Ionic liquids

Indian Journal of Chemistry Vol. 49A, May-June 2010, pp. 635-648

Ionic liquids: New materials with wide applications Nageshewar D Khupse* & Anil Kumar Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India Email: [email protected] (NDK)/ [email protected] (AK) Received 7 April 2010; accepted 21 April 2010 Ionic liquids have emerged as possible substitutes for volatile organic solvents and have found many applications in a variety of research areas. In this review, an effort has been made to discuss the special properties of ionic liquids that render these unique solvent media useful in chemical transformations, electrochemical applications, extractions, etc. Keywords: Ionic liquids, Physicochemical properties, Solvent properties, Viscosity, Chemical processes, Electrochemical devices

Ionic liquids have recently emerged as “green” and environment friendly solvents for their use in the industrial manufacture of chemicals. In the past decade, ionic liquids have been increasingly used for diverse applications such as organic synthesis, catalysis, electrochemical devices and solvent extraction of a variety of compounds.1-3 Ionic liquids are composed of cations and anions having low melting point ( [HMPy][NTf2] > [OMPy][NTf2]. However, in the case of the pyridinium ionic liquids the observed trend is [OP][NTf2] > [BP][NTf2] > [HP][NTf2] at 25 °C. It is important to understand the solute-solvent interactions in ionic liquids that cause such drastic variations with temperature. The temperaturedependence of the polarity parameters or thermosolvatochromism for all the ionic liquids was then studied at 298–353 K. Depending on the cationic and anionic species, the polarity values showed either a direct or an inverse relation with the change in

Fig. 8 — Temperature dependent ETN parameters for the pyridinium-based ionic liquids. {[BP][BF4](□), [OP][BF4](○), [BP][NTf2](), [HP][NTf2]() and [OP][NTf2]()}. [Figure taken from ref. 54].

Fig. 9 — The ETN-T plots for the pyrrolidinium-based ionic liquids, {[BMPyrr][NTf2](□), [HMPyrr][NTf2](●) and [OMPyrr][NTf2] ( )}. [Figure taken from ref. 54].

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temperature. The ETN value decreases with temperature for the pyridinium-based (Fig. 8) and pyrrolidinium-based ionic liquids (Fig. 9), but increases with temperature for the phosphonium ionic liquids (Fig. 10). This indicates that the choice of the cation can determine the response of polarity to a change in temperature. The betaine dye systems are known to show a negative solvatochromism due to differential solvation of more polar ground state (dipole moment of ground state µG = 15 D) as compared to the less polar excited state (dipole moment of excited state µE = 6 D). This explains the blue shift of absorption maxima for betaine dye with increasing solvent polarity. In polar solvents, the ground state is stabilized due to stronger solutesolvent interaction as compared to the excited state. When the temperature is increased, the ground state solvent interactions are weakened, thus reducing the energy gap between the ground state and the excited state of the betaine molecule. As a result, an increase in temperature should cause a red shift in the absorption maximum of betaine dye in polar solvents. This is precisely the case observed for the pyridinium and pyrrolidinium-based ionic liquids. The opposite effect of temperature in the case of the phosphoniumbased ionic liquids may then be explained along similar lines. The blue shifts in the solvatochromism indicate that the phosphonium ionic liquids solvate the excited state of betaine dye more efficiently as compared to the ground state. The greater stabilization of the excited state is also reflected in lower ETN value at 303 K in phosphonium-based ionic liquids as compared to the pyrrolidinium ionic liquids when the temperature increases.

Fig. 10 — Temperature dependent ETN parameters for the phosphonium-based ionic liquids. {[TBP][Ala] (□) and [TBP][Val] (○)}. [Figure taken from ref. 54].

