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Content uploaded by Mohamad Dinari
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1
A. Mohammad and Inamuddin (eds.), Green Solvents II: Properties
and Applications of Ionic Liquids, DOI 10.1007/978-94-007-2891-2_1,
© Springer Science+Business Media Dordrecht 2012
Abstract Volatile organic solvents (VOS) creating increasing air pollution are
common reaction media for many chemical processes. VOS cannot be easily sepa-
rated from the desired reaction products and diffi cult to recycle. In view of aware-
ness of the deteriorating environment, researches are directed on developing
alternative environmental friendly solvent systems to replace traditional volatile
organics. Within this context, the interest of ionic liquids (ILs) as “green” solvents
resides in their extremely low vapor pressure and high thermal stability, which offers
advantages such as ease of containment, product recovery, and recycling ability. In
addition, ILs show considerable variation in their stability to moisture and their
miscibility with molecular liquids. Properties such as density, melting point, water
and cosolvent miscibility, viscosity, polarity, acid/base character, and coordinating
ability can be tailored by the appropriate selection of the cation and/or anion com-
ponent. ILs have been implemented as solvent systems in chemical reactions, sepa-
rations, extractions, electroanalytical applications and chemical sensing, among
many others. Also, they have high ionic character that enhances the reaction rates to
a great extent in many reactions. These features allow ILs to be used as potential
alternative solvents to VOS in a wide variety of industrial chemical processes.
Furthermore, the use of ILs as industrial solvents can result in economical, social,
and ecological impact due to their effect on the human health and environment.
S. Mallakpour (*)
Organic Polymer Chemistry Research Laboratory, Department of Chemistry ,
Isfahan University of Technology , Isfahan , Iran
Nanotechnology and Advanced Materials Institute , Isfahan University of Technology ,
Isfahan , Iran
e-mail: mallak@cc.iut.ac.ir; mallak777@yahoo.com; mallakpour84@alumni.ufl .edu
M. Dinari
Organic Polymer Chemistry Research Laboratory, Department of Chemistry ,
Isfahan University of Technology , Isfahan , Iran
Chapter 1
Ionic Liquids as Green Solvents:
Progress and Prospects
Shadpour Mallakpour and Mohammad Dinari
2S. Mallakpour and M. Dinari
1.1 Introduction
Solvents are high on the list of damaging chemicals for two simple reasons:
(a) they are used in large quantity and (b) they are usually volatile liquids. Volatile
organic solvents, which have caused concerns on increasing air pollution and
worker’s health, are common reaction media for commercial production of differ-
ent chemicals. It is an enormous challenge to reduce the amount of volatile organic
compounds (VOCs) used in chemical and industrial processes. Governmental
policies for the control of emissions of different substances which are released
into the environment will become more restrictive as pollution increases world-
wide. Therefore, the development of more effi cient and environmentally friendly
processes will be obligatory in the coming years [
1– 6 ] . These developments must
be designed on the basis of two main characteristics: energy saving to avoid exces-
sive emission of carbon dioxide (CO
2 ) and reduction of emissions related to harm-
ful VOCs. Research on chemical manufacturing has focused on the investigation
of different approaches for diminishing the emission of VOCs including solvent-
free processes and the use of water, supercritical CO
2 , and, more recently, ionic
liquids (ILs) as the reaction media [ 7– 10 ] . Among solvents, ILs have been rather
sanguinely viewed as environmentally friendly or “green” solvents. Because of
multiplicity of their uses, attention in ILs suddenly increased. ILs are organic salts
that are liquid at ambient temperatures, preferably at room temperature. The rea-
son why ILs are liquid at room temperature is still not fully understood. From
recent X-ray crystal structure studies, we know that some of these tend to crystal-
lize into disordered solids, and, depending upon the rate of cooling, crystal poly-
morphism can be observed. On the basis of these observations, it has been
speculated that the gain in energy upon formation of the crystal is not as large as
in traditional inorganic salts and is not enough to compensate for the loss in
entropy that accompanies the formation of the crystal at room temperature.
Experiments show that several of these systems have a tendency toward glassy
behavior, and, depending upon the length of alkyl substituents in the cations, their
properties range from those of normal liquids to glassy or even liquid crystals
[ 11 ] . There are many synonyms including ionic fl uid, molten salt, liquid organic
salt, fused salt, or neoteric solvent [ 12– 15 ] that used for ILs, which can compli-
cate a literature search. “Molten salts” is the most common and most broadly
applied term for ionic compounds in the liquid state. It appears that the difference
between ILs and molten salts is just a matter of degree; however, the practical dif-
ferences are suffi cient to justify a separately identifi ed niche for the salts that are
liquid around room temperature.
Some useful properties of ILs are as following: they are relatively nonvolatile,
which means they do not produce atmospheric VOCs and can be used in low-
pressure environments. They possess good thermal stability and do not decom-
pose over a large temperature range, thereby making it feasible to carry out
reactions requiring high temperature conveniently in ILs. They can be consid-
ered both a polar and a noncoordinating solvents and show a high degree of
3
1 Ionic Liquids as Green Solvents: Progress and Prospects
potential for enantioselective reactions as a signifi cant impact on the reactivities
and selectivities. Chiral ILs have been used to control the stereoselectivity. ILs are
complex and versatile solvents capable to interact via hydrogen bonding, p – p ,
n– p , dispersive, dipolar, electrostatic, and hydrophobic interactions and serve as a
good medium to solubilize gases such as H
2 , CO, O
2 , and CO
2 . Many reactions are
now being performed using ILs and supercritical CO
2 . They can be immiscible
with nonpolar organic solvents and/or water. ILs have high ionic character that
enhances the reaction rates to a great extent in many reactions including micro-
wave-assisted organic synthesis as well as polymerization reactions. The solubil-
ity of ILs depends upon the nature of the cations and counter anions. They have
physicochemical properties that can be controlled by judicious selection of the
cation and/or anion. Many of them can be stored without decomposition for a long
period of time [ 16– 27 ] .
There are several important review articles on the synthesis, properties, and
applications of room temperature ILs (RTILs), including solvents for synthesis
and catalysis [ 28 ] , ILs – new solutions for transition metal catalysis [ 29 ] , a short
history of ILs [ 30 ] , IL (molten salt) phase organometallic catalysis [ 12 ] , RTILs as
replacements for conventional solvents [ 31 ] , biocatalysis in ILs – advantages
beyond green technology [ 32 ] , ILs and chirality: opportunities and challenges
[ 33 ] , biocatalytic transformations in ILs [ 34 ] , application of ILs as solvents for
polymerization processes [ 35 ] , chromatographic and spectroscopic methods for
the determination of solvent properties of RTILs [ 36 ] , development of ILs as
green reaction media and catalysts [ 37 ] , ILs for the convenient synthesis of func-
tional nanoparticles and other inorganic nanostructures [ 38 ] , ILs in catalysis [ 39 ] ,
non-haloaluminate RTILs in electrochemistry [ 40 ] , task-specifi c ILs (TSILs) [ 41 ] ,
application of ILs in analytical chemistry [ 42 ] , biodegradable ILs [ 43 ] , chiral ILs:
synthesis and applications [ 20 ] , effect of ions and other compatible solutes on
enzyme activity and its implication for biocatalysis using ILs [ 44 ] , IL crystals
[ 25 ] , ILs: green solvents for nonaqueous biocatalysis [ 45 ] , ILs in the synthesis
and modifi cation of polymers [ 46 ] , chemical and biochemical transformations in
ILs [ 21 ] , ILs in chemical analysis [ 47 ] , ILs solvent properties and organic reactiv-
ity [ 19 ] , metal-containing ILs and ILs crystals based on imidazolium moiety [ 48 ] ,
Pd-benzothiazol-2-ylidene complex in ILs [ 49 ] , practical considerations associ-
ated with voltammetric studies in RTILs [ 50 ] , sonochemistry and sonolumines-
cence in ILs, molten salts, and concentrated electrolyte solutions [ 51 ] , use of ILs
