A review of ionic liquids towards supercritical fluid applications

Transcription

J. of Supercritical Fluids 43 (2007) 150–180
Review article
A review of ionic liquids towards supercritical fluid applications
Seda Keskin, Defne Kayrak-Talay, Uğur Akman ∗ , Öner Hortaçsu
Department of Chemical Engineering, Boğaziçi University, Bebek 34342, İstanbul, Turkey
Received 2 August 2006; received in revised form 8 May 2007; accepted 29 May 2007
Abstract
Ionic liquids (ILs), considered to be a relatively recent magical chemical due their unique properties, have a large variety of applications in
all areas of the chemical industries. The areas of application include electrolyte in batteries, lubricants, plasticizers, solvents and catalysis in
synthesis, matrices for mass spectroscopy, solvents to manufacture nano-materials, extraction, gas absorption agents, etc. Non-volatility and nonflammability are their common characteristics giving them an advantageous edge in various applications. This common advantage, when considered
with the possibility of tuning the chemical and physical properties of ILs by changing anion–cation combination is a great opportunity to obtain
task-specific ILs for a multitude of specific applications. There are numerous studies in the related literature concerning the unique properties,
preparation methods, and different applications of ILs in the literature. In this review, a general description of ILs and historical background are
given; basic properties of ILs such as solvent properties, polarity, toxicology, air and moisture stability are discussed; structure of ILs, cation, anion
types and synthesis methods in the related literature are briefly summarized. However, the main focus of this paper is how ILs may be used in the
chemicals processing industries. Thus, the main application areas are searched and the basic applications such as solvent replacement, purification
of gases, homogenous and heterogeneous catalysis, biological reactions media and removal of metal ions are discussed in detail. Not only the
advantages of ILs but also the essential challenges and potentials for using ILs in the chemical industries are also addressed. ILs have become
the partner of scCO2 in many applications and most of the reported studies in the literature focus on the interaction of these two green solvents,
i.e. ILs and scCO2 . The chemistry of the ILs has been reviewed in numerous papers earlier. Therefore, the major purpose of this review paper is
to provide an overview for the specific chemical and physical properties of ILs and to investigate IL–scCO2 systems in some detail. Recovery of
solutes from ILs with CO2 , separation of ILs from organic solvents by CO2 , high-pressure phase behavior of IL–scCO2 systems, solubility of ILs
in CO2 phase, and the interaction of the IL–scCO2 system at molecular level are also included.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Ionic liquids; Supercritical carbon dioxide; Review
Contents
1.
2.
3.
4.
5.
∗
General description of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
History of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic properties of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.1. Solvent properties of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2. Polarity of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3. Toxicology of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.4. Air and moisture stability of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Structure and synthesis of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.1. Anions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.2. Cations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
4.3. Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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).
0896-8446/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.supflu.2007.05.013
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S. Keskin et al. / J. of Supercritical Fluids 43 (2007) 150–180
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5.2. Purification of gases . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.3. Homogenous and heterogeneous catalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.4. Biological reactions media . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
5.5. Removing of metal ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Challenges of ILs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ILs and scCO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1. High-pressure phase behavior of IL–CO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.1. The [bmim][PF6 ]–CO2 system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.1.2. Other IL–CO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.2. IL solubility in CO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.3. IL–CO2 interaction at the molecular level. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.4. Solute recovery from ILs with scCO2 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
7.5. Other applications of IL–scCO2 systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. General description of ILs
Ionic liquids (ILs) have been accepted as a new green chemical revolution which excited both the academia and the chemical
industries. This new chemical group can reduce the use of
hazardous and polluting organic solvents due to their unique
characteristics as well as taking part in various new syntheses.
The terms room temperature ionic liquid (RTIL), nonaqueous
ionic liquid, molten salt, liquid organic salt and fused salt have
all been used to describe these salts in the liquid phase [1]. ILs
are known as salts that are liquid at room temperature in contrast to high-temperature molten salts. They have a unique array
of physico-chemical properties which make them suitable in
numerous applications in which conventional organic solvents
are not sufficiently effective or not applicable. Short [2] pointed
out in 1980, that there were only a few patent applications for
ILs, in 2000, the number of patent applications increased to 100,
and finally by 2004, there were more than 800. This is a clear
indication of the high affinity of the academia and industry to
the ILs.
2. History of ILs
ILs have been known for a long time, but their extensive use
as solvents in chemical processes for synthesis and catalysis has
recently become significant. Welton [1] reported that ILs are
not new, and some of the ILs such as [EtNH3 ][NO3 ] was first
described in 1914 [3]. The earliest IL in the literature was created
intentionally in 1970s for nuclear warheads batteries [4]. During
1940s, aluminum chloride-based molten salts were utilized for
electroplating at temperatures of hundreds of degrees Celsius.
In the early 1970s, Wilkes tried to develop better batteries for
nuclear warheads and space probes which required molten salts
to operate [4]. These molten salts were hot enough to damage
the nearby materials. Therefore, the chemists searched for salts
which remain liquid at lower temperatures and eventually they
identified one which is liquid at room temperature. Wilkes and
his colleagues continued to improve their ILs for use as battery
electrolytes and then a small community of researchers began
to make ILs and test their properties [5,6]. In the late 1990s, ILs
became one of the most promising chemicals as solvents.
The first ILs, such as organo-aluminate ILs, have limited range of applications because they were unstable
to air and water. Furthermore, these ILs were not inert
towards various organic compounds [7]. After the first
reports on the synthesis and applications of air stable
ILs such as 1-n-butyl-3-methlyimidazolium tetrafluoroborate
([bmim][BF4 ]) and 1-n-butyl-3-methlyimidazolium hexafluorophosphate ([bmim][PF6 ]), the number of air and water stable
ILs has started to increase rapidly [7]. Recently, researchers have
discovered that ILs are more than just green solvents and they
have found several applications such as replacing them with
volatile organic solvents, making new materials, conducting heat
effectively, supporting enzyme-catalyzed reactions, hosting a
variety of catalysts, purification of gases, homogenous and heterogeneous catalysis, biological reactions media and removal of
metal ions [4].
Some of the basic physical properties of ILs such as density
and viscosity are still being evaluated by the researchers since
the study of the IL is a relatively young field [8]. The number
of research on ILs and their specific applications is increasing
rapidly in the literature. For example, the cation 1-n-ethyl-3methylimidazolium has been the most widely studied until 2001,
and nowadays, 1-3-dialkyl imidazolium salts are the most popularly used and investigated class of ILs. For the future of ILs, the
aim of research is the commercialization of ILs in order to use
them as solvents, reagents, catalysts and materials in large-scale
chemical applications.
3. Basic properties of ILs
ILs are made of positively and negatively charged ions,
whereas water and organic solvents, such as toluene and
dichloromethane, are made of molecules. The structure of ILs is
similar to the table salt such as sodium chloride which contains
crystals made of positive sodium ions and negative chlorine ions,
not molecules. While, salts do not melt below 800 ◦ C, most of ILs
remain liquid at room temperature. The melting points of sodium
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chloride and lithium chloride are known as 801 and 614 ◦ C,
respectively. Since these conventional molten salts exhibit high
melting points, their use as solvents in applications is severely
limited. However, RTILs are liquid generally up to 200 ◦ C. ILs
have a wide liquidus ranges. The adopted upper melting temperature limit for the classification as ‘IL’ is known as 100 ◦ C
and higher melting ion systems are generally referred as molten
salts.
Researchers explained that ILs remain liquid at room temperature due to the reason that their ions do not pack well
[9]. Combination of bulky and asymmetrical cations and evenly
shaped anions form a regular structure namely a liquid phase.
The low melting points of ILs are a result of the chemical composition. The combination of larger asymmetric organic cation
and smaller inorganic counterparts lower the lattice energy and
hence the melting point of the resulting ionic medium. In some
cases, even the anions are relatively large and play a role in lowering the melting point [10]. Most widely used ILs and their
structures are given in Table 1.
As solvents, ILs posses several advantages over conventional
organic solvents, which make them environmentally compatible
[1,4,8,10–15]:
• ILs have the ability to dissolve many different organic, inorganic 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 which
enables wide kinetic control.
• ILs have high thermal conductivity and a large electrochemical window.
• ILs are immiscible with many organic solvents.
• ILs are nonaqueous polar alternatives for phase transfer processes.
• The solvent properties of ILs can be tuned for a specific
application by varying the anion cation combinations.
Generally, the above statements are valid for the most commonly used ILs. However, one should note that there are many
ILs containing different anions and cations and their properties
cover a vast range. Therefore, the above statements should not
be generalized for all existing ILs and for those designed in the
future.
ILs exhibit the ability to dissolve a wide variety of materials
including salts, fats, proteins, amino acids, surfactants, sugars
and polysaccharides. ILs have very powerful solvent properties
such that they can dissolve a wide range of organic molecules,
including crude oil, inks, plastics, and even DNA [9].
Two important groups of ILs are those based on imidazolium
and pyridinium cations with PF6 − and BF4 − anions [13,14].
Figs. 1 and 2 illustrate the imidazolium and pyridinium derivatives of ILs and their possible anions which are extensively
investigated in literature.
ILs tend not to give off vapors in contrast to traditional
organic solvents such as benzene, acetone, and toluene. The
Fig. 1. Imidazolium derivatives of ILs (www.sigmaaldrich.com).
vapor pressures of the ILs are extremely low and are considered
as negligible. For example, Kabo et al. [16] gave the vapor pressure of [bmim][PF6 ] at 298.15 K as 10−11 Pa. ILs are introduced
as green solvents because unlike the volatile organic compounds
(VOCs) they replace, many of these compounds have negligible
vapor pressure, they are not explosive and it may be feasible
to recycle and repeatedly reuse them. It is more convenient to
work with ILs in the laboratory since the non-evaporating properties of ILs eliminate the hazardous exposure and air pollution
problems.
ILs are also known as ‘designer solvents’ since they give the
opportunity to tune their specific properties for a particular need.
The researchers can design a specific IL by choosing negatively
charged small anions and positively charged large cations, and
these specific ILs may be utilized to dissolve a certain chemical
or to extract a certain material from a solution. The fine-tuning
of the structure provides tailor-designed properties to satisfy the
specific application requirements. The physical and chemical
properties of ILs are varied by changing the alkyl chain length
on the cation and the anion. For example, Huddleston et al. [17]
concluded that density of ILs increases with a decrease in the
alkyl chain length on the cation and an increase in the molecular
weight of the anion.
Although ILs are studied by a great number of research
groups, there are still many questions that scientist are not
able to answer. For example, one of the basic rules of chemistry “like dissolves like” is seem to be broken by some ILs:
Nonpolar benzene is up to 50% soluble (by volume) in polar
tetrachloroaluminate-based ILs [9]. Therefore, studies on why
ILs are able to dissolve uncharged covalent molecules are continuing.
Until recently, ILs have been considered to be scarce but it
is now known that many salts form liquids at or close to room
temperature. There are literally billions of different structures
that may form an IL. The composition and the specific properties of these liquids depend on the type of cation and anion
in the IL structure. By combining various kinds of cation and
anion structures, it is estimated that 1018 ILs can be designed
[18,19].
Fig. 2. Pyridinium derivatives of ILs (www.sigmaaldrich.com).
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Table 1
Most 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][Tf2 N]
l-Hexyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide; R = C6 H17
[hmim][Tf2 N]
l-Octyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide; R = C8 H17
[omim][Tf2 N]
2,3-Dimethyl-1-hexylimidazolium bis(trifluoromethylsulfonyl) imide
[hmmim][Tf2 N]
Reprinted with permission from [121]. Copyright 2004 American Chemical Society.
3.1. Solvent properties of ILs
Both the chemical industry and academia search for alternative solvents to meet the cleaner technology requirements
since the most widely used solvents are volatile and damaging. ILs are good solvents for a wide range of substances;
organic, inorganic, organometallic compounds, bio-molecules
and metal ions. They are usually composed of poorly coordinat-
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ing ions which makes them highly polar but non-coordinating
solvents. ILs are immiscible with most of the organic solvents, thus they provide a nonaqueous, polar alternative for
two-phase systems [19]. Furthermore, 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 highvacuum systems. The negligible volatility is the basic property
which characterizes them as green solvents. Considering potential as solvents, ILs can easily replace other conventional
organic solvents which are used in large quantities in chemicals processing industries to eliminate major environmental
problems.
Many chemical reactions are carried out in conventional solvents. Upon the completion of reaction, chemical products must
be taken out of the solvent. There are several techniques to
recover a product from a solvent: For example, water-soluble
compounds may be extracted with water; distillation may be
used for chemicals with high vapor pressures. On the other
hand, for the chemicals with low vapor pressures, distillation
must be performed at low pressures, which may not be economical. In addition to this, there are some chemicals that can
decompose as a result of heating, such as pharmaceuticals.
Therefore, ILs seem to be potentially good solvents for many
chemical reactions in the cases where distillation is not practical, or water insoluble or thermally sensitive products are the
components of a chemical reaction. Although, ILs are not considered to be distilled due to their low volatility, Earle et al.
[20] showed that many ionic liquids, especially bistriflamide
ILs, can be distilled at 200–300 ◦ C and low pressure without
decomposition. It was once more understood that there is a
long way for total investigation of the properties of ILs. The
authors suggested that the possibility of IL distillation introduced a new method for IL purification, and also new application
areas (such as isolation of highly soluble products by hightemperature crystallization) could emerge. But, distillation still
cannot be applied when heat-labile products are encountered
in ILs.
In most chemical applications, extraction is used for separation since it is an energy efficient technique. Generally,
extraction consists of two immiscible phases such as an organic
phase and an aqueous phase. Many organic solvents used
in extractions are known with their flammable and toxical
properties. In order to improve the safety and environmental friendliness of this conventional technique, ILs may be
used as ideal substitutes due to their stability, nonvolatility and
adjustable miscibility and polarity [15].
