CChhaapptteerr 11 ______________________

Introduction

26

1.1 Need for Alternative Reaction Media

Some of the chemical reactions take place in the presence of solvents; they cannot be carried

out in isolation. Such reactions are highly influenced by surrounding molecules, atoms, and

ions. Such type of environment can be called as ‘medium’ and may contain molecules in gas

phase or present in crystal lattice. Such type of medium can also be called as solvent. The

other reacting molecules are known as solutes.

In general, compound used in excess can also be considered as solvent for chemical reactions,

and the other as solute. Solvents are widely used in all aspects of chemistry such as

reactions, separation, and analysis using GC (Gas Chromatography), HPLC (High

Performance Liquid Chromatography), crystal growth, and cleaning (Reichardt, 1998).

From environmental and economic point of view, the use of solvent is wasteful. The problem

with the solvent is not much with their use but the inefficiencies in recovering them

completely. In chemical reactions, solvents are used to facilitate the reaction, and after the

completion of the reactions, they are removed from the products or reused for the same

reaction. Removal of residual solvents from products is achieved by either evaporation or

distillation. Therefore, most of the solvents used are highly volatile. This volatility has lead

to major public concern for atmospheric pollution which may lead to dizziness, nausea, and

other long term effects including respiratory tract infection or sometimes cancer.

Government and Environmentalist agencies are implementing various laws to reduce such

types of pollutions. Many funding agencies initiate to aid and promote the development of

cleaner chemical technologies. Due to these, many industries have responded to these

regulations by reformulating products to reduce the content of the solvents or completely

eliminate the use of volatile solvents in the process (Riley, 1999).

The use of organic solvents and their emissions has been at the centre of major environmental

concern in recent years, and there is currently a great deal of interest in finding alternatives to

halogenated and volatile organic solvents for synthesis. Supercritical fluids, biphasic

reactions, ionic and fluorous liquids, and aqueous chemistry are alternative reaction media

which may be used to increase reaction efficiency, improve separation and catalyst recovery,

and reduce emissions to the environment.

27

1.2 Characteristic of Alternative Reaction Media

A solvent of some kind is used as reaction medium, and the reactants are solutes. A solvent

and solute are defined as two compounds which dissolve to give single homogenous phase.

A compound as a solvent can be selected following the criteria as follows:

1. The effect solvent has on the chemical reaction’s rate, kinetics, mechanism, and

equilibrium.

2. The stability of the substrate, catalyst used, and products formed, transition

intermediates, in the solvents.

3. Suitable liquid temperature range suitable for the reactions.

4. Sufficient solvent volatility for removal from products by evaporation or by

distillation.

5. Cost; this is considerably important while scaling up.

6. Reusability of the solvent must be possible with simple purification steps.

7. If reusability is not possible, it must be biodegradable or environmentally safe to

dispose.

8. Good stability over a reasonably long period of time for long storage purpose.

9. Easily available.

28

1.3 Possible Alternative Reaction Media

Alternative solvents strategies should allow for the efficient reusability and recovery of the

solvent after use. Compounds, which are used as alternative solvents are water, supercritical

CO2, ionic liquids, and fluorous compounds. Figure 1 shows the schematic representation of

the alternative solvents used in the reaction.

Water always cannot form homogenous mixture with all reagents used in reaction. Carbon

dioxide is widely used as extracting solvents and fluorous compounds forms homogenous

reaction mixture with similar polarity reagents while using in Chemical.

Whereas ionic liquids, can be tailor made and imparts tunable properties by changing the

composition of the salt used.

Figure 1.1 Diagrammatic Representation of Alternative reaction media (Dave et al., 2005)

29

1.4 What is Ionic Liquid?

The Structure and Properties of Ionic Melts” was the title of a Faraday Society Discussion

held in Liverpool in 1961; it dealt exclusively with molten inorganic salts Aberdeen (1962).

“Ionic Liquids” was the title of Chapter 6 of the textbook Modern Electrochemistry by

Bockris and Reddy, published in 1970. It discussed liquids ranging from alkali silicates and

halides to tetraalkylammonium salts (Bockris et al., 1970). The modern era of ionic liquids

stems from the work on alkylpyridinium and dialkylimidazolium salts in Colorado in the late

1970s (Wilkes, 2004). The term ionic liquids was introduced Seddon et al. (1997) to cover

systems below 100°C, one reason being to avoid the words “molten salts” in phrases such as

“ambient temperature molten salts,” another to create an impression of freshness and a third,

perhaps, for patent purposes. The first “Conference on Ionic Liquids” took place in Salzburg

in 2005. “Molten Salts 7” in Toulouse in 2005 had one of ten sessions devoted to ionic

liquids. However, the International Symposia on Molten Salts of ECS since 1976 to the

present have not shown discrimination on the basis of temperature.

Significant properties of ionic liquids such as, the low vapour pressures which contrast the

environmental problems of volatile organic solvents and moderate specific conductivities,

usually in the same range as those of aqueous electrolytes. It is found that many such

systems are excellent solvents or catalysts for organic reactions and some simple processes

such as electro deposition Zhu et al. (2002).

