Acidic Ionic Liquids - Chemical Reviews (ACS Publications)

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Acidic Ionic Liquids Ananda S. Amarasekara* Department of Chemistry, Prairie View A&M University, Prairie View, Texas 77446, United States ABSTRACT: Ionic liquid with acidic properties is an important branch in the wide ionic liquid field and the aim of this article is to cover all aspects of these acidic ionic liquids, especially focusing on the developments in the last four years. The structural diversity and synthesis of acidic ionic liquids are discussed in the introduction sections of this review. In addition, an unambiguous classification system for various types of acidic ionic liquids is presented in the introduction. The physical properties including acidity, thermo-physical properties, ionic conductivity, spectroscopy, and computational studies on acidic ionic liquids are covered in the next sections. The final section provides a comprehensive review on applications of acidic ionic liquids in a wide array of fields including catalysis, CO2 fixation, ionogel, electrolyte, fuel-cell, membrane, biomass processing, biodiesel synthesis, desulfurization of gasoline/diesel, metal processing, and metal electrodeposition.

CONTENTS 1. Introduction 2. Lewis Acidic Ionic Liquids 2.1. Lewis Acidic Ionic Liquids with Electron Accepting Ability in the Anion 2.1.1. Haloaluminate Lewis Acidic Ionic Liquids 2.1.2. Lewis Acidic Ionic Liquids with Zn, Sn, and Other Metallic Anions 2.2. Lewis Acidic Ionic Liquids with Electron Accepting Ability in the Cation 3. Brö nsted Acidic Ionic Liquids 3.1. Protic Acidic Ionic Liquids with Acidic Hydrogens on Cation 3.2. Protic Acidic Ionic Liquids with Acidic Hydrogens on Anion 3.3. Protic Acidic Ionic Liquids with Acidic Hydrogens on Cation and Anion 3.4. Brö nsted Acidic Ionic Liquids with Acidic Hydrogens on a Functional Group 3.5. Brö nsted Acidic Ionic Liquids with Acidic Hydrogens on Functional Groups and on an Anion or Cation 4. Brö nsted-Lewis Acidic Ionic Liquids 5. Polymeric Acidic Ionic Liquids 5.1. Acidic Ionic Liquid Groups Grafted onto an Existing Polymer 5.2. Polymerization of an Acidic Ionic Liquid Monomer 6. Immobilized Acidic Ionic Liquids 6.1. Immobilized on Inorganic Supports 6.2. Immobilized on Organic Supports 7. Physical Properties of Acidic Ionic Liquids 7.1. Solubility, Density, Viscosity, and Related Properties 7.2. Acidity 7.3. Thermo-Physical Properties © 2016 American Chemical Society

7.4. Ionic Conductivity 7.5. Spectroscopy 7.6. Miscellaneous Physical Properties 8. Computational Studies on Acidic Ionic Liquids 8.1. Computational Studies on Acidity and Other Physical Properties 8.2. Computational Studies on Acidic Ionic liquid Catalysis 9. Applications of Acidic Ionic Liquids 9.1. Catalysis Applications 9.1.1. Esterification and Saponification 9.1.2. Alkylation 9.1.3. Acetalization 9.1.4. Heterocyclic Synthesis 9.1.5. Carbonium Ion Generation and Rearrangements 9.1.6. Dehydration 9.1.7. Oxidation 9.1.8. Polymerization and Related Reactions 9.1.9. Miscellaneous Catalysis Applications 9.2. Carbon Dioxide Dissolution and Fixation 9.2.1. Cyclic Carbonate Synthesis 9.2.2. Urea Synthesis 9.3. Ionogels, Electrolyte, Battery, and Capacitor Applications 9.4. Fuel-Cell Applications 9.5. Electrophoresis Applications 9.6. Membrane Applications 9.7. Conversion of Biomass to Renewable Feedstock Chemicals 9.7.1. Renewable Furans, Levulinic Acid, and Related Compounds 9.7.2. Depolymerization of Cellulose 9.7.3. Depolymerization of Lignin

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Received: December 28, 2015 Published: May 13, 2016 6133

DOI: 10.1021/acs.chemrev.5b00763 Chem. Rev. 2016, 116, 6133−6183

Chemical Reviews 9.7.4. Pretreatment of Biomass 9.8. Biodiesel Synthesis 9.9. Desulfurization of Gasoline, Diesel, and Petroleum Refinery Applications 9.10. Metal Extractions and Processing 9.11. Electrodeposition of Metals 9.12. Miscellaneous Applications of Acidic Ionic Liquids 10. Concluding Remarks Author Information Corresponding Author Notes Biography Acknowledgments Abbreviations References

Review

Then, since 2006 there are five review articles published with the word “acidic ionic liquids” in the title.71−75 Of the more recent three reviews in this sub group; the article titled “Acidic Brönsted ionic liquids” with 123 references was published in 2010.73 The second with title “Structural effects on the physicochemical and catalytic properties of acidic ionic liquids: An overview” by Chiappe and Rajamani with 182 references appeared in 2011.74 The most recent review titled “The use of supported acidic ionic liquids in organic synthesis” with 79 references by Skoda-Foldes describes recent applications of acidic ionic liquids supported on organic and inorganic supports for catalysis applications.75 In addition a 2014 review titled “Halometallate ionic liquids-revisited” and a 2015 review titled “Acidic ionic liquids as sustainable approach of cellulose and lignocellulosic biomass conversion without additional catalysts” focused on specialized applications of acidic ionic liquids as well.76,77 The scope of this review is wider than the five previous reviews on this subdiscipline. This comprehensive review on acidic ionic liquids is an attempt to cover all types of acidic ionic liquids, their synthesis properties, and applications. It is important to define the nomenclature system used in this acidic ionic liquid review at the introduction. As the number of ionic liquids and the number of research groups working on ionic liquids have proliferated, especially in the neutral ionic liquid arena, there a number of deferent abbreviations for ionic liquids in the literature. However, the system defined by Hallett and Welton for neutral ionic liquids in their 2011 review article in “Chemical Reviews” is the widely used system today.14 Despite this many articles in this area still use their own abbreviations and naming systems, creating some confusion. Thus, it is about time to adopt a uniform nomenclature for ionic liquids as in other well developed branches such as organometallic complexes. For instance most abbreviations used in the literature derive from the name of the ion; for example: [bmim]+ and [BMIM]+ both refer to the 1butyl-3-methylimidazolium cation. Nevertheless, there are ambiguities in this naming system, such as [pmim]+, which can be used for the 1-propyl-3-methylimidazolium cation as well as for the 1-pentyl-3-methyllimidazolium cation. To avoid this complication, we have chosen to use an alphanumeric system to describe the alkyl chains, with an alphabetic abbreviation for the charged center. In this system the 1butyl-3-methylimidazolium cation becomes [C4C1im]+. If the alkyl chain is not linear, like for instance, the 1-tert-butyl-3methylimidazolium cation, it can be abbreviated as [tC4C1im]+. In the case of functionalized ionic liquids with a functional group in the side chain the type and position of the functional group can be noted as well. In the case of an alcohol group on the terminal carbon of the butyl chain, the abbreviation is [(HO)4C4C1im]+, where the position of the OH group is shown as 4 as a superscript between the group and the attached carbon. Following this system described in the 2011 Hallett and Welton’s review, this review extrapolates the same set of rules to functionalized class “acidic ionic liquids”. For example the most widely described acidic ionic liquid 1-(3-propylsulfonic)3 - m e t h y l im id az o li u m ch lo r id e i s a b br e vi a t e d a s [(HSO3)3C3C1im][Cl]. 1-(4-butylsulfonic)-3-methylimidazolium hydrogensulfate is [(HSO3)4C4C1im][HSO4]. Some common acidic ionic liquids and their nomenclature used in this review are shown in Figure 1. The AILs with one acid function can be classified according to the type of acid function in the molecule. The Lewis acidic ionic liquids (LAILs) display their acidity due to a deficiency in

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1. INTRODUCTION Since the 1999 review titled “Room-Temperature Ionic Liquids: Solvents for Synthesis and Catalysis” by Welton,1 there are about a dozen books2−13 published with ionic liquids (ILs) in the title, and ionic liquids have grown in to a major research area in the last 15 years. A March 30th, 2016 search using CAS SciFinder and Scopus for review articles with the keyword “Ionic liquids” in the title yielded 727 and 694 review papers, respectively. As a number of previous reviews have established the definition of ionic liquid as a salt in the liquid state, this review will also follow the same description. In some contexts, the term has been restricted to salts whose melting point is below some arbitrary temperature, typically 100 °C. Thus, it is important to keep in mind the terms such as roomtemperature ionic liquid, molten salt, liquid organic salt, fused salt, liquid electrolytes, ionic melts, ionic fluids, fused salts, and liquid salts have all been used to describe these salts in the liquid phase. Since we have come a long way from the initial understanding of ionic liquids as nonvolatile solvents for reactions; today the subject area of ionic liquids is diverse and wide, and it is impossible to analyze in a single review or a book. Hence, most recent reviews, book chapters, and books are evaluating a branch, subgroup, or application in the general subject area of ionic liquids. Numerous review articles on neutral ionic liquids can be referred to for information that is outside the scope of this article. These reviews have focused on a particular application of neutral nonfunctionalized or functionalized ionic liquids; for example, chemical catalysis−solvents,14−26 biocatalysis,27−34 chromatography and analysis,35−44 biomass pretreatment and processing45−50 electrochemical applications51−54,12,55−58 or engineering fluids,59−62 and other miscellaneous applications63−65,23,66−70 are covered in those articles. This review is designed to give a comprehensive coverage of a selected class of ionic liquids called acidic ionic liquids (AILs). An acidic ionic liquid can be defined as a low melting ionic salt with acidic characteristics. The acidic character can be Brönsted, Lewis, or a combination of Brönsted and Lewis acid type. Then the acidic function(s) or group(s) can be either in the cation, anion, or both. The AILs are a part of a wider group of functionalized ionic liquids. As described above, it is important to note that the majority of the ionic liquids known and described in the literature are neutral ionic liquids, and this review will concentrate only the synthesis, properties, and applications the acidic subgroup. 6134


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Lewis acidic ionic liquids (LAILs) this electron accepting ability may be located in the anion or cation. However, most of the known LAILs are in the electron accepting ability in the anion group. Some of these ILs are described as a part of a larger set in a 2014 review titled “Halometallate ionic liquids-revisited” as well.76 2.1. Lewis Acidic Ionic Liquids with Electron Accepting Ability in the Anion

A selected set of LAILs with electron accepting ability in the anion is shown in Figure 2; they are generally prepared by reacting a neutral ionic liquid with a Lewis acidic metal halide under anhydrous conditions.

Figure 2. Lewis acidic ionic liquids with electron accepting ability in the anion.

2.1.1. Haloaluminate Lewis Acidic Ionic Liquids. Lewis acidic ionic liquids with aluminum halide anions are a major subgroup in LAILs known for over 60 years.78,79 An early example of haloaluminates can be found in Hurley and Wier’s patents, where they used a mixture of 37 mol percent ethylpyridinium chloride and 63 mol percent aluminum chloride for the electrodeposition of aluminum.80,81 However, the systematic study of haloaluminate Lewis acidic ionic liquids started later when Osteryoung and co-workers rediscovered these salts and published their synthesis and electrochemical applications.82,83 In a follow up work Wilkes et al. reported the synthesis of dialkylimidazolium chloroaluminate Lewis acidic ionic liquids, their electrochemistry and spectroscopy.84 All chloroaluminate ionic liquids are moisture sensitive materials that require handling in dryboxes; however these materials generally exhibit good electrical conductivities and low viscosities. The widely studied chloroaluminate ionic liquids are with common cations of pyridinium, alkyl imidazolium and tetra-alkyl ammonium types. Depending on the molar ratio of aluminum chloride to neutral ionic liquid organic chloride salt [Cat]Cl, several negatively charged species are often present in equilibrium. The formation of these chloroaluminate anions AlCl4¯, Al2Cl7¯, and Al3Cl10¯ are shown in the equations below:

Figure 1. Common acidic ionic liquids.

electrons, whereas Brönsted acidic ionic liquids (BAILs) display their acidity due to ionizable protons. Then there are AILs with more than one Brö nsted or Lewis acid function and combinations of Brönsted and Lewis acid functions as well.

[Cat]Cl + AlCl3 ⇄ [Cat]AlCl4

[Cat]AlCl4 + AlCl3 ⇄ [Cat]Al 2Cl 7 [Cat]Al 2Cl 7 + AlCl3 ⇄ [Cat]Al3Cl10

A number of research groups have studied the synthesis and physical properties such as acidity,85,86 conductivity,87,88 electrodeposition,89 electrochemical reduction,90 density, and viscosity91−93 of these chloroaluminate ionic liquids. 2.1.2. Lewis Acidic Ionic Liquids with Zn, Sn, and Other Metallic Anions. One main disadvantage of chloroaluminate ionic liquids is their moisture sensitivity. The zinc

2. LEWIS ACIDIC IONIC LIQUIDS In the Lewis theory of acid−base reactions, the base donates a pair of electrons and the acid accepts a pair of electrons. For 6135


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based ionic liquids have been developed as a possible solution to this problem as well as a part of a search for lower melting LAILs.94−97 The initial examples of these LAILs were produced using zinc chloride with pyridinium, ethylphenylammonium,98 and imidazolium salts.99−102 However, these LAILS showed higher melting points than the corresponding chloroaluminate melts but are still fluids at ambient temperatures. During the same period it was also shown that FeCl2, FeCl3,103 and stannous salts104 form ionic liquids with 1-butyl-3-methylimidazolium chloride. The continuous search for moisture stable LAILs with wider electrochemical applications led to the development of Zn and Sn based quaternary ammonium type ionic liquids and especially salts such as choline chloride LAILs.94,105−108 The melting behavior of mixtures of various ammonium salts with zinc and tin chlorides is an interesting example; some of the results from Davies and co-workers study are shown in Table 1.94 As revealed in this study; with salts of symmetrical cations,

deposition,109 battery electrolytes, and as catalysts in Diels− Alder and Fisher indole synthesis.95,110 The identification of various metal halide species in the metal based LAILs is an important aspect in their studies and applications. The most widely used techniques are FAB-mass spectra and NMR spectroscopy. For example, three main chlorozincate anions in choline chloride (ChCl)/zinc chloride 1:2 composition ionic liquid have been identified as ZnCl3− (m/z 171), Zn2Cl5− (m/z 307), and Zn3Cl7− (m/z 443) by FAB-mass spectra. In addition higher clusters are also detectable at very low intensities. The formation of these species can be explained in a series of equilibriums as shown in the following equations:

Table 1. Freezing Points for the LAIL Materials Formed from Heating a Quaternary Ammonium Chloride and MCl2 (M = Zn, Sn) in a 1:2 mol Ratio94

In the case of Sn-based ionic liquids, FAB mass spectroscopy showed the presence of SnCl3− (m/z 225) and Sn2Cl5− (m/z 412). For the iron system the only anionic species detected was FeCl4− (m/z 197). Interestingly Sn3Cl7− or Fe2Cl7− clusters could not be detected in Sn and Fe systems; most likely these species are too unstable to be observed using FAB-MS.105 NMR spectroscopy is another technique used in investigating different species present in LAILs.111−115 Santini et al. examined the mixtures of ZnCl2 and 1-butyl-2,3-dimethyl imidazolium chloride [C4(C1)2im][Cl] using NMR and mass spectrometry.116 In this study they found that [C4(C1)2im][Cl] and [C4(C1)2im][ZnCl3] are present at χZnCl2 < 0.5 as well as pure [C 4(C 1 ) 2 im][ZnCl 3] at χZnCl2 = 0.5. Whereas a combination of [C4(C1)2im][ZnCl3] and [C4(C1)2im][Zn3Cl7] was found at χZnCl2 > 0.5.116 Terrade and co-workers have investigated the anionic speciation of chlorostannate(II) LAILs, by 119Sn NMR spectroscopy, X-ray photoelectron spectroscopy, and viscometry.114 In addition, they have examined the crystalline samples using single-crystal X-ray crystallography, Raman spectroscopy, and differential scanning calorimetry. Both liquid and solid systems (crystallized from the melt) contained [SnCl3]− in equilibrium with Cl− in samples at χSnCl2 < 0.50; [SnCl3]− in equilibrium with [Sn2Cl5]− at χSnCl2 > 0.50, and only [SnCl3]− at χSnCl2 = 0.50. In addition, Sn(II)chloride was found to precipitate when χSnCl2 > 0.63. Furthermore, the Lewis acidity of the Sn(II)chloride based systems expressed by their Gutmann acceptor number has been determined as a function of the composition χSnCl2, to reveal that Lewis acidity for χSnCl2 > 0.50 samples are comparable to the analogous systems based on zinc(II). Furthermore, it was possible to estimate the change in the Lewis basicity of the anion using 1H NMR spectroscopy, by comparison of the chemical shifts of the C-2 hydrogen in the imidazolium ring. Finally, compositions containing free chloride anions (χSnCl2 < 0.50) were found to oxidize slowly in air to form a chlorostannate(IV) ionic liquid containing the [SnCl6]2− anion.114 In another example Lungwitz and Spange studied the hydrogen-bond-accepting (HBA) and donating (HBD) parameters of LAILs with halogeno complex anions by 1H NMR spectroscopy.112 The imidazolium cation served both as part of the LAIL and as a 1H NMR probe. The HBA and HBD strengths were calculated in terms of the empirical polarity parameters β and α according to the Kamlet−Taft equation.

cation

M2+

freezing point/°C

H4N Me4N+ Et4N+ Me3N+Et Me3N+CH2CH2OH Me3N+CH2CH2Cl Me3N+CH2CH2OC(O)Me Me3N+CH2CH2OC(O)Ph Me3N+CH2CH2OH Me3N+CH2CH2Cl Me3N+CH2CH2OC(O)Me Me3N+CH2CH2OH

Zn Zn Zn Zn Zn Zn Zn Zn Sn Sn Sn Zn/Sna

>200 >200 90−92 53−55 23−25 23−25 30−32 46−48 43−45 69−71 13−15 21−23

+

a

2ZnCl 2 + Ch+Cl− ⇄ Zn2Cl5− + Ch+

2Zn2Cl5− ⇄ ZnCl3− + Zn3Cl 7−

Zn2Cl5− ⇄ ZnCl 2 + ZnCl3−

The ratio [Me3NCH2CH2OH]Cl:ZnCl2:SnCl2 was 1:1:1.

