Chapter 1Chapter 1Chapter 1Chapter 1
Introduction and Literature ReviewIntroduction and Literature ReviewIntroduction and Literature ReviewIntroduction and Literature Review
Chapter 1
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1.1 IONIC LIQUIDS: WHAT ARE THEY?
Ionic liquids (ILs) have been accepted as agents of green chemical revolution in
both the academia and the chemical industries. They have the potential to reduce
the use of hazardous and polluting organic solvents due to their unique
characteristics. The terms room temperature ionic liquid (RTILs), non-aqueous
ionic liquid, molten salt, liquid organic salt and fused salt have all been used to
describe these salts in the liquid phase.1 ILs are made up of positively and
negatively charged ions, whereas water and organic solvents (such as toluene and
dichloromethane) are made up solely of molecules. The structure of ILs is
similar to that of table salt (sodium chloride) which contains crystals made of
positive sodium ions and negative chlorine ions, not molecules. While, salts do
not melt below 800◦C, most of ILs remain liquid at room temperature. The
melting points of sodium chloride and lithium chloride are 801◦C and 614◦C,
respectively. Since these conventional molten salts exhibit high melting points,
their use as solvents is severely limited. However, RTILs are liquid generally up
to 200◦C, ILs have a wide liquidus ranges. The adopted upper melting
temperature limit for the classification as ionic liquid is 100◦C and higher
melting ion systems are generally referred as molten salts.2
Therefore, ILs are known as salts that are liquid at room temperature (that melt
below 100 degrees) in contrast to high-temperature molten salts. They have a
unique array of physico-chemical properties which make them suitable in
numerous applications in which conventional organic solvents are not
sufficiently effective or not applicable.
1.2 HISTORICAL BACKGROUND
ILs have been known for a long time, but their extensive use as solvents in chemical
processes for synthesis and catalysis has recently become very significant. Welton1
reported that ILs are not new and some of the ILs such as ethylammonium nitrate was
first described in 1914.3 The earliest IL in the literature was created intentionally in
1970s for nuclear warheads batteries4. During1940s, aluminum chloride-based molten
salts were utilized for electroplating at temperatures of 100°C. In the early 1970s,
Chapter 1
2
Wilkes tried to develop better batteries for nuclear warheads and space probes which
required molten salts to operate.4 These molten salts were hot enough to damage the
nearby materials. Therefore, the chemists searched for salts which remained liquid at
lower temperatures and eventually they identified one which was liquid at room
temperature. Wilkes and his colleagues continued to improve their ILs for use as battery
electrolytes and then a small community of researchers began to make ILs and test their
properties.5,6 In the late 1990s, ILs became one of the most promising chemicals as
solvents. The first ILs (such as organo-aluminate ILs) had limited range of applications
because they were unstable in air and water. Furthermore, these ILs were not inert
towards various organic compounds.7 Following the first reports on the synthesis and
applications of air stable ILs such as 1-n-butyl-3-methlyimidazolium tetrafluoroborate
([bmim]BF4) and 1- n-butyl-3-methlyimidazolium hexafluorophosphate ([bmim]PF6), the
number of air and water stable ILs has started to increase rapidly.7 Recently, researchers
have discovered that ILs are more than just green solvents and they have found several
applications such as replacement of volatile organic solvents, precursors for making new
materials, effective conductor of heat, support for enzyme-catalyzed reactions, host for a
variety of catalysts, agents for purification of gases, media for homogenous/
heterogeneous catalysis and biological reactions and reagents for removal of metal ions.4
Some of the basic physical properties of ILs such as density and viscosity are still being
evaluated by the researchers as the study of the ILs is still a relatively young field.8 It is
therefore evident that the amount of research on ILs and their specific applications is
increasing rapidly. As an example, though the cation 1-ethyl-3-methylimidazolium has
been the most widely studied until 2001, 1-3-dialkyl imidazolium salts are the most
investigated class of ILs in recent times. The future aim of research in ionic liquids
focuses towards their commercialization in order to use them as solvents, reagents,
catalysts and materials in large-scale chemical applications.
1.3 CLASSIFICATION OF IONIC LIQUIDS
Typical cations in ionic liquids are imidazolium, ammonium, pyridinium, pyrrolidinium,
phosphonium and sulfonium derivatives. The anions may be of inorganic or organic
origin. Common inorganic anions are halide, tetrachloroaluminate (also
tetrachloroferrate and tetrachloroindate), tetrafluoroborate, hexafluorophosphate and
Chapter 1
3
bis(trifluoromethylsulfonyl)imide and common organic anions are derivatives of
sulfate or sulfonate esters, trifluoroacetate, lactate, acetate or dicyanamide.9-11
Substituents (the R-group) on the cation are usually alkyl chains, but can contain
any of a variety of functional groups, such as fluoroalkyl, alkenyl, methoxy or
hydroxyl groups. Functionalized ILs are often designed for a particular use, e.g. for
specific reactions, extractions or separations and these ILs are then referred to as “task
specific ionic liquids” (TSILs).12 Table 1.1 shows the cations and anions organized
according to their relative acidity and basicity. Lewis basic and Lewis acidic families
of ionic liquids are expanding continually in terms of their discovery and utility.13,14
Table 1.1: Structures and nomenclature of the most common cations and anions
in ILs, and their acidis/ basic properties. 15
The relative acidity and basicity of the component ions imparts variety of physical
characteristics to the ionic liquids. A smart distinction can be gives as follows:
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1.3.1 Neutral Anions and Cations
Typical ionic liquid anions are those that can be described as neutral in the acid/ base
sense or very weakly basic; these exhibit only weak electrostatic interactions with the
cation and thus impart advantageously low melting points and viscosities. Included in this
class are anions such as PF6-, bis(trifluorometahnesulphonyl)imide (TFSI / Tf2N
-), BF4-,
methanesulfonate (mesylate), thiocyanate, tricyanomethide and p-toluenesulfonate
(tosylate). ILs formed from these anions typically exhibit good thermal and
electrochemical stability and thus, are often utilized as inert solvents in a wide range of
applications. 16-19
1.3.2 Acidic Cations and Anions
The simplest examples of slightly acidic ILs are those based on the protic ammonium,
pyrrolidinium and imidazolium ions, of which many are known. The well known
AlCl3 based ILs are Lewis acidic when they contain an excess of AlCl3.20-30
1.3.3 Basic Cations and Anions
There are a number of ionic liquid forming anions that can be called basic. These
include the lactate, formate, acetate (and carboxylates generally) and the dicyanamide
(dca) anion. The dicyanamides, in particular, have become readily available because
of their low viscosity. The basicity of these anions imparts different, advantageous
properties to the ILs, such as different solubilizing and catalytic properties.15 An
alternative to the design of ILs utilizing a basic anion is to incorporate a basic site into
the cation. This may afford more thermally stable ILs than those containing basic
anions, which frequently exhibit relatively low decomposition temperatures.13
1.3.4 Amphoteric Anions
There are a small number of ionic liquid anions that fall into the interesting class of
amphoteric anions, with the potential to both accept and donate protons depending on
the other substances present. The hydrogen sulfate (HSO4-) and dihydrogen phosphate
(H2PO4-) anions are simple examples of such anions.13
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1.4 PROPERTIES OF IONIC LIQUIDS, PHYSICAL AND CHEMICAL
Depending upon the application, properties of ionic liquids (whether physical or
chemical) can be tailored with the selection of suitable cationic and anionic
components. In this section an overview of the properties of ionic liquids ranging
from few physical and their common chemical behavior is given.
1.4.1 Melting point and liquidus range; Tm
Melting is said to occur when molecules or ions fall out of their crystal structure and
become disordered liquid. The two constituent of an ionic liquid- cation and its
corresponding anion, affect the melting point (Tm) of the former. In general, charge,
size, symmetry, intermolecular interaction and delocalization of charge are some of
the main factors that can influence the melting point.6,11,31-36 The melting point of ILs
is essential because it represents the lower limit of the liquidity and with thermal
stability it defines the interval of temperatures within which it is possible to use ILs as
solvents.37
Researchers have explained that ILs remain liquid at room temperature because their
ions do not pack well.38 Their low melting behavior is attributed to their chemical
composition. Combination of bulky and asymmetrical cations and evenly shaped
anions form a regular structure, lowering the lattice energy and hence the melting
point of the resulting ionic medium. In some cases, even the anions can be relatively
large and they can play a role in lowering the melting point.39
A comparative study here in Table 1.2 illustrates the trend in melting point of the
imidazolium ionic liquid system as affected by the size of anion. With increasing size
of the anion, the coulombic electrostatic interactions with the imidazolium cation in
the crystal lattice diminishes and melting point of the salt decreases. In combination
with a good charge delocalization, low solid-liquid phase transition temperatures can
be achieved. Melting point of a 1-ethyl-3-methyl imidazolium salt decrease from
87 °C to -14 °C in the order of Cl− > NO3− >BF4
− >CF3COO− (Table 1.2).5,6,16,40,41 The
melting temperature generally decreases with increasing anion radius except for anion
PF6−. This is because ILs with anions of PF6
− form strong hydrogen bonds due to
presence of fluorine atom and hence their melting points are comparatively higher.42
Chapter 1
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Table 1.2: Effect of anion size on the melting temperature of imidazolium ILs.43
Alkyl chain length also has a significant influence on the melting point. As an
example, melting of the ionic liquid based on 1-alkyl-3-methyl imidazolium cations
decreases with alkyl-chain length up to n = 8-10. However, beyond this point, van der
Waals interactions between the hydrocarbon chains gain more importance. The
melting point of an ionic liquid starts to rise with increasing alkyl chain length and its
symmetry decreases.11,44 Moreover, as branching on the alkyl chain increases the
melting point also increases.
