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J. of Supercritical Fluids 43 (2007) 150180
Review article
A review of ionic liquids towards supercritical fluid
applications
Seda Keskin, Defne Kayrak-Talay, Ugur Akman , Oner
HortacsuDepartment of Chemical Engineering, Bogazici University,
Bebek 34342, Istanbul, Turkey
Received 2 August 2006; received in revised form 8 May 2007;
accepted 29 May 2007
bstract
Ionic liquids (ILs), considered to be a relatively recent
magical chemical due their unique properties, have a large variety
of applications inll areas of the chemical industries. The areas of
application include electrolyte in batteries, lubricants,
plasticizers, solvents and catalysis inynthesis, matrices for mass
spectroscopy, solvents to manufacture nano-materials, extraction,
gas absorption agents, etc. Non-volatility and non-ammability are
their common characteristics giving them an advantageous edge in
various applications. This common advantage, when consideredith the
possibility of tuning the chemical and physical properties of ILs
by changing anioncation combination is a great opportunity to
obtain
ask-specific ILs for a multitude of specific applications. There
are numerous studies in the related literature concerning the
unique properties,reparation methods, and different applications of
ILs in the literature. In this review, a general description of ILs
and historical background areiven; basic properties of ILs such as
solvent properties, polarity, toxicology, air and moisture
stability are discussed; structure of ILs, cation, anionypes and
synthesis methods in the related literature are briefly summarized.
However, the main focus of this paper is how ILs may be used in
thehemicals processing industries. Thus, the main application areas
are searched and the basic applications such as solvent
replacement, purificationf gases, homogenous and heterogeneous
catalysis, biological reactions media and removal of metal ions are
discussed in detail. Not only thedvantages of ILs but also the
essential challenges and potentials for using ILs in the chemical
industries are also addressed. ILs have becomehe partner of scCO2
in many applications and most of the reported studies in the
literature focus on the interaction of these two green solvents,.e.
ILs and scCO . The chemistry of the ILs has been reviewed in
numerous papers earlier. Therefore, the major purpose of this
review paper is
2o provide an overview for the specific chemical and physical
properties of ILs and to investigate ILscCO2 systems in some
detail. Recovery ofolutes from ILs with CO2, separation of ILs from
organic solvents by CO2, high-pressure phase behavior of ILscCO2
systems, solubility of ILsn CO2 phase, and the interaction of the
ILscCO2 system at molecular level are also included.
2007 Elsevier B.V. All rights reserved.
eywords: Ionic liquids; Supercritical carbon dioxide; Review
ontents
1. General description of ILs . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . 1512. History of ILs . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 1513. Basic
properties of ILs . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 151
3.1. Solvent properties of ILs . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 1533.2. Polarity of ILs . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 1543.3. Toxicology of ILs . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . 1543.4. Air and
moisture stability of ILs . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
4. Structure and synthesis of ILs . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 1554.1. Anions . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . 156
4.2. Cations . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . .4.3. Synthesis . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . .
5. Major applications suggested for ILs . . . . . . . . . . . .
. . . . . . . . . . . . . . . . .5.1. Solvent replacement . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Corresponding author. Tel.: +90 212 3596867; fax: +90 212
2872460.E-mail address: [email protected] (U. Akman).
896-8446/$ see front matter 2007 Elsevier B.V. All rights
reserved.oi:10.1016/j.supflu.2007.05.013
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 156. . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . 156. . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 156
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . 157
mailto:[email protected]/10.1016/j.supflu.2007.05.013
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S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180
151
5.2. Purification of gases . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 1575.3. Homogenous and heterogeneous catalysis . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 1585.4. Biological reactions media . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 1595.5. Removing of metal ions . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . 159
6. Challenges of ILs . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 1607. ILs and scCO2 systems . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . 161
7.1. High-pressure phase behavior of ILCO2 systems . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 1637.1.1. The
[bmim][PF6]CO2 system . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 1637.1.2. Other ILCO2
systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 166
7.2. IL solubility in CO2 . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 1677.3. ILCO2 interaction at the molecular level .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 1687.4. Solute recovery from ILs with scCO2 . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 1687.5. Other applications of ILscCO2 systems . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . 170
8. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . 175Acknowledgements . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 176
. . . . .
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References . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . .
. General description of ILs
Ionic liquids (ILs) have been accepted as a new green chemi-al
revolution which excited both the academia and the
chemicalndustries. This new chemical group can reduce the use
ofazardous and polluting organic solvents due to their
uniqueharacteristics as well as taking part in various new
syntheses.he terms room temperature ionic liquid (RTIL),
nonaqueous
onic liquid, molten salt, liquid organic salt and fused salt
havell been used to describe these salts in the liquid phase [1].
ILsre known as salts that are liquid at room temperature in
con-rast to high-temperature molten salts. They have a unique
arrayf physico-chemical properties which make them suitable
inumerous applications in which conventional organic solventsre not
sufficiently effective or not applicable. Short [2] pointedut in
1980, that there were only a few patent applications forLs, in
2000, the number of patent applications increased to 100,nd finally
by 2004, there were more than 800. This is a clearndication of the
high affinity of the academia and industry tohe ILs.
. History of ILs
ILs have been known for a long time, but their extensive uses
solvents in chemical processes for synthesis and catalysis
hasecently become significant. Welton [1] reported that ILs areot
new, and some of the ILs such as [EtNH3][NO3] was firstescribed in
1914 [3]. The earliest IL in the literature was createdntentionally
in 1970s for nuclear warheads batteries [4]. During940s, aluminum
chloride-based molten salts were utilized forlectroplating at
temperatures of hundreds of degrees Celsius.n the early 1970s,
Wilkes tried to develop better batteries foruclear warheads and
space probes which required molten saltso operate [4]. These molten
salts were hot enough to damagehe nearby materials. Therefore, the
chemists searched for salts
hich remain liquid at lower temperatures and eventually they
dentified one which is liquid at room temperature. Wilkes andis
colleagues continued to improve their ILs for use as
batterylectrolytes and then a small community of researchers
began
scnr
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 176
o make ILs and test their properties [5,6]. In the late 1990s,
ILsecame one of the most promising chemicals as solvents.
The first ILs, such as organo-aluminate ILs, have lim-ted range
of applications because they were unstableo air and water.
Furthermore, these ILs were not inertowards various organic
compounds [7]. After the firsteports on the synthesis and
applications of air stableLs such as 1-n-butyl-3-methlyimidazolium
tetrafluoroborate[bmim][BF4]) and 1-n-butyl-3-methlyimidazolium
hexafluo-ophosphate ([bmim][PF6]), the number of air and water
stableLs has started to increase rapidly [7]. Recently, researchers
haveiscovered that ILs are more than just green solvents and
theyave found several applications such as replacing them
witholatile organic solvents, making new materials, conducting
heatffectively, supporting enzyme-catalyzed reactions, hosting
aariety of catalysts, purification of gases, homogenous and
het-rogeneous catalysis, biological reactions media and removal
ofetal ions [4].Some of the basic physical properties of ILs such
as density
nd viscosity are still being evaluated by the researchers
sincehe study of the IL is a relatively young field [8]. The
numberf research on ILs and their specific applications is
increasingapidly in the literature. For example, the cation
1-n-ethyl-3-ethylimidazolium has been the most widely studied until
2001,
nd nowadays, 1-3-dialkyl imidazolium salts are the most
popu-arly used and investigated class of ILs. For the future of
ILs, theim of research is the commercialization of ILs in order to
usehem as solvents, reagents, catalysts and materials in
large-scalehemical applications.
. Basic properties of ILs
ILs are made of positively and negatively charged ions,hereas
water and organic solvents, such as toluene andichloromethane, are
made of molecules. The structure of ILs is
imilar to the table salt such as sodium chloride which
containsrystals made of positive sodium ions and negative chlorine
ions,ot molecules. While, salts do not melt below 800 C, most of
ILsemain liquid at room temperature. The melting points of
sodium
-
1 ritical Fluids 43 (2007) 150180
crmlhpas
p[sTpahces
o[
mIcbf
iasi
aFti
o
vasa(vtwep
oTctoospocaw
gaiNtIt
itteia[18,19].
52 S. Keskin et al. / J. of Superc
hloride and lithium chloride are known as 801 and 614
C,espectively. Since these conventional molten salts exhibit
highelting points, their use as solvents in applications is
severely
imited. However, RTILs are liquid generally up to 200 C. ILsave
a wide liquidus ranges. The adopted upper melting tem-erature limit
for the classification as IL is known as 100 Cnd higher melting ion
systems are generally referred as moltenalts.
Researchers explained that ILs remain liquid at room tem-erature
due to the reason that their ions do not pack well9]. Combination
of bulky and asymmetrical cations and evenlyhaped anions form a
regular structure namely a liquid phase.he low melting points of
ILs are a result of the chemical com-osition. The combination of
larger asymmetric organic cationnd smaller inorganic counterparts
lower the lattice energy andence the melting point of the resulting
ionic medium. In someases, even the anions are relatively large and
play a role in low-ring the melting point [10]. Most widely used
ILs and theirtructures are given in Table 1.
As solvents, ILs posses several advantages over
conventionalrganic solvents, which make them environmentally
compatible1,4,8,1015]:
ILs have the ability to dissolve many different organic,
inor-ganic and organometallic materials.ILs are highly polar.ILs
consist of loosely coordinating bulky ions.ILs do not evaporate
since they have very low vapor pressures.ILs are thermally stable,
approximately up to 300 C.Most of ILs have a liquid window of up to
200 C whichenables wide kinetic control.ILs have high thermal
conductivity and a large electrochem-ical window.ILs are immiscible
with many organic solvents.ILs are nonaqueous polar alternatives
for phase transfer pro-cesses.The solvent properties of ILs can be
tuned for a specificapplication by varying the anion cation
combinations.