One property that is emphasized recently is the molarity of the liquid by Xiao et al. (2002), a

straightforward quantity except for mixed systems such as a basic chloroaluminate containing

both Cl and AlCl4 - in significant amounts. The molarity is important regarding kinetic

measurements, including conductivities a range of molarities of many liquids from 1 to 60,

with water at 55, liquid alkali halides up to 35 (LiCl) and most organic salts less than 10.

Specific conductivities span a far greater range from the metal sodium through molten

inorganic salts in the Scm–1 region to organic salts (the modern ionic liquids) and aqueous

solutions in mScm–1 region and finally to the near non-conducting but ionising acetic acid

�������������� –1. Combining these data into molar conductance is illuminating.

Comparable values are observed for simple inorganic salts alone and in aqueous solutions but

much smaller values for the low temperature semi-organic and organic systems. Thus these

30

modern ionic liquids must consist of IONS and ION PAIRS, (undissociated molecules);

while liquid alkali halides are purely IONIC and aqueous electrolytes behave as a mixture of

hydrated ions and the molecular solvent water. Figure 2 attempt to picture these differences.

Figure1. 2. Difference between solvated crystal lattice of NaCl and solid NaCl (Keith et al.,

2007)

To date, most chemical reactions have been carried out in molecular solvents. For two

millennia, most of chemistry has been based upon the behavior of molecules in the solution

phase in molecular solvents. Recently, however, a new class of solvent has emerged, namely,

ionic liquids. These solvents are often fluid at room temperature, and consist entirely of ionic

species. They have many fascinating properties which make them of fundamental interest to

all chemists, since both the thermodynamics and kinetics of reactions carried out in ionic

liquids are different to those in conventional molecular solvents, and then the chemistry is

different and unpredictable at current state of knowledge. However, in addition to the scope

for exciting new discoveries, ionic liquids have no measurable vapour pressure, and hence

can emit no volatile organic compounds (VOCs). Therefore, these have attracted, quite

justifiably, enormous attention as media for green synthesis.

As they are made up of at least two components which can be varied (anion and cation), these

solvents can be designed with a particular end use in mind, or to possess a particular set of

properties. Hence, the term “designer solvents” was used first used by Freemantle (1998).

The prospect of carrying out chemical reactions in ionic liquids may seem daunting to a

31

chemist who has not worked with them before, but it turns out that carrying reactions out in

ionic liquids can be exceptionally easy.

Ionic liquids have been described as designer solvents and this means that their properties can

be adjusted to suit the requirements of a particular process. Properties such as melting point,

viscosity, density, and hydrophobicity can be varied by simple changes to the structure of the

ions. For example, the melting points of 1-alkyl-3-methylimidazolium tetrafluoroborates, as

determined by Holbrey et al. (1999) and hexafluorophosphates by Gordon et al. (1998) are a

function of the length of the 1-alkyl group, and form liquid crystalline phases for alkyl chain

lengths over 12 carbon atoms. Another important property that changes with structure is the

miscibility of water in these ionic liquids. For example, 1-alkyl-3-methylimidazolium

tetrafluoroborate salts are miscible with water at 25 0C where the alkyl chain length is less

than 6, but at or above 6 carbon atoms, they form a separate phase when mixed with water.

This behavior can be of substantial benefit when carrying out solvent extraction or product

separation, as the relative solubility of the ionic and extraction phase can be adjusted to make

the separation as easy as possible.

1.5 History

Discovering a new ionic liquid is relatively easy, but determining its usefulness as a solvent

requires a much more substantial investment in determination of physical and chemical

properties. The best trick would be a method for predicting an ionic liquid composition with

a specified set of properties. That is an important goal that still awaits a better fundamental

understanding of structure-property relationships and the development of better

computational tool.

The historical answer to the nature of the present ionic liquids is somewhat in the eye of the

beholder. The very brief history presented here is just one of many possible ones, and is

necessarily biased by the point of view of just one participant in the development of ionic

liquids. The earliest material that would meet current definition of an ionic liquid was

observed in Friedel-Crafts reactions in the mid-19th century as a separate liquid phase called

the “red oil”. The fact that the red oil was a salt was determined more recently when NMR

spectroscopy became a commonly available tool. Early in the 20th century, Walden (1914)

discovered that some alkyl ammonium nitrate salts were found to be liquids, and more

32

recently liquid gun propellants have been developed using binary nitrate ionic liquids. In the

1960s John Yoke et al. (1963) at Oregon State University reported that mixtures of copper (I)

chloride and alkyl ammonium chlorides were often liquids. These were not as simple as they

might appear, since several chlorocuprous anions formed, depending on the stoichiometry of

the components. In the 1970s, Jerry Atwood et al. (1976) of the University of Alabama

discovered an unusual class of liquid salts he termed “liquid clathrates”. These were

composed of a salt combined with an aluminum alkyl, which then forms an inclusion

compound with one or more aromatic molecules. A formula for the ionic portion is

M(Al2(CH3)6X), where M is an inorganic or organic cation and X is a halide. None of the

interesting materials just described are the direct ancestors of the present generation of ionic

liquids. Most of the ionic liquids responsible for the burst of research publications in the last

several years evolved directly from high temperature molten salts, and the quest to gain the

advantages of molten salts without the disadvantages.

