Temperature-Responsive Ionic Liquids: Fundamental Behaviors and

Review pubs.acs.org/CR

Temperature-Responsive Ionic Liquids: Fundamental Behaviors and Catalytic Applications Yunxiang Qiao,†,§ Wenbao Ma,‡,§ Nils Theyssen,† Chen Chen,‡ and Zhenshan Hou*,‡ †

Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany Key Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, School of Chemistry & Molecular Engineering, East China University of Science and Technology, Shanghai 200237, People’s Republic of China

‡

ABSTRACT: Temperature-responsive ionic liquids (ILs), their fundanmental behaviors, and catalytic applications were introduced, especially the concepts of upper critical solution temperature (UCST) and lower critical solution temperature (LCST). It is described that, during a catalytic reaction, they form a homogeneous mixture with the reactants and products at reaction temperature but separate from them afterward at ambient conditions. It is shown that this behavior offers an effective alternative approach to overcome gas/liquid−solid interface mass transfer limitations in many catalytic transformations. It should be noted that IL-based thermomorphic systems are rarely elaborated until now, especially in the field of catalytic applications. The aim of this article is to provide a comprehensive review about thermomorphic mixtures of an IL with H2O and/or organic compounds. Special focus is laid on their temperature dependence concerning UCST and LCST behavior, including systems with conventional ILs, metal-containing ILs, polymerized ILs, as well as the thermomorphic behavior induced via host−guest complexation. A wide range of applications using thermoregulated IL systems in chemical catalytic reactions as well as enzymatic catalysis were also demonstrated in detail. The conclusion is drawn that, due to their highly attractive behavior, thermoregulated ILs have already and will find more applications, not only in catalysis but also in other areas.

CONTENTS 1. Introduction 1.1. Compositions and Applications of Ionic Liquids (ILs) 1.2. Mixed Solvent Systems Including IL 1.3. Thermomorphic Systems 2. Thermomorphic Systems Based on Ionic Liquids (ILs) 2.1. Mechanism of Thermoregulated Property 2.2. Conventional IL Systems with UCST and LCST Behavior 2.2.1. UCST Behavior 2.2.2. LCST Behavior 2.2.3. UCST and LCST Behavior 2.3. Metal-Containing IL Thermomorphic Systems 2.3.1. ILs with Simple Metal Salts 2.3.2. Polyoxometalate-Based ILs (POM−ILs) 2.4. Polymerized IL (PIL) Systems with UCST and LCST Behavior 2.4.1. UCST Behavior 2.4.2. LCST Behavior 2.4.3. UCST and LCST Behavior 2.5. Host−Guest Complexation Inducing Thermomorphic Systems 3. Thermoregulated IL Systems for Applications Other than Catalysis 4. Thermoregulated IL Systems for Catalysis © XXXX American Chemical Society

4.1. Chemical Catalysis 4.1.1. Hydroformylation 4.1.2. Reduction with H2 or CO 4.1.3. Coupling Reactions 4.1.4. Biomass Conversion 4.1.5. Oxidation Reaction 4.1.6. Hydrosilylation 4.1.7. Hydroaminomethylation Reaction 4.1.8. Hydroxyalkylation of Phenol with Formaldehyde 4.1.9. Stetter Reaction 4.1.10. Chloromethylation of Biphenyl 4.1.11. Condensation 4.1.12. Esterification 4.1.13. Atom Transfer Radical Polymerization 4.2. Enzyme Catalysis 5. Conclusion Author Information Corresponding Author ORCID Author Contributions Notes Biographies Acknowledgments

B B B B C C E E N T U U V V V W Z

AA AA AB AC AC AD AD AE AE AE AF AF AF AF AF AG AG AG AG AG AG AG AH

Z Special Issue: Ionic Liquids

Z AA

Received: September 23, 2016

A

DOI: 10.1021/acs.chemrev.6b00652 Chem. Rev. XXXX, XXX, XXX−XXX

Chemical Reviews Abbreviations References

Review

supported on solid surfaces to overcome these problems. The novel supported IL phase (SILP) catalysis concept does not only overcome the drawbacks but also allows the use of fixedbed reactors for the continuous flow reactions.32−38 However, there might still exist gas/liquid−solid interface mass transfer limitations or leaching of the active species in SILP catalytic systems.39 On the other hand, ILs with thermoregulated property, which means homogeneous reaction mixture at reaction temperature but heterogeneous separation after reaction, offers an alternative potential approach to overcome the problems of traditional IL catalysis.40−42

AH AJ

1. INTRODUCTION 1.1. Compositions and Applications of Ionic Liquids (ILs)

An ionic liquid (abbreviated as IL) is an organic salt with organic cation and suitable anion combination in the liquid state. The most common salts in use are those with alkylammonium, alkylphosphonium, N-alkylpyridinium, and N,N′-dialkylimidazolium cations with various organic or inorganic anions. Structures and abbreviations of some common IL cations and anions employed are illustrated in Figure 1.

1.2. Mixed Solvent Systems Including IL

Normally, an IL is classified as hydrophilic or hydrophobic depending on its composition, whether or not it is miscible with water. Nevertheless, this classification is ambiguous, since the miscibility of some ILs with water is strongly temperaturedependent. The first example and use of thermo responsive IL was already reported as early as 1998.43,44 The solubility increases slowly as the temperature increases or decreases, and then very rapidly within a narrow temperature range. The temperature at which the abrupt change in the solubility occurs is called the critical solution temperature (CST). In upper critical solution temperature (UCST) systems, the solubility of the IL in the solvent increases with increasing temperature, and a homogeneous system is formed above a certain point, namely the components are miscible at all concentrations above the UCST. The opposite happens for the lower critical solution temperature (LCST) systems (liquid−liquid demixing upon heating), in which the homogeneous phase is formed under the LCST. Typical phase behavior with UCST and LCST is shown in Figure 2.45,46 The existence of LCST is much less common

Figure 1. Common cations and anions of ILs.

ILs are quite famous due to the special properties of large liquid state range, low melting temperature, nonvolatility, nonflammability, relatively high thermal stability, favorable solvation behavior and high ionic conductivity, etc.1−10 They have been widely studied in catalysis,11−15 chemical industry,16 electrochemistry,17,18 biomass conversion,19−21 extraction22,23 and many other areas.24 The first IL, ethylammonium nitrate [(C2H5)NH3][NO3] with melting point of 12 °C, was reported in 1914 by Paul Walden.25 Until now, ILs have gone through three generations: (1) in 1980s, aluminum chloride based airsensitive ILs, which were widely used in acid catalysis, were reported; (2) in 1990s, common ILs based on imidazolium, pyridinium, and ammonium like 1-butyl-3-methylimidazolium tetrafluoroborate (abbreviated as [C4C1IM][BF4]) and 1-butyl3-methylimidazolium hexafluorophosphate ([C4C1IM][PF6]) were described.26 The best advantage of this generation is the air and moisture stability compared with the first generation;27 (3) since 20th century, functionalized ILs with task specific groups on either cations or anions or even both have been developed.28,29 Due to nearly unlimited combinations of cation and anion, huge kinds of tailor-made ILs can be obtained in principle. Applications of ILs to replace conventional solvents and being efficient catalysts in homogeneous catalysis have been extensively studied and widely utilized.12,30,31 Due to the intrinsic disadvantages of homogeneous catalytic systems like difficult product separation and catalyst recycling, ILs were

Figure 2. Typical phase behavior with UCST and LCST. Reprinted with permission from ref 45. Copyright 2015 American Chemical Society.

than the existence of UCST, but some cases do exist. The special thermoregulated property of IL system has already been applied to many fields such as extraction and separation, which further broadens the application of IL in catalysis. In the following, we will make a detailed summary of thermoregulated IL systems and their applications in catalytic reactions. 1.3. Thermomorphic Systems

In 2006, Behr summarized five thermomorphic systems: fluorous biphasic systems (FBS), thermoregulated phase transfer catalysis (TRPTC), soluble polymer-based catalysis, thermoregulated microemulsions and temperature−dependent multicomponent solvent systems (TMS).47 All these systems show the special property of being homogeneous under reaction temperature and two phase formation after cooling to room temperature. The main concepts, advantages, and disadvantages of each of these methods are listed in Table 1.47 B


Review

Figure 3. General principle of TRPTC system. Reproduced with permission from ref 48. Copyright 2004 Springer.

reaction system is heated, the catalyst would transfer from aqueous phase to substrate containing organic phase where the reaction takes place, while after reaction, the catalyst transfers back to the aqueous phase upon cooling leaving the products alone in the organic phase, which facilitates the separation of catalyst),48,49 TMS systems exhibit most advantages, e.g., the possibility to use common and cheap solvents as well as large amounts of classical molecular catalysts.50−52 Therefore, TMS system was expected to be one of the most promising thermomorphic recycling concepts for industrial scale, which have been adopted for many catalytic reactions by Behr et al., e.g., hydroformylation,53−57 hydroamination,58 and telomerization.59 Although reviews about recycling concepts for thermoregulated catalyst separation and recycling have already been given,60−64 IL-based thermomorphic systems are rarely summarized, especially those related to catalytic applications.40−42

2. THERMOMORPHIC SYSTEMS BASED ON IONIC LIQUIDS (ILS) Due to the advantages and practicability of thermomorphic systems, many ILs were designed to achieve thermomorphic behavior. Phase diagrams and applications of mixtures of designed ILs with H2O and/or organic solvents have been extensively studied. Many people might ask why ILs are chosen to be functionalized in a way that thermoregulated properties can be achieved. First of all, ILs are known as designable solvents and their properties can be conveniently manipulated by both the cations and anions according to requirement. Second, ILs possess higher polarity and they are not soluble in many organic solvents, which makes it easier to separate ILs from nonaqueous systems. Compared with thermomorphic polymers, which are well established in the literature,65−73 the structure of ILs can be adjusted with a higher flexibility allowing a more straightforward tailoring in many cases.

temperature-dependent multicomponent solvent systems (TMS)

thermoregulated microemulsions

thermoregulated phase transfer catalysis (TRPTC) soluble polymer-based catalysis

Although the TRPTC system was the most widely studied one (the general principle is shown in Figure 3, when the

enzyme catalysis, synthesis of nanoparticles, hydroformylation oligomerization, hydroaminomethylation, isomerizing hydroformylation

oxidation, C−C coupling, hydrogenation

applications

Heck reaction, hydroformylation, oxidation, polymerization, hydrosilylation hydroformylation, hydrogenation

disadvantages

expensive catalyst, fluorinated solvents catalyst leaching is too high for an industrial application complicated and expensive catalysts time−consuming phase separation catalyst leaching

advantages

high activity, very low catalyst leaching promising catalyst recycling, water can be used as solvent low leaching, good activity, high stability high interfacial areas and easy catalyst recycling cheap solvents and classical molecular catalysts can be used

concept

based on the ligands with a high affinity for perfluorinated solvents and a poor solubility in organic solvents to induce an inverse temperature-dependent solubility in water using thermomorphic ligands based on the phase-selectivity in a mixture of polar and nonpolar solvents ability to solubilize a broad range of substances with very different polarity combine homogeneous catalysis with an efficient catalyst recycling concept as in liquid−liquid catalysis

method

fluorous biphasic systems (FBS)

Table 1. Five Principles for Catalyst Recycling

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2.1. Mechanism of Thermoregulated Property

It is well-known that besides electrostatic forces, hydrogen bonding is also very important in ILs. One thing that has to be clear is that the hydrogen bonding in ILs is quite different from the already well-established hydrogen bonds.4,74 Hydrogen bonding in ILs is defined as “doubly ionic hydrogen bonds”, C


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Figure 4. Simplified 2D representation of the structure of a neat ionic liquid to its infinite dilution in the presence of other organic compounds. Most of these structures have been experimentally already observed (red spheres = anions, blue spheres = cations, black spots = solvent molecules, and the lines represent the hydrogen bonds and/or other weaker interactions). Reproduced with permission from ref 88. Copyright 2015 John Wiley & Sons, Inc.

solution (mainly long-range Columbic forces and short-range hydrogen bonding), both the IL and solvent can influence the phase phenomena dramatically.94−105 For example, normally, [NTf2]−-based ILs are considered to be more hydrophobic than the comparable [PF6]−-based ILs. The reason might be that the [NTf2]− anion with higher molecular weight has greater opportunity for van der Waals interactions with water or alcohol than the [PF6]− anion with lower molecular weight. For the same IL, van der Waals interactions are more important with alcohols than with water because of alcohols’ alkyl chains. Conversely, the [PF6]− anion has a greater charge densitiy than the [NTf2]− anion because it is smaller, so it can have stronger Coulombic or hydrogen bonding interactions, which are more important with water than with alcohols. Similarly, the effect of cation alkyl chain length on the phase behavior of IL-water systems is also opposite to that observed for IL-alcohol systems. For IL−water systems, longer alkyl chain length on the cation would decrease mutual solubility due to stronger hydrophobicity. On the other hand, like dissolves like; a high polar solvent has stronger interactions with ILs which also show high polarity; thus, the intramolecular interaction would be weekened and the intermolecular interaction would be strengthed. When the dielectric constant ε of the nonionic solvent is high (solvophobic systems), Coulombic forces are weak and phase transitions are driven by specific interactions (hydrogen bonds, hydrophobic interactions, etc.). Thus, Coulombic forces are responsible for phase separation in low dielectric constant media (Coulombic systems).96 Thermoregulated property of ILs can be understood as temperature-dependent solubility property, which is mainly attributed to hydrogen bonding or other possible interactions between IL and certain solvent(s). 1H NMR studies already probed that hydrogen bonding between the cation and anion are substantially weakened above the critical temperature for a UCST system.106 It means the intramolecular interaction in ILs is weakened, so the intermolecular interaction between IL and solvent is strengthed and a homogeneous solution can be formed. While for the LCST system, e.g., amino acid IL−water mixtures, it was reported that hydrogen bonding interaction between the anions of amino acid ILs can be increased with increasing temperature. This means the anion−H2O electrostatic interaction is weekened and directly causes separation of ILs from water showing LCST behavior.107 Meanwhile, solution theory can be used to predict the critical properties of mixed solutions even over the whole composition range.108,109 For example, Hildebrand and Scott theory

which forms between two charged species, cation and anion. It means a significant variety of distinct hydrogen bonds can be formed because of the different types and numbers of hydrogen bond donor and acceptor sites.75−79 Taking 1,3-dialkylimidazolium ILs as an example, they were regarded as hydrogen-bonded polymeric supramolecules (highly ordered hydrogen bonded materials) with one cation surrounded by at least three anions and one anion surrounded by at least three cations. The number of anions that surround the cation (and vice versa) can change depending on the anion size and N-alkyl group in the cation. The strong hydrogen bonding in imidazolium ring was originated normally from the most acidic H2 of the imidazolium cation, followed by the other two hydrogens (H4 and H5) of the imidazolium nucleus and/or the hydrogens of the N-alkyl radicals.80 Replacing C− H2 with a methyl group in imidazolium ILs which weakens hydrogen bonding was already proved by spectroscopic evidence.81 There are also reports with the opposite view that the hydrogen bonding indeed exist in IL (e.g., [C4C1IM][PF6]), but it is not so strong as published and far from dominating IL’s structure and dynamics.82 When other molecules were introduced into a pure IL, the hydrogen bonding network was disrupted and ion pairs were formed. In some cases nanostructures with polar and nonpolar regions were generated,83−87 and even larger ionic and neutral aggregates can be formed when dissolved in solvents with relatively high dielectric constants, such as acetonitrile and dimethyl sulfoxide (Figure 4).88 The stability and reactivity of the aggregates are dependent on the nature of the anion and imidazolium substituents and the aggregates are more abundant in ILs containing strong coordinating anions, in particular those that can form charge transfer complexes.89−91 Compared with imidazolium-based ILs, N-alkylpyridinium, N-alkylpiperidinium, alkylammonium, and alkylphosphiumbased ILs show weaker hydrogen bonding interactions between the hydrogen atoms in the cation and the electron rich center of the anion. However, when the alkyls are replaced by hydrogen atoms, the ILs become protic ILs which show much stronger hydrogen bonds. The key property that distinguishes protic ILs from other ILs is proton transfer, which leads to the presence of proton-donor and -acceptor sites and the formation of a hydrogen bonding network. Experiments and calculations have proved that there are stronger hydrogen bonds in protic ILs than in aprotic ones.92,93 Obviously, phase behavior of IL with H2O and/or organic solvents is the result of several competeing interactions in the D


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hydrophobicity, also with an increasing hydrophobicity of the anions in the order of [BF4]− < [CH3(C2H4O)2SO4]− < [C(CN)3]− < [PF6]− < [NTf2]−.146 Interestingly, kosmotropic salts have the effect of salting out water-miscible ILs to form salt−salt aqueous biphasic systems.155 According to phase diagrams, with increasing temperature, the degree of binodal shift becomes smaller in the order of [C4C1IM]Cl > [C4Py]Cl > [C4C1C1IM]Cl ≈ [(C4H9)4N]Cl, which correlates reasonably well with the decreasing in hydrogen bond donating abilities. Furthermore, UCST behavior of [C6C1IM][BF4]−water can be influenced by high molecular weight polymer poly(acrylic acid) (PAA).153 The phase-transition region of coexisting phases significantly shifts down with an increase in the concentrations of PAA. The CST values decrease linearly with increasing PAA concentrations in the system. Compared with commonly investigated [PF6]− and [BF4]− anions, [NTf2]− is superior, because it is hydrolytically stable, less viscous, and does not decompose to give HF in small but problematic amounts. Phase equilibria of binary and ternary mixtures of hydrophobic ILs with the common anion [NTf2]− were also widely investigated. Protonated betaine bis(trifluoromethylsulfonyl)imide ([Hbet][NTf2], Figure 5)

postulated that London dispersion forces (also called loosely van der Waals forces, they are a type of force acting between atoms and molecules) dominate, and polar interactions (such as dipole, ionic, and hydrogen bonding) are of secondary importance. The drawback of this theory is that polar effects usually outweigh all other effects in chemical systems which contain ions. To take into account all types of interactions, Hansen proposed a three-component system based on the division of intermolecular forces into dispersion, polar, and hydrogen-bonding contributions. He assumed that the total solubility parameter is made up of the additive contributions from nonpolar (dispersion) interactions, polar (dipole−dipole and dipole induced dipole) interactions, and hydrogen-bonding or other specific association interactions (including Lewis acid− base interactions). On the other hand, traditional approaches using the excess Gibbs energy models (NRTL, UNIQUAC) for correlating phase equilibriums or excess molar enthalpy of mixing in binary and ternary mixtures were used.110−128 Excellent agreement between the experimental and calculated results can be obtained using these models. Among these models, nonrandom two liquid (NRTL) model and the electrolyte-NRTL (eNRTL) model, or UNIQUAC model are quite widely used.129−136 The NRTL model is the most used model for equillibrium data correlation with ILs but not for systems involving electrolytes. In this model, the IL is considered to be completely associated, that is, anion and cation are paired and considered to be a single molecular species in the solution. The eNRTL model is a modification of the NRTL model, taking into account the low range forces, and so is more appropriate for systems containing electrolytes. Different from NRTL model, which only consists of enthalpy terms, UNIversal QUAsiChemical (UNIQUAC) model, which is an activity coefficient model used in description of phase equilibria, consists of both an entropy term and enthalpy term. 2.2. Conventional IL Systems with UCST and LCST Behavior

2.2.1. UCST Behavior. Generally speaking, the physicochemical properties of most ILs are strongly dependent on the type of cation and anion, while very small changes in the structure of IL can drastically affect the mutual solubility with other solvents including ILs.137−144 In most cases, the mutual solubility increases with an increase in the alkyl chain length attached on the cation in nonaqueous solutions. In many mixtures, the observations of USCT were limited by the boiling temperature of the solvent. Sometimes, it was nearly not possible to detect the mutual solubility of IL with the solvent by the visual method, and thus conductivity measurements,145 spectroscopic (FT-Raman, NMR, UV spectroscopy, inverse gas chromatography)108,114,120,145 or other techniques were necessary. 2.2.1.1. IL-H2O Binary Mixtures. IL-H2O binary mixtures have been studied extensively.146−154 Water was regarded as an “impurity” for ILs since water can interact with ILs (through hydrogen bonding) and further affect IL properties. Thus, the phase separation temperature depends on the composition of the IL and individually added proportions of IL and H2O. Solubility of water in ILs was determined by both the cation and anion, and the anion has greater impact on the phase behavior. Thus, it is quite interesting and meaningful to investigate the phase equilibria of IL-H2O binary mixtures. For widely studied imidazolium-based ILs, the mutual solubility between water and ILs decreases with an increasing alkyl chain length at the imidazolium cation due to the increased