as “green” solvents for extractions [ 52 ] , are ILs kosmotropic or chaotropic [ 53 ] ,
application of chromatographic and electrophoretic methods for the analysis of
imidazolium and pyridinium cations as used in ILs [
54 ] , analytical applications of
room-temperature ILs [ 55 ] , catalysis in ILs [ 56 ] , dissolution of cellulose with ILs
and its application [ 57 ] , electrophilic reactions of aromatic and heteroaromatic
compounds in ILs [ 58 ] , energetic nitrogen-rich salts and ILs [ 59 ] , enzyme-cata-
lyzed reactions in ILs [ 60 ] , functionalized imidazolium salts for task-specifi c ILs
and their applications [ 61 ] , ILs: an environmentally friendly media for nucleo-
philic substitution reactions [ 62 ] , ILs as solvents for catalyzed oxidations of
organic compounds [ 63 ] , physical properties of ILs: database and evaluation [ 64 ] ,
4S. Mallakpour and M. Dinari
review of ILs with fl uorine-containing anions [ 65 ] , supported IL phase catalysis
[ 66 ] , a review of ILs toward supercritical fl uid applications [ 67 ] , applications of
ILs in carbohydrate chemistry [ 68 ] , asymmetric synthesis in ILs [ 69 ] , biocatalysis
in nonconventional media [
70 ] , Brønsted acids in ILs [ 71 ] , catalysis in ILs [ 24 ] ,
design of sustainable chemical products – the example of ILs [ 72 ] , homogeneous
catalysis in ILs [
73 ] , enantioselective catalysis in ILs [ 74 ] , ionic green solvents
from renewable resources [ 6 ] , IL thermo: a free-access web database for thermo-
dynamic properties of ILs [ 75 ] , ILs in separations technique [ 76 ] , lanthanides and
actinides in ILs [ 77 ] , magnetic resonance spectroscopy in ILs [ 78 ] , novel process
options for the application of zeolites in supercritical fl uids and ILs [ 79 ] , reactiv-
ity of ILs [ 80 ] , self-assembled structures and chemical reactions in RTILs [ 81 ] ,
surface chemistry of RTILs [ 82 ] , transition metal-catalyzed reactions in noncon-
ventional media [ 83 ] , the path ahead for ILs [ 84 ] , the phosphorus aspects of green
chemistry [ 85 ] , toxicity of ILs [ 86 ] , utility of ILs in analytical separations [ 87 ] , a
review of ILs in chromatographic and electromigration techniques [ 88 ] , advances
in chiral ILs derived from natural amino acids [ 89 ] , applications of chiral ILs
[ 25 ] , applications of ILs in the chemical industry [ 90 ] , applications of ILs in elec-
trochemical sensors [ 91 ] , benzene alkylation with long-chain olefi ns catalyzed by
ILs [ 92 ] , biotransformations and organocatalysis with ILs [ 93 ] , catalysts with
ionic tag and their use in ILs [ 94 ] , chemistry in heterocyclic ammonium fl uorohy-
drogenate room-temperature IL [ 95 ] , dissolution and functional modifi cation of
cellulose in ILs [ 96 ] , electrochemical reactions in ILs [ 97 ] , hydroformylation in
RTILs: catalyst and process developments [ 98 ] , ILs in heterocyclic synthesis [ 27 ] ,
ILs and CE combination [ 99 ] , ILs as amphiphile self-assembly media [ 26 ] , mac-
romolecules in ILs: progress, challenges, and opportunities [ 100 ] , mutual solubil-
ity of hydrophobic ILs and water in liquid–liquid two-phase systems for analytical
chemistry [ 101 ] , predictive molecular thermodynamic models for liquid solvents
[ 102 ] , olefi n metathesis in ILs [ 103 ] , protic ILs: properties and applications [ 104 ] ,
palladium-catalyzed reactions in ILs [ 105 ] , recent advances in the synthesis and
application of chiral ILs [ 106 ] , recent developments on chiral ILs: design, synthe-
sis, and applications [ 107 ] , self-organization of imidazolium ILs in aqueous solu-
tion [ 108 ] , understanding ILs at the molecular level [ 109 ] , advanced applications
of ILs in polymer science [ 110 ] , application of ILs in high-performance reversed-
phase chromatography [ 10 ] , designing imidazole-based ILs and IL monomers for
emerging technologies [ 111 ] , electrochemical behavior of aluminum in 1-butyl-3-
methylimidazolium tetrafl uoroborate ([BMIm][BF
4 ]) IL electrolytes for capacitor
applications [ 112 ] , halogenation of organic compounds in ILs [ 113 ] , ILs as
advanced lubricant fl uids [ 114 ] , IL lubricants: designed chemistry for engineering
applications [ 115 ] , ILs as electrolytes for Li-ion batteries [ 116 ] , ILs as solvents
for polymerization processes [ 117 ] , ILs in tribology [ 118 ] , ILs used in and ana-
lyzed by capillary and microchip electrophoresis [ 119 ] , ILs and their derivatives
in polymer science and engineering [ 120 ] , promotion of atom transfer radical
polymerization and ring-opening metathesis polymerization in ILs [ 121 ] , the
design of polymeric ILs for the preparation of functional materials [ 122 ] , assessing
the greenness of some typical laboratory IL preparations [ 123 ] , biodegradation
5
1 Ionic Liquids as Green Solvents: Progress and Prospects
studies of ILs [ 124 ] , extraction of organic compounds with RTILs [ 125 ] , ILs in
analytical chemistry [ 126 ] , ILs and catalysis [ 127 ] , ILs for CO 2 capture [ 128 ] ,
methods for stabilizing and activating enzymes in ILs [ 129 ] , microwave-assisted
synthesis using ILs [
130 ] , recent advances of enzymatic reactions in ILs [ 131 ] ,
recent applications of ILs in separation technology [ 132 ] , task-specifi c ILs [ 133 ] ,
the Heck reaction in ILs [
134 ] , the roles of ILs in sorptive microextraction tech-
niques [ 135 ] and toward advanced ILs, and polar, enzyme-friendly solvents for
biocatalysis [ 136 ] .
1.2 History of Ionic Liquids (ILs)
Although Osteryoung, Wilkes, Hussey, and Seddon are pioneers in the fi eld of ILs,
the fi rst report on room-temperature molten salt was published by Welton in 1914
[ 28 ] . He reported the physical properties of ethylammonium nitrate ([C
2 H 5 NH 3 ]
NO
3 ), which is formed by the reaction of ethylamine with concentrated nitric
acid and has a melting point of 12°C. Later on, by mixing and warming 1-ethylpyri-
dinium chloride with aluminum chloride (AlCl
3 ), Hurley and Weir prepared another
molten salt [ 137 ] . With quaternization of the heterocycle and forming mixtures with
AlCl
3 , Osteryoung revolutionized this fi eld and reported new salts in 1978.
Consequently, the effect of changing the cation was investigated by Hussey and
Wilkes, and the dialkylimidazolium-based ILs were discovered [ 111 ] . The ILs based
on AlCl
3 can be considered as the fi rst generation of ILs, but the advancement in
their use was restricted due to hygroscopic nature. Thus, they were not found suit-
able for open-air applications. However, the second generation of ILs as named air
and water stable ILs were synthesized and attracted auxiliary attention in the use of
ILs in different fi elds. Wilkes and Zaworotko [ 138 ] reported the synthesis of water-
and air-stable dialkylimidazolium ILs. They revealed that upon anion exchange
with more hydrolytically stable anions such as BF
4 − , PF 6 − , NO 3 − , SO 4 −2 or acetate, the
resulting ILs could be prepared and safely stored outside of an inert atmosphere as
they are water insensitive. As regards the synthesis and applications of air-stable
ILs, [BMIm][BF
4 ] and 1- n -butyl-3-methylimidazolium hexafl uorophosphate
([BMIm][PF
6 ]) were the fi rst. After that, the magnitude of air- and water-stable ILs
has started to increase quickly [ 139 ] . Then ILs based on more hydrophobic anions
such as tri-fl uoromethanesulfonate (CF
3 SO 3 − ), bis-(trifl uoromethanesulfonyl)imide
[Tf
2 N] − , and tris-(trifl uoromethanesulfonyl)methide [(CF
3 SO 2 ) 3 C − ] were reported
[ 140 ] . This development led to the birth of the modern day ILs. Rogers focuses on
the synthesis and characterization of environmentally friendly ILs as green sol-
vents and found that cellulose could be dissolved in 1-butyl-3-methyl imidazole
chloride ([BMIm]Cl), which opened a new way for the development of a class of
cellulose solvent systems. Many papers dealing with the applications of ILs as
solvents for synthesis and catalysis have been published by Welton. He focused
his study on interaction of ILs with solute species and tried to replace environ-
mentally damaging solvents with more benign alternatives [ 28 ] . Preparation and
6S. Mallakpour and M. Dinari
characterization of ILs for use in the biphasic catalysis were reported by
Wasserscheid and Welton [ 141 ] . Jessop et al. indicated that functional groups can
be incorporated in IL and make them to behave not only as a reaction medium but
also as a reagent or catalyst in some reactions or processes. This group of ILs was
named as “task-specifi c ILs.” Addition of pressurized carbon dioxide into an
organic mixture transforms it into an IL, generating a safer solvent in situ.
Releasing of the pressure reverses the phenomenon, and the IL is retransformed
into the original mixture, thus removing completely the solvent and eliminating
tedious purifi cation and extraction steps [
142 ] .