H4NCl and Me4NCl, no liquid was formed below 200 °C, whereas with the longer ethyl chain in Et4NCl the freezing point was 90 °C. The unsymmetrical ammonium salt Me3NEt+ gave a freezing range of 53−55 °C, which is a reduction of 35 or 140 °C compared with Et4N+ and Me4N+ cations, respectively. In addition, they found that functionalized ethyl chains such as Me3N+C2H4Y give even lower freezing points; for example, if Y = OH or Cl, truly room temperature ionic liquids are observed with freezing points in the 23−25 °C range. Even when the substituent is significantly larger, for instance, Y = OC(O)Me or OC(O)Ph, the salts formed have lower freezing points than for Me3N+Et salt. These results suggest that both lower symmetry and the presence of a functional group may reduce the freezing points of the salts formed, though the exact role of the functional substituent is not yet clear. In all cases the liquids formed are viscous and hygroscopic but moisture-stable, so these materials can be easily prepared and stored without the need for special equipment.105 The Zn and Sn containing choline based LAILs are a particularly attractive group in ammonium salts as they are water and air insensitive. In addition, they are easy to prepare and relatively inexpensive, enabling their use in large-scale applications. The choline based zinc halide ionic liquids have been used in numerous applications including zinc alloy 6136


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the replacement of chlorides with bromides leads to a similar tendency toward ionic diffusion. It would seem that ion association is less pronounced in the FeBr4− salts, which could be rationalized on the basis of a more significant nephelauxetic effect together with the lower molar concentration in the FeBr4 salts.137

The study included 1-butyl-3-methylimidazolium ionic liquid with chloroaluminates of various mole fractions from 0 to 0.67 of AlCl3. The 1H NMR chemical shift of the C-2 proton in the imidazolium cation was found to be dependent on the HBA ability of the corresponding anion. For the chloroaluminates the HBA ability decreased and the HBD ability increased with increasing mole fraction of the Lewis acid. In general, the HBA strength of the LAILs studied increased in the following order: MoCl6− < Al2Cl7− < AlCl4− < WCl7− < I5− < SnCl5− < I3− < SbCl6− < TiCl5− < BBr4− < SnCl3−. The corresponding HBD ability showed a reverse trend.112 The Gutmann acceptor number (AN) is a quantitative measure of Lewis acidity and has been estimated for LAILs by 31 P NMR chemical shifts using triethylphosphine oxide as a probe molecule. In this study chlorometallate ionic liquids based on Group 3 metals (Al(III), Ga(III), and In(III)) with 1octyl-3-methylimidazolium cation were used and the results were compared with a range ANs of standard molecular solvents and acids.111 A few studies reported the X-ray structures of LAILs,114 where Zhou and Sasaki reported structures of several metal co ntainin g I Ls in clud ing [C 4 C 1 im] 2 [ S nCl 4 ] an d [C4C1im]2[ZnCl4].117 During this work they reported an interesting correlation between the symmetry of the metalcontaining anions and the melting points of the investigated ILs, for a series of LAILs with the same cation and molar ratio of anion to cation (1:2),117 where they found that melting points decreased in the order Oh > D4h > Td > D2d > C2v.117 Several iron based LAILs with common cations are known and these ionic liquids are especially useful in catalysis applications. These materials are a particularly attractive group due to moisture stability, magnetic liquid properties,118−125 and the ability to form ionogels.126,127 Some of the noticeable catalysis applications include [C4C1im]Cl1.5FeCl3 in glycosidation of 3,4,6-tri-O-acetyl-d-glucal with alcohols to give glycopyranosides,128 [C4C1im][FeCl4] in Michael addition,129 polystyrene-supported Lewis acidic ironcontaining ionic liquid for converting CO2 into cyclic carbonate,130 [C4C1im][FeCl4] for benzylation of various arenes/heteroarenes into the diarylmethanes,131 and glycolysis.132 In addition [C4C1im][FeCl4] system has been used as a desulfurizing agent133,134 and 1-ethyl-3-methylimidazolium chloride ([C2C1im]Cl) with iron chlorides FeCl2 and FeCl3, for rechargeable batteries135 as well. Xia and Taubert have reported another interesting property of FeCl4− anion containing LAILs.136 In this instance, 1-butyl-3-methylimidazolium tetrachloroferrate(III) [C4C1im][FeCl4] and 1-dodecyl3-methylimidazolium tetrachloroferrate(III) [C12C1im][FeCl4] [FeCl4] exhibited a thermally induced demixing with water (thermomorphism). Furthermore, they noticed that phase separation temperature varies with LAILs weight fraction in water and could be tuned between 100 °C and room temperature.136 Yoshida and Saito reported the influence of structural variations such as changing the alkyl chain (R) length in the cation and substituting the halides (X) in the anion on their thermal behavior, IR, VU−vis spectra, density, viscosity, ionic conductivity, and magnetic properties for a series of paramagnetic ionic liquids.137 These LAILs were comprised of 1alkyl-3-methylimidazolium cation (R = Et, nBu, nhexyl and n octyl) and tetrahalogenoferrate(III) FeX4− anion (X = Cl and Br). They found that the elongation of the alkyl chain leads to a pronounced reduction of fluidity and ionic conductivity, and

2.2. Lewis Acidic Ionic Liquids with Electron Accepting Ability in the Cation

Lewis acidic ionic liquids with electron accepting ability in the cation is not common in LAILs and a preparation of a boron containing example is shown in Figure 3.138

Figure 3. Synthesis of LAILs with electron accepting ability in the cation.

3. BRÖ NSTED ACIDIC IONIC LIQUIDS The Brönsted acidic property of a substance is based on the Brönsted−Lowry theory proposed independently by Johannes Nicolaus Brönsted and Thomas Martin Lowry in 1923. According to this theory Brönsted acid is a substance that donates a hydrogen ion (H+) or proton. Brönsted base is a substance that accepts a hydrogen ion (H+) or proton. Therefore, Brönsted acidic ionic liquid (BAIL) can be defined as an ionic liquid substance that can donate a hydrogen ion (H+) or proton. Those donating or easily releasable proton(s) in the BAIL can reside in a number of locations in the structure and can be further classified based on the location of the acidic proton(s). The most common locations you can find the acidic proton(s) are nitrogen or oxygen atoms, in the anion, and in acidic functional groups (e.g., SO3H, CO2H) attached to the cation. In addition to this BAILs can be classified based on the synthesis method as well. The acidic ionic liquids are prepared by reacting a Brönsted base and a Brönsted acid. The BAILs with one or more acidic hydrogen(s) residing on N or O atoms are known as protic ionic liquids (PILs) as well. In this review protic ionic liquids with H+ in cations and anions will be referred to as a part of the larger collection of Brönsted acidic ionic liquids. Then the third group is acid group functionalized ILs and in these compounds the acid group is generally tethered to the cation. Therefore, BAILs can be further classified into five different sub groups as shown below.

3.1. Protic Acidic Ionic Liquids with Acidic Hydrogens on Cation

The Brönsted acidic ionic liquids with acidic hydrogens residing on the cationic side can be considered as a major group; this sub group is commonly refered to as “protic ionic liquids” as well. However, in this review the classification is further refined as protic acidic ionic liquids with acidic hydrogens on cation and anion as two separate groups due to rapid growth in the field. Greaves and Grummond have reviewed the literature on 6137


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properties and applications of protic ionic liquids until 2008.139 This type of ionic liquid is formed by stoichiometric reactions between Brönsted acids and Brönsted bases. There are many combinations of Brönsted acids and bases that can give stable salts, but to obtain a true IL, it is necessary to have complete (conventionally ∼99.9%) transfer of the proton from the acid to the base. Generally, the aqueous pKa values of the precursor acids and bases are regarded as predictive of the behavior of the formed protic ILs; more accurately, ΔpKa values greater than 8−10 [ΔpKa = pKa(base) − pKa(acid)] are reported to produce protic ILs of ideal ionicity.140 As far as we are aware, the first ionic liquid was discovered by Gabriel in 1888, and this compound, ethanolammonium nitrate, is also a protic acidic ionic liquid with acidic hydrogens on the cation.141,142 As mentioned earlier this type of BAIL is synthesized by reacting equimolar amounts of Brönsted acids and Brönsted bases as shown in Figure 4.

plot.154−162 The Walden plot that makes allowance for differences in ion sizes is shown to be an improvement to this early approach, and in some cases it is feasible to directly quantify ionicity via the Nernst−Einstein equation, confirming the validity of the adjusted Walden plot approach as well.163 NMR spectroscopy is another tool used in the study of proton transfer in protic ionic liquids.164,150,153,165 In imidazolium PILs the chemical shift of the protons attached to the nitrogen atom of imidazolium cation and imide anions were used by Watanabe’s group.153 In another example, Burrell and co-workers studied 15N NMR spectra of a series of PAILs resulting from eight amine bases and six Brönsted acids.164 In this study it was possible to distinguish between neutral and ionized amine bases (ammonia vs ammonium-type ion), which indicated that the protic ionic liquids were completely ionized when made as a stoichiometric mixture. However, a Walden plot comparison of fluidity and molar conductivity indicated that the majority of PAILs had a much lower conductivity than predicted by viscosity, unless the base contained excess protondonating groups. This disparity is indicative of protic ionic molecules forming neutral aggregates or non-Newtonian fluid hydrogen-bonded networks with a secondary Grotthuss proton-hopping mechanism arising from polyprotic bases.164 In a recent development Moreira et al. reported the preparation of 28 PAILs from active pharmaceutical ingredients and their quantitative determinations of the degree of ionicity by 1H NMR in DMSO-d6 solutions.166

Figure 4. Synthesis of protic acidic ionic liquids with acidic hydrogens on cation by proton transfer from Brönsted acid to Brönsted base.

3.2. Protic Acidic Ionic Liquids with Acidic Hydrogens on Anion

Some selected examples of protic acidic ionic liquids with acidic hydrogens on cation are triethylammonium bromide,143 bis(tetrafluoromethylsulfonyl)amide,144 pyridinium bromide,143 1-methylimidazolium chloride, bromide, iodide,143 BF4−, CF3SO3−,145 1-butylimidazolium chloride,143 2-n‑butyl-1,1,3,3tetramethylguanidinium acetate,146 pyrolidonium, and lactam based BAILs.147 Some of these Brönsted acidic ionic liquids are shown in Figure 5. The tetramethylguanidinium compound is a particularly interesting Brönsted acidic ionic liquid because it act as a duel acid−base; H on N-2 acts as the Brönsted acid site, whereas the N1 and N3 are tertiary amine groups with basic properties. In an ideal protic acidic ionic liquid synthesis, the Brönsted acid and base react completely to produce the PAIL. However, in practice the proton transfer may be less than complete, resulting in the neutral acid and base species being present. In addition, aggregation or association of either ions or neutral species can also occur. Therefore, there are attempts to measure the ionic character or ionicity of the PAILs to evaluate the proton transfer in the final product.148−153 One of the widely used methods to measure ionicity for the characterization of ionic liquids is the log(equivalent conductivity) versus log(fluidity) plot known as the Walden

When the donating proton is located in the anion fragment of the acidic ionic liquid, these anions are generally derived from a polybasic acid. The common acids used in PAILs are dibasic sulfuric acid (H2SO4) and tribasic phosphoric acid (H3PO4); replacement of all the hydrogens by ionic liquid cations forms neutral ionic liquids if there are no acidic hydrogens on the cation. However, if all the hydrogens are not replaced in the anions, PAILs with proton donor anions are formed. For example 1-butyl-3-methylimidazolium hydrogensulfate can be prepared by reacting 1-butyl-3-methylimidazolium chloride with one equivalent of sulfuric acid and removal of volatile HCl to drive the reaction to produce the protic acidic ionic liquid as shown in Figure 6. In addition 1-butyl-3-methyl-

Figure 6. Synthesis of Brönsted acidic ionic liquids with proton donor anions.

Figure 5. Selected examples of protic ionic liquids with H+ on cation. 6138


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Figure 7. Protic acidic ionic liquids with acidic hydrogens on anion.

imidazolium hydrogensulfate ionic liquid has been prepared by using sodium bisulfate in place of concentrated sulfuric acid as well under microwave irradiation conditions with significantly reduced reaction times.167 Some selected examples for protic acidic ionic liquids with acidic hydrogens on the anion are dialkylimidazolium hydrogensulfate,168−171 dialkylimidazolium dihydrogen phosphate,170 and maleic and fumaric acid anions derived PAILs. A selected sample of this type of Brönsted acidic ionic liquids is shown in Figure 7.

tethers are easily prepared using propyl or butylsultones as shown in the first reaction in Figure 9. In the first step alkyl imidazole reacts with sultone to give a zwitterionic salt, which can be treated with a Brönsted acid like HCl or HBr to give the desired BAIL.175,176,173 Another method that has attracted recent attention is the sulfonation of an aromatic system in the neutral ionic liquid as shown in the second reaction in Figure 9.177 The carboxylic acid functionalized acidic ionic liquids are generally synthesized via the corresponding carboxylic acid esters.178 A wide variation in ionic core structures and multiple acidic group containing BAILs are known, and some common examples are shown in Figure 10.

3.3. Protic Acidic Ionic Liquids with Acidic Hydrogens on Cation and Anion

There are protic ionic liquids with H+ on the cation as well as on the anion such as diethylmethylammonium hydrogensulfate [(C2)2C1HN][HSO4] described by Watanabe and co-workers for prospective use as a fuel cell electrolyte.172 These are easier to prepare by partial neutralization of a polybasic acid with an organic base and some common examples are shown in Figure 8.

3.5. Brö nsted Acidic Ionic Liquids with Acidic Hydrogens on Functional Groups and on an Anion or Cation

The fifth type of BAILs can have two or more H+ on the acidic functional groups and on the anion or cation. There are a number of routes to synthesize this type of BAILs with multiple acidic sites as shown in Figure 11. The aryl sulfonic acid ionic liquid is prepared by direct sulfonation of the aromatic ring in N-benzyl methylimidazolium chloride.177 In the second reaction i‑butylamine is converted to BAILs in two steps using sultones.179 Meng and co-workers has extended this technique to produce a series of BAILs with multiple H+ on −SO3H and nitrogens.179 The third reaction is the synthesis of a caffeine based Brönsted acidic ionic liquid with a −SO3H group and an acidic anion.180 The anion exchange with polybasic acids such as H2SO4 and H3PO4 is also a valuable technique in introducing acidic anions in these BAILs.181,182 A selected sample of Brönsted acidic ionic liquids with H+ on acidic functional groups and on the anion or cation is shown in Figure 12. Butylsulfonic-1,1,3,3-tetramethylguanidinium triflate is an example of a BAIL with an acidic functional group and H+ on the cation.183 In addition acidic ionic liquids with several acidic H+ on multiple locations are also known. For example, N,N′,N″,N‴-tetrapropyl sulfonic hexamethylenetetramine tetrahydrosulfate is a BAIL with eight acidic hydrogens.184 The acidic ionic liquid (4-butylsulfonic)tris(4-phenylsulfonic)phosphonium hydrogensulfate is another example. This highly decorated molecule was described as a catalyst by Shaterian and co-workers and is an example of a BAIL with three different types of acidic protons.185

Figure 8. Protic ionic liquids with acidic hydrogens on cation and anion.

3.4. Brönsted Acidic Ionic Liquids with Acidic Hydrogens on a Functional Group

The fourth group of Brönsted acidic ionic liquids is the AILs with an acidic functional group tethered to the ionic core structure. This group can be seen as a type of functionalized ionic liquid as well. Forbes, Davis, and others first described the preparation of this type of strongly acidic BAILS with a −SO3H group tethered to an ionic group in 2002.173 Nevertheless, The synthetic approach used to assemble the zwitterionic precursors to these acidic IL was reported by Yoshizawa et al. 2001.174 They synthesized a series of imidazolium cations containing covalently bound anionic sites, such as sulfonate or sulfonamide groups. These zwitterionic imidazolium salts were found to form molten salts just like ordinary imidazolium salts. The acid group can be attached to the ionic core directly or via a carbon chain or a ring system. There are several methods of making this type of BAILs. One widely used approach is the sultone method; by which −SO3H functionalized ILs with C3 and C4

Figure 9. Two methods of preparation of −SO3H functionalized BAILs. 6139


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Figure 10. Brönsted acidic ionic liquids with acidic hydrogen containing functional groups attached to the cation.

[(HSO3)3C3C1im][(1/2.Zn)SO4],188 [(HSO3)4C4C1im][(1/ 2.Fe)SO4],189 [(HSO3)3C3C1im][ZnCl3],190,191 [(HSO3)3C3(C2)3N][FeCl4],192 [(C2)3N][AlCl4],192,193 and some of the structures are shown in Figure 13.

Figure 13. Brönsted−Lewis acidic ionic liquids. Figure 11. Synthesis of Brönsted acidic ionic liquids with acidic hydrogens on functional group as well as on cation/anion.177

Al, Zn, and Fe are the common Lewis acids used in Brö nsted−Lewis acidic ionic liquids.194 However, Cu(1) containing ionic liquids were also reported recently,195 where a series of Brönsted−Lewis acidic Ionic liquids were prepared by changing the Cu to BAIL mole ratio. In this method [(HSO3)3C3C1im][HSO4] and CuO were mixed in distilled water in the desired ratio, the solution was then stirred continuously for about 2 h at 308 K and dried in vacuum to produce a series of ILs with formulas such as [(HSO3)3C3C1im][(1/2Cu2+)SO42−] and [(HSO3)3C3C1im][(1/2H+.1/4Cu2+)SO42−].195 These task specific Brönsted− Lewis acidic ionic liquids are known in various applications including catalysis196−198 and desulfurization of diesel fuel.187

4. BRÖ NSTED-LEWIS ACIDIC IONIC LIQUIDS The group of acidic ionic liquids with both Brönsted and Lewis acidic characteristics in the same molecule is known as duelfunctionalized ionic liquids as well and is a relatively smaller class. This type of AILs is particularly useful as catalysts as some catalytic processes require both types of acidities in one or more steps of the reaction. Synthesis and applications of Brönsted-Lewis acidic ionic liquids have been reviewed in a short review of 37 references in 2012.186 Examples for Brönsted−Lewis acidic ionic liquids of Nmethylpyrrolidonium zinc chloride [C1HPyrr][ZnCl3],187

Figure 12. Brönsted acidic ionic liquids with acidic hydrogens on functional groups as well as on cation/anion. 6140


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Spectroscopic methods can provide the Lewis acidity in Brönsted−Lewis acidic ionic liquids. In one example Li and coworkers proved the Lewis acidity of [(HSO3)4C4C1im][Fe(SO4)3.HSO4] by using IR spectroscopy.189 They used the weak Lewis base acetonitrile as the probe, and the changes in acetonitrile IR peaks at 1000−1600 and 2200−2400 cm−1 were utilized to measure the Lewis acidity of the Brönsted−Lewis AIL.189

Figure 15. Synthesis of a polymeric acidic ionic liquid with acidic ionic liquid as pendent groups.211

5. POLYMERIC ACIDIC IONIC LIQUIDS Polymeric acidic ionic liquids are an emerging subgroup in AILs, which can combine all three: acidic, ionic liquid and polymer properties to a single material. The swift progress and explosive growth in polymeric ionic liquids area is reflected by the large body of recent publications and some recent reviews199−205 and particularly the imidazolium-based polymeric acidic ionic liquids are discussed in two recent dedicated reviews206,207 justifying the importance of these polymers. Polymeric acidic ionic liquids with ionic liquid and acid groups as pendant group in a polymer chain are the most common type. There are two basic synthetic approaches for assembling this type of materials. These two methods are grafting the acidic ionic liquid groups on to an existing polymer and polymerization of an ionic liquid or AIL precursor.

conductive films.223 In this approach ionic liquid crystals with hexagonal and lamellar phases were successfully fabricated by self-assembly based on intermolecular electrostatic interactions. The polymerizable amphiphilic zwitterions are formed by the reaction of 3-(1-vinyl-3-imidazolio)propanesulfonate (VIPS) with 4-dodecyl benzenesulfonic acid (DBSA).223

6. IMMOBILIZED ACIDIC IONIC LIQUIDS Immobilization of acidic ionic liquids can be carried out on soluble or insoluble supports. However, the most common examples are in the area of using insoluble supports to produce heterogeneous acid catalyst systems. In many cases these heterogeneous catalysts showed enhanced size and positional selectivity compared to homogeneous catalysts as the solid support imposes a specially designed environment around the catalytic site.75 In addition, the immobilization may prevent molecular aggregation or bimolecular self-destruction reactions, which leads to deactivation of the catalytic sites. The most important advantage is the ease of separation of the supported acidic ionic liquid from reactants and products for reuse. The commonly used supports are silica, alumina, zeolites and polystyrene type polymers. A 2014 review titled “The use of supported acidic ionic liquids in organic synthesis” with 79 references summarizes the recent progress in both inorganic and organic supported AILs in catalysis applications.75

5.1. Acidic Ionic Liquid Groups Grafted onto an Existing Polymer

A handful of examples of this type is known, which includes; immobilization of imidazolium - HSO4 on to a polymeric support based on calix[4]resorcinarene,208 carboxyl functionalized dication imidazolium-based ionic liquids209 and immobilization of pyridinium propanesulfonic acid catalyst on polychlorostyrene spheres.210 5.2. Polymerization of an Acidic Ionic Liquid Monomer

Polymerization of an acidic ionic liquid monomer or a precursor is a more widely investigated approach. The general examples include: imidazolium −SO3H heteropolyanion,211−215 poly(4-vinylpyridine),216−219 copolymerization of acidic ionic liquid oligomers and divinylbenzene (DVB),220 polymerization of 2-acrylamido-2-methyl-1-propanesulfonic acid salt of butylamine,221 and resorcinol-formaldehyde222 based polymeric acidic ionic liquids. Some of the common examples are shown in Figure 14.

6.1. Immobilized on Inorganic Supports

Silica is the most widely used, convenient, and acid stable solid support for BAILs, although there are a few examples in use of zeolites and metal organic frameworks as well.224 There are three basic techniques of immobilization, which involve grafting of catalyst on to a silica surface, synthesis of silica material by sol−gel formation, and adsorption on the solid surface. In 2010, Amarasekara and Owereh reported the grafting of the imidazolium propylsulfonic acid BAIL moiety onto a silica surface in two steps by using chloropropyl silica as shown in Figure 16.225 In addition this silica supported acid catalyst was shown to be effective in the hydrolysis of cellulose (DP ≈ 450) dissolved in 1-n-butyl-3-methylimidazolium chloride at 70 °C, producing glucose and total reducing sugars in 26 and 67% yields, respectively.225 Grafting the BAIL on to a silica surface using a longer carbon chain allows easier access to the acidic function. Yokoyama et al. grafted the BAIL group on to 3-mercaptopropyltrimethoxysilane modified silica gel by a free radical coupling reaction and this catalyst was used for the esterification and nitration of aromatic compounds.226 Similar BAIL catalysts have been used in the esterification of oleic acid,227 synthesis of bis(indolyl)methanes,228 conjugate addition of indoles,229 and thioacetalization of carbonyl compounds in water.230 Another common technique of immobilization of a catalyst on silica is sol−gel synthesis by hydrolysis of a mixture of silicates like tetraethyl orthosilicate (TEOS), and a silicate with covalently linked catalytic function. This method has been

Figure 14. Some common polymeric acidic ionic liquids prepared by polymerization of an acidic ionic liquid monomer.