Figure 5. Structure of betainium bis(trifluoromethylsulfonyl) imide ILs.

shows thermomorphic behavior with an UCST of around 55.5 °C with water.156−160 This IL has the ability to selectively dissolve large quantities of metal oxides after treatment with an acidic aqueous solution. The IL can be recycled for reuse after transfer of the metal ions to the aqueous phase. Furthermore, the IL can be used as a draw solute in forward osmosis: A 3.2 M solution of [Hbet][NTf2] was obtained by heating above 56 °C and this solution successfully drew water from high−salinity water (≤3.0 M) through forward osmosis. When the IL solution was cooled to room temperature, it spontaneously separated into a water-rich phase and an IL-rich phase. The ILrich phase could be used directly as the draw solution in the next cycle of the forward osmosis process.158 The influence of the cation on the thermomorphic property of ILs was also further explored. The analogues of betainium bis(trifluoromethylsulfonyl)imide with carboxyl group or longer alkyl chain instead of methyl group in betainium cation were synthesized.106 It was found that [C4Hbet][NTf2] and E


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Table 2. IL−H2O Systems with UCST Behaviora ILs

IL content

[C4C1IM][BF4] [C6C1IM][BF4] [C8C1IM][BF4] [C2H5(CH3)2(CH2CH2OH)N][BF4]

H2O content

7 mol % 93 mol % 30−60 wt % 70−40 wt % 30−60 wt % 70−40 wt %

[C2H5(CH3)2(CH2CH2OH)N]Br [C4C1IM][PF6] [C4C2IM][PF6] [C2C2IM][NTf2] [C4C2IM][NTf2] [C6C2IM][NTf2] [(C6H13OCH2)C1IM][NTf2] [(C6H13OCH2)2IM][NTf2] [(CH3)3(CH2CH2OH)N][NTf2] [(CH3)2(C2H5) (CH2CH2OH)N][NTf2] [(CH3)2(C3H7) (CH2CH2OH)N][NTf2] [(CH3)2(C4H9) (CH2CH2OH)N][NTf2] [(CH3)2(C5H11) (CH2CH2OH)N][NTf2] [choline][NTf2] [Hbet][NTf2] [HbetC1Mor][NTf2] [HbetC1IM][NTf2] [HbetPy][NTf2] [C8iQuin][NTf2] [CH3(CH3OCH2CH2)Mor][NTf2] [CH3(CH3OCH2CH2)Pip][NTf2] [CH3(CH3OCH2CH2)Pyr][NTf2] [CH3(CH3OCH2CH2)Mor][FAP] [CH3(CH3OCH2CH2)Pip][FAP] [CH3(CH3OCH2CH2)Pyr][FAP] [C2C1IM][FAP] [C2C1IM][EtSO4] [C4C1IM][TOS]

43,96,132,151,154,162,163 129,153 129 142

44 wt % 89 wt % 63.5 mol %

96

102,173

26 mol % 50 mol % 36 mol % 35 wt % 49 mol % 50 mol % 50 mol % 59 mol % 50 mol % 50 mol % 30 wt % 30 wt % 10 wt % 89 wt % 92 wt % 4 mol % 50 wt % 50 wt % 50 wt % 50 wt % 57 mol % 29 mol % 53 mol % 33 mol % 83 mol % 80 mol % 78 mol % 80 mol % 2 mol %

74 mol % 50 mol % 64 mol % 65 wt % 51 mol % 50 mol % 50 mol % 41 mol % 50 mol % 50 mol % 70 wt % 70 wt % 90 wt % 11 wt % 8 wt % 96 mol % 50 wt % 50 wt % 50 wt % 50 wt % 43 mol % 71 mol % 47 mol % 67 mol % 17 mol % 20 mol % 22 mol % 20 mol % 98 mol %

30 mol % 33 mol %

70 mol % 67 mol %

20 mol %

80 mol %

70 mol %

30 mol %

50 mol %

50 mol %

40 mol % 25 mol %

60 mol % 75 mol %

25 mol % 25 mol % 25 mol %

75 mol % 75 mol % 75 mol %

[C4C13Py][TOS] [C6C13Py][TOS] [C6C13Py][CF3SO3]

[C4C1Pip][N(CN)2] [C4C1Pyr][N(CN)2] [(CH3)2(C10H21)2N][NO3]

56 wt % [(PhCH2) (CH3)2(C12H25)N][NO3] (40 wt %) + [(PhCH2) (CH3)2(C14H29)N][NO3] 11 wt % (60 wt %) tetraisopentylammonium bromide 3.5 mol %

refs

4 60−65 67−70 154b −27 84 −25 137 101 72 94 94 55 44 69 94 92 86 88 72 55.5 52 64 55 79 76 74 84 44 71 78 70 −3 57b −37 −40 52b −37 51b 31 56b 38 63b 31 −18 27b −29 −31 −37 32b 16 88

[C4C14Py][TOS]

[C4C1Py][SCN] [C4C1Py][N(CN)2]

CST value/ °C

164 133

137 165

161 156−160 106

134 143

166 117 167 168 118 169 170 166 143

171 172

a

Note: For UCST systems, the mixture is biphasic below the critical solution temperature (CST) and homogeneous above CST, which varies with the composition of the two phases. For most systems, only CST values were provided, while UCST values were normally independent of composition and marked with the bold fonts (the same indication in the following tables). bThe phase diagram show UCST at the point of 100% IL.

[C6Hbet][NTf2] with butyl and hexyl chains showed more hydrophobicity than [Hbet][NTf2] and both were not miscible

with water even after heating, the same behavior happened also for both [HbetC1Pyr][NTf2] and [HbetC1Pip][NTf2]. HowF


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Review

results in an increase of the solubility. It is no doubt that the specific interaction occurs between the nitrogen atom of imidazole ring with solvent, as well as oxygen atom on the alkoxymethyl group of cation with solvent. A study on the miscibility of [CnC1IM][BF4]−dihydroxy alcohol systems shows the same behavior with UCST.182−184 The alkyl chain length in alcohol influences the miscibility as well but its impact is much more complex than in the case of monohydroxyalcohols. For example, with increasing chain length in the N−3 position of the [CnC1IM]+ cation, the UCST shifts toward higher temperatures for the systems with 1,2-ethanediol, while it was the opposite for the systems with monohydroxy alcohols (UCST shifts toward lower temperatures with increasing chain length). In contrast, for the systems of longer diols with hydroxyl group in the 1,2-position, improvement of miscibility was observed. The influence of the side chain of the imidazolium cation is different and depends on the structure of the diol. The mutual solubility of [C2C1IM][BF4] decreases with the increase in alkyl chain of 1,2dihydroxy alcohols, but the opposite effect was observed for the IL with phosphonium cation. For alkylimidazolium ILdihydroxy alcohols, miscibility follows the order [BF4]− > [NTf2]− > [PF6]− and is dependent on the hydrogen-bond basicity of the anion. Morevover, the relative position of the OH groups within an alcohol molecule affects the miscibility as well: the vicinal position favors the miscibility. Similar phenomena were observed also for the systems of [CnC1IM][PF6] (n = 4, 6, 8) and diols.185 Comparing the miscibility of 1,2-ethanediol with [CnC1IM][NTf2], the CST value increases in the orer of [C2C1IM]+ < [C4C1IM]+ < [C6C1IM]+, which is opposite to that observed earlier for systems with monohydroxy alcohols. Analyzing the influence of the polyhydroxy alcohol structure, the miscibility of the polyhydroxy alcohols with [C4C1IM][NTf2] or [C4C1IM][BF4] decreases when the polarity of the alcohol rises. For pyridinium-based ILs several effects are also involved in the mixing process.186,187 The pyridinium-based cation gives rise to larger immiscibility than the imidazolium-based cation, and over a slightly larger temperature range. However, the differences may not be so significant for practical application of these multiphasic and entirely ionic systems. Thus, the imidazolium based ILs might be preferable for applications at room temperature, as their melting points are considerably lower. In the [C4C1Py][BF4]−water mixture, volume expansion occurs with temperature. However, in the IL−alkanol mixtures, the rise in temperature resulted in volume contraction. The alkanol chain also has a clear influence on the mixture as expected, but this is more pronounced in the IL−MeOH mixtures. Similarly, piperidinium−based IL (1-methyl-1-propylpiperidinium bis{(trifluoromethyl)-sulfonyl}imide, [C 3 C 1 Pip][NTf 2 ]) with alcohols were showing UCST phenomena in most cases.188 For phosphonium-based ILs they are largely immiscible at ambient temperature with comparatively other ILs including 1,3-dialkylimidazolium or 1-alkylpyridinium cations. The miscibility of such pairs of ILs was observed to be temperature dependent, following an UCST behavior. Systematic studies of [NTf2 ]− anion based quinolinium189−191 and corresponding isoquinolinium134,192,193 ILs (Figure 6) were also found to show UCST characteristics. For mixtures of [CnQuin][NTf2] (n = 4, 6, or 8) with alcohols, it was observed that with increasing chain length of an alcohol the solubility decreases and the UCST increases. Complete

ever, [HbetC1Mor][NTf2] became miscible with water after heating to 52 °C, and [HbetC1IM][NTf2] and [HbetPy][NTf2] exhibit UCST behavior with water at temperatures of around 64 and 55 °C, respectively. Another similar IL, choline bistriflimide ([choline][NTf2], Figure 5), was found to be miscible with water above 72.1 °C and not miscible with water at room temperature.161 1H NMR studies showed that the hydrogen bonding between the choline cation and the [NTf2]− anion are substantially weakened above the critical temperature. Also the phase transition was not defined as sharp as [Hbet][NTf2]: instead of forming a homogeneous phase, a clouding was observed at the transition temperature, which slowly clarified after about 5 min. A brief summary of UCST behavior for conventional IL− H2O systems is listed in Table 2. Since various equilibrium diagrams can be obtained by varying the compositions, only part of the data were picked from the literature with a certain CST value to show the general thermoregulation property of the systems. 2.2.1.2. IL−Alcohol Systems. For IL−alcohol systems low solubility of IL in the alcohol and high solubility of alcohol in IL were observed.174−178 The effect of alkyl chain length on the cation and the choice of anion in ILs have significantly different effects on their mutual solubilities with alcohols than with water, e.g., the [BF4]− anion reduces the UCST greatly compared with [PF6]− anion.179 The relative alcohol affinity for the different anions observed was N(CN)2−> CF3SO3− > [NTf2]− > BF4− > PF6−.94,130 And alcohol solubility in ILs follows the order of alcohol branching: tertiary > secondary > primary alcohols, which correlates with the relative basicity of the alcohols (hydrogen bond acceptor capability). On the other hand, longer alkyl chain of the alcohol resulted in an increase in the UCST due to less interaction with the IL through hydrogen-bonding, dipolar and Coulombic forces.180 The influence of water (as an “‘impurity’”) on the cloud point temperature was also investigated for better understanding of some systems, it was found the system is not so sensitive to water, e.g. [C4C1IM][NTf2]/1,2-hexanediol.181 For imidazolium-based ILs an increase in the alkyl chain length on the cation resulted in decreased UCST in most cases due to the higher mutual solubilities (greater dispersion interactions between the alkyl chain of the cation and the chain of the alcohol).103 Moreover, replacement of the most acidic hydrogen at the C2 position of the ring with a methyl group resulted in an increase in the UCST due to a decrease in hydrogen bonding of the alcohol with the cation. A systematic study of the impact of different factors on the phase behavior of alkoxyimidazolium-based ILs with polar and nonpolar solvents was presented by Domańska.137 Most of the systems examined showed immiscibility with UCST behavior or complete solubility of the IL at room temperature in many solvents. Take [C4C1C1IM][BF4] for example, its solvation ability with alcohol was relatively weaker than that with [C4C1IM][BF4].115 It was believed that the hydrogen-bonding between alcohol and the hydrogen atom at C2 position of the imidazolium ring in [C4C1C1IM][BF4] was weaker than that with [C4C1IM][BF4]. By changing the anion from [BF4]− to [NTf2]−, the solubility dramatically increased and the UCST decreased. For the systems consisting of ILs [CnC1IM][NTf2] (2 ≤ n ≤ 10) and 1porponal or 1-butanol, UCST phenomena was observed also.116,178 By contrast, increasing hydrogen bonding interaction opportunities with the solvent by replacing a methyl group with the second alkoxy-group on the imidazolium ring G


Chemical Reviews

Review

tosylate) or [C4C13Py][TOS] (N-butyl-3-methylpyridinium tosylate) with alkane or alkanebenzene,118,169 Binary systems of [C 6 C 1 3 Py][CF 3 SO 3 ], [C 3 C 1 Pyr][CF 3 SO 3 ]) and [C4(CN)4Py][TCM] (N-butyl-4-cyanopyridinium tricyanomethanide) with organic solvents were reported to show UCST behavior for most systems.166,184 A comparison of phase diagrams indicated the possible extraction of sulfur compounds from heptane with high selectivity as the solubility of sulfur compounds in these ILs was complete at higher temperature in a wide range of mole fractions of the IL. Binary systems of tetra-n-butylphosphonium IL with different anions, specifically tosylate anion ([P4,4,4,4][TOS]), with organic solvents was reported by Domań s ka. 122 Changing the anion from [CH3SO3]− to [TOS]− increases the solubilities in binary systems with benzene and alkylbenzenes. Furthermore, since the UCST of IL [P6,6,6,14]Cl (trihexyl(tetradecyl)phosphonium chloride) + nonane is near ambient conditions (314 K under near anhydrous conditions), only moderate adjustment of temperature is required to induce phase separation, which was reported to have the ability to separate from n-nonane through moderate tuning of temperature or using water.216 Tetra-nbutylammonium picrate and 1-tridecaneol system was reported to have UCST at around 342 K.217 To get deep understanding, liquid−liquid equilibrium of the [NTf2]− anion based imidazolium ILs with arenes,218,219 haloalkanes221 and hydrocarbon222 have been extensively studied and were found to be good solvents for polymers with UCST behavior.223,224 For mixtures of [CnQuin][NTf2] (n = 4, 6, or 8) with benzene, alkylbenzene, or thiophene, the eutectic systems with mutual immiscibility in the liquid phase with very high UCSTs were observed.189−191 For the binary systems of [C6Quin][NTf2] with benzene and alkylbenzenes, eutectic diagrams with immiscibility gaps in the liquid IL phase (beginning from 0.13 to 0.28 mole fraction) with very high UCST were observed. For mixtures of [C8Quin][NTf2] with benzene and alkylbenzenes, an immiscibility gap in the liquid phase was detected for low mole fraction of the IL. The observed UCSTs were higher than the boiling point of the solvent. In the system with thiophene, the immiscibility gap is lower and an UCST behavior was measured also. [C4iQuin][NTf2] and [C6iQuin][NTf 2 ] show similar properties as the corresponding quinolinium-based ILs. Temperature-composition phase diagrams of [C8iQuin][NTf2] with aliphatic hydrocarbons, cyclic hydrocarbons, and an aromatic hydrocarbon were detected from ambient temperature to the boiling point of the solvent at ambient pressure.134,166 For such binary systems the immiscibility in the liquid phase with an UCST was observed in all mixtures. Phase equilibrium phenomena of systems, consisting of three dicyanamide ILs and four dienes, with UCST behavior was also reported.225 Generally, the shorter alkyl chain in the imidazolium ring, the better solubility of dienes. A brief summary of UCST behavior for conventional IL− hydrocarbon systems with typical equibilium is listed in Table 4. Interestingly, it was also found that small changes in the structure of an IL can drastically affect the mutual solubility with another IL. The liquid−liquid equilibrium data of these systems were acceptably correlated by means of the classical nonrandom two-liquid (NRTL) equation.123,131 The liquid− liquid equilibrium of the binary systems of [P6,6,6,14][NTf2] with [C2Py][NTf2] or [C2C1IM][NTf2] show UCST values at >100 °C. The ternary system ([C2C1IM][NTf2] + [C4C1IM][NTf2] + [P6,6,6,14][NTf2]) was studied at 25 °C. Despite [C4C1IM]-

Figure 6. Structures of [NTf2]− anion based quinolinium and corresponding isoquinolinium ILs.

miscibility of [C6Quin][NTf2] was observed for 1-butanol while other alcohols showed immiscibility with UCST behavior.190 For binary mixtures of [C8Quin][NTf2] with alcohols, complete miscibility in the liquid phase was observed for 1-butanol and 1-hexanol and immiscibility gaps with UCST behavior was noted for longer chain alcohols.191 Correspondingly, [CniQuin][NTf2] (n = 4, 6, or 8) were prepared and fully characterized. [C4iQuin][NTf2] and [C6iQuin][NTf2] show similar properties as the corresponding quinolinium-based ILs. Temperature-composition phase diagrams of [C8iQuin][NTf2] with alcohol were detected from ambient temperature to the boiling point of the solvent at ambient pressure.134,166 For such binary systems the immiscibility in the liquid phase with an UCST was observed in all mixtures. Complete solubility in the liquid phase was observed for 1-butanol and 2-phenylethanol. H/D isotope effect in the systems of tetraalkylammonium salts and D2O,102 dihydroxy alcohols with [BF4]− anion combined with imidazolium and phosphonium cations181 were also investigated. All systems showed diagram with UCST. In the case of investigated imidazolium ILs, the miscibility is better with shorter alkyl chain length in 1,2-diols. The opposite trend showed for the systems with [P6,6,6,14][BF4]: the longer alkyl chain in alcohol the lower CST value. The position of two hydroxyl groups in diols can change miscibility as well. Last but not the least, some ternary systems show UCST phenonema as well, e.g., IL/n-hexane/an organic compound,194 [CnC1IM][BF4] (n = 2,4)/H2O/CH3CH2OH or THF,195 [C4C1IM][PF6]/H2O/alcohol,174,196,197 [C8iQuin][NTf 2 ]/water/2-phenylethanol, 1 3 4 [(CH 3 ) 2 (C n H 2n + 1 ) (CH2 CH 2OH)N][NTf2 ] (n = 1−5)/water/1-octanol,165 [C4C1IM][NTf2]/2-methylpropanol/water,121 and [C2C1IM][EtSO4] with alcohol and water.117 The liquid−liquid equilibria of mixtures show that the behavior of this particular IL homologous family toward each of the molecular solvents is quite diverse and can be fine-tuned by taking into account the length of the alkyl side chain in IL. The presence of hydroxyl groups in the cation and the effect of temperature also play an important role. For [C4C1IM][NTf2]/1-butanol/water system, the mutual solubility of the ternary can be tuned by CO2 and the demixing pressure is strongly controlled by the water concentration.198 A brief summary of UCST behavior for conventional IL− alcohol systems is listed in Table 3. Similar as Table 2, only typical data have been listed for a rough understanding. 2.2.1.3. IL-Hydrocarbon Systems. IL−hydrocarbon binary systems were also widely studied, since aromatic hydrocarbons are well-known to form n-π interactions with many solutes.214 Binary systems composed of [C 1 C 1 IM][CH 3 SO 4 ] or [C4C1IM][CH3SO4] with hydrocarbons,215 [C2C1IM][TOS] (1-ethyl-3-methylimidazolium tosylate) with hexane or cycloheptane and [C2C1IM][EtSO4] with hexane or benzene,138,220 [C4C1IM][TOS] with n-hexane or an aromatic hydrocarbons,167 [C4C14Py][TOS] (N-butyl-4-methylpyridinium H