1.3 Structure of Ionic Liquids (ILs)
Similar to all salts, ILs are made up of separate cationic and anionic species, but
unlike common salts, ILs have a low tendency to crystallize due to their bulky and
asymmetrical cation structure. The nearly infi nite combinations of suitable cat-
ions and anions lead to the possibility of tailoring the IL properties, with the anion
responsible for such qualities as air and water stability and the cation responsible
for melting temperature and organic solubility. They are also known as “designer
solvents” since they give the opportunity to tune their specifi c properties for a
particular need. The researchers can design a specifi c IL by choosing negatively
charged small anions like [Tf
2 N] − , PF 6 − , or PF 4 − and positively charged large cat-
ions of alkylimidazolium, alkylpyridinium, alkylpyrrolidinium, alkylphospho-
nium, or alkylmorpholinium. These specifi c ILs may be utilized to dissolve a
particular chemical or to extract a certain material from a solution. The fi ne-tun-
ing of the structure provides tailor-designed properties to satisfy the specifi c
application requirements. Although these particular cations and anions and their
various combinations have already been studied extensively for their potential
applications in numerous chemical and physical processes, every year more and
more cations and anions forming liquid salts at room temperature are reported
[ 54, 143– 145 ] . Plechkova and Seddon estimated that there may be in excess of 10
6
possible ILs if all currently known IL cations and anions were to be paired and as
many as 10
18 if all ternary systems were to be investigated [ 90 ] . Their structures
play a key role in several interesting and useful ways because their unique proper-
ties that departs from those of conventional solvents. Their coulombic nature
imposes a degree of order on the short-range scale, and their amphiphilic combi-
nation of polar and nonpolar components leads to different types of correlations
on longer scales [ 146 ] .
The structure of cation–cation and anion–anion peaks and valleys in the pair
distribution is diametrically out of phase with the cation–anion pair distribution
[ 147 ] . The overall properties of ILs result from the composite properties of the cat-
ions and anions and include those that are superacidic, basic, hydrophilic, water
miscible, water immiscible, and hydrophobic. The structures of most normally used
cations and some possible anion are represented in Fig. 1.1 .
7
1 Ionic Liquids as Green Solvents: Progress and Prospects
1.3.1 Cations
The cation of IL is usually a bulk organic structure with low symmetry which gives
salts having low melting points even though dibutyl, dioctyl, dinonyl, and didecylimi-
dazolium hexafl uorophosphates are liquid at room temperature [ 148 ] . The majority of
ILs are based on imidazolium, pyridinium, ammonium, phosphonium, sulfonium, thi-
azolium, pyrazolium, and oxazolium cations. The research mainly focuses on RTILs
composed of asymmetric N , N -dialkylimidazolium cations associated with a variety of
anions. The melting points of the most ILs are indecisive. For that reason, by examin-
ing the properties of a series of imidazolium-cation-based ILs, it has been concluded
that as the size and asymmetry of the cation increases, the melting point decreases.
Further, an increase in the branching on the alkyl chain increases the melting point.
ILs with specifi c functional groups on the cation have been prepared by different
groups. For example, ILs bearing a fl uorous tail have been synthesized to facilitate the
emulsifi cation of perfl uorocarbons in ILs. These ILs perform as surfactants and appear
to self-aggregate within imidazolium ILs [ 149 ] . A free amine group or a urea or thio-
urea has been inserted to capture H
2 S or CO
2 or heavy metals, respectively [ 150 ] .
Moreover, ether and alcohol functional groups have been attached to imidazolium cat-
ions to promote the solubility of inorganic salts. The presence of these extra potential
complexing groups makes these ILs suitable for specifi c applications [ 11, 151, 152 ] .
Fig. 1.1 Most commonly used cation structures and possible anion types
N
N
N
NN
RR
R R
P
R R
R R
R
R
R
RR
N-Alkyl-
pyridinium
1,3-Dialkyl-
imidazolium
Tetraalkyl-
ammonium
Tetraalkyl-
phosphonium
Most Commonly Used Cations:
Most Commonly Used Anions:
Water
Immiscible
Water
Miscible
PF
6
N(SO
2
CF
3
)
2
BR
4
BF
3
CF
3
SO
2
CH
3
CO
2
CH
3
SO
2
NO
3
, Cl
N,N-Dialkyl-
pyrrolidinium
8S. Mallakpour and M. Dinari
1.3.2 Anions
Since the nature of anion has a great effect on the properties of IL, there are major
differences between ILs with different anions. The introduction of different anions
results in an increasing number of alternative ILs with various properties [
153 ] .
The physical and chemical properties of the ILs can be determined by different
ion pairs. IL with 1- n -butyl-3-methylimidazolium cation and PF
6 − anion is immis-
cible with water, whereas IL with same cation and BF
4 − anion is water soluble.
This example represents the “designer solvent” property of ILs. By changing the
anion, the hydrophobicity, viscosity, density, and solvation of the IL system may
be changed [ 67 ] .
On the basis of the anion, ILs may be divided into four groups: (a) systems based
on AlCl
3 and organic salts such as [BMIm]Cl; (b) systems based on anions like PF
6 − ,
BF 4 − , and SbF 6 − ; (c) systems based on anions such as [CF
3 SO 3 ] − , [(CF 3 SO 2 ) 2 N] − ,
[Tf
2 N] − , and similar; and (d) systems based on anions such as alkylsulfates and
alkylsulfonates [ 19, 154, 155 ] . The fi rst group represents the ILs of “fi rst genera-
tion,” whose Lewis acidity can be varied by the relative amounts of organic salt/
AlCl
3 . With a molar excess of AlCl
3 , these ILs are Lewis acidic; with an excess of
organic salt, they are Lewis basic; and Lewis neutral liquids contain equimolar
amounts of organic salt and AlCl
3 . These ILs are, however, extremely hygroscopic,
and handling is possible only under a dry atmosphere. The systems mentioned in (b)
are nearly neutral and air stable, although they have the drawback of reacting exo-
thermically with strong Lewis acids, such as AlCl
3 , and with water [ 141 ] . ILs based
on anions mentioned in (c) are much more stable toward such reactions and are
generally characterized by low melting points, low viscosities, and high conductivi-
ties. Structural studies of organic [Tf
2 N] − salts have shown only weak coulombic
interactions between [Tf
2 N] − and weak Lewis acids, attributable to delocalization of
the negative charge within the S–N–S core [ 156, 157 ] .
Probably, the metal enhances the contribution of the resonance structure bear-
ing the negative charge on the nitrogen atom. Lately, the synthesis of numerous
ILs based on the bis(methanesulfonyl)amide ([Ms
2 N] − ) anion has provided new
insights into the effect of anion fl uorination on the properties of ILs. The substi-
tution of [Tf
2 N] − anion with [Ms
2 N] − induces a signifi cant increase in hydrogen
bonding, which determines a signifi cant rise in the glass transition temperature
with concurrent increase in viscosity, which in turn produces a drop in conduc-
tivity [ 158 ] .
ILs bearing perfl uorinated anions are expensive (in particular, those having
[Tf
2 N] − as counter anion), and the presence of fl uorine makes the disposal of
spent ILs more complicated. For these reasons, research on new ILs bearing inert
low-coordinating and nonfl uorinated anions represents a fi eld of intense investi-
gation in the chemistry of ILs. Among the possible alternatives recently proposed
are the ILs having as carboranes and orthoborates counter anions [ 4 ] . ILs based on
anions mentioned in (d) may conquer at least some of the above-mentioned
problems. These anions are relatively cheap, do not contain fl uorine atoms, and
9
1 Ionic Liquids as Green Solvents: Progress and Prospects
often the corresponding ILs can be easily prepared under ambient conditions
by reaction of organic bases with dialkyl sulfates or alkyl sulfonate esters.
Moreover, these new ILs are characterized by a wide electrochemical window
and air stability [
153 ] .
1.4 Synthesis of Ionic Liquids (ILs)
The growing attention in ILs, especially in the light of their current common com-
mercial applicability, has resulted in further progresses in their synthesis and purifi -
cation. Above all, this has required a shift toward improving the standard of synthetic
procedures to ensure consistency in the quality of the materials. Furthermore, in
order to improve the chances of large-scale commercial applications, the effi ciency
of synthetic procedures, IL toxicity, and biodegradation have all become important
topics [ 141 ] . Notably, the inherent synthetic fl exibility afforded by pairing different
cations with any of a growing number of anions provides the possibility for “fi ne-
tuning” certain IL solvent properties to the particular task at hand.
There are three basic methods to synthesize ILs: metathesis reactions, acid–
base neutralization, and direct combination. Many alkylammonium halides are
commercially available which can be organized simply by the metathesis reaction
of the appropriate halogenoalkane and amine. Pyridinium and imidazolium halides
are also synthesized by metathesis reaction. On the other hand, monoalkylammo-
nium nitrate salts are best prepared by the neutralization of aqueous solutions of
the amine with nitric acid. After neutralization reactions, ILs are processed under
vacuum to remove the excess water [ 28 ] . Tetraalkylammonium sulfonates are also
prepared by mixing sufonic acid and tetraalkylammonium hydroxide [ 159 ] . In
order to obtain pure IL, products are dissolved in an organic solvent such as ace-
tonitrile and treated with activated carbon, and the organic solvent is removed
under vacuum. The fi nal method for the synthesis of ILs is the direct combination
of halide salt with a metal halide. Halogenoaluminate and chlorocuprate ILs are
prepared by this method. The synthesis methods of ILs have been given in numerous
articles [ 89, 138, 160, 161 ] .