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 [1,21]. Charge distribution on the anions, H-bonding
ability, polarity, dispersive interactions are the main factors
that influence the physical properties of ILs [22]. For example, imidazolium-based ILs are highly ordered hydrogen-bonded
solvents and they have strong effects on chemical reactions and
processes.
3.2. Polarity of ILs
Polarity of chemicals is commonly used to classify the solvents. The terms used as polar, nonpolar and apolar are generally
related to the values of dielectric constants, dipole moments,
polarizabilities. If a solvents has the ability to dissolve and stabilize dipolar or charged solutes, it is defined as a polar solvent.
Under this simple definition, ILs are highly polar solvents, but
it is not completely true to make such strict conclusions since
there ILs can be designed in a vast range.
The existences of polar and nonpolar domains, believed to
be associated with the unique “amphiphilic” solvent properties of ILs, are found in the structures of PF6 − and BF4 −
salts [23]. Since polarity is the simplest indicator of solvent
strength, researchers compared polarities of ILs and conventional solvents: Carmichael and Seddon [24] showed that
1-alkyl-3-methylimidazolium ILs with anions [PF6 ], [BF4 ],
[(CF3 SO2 )2 N], and [NO3 ] are in the same polarity region as
2-aminoethanol and lower than alcohols such as methanol,
ethanol and butanol. Aki et al. [25] indicated that [bmim][PF6 ],
[C8 mim][PF6 ], [bmim][NO3 ] and [N-bupy][BF4 ] are more polar
than acetonitrile and less polar than methanol and these ILs are
expected to be at least partially miscible with water. ILs based
on [PF6 ] anion is preferred as solvents in most extraction application to form biphasic systems due to their immiscibility with
water.
3.3. Toxicology of ILs
The green character of ILs has been usually related with their
negligible vapor pressure; however their toxicology data have
been very limited until now. Several authors [26–29] already
mentioned this lack of toxicological data in the literature [30].
Although ILs will not evaporate and thus will not cause air pollution, it does not mean that they will not harm the environment if
they enter. Most of ILs are water soluble and they may enter the
aquatic environment by accidental spills or effluents. The most
commonly used ILs [bmim][PF6 ] and [bmim][BF4 ] are known
to decompose in the presence of water and as a result hydrofluoric and phosphoric acids are formed [31]. Therefore, both
toxicity and ecotoxicity information which provide metabolism
and degradability of ILs are also required to label them as green
solvents or investigate their environmental impact.
The ecotoxicological studies performed to understand the
effects of different ILs on enzymatic activities, cells and microorganisms are utilized to obtain LC50 levels (lethal concentration).
Decreasing LC50 values indicate higher toxicities according to
the toxicity classes of Hodge and Sterner scale (1956) [32]. This
scale indicates that the LC50 value (in terms of mg/L) of 10
or less shows that the chemical is extremely toxic, LC50 value
between 10 and 100 shows that chemical is highly toxic, LC50
value between 100 and 1000 shows that chemical is slightly
toxic, and finally LC50 value between 1000 and 10,000 means
that chemical is practically nontoxic.
The impact of ILs on aquatic ecosystems is highly important
since some of ILs have a high solubility in water. Maginn [33]
provided the LC50 levels for two imidazolium-based ILs with
Table 2
LC50 values for certain solvent [33]
Compound
LC50 (mg/L)
[bmim][PF6 ]
[bmim][BF4 ]
Acetone
Dichloromethane
Toluene
Benzene
Chlorobenzene
Phenol
Ammonia
Chlorine
250–300
225–275
30,642
310
60–313
203
5–86
5
0.53–4.94
0.028
Daphnia magna, common fresh water crustaceans. Due to the
reason that D. magna are filter feeders at the base of the aquatic
food chain, their responses to ILs are essential to understand how
these new solvents may impact an environmental ecosystem. As
ILs, 1-n-butyl 3-methylimidazolium cation with PF6 − and BF6 −
anions are used and the results are tabulated in Table 2: These
two ILs are as toxic to Daphnia as benzene and even far more
toxic than acetone, but much less toxic than ammonia, chlorine,
phenol, etc. Wells and Coombe [34] also provided the results
of freshwater ecotoxicity tests of some common ILs with imidazolium, ammonium, phosphonium and pyridinium cations on
invertebrate D. magna and the green alga Pseudokirchneriella
subcapitata (formerly known as Selenastrumcapricornutum).
The results were reported using medium effective concentration (EC50 ) values. The toxicity values of the most toxic IL
were four orders of magnitude more than the least toxic IL.
There was a relation between the order of toxicity and alkyl
side chain length of the cation. For alkyl methylimidazolium
ILs with C4 side chain constituents showed moderate toxicity,
whereas the C12 , C16 , and C18 species were very highly toxic
to both organisms under investigation. Pyridinium, phosphonium, and ammonium species with C4 side chain constituents
had also only moderate toxicity, whereas C6 and longer side
chains showed significant increases in toxicity. It was shown
that the least toxic ionic liquids’ ecotoxicity were comparable to
hydrocarbons’ such as toluene and xylene. The most toxic ionic
liquids are many orders of magnitude more acutely ecotoxic
than organic solvents such as methanol, tert-butyl methyl ether,
acetonitrile, and dichloromethane. The authors also emphasized
that simple acute ecotoxicity measurements did not enough to
fully characterize the full impact of a solvent released to the
environment but were only part of the environmental impact
assessment.
There were also some studies on investigation of toxicity of
ILs performed on animals such as the nematode model organism (Caenorhabditis elegans) [35], freshwater pulmonate snails
(Physa acuta) [36], Fischer 344 rats [37] and zebra fish (Danio
rerio) [38].
One of the most important points that must be taken into
account during the toxicological study of the ILs is to pay attention to the purity of the IL studied. Therefore, different authors
[35,39] attached significant importance to proper analyzing techniques [30]. ILs are introduced under the concept of green
chemistry in all research papers due to their nonvolatile nature.
155
The environmental persistence [34,40] of commonly used ILs,
along with their possible toxicity should be taken into account.
There are still very few results about the (eco)toxicological effect
of ILs and they can be evaluated more satisfactorily as green
solvents or not after more data on the subject will be provided.
The possible toxic and non-biodegradable nature of the existing
ILs also led to the development of new types of nontoxic and
biodegradable ILs [40–46].
3.4. Air and moisture stability of ILs
The stability of ILs is crucial for optimum performance. Many
of ILs are both air and moisture stable, some are even hydrophobic. On the other hand, most imidazolium and ammonium salts
are hydrophilic and if they are used in open vessels, hydration
will certainly occur. The hydrophobicity of an IL increases with
increasing length of the alkyl chain [25]. Despite their wide
spread usage, ILs containing PF6 − and BF4 − have been reported
to decompose in the presence of water, giving off HF. Wasserscheid et al. [47] pointed out that ILs containing halogen anions
generally show poor stability in water, and also give off 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 degree to which this hydration is a problem depends
on the application. For instance, small amounts of highly reactive species which are used as catalysts may be deactivated by
even very small amounts of water. For this kind of application,
ILs must be handled under an inert atmosphere. Moreover, the
solutes used may be sensitive for air or moisture, thus an inert
atmosphere is required for the IL–solute systems.
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 BF4 − and lowest in
PF6 − [48]. However, Tf2 N− is much more stable in the presence of water as well as having the advantage of an increased
hydrophobic character.
ILs immiscible with water tend to absorb water from the
atmosphere. The infra-red (IR) studies of Cammarata et al. [31]
demonstrated that the water molecules absorbed from the air
are mostly present in the free state, bonded via H-bonding with
the PF6 − and BF4 − anions. The presence of water may have
dramatic effect on IL reactivity. Since water is present in all ILs,
they are usually utilized after a moderate drying process.
The new ILs synthesized are more stable than the old
halogenoaluminate systems. Certain ILs incorporating 1-3dialkyl imidazolium cations are generally more resistant than
traditional solvents under harsh process conditions, such as those
occurring in oxidation, photolysis and radiation processes [10].
4. Structure and synthesis of ILs
There are a great number of different cation and anion
combinations to synthesize IL. Different types of ILs give an
opportunity to modify the physical and chemical properties of
the IL. The most widely used cations are imidazolium, pyridinium, phosphonium and ammonium. The properties of ILs
156
Fluorinated anions tend to be expensive and in response to
cost and safety concerns new ILs with non-fluorous ions have
been introduced. In the synthesis of these ILs, anions are derived
from inexpensive bulk chemicals. Alkylsulfate anions are the
most popular non-fluorous anions due to their nontoxic and
biodegradable structures. The first commercially available IL for
which toxicology data are available contains alkylsulfate anion
(methosulfate) [50].
4.2. Cations
Fig. 3. Most commonly used cation structures and possible anion types [50].
are determined by mutual fit of cation and anion, size, geometry, and charge distribution. Among the similar class of salts,
small changes in types of ions influence the physico-chemical
properties. The overall properties of ILs result from the composite properties of the cations and anions and include those
that are superacidic, basic, hydrophilic, water miscible, water
immiscible and hydrophobic. Usually, the anion controls the
water miscibility, but the cation also has an influence on the
hydrophobicity or hydrogen bonding ability [49]. The structures
of most commonly used cations and some possible anion types
are tabulated in Fig. 3 [50].
4.1. Anions
The properties of ILs are determined by the anion type. The
introduction of different anions results in an increasing number of alternative ILs with various properties. There are two
different types of IL anions: ILs containing fluorous anions
such as PF6 − , BF4 − , CF3 SO3 − , (CF3 SO3 )2 N− and ILs with
non-fluorous anions such as AlCl4 − .
In designing ILs, fluorous anions are usually used. The
most popular anions consist of chloride, nitrate, acetate, hexafluorophosphate and tetrafluoroborate [13]. The most widely
investigated ILs are the ones with anions PF6 − and BF4 − . Especially PF6 − is the most prominent anion used in IL research.
Since the anion chemistry has a large effect on the properties
of IL, although the cations are the same, there are significant
differences between ILs with different anions. For example IL
with 1-n-butyl-3-methylimidazolium cation and PF6 − anion is
immiscible with water, whereas IL with same cation and BF4 −
anion is water soluble. This example represents the ‘designer
solvent’ property of ILs: different ion pairs determine physical
and chemical properties of the liquid. By changing the anion the
hydrophobicity, viscosity, density and solvation of the IL system
may be changed [8].
Although PF6 − and BF4 − are the two anion types that are
utilized in most of IL applications, they have an important disadvantage: these two anions may decompose when heated in the
presence of water and liberate HF. After the researchers realized
the production of HF in the presence of water, the bonding style
of anion was altered and fluorous anions inert to hydrolysis were
used. The fluorine of the anion is bonded to carbon and C–F bond
becomes inert to hydrolysis. In this way, ILs such as CF3 SO3 −
and (CF3 SO3 )2 N− are produced [50].
The cation of IL is generally a bulk organic structure with
low symmetry. Most ILs are based on ammonium, sulfonium,
phosphonium, imidazolium, pyridinium, picolinium, pyrrolidinium, thiazolium, oxazolium and pyrazolium cations. The
research mainly focuses on RTILs composed of asymmetric N,N-dialkylimidazolium cations associated with a variety
of anions. 1-n-butyl-3-methylimidazolium and 1-n-ethyl-3methylimidazolium are the most investigated structures of this
class.
Chiappe and Pieraccini [18] indicated that the melting points
of the most ILs are uncertain since ILs undergo considerable
supercooling. Therefore, by examining 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. The melting point of ILs
is essential because it represents the lower limit of the liquidity
and with thermal stability it defines the interval of temperatures
within which it is possible to use ILs as solvents [18].
4.3. Synthesis
There are three basic methods to synthesize ILs: metathesis reactions, acid–base neutralization, direct combination [1].
Many alkylammonium halides are commercially available; they
can also be prepared 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, monoalkyllammonium 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 [1]. Tetraalkylammonium sulfonates are also prepared by mixing sufonic acid and
tetraalkylammonium hydroxide [51]. In order to obtain pure IL,
products are dissolved in an organic solvent such as acetonitrile and treated with activated carbon, and the organic solvent
is removed under vacuum. The final 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 [52–56].
5. Major applications suggested for ILs
The research areas on ILs are growing very rapidly and the
potential application areas of ILs are numerous. The unique
Fig. 4. Major application areas of ILs.
chemical and physical properties of ILs bring about several
application areas including reaction and synthesis media. The
application areas of ILs can be expressed as solvents for organic,
organometallic synthesis and catalysis; electrolytes in electrochemistry, in fuel and solar cells; lubricants; stationary phases
for chromatography; matrices for mass spectrometry; supports
for the immobilization of enzymes; in separation technologies;
as liquid crystals; templates for synthesis nano-materials and
materials for tissue preservation; in preparation of polymer–gel
catalytic membranes; in generation of high conductivity materials [7]. Fig. 4 represents the major applications suggested for ILs
and these essential applications are discussed in detail below.
5.1. Solvent replacement
A majority of common solvents have potential health hazards
although they are extensively utilized. For example, approximately half of 189 hazardous air pollutants regulated by Clean
Air Act Amendment of U.S. (1990) are VOCs including solvents
such as dichloromethane [57]. The VOCs are the workhorses of
industrial chemistry in the pharmaceutical and petrochemical
areas. The use of VOCs by these industries can be assessed
using the Sheldon E-factor. This factor is responsible to measure process by-products as a proportion of production on the
mass basis. Researchers investigated how VOC use is distributed
across the chemical industry and found that the value of E-factor
is between 25 and 100 for pharmaceuticals industries with a production of 10 to 103 t/year although oil refining industries with
a production of 106 to 108 t/year have an E-factor of 0.1 [9]
(adapted from [58]). These values suggest that pharmaceuticals
industries use inefficient and dirty processes although on smaller
scale as compared to the oil refining industries. The oil and bulk
chemicals industries which are commonly regarded as dirty are
apparently remarkably waste conscious when the E-factors are
analyzed [9].
As the introduction of cleaner technologies has become
a major concern throughout both industry and academia, the
search for the alternative solvents has become a high priority.
Therefore, environmentally friendly ILs can easily replace the
hazardous VOCs in large scale to reduce E-factors.