Generally, the above statements are valid for the most com-only
used ILs. However, one should note that there are many
Ls containing different anions and cations and their
propertiesover a vast range. Therefore, the above statements should
note generalized for all existing ILs and for those designed in
theuture.
ILs exhibit the ability to dissolve a wide variety of
materialsncluding salts, fats, proteins, amino acids, surfactants,
sugarsnd polysaccharides. ILs have very powerful solvent
propertiesuch that they can dissolve a wide range of organic
molecules,ncluding crude oil, inks, plastics, and even DNA [9].
Two important groups of ILs are those based on imidazoliumnd
pyridinium cations with PF6 and BF4 anions [13,14].igs. 1 and 2
illustrate the imidazolium and pyridinium deriva-
ives of ILs and their possible anions which are
extensivelynvestigated in literature.
ILs tend not to give off vapors in contrast to traditionalrganic
solvents such as benzene, acetone, and toluene. The
Fig. 1. Imidazolium derivatives of ILs
(www.sigmaaldrich.com).
apor pressures of the ILs are extremely low and are considereds
negligible. For example, Kabo et al. [16] gave the vapor pres-ure
of [bmim][PF6] at 298.15 K as 1011 Pa. ILs are introduceds green
solvents because unlike the volatile organic compoundsVOCs) they
replace, many of these compounds have negligibleapor pressure, they
are not explosive and it may be feasibleo recycle and repeatedly
reuse them. It is more convenient toork with ILs in the laboratory
since the non-evaporating prop-
rties of ILs eliminate the hazardous exposure and air
pollutionroblems.
ILs are also known as designer solvents since they give
thepportunity to tune their specific properties for a particular
need.he researchers can design a specific IL by choosing
negativelyharged small anions and positively charged large cations,
andhese specific ILs may be utilized to dissolve a certain
chemicalr to extract a certain material from a solution. The
fine-tuningf the structure provides tailor-designed properties to
satisfy thepecific application requirements. The physical and
chemicalroperties of ILs are varied by changing the alkyl chain
lengthn the cation and the anion. For example, Huddleston et al.
[17]oncluded that density of ILs increases with a decrease in
thelkyl chain length on the cation and an increase in the
moleculareight of the anion.Although ILs are studied by a great
number of research
roups, there are still many questions that scientist are notble
to answer. For example, one of the basic rules of chem-stry like
dissolves like is seem to be broken by some ILs:onpolar benzene is
up to 50% soluble (by volume) in polar
etrachloroaluminate-based ILs [9]. Therefore, studies on whyLs
are able to dissolve uncharged covalent molecules are
con-inuing.
Until recently, ILs have been considered to be scarce but its
now known that many salts form liquids at or close to
roomemperature. There are literally billions of different
structureshat may form an IL. The composition and the specific
prop-rties of these liquids depend on the type of cation and anionn
the IL structure. By combining various kinds of cation andnion
structures, it is estimated that 1018 ILs can be designed
Fig. 2. Pyridinium derivatives of ILs
(www.sigmaaldrich.com).
http://www.sigmaaldrich.com/http://www.sigmaaldrich.com/
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S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180
153
Table 1Most widely used ILs, their structures and short
names
Ionic liquid Structure Short name
1-Butyl-3-methylimidazolium tetrafluoroborate [bmim][BF4]
1-Butyl-3-methylimidazolium triflate [bmim][TfO]
1-Butyl-3-methylimidazolium methide [bmim][methide]
1-Butyl-3-methylimidazolium dicyanamide [bmim][DCA]
1-Butyl-3-methylimidazolium hexafluorophosphate [bmim][PF6]
1-Butyl-3-methylimidazolium nitrate [bmim][NO3]
1-Butyl-3-methylimidazolium bis(trifluoromethylsulfony1) imide
[bmim][Tf2N]
l-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide;
R = C6H17 [hmim][Tf2N]
l-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide;
R = C8H17 [omim][Tf2N]
2,3-Dimethyl-1-hexylimidazolium bis(trifluoromethylsulfonyl)
imide [hmmim][Tf2N]
R ciety.
3
n
eprinted with permission from [121]. Copyright 2004 American
Chemical So
.1. Solvent properties of ILs
Both the chemical industry and academia search for alter-ative
solvents to meet the cleaner technology requirements
sioa
ince the most widely used solvents are volatile and damag-ng.
ILs are good solvents for a wide range of substances;rganic,
inorganic, organometallic compounds, bio-moleculesnd metal ions.
They are usually composed of poorly coordinat-
-
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vbrcuhmndTcccs[Idladatci
aepiptua
aaaatpsp
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egDtsobvt
54 S. Keskin et al. / J. of Superc
ng ions which makes them highly polar but
non-coordinatingolvents. ILs are immiscible with most of the
organic sol-ents, thus they provide a nonaqueous, polar alternative
forwo-phase systems [19]. Furthermore, ILs which are not mis-ible
with water can be used as immiscible polar phasesith water.
Although all other conventional solvents evap-rate to the
atmosphere, ILs do not evaporate and theironvolatility gives an
opportunity to utilize them in high-acuum systems. The negligible
volatility is the basic propertyhich characterizes them as green
solvents. Considering poten-
ial as solvents, ILs can easily replace other conventionalrganic
solvents which are used in large quantities in chem-cals processing
industries to eliminate major environmentalroblems.
Many chemical reactions are carried out in conventional
sol-ents. Upon the completion of reaction, chemical products muste
taken out of the solvent. There are several techniques toecover a
product from a solvent: For example, water-solubleompounds may be
extracted with water; distillation may besed for chemicals with
high vapor pressures. On the otherand, for the chemicals with low
vapor pressures, distillationust be performed at low pressures,
which may not be eco-
omical. In addition to this, there are some chemicals that
canecompose as a result of heating, such as
pharmaceuticals.herefore, ILs seem to be potentially good solvents
for manyhemical reactions in the cases where distillation is not
practi-al, or water insoluble or thermally sensitive products are
theomponents of a chemical reaction. Although, ILs are not
con-idered to be distilled due to their low volatility, Earle et
al.20] showed that many ionic liquids, especially bistriflamideLs,
can be distilled at 200300 C and low pressure withoutecomposition.
It was once more understood that there is aong way for total
investigation of the properties of ILs. Theuthors suggested that
the possibility of IL distillation intro-uced a new method for IL
purification, and also new applicationreas (such as isolation of
highly soluble products by high-emperature crystallization) could
emerge. But, distillation stillannot be applied when heat-labile
products are encounteredn ILs.
In most chemical applications, extraction is used for sep-ration
since it is an energy efficient technique. Generally,xtraction
consists of two immiscible phases such as an organichase and an
aqueous phase. Many organic solvents usedn extractions are known
with their flammable and toxicalroperties. In order to improve the
safety and environmen-al friendliness of this conventional
technique, ILs may besed as ideal substitutes due to their
stability, nonvolatility anddjustable miscibility and polarity
[15].
The solvent properties of ILs are mainly determined by thebility
of the salt to act as a hydrogen bond donor and/orcceptor and the
degree of localization of the charges on thenions [1,21]. Charge
distribution on the anions, H-bondingbility, polarity, dispersive
interactions are the main factors
hat influence the physical properties of ILs [22]. For exam-le,
imidazolium-based ILs are highly ordered hydrogen-bondedolvents and
they have strong effects on chemical reactions androcesses.
t
sp
l Fluids 43 (2007) 150180
.2. Polarity of ILs
Polarity of chemicals is commonly used to classify the sol-ents.
The terms used as polar, nonpolar and apolar are generallyelated to
the values of dielectric constants, dipole moments,olarizabilities.
If a solvents has the ability to dissolve and sta-ilize dipolar or
charged solutes, it is defined as a polar solvent.nder this simple
definition, ILs are highly polar solvents, but
t is not completely true to make such strict conclusions
sincehere ILs can be designed in a vast range.
The existences of polar and nonpolar domains, believed toe
associated with the unique amphiphilic solvent proper-ies of ILs,
are found in the structures of PF6 and BF4alts [23]. Since polarity
is the simplest indicator of solventtrength, researchers compared
polarities of ILs and conven-ional solvents: Carmichael and Seddon
[24] showed that-alkyl-3-methylimidazolium ILs with anions [PF6],
[BF4],(CF3SO2)2N], and [NO3] are in the same polarity region
as-aminoethanol and lower than alcohols such as methanol,thanol and
butanol. Aki et al. [25] indicated that [bmim][PF6],C8mim][PF6],
[bmim][NO3] and [N-bupy][BF4] are more polarhan acetonitrile and
less polar than methanol and these ILs arexpected to be at least
partially miscible with water. ILs basedn [PF6] anion is preferred
as solvents in most extraction appli-ation to form biphasic systems
due to their immiscibility withater.
.3. Toxicology of ILs
The green character of ILs has been usually related with
theiregligible vapor pressure; however their toxicology data
haveeen very limited until now. Several authors [2629]
alreadyentioned this lack of toxicological data in the literature
[30].lthough ILs will not evaporate and thus will not cause air
pollu-
ion, it does not mean that they will not harm the environment
ifhey enter. Most of ILs are water soluble and they may enter
thequatic environment by accidental spills or effluents. The
mostommonly used ILs [bmim][PF6] and [bmim][BF4] are knowno
decompose in the presence of water and as a result hydroflu-ric and
phosphoric acids are formed [31]. Therefore, bothoxicity and
ecotoxicity information which provide metabolismnd degradability of
ILs are also required to label them as greenolvents or investigate
their environmental impact.