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The weakening of stabilizing excited state-solvent interactions should lead to an increase in energy gap between the ground state and excited state of betaine molecules. This causes the observed blue shift in the thermosolvatochromism. The results present an interesting contrast to Suppan’s generalization that “absorption bands which give solvatochromic blue shifts are expected, on this basis to show thermochromic red shift”. The present line of reasoning implies that while pyridinium- and pyrrolidinium-based ionic liquids are perceived as “polar” solvent by the betaine molecule, the phosphonium-based ionic liquids acts as “nonpolar” solvents for the same probe molecule. It would be interesting to validate this implication further by analyzing the reasons for the apparent contrast. Since the betaine dye is neither a hydrogen bond donor nor a Lewis acid, it is reasonable to assume that there is no direct interaction of the molecule with the anions of the ionic liquids. The betaine dye is capable of exhibiting strong dipole-dipole, dipole-induced dipole and H-bond acceptor and dispersion interactions (due to large polarizability). The cation-betaine molecule interaction should then be the primary interaction, while the influence of the anion is accounted for only in an indirect manner. An increase of solvent polarity for probe (2) leads to a bathochromic shift of υmax. This is consistent with series of π-π* transitions, which go from a relatively charge diffused ground state to an excited state, wherein electronic charges are more concentrated and charge centers are more separated. Hence, more polar solvents stabilize the electronic ground state with the effect of shifting υmax to lower energy. As the temperature increases, the solute-solvent interactions are weakened. In the case of dye (2), this entails that the relative stabilization of the excited state decreases as the temperature increases. Increase in excited energy is reflected by decreasing π* value with temperature for all the ionic liquids studied. It is difficult to discuss the variation in α and β parameters along similar lines since these parameters are determined by the combination of responses of two or more solute probes. A thorough computational investigation into the interesting variations in thermochromic behavior of the dye molecules with the structure of the ionic liquids would be desirable in the future. Applications Chemical processes

Ionic liquids are found to be promising solvents in many of the organic reaction such as Diels-Alder,

Bails-Hillman, Heck Reaction, esterification, isomerization reactions and many coupling reaction.55,56 Pressure, temperature and concentrations of reactants govern the progress of the reaction. However, it has been observed that viscosity can also play an important role in reaction kinetics. The rates of Diels-Alder reaction increase with an increase in viscosity up to ∼1 cP. A sharp downfall in the rates of these reactions is witnessed in solvents possessing viscosities greater ∼1 cP. The increase in the rates was attributed to the gain of vibrational mode at the expense of translational modes up to ∼1 cP, thus facilitating the bond formation.57 However, the reactions slow down considerably in viscous solvents owing to the diffusion problems in highly viscous environment thus causing retardation in the reaction rates. Reports are available on the rate of reaction being faster in the ionic liquids possessing higher viscosities than in the ones with lower viscosities.58 This means that the high viscosity favors the above reaction. In another study, the rate constants for a diffusion-controlled reaction involving neutral reactants were measured in ionic liquids with different viscosities at varying temperatures. The overall bimolecular rate constant, k2, was noted to increase with an increase in the viscosities of ionic liquids. However, the data reported in the work appeared to be insufficient for conclusive evidence of the role of viscosity in Diels-Alder reactions. In our laboratory, we discovered that the k2 values for a number of Diels-Alder reactions in ionic liquids with varying viscosities were inhibited in the ionic liquids with high viscosities. The observations were attributed to the diffusion problems arising out of high viscosities. However, efforts to correlate k2 values with different properties such as surface tension, solvent properties, etc., of ionic liquids, were not successful. The results were interpreted in terms of the restricted diffusion of reactants in the encountercontrolled regime. The order for the rates of reaction of cyclopentadiene with the three acrylates used was as follows: methyl acrylate > ethyl acrylate > butyl acrylate. The difference may arise due to the different microviscosity experienced by each of these acrylate molecules, which will be a complex function of the viscosity of the medium, the molecular volume of the reacting moiety and the viscosity of the acrylate itself. The enhanced steric effect will also play an important role in governing the reactions. The reaction of cyclopentadiene with methyl acrylate showed Arrhenius behavior as evident from

KHUPSE & KUMAR: IONIC LIQUIDS: NEW MATERIALS WITH WIDE APPLICATIONS

the temperature dependence of rate constants in [BMIM][PF6] and [EMIM][BF4]. The activation energy, Ea, was reported to be 63.4 kJ mol-1 and 57.7 kJ mol-1 for the reaction of cyclopentadiene with methyl acrylate in [BMIM][PF6] and [EMIM][BF4] respectively. However, any change in temperature led to a change in the η values of the ionic liquids, which affected the rate of the reaction. The “intrinsic” activation energy, Eo, was reported to be 51.8 kJ mol-1 implying a difference (Ea-Eo) of 5.9 kJ mol-1 for [EMIM][BF4] and of 11.6 kJ mol-1 for [BMIM][PF6]. These values were in agreement with the qualitative prediction that the reactants would have to overcome a “higher barrier” in a more viscous medium, leading to a decrease in the rate of the reaction. A detailed study of the isoviscosity relationships, Arrhenius parameters and determination of microviscosity are some of the most challenging problems one needs to address in this area. The effect of a “viscosity reducer” in the presence of ionic liquid was also checked on the rates of reaction. For this purpose, the reaction of cyclopentadiene with methyl acrylate was carried out at 298 K in a mixture of [BMIM][BF4] with dichloromethane (45 mol % of [BMIM][BF4] in 55 mol % of dichloromethane). Here, dichloromethane was used as a “viscosity reducer”, with η = ~18 cP as compared to the value of η = 233 cP for [BMIM][BF4]. The resulting rate constant, k2, was much higher than that in pure [BMIM][BF4] or dichloromethane alone. Although a simple correlation between a given solvent property and the magnitude of a particular rate constant does not conclusively imply a dependence of rate on that solvent property, the evidences do merit further investigation. It is important to understand the structure-property correlations with reference to their effects on the chemical processes. The highly viscous nature of ionic liquid can slow down the rate of a bimolecular Diels-Alder reaction by an order of magnitude as compared to that in water. However, there have been very few attempts to correlate the physico-chemical properties of ionic liquids with the kinetic and stereochemical outcome of the reactions.59 Towards this end, an extensive collection of kinetic data for a variety of organic reactions carried out in a range of ionic liquids and subsequent comparison of the results with theoretical models is essential. In this context, our group studied the

