This may be a problem for applications such as the extraction of metal ions from an aqueous phase or the electrodeposition of reactive metals (aluminium, magnesium, tantalum..). 

Cost and biodegradabiUty have also been major concerns, and new families of ILs derived from renewable feedstock or from low-cost starting materials have beat described. These Bio-ILs are entirely composed of biomaterials [183]. An example to be cited is the development of the deep eutectic mixtures liquid systems based on choline chloride [ 184] for which the qualification of ILs is stiU the subject of controversies. Choline can be used as alternative cation in combination with suitable anion to generate ILs. The biodegradable properties of these ILs have been reported [185]. 

Li, J., Zhao, Y, Wang, N., and Guan, L. (2011]. Frontal polymerizations carried out in deep-eutectic mixtures providing both the monomers and the polymerization medium, Chem. Commun., 47, pp. 5328-5330. 

Just taking a quick look at the different chapters, one can realize which areas of each field are mature and which can evolve immediately. Although it is very difficult to venture a general trend for the whole area, we could emphasize that there is not a tmly enantioselective version for the mono-activated hydrogen autotransfer reaction. The use of first-row transition metals is almost an unexplored Blue Ocean , in all aspects. Furthermore, the possible impact of green neoteric solvents, including deep eutectic mixtures, supercritical fluids, and bio-based solvents, on the hydrogen transfer process is definitely unknown. 

Certainly, the study of new synthesis and transformations in deep eutectic mixtures media will continue to be a fast-moving topic for the next several years, with the discovery of new applications being expected in the near future. In this sense, the easy fine-tuning of their physicochemical properties through the adequate selection of components [hydro-gen-bond-donor (HBD) and hydrogen-bond-acceptor (HBA)], opens a new gate for the preparation of a la carte DESs. 

Related to ionic liquids are substances known as deep eutectic solvents or mixtures. A series of these materials based on choline chloride (HOCH2CH2NMe3Cl) and either zinc chloride or urea have been reported (Abbott et al., 2002 2003). The urea/choline chloride material has many of the advantages of more well-known ionic liquids (e.g. low volatility), but can be sourced from renewable feedstocks, is non-toxic and is readily biodegradable. However, it is not an inert solvent and this has been exploited in the functionalisation of the surface of cellulose fibres in cotton wool (Abbott et al, 2006). Undoubtedly, this could be extended to other cellulose-based materials, biopolymers, synthetic polymers and possibly even small molecules. 

The electrodeposition of chromium in a mixture of choline chloride and chromium(III) chloride hexahydrate has been reported recently [39]. A dark green, viscous liquid is obtained by mixing choline chloride with chromium(III) chloride hexahydrate and the physical properties of this deep eutectic solvent are characteristic of an ionic liquid. The eutectic composition is found to be 1 2 choline chloride/chromium chloride. From this ionic liquid chromium can be electrode-posited efficiently to yield a crack-free deposit [39]. Addition of LiCl to the choline chloride-CrCl3-6H20 liquid was found to allow the deposition of nanocrystalline black chromium films [40], The use of this ionic liquid might offer an environmentally friendly process for electrodeposition of chromium instead of the current chromic acid-based baths. However, some efforts are still necessary to get shining... 

Abbott et al. [98-103] reported the synthesis and characterization of new moisture-stable, Lewis acidic ionic liquids made from metal chlorides and commercially available quaternary ammonium salts (see Chapter 2.3). They showed that mixtures of choline chloride (2-hydroxyethyltrimethylammonium chloride, [Me3NC2H40H]Cl and MCU (M=Zn, Sn) give conducting and viscous liquids at or around room temperature. These deep eutectic solvents/ionic liquids are easy to prepare, are water-and air-stable, and their low cost enables their use in large-scale applications. Furthermore, they reported [104] that a dark green, viscous liquid can be formed by mixing choline chloride with chromium(III) chloride hexahydrate and that the... 

Jefremov and Khaibashev [60] have also investigated melts of TNT with other nitro compounds. They observed deep minima on the isotherms of plastic flow of mixtures of TNT and picric acid, trinitroxylene or 1,8-dinitronaphthalene. corresponding to eutectic mixtures. Unlike those the corresponding curves for mixtures of TNT and 2,4-dinitrotoluene, m- dinitrobenzene, and tetiyl, showed an additive character. 

Most inorganic salts, when they melt, are found to flow and conduct electricity according to a simple Arrhenius law at all temperatures down to their melting points. For instance, unless measurements of high precision are used, the alkali halides appear to remain obedient to the Arrhenius equation even down to the deep eutectic temperatures of their mixtures with other salts. LiCl and KCl form a eutectic mixture with a freezing point of 351°C, some 300 K below either pure salt freezing point, yet the viscosity of the melt barely departs from Arrhenius behavior before freezing. 

Patino J, Gutierrez MC, Carriazo D, Ania CO, Parra JB, Ferrer ML, Monte Fd (2012) Deep eutectic assisted synthesis of carbon adsorbents highly suitable for low-pressure separation of CO2-CH4 gas mixtures. Energy Environ Sci 5 8699-8707... 

Smith et al. [67] studied the Zn electrodeposition from deep eutectic solvents (DES) by an AFM-EQCM system. The DES chosen in this study were the solvents based on a eutectic mixture of choline chloride (ChCl) and ethylene glycol (EG). The peculiarity of the DES is their very high viscosity at room temperature when compared to that of water. The authors proved by combining chronoamperometry, AFM, and EQCM techniques that the initial phases of Zn nucleation from DES resemble the 3D progressive model, as described mathematically by Sharifker and Hills [68]. 