Methylation at C-2 increases the melting point of alkylimidazolium based ionic
liquids. As an example, the melting point of 1-ethyl-2-methyl imidazolium chloride is
181°C, which is much higher than that of 1-ethyl-3-methylimidazolium chloride
87.15°C. This implies that the effect of the van der Waals interaction via a methyl
group dominates over the electrostatic interaction via proton on C-2.45
1.4.2 Glass transition temperature Tg
The glass transition is another important physical parameter important to ILs.
The glass transition temperature (Tg) is defined as the temperature at which transition
happens from a solid crystalline state to an amorphous solid state. However, it should
Chapter 1
7
be noted that even crystalline solids may have some amorphous portion in them. Due
to this reason some ILs may have both- a glass transition temperature as well as a
melting temperature. In the case of most ILs, cooling from the liquid state leads to
glass formation at low temperatures as a result of the extremely unfavorable packing-
efficiency in the solid state. Usually, the glass transition temperature (Tg) is found to
be lower than -50°C46 and particularly in the range between -70°C and -90°C for
1- alkyl-3-methyl imidazolium salts.1
On the average, ionic liquids have a wide temperature range for the liquid state,
frequently found from-80 °C up to 300 °C. The melting point represents the lower
limit of the liquid range within which it is possible to use the salt as a liquid. The
upper limit is usually related to the thermal decomposition of ILs, as most of them are
non-volatile. Until now, the statement that ILs have no vapor pressure has not only
theoretically been refuted, in some cases distillation of ILs in vacuum is also
possible.47,48
1.4.3 Decomposition temperature Td
Thermal decomposition of an ionic liquid is strongly dependent on its structure. With
certain ions it is also dependent on the sample pan composition.33 Different from
organic solvents, many kinds of ILs can be kept in the liquid state above 400 °C and
this makes them have good dynamic properties and excellent catalytic activities.
Generally, the imidazolium cations tend to be thermally more stable than the tetra-
alkyl ammonium cations. High thermal stability is provided by certain kinds of anions
such as TFSI-. The relative anion stabilities follows the order; PF6- > TFSI- > CF3SO3
-
> BF4- >> I-, Br-, Cl-. The decomposition temperature (Td) is mainly influenced by the
strength of the incorporated heteroatom-carbon and heteroatom-hydrogen bond.49
High decomposition temperatures can be provided by ILs whose cations are obtained
by quaternization reaction using an alkylating agent and in special cases Td up to
450 °C can be obtained.50 In general, the temperature stability is higher when weakly
coordinating anions are used (Table 1.3).16,33,40,51,52
Chapter 1
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Table 1.3: Influence of the anion on the decomposition temperature (Td) for 1-
ethyl-3-methyl imidazolium based ILs.51
1.4.4 Viscosity
ILs can be classified generally in terms of their Newtonian or in some cases
thixotropic characteristics.53 Their viscosities range from 10 mPas to 500 mPas at
ambient temperature,1 which is two or three orders of magnitude higher than
viscosities of traditional organic solvents.54 This is quite higher than viscosity of
water; 0.89 mPas. The high viscosities of ILs are therefore one of the major limiting
factors for their large-scale use. In most cases, viscosity is influenced by the tendency
of the constituents to form hydrogen bonds and by the strength of their van der Waals
interactions.16 The ability of hydrogen bonding is mostly affected by the anions
present. Within a series of imidazolium based ILs carrying the same cation, variation
of the anion clearly changes the viscosity in the general order Tf2N⎯ < BF4⎯< PF6
⎯<
halides. Furthermore, for ILs with the same anion, the trend of increasing viscosity
with increasing chain length of the alkyl substituent (by means of stronger van der
Waals interactions) has also been cited.11,16 Lengthening of alkyl chain or fluorination
can make the salt more viscous, due to an increase in van der Waals interactions and
hydrogen bonds.16 Similarly, methylation at C(2), but not at C(3), increases the
viscosity as it does for the melting point.
The viscosity of many ILs is strongly dependent on the temperature also. The
empirical equation 1.1, is also applicable in ionic liquid systems to describe the
Chapter 1
9
temperature dependence of the dynamic viscosity for unassociated liquid
electrolytes.16
η = A eε/ RT (1.1)
Temperature and also the presence of additives are important factors in influencing
the viscosity of ILs. The viscosity will decrease when the temperature is slightly
increased6,55,56 or little organic solvent55,56 is added to ILs.
1.4.5 Density
The densities of most of the ILs are higher than water except for pyrrolidinium
dicyanodiamide and guanidinium (where density ranges from 0.9gcm-3 to 0.97gcm-3).
Density of ILs decreases as the number of carbon atoms in the alkyl group and the
sum of carbon numbers for the quaternary ammonium ILs increases.42 It is interesting
to note that the density of 1-methylimidazolium ionic liquids decreases linearly with
increasing temperature but at a rate less than that for molecular organic solvents.40
1.4.6 Surface tension
Data available on the surface tension of ILs is very limited. Their liquid/air surface
tension values are somewhat higher than conventional solvents (e.g., hexane: 1.8 Pa
cm), but not so high as water (7.3 Pa cm).51 Dzyuba and Bartsch have reported the
influence of the 1-alkyl group on the surface tension of [Cnmim]PF6 and
[Cnmim]TFSI and have pointed out that the surface tension decreases with the
increase of the carbon number and a lower surface tension is found for TFSI− salt than
the corresponding PF6−.57
1.4.7 Purity; Anionic impurity
Impurities, such as water, halides, unreacted organic salts and organics, are usually
retained in ILs during synthesis or catalytic applications.58 These expected impurities
may influence the solvent properties53,59 and/or interfere with the catalyst or
biocatalyst.60
Chapter 1
10
It is therefore of utmost importance to assess the purity of the ILs. The Vollhard
method or an ion-selective electrode method can be used to measure chloride, and the
later method can be applied for the measurement of sodium also. Water can be found
to be present in ILs either due to ineffective drying after preparation or due to
absorption from the atmosphere due to the hygroscopic properties of the synthesized
ILs. However, even water immiscible ILs are known to absorb moisture from
atmosphere. Indeed, [C4mim]PF6 can absorb up to 0.16 mole fraction of water from
atmospheric air (measurement through Karl-Fischer titration).
Both water and chloride impurities can alter physical properties of ILs considerably.
The presence of contamination with chloride can increase the viscosity of the ILs,
whereas the presence of water, or other co-solvents, can reduce the viscosity. The
addition of co-solvents in general reduces the viscosity, with the effect being stronger
for co-solvents with higher dielectric constant. The structural changes affecting
majority of properties at an equimolar concentration of water and ionic liquid
indicates the possible formation of a hydrogen-bonded complex with water.53
1.4.8 Solvent properties of ILs
1.4.8.1 Polarity
Polarity behavior of any chemical helps in classifying it as a solvent. Under the
definition of polar solvent, i.e. a solvent having the ability to dissolve and stabilize
dipolar or charged solutes, ILs are highly polar solvents. But this cannot be strictly
concluded as ILs can be designed in a vast range. Ionic liquids can therefore, be
classified as dipolar, protic or aprotic solvents respectively. The solvent polarity for
experimental and theoretical studies is determined by the values of dielectric
constants, dipole moments and polarizabilities.1 However, a direct measurement of
the dielectric constant which requires a non-conducting medium is not available for
ionic liquids.