The ecotoxicological studies performed to understand theffects
of different ILs on enzymatic activities, cells and microor-anisms
are utilized to obtain LC50 levels (lethal concentration).ecreasing
LC50 values indicate higher toxicities according to
he toxicity classes of Hodge and Sterner scale (1956) [32].
Thiscale indicates that the LC50 value (in terms of mg/L) of 10r
less shows that the chemical is extremely toxic, LC50 valueetween
10 and 100 shows that chemical is highly toxic, LC50alue between
100 and 1000 shows that chemical is slightlyoxic, and finally LC50
value between 1000 and 10,000 means
hat chemical is practically nontoxic.
The impact of ILs on aquatic ecosystems is highly importantince
some of ILs have a high solubility in water. Maginn [33]rovided the
LC50 levels for two imidazolium-based ILs with
-
S. Keskin et al. / J. of Supercritica
Table 2LC50 values for certain solvent [33]
Compound LC50 (mg/L)
[bmim][PF6] 250300[bmim][BF4] 225275Acetone
30,642Dichloromethane 310Toluene 60313Benzene 203Chlorobenzene
586PAC
DrftIattpodisTtwTsIwtnhcthltatfea
Ii(r
at[nc
TaTosTIb
3
obawistsgcua
oteIsa
haPeh
adatdt
hdto
4
henol 5mmonia 0.534.94hlorine 0.028
aphnia magna, common fresh water crustaceans. Due to theeason
that D. magna are filter feeders at the base of the aquaticood
chain, their responses to ILs are essential to understand howhese
new solvents may impact an environmental ecosystem. AsLs, 1-n-butyl
3-methylimidazolium cation with PF6 and BF6nions are used and the
results are tabulated in Table 2: Thesewo ILs are as toxic to
Daphnia as benzene and even far moreoxic than acetone, but much
less toxic than ammonia, chlorine,henol, etc. Wells and Coombe [34]
also provided the resultsf freshwater ecotoxicity tests of some
common ILs with imi-azolium, ammonium, phosphonium and pyridinium
cations onnvertebrate D. magna and the green alga
Pseudokirchneriellaubcapitata (formerly known as
Selenastrumcapricornutum).he results were reported using medium
effective concentra-
ion (EC50) values. The toxicity values of the most toxic ILere
four orders of magnitude more than the least toxic IL.here was a
relation between the order of toxicity and alkylide chain length of
the cation. For alkyl methylimidazoliumLs with C4 side chain
constituents showed moderate toxicity,hereas the C12, C16, and C18
species were very highly toxic
o both organisms under investigation. Pyridinium, phospho-ium,
and ammonium species with C4 side chain constituentsad also only
moderate toxicity, whereas C6 and longer sidehains showed
significant increases in toxicity. It was shownhat the least toxic
ionic liquids ecotoxicity were comparable toydrocarbons such as
toluene and xylene. The most toxic ioniciquids are many orders of
magnitude more acutely ecotoxichan organic solvents such as
methanol, tert-butyl methyl ether,cetonitrile, and dichloromethane.
The authors also emphasizedhat simple acute ecotoxicity
measurements did not enough toully characterize the full impact of
a solvent released to thenvironment but were only part of the
environmental impactssessment.
There were also some studies on investigation of toxicity ofLs
performed on animals such as the nematode model organ-sm
(Caenorhabditis elegans) [35], freshwater pulmonate snailsPhysa
acuta) [36], Fischer 344 rats [37] and zebra fish (Danioerio)
[38].
One of the most important points that must be taken intoccount
during the toxicological study of the ILs is to pay atten-
ion to the purity of the IL studied. Therefore, different
authors35,39] attached significant importance to proper analyzing
tech-iques [30]. ILs are introduced under the concept of
greenhemistry in all research papers due to their nonvolatile
nature.
cotd
l Fluids 43 (2007) 150180 155
he environmental persistence [34,40] of commonly used ILs,long
with their possible toxicity should be taken into account.here are
still very few results about the (eco)toxicological effectf ILs and
they can be evaluated more satisfactorily as greenolvents or not
after more data on the subject will be provided.he possible toxic
and non-biodegradable nature of the existing
Ls also led to the development of new types of nontoxic
andiodegradable ILs [4046].
.4. Air and moisture stability of ILs
The stability of ILs is crucial for optimum performance. Manyf
ILs are both air and moisture stable, some are even hydropho-ic. On
the other hand, most imidazolium and ammonium saltsre hydrophilic
and if they are used in open vessels, hydrationill certainly occur.
The hydrophobicity of an IL increases with
ncreasing length of the alkyl chain [25]. Despite their
widepread usage, ILs containing PF6 and BF4 have been reportedo
decompose in the presence of water, giving off HF. Wasser-cheid et
al. [47] pointed out that ILs containing halogen anionsenerally
show poor stability in water, and also give off toxic andorrosive
species such as HF or HCl. Therefore, they suggest these of
halogen-free and relatively hydrolysis-stable anions suchs
octylsulfate-compounds.
The degree to which this hydration is a problem dependsn the
application. For instance, small amounts of highly reac-ive species
which are used as catalysts may be deactivated byven very small
amounts of water. For this kind of application,Ls must be handled
under an inert atmosphere. Moreover, theolutes used may be
sensitive for air or moisture, thus an inerttmosphere is required
for the ILsolute systems.
The interaction between water and ILs and their degree
ofydroscopic character are strongly dependent on anions. Themount
of absorbed water is highest in the BF4 and lowest inF6 [48].
However, Tf2N is much more stable in the pres-nce of water as well
as having the advantage of an increasedydrophobic character.
ILs immiscible with water tend to absorb water from
thetmosphere. The infra-red (IR) studies of Cammarata et al.
[31]emonstrated that the water molecules absorbed from the airre
mostly present in the free state, bonded via H-bonding withhe PF6
and BF4 anions. The presence of water may haveramatic effect on IL
reactivity. Since water is present in all ILs,hey are usually
utilized after a moderate drying process.
The new ILs synthesized are more stable than the
oldalogenoaluminate systems. Certain ILs incorporating 1-3-ialkyl
imidazolium cations are generally more resistant thanraditional
solvents under harsh process conditions, such as thoseccurring in
oxidation, photolysis and radiation processes [10].
. Structure and synthesis of ILs
There are a great number of different cation and anion
ombinations to synthesize IL. Different types of ILs give
anpportunity to modify the physical and chemical properties ofhe
IL. The most widely used cations are imidazolium, pyri-inium,
phosphonium and ammonium. The properties of ILs
-
156 S. Keskin et al. / J. of Supercritica
F
aespptiwhoa
4
ibdsn
maicSodwiasahm
uaptouba
cbfmbw(
4
lpdrromc
osoapaiaw
4
sMcaztpwuntptiohbi
ig. 3. Most commonly used cation structures and possible anion
types [50].
re determined by mutual fit of cation and anion, size, geom-try,
and charge distribution. Among the similar class of salts,mall
changes in types of ions influence the physico-chemicalroperties.
The overall properties of ILs result from the com-osite properties
of the cations and anions and include thosehat are superacidic,
basic, hydrophilic, water miscible, watermmiscible and hydrophobic.
Usually, the anion controls theater miscibility, but the cation
also has an influence on theydrophobicity or hydrogen bonding
ability [49]. The structuresf most commonly used cations and some
possible anion typesre tabulated in Fig. 3 [50].
.1. Anions
The properties of ILs are determined by the anion type.
Thentroduction of different anions results in an increasing num-er
of alternative ILs with various properties. There are twoifferent
types of IL anions: ILs containing fluorous anionsuch as PF6, BF4,
CF3SO3, (CF3SO3)2N and ILs withon-fluorous anions such as
AlCl4.
In designing ILs, fluorous anions are usually used. Theost
popular anions consist of chloride, nitrate, acetate, hex-
fluorophosphate and tetrafluoroborate [13]. The most
widelynvestigated ILs are the ones with anions PF6 and BF4.
Espe-ially PF6 is the most prominent anion used in IL research.ince
the anion chemistry has a large effect on the propertiesf IL,
although the cations are the same, there are significantifferences
between ILs with different anions. For example ILith
1-n-butyl-3-methylimidazolium cation and PF6 anion is
mmiscible with water, whereas IL with same cation and BF4nion is
water soluble. This example represents the designerolvent property
of ILs: different ion pairs determine physicalnd chemical
properties of the liquid. By changing the anion theydrophobicity,
viscosity, density and solvation of the IL systemay be changed
[8].Although PF6 and BF4 are the two anion types that are
tilized in most of IL applications, they have an important
dis-dvantage: these two anions may decompose when heated in
theresence of water and liberate HF. After the researchers
realizedhe production of HF in the presence of water, the bonding
style
f anion was altered and fluorous anions inert to hydrolysis
weresed. The fluorine of the anion is bonded to carbon and CF
bondecomes inert to hydrolysis. In this way, ILs such as CF3SO3nd
(CF3SO3)2N are produced [50].
5
p
l Fluids 43 (2007) 150180
Fluorinated anions tend to be expensive and in response toost
and safety concerns new ILs with non-fluorous ions haveeen
introduced. In the synthesis of these ILs, anions are derivedrom
inexpensive bulk chemicals. Alkylsulfate anions are theost popular
non-fluorous anions due to their nontoxic and
iodegradable structures. The first commercially available IL
forhich toxicology data are available contains alkylsulfate
anion
methosulfate) [50].