In one example, Leng and co-workers reported the synthesis of a −SO3H functionalized heteropolyanion containing polymeric acidic ionic liquid by polymerization of the zwitterionic monomer as shown in Figure 15.211 In addition they used the new polymer as a heterogeneous catalyst for an esterification reaction.211 The photopolymerization of acidic ionic liquid components has been used in the fabrication of nanostructured proton6141


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Figure 16. Synthesis of sulfonic acid functionalized acidic ionic liquid modified silica catalysts by grafting method.

ILs were calculated by using the van’t Hoff equation. This study provides some useful information for further research and applications of AILs.303 In 2014 the physical propertiesdensity, viscosity, molecular volume, standard entropy, thermal expansion coefficient, and crystal energyof four common Brönsted acidic ILs, 1-(3propylsulfonic)-3-methylimidazolium hydrogensulfate [(HSO3)3C3C1im][HSO4], 1-(4-butylsulfonic)-3-methylimidazolium hydrogensulfate [(HSO3)4C4C1im][HSO4], 1-(3-propylsulfonic)-3-butylimidazolium hydrogensulfate [(HSO3)3C3C4im][HSO4], and 1-(4-butylsulfonic)-3-butylimidazolium hydrogensulfate [(HSO3)4C4C4im][HSO4], were reported by Muhammad and co-workers.304 The density and viscosity of these ILs are shown in Table 4, whereas molecular volume Vm, standard entropy S0, thermal expansion coefficient (αp) and crystal energy UPOT are shown in Table 5. Standard entropy S0 was calculated using the equation

successfully applied for the immobilization of acidic ionic liquids. For example Miao and co-workers immobilized 1-(3propylsulfonic) imidazolium hydrogensulfate on silica−gel using tetraethyl orthosilicate as the primary silica source as shown in Figure 17.231 Selected examples of immobilizations of AILs on inorganic supports and catalytic applications of the immobilized AILs are shown in Table 2.

Figure 17. Synthesis of sulfonic acid functionalized acidic ionic liquid modified silica catalyst by sol−gel synthesis method.231

Immobilization by adsorption or entrapment in an inorganic support is another approach of immobilization of a catalyst, which has been applied to AILs as well.281 Marr and co-workers prepared a catalyst with (3-propylsulfonic)-trimethylammonium type ionic liquid [(HSO3)3C3(C2)3N] ][NTf2] entrapped within silica gel. This silica gel containing functionalized acidic ionic liquid was used as a recyclable liquid phase catalyst for the dehydration of rac-1-phenyl ethanol. Furthermore, hot filtration tests showed that the activity was within the gel.281

S 0 = 1246.5(Vm) + 29.5

where Vm is the molecular volume in nm3 and S0 is the standard entropy in J K−1 mol−1. Lattice energy (UPOT) is reflected by the strength of the interactions between its ions. This was calculated using the following equation based on the Glasser theory.305 UPOT = 1981.2(ρ /M) + 103.8

6.2. Immobilized on Organic Supports

The thermal expansion coefficient (αp) values were calculated using the equation304 αp = −A1/(A0 + A1T) where A0 and A1 are the fitting parameters that have been determined from the plot between density and temperature, while T is the absolute temperature. The thermal expansion coefficient values reflected the free volume of ionic liquid; that is, higher thermal coefficient values correspond to higher free volume of ionic liquid, similar to other types of imidazolium-based ILs.306,307 It is interesting to note that relatively low values of thermal expansion coefficients were obtained for these ILs, and this could be due to more compact spatial molecular structures of the ILs. Viscosity and particularly the temperature dependence of the viscosity or rheology properties of protic ionic liquids is an area of particular interest as these measurements provides information regarding H-bonding network in the PAILs.308−311 Atkin and co-workers studied the rheological properties of a series of protic ionic liquids, ethylammonium nitrate [C 2 (H) 2 N][NO 3 ], propylammonium nitrate [C3(H)2N][NO3], ethanol ammonium nitrate [HO2C2(H)2N][NO3], ethylammonium formate [C2(H)2N][HCO2], and dimethylethylammonium formate [C2(C1) 2H3N][HCO2]. They interpreted the viscosity data by considering the effects of both the H-bond network and the solvophobic nanostructure of the liquids.309 At 20 °C, [C2(H)2N][NO3] has the highest zero shear viscosity of 156.1 mPa s, followed by [C3(H)2N][NO3]. They found that primary ammonium ILs behave as Newtonian fluids at low shear rates but shear thin at

Acidic ionic liquid immobilized on organic polymeric structures are also known. Polystyrene immobilized acidic ionic liquids are generally prepared by copolymerization of a mixture of a BAIL attached vinylbenzene and divinylbenzene as shown in Figure 18.282 The polystyrene based systems are the most widely used organic supports, in addition poly(vinylpyridine)216 and melamine resins283 have also been used as solid supports for AILs. The recent examples of immobilization on organic supports are listed in Table 3.

7. PHYSICAL PROPERTIES OF ACIDIC IONIC LIQUIDS 7.1. Solubility, Density, Viscosity, and Related Properties

Interest in physical properties of AILs has seen a rapid growth as the applications have widened in the last 5−6 years. Wang and co-workers studied the solubilities of benzothiazolium acidic ionic liquids in common alcohols (methanol, ethanol, 1propanol, 2-propanol, 1-butanol, and 2-methyl-1-propanol) at temperatures 253−384 K using a static equilibrium method under atmospheric pressure.303 The modified Apelblat equation and λh equation were employed to correlate the experimental data with good agreement. It was interesting to find that the solubilities of some ILs in alcohols were with “temperaturesensitive” properties. The solubility is related to the polarity and molecular structures of the solvent, as well as the strength of hydrogen bonding between alcohols and anionic groups of ILs. During these studies the dissolution enthalpy and entropy of 6142


1-benzyl-3-methylimidazolium hydrogensulfate propylsulfonic acid imidazolium propylsulfonic acid imidazolium hydrogensulfate methylimidazolium hydrogen sulfate 3-propylsulfonic methylimidazolium hydrogensulfate 1-methyl-3-propyl-imidazolium hydrogensulfate. propylsulfonic acid imidazolium hydrogensulfate N-propyl-2-pyrrolidonium hydrogen sulfate 1-methyl-3-propyl-imidazolium hydrogen sulfate propylsulfonic acid imidazolium hydrogen sulfate 1-methyl-3-propyl-imidazolium hydrogen sulfate 1-butylsulfonic imidazolium trifluoromethanesulfonate propylsulfonic acid imidazolium hydrogen sulfate 1-(4-butylsulfonic)-3-propylimidazolium hydrogen sulfate 1-(4-butylsulfonic)-3-propylimidazolium hydrogen sulfate 1-(4-butylsulfonic)-3-vinylimidazolium hydrogen sulfate - Pt 1-(4-butylsulfonic)-3-methylimidazolium hydrogen sulfate 1-(3-propylsulfonic)-3-vinylimidazolium hydrogen sulfate 1-(4-butylsulfonic)-3-allylimidazolium trifluoromethanesulfonate 1-butylsulfonic-3-methylimidazolium aluminum chloride 1-(3-propylsulfonic)-3-vinylimidazolium hydrogen sulfate 1-butyl-3-methylimidazolium aluminum chloride 1-butyl-3-methylimidazolium iron(III) chloride triethylammonium aluminum chloride tetraalkylammonium or pyridinium chloride tin(IV) chloride 1-propyl-3-methylimidazolium aluminum chloride butylsulfonic pyridinium hydrogen sulfate 1-(4-butylsulfonic)-3-methylimidazolium hydrogen sulfate and triflate 1-(3-propylsulfonic)-3-methylimidazolium hydrogen sulfate 1-(3-[ropylsulfonic)-3-methylimidazolium hydrogen sulfate

acidic ionic liquid

243 244 245,246 247

Knoevenagel−Michael synthesis of tetrahydrochromenes and hexahydroquinoline carboxylates esterification of oleic acid with straight-chain alcohols benzaldehyde ethane diol acetal and n-butyl acetate synthesis esterification and acetalization synthesis of amidoalkyl naphthols

nanoporous silica SBA-15 cobalt ferrite nanoparticles embedded in silica silica tetraethoxysilane (TEOS) as silica source silica silica

6143

Friedel−Crafts acylation reaction of epoxypropane with POCl3 synthesis of 3-methyl-3-buten-1-ol by Prins reaction

silica and MCM-41 5 Å molecular sieves silica

magnetic Fe3O4 nanoparticles with a silica shell

synthesis of pyrano[3,2-b]indole derivatives, 2,9-dihydro-9-methyl-2-oxo-4-aryl-1H-pyrido[2,3-b]indole-3-carbonitrile compounds and 5-amino-7-aryl-6-cyano-4H -pyrano[3,2-b ]pyrroles biginelli synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones

trimerization of isobutene

silica

silica

Baeyer−Villiger oxidation

synthesis of 2-aryl-2,3-dihydroquinazolin-4(1H)-ones oligomerization of isobutene

Friedel−Crafts alkylation of benzene, cumene, toluene, naphthalene, with alkenes

impregnation on silica and MCM-41 silica with meso and macropores

silica silica

257

dehydration of fructose to HMF

silica

benzylation of aromatic substrates

253−256

esterification of acetic acid with n-butyl alcohol

silica

silica and MCM-41

251

esterification of hexanoic acid with ethanol

silica

273

267−272

263,264 265,266

262

259 260 261

258

252

250

hydrogenation of nitrobenzene to p-aminophenol

silica

249

248

238 239 240 241 242

preparation of 1-(benzothiazolylamino)phenylmethyl-2-naphthols esterification of oleic acid with short-chain alcohols and trans-esterification of soybean oil. diazotization-iodination of aromatic amines N-formylation of amines using formic acid benzaldehyde ethylene glycol acetal synthesis and rape seed oil biodiesel synthesis

refs 232 233,234 235 236 237

amorphous silica surface of Fe3O4@SiO2 nanosized silica-coated magnetite rice husk ash amorphous silica silica

application esterification of fatty acids hydrolysis of cellulose in water, N-Boc protection of amines synthesis of indazolophthalazine-triones and bis-indazolophthalazine-triones esterification adsorption of Cr6+ and Cr3+

silica silica nanosilica silica silica

support

Table 2. Selected Examples of Immobilizations of AILs on Inorganic Supports and Their Catalytic Applications

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276 244

277

278

279,280

acetal formation of benzaldehyde with ethylene glycol esterification of oleic acid with straight-chain alcohols

ketalization of cyclohexanone with glycol, 1,2-propylene glycol and 1,3-butylene glycol

absorption of ethane and ethylene.

synthesis of 1H-pyrazol-5-ol derivatives and N-formylation of amines

Fe3O4 nanoparticles cobalt ferrite nanoparticles embedded in silica MOR zeolite

MCM-22

nanoporous Na+-montmorillonite clay

application

refs

synthesis of benzoxanthenes support

silica-coated Fe3O4 nanoparticles

acidic ionic liquid

Table 2. continued

Review

1-(3-propylsulfonic)-3-methylimidazolium hydrogen sulfate tolylsulfonic phosphonium chloride 1-(4-butylsulfonic)-3-allylimidazolium trifluoromethanesulfonate 1-(4-butylsulfonic)-3-methylimidazolium hydrogen sulfate 1-(4-butylsulfonic)-3-methylimidazolium hydrogen sulfate 1-propyl-3-methylimidazolium hydrogen sulfate

274,275

Chemical Reviews

high shear. In addition they fit the Vogel−Fulcher−Tammann model, which revealed that nanostructure is not affected appreciably by temperature and that all the ILs studied is of intermediate fragility. The rheology of binary mixtures of these ILs were also analyzed and used to demonstrate fundamental differences in the interactions of cations and anions.309 Broadband dielectric spectroscopy and pressure−temperature−volume methods are other methods used in physicochemical studies on acidic ionic liquids.311 These techniques were employed to investigate the effect of hydrostatic pressure on the conductivity relaxation time (τσ), both in the supercooled and glassy states of protic ionic liquid lidocaine hydrochloride monohydrate. Due to the decoupling between the ion conductivity and structural dynamics, a change in behavior of τσ(T) dependence (from Vogel−Fulcher−Tammann like to Arrhenius like behavior) was observed for lidocaine HCl. This crossover is a manifestation of the liquidglass transition of lidocaine HCl. Additionally, Swiety-Pospiech et al. analyzed the changes of conductivity relaxation times of the AIL sample and found that compression enhances the decoupling of electrical conductivity from the structural relaxation of these protic ionic liquids.311 7.2. Acidity

Many research groups have studied acidity properties as this character is critical in many applications.312 The acid dissociation constant measurement is a one key experiment. This includes the studies on effects of alkyl chain lengths, solvents on thermodynamic dissociation constants.313,178 The acidity of BAILs can be measured using UV−visible spectrophotometry by means of a basic indicator, such as 4nitroaniline in water. Hammett acidity function H0 can be calculated using the following equation. The [I]/[IH+] (I represents indicator) ratio can be determined from the absorbance measured before and after the addition of BAILs. ⎡ (I) ⎤ H0 = pK (I)aq + log⎢ ⎥ ⎣ (IH+) ⎦

In one illustration Xing and co-workers have investigated the Brönsted acidities of sulfonic acid-functionalized BAILs with pyridinium cations.314 For these BAILs the Hammett function values are between −1.5 and −3.6 at 110 °C as shown in Table 6. The absorption spectra of 0.025 mg/mL 2,4-dichloro-6nitroaniline (A) and [(HSO3)3C3pyr][BF4] were used in the calculation of H0 as shown in Figure 19. Additionally they found that the minimum-energy geometries of sulfonic acidfunctionalized BAILs manifest in anions that have strong interactions with the sulfonic acid proton. It is accepted that in addition to the alkyl sulfonic acid group, the anion is likely to serve as an available acid site. Hence the acidities and catalytic activities of −SO3H functionalized ILs depend on the kinds of anions as well.314 Kang and co-workers have studied Brönsted−Lewis acidic ionic liquids [(HSO3)4C4C1im][Cl-.xZnCl2] with different molar fractions of zinc chloride using Hammett method combined with UV−visible spectroscopy. In this work the catalytic performance of [(HSO3)4C4C1im][Cl-.xZnCl2] for the synthesis of methylene diphenyl dimethyl carbamate (MDC) from methyl N-phenyl carbamate (MPC) and formaldehyde (HCHO) was corelated to the ZnCl2 content. The results showed that [(HSO3)4C4C1im][Cl.0.7ZnCl2] had the highest catalytic activity.191 In another study the ionicity of −SO3H functionalized ionic liquids [(HSO3)3C3iC4(H)2N][HSO4] and 6144


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Figure 18. Synthesis of a polystyrene immobilized Brönsted acidic ionic liquid catalyst.

Table 3. Selected Examples of Immobilizations of AILs on Organic Supports and Their Catalytic Applications acidic ionic liquid

support/technique

application

methylimidazolium iron(III)tetrachloride

polystyrene

carboxyl-isobutyl-imidazolium bromide 1-butyl-4-vinylpyridinium chloroaluminate, 1-butyl-3vinylimidazolium chloroaluminate, and 1-ethyl-3vinylimidazolium chloroaluminate 1-butyl-3-vinylimidazolium chlorogallate

polystyrene polymerization to branched polyethylene

1-butyl-3-vinylimidazolium bromozincate and 1-ethyl-3vinylimidazolium bromozincate 1-N-ferrocenyl methylimidazolium hydrogensulfate

polymerization

1-(3-propylsulfonic) imidazolium hydrogensulfate (4-butylsulfonic)-pyridinium hydrogensulfate

grafted to chloromethylated polystyrene propyl sulfonation of poly(4-vinylpyridine)

(4-butylsulfonic)-melaminium hydrogensulfate

melamine-formaldehyde resin with 1,4butanesulfonate reaction of 4-vinylpyridine with 1,3-propane sultone, followed by the polymerization and the addition of the heteropolyacid. grafted on cellulose

(3-propylsulfonic)-pyridinium [PW12O40] [1-butyl-3-(3-trimethoxypropyl)- imidazolium hydrogensulfate 1-(3-propylsulfonic) imidazolium dihydrogen phosphate 1-(3-propylsulfonic) imidazolium hydrogensulfate

copolymerization with styrene

Merrifield resin

polystyrene polystyrene

1-methylimidazolium aluminum chloride

polystyrene

1-(3-propylsulfonic) imidazolium hydrogensulfate 1-(4-autylsulfonic) imidazolium hydrogensulfate alkyl imidazolium hydrogensulfate

polystyrene grafted silica gel polystyrene poly calix[4]resorcinarene

1-(3-propylsulfonic) imidazolium dihydrogen phosphate 1-(3-butylsulfonic)-3-vinylimidazolium triflate and 1-(3propylsulfonic)-3- vinylimidazolium hydrogensulfate

polystyrene copolymerization with styrene

BAIL

density (ρ) g/cm3

viscosity (η) mPa·s

[(HSO3) C3C1im][HSO4] [(HSO3)4C4C1im][HSO4] [(HSO3)3C3C4im][HSO4] [(HSO3)4C4C4im][HSO4]

1.5017 1.4647 1.3469 1.3611

1296 1427 1000 1592

130

acetalization of carbonyl compounds with methanol coupling reactions of alkylene oxide and CO2 to produce alkylene carbonates homoallylic alcohols from Ar−CHO and allyltrimethylsilane esterification synthesis of 4,4′-(arylmethylene)bis(3methyl-1-phenyl-1H-pyrazol-5-ols) acetal formation with diols

286

284 285

287 288

289,219,290 283

2,3-dihydro-4(1H)-quinazolinones

216

hydroxylated pyridines

291

biodiesel from vegetable oil esterification of n-BuOH with acetic acid. Nitration of toluene with HNO3. Oxidation of benzothiophene Knoevenagel condensation of aromatic and aliphatic aldehydes with ethyl cyanoacetate. esterification Hantzsch 1,4-dihydropyridine synthesis condensation of amides with aldehydes to give bisamides biodiesel from vegetable oil acetalization of aldehydes

220 292−296,212

297,298

299 300 208 220 301,302

Table 5. Molecular Volume, Vm, Crystal Energy (UPOT), Thermal Expansion Coefficient (αp), and Standard Entropy (S0) of Selected BAILs at 20 °C304

Table 4. Density and Viscosity Data for Some Common BAILs (at 20 °C and p = 0.1 MPa)a304 3

refs

reactions of aziridines/propargyl amines with CO2 cycloaddition reaction of CO2 with epoxides Diels−Alder reaction of cyclopentadiene with methyl methacrylate

BAIL [(HSO3)3C3C1im] [HSO4] [(HSO3)4C4C1im] [HSO4] [(HSO3)3C3C4im] [HSO4] [(HSO3)4C4C4im] [HSO4]

Standard uncertainty u is u(T) = 0.01 °C and the combined expanded uncertainty is uc(ρ) = 3 × 10−4 g/cm3, uc(η) = 0.4% mPa·s (level of confidence = 0.95).304 a

[(HSO3)4C4iC4(H)2N][HSO4] have been assessed by the product of molar conductivity and viscosity.179 In addition to UV−vis spectroscopic methods, NMR can also be used in the determination of acidities of ionic liquids. Welton et al. introduced this technique as a quick, simple, robust method to measure acidity in ionic liquid systems by the use of the NMR-probe mesityl oxide. Acidity corresponding to a Hammett acidity of 1−9 could be measured reliably using this technique, a range that vastly exceeds that of any single UV−vis

Vm (nm3)

UPOT (kJ mol−1)

αp 104/ (K−1)

S0 (J K−1 mol−1)

0.333

442.19

3.81

445.03

0.357

434.24

3.91

474.95

0.423

416.21

4.21

557.21

0.436

413.12

4.25

573.42

probe.315 The Gutmann acceptor no. (AN), which is a quantitative measure of Lewis acidity, has also been estimated using the 31P NMR chemical shift of a probe molecule triethylphosphine oxide, for a range of chlorometallate(III) ionic liquids based on Group 13 metals (aluminum(III), gallium(III), and indium(III).316 There are attempts to correlate calculated H0 values of BAILs with catalysis activities in chemical reactions such as, Pechmann 6145


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Table 6. H0 Values of the Sulfonic Acid-Functionalized BAILs with Different Anions at 110 °C314

7.3. Thermo-Physical Properties

sulfonic acid functionalized BAIL

water content of BAIL (%)

[I] (%)

[HI] (%)

H0

[(HSO3)3C3pyr][BF4] [(HSO3)3C3pyr][HSO4] [(HSO3)3C3pyr][p-TSA] [(HSO3)3C3pyr][H2PO4] [(HSO3)3C3pyr][ H2PO4]

5.7 5.0 6.1 6.0 2.0

33 51 36 81 65

67 49 64 19 35

−3.6a −3.3a −2.1b −1.2b −1.5b

a

Indicator: 2,4-dichloro-6-nitroaniline. nitroaniline.

b

In 2011, Amarasekara and Owereh first reported the thermal decomposition onset temperatures for a total of 24 methylimidazolium, triethanolammonium, and pyridinium type sulfonic acid group functionalized Brönsted acidic ionic liquids with Cl−, Br−, SO42−, PO43−, BF4−, CH3CO2−, and CH3SO3− anions.331 Thermal stabilities of these sulfonic acid group functionalized ionic liquids decreased in the order methylimidazolium > triethanolammonium > pyridinium. The methylimidazolium, pyridinium, and triethanolammonium ionic liquids investigated showed decomposition onset temperatures (air) in the 213−353, 167−240, and 230−307 °C ranges, respectively. Additionally, they found that the decomposition temperatures of these ionic liquids are highly dependent on the nature of the anion.331 The thermophysical properties of acidic ionic liquid 1-(4-butylsulfonic)-3-methylimidazolium chloride [(HSO3)4C4C1im][Cl] has been compared with that of the corresponding neutral ionic liquid butylmethylimidazolium chloride [C4C1im][Cl] as shown in Figure 20. The acidic ionic liquid decomposed in three steps with DTG peaks centered at 269, 373, and 387, whereas the neutral [C4C1im][Cl] decomposed in one step.331

Indicator: 2,5-dichloro-4-

Figure 19. Absorption spectra of 0.025 mg/mL 2,4-dichloro-6nitroaniline before (A) and after (B) addition of [(HSO3)3C3pyr][BF4]. Reprinted with permission from ref 314. Copyright 2007 Elsevier.

condensation,317 hydration of alkynes,177 Fischer indole synthesis,318 coupling of epoxides and CO2,319 alcoholysis of furfural alcohol to alkyl levulinates,320 esterification,321−324 fructone synthesis,325 2-phenylbenzimidazole synthesis,326 degradation of lignin model compounds,327 acetal synthesis328 and dehydration of fructose.329 In one instance H0 values of a series of BAILs have been corelated to the yields and conversions in the synthesis of methylene diphenyl dimethyl carbamate (4,4′-MDC) from methyl N-phenyl carbamate (MPC) and formaldehyde (HCHO) as shown in Table 7.330 These experiments show that CF3SO3 and HSO4 anion containing BAILs with lower H0 values gives higher MPC conversions and MDC yields.