Attempts have been made to develop empirical solvent polarity scales for ILs as a
means of explaining differences in solvent-mediated reaction pathways, reaction yields,
synthesis product ratios, chromatographic retention, and extraction coefficients. Based
Chapter 1
11
on the comparison of the effects on the UV-visible spectra for sets of closely related
dyes, Abboud, Kamlet, and Taft evaluated some typical properties, dipolarity or
polarizability (π*), H-bond basicity (β), and H-bond acidity (α).61-63 Different
investigations of solvent-solute interactions in ILs using solvatochromic dyes have been
reported.61,62,64 Crowhurst et al.65 applied the Abboud-Kamlet-Taft method using three
solvatochromic dyes (Richardt’s66, N,N-diethyl-4-nitroaniline, and 4-aniline) to
determine the solvent parameters π*, β and α of imidazolium ILs (Table 1.4). The π*
values found by Crowhurst et al. for the investigated ILs indicated higher values of
dipolarity or polarizability than that of alkyl chain alcohols. Although differences
between the ILs are small, both the cation and the anion have been found to affect this
parameter. On the contrary, the H-bond basicity of the examined ILs covers a large
range, from a similar value to that of acetonitrile to lower β-values. The parameter β for
H-bond basicity is determined by the nature of the anion while H-bond acidity is
determined by the cation, (even if a smaller anion effect is there). In particular, it has
been suggested that α values are controlled by the ability of the cation to act as an H-
bond acceptor; a strong anion-cation interaction reduces the ability of the cation to
hydrogen bond with the substrate. The acidity of the investigated ILs is generally less
than those of water and most short-chain alcohols but greater than those of various
organic solvents.20 The polarity scale of several organic solvents including different
groups of ionic liquids is illustrated in Figure 1.1.43
The solvent properties of the ILs have also been investigated using chromatographic
techniques.67-72 The solvent properties of ILs i.e. their ability to act as a hydrogen-
bond donor or acceptor, have been measured by Anderson et al.73 GC retention times
of a range of probe solutes on a variety of columns using ILs as the stationary phases
were studied. The ILs were found to interact with solutes via high dipolar and
dispersion forces and also acted as strong hydrogen bond bases.
A different approach towards measurement of solvent polarity is based upon the
measurement of keto-enol equilibria (as this is known to be affected by the polarity of
the medium). This particular methodology, when applied to probe the polarity of ionic
liquids, indicated that [bmim]BF4, [bmim]PF6, and [bmim]NTf2 are more polar than
organic solvents such as methanol or acetonitrile.74
Chapter 1
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Table 1.4: Kamlet-Taft parameters for few ILs.20
Ionic liquid ET a Π* α β
[bmim]BF4 0.670 1.047 0.627 0.376
[bmim]PF6 0.669 1.032 0.634 0.207
[bmim]OTf 0.656 1.006 0.625 0.464
[bmim]NTf2 0.644 0.984 0.617 0.243
[omim]PF6 0.633
Ethanol 0.650
aET = 28592/ (the wavelength maximum of the lowest energy π-π* absorption band of the zwitter ionic Richardt’s dyes).
Figure 1.1: Normalized solvent polarity scale for several organic solvents and different
groups of ionic liquids.75
In the case of ILs based on 1-alkyl-3-methyl imidazolium cations, the polarity is
influenced by the anion, for shorter alkyl chains, whereas for longer alkyl chains the
influence of the anion present is less. The polarity typically decreases in the order of
NO2⎯ >NO3
⎯ >BF4⎯ >Tf2N⎯ >PF6
⎯ and with anion size (more particular with the
effective charge density of the anion).76
Chapter 1
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1.4.8.2 Miscibility behavior of ILs
The search for alternative solvents to meet the cleaner technology requirements is
always under exploration since the most widely used solvents are volatile and
damaging. ILs are solvents of choice for a wide range of substances; organic,
inorganic, organometallic compounds, bio-molecules and metal ions. Generally, they
are composed of poorly coordinating ions which makes them highly polar but non-
coordinating solvents.2
Most of the listed categories of compounds are sufficiently soluble in ILs towards
their performance in organic transformations. With regard to their general solvent
properties, it has been concluded (on the basis of the Abraham free energy
relationship) that ILs resemble polar organic solvents such as acetonitrile,
N-methylpyrrolidone, or methanol.77 A potential application of polar aprotic ILs is to
use them as a medium for solublizing biomolecules such as proteins and
carbohydrates (that are sparingly soluble in common organic media). However, it has
been found that even simple sugars do not dissolve to an appreciable degree in water-
miscible ionic liquids, such as [bmim]BF4. In contrast, [bmim]Cl can dissolve
massive amounts of cellulose.78 The ability of ionic liquids to act as solvents or to
dissolve complex compounds, such as sugars and proteins, mainly depends on the
ability of the salt to act as a hydrogen bond donor and/or acceptor and the degree of
localization of the charges on the anions.1,79 Charge distributions over anions, H-
bonding ability, polarity, dispersive interactions are the major factors that influence
the physical properties of ILs.80 As an example, imidazolium-based ILs are highly
ordered, hydrogen-bonded solvents and they have strong effects on chemical reactions
and processes.
Miscibility of ILs with water also varies unpredictably. [bmim]BF4 and
[bmim]MeSO4 are water-miscible, while [bmim]PF6 and [bmim]Tf2N are not. These
ionic liquids are of similar polarity on the Reichardt scale,75 and the coordination
strengths of the BF4- and PF6
- anions are also comparable.62 A measurement of the H-
bond accepting properties of such ionic liquids has revealed that BF4- and MeSO4
- are
better H-bond acceptors (β= 0.61 and 0.75, respectively) than PF6- (β= 0.50),81 which
could explain the difference in water miscibility. It must be taken into consideration
Chapter 1
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that aqueous mixtures of ILs may not be homogeneous at molecular scale as, at this
level even water does not mix in methanol and is present as strings or clusters of
molecules.82 As discussed before, even water-immiscible ionic liquids can be
hygroscopic, as they can readily absorb water.53 IR spectroscopic analysis has
confirmed that water interacts mainly with the anion83 via the formation of double H-
bonds,84 at least in case where the cation is a weak hydrogen bond donor.
The miscibility behavior of ILs and organic solvents however is not well documented. A
relationship with the dielectric constant has been proposed, as lower alcohols and ketones,
dichloromethane and THF (ε = 7.58) mix with for example, [bmim]Tf2N, whereas
alkanes and ethers do not.16 Most of the ILs are immiscible with most of the organic
solvents and thereby provide a non-aqueous, polar alternative for two-phase systems.85
1.4.8.3 Volatility; Low vapor pressure
Due to their extremely low vapor pressure, ILs do not tend to give off vapors in
contrast to traditional organic solvents such as benzene, acetone, and toluene.
Kabo et al.86 have reported the vapor pressure of [bmim]PF6 at 298.15 K as
10Pa−11Pa. ILs can be introduced as green solvents because unlike the volatile
organic compounds (VOCs) they have negligible vapor pressure, are not explosive
and in certain cases may be feasibly recycled and used repeatedly. Moreover, these
non-evaporating ILs eliminate the hazardous exposure and air pollution problems.
Unlike conventional solvents ILs do not evaporate into atmosphere and their non-
volatility gives an opportunity to utilize them in high vacuum systems. In addition,
ILs are potentially good solvents for many chemical reactions in the cases where
distillation is not practical, or water insoluble or thermally sensitive products (e.g.
certain pharmaceutical compounds) are the components of a chemical reaction.2
Although, reported earlier ILs were not considered to be distilled due to their low
volatility, Earle et al.47 showed that many ILs, especially bistriflamide ILs can be
distilled at 200◦C–300◦C and low pressure without decomposition.
Thus, due to their stability, non-volatility, adjustable miscibility and polarity, ILs may
be used as ideal substitutes for conventional organic solvents.87
Chapter 1
15
1.5 SYNTHESIS METHODS OF IONIC LIQUIDS
ILs are ‘designer solvents’ since their specific properties can be tuned for a particular
need. A specific IL can be designed by choosing negatively charged small anions and
positively charged large cations and these specific ILs can be utilized to dissolve
certain chemicals or to extract them from a solution. The fine-tuning of the structure
provides tailor-designed properties so as to satisfy the requirements for a specific
application. The physical and chemical properties of ILs can be varied by changing
the alkyl chain length on the cation and the anion. As an example, Huddleston et al.88
concluded that density of ILs increases with a decrease in the alkyl chain length on
the cation and an increase in the molecular weight of the anion. It is estimated that by
combining various kinds of cation and anion structure, 1018 ILs can be designed.37,85
The most widely used cations are imidazolium, pyridinium, phosphonium and
ammonium ions. The overall properties of ILs result from the composite properties of
the cations and anions, where the anion controls the water miscibility and the cation
also has an influence on the hydrophobicity or hydrogen bonding ability of the ionic
liquid.89
1.5.1 Anion
ILs with varied properties can be obtained by introducing different anions. IL anions
can be of two types: fluorous anions such as PF6−, BF4
−, CF3SO3−, (CF3SO3)2N
− and
non-fluorous anions such as AlCl4−. Anions most commonly encountered in an IL are;
chloride, nitrate, acetate, hexafluorophosphate and tetrafluoroborate.90 However, in
designing ILs, fluorous anions are usually opted because of the distinct properties
they impart. As an example, as already discussed, IL with 1-n-butyl-3-
methylimidazolium cation and PF6- anion is water-immiscible, whereas IL with same
cation and BF4− anion is water soluble. This exemplifies the ‘designer solvent’
property of ILs, i.e. by changing the anion the density, hydrophobicity, viscosity and
solvation properties of the IL system can be altered.8 Although PF6- and BF4
− are the
two anion types that are utilized in most of IL applications, they suffer from a serious
Chapter 1
16
disadvantage. These anions undergo decomposition when heated in the presence of
water and liberate HF. Following the discovery of this phenomenon, fluorous anions
containing C-F bond which is inert to hydrolysis were started to be used.