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kinetic studies of an intramolecular Diels-Alder (IMDA) reaction of (E)-1-phenyl-4-[2-(3-methyl-2butenyloxy) benzylidene]–5-pyrazolone in a series of pyridinium-based ionic liquids. The IMDA reactions were carried out in different pyridinium based ionic liquids. The alkyl substituents on the pyridinium cation were varied to give different cations, viz., 1-butyl, 1-hexyl and 1-octyl. The anions used were [BF4]¯ and [NTf2]¯ . The use of two anions resulted in a homologous series of ionic liquids, differing only with respect to the alkyl substituent on the pyridinium cations. Changing the alkyl substituents, the anion or the temperature, could thus vary the viscosity of the ionic liquids (Fig. 11). The rate constants within the [BF4]¯ series of ionic liquids show a definite correlation with the viscosity of the medium. The rate of reaction decreased from 4.45 × 10-5 s-1 in [BP][BF4] to 1.68 × 10-5 s-1 in [3MOP][BF4] for a corresponding change in viscosity from 175.4 cP to 66.1 cP at 308 K. The correlation between k and η was not very obvious for the [NTf2]¯ series. Surprisingly, the magnitude of k in the [NTf2]¯ ionic liquids was very close to that in [BF4]¯ based ionic liquids. If the rate constants are indeed dependent on the viscosity of the medium, then the lower viscosity of the [NTf2]¯ based ionic liquids should have led to higher rates. In order to access the correlation between the rates and viscosity in greater detail, temperature dependent studies were carried out in different ionic liquids. The variation in temperature served to control the

Fig. 11 — Plot of ln k for the IMDA reaction against the viscosity, η, of pyridinium ionic liquids with [BF4]¯ anion (□) and [NTf2]¯ anion (○) at different temperatures. [Figure taken from ref. 53a].

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viscosity of the medium and the results plotted are a compilation of the rates in different ionic liquids at different temperatures. For the [BF4]¯ series of ionic liquids, the uniform variation in the experimental data showed that the rate constants at the same viscosity were nearly the same (independent of the means by which that viscosity value was attained) on changing the alkyl substituent or the temperature. The rate constants decreased uniformly with increasing viscosity, thus indicating “universal” viscosity dependence. It was obvious however, that this “universal” trend failed to extend to the [NTf2]¯ series. The [NTf2]¯ based ionic liquids also showed a similar but independent trend with the changing viscosity; in fact, the decrease in rates was sharper for [NTf2]¯ ionic liquids for a similar magnitude of increase in viscosity. At a given value of viscosity, the rate of the IMDA reaction in [NTf2]¯ was much lower than that in the [BF4]¯ ionic liquid. The rate constant decreased from ~13 × 10-5 s-1 to ~3 × 10-5 s-1 when the viscosity increased from 50 cP to 100 cP for the [BF4]¯ based ionic liquids, i. e., a decrease to onefourth of the original rate constant. For a viscosity of 100 cP for [NTf2]¯ ionic liquids, the rate decreased to one-tenth of its value at 50 cP, i. e., from ~2.10 × 10-5 s-1 to ~0.21 × 10-5 s-1 The results indicate that in addition to viscosity, the rates are also influenced by a solvent property that varied, independent of viscosity, on changing from one anion series to another. Within the homologous series, this effect might still be operative but was either masked by a greater competing influence of viscosity or it changed in proportion to the viscosity.