Figure 20. Thermogravimetric analysis (TG) and derivative thermogravimetric analysis (DTG) curves for 1-(4-butylsulfonic)-3methylimidazolium chloride [(HSO3)4C4C1im][Cl] and 1-butyl-3methylimidazolium chloride [C4C1im][Cl] under N2 atmosphere.331 Reprinted with permission from ref 331. Copyright 2010, Springer.

Later Muhammad and co-workers reported a comparison of the thermal decomposition temperatures of Brönsted acidic ILs and their corresponding zwitterions of 1-(3-propylsulfonic)-3methylimidazolium chloride [(HSO3)3C3C1im][Cl], 1-(3-propylsulfonic)-3-butylimidazolium chloride [(HSO3)3C3C4im][Cl], 1-(4-butylsulfonic)-3-methylimidazolium chloride [(HSO3)4C4C1im][Cl], and 1-(4-butylsulfonic)-3-butylimidazolium chloride [(HSO3)4C4C4im][Cl].304 The effect of alkyl groups on thermal degradation and viscosity is a common theme in many studies on ILs.332 Santos and co-workers studied several physicochemical properties including glass transition temperatures of a series of systems of the type [C1Him][NTf2], [C2Him][NTf2], [1C12C1Him][NTf2], and [1C42C13C1im][NTf2].333 They established a linear correlation between the glass transition temperature Tg and the alkyl chain size. It was found that the most viscous ILs

Table 7. Corelation between Catalytic Activities of Ionic Liquids and Their H0 Values330 sulfonic acid functionalized BAIL [(HSO3)4C4C1im] [CF3SO3] [(HSO3)4C4C1im][ HSO4] [(HSO3)4C4C1im][ p-TSA] [(HSO3)4C4C1im] [CH3SO3] [(HSO3)4C4C1im] [CF3 COO]

conversion of MPC (%)

yield of MDC (%)

selectivity MDC/MPC

H0

54.9

61.6

55.8

−3.6

60.3

42.7

34.3

−3.3

33.2

7.4

11.1

−0.8

34.3

4.6

6.7

−0.7

18.5

+0.9

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([1C1Him][NTf2], [1C2Him][NTf2], and [1C12C1Him][NTf2]) have an acidic N−H group in the imidazolium ring, which is in agreement with the observed increase in energy barrier of flow. The methylation in C-2 position as well as the N−H acidic group in the imidazolium ring contributes to a significant variation in the cation−anion interactions and their dynamics, which is reflected in their charge distribution and polarizability leading to a significant differentiation of the refractive indices, surface tension, and heat capacities.333

Table 8. Selected Set of References for Spectroscopy of Common Acidic Ionic Liquids acidic ionic liquid type 1-(alkyl sulfonic)-3-alkylimidazolium and imidazolium PAILs pyridinium hydrogensulfate alkyl sulfonic acid benzimidazolium 1,1′-disulfo-[2,2′-bipyridine]-1,1′-diium chloride succinimidinium N-sulfonic acid

7.4. Ionic Conductivity

Ionic conductivity of AILs in relation to structure is another useful tool in extracting structural information.334−336 The selfassembled ordered structures of AILs are known to have improved conductivities as well.337 The energetics of transfer of a proton from one member of the pair to the other is a common approach applicable in protic ionic liquids.338 In a recent investigation Wu and co-workers studied the ionic conductivities of a series of protic ionic liquids based on acetamide and Brönsted acids (HX, where X is CF3COO−, CH 3 COO −, or HSO 4 − ). They found that the ionic conductivities for most of the samples are between 10−3 and 10−1 S/m at room temperature. Acetamide trifluoroacetate with acetamide: trifluoroacetic acid 1:1 mol ratio showed an ionic conductivity of 0.25 S/m, and a viscosity of 10 cP at 25 °C. Moreover, most of the AILs studied possessed relatively moderate thermal stabilities (up to 106 °C for acetamide trifluoroacetate) and a wide liquid range (down to −69 °C for acetamide trifluoroacetate). The ionic conductivity properties observed in this study may perhaps make these acetamidebased PILs of interest as reaction media, catalysts in organic synthesis, or as electrolytes in fuel cells.334

alkylsulfonic ammonium, [(C2)2H2N][OTf] and [(C2)3HN][OAc]

spectroscopy IR, 1H, 13C NMR, mass IR, 1H, 13C NMR IR, 1H, 13C NMR IR, 1H, 13C NMR IR, 1H, 13C NMR 1 H, 13C NMR

refs 344−350 351−353 354 355 356,357 358−362

8. COMPUTATIONAL STUDIES ON ACIDIC IONIC LIQUIDS 8.1. Computational Studies on Acidity and Other Physical Properties

Several research groups have used computational methods to predict and explain acidity related parameters, hydrogen bonding characteristics and molecular geometries of Lewis and Brönsted acidic ionic liquids.376−378 Liu and co-workers used density functional theory to study the −SO3H functionalized acidic ionic liquid 1-(3-propylsulfonic)-3-methylimidazolium hydrogen sulfate [(HSO3)3C3C1im][HSO4] and its precursor zwitterion compound.379 These species were optimized systematically by the DFT theory at B3LYP/6311++G** level, and their most stable geometries were obtained. The calculation results indicated a great tendency to form strong intramolecular hydrogen bonds in the zwitterion, and this tendency was weakened in the cation, which was the protonation product of the zwitterion. The intramolecular hydrogen bonds and intermolecular hydrogen bonds coexisted in the ionic liquid, and they played an important role in the stability of the systems. The strongest interaction in the ionic liquid was found between the anion and the functional group. The transition state research and the intrinsic reaction coordinate analysis of the hydrogen transfer reaction showed that when the cation and the anion interacted near the functional group by double O−H···O hydrogen bonds, the ionic liquid was inclined to exist in a form of the zwitterion and H2SO4.379 In addition the acidity in imidazolium bromoalumanate LAILs have also been examined using DFT methods.376 In a recent study dynamic simulation based on the ion pair charge approach has been performed to investigate the structural characteristic of −SO3H functionalized ILs in the development of new proton conductive materials.380 In this study Shan and co-workers found that there are significant aggregations of sulfonic acid side chains due to strong interactions between different sulfonic acid groups. Furthermore, they observed that after the introduction of a sulfonic acid group to the side chain of the cation, the properties of the anions have a remarkable influence on the preferential location of anions in the interfacial region.380

7.5. Spectroscopy

The NMR characterizations of acidic ionic liquids are reported in some publications on synthesis and applications of AILs and in most cases the NMR spectra are recorded in D2O or DMSOd6. In addition to the structural characterization, NMR spectroscopy have been used in the determination of acidity in ionic liquids,315,339 study AILs in aqueous environments,340 ion-pairing behavior,341 anion speciation by 119Sn342 and 27Al343 NMR spectroscopy. For instance Lewis and Brönsted acidic species of chloroaluminate ILs have been investigated by 27Al NMR spectroscopy. These experiments revealed that Lewis acidity arises mainly from Al2Cl7−, having a chemical shift at 102 ppm in the 27Al NMR spectrum, while Brönsted acidity arises from Al2Cl6OH− (chem. shift at 97 ppm). The peak at 94 ppm in the 27Al NMR spectrum is related to Al2Cl5O−.339 A selected set of references for UV−vis, IR, and 1H and 13C NMR, mass spectroscopy of common acidic ionic liquids are shown in Table 8. 7.6. Miscellaneous Physical Properties

In addition to the physical properties discussed, a number of other interesting characteristics of AILs have been explored in recent years. These include studies on aggregation, crystal structure, chiral recognition and some of these examples are shown in Table 9.

8.2. Computational Studies on Acidic Ionic liquid Catalysis

There are a handful of cases where computational methods have been used for explaining the mechanisms, product distributions, and stabilities by computational methods in AIL catalyzed reactions. These studies include 1-ethyl-3-methyl6147


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Table 9. Miscellaneous Physical Properties Studies of Some Common Ionic Liquids ionic liquid(s) amines neutralized by bis(trifluoromethanesulfonic) amide, Alkanol amines neutralized by organic acids, 1,8-diazabicyclo[5,4,0]undec-7-ene neutralized by Brönsted acids and [(C4)3HP][BF4] decahydroisoquinoline neutralized by Brönsted acids heterocyclic amines neutralized by trifluoroacetic acid [Pyrr][HSO4] and [Pyrr][OTf] [(HO2C2)H3N][C4−COO] ephedrines neutralized by bis(trifluoromethanesulfonyl)amide and bis(pentafluoroethanesulfonyl) amide [(HO2C2)H3N][oleate] [Hbim][Tfsa] (Tfsa= bis(trifluoromethanesulfonyl)amide) N-(2-hydroxyethyl)piperazinium propionate [(HO2C2)H3N][NO3] [C2Him][OTf] 1-ethyl-2-butyl-benzimidazolium tetra-fluoroborate

imidazolium cation [C2C1im]+ and chloroaluminate (AlCl4 − and Al2Cl7 −) catalyzed Diels−Alder cyclo-addition of methyl acrylate to cyclopentadiene,381 removal of dibenzothiophene and 4,6-dimethyldibenzothiophene from model diesel,382 glucose transformations, 383 extractive desulfurization in [C4C1im][AlCl4],384 ether formation,385 and biodiesel synthesis.386 In a recent development Li and co-workers reported a DFT study on the mechanism for 1-(4-butylsulfonic)-3methylimidazolium hydrogen sulfate [(HSO3) 4C 4C 1im][HSO4] catalyzed conversion of glucose into 5-hydroxymethylfurfural. They found that the conversion may proceed via two potential pathways and that throughout most elementary steps; the cation of BAIL plays a substantial role, functioning as a proton shuttle to promote the reaction. The chloride anion interacts with the substrate and the acidic proton in the imidazolium ring via H-bond, as well as provides a polar environment together with the imidazolium cation to stabilize intermediates and transition states. The calculated overall barriers of the catalytic conversion along two potential pathways are 32.9 and 31.0 kcal/mol respectively, which are compatible with the observed catalytic performance of the AIL under mild conditions (100 °C).387 The fixation of carbon dioxide with epoxides catalyzed by a carboxylic acid functionalized IL catalyst has also been investigated using computational methods. In this case, catalysis mechanism of 1-(2carboxyethyl)pyridinium-4-carboxylic acid bromide was explored using density functional theory techniques.388

physicochemical properties studied

refs

temperature dependencies of density, viscosity, conductivity, and electrochemical activity ionicity and fragility capacitance with a RuO2 electrode and energy storage mechanisms self-diffusion coefficients and conductivities aggregation by 1H, and 13C NMR spectroscopy chiral recognition

363−366

effect of temperature on density, speed of sound, viscosity, and refractive index proton conductivity by Raman and NMR spectroscopy molar volume, viscosity and electrical conductivity thermal conductivity ion−ion association by molar conductivity crystallographic structure by X-ray diffraction analysis

159 367 336 368 369 370 371 372 373 374 375

cases the specific acid is the protonated form of the solvent in which the reaction is being performed. General acid catalysis is a system in which more than one species behaves as an acid. Acidic ionic liquids are used in applications where they behave as Lewis and Brönsted acids as well as in reactions involving both mechanisms. In addition AILs are known in the form of homogeneous and heterogeneous acid catalysts as well. In numerous cases the substitution of a conventional acid with an AIL has resulted in improvements in yield, turnover number (TON), or turnover frequencies (TOF) and catalyst recyclabilities, outperforming their traditional counterparts when used in classical reactions. 9.1.1. Esterification and Saponification. Esterification and saponification are industrially important and widely studied acid catalyzed reactions. A fair number of researchers have studied the possibility of substitution of classical acid catalysts with AILs for these reactions.389 The biodiesel synthesis is in fact a good example of saponification-esterification process; however, biodiesel synthesis is presented as a separate subtopic in this review as it is an extensively studied current application that needs special attention. Therefore, only esterifications and saponifications which are not directly related to biodiesel production are covered in this section. Only about a dozen examples are known in homogeneous catalysis area, however some trans-esterification390,391 and considerable variations in acidic ionic liquid types are known. For instance hydrophobic 1-(4-butylsulfonic)-3-octylimidazolium hydrogen sulfate [(HSO3)4C4C8im][HSO4] has been used as an efficient catalyst for esterification of oleic acid with methanol392 with excellent results, and some selected examples from recent applications are shown in Table 10. In addition to the homogeneous acid catalysis, AILs have been used as supported heterogeneous catalysts as well in esterification. In one example, a series of polystyrene-supported 1-(3-propylsulfonic)-3-methylimidazolium hydrogensulfate catalysts with different acidic ionic liquid contents have been tested for esterification of n‑butyl alcohol with acetic acid. In this study, the reactivity of the catalyst increased with increasing [(HSO3)3C3C1im][HSO4] content, and a 98% yield of n‑butyl acetate was obtained with the best catalyst.292 9.1.2. Alkylation. Alkylation or the attachment of an alkyl group can be achieved through electrophilic, nucleophilic and

9. APPLICATIONS OF ACIDIC IONIC LIQUIDS 9.1. Catalysis Applications

In acid catalysis there are typical Lewis acid catalyzed reactions like Friedel−Crafts alkylation and Brönsted acid catalyzed reactions like esterification. Lewis acid catalyzed reactions are noticeable by electron transfer toward electron deficient Lewis acid and formation of a charge separated complex. Brönsted acid catalyzed reactions are marked by proton transfer, and in these reactions proton transfer can make one reacting partner more electrophilic. Then acid catalysis can occur in two different ways: specific acid catalysis and general acid catalysis. Specific-acid catalysis refers to a process in which the reaction rate depends upon the specific acid in the solution. In most 6148


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Table 10. Esterification and Saponification Using Acidic Ionic Liquid Catalysts acidic ionic liquid catalyst(s)

esterification/saponification reaction

[C6C1im][HSO4] [(HSO3)4C4C1im][HSO4] [(HSO3)4C4(CC)im][OTf] on magnetic mesoporous silica [(HSO3)4C4C1im][HSO4] [(HSO3)3C3C1im][HSO4] on silica [(C2)3N][HSO4], [(C2)3N][H2PO4], [(C2)3N][OTs], [Bz(C2)2N][HSO4], [(C4)3N] [HSO4], [(C8)3N][HSO4], [(C2)2HN][HSO4], and [(C2)2HN][H2PO4] [(HSO3)3C3(CC)im][HSO4] on silica [(HSO3)3C3(C2)3N][HSO4], and [(HSO3)3C3(C2)3N][H2PO4] [(HSO3)4C4C1im][HSO4], [(HSO3)3C3pyr][HSO4], and [(HSO3)3C3(C2)3N] [HSO4] [(HSO3)4C4C1im][OTf] guanidinium tetrafluoroborate SO3H-functionalized IL polymer with heteropolyacid (HPA) polyoxometalate-based sulfonated ionic liquid choline chlorostanate, chlorozincate and chloroferrate −SO3H functionalized imidazolium BAILs [(HSO3)3C3C1im][HSO4], [(HSO3)3C3C1im][OTs], [(HSO3)3C3C1im][H2PO4], and [(HSO3)3C3C1im][OTf] [(HSO3)3C3C1im][HSO4], [(HSO3)3C3C1im][BF4], [(HSO3)3C3C1im][PF6], and [(HSO3)3C3C1im][OTf] [(HSO3)3C3C1im][HSO4] and [(HSO3)4C4C1im][HSO4] [(HSO3)3C3Ph3P][OTs] [(HSO3)3C3C1im][HSO4] and [(HSO3)pyr][HSO4] [C1Pyrr][HSO4] 1-[(1,1,1,2,3,3-hexafluoro-2-hydroxysulfonyl)propyloxyethyl]-3-methylimidazolium chloride [C6C1im][HSO4] and [C4C1im][H2PO4] BAIL with alkanesulfonic acid groups and a polyether [(HSO3)4C4C1im][HSO4] and [(HSO3)4C4(C2)3N][HSO4] [(HSO3)3C3Ph3P][OTs] and [(HSO3)3C3pyr][HSO4] [C1Pyrr][HSO4] [C4C1im][HSO4] [(HSO3)3C3(C2)3N][HSO4] and [(HSO3)3C3(C1)2H N][OTs] [BzC1im][HSO4] 1-(4-butylsulfonic)-caprolactamium hydrogensulfate and [(HSO3)4C4C1im][HSO4] [(HSO3)4C4C1im][OTs] [(COOH)C1C1im][HSO4]

free radical alkylation reactions. Acid catalyzed alkylation is generally an electrophilic alkylation. One common example is the Friedel−Crafts type alkylation, in which a Lewis acid catalyst is used to generate the carbocation, which acts as the electrophile in the reaction. Lewis and Brönsted acidic ionic liquids can be used to generate carbocations and the acidic ionic liquids are often more effective than common Lewis and mineral acids in these reactions. In 2013 Zhang et al. reviewed the application of acidic ionic liquids as a catalyst in the alkylation reaction.418 The methods used in the determination of acidity, application of acidic ionic liquids in alkylation, acidic adjustment methods, and novel acidic ionic liquids are discussed in this review article with 38 references.418 Earlier Qiao wrote a review on benzene alkylation with long chain olefins catalyzed by ionic liquids as well, and this review is particularly focused on the applications of chloroaluminate ionic liquids in benzene alkylation of special interest to petrochemical and detergent industry.419

refs

oleic acid with lauryl alcohol in TX-100/cyclohexane oleic and caprylic acid with methyl alcohol oleic acid with straight-chain alcohols n-butyric acid with methanol acetic acid with ethanol, n-octanol, n-decanol acetic acid with 1-octanol

393 394,346 244 324 226 395

n-butanol and acetic acid α-pinene with acetic acid n-caproic, benzoic acid with 1-butanol, 1-octanol and iso-amyl alcohol acetic acid with 1-heptene formic and butaric acids with alcohols acetic acid with n-butanol, ethylene glycol, dodecanol palmitic acid with ethanol phytosterols with fatty acids acetic and benzoic acids with benzyl alcohol n-butyric acid with n-butanol

246 396 175 397 398 211 399 400 321 401

benzoic acid with ethanol

402

citric acid with n-butanol acetic acid with ethanol phthalic anhydride and sebacic acid with n-octanol and nbutanol gallic acid with n-propanol n-propanol with octanoic acid

403 404 405

neo-pentan-1-ol, hexan-1-ol, heptan-1-ol and decan-1-ol with acetic acid, methoxyacetic acid and methylmalonic acid oleic acid with methanol salicylic acid with isoamyl alcohol acetic acid with ethanol benzoic acid with methanol, ethanol, and butanol transesterification of dimethyl carbonate with phenol to methylphenyl carbonate and diphenyl carbonate oleic acid with methanol acetic, metacetonic and benzoic acid with ethanol, butanol, and benzyl alcohol stearic, palmitic and lauric acid with ethanol. saponification of vegetable oil palladium-catalyzed hydroesterification of olefins with isosorbide arene carboxylic acids with alcohols