Consequently, ILs bearing CF3SO3− and (CF3SO3)2N
− anions in which the fluorine is
bonded to carbon have been produced.91 However, fluorinated anions tend to be
expensive and toxic to the environment. Hence, alkylsulfate anions derived from
inexpensive bulk chemicals have been found as the most popular non-fluorous anions
due to their non-toxic and biodegradable structures.91
1.5.2 Cations
The preferable cation for any ionic liquid is one having a bulky structure with low
symmetry. Most of the ILs currently in research are based on ammonium, sulfonium,
phosphonium, imidazolium, pyridinium, picolinium, pyrrolidinium, thiazolium,
oxazolium and pyrazolium cations.2 Properties of ILs, such as melting temperature,
density, viscosity etc. are affected differently with variation in size, symmetry and
alkyl chains attached to cation (as already discussed in Section 1.4). Not only this,
different ILs can be designed by introducing a suitable functionality into the cation
leading to the formation of third generation ILs-Task specific ILs.12 An essential
target to chemists involved in organic transformations and total synthesis is tuning the
stereochemistry of the product. In this respect, chiral ILs have been suitably designed
to carry out the work of asymmetric synthesis.92
1.5.3 Synthesis
The synthesis of ILs generally proceeds in two steps: formation of the cation followed
by anion exchange (metathesis). Typical synthetic pathways for the preparation of ILs
are shown in Figure 1.2, where the preparation of imidazolium based ILs is taken as
an example.
The cation formation step, most often described as a quaternization reaction, imparts
ionic nature to the compound. The starting material, imidazolium (or amine,
Chapter 1
17
pyrimidine, etc.), is alkylated with an appropriate alkyl halide (RX) and in halogen
based ILs, this is the only step which is required. However, quaternization with alkyl
halides sometimes may leave traces of halide ion in the ionic liquid. Not always, but
halide ions can also interfere with metal catalysts, can cause corrosion problems in
chemical plants and interfere with measurements of physical property of ILs.93 As an
alternative, ILs can be synthesized via a “halogen- free” route, where an alkyl
alkylsulfonate, usually alkyl methylsulfonate (mesylate)94 or alkyl toluenesulfonate
(tosylate)95, is used for the quaternization reaction. The quaternization reaction can
also proceed by protonation with a Brønsted acid.
The anion exchange reactions can be brought about in two possible ways: a halide salt
can be treated with a Lewis acid to form a Lewis acidic ionic liquid, or an exchange
reaction can be carried out by anion metathesis.11 Typical Lewis acids that can be
used in this context are AlCl3, BCl3, CuCl2, FeCl2, or SnCl2.
Figure 1.2: Synthesis routes for the preparation of methylimidazolium based ionic liquids.11
Chapter 1
18
The beginning of IL preparation dates back to 1914, where ethylammonium nitrate
([EtNH3]NO3, mp 13°C-14°C) was prepared by neutralization of ethylamine with
concentrated nitric acid. The discovery did not attract much scientific interest and
these new materials went largely unrecognized till the 1970s when organic
chloroaluminates (first- generation ILs, Figure 1.3) were investigated. In the 1990s,
Wilkes and Zaworotko reported the preparation of air- and moisture-stable ILs
(second- generation ILs, Figure 1.3) using new combinations of cations and anions.
Since then, a wide range of ILs have been developed including TSILs (third-
generation ILs, Figure 1.3), which were introduced by Davis12 in 2004.9,10
Figure 1.3: The three generations of ionic liquids.10
Below is thus summarized, generation vise synthesis of ILs; ranging from ammonium
ILs, non- functionalized ILs, functionalized task specific ILs (TSILs) and chiral ILs.
1.5.3.1 Ammonium cation based ILs
The aliphatic quaternary ammonium (AQA) cation is a useful cationic component of
room temperature ILs (Figure 1.4), since the salts containing AQA cations and
appropriate oxidation resistant anions such as ClO4-, BF4
- or PF6- are
electrochemically stable and may be used as a supporting electrolyte. The asymmetric
amide anion (CF3SO2-N-COCF3)- has an excellent ability to lower both the melting
points and viscosities of room temperature ionic liquids, combining with the small
aliphatic cations.96
Chapter 1
19
Figure 1.4: AQA cations used in ILs97
There is, however, a limitation on the reduction of the viscosity of the AQA-based
room temperature ionic liquids, compared with the imidazolium systems as the
molecular weight of the AQA cations cannot be reduced to below a threshold value.96
1.5.3.2 Non-functionalized ILs
As already discussed, room temperature ionic liquids are prepared by direct
quarternisation of the appropriate amines or phosphines.98 Dialkylimidazolium and
alkylpyridinium cation-based ionic liquids have been easily prepared by alkylation of the
commercially available N-methylimidazole or pyridine with an alkyl halide to give the
corresponding 1-alkyl-3-methylimidazolium or 1-alkylpyridinium halide. Different
anions have subsequently been introduced by anion exchange (metathesis), although, due
to their non-volatile nature, they cannot be purified by distillation. Purification is
therefore, usually carried out by dissolving the ionic liquid in acetonitrile or
tetrahydrofuran (THF), treating it with activated charcoal for more than 24 h and finally,
removing the solvent in vacuuo.
A more recent method involves the microwave-assisted solvent-less synthesis of
imidazolium ionic liquids.99 The microwave heating reduces the reaction time from
several hours to minutes and avoids the use of a large excess of alkyl halides/ organic
solvents as the reaction medium. Dialkylimidazolium tetrachloroaluminates are
Chapter 1
20
prepared in a few minutes by the reaction of the appropriate N,N´-dialkylimidazolium
chloride and aluminium chloride under microwave irradiation.100 A new series of salts
based on the dicyanamide anion (dca), most of which are liquids at room temperature,
have been synthesized (Figure 1.5). These ILs have potential donor characteristics, as
the anion is a powerful ligand, and possess a lower viscosity.101
Figure 1.5: Dicyanamide anion based ionic liquids.101
1.5.3.3 Functionalised ILs; TSILs (Task Specific Ionic Liquids)
These have been described as a class of ILs, which incorporates functional groups
designed to impart to them particular properties or reactivities. Two basic rationales
have been given by Davis12 for the inclusion of functionality into an IL. First, the
inclusion of the functional group will undouubtedly alter the solvent parameters of an
IL relative to an analog bearing a simple hydrocarbon appendage.102 These
parameters-dipolarity, H-bond acidity and basicity, polarizability, etc. are the
attributes which make any chemical a good or poor solvent for a given solute.103 A
second rationale for a functional group being incorporated into an IL is to make the
salt with a capacity to covalently bind to or catalytically activate a dissolved substrate.
This application also parallels with the solid support catalysis using ILs. Moreover,
Scammells et al. have shown that the incorporation of certain functional groups
(especially esters) increases the rate of breakdown of an IL in the environment (a
factor of considerable practical importance).104,105
The conventional method (Figure 1.6) to synthesize TSILs involves displacement of
halide from a functionalized organic compound by a parent imidazole, phosphine etc.
in the quaternization step. This is followed by usual anion exchange step to yield the
desired task specific ionic liquid.12
Chapter 1
21
Figure 1.6: Conventional method for the synthesis of TSILs 12
Michael reaction has also been cited as a complimentary method for synthesis TSIL
(Figure 1.7).106 In this approach, the imidazole or other nucleophile of interest is
protonated using the acid form of the anion which will eventually be incorporated into
the IL, e.g., HPF6 for PF6-. To this salt is added the desired Michael acceptor, which
inserts into the N–H (or element–H) bond. The approach is broadly effective, giving
TSIL in good yields. Moreover, the procedure eliminates the need for an anion
metathesis step and provides an IL free of halide. The latter is an important factor if
the IL is to be used with a transition metal catalyst. The only apparent drawback
however, is the limited thermal stability of the cations, which at moderately elevated
temperatures can undergo a retro-Michael reaction. Various other methods have also
been cited by Davis, including the works from other research groups for the
incorporation of functionality into an IL.12
Figure 1.7: Michael reaction for synthesis of TSILs.106
Chapter 1
22
As far as applications of TSILs are concerned, by virtue of their incorporated
functionalities, these unique salts can act not only as solvents but also as catalysts and
reagents in an array of synthetic, separation and electrochemical applications.