.2. Cations
The cation of IL is generally a bulk organic structure withow
symmetry. Most ILs are based on ammonium, sulfonium,hosphonium,
imidazolium, pyridinium, picolinium, pyrroli-inium, thiazolium,
oxazolium and pyrazolium cations. Theesearch mainly focuses on
RTILs composed of asymmet-ic N,N-dialkylimidazolium cations
associated with a varietyf anions. 1-n-butyl-3-methylimidazolium
and 1-n-ethyl-3-ethylimidazolium are the most investigated
structures of this
lass.Chiappe and Pieraccini [18] indicated that the melting
points
f the most ILs are uncertain since ILs undergo
considerableupercooling. Therefore, by examining the properties of
a seriesf imidazolium cation based ILs, it has been concluded thats
the size and asymmetry of the cation increases, the meltingoint
decreases. Further, an increase in the branching on thelkyl chain
increases the melting point. The melting point of ILss essential
because it represents the lower limit of the liquiditynd with
thermal stability it defines the interval of temperaturesithin
which it is possible to use ILs as solvents [18].
.3. Synthesis
There are three basic methods to synthesize ILs: metathe-is
reactions, acidbase neutralization, direct combination [1].
any alkylammonium halides are commercially available; theyan
also be prepared simply by the metathesis reaction of theppropriate
halogenoalkane and amine. Pyridinium and imida-olium halides are
also synthesized by metathesis reaction. Onhe other hand,
monoalkyllammonium nitrate salts are best pre-ared by the
neutralization of aqueous solutions of the amineith nitric acid.
After neutralization reactions, ILs are processednder vacuum to
remove the excess water [1]. Tetraalkylammo-ium sulfonates are also
prepared by mixing sufonic acid andetraalkylammonium hydroxide
[51]. In order to obtain pure IL,roducts are dissolved in an
organic solvent such as acetoni-rile and treated with activated
carbon, and the organic solvents removed under vacuum. The final
method for the synthesisf ILs is the direct combination of halide
salt with a metalalide. Halogenoaluminate and chlorocuprate ILs are
preparedy this method. The synthesis methods of ILs have been
givenn numerous articles [5256].
. Major applications suggested for ILs
The research areas on ILs are growing very rapidly and
theotential application areas of ILs are numerous. The unique
-
S. Keskin et al. / J. of Supercritica
caaocffamcaa
5
amAsiausmaida(iscaa
asTh
ihp[wzo
umttaIwb
uaaelAbrr
5
nidbtufa
ga[seaitiwtwoaCellowlw
Fig. 4. Major application areas of ILs.
hemical and physical properties of ILs bring about
severalpplication areas including reaction and synthesis media.
Thepplication areas of ILs can be expressed as solvents for
organic,rganometallic synthesis and catalysis; electrolytes in
electro-hemistry, in fuel and solar cells; lubricants; stationary
phasesor chromatography; matrices for mass spectrometry; supportsor
the immobilization of enzymes; in separation technologies;s liquid
crystals; templates for synthesis nano-materials andaterials for
tissue preservation; in preparation of polymergel
atalytic membranes; in generation of high conductivity materi-ls
[7]. Fig. 4 represents the major applications suggested for ILsnd
these essential applications are discussed in detail below.
.1. Solvent replacement
A majority of common solvents have potential health
hazardslthough they are extensively utilized. For example,
approxi-ately half of 189 hazardous air pollutants regulated by
Cleanir Act Amendment of U.S. (1990) are VOCs including
solvents
uch as dichloromethane [57]. The VOCs are the workhorses
ofndustrial chemistry in the pharmaceutical and petrochemicalreas.
The use of VOCs by these industries can be assessedsing the Sheldon
E-factor. This factor is responsible to mea-ure process by-products
as a proportion of production on theass basis. Researchers
investigated how VOC use is distributed
cross the chemical industry and found that the value of
E-factors between 25 and 100 for pharmaceuticals industries with a
pro-uction of 10 to 103 t/year although oil refining industries
withproduction of 106 to 108 t/year have an E-factor of 0.1 [9]
adapted from [58]). These values suggest that
pharmaceuticalsndustries use inefficient and dirty processes
although on smallercale as compared to the oil refining industries.
The oil and bulkhemicals industries which are commonly regarded as
dirty arepparently remarkably waste conscious when the E-factors
arenalyzed [9].
As the introduction of cleaner technologies has becomemajor
concern throughout both industry and academia, the
earch for the alternative solvents has become a high
priority.herefore, environmentally friendly ILs can easily replace
theazardous VOCs in large scale to reduce E-factors.
ILs are able to dissolve a variety of solutes. They can be
usednstead of traditional solvents in liquidliquid extractions
whereydrophobic molecules such as simple benzene derivatives
willartition to the IL phase. Huddleston et al. [17] showed
that
bmim][PF6] could be used to extract aromatic compounds fromater.
Fadeev and Meagher [59] demonstrated that two imida-
olium ILs with PF6 anion could be used for the extractionf
butanol from aqueous fermentation broths. Selvan et al. [60]
saCi
l Fluids 43 (2007) 150180 157
sed ILs for the extraction of aromatics from
aromatic/alkaneixtures, whereas Letcher et al. [61] used ILs for
the extrac-
ion of alcohols from alcohol/alkane mixtures. Moreover,
binaryemperaturecomposition curves of ILs with alcohols,
alkanes,romatics and water; ternary temperaturecomposition curves
ofLs with alcohols and water; solubilities of some organics andater
in ILs are all investigated by various groups to completelyenefit
from the solvent properties of ILs [6264].
Arce et al. [65] studied essential oil terpenless by
extractionsing organic solvents or ILs. Citrus essential oil is
simulateds a mixture of limonene and linalool and
2-butene-1,4-diolnd ethylene glycol are used as solvents. They
choose 1-thyl-3-methylimidazolium methanesulfonate as the IL
andiquidliquid equilibria data for the ternary systems are
reported.rce et al. [65] concluded that IL presents the highest
selectivityut close to the other organic solvents and they reported
that theesults for solute distribution ratio depend on the
concentrationange of extraction.
.2. Purification of gases
Reliable information on the solubility of gases in ILs iseeded
for the design and operation of any possible processesnvolving IL.
Processes using ILs to purify gas streams wereeveloped after
solubilities of various gases in ILs were reportedy some
researchers [6668]. These experimental studies showhat some gases,
especially CO2 is highly soluble in ILs. The sim-lations performed
explain that the anion of the IL is responsibleor high gas
solubility. With this property ILs, can be replaceds solvents in
reactions involving gaseous species.
Anthony et al. [69] investigated solubility of nine
differentases up to 13 bar: carbon dioxide, ethylene, ethane,
methane,rgon, oxygen, carbon monoxide, hydrogen, and nitrogen
inbmim][PF6]. These gases were chosen for several reasons:
CO2olubility is important due to the possibility of using scCO2
toxtract solutes from ILs; ethylene, hydrogen, carbon monoxide,nd
oxygen are reactants in several types of reactions studiedn IL such
as hydroformylations, hydrogenations, and oxida-ions. Due to the
nonvolatile nature of IL, the gas solubilitiesn IL were measured
using a gravimetric technique, usuallyith a microbalance. The study
of Anthony et al. [69] showed
hat CO2 has the highest solubility and strongest interactionsith
[bmim][PF6], followed by ethylene and ethane. Argon andxygen had
very low solubilities and immeasurably weak inter-ctions. Fig. 5
demonstrates the solubility of various gases (CO2,2H4, C2H6, CH4,
Ar, O2) in [bmim][PF6] at 25 C and at differ-nt pressures. Except
for CO2, all gases remained in the Henrysaw regime up to 13 bar.
However, CO2 showed some non-inearity, indicating some degree of
saturation. Henrys constantsf these gases in various organic
solvents and in [bmim][PF6]ere compared and the results showed that
the gases that are
ess soluble in the IL are less soluble in the other solvents
asell. However, CO2 is more soluble in the IL than in the other
olvents. The relatively high solubility of CO2 was explaineds a
result of its large quadrapole moment. The solubility ofO2 in
[bmim][PF6] at different temperatures is demonstrated
n Fig. 6.
-
158 S. Keskin et al. / J. of Supercritica
Fp
Cm(htsoRetatettae
wso
Fw
ttTgadcaatombdIl
c(mbsetapd
5
tnito
ig. 5. Solubility of various gases in [bmim][PF6] at 25 C.
(Reprinted withermission from [69]. Copyright 2002 American
Chemical Society)
Camper et al. [66] measured the solubility of CO2 and2H4 in
[bmim][PF6], [emim][Tf2N], [emim][CF3SO3] (ethyl-ethylimidazolium
trifluoromethanesulfone), [emim][dca]
ethylmethylimidazolium dicyanamide) and [thtdp][Cl]
(tri-exyltetradecylphosphonium chloride) to demonstrate thathe
regular solution theory can be used to model the gasolubilities in
RTILs at low pressures and studied the effectsn pressure and the
temperature on the solubility of gases inTILs. The previous works;
Blanchard et al. [70] and Anthonyt al. [71] related that the
solubility of the gases in ILs tohe intermolecular interactions
between the anion of the ILnd the gas. On the other hand, Camper et
al. [66] indicatedhat at low pressures, the solubility of CO2 and
C2H4 may bexplained using the regular solution theory without
consideringhe intermolecular interactions between the anion of the
IL andhe gas. At higher pressures, regular solution theory is
limitednd Camper et al. [66] attributed this limitation to the
dominantntropic effects.
Recently, the results of solubility of hydrogen in
[bmim][PF6]
as presented for temperatures from 313 to 373 K and pres-
ures up to 9 MPa. The results demonstrated that the solubilityf
hydrogen in [bmim][PF6] is low and increases slightly with
ig. 6. Solubility of CO2 in [bmim][PF6] at different
temperatures. (Reprintedith permission from [69]. Copyright 2002
American Chemical Society)
awcmsuMois
Salh[cnrfl
l Fluids 43 (2007) 150180
emperature [72]. Since ILs can dissolve certain gaseous
species,hey may be used in conventional gas absorption
applications.he nonvolatility of ILs prevent any cross
contamination of theas stream by the solvent during the process.