ILs are able to dissolve a variety of solutes. They can be used
instead of traditional solvents in liquid–liquid extractions where
hydrophobic molecules such as simple benzene derivatives will
partition to the IL phase. Huddleston et al. [17] showed that
[bmim][PF6 ] could be used to extract aromatic compounds from
water. Fadeev and Meagher [59] demonstrated that two imidazolium ILs with PF6 − anion could be used for the extraction
of butanol from aqueous fermentation broths. Selvan et al. [60]
157
used ILs for the extraction of aromatics from aromatic/alkane
mixtures, whereas Letcher et al. [61] used ILs for the extraction of alcohols from alcohol/alkane mixtures. Moreover, binary
temperature–composition curves of ILs with alcohols, alkanes,
aromatics and water; ternary temperature–composition curves of
ILs with alcohols and water; solubilities of some organics and
water in ILs are all investigated by various groups to completely
benefit from the solvent properties of ILs [62–64].
Arce et al. [65] studied essential oil terpenless by extraction
using organic solvents or ILs. Citrus essential oil is simulated
as a mixture of limonene and linalool and 2-butene-1,4-diol
and ethylene glycol are used as solvents. They choose 1ethyl-3-methylimidazolium methanesulfonate as the IL and
liquid–liquid equilibria data for the ternary systems are reported.
Arce et al. [65] concluded that IL presents the highest selectivity
but close to the other organic solvents and they reported that the
results for solute distribution ratio depend on the concentration
range of extraction.
5.2. Puriﬁcation of gases
Reliable information on the solubility of gases in ILs is
needed for the design and operation of any possible processes
involving IL. Processes using ILs to purify gas streams were
developed after solubilities of various gases in ILs were reported
by some researchers [66–68]. These experimental studies show
that some gases, especially CO2 is highly soluble in ILs. The simulations performed explain that the anion of the IL is responsible
for high gas solubility. With this property ILs, can be replaced
as solvents in reactions involving gaseous species.
Anthony et al. [69] investigated solubility of nine different
gases up to 13 bar: carbon dioxide, ethylene, ethane, methane,
argon, oxygen, carbon monoxide, hydrogen, and nitrogen in
[bmim][PF6 ]. These gases were chosen for several reasons: CO2
solubility is important due to the possibility of using scCO2 to
extract solutes from ILs; ethylene, hydrogen, carbon monoxide,
and oxygen are reactants in several types of reactions studied
in IL such as hydroformylations, hydrogenations, and oxidations. Due to the nonvolatile nature of IL, the gas solubilities
in IL were measured using a gravimetric technique, usually
with a microbalance. The study of Anthony et al. [69] showed
that CO2 has the highest solubility and strongest interactions
with [bmim][PF6 ], followed by ethylene and ethane. Argon and
oxygen had very low solubilities and immeasurably weak interactions. Fig. 5 demonstrates the solubility of various gases (CO2 ,
C2 H4 , C2 H6 , CH4 , Ar, O2 ) in [bmim][PF6 ] at 25 ◦ C and at different pressures. Except for CO2 , all gases remained in the Henry’s
law regime up to 13 bar. However, CO2 showed some nonlinearity, indicating some degree of saturation. Henry’s constants
of these gases in various organic solvents and in [bmim][PF6 ]
were compared and the results showed that the gases that are
less soluble in the IL are less soluble in the other solvents as
well. However, CO2 is more soluble in the IL than in the other
solvents. The relatively high solubility of CO2 was explained
as a result of its large quadrapole moment. The solubility of
CO2 in [bmim][PF6 ] at different temperatures is demonstrated
in Fig. 6.
158
Fig. 5. Solubility of various gases in [bmim][PF6 ] at 25 ◦ C. (Reprinted with
permission from [69]. Copyright 2002 American Chemical Society)
Camper et al. [66] measured the solubility of CO2 and
C2 H4 in [bmim][PF6 ], [emim][Tf2 N], [emim][CF3 SO3 ] (ethylmethylimidazolium trifluoromethanesulfone), [emim][dca]
(ethylmethylimidazolium dicyanamide) and [thtdp][Cl] (trihexyltetradecylphosphonium chloride) to demonstrate that
the regular solution theory can be used to model the gas
solubilities in RTILs at low pressures and studied the effects
on pressure and the temperature on the solubility of gases in
RTILs. The previous works; Blanchard et al. [70] and Anthony
et al. [71] related that the solubility of the gases in ILs to
the intermolecular interactions between the anion of the IL
and the gas. On the other hand, Camper et al. [66] indicated
that at low pressures, the solubility of CO2 and C2 H4 may be
explained using the regular solution theory without considering
the intermolecular interactions between the anion of the IL and
the gas. At higher pressures, regular solution theory is limited
and Camper et al. [66] attributed this limitation to the dominant
entropic effects.
Recently, the results of solubility of hydrogen in [bmim][PF6 ]
was presented for temperatures from 313 to 373 K and pressures up to 9 MPa. The results demonstrated that the solubility
of hydrogen in [bmim][PF6 ] is low and increases slightly with
Fig. 6. Solubility of CO2 in [bmim][PF6 ] at different temperatures. (Reprinted
with permission from [69]. Copyright 2002 American Chemical Society)
temperature [72]. Since ILs can dissolve certain gaseous species,
they may be used in conventional gas absorption applications.
The nonvolatility of ILs prevent any cross contamination of the
gas stream by the solvent during the process. Moreover, regeneration of the solvent may be performed easily by a simple flash or
distillation to remove the gas from the solvent without any cross
contamination. The other advantages of ILs as separating agents
are no solvent loss and no air pollution. Currently, researchers
are interested in examining the potential of ILs for the separation of CO2 from flue gases emitted from fossil–fuel combustion
operations [73]. ILs may also be utilized as supported liquid
membranes. In conventional membranes, gas dissolves in liquid
but then the liquid in which the gas dissolved evaporates rendering the membrane useless [50]. Due to the nonvolatility of
ILs, they can be immobilized on a support and used in supported
liquid membranes.
ILs are also used for storage and delivery of hazardous specialty gases such as phosphine (PH3 ), arsine (AsH3 ) and stibine
(SbH3 ). GASGUARD® Sub-Atmospheric Systems supply the
major ion implant gases: AsH3 , boron trifluoride (BF3 ), enrich
boron trifluoride (11 BF3 ) and PH3 sub-atmospherically [74]. The
system is combined with gas supply technologies for the delivery of the gases when needed. In the complexed gas technology,
the desired gases (BF3 and PH3 ) are chemically bond to ILs subatmospherically, then pulling the vacuum on the IL–gas complex
provides the mechanism to evolve high purity gas, similar to
desorbing a gas from active carbon.
5.3. Homogenous and heterogeneous catalysis
One of the most important targets of modern chemistry is
to combine the advantages of both homogenous and heterogeneous catalysis [75]. Greater selectivity is generally observed
in homogenous catalysis compared to its heterogeneous counterparts, but separation of the catalyst from the product stream
or from the extract stream causes a problem [8]. ILs offer the
advantages of both homogenous and heterogeneous catalysts
with their two main characteristics: A selected IL may be immiscible with the reactants and products, but on the other hand the IL
may also dissolve the catalysts. ILs combine the advantages of a
solid for immobilizing the catalyst, and the advantages of a liquid for allowing the catalyst to move freely [76]. Brennecke and
Maginn [8] indicated that the ionic nature of the IL also gives an
opportunity to control reaction chemistry, either by participating
in the reaction or stabilizing the highly polar or ionic transition
states.
ILs have an active role in chemical reactions and catalysis.
Some of the examples where ILs are utilized are: reactions of
aromatic rings; clean polymerization [77]; Friedel Crafts alkylation [78]; reduction of aromatic rings [79]; carbonylation [80];
halogenation [81]; oxidation [82]; nitration [83]; sulfonation
[84]; solvents for transition metal catalysis; immobilization of
charged cationic transition metal catalysis in IL phase without
need for special ligands [85]; in situ catalysis directly in IL
rather than aqueous catalysis followed by extraction of products
from solution: this process eliminates washing steps, minimizes
losses of catalysis and enhances purity of the products [86].
Many applications of ILs in catalytic reactions can be found in
various articles in the literature [1,12,85,87–89].
Holbrey and Seddon [90] described many of the catalytic
processes which use low temperature ILs as reaction media and
indicated that the classical transition metal catalyzed hydrogenation, hydroformylation, isomerization, dimerization and
coupling reactions can be performed in IL solvents. In their
review, Holbrey and Seddon [90] concluded that ILs may be used
as effective solvents and catalysts for clean chemical reactions
instead of the volatile organic solvents.
Brennecke and Maginn [8], concluded that ILs have been
used successfully for hydrogenations, hydroformylations, isomerizations, dimerizations, alkylations, Diels-Alder reactions
and Heck and Suzuki coupling reactions, and in general
researchers have concluded that the reaction rates and selectivities are as good or better in ILs than in conventional
organic solvents. The catalytic hydrogenation of cyclohexene using rhodium-based homogenous catalysts [91] and
hydrogenation of olefins using ruthenium and cobalt-based
homogenous catalyst [92] in various ILs are studied and the
results indicated that there is a certain increase in the reaction rates and selectivity compared to the other normal liquid
solvents.
Lagrost et al. [12] used immidazolium and ammoniumbased ILs ([emim][NTf2 ], [bmim][NTf2 ], [bmim][PF6 ],
[(C8 H17 )3 NCH3 ][NTf2 ]) as reaction media for different types
of electrochemical reactions and investigated the oxidation
of organic molecules (anthracene, naphthalene, durene, 1,4dithiafulvene and veratrole) in ILs. Their results suggest that
the nature of investigated mechanisms is almost unchanged in
ILs as compared with the conventional organic media although
the structure of molecular solvents and ILs are expected to
be quite different. Lagrost et al. [12] also concluded that the
diffusion coefficient of the organic compounds are about 100
times smaller than those in conventional media as expected
from the lower viscosity of RTILs versus organic solvents. The
positive results of this study demonstrated that ILs can be used
as a new media for organic electrochemistry [12].
5.4. Biological reactions media
ILs are used in biological reactions such as the synthesis
of pharmaceuticals due to the stability of enzymes in ILs, and
in separation processes such as the extraction of amino acids
[15]. IL biphasic systems are used to separate many biologically important molecules such as carbohydrates, organic acids
including lactic acid [93,94]. Carbohydrates are renewable and
inexpensive sources of energy and raw material for the chemical industry. The underivatized carbohydrates are not soluble
in most of the conventional solvents although they are soluble in water. Their insolubility in most solvents prevents the
transformation of carbohydrates. Therefore, the ability of ILs
to dissolve carbohydrates enables transformation possibilities
[15,93,95,96].
Lau et al. [95] studied the alcoholysis, ammoniolysis, and
perhydrolysis reactions by Candida antarctica lipase catalysis using the [bmim[[PF6 ] and [bmim[[BF4 ] as reaction media.
159
Reaction rates were generally comparable with, or better than,
those observed in organic media. Park and Kazlauskas et al. [96]
studied the acetylation of 1-phenylethanol catalyzed by lipase
from Pseudomonas cepacia (PCL) in several ILs and the reaction was as fast and as enantioselective in ILs as in toluene. They
also investigated the acetylation of glucose catalyzed by lipase
B from C. antarctica (CALB) and found that the transformation was more regioselective in ionic liquids because glucose
is up to one hundred times more soluble in ionic liquids. Liu
et al. [93] stated that carbohydrates are only sparingly soluble
in common organic solvents as well as in weakly coordinating ionic liquids, such as [bmim][BF4 ]. They found that ILs that
contain the dicyanamide anion could dissolve approx. 200 g L−1
of glucose, sucrose, lactose and cyclodextrin and the esterification of sucrose with dodecanoic acid in [bmim][dca] could be
performed with CALB.
Swatloski et al. [97] showed that ILs can also be used as
non-derivatizing solvents for cellulose, the most abundant biorenewable material. Cellulose, which is insoluble in water and
in most of the common organic solvents, has many derivitized
products in many applications of the fiber, paper, and polymer
industries. ILs incorporating anions which are strong hydrogen
bond acceptors are most effective solvents for cellulose, whereas
ILs containing non-coordinating anions including PF6 − and
BF4 − are not effective. Furthermore, Przybysz et al. [98] examined the influence of ILs on a cellulose product, paper and found
that the wettability of paper is improved, whereas the strength
decreased as a result of weakening of cellulose hydrogen
bonds.
Finally, Pfruender et al. [99] tested the water immiscible ILs
namely; [bmim][PF6 ], [bmim][Tf2 N] and [oma][Tf2 N] (methyltrioctylammonium bistrifluoromethanesulfonylimide) for their
biocompatibility towards Escherichia coli and Saccharomyces
cerevisiae. The results of this study showed that these water
immiscible ILs do not damage microbial cells and therefore
one can utilize these water immiscible ILs as substrate reservoirs and in situ product extracting agents for biphasic whole
cell biocatalytic processes. Generally, toxic organic solvents
have been used as substrate reservoirs and with this study it
is shown that water immiscible ILs may be used as biocompatible solvents for microbial biotransformations. The experimental
results demonstrated that there is an increase of chemical yield
from <50% to 80–90% in simple batch processes and (R)-1-(4chlorophenyl) ethanol was produced at a higher initial reaction
rate in the biphasic system (>50 ␮M s−1 L−1 ) compared to the
aqueous system [99]. Although ILs are known by their highly
viscous characteristics, good mass transfer rates were obtained
in their study.
5.5. Removing of metal ions
Dai et al. [100] studied the effects of ILs (with PF6 − and
Tf2 N− anions) on improving the ability of crown ethers to
remove metal ions from aqueous solutions. Strontium nitrate,
a fission product for which there is no available extraction technique for its removal from radioactive waste sites, was used in
this study.