In 1963, Major (Dr.) Lowell A. King at the U.S. Air Force Academy initiated a research

project aimed at finding a replacement for the LiCl-KCl molten salt electrolyte used in

thermal batteries. Then there was a continuous molten salts/ionic liquids research program at

the Air Force Academy, with only three principal investigators – King, John Wilkes, and

Richard Carlin. Even though the LiCl-KCl eutectic mixture has a low melting temperature

(355 0C) for an inorganic salt, the temperature causes materials problems inside the battery,

and incompatibilities with nearby devices. The class of molten salts, known as

chloroaluminates, which is mixture of alkali halides and aluminum chloride, has melting

temperatures, much lower than boiling point of water as compared to, nearly all other

inorganic eutectic salts. In fact NaCl-AlCl3 has a eutectic composition with a melting point

of 107 0C, very near to that of an ionic liquid reported by Murphy et al. (1980).

Chloroaluminates are another class of salts that are not simple binary mixtures, because the

Lewis acid-base chemistry of the system results in the presence of the series of anions Cl–,

(AlCl4)–, (Al2Cl7)–, and (Al3Cl10)– although, fortunately, not all of these in the same mixture.

If a new material is to be accepted as a technically useful material, the chemists must present

reliable data on the chemical and physical properties needed by engineers to design processes

and devices. Hence, the group at the Air Force Academy in collaboration with several other

groups determined the densities, conductivities, viscosities, vapour pressures, phase

equilibria, and electrochemical behavior of the salts. The research resulted in a patent for a

33

thermal battery using the NaCl-AlCl3 electrolyte, and a small number of the batteries were

manufactured. Early in their work on molten salt electrolytes for thermal batteries, the Air

Force Academy researchers surveyed the aluminum electroplating literature for electrolyte

baths that might be suitable for a battery with an aluminum metal anode and chlorine cathode.

They found one 1948 patent describing ionically conductive mixtures of AlCl3 and 1-

ethylpyridinium halides, mainly bromides. Subsequently the salt 1-butylpyridinium chloride-

AlCl3 (another complicated pseudo-binary) was found to be better behaved than the earlier

mixed halide system, so the chemical and physical properties were measured and published

by Gale et al. (1978). This was the modern era for ionic liquids, because for the first time a

wider audience of chemists started to take interest in these totally ionic, completely

nonaqueous new solvents. The alkylpyridinium cations suffer from being relatively easy to

reduce, both chemically and electrochemically. The classes of cations that were the most

attractive candidates were the dialkylimidazolium salts, and the 1-ethyl-3-

methylimidazolium, [EMIM], whereas [EMIM]Cl mixed with AlCl3 formed ionic liquids

with melting temperatures below room temperature over a wide range of compositions, as

was observed by Wilkes et al. (1982). Chemical and physical properties were once again

determined, and demonstrated some new battery concepts based on this well-behaved new

electrolyte. For some organic reactions, such as Friedel-Crafts chemistry, it was found by

Boon et al. (1986) that ionic liquids were excellent both as solvents and catalysts. They

appeared to act like acetonitrile, except that they were totally ionic and nonvolatile.

The pyridinium- and the imidazolium-based chloroaluminate ionic liquids share the

disadvantage of being reactive with water. In 1990, Mike Zaworotko, took a sabbatical leave

at the Air Force Academy, where he introduced a new dimension to the growing field of ionic

liquid solvents and electrolytes. His goal for that year was to prepare and characterize salts

with dialkylimidazolium cations with water-stable anions. Wilkes et al. (1992) proposed an

easy formation of the chloroaluminate salts, which could be done outside the glove box. The

new tetrafluoroborate, hexafluorophosphate, nitrate, sulphate, and acetate salts were stable (at

least at room temperature) towards hydrolysis. Zaworotko left, and Joan Fuller came to the

Air Force Academy, and spent several years extending the catalog of water stable ionic

liquids, discovering better ways to prepare them, and testing the solids for some optical

properties. She made a large number of ionic liquids from the traditional dialkylimidazolium

cations, plus a series of mono- and tri-alkylimidazoliums. She combined those cations with

34

the water stable anions mentioned above plus the additional series bromide, cyanide,

bisulphate, iodate, trifluoromethanesulfonate, tosylate, phenylphosphonate and tartrate.

This resulted in a huge array of new ionic liquids with anion sizes ranging from relatively

small to very large.

1.6 Different properties of IL

Over the last few years there has been a dramatic increase in research relating to the use of

ionic liquids as potential replacements for organic solvents in chemical processes as reported

by Pârvulescu et al. (2007). More recently, specialized areas such as lubricants were

investigated by Holbrey (2007), while heat transfer fluids and analytical applications were

investigated by Liu et al. (2005). These materials are generally organic salts which have a

relatively low melting point when compared to inorganic salts. For example, many are fluid

at temperatures below 298 K and these are often described as room temperature ionic liquids

(RTILs). However, the term ionic liquid does not exclude those salts which have higher

melting points and although this description is associated with salts which melt below 373 K,

in reality there is no clear distinction between the term molten salt (often used for high

temperature liquids) and the term ionic liquid. Katritzky et al. (2002) explains that expanding

range of applications is not surprising given that approximately 1018 anion-cation

combinations exist which could generate ionic liquids and thus these liquids could be

classified as true designer materials, particularly since many of these designs include in-built

functionality. Therefore, given the potential range available it is possible to have properties

suited to a particular application or, if desired, contradict some of the earlier perceived

advantages of dealing with fluids consisting of only ionic species. For example, ionic liquids

are generally regarded as having negligible vapour pressure. Yet, recently Widegren et al.