1.5.3.4 Chiral ILs (CILs)
Chiral discrimination can be accomplished by using wide variety of chiral ionic liquids.
Although, CILs are at a premature phase of development, they have already found
promising applications as solvents for chiral separation techniques107, asymmetric
synthesis,108-112 stereoselective polymerization,113-115 chromatography,116,117 liquid
crystals118,119 and as NMR shift reagents.107,120-123
The CILs are supposed to meet the criteria of easy preparation by direct synthesis in
enantiopure form and have low melting points, good chemical stability towards water
and common organic substrates together with relatively low viscosity and good
thermal stability. Preparation and applications of CILs have been compiled by
Chauhan et.al.97
1.6 CHARACTERIZATION OF IONIC LIQUIDS
Characterization of the chemical properties of ILs is desired to gain a better
understanding of their fundamental characteristics. These properties on comparison
with those of conventional solvents allow determination of unique characteristics of
ILs. Viscosity and density allow for an understanding of bulk properties of the system
while several other internal properties can also been investigated.51,54,124,125 The effect
of constituent cation and anion of an IL over properties like density, viscosity, surface
tension, melting temperature and solvent properties have already been discussed.
However, a careful study involving conductivity measurements, thermal analysis,
measurement of decomposition temperature, structural analysis and toxicology studies
need to be done on these systems.
Electrochemical analysis of ILs have shown that the conductivity (and related
impedance) and of diffusion coefficients can be helpful in understanding the transport
properties and solvent-solute interactions within the ILs. Examination of imidazolium
Chapter 1
23
based ILs has shown that they exhibit low electrical resistance values.126 This should
be expected due to the charged nature of the ions and the corresponding high
concentration of such charges within a pure ionic liquid.
Thermochemical analysis of ILs such as phase equilibrium studies 127 have provided
an insight into the stability and solubility of ionic liquids and their interactions with
their surroundings. Since ILs consist of charged species, it would be expected that
lattice formation or similar structuring that may be present within these liquids can
directly affect their characteristics such as melting point. As already discussed in
Section 1.4.1, melting temperature is therefore a critical parameter, as the practical
use of ILs depends on the temperature range in which they remain in a liquid form.
Differential scanning calorimetry (DSC) has allowed for the determination of ionic
liquid melting, crystallization and glass transition (Tg) points and can help in
rationalizing the relationship between ionic liquid structuring and physical
characteristics.42 In general it has been found that increasing ion size (producing
weaker coulombic interactions in the crystal lattice) can significantly change the
melting point of ionic liquids. Moreover, with increasing side chain lengths, the
resulting weak van der Waals interactions can reduce the stronger hydrogen bonds
present in the system.11
Thermogravimetric analysis (TGA), coupled with DSC data, allows for examination
of decomposition temperatures, determination of limits of practical use and the overall
stability of ILs.128,129 Studies have identified that in certain cases perfluorination of
the anions (with stable C-F bonds such as those in NTf2-) can also enhance the
thermal stability of ILs (an important factor for their implementation in batteries). 130
Spectroscopic methods of analysis have been employed to probe the structure of ILs.
Techniques such as infrared (IR) and Nuclear Magnetic Resonance (NMR)
spectroscopy have allowed for diffusional131 and structural analysis132 of ILs, which
further help in understanding solvent-solvent interactions and their effects upon the
transport properties of the liquids. Mass spectrometry has allowed for more detailed
investigation of decomposition fragments. The pattern of such processes has enabled
the determination of physical properties such as enthalpy of vaporization133
Chapter 1
24
(a property that is related to the thermal stability of the liquids). In addition,
immobilization in HPLC stationary phases facilitates the investigation of ion
exchange properties of ILs and more specific interactions such as those with aromatic
compounds.134
Though not many studies have been done, ILs have been found to exhibit
considerable levels of toxicity.135-137 It has been reportedly found that phosphonium
based ILs can be labeled as corrosive in addition to irritating. Recent studies have also
shown that the length of alkyl chain (particularly imidazolium cations) can have a
direct and substantial impact upon the toxicity of ILs.138 However, biologically
compatible inert ions can serve as promising alternative to current ILs (which have
considerable levels of toxicity). Tao et al. have reported the synthesis of amino acid
based ILs with biodegradable characteristics.139 In addition to this, other bio-
compatible molecules such as the sugar based anions, succinate and lactate, can also
be used for the formation of ILs with much lower toxicity.140
1.7 MAJOR APPLICATIONS OF ILs
The outstanding physicochemical properties of ILs, especially room temperature ionic
liquids (RTILs), render them excellent candidates for a broad range of
applications.141-145 At the current level of development, ILs can even replace
conventional organic solvents in numerous different applications.9 ILs have already
been used as catalyst,146,147 reagents148 or solvents149,150 in several chemical reactions.
Furthermore, ILs can be used in separation processes151,152 and as electrolyte materials
in catalytic processes.153,154 Great efforts have been made in utilizing ILs as solvents
for biopolymers. Especially cellulose (the most abundant natural polymer in nature)
can be dissolved in rather high concentrations (up to 25 wt%) in ILs (which is not
possible in conventional organic solvents).78 The most efficient solubility can be
obtained when imidazolium based ionic liquids with chloride or acetate anions are
used, e.g. 1-ethyl-3-methyl imidazolium acetate or 1-n-butyl-3-methyl imidazolium
chloride. These anions are non-hydrated and can disrupt and break the intramolecular
hydrogen bonds of the cellulose network without derivatization.78,155,156 Beside the
usage of ILs as solvents for organic reactions, their applications as electrolytes in
Chapter 1
25
lithium batteries,157,158 in electroplating processes,159 and solar cells160-163 reflects their
applicability in electrochemistry. Remarkable are also the investigations of ILs with
regard to their advantages in formulation technology, in colloid science and in
tribology during the last years. ILs can also be utilized as additives in paints (for
improved finish and drying processes),164 as templates in nanotechnology165-172 or as
innovative lubricants for steel on aluminium applications.173
Interestingly, while on one hand ILs are known to pose a threat to nature, their
inherent cytotoxicity may have the potential for beneficial use in certain cases.
Current antifouling coatings, typically containing organic derivatives of heavy metals
such as tributyltin, have been found to leech from these coatings over time into the
environment. Ionic liquids have been successfully immobilized in polymers and found
to be sufficiently trapped so as to prevent leaching. The toxic nature of ILs can inhibit
growth, as desired, upon the polymer surface and so could potentially be used as
durable thin film coatings for filters or equipment exposed to potential bio-fouling
agents.174 An overview of the diversity of IL applications is given in Figure 1.8.
Figure 1.8: Major applications of ILs43
Chapter 1
26
1.8 ILs IN ORGANIC TRANSFORMATIONS
There has been a continuous and sustained research focusing on the use of ILs in
organic reactions and significant improvement in terms of products yields, reaction
times, reaction work-up have been obtained. However, the role of ILs in organic
transformation is still not very clear. Some authors have suggested that ILs can act as
an organocatalyst.175,176 One of the promising approach to organocatalysis is proposed
via hydrogen-bonding interactions and the results obtained with certain ILs have
confirmed this statement. On the other hand, Welton1,177 has studied catalytic
reactions in ionic liquids and has postulated that a potentially more powerful way in
which an IL can be used in catalysis can be a combination of both solvent and
catalyst. It is based on this postulate that whenever changing a solvent leads to an
accelerated reaction, the new solvent can be argued to behave like a catalyst. This is
simply because the reaction rate can be enhanced with the solvent remaining
unchanged in the process.
1.8.1 ILs as Solvents
In Section 1.4.8, the determination of solvent polarity parameters for ILs using
solvatochromic dyes, chromatographic techniques and the ability of ILs to affect the
keto-enol equilibria had been discussed. However, it turns out that ‘polarity’ or the
‘solvent strength’ alone can be insufficient in explaining the variation in experimental
results in many solvent-mediated processes. A reasonable postulate that has been
proposed by Bonacorso et al. as a general ionic liquid effect is that the accelerated
reaction rates can be a result of the decrease of activation energy of the slow step.20
ILs have been expected as a solvent media for the stabilization of highly polar or
charged intermediates, such as carbocations, carbanions and activated complexes.178
The influence of solvents on rate constants has been explained in terms of transition-
state theory. Solvents can thus help in modifying the Gibbs energy of activation (as
well as the corresponding activation enthalpies, activation entropies, and activation
volumes) by differential solvation of the reactants and the activated complex. The
effect of solvent on reactions has been investigated by Hughes and Ingold. They used
a simple qualitative solvation model considering only pure electrostatic interactions
Chapter 1
27
between ions or dipolar molecules and solvent molecules in the initial and transition
states179 and postulated that a change to a more polar solvent will increase or decrease
the rate of the reaction, depending on whether the activated reaction complex is more
or less dipolar than the initial reactants (Figure 1.9). In this respect, the term “solvent
polarity” has been used synonymously with the power to solvate solute charges.