160
Visser et al. [57] designed and synthesized several ILs to
remove cadmium and mercury from contaminated water. The
hydrophobic ILs come into contact with contaminated water and
they snatch the metal ions out of water. Task-specific ionic liquids (TSIL) concept is introduced in order to synthesize ILs with
desired properties to extract metal ions. Visser et al. [57] produced TSIL cations by appending different functional groups
(namely thiother, urea and thiourea) to imidazolium cations.
These IL cations can be considered as a new IL class, or a
novel class of IL extractants. Synthesized TSIL cations were
combined with PF6 − anion and used alone or in a mixture with
[bmim][PF6 ]. The results of the study gave significant distribution ratios for mercury and cadmium in liquid–liquid separations
and minimized the reliance on traditional organic solvents for
this process. Davis [101] gave a detailed analysis and related
information on TSILs.
In traditional solvent extraction technologies, adding extractants that reside quantitatively in the extracting phase increases
the metal ion partitioning to the more hydrophobic phase. The
added extractant molecules dehydrate the metal ions and provide a more hydrophobic environment enabling their transport
to the extracting phase [50].
In TSILs, attaching a metal ion coordinating group directly
to the imidazolium cation makes the extractant an integral part
of the hydrophobic phase and in this way the chance for IL
loss to the aqueous phase is reduced. Therefore, TSILs act as
the hydrophobic solvent and the extractant at the same time
[50]. However, the cost of TSIL is generally high. In order to
eliminate this drawback, TSILs may be added to the mixtures
of less expensive ILs. Furthermore, Davis [101] and Zhao et al.
[15] stated in their recent reviews that TSILs are not limited to
extraction processes; they can be also used as versatile solvents
in organic catalysts, solid phase synthesis and even in production
of liquid Teflon.
The extraction of radioactive metals (lanthanides and
actinides) has particular industrial significance among IL extraction of metal ions for the handling of nuclear materials [15]. The
behaviors of uranium species in various ILs were investigated
in early studies [102–105]. Recently, researchers have focused
on the fundamental understanding of ILs in nuclear chemistry
such as radiochemical stability of ILs [106].
Recently, Nakashima et al. [107] examined the feasibility
of extracting of rare earth metals into ILs from aqueous solutions and stripping of metal ions from ILs into an aqueous phase
by complexing agents. They successfully accomplishing to
recycle the extracting IL phase. In this study, octyl(phenyl)-N,Ndiisobutylcarbamoylmethyl phosphine oxide (CMPO) dissolved
in [bmim][PF6 ] showed an extremely high extraction ability
and selectivity of metal ions as compared to in an ordinary
diluent, n-dodecane. The results of this study indicate that
ILs are a promising medium for actinide and fission product
separation.
In the literature, various studies were performed to extract
metal ions using ILs [108–113]. Different metal ions including
alkali, alkaline earth metals, heavy metals and radioactive metals
are researched by using different ILs. Generally, the side chain
of the IL on the cation is varied and the effect of structure of the
IL on the extraction efficiency of the metal ions is investigated.
The side chain of the cation influences the hydrophobic character
of the IL and thus the partition coefficient of the metal ions is
affected.
Visser et al. [108] extracted Na+ , Cs+ using [Cn mim][PF6 ]
(n = 4, 6, 8); Chun et al. [109] investigated extraction of
other alkali metals such as Li+ , K+ , Rb+ using [Cn mim][PF6 ]
(n = 4–9); Luo et al. [112,113] studied the extraction of Na+ ,
K+ , Cs+ ions using [Cn mim][Tf2 N] (n = 2, 4, 6, 8). Not only
the alkali metals but also extraction of alkaline earth metals
were studied by various groups: Visser et al. [108] removed Sr2+
using [Cn mim][PF6 ] (n = 4,6,8); Bartsch et al. [110] studied the
removal of Mg2+ , Ca2+ , Sr2+ , Ba2+ ; Luo et al. [112,113] utilized
[Cn mim][Tf2 N] (n = 2, 4, 6, 8) to extract Sr2+ . The extraction of
heavy and radioactive metals such as Cu2+ , Ag+ , Pb2+ , Zn2+ ,
Cd2+ , Hg2+ were studied by using [Cn mim][PF6 ] (n = 4–9) and
TSILs [110,111,114].
6. Challenges of ILs
The unique properties of ILs and the ability to design
their properties by choice of anion, cation and substituents
create many more processing options, alternative to the ones
with conventional solvents. However, high cost, lack of physical property and toxicity data restrict the advantageous use
of ILs as process chemicals and processing aids at the
present.
The challenges in the use of ILs must be also addressed as
well as their advantages. The major challenge is the cost. A
kilogram of IL costed about 30,000-fold greater than a common
organic solvent such as acetone. Renner [9] reported that this cost
could be reduced to approximately 1000-fold greater depending
on the composition of IL and the scale of production. Wagner
and Uerdingen [115] anticipated that the price of cation systems
based on imidazole will be in the range of D 50–100 kg−1 , if
larger quantities of ILs are produced. The price can be lowered
even below D 25 kg−1 if ILs are prepared with cheaper cation
sources on a ton scale. Another estimation was done by Wassersheid and Haumann [116]. They expected that for ‘bulk ionic
liquids” choosing proper (relatively cheap) cations and anions
lead to prices approximately D 30 l−1 for production rates of
multi-ton. Moreover, scientists emphasized that the price of the
ILs may look still discouraging however, the essential factor
is the price to performance ratio. If the performance of an IL
is extremely higher than that of the material (solvent) it aims
to replace, less amounts of the IL may be needed for a given
specific job [2], thus totally or partially overcoming the price
disadvantage.
The second problem is associated with the manufacturing
method of ILs. In manufacturing ILs environmental issues also
need to be tackled since some VOCs are used to manufacture
ILs. Recently, some advanced methods have been developed
in the solventless syntheses of ILs. For example, 1-alkyl3-methylimidazolium halides have been synthesized in open
containers in a microwave oven without any VOCs by Varma and
Namboodiri at the Environmental Protection Agency of U.S.,
2001 [9].
Researchers need to find alternative ways to recycle ILs due to
the reason that many processes for cleaning up ILs involve washing with water or VOCs which creates another waste stream. This
problem has been solved by adopting supercritical extraction
technologies to recover the dissolved organic compounds from
ILs or using membrane separation processes. However, there are
other solid matrixes which adsorb some part of the ILs. Thus, a
second rinse would be required and this would create an aqueous waste stream that contains ILs. Despite this disadvantage
there may be some cleaning applications where ILs would be
attractive [8].
Incomplete physico-chemical data are another challenge for
the application of ILs. At the present most available data are
focused on bulk physical properties such as viscosity, density
and phase transitions. Relatively little is known about the microscopic physical properties of ILs. After these properties are
investigated properly and the influence of ILs on chemical reaction rates is found, new ILs with precisely tailored properties
can be synthesized.
It is extremely important to obtain reliable thermophysical
data and transport properties of ILs in order to make them
available for many applications and to design IL-based processes efficiently. Harris et al. [117] measured the viscosities
of two members of one of the most commonly studied IL
groups, that are based on imidazolium cations; [omim][PF6 ]
and [omim][BF4 ] between 0 and 80 ◦ C and at pressures to
176 MPa ([omim][PF6 ]) and 224 MPa ([omim][BF4 ]) with a
falling body viscometer and densities between 0 and 90 ◦ C
at atmospheric pressure. The bulk physical properties of most
widely used ILs at wider temperature and pressure ranges are
essential.
Another barrier to the large-scale application of ILs arises
from their high viscosities. The viscosities of ILs are higher
than most organic solvents and water, usually similar to viscosity of oils. This high viscosity may be responsible to produce
a reduction in the rate of many organic reactions and even a
reduction in the diffusion rates of species. Also, handling of ILs
with high viscosities is difficult however; increasing temperature, changing anion–cation combinations may yield ILs with
lower viscosities. To overcome mass transfer limitations in gasIL systems resulting from high viscosity reactions using ILs may
be run at high pressures and in efficient gas–liquid contacting
equipment.
In chemical processing, pharmaceuticals, fine chemicals,
petroleum refining, metal refining, polymer processing, pulp and
paper, and textiles where a nonvolatile liquid with a wide liquidus range could work better, ILs are the best choice however,
the challenges of turning ILs into useful and environmentally
benign fluids must be overcome.
7. ILs and scCO2 systems
Green chemistry, also known as sustainable chemistry,
describes the search for reducing or even eliminating the use
of substances in the production of chemical products and reactions which are hazardous to human health and environment.
The goal of green chemistry is to create a cleaner and more
161
sustainable chemistry and it has received more and more attention in recent years. Green chemistry searches for alternative,
environmentally friendly reaction media as compared to the traditional organic solvents and at the same time aims at increased
reaction rates, lower reaction temperatures as well higher
selectivities.
The ideal situation for a safe and green chemical process
is using no solvent, however most of the chemical processes
depend on solvents. Some of these solvents are soluble in water
and therefore they must be stripped from water before it leaves
the process not only for ecological but also for economic reasons.
Solvents must be recovered for recycle and reuse for an economically viable process. Water, perfluorinated hydrocarbons and
supercritical fluids (SCFs) are alternative solvents which may
be used in green chemistry. Among these, the most promising
elements of green chemistry are ILs and scCO2 .
The low volatility of ILs is the key property that makes them
green solvents. However, this advantage also causes a problem
for product separation and recovery [15]. Several techniques for
solute recovery from ILs exist: volatile products can be extracted
from IL by distillation or simply by evaporation. However, nonvolatile or thermo-sensitive products cannot be separated from
ILs with these methods. ILs exhibiting immiscibility with water
can be extracted with water to separate water-soluble solutes
from IL into the aqueous phase; but this method is not suitable
for hydrophilic ILs [17]. Of course, organic solvents such as hexane and toluene may be effective to recover the products from
IL but this approach obviously compromises the ultimate goal
of ‘green’ technologies [15]. Furthermore, the cross contamination between the phases presents another problem. Finally,
another green solvent is discovered which solves all the problems and recovers various kind of solutes from ILs without cross
contamination: supercritical fluids (SCFs).
SCFs are compounds which are above their critical temperature and pressure and they can be manipulated from gas like
to liquid like densities due to their unusual properties near the
critical point. They are commercially viable solvents in several
applications such as dry cleaning and polymer impregnation.
scCO2 is the most widely used SCF as a result of nontoxic and
non-flammable characteristics. scCO2 has low critical temperature and pressure and it is not expensive.
The advantages of using SCFs as extraction medium include
low cost, nontoxic nature, recoverability and ease of separation
from the products. SCFs have been adapted for product recovery
from ILs and supercritical fluid extraction (SCFE) is shown to
be a viable technique with the additional benefits of environmental sustainability and pure product recovery [118]. Among
the SCFs, an inexpensive and readily available one, scCO2 has
become a partner of IL and two environmentally benign solvents are utilized together in several applications. The volatile
and nonpolar scCO2 forms different two-phase systems with
nonvolatile and polar ILs. The product recovery process with
these systems is based on the principle that scCO2 is soluble in
ILs, but ILs are not soluble in scCO2 [70]. Since most of the
organic compounds are soluble in scCO2 , with the high solubility of scCO2 in ILs, these products are transferred from the IL
to the supercritical phase.
