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A Review of Ionic Liquids Their Limits and Applications

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Green and Sustainable Chemistry
 Vol.iv No.i(2014), Article ID:43349,10 pages DOI:10.4236/gsc.2014.41008

A Review of Ionic Liquids, Their Limits and Applications

Department of Chemical science, Mount Allison Academy, Sackville, Canada

Email: kghandi@mta.ca

Copyright (c) 2014 Khashayar Ghandi. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted utilize, distribution, and reproduction in any medium, provided the original work is properly cited. In accord of the Creative Commons Attribution License all Copyrights (c) 2014 are reserved for SCIRP and the possessor of the intellectual property Khashayar Ghandi. All Copyright (c) 2014 are guarded by law and by SCIRP equally a guardian.

Received May 2, 2013; revised September 7, 2013; accepted January 4, 2014

Keywords: Technology; Preference for Quality; Volume of Trade; Vertical Intra-Industry Trade

ABSTRACT

Since ecology pollution caused past chemic and free energy industries has increased for several decades, there is a social expectation that scientists and engineers try to design sustainable chemical processes, to generate less hazardous materials and more than environmentally friendly sources of energy production. In this review the roles of Ionic Liquids (ILs) and IL based solvent systems every bit proposed culling for conventional organic solvents are described. Since there are already many reviews on benefits of ILs, subsequently a very brief review of ILs we focus mostly on aspects that are non covered in other reviews, in particular the known limits of these solvents. In add-on, unlike methods to measure out the physicochemical properties relevant to their apply in energy storage applications such as fuel cells and batteries are introduced. The physicochemical properties that are reviewed are thermal properties, conductivity and chemical reactivity. The focus of the review is on the literature afterwards 2008, with the exception of some important historic articles on ILs.

i. Introduction

I of the major sources of waste product is solvent losses that terminate up in the atmosphere or in ground h2o [ane-6]. Solvent use has been reported to account for about sixty% of the overall energy in pharmaceutical product, and information technology has been responsible for 50% of post-treatment greenhouse gas emissions [7,8]. Therefore, solvent selection should be considered systematically to improve synthesis conditions within the framework of green chemistry principles.

Various methods and tools have been developed for the identification and selection of appropriate solvents for synthesis. Consequently, there are a number of solvent selection guides available in the literature [one-8]. We have categorized the common solvents in iii different classes of preferred, usable, and undesirable (Table 1) [i-8]. The first three items in the preferred column are the most desired solvents and ILs on their own are a large class of solvents. However not all ILs are greenish solvents.

The focus of this review is on the literature after 2008, with the exception of some important historic manufactures. We will offset accept a very brief review of ILs and so we will discuss the limits of these solvents. Most of the limits in the literature are reported for non protic ILs therefore in our review of protic ionic liquids instead of discussion of their limits we focused by and large on their applications in energy industry. Consequently several methods to measure out the physicochemical backdrop relevant to their apply in energy industry applications are reviewed.

two. Ionic Liquids

Room-temperature ILs, organic salts that are liquid beneath 100˚C, have received considerable attention as substitutes for volatile organic solvents. Since they are nonflammable, not-volatile and recyclable, they are classified as green solvents. Due to their remarkable properties, such as outstanding solvating potential [9], thermal stability [10] and their tunable propertiesby suitable choices of cations and anions [11], they are consideredfavourable medium candidates for chemic syntheses.

ILsare unremarkably categorized into 4 types based on their cation segment: ane) alkylammonium-, 2) dialkylimidazolium-, three) phosphoniumand 4) N-alkylpyridiniumbased ILs (Figure 1).

Table 1. Guide for solvent selection.

Figure 1. Alkylammonium, phosphonium, dialkylimidazolium and N-alkylpyridinium cations.

Although these ILs are used successfully as solvents and catalysts in many reactions, there are some limitations in their use. In the following, we will depict the known advantages and disadvantages of each class of ILs.

The first synthesized IL was an ammonium-based one (ethanolammonium nitrate, EOAN), which was reported by Gabriel in 1888 [12]. Ammonium-based ILs accept been used widely every bit electrolytes in loftier-free energy electrochemical devices owing to their expert electrochemical cathodic stabilities, low melting points and low viscosities [13-15].

Popular imidazolium-based ILs are among the most studied ILs. Selection of the imidazolium ring as a cation (Figure 2) is often due to its stability within oxidative and reductive conditions [16], low viscosity of imidazolium ILs and their ease of synthesis [17]. There are also several reports regarding the application of imidazolium-based ILs as catalysts for the comeback of reaction fourth dimension, yield and chemoselectivity of many organic reactions [18-21].