Solvent polarity is thus assumed to increase with the dipole moment of the solvent
molecules and to decrease with the increased thickness of shielding of the dipole
charges.
Figure 1.9: Schematic Gibbs energy diagram for a general nucleophilic addition to
carbonyl carbon: (a) non-polar solvents; (b) polar solvents.20
Finally it is assumed that ILs as solvents can stop the use of volatile organic compounds
(VOCs) in pharmaceutical and petrochemical industries. The use of VOCs can be
assessed using a factor that measures process by-products as a proportion of production
on the mass basis - ‘Sheldon E- factor’. Researchers have analyzed that E-factor is
between 25 to 100 for pharmaceuticals industries with a production of 10 t/year to 103
t/year although oil refining industries with a production of 106 t/year to 108 t/year have an
E-factor of 0.1.31 These values suggest that pharmaceuticals industries use inefficient and
dirty processes although on smaller scale as compared to the oil refining industries.
Environmentally friendly ILs can presumably replace the hazardous VOCs in a large
scale context so as to reduce the E-factors.
Chapter 1
28
Since, ILs are able to dissolve a variety of solutes, they can be used instead of
traditional solvents in liquid–liquid extractions where hydrophobic molecules such as
simple benzene derivatives will partition to the IL phase. Huddleston et al.180-182
showed that [bmim]PF6 could be used to extract aromatic compounds from water.
Selvan et al.183 have used ILs for the extraction of aromatics from aromatic/ alkane
mixtures, whereas Letcher et al.184 have used ILs for the extraction of alcohols from
alcohol/ alkane mixtures. Moreover, binary temperature–composition curves of ILs
with alcohols, alkanes, aromatics and water; ternary temperature–composition curves
of ILs with alcohols and water; solubilities of some organics and water in ILs have all
been investigated by various groups so as to completely understand the solvent
properties of ILs.185-187
1.8.2 ILs as Catalyst
ILs can play an active role in chemical reactions and catalysis. Some of the examples
where ILs have been utilized are: reactions of aromatic rings, clean polymerization,188
Friedel Crafts alkylation,189 reduction of aromatic rings,190 carbonylation,191
halogenation,192 oxidation,193 nitration194 and sulfonation reactions.195 ILs can be
utilized as:
• Solvents/ co-catalyst/ catalyst activator for transition metal catalysis.
• Immobilization of charged cationic transition metal catalysis in ionic liquid
phase without need for special ligands178
• Immobilization of ionic liquid over a solid support
• In situ catalysis directly in ionic liquid rather than aqueous catalysis followed by
extraction of products from solution (this process eliminates washing steps,
minimizes losses of catalysis and enhances purity of the products).189
1.8.2.1 Ionic liquid as reaction media: Co-catalyst/ Catalyst activator
ILs can act as reaction media in both homogenous and heterogeneous systems. They
offer the advantages of both homogenous and heterogeneous catalysts with their two
main characteristics: A selected ionic liquid may be immiscible with the reactants and
products, but on the other hand the ionic liquid may be able to dissolve the catalysts.
Chapter 1
29
ILs therefore, combine the advantages of a solid for immobilizing the catalyst/ and the
advantages of a liquid for allowing the catalyst to move freely.196 The ionic liquids
have been shown to be superior solvents, with an enhancement of catalyst activity and
stability for transition-metal catalyzed reactions, in comparison to water and common
organic solvents, especially when ionic complexes of transition metals are used as the
catalysts.97 Brennecke and Maginn8 have indicated that the ionic nature of the ionic
liquid can give an opportunity to control reaction chemistry, either by participating in
the reaction or by stabilizing the highly polar or ionic transition states. Holbrey and
Seddon191 have described many of the catalytic processes which use low temperature
ILs as reaction media and have indicated that the classical transition metal catalyzed
hydrogenation, hydroformylation, isomerization, dimerization and coupling reactions
can be performed in IL solvents. In their review, they have concluded that ILs may be
used as effective solvents and catalysts for clean chemical reactions instead of the
volatile organic solvents.
In general it can be said that the reaction rates and selectivities are as good or better in ILs
than in conventional organic solvents. The catalytic hydrogenation of cyclohexene using
rhodium-based homogenous catalysts197 and hydrogenation of olefins using ruthenium
and cobalt-based homogenous catalyst198 in various ILs have been studied and the results
indicate that there is a certain increase in the reaction rates and selectivity compared to the
other normal liquid solvents. Lagrost et al. have shown that the diffusion coefficient of
the organic compounds are about 100 times smaller than those in conventional media as
expected from the lower viscosity of RTILs. The positive results of this study
demonstrated that ILs can be used as a new media for organic electrochemistry.199
ILs can also be employed as biological reaction media due to the stability of enzymes in
these liquids.200-203 Yet another important attribute of ILs in these reactions is their ability
to dissolve a variety of bio-molecules or substrates such as carbohydrates, amino acids,
organic acids such as lactic acids and in certain cases cellulose. According to Swatloski et
al.78 ILs incorporating anions which are strong hydrogen bond acceptors are most
effective solvents for cellulose, whereas ILs containing non coordinating anions
(including PF6− and BF4
−) are not that effective. Pfruender et al.204 tested the water
immiscible ILs-[bmim]PF6, [bmim]Tf2N and [oma]Tf2N (methyl-trioctylammonium
Chapter 1
30
trifluoromethanesulfonylimide) for their bio-compatibility towards Escherichia coli and
Saccharomyces cerevisiae. Results of this study indicated that these water immiscible ILs
did not damage the microbial cells and could be used as biocompatible solvents for
microbial bio-transformations as an alternative to toxic organic solvents.
1.8.2.2 Functional ILs and TSILs as catalyst; Catalysis in-situ.
Recently, ILs have been prepared so that one of the ions could serve as a catalyst for
the reaction.205,206 Functionalized ionic liquids that are able to act as catalysts
(particularly imidazolium salts containing anionic selenium species, SeO3Me-) have
been prepared.207 These salts have been used as selenium catalysts for the oxidative
carbonylation of anilines. Similarly, ILs bearing acid counter-anions (HSO4- and
H2PO4-) have been used as catalyst in recyclable reaction media for esterification
reactions.208 Similar results have also been obtained using zwitter-ionic ILs bearing a
pendant sulfonate group (which can be converted into corresponding Brønsted acid
ILs) by reaction with an equimolar amount of an acid that has a sufficiently low pKa
(TsOH, TfOH).209 ILs, containing the SO3H as a functionality, have recently been
employed in the oligomerization of various alkenes to produce branched alkene
derivatives with high conversions and excellent selectivity.210
Protonated ILs have been synthesized by direct neutralization of alkylimidazoles,
imidazoles, and other amines with acids and their physical properties (thermal stability,
conductance, viscosity) are currently under investigation. Brønsted basic ILs have also
been described as catalysts for organic reactions. As an example, Ranu and Banerjee
have demostrated the use of a tailor-made, task-specific, and stable ionic liquid
[bmim]OH as basic catalyst for Michael addition.211 On the other hand, the asymmetric
synthesis of ILs is still at a preliminary stage. Chiral ILs, for example, have been
synthesized and their use in asymmetric synthesis is under investigation.45,212-214
1.8.2.3 Immobilized ILs for catalysis
Immobilization of ILs is important as it utilizes only small amount of catalyst, which
could be easily recycled. A typical process for support or immobilization of ionic liquid
catalysts has been reported recently,215 in which the ionic liquid fragment (such as
dialkylimidazolium cation) was covalently anchored to the surface of silicon dioxide
Chapter 1
31
where this chemical bonding could limit the degree of freedom of the dialkylimidazolium
cation and even change the physicochemical properties of the ionic liquid. In another
method, immobilization of the ionic liquid has been proposed by dipping the porous
silicon dioxide in the mixture of ionic liquid containing the catalysts216 and in this case the
obvious leaching of the ionic liquid could not be avoided.