162
Table 3
IL–gas systems phase behaviors
System
Pressure
Findings
Reference
[bmim][PF6 ]–CO2
[C8 -mim][PF6 ]–CO2
[C8 -mim][BF4 ]–CO2
[bmim][NO3 ]–CO2
[emim][EtSO4 ]–CO2
[N-bupy][BF4 ]–CO2
40, 50, and
60 ◦ C
Up to 93 bar
As pressure increases, solubility of CO2 in the IL increases
Solubility of CO2 in IL-rich phase decreases with temperature
CO2 solubility depends on the nature of the anion and cation
The solubility of CO2 in IL-rich phase is highest for ILs with fluorinated anions
The general trend of the phase behavior is almost identical for all ILs
[70]
[bmim][PF6 ]–CO2
[bmim][PF6 ]–C2 H4
[bmim][PF6 ]–C2 H6
[bmim][PF6 ]–CH4
[bmim][PF6 ]–Ar
[bmim][PF6 ]–O2
[bmim][PF6 ]–CO
[bmim][PF6 ]–N2
[bmim][PF6 ]–H2
10, 25, and 50 ◦ C
Up to 13 bar
Water and carbon dioxide exhibited the strongest interactions and the
Highest solubilities in [bmim][PF6 ], followed by ethylene, ethane, and methane
Argon and oxygen both had very low solubilities and essentially no
interactions with the IL
[69]
[bmim][PF6 ]–CO2
20, 40, 60, 80,
100, and 120 ◦ C
30–70 ◦ C
Up to 9.7 MPa
Total pressure increases linearly with increasing amount of CO2 in IL
[119]
Atmospheric pressure
CO2 is found to be one order of magnitude more soluble in the IL than O2
The solubility of CO2 in the IL decreases with temperature
The solubility of O2 in the IL slightly increases with temperature
Similar results are obtained in [bmim][PF6 ]
[131]
[bmim][PF6 ]–CO2
[bmim][BF4 ]–CO2
[bmim][TfO]–CO2
[bmim][NO3 ]–CO2
[bmim][methide]–CO2
[bmim][DCA]–CO2
[bmim][Tf2 N]–CO2
[hmim][Tf2 N]–CO2
[omim][Tf2 N]–CO2
[hmmim][Tf2 N]–CO2
25, 40, and 60 ◦ C
Up to 150 bar
Solubility of CO2 in ten different IL is reported
The solubility of CO2 is strongly dependent on the choice of the anion
Increasing the alkyl chain length, increases the solubility of CO2 in IL
All of the ILs expand a relatively small amount when CO2 is added
[121]
[emim][PF6 ]–CHF3
36.15–94.35 ◦ C
1.6–51.6 MPa
The solubility of supercritical CHF3 in [emim][PF6 ] is very high
At low CHF3 concentrations (mole fraction <0.5), the equilibrium pressure
increases almost linearly with CHF3 concentration, whereas further increase in
CHF3 concentration causes a sharp increase in the equilibrium pressure
The Peng-Robinson EoS is capable of describing the experimental bubble
point data of the system satisfactorily and qualitatively predicting the solubility
of the ionic liquid in supercritical CHF3
The high-pressure phase behavior of [emim][PF6 ]–CHF3 system was
completely different from that of [bmim][PF6 ]–CO2
[142]
[emim][PF6 ]–CO2
35–93 ◦ C
1.49–97.10 MPa
CO2 is more soluble in [bmim][PF6] than in [emim][PF6 ]
The general phase behavior of [emim][PF6]–CO2 and [bmim][PF6]–CO2
systems are found to be completely similar
CO2 is more soluble in [bmim][PF6 ] than in [emim][PF6 ]
The phase behaviors of the systems [emim][PF6 ]–CO2 and
[emim][PF6 ]–CHF3 are different
CHF3 is more soluble in the IL than CO2 at higher pressures and the IL is also
more soluble in supercritical CHF3 than in supercritical CO2
[129]
[hmim][PF6 ]–CO2
25.16–90.43 ◦ C
0.64–94.60 MPa
CO2 is more soluble in [hmim][PF6] than in [emim][PF6 ]
The general phase behaviors of [emim][PF6]–CO2 and [hmim][PF6]–CO2 are
similar
The solubility of the IL in scCO2 phase is very low and cannot be detected
[130]
[bmim][PF6 ]–CO2
[C6 mim][PF6 ]–CO2
[emim][BF4 ]–CO2
[C6 mim][BF4 ]–CO2
[emim][Tf2 N]–CO2
[C6 mim][Tf2 N]–CO2
25 ◦ C
Up to 1 MPa
A group contribution form of a non-random lattice fluid model is applied to
predict the solubility of CO2 in ILs
[123]
[bmim][BF4]–CO2
[bmim][BF4]–O2
Temperature
163
Table 3 (Continued )
System
Temperature
Pressure
Findings
Reference
[bmim][PF6 ]–CO2
[emim][PF6 ]–CO2
[hmim][PF6 ]–CO2
40–90 ◦ C
Up to 97 MPa
CO2 has good solubilities in these ILs at lower pressures
There is a linear relationship between the alkyl chain length and solubility
of CO2
[13]
[hmim][BF4 ]–CO2
[hmim][PF6 ]–CO2
20–95 ◦ C
0.54–100 MPa
CO2 was found to be more soluble in [hmim][PF6 ] than in [hmim][BF4 ]
[133]
[bmim][BF4 ]–CO2
5.32–95.07 ◦ C
0.587–67.62 MPa
CO2 has a high solubility in [bmim][BF4 ] at lower pressures, but the solubility
decreases dramatically at higher pressures
The phase behavior of the system [bmim][BF4 ]–CO2 shows similarities with
the phase behavior of the system [hmim][BF4 ]–CO2
CO2 is more soluble in [hmim][BF4 ] than in [bmim][BF4 ]
[132]
[omim][BF4 ]–CO2
29.85–89.95 ◦ C
0.1–100 MPa
High solubilities of CO2 for low CO2 mole fractions (mole fraction < 0.6) are
attained at relatively low pressure, whereas for mole fraction >0.6, the pressure
needed for completely dissolving the CO2 increases drastically
Increase in temperature slightly decreases the solubility of CO2
The CO2 solubility increases in the IL with increasing chain length of the alkyl
group
Suggested for further study to investigate the effect of the anion on the
solubility of CO2 .when the cation is fixed as [omim]
[139]
7.1. High-pressure phase behavior of IL–CO2 systems
Preliminary works have shown that scCO2 extraction is a
viable method for solute recovery from an IL. However, the
knowledge of phase behavior of IL–CO2 systems is an essential
aspect of this methodology. scCO2 dissolution in the IL phase is
not only necessary for contact with the solute but it also reduces
the viscosity of the IL and therefore enhancing the mass transfer
process.
Early studies of IL–CO2 phase behavior indicated that these
systems are very unusual biphasic systems. No measurable
amount of [bmim][PF6 ] was soluble in the CO2 -rich phase,
although a large amount of CO2 dissolved in the IL-rich phase,
reducing the viscosity of IL [70]. Blanchard and Brennecke
[118] concluded that the system remained as two distinct phases
even under pressures up to 400 bar. Therefore, high-pressure
phase behavior of [bmim][PF6 ]–CO2 is totally different from
that of any ordinary organic liquid–CO2 systems. This different
phase behavior is the key phenomena which makes extraction
of solutes from IL with CO2 attractive.
Table 3 summarizes the phase behavior studies performed
for IL–gas systems, demonstrates the type of IL and gases used
in these studies, the experimental conditions, the basic findings
and the related references.
7.1.1. The [bmim][PF6 ]–CO2 system
The phase behaviors of IL–scCO2 systems are studied very
extensively in the literature for a better understanding of the processes involving both IL and scCO2 . Since [bmim][PF6 ] is the
most widely studied IL in the literature, many researchers studied the high-pressure phase behavior of the [bmim][PF6 ]–CO2
system [13,69,70,119–122].
Blanchard et al. [70] measured the high-pressure
vapor–liquid phase behavior of [bmim][PF6 ]–CO2 system
by using two different apparatus sets: a static high-pressure
phase equilibrium apparatus and a dynamic flow apparatus. In
the static high-pressure vapor–liquid equilibrium apparatus, a
glass cell was loaded with a known amount of IL sample and
known amounts of CO2 were metered into the cell while the
sample within was vigorously stirred to ensure equilibrium.
With the assumption of pure CO2 vapor phase, the composition
of the IL-rich phase was calculated by knowing the amount of
CO2 added to the cell. At the end of the equilibration period,
CO2 was completely removed from IL phase upon depressurization. The same group also used a dynamic apparatus, i.e. a
high-pressure extractor to determine the solubility of the IL in
the CO2 phase. The detailed description of these apparatus sets
and experimental procedures can be found in the literature [70].
Different experimental set-ups were used by other researchers:
A schematic diagram of a general IL–scCO2 experimental
apparatus, which was used to measure the solubility of CO2
in IL ([bmim][PF6 ]) is given by Kim et al. [123]. Shiflett and
Yokozeki [124] measured the gas solubility and diffusivity
of CO2 in [bmim][PF6 ] using a gravimetric microbalance for
which the details of the experimental set-up is given in the
related reference.
The solubility of CO2 in [bmim][PF6 ] was determined at 40,
50 and 60 ◦ C and pressures up to 93 bar [70]. As the pressure
increases, the solubility of CO2 in the IL-rich phase increases
dramatically and the solubility value reaches a mole fraction of
0.72 at 40 ◦ C and 93 bar. A general rule suggests that an increase
in temperature results with a decrease in the solubility of gases
in liquids. As expected, the solubility of CO2 in [bmim][PF6 ]
rich phase decreases with temperature. However, they noticed
that the temperature dependence of the solubility is quite small
in this temperature and pressure range. Another crucial point
is the effect of large degree of CO2 solubility on the viscosity
of IL. The viscosity of IL decreases when a certain amount of
CO2 is dissolved in IL and this effect can easily be observed
by the reduced drag on the stirring magnet when a static highpressure vapor–liquid equilibrium set-up is used. This reduction
in viscosity of the liquid facilitates the solution process.
164
Fig. 7. Total pressure versus molality of gas for [bmim][PF6 ]–CO2 system.
(Reprinted with permission from [119]. Copyright 2003 American Chemical
Society)
Kamps et al. [119] presented the solubility of CO2 in
[bmim][PF6 ] for temperatures 293–393 K in 20 K intervals and
pressures up to about 9.7 MPa. The total pressure is plotted versus the stoichiometric molality of the gas (number of moles
per kilogram of the IL) in Fig. 7. The total pressure increases
linearly with increasing amount of the gas in IL. The solubility
data represented by Kamps et al. [119] differ from the previously
reported solubility data of Blanchard et al. [70]. The comparison
of experimental data of two studies is given in Fig. 8.
There are several [bmim][PF6 ]–CO2 high-pressure
vapor–liquid equilibrium data sets available in the literature. However, these sets differ from each other considerably
in the values they report for similar conditions. The reason
of differing solubility data reported may be due to the small
amounts of water dissolved in the IL sample used. For example,
Blanchard et al. [125] presented a solubility data of CO2
in [bmim][PF6 ] which is different than the data reported by
the same group in 2001. In the first study, this group used
[bmim][PF6 ] which was saturated with water at 22 ◦ C, containing 2.3 wt.% water. In the second study, they used [bmim][PF6 ]
Fig. 8. Comparison of experimental data of Blanchard et al. [70] (䊉, , and )
and Kamps et al. [119] ( and ) for [bmim][PF6 ]–CO2 system. (Reprinted
with permission from [119]. Copyright 2003 American Chemical Society)
Fig. 9. [bmim][PF6 ]–CO2 liquid phase compositions for dried and wet IL samples at 40 ◦ C. (Reprinted with permission from [70]. Copyright 2001 American
Chemical Society)
which was dried to approximately 0.15 wt.% water. Drying of
IL has a significant effect on the phase behavior with CO2 .
Thus, Blanchard et al. [70] showed that the solubility of CO2
in ILs was decreased in the presence of water. In Fig. 9,
[bmim][PF6 ]–CO2 liquid phase compositions are given for
dried and wet IL samples.
In order to observe the effect of water impurity in IL, phase
behaviors of two IL samples (dry and wet) with CO2 were compared. The effect of water impurity in IL is significant at 57 bar.
For dried IL sample, the mole fraction of CO2 is 0.54, whereas
for the wet (water saturated) IL sample it is only 0.13. The effect
of water in IL may be explained by CO2 -phobic nature of water.
Even at high pressures, mutual solubilities of water and CO2 are
very low [126]. Another point is the formation of carbonic acid
from the reaction of CO2 with water that can result in a reduction
of the aqueous phase pH to as little as 2.80 [127].
Rubero and Baldelli [128] investigated gas–liquid interface
of imidazolium ILs using surface-sensitive vibrational spectroscopy sum frequency generation. The results indicated that
when the IL is dry, the cation is oriented with the imidazolium ring parallel to the surface plane for both hydrophilic
and hydrophobic ILs. But the cation reorients itself with respect
to the surface for the hydrophobic liquid when water is added,
while the orientation in the hydrophilic liquid is unaffected.
After the influence of water is noticed, researchers working
on IL–CO2 solubility and equilibrium have started to dry and
degas all ILs under vacuum at room temperature for several
days prior to use. After ILs are dried and degassed, the water
contents are estimated by the Karl Fischer analysis before solubility data are taken. Measurements show that the most widely
studied IL; [bmim][PF6 ] absorbs a couple wt.% water when left
to the atmosphere. The estimated water content of [bmim][PF6 ]
after drying was approximately 0.15 wt.% water as measured by
Karl Fischer analysis [70].
A number of high and low-pressure [bmim][PF6 ]–CO2 solubility studies have appeared in the literature. Although consistent
results have been established for low-pressure solubility data of
CO2 in [bmim][PF6 ], there are large discrepancies among highpressure solubility data of several researchers [13,70,119–121].
These large solubility differences in the literature are most
probably due to the differences in the purities of the ILs
used.
Since the behaviors of IL–CO2 systems are different from
other organic liquid–CO2 systems, full phase diagrams of
IL–CO2 systems are investigated. Blanchard and co-workers
[125] found two-phase immiscibility regions with three cloud
point measurements of 1.31, 4.92 and 7.15 mole% IL mixtures with the balance being CO2 . Although, these experiments
were conducted with water-saturated ILs, qualitatively a similar
behavior is expected with dried samples.
Blanchard et al. [70] gave a qualitative phase behavior of
[bmim][PF6 ]–CO2 system over a wide pressure range. They
noticed that the phase behavior where a large miscibility gap
exists even at extremely high pressures. Blanchard et al. [70]
reported and referred that as a complementary work, McHugh
and co-workers studied [bmim][PF6 ]–CO2 phase behavior at
higher pressures up to 3100 bar and found two distinct phases
at all conditions. The existence of large immiscibility gap even
at very high pressures is not expected for organic liquid–CO2
systems and the existence of two distinct phases is explained
by the following discussion: At high-pressures density of pure
CO2 phase increases but since the liquid phase does not expand,
the two phases will never become identical and a mixture critical point will never be reached. Therefore, the IL–CO2 system
remains as two phases even at very high pressures, although the
CO2 solubility is quite high, the mixtures never become a single
phase [70].
Anthony et al. [69] reported the solubilities and Henry’s
constants of different gases (carbon dioxide, ethylene, ethane,
methane, argon, oxygen, carbon monoxide, hydrogen, and nitrogen) in [bmim][PF6 ] and showed that CO2 has the highest
solubility and strong interaction with [bmim][PF6 ]. The solubility data of CO2 in [bmim][PF6 ] is in good agreement with
the published results of Blanchard et al. [70] although different
techniques were used in these studies. Furthermore, Baltus et al.
[68] reported that Henry’s constants for Kamps et al. [119] data
are in reasonable agreement with those obtained by Anthony et
al. [69].
Aki et al. [121] studied the high-pressure phase behavior of
CO2 in imidazolium-based ILs and compared the phase behavior of the system [bmim][PF6 ]–CO2 with the other solubility
data present in the literature. The solubility results of CO2 in
[bmim][PF6 ] at 25 ◦ C measured by Aki et al. [121] agreed
remarkably well with the solubility results of Anthony et al.
[69] at low pressures and with the solubility results of Kamps et
al. [119]. Aki et al. [121] investigated the solubility of CO2 in
[bmim][PF6 ] at 40 ◦ C and compared the results with the previous studies of Kamps et al. [119], Blanchard et al. [70] and Liu et
al. [120]. As expected the agreement between the data points is
good at low pressures but the discrepancy is obvious at high pressures. Aki et al. [121] explained that in their previous work [70],
they were not aware of the various impurities and degradation
products that were present in the samples they used. There-
165
fore, they attributed the difference between their study [121]
and the previous study of the same group [70] to the purity of
the IL.