406 407 408 409 322 173 410 411 412 413,414 232 415 416 417

The Friedel−Crafts type alkylation at a carbon atom is the most common application in acidic ionic liquid catalyzed alkylations.420−424 However, there are examples of alkylations at N, O and S atoms as well.425,426 Acidic ionic liquid catalyzed alkylation of phenol and phenol derivatives like cresols are wellknown.427 Alkenes or tertiary alcohols such as t‑butanol are common alkylating agents used in this reaction. For example, Elavarasan and co-workers tested catalytic activities of three BAILs in the t‑butylation of phenol with t‑butyl alcohol (TBA) as shown in Figure 21.428 The reaction time, temperature, and reactant/catalyst mole ratio were optimized in this study. Among the three ionic liquids studied, triethylammonium based sulfonic acid functionalized ionic liquid was found to be the most promising and gave the highest phenol conversion. A comparison of catalyst performances in the alkylation of phenol with t‑butyl alcohol is shown in Table 11. Additionally they found that the catalyst retained its activity even after 5 recycles. The activation energy for ionic liquid catalyzed alkylation of 6149


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Another widely studied application of acidic ionic liquids is the alkylation of iso‑butane with 1-butene, and this is an important process in the petroleum industry. The products are often used as premium blending stock for reformulated gasoline since they have a high octane number, low vapor pressure, and low aromatic contents, alkenes and sulfur. Concentrated sulfuric acid or hydrogen fluoride based methods are currently being used in this process; however, these technologies are gradually being restricted because of environmental pollution and safety problems, creating a genuine quest for safer acid catalysts. A number of research groups have studied chloroaluminate-based ionic liquids due to their standout acidity as catalysts in iso‑ butane 432−438 and isohexane alkylation.439 In 1994, Chauvin et al.432 reported that the alkylation of iso‑butane with 2-butene could be efficiently catalyzed by the ionic liquid 1-butyl-3-methylinidazolium chloroaluminate. Since then, various chloroaluminate-based ionic liquids have been investigated as catalysts for iso‑butane− butane alkylation by Yoo et al.,433 Xu et al.,434,435 Zhang et al.,436 Bui et al.,437 and so on. However, Lewis acidic chloroaluminate ionic liquids are extremely sensitive to moisture and hydrolyze to release HCl in contact with traces of water. Brönsted acidic ionic liquids have also been utilized for iso‑butane-butene alkylation.440,441 In one example, Cui and co-workers studied the alkylation of iso‑butane with 1-butene catalyzed by triflic acid (TFOH) coupled with a series of protic ammonium-based ionic liquids (AMILs), and they found that addition of the AMILs can dramatically enhance the efficiency of TFOH for the alkylation reaction. 442 The alkylate compositions produced by the optimized TFOH/IL with HSO4− anions are shown in Table 12. Under these conditions

Figure 21. t‑Butylation of phenol with t‑butyl alcohol (TBA) using different acidic ionic liquid catalysts: [(HSO3)4C4C1im][HSO4], [(HSO3)4C4pyr][ HSO4] and [(HSO3)4C4(C2)3N][ HSO4]428

Table 11. Comparison of Catalyst Performances in the Alkylation of Phenol with t‑Butyl Alcohola selectivity (%) catalyst [(HSO3)4C4(C2)3N] [HSO4] [(HSO3)4C4pyr] [HSO4] [(HSO3)4C4C1im] [HSO4]

conversion of phenol (%)

2TBP

4TBP

2,4DTBP

2,6DTBP

84.5

23.8

10.2

57.6

7.4

79.6

25.7

9.7

57.0

7.6

78.2

23.2

12.7

55.1

8.9

a

Reactant to catalyst ratio (phenol: IL) 1:1, reactant mole ratio (TBA: phenol) 2:1, temperature: 70 °C, reaction time: 8 h.428

phenol was found to be 11.13 kcal/mol in the 50−90 °C temperature range.428 In another type of an alkylation of a phenol involving a multicomponent reaction, Hajipour et al. demonstrated that 2naphthol can be alkylated with aldehydes and an amide or urea in the presence of a catalytic amount of Brönsted acidic ionic liquid (4-butylsulfonic)-triethylammonium hydrogensulfate [(HSO3)4C4(C2)3N][HSO4] under thermal, solvent-free conditions in high yields, as shown in Figure 22.429 In addition,

Table 12. Alkylate Compositions Produced by the Optimized TFOH/ILs with HSO4− Anions442a ionic liquid [HO2C2(C2)2HN] [HSO4] (15 vol %) [(HO2C2)2C1HN] [HSO4] (15 vol %) [(HO2C2)2C2HN] [HSO4] (15 vol %) [HOC1(C2)2HN] [HSO4] (15 vol %) [[HO2C2C1H2N] [HSO4] (15 vol %) [(C2)3HN] [HSO4] (25 vol %)

Figure 22. Brönsted acidic ionic liquid catalyzed one-pot synthesis of 1-amidoalkyl 2-naphthols429

Kotadia and Soni also used a silica supported −SO3H functionalized benzimidazolium based ionic liquid for the one-pot synthesis of a similar series of 1-amidoalkyl 2naphthols.430 A series of acidic ionic liquids have been shown as excellent catalysts for the high-yielding synthesis of diphenolic acid (DPA)431 as shown in Figure 23. Importantly, p,p′-DPA was obtained as the preferential product over o,p′-DPA with an isomer ratio over 100. Moreover, diphenolic esters could also be prepared in high yield through a one-pot method.431

C5−C7 (%)

C8 (%)

C9+ (%)

TMP/ DMH

RON

3.8

88.9

7.3

12.1

97.4

2.5

92.1

5.4

12.0

97.8

3.5

87.3

9.1

13.5

97.4

3.8

89.6

6.6

12.9

97.7

2.9

87.9

9.1

10.4

97.0

1.9

91.5

6.6

13.5

98.0

Reaction conditions: temperature, 10 °C; pressure, 0.5 MPa; reaction time, 10 min; stirring speed, 1000 rpm.442

a

up to 85.1% trimethylpentanes (TMP) selectivity and 98 research octane number (RON) were achieved with the optimized TFOH/AMIL catalyst (75 vol % triflic acid and 25

Figure 23. BAIL catalyzed synthesis of diphenolic acid (DPA). BAIL: [(HSO3)3C3C1im][CF3SO3], [(HSO3)3C3(C2)3N][CF3SO3], and [(HSO3)3C3pyr][CF3SO3]. 6150


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Table 13. Selected Examples of Application of Acidic Ionic Liquids in Alkylation Reactions acidic ionic liquid catalyst(s) [C4C1im][HSO4], [C4C1im][HCO2] prolinium triflate (ProTf) and [((HO)2C2)3N][HSO4] [(C2)3NH][AlCl4] [(HSO3)4C4 C1im][OTf] [(HSO3)4C4(C2)3N][HSO4], [(HSO3)4C4Py][HSO4] and [(HSO3)4C4 C1im][HSO4] [C4C1im][HSO4] [C4C1im][AlCl4], [(C2)3NH][AlCl4], [[(C2)3NH][AlCl4−CuCl], [(C2)3NH][GaCl3] and [(C2)3NH][GaCl3−CuCl] [(C2)3NH][AlCl4] [(HSO3)4C4 C1im][OTf], [(HSO3)4C4 C1im][OTs] and [(HSO3)4C4 C1im][HSO4] [(HSO3)4C4 C1im][OTf] [(HSO3)3C3C1im][OTf], [(HSO3)3C3C1im][OMs], [(HSO3)3C3C1im][OTs], [(HSO3)3C3C1im][OTs] and [(HSO3)3C3C1im][HSO4] [(HSO3)3C3(C2)3N][HSO4], [(HSO3)3C3pyr][HSO4] and [(HSO3)4C4pyr][HSO4] 4

[(HSO3) C4 C1im][OTf] silica immobilized phosphonium ionic liquid with In2Cl7− or InCl4− [C4C1im][HSO4] [(HSO3)4C4 C1im][HSO4] [C4C1im][FeC14] [(HSO3)4C4(C1)3N][HSO4], [(HSO3)4C4(C1)3N][OTs] and [(HSO3)4C4 Py][HSO4] N-methyl-2-pyrrolidone hydrogensulfate [(C2)3HN][AlCl4], [(C2)3HN][FeCl4], [(C2)3HN][ZnCl3], [(C2)3HN][CuCl3] and [(C2)3HN][SnCl3], [(C2)3HN][SnCl5] [(HSO3)4C4(C2)3N][HSO4] [(C2)3HN][AlCl4] and [(C2)3HN][FeCl4] [C4 C1im][AlCl4] [(HSO3)4C4 C1im][HSO4] [(HSO3)3C3C1im][OTf] and [(HSO3)3C3(C2)3N][OTf] [C4 C1im][ZnCl3], [C4 C1im][FeCl4] and [C4 C1im][FeCl3] [(HSO3)3C3(C2)3N][ZnCl3] 4-(3-propylsulfonic)-4-butylthiomorpholinium 1,1-dioxide trifluoromethanesulfonate 1-(3-propylsulfonic)-3-alkylimidazolium-divinylbenzene copolymer with PW12O403− anions [C4C1im][AlCl4] [(HO2C2)3HN][ CF3COO] [(HSO3)3C3C1im][HSO4] [(HSO3)4C4Him][OTf], [(HSO3)3C3Him][HSO4] and [(HSO3)3C3Him][H2PO4] N-methyl-2-pyrrolidone hydrogensulfate [(HSO3)4C4C1im][OTf] [(HSO3)2C2(SO3)H2N][HSO4], [(HSO3)2C2(SO3)H2N][OTf], [(HSO3)2C2(SO3)H2N][NO3] and [(HSO3)2C2(SO3)H2N] [Tfa] [(HSO3)4C4(C2)3N][OTs] 1-ethyl-1,2,4-triazolium methanesulfonate [C1Him][H2PO4]

reaction(s)

refs

indole + aldehydes/ketones → bis(indolyl)methanes

169,443,444

napthalene + iPrBr → 2-iPr napthalene phenol or anisole + alkene → alkylphenol or alkylanisole phenol + tBuOH → 2-TBP + 4-TBP + 2,4-DTBP + 2,6DTBP hydroxybenzene + α,β-unsaturated compounds → oxaMichael adducts and Friedel−Crafts alkylated products i butane/butene → octanes

445 446 428

448,449,193,435,450,451,440,433,442,452

toluene +2-chloro-2-methylpropane → p-tbutyltoluene glycerol + tBuOH → glycerol-tBu ethers

453,454 455

benzothiazole-2-thiole, aromatic amine, azole compounds, indoles + benzylic alcohols → O, N, S, C benzylated products arene + aromatic aldehyde → triarylmethane

425

456

p-cresol + tbutanol →2-tBu-p-cresol and 2,6-ditBu-p-cresol

457−461

m-cresol + tbutanol → alkylated cresols benzene +1-dodecene →2-phenyldodecane phenol + iButene →2-TBP, 4-TBP, 2,4-DTBP, 2,6-DTBP isoprene + thiophene → desulfurization products phenol + tbutanol →2-TBP, 4-TBP, 2,4-DTBP, 2,6-DTBP benzene +1-octadecene octadecylbenzene catechol + tbutanol →4-TBC + 3-TBC + TBCE

462,463 464 465 466,467 468 469,470

phenol or thiol + alkene alkylated products. Coupling of alcohols with styrenes benzene + CH2Cl2 → diphenylmethane

447

471,472 473

2-naphthol+ aldehydes + amides or urea →1-amidoalkyl 2naphthols benzene +1-hexene → hexylbenzene Ar−H + R-NCS → N-substituted thioamides PhCOCH3 + ArCHO + CH3COCl + CH3CN → βacetamido ketones levulinic acid + phenol → diphenolic acid BzCl + Ar−H → diphenylmethanes abietic acid → abietic acid dimer indoles + ketone →3-vinyl indoles

431 476 477,478 479

Ar−H + BzOH → Ar- Bz

480

diphenyl oxide +1-dodecene → monododecyl diphenyl oxides knoevenagel reaction aldehydes +2-naphthol +2-aminobenzothiazole →1(Benzothiazolylamino) methyl −2-naphthols aldol condensations

481 482 483

1,8-dioxo-octahydroxanthene derivatives 1,3-dioxanes via Prins reaction nitrostyrene from Henry reaction. α-amino-phosphonates

486 487 488,489

condensation of n-butyraldehyde to 2-ethyl-2-hexenal β-amino carbonyl compounds alkyl aza-arene addition to aldehydes

490 491 492

vol % triethylammonium hydrogen sulfate), which were much

429 474 475 264

484,485

As described earlier there are numerous applications of acidic ionic liquids in alkylation reactions and some selected examples and their references are shown in Table 13. 9.1.3. Acetalization. Acetalization is another acid catalyzed reaction tested with BAILs, where (4-butylsulfonic)-triethy-

better than those obtained with the commercial H2SO4 catalyst (65% TMP selectivity, 97 RON) and pure triflic acid.442 6151


Chemical Reviews

Review

lammonium hydrogensulfate [(HSO3)4C4(C2)3N][HSO4] was reported to give excellent yields in the production of fructone via the acetalization reaction of ethyl acetoacetate with ethylene glycol.325,493 The condensation of glycerol, a platform chemical from renewable materials with benzaldehyde to generate cyclic acetals has been investigated using BAIL catalysts. Evidence was presented for the formation of a product mixture with 4hydroxymethyl-2-phenyl-1,3-dioxolane and 5-hydroxyl-2-phenyl-1,3-dioxane as cis and trans stereoisomers. Under optimum reaction conditions butyl-pyridinium hydrogensulfate [C4pyr][HSO4] catalyst produced a 99.8% yield of acetals at room temperature.494 9.1.4. Heterocyclic Synthesis. The use of Brönsted acidic ionic liquids in heterocyclic synthesis is a well documented application. A fair number of classical heterocyclic syntheses done with Lewis and Brönsted acids have been tested with BAILs and in many cases BAILs have proven to be better catalysts than equivalent dose of traditional acids. Some of the initial applications are discussed in a few earlier ionic liquid review articles495−498 and in a book titled “Ionic Liquids in Synthesis”.499 The 2008 review titled “Ionic Liquids in Heterocyclic Synthesis” with 367 references gives a comprehensive account on the use of neutral, acidic and basic ILs as solvents and catalysts in the synthesis of many classes of heterocyclic compounds.500 In a recent application, 1-(3-propylsulfonic)-3-methylimidazolium chloride [(HSO3)3C3C1im][Cl] and 1-(4-butylsulfonic)-3-methylimidazolium chloride [(HSO3)4C4C1im][Cl] BAILs are shown as excellent catalysts and reaction mediums for Skraup synthesis of quinolines under microwave conditions501 (Figure 24). There are a number of advantages to

Brönsted acidic ionic liquids to generate the carbonium ions and subsequent reactions in the ionic liquid media is known in named reactions like the pinacol rearrangement173 and Ritter reaction558 as well as in isomerizations.559 Kalkhambkar558 and co-workers have used imidazolium ionic liquid 1-(4-butylsulfonic)-3-methylimidazolium triflate [(HSO3)4C4 C1im][OTf] as catalyst for the high yield synthesis of a wide variety of amides under mild conditions via the Ritter reaction of alcohols with nitriles as shown in Figure 25. In another recent work efficient and selective rearrangements of stilbene epoxides were observed with 0.1−0.3 equiv of [(HSO3)4C4C1im][OTf] in dichloromethane as shown in Figure 26. A substituent effect study was performed with a series of singly substituted 1,2-diphenyl oxiranes and competing formation of ketones was observed in acidic ionic liquids.560 In another application propargylic alcohols were converted to cyclic and acyclic α,β-unsaturated enones using [(HSO3)4C4C1im][OTf] as the catalyst and [C4C1im][PF6] as the solvent via the Rupe rearrangement as shown in the example in Figure 27.561 Pinacol rearrangement of hydrobenzoin using triethylammonium hydrogensulfate [(C2)3NH][HSO4] as the catalyst is another example for carbonium ion generation and rearrangement as shown in Figure 28.562 The optimal conditions were found to be heating at 80 °C for 5 min under low power microwave (20 W) conditions to avoid ionic liquid degradation. Lewis acidic ionic liquids like pyridinium chloroaluminate has been used in isomerization of exo-tetrahydrodicyclopentadiene to adamantane.563,564 Rearrangements of terpenes is another area of interest, where 1-(3-propylsulfonic)-3-methylimidazolium dihydrogen phosphate [(HSO3)3C3C1im][H2PO4] has been used as the catalyst in the synthesis of α-terpineol from αpinene.565 The same group reported 1-(1-ethyl acetate-yl)-3methylimidazolium chloroaluminate catalyzed isomerization of α-pinene to camphene as well.566 9.1.6. Dehydration. The acid catalyzed dehydration of an alcohol to yield an alkene is another well established synthetic operation and has been known with acidic ionic liquid catalysts as well. Dehydration of glycerol to acrolein is a new renewable resources based route to this C3 aldehyde; liquid phase170 as well as solid supported acidic ionic liquid catalyzed methods are reported to this important feedstock in the recent literature.567,568 Shen and co-workers have investigated a liquid phase dehydration of glycerol to acrolein catalyzed by BAIL using the semibatch reaction technique. They found that for a series of BAIL catalysts, the acrolein yields were in the order of [C4C1im][H2PO4] > [C4C1im][HSO4] > [C4Py][HSO4] > [(HSO3)3C3Py][HSO4] > [C4(C2)3N][HSO4] > [(HSO3)3C3Py][H2PO4] > [C4Py][H2PO4] > [C4(C2)3N][H2PO4]. When [C4C1im][H2PO4] and [C4C1im][HSO4] were used as the catalysts at 270 °C with the molar ratio of catalyst to glycerol of 1:100, the acrolein yields were 57.4% and 50.8%, respectively, at complete conversion of glycerol.170 Nevertheless in the dehydration of ethanol to ethylene, Gong et al. found that the catalytic activity of 1-(4-butylsulfonic)-3methylimidazolium hydrogen sulfate [(HSO 3) 4C4 C1 im][HSO4] is similar to H2SO4.569 9.1.7. Oxidation. In another application BAIL catalysts have been used in the oxidation of alcohols to aldehydes and ketones. In this instance, −SO3H functionalized heteropolyanion-based ionic liquids with 35% aqueous H2O2 were used without adding any phase transfer catalyst as shown in Figure 29.570 For the aliphatic alcohols corresponding aldehydes and

Figure 24. 1-(Alkylsulfonic)-3-methylimidazolium chloride Brönsted acidic ionic liquid catalyzed Skraup synthesis of quinolines under microwave heating.

using the BAILs as reaction mediums and catalysts in the Skraup synthesis. These include: shorter reaction time, better yields, ease of isolation of the quinoline product, and most importantly elimination of the use nitrobenzene or other oxidizing agents and additional metal catalysts.501 The BAILs on solid supports have been used in heterocyclic synthesis.502 In this application, Fe2O3-MCM-41 materials with 1-(4-butylsulfonic)-3-propylimidazolium hydrogensulfate has been used as the catalyst for the one-pot synthesis of pyrimido[4,5-d]pyrimidine derivatives as well as 2-amino-4Hchromenes503 using 6-aminouracil, aldehydes, and urea or thiourea under solvent free conditions.504 Some selected examples of heterocyclic synthesis using acidic ionic liquids as catalysts are shown in Table 14. 9.1.5. Carbonium Ion Generation and Rearrangements. Acidic ionic liquid catalyzed carbonium ion generation and rearrangements of the resulting carbonium ions is another reaction receiving attention. In many examples the carbonium ion rearrangement product distributions are influenced by the AIL catalysts as compared with traditional acids. The use of 6152