Moreover, regener-tion of the solvent may be performed easily by a
simple flash oristillation to remove the gas from the solvent
without any crossontamination. The other advantages of ILs as
separating agentsre no solvent loss and no air pollution.
Currently, researchersre interested in examining the potential of
ILs for the separa-ion of CO2 from flue gases emitted from
fossilfuel combustionperations [73]. ILs may also be utilized as
supported liquidembranes. In conventional membranes, gas dissolves
in liquid
ut then the liquid in which the gas dissolved evaporates
ren-ering the membrane useless [50]. Due to the nonvolatility ofLs,
they can be immobilized on a support and used in supportediquid
membranes.
ILs are also used for storage and delivery of hazardous
spe-ialty gases such as phosphine (PH3), arsine (AsH3) and
stibineSbH3). GASGUARD Sub-Atmospheric Systems supply theajor ion
implant gases: AsH3, boron trifluoride (BF3), enrich
oron trifluoride (11BF3) and PH3 sub-atmospherically [74].
Theystem is combined with gas supply technologies for the deliv-ry
of the gases when needed. In the complexed gas technology,he
desired gases (BF3 and PH3) are chemically bond to ILs
sub-tmospherically, then pulling the vacuum on the ILgas
complexrovides the mechanism to evolve high purity gas, similar
toesorbing a gas from active carbon.
.3. Homogenous and heterogeneous catalysis
One of the most important targets of modern chemistry iso
combine the advantages of both homogenous and heteroge-eous
catalysis [75]. Greater selectivity is generally observedn
homogenous catalysis compared to its heterogeneous coun-erparts,
but separation of the catalyst from the product streamr from the
extract stream causes a problem [8]. ILs offer thedvantages of both
homogenous and heterogeneous catalystsith their two main
characteristics: A selected IL may be immis-
ible with the reactants and products, but on the other hand the
ILay also dissolve the catalysts. ILs combine the advantages of
a
olid for immobilizing the catalyst, and the advantages of a
liq-id for allowing the catalyst to move freely [76]. Brennecke
andaginn [8] indicated that the ionic nature of the IL also gives
an
pportunity to control reaction chemistry, either by
participatingn the reaction or stabilizing the highly polar or
ionic transitiontates.
ILs have an active role in chemical reactions and catalysis.ome
of the examples where ILs are utilized are: reactions ofromatic
rings; clean polymerization [77]; Friedel Crafts alky-ation [78];
reduction of aromatic rings [79]; carbonylation [80];alogenation
[81]; oxidation [82]; nitration [83]; sulfonation84]; solvents for
transition metal catalysis; immobilization ofharged cationic
transition metal catalysis in IL phase without
eed for special ligands [85]; in situ catalysis directly in
ILather than aqueous catalysis followed by extraction of
productsrom solution: this process eliminates washing steps,
minimizesosses of catalysis and enhances purity of the products
[86].
-
ritica
Mv
pigcrai
umartoehhrts
b[oodtItbdtfpa
5
oi[ciiiibtt[
ps
RtsftaBtieiicotp
nnipibIBitdb
ntbciovchibrfcravi
5
T
S. Keskin et al. / J. of Superc
any applications of ILs in catalytic reactions can be found
inarious articles in the literature [1,12,85,8789].
Holbrey and Seddon [90] described many of the catalyticrocesses
which use low temperature ILs as reaction media andndicated that
the classical transition metal catalyzed hydro-enation,
hydroformylation, isomerization, dimerization andoupling reactions
can be performed in IL solvents. In theireview, Holbrey and Seddon
[90] concluded that ILs may be useds effective solvents and
catalysts for clean chemical reactionsnstead of the volatile
organic solvents.
Brennecke and Maginn [8], concluded that ILs have beensed
successfully for hydrogenations, hydroformylations, iso-erizations,
dimerizations, alkylations, Diels-Alder reactions
nd Heck and Suzuki coupling reactions, and in generalesearchers
have concluded that the reaction rates and selec-ivities are as
good or better in ILs than in conventionalrganic solvents. The
catalytic hydrogenation of cyclohex-ne using rhodium-based
homogenous catalysts [91] andydrogenation of olefins using
ruthenium and cobalt-basedomogenous catalyst [92] in various ILs
are studied and theesults indicated that there is a certain
increase in the reac-ion rates and selectivity compared to the
other normal liquidolvents.
Lagrost et al. [12] used immidazolium and ammonium-ased ILs
([emim][NTf2], [bmim][NTf2], [bmim][PF6],(C8H17)3NCH3][NTf2]) as
reaction media for different typesf electrochemical reactions and
investigated the oxidationf organic molecules (anthracene,
naphthalene, durene, 1,4-ithiafulvene and veratrole) in ILs. Their
results suggest thathe nature of investigated mechanisms is almost
unchanged inLs as compared with the conventional organic media
althoughhe structure of molecular solvents and ILs are expected toe
quite different. Lagrost et al. [12] also concluded that
theiffusion coefficient of the organic compounds are about 100imes
smaller than those in conventional media as expectedrom the lower
viscosity of RTILs versus organic solvents. Theositive results of
this study demonstrated that ILs can be useds a new media for
organic electrochemistry [12].
.4. Biological reactions media
ILs are used in biological reactions such as the synthesisf
pharmaceuticals due to the stability of enzymes in ILs, andn
separation processes such as the extraction of amino acids15]. IL
biphasic systems are used to separate many biologi-ally important
molecules such as carbohydrates, organic acidsncluding lactic acid
[93,94]. Carbohydrates are renewable andnexpensive sources of
energy and raw material for the chem-cal industry. The
underivatized carbohydrates are not solublen most of the
conventional solvents although they are solu-le in water. Their
insolubility in most solvents prevents theransformation of
carbohydrates. Therefore, the ability of ILso dissolve
carbohydrates enables transformation possibilities
15,93,95,96].
Lau et al. [95] studied the alcoholysis, ammoniolysis,
anderhydrolysis reactions by Candida antarctica lipase cataly-is
using the [bmim[[PF6] and [bmim[[BF4] as reaction media.
rant
l Fluids 43 (2007) 150180 159
eaction rates were generally comparable with, or better
than,hose observed in organic media. Park and Kazlauskas et al.
[96]tudied the acetylation of 1-phenylethanol catalyzed by
lipaserom Pseudomonas cepacia (PCL) in several ILs and the reac-ion
was as fast and as enantioselective in ILs as in toluene. Theylso
investigated the acetylation of glucose catalyzed by lipase
from C. antarctica (CALB) and found that the transforma-ion was
more regioselective in ionic liquids because glucoses up to one
hundred times more soluble in ionic liquids. Liut al. [93] stated
that carbohydrates are only sparingly solublen common organic
solvents as well as in weakly coordinat-ng ionic liquids, such as
[bmim][BF4]. They found that ILs thatontain the dicyanamide anion
could dissolve approx. 200 g L1f glucose, sucrose, lactose and
cyclodextrin and the esterifica-ion of sucrose with dodecanoic acid
in [bmim][dca] could beerformed with CALB.
Swatloski et al. [97] showed that ILs can also be used
ason-derivatizing solvents for cellulose, the most abundant
biore-ewable material. Cellulose, which is insoluble in water andn
most of the common organic solvents, has many derivitizedroducts in
many applications of the fiber, paper, and polymerndustries. ILs
incorporating anions which are strong hydrogenond acceptors are
most effective solvents for cellulose, whereasLs containing
non-coordinating anions including PF6 andF4 are not effective.
Furthermore, Przybysz et al. [98] exam-
ned the influence of ILs on a cellulose product, paper and
foundhat the wettability of paper is improved, whereas the
strengthecreased as a result of weakening of cellulose
hydrogenonds.
Finally, Pfruender et al. [99] tested the water immiscible
ILsamely; [bmim][PF6], [bmim][Tf2N] and [oma][Tf2N]
(methyl-rioctylammonium bistrifluoromethanesulfonylimide) for
theiriocompatibility towards Escherichia coli and
Saccharomyceserevisiae. The results of this study showed that these
watermmiscible ILs do not damage microbial cells and thereforene
can utilize these water immiscible ILs as substrate reser-oirs and
in situ product extracting agents for biphasic wholeell
biocatalytic processes. Generally, toxic organic solventsave been
used as substrate reservoirs and with this study its shown that
water immiscible ILs may be used as biocompati-le solvents for
microbial biotransformations. The experimentalesults demonstrated
that there is an increase of chemical yieldrom 50 M s1 L1) compared
to thequeous system [99]. Although ILs are known by their
highlyiscous characteristics, good mass transfer rates were
obtainedn their study.
.5. Removing of metal ions
Dai et al. [100] studied the effects of ILs (with PF6 andf2N
anions) on improving the ability of crown ethers to
emove metal ions from aqueous solutions. Strontium
nitrate,fission product for which there is no available extraction
tech-ique for its removal from radioactive waste sites, was used
inhis study.
-
1 ritica
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60 S. Keskin et al. / J. of Superc
Visser et al. [57] designed and synthesized several ILs toemove
cadmium and mercury from contaminated water. Theydrophobic ILs come
into contact with contaminated water andhey snatch the metal ions
out of water. Task-specific ionic liq-ids (TSIL) concept is
introduced in order to synthesize ILs withesired properties to
extract metal ions. Visser et al. [57] pro-uced TSIL cations by
appending different functional groupsnamely thiother, urea and
thiourea) to imidazolium cations.hese IL cations can be considered
as a new IL class, or aovel class of IL extractants. Synthesized
TSIL cations wereombined with PF6 anion and used alone or in a
mixture withbmim][PF6]. The results of the study gave significant
distribu-ion ratios for mercury and cadmium in liquidliquid
separationsnd minimized the reliance on traditional organic
solvents forhis process. Davis [101] gave a detailed analysis and
relatednformation on TSILs.