Chemical Reviews

Review

Table 3. IL−Alcohol Systems with UCST Behavior ILs

IL content

alcohols t-butanol, t-butanol, 1decanol t-butanol 1-decanol 1-dodecanol 1,2-ethanediol 1,2-propanediol 1,2-butanediol 1,2-hexanediol 1,2-butanodiol-d2 1,2-hexanediol-d2 1,3-propanediol 1,3-propanediol 1-butanol 1-butanol-d1 i-butanol i-butanol-d1 2-butanol 2-butanol-d1 t-butanol t-butanol-d1 1-propanol 1-butanol 1-pentanol glycerol 1,2-propanediol 1,3-propanediol 1,2-ethanediol 1,2- butanediol 1,2-butanediol-d2 1,2-Hexanediol-d2 1,3-butanediol 1,4-butanediol 2,3-butanediol 1,5-pentanediol 1,2-hexanediol 1,2-ethanediol 1,2-propanediol 1,2-butanediol 1,2-pentanediol 1,2-hexanediol 1-hexanol 1-octanol 1-pentanol 1,2-ethanediol 1,3-propanediol 1,2-butanediol 1,3-butanediol 1,4-butanediol 2,3-butanediol 1,5-pentanediol 1,2-hexanediol 1-propanol 1-butanol 1-pentanol 1-hexanol 1-butanol 1-pentanol 1-hexanol

[C12C1IM][Cl]

[C2C1IM][BF4]

[C4C1IM][BF4]

[C6C1IM][BF4]

[C8C1IM][BF4]

[C4C2IM][BF4]

[C4C3IM][BF4]

17 mol % 3.6 mol % 1.3 mol % 25 mol % 22 mol % 23 mol % 28 mol % 24 mol % 28 mol % 26 mol % 23 mol % 23 mol % 15−25 mol % 15−25 mol % 15−25 mol % 7−20 mol % 12 mol % 20 mol % 20 mol % 6 mol % 13 mol % 6 mol % 26 mol % 22 mol % 26 mol % 24 mol % 22 mol % 22 mol % 24 mol % 24 mol % 25 mol % 19 mol % 27 mol % 25 mol % 21 mol % 21 mol % 20 mol % 19 mol % 21 mol % 5−15 mol % 13 mol % 33 wt % 23 mol % 21 mol % 23 mol % 22 mol % 25 mol % 18 mol % 26 mol % 18 mol % 19 mol % 15 mol % 20 mol % 22 mol % 15 mol % 15 mol % 26 mol %

I

alcohol content

CST value/ °C 97a

83 mol % 96.4 mol % 98.7 mol % 75 mol % 78 mol % 77 mol % 72 mol % 76 mol % 72 mol % 74 mol % 77 mol % 77 mol % 85−75 mol % 85−75 mol % 85−75 mol % 93−80 mol % 88 mol % 80 mol % 80 mol % 94 mol % 87 mol % 94 mol % 74 mol % 78 mol % 74 mol % 76 mol % 78 mol % 78 mol % 76 mol % 76 mol % 75 mol % 81 mol % 73 mol % 75 mol % 79 mol % 79 mol % 80 mol % 81 mol % 79 mol % 95−85 mol % 87 mol % 67 wt % 77 mol % 79 mol % 77 mol % 78 mol % 75 mol % 82 mol % 74 mol % 82 mol % 81 mol % 85 mol % 80 mol % 78 mol % 85 mol % 85 mol % 74 mol %

−3 6 23 4 26 53 101 50 98 37 26 62 57 59 55 60 54 48 41 39 57−62 73 75 7 26 4 25 24 63 32 37 20 50 67 12 1 8 22 38 28 42 19 30 50 -1 26 37 2 38 18 47 64 78 86 35 49 61

refs 199

182

179 200

176,201

184

182 183

175 174 183

113

113


Chemical Reviews

Review

Table 3. continued ILs [C4C4IM][BF4] [C4C1C1IM][BF4]

IL content

alcohols

15 mol % 16 mol % 13 mol % 9 mol % 15 mol % 20 mol % 19 mol %

1-pentanol 1-hexanol 1- propanol 1-butanol 1-pentanol 1-hexanol 2-propanol ethanol, 1-octanol ethanol 1-octanol 1-butanol 1-hexanol 1-octanol 1-hexanol, 1-octanol, 1decanol 1-hexanol 1-octanol 1-decanol 1-butanol, 2-butanol, 1octanol, 1-dodecanol 1-butanol 2-butanol 1-octanol 1-dodecanol 1-dodecanol ethanol, 1-butanol, 1hexanol, 1-dodecanol ethanol 1-butanol 1-hexanol 1-dodecanol 1-butanol ethanol 1-propanol 1-butanol ethanol 1-propanol 1-butanol 1-pentanol 1,2-propanediol 1,2-butanediol 1,3-butanediol 2,3-butanediol 1,2-pentanediol 1,2-butanediol-d2 methanol ethanol 1-propanol 2-propanol 1-butanol 2-butanol t-butanol 3-methyl-1-butanol 1,3-propanediol ethanol ethanol/H2O

[(C4H9OCH2)2IM][BF4]

[C6H13OCH2C1IM][BF4]

50 mol % 50 mol % 20 mol %

[(C6H13OCH2)2IM][BF4] 40 mol %

[C2H5(CH3)2(CH2CH2OH)N][BF4] 50 mol %

[C6H13(CH3)2(CH2CH2OH)N][BF4] [Pyr][BF4]

50 mol %

50 mol %

[C4C1Py][BF4] [C4C13Py][BF4]

[C4C14Py][BF4]

[P6,6,6,14][BF4]

31 mol % 9 mol % 14 mol % 13 mol % 9 mol % 12 mol % 14 mol % 17 mol % 8 mol % 8 mol % 7 mol % 7 mol % 9 mol % 9 mol %

[C2C1IM][PF6]

[C4C1IM][PF6]

50 51 51 51 51 52 70 26 11 37

mol % mol % mol % mol % mol % mol % mol % mol % mol % wt %

79 mol % 23 mol % 23 mol %

1-butanol 1,2-ethanediol 1,2-hexanediol J

alcohol content 85 84 87 91 85 80 81

mol mol mol mol mol mol mol

% % % % % % %

50 mol % 50 mol % 80 mol %

60 mol %

50 mol %

50 mol %

50 mol %

69 91 86 87 91 88 86 83 92 92 93 93 91 91

mol mol mol mol mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % % % % % %

50 mol % 49 mol % 49 mol % 49 mol % 49 mol % 48 mol % 30 mol % 74 mol % 89 mol % 47.7 wt %/15.3 wt % 21 mol % 77 mol % 77 mol %

CST value/ °C 41 47 50 53 65 79 104 8a −5 −2 18 34 54 36a

refs

115

142

137

16 18 22 154a

142

100 76 145 139 22 89a

138 142

79 74 72 75 49 5 31 48 3 30 45 57 93 37 84 47 6 37 60a 49 83 75 116 95 81 109 87 52 15 100 69 125

176 95

186

182

202

179 121,163,164 164 108,203 185


Chemical Reviews

Review

Table 3. continued ILs

[C5C1IM][PF6] [C6C1IM][PF6]

[C7C1IM][PF6] [C8C1IM][PF6]

[C4C2IM][PF6] [CnC1IM][NTf2] (n = 1, 3) [C1C1IM][NTf2]

[C2C1IM][NTf2]

[C3C1IM][NTf2] [C4C1IM][NTf2]

IL content

alcohols

50 mol % 60 mol % 70 mol % 24 mol % 88 mol % 90 mol % 25 mol % 22 mol % 24 mol % 20 mol % 18 mol % 94 mol % 92 mol % 20 mol % 22 mol % 22 mol % 23 mol % 28 mol % 28 mol % 21 mol % 18 mol % 23 mol % 20 mol % 20−30 mol %

ethanol 1-propanol 1-butanol 1,2-propanediol 1-butanol 1-butanol 1,2-ethanediol 1,2-propanediol 1,3-propanediol 1,2-butanediol 1,2-hexanediol 1-butanol 1-butanol 1,2-ethanediol 1,2-propanediol 1,3-propanediol 1,2-butanediol 1,3-butanediol 1,4-butanediol 2,3-butanediol 1,2-pentanediol 1,5-pentanediol 1,2-hexanediol 1-octanol n-alkyl alcohols (3 ≤ n ≤ 8) 1-propanol 1-butanol 1-pentanol 1-hexanol 1-heptanol 1-octanol 1-propanol 1-butanol 1-pentanol 1-hexanol 1-heptanol 1-octanol 1,2-ethanediol 1,2-butanediol 1,3-butanediol 1,4-butanediol 1-propanol 1-butanol 1-hexanol 1-octanol 1-butanol 1-pentanol 1-heptanol 1-nonane 1-decanol 1-undecanol 2- methylpropanol 1-butanol 1-hexanol 1,2-propanediol 1,3-propanediol 1,2-ethanediol 1,2-butanoediol cyclohexanol

14 mol % 15 mol % 16 mol % 18 mol % 20 mol % 22 mol % 14 mol % 15 mol % 16 mol % 17 mol % 16 mol % 19 mol % 26 mol % 24 mol % 26 mol % 34 mol % 13 mol % 14 mol % 16 mol % 18 mol % 30 mol % 15 mol % 18 mol % 20 mol % 19 mol % 21 mol % 12 mol % 14 mol % 20 mol % 24 mol % 23 mol % 20 mol % 41 mol % 30−50 wt % K

alcohol content 50 mol % 40 mol % 30 mol % 76 mol % 12 mol % 10 mol % 75 mol % 78 mol % 76 mol % 80 mol % 82 mol % 6 mol % 8 mol % 80 mol % 78 mol % 78 mol % 77 mol % 72 mol % 72 mol % 79 mol % 82 mol % 77 mol % 80 mol % 80−70 mol % 86 mol % 85 mol % 84 mol % 82 mol % 80 mol % 78 mol % 86 mol % 85 mol % 84 mol % 83 mol % 84 mol % 81 mol % 74 mol % 76 mol % 74 mol % 66 mol % 87 mol % 86 mol % 84 mol % 82 mol % 70 mol % 85 mol % 82 mol % 80 mol % 81 mol % 79 mol % 88 mol % 86 mol % 80 mol % 76 mol % 77 mol % 80 mol % 59 mol % 70−50 wt %

CST value/ °C 20 70 45 69 93 77 76 53 86 58 95 62 53 87 55 93 49 71 88 46 59 87 75 83 7−142 31 57 77 97 117 142 22 49 68 87 104 127 50 30 54 73 8 36 72 105 27 42 74 101 117 134 30 27 60 65 77 63 186 46

refs 114

184 203 203 185

203 203 185

133 204 204

177,181,205,206

184 207

204

176,205 208

163 205 184

207 181


Chemical Reviews

Review

Table 3. continued ILs [C6C1IM][NTf2]

[C8C1IM][NTf2]

[C10C1IM][NTf2] [C12C1IM][NTf2]

[C2C2IM][NTf2] [C4C2IM][NTf2] [C6C2IM][NTf2] [C6C1C1IM][NTf2] [(C6H13OCH2)C1IM][NTf2]

IL content

alcohols

40−60 wt % 10−20 mol % 10−20 mol % 12 mol % 14 mol % 13 mol % 19 mol % 16 mol % 21 mol % 19 mol % 19 mol % 19 mol % 10−15 mol % 15 mol % 17 mol % 16 mol % 19 mol % 17 mol % 17 mol % 19 mol % 20 mol % 23 mol % 39 mol % 35 mol % 13 mol % 15 mol % 14 mol % 13 mol % 14 mol % 15 mol % 16 mol % 22 mol % 10−20 mol % 13 mol % 10−15 mol % 39 mol % 39 mol %

1,2-hexanediol 1-hexanol 1-octanol 1-butanol 1-heptanol 1-pentanol 1-nonane 1-decanol 1-undecanol 1-dodecanol 1-tetradecanol 1,2-ethanediol 1-octanol 1-octanol 1-octanol 1-decanol 1-undecanol 1-dodecan-ol 1-tetradecan-ol 1-hexadecan-ol 1-octadecan-ol 1-eicosan-ol 1,2-butanediol 1,2-butanediol 1-decanol 1-undecanol 1-dodecan-ol 1-tetradecan-ol 1-hexadecan-ol 1-octadecan-ol 1-eicosan-ol 1-octanol

1-hexanol 1-hexanol 1-octanol ethanol, 1-octanol ethanol 1-octanol ethanol 1-hexanol 1-octanol

[(C8H17OCH2)2IM][NTf2] 50 mol % [(C10H21OCH2)2IM][Tf2N] [CH3OCH2CH2OCH2CH2)2IM][NTf2] [(CH3)3(CH2CH2OH)N][NTf2] [(CH3)2(C2H5) (CH2CH2OH)N][NTf2] [(CH3)2(C3H7) (CH2CH2OH)N][NTf2] [(CH3)2(C4H9) (CH2CH2OH)N][NTf2] [(CH3)2(C5H11) (CH2CH2OH)N][NTf2] [C4C1Py][NTf2] [C4C14Py][NTf2]

[C3C1Pip][NTf2]

50 mol % 64 mol % 85−50 mol 85−60 mol 85−60 mol 85−65 mol 80 mol % 74 mol % 88 mol % 85 mol % 67 mol % 85 mol % 91 mol % 89 mol % 87 mol % 86 mol % 85 mol % 40 mol % 40 mol %

% % % %

CST value/ °C 55 33 60 -4 17 16 60 83 91 104 127 77 36 36 47 58 64 76 91 107 124 162 168 117 15 20 33 48 60 70 78 109 80 55 58 5 35 15a 7 10 27 40−52 99 88 66 48 27 30 19 49 76 143 62 80 97 113 129 113 136

refs

175,205,209

184 175 205

207 207 210

133

175 137 137 142

145 165

176 211

188


Chemical Reviews

Review


IL content

alcohols

[C4Quin][NTf2] 14 14 14 14 14

mol mol mol mol mol

ethanol, 1-butanol ethanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-dodecanol 1-hexanol 1-octanol 1-decanol 1-dodecanol 1-octanol ethanol, 1-butanol ethanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol, 1-hexanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol, 1-hexanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-butanol, 1-hexanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-decanol, 1-dodecanol 1-decanol 1-dodecanol 1-octanol, 2-phenylethanol 1-octanol 2-phenylethanol 1-propanol, 1-butanol 1-propanol 1-butanol

% % % % %

[C4iQuin][NTf2] 12 mol % 11 mol % 13 mol % [C6Quin][NTf2] 15 15 14 10

mol mol mol mol

% % % %

[C6iQuin][NTf2] 50 mol % 50 mol % [C8Quin][NTf2]

[C8iQuin][NTf2]

[C2C1IM][FAP] [C4C1IM][TOS]

10 mol % 11 mol % 17 mol % 9 mol % 10 mol % 10 mol % 10 mol % 82 mol % 60 49 50 70 50

mol mol mol mol mol

% % % % %

50 48 18 20

mol mol mol mol

% % % %

[C4C14Py][TOS]

[C4C13Py][TOS] 60 mol %

[C6C13Py][TOS] 40 mol % 40 mol % 8 mol % 17 mol % [P4,4,4,4][TOS] 21 mol % 29 mol % [C6C13Py][CF3SO3] 50 mol % 50 mol % [(CH3)2(C10H21)2N][NO3] 50 wt %

M

alcohol content 86 86 86 86 86

mol mol mol mol mol

% % % % %

88 mol % 89 mol % 87 mol % 85 85 86 90

mol mol mol mol

% % % %

50 mol % 50 mol %

90 89 83 91 90 90 90 18

mol mol mol mol mol mol mol mol

% % % % % % % %

40 51 50 30 50

mol mol mol mol mol

% % % % %

50 52 82 80

mol mol mol mol

% % % %

40 mol %

60 60 92 83

mol mol mol mol

% % % %

79 mol % 71 mol % 50 mol % 50 mol % 50 wt %

CST value/ °C 56a 24 34 65 98 169 48a 58 83 108 44a 27 47 73 99 54a 54a 43 47 48a 48a 52 74 94 18 41 62 79 87 57a 32 22 27 46 38 52a 25 27 −18 3 51a 23 28 33 37 56a 20 24 −16 4 62a 0 13 63a 44 31 32a 22 19

refs 189

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118

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[(PhCH2) (CH3)2(C12H25)N][NO3] (40 wt %) + [(PhCH2) (CH3)2(C14H29)N][NO3] (60 wt %)

[C4C1Pyr][C(CN)3] [C4C1Mor][C(CN)3]

[C6H13(CH3)2(CH2CH2OH)N]Br

IL content

77 90 80 60 80 44 45 22 45 53 60

alcohols 1-octanol 1-decanol 1-propanol 1-butanol 1-hexanol 1-octanol 1-decanol 1-octanol 1-decanol 1-hexanol 1-octanol 1-decanol ethanol 1-dodecanol 1-hexanol, 1-heptanol 1-hexanol 1-heptanol 1-octanol 1-nonanol 1-decanol 1-undecanol 1-dodecanol 1-octanol 1-octanol tridecanol

wt % wt % wt % wt % wt % mol % mol % mol % mol % mol % mol %

[C1C1IM][CH3SO4]

[C2C1IM][EtSO4] [(C11H23OCH2) (CH3)2(CH2CH2OH)N][N (CN)2] ethylammonium nitrate tetra-n-butylammonium picrate a

50 mol % 50 mol % 50−60 mol % 48 mol % 60 mol % 70 mol % 86 mol % 70 mol % 77 mol % 16 mol %

alcohol content

23 10 20 40 20 56 55 78 55 47 40

wt % wt % wt % wt % wt % mol % mol % mol % mol % mol % mol %

50 mol % 50 mol % 50−40 mol % 52 mol % 40 mol % 30 mol % 14 mol % 30 mol % 23 mol % 84 mol %

CST value/ °C 20 31 13 22 23 28 21 19 52 27 64 114 7 70 36a 24 26 73 82 83 29 51 40 42 40

refs

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212

138 138 213

117 117 138 97,98 97,98

The phase diagram show UCST at the point of 100% IL.

was believed that the LCST-type phase transition property was originated from the dissociation state of the carboxylic group, which is partially dissociated in water, but could be suppressed by heating. Further studies show that mixing two amino acid ILs with −NH2 and −COOH groups on their side chains could reach very strong hydrogen bonding interaction between the anions. This results in a decreased anion−H2O electrostatic interaction upon increase of temperature, and LCST behavior is observed.107 To further clarify the factors which facilicate the LCST-type phase separation of ILs with water, ILs with diverse hydrophobicity, combining phosphonium or ammonium cations with several anions, were prepared.236,237 The LCST value of IL−water mixtures was found to increase with increasing hydrophilicity of the component ions. This indicated that the LCST-type IL−water mixture could be suitable for evaluating the hydrophilicity of target ions by determining the CST value after addition of corresponding ionic species.238 Furthermore, [P4,4,4,4][CF3COO] molecules can form some kind of long-living aggregates in aqueous solution under certain conditions before the phase separation occurs.239,240 These aggregates displayed characteristic properties of microemulsions, although no surfactants are used. This special system can be regarded as surfactant-free microemulsion-like aggregates and should be an effective platform to provide novel extraction or separation media. Of course, microemulsions can be created through addition of suitable amount of a surfactant (Triton X− 100) to [P4,4,4,4][CF3COO] at high temperature.241 When the temperature was lower than LCST, Triton X-100 molecules are aggregated into micelles and [P4,4,4,4][CF3COO] are dissolved in the surrounding aqueous water. A reversible phase transition

[NTf2] being entirely miscible with the other two ILs, immiscibility was still observed in the system even at relatively wide mole fractions of [C4C1IM][NTf2]. 2.2.2. LCST Behavior. Compared with UCST, LCST behavior is a relatively uncommon entropy-driven phenomenon. In the LCST-type phase transition, the homogeneous mixture underwent phase separation upon heating, but becoming homogeneous again upon cooling.110,228,229 The phase separation behavior of poly(ethylene oxide) (PEO) in [C2C1IM][BF4] was investigated and LCST phase separation was found.230 Interestingly, out of many different cation−anion combinations, the majority of the LCST-type ILs reported are based on variations of phosphonium cations, such as tetrabutyl or tributyl-hexyl phosphonium cations ([P 4,4,4,4 ] + and [P4,4,4,6]+). The most common anions employed to make LCST ILs so far have been [CF3COO]¯ and derivatives of benzenesulfonic acid. A particularly interesting development was implemented by the use of 4-styrenesulfonate as an anion.231,232 Some typical IL systems with LCST behavior are summerized in Table 5. Ohno and co-workers made seminal contributions to LCST phase changes in ILs.40−42 The first LCST−type phase change of a mixture of water and ILs derived from amino acids was reported by Ohno in 2007.233 The phase separation temperature of these mixtures depends reproducibly on the ion structure and water content. Among them, the system of water and [P4,4,4,4][Tf−Leu] (tetrabutylphosphonium N-trifluoromethanesulfonyl leucine, Figure 7) with LCST bahaviour was utilized to extract proteins.234 Furthermore, it was found that the reversible phase transition of the [P4,4,4,4][Tf−Leu]/water mixture can also be realized by bubbling CO2 and N2 through it, which was driven by a slight pH change in the mixture.235 It N