Chemical Reviews

Review

Table 14. Applications of Acidic Ionic Liquids in Heterocyclic Synthesis acidic ionic liquid catalyst(s) [C4C1im][HSO4] [C1Him][Tfa] and Cu(OAc)2/sodium ascorbate choline chloride · 2ZnCl2 [C4C1im][HSO4] and [(HOOC)C1 C1im][HSO4] [C1Him][Tfa] [C4C1im][ZnCl3] [C4C1im][HSO4] and [C4C1im][OTf] polymer supported [(HSO3)3C3pyr][1/3. PW12O40] [(HSO3)4C4 C1im][HSO4], [(HSO3)4C4 C1im][OTs], [(HSO3)4C4 C1im] [OMs] and N-methyl-2-pyrrolidonium tosylate [(HSO3)4C4(C2)3N][HSO4] 3-carboxypyridinium hydrogensulfate [C1Him][HSO4] [(HSO3)3C3C1im][HSO4] and [(HSO3)3C3C1im][OTf] [(HOOC)C1 C1im][OTf] [(HSO3)3C3(C2)3N][HSO4] [C4C1im][HSO4], [C4C1im][H2PO4] and [C6C1im][BF4] caprolactam tetrafluoroborate [(HSO3)3C3(C2)3N][HSO4] [(HSO3)3C3C1im][Cl], [(HSO3)3C3C1im][H2PO4], [(HSO3)3C3C1im] [HSO4], [(HSO3)4C4C1im][Cl], [(HSO3)4C4C1im][H2PO4], and [(HSO3)4C4C1im][HSO4] 1,1′-butylenebis(3-methylimidazolium) hydrogen sulfate [(HSO3)4C4pyr][HSO4] 1,4-dimethyl-1,4-bis(4-butylsulfonic)piperazinium hydrogensulfate [(C2)3HN][ZnCl3] [(HSO3)4C4pyr][AlCl4] polymer supported [(HSO3)C4im][Cl] [(HSO3)4C4pyr][HSO4] polymer supported hexamethyltetrammonium sulfonic acid-phenol-formaldehyde resin [C1C3im][HSO4] immobilized on nanoporous Na+-montmorillonite N-methyl-2-pyrrolidonium hydrogensulfate [(HSO3)3C3Ph3P][OTs] N-methyl-2-pyrrolidonium hydrogensulfate [(HSO3)4C4C1im][HSO4]

N-methyl-2-pyrrolidonium hydrogensulfate [(HSO3)4C4C1im][OTf] [C1Him][HSO4] [(HSO3)4C4C1im][OBs] N-methyl-2-pyrrolidonium dihydrogen phosphate [(HSO3)4C4C1im][HSO4] [C6C1im][HSO4] [(C2)2H2N][ClSO3] and [(C4)2H2N][ClSO3] [(Py)2SO][HSO4]2 [(C2)3HN][HSO4] [(HSO3)2im][HSO4]

reaction

refs

hydrazines +1,3-dicarbonyls → pyrazoles. 4-hydroxyquinoline-2-one + Meldrum’s acid + aldehyde → pyrano[3,2-c]quinoline-2,5-diones propargylated aldehydes + azides +2-aminobenzophenone + ammonium acetate → phenylquinazolines quinolines via Friedländer annulation aldehydes + methyl acetoacetate + urea or thiourea →3,4dihydropyrimidin-2(1H)-one and 3,4-dihydropyrimidine-2(1H)thione (Biginelli reaction) dimedone + aldehyde →1,8-dioxooctahydroxanthenes dimedone + aldehyde + R-NH2 → 1,8-dioxodecahydroacridine o-amino aromatic carbonyls + ketones → quinolines (Friedländer synthesis) phenol + methyl acetoacetate → coumarins anthranilamide + aldehydes →2,3-dihydro-4(1H)-quinazolinones pyrrole + cyclohexanone → N-confused meso-tetraspirocyclohexyl calix[4]pyrrole Ar−CHO + cyclopentanone + urea/thiourea → pyrimidinone 1,3-dicarbonyl + Ar−CHO and urea/thiourea →3,4dihydropyrimidin-2(1H)- ones ethylacetoacetate + R-CHO + urea/thiourea →3,4-dihydropyrimidin2(1H)-ones and thiones and piperidines ketones + pyrrole → calix[4]pyrroles and N-confused calix[4] pyrroles, 21-thia-5,10,15,20-tetraarylporphyrins, 2-aryl-1-arylmethyl1H-benzimidazoles ArCHO + dimedone + NH4OAc → Acridinedione ArCHO + dimedone →1,8-dioxo-octahydroxanthenes resorcinol + methyl acetoacetate →7-hydroxy-4-methylcoumarin cyclohexanone oxime → caprolactam α-oxothioformanilide + o-phenyldiamine → quinoxaline Ph-NHNH2 + cyclohexanone → indoles

505,506

2-amino-3-pyridinecarbonitrile → tacrine analogue and 13-(aryl)12H-benzo[f]indeno[1,2-b]quinoline-12-one 2-aminobenzamide + ArCHO → 2-aryl-2,3-dihydroquinazolin-4 (1H)-ones pyranopyrazoles, benzopyrans, amino-2-chromenes and dihydropyrano[c]chromenes condensation of isophytol and trimethylhydroquinone in the synthesis of vitamin E biscoumarins by domino Knoevenagel-Michael addition hexahydroquinolines 1-amidoalkyl-2-naphthols and substituted quinolines Peckmann reaction 4,4-(arylmethylene)-bis(3-methyl-1-phenyl-1H-pyrazol-5-ol) pyrano[2,3-c]pyrazoles 14-aryl-14H-dibenzo[a,i]xanthene-8,13-dione, 3,4-dihydro-2H-benzo [b]xanthene-1,6,11(12H)-trione spiro[diindenopyridine-indoline]-triones by condensation of 1,3indandione, isatin and aniline. benzothiazoloquinazolines by reaction of 2-aminobenzothiazoles, αtetralone, and furan-2-carboxaldehyde furan-2(5H)-ones. 2,3-dihydroquinazolin-4(1H)-ones 1,3-dioxanes via the Prins reaction Knoevenagel-hetero-Diels−Alder reaction. pyrano[3,2-c]quinolin-2-one derivatives by the tandem cyclization of 4-hydroxy-1-methyl-2-quinolone with chalcones 12-aryl-8,9,10,12-tetrahydrobenzo[a]xanthene-11-ones. Spiro [diindenopyridine-indoline]triones pyrido[2,3-d]pyrimidines N-substituted pyrroles by Clauson Kaas reaction 2-mercaptonaphthalen-1-yl-methyl-3-hydroxy-5,5-dimethylcyclohex2-enones ketone + thiosemicarbazide → spiro-1,2,4-triazolidine-3-thiones azlactones/oxazolones pyrimido[4,5-b]quinolines

529,530

6153

507 508 509,510 511 512 167,513 216 514,515 516 517 518,519 520−524 525 526 317 527 528 318

263 531 532 533,289,298 534 218 535 279 197 536,537 538,539

540−544 487,545 546 547 548,549 550 551 552,553 554 555 556


Chemical Reviews

Review

Table 14. continued acidic ionic liquid catalyst(s) 3-sulfonic-1-imidazolopyridinium hydrogensulfate [(HSO3)4C4(2-HSO3−Ph)3P][HSO4]

reaction

refs

2H-indazolo[2,1-b]phthalazine-1,6,11(13H)-triones pyrano pyrimidinones

180 557

After the completion, two ionic liquids could be reused five times after simple removal and without significant loss of their catalytic activities. In another oxidation by silver tungstate nanorods in a Brönsted acidic ionic liquid (1,2-dimethyl-3-dodecylidazolium hydrogensulfate) cyclohexene has been converted to adipic acid using 30% H2O2 as the oxidant.571 9.1.8. Polymerization and Related Reactions. Acidic ionic liquids can be used as catalysts for polymerization as well as synthetic modifications of polymers like cross-linking and functionalizations.572 Methylimidazolium hydrogensulfate has been used as the catalyst for polymerization of L-aspartic acid to polyaspartic acid under microwave irradiation.573 Tian et al. demonstrated that 1-vinyl, 3-(3-propylsulfonic) imidazolium hydrogensulfate works as a “quasi-homogeneous” catalyst for the acetylation of cellulose.574 Unlike existing techniques that use a large amount of ILs as solvent to dissolve and acetylate cellulose, a small amount of acidic IL was used as the catalyst in this study to overcome the low efficiency associated with relatively high viscosity and costs of ILs during homogeneous acetylation. Fully substituted cellulose acetate with a conversion of 88.8% was obtained by using only 9 mol % of the catalyst, which is still higher than common commercialized solid acid catalysts.574 Some other applications include BAIL catalyzed synthesis of 2-dimethylaminoethyl methacrylate-dicarboxylate polymer adhesives,575 1-(3-propylsulfonic)-3-methylimidazolium hydrogen sulfate [(HSO3)3C3C1im][HSO4] catalyzed methanolysis of poly(lactic acid),576 glycolysis of PET,577,578 depolymerization of PET using 1-hexyl-3-methylimidazolium hydrogen sulfate [C6C1im][HSO4],579 and degradation of chitosan.580 Selected applications of the use of acidic ionic liquid catalysis to prepare polymers and oligomers are outlined in Table 15. 9.1.9. Miscellaneous Catalysis Applications. The preparation of glycerol dimethyl ether (2,3-dimethoxy-1propanol), a potential fuel additive, from glycerol using AIL catalysts is a noteworthy application.586 This route uses both glycerol and methanol as starting materials, by way of epichlorohydrin as an intermediate product, and utilizes HCl as a recycling agent. Among various BAIL catalysts investigated, the acidic ionic liquid (4-butylsulfonic)-trimethylammonium hydrogensulfate [(HSO3)4C4(C1)3N][HSO4] exhibited the highest activity and selectivity in the process.586 N-Boc protection of amines is a common synthetic operation. Shirini and co-workers have introduced a new class of BAIL,

Figure 25. [(HSO3)4C4C1im][OTf] catalyzed Ritter reaction558

Figure 26. [(HSO3)4C4 C1im][OTf] catalyzed rearrangements of stilbene epoxides560

Figure 27. [(HSO3)4C4C1im][OTf] catalyzed rearrangement of a propargylic alcohol to α,β−unsaturated enone via the Rupe rearrangement.561

Figure 28. Pinacol rearrangement of hydrobenzoin under microwave irradiation using [(C2)3NH][HSO4] catalyst562

Figure 29. Oxidation of alcohols with H2O2 catalyzed by long chain −SO3H functionalized heteropolyanion-based ionic liquids under solvent-free conditions.570

ketones were obtained in 63%−100% yields using SiW12O404− anions. For benzyl alcohols, the corresponding benzoic acids were obtained in 64%−94% yields using PW12O403− anions.

Table 15. Application of Acidic Ionic Liquids in Polymerization and Oligomerization Reactions acidic ionic liquid catalyst(s)

polymerization/oligomerization reaction

[C4C1im][AlCl4] and [(EtOCOCH2)C1im][AlCl4] [(HSO3)4C4 C1im][OTf], [(HSO3)4C4 C1im][BF4], [(HSO3)4C4 C1im][OTs], [(HSO3)4C4 C1im][HSO4], [(HSO3)4C4 Py][ HSO4] and [(HSO3)4C4 (alkyl)im][HSO4] supported on silica gel [HPyrr][HSO4] Multi-SO3H-functionalized ionic liquid

6154

α-pinene polymerization copolymerization of lactic acid and εcaprolactone oligomerization of isobutene homo and graft polymerization of εcaprolactone polymerization of lactic acid-ethylene glycol

refs 581,582 583 265 584 585


Chemical Reviews

Review

9.2.1. Cyclic Carbonate Synthesis. Cyclic carbonates are valuable industrial raw materials and acidic ionic liquids are excellent catalysts for the preparation of cyclic carbonates from epoxides and carbon dioxide.143,597 A variety of acidic ionic liquids have been shown to be good catalysts for this process, which includes; imidazolium, pyridinium, triethanolammonium protic and −SO 3 H functionalized ionic liquids, 143,598 [(HO2C)2C2Ph3P][Br],599 2-(N,Ndimethyldodecylammonium)acetic acid bromide,319 and carboxyl-isobutyl-functionalized immobilized ionic liquids.284 Xiao and co-workers studied a series of protic ionic liquids for the synthesis of propylene carbonate from CO2 and propylene oxide as shown in Figure 31.143 The effect of protic acid ionic liquid catalysts on the synthesis of propylene carbonate is shown in Table 16.

succinimidinium hydrogensulfate, as efficient catalysts for the room temperature amine protection reaction under neat conditions.587 The miscellaneous catalysis applications of BAILs include: the insertion of α-diazoacetate into the N−H bonds of amines,485 regioselective thiocyanation of aromatic and heteroaromatic compounds at room temperature,588 and conversion of alcohols to azides.589 Selective nitration of phenols is another instance where nitration with sodium nitrate could be carried out in the presence of acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate [C4C1im][HSO4] at room temperature, in good to high yields and short reaction times.590 Furthermore, regioselective mononitration of chlorobenzene has been investigated in the HNO3−Ac2O system with caprolactam based BAILs as shown in Figure 30.591 The sequence of

Figure 31. Protic acid ionic liquid catalyzed synthesis of propylene carbonate from propylene oxide and carbon dioxide.143 Figure 30. Caprolactam based BAIL catalyzed regioselective mononitration of chlorobenzene591

Table 16. Effect of Protic Acid Ionic Liquid Catalysts on the Synthesis of Propylene Carbonatea143

nitrification activity is [CP][HSO4] > [CP][p-TSA] > [CP][BSA]. The mole ratio of chlorobenzene to ionic liquid was 10:1.5, at 60 °C and the reaction time was 2.5 h, the yield of mononitro-chlorobenzene could reach 71.22%; and the mass ratio of para/ortho isomer was 7.74, which was much more than 2.0 of para/ortho isomer mass ratio obtained by nitricsulfuric mixed acids as catalyst. Besides, the ionic liquid could be used repeatedly.591 Acidic ionic liquid catalyzed direct nitration of 1,4,5,8-tetraaza-bicyclo-[4,4,0]-decane to synthesize 1,4,5,8-tetranitro-1,4,5, 8-tetraazabicyclo-[4,4,0]-decane (TNAD) with N2O5 is also another interesting application.592

entry

protic ionic liquid catalyst

yieldc (%)

1 2 3 4 5 6 7 8 9 10b

methylimidazole (C1im) methylimidazolium bromide [C1Him][Br] imidazoliumiodide [Him][I] triethylammonium bromide [(C2)3HN][Br] pyridinium bromide [HPy][Br] methylimidazolium chloride [C1Him][Br] methylimidazoliumiodide [C1Him][I] ethylimidazolium bromide [C2Him][Br] butylimidazolium bromide [C4Him][Br] methylimidazolium bromide [C1Him][Br]

trace 94.5 93.3 34.9 31.6 47.4 91.8 85.8 77.7 59.8

a

Reaction conditions: propylene oxide 5.0 mL, catalyst 1.0 mol %, CO2 1.5 MPa, 120 °C, 2.0 h. bCatalyst 0.5 mol %. cIsolated yield and the selectivity >99%.

9.2. Carbon Dioxide Dissolution and Fixation

Dissolution and fixation of CO2 is an important green chemistry topic of current interest, which include the reduction and utilization of captured CO2 for the synthesis of valuable chemicals.593 Amide-based Brönsted acidic ionic liquids are well-known for their good CO2 absorption characteristics.594,595 Deng et al. has prepared a series of amide-based Brönsted acidic ionic liquids by acid−base neutralization reaction of N,Ndimethylformamide (DMF), N,N-dimethylacetamide (DMAc), and N-methylpyrrolidone (NMP) with trifluoroacetic acid (TFA) or tetrafluoroboric acid (FBA). The solubility data of CO2 in these BAILs was determined at different temperatures and sub atmospheric pressure using isochoric saturation method. In these experiments they found that, with the same cation CO2 solubility in TFA-based BAILs are higher than that in FBA-based ones.594 Mumford et al. recently evaluated the protic ionic liquid ethylenediamine N,N-dimethylaminoethylammonium formate for the removal of CO2 from N2 mixed gas streams.596 This evaluation was conducted by comparison with the industrial standard 30 wt % monoethanolamine (MEA). These studies demonstrated that the sorption process included a chemical reaction component and is comparable to MEA; however, the mass transfer coefficient is an order of magnitude lower than MEA, which was likely due to the higher viscosity of the ionic liquid and its impact on carbon dioxide diffusivity.596

In another example Gao and co-workers proved that a polystyrene-supported Lewis acidic iron-containing ionic liquid is a recyclable heterogeneous catalyst for converting CO2 into cyclic carbonates.130 Notably, the catalyst could be readily recovered and reused over five times without appreciable loss of catalytic activity. Additionally the protocol has been applied to reactions of aziridines/propargyl amines with CO2.130 Carboxyl functionalized dication imidazolium-based ionic liquids have also been evaluated as efficient catalysts for the synthesis of cyclic carbonates from epoxides and CO2 in the absence of a cocatalyst and a solvent.600 9.2.2. Urea Synthesis. Lewis acidic ionic liquids are known to catalyze the reaction of CO2 with amines to give ureas. Yao et al. reported the use of [C4C1im][AlCl4] as the catalyst and solvent, for the reaction of CO2 with aniline at 160 °C, producing 1,3-diphenyl urea in 17.9% yield and 98.9% selectivity as shown in Figure 32.601 9.3. Ionogels, Electrolyte, Battery, and Capacitor Applications

Mizumo and co-workers reported a thermally stable, water-free proton conductor based on Brönsted acidic ionic liquid and 6155


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Protic ionic liquid based membranes have been tested in polymer electrolyte membrane fuel cells. In this instance Nafion membranes impregnated with the protic ionic liquids 1-(4butylsulfonic)-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [(HSO3)4C4C1im][NTf2], 1-(4-butylsulfonic)3-methylimidazolium bis trifluoromethanesulfonate [(HSO3)4C4 C1im][OTf] and membranes based on the polymerization of 1-(4-butylsulfonic)-3-vinylimidazolium trifluoromethanesulfonate [(HSO3)4C4(CC)im][OTf] were tested. This is the first report that describes the application of a polymerized protic ionic liquid membranes for fuel cells.630 An alternative approach is the immersing of the membranes based on imidazolium ionic liquids with 1-butyl-3-methylimidazolium dihydrogen phosphate [C4C1im][H2PO4] and polymers of sulfonated poly(ether−ether) ketone (SPEEK) or polyvinylidenefluoride (PVDF) into pure phosphoric acid at room temperature. A composite membrane fabricated by this method has achieved proton conductivity of 3.0 × 10−2 S cm−1 at 160 °C under anhydrous conditions.631

Figure 32. Lewis acidic ionic liquid catalyzed synthesis of 1,3-diphenyl urea.

silicate network.602 This material was prepared with a mixture of bis(trifluoromethyl sulfonyl)imide and 1-(4-butylsulfonic)-3methylimidazolium hydrogen sulfate by using the sol−gel process. The ionic conductivity of the ionogel reached 10−3 S cm−1 at 150 °C without any humidification.602 Acidic ionic liquids have been studied as electrolytes for several applications like battery electrolytes, electrophoresis media and electroplating media.603 Rezaei et al. studied the electrochemical and corrosion behaviors of alkyl ammonium hydrogensulfates as electrolyte additives on Pb alloy electrodes of the lead-acid battery.604 A few acidic ionic liquids such as [(C6H5CH2)(C1)2HN][AlCl4] have been tested as electrolytes for batteries.605,606 Then there is an example in the use as a solar cell electrolyte, where the acidic ionic liquid polymer poly[((3-(4-vinylpyridine) propanesulfonic acid) iodide)-co(acrylonitrile)] works as an ionic liquid electrolyte for dyesensitized solar cells.607 The cell based on electrolyte containing 20 wt % poly-AIL yielded an overall energy conversion efficiency of 6.95% under AM 1.5 illumination at 100 mW cm−2.607 A number of acidic ionic liquids have been used as neat liquids as well as additives in electrodeposition experiments, which include: [C4C1im][HSO4],608,609 [C2py][HSO4], [C4py][HSO4] and [C6py][HSO4] in the electrodeposition of zinc,610 [(C2)4N][HSO4], [(C4)4N][HSO4],611 [C4py][HSO4] and [C 6 py][HSO 4 ] in copper electrodeposition, 612,613 [(C1)3HN][Cl],614 [(C2)3HN][Cl]615 in the electrodeposition of Al, [C2C1im][AlCl4] in the electrodeposition of Al−W alloy616−618 and octyl-3-methylimidazolium chlorogalate in the electrodeposition of Ga.619

9.5. Electrophoresis Applications

In an interesting chromatography application N-methyl-2pyrrolidonium methyl sulfonate acidic ionic liquid has been used as a new dynamic coating for separation of basic proteins by capillary electrophoresis.632 The authors found that AIL modified capillary not only generated a stable suppressed electro osmotic flow, but also effectively eliminated the wall adsorption of proteins. 9.6. Membrane Applications