However, Olofson et al., in 1964 [22], reported on a kinetics studydemonstrating that the proton sandwiched betwixt the two nitrogen atoms (H2)in the imidazolium cation undergoes deuterium substitution in deuterated solvent because of its acidic nature. Ii after studiesreported that deprotonation of the imidazolium cation to the highly reactive carbene and hence showedthe noninnocent nature of immidazolium-based ILs nether basic atmospheric condition [23,24].

In some other investigation [25], the low yield of a basecatalyzed Baylis-Hillman reaction in the presence of imidazolium-based ILs was attributed to a side reaction involving the imidazolium-based IL (Scheme one). This observation likewise confirmed that using this type of IL nether bones conditions needs to be considered with more precaution to avoid unexpected side reactions (Scheme 2).

Pyridinium-based ILs are more novel in comparison with their imidazolium-based counterparts, and inquiry on their stability, reactivity and catalytic role in organic synthesis is still in progress. Although they prove poor regioselectivity in palladium-catalyzed telomerization of butadiene with methanol [26], and they have a negative effect on the rate of some Diels-Alder reactions [27], applications of this type of ILs are quite successful in reactions such every bit Friedel-Crafts [28] and Grignard [29]. The catalytic role of pyridinium-based ILs has been shown to exist remarkablein the synthesis of some pharmaceutical agents such as 1,iv-dihydropyridine [xxx], dihydropyrimidinones [31] and iii,5-bis(dodecyloxycarbonyl)- i,4-dihydropyridine derivatives [32].

Phosphonium-based ILs aremore novel than the imidazoliumand pyridinium-based ILs. They are more than thermally stable (in some cases up to virtually 400˚C!) [33] in comparison with ammonium and imidazolium salts, and this remarkable property makes them suitable for reactions that are carried out at greater than 100˚C. Phosphonium-based ILs are used as the catalyst and solvent for hydroformylation [34], palladium-catalyzed Heck re-

Figure 2. The imidazolium cation.

Scheme ane. Base-catalyzed Baylis-Hillman reaction.

Scheme 2. The side reaction of an imidazolium-based ionic liquid with benzaldehyde in the Baylis-Hillman reaction.

deportment [35] and palladium-mediated Suzuki crosscoupling reactions [35]. In add-on, they arealso powerful phase-transfer catalysts for the Halex reaction [36].

Recently, phosphonium-based ILs take been used for CO2 capture [37]. Along with their application in the synthesis of a novel polystyrene-based material [38], the styrenic derivatives of phosphonium-based ILs are used as monomers in the synthesis of phosphonium-containing random copolymers [39]. Thecyclohexadieneylradical in trihexyl (tetradecyl) phosphonium chloride (IL101) [40] has been studied, and the effects of temperature and solvent on the reaction have been investigated using muon techniques at the TRIUMF National Laboratory of Canada [41]. These studies showed [twoscore,41] reactive free radicals exercise not react with phosphonium ionic liquids.

Although phosphonium-based ILs showed good stability in the presence of bases (even in reactions involving strong bases such equally Grignard reagents [42]), they are however susceptible to reaction with modest bases [43] (Scheme 3).

Unlike ammonium-based ILs, which undergo Hoffman or -elimination in the presence of a base at high temperature, phosphonium-based ILs tend to decompose to 3rd phosphine oxides and alkanes nether alkaline conditions[44] (equation (one)).

(i)

3. Protic ILs

ILs can exist divided into two wide categories: protic ILs (PILs) and aprotic ILs (APILs). PILs are produced through proton transfer from a Brønsted acid to a Brønsted base.

Historically, the beginning PIL, EOAN was reported in 1888 by Gabriel [12]. There are a large number of reports on the backdrop of APILs and their applications in different fields [9,45-51]; withal, in that location are few reviews on PILs [52,53].

In comparing with APILs, PILs ofttimes have college conductivity and fluidity likewise equally lower melting points [54]. They are also cheaper and more convenient to prepare every bit their synthesis does not involve the formation of byproducts [52]. However, there are several reports of the power of PILs to form a hydrogen bond network, which tin limit the ionicity of PILs in comparison with APILs [55-57]. The hydrogen bonding of PILs has been identified by NMR [55], 10-ray diffraction [58] and neutron diffraction [58]. The most studied example of the power of a PIL to course supramolecular networks through hydrogen bonding is related to ethylammonium nitrate (EAN) [59-61].

Normally, PILs are prepared through the neutralization of a base by an acrid [62,63] or the mixture of equimolar amounts of acid and base [54,55]. Ideally, the proton transfer is completed from Brønsted acid to Brønsted base, but, in most cases, a neutral species is formed attributable to incomplete proton transfer. Aggregation or the formation of ion complexes also can happen to prevent complete proton transfer, which limits the ionicity of the PILs [51]. Although in that location is notwithstanding no standard method to measure the ionicity of PILs, some qualitative techniques such equally NMR spectroscopy [56,64], changes in thermal backdrop every bit a function of stoichiometry [56,64], IR spectroscopy [64], Raman spectroscopy [64] and ionic conductivity by using Walden plots [65,66] have been used to provide information about the ionicity of PILs. Apparently, proton transfer improves past using stronger acids and bases.