Chemical Reviews

Review

Table 4. IL−Hydrocarbon Systems with UCST Behavior IL content

ILs [(C4H9OCH2)2IM][BF4] [(C6H13OCH2)C1IM][BF4]

[C6Quin][NTf2]

45 mol % 80 mol % 80 mol % 85 mol % 85 mol % 70 mol % 13 mol % 34 mol % 50 mol % 40 mol % 50 mol % 51 mol % 50 mol % 15 mol % 45 mol % 60 mol % 70 mol % 70 mol % 79 mol % 80 mol % 34 mol % 56 mol % 70 mol % 70 mol % 70 mol % 70 mol % 90 mol % 90 mol % 90 mol % 91 mol % 80 mol % 80 mol % 71 mol % 80 mol % 51 mol % 2 mol % 3 mol % 7 mol % 8 mol % 8 mol % 9 mol % 50 mol % 30 mol % 33 mol % 50 mol % 50 mol % 50 mol % 50 mol % 50 mol % 40 mol % 50 mol %

[C8Quin][NTf2]

50 mol %

[C4iQuin][NTf2]

50 mol %

[C6H13(CH3)2(CH2CH2OH)N][BF4] [Pyr][BF4] [C2C1IM][PF6]

[C4C1IM][PF6]

[(C6H13OCH2)C1IM][NTf2]

[(C6H13OCH2)2IM][NTf2] [(C8H17OCH2)2IM][NTf2] [(C10H21OCH2)2IM][NTf2] [C4Quin][NTf2]

O

hydrocarbons bezene n-pentane n-hexane n-heptane n-octane cyclohexane benzene toluene ethylbenzene o-Xylene m-Xylene p-xylene n-hexane bezene benzene toluene ethylbenzene o-xylene m-xylene p-xylene benzene toluene ethylbenzene o-xylene m-xylene p-xylene n-pentane n-hexane n-heptane n-octane cycloheptane cyclohexane n-hexane n-heptane cyclohexane benzene toluene ethylbenzene o-xylene m-xylene p-xylene n-hexane cyclohexane bezene bezene benzene toluene ethylbenzene propylbenzene thiophene benzene toluene ethylbenzene propylbenzene benzene toluene ethylbenzene propylbenzene thiophene benzene

hydrocarbon content

CST value/°C

55 mol % 20 mol % 20 mol % 15 mol % 15 mol % 30 mol % 87 mol % 66 mol % 50 mol % 60 mol % 50 mol % 49 mol % 50 mol % 85 mol % 55 mol % 40 mol % 30 mol % 30 mol % 21 mol % 20 mol % 66 mol % 44 mol % 30 mol % 30 mol % 30 mol % 30 mol % 10 mol % 10 mol % 10 mol % 9 mol % 20 mol % 20 mol % 29 mol % 20 mol % 48 mol % 98 mol % 97 mol % 93 mol % 92 mol % 92 mol % 91 mol % 50 mol % 70 mol % 67 mol % 50 mol % 50 mol % 50 mol % 50 mol % 50 mol % 60 mol % 50 mol %

−13 28 64 81 124 45 371 48 33 122 68 25 116 59 54 105 133 140 98 83 80 110 82 80 98 108 129 56 94 111 50 72 55 49 79 78 145 267 293 296 295 27 58 −2 22 29 29 32 33 18 25 25 25 28 28 30 30 30 28 24

50 mol %

50 mol %

refs 142 130

138 142 214

130

137 137 142 142 189

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Table 4. continued IL content

ILs

[C6iQuin][NTf2]

50 mol %

[C8iQuin][NTf2]

[C1C1IM][CH3SO4]

66 mol % 70 mol % 50 mol % 60 mol % 5 mol % 90 mol % 90 mol % 94 mol % 89 mol % 90 mol % 84 mol % 91 mol % 90 mol % 79 mol % 80 mol % 81 mol %

[C4C1IM][CH3SO4]

85 79 70 80 82 90 80 80 80 86

[C3C1Pip][NTf2]

[C4C1Pip][NTf2]

mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % %

30 mol % 60 mol % 60 mol % 70 mol % 60 mol % 60 mol % 60 mol % 73 mol % 40 mol % 15 wt % 18 wt % 10 wt % 17 wt % 12 wt % 2 mol %

[C2C1IM][EtSO4] [C8C1IM][NTf2] [C10C1IM][NTf2]

P

hydrocarbons toluene ethylbenzene propylbenzene benzene toluene ethylbenzene propylbenzene n-hexane n-heptane cyclohexane cycloheptane thiophene n-hexane n-heptane n-octane cyclohexane cycloheptane n-hexane n-heptane n-octane cyclohexane cycloheptane n-pentane n-hexane n-heptane n-octane n-decane cyclohexane cycloheptane benzene toluene ethylbenzene propylbenzene o-Xylene m-Xylene p-Xylene n-pentane n-hexane n-heptane n-octane n-decane cyclohexane cycloheptane benzene toluene ethylbenzene propylbenzene o-xylene m-xylene p-xylene hexane benzene benzene α-methylstyrene benzene toluene α-methylstyrene benzene benzene-d6 toluene

hydrocarbon content

50 mol %

34 mol % 30 mol % 50 mol % 40 mol % 95 mol % 10 mol % 10 mol % 6 mol % 77 mol % 10 mol % 16 mol % 9 mol % 10 mol % 21 mol % 20 mol % 19 mol %

15 21 30 20 18 10 20 20 20 14


% % % % % % % % % %

70 40 40 30 40 40 40 27 60 85 82 90 83 88 98

mol % mol % mol % mol % mol % mol % mol % mol % mol % wt % wt % wt % wt % wt % mol %

CST value/°C 24 22 30 32 32 34 35 63 70 59 56 27 53 40 78 34 36 79 49 86 38 70 28 58 80 85 109 64 80 31 40 136 95 56 60 56 52 58 75 80 102 41 92 56 38 82 125 133 137 128 49 37 101 155 45 68 70 42 36 52

refs

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215

215

215

138 218

219


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ILs [C12C1IM][NTf2]

2 mol %

[oleyl-C1IM][NTf2]

[C4C14Py][NTf2] [(PhCH2) (CH3)2(C12H25)N][NO3] (40 wt %) + [(PhCH2) (CH3)2(C14H29)N] [NO3] (60 wt %)

[C1C1IM][CH3SO4]

[C4C1IM][CH3SO4]

[C2C1IM][EtSO4]

[C3C1Pyr][CF3SO3]

[C4(CN)4Py][C(CN)3]

[C2C1IM][TOS] [C4C1IM][TOS]

20 mol % 40 mol % 40 mol % 20 mol % 10 mol % 17 mol % 8 mol % 93 wt % 93 wt % 88 wt % 97 wt % 70 mol % 70 mol % 70 mol % 80 mol % 50 mol %

60 50 50 60 80 82 80 70 80 73 40 70 75 60 60 60 80 90 79

mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % % % % % % % % % % %

81 50 40 90 50 50 40 80 51 50 50 50

mol mol mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % % % %

93 mol % 49 mol % Q

hydrocarbons toluent-d6 benzene benzene-d6 toluene toluent-d6 n-hexane n-octane n-decane cyclohexane methylcyclohexane 1-octene cyclohexane benzene toluene hexane hexadecane 2-pentanone 3-pentanone 2-hexanone 4-heptanone dipropyl ether dibutyl ether MTBE MTAE cyclopentanone 2-pentanone 3-pentanone 2-hexanone 4-heptanone dipropyl ether dibutyl ether MTBE MTAE 1-hexane benzene toluene ethylbenzene 2-pentanone 3-pentanone 2-hexanone 4-heptanone dibutyl ether tert-butyl-methyl ether tert-butyl-ethyl ether tetrahydrofuran DMSO n-heptane benzene thiophene benzothiophene n-heptane toluene 2-methylthiophene benzothiophene hexane cycloheptane hexane benzene

hydrocarbon content 98 mol %

80 mol % 60 mol % 60 mol % 80 mol % 90 mol % 83 mol % 92 mol % 7 wt % 7 wt % 12 wt % 3 wt % 30 mol % 30 mol % 30 mol % 20 mol % 50 mol %

40 50 50 40 20 18 20 30 20 27 60 30 25 40 40 40 20 10 21

mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % % % % % % % % % % %

19 50 60 10 50 50 60 20 49 50 50 50

mol mol mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % % % %

7 mol % 51 mol %

CST value/°C 43 8 3 −3 −10 2 3 5 −1.5 33 18 391 60 82 26 32 88 87 116 120 59 115 46 69 63 8 3 42 90 57 76 70 90 49 36 53 102 31 46 120 126 135 95 99 85 −17 77 46 37 35 88 77 66 62 44 44 56 26

refs 219

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ILs

50 50 70 70 80 90 97 51 50 62 67 51 93 93 98 90 90 90 90 90 90 94 88 91 91 90

[C4C14Py][TOS]

[C4C13Py][TOS]

[C6C13Py][TOS]

mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol mol

% % % % % % % % % % % % % % % % % % % % % % % % % %

[P4,4,4,4][TOS] 25 mol % 2 mol % 3 mol % 3 mol % 3 mol % 2 mol % 3 mol % 3 mol % 3 mol % 50 mol %

[P6,6,6,14]Cl

[P6,6,6,14]Br

[C4H9(CH3)2(CH2CH2OH)N]Br

[C6H13(CH3)2(CH2CH2OH)N]Br [(CH3)2(C10H21)2N][NO3]

50 mol %

50 mol % 50 mol % 50 mol % 90 mol % 2 mol %

tetra-n-butylammonium naphthylsulfonate a

hydrocarbons toluene ethylbenzene propylobenzene thiophene hexane heptane octane benzene toluene ethylbenzene propylobenzene tetrahydrofuran n-hexane n-heptane n-octane benzene toluene ethylbenzene n-propylbenzene n-hexane n-heptane n-octane benzene toluene ethylbenzene n-propylbenzene benzene benzene n-heptane n-octane n-nonane n-decane n-heptane n-octane n-nonane n-decane n-hexane n-heptane cyclohexane benzene toluene, propylbenzene toluene propylbenzene hexane hexadecane toluene

hydrocarbon content

CST value/°C

50 mol % 50 mol % 30 mol % 30 mol % 20 mol % 10 mol % 3 mol % 49 mol % 50 mol % 38 mol % 33 mol % 49 mol % 7 mol % 7 mol % 2 mol % 10 mol % 10 mol % 10 mol % 10 mol % 10 mol % 10 mol % 6 mol % 12 mol % 9 mol % 9 mol % 10 mol %

36 43 47 39 112 99 51 28 31 37 100 31 47 49 49 45 47 44 46 55 55 55 55 55 55 55 62a −5 22 27 33 40 69 72 78 85 80 73 68 78 32a

75 98 97 97 97 98 97 97 97 50


% % % % % % % % % %

50 mol %

50 50 50 10 98

mol mol mol mol mol

% % % % %

10 19 27 31 56

refs

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169

170

122

128

142

138

171

101

The phase diagram show UCST at the point of 100% IL.

between micelles and microemulsions can be switched by using temperature as a trigger. ILs consisting of tetrapentylphosphonium cations and longalkyl carboxylate anions, with hydrophobic and highly polar properties, showed LCST-type phase transition after water addition and can be applied to cellulose dissolvation.242 This is the first report on the cellulose dissolving in hydrophobic ILs, and it was clarified that not only polarity but also density of ILs is an important factor in designing the ILs for cellulose dissolution.

A thermoresponsive IL tributylhexylphosphonium 3-sulfopropyl methacrylate ([P4,4,4,6][MC3S]) itself243 and its combination with two thermoresponsive polymers, poly(N-isopropylacrylamide) (PNIPAM) and poly(N-vinylcaprolactam) (PVCL), respectively,244 was investigated. An obvious distinction was observed in the LCSTs and morphologies of [P4,4,4,6][MC3S]−PNIPAM and [P4,4,4,6][MC3S]−PVCL aqueous solutions. It is believed that the formation of strong intra-/ intermolecular hydrogen bonds in PNIPAM is the driving force for the LCST phenomenon of [P4,4,4,6][MC3S]−PNIPAM solution, while it is [P4,4,4,6][MC3S] that dominates the phase R


Chemical Reviews

Review

Table 5. IL Systems with LCST Behavior ILs

solvents

[P4,4,4,4][SS] (20% w/v) [P4,4,4,4][SS] (20% w/v) [P4,4,4,4][Tf-Leu] (40−45 wt %) [P4,4,4,4][CF3COO] (2.5 mol %) [P4,4,4,4][CF3COO] (50 wt %)

H2O H2O H2O (60−55 wt %) H2O (97.5 wt %) H2O (50 wt %)

[P6,6,6,6][HCOO] (50 wt %) [P5,5,5,5][C5COO] (50 wt %) [P5,5,5,5][C7COO] (50 wt %) [P5,5,5,5][C9COO] (50 wt %) [P5,5,5,5][C11COO] (50 wt %) [P4,4,4,6][MC3S] (20% w/v)

H2O (50 wt %)

[P6,6,6,6][(EtO)HPO2] (50 v%) [P4,4,4E3][DEHP] (50 wt %) [C2C1IM][NTf2] (95 mol %) [P4,4,4,4][maleate] (5 mol %)

H2O (50 v%) H2O (50 wt %) 1,2,4-trifluorobenzene (5 mol %) H2O (95 mol %)

additives BSA (20 wt %)

[P4,4,4,4][CH3CO3] [P4,4,4,4]Cl [P4,4,4,4]Br [P4,4,4,4][NO3] [P4,4,4,4][TsO] [C4C1IM][CF3COO] [C4Pyr][CF3COO] [N4,4,4,4][CF3COO] (0.20 M) cellulose (0.1 wt %)

H2O

none PNIPAM PVCL (20 wt % in [P4,4,4,6][MC3S])

LCST value/°C 36 39 22 29 50 47 39 34 33 41 40 35 10 18 20 12 14 37 25 39 35 44 81 22

refs 232 233,234 238,239,241 238

242

244

246 247 248 255

Brønsted acid such as trifluoromethanesulfonic acid (HTfO) showed LCST phenomena also, and the CST value was controllable by water content.245 Other phosphonium styrenesulfonate-type ILs, based on [P4,4,4,4][SS] (tetrabutylphosphonium 4−styrenesulfonate), were also confirmed to undergo an LCST-type phase transition after mixing with water.231 This LCST behavior can be maintained after polymerizing the [P4,4,4,4][SS]. The transition temperature of the mixture with water was dramatically lowered by the copolymerization of [P4,4,4,4][SS] with more hydrophobic [P4,4,4,6][SS]. Hydrophobicity is the essential factor to

Figure 7. Structure of IL [P4,4,4,4][Tf−Leu].

separation of [P4,4,4,6][MC3S]−PVCL solution. Another ammonium-type zwitterion, N,N-dihexyl-N-monopentyl-3-sulfonyl-1-propaneammonium (C5 H11)(C6H13) 2N(C 3H6SO3) with certain hydrophobicity showed reversible and highly temperature-sensitive LCST-type phase transitions after being mixed with pure water. Its mixture with equimolar amounts of a

Figure 8. Dynamic phase transition and denaturation mechanism of [P4,4,4,4][SS]−BSA−D2O solution during the heating and cooling processes. Reproduced with permission from ref 232. Copyright 2014 Royal Society of Chemistry. S


Chemical Reviews

Review

[CH3(C8H17)3N] or [P6,6,6,14], were selected to investigate their mutual miscibility. LCST-like behaviors were found for the systems with the phosphonium cation, and hourglassshaped liquid−liquid domains for the systems with the ammonium cation. Enthalpies and entropies of mixing, calculated from the liquid−liquid equilibrium values, revealed that the first type of liquid liquid equilibria was entropically driven, whereas for the second type a transition from enthalpically to entropically driven occurred with an increase in temperature.254 2.2.3. UCST and LCST Behavior. Theoretically, partially miscible IL solutions can exhibit both UCST and LCST phenomena. At temperatures below LCST or above UCST, the system is completely miscible in all proportions, whereas at temperatures above LCST or below UCST, partial liquid miscibility occurs. For example, [P4,4,4,4]-type ILs with fumarate anion and maleate anion (Figure 10) exhibit different

control the LCST behavior of IL-based polyelectrolytes. Influence of bovine serum albumin (BSA) on the phase transition behavior of [P4,4,4,4][SS] together with the interactions between [P4,4,4,4][SS] and BSA was further investigated.232 It was found that the addition of BSA would increase the phase transition temperature but weaken the transition behavior of [P4,4,4,4][SS] solution. Interactions between [P4,4,4,4][SS] and BSA together with the phase transition behavior of [P4,4,4,4][SS] are responsible for the denaturation of BSA upon heating. As shown in Figure 8, the [P4,4,4,4][SS] molecules tend to aggregate together under the driving of hydrophobic interactions upon heating while BSA would turn into β-sheet and β-turn conformation after heating. Polar ILs with special properties by individual design of both the cation and anion can be achieved.246 A series of alkylphosphonium−type ILs, in particular, ethylphosphonate anion [(EtO)HPO2]¯ coupled with tetra-n-hexylphosphonium ([P6,6,6,6]+) and tri-n-hexyl-n-octylphosphonium ([P6,6,6,8]+) cations, had LCST behavior after mixing with water. Moreover, tetra-n-octylphosphonium ethylphosphonate ([P8,8,8,8][(EtO)HPO2]) and [P4,4,4,4][(EtO)HPO2] exhibited stable phase separation and strong hydrogen bonds between each other even after mixing with water. Phosphonium ILs having the bis(2-ethylhexyl) phosphate ([DEHP]) as anion show LCST phase behavior also.247 The obtained IL [P4,4,4,E3][DEHP] (Figure 9) was used in the first example of homogeneous

Figure 10. Structure of tetra-n-butylphosphonium type ILs with fumarate anion and maleate anion.

physicochemical properties and different solubilities in water in their cis and trans conformations; fumarate showed UCST behavior, whereas maleate had highly unusual LCST behavior after mixing with water.255 Their UCST and LCST behavior can be easily controlled by choosing the anion structure and specifying the amount of water added, while their equimolar mixture did not show any phase transition after mixing with water. Ammonium based zwitterions (Figure 11) in combination with water also show LCST and UCST phase transitions.256

Figure 9. Structure of IL [P4,4,4E3][DEHP].

liquid−liquid extraction of metal ions using an LCST thermomorphic system. The LCST value is tunable by making modifications in the structure of the IL. Longer oligo-ethylene chains induce higher CST. Binary equilibria with LCST behavior in mixtures of ILs ([CnC1IM][NTf2], n = 2 and 4) with fluorinated benzenes248 and amines (diethylamine and triethylamine)249 were reported. Cholinium-based ILs, N-alkyl-N,N-dimethylhydroxyethylammonium bis(trifluoromethane) sulfonylimide [(CH3)2(CnH2n+1) (CH2CH2OH)N][NTf2], with different ethers also exhibited a LCST behavior.250 The mutual solubility of the ILs and ethers are larger (higher LCST values) for ILs with longer alkyl side chains attached on the cations. Additionally, the LCST property for the binary system of thiophene with [C4C1IM][NTf2] is also reported, which made it easy for aromatic−aliphatic separation or removal of sulfurcontaining compounds.251 Interestingly, solubility behaviors of threo- and erythrodiastereomers of HFC-4310mee, including their deuterated isomers, with RTILs ([C4C1IM][PF6], [C4C1IM][BF4], and [C2C1IM][BF4]) were also reported to show LCST behavior.252,253 The threo-isomers are more miscible than the erythroisomers in the investigated ILs. The deuterated threo-isomer systems with the ILs show slightly better solubility than the nondeuterated cases and have about 2 K higher LCSTs. What’s more, pairs of ILs with chloride or acetate as anion and largely different cations, namely a [CnC1IM]Cl (n = 2, 4) or

Figure 11. Structure of ammonium based zwitterions.