The protonation of suitable starting materials (generally amines and phosphines)
still represents the simplest method for the formation of such materials, but unfortu-
nately, it is restricted to the small range of useful salts. The possibility of decomposi-
tion via deprotonation has adversely affected the use of such salts. Probably, the most
widely used salt of this type is pyridinium hydrochloride as evident from literature
reviewed by Pagni [
162 ] .
The majority of ILs are formed from cations that have not been obtained by
protonation of a nucleophile. A summary of the applications and properties of ILs
may be found in a number of review articles [ 12, 19, 41, 163 ] . The synthesis of ILs
can generally be divided into two steps: the formation of the desired cation, and
anion exchange to form the desired product. In some cases, only the fi rst step is
required, as with the formation of [C
2 H 5 NH 3 ]NO 3 . In many cases, the desired cation
10 S. Mallakpour and M. Dinari
is commercially available at reasonable cost, most commonly as a halide salt, thus
requiring only the anion exchange reaction. The steps involved in the synthesis of ILs
are described below.
1.4.1 Quaternization Reactions
The formation of the cations may be carried out either via protonation with a free
acid or by quaternization of an amine, phosphine, or sulfi de, most commonly using
a haloalkane or dialkylsulfates. The protonation reaction, as used in the formation
of salts such as [C
2 H 5 NH 3 ]NO 3 , involves the addition of nitric acid to a cooled aque-
ous solution of ethylamine [ 164 ] . The excess amine is removed along with the water
by heating to 60°C in vacuum. The same general process may be employed for the
preparation of all salts of this type, but when amines of higher molecular weight are
employed, there is clearly a risk of contamination by residual amine. A similar
method has been reported for the formation of low-melting, liquid crystalline, long-
alkyl-chain-substituted 1-alkylimidazolium chloride, nitrate, and tetrafl uoroborate
salts [ 165 ] . At this point, a slight excess of acid could be employed as the products
are generally crystalline at room temperature.
The quaternization of amines and phosphines with haloalkanes has been known
for many years. In general, the reaction may be carried out using chloroalkanes,
bromoalkanes, and iodoalkanes, with the milder reaction conditions in the order
Cl → Br → I, as is expected for nucleophilic substitution reactions. Fluoride salts
cannot be formed in this manner.
In theory, the quaternization reactions are extremely simple: the amine (or phos-
phine) is mixed with the desired alkylating agent, and the mixture is then stirred and
heated. The following section refers to the quaternization of 1-alkylimidazoles, as these
are the most common starting materials. The common methods are similar, but for other
amines such as pyridine, isoquinoline, 1,8-diazabicyclo[5.4.0]-7-undecene, 1-meth-
ylpyrrolidine, and trialkylamines, as well as for phosphines, it is not popular [ 140, 166–
168 ] . The reaction temperature and time varied according to the nature of the alkylating
agent employed, chloroalkanes being the least reactive and iodoalkanes the most. The
reactivity of the haloalkanes generally decreases with increasing alkyl chain length.
Several different protocols have been reported, but most researchers use a simple round-
bottomed fl ask/refl ux condenser experimental setup for the quaternization reaction. If
possible, the reaction should be carried out under dinitrogen or some other inert gas in
order to exclude water and oxygen during the quaternization reactions [ 141 ] .
1.4.2 Anion-Exchange Reactions
The anion-exchange reactions of ILs can be alienated into two distinct categories:
direct reaction of halide salts with Lewis acids and the formation of ILs via anion
metathesis.
11
1 Ionic Liquids as Green Solvents: Progress and Prospects
1.4.2.1 Lewis-Acid-Based Ionic Liquids (ILs)
The formation of ILs by the reaction of halide salts with Lewis acids (most notably
AlCl
3 ) dominated in the infancy of this area of chemistry. The great breakthrough
came in 1951 with the report by Hurley and Weir on the formation of a salt that was
liquid at room temperature based on the combination of 1-butylpyridinium with
AlCl
3 in the relative molar proportions 1:2 [ 169 ] . The technology of room-temper-
ature chloroaluminate melts based on 1-alkylpyridinium and 1-alkyl-3-methyl-
imidazolium [RMIm]
+ cations has been developed in recent years [ 170 ] . In general,
the reaction of a quaternary halide salt Q
+ X − with a Lewis acid MX
n
results in the
formation of more than one anion species, depending on the relative proportions of
Q
+ X − and MX
n
. The chloroaluminates are not the only ILs prepared in this manner.
Other Lewis acids employed include AlEtCl
2 , BCl 3 , CuCl, SnCl
2 , and FeCl
3 . In
most cases, the preparative methods employed for all of these salts are similar to
those indicated for AlCl
3 -based ILs [ 141 ] .
The most common method for the formation of such liquids is simple mixing of the
Lewis acid and the halide salt, with the IL forming on contact of the two materials.
The reaction is generally quite exothermic, which means that care should be taken
when adding one reagent to the other. Although the salts are relatively thermally sta-
ble, the buildup of excess local heat can result in decomposition and discoloration of
the IL. This may be prohibited either by cooling the mixing vessel or by adding one
component to the other in small portions to allow the heat to dissipate. Considering the
water-sensitive nature of most of the starting materials, the reaction be carried out in
a drybox and the ILs should preferably be stored in a drybox until use [ 141 ] .
1.4.2.2 Anion Metathesis
The fi rst report on the development of air- and water-stable ILs was reported by
Wilkes and Zaworotko [ 138 ] which was based on 1,3-dialkylmethylimidazolium cat-
ions. The preparation involved a metathesis reaction between 1-ethyl-3 methylimida-
zolium iodate [EMIm]I and a range of silver salts (Ag[NO
3 ], Ag[NO 2 ], Ag[BF 4 ],
Ag[CH
3 CO 2 ], and Ag
2 [SO 4 ]) in methanol or aqueous methanol solution. The very
low solubility of silver iodide in these solvents allowed its separation simply by fi l-
tration, and removal of the reaction solvent allowed isolation of the ILs in high yield
and purity. This method remains the most effi cient for the synthesis of water-miscible
ILs, but is obviously limited by the relatively high cost of silver salts, not to mention
the large quantities of solid by-product produced [
141 ] . The fi rst report on a water-
insoluble IL appeared 2 years later, with the preparation of [EMIm][PF
6 ] from the
reaction of [EMIm]Cl and HPF
6 in aqueous solution [ 171 ] . Though the protocols
reported in the above two papers were sound, subsequent authors suggested refi ne-
ments in the methods employed. Most of the [EMIm]
+ -based salts are solid at room
temperature, facilitating purifi cation via recrystallization. In many applications,
however, a product is required that is liquid at room temperature. Therefore, it is use-
ful to employ cations with 1-alkyl substituents of chain length 4 or greater, which
12 S. Mallakpour and M. Dinari
results in a considerable lowering in melting point. The main goal of all anion
exchange reactions is the formation of the desired IL uncontaminated with unwanted
cations or anions, a task that is easier for water immiscible ILs.
It should be noted, however, that low-melting salts based on symmetrical onioum
cations have been prepared using anion-exchange reactions for many years. For
instance, the preparation of tetrahexylammonium benzoate (a liquid at 25°C) from
tetrahexylammonium iodide, silver oxide, and benzoic acid was reported in 1967.
The same authors also commented on an alternative approach involving the use of
an ion-exchange resin for the conversion of the iodide salt to hydroxide, but con-
cluded that this approach was less desirable. Low-melting salts based on cations
such as tetrabutylphosphonium [
172 ] and trimethylsulfonium [ 173 ] have also been
produced using very similar synthetic methods.
As the preparation of water-immiscible ILs is considerably more straightforward
than the preparation of their water-soluble analogues, these methods will be consid-
ered fi rst. The water solubility of the ILs is highly dependent on the nature of both the
anion and cation present and, in general, will decrease with increasing organic char-
acter of the cation. The most common approach for the preparation of water-immisci-
ble ILs is fi rst to prepare an aqueous solution of a halide salt of the desired cation. The
cation exchange is then carried out using either the free acid of the appropriate anion
or a metal or ammonium salt. Where available, the free acid is probably to be favored,
as it leaves only HCl, HBr, or HI as the by-product which can be easily removed from
the fi nal product by washing with water. It is recommended that these reactions are
carried out with cooling of the halide salt in an ice bath, as the addition of a strong acid
to an aqueous solution is often exothermic [ 141 ] . In cases where the free acid is incon-
venient to use, however, alkali metal or ammonium salts may be substituted without
major problems. It may also be preferable to avoid using the free acid in systems
where the presence of traces of acid may cause problems. A number of authors have
outlined broadly similar methods for the preparation of [PF
6 ] − and [Tf
2 N] − salts that
may be adapted for most purposes [ 170, 174 ] . When free acids are used, the washing
should be continued until the aqueous residues are neutral, as traces of acid can cause
decomposition of the IL. The most common approach is to perform the exchange in
aqueous solution using either the free acid of the appropriate anion, the ammonium
salt, or an alkali metal salt. When using this approach, it is signifi cant that the desired
IL can be isolated without excess contamination from unwanted halide-containing
by-products. A reasonable compromise has been suggested by Welton et al. for the
preparation of [BMIm][BF
4 ] [ 175 ] . In this method, which could in principle be adapted
to many other water-miscible systems, the IL is formed by metathesis between
[BMIm]Cl and HBF
4 in aqueous solution. The product is extracted into CH
2 Cl 2 , and
the organic phase is then washed with successive small portions of deionized water
until the pH of washings reach to neutral value. Although the water wash can result in
a lowering of the yield, the aqueous wash solutions may ultimately be collected
together, the water removed, and the crude salt added to the next batch of IL prepared.