(2007) have reported volatile ionic liquids and the distillation of ionic liquids have been

demonstrated by Earle et al. (2006). Their biodegradability and toxicity has been questioned

by Hough et al. (2007), and yet nutritional or pharmacological ionic liquids are feasible.

Similarly, while some ionic liquids could be used as flame retardants, as reported by Xue et

al. (2005), some others are combustible and energetic ionic liquids are a reality.

Very few works such as Deetlefs et al. (2006) have systematically studied the qualitative

and/or quantitative relationships between the structures of ILs and their fundamental

35

properties such as melting point, viscosity, density, surface tension, thermal and

electrochemical conductivity, solvent properties and speed of sound. At present, data for

many other important physico-chemical properties of ionic liquids are in short supply, or are

currently too unreliable to allow for similar structure-property relationship studies.

1.6.1 Liquidus range:

The liquidus range relates to the temperature range where the ionic liquid is in liquid form.

In general, this is the difference between the melting point and the decomposition

temperature. However, it could also represent the temperature difference between glass

transition point and boiling point, etc. Accurate values for melting points for ionic liquids are

scarce as, like in the case of inorganic salts, melting point and glass transition temperatures

can be strongly affected by the presence of impurities. Figure 3 shows the phase transition

temperature as a function of chain length, n. It is observed from the figure that melting point

is initially high and then decreases as chain length increases.

Figure 1.3 Phase transition temperature as a function of chain length, n (Kichner et al., 2001)

As the chain length increases, asymmetry of the molecule also increases which restrict the

molecule to fit in proper crystal lattice structure, and hence, these IL remains in liquid state.

At chain length n = 9, the melting point again increases and liquid crystalline region is

36

observed to form at higher chain length. The liquidus range exhibited by ILs can be much

greater than that found in common molecular solvents. For example, water has a liquid range

of 100 0C (0 to 100 0C), and dichloromethane has a liquid range of 145 0C (�95 to 40 0C) at

ambient pressure. Lower the temperature limit, it tends to the solidification (either as

crystallisation or glassification), it is governed by the structure and interactions between the

ions. Ionic liquids, comprised of totally ionised components and having relatively weak ion-

ion pairing (in comparison to molten salts), have little measurable vapour pressure and thus,

in contrast to molecular solvents, the upper limit of the liquid phase for fully ionic liquids is

usually that of thermal decomposition rather than vaporisation.

It is important that the forces and interactions that govern the melting points of ionic liquids

are not considered in isolation. These interactions also control the dissolution and solubility

of other components in the ionic liquids. For example, if there is a requirement for an ionic

liquid to have strong H-bond accepting character (in the anion), then it should be anticipated

that this will also lead to hydrogen bonding interactions between ions, resulting in greater

attractive forces and elevated melting points.

1.6.2 Density:

Density as a function of temperature has been measured for a range of imidazolium,

pyridinium, ammonium, phosphonium, and pyrrolidium based ionic liquids. For pure ILs,

the values vary depending on the choice of anion and cation. Typical values range from 1.05

to 1.64 g cm�3 at 293 K, which decreases with temperature to between 1.01 and 1.57 g cm�3

at 363 K. As with molecular solvents, the densities are closely related to the molar mass of

the liquid with ILs containing heavy atoms found to be most dense. Figures 4 and 5 show a

range of measured densities of dried ionic liquids as a function of temperature, at 0.1 MPa,

where it can be seen that the density is a strong function of anion type and decreases with

increasing the alkyl chain length.

37

Figure 1.4 Effect of the anion on the densities of (C4mim) + based IL: filled circles (NTf2)–;

filled square (PF6)–; filled triangles (OTf)–; inverted open triangles (BF4)– (Rooney et al.,

2010)

Figure 1.5 Effect of cation on the densities of (NTf2)- based IL : filled squares, (C2mim)+ ;

filled circles , (C4mim)+ ; inverted filled triangles , (C6mim)+ ; filled triangles (C8mim)+ ;

filled diamonds (C10mim)+ ( Rooney et al., 2010)

38

1.6.3 Viscosity:

Viscosity relates to the internal friction within the fluid which is caused by intermolecular

interactions, and is therefore important in all physical processes which involve the movement

of the fluid or components dissolved within it. Therefore, the design of liquid-liquid

extractors, distillation columns, heat-transfer equipment, process piping, reactors, and other

units found in various chemical and pharmaceutical industries requires the knowledge of the

viscosity of fluids and their mixtures. Viscosity is arguably one of the most important

physical properties while considering any scale-up of ionic liquid applications. In general,

low viscosity is desired for solvent applications in order to minimize pumping costs and

increase mass transfer rates while higher viscosities may be favorable for other applications

such as lubrication or use in supported membrane separation processes. It is known that the

viscosity of ionic liquids vary widely depending on the type of cation and anion and are

relatively high when compared to those of common organic solvents.