Hydrophilic N,N,N-tripentyl-3-sulfonyl-1-propaneammonium ([N5,5,5][C3S]) was miscible with water and relatively more hydrophobic N,N,N-trihexyl-3-sulfonyl-1-propaneammonium ([N 6, 6, 6 ][C3S]) was immiscible with water. When [N5,5,5][C3S] and [N6,6,6][C3S] were mixed in a suitable molar ratio, the mixture showed both LCST and UCST phase transitions after mixing with water. Both phase transition temperatures depended on the ratio of zwitterions to water molecules. Furthermore, the water content of the biphasic system composed of the [N5,5,5][C3S]−[N6,6,6][C3S]−water mixture varied dramatically with small temperature changes. Another tetraalkylammonium salt n-propyl-tri-n-butylammonium iodide was reported also to show both LCST (44.6 wt %, 331.72 K) and UCST (43.8 wt %, 345.73 K) with water.257 Binary systems of N-octylisoquinolinium thiocyanate ([C8iQuin][SCN]) and aliphatic hydrocarbon (n-hexane, nheptane), cyclohexane, aromatic hydrocarbon (benzene, T


Chemical Reviews

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toluene, ethylbenzene, n-propylbenzene), and thiophene have been investigated.258 Mutual immiscibility with an UCST for IL with aliphatic hydrocarbon, or thiophene, were observed. The immiscibility gap with LCST for IL with aromatic hydrocarbon was determined. Phase equilibrium data for mixtures of pyridinium tosylatebased ILs, or imidazolium thiocyanate-based ILs, or sulphonium-based ILs with thiophene were reported also.259 For the systems containing pyridinium or sulphonium IL with thiophene, the mutual immiscibility with an UCST was detected for a very narrow and low mole fraction of the IL. For the binary systems containing imidazolium thiocyanate with thiophene, the mutual immiscibility with the LCST was detected at higher mole fractions of the IL. This is largely due to the high melting point of the tosylate-based ILs. Binary systems of [C4C1IM][SCN] with water and various organic solvents have been investigated at ambient pressure.260 In the systems of IL with alkane, cycloalkane, or ether, the mutual immiscibility with an UCST was detected, and in the systems of IL with benzene, alkylbenzene, or THF, the mutual immiscibility with a LCST was observed. For the binary systems containing alcohol, it was noticed that increased alcohol chain length caused decreased solubility. For [C6C1IM][SCN] combined with water or organic solvent, simple eutectic systems were observed in the case of water with complete miscibility in the liquid phase.261 UCSTs were observed for systems with n-alkanes and the LCSTs for systems with aromatic hydrocarbons. The solubility decreases with an increase of the molecular weight of the solvent. Interesting and complicated miscibility behavior was observed in the case of [CnC1IM][NTf2] (n = 2, 4, and 5) combined with chloroform or chloroform/carbon tetrachloride mixtures.262 Phase diagrams with the UCST were obtained in most cases. However, under some conditions phase diagrams with LCST or even with both (UCST and LCST) behavior were observed, too. The impact of the cation on the miscibility supports a rather general rule for such systems: longer alkyl side chains increase the miscibility. The results obtained for the [C5C1IM][NTf2]−CHCl3/CCl4 system show that the miscibility depends very strongly on the composition of the mixed solvent. For pure CHCl3 the system is miscible in a broad temperature range, at least from 250 K up to 450 K and upon the addition of 7.5% of CCl4, a large immiscibility region was observed (“hourglass” type diagram). Due to the thermoregulated property of PEG, solubility of PEG-based ILs [PEGm(C1IM)2][NTf2]2 (m = 200, 400, 600, 800, 1000) in aliphatic alcohol (ethanol, 1-propanol, and isopropanol) and in aliphatic alcohol/water mixtures was investigated.263 It was found that these ILs exhibited UCST phase behavior in ethanol, 1-propanol, and isopropanol. The CST of [PEGm(C1IM)2][NTf2]2/alcohol mixtures increased with the increase of the alkyl chain length of the aliphatic alcohols, but decreased with the increase of the molecular weight of the PEG middle block of the ILs. Interestingly, the liquid−liquid phase transition temperature of the IL/alcohol mixtures decreased significantly with the addition of a few percent of water and could be tuned from UCST to LCST by the addition of more than 41 wt % water. It was believed that, for the IL/alcohol mixtures, the hydrogen bonding interactions among ethanol molecule themselves were strengthened upon cooling and then dominated the IL−ethanol interactions, leading to the desolvation of the PEG chain and finally UCST macroscopic phase separation. In the IL/alcohol/water

mixtures, phase transition of the ILs from UCST to LCST is likely ascribed to the change in conformation of the PEG chain of the IL upon variation of water content and system temperature. In addition, the tunable phase behavior and phase separation temperature can be used to meet the demand of specific applications such as product separation, efficient homogeneous liquid−liquid extraction and so on. 2.3. Metal-Containing IL Thermomorphic Systems

2.3.1. ILs with Simple Metal Salts. The first examples of metal-containing IL exhibiting thermomorphism were reported by Taubert in 2011.264 Simple alkylimidazolium based ILs, [C4C1IM][FeCl4] and [C12C1IM][FeCl4], exhibit a thermally induced demixing with water. The phase separation temperature varies with IL weight fraction in water and can be tuned between 100 °C and room temperature. The reversible LCST is only observed at IL weight fractions below ca. 35% in water. However, the process is not very efficient with about 40% of iron remaining in the water phase upon phase separation.264 Later, a magnetic IL, N-butylpyridinium tetrachloroferrate ([C4Py][FeCl4]), was reported by Li and co-workers in 2013.265 This IL was applied in oxidative desulfurization, and it can be reused for five cylces without significant loss of activity due to the easy recovery. Another temperature−responsive IL, N-butyl-N-methylpiperidinium tetrachloroferrate ([C4C1Pip][FeCl4]), was reported by Zhu and Li in 2015.266 It was used as an extractant for aromatic sulfur compounds from model oil. After the extraction, the system was cooled to room temperature, whereby the IL became solid (CST value: 45 °C) which made an easy separation of the aromatic sulfur components possible (extraction efficiency up to 45.5%). Mixtures of water and dodecyltrimethylammonium tetrachloroferrate [N1,1,1,12][FeCl4] or DBU−Bu[FeBrCl3] (Figure 12) showed LCSTs of

Figure 12. Structure of ILs [N1,1,1,12][FeCl4] and DBU−Bu[FeBrCl3].

60 and 50 °C, respectively.267 Through investigations of different kinds of [FeCl4]− based magnetic ILs with different cation structures, it was found that LCST behavior is essentially determined by the disruption of the hydrogen bonds between the [FeCl4]− anion and water.268 The LCST behavior of these magnetic ILs in water can be significantly influenced by the cation structure in terms of alkyl chain length and through variation of the molar ratio of added FeCl3. Thus, the phase separation temperature can be tuned in a rather broad range. The cobalt(II)-based metal-containing IL [Co2(HisCH3)4IM][NTf2]4 was reported by Gin and Noble in 2015.269 It is a neat, liquid-state material and can be synthesized using a simple, scalable method. Due to the extremely high concentration of O2−binding sites, it reversibly binds O2 over N2 with high selectivity and an associated color change from room temperature to 60 °C. Other thermo− responsive polyether−substituted imidazolium cobalt tetracarbonyl salts, [H(OCH2CH2)nC4IM][Co(CO)4)] (n = 8, 15, and 22), were reported by Lv and Guo in 2016.270 At room U


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PNIPAM supported POM hybrids were reported by the same group.278 The obtained hybrids ([C16H33N(CH3)3]3[PO4{MO(O2)2}4]/PNIPAM (M = Mo and W, abbreviated as C16PM(O2)2/PNIPAM)) exhibit novel switchable property based on the change of temperature, while its solubility in organic solvent is reversibly controllable through an external temperature stimulus linking the gap between heterogeneous catalysis and homogeneous one. Similar POM-ILs (Figure 15) with poly(ethylene glycol)functionalized alkylimidazolium cations and heteropolyacids as

temperature, the IL catalysts are insoluble in organic solvent, and the system is biphasic. When heated to temperatures above the CST, the IL catalysts are completely soluble in organic solvent, and the system becomes homogeneous. After cooling to a temperature below the CST, the system becomes biphasic again, accompanied by separation of the products in the organic phase from the IL catalysts. This highly attractive performance can be further utilized for catalyst separation in hydroesterification of olefins. 2.3.2. Polyoxometalate-Based ILs (POM−ILs). A surfactant-encapsulated terbium-substituted heteropolyoxotungstate complex bearing mesomorphous groups ([L]13[Tb(SiW11O39)2]·30H2O) has been reported by Wu in 2005,271 which showed thermomorphic behavior (please see the structure of L cation in Figure 13), where mesostructured

Figure 15. Structure of poly(ethylene glycol)-functionalized alkylimidazolium as cation and heteropolyacids as anion.

anion were further reported by Hou and co-worker.279−281 Due to the special thermo-regulated behavior, they have proved to be efficient catalysts both in esterification and epoxidation reactions. Interestingly, silica immobilized tungsten peroxo complex was also proven to be thermoresponsive by the same group in 2015.282 During the one-pot synthesis of alkoxy alcohols from olefin and methanol with H2O2, it was found that the catalytically active tungsten peroxo complex can dissociate from silica under the reaction temperature, resulting in a homogeneous reaction. However, the soluble W-based species was anchored robustly on silica as the temperature was decreased to 0 °C.

Figure 13. Structure of cation L.

supramolecular assemblies combining giant mixed-valent polyoxomolybdates and surfactants are formed, and the inorganic clusters were organized into liquid crystal like alignment with long-range ordered structure. A series of vanadium-substituted POM−IL were reported to show the property of reversible thermal-response also: gel−type compounds bearing sulfo-group grafted ammonium (PyPS), [PyPS]6PW9V3O40, [PyPS]4PW11VO40, [PyPS]4PMo11VO40 and [PyPS]7P2W17VO62.272 As the temperature increases, a phase transition has been observed from a quasi-solid-state gel phase to an isotropic sol phase, accompanying an apparent increase in conductivities of these compounds. Furthermore, a similar series of POM-ILs combining 12-tungstophosphoric acid with different sulfonated organic cations were reported (Figure 14).273−275 They were used as a thermoregulated

2.4. Polymerized IL (PIL) Systems with UCST and LCST Behavior

Polymerized ILs (PILs) have emerged recently in the fields of polymer and materials science.283,284 They are commonly prepared from the polymerization of a monomeric IL and resemble a linear connection of IL species through a polymeric backbone. PILs keep some particular properties of ILs and can be processed into well-defined shapes and morphologies, which are not accessible by ILs. A common property of the thermoregulated PILs is their concentration dependence in water, which allows an easy tuning of their CST values. 2.4.1. UCST Behavior. PILs with UCST behavior were also reported by many interesting examples: thermo-responsive poly[1-(4-vinylbenzyl)-3-methylimidozolium tetrafluoroborate] trithiocarbonate (P[VBIM][BF4]−TTC) shows UCST in methanol−water mixtures. Same holds for the further polymerized diblock copolymer poly[1-(4-vinylbenzyl)-3-methylimidozolium tetrafluoroborate]-b-polystyrene (P[VBIM][BF4]-b-PS).285 Several parameters including the polymerization degree, the polymer concentration and the water content in the methanol−water mixture were found to influence the UCST of the PIL. Nanogels prepared via one-step cross-linking copolymerization of ethylene glycol dimethacrylate, divinylbenzene and ILbased monomers, 1-n-dialkyl-3,3′-bis-1-vinyl imidazolium bromides ([CnVIm]Br; n = 6, 8, and 12), showed phase transformation with UCST behavior with methanol in the range of 5−25 °C.286 The obtained nanogels could also be used as highly active catalysts in the cycloaddition reaction of CO2 and epoxides. Besides, poly(triphenyl-4-vinylbenzylphosphonium chloride) (P[VBTP][Cl]) of varying and controllable

Figure 14. Structure of POM-ILs combining of 12-tungstophosphoric acid with different sulfonated organic cations.

recoverable homogeneous catalysis system for esterification and biodiesel production, respectively. Protic N-dodecylimidazolium peroxotungstate ([HDIm]2[{WO(O2)2}2(μ-O)]) was reported by Hou in 2009.276 This protic IL was found to be a room temperature liquid molten salt and proved to be efficient reaction-induced phase-separation catalyst for olefin epoxidation reaction. Combination of choline chloride with H3PW12O40 affords a series of heteropolyacids (HOCH2CH2N(CH3)3)xH3−xPW12O40, abbreviated as cholinexH3−xPW12O40, x = 1−3).277 They proved to be efficient heterogeneous catalysts for HMF directly from cellulose. Thermoregulated polymer V


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efficient draw agents in forward osmosis by Wu for the first time.297 Also, chemically cross-linked, sufficiently hydrated PIL membrane, poly([P4,4,4,4][SS]0.3 -co-[P 4,4,4,8][SS]0.7 , shows switchable hydrated states via LCST-type phase behavior, enables concentration of some water-soluble proteins from aqueous media.298 Furthermore, [P4,4,4,4][SS] can be introduced to PNIPAM by two different ways, mixing and copolymerization, respectively.299 Interestingly, the obtained materials show distinct thermo-responsive phase transition behaviors, exhibiting a sharp and drastic phase transition in case of mixing, similar to pure PNIPAM. While in the statistical copolymer, PNIPAM-co-P[P4,4,4,4][SS], the thermo-sensitivity of P[P4,4,4,4][SS] is largely suppressed resulting in a linear, mild and incomplete phase transition. This might be arisen from the outstanding hydration ability of P[P4,4,4,4][SS]. Besides [P4,4,4,4][SS], tributyl−hexylphosphonium 3−sulfopropyl acrylate ([P4,4,4,6][SPA]) was also photopolymerized with various cross-linkers to form thermoresponsive gels.300 P[P4,4,4,6][SPA] hydrogel was found to be capable of responding to multiple stimuli, namely temperature, ionic strength, and white light irradiation.301 Similar to [P 4 , 4 , 4 , 4 ][SS], poly[1,8-octanediylbis(tributylphosphonium) 4-styrenesulfonate] (poly(SS−P2), Figure 16) as a new type of gemini PIL was synthesized via free radical polymerization of the dicationic IL monomer in DMF, reported by Schlaad and Yuan.302 This PIL presented LCST transition in aqueous solution. Notably, this phase transition can be observed even at concentrations as low as 0.1 wt %. Moreover, the CST value can be influenced by the type and concentration of external salts. Interestingly, LCST behavior in aqueous media can be precisely tuned by blending. For instance with PILs having different hydrophobicity, such as homopolymers comprised of alkyltributylphosphonium cations ([P4,4,4,n]+, n = 4, 6, or 8) and the 3-sulfopropyl methacrylate ([MC3S]) anion.303 Such a blending with two or more PILs allows a fine-tuning of phase-transition temperatures as well as phase behavior of the mixture in an aqueous phase which is much easier to achieve than copolymerization of corresponding monomeric ILs. Thermo-responsive PIL hydrogel with LCST behavior has been prepared by utilizing an adequate amount of IL as a monomer (tetra-alkylphosphonium 3-sulfopropyl methacrylate), a cross-linker, and a radical polymerization initiator.304 The prepared hydrogels show reversible water uptake and release, in which the gels absorb and desorb water for at least ten cycles by a small temperature change. Strongly temperature-sensitive LCST transitions were also observed in poly([P4,4,4,6][C3S])-aqueous mixed systems.305 The LCST transition varied widely with both P[P4,4,4,6][C3S] and salt concentration. Furthermore, P[P4,4,4,6][C3S] was precipitated from the aqueous media to form a polymer-rich phase. The resulting polymer-rich phase underwent gelation upon a small increase in temperature and this gel-to-liquid transition was thermally reversible. Phase behavior of poly([tri-n-alkyl(vinylbenzyl)phosphonium]chloride) (poly([Pn,n,nVB]Cl, n = 4, 5, or 6) is reported after mixing with aqueous sodium chloride solutions.306 Both monomeric [P5,5,5VB]Cl and the resulting P[P5,5,5VB]Cl linear homopolymer show a LCST-type phase behavior in aqueous NaCl solutions. The phase transition temperature of the PIL shifts to lower value by increasing concentration of NaCl. At the same time the swelling degree of cross−linked P[P5,5,5VB]Cl gel decreases, clearly suggesting that the “salting-out” effect of NaCl results in a significant

molecular weights in aqueous solution exhibits a distinct UCST transition and transforms into a one-phase transparent solution due to the disruption of ion bridges upon heating.287 PILs prepared from copolymerization of 1-vinyl-3-butylimidazolium bromide and N-isopropylacrylamide, followed by anion exchange of bromide to deprotonated amino acid were thermo-responsive in acetonitrile with UCST varying from 25.7 to 34.8 °C and can be completely precipitated out by lowering the solution temperature.288 PIL-acetonitrile solutions were used as an extracting agent for separation of tocopherol homologues in hexane. Here, PILs could be reused for multiple extraction cycles with negligible change. UCST−type phase separations in water can also be achieved using PILs with imidazolyl groups in their side chains. Aqueous solutions of the polymers with [BF4]− as counteranions showed sharp and reversible UCST−type phase separation at 5−15 °C.289 The effects of polymer concentration, chain−end groups and molecular weight on the phase separation temperature suggest a crucial influence of interpolymer electrostatic interactions. Other PILs with the [SbF6]− anion also showed LCST-type phase separations in various organic solvents. Besides the conventional ILs, a new family of thermoresponsive polypeptides with Y-shaped and IL pendants (PPLG(IMX)2, X = Br, I, or BF4) were reported. PPLG-(IMI)2 showed a reversible UCST-type phase behavior in ethanol, whereas PPLG-(IMBF4)2 showed reversible UCST-type phase behaviors in both methanol and water.290 In some cases, ILs can be used as solvent for IL−polymer systems with UCST behavior.291−293 Interesting phenomena were observed for example in the case of polymer-brush-grafted silica particles (“hairy” particles) in hydrophobic 1-ethyl-3methylimidazolium bis(trifluoromethylsulfonyl)amide ([C2C1IM][TFSA]): The hairy particles move spontaneously from the aqueous phase to the [C2C1IM][TFSA] phase upon heating to 80 °C and returned to the aqueous layer upon cooling to 10 °C. This process can be repeated many times.294 2.4.2. LCST Behavior. P[P4,4,4,4][SS] (Figure 16) was found to be a unique PIL showing tunable LCST in aqueous

Figure 16. Structures of [P4,4,4,4][SS], P[P4,4,4,4][SS], and poly(SS-P2).

solution, depending on the concentration and ionic strength.295 An obvious distinction was observed between the LCST of the monomer [P4,4,4,4][SS] and the polymer P[P4,4,4,4][SS] solutions, indicating their largely different dynamic transition process.296 Further studies revealed that the aggregation of [P4,4,4,4][SS] is mainly driven by the synergetic variations of cations and anions while the separation process of the P[P 4,4,4,4 ][SS] solution is found to be dominated by conformation changes of the anions. Thermally responsive hydrogels based on P[P4,4,4,4][SS] and P[P4,4,4,6][SS] (tributylhexylphosphonium p−styrenesulfonate) were reported to be W