In this manner, the amount of product loss is minimized, and the purity of the IL pre-
pared appears to be reasonable for most applications.
Alternatively, the metathesis reaction may be carried out entirely in an organic
solvent such as CH
2 Cl 2 , as described by Cammarata et al. [ 176 ] , or acetone, as
13
1 Ionic Liquids as Green Solvents: Progress and Prospects
described by Fuller et al. [ 177 ] . In both of these systems, the starting materials are
not fully soluble in the reaction solvent, so the reaction is carried out in a suspension.
In the case of the CH
2 Cl 2 process, the reaction was carried out by stirring the 1-alkyl-
3-methylimidazolium halide salt with the desired metal salt at room temperature for
24 h. Although the halide by-products have limited solubility in CH
2 Cl 2 , they are
reasonably soluble in the IL/CH
2 Cl 2 mixture. Thus, when this method is employed,
it is important that the CH
2 Cl 2 extracts are washed with water to minimize the halide
content of the fi nal product. This approach clearly results in a lowering of the yield
of the fi nal product. Therefore, care must be taken that the volume of water used to
carry out the washing is low. Lowering the temperature of the water to near 0°C can
also reduce the amount of IL loss. The fi nal product can be purifi ed by stirring with
activated charcoal followed by passing through an alumina column, as described in
the previous paragraph. This process was reported to give fi nal yields in the region
of 70–80% and was used to prepare ILs containing a wide variety of anions ([PF
6 ] − ,
[SbF
6 ] − , [BF 4 ] − , [ClO 4 ] − , [CF 3 SO 3 ] − , [NO 3 ] − , and [CF
3 CO 2 ] − ). In the case of the ace-
tone route, [EMIm]Cl was stirred with [NH
4 ][BF 4 ] or [NH
4 ][CF 3 SO 3 ] at room tem-
perature for 72 h. In this case, all starting materials were only slightly soluble in the
reaction solvent. The insoluble [NH
4 ]Cl by-product was removed by fi ltration. No
water wash was carried out, but trace organic impurities were removed by stirring
the acetone solution with neutral alumina for 2 h after removal of the metal halide
salts by fi ltration. The salts were fi nally dried by heating at 120°C for several hours,
after which they were analyzed for purity by electrochemical methods, giving
quoted purities of at least 99.95% [ 177 ] .
1.5 Properties of Ionic Liquids (ILs)
ILs with many interesting properties are currently enjoying popularity among
chemists. Overall, generalizing and reporting their properties is not easy because
some of the properties such as electrochemical window, long-term thermal stability,
polarity, and volatility have been the subject of debate. This happens due to better
understanding and adequate characterization of ILs with improved quantifi cation of
their impurities which are well known to affect the thermophysical properties of
them [ 127 ] . Water solubility of an IL can be tuned by changing the R group on the
cation component. In addition, by the choice of the anion, chemical and physical
properties can be changed signifi cantly. The generic properties of ILs have been
described in literature [ 146 ] and can be easily found in a database.
1.5.1 Melting Point
The key criterion for the evaluation of an IL is its melting point. Although ILs have
been defi ned to have melting points below 100°C and most of them are liquid at room
temperature, data must be considered with caution. The melting point of many ILs
14 S. Mallakpour and M. Dinari
may be uncertain because they undergo considerable supercooling, the temperature of
the phase change which can differ considerably depending on whether the sample is
heated or cooled, and also because of the potential presence of impurities [ 127 ] . There
is a signifi cant relationship between the structure and chemical composition of an IL
and its melting point. This physical property can be adjusted through variations on the
cation and/or anion, and both cations and anions contribute to the low meting points
of ILs. The increase in anion size leads to a decrease in melting point. For instance,
the melting points of 1-ethyl-3-methylimidazolium-type ILs with different anions,
such as [BF
4 ] − and [Tf
2 N] − , are 15°C and −3°C, respectively [ 178 ] . Comparison of the
melting points of different chlorine salts illustrates the infl uence of the cation clearly.
High melting points are characteristic for alkali metal chloride, whereas chloride with
suitable organic cation melts at temperatures below 150°C. In the literature, the fol-
lowing features are discussed for the cations of low-melting salt: low symmetry, weak
intermolecular interaction such as avoidance of hydrogen bonding, and a good distri-
bution of charge in the cation. For IL prepared by reaction of halide [cation]
+ X − with
a Lewis acid MX
l
, the molar ratio of two reactants infl uences the melting point [ 29 ] .
Further, an increase in the branching on the alkyl chain increases the melting point.
1.5.2 Volatility
In addition to the favorable physical and chemical properties of ILs, an important
property of ILs that stimulates interest in using them in the context of so-called
green chemistry is their essentially zero vapor pressure even at elevated tempera-
tures. ILs are indeed nonvolatile in that sense that at near ambient temperatures their
vapor pressure is negligible. For typical ILs, normal boiling temperatures (T
b ),
which correlate with their vapor pressure at one atmosphere, cannot be experimen-
tally determined as ILs decomposes at a lower temperature. It has nevertheless been
reported that ILs can be distilled at 200–300°C but under signifi cantly reduced pres-
sure and at very low distillation rate (<0.01 g h
−1 ). Vapor pressure of ILs remains,
however, negligible at near ambient conditions; thus, for all practical purposes, they
may be considered as nonvolatile solvents [ 35 ] . On the whole, the negligible volatil-
ity of these ILs denotes that air pollution by gaseous release is not a concern. ILs are
considered as nonvolatile and, consequently, nonfl ammable at ambient and higher
temperatures. However, the potential release of IL vapors (or decomposition prod-
ucts) must be considered when ILs are used at elevated temperatures.
1.5.3 Thermal Stability
Thermogravimetric analysis indicates high thermal stability for many ILs, generally
>350°C. For example, ILs [EMIm][BF
4 ], [BMIm][BF 4 ], and 1,2-dimethyl-3-propy-
limidazolium bis(trifl uorosulfonyl)imide are stable up to temperatures of 445°C,
15
1 Ionic Liquids as Green Solvents: Progress and Prospects
423°C, and 457°C, respectively [ 127 ] . Such high temperatures are only tolerated by
most liquids for a short time. For example, after 10 h, even at temperatures as low
as 200°C, [RMIm][PF
6 ] and 1-decyl-3-methylimidazolium trifl ate show an appre-
ciable mass loss [
179 ] . The ILs with low thermal stability are [EMIm][X], where
X = [Tf 2 N] − , [M S N] − , and Br
− [ 68 ] . Phosphonium ILs with [Tf
2 N] − or [N(CN)
2 ] −
anions decompose completely to volatile products in a single step. The degradation
products indicate that Hofmann elimination process and/or dealkylation reactions
occurred. Conversely, ILs based on nitrogen cations do not decompose completely
[ 127 ] . The start of thermal decomposition is furthermore similar for the different
cations but appears to decrease as the anion hydrophilicity increases. It has been
suggested that the stability dependency on the anion is in the order [PF
6 ] − > [Tf 2 N] −
[BF
4 ] − > halides. An increase in cation size, at least from 1-butyl to 1-octyl, does not
offer large effect [ 19 ] .
1.5.4 Viscosity
The viscosity of many ILs is relatively high compared to conventional solvents, one
to three orders of magnitude higher. The viscosity is determined by van der Waals
forces, hydrogen bonding, and electrostatic forces. The ability of fl uorinated anions
such as BF
−4 and PF
−6 to the formation of hydrogen bonding results in the formation
of viscous ILs [ 178 ] . The presence of AlCl
4 − and Al 2 Cl 7 − in acidic mixture leads to
formation of weaker hydrogen bond and much lower viscosity. The transition from
trifl ate ion to n-C
4 F 9 SO 3 and from the trifl ouroacetate ion to the n-C
3 F 7 COO − ion
shows an obvious increase in viscosity. In fact, stronger van der Waals forces lead
to increase in the energy required for molecular motion. The case of n-C
4 F 9 SO 3 and
n-C
3 F 7 COO − ions results in a higher viscosity of IL. Comparison of the viscosity of
the [BMIm]CF
3 SO 3 with [BMIm][Tf
2 N] reveals a lower viscosity despite stronger
van der Waals interaction. In this case, the almost complete suppression of hydro-
gen bond is responsible for expected increase viscosity [ 29 ] . For a variety of ILs,
viscosity has been reported in the range 10–500 mPa s
−1 at room temperature. The
viscosity of ILs can affect transport properties such as diffusion and plays a major
role in stirring, mixing, and pumping operations.