The increment with the alkyl chain length of imidazolium cation is more pronounced in case

of ionic liquids containing the Cl� anion, and seems to decrease with the symmetry of anion,

showing the trend Cl� > (CH3COO)� > (PF6)� > (C1SO4)� > (C2SO4)� > (BF4)� > (OTf)� >

(NTf2)�. In general, ionic liquids having highly symmetric or almost spherical anions are

more viscous and viscosity decreases with increasing anion asymmetry. For ionic liquids

having a common anion and a similar alkyl chain length on the cation, it is observed that the

viscosity increases with cations following the order imidazolium < pyridinium <

pyrrolidinium. This is in agreement with the results of Crosthwaite et al. (2005), which show

that pyridinium salts are generally more viscous than the equivalent imidazolium salts.

1.6.4 Surface Tension:

The versatility of ILs has generated increasing interest in using them in extraction and

multiphase or homogeneous catalytic reactions. Leclercq et al. (2008) worked on a system

containing one phase for dissolving the catalyst and immiscible with the second phase that

contains the reactant and products. Such processes occur at the interface between the IL and

the overlying aqueous or organic phase, and are dependent on the access of the material to the

surface and the transfer of material across the interface. A clearer understanding of the

39

mechanisms behind these processes requires a more detailed examination of the surface

properties of the ionic liquids.

Surface tension is a significant property in the study of physics and chemistry of free surfaces

as it affects the transfer rates of vapour absorption at the vapour-liquid interface. Such data

are of importance to scientists, engineers, and practitioners in many fields such as chemical

process and reactor engineering, flow and transport in porous media, materials selection and

engineering, biomedical and biochemical engineering, electronic and electrical engineering,

etc. The surface of a liquid is not only interesting for the fundamental aspects but also for its

relevance in environmental problems, biological phenomena, and industrial applications.

Experimental data for surface tensions of ionic liquids is very scarce and currently limited to

imidazolium based ionic liquids. Typical values for surface tension are shown in Figure 6

which shows that these ionic liquids have a lower surface tension than water (71.97 mN m�1

at 298 K) but higher than many organics.

Figure 1.6 Surface tension at 298 K as a function of chain length n, for series of imidazole IL

having anions; inverted open triangles, (C1SO4)- ; filled circles, (C2SO4)- ; filled squares,

(PF6)- ; open circles (CH3COO)- ; filled diamond , (BF4)- ; open squares, (OTf)- ; open

triangles , (NTf2)- (Knotts et al., 2001)

40

For the ILs having similar anion, the surface tension decreases with an increase in alkyl chain

length of imidazolium cation and as is observed with organic solvents decreases with

increasing temperature as observed by Knotts et al. (2001).

1.6.5 Specific Heat Capacity:

Heat capacity represents the relationship between energy and temperature for a specified

quantity of material. In general, this value relates to the kinetic energy stored within the

vibrations of the molecule of interest and can be correlated to such. For example, Strechan et

al. (2008) reported a predictive method for determining heat capacities of six different ionic

liquids by correlating this property with the intramolecular vibrational contribution where

they reported a relative deviation of 0.9%. The fact that these fluids are ionic should not have

a significant effect on the specific heat capacity of ionic liquids and indeed reported values

are in line with those one would expect for organic molecules. For example, the heat

capacity for chlorobenzene is 152.1 J mol�1 K�1 or 1.36 kJ kg�1 K�1 when written in terms of

weight, is similar to that reported for (C2mim) (NTf2), i.e., 525 J mol�1 K�1 or 1.34 kJ kg�1

K�1. When written in a molar basis the heat capacities of ionic liquids are generally higher

than typical organic solvents. This is expected given the relatively large molecular weights

of ILs. For example, at 298 K, the heat capacities of water, ethanol, nitromethane, benzene

are between 75 and 292 J mol�1 K�1 reported by García-Miaja et al. (2007).

41

1.7 Different types of IL

Following their discovery in late nineteenth century and early twentieth century, various

synonyms and abbreviations have been and are used in the scientific literature for organic

salts with low melting points or low glass-transition temperatures:

Ionic LIQUIDS (IL)

Room-temperature ionic liquids (RTIL)

Ambient-temperature ionic liquids

Non-aqueous ionic liquids (NAIL)

Molten organic salts

Fused organic salt

Low melting salt

Neoteric solvent

Designer solvent

1.7.1 Attraction of ILs

Since the late 1990s, ILs has attracted the attention of chemist around the world for various

reasons. ILs has opened up a new face of chemistry. Before 1998, there were relatively

fewer studies of chemistry at temperature below 100 0C in a liquid environment that was

entirely ionic compared with the chemistry in a molecular environment. The scientific

potential for research on ILs is virtually unlimited. Till date, more than 1500 IL have been

reported in the literature. In theory at least, a million or so ILs are possible. An almost

limitless number of IL systems are possible by mixing two or more simple ILs. Unlike

organic molecular solvents, ILs has negligible vapour pressure and therefore do not evaporate

under normal conditions. They are non flammable and remains stable at temperature higher

than conventional organic molecular solvents, they shows wide electrochemical window.