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addition of salts. A detailed study on the monovalent salt addition process revealed two parallel effects, the anion exchange and the salting out effect. The unique solution property of this cationic polyelectrolyte can be applied as a smart stabilizer for nanomaterials to manipulate their solution state. Besides, ILs were also used as components for copolymer preparations showing thermo-responsive properties.311−315 IL [C2VinylIM][Br] was used as a monomer to prepare star block copolymers with poly(NIPAAm) stars (poly(VEI-Br-b-NIPAAm) star and poly(NIPAAm-b -VEI-Br) star shown in Figure 19).312 Core−shell micelle structure consisting of

dehydration of the P[P5,5,5VB]Cl gel. The absorbed water in the PIL gel is desorbed by moderate heating via the LCST behavior. Suitably designed polyelectrolytes derived from tributyl-n-alkylphosphonium 3-sulfopropyl methacrylate-type IL monomers also undergo LCST transition, and their transition temperature is a function of the alkyl chain length of the phosphonium cations. The hydrated state of these PIL gels varied widely with temperature. They desorbed water by elevating the temperature only by a few degrees and the transition temperature was finely controlled by mixing various compositions of IL monomers with different alkyl chain le n g t h s . 3 0 7 P o l y 1 - b ut y l -3 - v i n y l im i d a z o l i um b is (trifluoromethylsulfonyl) imide ([C4VIM][NTf2]) was found to have a pseudo-LCST effect in the presence of cyclodextrin.308 It was reported that the cyclodextrin ring complexes only the polymer anion and slips off at higher temperature when the PIL becomes insoluble in water. Based on this novel discovery, thermo-responsive PIL-based hydrogels with surprisingly broad LCST and volume transition behavior could be prepared. Covalently grafting of a PIL functionalized temperature-responsive copolymer onto the supporting membrane can yield a temperature-responsive composite membrane (Figure 17).309 The “switch” effect of the composite membrane can be

Figure 19. Structure of thermoresponsive-ionic star block copolymers.

relatively hydrophobic core of poly(vinylimidazolium bromide) and a hydrophilic shell of PNIPAM was also reported (poly(NIPAM)m-b-poly(BVIMBr)n and NP-g-PNIPAM shown in Figure 20).313−315 It was observed that the collapse of PNIPAM regarded as the shell of micelle and the dehydration of PIL as the core of the micelle take place above the phase transition temperature, which induced that a compact and regular core−shell structure formed. PIL consisting of IL 1benzyl-4-vinylpyridine bromide (4-VPBn+Br−) and thermoresponsive polymer NIPAM has been synthesized by Wu et al.316 Thermo-responsive nanogels through reversible additionfragmentation transfer (RAFT) cross-linking copolymerization (one of several kinds of reversible-deactivation radical polymerization) of IL−based monomers is demonstrated.317 The use of chain transfer agents (CTAs) containing a carboxyl group in the RAFT polymerizations is the key to produce highly thermoresponsive nanogels. Experimental results demonstrate that the critical gelation temperature of the as−prepared nanogels can be tuned by adjusting the feed ratio of monomer and CTA. Hydrogen-bonding interactions between the carboxyl groups of CTAs are responsible for the thermoresponsive behaviors of PIL-based nanogels. The self-assembly of linear polymers containing chiral IL units generates a high-order supramacromolecular structure with a complex hierarchical architecture, which is able to exhibit LCST behavior with different structural elements that can be used to fine-tune this LCST.318 It is confirmed that introduction of IL into the PNIPAM offers a wide range of LCST behaviors with a synergism between the hydrophobic part of the IL and the basic strength of the counteranion, probably by varying the hydrogen bonding abilities of the copolymer.319−322 One good example would be PIL based electrolytes which was systhesized by copolymerization of NIPAM with (or without) 3-butyl-1-vinyl-imidazolium bromide ([BVIM][Br]). Diallyl-viologen (DAV) is used as

Figure 17. Illustration of temperature-responsive composite membrane. Reproduced with permission from ref 309. Copyright 2015 Royal Society of Chemistry.

facilely adjusted by tuning the counter-ion of PIL units based on variations on hydrophilicity and hydrophobicity. The response temperature of the composite membrane was decreased with increasing the content of hydrophobic PIL units. A cationic polyelectrolyte based on the styrenic IL tributyl-4vinylbenzylphosphonium pentanesulfonate (Figure 18) was found to undergo a LCST-type phase transition in aqueous solutions.310 The CST value could be tuned in a wide temperature window in terms of polymer concentration and

Figure 18. Structure of IL tributyl-4-vinylbenzylphosphonium pentanesulfonate. X


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Figure 20. Copolymerization of NIPAM with (or without) 3-butyl-1-vinyl-imidazolium bromide ([BVIM][Br]). Reproduced with permission from ref 322. Copyright 2017 Royal Society of Chemistry.

Table 6. Properties and Potential Applications of PILs PIL P[P4,4,4,4][SS] (100 g/L) [P4,4,4,4][SS] P[P4,4,4,4][SS] (20 wt/v %) poly(SS−P2) (7.6 wt %) [P4,4,4,6][MC3S] and [P4,4,4,8][MC3S] (mole fraction: 10:1) poly([C4VIM][NTf2])

additives H2O H2O H2O (92.4 wt %) H2O (95 wt %) cyclodextrin (0.18 M)

LCST point/°C

application

refs

61 36 54 36

stabilize reduced graphene facilitate the design of ILs and PILs for thermo−sensitive functional materials and drug delivery applications

295 296

Preparation of thermoresponsive poly hydrogel

302

∼45

fine-tuning of their phase−transition temperature and phase behavior for certain application design of temperature sensitive gels

303

57

both the cross-linking agent and electrochromic material. The LCST of P(NIPAM−DAV) IL gels (22.8−30.1 °C) can be improved by incorporation of hydrophilic [BVIm][Br] (Figure 20). The LCST values of the electrolytes can be adjusted either by changing the weight ratio of the IL/H2O mixture or the molar ratio of NIPAM/[BVIm][Br] units.322 Meanwhile, some hydrophobic ILs were also found to be suitable solvents or even good modifiers for polymers showing LCST behavior.323−330 Among them, systems composed of poly(benzyl methacrylate) (PBzMA) and/or poly(styrene-coMe methacrylate) (P(St-co-MMA)) in [CnC1IM][NTf2] are mostly discussed.331−341 It has been demonstrated that the LCST-type phase separation of the polymers in an IL is greatly affected by the distribution of the solvophilic and solvophobic groups on the polymer chains. An increase in alkyl chain length between the Ph and ester groups in the polymer side chain decreased the CST value; alternatively, substitution of the imidazolium cation with a longer alkyl chain increased the CST value. When the same anion was used, the miscibility of the polymer−IL system was mainly determined by the alkyl chain length. CST value could also be varied by mixing two ILs in an appropriate ratio. It is also found that the imidazolium based

308

ILs can significantly affect the phase transition of PNIPAM and ruptured the hydrogen bonding between polymer and water molecules and destabilize the hydrated structure.342 Similar systems, as poly(benzyl methacrylate) in a room temperature IL, [C2C1IM][TFSA], also exhibits LCST-type phase separation.343 IL−polyether binary mixtures are another widely studied class that exhibits an LCST-type phase separation behavior.344−351 It is found that the miscibility of polymers and ILs was enhanced by the following factors: (1) the polarity of the polyethers, (2) the presence of acidic hydrogen atoms on the cationic structures, (3) the length of the N-substituted alkyl chain of the imidazolium cations and (4) the Lewis basicity of the anions. For poly(ethyl glycidyl ether) (PEGE)−[C2C1IM][NTf2] system, it is found that the hydrogen bonds between the protons of the [C2C1IM] cation ring and the oxygen atoms of the PEGE chain appeared to be the primary driving force of the LCST behavior.333,346 For poly(ethylene oxide) (PEO, MW = 5.000 and 20.000) and [CnCnIM][BF4] systems, good phase separation is observed from 130 to 170 °C.344,345,352,353 Unlike typical LCST phase diagrams of polymer solutions, the PEO− IL phase diagram is either roughly symmetric with a critical Y


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anthracene pillar[5]arene (MAP5) can also be bound to imidazolium based IL with a thiol group through host−guest interactions. This was done to achieve a modification of a gold interface.362 The bonding and release of MAP5 was reversibly controlled by temperature regulation. Based on the reported results, preparation of thermally sensitive supramolecular materials, such as temperature controlled release systems and temperature-sensitive gels might be possible due to the controllability of the host−guest interactions and the diversity of host−guest complexes. Similar host−guest interactions was also reported in [C1C1IM]I/acetone/macrocyclic compounds (Figure 21).363,364 Considering the easy commercial availability and greenness of both acetone and IL, this binary system is thus of practical importance.

composition near 50% polymer or asymmetric with a critical composition shifted to an even higher concentration of PEO. The values of the LCST increase as the length of the alkyl chain in the imidazolium cation increases. Furthermore, replacing the most acidic proton of the imidazolium ring (in the C2 position) with a methyl group lowers the CST and changes the shape of the phase diagram significantly, suggesting that the hydrogen bonds between the H atoms on the C2 position of the imidazolium ring and the O atoms of PEO play an important role in determining the LCST phase behavior of this system. It is also reported that PEO homopolymer has the ability to “shuttle” between water and hydrophobic [EMIM][TFSI].354 The reversible phase transfer between water and [C2C1IM][NTf2] was used to prepare thermo-sensitive polymer-grafted carbon nanotubes.355 It is believed that the temperatureinduced phase transfer behavior between water and IL is due to the relative affinity of the two solvents for the PEO units grafted on the carbon nanotubes. Properties and potential applications of PILs with LCST behavior are summarized in Table 6. 2.4.3. UCST and LCST Behavior. Doubly thermosensitive aggregation behavior was also observed in IL−polymer systems.356−358 Typical hydrophobic ILs [C2C1IM][NTf2], [C4C1IM][PF6] and a 1:1 mixture were selected as solvents.357 Both PNIPAm and P(NIPAm-r-AAm) (AAm refer to acrylamide) exhibit an UCST phase diagram in these ILs, whereas poly(benzyl methacrylate) (PBnMA) has a LCST. Consequently, at low temperature, the copolymers formed micelles with PNIPAm cores, whereas above the LCST of the PBnMA, inverse micelles with PBnMA cores were formed. According to the study of binary systems of PNIPAM in imidazolium ILs and their mixtures with water, the IL anion has a strong influence: [BF4]− anion yields a complex phase diagram with both LCST and UCST-type regimes. 358 Copolymer composed of NIPAm and cationic (3-acrylamidopropyl) trimethylammonium chloride (AMPTMA) with nearly equal amounts also show both LCST and UCST phenomena.359 A novel nanogel, thermosensitive diblock copolymer, exhibited LCST phase behavior in the aqueous phase and the opposite UCST phase behavior in hydrophobic ILs was reported.360 The obtained nanogel can reversibly shuttle between a hydrophobic IL phase and an aqueous phase in response to temperature changes. Upon increasing the temperature, the nanogel underwent a swollen-to-shrunken phase change in the aqueous phase, a transfer from the aqueous phase to the IL phase, and a shrunken-to-swollen phase change in the IL phase. These processes were thermally reversible, which made the round-trip shuttling of the nanogel between the aqueous and IL phases possible.

Figure 21. Chemical structures of ionic liquids, and two macrocyclic compounds (pillar[5]arene P5 and crown ether DB24C8) which are known hosts for the imidazolium cation. Reproduced with permission from ref 363. Copyright 2016 Royal Society of Chemistry.

3. THERMOREGULATED IL SYSTEMS FOR APPLICATIONS OTHER THAN CATALYSIS Among possible fields of applications, extraction with ILs as solvents holds an important position. Hydrophobic ILs are frequently used as the organic phase in biphasic IL/H2O solvent extraction studies. In the case of thermomorphic systems, the extraction process can be easily achieved by temperature tuning. There have already been many examples of thermoregulated ILs for extraction applications (Table 7): • The [P4,4,4,4][Tf−Leu]/H2O system with a LCST of 22 °C (1:1 wt/wt) was utilized to extract proteins.234 • The [P4,4,4E3][DEHP]/H2O system with a LCST at 44 °C (1:1 wt/wt) is the first example of homogeneous liquid−liquid extraction of metal ions with a thermomorphic system.247 • ILs based on Girard’s reagents show thermomorphic UCST behavior with water, and this property was used to extract metals from water.150 • [C6iQuin][SCN] with an UCST behavior with aliphatic hydrocarbon, cyclohexane or water, and LCST behavior with aromatic hydrocarbon or thiophene, was applied for separation of aromatic from aliphatic hydrocarbons or for the desulphurization process.366 • The ternary system of [C8iQuin][NTf2]/water/2-phenylethanol made possible to use the IL as a solvent to perform the extraction of 2-phenylethanol from a mixture with water.134 • The [Hbet][NTf2]/H2O system (1:1 wt/wt), with an UCST at 55−57 °C, has been used in the homogeneous extraction of metal ions, mainly trivalent rare-earth

2.5. Host−Guest Complexation Inducing Thermomorphic Systems

Special LCST−type phase transition induced by the host−guest complexation between IL [C1C1IM]I and dipropoxypillar[5]arene (DPP5) was reported by Yuan and Huang in 2013.361 Interestingly, neither of theim shows any thermo-responsive behavior in chloroform solution individually. Due to the host− guest interactions, DPP5 can bind IL to form a stable host− guest complex at low temperature, while at high temperature, host−guest interactions between DPP5 and IL weakened dramatically, which led to the release of IL from the host−guest complex to the solvent. The mixture gradually became turbid and finally turned into two phases. Mono-functionalized Z


Chemical Reviews

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Table 7. Examples of Extraction Applications Using Thermoregulated IL Systems ILs

cosolvent

CST/°C

property

extracted

[P4,4,4,4][Tf-Leu] [P4,4,4E3][DEHP] [P4,4,4,14][Cl] [Hbet][NTf2] [Hbet][NTf2] [Hbet][NTf2] [Hbet][NTf2] [choline][NTf2] [HbetC1Mor][NTf2] [HbetC1IM][NTf2] [HbetPy][NTf2]

H2O H2O H2O H2O H2O H2O H2O H2O H2O

22 44 0−40 55−57 55 55 55 72 52 64 55

LCST LCST LCST UCST UCST UCST UCST UCST UCST

cytochrome c transition metals (Co(II), Ni(II), Cu(II), Zn(II)) cobalt(II) and nickel(II) trivalent rare-earth (indium, gallium ions) rare-earth oxides U(VI) platinum group metals (Ru(III) and Rh(III)) neodymium(III) metal oxides, metal hydroxides

additives

NaCl (2−11 wt %)

HNO3 HNO3 [choline][hfac]

refs 234 247 365 159,367 368 369 370 371 106

4.1. Chemical Catalysis

ions,159,160,367 rare-earth oxides,368 U(VI),369 and plati-

4.1.1. Hydroformylation. Hydroformylation, an atom economic and environmentally friendly reaction, is one of the most versatile methods for the synthesis of intermediates and is a very important industrial process.375 To solve the limited mass transfer and difficult hydroformylation of higher olefins, Jin reported a series of thermoregulated nonionic phosphine ligands containing poly(ethylene glycol) (PEG) chains as the hydrophilic group (Figure 22), mainly PETPP, 376−378

num group metals.370 • [Choline][NTf2]/H2O, with an UCST of 72 °C, was used for the extraction of Nd(III) with choline hexafluoroacetylacetonate ([Choline][hfac]) as extractant.371 • Last but not the least, [HbetC 1 Mor][NTf 2 ], [HbetC1IM][NTf2], and [HbetPy][NTf2] bearing carboxylic groups show UCST behaviors with water.106 They show potential ability for extraction of metal oxides and metal hydroxides from water solution. Furthermore, it was found that the cloud point temperature of thermomorphic systems can be affected by the presence of metal salts to a great extent. And the metals have different effcts on UCST and LCST systems. General effects and guidelines was illustrated by Binnemans in 2015.45 Thus, hydrophilic IL can also be used for extraction from aqueous solution, e.g., [P4,4,4,14][Cl]/H2O system with a LCST of ca. 25 °C (40:60 wt/wt) (together with NaCl (5%) as salting out agent) shows promising results for the extraction of Co(II) and Ni (II).365 ILs for battery applications have already been summaried by Scrosati and Passerini.372 Interestingly, LCST behavior can also be used to control the function of energy storage devices, through modulating the electrochemical activity of electrode interfaces.373 In the electrolyte system of [C2C1IM][BF4], PEO, and Li+ salts, the solution conductivity was found to decrease when the temperature of the system increases beyond the LCST.

Figure 22. Structures of thermoregulated nonionic phosphine ligands containing polyoxy-ethylene chains.

OPGPP, 3 7 9 − 3 8 1 PEO-TPP, 3 8 2 , 3 8 3 AEOPP, 3 8 4 PEO− DPPSA,385 PEO−DPPPA,386 TMPGP387,388 and CH3(OCH2CH2)mPPh2.389,390 These ligands were used to stabilize rhodium nanoparticles and further applied for two− phase hydroformylation of olefins with desirable results. It was found that the thermomorphic property was originated from the hydrogen bonds formed between the polyether chain and water molecules. By introducing an additional organic phase containing a water−immiscible substrate into the reaction system, the catalyst would transfer into the organic phase after heating to the critical temperature, and the reaction actually takes place in the homogeneous phase.48 Meanwhile, the catalysts can be easily separated from the product by phase separation after cooling and can be used for several times without evident loss in both activity and selectivity. Besides Rh NPs, cobalt complexes combined with phosphine ligands were also applied to hydroformylation of olefins in TRPTC systems

4. THERMOREGULATED IL SYSTEMS FOR CATALYSIS Catalysis might be the most important application of thermoregulated IL systems. According to the designability of ILs, the thermoregulated structure unit and the catalytic center can be simultaneously coupled into the IL as demanded. The integration of thermoregulation and catalysis has already found many applications and certainly will endow ILs as reaction media in the future. In the following, applications of thermoregulated IL systems for different reactions will be illustrated in detail. It should be noted that all previous reviews are mainly related to systems with thermoregulated molecular catalysts or thermoregulated polymer-based catalysts as well as thermoregulated microemulsions and temperature-dependent multicomponent solvent systems.47−49,60,61,70,374 However, ILbased thermomorphic systems are rarely elaborated, especially in the field of catalytic applications.40−42 AA


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The first hydrogenation system using thermomorphic IL was reported by Dyson and co-workers in 2001.409 An IL phase based on [C8C1Im][BF4], which contained [Rh(η4-C7H8) (PPh3)2][BF4] as a catalyst was successfully applied for the hydrogenation of water-soluble substrate 2-butyne-1,4-diol. Here, a single phase is formed after heating to reaction temperature (80 °C) while two phases reform after cooling. At room temperature the IL phase contains the catalyst whereas the aqueous phase contains a mixture of the products. Thus, the products can be removed simply without catalyst contamination. Ammonium IL with similar structure as ILPEG-3 (Figure 23) but with HSO4− as anion was used as a thermomorphic solvent for Ru/TPPTS-catalyzed hydrogenation of polymer SBS (polystyrene-b-polybutadiene-b-polystyrene) in a mixture of toluene and tetrahydrofuran.410 Thermoregulated ILPEG750-3/ organic biphasic systems were also reported for olefin hydrogenation.411,412 The use of transition-metal NPs/IL as a thermoregulated and recyclable catalytic system for hydrogenation were also reported by Hou.413,414 Due to a cooperative effect regulated by both the cation (PEG-functionalized alkylimidazolium) and anion (tris(meta-sulfonatophenyl)-phosphine, [P(C6H4-mSO3)3]3−) of the IL (abbreviated as [PEG-2000-C12C1IM]1.5[P(C6H4-m-SO3)3], Figure 24), the NPs displayed distin-

due to its good reactivity toward internal olefins and cheaper prices.391,392 Interestingly, pure PEG 4000 shows thermomorphic property as well, and thus PEG 4000 stabilized Rh NPs can be used as an efficient and recyclable catalyst for hydroformylation of olefins in biphasic system with both conversions and selectivities ≥98%.393 To make the system more simple, PEG chains were incorporated into the IL structure to form ILPEG-1,394 ILPEG-2395 and ILPEG-3 [CH3(OCH2CH2)16N+Et3][CH3SO3−]396,397 (Figure 23). These materials were used as

Figure 23. Structures of ILPEG-1, ILPEG-2, and ILPEG-3.

thermoregulated phase−separable catalysts for hydroformylation of higher olefins. Although excellent results can be achieved with both conversions and selectivities higher than 95%, the disadvantages is the usage of polar ligands which also improve the unwanted interaction between the IL and Rh NPs. IL-type imidazolium surfactants combined with α-cyclodextrins microemulsion system was also found to be an efficient thermoregulated catalytic hydroformylation approach.398 Applications of thermoregulated IL systems for hydroformylation are summarized in Table 8. 4.1.2. Reduction with H2 or CO. Using similar principles as for the hydroformylation reaction, hydrogenations were also widely studied applying thermoregulated biphasic catalysis: Ru3(CO)12/PETPP gave full conversion of styrene hydrogenation in toluene399 and Rh/CH3(OCH2CH2)mPPh2 (m = 16, 22) showed promising activity for hydrogenation in aqueous/1-alkanol biphasic system.400−403 Furthermore, RhCl[PPh[(OCH2CH2)5≤n≤6CH3]2]3 (Rh/AEOPP),404 Rh/cyclodextrin,405 Rh/PEG,406,407 and Pd/PEG408 were also reported as beeing efficient and recyclable thermomorphic hydrogenation catalysts.