Alternative approaches that have been in practice to facilitate the catalyst re-use (and in
the context of continuous flow processes) is supported ionic liquid phase (SILP)
catalysis and this has been quite extensively studied.217,218 The general concept involves
the immobilization of imidazolium (together with other ionic fragments) onto solid
supports using appropriate functional groups attached to the cation and a charged
catalyst is then supposed to reside within the ionic matrix. The concept is illustrated in
Figure 1.10 for the racemic epoxidation of olefins using a peroxotungstenate catalyst,
supported on IL modified silica.219 The solid support was reacted with 1-octyl-3-
(3-triethoxysilylpropyl)-4,5-dihydroimidazolium, affording a SiO2 surface on which the
IL was covalently bound. This heterogeneous catalyst was used to successfully
epoxidise olefins using H2O2 as an oxidant and the reaction rates were comparable to
those observed under homogeneous conditions.
Figure 1.10: A tungstenate catalyst immobilized on IL modified silica.220
ILs have further proven to be excellent solvents to both immobilize and stabilize
nanoparticle catalysts. Nanoparticles were first identified in ILs as species formed
Chapter 1
32
during Heck reactions using Pd(II) compounds as catalyst precursors.221 Dupont et al.
reported the controlled preparation of transition metal nanoparticles in ILs by
reduction of the metal complex with molecular hydrogen in the absence of stabilizers
and demonstrated their application in hydrogenation and C-C coupling reactions.222,223
It is believed that both electrostatic and coordination effects of imidazolium cations
can contribute to nanoparticle stabilization by ILs.224 However, particularly more
forcing reaction conditions may nevertheless require the presence of additional
stabilizers to avoid aggregation of the nanoparticles in the ionic liquid. As an
example, PVP (poly-N-vinyl-2-pyrrolidone) has been used for nanoparticles synthesis
in ILs.225 In addition, thiol-functionalized ionic liquids226,227 and ionic liquid-like
copolymers228 have been developed to stabilize IL soluble nanoparticles. It has also
been demonstrated that nanoparticles stabilized by an IL polymer can be efficiently
transferred between phases via anion exchange229 and this could have important
applications in catalysis with respect to product separation.
1.8.2.4 Role of ILs in organic transformations
Based on the above discussion, it is clear that not all the ILs work in a similar manner
for a given organic transformation.37,230 However, the ILs can be tailor made for a
given reaction with ideal combination of cation and anion. Taking a view over the
most commonly encountered reactions in ILs (i.e. of SN2 type) a plausible explanation
has been proposed on the basis of Hughes-Ingold approach. In case of reactions of
highly associating anions (such as halides) reaction rates are greater in ILs composed
of the least coordinating cations (poor hydrogen bond acids) and the rate of the
reaction is affected by H-bonding ability and ion pairing property of the ionic liquid.
The relative rates of reaction can be therefore, compared. The H-bonding ability and
ion pairing property of ILs have been also used to explain the increase of reactivity
and selectivity in eletrophilic additions. It has been proposed that ILs can affect the
lifetime of reaction intermediates, affecting their stability or modifying the
nucleophilicity of the attacking anion. Furthermore, it can also affect the syn/anti ratio
by decreasing the rate of isomerization of the ionic intermediates through rotation
around the C-C bond.20
In electron transfer reactions, the enhancement of reactivity has been attributed to the
effect of cation-anion association and the presence of cavities in the ILs. It has been
Chapter 1
33
suggested that the highly ordered structure of these salts may contain voids and these
voids may be able to accommodate small solute molecules. Thus, the presence of
voids and the ability of small molecules to move within them have also been proposed
recently to explain the reactivity of hydrogen radical (H•) atoms with aromatic solutes
in ILs.231-233
Variety of catalytic reactions have been studied in ILs145,177 and ILs have shown
significant advantages over conventional solvents (especially in the case of
homogeneously catalyzed reactions).31 In these cases, the ionic liquid can be used in
“biphasic catalysis” or the catalyst can be entrapped or “immobilized” allowing
extraction or distillation of the organic product and the ionic liquid/catalyst system
can be reused. However, in order to achieve sufficient solubility of the metal complex,
a solvent of higher polarity is required and this may compete with the substrate for the
coordination sites at the catalytic center. Consequently, the use of an inert, weakly
coordinating IL in these cases can result in a clear enhancement of catalytic activity as
some ILs are known to combine high solvation power of polar catalyst complexes
(polarity) with the weak coordination (nucleophilicity).145 ILs formed by treatment of
a halide salt with a Lewis acid (such as chloroaluminate or chlorostannate melts)
generally act both as solvent and as co-catalyst in transition metal catalysis. Both the
cation and the anion of an IL can act as a ligand or ligand precursor for a transition
metal complex dissolved in the ionic liquid.31,234
1.9 ILs FOR SYNTHESIS AND STABILIZATION OF METAL NANOPARTICLES
Nanoparticles (NPs) can be considered as assemblies of hundreds to thousands of atoms
and a size in the range of 1nm- 50nm. Metal nanoparticles (M-NPs) are of significant
interest for technological applications in several areas of science and industry, especially
in catalysis due to their high surface activity. The controlled and reproducible synthesis of
defined and stable M-NPs with a small size distribution is very important for a range of
applications.235-241 Kinetically stable small (<5nm) M-NPs agglomerate to
thermodynamically favored larger metal particles. Tendency of M-NPs for aggregation
arises due to the high surface energy and the large surface area. To avoid this coagulation,
M-NPs need to be stabilized with strongly coordinating protective ligand layers that can
Chapter 1
34
provide electrostatic and/or steric protection like polymers and surfactants.242-245 ILs can
be an alternative to such ligand layers (Figure 1.11). ILs may be seen to act as a “novel
nanosynthetic template”246 that can help in stabilizing M-NPs on the basis of their ionic
nature,247 high polarity, high dielectric constant and supramolecular network (without the
need of additional protective ligands) (Figure 1.12)7,248-251.
Figure 1.11: Stabilization of metal nanoparticles (M-NP)through protective ligand
stabilizers or using ILs.252
When mixed with other molecules or M-NPs, ILs become nanostructured materials
with polar and non-polar domains.253-255 This kind of nanometer-scale structuring
in RTILs has been observed by molecular simulation for ILs belonging to the
1-alkyl-3-methylimidazolium family with hexafluorophosphate or with
bis(trifluoromethylsulfonyl)amide as the anions. In case of ILs bearing alkyl side
chains longer than or equal to C4, aggregation of the alkyl chains in non-polar
domains has been observed. These domains generate a three-dimensional network
of charged or polar ionic channels (formed by anions and by the imidazolium
cation rings) (Figure 1.12). As the length of the alkyl chain increases, the non-
polar domains become larger and more connected and cause swelling of the ionic
network.256 In other words, ILs are nanostructurally organized with non-polar regions
arising from clustering of the alkyl chains and ionic networks arising from the ordering of
the charged anions and imidazolium rings of the cations.257 The combination of
Chapter 1
35
undirected Coulomb forces and directed hydrogen bonds leads to a high attraction of the
IL building units. This phenomenon also forms the basis for their (high) viscosity,
negligible vapor pressure and three-dimensional constitution. The IL network properties
are well suited for the synthesis of defined nano-scaled metal colloid structures even in
the absence of stabilizing ligands (Figure 1.12).7,248,249
Figure 1.12: The inclusion of M-NPs in the supramolecular IL network with
electrostatic and steric (= electrosteric) stabilization is indicated through
the formation of the suggested primary anion layer forming around the
M-NPs.252
Chapter 1
36
1.9.1 Synthesis of metal nanoparticles (M-NPs) in ILs
M-NPs can be synthesized in ILs258 through chemical reduction,259-265
decomposition221,266-268 or by means of photochemical reduction269,270 or electro-
reduction271 of metal salts, where the metal atom is in a formally positive oxidation
state. They can also be generated by the decomposition of metal carbonyls with zero-
valent metal atoms246,265,272,273 (without the need of extra stabilizing molecules or
organic solvents).242,243,248,249,274,275 A range of M-NPs have been prepared in ILs from
compounds where the metal is in a formally positive oxidation state Mn+. Such M-NPs
then include, for example, the main-group metals and metalloids Al,276,277 Te278,279
and the transition metals Ru,280 Rh,262 Ir,222 Pt,223 Ag259,281 and Au.226
Alternatively, functionalized ILs have also been used where the IL could work as both
a reducing agent as well as stabilizer for the synthesis of M-NPs. Moreover, in
comparison with non-functionalized imidazolium-ILs, functionalized imidazolium-
ILs can stabilize aqueous dispersion of metal NPs much more efficiently because of
the special functional group. Thiol-functionalized,226,282,283 ether-functionalized,284
carboxylic acid– functionalized,285 amino-functionalized,285,286 and hydroxyl-
functionalized227 imidazolium-ILs have been used to synthesize aqueous dispersion of
noble (primarily gold) metal NPs.