is that it should be able to offer a wide electrochemical window. While aqueous systems possess about 1.23 V as the electrochemical window, propylene carbonate and acetonitrile can offer an electrochemical window as high as 4 V. Almost many the ionic liquids offer electrochemical window in the range of 4-5 V, while the most common and popular ionic liquid, [BMIM][BF4], possesses an electrochemical window as high as 6 V as shown in Fig. 12. Another ionic liquid, [BMP][CF3SO3], is reported to have an electrochemical window of 6 V. One notes that the selection of reference electrode can be very important criteria in enhancing electrochemical stability. Ionic liquids have been found to be useful for electric double layer capacitors.65,66 Several applications in this regard have been critically examined. For example, many ionic liquids composed of [EMIM] cation with a variety of anions have been found effective. Another popular ionic liquid, [BMIM][BF4], with quite a high electrochemical window is also very useful for this application. In general, the capacitance changes in the order: aqueous solutions > ionic liquids > organic solutions. The high viscous ionic liquid has also been mixed with propylene carbonate to decrease the viscosity and to provide output data better than conventional systems. Another important area of application of ionic liquids is lithium batteries. The main interest in using ionic liquids for this application has been to find

Electrochemical devices

Excellent reviews are published demonstrating the applications of ionic liquids to electrochemical devices such as super capacitors, lithium ion batteries, polymer-electrolyte fuel cells and dye-sensitized solar cells. In an electrochemical device, an ionic liquid acts as electrolyte.60-64 Ionic liquids should be resistant to any electrochemical reduction and oxidation. Two main advantages of using ionic liquids in electrochemical devices include non-volatility and prevention of electrolytes from drying during the operation. The transport properties, like viscosity, conductance and diffusion gain significance when one considers diffusion process or conductivity for making an effective electrochemical device. However, the fundamental requirement for an ionic liquid to be useful in developing applications in electrochemistry

Fig. 12 — Electrochemical window of [BMIM][BF4] measured by linear sweep voltammetry using W, Pt electrodes which require a high conductivity and a wide electrochemical window.


substitutes for organic solvents. Also, if ionic liquids are used instead of organic solvents, the batteries will be safer to use. Chloroaluminate ionic liquids have been used from the very beginning in lithium batteries. In fact, this is the reason that one finds a large number of data on physico-chemical and electrochemical properties of chloroaluminates. Normally, a combination of two species has been employed for this work. Also, ionic liquids with [BF4] and [NTf2] anions coupled with lithium salts of the same anion have been frequently employed for this purpose. For example, [EMIM][BF4] mixed with LiBF4 acts as a good substance for lithium battery with LiCoO2 as a cathode material. Also, one gets good results with LiCoO2 and Li-Al as cathode and anode materials, respectively with the same ionic liquid. In order to avoid cathodic reduction, pyrrolidinium and piperidinium based ionic liquids are preferable. Interestingly, some additives like water and SOCl2 in traces with ionic liquids have been reported to be effective for this application. Also, ionic liquids based on guanidinium cation with [NTF2]¯ anion are found to be potential electrolytes for electrochemical devices. Though exhaustive reports are available, reviewing the use of ionic liquids in lithium batteries, a greater need exists to develop a model to predict the composition of a mixture of ionic liquids with some solvent that will provide the optimum results on lithium batteries. It is now certain that ionic liquids with sufficient conductivity and cycle stability in the lithium deposition/dissolution process can be designed by introducing appropriate functional groups. Ionic liquids are non-volatile due to their insignificant vapor pressure. This property of ionic liquid renders them a potential candidate for solar cell. An important point has emerged in this regard. Ionic liquids based on [HMIM] cation and [NTf2] anion are noted to be effective in solar cells. Further, addition of a dye in ionic liquid system has been noted to enhance the efficiency of a solar cell. Ionic liquids have appeared to be potential substitute for smooth operation of polymer electrolyte membrane fuel cell at high temperatures. A mixture of ionic liquids with additives like amines is the most promising candidate for this work. Proton conducting gelatinous electrolyte with [BMIM][BF4] are thermally mechanically stable up to 300 ºC. It is now possible to transport hydrogen via fluorohydrogenate conduction in ionic liquids system. Quite interestingly, ionic liquids have found a useful role in actuators. The

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research group of Asaka67 has shown that a dry actuator can be fabricated layer to layer casting using bucky gel, which is a gelatinous room temperature ionic liquids containing single wall carbon nanotubes. The solid polymer electrolyte layer is sandwiched between polyaniline (PANI) and polypyrrole (PPY), etc. Unfortunately, the solid polymer electrolyte layer possessing low ionic conductivity leads to slow response system. The electrolyte solution gives rise to swelling polymer electrolyte thereby leading to poor performance of the system. Figure 13 depicts a cross section of PPy double sided actuators with ionic liquids. [BMIM][PF6] showed PPy was stable and was able to withstand continuous functioning for more than 3600 cycles without degradation.68 Extraction technology