Figure 24. Structure of [PEG-2000-C12MeIm]1.5[P(C6H4-m-SO3)3].

guished temperature-dependent phase behavior and excellent catalytic activity and selectivity, coupled with high stability.413 It is believed that the anion plays a role in changing the surrounding electronic characteristics of the NPs through its coordination capacity, whereas the cation is responsible for the thermomorphic properties. Another sulfonated chiral diamine Ru complexes being the anion of a functionalized IL (Figure 25) were employed as efficient catalysts for the asymmetric transfer hydrogenation of various ketones.414 The catalysts displayed both excellent activity and enantioselectivity for a wide range of substrates bearing different functional groups. Moreover, excellent thermoregulated phase-separation behavior in ethyl acetate, resulting in highly clean separation and an effective recycling of the catalyst was reported. Besides hydrogenation, CO selective reduction of nitroarenes catalyzed by the thermoregulated phase-transfer catalyst Ru3(CO)9(PEO−DPPSA)3 (PEO−DPPSA: Figure 22) was

Table 8. Thermo-Regulated IL Systems for Hydroformylation

substrates

solvents

thermomorphic IL

catalysts

1-tetradecene 1-octene 1-dodecene 1-octene 1-allyl-3-methoxybenzene

n-heptane cyclohexane n-heptane and toluene n-heptane and toluene toluene

ILPEG-1 ILPEG-2 ILPEG-3 ILPEG-3 [C16C1IM][Br]

Rh−TPPTS Rh Rh/TPPTS (triphenylphosphine trisulfonate) Rh [Rh(acac) (CO)2]-TPPTS with α-cyclodextrins AB

recycling performed 7 6 8 8 5

runs runs runs runs runs

refs 394 395 396 397 398


Chemical Reviews

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and ligand free conditions. Moreover, the recovery of this system is really simple: H5PMo10V2O40-ILs was recovered through a single filtration after cooling to 5 °C and Fe3O4@ OA−Pd was retrieved by magnetic separation. 4.1.4. Biomass Conversion. In the past decade, conversion of biomass to fuels was a very hot topic because biomass can be used as a source of energy. Among the three components, cellulose received wide attention (Scheme 1). There are also some examples of thermoregulated IL systems used in biomass conversion. A POM−IL, namely [(CH3)3NCH2CH2OH]5[PV2Mo10O40], proved to be effective in catalyzing the oxidation of starch.425 It showed nearly identical or even higher performance with traditional catalysts, such as FeSO 4 . Furthermore, the special temperature responsive property made the recovery of POM−IL very easy and it can be run for at least six times without appreciable loss of its performance, highlighting the practical application of this catalyst class. Considering the temperature−dependent solubility of [(HOOCCH2)C1IM]Cl in isopropanol, a thermoregulated recyclable catalytic system consisting of [(HOOCCH2)C1IM]Cl as an acidic IL catalyst and isopropanol as solvent for 5−hydroxymethylfurfural (HMF) production from fructose was reported.426 [HO2CC1C1IM]Cl is completely insoluble in isopropanol at room temperature, while it becomes soluble when the solution is heated to temperatures higher than 80 °C. Cooling the solution to room temperature resulted in IL precipitation. Additionally, the solubilization and precipitation processes are reversible. This catalytic system can be envisioned to find more applications in acid catalyzed reactions with thermoregulated catalyst recovery features. Direct production of HMF from cellulose was reported using cholinexH3−xPW12O40 catalysts.281 An HMF yield of 75.0% was achieved catalyzed by cholineH2PW12O40 within 8 h at 140 °C in mixture of methyl isobutyl ketone and H2O, which was almost the highest yield of HMF from cellulose reported by now. In addition, another heteropolyacids [(CH3)3NCH2CH2OH]nH5‑nAlW12O40 have been synthesized using choline chloride and H5AlW12O40 as precursors, and the resulting HPA nanohybrids exhibited novel switchable properties based on temperature variation due to the incorporation of the choline cation, which dissolved in the reaction mixture at higher temperatures to form a homogeneous catalytic system and then precipitated spontaneously from the mixture at room temperature.427 The temperature-responsive catalyst combined with its dual Lewis and Brønsted acidity can catalyze the

Figure 25. Structure of the functionalized IL bearing a sulfonated chiral diamine Ru complex as the anion [PEG-4000-C8C1Im][RuTsDPENDS].

also investigated.415 The reduction proceeded with good activity and selectivity toward the nitro group when halogen, carbonyl or cyano groups are present in the substrates. The catalyst in the water phase could be recycled three times with trace loss of catalyst activity. Table 9 summarizes mainly the applications of thermoregulated IL systems in hydrogenation reduction. 4.1.3. Coupling Reactions. Coupling reactions are another really important class of organic transformations in which two hydrocarbon fragments are coupled with the aid of a metal catalyst. Combined with Pd as an active species, PEG modified phosphine Ph2P(CH2CH2O)nCH3 was again proven to be a very efficient and versatile thermoregulated ligand for palladium-catalyzed Suzuki reaction416−418 and Sonogashira coupling.419,420 Similarly, an amphiphilic dipyridyl-based ligand incorporated in a PEG chain was reported to be temperature responsive for Pd-catalyzed Heck reactions.421 Furthermore, N-heterocyclic carbene (NHC) and phosphine-chelated Pd catalysts with PEG chain for Suzuki− Miyaura cross-coupling reaction was reported.422 In situ formed active NHC-supported palladium complexes joined by phosphine were highly efficient for coupling of aryl bromides with phenylboronic acid. ILPEG750-3 (Figure 23) combined with stabilized Pd NPs was also successfully employed for the Heck reaction.423 This system is free of phosphines or other ligands and also exhibited high efficiency, all the cross-coupling products are detected to be trans isomers. Due to the thermoregulated separation of the catalyst, the system can be reused for at least seven runs. Another system consisted of palladium and deprotonated oleic acid on Fe3O4 NPs (Fe3O4@ OA−Pd) combined with H5PMo10V2O40-ILs as a cocatalyst was reported to be efficient, heterogeneous, and recyclable for Heck coupling reactions.424 The reaction was performed under base

Table 9. Thermo-Regulated IL Systems for Hydrogenation Reduction

substrates

solvents

thermomorphic IL

2-butyne-1,4-diol

H2O

[C8C1IM][BF4]

SBS cyclohexene 1,5-cyclooctadiene α,β-unsaturated aldehydes ketones

toluene/THF toluene/n-heptane toluene/n-heptane ethyl acetate

ILPEG ILPEG-3 ILPEG-3 [PEG-2000-C12C1IM]1.5 [P(C6H4-mSO3)3] [PEG-4000-C8C1IM] [Ru-TsDPENDS]

ethyl acetate/HCOOH-NEt3 azeotrope

AC

catalysts [Rh(η4-C7H8) (PPh3)2] [BF4] Ru/TPPTS/TPP Rh Rh Pd or Rh

recycling performed

ref

not mentioned

409

3 runs 10 runs 11 runs 10 runs

410 411 412 413

5 runs


Chemical Reviews

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Scheme 1. Reaction Pathway of Cellulose

gradually slight drop in the catalyst recycling runs. Polymer supported POM (C16PM(O2)2/PNIPAM) was also successfully used for catalyzing the oxidation of refractory sulfur-containing compound dibenzothiophene into its corresponding sulfone with high selectivity in the presence of H2O2.282 Application of this catalyst brings about an efficient, useful, and green process in desulfurization through extraction and oxidation simultaneously. On the other hand, epoxidation is also an important reaction in industry. Protic and PEG-functionalized imidazolium POM− ILs were reported to be efficient thermoregulated catalysts for epoxidation reactions.272,278 Besides the homogeneous catalysis, a tungsten peroxo complex was immobilized successfully on imidazole-functionalized silica.280 The obtained material was employed as an efficient catalyst for the hydroxymethoxylation of olefins. It was demonstrated that the immobilization of tungsten peroxo complex was highly temperature-dependent. The tungsten peroxo complex can dissociate and diffuse into the liquid phase at the reaction temperature, resulting in a homogeneous reaction. Nevertheless, the catalytically active species was rebound on the imidazole-functionalized silica by hydrogen bonding as the temperature was lowered to 0 °C after reaction, which thus offered a highly effective approach for recycling the catalyst for consecutive cycles. In addition, various olefins can be converted with good conversion and selectivity. PEG-based imidazolium ILs were further used as support for well-dispersed Au NPs, the resultant catalysts showed a unique thermoregulated self-separating nature in the epoxidation of styrene in toluene.433 4.1.6. Hydrosilylation. Hydrosilylation, also called catalytic hydrosilylation, is one of the most important Si−C bond forming reactions in organosilicon chemistry. Olefin hydrosilylation was also described on the basis of thermoregulated IL concept (Table 10).434−437 The Fluorous room-temperature IL, 1-butyl-3-methylimidazolium tetrakis[p-{dimethyl(1H,1H,2H,2H-perfluorooctyl)silyl} phenyl]-borate ([C4C1Im][B(Arf)4], Figure 28), was used as a solvent for the homogeneous hydrosilylation of 1−octene catalyzed by a fluorous version of Wilkinson’s catalyst.434 The IL showed a critical temperature of 62 °C at a toluene mole fraction of 90%, which makes this solvent especially attractive for use in catalytic processes that suffer from phase transfer limitations in other biphasic solvent systems. Organic molten N-alkylpyridinium or N,N-dialkylimidazolium salts were also used as thermoregulated supports for the Rh(PPh3)3Cl complex. These systems were applied for

cellulose into levulinic acid directly in a highly effective single phase strategy. 4.1.5. Oxidation Reaction. Desulfurisation is a chemical process for the removal of sulfur from a material, which is very important for industry and environmental protection. A series of organic−inorganic POM−IL with structures similar to [PyPS]PW, including [BuPyPS]PW, [PhPyPS]PW, and [BzPyPS]PW, were synthesized and subsequently tested in the oxidation of methyl phenyl sulfide with hydrogen peroxide by Rafiee (Figure 26).428 This group also prepared some similar

Figure 26. Oxidation of sulfide catalyzed by the different [PyPS]PW.

POM-IL with longer alkyl group attached on the N atom in pyridinium, which is even more active for the catalytic oxidation desulfurization of a synthetic mixture of model oil.429 On the other hand, [C8C1IM][PF6] was proved to be a good solvent for [PyPS]PW catalyzed desulfuriyation.430 Imidazolium ILs were also combined with PEO chains as the thermo−regulated structure unit and POM as the catalytic oxidation center (Figure 27a).431 Similar ammonium oxidative thermoregulated bifunctional IL catalysts have been synthesized (Figure 27b).432 Both POM-ILs were proved to be efficient for catalytic H2O2 oxidative desulfurization of model fuels. The recycle of the IL catalyst is easily achieved. The desulfurization rate only keeps a

Figure 27. Structures of imidazolium- and ammonium-based POMILs. AD


Chemical Reviews

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Table 10. Thermo-Regulated IL Systems for Hydrosilylation Reactions

substrates

thermomorphic IL

catalysts

recycling performed

ref

1-octene with dimethylsilane alkenes with triethoxysilane styrene with triethoxysilane styrene with triethoxysilane

[C4C1Im][B(Arf)4] N-alkylpyridinium or N-alkylimidazolium hexafluorophosphate

RhCl(P(Arf)3)3 Rh(PPh3)3Cl Rh(PPh3)3Cl Rh(PPh3)3Cl

15 runs 10 runs 4 runs 10 runs

434 435 436 437

{[(C4H9)2N]3[(C8H17)2N]P}PF6

different anions of dihydrogen phosphate [H2PO4]−, acetate [CH3COO]−, and hydrogen sulfate [HSO4]− were also found to be thermoregulated solvents.440 Among them, [C6C1IM][HSO4] was found to be very suitable for the synthesis of bisphenol F (Scheme 3). Both the formation of a thermo-

Figure 28. Structure of fluorous room-temperature IL, 1-butyl-3methylimidazolium tetrakis[ p-{dimethyl(1H,1H,2H,2H− perfluorooctyl)silyl}phenyl]-borate.

Scheme 3. Hydroxyalkylation of Phenol with Formaldehyde

435,436

hydrosilylation of alkenes and triethyloxysilane. The catalyst phase is liquid under the reaction conditions, whereas it is solid at room temperature. Therefore, it can be reused simply by decantation. Furthermore, tetrakis(dialkylamino)phosphonium salts were also proved to be ideal supports for Rh complexes.437 Due to the thermomorphic property, the system consisting of a molten salt, {[(C4H9)2N]3[(C8H17)2N]P}PF6, and Rh(PPh3)3Cl can be reused more than 10 times without noticeable loss of catalytic activity and selectivity. 4.1.7. Hydroaminomethylation Reaction. Hydroaminomethylation has been used as an important tool in the synthesis of amines. It is a tandem reaction involving hydroformylation of olefins followed by the reaction of resulting aldehydes with a primary or secondary amine to produce the corresponding imines or enamines. Finally, the imines or enamines were hydrogenated into the desired amines. Again, the thermoregulated ligand Ph2P(CH2CH2O)16CH3 was used to stabilize Rh nanoparticle in the aqueous/1-butanol biphasic system for the hydroaminomethylation of olefins (Scheme 2).438 Similarly to the many other examples mentioned above, it allows a homogeneous catalytic reaction and an easy biphasic separation. Both the conversion of 1-octene and the product amine selectivity were as high as 99% and 97%, respectively. The thermomorphic IL, [CH 3 (OCH 2 CH 2 ) 22 N + Et 3 ][CH3SO3−] (abbreviated as ILPEG1000), in combination with ILPEG1000-stabilized Rh NPs and organic solvent were also reported to be an efficient hydroaminomethylation system for 1-octene with 100% conversion and 90% selectivity.439 After reaction, the Rh NP catalyst could be easily separated by simple decantation and reused for four times without evident loss in activity. 4.1.8. Hydroxyalkylation of Phenol with Formaldehyde. Brønsted acidic catalysts of imidazolium-, ammonium-, phosphonium- and pyridinium-based water-soluble ILs with

regulated monophasic reaction system at 90 °C to enhance the reaction efficiency and a thermoregulated phase−transition solvent to facilitate its recovery from the reaction system are realized also in this case. [C6C1IM][HSO4] could be recovered by simple decantation and could retain its original activity even after six recycling runs. 4.1.9. Stetter Reaction. Another type of a thermoregulated thiazolium IL with a polyether chain (Figure 29) has been

Figure 29. Stetter reaction of ethyl acrylate with furfural or butanal catalyzed by the thermoregulated polyether-substituted thiazolium IL.

synthesized and used as catalysts in the Stetter reaction of ethyl acrylate with furfural or butanal (Figure 29).441,442 The catalytic system possess the properties of CST and an inverse temperature-dependent solubility in toluene−heptane. There is no obvious decrease of both the conversion and yield after five cyclings. The novel thermoregulated system has widened

Scheme 2. Reaction Pathway of Hydroaminomethylationa

a

Reproduced with permission from ref 438. Copyright 2013 Elsevier. AE


Chemical Reviews

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the application scope of liquid/liquid biphasic catalysis and provided a promising route for environmentally benign synthesis. 4.1.10. Chloromethylation of Biphenyl. Chloromethylation is the chemical reaction of aromatic rings with formaldehyde and hydrogen chloride catalyzed by zinc chloride to form chloromethyl arenes, which was discovered by Blanc in 1923. The PEG 1000 linked, dicationic acidic IL (PEG 1000DIL, Figure 30) was successfully applied in the chloromethy-

Table 11. Thermo-Regulated IL Systems for Esterification Reactions

substrates citric acid with n-butanol oleic acid with methanol aldehydes with methanol palmitic acid with methanol benzoic acid and ethanol

Figure 30. Chloromethylation reaction catalyzed by the thermoregulated PEG 1000-DIL.

solvents

thermomorphic IL

recycling performed

ref

none

[C1IMPs]3PW12O40

4 runs

273

none

[QPS]PW

5 runs

275

none

[PEG-2000-C4IM] [HPW12O40] [(CH3)3(CH2CH2OH) N]H2PW12O40 PEG1000-DIL

4 runs

279

6 runs

448

7 runs

449

None toluene

prepared by polymerization was successfully applied in catalystic atom transfer radical polymerization.451 Even after 10 times of recycling experiments, the recycling efficiency of the system remained more than 95%, and the transition metal catalyst residual in polymer solution was just about 1.5 ppm.

lation of biphenyl in aqueous media, giving up to 85% yield.443 Another similar PEG-IL in combination with methylcyclohexane also proved to be efficient for chloromethylation of aromatic hydrocarbons with good to excellent yields (Figure 30).444 The novel method does not only enhance the yield but also allows an easy workup. Simple reaction conditions, good thermoregulated biphasic behavior of the IL and facile product isolation steps are the attractive features of this approach. Moreover, the excellent recyclability of the catalytic system makes this procedure cleaner, being therefore a good example of green chemistry technology. Similar thermoregulated IL like [PEG 1000-(C1IM)2][NTf2]2 was also reported to be good solvent for cycloaddition reaction.445 4.1.11. Condensation. PEG 1000-DIL was also used for polyfunctionalized 4H-pyrans preparation via one-pot threecomponent condensation reaction with yields between 81− 93%.446 This catalytic system has the advantages of both homogeneous and heterogeneous and offering the high activity as well as practical convenience in the product separation from the IL system. Another similar PEG bridged tertiary amine functionalized ionic liquid PEG 800-DPIL(Cl) was synthesized and applied in Knoevenagel condensation of aromatic aldehydes with active methylene compounds to afford substituted benzylidenes with up to 99% yield.447 In the presence of cyclohexane and isopropanol it can also form a temperature driven reversible biphasic solvent system which allow recovery by simple decanting for several catalytic runs. 4.1.12. Esterification. POM−ILs were proven to be efficient catalysts for esterifications also (Figures 14 and 15 and Table 11).273−275,279,448 Similar as other thermoregulated catalytic systems, they were found to be “temperature controlled self-separation catalyst” and made the recovery and reuse really convenient. PEG 1000-DIL (Figure 30) was proven to be potent in the esterification of aromatic acids and in the acetalization of aromatic aldehydes with good to excellent yields in both transformations.449 4.1.13. Atom Transfer Radical Polymerization. The first atom transfer radical polymerization using a thermo-regulated IL (IL [CH3(OCH2CH2)16(C2H5)3N][CH3SO3]) was reported by Cheng and Zhu recently in 2014.450 The catalyst could be reused five times with negligible loss of catalytic activity due to the self-precipitaion. Another PIL which was

