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.