In traditional solvent extraction technologies, adding
extrac-ants that reside quantitatively in the extracting phase
increaseshe metal ion partitioning to the more hydrophobic phase.
Thedded extractant molecules dehydrate the metal ions and pro-ide a
more hydrophobic environment enabling their transporto the
extracting phase [50].
In TSILs, attaching a metal ion coordinating group directlyo the
imidazolium cation makes the extractant an integral partf the
hydrophobic phase and in this way the chance for ILoss to the
aqueous phase is reduced. Therefore, TSILs act ashe hydrophobic
solvent and the extractant at the same time50]. However, the cost
of TSIL is generally high. In order toliminate this drawback, TSILs
may be added to the mixturesf less expensive ILs. Furthermore,
Davis [101] and Zhao et al.15] stated in their recent reviews that
TSILs are not limited toxtraction processes; they can be also used
as versatile solventsn organic catalysts, solid phase synthesis and
even in productionf liquid Teflon.
The extraction of radioactive metals (lanthanides andctinides)
has particular industrial significance among IL extrac-ion of metal
ions for the handling of nuclear materials [15]. Theehaviors of
uranium species in various ILs were investigatedn early studies
[102105]. Recently, researchers have focusedn the fundamental
understanding of ILs in nuclear chemistryuch as radiochemical
stability of ILs [106].
Recently, Nakashima et al. [107] examined the feasibilityf
extracting of rare earth metals into ILs from aqueous solu-ions and
stripping of metal ions from ILs into an aqueous phasey complexing
agents. They successfully accomplishing toecycle the extracting IL
phase. In this study, octyl(phenyl)-N,N-iisobutylcarbamoylmethyl
phosphine oxide (CMPO) dissolvedn [bmim][PF6] showed an extremely
high extraction abilitynd selectivity of metal ions as compared to
in an ordinaryiluent, n-dodecane. The results of this study
indicate thatLs are a promising medium for actinide and fission
producteparation.
In the literature, various studies were performed to extract
etal ions using ILs [108113]. Different metal ions including
lkali, alkaline earth metals, heavy metals and radioactive
metalsre researched by using different ILs. Generally, the side
chainf the IL on the cation is varied and the effect of structure
of the
3cN2
l Fluids 43 (2007) 150180
L on the extraction efficiency of the metal ions is
investigated.he side chain of the cation influences the hydrophobic
characterf the IL and thus the partition coefficient of the metal
ions isffected.
Visser et al. [108] extracted Na+, Cs+ using [Cnmim][PF6]n = 4,
6, 8); Chun et al. [109] investigated extraction ofther alkali
metals such as Li+, K+, Rb+ using [Cnmim][PF6]n = 49); Luo et al.
[112,113] studied the extraction of Na+,
+, Cs+ ions using [Cnmim][Tf2N] (n = 2, 4, 6, 8). Not onlyhe
alkali metals but also extraction of alkaline earth metalsere
studied by various groups: Visser et al. [108] removed Sr2+
sing [Cnmim][PF6] (n = 4,6,8); Bartsch et al. [110] studied
theemoval of Mg2+, Ca2+, Sr2+, Ba2+; Luo et al. [112,113]
utilizedCnmim][Tf2N] (n = 2, 4, 6, 8) to extract Sr2+. The
extraction ofeavy and radioactive metals such as Cu2+, Ag+, Pb2+,
Zn2+,d2+, Hg2+ were studied by using [Cnmim][PF6] (n = 49) andSILs
[110,111,114].
. Challenges of ILs
The unique properties of ILs and the ability to designheir
properties by choice of anion, cation and substituentsreate many
more processing options, alternative to the onesith conventional
solvents. However, high cost, lack of phys-
cal property and toxicity data restrict the advantageous usef
ILs as process chemicals and processing aids at theresent.
The challenges in the use of ILs must be also addressed asell as
their advantages. The major challenge is the cost. Ailogram of IL
costed about 30,000-fold greater than a commonrganic solvent such
as acetone. Renner [9] reported that this costould be reduced to
approximately 1000-fold greater dependingn the composition of IL
and the scale of production. Wagnernd Uerdingen [115] anticipated
that the price of cation systemsased on imidazole will be in the
range of D 50100 kg1, ifarger quantities of ILs are produced. The
price can be loweredven below D 25 kg1 if ILs are prepared with
cheaper cationources on a ton scale. Another estimation was done by
Wasser-heid and Haumann [116]. They expected that for bulk
ioniciquids choosing proper (relatively cheap) cations and
anionsead to prices approximately D 30 l1 for production rates
ofulti-ton. Moreover, scientists emphasized that the price of
the
Ls may look still discouraging however, the essential factors
the price to performance ratio. If the performance of an ILs
extremely higher than that of the material (solvent) it aimso
replace, less amounts of the IL may be needed for a givenpecific
job [2], thus totally or partially overcoming the
priceisadvantage.
The second problem is associated with the manufacturingethod of
ILs. In manufacturing ILs environmental issues also
eed to be tackled since some VOCs are used to manufactureLs.
Recently, some advanced methods have been developedn the
solventless syntheses of ILs. For example, 1-alkyl-
-methylimidazolium halides have been synthesized in
openontainers in a microwave oven without any VOCs by Varma
andamboodiri at the Environmental Protection Agency of U.S.,001
[9].
-
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S. Keskin et al. / J. of Superc
Researchers need to find alternative ways to recycle ILs due
tohe reason that many processes for cleaning up ILs involve wash-ng
with water or VOCs which creates another waste stream. Thisroblem
has been solved by adopting supercritical extractionechnologies to
recover the dissolved organic compounds fromLs or using membrane
separation processes. However, there arether solid matrixes which
adsorb some part of the ILs. Thus, aecond rinse would be required
and this would create an aque-us waste stream that contains ILs.
Despite this disadvantagehere may be some cleaning applications
where ILs would bettractive [8].
Incomplete physico-chemical data are another challenge forhe
application of ILs. At the present most available data areocused on
bulk physical properties such as viscosity, densitynd phase
transitions. Relatively little is known about the micro-copic
physical properties of ILs. After these properties arenvestigated
properly and the influence of ILs on chemical reac-ion rates is
found, new ILs with precisely tailored propertiesan be
synthesized.
It is extremely important to obtain reliable thermophysicalata
and transport properties of ILs in order to make themvailable for
many applications and to design IL-based pro-esses efficiently.
Harris et al. [117] measured the viscositiesf two members of one of
the most commonly studied ILroups, that are based on imidazolium
cations; [omim][PF6]nd [omim][BF4] between 0 and 80 C and at
pressures to76 MPa ([omim][PF6]) and 224 MPa ([omim][BF4]) with
aalling body viscometer and densities between 0 and 90 Ct
atmospheric pressure. The bulk physical properties of mostidely
used ILs at wider temperature and pressure ranges are
ssential.Another barrier to the large-scale application of ILs
arises
rom their high viscosities. The viscosities of ILs are higherhan
most organic solvents and water, usually similar to viscos-ty of
oils. This high viscosity may be responsible to produce
reduction in the rate of many organic reactions and even
aeduction in the diffusion rates of species. Also, handling of
ILsith high viscosities is difficult however; increasing
tempera-
ure, changing anioncation combinations may yield ILs withower
viscosities. To overcome mass transfer limitations in gas-L systems
resulting from high viscosity reactions using ILs maye run at high
pressures and in efficient gasliquid contactingquipment.
In chemical processing, pharmaceuticals, fine chemicals,etroleum
refining, metal refining, polymer processing, pulp andaper, and
textiles where a nonvolatile liquid with a wide liq-idus range
could work better, ILs are the best choice however,he challenges of
turning ILs into useful and environmentallyenign fluids must be
overcome.
. ILs and scCO2 systems
Green chemistry, also known as sustainable chemistry,
escribes the search for reducing or even eliminating the usef
substances in the production of chemical products and reac-ions
which are hazardous to human health and environment.he goal of
green chemistry is to create a cleaner and more
Ioit
l Fluids 43 (2007) 150180 161
ustainable chemistry and it has received more and more atten-ion
in recent years. Green chemistry searches for
alternative,nvironmentally friendly reaction media as compared to
the tra-itional organic solvents and at the same time aims at
increasedeaction rates, lower reaction temperatures as well
higherelectivities.
The ideal situation for a safe and green chemical processs using
no solvent, however most of the chemical processesepend on
solvents. Some of these solvents are soluble in waternd therefore
they must be stripped from water before it leaveshe process not
only for ecological but also for economic reasons.olvents must be
recovered for recycle and reuse for an econom-
cally viable process. Water, perfluorinated hydrocarbons
andupercritical fluids (SCFs) are alternative solvents which maye
used in green chemistry. Among these, the most promisinglements of
green chemistry are ILs and scCO2.
The low volatility of ILs is the key property that makes
themreen solvents. However, this advantage also causes a problemor
product separation and recovery [15]. Several techniques forolute
recovery from ILs exist: volatile products can be extractedrom IL
by distillation or simply by evaporation. However, non-olatile or
thermo-sensitive products cannot be separated fromLs with these
methods. ILs exhibiting immiscibility with wateran be extracted
with water to separate water-soluble solutesrom IL into the aqueous
phase; but this method is not suitableor hydrophilic ILs [17]. Of
course, organic solvents such as hex-ne and toluene may be
effective to recover the products fromL but this approach obviously
compromises the ultimate goalf green technologies [15].