4.2. Enzyme Catalysis

Enzyme catalysis is a very important part of catalysis, which uses protein as a catalyst and has very similar mechanism as chemical catalysis.452,453 Lozano and co-workers have developed a series of enzymatic process taking advantage of the thermo property of the ILs (Table 12),454,455 and a new concept of sponge-like ILs was introduced by them.456,457 Basically, sponge-like ILs are hydrophobic ILs with long alkyl chains and they behave as temperature switchable IL/solid phases which is quite similar to sponge like systems. The sponge-like ILs were able to “soak up” biodiesel as a liquid phase and then could be “wrung out” by centrifugation in a solid phase; one example is shown in Figure 31.457 Biodiesel production was the most investigated subject. [C18C1IM][NTf2]/Novozym 435 (immobilized Candida Antarctica lipase) system458 and [C16C1IM][NTf2]/Novozym 435459,460 proved to be efficient for producing biodiesel with easy and efficient recycling. The appropriate physical properties of the ILs permit the full solubilization of all substrates over a wide range of IL/triolein concentration ratios that results in monophasic systems. After reaction, three phases existed with a lower layer with enzyme dissolved in IL, which can further be solidified by decreasing the media reaction temperature, thus facilitating the extraction of the biodiesel product. Both [C18C1IM][NTf2] and [(CH3)3(C18H37)N][NTf2] were used to prepare oxygenated biofuels from vegetable and waste cooking oils with product yields higher than 94%.461 After reaction, the reaction mixture can be easily fractionated by iterative centrifugations at controlled temperature into three phases, leading to a straightforward and clean approach allowing the full recovery of the biocatalyst/IL system for further reuse and the simple product isolation. Furthermore, the enzyme did not shown any loss in activity during reuse in these reaction systems after six operation cycles. The [C16C1IM][NTf2]/Novozym 435 system was used for successful production of 16 different flavor esters by esterification of an aliphatic carboxylic acid (acetic, propionic, butyric, or valeric) and a flavor alcohol (isoamyl alcohol, nerol, citronellol, or geraniol).462 It is also quite a catalytic system for AF


Chemical Reviews

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Table 12. Thermo-Regulated IL Systems for Enzymatic Catalytic Reactions substrates

solvents/thermomorphic ILs

triolein/methanol triglycerides with methanol triolein/methanol vegetable and waste cooking oils triolein/methanol aliphatic carboxylic acid and flavor alcohol acetic acid with anisyl alcohol fatty acids with glycerol

[C18C1IM][NTf2] [C16C1IM] [NTf2] [C16C1IM] [NTf2] [(CH3)3(C16H33)N][NTf2]/[C18C1IM][NTf2] [(CH3)3(C16H33)N][NTf2]/[(CH3)3(C18H37)N][NTf2] [C16tma][NTf2] [(CH3)3(C16H33)N][NTf2]/[C18C1IM][NTf2] [C12C1IM][BF4]

catalysts Novozym Novozym Novozym Novozym Novozym Novozym Novozym Novozym

recycling preformed

refs

7 runs not mentioned 9 runs 6 runs 15 runs 7 runs 10 runs 8 runs

456 457 458 459 455,465 462 463 464

435 435 435 435 435 435 435 435

IL-water and/or organic compound as well as IL−IL mixtures have been investigated, not only for imidazolim ILs but also for nonimidazolium ILs. It is believed that the special thermoregulated property is originated from the highly ordered hydrogen bonding (which is different from the conventional hydrogen bonding), as well as further ion pairs and ionic aggregates. Different spectroscopic methods were used to better understand the interactions between investigated ILs and the solvents. Many theoretical models were also applied to the studied binary or ternary mixtures. In most cases, the experimental data correlates quite well with the model calculation. Different kinds of ILs including conventional ILs, metal containing ILs, PILs, and host−guest complexations, with either UCST or LCST as well as both UCST and LCST behavior have been demonstrated in detail. Further applications of thermoregulated IL systems in both chemical and enzymatic catalytic reactions were also illustrated accordingly. Due to the special property of “homogeneous reaction and heterogeneous separation” by easy tuning of reaction temperature, thermoregulated ILs can be recovered easily and reused for many recycles. To some extent, thermoregulated IL systems serve as briges between homogeneous and heterogeneous catalysis. The obtained results not only open up a new way in green chemistry for separating products from reaction media based on ILs but also open up new opportunities to develop green industrial processes. It is believed that this kind of catalytic system could be extended for many other reactions dealing with the separation of catalyst in homogeneous catalysis. Until now, only limited reactions have been performed using the advantages of thermoregulated property. In the future, many more applications will be found not only in catalysis but also in other areas.

Figure 31. Schematic representation of the sponge-like IL hypothesis, showing the IL net as a dry sponge (A), a sponge swollen with methyl oleate (B), and a “wet” sponge after wringing out by centrifugation (C). Reproduced with permission from ref 457. Copyright 2013 Royal Society of Chemistry.

production of anisyl acetate fragrance by esterification of acetic acid with anisyl alcohol under both conventional and microwave (MW) heating.463 Interestingly, [C16C1IM][NTf2]/Novozym 435 system was not so efficient for production of monoacylglycerides (MAGs) by esterification of fatty acids (i.e., capric, lauric, myristic, palmitic, and oleic acids, respectively) with glycerol, whereas the [C12C1IM][BF4]/Novozym 435 system showed the promising result with up to 100% selectivity and 100% yield for monolaurin. Furthermore, easy and efficient of IL-free MAG recovery can also be obtained through iterative centrifugations at controlled temperature.464

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

5. CONCLUSION The IL field has grown into a wide subject area in the past decades, while the thermoregulated IL systems seem to be extremely attractive for clean separation and green synthesis on the basis of the fact that ILs are an interesting class of tunable and designable solvents. An overview of ILs-solvent mixture with respect to the phase behavior has been afforded in detail. In UCST systems, the solubility of IL in the solvent increases with increasing temperature and a homogeneous phase forms after heating to a certain temperature. While in LCST systems, the solubility of IL in the solvent decreases with increasing temperature, and homogeneous solution changes to two phases. Interestingly, some systems show both UCST and LCST phenomena due to the designability of ILs. Lots of phase equilibrium studies of binary and ternary systems consisted of

ORCID

Zhenshan Hou: 0000-0003-1545-6804 Author Contributions §

These authors contributed equally.

Notes

The authors declare no competing financial interest. Biographies Yunxiang Qiao got her Diploma in Applied Chemistry from Nanjing University of Technology in 2007. Then she moved to East China University of Science and Technology and had her combined study of Master’s and Ph.D. under the supervision of Professor Zhenshan Hou from 2007 to 2012. After finishing her Ph.D., she came to Max-PlanckAG


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[C4C14Py][BF4] = 1-butyl-4-methylpyridinium tetrafluoroborate [BA][NO3] = (benzyl)dimethylalkylammonium nitrate [BCN4Py][TCM] = N-butyl-4-cyanopyridinium tricyanomethanide [BMPy][TOS] = N-butyl-4-methylpyridinium tosylate (ptoluenesulfonate) [BPy] = N-butylpyridinium [BVIm][Br] = 3-butyl-1-vinylimidazolium bromide [bvim][NTf 2 ] = 1-butyl-3-vinylimidazolium bis(trifluoromethylsulfonyl) imide C12MeIm = 1-dodecyl-2-methylimidazolium C2Py = 1-ethylpyridinium C4Py = 1-butylpyridinium C8iQuin = N-octylisoquinolinium CST = critical solution temperature CTAs = chain transfer agents [C 11 OMEtOHMMN][(CN) 2 N] = (2-hydroxyethyl)dimethylundecyloxymethylammonium dicyanamide [C 18 tma][NTf 2 ] = octadecyltrimetylammonium bis(trifluoromethylsulfonyl)imide [C 2 C 1 IM][FAP] = 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate [C2 C1IM][TFSA] = 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide [C2C1IM][TOS] = 1-ethyl-3-methylimidazolium tosylate [C2py][NTf2] = 1-ethylpyridinium bis{(trifluoromethyl)sulfonyl} imide [C3C1Pip] = 1-propyl-1-methylpiperidinium [C3C1Pyr][CF3SO3] = 1-methyl-1-propylpyrrolidinium triflate [C4(CN)4Py] = N-butyl-4-cyanopyridinium [C4C13Py][TOS] = N-butyl-3-methylpyridinium tosylate (ptoluenesulfonate) [C4C14Py][TOS] = N-butyl-4-methylpyridinium tosylate (ptoluenesulfonate) [C4C1C1IM][BF4] = 1-butyl-2,3-dimethylimidazolium tetrafluoroborate [C4C1IM][BF4] = 1-butyl-3-methylimidazolium tetrafluoroborate [C4C1IM][PF6] = 1-butyl-3-methylimidazolium hexafluorophosphate [C4C1Mor] = 1-butyl-1-methylmorpholinium [C4C1pip][FeCl4] = N-butyl-N-methylpiperidinium tetrachloroferrate [C4C1Py] = 1-butyl-3-methyl-pyridinium [C4C1Pyr] = 1-methyl-1-butylpyrrolidinium [C 4 HBet][NTf2] = N-butyl-N-dimethylbetainium bis(trifluoromethylsulfonyl)imide [C4Py][FeCl4] = N-butylpyridinium tetrachloroferrate [C4pyr] = 1-butylpyrrolidinium [C6C1Py][CF3SO3] = 1-hexyl-3-methylpyridinium triflate [C 6HBet][NTf 2] = N-hexyl-N-dimethylbetainium bis(trifluoromethylsulfonyl)imide [C8iQuin][SCN] = N-octylisoquinolinium thiocyanate [CH3OCH2CH2OCH2CH2)2IM][NTf2] = 1,3-bis-[2-(2methoxyalkoxy)ethyl]imidazolium bis{(trifluoromethyl)sulfonyl}imide [choline][NTf2] = choline bistriflimide [CnC1IM][BF4] = 1-n-alkyl-3-methylimidazolium tetrafluoroborate [CnQuin][NTf2] = N-n-alkyl-quinoliniumbis{(trifluoromethyl)sulfonyl}imide

Institut für Kohlenforschung as a postdoc in the joint group of Dr. Nils Theyssen and Professor Walter Leitner since 2013. Her current research interests are focused on catalytic biomass conversions. Wenbao Ma was born in Heilongjiang, China, in 1991. He received his B.S. degree from East China University of Science and Technology in 2014. After that, he joined Prof. Zhenshan Hou’s group in the Key Laboratory for Advanced Materials Research Institute of Industrial Catalysis at East China University of Science and Technology for his Master’s. Since 2016, he has been working on a Ph.D. in the same laboratory. His present research interests focus on green catalysis by designing new efficient ionic liquid catalysts. Nils Theyssen, born in 1974, got his Diploma in Environmental Chemistry from the University of Jena in 2000. He then switched to the Max-Planck-Institut für Kohlenforschung, were he worked as a Ph.D. under the supervision of Professor Walter Leitner. In 2002 he became leader of the technical laboratories and the leader of a local scientific subgroup from Professor Walter Leitner, RWTH Aachen University. In 2008 he was appointed as the safety commissioner of the institute. His research interests are located in the application of supercritical carbon dioxide as an alternative reaction medium for catalyzed transformations. Chen Chen was born in Jiansu province, China, in 1989. She completed each of her academic degrees in Industrial Catalysis from the East China University of Science and Technology, receiving her Ph.D. in 2016 in Professor Zhenshan Hou’s group. Her Ph.D. research was focus on designing ionic liquid catalysts for epoxidation under the mild conditions. Now she works for BASF Catalysts (Shanghai) Co. Ltd. Zhenshan Hou earned a B.A. in chemistry at Shannxi Normal University and an M.S. in physical chemistry with Professor Qin Xin at Dalian Institute of Chemical Physics (DICP) of the Chinese Academy of Sciences (CAS) and completed his Ph.D. with Professor Hanqing Wang at the Lanzhou Institute of Chemical Physics of Chinese Academy of Sciences. He was a postdoctoral fellow at the Institute of Chemistry of Chinese Academy of Sciences (1999−2002) with Professor Buxing Han and at Max-Planck-Institut für Kohlenforschung (2002−2006) with Professor Walter Leitner. He has been appointed Full Professor at East China University of Science and Technology since 2006. His current and main scientific interests include green chemistry and catalysis.

ACKNOWLEDGMENTS The authors are grateful for financial support from the National Natural Science Foundation of China (21373082), the innovation Program of Shanghai Municipal Education Commission (15ZZ031), the Fundamental Research Funds for the Central Universities, and the Max−Planck−Institut für Kohlenforschung. The authors also acknowledge greatly Prof. Walter Leitner’s valuable suggestions. ABBREVIATIONS AAm = acrylamide AMPTMA = (3-acrylamidopropyl) trimethylammonium chloride Arf = [p-{dimethyl(1H,1H,2H,2H-perfluorooctyl)silyl}phenyl Bn2ImPF6 = 1,3-dibenzylimidazolium hexafluorophosphate BnHexImPF6 = 1-benzyl-3-hexylimidazolium hexafluorophosphate BSA = bovine serum albumin AH


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[P4,4,4,4][Tf−Leu] = tetrabutylphosphonium N-trifluoromethanesulfonyl leucine [P4,4,4,6][MC3S] = tributylhexylphosphonium 3-sulfopropyl methacrylate [P4,4,4,6][SPA] = tributylhexyl phosphonium 3-sulfopropyl acrylate [P4,4,4,6][SS] = tributylhexylphosphonium p-styrenesulfonate [P4,4,4,E3][DEHP] = tri-n-butyl-{2-[2-(2-methoxyethoxy)ethoxy]ethyl}phosphonium bis(2-ethylhexyl)phosphate [P6,6,6,14]Cl = trihexyl(tetradecyl)phosphonium chloride [P6,6,6,6]+ = tetra-n-hexylphosphonium [P6,6,6,8]+ = tri-n-hexyl-n-octylphosphonium [P8,8,8,8][(EtO)HPO2] = tetra-n-octylphosphonium ethylphosphonate [PMPIP][NTf2] = 1-methyl-1-propylpiperidinium bis{(trifluoromethyl)sulfonyl}imide [PMPyr][CF3SO3] = 1-methyl-1-propylpyrrolidinium triflate [Pyr][BF4] = N-decyloxymethyl-3-amidopyridinium tetrafluoroborat P(C6H4-m-SO3)3 = tris(meta-sulfonatophenyl)phosphine P(St-co-MMA) = poly(styrene-co-Me methacrylate) P[P4,4,4,4][SS] = poly(tetrabutylphosphonium 4-styrenesulfonate P[P4,4,4,6][C3S] = tributylhexylphosphonium 3-sulfopropyl methacrylate P[VBIM][BF4]-b-PS = diblock copolymer poly [1-(4vinylbenzyl)-3-methyl-imidozolium tetrafluoroborate]-b-polystyrene P[VBTP][Cl] = poly(triphenyl-4-vinylbenzylphosphonium chloride) PAA = polymer poly(acrylic acid) PBnMA = poly(benzyl methacrylate) PBzMA = poly(benzyl methacrylate PEG = poly(ethylene glycol) PEGE = poly(ethyl glycidyl ether) PEO = poly(ethylene oxide) PEO−DPPSA = poly(ethylene oxide)-substituted 4(diphenylphosphino)benzenesulfonamide PIL = polymerized IL PNIPAM = poly(N-isopropylacrylamide) poly([P n,n ,n VB]Cl = poly([tri-n-alkyl(vinylbenzyl)phosphonium]chloride) poly(4-VPBn+Br−) = 1-benzyl-4-vinylpyridine bromide poly(SS−P2) = poly[1,8-octanediylbis(tributylphosphonium) 4-styrenesulfonate] POM = polyoxometalates POM−IL = polyoxometalate-based IL PPLG-(IMBr)2 = poly(γ-propyl-L-glutamate) bis(N-butylimidazolium bromide) PVCL = poly(N-vinylcaprolactam) PyPS = sulfo-group grafted ammonium [QPS]PW = [(3-sulfonic acid)propyl-isoquinoline]3PW12O40 RAFT = reversible addition-fragmentation transfer SBS = polystyrene-b-polybutadiene-b-polystyrene SILP = supported IL phase TMS = temperature-dependent multicomponent solvent systems TOS = p-toluenesulfonate (tosylate) anion TRPTC = thermoregulated phase-transfer catalysis TsDPENDS = sulfonated N-(p-tolylsulfonyl)-1,2-diphenylethylene-diamine UCST = upper critical solution temperature v% = volume percentage

[CnVIm]Br = 1-n-dialkyl-3,3′-bis-1-vinylimidazolium bromides [COC2Mor] = 4-(2-methoxyethyl)-4-methylmorpholinium [COC2Pip] = 1-(2-methoxyethyl)-1-methylpiperidinium [COC2Pyr] = 1-(2-methoxyethyl)-1-methylpyrrolidinium [Cxmim][TFSI] = 1-alkyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide DAV = diallyl-viologen DBU−Bu = 8-butyl-1,8-diazabicyclo[5.4.0]undec-7-ene DPP5 = dipropoxypillar[5]arene [DEHP] = bis(2-ethylhexyl) phosphate eNRTL = electrolyte-NRTL EtPyPF = N-ethylpyridinium hexafluorophosphate [EMIM][FAP] = 1-ethyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate FBS = fluorous biphasic systems Fe3O4@OA−Pd = palladium deposited oleic acid coatedFe3O4 nanoparticles HFC-4310mee = threo-2,3-dihydrodecafluoropentane and erythro-2,3-dihydrodecafluoropentane His = histidine HMF = 5-hydroxymethylfurfural HTfO = trifluoromethanesulfonic acid [Hbet][NTf2] = protonated betaine bis(trifluoromethylsulfonyl)imide [Hbetmim][NTf2] = N-carboxymethyl-N-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide [HbetmMor][NTf2] = N-carboxymethyl-N-methylmorpholinium bis(trifluoromethylsulfonyl)imide [HbetmPip][NTf2] = N-carboxymethyl-N-methylpiperidinium bis(trifluoromethylsulfonyl)imide [HbetmPyr][NTf2] = N-carboxymethyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl) imide [HbetPy][NTf 2 ] = N-carboxymethylpyridinium bis(trifluoromethylsulfonyl)imide [HDIm]2[{WO(O2)2}2(μ-O)] = protic N-dodecylimidazolium peroxotungstate [HOOCC1C1IM]Cl = N-carboxymethyl-N-methylimidazolium chloride IL = ionic liquid ILPEG750 = [CH3(OCH2CH2)16N+Et3][CH3SO3−] LA = levulinic acid LCST = lower critical solution temperature MAGs = monoacylglycerides MAP5 = anthracene pillar[5]arene mol % = molar percentage MW = microwave [MC3S] = 3-sulfopropyl methacrylate NHC = N-heterocyclic carbene NIPAM = N-isopropylacrylamide NRTL = nonrandom two-liquid [N1,1,1,12][FeCl4] = dodecyltrimethylammonium tetrachloroferrate [N11n2OH][NTf2] = N-alkyl-N,N-dimethylhydroxyethylammonium bis(trifluoromethane) sulfonylimide [N555][C3S] = N,N,N-tripentyl-3-sulfonyl-1-propaneammonium [N666][C3S] = N,N,N-trihexyl-3-sulfonyl-1-propaneammonium [NTf2]− = bis(trifluoromethylsulfonyl)imide [oleyl-C1 IM][NTf2 ] = 1-methyl-3-(Z-octadec-9-enyl)imidazolium bistriflimide [P4,4,4,4][SS] = tetrabutylphosphonium 4-styrenesulfonate AI


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[VBIM][BF4]−TTC = [1-(4-vinylbenzyl)-3-methylimidozolium tetrafluoroborate] trithiocarbonate wt % = weight percentage 4-= VPBn+Br− = 1-benzyl-4-vinylpyridine bromide

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Switchable Ionic Liquid/Solid Phases. Green Chem. 2012, 14, 3026− 3033. (463) Lozano, P.; Bernal, J. M.; Lajarin, A.; Romera, D.; GarciaVerdugo, E.; Sanchez-Gomez, G.; Pucheault, M.; Vaultier, M.; Burguete, M. I.; Luis, S. V. A Green Approach for Producing Solvent−free Anisyl Acetate by Enzyme catalyzed Direct Esterification in Sponge−like Ionic Liquids Under Conventional and Microwave Heating. Curr. Green Chem. 2014, 1, 145−154. (464) Lozano, P.; Gomez, C.; Nieto, S.; Sanchez-Gomez, G.; GarciaVerdugo, E.; Luis, S. V. Highly Selective Biocatalytic Synthesis of Monoacylglycerides in Sponge−Like Ionic Liquids. Green Chem. 2017, 19, 390−396. (465) Lozano, P.; Bernal, J. M.; Gomez, C.; Garcia-Verdugo, E.; Isabel Burguete, M.; Sanchez, G.; Vaultier, M.; Luis, S. V. Green bioprocesses in sponge−like Ionic Liquids. Catal. Today 2015, 255, 54−59.

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