Acidic ionic liquid incorporated membranes are known and these materials are particularly useful for the separation of gases633,634 as well as incorporation of ion exchange and antibiofouling properties.635 Huang et al. reported the preparation and use of dicarboxylate-based IL incorporated membranes as a class of tunable media for the selective separation of acidic gases.633 When the anions of dicarboxylatebased ILs are fully deprotonated, they could be used as effective carriers for the selective separation of CO2. The permeabilities of CO2 in triethylbutylammonium malonate [C4(C2)3N]2[malonate] and triethylbutylammonium maleate [C4(C2)3N]2[maleate] under the partial pressure of 0.1 bar range from 2147 to 2840 barrers and the permselectivities of CO2/N2 and CO2/CH4 in them approach to 178−265 and 98−221, respectively. However, when the anions of dicarboxylate-based ILs are half deprotonated, they are efficient solvents for the selective separation of SO2.633 The application of encapsulated ionic liquids within porous moieties in the proton exchange membranes field is described in a book chapter by Eguizábal and Pina as well.636 Nafion-polymeric acidic ionic liquid composite membranes with different cations have been studied for their physical properties.637 In general, the Nafion/polymeric IL composite membranes exhibit a significant increase in the ionic conductivities than Nafion under anhydrous conditions. The interactions between the Nafion ionomer and different geometric cations of polymeric ILs were also discussed by Lu et al. in the comparison of nanostructures, dynamic-mechanical properties, and thermal stabilities of the Nafion/polymeric IL composite membranes.637 The molecular dynamics and ion transport studies on polymerized imidazolium-based protic ionic liquid [(HSO3)4C4(CC)im][OTf] have indicated a strong decoupling between conductivity relaxation times σσ (related to the ions migration through the polymer matrix) and

9.4. Fuel-Cell Applications

The use of acidic ionic liquids as electrolytes in fuel-cells is an emerging field due to their high conductivity, as well as their thermal, chemical and electrochemical stability.620,621,338,622 Yasuda et al. investigated diethylmethylammonium trifluoromethanesulfonate [(C2)2C1HN][OTfs], which exhibits favorable bulk properties and electrochemical activity for fuel-cell applications. In this case solid thin films containing [(C2)2C1HN][OTfs] were fabricated using sulfonated polyimide as a matrix polymer and used in nonhumidifying fuel cell operation at 120 °C.621,623 Many research groups have studied the use of protic ionic liquids as a component in composite proton exchange membranes (PEMs) for the next generation fuel cells.624−626 The protic ionic liquids studied in this application include 1vinylimidazolium trifluoromethanesulfonate [(CH2CH)Him][OTf],627 1-methyl-imidazolium trifluoromethanesulfonate [C1Him][OTf],628 and polymer electrolyte membranes based on poly(ethylene oxide)/1-butyl-3-methylimidazolium hydrogen sulfate [4C4C1im][HSO4].629 In another approach the acid retention capability of poly(4,4′-diphenylether-5,5′bibenzimidazole) (OPBI) nanocomposite membranes in fuel cells could be enhanced by incorporating protic ionic liquids.625 In this case silica nanoparticles of 25 nm size were prepared, successfully modified with phosphate anion containing imidazolium AILs and were incorporated in OPBI by solution blending method.625 6156


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comprising acidic ionic liquid N,N-dimethylacetamide methanesulfonate, Ru/C, and formic acid as shown in Figure 34.658

segmental dynamics when the ionic transport is controlled by fast proton hopping through the dense hydrogen-bond network.638 Some selected examples of the use of acidic ionic liquids in ion conducting membranes are shown in Table 17. Table 17. Use of acidic ionic liquids ion conducting membranes AIL membrane application diethylmethylammonium triflate [(C2)2C1HN][OTf] doped Nafion membrane triethylammmonium triflate [(C2)3HN][OTf] doped Nafionmembranes diethylmethylammonium triflate [(C2)2C1HN][OTf] proton conductivity alkylammonium hydrogensulfate [RH3N][HSO4] in permeation of chromium(III) using ionic liquid and pseudoemulsion hollow fiber strip dispersion. N,N-dimethylpyrrolidinium methylphosphite membrane [(C1)2Pyrr][MeHPO3] for the separation of acetylene/ olefin mixtures

Figure 34. One-pot conversion of lignocellulosic and algal biomass into 2,5-dimethylfuran (DMF)658

refs 639 640,641,639

Cellulose can be depolymerized to glucose and then dehydrated to HMF in one-pot using acidic ionic catalysts.659 Ding et al. reported the catalytic conversion of microcrystalline cellulose (MCC) to HMF using CuCl2 in 1-(4-butylsulfonic)-3methylimidazolium hydrogen sulfate [(HSO 3) 4C4 C1 im][HSO4] in 69.7% yield.660 In an example using raw biomass, the carbohydrate-rich weed species foxtail weed was directly converted to platform chemicals HMF and 5-ethoxymethyl-2furfural using dimethylacetamide methanesulfonate and Nmethylpyrolidinium methanesulfonate as catalysts giving HMF in 58 and 52% yields, respectively.661 In the C5 sugar platform, Serrano-Ruiz et al. reported the dehydration of xylose to furfural using −SO3H functionalized acidic ionic liquids under microwave heating.662 In another example, Tao and co-workers reported the 1-(4-butylsulfonic)3-methylimidazolium hydrogen sulfate [(HSO3)4C4C1im][HSO4] catalyzed dehydration of xylose into furfural under mild conditions. A xylose conversion of 95.3% with a furfural yield of 91.45% was achieved in a 25 min reaction time at 150 °C.663 4-Oxopentanoic acid or levulinic acid is a renewable generation key chemical that can be derived from C5 and C6 carbohydrates via a series of dehydration/hydration reactions and acidic ionic liquids have been used as catalysts for the preparation of levulinic acid and related compounds. High molecular weight cellulose has been converted to a mixture of ethyl levulinate and levulinic acid by using 1-(3-propylsulfonic)3-methylimidazolium chloride [(HSO3)3C3C1im][Cl] as the catalyst in aqueous ethanol medium in a one-pot operation under mild conditions as shown in Figure 35.664 The highest ethyl levulinate yield of 19.0% was obtained for a reaction carried out at 170 °C, for 12 h, in water−ethanol medium containing 38.5% water. The levulinic acid yields continues to increase with increasing water content up to about 54% water in aqueous ethanol for reactions carried out at 150 °C for 48 h, and the highest levulinic acid yield was 23.7%. The BAIL catalyst could be efficiently recovered (96%) from the water phase with negligible contamination and the stability of the catalyst was confirmed by comparison of the 1H NMR spectrum of the recovered catalyst with fresh catalyst.664 In another example cellulose has been liquefied in ethylene glycol at 180 °C, using 6.7 mol % 1-(3-propylsulfonic)-3methylimidazolium chloride [(HSO3)3C3C1im][Cl] and 1-(4butylsulfonic)-3-methylimidazolium hydrogen sulfate

621 642,643 644

9.7. Conversion of Biomass to Renewable Feedstock Chemicals

9.7.1. Renewable Furans, Levulinic Acid, and Related Compounds. Acidic ionic catalyzed dehydration of carbohydrates to 5-hydroxymethylfurfural, furfural, and levulinic acid and related compounds has emerged as an active research area in the last 4−5 years due to the current interest in renewable feedstocks and fuels. Dehydration of carbohydrates to furan derivatives is the most widely studied reaction, and the common dehydration of fructose to 5-hydroxymethylfurfural (HMF) occurs via elimination of three molecules of water as shown in Figure 33. 645,646 Acidic ionic liquids 1-(4butylsulfonic)-3-methylimidazolium hydrogen sulfate,647,648,350 1-(4-butylsulfonic)-3-allylimidazolium hydrogen sulfate trifluoromethanesulfonate,649 N-methyl-2-pyrrolidonium hydrogen sulfate,329 1-methyl-3-(butyl-4-chlorosulfonyl) imidazolium chlorosulfate,650 1-ethyl-3-methylimidazolium hydrogen sulfate,651 dimethylimidazolium hydrogensulfate,652,653 dimethylbenzaimidazolium hydrogensulfate,652 1-carboxymethyl-3methyl imidazolium chloride,654 1-carboxypropyl-3-methyl imidazolium chloride,655 and Cr3+ containing SO3H-functionalized polymeric BAILs656 have been reported as catalysts. Acidic ionic catalyzed dehydration of D-glucose to 5hydroxymethylfurfural is also known.657 Amarasekara and Razzaq have proposed a mechanism for 1-(1-propylsulfonic)3-methylimidazolium chloride catalyzed transformation of Dglucose to HMF using NMR studies of C-1 and C-2 13C labeled D-glucose. The proposed mechanism involves an isomerization of D-glucose to D-fructose via open chain forms during the dehydration reaction.645 In a related example, one-pot conversion of lignocellulosic and algal biomass into a liquid fuel 2,5-dimethylfuran (DMF) has been achieved by using a multicomponent catalytic system

Figure 33. Brönsted acidic ionic liquid catalyzed transformation of D-fructose to 5-hydroxymethylfurfural (HMF). 6157


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Figure 35. Brönsted acidic ionic liquid 1-(3-propylsulfonic)-3-methylimidazolium chloride [(HSO3)3C3C1im][Cl] catalyzed conversion of cellulose to 5-hydroxymethylfurfural (HMF), levulinic acid (LA), and ethyl levulinate (EL) in water−ethanol solvent system.664

The catalytic performances of AIL containing HSO4− anion and acidic functionalized imidazolium cations have been investigated for biomass liquefaction in polyethylene glycol 400-glycerol mixtures as well as in monohydric alcohol like 1octanol666,667 in order to explore green and efficient ways to convert woody biomass into sustainable energy and chemicals as shown in Figure 37. These results showed that fir sawdust could be liquefied up to 99% using 0.3 mol·L−1 1-(3propylsulfonic)-3-methylimidazolium chloride as the catalyst at a temperature of 423 K, in 60 min. Furthermore, Lu and coworkers reported that more than 90 wt % of the bio-oil was the heavy oil that derives from lignin.668 Single reactor conversion of lignocellulosic biomass to C5− C20 furanic biocrude oils using sulfonic acid functionalized Brönsted acidic ionic liquid catalysts is another application of BAILs.664 In this biomass liquefaction untreated switch grass was liquefied to a mixture of biomass derived furan-acetone aldol condensation products using [(HSO3)3C3C1im][Cl] or [(HSO3)4C4C1im][Cl] as the BAIL catalyst under moderate temperature−pressure conditions.669 As described earlier there are numerous examples of applications of acidic ionic liquids in the preparation of levulinic acid and related biomass derived compounds, some selected examples and their references are shown in Table 18. 9.7.2. Depolymerization of Cellulose. Ionic liquids with built-in −SO3H acid functionality is an emerging class of biomass processing systems,676 whereas relatively milder acidic ionic liquids like 1-butyl-3-methylimidazolium hydrogensulfate are used in partial depolymerization and preparation of

[(HSO3)4C4 C1im][Cl]. The liquefied oil produced contained only three compounds as shown in Figure 36, which were

Figure 36. 1-(3-Propylsulfonic)-3-methylimidazolium chloride [(HSO3)3C3C1im][Cl] and 1-(4-butylsulfonic)-3-methylimidazolium hydrogen sulfate [(HSO3)4C4 C1im][Cl] catalyzed liquefaction of cellulose in ethylene glycol.

identified as 2-hydroxyethyl levulinate, 2-hydroxyethyl levulinate ethylene ketal, and 2,3,6,7-tetrahydro-cyclopenta[1,4]dioxin-5-one.665 The composition of the three components reaches a steady state after 20 h reaction at 180 °C with 2hydroxyethyl levulinate: 2-hydroxyethyl levulinate ethylene ketal: 2,3,6,7-tetrahydro-cyclopenta[1,4]dioxin-5-one molar percentage ratio of approximately 47:22:31.665

Figure 37. Application of acidic ionic liquids in liquefaction of sawdust in polyethylene glycol 400-glycerol mixtures668 6158


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Table 18. Selected Examples of Application of Acidic Ionic Liquids in Preparation of Levulinic Acid and Related Biomass Based Compounds preparation of levulinic acid and related compounds.

acidic ionic liquid catalyst(s) [(HSO3)3C3C1im][HSO4], [(HSO3)4C4C1im][HSO4], [(HSO3)3C3C1im][Cl], [(HSO3)3C3C1im] [H2PO4], [(HSO3)3C3C1im][OMs], [(HSO3)3C3(C2)3N][HSO4], [(HSO3)3C3Py[HSO4] and [(HSO3)3C3Ph3P][HSO4] [C4C1im][HSO4] [(HSO3)4C4C1im][HSO4] [(HSO3)4C4C1im][HSO4] [((HSO3)3C3)2im][HSO4]

cellulose nanowhiskers.677 These Brönsted acidic ionic liquids can behave as the solvent as well as the catalyst; additionally, no neutralization and separation of the acid catalyst is required, and there is no waste in acid, as the acid is in the solvent itself. Furthermore, a higher concentration of −SO3H active sites is expected to accelerate the reaction and lower the operating temperature, thus saving energy. In March 2015 da Costa Lopes and Bogel-Łukasik published a review article with 131 references on perspectives of hydrolysis and conversion of cellulose and lignocellulosic biomass using acidic ILs, justifying the fast growth in the field.77 The first use of this class of Brönsted acidic ionic liquids were reported by Amarasekara and Owereh in 2009.678 In this work they reported that cellulose rapidly dissolves in Brönsted acidic ionic liquids 1-(3-propylsulfonic)-3-methylimidazolium chloride and 1-(4-butylsulfonic)-3-methylimidazolium chloride up to 20 g/100 g ionic liquid by gentle mixing at room temperature. Optical microscope images (x 400) of dissolution of Sigmacell cellulose (DP ≈ 450) are shown in Figure 38.678

cellulose → levulinic acid bamboo shoot shell → levulinic acid furfural + ROH (MeOH, EtOH, nBuOH and iPrOH) → levulinate esters cellulose + butanol → butyl levulinate furfural alcohol → alkyl levulinates

refs hydrothermal670 and microwave conditions671 672,673 674 675 320

Table 19. Average % Yields of TRS and Glucose Produced in 10% w/w Cellulose in Brönsted Acidic Ionic Liquid Solutionsa temp. (°C)/time (min)

entry

BAIL/cellulose

1

[(HSO3)3C3C1im][Cl] /αcellulose [(HSO3)3C3C1im][Cl]/ MC-cellulose [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)3C3C1im][Cl]/ Sigmacell [(HSO3)4C4C1im][Cl]/αcellulose [(HSO3)4C4C1im][Cl ]/MC-cellulose [(HSO3)4C4C1im][Cl]/ Sigmacell [(HSO3)4C4Py][Cl]/αcellulose [(HSO3)4C4Py][Cl]/MCcellulose [(HSO3)4C4Py][Cl]/ Sigmacell [(HSO3)4C4(HO2C2)3N] [Cl] /α-cellulose [(HSO3)4C4(HO2C2)3N] [Cl]/MC-cellulose [(HSO3)4C4(HO2C2)3N] [Cl]/Sigmacell

2 3 4 5 6 7 8 9 10 11 12 13

Figure 38. Optical microscope images (×400) of dissolution of Sigmacell cellulose (DP ≈ 450) in 1-(3-propylsulfonic)-3-methylimidazolium chloride at room temperature (23 °C) and atmospheric pressure, after 0, 60, and 150 s. Reprinted with permission from ref 678. Copyright 2009 American Chemical Society.

14 15 16 17

Hydrolysis of cellulose was tested with three cellulose-ionic liquid systems with imidazolium, pyridinium and triethanol ammonium cations by the addition of 2.0 equiv of water per glucose unit of cellulose and heating the solution at 70 °C, and at atmospheric pressure with or without preheating to give glucose along with other reducing sugars. The average % yields of total reducing sugar (TRS) and glucose produced in a series of cellulose hydrolysis experiments using Brönsted acidic ionic liquids are shown in Table 19. Hydrolysis of Sigmacell cellulose (DP ≈ 450) in 1-(3-propylsulfonic)-3-methylimidazolium chloride produced the highest total reducing sugar (62%) and

18 19 20

yield (%)

before adding H2O

after adding H2O

TRS

glucose

70/60

70/30

59

15

70/60

70/30

12

4

70/60

70/30

62

14

70/30

39

12

70/40

70/30

56

12

70/60

70/60

42

7

70/60

70/240

29

4

50/960

32

3

90/30

34

3

90/30

26

2

90/240

15

2

70/60

70/30

32

70/60

70/30

7

70/60

70/30

12

70/60

70/30

14

70/60

70/30

8

70/60

70/30

16

70/60

70/30

5

70/60

70/30

2

70/60

70/30

10

90/30

a 2.0 equiv of H2O per glucose unit of cellulose were added in all hydrolysis experiments678.

glucose (14%) yields, and was attained with 1 h. of preheating at 70 °C and 30 min heating at 70 °C, after adding water. 6159


Chemical Reviews

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Table 20. Selected Examples of Application of Acidic Ionic Liquids in Hydrolysis of Cellulose and Cellulosic Biomass acidic ionic liquid catalyst(s)

hydrolysis of polysaccharide/biomass and conditions

1-propyl sulfonic acid-2-phenyl imidazoline hydrogensulfate and 1- butyl sulfonic acid-2phenyl imidazoline hydrogensulfate [(HSO3)3C3C1im][HSO4], [(HSO3)3C3C1im][OTs] and [(HSO3)3C3C1im][Cl] [(C1Him][Cl] [(HSO3)3C3(C2)3N][HSO4] [(HSO3)4C4C1im][HSO4] [(HSO3)3C3C1im][Cl] immobilized on silica [(HSO3)3C3(CC)im][OTf] polymer [(HSO3)4C4C1im][Cl] 1,1,3,3-tetramethylguanidinium hydrogensulfate [(HSO3)3C3C1im][Cl] immobilized on polystyrene [(HSO3)3C3C1im][Cl] and [(HSO3)4C4C1im][Cl] [(HSO3)4C4N2(CH2)3]3−nHnPW12O40 [(HSO3)3C3C1im][HSO4].1/3Cr [(HSO3)3C3C1im][Cl] immobilized on biochar with ZnCl2 and CuCl2

refs

cellulose, 100 °C, 60 min. 85.1% TRS yield

687

hardwood hemicellulose, 160 °C to give C5 sugars (xylose + arabinose) in 87% yield soybean straw and corn straw, 70 °C, 120 min. cellulose, 100 °C, 99% TRS yield cellulose, sawdust, 80−120 °C cellulose, 70 °C, glucose and TRS in 26 and 67% yields respectively cellulose, 100 °C, 77% yield switchgrass, 70 °C for 2 h, 58. One %TRS and 15. Three % glucose yields cellulose, 26% glucose and 72% TRS yields cellulose, 160 °C, 3 h, glucose, TRS in 21.7 and 50.1% yields respectively cellulose, 140−180 °C, 3 h cellulose cellulose, 94% TRS yield cellulose and bamboo

688 689 679 690,691 225 692 693 694 282,695 683 696 695 697,698

series of −SO3H functionalized imidazolium, pyridinium, trialkylammonium BAILs have revealed that single −SO3H attached imidazolium catalysts are more effective than pyridinium and trialkylammonium types in cellulose hydrolysis in water at moderate temperature−pressure conditions.683 A number of research groups have studied acidic ionic liquids immobilized on a solid supports, which include; graphene-like nanoporous carbons,684 silica,225,233 and polystyrene282 for the depolymerization of cellulose as well as various lignocellulosic biomass forms. Acidic ionic liquid catalysts have been tested for liquefaction of untreated lignocellulosic biomass samples as well, and these experiments often produced complex mixtures of products. Long and co-workers used a −SO3H, −COOH functionalized, and HSO4-paired imidazolium ionic liquid as catalysts for bagasse liquefaction in hot compressed water. Using −SO3H functionalized ionic liquid at 542 K, 96.1% of bagasse could be liquefied and 50.6% was selectively converted to low-boiling biochemicals.685 In another experiment a series of AILs containing HSO4− and imidazolium cation were used in the liquefaction of Chinese fir sawdust in 1-octanol. This experiment showed that the liquefaction rate could be gradually improved with the AILs of increasing acidity, and reached 71.5% when 1-(4-butylsulfonic)-3-methylimidazolium hydrogen sulfate was used as the catalyst at 423 K, with the 6:1 mass ratio of 1-octanol to sawdust.667 Zhuo et al. recently reported the synthesis of a series of 2-phenyl-2-imidazoline based acidic ionic liquids and used as catalysts for the hydrolysis of cellulose in 1-butyl-3-methylimidazolium chloride.686 They noted that BAILs with anions HSO4− and Cl− showed better catalytic performance for the hydrolysis of cellulose than those with H2PO4−.686 Selected recent examples in the hydrolysis of cellulose and cellulosic biomass using acidic ionic liquids are shown in Table 20. The addition of some metal ions699−701 and zeolites702 as cocatalysts are also known to enhance the BAIL catalyzed hydrolysis of cellulose in neat and in aqueous media. In a recent development the effect of eight metal ions, Cr3+, Mn2+, Fe3+, Co2+ Ni2+, Cu2+, Zn2+, and La3+ on 1-(1-propylsulfonic)-3methylimidazolium chloride acidic ionic liquid catalyzed hydrolysis of cellulose in water at 140−170 °C was reported.700