Amongst the methods mentioned above, the reduction of metal salts is the most
utilized method for generating M-NPs in solution and also in ILs in general. Many
different types of reducing agents are used, like gases (molecular hydrogen) organic
(citrate, ascorbic acid, imidazolium cation of IL) and inorganic (NaBH4, SnCl2)
materials.252 However, the applicability of any IL in chemical reduction method
depends upon its stability in the reaction mixture. ILs are quite often known to react
with strong bases, acids, some reducing agents (e.g. NaBH4, LiAlH4, R3Al, etc.) or
may decompose at relatively high reaction temperatures. In this context mild reducing
agents (such as alcohols or molecular hydrogen) may be opted for.252
Chapter 1
37
In the direct route of chemical reduction (Figure 1.13), a metal precursor is dissolved
in an ionic liquid and is reduced with a suitable reducing agent (which produces lesser
by-products that can decompose an IL). Heating if required, is carried out at the
temperatures below the decomposition temperature of the ionic liquid. It has been
postulated that if a metal precursor is completely soluble in an ionic liquid, use of an
additional solvent may no longer be required. In such cases, the reaction rate can be
accelerated by stirring, heating, ultrasonic treatment or addition of a few drops of a
suitable solvent.287 Some examples describing the preparation of NPs in ILs via the
chemical route can be found in the literature.222,223,225,229,242,288-290
Figure 1.13: Preparation of NPs in ILs via direct route252
Using the direct method, not all the requirements for the formation of NPs can be met
in an IL. Hence, an indirect method for the synthesis of NPs has been proposed. The
NPs of interest are prepared using a convenient and suitable combination of stabilizer,
metal precursor and reducing agent in an organic solvent and then transferred into the
required IL(Figure 1.14).291
Figure 1.14: Preparation of NPs in ILs via indirect route252
However, in this indirect method, the addition of the stabilizer can be replaced with
the addition of a suitable IL which can prevent the particle agglomeration and provide
extended stability. As an example, carboxylic acid and amino-functionalized ILs
Chapter 1
38
[C1mim]Cl (1-carboxylmethyl-3-methylimidazolium chloride) and [Aemim]Br
(1-aminoethyl-3-methylimidazolium bromide) were used as the stabilizer for the
synthesis of gold and platinum metal nanoparticles in aqueous solution.285 The
mechanism of stabilization has been proposed due to the interactions between
imidazolium ions/functional groups in ILs and the metal atoms.
1.9.2 Stabilization of M-NPs using ILs: DLVO theory and other effects
The basic and most common theory for interaction of two particles in a dispersion is the
DLVO theory (Derjaguin, Landau, Verwey and Overbeek theory) considered as a
combination of repulsive coulombic and the attractive van der Waals forces. The DLVO
theory considers initially charged colloidal particles whereby the electric charges are
uniformly distributed over their surface. The total energy potential VT (or the DLVO
potential) of the interaction between two particles is then described as the sum of
attractive (van der Waals) forces and repulsive forces (due to a double layer of counter
ions). The height of the overall potential barrier VT determines, whether the particles are
stable (the kinetic energy Ek of particle motion is less than VT i.e. Ek>VT ) or not
(Ek>VT).287
Some assumptions and simplifications involved with DLVO theory are often
introduced. It is assumed that the particle surfaces are flat and the charge density is
homogeneous and remains so, even when particles approach each other. Moreover,
there is no change in the concentration of the counter ions which cause the electric
potential. The solvent itself influences only through its dielectric constant.
It is quite clear that the surface of a particle is not flat and the charge density changes
when two particles approach each other. Thus it is evident that the theory is being
limited to certain assumptions and can thus, only approximate the real-life interactions
of two particles.252
Concerning NPs and their interactions, the anion has been considered to interact with
the unsaturated surface of the electrophilic NPs.292 Thus, the NPs with their anion
layer assume a negative charge and turn into a large multi-negative anion. The
repulsion between two such negatively charged NPs is the Coulomb part of the
DLVO theory. The stability of colloids is a balance between Coulomb forces and
van der Waals attraction. A measure of the stability of a colloid is the thickness of the
Chapter 1
39
Debye layer, which is the sum of the layers of counter ions surrounding the particle.
The thicker the Debye layer, the more stable is the particle because the distance to the
next particle is greater and the van der Waals attraction is reduced. Finke et al. studied
the stability of colloids in different solvents and found that higher the dielectric
constant of the medium, the better is the stabilization of the colloid.261
The DLVO theory has certain limitations. It can only be applied to dilute systems
(<5× 10−2mol/l) and not even for higher concentrations. It cannot be applied to ions
with multiple charges and sterically stabilized systems.293 Nowadays, the DLVO
theory has been supplemented with “extra-DLVO” forces which include effects such
as hydrogen bonding, hydrophobicity, steric interactions and viscous forces.252
1.10 CHALLENGES WITH IONIC LIQUIDS
Unique properties of ILs can be exploited for innumerable applications. However
there are few disadvantages that restrict their use for certain specific applications.
1.10.1 Cost/ Economic perspective
Cost is a major challenge to be encountered in synthesizing ILs on an industrial scale. A
kilogram of ionic liquid used to cost about 30,000-fold greater than a common organic
solvent such as acetone. Renner38 reported that this cost could be reduced to
approximately 1000-fold depending on the composition of ionic liquid and the scale of
production. Wagner and Uerdingen294 anticipated that the price of cation systems based
on imidazole will be in the range of € 50 kg−1–100 kg−1, if larger quantities of ILs are
produced. The price can be lowered even below € 25 kg−1 if ILs are prepared with cheaper
cation sources on a ton scale. Further, another estimation was done by Wassersheid and
Haumann.295 They expected that for ‘bulk ILs” choosing proper (relatively cheap) cations
and anions lead to prices approximately € 30 l−1 for production rates of multi-ton.
Moreover, scientists emphasize that although the price of the ILs may look
discouraging but still, the essential factor is the price to performance ratio. If the
performance of an IL is extremely high as compared to that of the material (solvent) it
aims to replace, less amounts of the IL may be needed for a given specific job296,
thereby totally or partially overcoming the price disadvantage.
Chapter 1
40
1.10.1 Green aspects of ILs; Recyclability and Disposal
A problem is faced while manufacturing ILs and that needs to be tackled. This is the
use of VOCs in the manufacture of ILs. Recently, some advancement has been
achieved in the solventless syntheses of ILs. For example, 1-alkyl-3-
methylimidazolium halides have been synthesized in open containers in a microwave
oven without any VOCs by Varma and Namboodiri at the Environmental Protection
Agency of U.S.,2001.38
Recycling of ILs is another important issue that concerns the researchers working with
them. Many processes for cleaning up ILs involve washing with water or VOCs which
creates another waste stream. This problem has been solved by adopting supercritical
extraction technologies to recover the dissolved organic compounds from ILs or using
membrane separation processes. A green solvent, which has been discovered and solves
all the problems and recovers various kinds of solutes from ILs without cross
contamination, are supercritical fluids (SCFs).The advantages of using SCFs as extraction
medium include low cost, nontoxic nature, recoverability and ease of separation from the
products. SCFs have been adapted for product recovery from ILs and supercritical fluid
extraction (SCFE) is shown to be a viable technique with the additional benefits of
environmental sustainability and pure product recovery.297 Among the SCFs, an
inexpensive and readily available one, scCO2 has become a partner of IL and two
environmentally benign solvents have been utilized together in several applications. The
volatile and non-polar scCO2 forms different two-phase systems with non-volatile and
polar ILs. The product recovery process with these systems is based on the principle that
scCO2is soluble in ILs, but ILs are not soluble in scCO2.297 Since most of the organic
compounds are soluble in scCO2, with the high solubility of scCO2 in ILs, these products
are transferred from the ionic liquid to the supercritical phase.
Apart from general green credentials of ILs (low vapor pressure, low flammability
and causing lesser air pollution problems) eco-toxicological risk profiles must also be
addressed. According to Jastorff et al.,298 the toxicity of ILs is roughly driven by the
head group, the side chain, and the anion. Currently, the biological effects of ILs have
resulted in increasing reports, which have dealt mainly with the influence of the alkyl
side chain length of various head groups of ILs. Pernak et al.299-303 pointed out that
their antimicrobial activity increased within increasing alkyl chain length on
Chapter 1
41
pyridinium, imidazolium, and quaternary ammonium salts. Some studies have
reported that varying the anion had minimal effects on the toxicities of several
pyridinium and imidazolium compounds and indicated that the toxicity of ILs was
largely driven by the alkyl chain branching and hydrophobicity of the cation.301,304-306
However, in particular, some ILs with fluorine-containing anions were suggested to
be relatively toxic because the anions were hydrolyzed to fluoride in the aqueous
solution and the fluoride had a toxic effect.307,308
Thus, designing of ILs and their applications in the field of research at every level
should deal with toxicity and biodegradability issues wherever possible.
Chapter 1
42
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