At 40 ◦ C and at all pressures, there is a very good agreement within the solubility values reported by Aki et al. [121]
and Liu et al. [120]. However, this statement is not correct for
the reported solubility data of Aki et al. [121] and Kamps et al.
[119]. The results of two studies agree at low pressures, but at
high pressures, there is a significant difference: At about 43 bar,
the solubility of CO2 in [bmim][PF6 ] was measured as 0.43
(mole fraction) by Aki et al. [121], however, at the same point
Kamps et al. [119] reported the solubility of CO2 as 0.38. By considering the studies mentioned above, it may thus be concluded
that the solubility of CO2 in one of the most widely studied IL,
[bmim][PF6 ], varies among different groups in the literature and
is not a good agreement especially at higher pressures.
The solubility of CO2 in [bmim][PF6 ] was experimentally studied at 298.15 K and up to 1.0 MPa by Kim et al.
[123]. A group contribution form of a non-random lattice–fluid
model (GC-NLF) was applied to predict solubility of CO2 in
[bmim][PF6 ]. They used the solubility data of Kamps et al. [119]
for a wider pressure range for the group parameter determination. Comparisons of calculated solubility data with Kamps et
al. data [119] for [bmim][PF6 ] demonstrated that the method
applied is fairly accurate except for regions close to the critical
conditions of CO2 . Kim et al. [123] also compared the calculated values of solubility of CO2 in different ILs ([emim][PF6 ],
[bmim][PF6 ], and [C6 mim][PF6 ]) with the experimental solubility data reported by other groups for the same ILs. The
agreements are generally good up to 10 MPa pressure, however,
further comparisons for higher pressure shows some degree of
discrepancy between the calculated solubility data of Kim et al.
[123] and the experimental solubility data of Shariati and Peters
[129,130].
Finally, Shariati and Peters [13] studied the comparison of
the phase behavior of [bmim][PF6 ]–scCO2 system with the
other studies present in the literature. The solubility of CO2
in [bmim][PF6 ] was determined by measuring the bubble point
pressure of the binary system at different temperatures for several isopleths and pressures up to 97 MPa. The solubility data
of CO2 in [bmim][PF6 ] at 323.15 K was compared with that of
Blanchard et al. [70] and Anthony et al. [71] and the results of
Shariati and Peters [13] were in a good agreement with those
of Anthony et al. [71]. The solubility data taken at 333.15 K
was compared with that of Blanchard et al. [70], Kamps et
al. [119], Liu et al. [120]. Although the experimental methods were completely different, there is also a good agreement
between the results of Shariati and Peters [13] and Kamps et
al. [119] at a temperature of 333.15 K. The solubility data of
Blanchard et al. [70] and Liu et al. [120] show greater deviations from the data of Shariati and Peters [13] especially at
higher pressures. Shariati and Peters [13] reported the existence
of the three-phase equilibrium liquid–liquid–vapor (LLV). As
the other studies demonstrated, this study also indicated that
CO2 has a high solubility in [bmim][PF6 ] and there is a linear
relationship between the alkyl chain length and the solubility
of CO2 .
166
7.1.2. Other IL–CO2 systems
High-pressure phase behavior of different types of
ILs are similar to that of [bmim][PF6 ]. Blanchard et
al. [70] investigated the high-pressure phase behavior of
CO2 with six different ILs: 1-n-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF6 ]), 1-n-octyl-3 methylimidazolium hexafluorophosphate ([C8-mim][PF6]), 1-n-octyl3-methylimidazolium tetrafluoroborate ([C8 -mim][BF4 ]), 1n-butyl-3-methylimidazolium nitrate ([bmim][NO3 ]), 1-ethyl3-methylimidazolium ethyl sulfate ([emim][EtSO4 ]), and
N-butylpyridinium tetrafluoroborate ([N-bupy][BF4 ]). They
investigated the solubility of CO2 in different ILs at 40,
50, 60 ◦ C and pressures up to 93 bar. The focus of the
work was to develop an insight into the physical interaction
between CO2 and ILs with different cation–anion configurations. The solubility of CO2 in the IL-rich phase was
greatest for ILs with fluorinated anions, following the trend
of [bmim][PF6 ] and [C8 -mim][PF6 ] > [C8 -mim][BF4 ] > [Nbupy][BF4 ] > [bmim][NO3 ] > [emim][EtSO4 ] [70]. The solubility of CO2 was found to be greatest in ILs with PF6 − , anion
and the IL that exhibited the lowest solubility of CO2 was
[emim][EtSO4 ].
The solubility of CO2 in ILs increases with increasing pressure but the exact amount of CO2 dissolved in the liquid phase
varies significantly. In the study of Blanchard et al. [70], it is
found that at 70 bar the solubility of CO2 in [emim][EtSO4 ] was
0.36 (mole fraction) whereas, it was 0.63 in [C8 -mim][PF6 ].
Although there are some numerical differences in the mole fraction of CO2 dissolved, the general trend of the phase behavior is
nearly identical for all ILs. The solubility of CO2 in the IL-rich
phase changes slightly with the temperature as in the case of
[bmim][PF6 ]–CO2 system. Since the qualitative phase behavior
of almost all ILs seems similar, it can easily be concluded that
scCO2 can be used to recover solutes not only from [bmim][PF6 ]
but also from all kind of ILs.
In order to understand the effect of the anion on the phase
behavior of IL–CO2 systems, two pairs of ILs with the same
cations were compared: [C8 mim][PF6 ]–[C8 mim][BF4 ] and
[bmim][PF6 ]–[bmim][NO3 ]. Changing the anion from [PF6 ]−
to [BF4 ]− in the [C8 mim] salts, results in an approximately
8% decrease in CO2 solubility at 40 ◦ C over the range of pressures studied by Blanchard et al. [70]. The solubility of CO2 in
[bmim][NO3 ] is about 25% less than in [bmim][PF6 ]. Experimental and molecular simulation studies found that the anions of
IL dominate the interaction with CO2 , with the cation playing a
secondary role. Cadena et al. [122] pointed out that the changes
in the imidazolium cation involving alkyl groups have relatively
little influence on the solubility of CO2 in IL.
[bmim][BF4 ] is one of the popular ILs which has been very
widely used in most of the studies: Husson-Borg et al. [131]
reported the solubility of CO2 in [bmim][BF4 ] as a function
of temperature between 303 and 343 K and at atmospheric
pressure. This group used a new type of experimental apparatus based on a saturation method. The equilibrium cell is
specially designed for viscous solvents like the IL, and an
appropriate gas–liquid contact is obtained by good agitation.
Kroon et al. [132] studied the phase behavior of different
IL-supercritical fluid systems including [bmim][BF4 ]. They
investigated the phase behavior of [bmim][PF6 ]–CO2 binary
system experimentally and reported its bubble point pressures for CO2 concentrations between 10.22 and 60.17 mole%
and in a temperatures range of 278.47–368.22 K. They found
that CO2 has a high solubility at lower pressures, but the
solubility decreases dramatically at higher pressures. The
experimental results for the [bmim][BF4 ]–CO2 system were
compared with the available phase behavior data of the
binary system 1-hexyl-3-methylimidazolium tetrafluoroborate
([hmim][BF4 ])–CO2 [133] to investigate the effect of the alkyl
group length on the phase behavior. The results showed that
a larger alkyl group led to lower bubble-point pressures and,
therefore, to higher solubilities of CO2 in the imidazolium-based
ionic liquid. Thus, CO2 was more soluble in [hmim][BF4] than
in [bmim][BF4 ].
Shariati and Peters [129] studied the phase behavior of
binary system [emim][PF6 ]–CO2 experimentally by measuring its bubble point pressures at temperatures and pressure
ranges of 308.14–366.03 K and 1.49–97.10 MPa, and compared
it with [bmim][PF6 ]–CO2 system to understand the effect of
the length of alkyl chain on the solubility of CO2 . The solubility of CO2 in [bmim][PF6 ] is higher than in [emim][PF6 ].
Since the butyl group in [bmim][PF6 ] is bulkier than the
ethyl group in [emim][PF6 ], Shariati and Peters [129] emphasized that CO2 can dissolve better in [bmim][PF6 ] than in
[emim][PF6 ], the latter component being denser. Moreover, it
was observed that the experimentally determined phase behavior of the [emim][PF6 ]–CO2 is similar to the phase behavior of
[bmim][PF6 ]–CO2 at 333.15 K.
Aki et al. [121] also presented the solubility of CO2 in
ten different imidazolium-based ILs at 25, 40, and 60 ◦ C
and pressures to 150 bar. They concluded that the solubility of CO2 in imidazolium-based ILs increases with
increasing pressure and decreases with increasing temperature for all the ILs investigated. Furthermore, Aki et al.
[121] investigated the influence of the different anions,
namely; dicyanamide ([DCA]), nitrate ([NO3 ]), tetrafluoroborate ([BF4 ]), hexafluorophosphate ([PF6 ]), trifuoromethanesulfonate ([TfO]), bis(trifluoromethylsulfonyl) imide ([Tf2 N]), and
tris(trifluoromethylsulfonyl)methide ([methide]). The results of
their study showed that the solubility of CO2 is strongly dependent on the choice of anion. Solubility measurements of Anthony
et al. [69] and Cadena et al. [122]; spectroscopic studies of
Kazarian et al. [134] also all showed that the solubility of CO2
in ILs depend on the anion, especially the interaction between
the anion of the IL and CO2 .
CO2 is the least soluble in the two ILs with non-fluorinated
anions, [NO3 ] and [DCA], and it has the highest solubility in
ILs with anions containing fluoroalkyl groups, [TfO], [Tf2 N],
and [methide]. Aki et al. [121] attributed the high solubility of
CO2 in ILs to the anions containing fluoroalkyl groups as a
result of favorable interactions between CO2 and the fluoroalkyl
substituents on the anion since fluoroalkyl groups are known as
CO2 -philic ones [135–137].
Aki et al. [121] also compared the solubility of CO2 in
[bmim][Tf2 N], [hmim][Tf2 N], and [omim][Tf2 N] at 25, 40, and
60 ◦ C to investigate the influence of cation alkyl chain length.
The solubility of CO2 increases with an increase in the alkyl
chain length at all pressures, with the increase being more
apparent at higher pressures. This result of Aki et al. [121] is
consistent with the results of previous studies of Shariati and
Peters [129,130]. Furthermore, Blanchard et al. [70] reported
that the CO2 solubility increases when the alkyl chain length
was increased from butyl to octyl for ILs containing [PF6 ].
It is known that the densities of the imidazolium-based ILs
decrease as the alkyl chain length increases [49,138]. Therefore,
imidazolium-based ILs with longer alkyl chains have greater free
volume and thus greater solubility of CO2 is expected in these
ILs. Therefore, the higher solubility of CO2 in ILs with longer
cation alkyl chains may be attributed to the entropic rather than
enthalpic arguments [121]. Finally, it may be concluded that it
is possible to increase the solubility of CO2 in ILs by increasing
the alkyl chain length on the cation.
Constantini et al. [133] investigated the phase behavior of
a binary mixture of 1-hexyl-3-methylimidazolium tetrafluoroborate ([hmim][BF4 ])–scCO2 system and compared it with
the experimental data of the binary system of 1-hexyl3-methylimidazolium hexafluoroborate ([hmim][PF6 ])–scCO2
system, in order to demonstrate the anion effect. Although the
phase behaviors of the binary systems are similar, the solubility of the scCO2 is found to be higher in [hmim][PF6 ] than in
[hmim][BF4 ]. This higher solubility may be explained with the
greater interaction between CO2 and the [PF6 ] anion, although
the former is denser than the latter [133].
In another study [139], the high-pressure phase behavior of
the binary system 1-octyl-3-methylimidazolium tetrafluoroborate ([omim][BF4 ])–CO2 was studied in the liquid-phase CO2
mole fraction range 0.1–0.75, and in the pressure and temperature range of 0.1–100 MPa and 303–363 K. They compared
their experimental data with the data given by Blanchard et al.
[70]. Although there was a discrepancy between two studies for
CO2 mole fractions higher than 0.6, they both suggested a peculiar behavior of a rapid pressure increase at higher CO2 mole
fractions [139]. Kroon et al. [140] presented the phase behavior
of several IL–CO2 binary systems experimentally, and finally
developed an equation of state (EoS) to predict the phase behavior of IL–CO2 systems based on the truncated perturbed chain
polar statistical associating fluid theory (tPC-PSAFT) EoS. The
EoS was used to describe the CO2 solubility in several 1-alkyl3-methylimidazolium-based ionic liquids with different alkyl
chain lengths within a pressure and CO2 mole fraction range
of 0–100 MPa and 0–75%, respectively. The binary interaction
parameter was fitted to V–L equilibrium data.
In general when a gas is dissolved in a liquid phase, dilation
of the liquid occurs. ILs show only slight dilations in volume
with CO2 dissolution. This behavior is different than normal
liquid–CO2 systems. In fact, the dissolution of CO2 in liquids to
expand them and reduce their solvent strength is a well-known
phenomena and it is the basis of gas anti-solvent (GAS) process
to precipitate solutes from liquids [141]. However, ILs do not follow this trend due to the strong Coulombic forces associated with
the ionic nature of ILs. The lack of expansion of ILs seems to
derive from the fact that dissolved CO2 does not greatly affect the
167
strength of interionic interactions in IL. Therefore, the IL–CO2
system is extremely different from other organic liquid–CO2
systems with the large immiscibility region and lack of dilation
of the liquid phase [70].
Although there are not so many available studies concerning
the IL/‘SCF other than CO2 systems, for the completeness of the material they must be mentioned. Shariati and
Peters [142] investigated the high-pressure phase behavior of
[emim][PF6 ]–supercritical fluoroform, the experimental conditions and the results of this study is given on Table 3.