Later, Liu et al. as well as Feng et al. also reported the use of Brö nsted acidic ionic liquids for catalysis of cellulose depolymerization under mild conditions.679,680 However, their approach is somewhat different from the earlier group, where Liu et al. first dissolved cellulose in the neutral ionic liquid 1-nbutyl-3-methylimidazolium chloride to make a 5% solution, and then Brönsted acidic ionic liquid and a controlled amount of water was added to hydrolyze cellulose. Furthermore, a series of −SO3H group functionalized imidazolium and triethylammonium Brönsted acidic ionic liquids were used as catalysts in the Liu et al.’s study.679 They found that all of the Brönsted acidic ionic liquids studied are effective in hydrolysis of cellulose, with the maximum total reducing sugar (TRS) yields over 83% at 100 °C. Acidic ionic liquids with analogous structures showed similar catalytic activities and (3-propylsulfonic)-triethylammonium hydrogen sulfate showed the highest activity in this study. Interestingly, acid group functionalized acidic ionic liquids can be used as catalysts in aqueous media as well under moderately high temperature−pressure conditions with or without a cocatalyst. Amarasekara and Wiredu studied the catalytic activities of dilute aqueous solutions of 1-(3propylsulfonic)-3-methylimidazolium chloride for the hydrolysis of pure cellulose by comparison with p-toluenesulfonic acid and sulfuric acid.681 In this study dilute aqueous solutions of 1(3-propylsulfonic)-3-methylimidazolium chloride and p-toluenesulfonic acid are shown to be better catalysts than aqueous sulfuric acid of the same H+ ion concentration for the degradation of cellulose at moderately high temperatures and pressures. For example, Sigmacell cellulose (DP ∼ 450) in aqueous solutions of 1-(1-propylsulfonic)-3-methylimidazolium chloride, p-toluenesulfonic acid, and sulfuric acid of the same acid strength (0.0321 mol H+ ion/L) produced total reducing sugar (TRS) yields of 28.5, 32.6, and 22.0% respectively, after heating at 170 °C for 3.0 h. In the same set of experiments 22.2, 21.0, and 16.2% glucose yields were attained in 1-(1propylsulfonic)-3-methylimidazolium chloride, p-toluenesulfonic acid, and sulfuric acid mediums, respectively.681 In addition, hydrolysis of a cellulose model compound in water also supported the observation of enhanced catalytic activity of −SO3H functionalized imidazolium BAILs in water in comparison to H2SO4.682 Structure−activity relationships in a 6160


Chemical Reviews

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Table 21. Recent Applications of Acidic Ionic Liquids in Biodiesel Synthesis acidic ionic liquid catalyst(s)

starting material, biodiesel % yield

[C4C1im][HSO4] and [C4C1im][OTf] [(HSO3)3C3(CC)im][H2PO4] polymer [(HSO3)3C3C1im][HSO4] immobilized on magnetic nanoparticles [(HSO3)4C4C1im][HSO4] -Fe2(SO4)3 [(HSO3)4C4C1im][Cl] 3-butyl-6-sulfo-1-(4-benzylsulfonic)-1H-imidazolium hydrogen sulfate [(HSO3)3C3pyr][HSO4] immobilized on polymer [(HSO3)4C4 (alkyl)im][OTf] immobilized on silica [(HSO3)3C3(C2)3N][Cl].0.67FeCl3 [(HSO3)4C4C1im][HSO4], [(HSO3)3C3C1im][HSO4] and 1-methyl-2-(4-butylsulfonic)pyrazolium hydrogensulfate [(HSO3)3C3(C2)3N][HSO4] [(HSO3)4C4pyr][HSO4] [(C2)3HN][HSO4] and [C4Him][HSO4]

refs

waste cooking oil, 95.65%. Sewage sludge lipids, 90% vegetable oil, 94.2% soybean oil with short-chain alcohols Camptotheca acuminata seed oil, 95.7% jatropha curcas oil, 93.9% Nigella sativa seed oil with methanol waste cooking oil, 99.0% glycerol trioleate with methanol waste cooking oil, 95% soybean oil and citrullus colocynthis oil

713,714

soybean oil with methanol, 93.2% soybean oil and methanol, 94.5% palm oil

720 721 722,723

220 239 189 715 716 215,213,214 227 192 717−719

depolymerized and subsequently dissolved in the ionic liquid. This process occurred more quickly at higher temperatures, although at the highest temperatures tested significant cellulose degradation also occurred.707 Then there are few other biomass forms that have been tested with aqueous acidic liquid pretreatments, which include pretreatment of corn stover using [C1C2im][HSO4],171 microalgae using n-butyl-3-methylimidazolium hydrogen sulfate,708 hybrid aspen and Norway spruce using [C1C4im][HSO4],709 and microwave pretreatment of eucalyptus [C1C4im][HSO4].710

It is interesting to note that certain transition metal ions like Mn2+, Fe3+, and Co2+ as cocatalysts produced significant enhancements in total reducing sugar (TRS) yields, with Mn2+ showing the highest activity. During these BAIL catalyzed cellulose hydrolysis in water, using Mn2+ as cocatalyst produced 91.8, and 91.9% TRS yields, whereas samples without Mn2+ gave 28.0 and 28.7% yields at 160 and 170 °C, respectively.700 9.7.3. Depolymerization of Lignin. In addition to the hydrolysis of polysaccharide components acidic ionic liquids have been used as acid catalysts for the depolymerization of lignin fraction in a number of biomass materials. Ekerdt and coworkers separated the lignin from Oak wood by dissolving in the neutral ionic liquid 1-methyl-3-ethylimidazolium acetate and subsequent precipitation. This was successfully depolymerized in the acidic ionic liquid 1-H-3-methylimidazolium chloride under mild conditions (110−150 °C).703 Furthermore, gel permeation chromatography results, showed that an increase in temperature from 110 to 150 °C increased the rate of reaction, but did not significantly change the final size of the lignin fragments.703 In model compound studies, the same research group showed that β-O-4 bonds of both guaiacylglycerol-β-guaiacyl ether and veratrylglycerol-β-guaiacyl ether undergoes catalytic hydrolysis to produce guaiacol as the primary product with more than 70% yield at 150 °C.704,327 Long and co-workers examined the liquefaction of sugar cane bagasse lignin using acidic ionic liquid 1-(4-butylsulfonic)-3methyl imidazolium hydrogen sulfate [(HSO3)4C4C1im][HSO4]. They showed that more than 65% of liquefaction can be achieved under optimized conditions, yielding 13.5% of useful aromatic fine chemicals such as phenol, 4-ethylphenol, and guaiacol.705 9.7.4. Pretreatment of Biomass. Pretreatment of biomass as a preparation for enzymatic hydrolysis is another application of acidic ionic liquids. In this application dilute aqueous solutions of acidic ionic liquids are often used under mild conditions. The selective hydrolysis of hemicellulose fraction is known in using BAIL based pretreatments.706 Cox and Ekerdt have shown that acidic ionic liquid 1-H-3-methylimidazolium chloride can be used for effective pretreatment of yellow pine wood chips under mild conditions.707 In this study wood samples were treated between 110 and 150 °C for up to 5 h in the ionic liquid and three fractions were collected; a cellulose rich fraction, lignin, and an aqueous fraction. This treatment caused the hemicellulose and the lignin to be degraded and dissolved from the cell walls of the pine wood. The lignin was

9.8. Biodiesel Synthesis

Biodiesel is the fatty acid methyl ester product formed by transesterification of vegetable oils or animal fats with methanol. The trans-esterification reaction can be catalyzed by bases or acids.711 A fair number of researchers have studied the option of using homogeneous and immobilized forms of Brönsted acidic ionic liquid catalysts for this trans-esterification reaction due to possibilities like low catalyst loading and recycling of the catalyst. In 2012 Amin, Fauzi and Hafiidz published a review article titled “An overview of ionic liquids as solvents in biodiesel synthesis” with 138 references summarizing the progress in this area until late 2011.712 More recent and after 2012 applications of acidic ionic liquids in biodiesel synthesis are shown in Table 21. 9.9. Desulfurization of Gasoline, Diesel, and Petroleum Refinery Applications

The removal of cyclic sulfur compounds and nitrogen compounds such as thiophene (TS), dibenzothiophene (DBT), carbazole, pyridine, and their derivatives from gasoline and other fuel oils is another application in acidic ionic liquids.212,724−726 A number of common Brönsted and Lewis acid ionic liquids have been tested for hydrodesulfurization and hydrodenitrogenation processes, which includes dialkylpyridinium tetrachloroferates and [C1C6im][FeCl4],727,728 1-methylimidazolium hydrogen sulfate [C1Him][HSO4], N-methylpyrrolidinium hydrogen sulfate [C1HPyrr][HSO4],729 [C1C4im][HSO4],730,731 [(HSO3)2C2C1im][HSO4].732,733 N-methylpyrrolidonium tetraflouoroborate [C 1 HPyrr][BF 4 ], 734 [C4C1im][FeCl4],735 and [(COO)C1C1im][HSO4].382 Chen and co-workers demonstrated that Lewis acidic ILs 1butyl-3-methylimidazolium chloride with IL: ZnCl2 1:1 and 1:2 mol ratios as well as Brönsted acidic ILs [C4C1im][HSO4] and [C4C1im][HSO4], Lewis-Brönsted acidic IL N-methylpyrrolidonium zinc chloride with IL: ZnCl2 mole ratios 2:1 to 1:2 can 6161


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be used to extract thiophene, dibenzothiophene, carbazole, and pyridine from their hexane (model gasoline) or octane (model diesel fuel) mixtures. Typically, 93.8% TS removal (S-content drops from 500 to 31 ppm) and 95.9% DBT removal (Scontent from 516 to 21 ppm) by [C4C1im][ZnCl3] are achieved after 6-stage extraction at 25 °C; while 93.8% carbazole removal (N-content from 279 to 17 ppm) and 97.8% pyridine removal (N-content from 495 to 11 ppm) were realized just after one stage extraction and the N-content is undetectable after 2-stage extraction.736,187,737 In another experiment using a series of FeCl3-containing 1-hexyl-3methylimidazolium chloride Lewis-acidic ionic liquids the selectivity of sulfur compounds by extraction process followed the order of dibenzothiophene (DBT) > benzothiophene (BT) > 4,6-dimethyldibenzothiophene (4,6-DMDBT).738

Table 22. Electrodeposition of Metals Using Lewis Acidic Ionic Liquids with Halometalate Anions acidic ionic liquids [C2C1im][AlCl4]

[C2C1im][AlCl4] [C4C1im][AlCl4] [C2C1im][ZnCl3] - NiCl2 [C1(CC)im][AlCl4] and [C4C1im][AlCl4] [C2C1im][AlCl4] - K3[W2Cl9] [C8C1im][GaCl4] [(C1)3HN][AlCl4] [C4C1im][ZnCl3] and [C2C1im][ZnCl3] - CuCl [C2C1im][AlCl4] [C2C1im][AlCl4] - MgCl2

9.10. Metal Extractions and Processing

In the last 5−6 years a fair number of research groups have explored the use of ionic liquids for the extraction and processing of metals and particularly rare earth metals.739−741 Among the ionic liquids used in this applications, there are protic ionic liquids and carboxyl acid group functionalized ILs. The representative examples are the use of protic ionic liquids trioctylammonium bis(trifluoromethanesulfonyl)amide in the extraction of Pd and Pt,742 carboxyl-functionalized ionic liquid: betainium bis(trifluoromethylsulfonyl)imide, for a combined leaching/extraction of Nd from a neodymium−iron-boron magnets (NdFeB) in the recycling process,743 mixtures of trioctylammonium bis(trifluoromethanesulfonyl)amide and trioctylammonium nitrate for the extraction of aluminum(III), gallium(III), and indium(III) from hydrochloric acid solutions.744 In addition immobilized AILs have been used in selective absorption and removal of Cr6+ as well.237 The protic ionic liquids have been used to extract metal ions by leaching from metal ores. Dong et al. used an aqueous solution of 1-butyl-3-methylimidazolium hydrogensulfate for the leaching of chalcopyrite concentrate at ambient pressure at 50 to 90 °C in air. The copper extraction increased from 52% to 88% as the ionic liquid concentration in solution increased from 10% (v/v) to 100%. Copper extraction was very low at temperatures below 70 °C, but increased significantly at temperatures from 70 to 90 °C, suggesting a high activation energy for the chemical reaction.745 Recovery of zinc and copper with the BAIL as leachate is another recent application.746−748 In experiments with brass waste it was found that all zinc in brass waste could be dissolved using Brönsted ionic liquid 1-butyl-3-methyl-imidazolium hydrogensulfate. The metallic zinc could be recovered by the electrowinning method without a purification step.746

[C4Py][AlCl4] - NiCl2 [C2C1im][AlCl4] - HfCl4

electrodeposition/ electroplating metal/alloy

refs

Al on a Mg alloy AlInSb semiconductor alloys Al and Zn on AZ91D magnesium alloy Pb on Au NiZn nanofilaments on W Al,

752−755

758 759 760,761

Al−W alloy Ga Al wires Zn and Zn−Cu alloy

618,617,616 619 614 762,763

Ternary Al−W−Mn alloys Al−Mg alloys on to Pt and Cu Al−Ni alloys on to Pt and mild steel Al−Hf alloys onto Cu

764 765

756,757

89 766

In an electrochemical etching study, anodic behavior of Mg in 1-ethyl-3-methylimidazolium chloride-AlCl3 ionic liquid was investigated by Xu and co-workers.767 During this experiment dissolution of Mg under anodic polarization occurred after the breakdown of the oxide film and a formation of a viscous layer was observed at the Mg-ionic liquid interface during the dissolution process. 9.12. Miscellaneous Applications of Acidic Ionic Liquids

In addition to the AIL applications discussed, the oxygen reduction reaction has been studied at platinum, gold, and glassy carbon electrodes using cyclic voltammetry and potential-step chronoamperometry in 11 room temperature protic ionic liquids.768 Protic chiral ionic liquids based on ephedrines have been used in chiral recognition. Interestingly ephedrinium protic chiral ionic liquids displayed strong chiral recognition capabilities as evidenced by peak splitting in the 19 F-NMR spectrum of the Mosher’s salt. In addition, these protic chiral ionic liquids demonstrated enantiomeric recognition capabilities toward a range of structurally diverse analytes as well.369 The use as a structure directing media is another new application of AILs, where 1-butyl-3-methylimidazolium hydrogensulfate was used as an acidic, hydrolyzing and templating/ structure directing agent and where BAILs were used in the synthesis of nanocrystalline anatase TiO2769 and shapecontrolled biochar materials.770

9.11. Electrodeposition of Metals

10. CONCLUDING REMARKS The ionic liquid field has grown into a wide subject area in the past decade, and acidic ionic liquid is an important branch in this subject. The central theme of this review is to show the structural diversity of AILs and to highlight the range of complex applications of acidic ionic liquids. The combination of acidic and ionic liquid properties into a single molecule has opened new opportunities. The most recent advancements in using the new AILs appears to be in catalysis, lignocellulosic biomass processing, and electrolyte applications. In the catalysis area many organic reactions first discovered with mineral acids like concentrated H2SO4 were later found to work better with

Electrodeposition or electroplating of metals is an important manufacturing process. In some metals, molten salts are required in electrodeposition. In these cases the use of metal containing low melting acidic ionic liquids as the electrolyte has certain advantages as these metalo ionic liquids have much lower melting points than metal salts and requires less energy in maintaining the liquid electrolyte. The most common type of ionic liquids used for this application are Lewis acidic ionic liquids with halometallate anions, and some of the electrodeposition applications are described in the patent literature.749−751 A selected list of electrodeposition of metals using acidic ionic liquids is presented in Table 22. 6162


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organic soluble sulfonic acids like p-toluene sulfonic acid and fluorinated super acids. Now the same reactions are being tested again with BAILs and in many cases the BAILs have shown superior catalytic properties in comparison to organic sulfonic acids and mineral acids. This may be due to organic soluble nature and interactions of charged intermediates with cation/anion in BAIL that allows facile proton or electron transfers with acidic ionic liquids. The use of BAILs in biomass processing research is an important turning point. The introduction of sulfonic acid group functionalized acidic ionic liquids in the depolymerization of cellulose in 2009 has opened a new branch of research with a vast potential and industrial applications.678 The rapid progress in this arena since the introduction of AILs in biomass processing has been reviewed by da Costa Lopes and BogelŁukasik in the March 2015 review titled “Acidic ionic liquids as sustainable approach of cellulose and lignocellulosic biomass conversion without additional catalysts”.77 The future of acidic ionic liquids as biomass processing catalysts is bright, and one possibility is the development of an ionic liquid based small molecular mimic for cellulase enzyme. The BAIL catalyst has the features of the enzyme; it can bind with cellulose via hydrogen bonds, and if the orientation is correct it can deliver the acidic proton to glycosidic bond of cellulose promoting the hydrolysis, like the cellulase enzyme. However, a lot more work is needed in the area of BAIL−cellulose binding mechanisms using experimental and computational tools. The second highlighted area is the ionic conductivity aspects of acidic ionic liquids. The earliest known application of this property is the use of Lewis acidic ionic liquids with metal salts like AlCl3 for the electrodeposition of Al, which generally require high melting salt baths without the use of ionic liquid based systems. Then more recent developments are in ion conducting membranes and electrolytes in fuel cells as well as in batteries, and these uses are in the verge of commercial applications. Overall, acidic ionic liquids have grown onto their own sub discipline with vast array of applications. Some of the research in this area is geared toward technologies likely to replace the use of noble metal on chlorinated alumina for reforming and CoMo/NiMo catalysts for desulfurization of gasoline, by the use of acidic ionic liquids as catalysts. Another possible application is replacement of mineral acids in alkylation processes. However, the cost of these new materials are much higher than H2SO4 or HF; hence, the overall benefits of using AILs depends on efficiency improvements as well as the recyclability of AILs. The future of acidic ionic liquids in industrial applications is promising as many indications have shown the advantages AILs over traditional acids in numerous processes.

Lanka, his Ph.D. in organic chemistry from City University of New York (with William. F. Berkowitz, 1985), completed his postdoctoral training at Bar-Ilan University in Israel (with Alfred Hassner, 1985-87), and spent two years at the Center for Molecular Architecture at Central Queensland University in Rockhampton, Australia as a visiting scientist with Ronald Warrener. In 2009, professor Amarasekara’s research group introduced the use of sulfonic acid group functionalized Brönsted acidic ionic liquids for the direct hydrolysis of untreated cellulose and lignocellulosic biomass to glucose and its oligomers. The current research interests of his group are acidic ionic liquid based catalysis and the development of chemocatalytic methods for the processing of biomass to renewable fuels and feedstock chemicals. He is the author of the book “Handbook of Cellulosic Ethanol” (Wiley Scrivener 2013) and has published more than 100 peer reviewed research articles.

ACKNOWLEDGMENTS Author thanks NSF Grants CBET-0929970, CBET-1336469, and HRD-1036593 for financial support. ABBREVIATIONS AIL acidic ionic liquid BAIL Brönsted acidic ionic liquid DP degree of polymerization DPA diphenolic acid DTBP di tbutyl phenol EL ethyl levulinate HMF 5-hydroxymethylfurfural LA levulinic acid LAIL Lewis acidic ionic liquid MEA mono ethanol amine MPa mega Pascal OBs benzenesulfonate OTs toluene sulfonate OTf trifluoromethanesulfonate PAIL protic acidic ionic liquid TBC tbutyl catechol TBP tbutyl phenol TEOS tetraethyl orthosilicate Tfa trifluoroacetate TOF turnover frequency TON turnover number TRS total reducing sugar REFERENCES (1) Welton, T. Room-Temperature Ionic Liquids. Solvents for Synthesis and Catalysis. Chem. Rev. 1999, 99 (8), 2071−2083. (2) Freemantle, M. An Introduction to Ionic Liquids; Royal Society of Chemistry: London, 2010. (3) Ohno, H. Electrochemical Aspects of Ionic Liquids; John Wiley & Sons: Hoboken, 2011. (4) Gutel, T. Ionic Liquids, Use and Specific Task as Solvent in Catalytic Reaction; Lambert Academic Publishing: Saarbrücken, 2010. (5) De Los Rios, A. P.; Fernandez, F. J. H. Ionic Liquids in Separation Technology; Elsevier: Amsterdam, 2014. (6) Plechkova, N. V.; Seddon, K. R. Ionic Liquids further UnCOILed: Critical Expert Overviews; John Wiley & Sons: Hoboken, 2014. (7) Torrecilla, J. S. The Role of Ionic Liquids in the Chemical Industry; Nova Science Publishers: Hauppauge NY, 2012. (8) de María, P. D. Ionic Liquids in Biotransformations and Organocatalysis: Solvents and Beyond; John Wiley & Sons: Hoboken, 2012. (9) Mun, J.; Sim, H. Handbook of Ionic Liquids: Properties, Applications and Hazards; Nova Science Publishers: Hauppauge NY, 2012.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +1 936 261 3107. Fax: +1 936 261 3117. Notes

The authors declare no competing financial interest. Biography Ananda S. Amarasekara is a professor of chemistry at Prairie View A&M University in Texas, where he has been a faculty member since 2003. He received his B.Sc. degree from University of Colombo, Sri 6163


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