Their physical, chemical, and biological properties can be “tuned “or “tailored” by simply (a)

switching anion or cation, (b) by designing specific functionalities in to the cation and/or

anion, and (c) by mixing two or more simple IL, because IL consist of cation and anion , they

have dual functionality. Therefore, they impart a unique architectural platform compared

with molecular liquids. Consequently, ILs can potentially be exploited as solvents and new

42

material for wide ranging applications such as, electrochemistry, organic and inorganic

chemistry, biochemistry, material science and pharmaceuticals. ILs could contribute

significantly to the development of green chemistry and green technology by replacing toxic,

flammable volatile organic solvents, reducing or preventing chemical wastage and pollution

and improving the safety of chemical process and products.

1.7.2 Cations and Anions:

Room temperature Ionic liquids are typically salts with large nitrogen or phosphorus

containing organic cations with linear alkyl chains. Many researchers have worked

imidazolium cations. Figure 1.7 shows the ring numbering systems for these cations, along

with the structure of other widely studied cations.

N S+

R2

R1

R3

R4

N O+

+

R2

R1

R3

R4

N N2+

R2

R5

R4

R1

R3

N+

N+

R2

R5R

1

R3

R4

N+

N

N+

R2

R1

R3

R4

R5

Imidazolium

Thiazolium Oxazolium Parazolium

Triazolium

43

N+

RPyridinium

N+

N+

N

R1

R2

R3

Benzotriazolium

N+

R1

Isoqunolinium

N+

NCH3 R

Figure 1.7 Cations for ionic liquids (Clare et al., 2010)

The following are some of the most common anions cited in the ionic liquids literature:

� Halide: bromide Br- ; chloride Cl–

� Nitrate: (NO3)–

� Chloroaluminates: (AlCl4)–; (Al2Cl7)–

� Hexafluorophosphates (PF6)–

� Tetrafluoroborates (BF4)–

� Alkyl sulphates (RSO4)–, for example, ethyl sulphate (C2H5SO4)–

� Alkylcarboxylates (RCO2)– , for example, acetate (CH3CO2)– also written as (OAc)

� p-toulenesulphonates (CH3C6H4SO3)– is also known as tosylate (OTs)– or (Ts)–

� trifluoromethylsulphonates (CF3SO3)– is also known as trfilate (OTf)–

� bis(trifluoromethylsulphony)amides (N(SO2CF3)2)– also known as bistriflamide or

sometimes bistriflimide(NTf2)–

� bis(perfluoroethylsulphonyl)amides (N(SO2C2F5)2)–

� Dicyanamides (N(CN)2)–

� tris(pentafluoroethyl)trifluorophosphates ((C2F5)3PF3)–

� Metal complexes, for example, (Co(CO)4)– and (SbF6)–

44

1.7.3 Aprotic and Protic IL

Most research on IL has focused on aprotic ILs. These are generally salts consisting solely of

cations, which are not protonated, and anions. Example are (C2mim) (BF4) and (C4mim)

(NTf2). Protic IL are formed by proton transfer from an acid that can donate a proton i.e. a

Bronsted acid (HA) , to a base that can accept a proton , i.e. a Bronsted base (B):

� �HA B BH A�� � �

The classic example of a protic IL is ethylammonium nitrate, (C2H5NH3)(NO3), which is

formed by the protonation of ethylamine:

� � � �2 5 2 3 2 5 3 3C H NH HNO C H NH NO� �� � �

1.7.4 Binary Mixture:

Many IL of interest to chemist are binary mixture of an organic salt and an inorganic salt.

They are typically binary haloaluminate systems.

Binary mixtures of the organic salt (C2mim)Cl and aluminium chloride, AlCl3, denoted

(C2mim)Cl-AlCl3, are classic examples of such systems. A mixture containing (C2mim)Cl

and the Lewis acidic aluminium, AlCl3 in the mole ratio 1:2 is liquid at room temperature.

This mixture is acidic. The cation and anion in this mixture are (C2mim)+ and the Lewis

acidic (Al2Cl7)-, respectively.

A (C2mim)Cl-AlCl3 mixture with a mole ratio of 2:1 is also a room temperature IL. In this

case, the mixture is basic. The cation is (C2mim)+, but there are two anion ; Cl- and (AlCl4)-.

(C2mim)Cl-AlCl3 is an aprotic IL. Like any other haloaluminate, it is highly sensitive.

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1.7.5 Organic and Inorganic ILs

Most ILs reported in the literature consists of organic cations and organic or inorganic anions.

For example, (C4mim)(NTf2) consists of organic cations and organic anions, and (C2py)(BF4)

of organic cations and inorganic anions.

Inorganic ILs with low melting points are also known. For example, hydrazinium bromide,

(N2H5)Br, and hydrazinium nitrate, (N2H5)(NO3), melt at 86.5 and 70 0C, respectively. A

binary mixture of lithium nitrate and ammonium nitrate, LiNO3-NH4NO3, has a temperature

of 98 0C.