7.2. IL solubility in CO2
The majority of the research focus on the solubility of CO2 in
IL, on the other hand the concentration of IL in CO2 -rich phase
is also important. A dynamic apparatus with a high-pressure
cell, was used to measure the solubility of [bmim][PF6 ] in the
CO2 -rich phase [70]. The solubility of [bmim][PF6 ] in CO2 was
determined at 40 ◦ C and 137.9 bar by flowing 0.5866 mole of
CO2 through a cartridge loaded with [bmim][PF6 ]. UV–visible
analysis gave no appreciable IL absorption peak, indicating
[bmim][PF6 ] solubility of less than 5 × 10−7 (mole fraction of
[bmim][PF6 ]) of in the CO2 phase. Due to the fact that no measurable IL dissolves in CO2 , a solute dissolved in an IL can be
easily recovered with scCO2 without any cross contamination.
The lack of solubility of IL in the CO2 phase can be attributed to
two reasons: extremely low vapor pressure of IL and the inability
of CO2 to adequately solvate ions in the gaseous phase.
Although the solubility of IL in scCO2 is extremely low
and not measurable, in industrial applications, the scCO2 phase
may contain some other components such as reactants, products
which may act as cosolvents to enhance the ability of scCO2 to
dissolve IL significantly. In this case, the amount of IL dissolved
in CO2 -rich phase may not be negligible under some conditions.
In order to decide on the conditions to avoid cross contamination, solubility of IL in scCO2 /organic compound mixtures
must be known. Wu et al. [143] conducted the first study in
the literature to observe the effect of organic compounds in
scCO2 on the solubility of an IL in CO2 phase. The solubility of
[bmim][PF6 ] in scCO2 , and in scCO2 –ethanol, scCO2 –acetone,
scCO2 –n-hexane mixtures was investigated quantitatively. The
solubility of IL in scCO2 is extremely low as the earlier studies
suggested, however by addition of ethanol and acetone, the solubility of IL increases dramatically as the concentration of the
organic compounds in scCO2 exceeds 10 mole%. This enhancement of the IL solubility by addition of ethanol and acetone
results mainly from strong interaction of the two compounds
with the IL due to their strong polarity. The polarity of acetone is stronger than ethanol; therefore, the solubility of IL is
higher in the scCO2 –acetone system than in the scCO2 –ethanol
system. Since n-hexane is a nonpolar substance its influence
on the solubility of IL in scCO2 phase is very limited. A further study of Wu et al. [144] also showed that the ability
of cosolvents to increase the solubility of ILs ([bmim][PF6 ]
and [bmim][BF4 ]) in scCO2 follows the order: acetonitrile > acetone > methanol > ethanol > n-hexane. This order is
same with the order of dipole moments of cosolvents. With this
168
Table 4
IL solubility in CO2
System
Temperature
Pressure
Findings
Reference
[bmim][PF6 ]–CO2
40 ◦ C
137.9 bar
The solubility of [bmim][PF6] in scCO2 is extremely low
scCO2 may be used to recover solutes dissolved in IL
without any cross contamination
[70]
[bmim][PF6 ]–CO2
[bmim][PF6 ]–CO2 –ethanol
[bmim][PF6 ]–CO2 –acetone
[bmim][PF6 ]–CO2 –n-hexane
40 and 55 ◦ C
12–15 MPa
By addition of ethanol and acetone, the solubility of the IL
in scCO2 phase enhances
[143]
[bmim][PF6 ]–CO2
[bmim][PF6 ]–CO2 –acetonitrile
[bmim][PF6 ]–CO2 –methanol
[bmim][PF6 ]–CO2 –ethanol–n-hexane
[bmim][PF6 ]–CO2 –acetone–n-hexane
[bmim][PF6 ]–CO2 –ethanol–acetone
[bmim][BF4 ]–CO2
[bmim][BF4 ]–CO2 –ethanol
40 ◦ C
15 MPa
By the addition of organic cosolvents, the solubility of ILs
in scCO2 phase may be enhanced significantly
[144]
discussion, it is clear that the polarity of the organic compound is
a dominant factor in influencing solubility. With this study Wu et
al. [144] emphasized that the amount of IL dissolved in scCO2 rich phase may be significant if the system contains sufficiently
polar organic compounds in sufficient concentrations. Table 4
shows the studies of Blanchard et al. [70], Wu et al. [143] and
Wu et al. [144], the components of their systems, experimental
conditions and basic results.
The phase behavior of the IL–CO2 –methanol system and
the viscosity of the mixtures were studied previously by
Liu et al. [120] at different conditions. The phase behavior
of IL–CO2 –water system was investigated by Zhang et al.
[145] and finally this group studied the phase behavior of
[bmim][PF6 ]–CO2 –acetone system in detail at 313.15 K over a
wide pressure range. Zhang et al. [146] determined the distribution coefficients of the components between different phases and
found that CO2 distribution coefficient decreases with increasing pressure while the acetone distribution coefficient increased
with pressure.
7.3. IL–CO2 interaction at the molecular level
An in situ attenuated total reflectance–infra red (ATR–IR)
study of CO2 dissolved in two ILs ([bmim][PF6 ] and
[bmim][BF4 ]) at high pressures has demonstrated the effects
of anionic species of the ILs on the molecular state of the dissolved CO2 . Kazarian et al. [147] showed that CO2 forms weak
Lewis acid–base complexes with the anions in [bmim][PF6 ] and
[bmim][BF4 ]. Furthermore, they demonstrated that this interaction is stronger with [bmim][BF4 ]. BF4 − acts as a stronger Lewis
base towards CO2 than PF6 − . In addition to this, as the size of
anion increases the strength of interaction decreases. However,
the solubility data of various group indicated that CO2 has a
higher solubility in [bmim][PF6 ] than in [bmim][BF4 ]. Thus,
the strength of these interactions cannot be solely responsible
for the solubility of CO2 in these ILs, and presumably, a free
volume contribution in IL plays a significant role [148]. The
strength of the anion–cation interactions in IL affects the avail-
able free volume and one can expect that a weaker interacting
anion leads to more free volume being available.
ILs are generally distinguished with their low melting points.
Another in situ ATR–IR spectroscopic study of Kazarian et al.
[134] showed that high-pressure CO2 reduces the melting temperature of ILs. This possibility of reducing melting temperature
of ILs further under high pressure CO2 provides a new opportunity to use ILs as solvents at milder temperatures. Kazarian et
al. [134] investigated the effect of CO2 pressure on the phase
behavior of 1-hexadecyl-3-imidazolium hexafluorophosphate.
The data presented indicated that 70 bar CO2 reduces the melting point of this IL from 75 to 50 ◦ C. This result was assigned
to a weak Lewis acid–base type interaction between anion and
CO2 , with the P–F bonds perpendicular to O C C axis thereby
reducing the rather stronger interactions between the P–F bonds
and the cations. CO2 disrupts the cation–anion and the tail-tail
interactions in IL rather than simply playing an impurity role in
the mechanism of induced melting. Preliminary results obtained
by Kazarian et al. [134] also indicated that melting temperatures
of other analogous ILs decrease by high pressure CO2 allowing
their use in many applications at mild temperatures.
7.4. Solute recovery from ILs with scCO2
Researchers found that nonvolatile organic compounds can
be extracted from ILs using scCO2, which is widely used to
extract large organic compounds with minimal pollution. Blanchard et al. [125] showed that CO2 can be used to extract
naphthalene, a low volatility model solute, from an IL. They
synthesized [bmim][PF6 ] which was stable in the presence of
water and oxygen. The model compound naphthalene was readily soluble in [bmim][PF6 ] (maximum solubility of 0.30 mole
fraction at 40 ◦ C) and in CO2 . Their study investigated the phase
behavior of [bmim][PF6 ] with CO2 , as well as with naphthalene
and finally that of the [bmim][PF6 ]–CO2 –naphthalene ternary
system. The results showed that CO2 -rich phase was not significantly contaminated by IL, as would be expected during contact
of CO2 with any conventional organic solvent. Blanchard et al.
[125] demonstrated that CO2 is highly soluble in [bmim][PF6 ]
reaching a mole fraction of 0.6 at 8 MPa. After extracting IL with
CO2 at 13.8 MPa and 40 ◦ C there was no detectable [bmim][PF6 ]
in the extract, indicating that the solubility is less than 10−5 mole
fraction. In contrast to this result, a mixture of CO2 with conventional organic liquid results in significant solubility of the liquid
in the CO2 phase. A mixture of 0.12 mole fraction naphthalene
in [bmim][PF6 ] was extracted with CO2 at 13.8 MPa and 40 ◦ C
with recoveries of 94–96% and therefore it was concluded that
it is possible to quantitatively extract an organic solute having
a reasonable molecular weight from IL using scCO2 without
any cross contamination. Furthermore, the dissolution of scCO2
in IL was completely reversible and pure IL remained after the
extraction of naphthalene and depressurization [125].
The experimental work of Blanchard and Brennecke [118],
which showed that a wide variety of solutes can be extracted from
[bmim][PF6 ] with scCO2 , with recovery rates greater than 95%,
provided a significant step to visualize the partnership established by the IL–CO2 system. One of the essential problems
of ILs, namely product recovery, was solved by applying the
SCFE technique. Hexane and benzene were chosen as the roots
to which numerous substituents groups were added to explore the
effect of chemical structure on the solubility and extractability
of an organic solute in and from an IL. The substituent groups
represented were halogen, alcohol, ether, amide, ketone, carboxylic acid, ester, and aldehyde with a wide range of polarity.
The extraction experiments were conducted at 40 ◦ C and 138 bar.
The authors noted that for some organic solutes, scCO2 extraction achieved greater than 98% recoveries before the extraction
test was terminated. These high recovery rates clearly indicated
that although the ionic nature of IL might lead to an interaction with solute, it did not limit the extent of reaction. Benzene
and chlorobenzene which exhibited phase immiscibility with
[bmim][PF6 ] required the least amount of CO2 for recovery.
Phenols, benzoic acid and benzamide which are solids at room
temperature, required the largest amount of CO2 . Figs. 10 and 11
illustrate % recovery of solutes as a function of molar ratio of
CO2 passed through the extractor to organic solute loaded in the
reactor (solute dipole moments are also given in these figures in
Fig. 10. Extraction of aromatic solutes from [bmim][PF6 ] with scCO2 at 40 ◦ C
and 138 bar. (Reprinted with permission from [118]. Copyright 2001 American
Chemical Society)
169
Fig. 11. Extraction of aliphatic solutes from [bmim][PF6 ] with scCO2 at 40 ◦ C
and 138 bar. (Reprinted with permission from [118]. Copyright 2001 American
Chemical Society)
terms of Debye). In this paper they showed that there was a relation between the dipole moments of solutes and the amount of
CO2 required for extraction, concluding that the organics with
a dipole moment of zero (such as benzene, hexane, cyclohexane, etc.) are easily extracted compared to solutes with nonzero
dipole moments. Fig. 12 illustrates the number of CO2 per mole
of organic solute as a function of dipole moment.
Studies on the solubilities of organics in [bmim][PF6 ] were
carried out under ambient conditions, 22 ◦ C and 0.98 bar [118],
where IL–solute mixtures were stirred in closed containers to
avoid contamination with air and water vapor. Their results
demonstrated that organics with the potential for strong intermolecular interactions, those with a large dipole moment for
example, generally exhibited complete miscibility or a large
degree of solubility in [bmim][PF6 ]. Solubilities of the solid
organic solutes were considerably less than the solubilities of the
liquid organics, with the exception of the phenol. The authors
Fig. 12. Effect of solute dipole moment on ease of extraction of [bmim][PF6 ]
with scCO2 at 40 ◦ C and 138 bar. (Reprinted with permission from [118]. Copyright 2001 American Chemical Society)
170
concluded that the solubilities of the benzene-based organics were significantly greater than those of their hexane-based
counterparts. Solubility measurement results indicated that aromatic compounds are more soluble in [bmim][PF6 ] than in
non-aromatic compounds of equivalent molecular weight and
polarity.
Distribution coefficient (K), may be defined as the ratio of
solute mole fractions in the supercritical and in the IL phases,
respectively. Since ILs do not dissolve in scCO2 appreciably,
supercritical phase is essentially CO2 and organic solute. The
distribution coefficient is an important thermodynamic property
to comprehend the IL–scCO2 –solute systems. Theoretically, one
can anticipate the trend of distribution coefficient between the IL
and the scCO2 phases by considering the volatility and polarity
characteristics of the solute. For example, a solute with a high
volatility and low polarity will have a large affinity for CO2 ,
whereas a solute with high polarity and aromaticity will have
a large affinity for the IL-rich liquid phase. Therefore, solutes
with high polarities give a small K value due to high affinity
for IL and low affinity for CO2 . Conversely, nonpolar solutes
give large K values as a result of high affinity for CO2 . With this
discussion, it is clear that the compounds that have high affinity
for CO2 can be more easily extracted from the IL mixture.
The phase behaviors of the organic solute–CO2 binary systems also affect the ease of extraction of a compound from ILs
with scCO2 . Investigating the liquid phase data at low pressures,
a measure of affinity of compounds for CO2 was determined, and
also that compounds in which CO2 readily dissolves at low pressures have a greater attraction for CO2 [118]. Furthermore, the
mole ratio of CO2 to solute for 95% recovery is determined as
1840 for CO2 –cyclohexane binary system [149] however, the
same ratio is as high as 20,300 for CO2 –acetophenone system
[150].
Using scCO2 to recover products from IL is a good alternative
if the products are thermally sensible or nonvolatile. A high boiling point solute can also be recovered from IL with scCO2 , where
distillation is not a desirable option. Blanchard and Brennecke
[118] extracted a high-boiling point (230 ◦ C) organic liquid
namely 1,4-butanediol from [bmim][PF6 ]. This high-boiling
point solute followed the same solubility and extractability trend
as the other liquid solutes. Therefore, SCFE technology is readily applicable for the recovery of various kinds of compounds
from ILs.