Furthermore, the cross contami-ation between the phases presents
another problem. Finally,nother green solvent is discovered which
solves all the prob-ems and recovers various kind of solutes from
ILs without crossontamination: supercritical fluids (SCFs).
SCFs are compounds which are above their critical temper-ture
and pressure and they can be manipulated from gas likeo liquid like
densities due to their unusual properties near theritical point.
They are commercially viable solvents in severalpplications such as
dry cleaning and polymer impregnation.cCO2 is the most widely used
SCF as a result of nontoxic andon-flammable characteristics. scCO2
has low critical tempera-ure and pressure and it is not
expensive.
The advantages of using SCFs as extraction medium includeow
cost, nontoxic nature, recoverability and ease of separationrom the
products. SCFs have been adapted for product recoveryrom ILs and
supercritical fluid extraction (SCFE) is shown toe a viable
technique with the additional benefits of environ-ental
sustainability and pure product recovery [118]. Among
he SCFs, an inexpensive and readily available one, scCO2
hasecome a partner of IL and two environmentally benign sol-ents
are utilized together in several applications. The volatilend
nonpolar scCO2 forms different two-phase systems withonvolatile and
polar ILs. The product recovery process withhese systems is based
on the principle that scCO2 is soluble in
Ls, but ILs are not soluble in scCO2 [70]. Since most of
therganic compounds are soluble in scCO2, with the high solubil-ty
of scCO2 in ILs, these products are transferred from the ILo the
supercritical phase.
-
162 S. Keskin et al. / J. of Supercritical Fluids 43 (2007)
150180
Table 3ILgas systems phase behaviors
System Temperature Pressure Findings Reference
[bmim][PF6]CO2 40, 50, and 60 C Up to 93 bar As pressure
increases, solubility of CO2 in the IL increases
[70][C8-mim][PF6]CO2 Solubility of CO2 in IL-rich phase decreases
with temperature[C8-mim][BF4]CO2 CO2 solubility depends on the
nature of the anion and cation[bmim][NO3]CO2 The solubility of CO2
in IL-rich phase is highest for ILs with fluorinated
anions[emim][EtSO4]CO2 The general trend of the phase behavior is
almost identical for all ILs[N-bupy][BF4]CO2
[bmim][PF6]CO2 10, 25, and 50 C Up to 13 bar Water and carbon
dioxide exhibited the strongest interactions and the
[69][bmim][PF6]C2H4 Highest solubilities in [bmim][PF6], followed
by ethylene, ethane, and methane[bmim][PF6]C2H6 Argon and oxygen
both had very low solubilities and essentially no
interactions with the
IL[bmim][PF6]CH4[bmim][PF6]Ar[bmim][PF6]O2[bmim][PF6]CO[bmim][PF6]N2[bmim][PF6]H2
[bmim][PF6]CO2 20, 40, 60, 80,100, and 120 C
Up to 9.7 MPa Total pressure increases linearly with increasing
amount of CO2 in IL [119]
[bmim][BF4]CO2 3070 C Atmospheric pressure CO2 is found to be
one order of magnitude more soluble in the IL than O2
[131][bmim][BF4]O2 The solubility of CO2 in the IL decreases with
temperature
The solubility of O2 in the IL slightly increases with
temperatureSimilar results are obtained in [bmim][PF6]
[bmim][PF6]CO2 25, 40, and 60 C Up to 150 bar Solubility of CO2
in ten different IL is reported [121][bmim][BF4]CO2 The solubility
of CO2 is strongly dependent on the choice of the
anion[bmim][TfO]CO2 Increasing the alkyl chain length, increases
the solubility of CO2 in IL[bmim][NO3]CO2 All of the ILs expand a
relatively small amount when CO2 is
added[bmim][methide]CO2[bmim][DCA]CO2[bmim][Tf2N]CO2[hmim][Tf2N]CO2[omim][Tf2N]CO2[hmmim][Tf2N]CO2
[emim][PF6]CHF3 36.1594.35 C 1.651.6 MPa The solubility of
supercritical CHF3 in [emim][PF6] is very high [142]At low CHF3
concentrations (mole fraction
-
S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150180
163
Table 3 (Continued )
System Temperature Pressure Findings Reference
[bmim][PF6]CO2 4090 C Up to 97 MPa CO2 has good solubilities in
these ILs at lower pressures [13][emim][PF6]CO2 There is a linear
relationship between the alkyl chain length and solubility
of CO2[hmim][PF6]CO2
[hmim][BF4]CO2 2095 C 0.54100 MPa CO2 was found to be more
soluble in [hmim][PF6] than in [hmim][BF4] [133][hmim][PF6]CO2
[bmim][BF4]CO2 5.3295.07 C 0.58767.62 MPa CO2 has a high
solubility in [bmim][BF4] at lower pressures, but the
solubilitydecreases dramatically at higher pressures
[132]
The phase behavior of the system [bmim][BF4]CO2 shows
similarities withthe phase behavior of the system [hmim][BF4]CO2CO2
is more soluble in [hmim][BF4] than in [bmim][BF4]
[omim][BF4]CO2 29.8589.95 C 0.1100 MPa High solubilities of CO2
for low CO2 mole fractions (mole fraction < 0.6) areattained at
relatively low pressure, whereas for mole fraction >0.6, the
pressureneeded for completely dissolving the CO2 increases
drastically
[139]
Increase in temperature slightly decreases the solubility of
CO2The CO2 solubility increases in the IL with increasing chain
length of the alkylgroup
for fof CO
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.1. High-pressure phase behavior of ILCO2 systems
Preliminary works have shown that scCO2 extraction is aiable
method for solute recovery from an IL. However, thenowledge of
phase behavior of ILCO2 systems is an essentialspect of this
methodology. scCO2 dissolution in the IL phase isot only necessary
for contact with the solute but it also reduceshe viscosity of the
IL and therefore enhancing the mass transferrocess.
Early studies of ILCO2 phase behavior indicated that theseystems
are very unusual biphasic systems. No measurablemount of
[bmim][PF6] was soluble in the CO2-rich phase,lthough a large
amount of CO2 dissolved in the IL-rich phase,educing the viscosity
of IL [70]. Blanchard and Brennecke118] concluded that the system
remained as two distinct phasesven under pressures up to 400 bar.
Therefore, high-pressurehase behavior of [bmim][PF6]CO2 is totally
different fromhat of any ordinary organic liquidCO2 systems. This
differenthase behavior is the key phenomena which makes extractionf
solutes from IL with CO2 attractive.
Table 3 summarizes the phase behavior studies performedor ILgas
systems, demonstrates the type of IL and gases usedn these studies,
the experimental conditions, the basic findingsnd the related
references.
.1.1. The [bmim][PF6]CO2 systemThe phase behaviors of ILscCO2
systems are studied very
xtensively in the literature for a better understanding of the
pro-esses involving both IL and scCO2. Since [bmim][PF6] is theost
widely studied IL in the literature, many researchers stud-
ed the high-pressure phase behavior of the [bmim][PF6]CO2ystem
[13,69,70,119122].
Blanchard et al. [70] measured the high-pressureaporliquid phase
behavior of [bmim][PF6]CO2 systemy using two different apparatus
sets: a static high-pressurehase equilibrium apparatus and a
dynamic flow apparatus. In
Cbpi
urther study to investigate the effect of the anion on the
2.when the cation is fixed as [omim]
he static high-pressure vaporliquid equilibrium apparatus, alass
cell was loaded with a known amount of IL sample andnown amounts of
CO2 were metered into the cell while theample within was vigorously
stirred to ensure equilibrium.
ith the assumption of pure CO2 vapor phase, the compositionf the
IL-rich phase was calculated by knowing the amount ofO2 added to
the cell. At the end of the equilibration period,O2 was completely
removed from IL phase upon depressur-
zation. The same group also used a dynamic apparatus, i.e.
aigh-pressure extractor to determine the solubility of the IL inhe
CO2 phase. The detailed description of these apparatus setsnd
experimental procedures can be found in the literature
[70].ifferent experimental set-ups were used by other
researchers:
schematic diagram of a general ILscCO2 experimentalpparatus,
which was used to measure the solubility of CO2n IL ([bmim][PF6])
is given by Kim et al. [123]. Shiflett andokozeki [124] measured
the gas solubility and diffusivityf CO2 in [bmim][PF6] using a
gravimetric microbalance forhich the details of the experimental
set-up is given in the
elated reference.The solubility of CO2 in [bmim][PF6] was
determined at 40,
0 and 60 C and pressures up to 93 bar [70]. As the
pressurencreases, the solubility of CO2 in the IL-rich phase
increasesramatically and the solubility value reaches a mole
fraction of.72 at 40 C and 93 bar. A general rule suggests that an
increasen temperature results with a decrease in the solubility of
gasesn liquids. As expected, the solubility of CO2 in
[bmim][PF6]ich phase decreases with temperature. However, they
noticedhat the temperature dependence of the solubility is quite
smalln this temperature and pressure range. Another crucial points
the effect of large degree of CO2 solubility on the viscosityf IL.
The viscosity of IL decreases when a certain amount of
O2 is dissolved in IL and this effect can easily be observedy
the reduced drag on the stirring magnet when a static high-ressure
vaporliquid equilibrium set-up is used. This reductionn viscosity
of the liquid facilitates the solution process.
-
164 S. Keskin et al. / J. of Supercritical Fluids 43 (2007)
150180
F(S
[pspldro
vtioaBit[i
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wITi[d
bpFfoEv
ig. 7. Total pressure versus molality of gas for [bmim][PF6]CO2
system.Reprinted with permission from [119]. Copyright 2003
American Chemicalociety)
Kamps et al. [119] presented the solubility of CO2 inbmim][PF6]
for temperatures 293393 K in 20 K intervals andressures up to about
9.7 MPa. The total pressure is plotted ver-us the stoichiometric
molality of the gas (number of moleser kilogram of the IL) in Fig.