The protic molten salt (NH4)(HF2) almost falls within the modern IL. It has a melting point

of 125 0C. Another example is ammonium hydrogen sulphate (NH4)(HS04), which has a

melting point of 116 0C.

1.7.6 Deep Eutectic Solvents:

Deep eutectic solvent are also known as eutectic-based ILs. These show a marked depression

of freezing point when the two components of eutectic mixture are mixed. They are formed

typically by mixing a simple quaternary ammonium halide with an inorganic metal salt or an

organic hydrogen bond donor such as an amide or an alcohol. The inorganic salt or hydrogen

bond donors form a complex with the halide anion. As a result, the charge on the anion is

delocalised and the freezing point of the mixture decreases.

Deep eutectic solvents were first reported by Abbott et al. (2003). The team showed that

when choline chloride and urea, both of which are solids, are mixed together in a molar ratio

of 1:2, a liquid mixture is formed that freezes at 12 0C. As pure compounds, choline chloride

(2-hydroxyethyltrimethylammonium chloride, HOCH2CH2N(CH3)3)Cl and urea ((NH2)2CO)

melt at 302 and 133 0C , respectively.

Most deep eutectic solvents reported in the literature have been formed by mixing choline

chloride, or other substituted by ammonium salts, and metal halides. For example, eutectic

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mixtures of zinc chloride and substituted by ammonium salts have freezing point depression

of up to 270 0C. The fluid mixtures consist of the ammonium cation and a complex of the

chloride ion. The ionic mixtures dissolve metallic compounds such as nickel, copper, and

zinc oxides and have been exploited for the electrodeposition of metals and alloys by Abbott

et al. (2007).

1.7.7 Task-specific ILs

Davis et al. (2004) introduced task specific ILs. These are ILs designed with functionalised

cations and or anions that imparts specific properties or reactivities to the ILs. Task specific

ILs is also known as functionalised ILs. They are, in effect, designer ILs. ILs with

imidazolium or triphenylphosphine cations functionalised with sulphonic acid (-SO3H)

groups are some of the examples. These Brönsted acidic ILs were first reported by Frobes

and co-workers (2002) and used them as dual solvent- catalyst for a range of acid catalysed

organic reaction such as esterification.

1-butyl -3-methyl-imidazolium cobalt tetracarbonyl, (C4mim)(Co(CO4)), was one of the first

example of a task specific IL with functionalised anion to be synthesised. Dyson, et al.

(2007) described its preparation and use as a transition metal carbonyl catalyst for the

debromination of 2-bromoketones. Also Brown et al. (2001) have reported similar ILs.

In 2006, Dyson and colleagues research that, numerous IL with task specific cation have been

synthesised but less efforts has been devoted to the synthesis of IL with task specific anions.

1.7.8 Chiral IL

Numerous ILs have been synthesised by Baudequin et al. (2005) with either chiral cations or

chiral anion. Chiral IL is potentially useful as solvents or chiral catalyst for asymmetric

organic synthesis. Other potential application includes resolution of racemates by co-

crystallisation or extraction and their use as mobile or stationary phases in chromatography.

Research activity, on chiral ILs by Carmicheal et al. (2003) has focused on ILs with chiral

cations. One example is the chiral IL di(1-phenyl) imidazolium nitrate, (dpeim)(NO3), the

cation of which can exist as the optical isomer or the (S)-isomer as shown in Figure 9. There

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have been relatively few reports of ILs with chiral anions. The earliest example,

(C4mim)(lactate), was reported by Seddon et al. (1999). One more similar IL was prepared

by anion exchange between (C4mim)Cl and sodium (S)-2-hydroxypropionate in acetone by

Machado et al. (2005).

N+

OH

CH3

CH3

CH3

CH3

(1S, 2R)-(+)-N,N-dimethylephedrinium ion

Figure 1.8 Cation which can exist as the optical isomer (Carmicheal et al. (2003)

1.8 Various application of Ionic Liquids

Depending on different properties of ionic liquids, various applications are possible. The first

major industrial application of ILs was the BASIL (Biphasic Acid Scavenging utilising Ionic

Liquids) process by BASF, in which a 1-alkylimidazole was used to scavenge the acid from

an existing process. This results in the formation of an IL which can easily be removed from

the reaction mixture. But the easier removal of an unwanted side-product (as an IL rather

than as a solid salt) is not the only advantage of the IL based process. By using an IL it was

possible to increase the space/time yield of the reaction by a factor of 80,000 as stated by

Hermanutz et al. (2006). It should, however, be kept in mind that improvements of such

scale are rare.

Occurring at a volume of some 700 billion tons, cellulose is the earth’s most widespread

natural organic chemical and, thus, highly important as a bio-renewable resource. But even

out of the 40 billion tons nature renews every year, only approx. 0.2 billion tons are used as

feedstock for further processing. A more intensive exploitation of cellulose as a bio-

renewable feedstock has to date been prevented by the lack of a suitable solvent that can be

used in chemical processes. Robin Rogers and co-workers at the University of Alabama have

found that by means of ionic liquids, however, real solutions of cellulose can now be

produced for the first time at technically useful concentrations (Richard et al., 1988). This

new technology therefore opens up great potential for cellulose processing.