1.5.5 Density
ILs are generally denser than either organic solvents or water, with typical density
values ranging from 1 to 1.6 g cm
−3 . Their densities were found to decrease with
increase in the length of the alkyl chain in the cation. For example, for ILs com-
posed of substituted imidazolium cations and CF
3 SO 3 − anion, the density decreases
from 1.39 g cm
−3 for [EMIm] 1 to 1.33 g cm
−3 for [E
2 Im], to 1.29 g cm
−3 for
[BMIm], and to 1.27 g cm
−3 for [BEIm] 1.22. Density of an IL depends on the type
16 S. Mallakpour and M. Dinari
of cation and anion. The density of comparable ILs decreased as the bulkiness of
the organic cation increase. The order of increasing density for ILs composed of
a single cation is [CH
3 SO 3 ] − » [BF 4 ] − < [CF 3 CO 2 ] − < [CF 3 SO 3 ] − < [C 3 F 7 CO 2 ] − < [(CF 3
SO
2 ) 2 N] − [ 178 ] . The molar mass of the anion signifi cantly affects the overall density
of ILs. The [Ms
2 N] − species have lower densities than the [Tf
2 N] − salts, in agree-
ment with the fact that the molecular volume of the anion is similar but the mass
of the fl uorine is greater. In the case of orthoborates, with the exception of
bis(salicylato)borate, the densities of the ILs having the [BMIm] cation decrease
with increase in anion volume. This order is also followed in ILs, those having
[Tf
2 N] − , [TfO] − , or [BF
4 ] − as anion. This behavior has been attributed to the fact
that packing becomes more compact as the alternating positive and negative spe-
cies are more even in size [ 19 ] . Density measurement of IL with trifl ate or trifl uo-
roacetete ions confi rms the more general trend. Furthermore, a certain density
range is established by the choice of anion, within which a fi ne adjustment is
possible by careful choice of cation [ 29 ] .
1.5.6 Polarity
To classify the solvents, the polarity is one of the most important properties for
characterizing the solvent effect in chemical reactions. In IUPAC document,
polarity is defi ned as “the sum of all possible, nonspecifi c interactions between
the solute ions and molecules and solvent molecules, excluding such interactions
leading to defi nite chemical alterations of the ions or molecules of the solute”
[ 67 ] . The subject of IL polarity has been addressed by a variety of methodologies.
As an example, the property of solvents to stabilize a charge is usually determined
from the absorption maximum of a solvatochromic dye. By this measurement, the
polarity, archetypical [BMIm][BF
4 ], is in the range of the lower alcohols. A sol-
vatochromic test for the coordination strength (nucleophilicity) of the anion, in
contrast, indicates that the often used [PF
6 ] and [Tf
2 N] anions are much less
nucleophilic than the lower alcohols [ 32 ] . Although solvatochromic dyes can be
used to determine empirical polarity parameters, these parameters are probably
not truly independent on the probe molecule used. The diffi culty in the case of ILs
is to fi nd a suitable soluble probe which measures the polarity parameters as inde-
pendently as possible from other infl uences of the solvent [ 127 ] . Measurement of
keto-enol equilibria is another approach to solvent polarity, which is dependent on
the polarity of the medium. Based on this methodology, the polarity of [BMIm]
[BF
4 ], [BMIm][PF 6 ], or [BMIm][NTf
2 ] was higher than methanol or acetonitrile.
Microwave dielectric spectroscopy measurements can also be used for the remain-
ing measurement of dielectric constants of a number of ILs. By this measurement,
the polarities of [BMIm][BF
4 ] and [BMIm][PF
6 ] are in the range of a medium-
chain alcohol, such as 1-hexanol or 1-octanol, with marked contributions from the
anion as well the cation [ 32 ] .
17
1 Ionic Liquids as Green Solvents: Progress and Prospects
1.5.7 Conductivity and Electrochemical Window
An attractive aspect of ILs is their conductivity by virtue of which they are very
useful solvents and electrolytes in electrochemical reactions. Based on the fact that
ILs are composed solely of ions, it would be expected that ILs have high conductivi-
ties. However, the conductivity of any solution depends not only on the number of
charge carriers but also on the viscosity, density, ion size, anionic charge delocaliza-
tion, aggregations, and ionic mobility. The large size of ions of ILs reduces the ion
mobility which, in turn, leads to lower conductivities. Furthermore, ion pair forma-
tion and/or ion aggregation leads to reduced conductivity. The conductivity of ILs is
inversely linked to their viscosity. Hence, ILs of higher viscosity exhibit lower con-
ductivity [ 178 ] . In general, higher conductivities are found for imidazolium-based
ILs in comparison with the ammonium-based ILs. Strong ion-pair associations have
been invoked in the case of [Tf
2 N] − -based ILs, to understand their lower conductivity
in comparison with [BF
4 ] − -based ILs. ILs exhibit broad range of conductivities
spanning from 0.1 to 20 mS cm
−1 [ 127 ] . ILs by their electrochemical window play
a key role in electrodeposition of metals and semiconductors. By defi nition, the
electrochemical window is the electrochemical potential range over which the elec-
trolyte is neither reduced nor oxidized at an electrode. Because of its low electro-
chemical window, the electrodeposition of elements and compounds in water is
restricted. On the contrary, ILs have signifi cantly larger electrochemical windows,
found in the range 4.5–5 V, which is similar to or slightly larger than that found in
conventional organic solvents, but larger than that of aqueous electrolytes. In gen-
eral, the wide electrochemical windows of ILs have opened the door for electrode-
posit ion of metals and semiconductors at room temperature which were formerly
obtained only from high temperature molten salts. For example, Al, Mg, Si, Ge, and
rare earth elements can be obtained from RTILs. The thermal stability of ILs allows
to electrodeposit Ta, Nb, V, Se, and presumably many other ones at elevated tem-
perature [ 19, 178, 180 ] .
1.5.8 Toxicity
The main reason for believing ILs to be nontoxic results from their nonvolatile prop-
erties, which makes them potential green substitutes for conventional volatile organic
solvents. Unfortunately, this green image is misplaced and has recently aroused the
awareness of chemists, especially those working in the area of green chemistry. It is
easy to found that some of cations and anions for preparing IL are hazardous, so it
is wrong to assume that the risk hazards of these precursors will fade away following
their conversion into ILs. On the other hand, incorporating different functional groups
makes it complicated to investigate the toxicity of ILs because of the potential
virulence of the incorporated functionalities. Though ILs may help in reducing the
risk of air pollution, their release to aquatic environments could cause severe water
18 S. Mallakpour and M. Dinari
contamination because of their potential toxicity and inaccessible biodegradability.
Because of the relative stability features of ILs, their accumulation in the environ-
ment becomes feasible, if they are applied in operational use. Thus, the fundamentals
of IL biodegradability turn out to be an important issue for the reduction of ignition
and landfi ll-waste risks. Therefore, various efforts to produce biodegradable and
biorenewable ILs that can be obtained through modifi cation of natural sources have
been undertaken [
67, 127 ] .
1.5.9 Air and Moisture Stability
Many of ILs are both air and moisture stable. Conversely, most imidazolium and
ammonium salts are hydrophilic, and if they are used in open vessels, hydration will
certainly take place. The hydrophobicity of an IL increases with increasing length of
the alkyl chain. Despite their widespread usage, ILs containing PF
6 − and BF 4 − h a v e
been reported to decompose in the presence of water, giving off HF. Wasserscheid
et al. [ 181 ] pointed out that ILs containing halogen anions generally show poor stabil-
ity in water and also produce toxic and corrosive species such as HF or HCl. Therefore,
they suggest the use of halogen-free and relatively hydrolysis-stable anions such as
octylsulfate compounds. The interaction between water and ILs and their degree of
hydroscopic character are strongly dependent on anions. The amount of absorbed
water is highest in the BF
4 − and lowest in PF
6 − [ 182 ] . However, [Tf 2 N] − is much more
stable in the presence of water as well as having the advantage of an increased hydro-
phobic character. ILs immiscible with water tend to absorb water from the atmo-
sphere. The infrared studies demonstrated that the water molecules absorbed from the
air are mostly present in the free state and bonded via H-bonding with the PF
6 − and
BF 4 − anions. The presence of water may have dramatic effect on the reactivity of ILs.
Since water is present in all ILs, they are usually utilized after a moderate drying pro-
cess. The newly synthesized ILs are more stable than the old halogenoaluminate
systems. Certain ILs incorporating 1-3-dialkylimidazolium cations are generally more
resistant than traditional solvents under harsh process conditions, such as those occur-
ring in oxidation, photolysis, and radiation processes [ 167 ] .