Solvent extraction is used in nuclear reprocessing, ore processing, the production of fine organic compounds, processing of perfumes and other industries. The use of ionic liquids to separate toxic metal ions and organic molecules has been investigated.69,70 At present, ionic liquids have been used as extraction solvents for the separation of metal ions by crown ethers. Hence, ionic liquids have got much more attention towards their use in extraction processes. Earlier, crown ethers found use in designing novel solvent extraction systems that are selective for certain metal ions which are based on the size of the crown-ether rings. Crown ether extract metal ions from aqueous solutions by complexation. The efficiency of such extraction processes is strongly dependent not only on cations but also on counter anions. The extraction process is more favored by hydrophobic counter anions in aqueous solutions than

Fig. 13 — Cross-section of PPy double sided PVDF electrochemical actuator using polymer in ionic liquids as electrolyte. [Published from ref. 69, with copyright permission from Elsevier Limited, UK].

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with hydrophilic anions. Due to low efficiency in using crown ether for extraction, ionic liquids have been used as extraction solvents to remove strontium nitrate from aqueous phase into ionic liquids by crown ether.71 The distribution coefficients for the ionic liquid extraction systems are higher than extraction systems based on organic solvents. Successful extraction of organic acids from aqueous solutions into an ionic liquid, 1-butyl3-methylimidazolium hexafluorophosphate, has been reported. The solvation of ionic species (such as crown-ether complexes, NO3 and SO4) in the ionic liquids is more favored thermodynamically than in conventional solvent extractions. The unique solvation capability of the room-temperature ionic liquid has not been used to full advantage. In many nuclear plants, large volumes of liquid waste are produced, called “tank waste.” This liquid waste, which may be acidic or basic depending on the treatment process, contains many long-lived radioactive products, including 137Cs, 129I, 90Sr and 99 Tc and a variety of transuranic elements, which are normally α-emitters.72 The 137Cs+ and 90Sr2+ in tank waste are normally present at low concentrations in a large volume of liquid and any treatment process for this waste must therefore produce a significant reduction in the volume of this liquid. So to extract these radioactive products, ionophores such as calix[4]arene-bis(t-octylbenzocrown-6, BOBCalixC-6 and dicyclohexano-18-crown-6, dissolved in hydrophobic solvents, including hydrophobic ionic liquids have been used.73 Hence, electrochemistry can play a useful role in this recycling process. This proposed electrochemical process for remediation of the extraction solvent preserves both the ionophore and ionic liquids. Conclusions From the above discussion, it is clear that ionic liquids can be very effective solvent media for obtaining optimum output in several applications with minimum possible environment pollution. It is hoped that demerits of ionic liquids such as high viscosity will be tackled by chemical or physical means to enable them to emerge as very powerful solvents. It is also hoped that simple semi-empirical models if developed, will be able to help many chemical engineers to predict thermal and transport properties of ionic liquids on the basis of the information on the structures of ionic liquids.

Acknowledgement NDK thanks CSIR, New Delhi, for providing a research fellowship to carry out this work. AK thanks DST, New Delhi, for providing financial assistance in the form of many research grants-in-aid during which this work was carried out. Nomenclature Abbreviation [MMIM] [EMIM] [BMIM] [HMIM] [OMIM] [BMPyrr] [HMPyrr] [OMPyrr] [BP] [HP] [OP] [4-MBP] [TBP] BF4 PF6 NTf2 C2H5SO4 CF3CO2 CF3SO3 (C2F5SO2)2N [Ala] [Val] ClO4

∆η Vf2

Name 1,3-Dimethyl imidazolium 1-Ethyl-3-methyl imidazolium 1-Butyl-3-methyl imidazolium 1-Hexyl-3-methyl imidazolium 1-Octyl-3-methyl imidazolium 1- Butyl-1-methyl pyrrolidinium 1-Hexyl-1-methyl pyrrolidinium 1-Octyl-1-methyl pyrrolidinium 1-Butyl-pyridinium 1-Hexyl-pyridinium 1-Hexyl-pyridinium 1-Butyl-4-methyl pyridinium Tetrabutyl phosphonium Tetrafluoroborate Hexafluorophosphate Bis(trifluoromethane sulfonyl)imide Ethyl sulfate Trifluoromethyl acetate Trifluoromethyl sulfate Bis(pentafluoroethanesulfonyl)imide Alanate Valinate Perchlorate Excess viscosity Volume fraction of ionic liquids

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