PILs have broad applications in biological systems [52] and chromatography [52,67]. In improver, they have been applied as proton-conducting electrolytes for polymer membrane fuel cells [68-72], because of the reward of having a defined proton activity also as loftier proton electrical conductivity, allowing the fuel cell to operate under nonhumidified and loftier-temperature weather condition. PILs also have been widely used equally a Brønsted acid or base of operations [72] in many acid-base of operations-catalyzed organic reactions such as Knoevenagelcondensation [73], the Diels-Alder reaction [74], aldol condensation[75], Fischer esterification [76] and pinacol rearrangement [77] owing to their not-corrosive, non-volatile and recyclable nature in comparison with mineral acids.

Since ILs, including PILs, are bang-up microwave absorbents, they are proficient candidates for application as a medium or catalyst in many microwave-assisted reactions. Henderson and Byrne [77] used several ammoniumbased PILs as potential mediators for pinacol rearrangements under microwave irradiation; complete conversion was observed in optimized conditions in a pinacol rearrangement of hydrobenzoin (Scheme four).

Esterification of benzoic acrid with a variety of alcohols and a multifariousness of acids with benzoic alcohols are also reported to exist efficient when used with some types of imidazoliumor pyridinium-based PILs under microwave irradiation[78] (Scheme 5).

The solvent-free synthesis of coumarins [79], the estrification of salicylic acid [eighty] and the dehydration of dfructose and glucose [81] are other examples of using PILs as acidic catalysts under microwave irradiation.

Compared to the disadvantages of ordinary ILs (mostly their reactivity under certain reaction conditions where ILs are used equally medium for reaction) there are limited disadvantages of PILs including side reactions during the planned reactions. On the other hand PILs have been considered mostly for their applications in free energy industry not as medium for chemical science. For such applications noesis of their physicochemical properties is important.

Scheme 3.Reactions of benzoate salts with trihexyl(tetradecyl)phosphonium chloride(IL101)under microwave irradiation.

Scheme four.Microwave-assisted hydrobenzoin pinacol rearrangements mediated by protic ionic liquids (PILs) [78].

Scheme 5.Fisher esterification reaction in Brønsted acidic ionic liquids under microwave irradiation [79].

3.i. Physicochemical Properties of PILs

Different potential applications of PILs rely on their physicochemical properties, which vary based on the structures of the cation and anion used in the system. In the following sections, different methods for the investigation of thermal and ionic properties of PILs are described briefly.

3.1.1. Thermal-Stage Behaviour

Differential scanning calorimetry (DSC) is one of the thermoanalytical techniques used to study stage transitionsof materials (Effigy 3). In DSC, both the sample and the reference are maintained at the same temperature (∆T = Ts – Tr = 0) and whatsoever rut transfer between the sample and reference materials is recorded against the temperature [82,83]. The reference in the DSC method is a material that does non show phase modify over a broad temperature range, Alumina (Al2O3) and silicon carbide (SiC) are generally used every bit the reference materials in DSC. The DSC trace unremarkably is plotted as the heat flow versus temperature; difference from the baseline of the DSC trace is representative of a phase change such as melting of the sample. Steps in the baseline position of the DSC curves usually refer to the glass-transition temperature (Tg) of the materials, which is a transition that happens for amorphous and semi-crystalline materials including some ILs. For ILs, the Tg indicates the cohesive energy inside the table salt which is decreased by repulsive Pauli forces and increased through bonny Coulomb and van der Waals interactions [52]. Therefore, information technology is feasible to attain a lower Tg past decreasing the cohesive free energy of the ILs through the modification of the their cations and anions.

Usually the dependence of logarithm of viscosity on inverse temperature, i.due east. log(η) vs. Tg/T are used to show the fragility of materials. More than deviation from a linear trend indicates more than fragility, which means that as the temperature goes upward, the viscosities will decrease at a faster rate than the Arrhenius human relationship. Almost all the data on room temperature PILs provide show that they evidence delicate behavior [52,84-88].

3.i.2. Thermal Stability

Thermogravimetry assay (TGA) is a type of thermal analysis that examines the mass loss of the sample as a function of temperature in a controlled temper [83] (Effigy 4).

Figure 3. Unproblematic schematic diagram of differential scanning calorimetry (DSC).