1.5.10 Cost and Biodegradability
Cost and biodegradability have also been major concerns, and new families of ILs
derived from renewable feedstock or from “low-cost” starting materials have been
described. These “Bio-ILs” are entirely composed of biomaterials [ 183 ] . An example
to be cited is the development of the “deep eutectic mixtures” liquid systems based on
choline chloride [ 184 ] for which the qualifi cation of “ILs” is still the subject of contro-
versies. Choline can be used as alternative cation in combination with suitable anion
to generate ILs. The biodegradable properties of these ILs have been reported [ 185 ] .
19
1 Ionic Liquids as Green Solvents: Progress and Prospects
Very recently, it was shown that the introduction of an ester group into long alkyl
chains leads to reduce toxicity and improve ecotoxicity of ILs. Further, incorporation
of ether groups into the side chain improves the biodegradability of imidazolium-
based ILs, while the introduction of the biodegradable octylsulfate anion has a fur-
ther benefi cial effect. Recent work on pyridinium-based ILs demonstrated how the
heteroaromatic cationic core can be modifi ed to produce biodegradable ILs. As with
the imidazolium examples, the inclusion of an ester group in the cation side chain led
to improved biodegradability. High levels of biodegradability have also been reported
in cases where environmentally benign anions such as saccharinate and acesulfamate
are included. Several ammonium ILs based on choline have been introduced which
are biodegradable and can be readily prepared [
124 ] .
1.6 Solvent Properties and Solvent Effects
Solvents are commonly characterized by macroscopic physical constants such as
vapor pressure, surface tension, boiling point, density, relative permittivity e r (dielec-
tric constants), cohesive pressure, and refractive index. Many chemical reactions are
carried out in homogenous media using conventional solvents. However, it is diffi -
cult to fi nd solvents in which covalent organic compounds and ionic inorganic
reagents as well as catalysts are adequately soluble to achieve a homogeneous reac-
tion mixture. Furthermore, upon the completion of reaction, chemical products have
to be removed from the solvent. There are some methods to recover a product from
a solvent. For example, water-soluble compounds may be extracted in water and the
distillation may be used for chemicals with high vapor pressures [ 67 ] . Alternatively,
for chemicals with low vapor pressures, distillation must be performed at low pres-
sures. In addition to this, there are some chemicals that can decompose as a result of
heating, such as pharmaceutical compounds. Additionally, isolation of the product(s)
requires a fairly lengthy work-up procedure. ILs seem to be potentially good sol-
vents for many chemical reactions in cases where distillation is not practical, or
water insoluble or thermally sensitive products are the components of a chemical
reaction. They are immiscible with most of the organic solvents, thus they provide
a nonaqueous, polar alternative for two-phase systems [ 67 ] . Moreover, ILs which
are not miscible with water can be used as immiscible polar phases with water.
Although all other conventional solvents evaporate to the atmosphere, ILs do not
evaporate and their nonvolatility gives an opportunity to utilize them in high vac-
uum systems. The negligible volatility is the basic property which characterizes
them as green solvents. Considering as potential solvents, ILs can easily replace
other conventional organic solvents which are used in large quantities in chemical
processing industries to eliminate major environmental problems. Many studies
have been directed to the characterization of ILs and their “bulk” physicochemical
properties [ 56 ] .
The dielectric constant is an important parameter of solvent that refl ects its
molecular symmetry. It is worth to emphasize that the favorable methods for
20 S. Mallakpour and M. Dinari
determining dielectric constants fail in case of ILs because of their high electrical
conductivities. However, it could be measured for a series of imidazolium-based
ILs using microwave dielectric spectroscopy. ILs can be classifi ed as moderately
polar solvents. Dielectric constant ( e ) values are found in the range of 8.8–15.2,
decreasing with increasing the length of the alkyl chain on the imidazolium cat-
ion. However, the variation is small compared to the wide range variation in ( e )
values of molecular solvents (2–180) [
85, 186 ] . The dielectric constants were
found to depend mainly on the nature of the ILs anions. However, the abundant
different interactions acting together in ILs make them very complex so that it is
not surprising that a single physical parameter such as the dielectric constant is
incapable of adequately modeling the solvent–solute interactions. This parameter
has often failed in correlating solvent effects qualitatively and quantitatively.
An example is given by the solvent effect study on nucleophilic reactions in ILs
compared to molecular solvents where Hugues–Ingold viewpoint using dielectric
constant as a measure of solvent polarity proved to be insuffi cient to explain the
IL system [ 187 ] . For a rational design and a better choice of ILs, better under-
standing of their properties is required.
1.6.1 Solute–Ionic Liquids (ILs) Interactions
The solvent properties of ILs are mainly determined by 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 [ 28 ] . Charge distribution on the anions, H-bonding ability, polarity, and
dispersive interactions are the main factors that infl uence the physical properties of
ILs. For example, imidazolium-based ILs are highly ordered hydrogen-bonded sol-
vents and they have strong effects on chemical reactions and processes [ 67 ] .
Many studies have focused on the cation–anion interactions (solvent–solvent
interactions) rather than ions–solute interactions (solvent–solute interactions or
solvent solvation). In conventional medium, solvent–solute interactions are gener-
ally predominant, while in ILs, interactions inside the solvent are more important.
The Diels–Alder reaction is an interesting example because it is a key step in many
syntheses. In the case of the reaction of cyclopentadiene and methyl acrylate, the
ability of the IL to act as H-bond donor (cation effect) appeared to be a key crite-
rion to explain the enhancement of reaction rate and endoselectivity [ 188 ] . This
effect has to be moderated by the H-bond acceptor ability of the IL (anion effect).
ILs with strong H-bond interaction between the cation and the anion (contact pair
ions) are poor solvents for Diels–Alder due to competition between the anion and
the H-bond acceptor dienophile for H-bonding with the cation. It is not surprising
that low yields have been reported for dialkylimidazolium bromide and trifl uoroac-
etate ILs. This H-bonding with the substrate can be infl uenced by p-stacking of the
imidazolium cations and H-bonding interactions between the cation and the anion
of the IL [ 127 ] .
21
1 Ionic Liquids as Green Solvents: Progress and Prospects
Another good example of model reaction to examine the IL effect is the
nucleophilic substitution reactions [ 127 ] . In molecular solvents, the Hughes–
Ingold qualitative model describes the solvent effect considering the pure electro-
static interactions between ions or dipolar molecules in initial and transition states
(solvent polarity). This model does not take into account the H-bond interactions
and proved to be limited to describe ILs effect. Kamlet–Taft linear solvation
energy relationship has also been utilized to describe ILs effect on nucleophilic
reactions. The characteristic values of a , b , and p * have been collected for ILs
[ 189 ] . The solvent’s hydrogen donor ability ( a value) appears as the dominant
effect in reducing the nucleophilicity of the nucleophile and slowing the reaction
rate [ 190 ] . Hard and soft natures of the nucleophile also proved to be important,
the [BMIm] cation of the ILs acting as a hard “solvent” in interacting more
strongly with hard anions (e.g., [Cl]
− ) than soft ones (e.g., [CN]
− ). Compared to
molecular solvents, the nucleophilicity of halide anions (e.g., [Br]
− ) in the S
N 2
reactions of methyl- p -nitrobenzenesulfonate was reduced in ILs (the reaction is
15 times slower in N -butyl-3-methylpyridinium ([BMP])[Tf 2 N] than in dichlo-
romethane, and it is roughly two times slower in [BMIm][Tf
2 N] than in [BMP]
[Tf
2 N] ). This can be ascribed to the existence of strong H-bond between the
nucleophile (the anion, particularly the chloride) and the [BMIm] cation. This dif-
ference arises largely from the a value [ 127 ] .
Another systematic study on nucleophilicity of a series of anions on the substi-
tution reaction of methanesulfonic group was conducted in different ILs and com-
pared with that obtained in organic solvents (PhCl, DMSO, and MeOH). The
results emphasize the predominant role of water for hydrophilic anions ([Cl]
− and
[PhCO
2 ] − ). In this case, the interaction of the anion with the imidazolium has a
lower effect. These results confi rm that water molecules create H-bond with the
imidazolium cation replacing the cation–anion interactions present in the ILs
[ 191 ] . Higher reactivity is also observed in C(2)-protected imidazolium-based
ILs. The case of charged electrophiles within the framework of S
N 2 reactions is
also of fundamental interest since many catalytic centers carry positive charges.
The reaction of chloride ion with an ionic electrophile (sulfonium associated with
[Tf
2 N] − or [TfO]
− ) was studied in several molecular solvents and ILs. The nucleo-
philic substitution reaction does not take place in either strong dissociating molec-
ular water solvents or in methanol. In nondissociating solvent, the reaction is
supposed to occur via a solvated ion pairs. The behavior of this reaction in ILs is
different from that in molecular solvents; the kinetic experiments in ILs are in
favor of reaction via dissociated ions. The results show that ILs can be considered
as superdissociating solvents, this effect arising from the fact that ILs are at the
same time liquid and ionic. The reaction mechanism would proceed through a true
S
N 2 reaction of free solvated ions rather than with a ion-pair mechanism seen in
molecular solvents [ 192 ] . Based on these model reactions, we can assume that the
interactions between ILs and different species are of diverse nature and complex
which signifi cantly modify their reactivity. The interactions of ILs with selected
solutes will be discussed below.