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RTILs are extensively explored for various innovative applications in nuclear industry. It

includes application of ionic liquid as extractants / diluents in solvent extraction systems, as

alternate electrolyte media for the high temperature pyrochemical processing, etc.

Fundamental studies on the extraction methods for electrodeposition of fission products like

uranium, palladiums etc., from spent nuclear fuel using RTILs as extractants are reported.

Reports on using ionic liquids as non-aqueous electrolyte media for the recovery of uranium

was reported by Giridhar et al. (2007), lanthanides was recorded by Rao et al. (2007), and

useful fission products like palladium was reported by Jayakumar et al. (2007) and rhodium

was again reported by Jayakumar et al. (2008) from spent nuclear fuel are also available .

Studies on the electrochemical behaviour of uranium(VI) in ionic liquid, 1-butyl-3-

methylimidazolium chloride and also the recovery of valuable fission products from tissue

paper waste was studied in room temperature ionic liquids by Rao et al. (2007). The

dissolution properties of uranium oxides, UO3, UO2, and U3O8 and their individual separation

was studied using a task-specific ionic liquid, namely protonated betaine

bis(trifluoromethanesulphonyl) imide, (Hbet)(NTf2).

1.8.1 Solar Energy Applications

Ionic liquids show great potential for use as a heat transfer and storage medium in solar

thermal energy systems. Solar thermal power concentration facilities such as parabolic

troughs and solar power towers utilise the energy of the sun by focusing it onto a receiver

which can generate temperatures of around 60 0C. This heat can then be used to generate

electricity in a steam or other cycle. For buffering during cloudy periods or to enable

generation overnight, some of this energy can be stored by heating an intermediate fluid.

Although nitrate salts have been the medium of choice since the early 1980s, they freeze at

220C and thus require heat tracing overnight to prevent solidification. Ionic liquids such as

(C4mim)(BF4) have been identified with more favourable liquid-phase temperature ranges (–

75 to 459 0C), and could therefore be excellent liquid thermal storage media and heat transfer

fluids in solar thermal power plants reported by Banqui et al. (2001).

1.8.2 Hydrogen Storage

Ionic liquids have several properties that make them viable options for hydrogen storage

systems. For instance, the vapour pressure of ionic liquids is very low and is negligible in

most situations. These liquid are also stable at high temperatures. In addition, ionic liquids

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are able to act as solvents for a wide variety of compounds and gases; they also have weakly

coordinating anions and cations which are able to stabilise polar transition states. Finally, the

liquids are able to be reused with minimal loss of activity. In their research Karkamkar et al.

(2008) used 1-butyl-3-methylimidazolium chloride (BmimCl) in the dehydrogenation of

ammonia borane. Immediately upon heating the sample, hydrogen evolution took place with

a final value of hydrogen evolution as high as 5.4 wt% H2, as was observed by Karkamkar et

al. (2007).

1.8.3 Natural Product Extraction

Ionic liquids are proving superior to conventional solvents in the extraction of specific natural

compounds from plant biomass for pharmaceutical, nutraceutical, and cosmetic applications.

For example, a series of protic ionic liquids have been evaluated as solvents for the isolation

of the important antimalarial drug artemisinin from the plant Artemisia annua. Lapkin et al.

(2006) conducted a benchmarking study taking into consideration of operational parameters,

in which the ionic liquid equalled or outperformed the alternatives.

1.8.4 Waste Recycling

Ionic liquids can be developed for the recycling of synthetic goods, plastics, and metals.

They offer the specificity required to separate similar compounds from each other, such as in

the separation of polymers from plastic waste streams. This has been achieved this using

lower temperature extraction processes than current approaches and could be the answer to

avoiding tonnes of plastics being incinerated or consigned to landfill each year.

1.8.5 Safety

Due to their non-volatility, thus effectively eliminating a major pathway for environmental

release and contamination, ILs have been considered as having a low impact on the

environment and human health, and thus recognised as solvents for green chemistry.

However, this is distinct from toxicity, and it remains to be seen how ‘environmentally-

friendly’ ILs will be regarded once widely used by industry. Research into IL aquatic

toxicity has shown them to be as toxic as or more so than many current solvents already in

use. Review papers on this aspect have been published in 2007 by Ranke et al. Available

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research also shows that mortality isn't necessarily the most important metric for measuring

their impacts in aquatic environments, as sub-lethal concentrations have been shown to

change organisms’ life histories in meaningful ways. According to these researchers,

balancing between zero VOC emissions, and avoiding spills into waterways (via waste

ponds/streams, etc.) should become a top priority. However, with the enormous diversity of

substituents available to make useful ILs, it should be possible to design them with useful

physical properties and less toxic chemical properties.

With regard to the safe disposal of ionic liquids, Chiappe et al. (2006) and Xuehui et al.

(2007) have reported the use of ultrasound to degrade solutions of imidazolium-based ionic

liquids with hydrogen peroxide and acetic acid to relatively innocuous compounds.

Despite their low vapour pressure, many ionic liquids, as suggested by Smiglak et al. (2006)

have also found to be combustible and therefore require careful handling. Brief exposure

(about 5 to 7 seconds) to a flame torch will ignite these ILs and some of them are even

completely consumed by combustion.