7. The total pressure increasesinearly with increasing amount of
the gas in IL. The solubilityata represented by Kamps et al. [119]
differ from the previouslyeported solubility data of Blanchard et
al. [70]. The comparisonf experimental data of two studies is given
in Fig. 8.
There are several [bmim][PF6]CO2 high-pressureaporliquid
equilibrium data sets available in the litera-ure. However, these
sets differ from each other considerablyn the values they report
for similar conditions. The reasonf differing solubility data
reported may be due to the smallmounts of water dissolved in the IL
sample used. For example,lanchard et al. [125] presented a
solubility data of CO2
n [bmim][PF6] which is different than the data reported byhe
same group in 2001. In the first study, this group usedbmim][PF6]
which was saturated with water at 22 C, contain-ng 2.3 wt.% water.
In the second study, they used [bmim][PF6]
ig. 8. Comparison of experimental data of Blanchard et al. [70]
(, , and )nd Kamps et al. [119] ( and ) for [bmim][PF6]CO2 system.
(Reprintedith permission from [119]. Copyright 2003 American
Chemical Society)
fo
otwzatw
oddcbstaK
b
ig. 9. [bmim][PF6]CO2 liquid phase compositions for dried and
wet IL sam-les at 40 C. (Reprinted with permission from [70].
Copyright 2001 Americanhemical Society)
hich was dried to approximately 0.15 wt.% water. Drying ofL has
a significant effect on the phase behavior with CO2.hus, Blanchard
et al. [70] showed that the solubility of CO2
n ILs was decreased in the presence of water. In Fig.
9,bmim][PF6]CO2 liquid phase compositions are given forried and wet
IL samples.
In order to observe the effect of water impurity in IL,
phaseehaviors of two IL samples (dry and wet) with CO2 were
com-ared. The effect of water impurity in IL is significant at 57
bar.or dried IL sample, the mole fraction of CO2 is 0.54, whereasor
the wet (water saturated) IL sample it is only 0.13. The effectf
water in IL may be explained by CO2-phobic nature of water.ven at
high pressures, mutual solubilities of water and CO2 areery low
[126]. Another point is the formation of carbonic acidrom the
reaction of CO2 with water that can result in a reductionf the
aqueous phase pH to as little as 2.80 [127].
Rubero and Baldelli [128] investigated gasliquid interfacef
imidazolium ILs using surface-sensitive vibrational spec-roscopy
sum frequency generation. The results indicated thathen the IL is
dry, the cation is oriented with the imida-
olium ring parallel to the surface plane for both hydrophilicnd
hydrophobic ILs. But the cation reorients itself with respecto the
surface for the hydrophobic liquid when water is added,hile the
orientation in the hydrophilic liquid is unaffected.After the
influence of water is noticed, researchers working
n ILCO2 solubility and equilibrium have started to dry andegas
all ILs under vacuum at room temperature for severalays prior to
use. After ILs are dried and degassed, the waterontents are
estimated by the Karl Fischer analysis before solu-ility data are
taken. Measurements show that the most widelytudied IL; [bmim][PF6]
absorbs a couple wt.% water when lefto the atmosphere. The
estimated water content of [bmim][PF6]
fter drying was approximately 0.15 wt.% water as measured byarl
Fischer analysis [70].A number of high and low-pressure
[bmim][PF6]CO2 solu-
ility studies have appeared in the literature. Although
consistent
-
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S. Keskin et al. / J. of Superc
esults have been established for low-pressure solubility data
ofO2 in [bmim][PF6], there are large discrepancies among
high-ressure solubility data of several researchers
[13,70,119121].hese large solubility differences in the literature
are mostrobably due to the differences in the purities of the
ILssed.
Since the behaviors of ILCO2 systems are different fromther
organic liquidCO2 systems, full phase diagrams ofLCO2 systems are
investigated. Blanchard and co-workers125] found two-phase
immiscibility regions with three cloudoint measurements of 1.31,
4.92 and 7.15 mole% IL mix-ures with the balance being CO2.
Although, these experimentsere conducted with water-saturated ILs,
qualitatively a similarehavior is expected with dried samples.
Blanchard et al. [70] gave a qualitative phase behavior
ofbmim][PF6]CO2 system over a wide pressure range. Theyoticed that
the phase behavior where a large miscibility gapxists even at
extremely high pressures. Blanchard et al. [70]eported and referred
that as a complementary work, McHughnd co-workers studied
[bmim][PF6]CO2 phase behavior atigher pressures up to 3100 bar and
found two distinct phasest all conditions. The existence of large
immiscibility gap event very high pressures is not expected for
organic liquidCO2ystems and the existence of two distinct phases is
explainedy the following discussion: At high-pressures density of
pureO2 phase increases but since the liquid phase does not
expand,
he two phases will never become identical and a mixture criti-al
point will never be reached. Therefore, the ILCO2 systememains as
two phases even at very high pressures, although theO2 solubility
is quite high, the mixtures never become a singlehase [70].
Anthony et al. [69] reported the solubilities and Henrysonstants
of different gases (carbon dioxide, ethylene, ethane,ethane, argon,
oxygen, carbon monoxide, hydrogen, and nitro-
en) in [bmim][PF6] and showed that CO2 has the highestolubility
and strong interaction with [bmim][PF6]. The solu-ility data of CO2
in [bmim][PF6] is in good agreement withhe published results of
Blanchard et al. [70] although differentechniques were used in
these studies. Furthermore, Baltus et al.68] reported that Henrys
constants for Kamps et al. [119] datare in reasonable agreement
with those obtained by Anthony etl. [69].
Aki et al. [121] studied the high-pressure phase behavior ofO2
in imidazolium-based ILs and compared the phase behav-
or of the system [bmim][PF6]CO2 with the other solubilityata
present in the literature. The solubility results of CO2
inbmim][PF6] at 25 C measured by Aki et al. [121] agreedemarkably
well with the solubility results of Anthony et al.69] at low
pressures and with the solubility results of Kamps etl. [119]. Aki
et al. [121] investigated the solubility of CO2 inbmim][PF6] at 40
C and compared the results with the previ-us studies of Kamps et
al. [119], Blanchard et al. [70] and Liu etl. [120]. As expected
the agreement between the data points is
ood at low pressures but the discrepancy is obvious at high
pres-ures. Aki et al. [121] explained that in their previous work
[70],hey were not aware of the various impurities and
degradationroducts that were present in the samples they used.
There-
tCro
l Fluids 43 (2007) 150180 165
ore, they attributed the difference between their study [121]nd
the previous study of the same group [70] to the purity ofhe
IL.
At 40 C and at all pressures, there is a very good agree-ent
within the solubility values reported by Aki et al. [121]
nd Liu et al. [120]. However, this statement is not correct
forhe reported solubility data of Aki et al. [121] and Kamps et
al.119]. The results of two studies agree at low pressures, but
atigh pressures, there is a significant difference: At about 43
bar,he solubility of CO2 in [bmim][PF6] was measured as 0.43mole
fraction) by Aki et al. [121], however, at the same pointamps et
al. [119] reported the solubility of CO2 as 0.38. By con-
idering the studies mentioned above, it may thus be concludedhat
the solubility of CO2 in one of the most widely studied
IL,bmim][PF6], varies among different groups in the literature ands
not a good agreement especially at higher pressures.
The solubility of CO2 in [bmim][PF6] was experimen-ally studied
at 298.15 K and up to 1.0 MPa by Kim et al.123]. A group
contribution form of a non-random latticefluidodel (GC-NLF) was
applied to predict solubility of CO2 in
bmim][PF6]. They used the solubility data of Kamps et al.
[119]or a wider pressure range for the group parameter
determina-ion. Comparisons of calculated solubility data with Kamps
etl. data [119] for [bmim][PF6] demonstrated that the methodpplied
is fairly accurate except for regions close to the
criticalonditions of CO2. Kim et al. [123] also compared the
calcu-ated values of solubility of CO2 in different ILs
([emim][PF6],bmim][PF6], and [C6mim][PF6]) with the experimental
sol-bility data reported by other groups for the same ILs.
Thegreements are generally good up to 10 MPa pressure,
however,urther comparisons for higher pressure shows some degree
ofiscrepancy between the calculated solubility data of Kim et
al.123] and the experimental solubility data of Shariati and
Peters129,130].
Finally, Shariati and Peters [13] studied the comparison ofhe
phase behavior of [bmim][PF6]scCO2 system with thether studies
present in the literature. The solubility of CO2n [bmim][PF6] was
determined by measuring the bubble pointressure of the binary
system at different temperatures for sev-ral isopleths and
pressures up to 97 MPa. The solubility dataf CO2 in [bmim][PF6] at
323.15 K was compared with that oflanchard et al. [70] and Anthony
et al. [71] and the results ofhariati and Peters [13] were in a
good agreement with thosef Anthony et al. [71]. The solubility data
taken at 333.15 Kas compared with that of Blanchard et al. [70],
Kamps et
l. [119], Liu et al. [120]. Although the experimental meth-ds
were completely different, there is also a good agreementetween the
results of Shariati and Peters [13] and Kamps etl. [119] at a
temperature of 333.15 K. The solubility data oflanchard et al. [70]
and Liu et al. [120] show greater devi-tions from the data of
Shariati and Peters [13] especially atigher pressures. Shariati and
Peters [13] reported the existencef the three-phase equilibrium
liquidliquidvapor (LLV). As
he other studies demonstrated, this study also indicated thatO2
has a high solubility in [bmim][PF6] and there is a linear
elationship between the alkyl chain length and the solubilityf
CO2.
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