22 S. Mallakpour and M. Dinari
1.6.1.1 Interaction of Ionic Liquids (ILs) with Water
The hydrophilic/hydrophobic behavior is signifi cant for the solvation properties of
ILs as it is essential to dissolve reactants, but it is also relevant for the recovery of
products by solvent extraction. In addition, the water content of ILs can affect the
rates and selectivity of reactions. One potential problem with ILs is the possible
pathway into the environment through wastewater. The effect of water in modify-
ing IL properties has been a central focus of recent interest [
127 ] . This factor has
been illustrated by the examination of water solvation at low and high concentra-
tions. IR [ 176 ] and dielectric constant [ 193 ] studies have revealed that water is
molecularly dispersed in 1-alkyl-3-methyl imidazolium-based ILs ([H
2 O < 2 M] ) at
low concentrations. When the water concentration is increased, water molecules
aggregate and form a well-defi ned water hydrogen-bonds network [ 193 ] . It is found
that the local organization between ions precludes any specifi c interactions between
water and the proton of the imidazolium cation. Water would be doubly hydrogen-
bonded with two anions, in symmetric 2:1 [ Anion–H–O–H–Anion] structures.
PCl
3 and POCl
3 show unexpectedly high hydrolytic stability in wet ILs. For exam-
ple, in the [Tf
2 N] − -based ILs, PCl
3 was soluble at concentrations up to 0.20 M, and
in [BMP][Tf
2 N], it was found to be hydrolytically stable for weeks, even when
stirred in air. The ability of even wet ILs to stabilize hydrolytically unstable solutes
may be understood by considering the interaction of water with IL. The nucleophi-
licity of water and therefore its hydrolysis tendency can be reduced due to its inter-
action with IL anions. In hydrophilic ILs, the higher water content results in higher
rate of hydrolysis [ 194 ] . Interestingly, ILs have been used to improve the solubility
of hydrophobic compounds in water. For example, the solubility of acetophenone
in aqueous solution can be increased by a factor of 10 by addition of [BMIm][BF
4 ],
which can be important for application in biocatalysis. This phenomenon can
be explained by the ability of ILs to form small aggregates which are solvent
dependent [ 195 ] .
1.6.1.2 Interaction of Ionic Liquids (ILs) with Acid and Base
The importance of ILs as solvents to perform acid–base reactions has been
recently realized. A straightforward way to create and modulate the acidity is to
add a Brønsted acid into the IL. Therefore a new level of acidity can be found
either by varying the acid concentration in the IL or by changing the nature of the
IL [ 161 ] . A lot of acid-catalyzed reactions have been reported in ILs, but only
few studies have been devoted to the quantifi cation of the acidity level of the
proton in these media. However, as the acidity of protons is mainly determined
by their solvation state, the properties of protons will depend strongly on the
nature of the IL and the nature and concentration of the acid. Relative estimation
of the proton acidity level has been reported using the determination of the
Hammett acidity functions, by UV–vis spectroscopy [ 160 ] . For the same content
23
1 Ionic Liquids as Green Solvents: Progress and Prospects
of added strong acid, the anion of ILs plays a fundamental role; the acidity levels
are in the order: [PF
6 ] − > [BF
4 ] − > [Tf
2 N] − > [TfO]
− , thus implying that the solvat-
ing power (or basicity) of the anions follows the reverse order. The presence of
basic impurities in the ILs can also have a dramatic effect. Even if absolute acidi-
ties cannot be determined with this method, global acidity must be higher than
that observed in water [
161 ] .
1.6.1.3 Interaction of Ionic Liquids (ILs) with Aromatic Hydrocarbon
Aromatic hydrocarbons show unusual high solubility in ILs as compared to ali-
phatic compounds. This extent of solubility decreases with an increase in the molec-
ular weight of the hydrocarbon, but the differences of solubilities of o -, m -, and
p -xylenes are not signifi cant [ 161 ] . Previous studies show that imidazolium-based
ILs can form liquid clathrates in the presence of aromatic hydrocarbons [ 160 ] .
Dialkylimidazolium cations are able to form specifi c and oriented interactions with
arenes. For instance, in the salt crystal [BMIm][PF
6 ], 0.5 benzene, a three-dimen-
sional network has been observed with H-bonds between anion and cation. This
results in the formation of channels containing the benzene molecules. Short inter-
actions between methyl hydrogen of the cation and aromatic hydrogen are present
[ 196 ] . Interactions between p -aromatic systems and inorganic cations (Li
+ , Na + , K + ,
or Ag
+ ) or organic cations (ammonium) are already well known as the “p-cation
interaction,” important in biochemistry, and experimentally evidenced [ 197 ] . A
detailed study conducted with the aid of NMR and molecular simulation shows dif-
ference in interaction of toluene with the ILs due to the substitution of the C(2) of the
imidazolium cation. In the case of the [BMIm] cation, toluene is located closer to
the methyl group at the end of the butyl chain, whereas in the case of [BMMiI] cat-
ion, toluene was closer to C(2)-Me of the imidazolium. The H-bonding association
between [BMI]
+ and [Tf
2 N] − is too strong to be cleaved by toluene. In the case of
C(2)-Me cation, the less strongly bonded IL network renders possible the penetra-
tion and interaction of toluene [ 197 ] .
1.6.1.4 Interaction with Chiral Substrates
Some chiral ILs have been designed and synthesized. They have already been
applied in different fi elds such as asymmetric synthesis, stereoselective polymeriza-
tion, chiral chromatography, liquid crystals, chiral resolution, and NMR shift
reagents [ 20, 106, 107 ] . Chiral solvents have been reported in asymmetric synthe-
ses. In the Baylis–Hillman reaction of benzaldehyde and methyl acrylate in the
presence of bases, chiral ILs demonstrate their ability in the transfer of chirality,
even if the enantiomeric excesses (ee) are still moderate. The presence of an alcoholic
functional group on the N -alkyl- N methylephedrinium is primordial and acts as a
fi xing point of the chiral IL on the reactants. It is assumed that the OH is connected
24 S. Mallakpour and M. Dinari
with a carbonyl group of the substrate (from either benzaldehyde or methyl acrylate)
via H-bonding [ 127, 198 ] . However, with N -methylephedrine, very low ee are
obtained which also indicate that the ammonium group plays a crucial role in the
chirality induction. Even if not directly demonstrated, it seems that the key of effec-
tive asymmetric induction is the existence of both strong intermolecular interac-
tions, like electrostatic attraction and hydrogen bonding, between ionic solvents and
intermediates or transition states of the diastereoselective reaction step. The need of
H-bonding in the transfer of chirality has also been confi rmed in the case of borate-
based chiral IL bearing maleic acid functions. In this latter case, by incorporating
the acidic center into the chiral anion of the solvent, the IL offers the possibility of
establishing a bifunctional interaction, which allows monofunctional achiral nucleo-
philes to be used as catalysts [
199– 201 ] .
1.7 Conclusions
VOCs are often diffi cult to separate from the desired reaction products, problematic
to recycle, and challenging to dispose of without encountering extra costs and/or
adversely affecting the environment. Therefore, the interest of ILs as “green” sol-
vents resides in their extremely low vapor pressure and high thermal stability, which
offer advantages such as ease of containment, product recovery, and recycling abil-
ity. Due to their attractive properties, ILs are being used for a wide variety of appli-
cations. Current research indicates that replacing an organic solvent with an IL can
bring about remarkable improvements in well-known chemical processes. ILs are
being used extensively as solvent systems for chemical and polymerization reac-
tions in addition to their use as biocatalysis. ILs have also been increasingly used in
separation science, gas chromatography, liquid chromatography, and capillary elec-
trophoresis. ILs have also found uses in liquid–liquid extraction, immunoassays,
lubricants, and embalming/tissue preservation. Additionally, the change of the cat-
ion and/or anion component of the IL provides a way to adjust all properties, allow-
ing the potential to fi ne-tune an IL for specifi c tasks. These features allow ILs to be
used as potential alternative solvents to VOCs in a wide variety of industrial chemi-
cal applications.
It is hoped that successful commercialization of technologies utilizing these neo-
teric solvents will be a key driver for their continued development and integration
into the chemical industry. Large-scale industrial manufacture of the ILs themselves
is clearly a necessary precursor for this process. We also believe that due to the
extensive variety in ILs formation and applications, they will play an important role
in future environmentally friendly science and technology.
Acknowledgments We wish to express our gratitude to the Research Affairs Division Isfahan
University of Technology (IUT), Isfahan, for partial fi nancial support. Further fi nancial support
from National Elite Foundation (NEF) and Center of Excellence in Sensors and Green Chemistry
Research (IUT) is gratefully acknowledged.
25
1 Ionic Liquids as Green Solvents: Progress and Prospects
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