Deep Eutectics. Some years ago it was discovered that by mixing salts, e.g., choline chloride (melting point 302 °C), with urea (melting point 133 °C) in a molar ratio of 1 2, the melting point of the mixture drops down to 12 °C [7], These novel systems are somewhere in between Ionic Liquids and electrolyte solutions and have promising properties. 

In another study, chitin nanofibers were prepared by using deep eutectic solvents (DESs), including mixtures of choline halide (chloride/bromide)-urea and choline chloride-thiourea. The produced chitin nanofibers were utilized for manufacturing calcium alginate bionanocomposite gel beads to improve elasticity and retard drug release [21]. 

Recently it has been shown the possibility of formation of ionic liquids from eutectic mixtures of quaternary ammonium salt such as choline chloride (2-hydroxy-ethyl-trimethyl ammonium chloride) with a hydrogen bond donor species such as amides, glycols or carboxylic adds ( Abbott et al., 2003 Endres, 2002 Endres et al., 2008 Mukhopadhyay et al., 2005 Wasserscheid Welton, 2007). These media, also known as "deep eutectic solvents", have been further used to electrodeposit a large range of metals and alloys induding Zn and Zn alloys, Cr, Sn and Sn alloys, Cu and Ag ( Abbott et al., 2003 Abbott et al., 2006 Abbott et al, 2007 Cojocaru et al., 2009 Endres et al., 2008 Florea et al, 2010). 

Kroon, M. C., Casal, M. F., Den Bruinhorst, A. V. Pretreatment of lignocellulosic biomass and recovery of substituents using natural deep eutectic solvents/compound mixtures with low transition temperatures. Patent WO 2013153203 Al, 2013. 

Deep eutectic solvents have been extensively used as an alternative green and biorenewable solvents to traditional VOCs in organic synthesis. Moreover, the addition of water to the reaction crade allows the easy separation (layer separation or precipitation) of the desired organic product. In most of these reactions, DESs were not only solvents for reactants, but also catalysts. In this sense, Shankarling and co-workers reported the reaction rate improvement of the electrophilic substitution of l-aminoanthra-9,10-quinone derivatives in the eutectic mixture lChCl/2Urea as compared wifli conventional VOCs solvents such as methanol or chloroform (see reaction (a) in Scheme 20.3.1). This rate enhancement could be related to the aforementioned basic nature of this eutectic mixture (Hammet function (H ) value 10.86). At the end of the reaction, die addition of water precipitates the dibrominated product (84-95% yield). The system was easily recycled (up to five consecutive times) by evaporation of water without appreciable lost in activity. 

Other eutectic mixtures apart from ChCl-based DESs have been used as reaction media in organic synthesis. In this sense, sweet deep eutectic solvents (also ealled low melting mixtures) based on carbohydrates (i.e., maltose) have been applied as green and biorenewable solvent in the high-yield synthesis of quinazoline derivatives via one-pot three-component coupling of 2-aminoaryl ketones, aldehydes and ammonium aeetate (see Scheme 20.3.3). However, the use of these carbohydrate-based DESs is limited because i) they present relatively high melting points, and ii) their components are rather reactive. 

Deep eutectic solvents based on choline acetate (ChOAc), which have lower viscosities as compare to the ChCl/Urea eutectic mixture, have been also used as reaction media in several biocatalyzed transesterification reactions. In this sense, Zhao et al. reported the transesterification of ethyl sorbitate with 1-propanol by the lipase Novozym 435 Candida Antarctica lipase B immobilized on acrylic resins), achieving high initial rates (1 pmolmin g ) and selectivity (99%). Furthermore, in a model biodiesel synthesis system, the authors examined the transeterification of the lipid Miglyol oil 812 (a mixture of triglycerides of caprylic acid (C8) and capric acid (CIO)) with methanol, catalyzed by Novozym 435 in ChOAc/Gly (1 1.5 molar ratio). The biocatalytic reaction was very rapid in this eutectic mixture, with 97% conversion achieved after only 3 hours. 

Most recently, the lipase from Rhizopus oryzae was used as biocatalyst in the Bigi-nelly reactions in the eutectic mixture ChCl/Urea yielding the corresponding dihydropyrimidines as a racemic mixture (see Scheme 20.3.6). The reaction was characterized by high efficiency and selectivity, short reaction time, and mild and environmentally fiiendly reaction conditions. It is important to note that the reuse of both the lipase and deep eutectic solvent was possible in four consecutive cycles. ... 

Analogous to traditional ILs, deep eutectic solvents have been successfiilly applied in electrochemistry as electrolytes for i) electrodeposition of metals, ii) dyes-sensitized solar cells,iii) electropolishing of metals, " and iv) metal separations. In this section we will focus our attention on the electrodeposition of metals using different ChCl-based eutectic mixtures (ChCl/Urea and ChCl/EG). 

Owing to these superior performances of DESs in polymer synthesis, in 2012, Fernandes reported the electrochemical synthesis of conducting polymers (polyaniline) in the eutectic mixture ChCl/EG " Ramesh and co-workers have also described other conducing polymers composed of cornstarch or cellulose acetate, mixed with lithium bis(trifluo-romethanesulfonyl)imide and the deep eutectic solvent ChCl/Urea. " " Finally, Leroy et al. used the eutectic mixtures ChCl/Urea and ChCl/Gly as functional additives to develop an efficient polymer blend of thermoplastic starch. " ... 

This chapter clearly exemplifies the maturity gained by Deep Eutectic Solvents as new green and biorenewable solvents in different applications of modem synthesis. As the reader have noticed, a huge number of synthetic reactions and chemical processes can be presently performed in eutectic mixtures as effectively as in classical organic media. More importantly, enhanced or completely new reactivities have been in some cases observed using DESs as a solvent. Despite all these promising applications, much effort is still needed in order to widen the utilization of DESs as green, low toxic and biocompatible reaction medium. 

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