Information technology has been established that PILs with large protontransfer energies decompose before reaching their boiling points [52,84,89]. The decomposition temperature varies between 100˚C and 360˚C [52,68,85,90]. PILs with a bis (trifluoromethane) sulfonamide anion (TFSI) with alkylammonium cations, the imidazolium cation, and a variety of heterocyclic cations are known as the almost stable PILs with decomposition temperatures of >200˚C [52, 90].

three.i.3. Viscosity

Viscosity is an important belongings of ILs for different applications. Normally, materials with greater van der Waals interactions and hydrogen bonding have higher viscosities [52,86,87,90].

Although it has been observed that the size of the PIL components has little issue on viscosity, the construction of the anion has a large effect on the viscosity and usually more than the structure of the cation [88]. This is an interesting ascertainment that needs to exist further substantiated by theoretical modeling. Specific structural features can bear on viscosity of ILs. Eastward.m. in substituted imidazolium PILs, stacking of the aromatic rings leads to higher viscosity [52,90]. Increasing the cation size by increasing the ring number in lactam-based PILs increases the viscosity past enhancing the cation-anion interactions [68]. Analysis of the temperature dependence of viscosity for PILs with unlike glass transitions, Tg, suggests that PILs in general are among fragile material and the PILs with college fragility accept lower viscosity (Figure five) [xc].

3.i.four. Electrical conductivity

The ionic conductivity, which depends on the available accuse carriers and their mobility (which depends on viscosity), varies with the molecular weight, and size of the ion. The conductivity of PILs is limited usually by their ion mobility resulting from aggregation [52,88]. Therefore, less ionic interaction and more delocalized accuse atomic number 82 to college conductivity; therefore, high ionic electrical conductivity values will be expected for the stronger Brønsted acids and bases [69,89]. Ion conductivity decreases by increasing the size of the cation (less mobility);

Figure 4. Thermogravimetry analysis (TGA) sample curve for a one-stage procedure. Ti, intial temperature; Tf, final temperature.

Figure 5. Logarithm of viscosity vs. Tg/T (in Kelvin) for several materials with different glass temperatures (in Kelvin, presented in the legend) equally well every bit for several known PILs [84]. The ellipse shows the range of information for PILs.

consequently, the electrical conductivity of PILs with longer alkyl chains decreases [52]. Therefore, the higher ionic conductivity of the i-methyl-ii-methyl imidazolium-based PILs compared with the 1-benzyl-2 methyl imidazoliumbased PILs [81], and the college conductivity value for methyl formate over butyl ammonium formate [84], should be due to the increase in the size of the cation. The ionic electrical conductivity of heterocyclic PILs increases with less symmetrical cation structure and smaller molecular weight [52,68]. No obvious tendency can exist found for the anions used in the system: equally an instance, nitrate has the highest ionic conductivity in ethyl ammonium-based PILs in comparison with formate, acetate, but charge per unit and lactate anions, but in the serial of ethanol ammonium-based PILs, nitrate has the lowest ionic electrical conductivity compared with the same anions [84]. There are a few alkylammonium-based PILs, such as methylammoniumformate (MAF), EAN, ethylammonium acetate (EAA), and ethylammonium formate (EAF), that have a loftier ion conductivity over ten mS∙cm–1 at 25˚C [52].

A Waldon plot, which is of the equivalent conductivity against the log of the fluidity (changed viscosity), is a good indication of the ionicity of the ILs. The Walden rule is shown in equation (ii), where ᴧ is the molar conductivity and η is the viscosity.

(2)

Negative divergence from the straight line can be representative of an incomplete proton transfer or assemblage of PILs [65] (Figure 6). The solid platonic line corresponds to a dilute aqueous KCl solution in which the system is known to be fully dissociated and to accept ions of equal mobility. The presence of parent acid and base molecules in the organisation can be indicated by vertical difference from the Walden line as well. The PILs are commonly categorized as poor ILs, except for those with a BF4 anion, which accept good ionicity [52].

iv. Conclusions

Over the past five years, ILs have connected to be used significantly as a medium and goad for many reactions. Despite the significant potential of ILs in this regard, at that place are reports that show in some cases ILs react with reactants and therefore they cannot be considered as inert solvents. Therefore synthetic chemists should be cautious when designing reactions in ILs depending on reactions they want to practice. Overall PILs are less studied compared to aprotic ILs, however amidst PILs that have been studied so far at that place is no written report of reactivity of PILs with reagents used for chemic reactions. Moreover PILs have an hands tunable and interesting range of physicochemical backdrop that make them potential candidates for applications in alkaline and alkaline earth ion batteries and fuel cells.

Effigy 6. Walden plot of log (equivalent conductivity) against log (fluidity).

There is a significant need for modelling IL and PIL chemical and physicochemical properties to guide the applications of these materials more efficiently. On the experimental side, the to the lowest degree understood reactions in ILs and PILs are the free radical reactions. There is a need for a microscopic understanding of costless radical chemical science in ILs and PILs.

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