Scurto et al. [151] studied the use of scCO2 as a separation switch for IL–organic mixtures. They demonstrated that the
solutions of methanol and 3-butyl-1-methyl-imidazolium hexafluorophosphate ([C4 mim][PF6 ]) can be induced to form three
phases in the presence of scCO2 . Although the original solution
is quite dilute in IL, application of scCO2 induces the formation of an additional liquid phase which is rich in IL. This study
showed that there is an alternative way that CO2 may be used
to separate ILs from organic compounds and the extraction of
organic materials can be achieved without any IL cross contamination in the recovered product. In their following study, Scurto
et al. [152] demonstrated that separation of hydrophobic and
hydrophilic imidazolium-based ILs from aqueous solutions by
the application of scCO2 is possible and [bmim][PF6 ] is sepa-
rated from an IL-saturated aqueous solution at 293 K and at a
CO2 pressure of 4.9 MPa.
The result of the mentioned studies presented that organic
compound–IL–scCO2 mixture has a complex phase behavior.
According to the results of their research Scurto et al. [151]
emphasized that during the IL–scCO2 reaction studies in which
larger amounts of organic reactant and products are present,
one must be aware of the possible formation of the additional
liquid phases that might contain only part of the components
necessary for the desired reaction. After showing ILs could be
recovered from methanol and water using CO2 -induced separation, the same research group [153] investigated the factors
that control the vapor–liquid–liquid equilibrium in IL–organic
compound–CO2 ternary systems via studying on several homogeneous IL–organic compound mixtures. The experiments were
conducted at 40 ◦ C. The results showed that the lowest critical
endpoint pressure (LCEP) was dependent on the choice of both
organic, IL and the initial concentration of IL in the organic. The
K-point pressure was however independent of the type of IL, was
identical with the organic compound–CO2 mixture critical point.
Najdanovic-Visak et al. [154] investigated the vapor–liquid
equilibrium of ternary (1-butanol–water–CO2 ) and quaternary
([C4 mim][NTf2 ]–1-butanol–water–CO2 ) systems. The demixing pressures of both mixtures were strongly controlled by the
water concentration.
The studies concerning the solute recovery from ILs by
scCO2 , the system components, experimental conditions, major
results of the studies and related references are given in Table 5.
7.5. Other applications of IL–scCO2 systems
Brennecke and Maginn [8] discussed the potential industrial
applications of ILs in many areas such as catalytic reactions, liquid–liquid extractions, gas separations etc. After the
IL–scCO2 systems and the advantages of these systems are realized, a number of studies have been done for IL–CO2 biphasic
systems: It is shown that a desired solute may be extracted
from an IL using scCO2 without any cross contamination
[118,125]. The use of scCO2 to separate ILs from their organic
solvents [151]; the addition of CO2 to separate hydrophobic
and hydrophilic imidazolium-based ILs from aqueous solutions
[152] have all been important applications of IL–scCO2 systems.
Dzyuba and Bartsch [155] demonstrated the recent applications of room temperature IL–scCO2 systems in metal catalyzed
organic reactions and enzyme-catalyzed transformations. The
solubility or stability of organometallic or enzymatic catalysts in
ILs and their negligible solubility in scCO2 is the basic advantage
of IL–scCO2 systems.
Several groups have studied the IL–scCO2 reaction systems
[156–168]. In these studies, mostly IL was used as reaction
media and scCO2 was used as transport media for reactants and
products. Cole-Hamilton et al. [169] summarized the continuous
flow homogeneous catalysis using IL–scCO2 biphasic systems
in detail according reaction types. Also, Gordon and Leitner
[170] mentioned some of IL–scCO2 biphasic reaction systems.
In the following paragraphs, IL–scCO2 biphasic reaction systems are summarized.
171
Table 5
Recovery of organic compounds from IL by scCO2
System
Solute
Temperature
Pressure
Findings
Reference
[bmim][PF6 ]–CO2
Naphthalene
25–40 ◦ C
Up to 40 MPa
CO2 is highly soluble in [bmim][PF6]
Two phases are not completely miscible
CO2 may be used to extract naphthalene, model solute, from an IL
The dissolution of CO2 in IL is completely reversible: pure IL remains
after extraction of naphthalene and depressurization
CO2 can extract a wide variety of organic solutes from an IL
IL contamination in the recovered product is eliminated by using CO2
All organic solutes exhibited recovery greater than 95%
Intermolecular interactions between organic solutes and [bmim][PF6]
have an effect on the solubility of solute in IL, but these interactions do
not limit the degree of recovery
[125]
[bmim][PF6 ]–CO2
Benzene
Chlorobenzene
Phenol
Anisole
Aniline
Acetophenone
Benzoic acid
Methyl benzoate
Benzamide
Benzaldehyde
Hexane
1-Chlorohexane
1-Hexanol
Butyl ethyl ether
Cyclohexane
2-Hexanone
Hexanoic acid
Methylpentanoate
Hexanamide
1,4-Butanediol
40 ◦ C
138 bar
CO2 can extract a wide variety of organic solutes from an IL
IL contamination in the recovered product is eliminated by using CO2
All organic solutes exhibited recovery greater than 95%
Intermolecular interactions between organic solutes and [bmim][PF6]
have an effect on the solubility of solute in IL, but these interactions do
not limit the degree of recovery
[118]
Sellin et al. [156] and Webb et al. [157] studied the hydroformylation of alkenes in IL–scCO2 biphasic reaction media
and described the continuous flow homogeneous catalysis in
IL–scCO2 biphasic system. They dissolved the catalyst in ionic
liquid and used scCO2 as the transport medium for the substrates and products. In Sellin et al. [156], they demonstrated
the hydroformylation of several alkenes such as 1-hexene, 1nonene and 1-octene in IL–scCO2 biphasic mixture. First, the
hydroformylation of 1-hexene was studied using triphenylphosphite as the rhodium-based ligand ([Rh2 (OAc)4 ]/P(OPh3 )) in
[bmim][PF6 ] using scCO2 . The hydroformylation of 1-hexene
was also performed in [bmim][PF6 ] without scCO2 . The results
showed that addition of scCO2 to the reaction mixture lowers
the conversion from >99–40%, but the selectivity and linear
to branched (l:b) ratio were enhanced from 15.7 to 83.5% and
2.4 to 6.1, respectively. The success of this system encouraged the authors to carry out similar reactions of 1-hexene
and 1-nonene with repetitive uses of the same catalyst to
search for the possibility of continuous flow homogeneous
catalysis. They found that the catalyst retained its activity and
selectivity for only three to three runs, so in order to eliminate this drawback, the ligand–catalyst system was changed.
[Ph2 P(C6 H4 SO3 )]–[bmim] was used together with [Rh2 (OAc)4 ]
as the catalyst precursor for the hydroformylation of 1-nonene
in the [[bmim][PF6 ]–scCO2 biphasic system. The products were
flushed from the reactor with scCO2 . The activity of this cata-
lyst system remained high for 12 runs with an acceptable l:b
ratio. However, after the ninth run Rh leaching became important which was attributed to ligand oxidation. Contamination
with air during the many openings of the reactor may cause
the oxidation, which will be eliminated during continuous flow.
Finally, Sellin et al. [156] demonstrated the continuous hydroformylation of 1-octene using [PhP(C6 H4 SO3 )2 ]–[pmim]2 and
[Rh2 (OAc)4 ] dissolved in [bmim][PF6 ]. The reactants and
the products were transported into and out of the reaction
medium via scCO2 . The total pressure, temperature and reaction time were 200 bar, 100 ◦ C and 33 h. The results showed
that the catalyst was stable at least between 8 and 10 h reaction time, no ligand oxidation occurred and the l:b ratio
of the product aldehydes was 3.1. In another study, Webb
et al. [157] investigated the hydroformylation 1-dodecene
which was representative for hydroformylation of relatively
low volatility alkenes. The reactions were catalyzed by either
Rh/[prmim][Ph2 P(3-C6 H4 SO3 )] or Rh/[prmim][TPPMS]. The
latter one was used for its easiness in crystallization and purification. They performed optimization reactions in order to obtain
higher rates. They investigated the effect of the ionic liquid,
substrate flow rate, temperature, gas composition on the rate of
hydroformylation of 1-octene. Among several ionic liquids used
([bmim][PF6 ] [bmim][NTf2 ], [octmim][PF6 ], [octmim][NTf2 ],
[decmim][NTf2 ]), [octmim]NTf2 was found to be the most
effective in terms of high conversion rates attained (>80%).
172
The authors found that the catalyst turnover frequency (TOF),
which indicates productivity, could be >500 h−1 as the substrate
(1-octene) flow rate was increased (>0.3 mL/min) at 200 bar
and 100 ◦ C. They concluded that continuous flow homogeneous catalysis in IL–scCO2 biphasic system can be used for
hydroformylation of long chain alkenes at rates comparable
with the ones found in commercial systems. In the latest study
conducted by Cole-Hamilton and co-workers [158] on hydroformylation of alkenes, they achieved rapid hydroformylation of
1-octene (rates up to 800 h−1 ) with the catalyst remaining stable for at least 40 h and with very low rhodium leaching levels
(0.5 ppm). A new system, in which the substrate, reacting gases
and products dissolved in scCO2 and were flowing over a fixed
bed “supported ionic liquid phase (SILP)” catalyst, was introduced. The reactants in scCO2 (1-octene, CO and H2 ) flowed
upwards through a tubular reactor containing a catalyst composed of [prmim][Ph2 P(3-C6 H4 SO3 )], [Rh(acac)(CO)2 ] and
[octmim][Tf2 N] supported on silica gel at 100 ◦ C, and 100 bar.
The authors claimed that this new system provided excellent diffusion of the substrate and gases to the catalyst surface, excellent
solubility of the substrates and gases within the supported ionic
liquid and extraction of heavy products that might otherwise foul
the catalyst by filling the pores.
There were two other studies on SILP catalysis [171,172],
but both studies were conducted in batch mode. Wang et al.
[171] described the synthesis of cyclic carbonates from CO2 and
epoxides over silica-supported quaternary ammonium salts and
Ciriminna et al. [172] studied the aerobic oxidation of alcohols
over a perruthenate catalyst.
Brown et al. [159] studied the asymmetric hydrogenation
of tiglic acid catalyzed by Ru(O2 CMe)2 ((R)–tolBINAP) in
[bmim]PF6 with addition of water as cosolvent. In this study,
scCO2 was not involved in the reaction, but it was used to
recover the products from the IL after the reaction time was
over. The results were evaluated in terms of enantioselectivity
and conversion. The catalyst/ionic liquid solution was reused
repeatedly (five cycles) without significant loss of enantioselectivity or conversion. In another work of the same group [160],
a number of different solvents were studied in order to evaluate
the most effective system for the enantioselective hydrogenation
of ␣,␤-unsaturated acids. Different solvent systems comprised
scCO2 , different ILs, ILs with cosolvents and also CO2
expanded ionic liquids. They studied two types of substrates,
namely class I (atropic acid) and class II (tiglic acid) substrates.
Class I substrates were hydrogenated in high enantioselectivity
at high H2 concentration whereas Class II substrates were
hydrogenated in high enantioselectivity at low H2 concentration. For both substrates, Ru(O2 CMe)2 (R–tolBINAP) was
used as the catalyst. Atropic acid was hydrogenated with
the highest enantioselectivity (92%) in methanol (50 bar H2
pressure). Lower enantioselectivity values were obtained for
reactions in ILs ([bmim][PF6 ]—32%, [bmim][BF4 ]—15%,
[emim][O3 SCF3 ]—25%,
[emim][N(O2 SCF3 )2 ]—31%,
[dmpim][N(O2 SCF3 )2 ]—39%) and IL–cosolvent systems
[bmim][PF6 ]–PrOH—33%,
([bmim][PF6 ]–toluene—19%,
[bmim][PF6 ]–MeOH—54%) at 50 bar H2 pressure. [bmim]
[PF6 ], [bmim][PF6 ]–toluene, [bmim][PF6 ]–PrOH, [bmim]
[PF6 ]–MeOH systems were expanded by CO2 (50 bar CO2 ).
Expansion caused increases in enantioselectivity consistent
with greater H2 solubility and mass transfer rate in all mediums
except for [bmim][PF6 ]–toluene. Tiglic acid was hydrogenated
with the highest enantioselectivity (95%) in [emim]N(O2 SCF3 )2
(5 bar H2 ). Reasonably high enantioselectivity values were also
obtained for other ionic liquids, as the authors expected, because
of low H2 concentration ([dmpim][N(O2 SCF3 )2 ]—93%,
[bmim][PF6 ]—93%, [mbpy][BF4 ]—88%, [bmim][BF4 ]—
88%, [emim][O3 SCF3 ]—84%). IL–cosolvent systems
([bmim][PF6 ]–toluene, [bmim][PF6 ]–PrOH) were less
selective than hydrogenation in IL ([bmim][PF6 ]) alone. Tiglic
acid hydrogenations were also conducted in [bmim][PF6 ],
[bmim][PF6 ]–toluene, [bmim][PF6 ]–PrOH expanded systems.
In these CO2 (70–bar) expanded systems, decreases in enantioselectivity were observed when compared to non-expanded
systems. The decrease in enantioselectivity for [bmim][PF6 ]
system was from 93% (non-expanded) to 85% (expanded).
Another example of hydrogenation reaction in IL–scCO2
biphasic system was reported by Liu et al. [161]. IL phase was
used to immobilize the transition metal catalyst, and scCO2
phase was used to recover the products. They examined the
hydrogenation of 1-decene and cyclohexene using Wilkinson’s
catalyst RhCl(PPh3 )3 , and hydrogenation of carbon dioxide in
the presence of dialkylamines to form N,N-dialkylformamides
using RuCl2 (dppe)2 in [bmim][PF6 ]–scCO2 biphasic system.
98% conversion was attained for hydrogenation of 1-decene at
48 bar H2 and 207 bar total pressure (TOF: 410 h−1 ) at the end
of 1 h reaction time. Hydrogenation of cyclohexene under the
same experimental conditions mentioned above proceeded more
slowly (82% conversion after 2 h, 96% conversion after 3 h).
They also performed four repetitive batch runs for hydrogenation
of 1-decene, and demonstrated the efficient catalyst recycling via
immobilization in ionic liquid. The hydrogenation of 1-decene
and cyclohexene were also done with [bmim][PF6 ]–n-hexane
biphasic system. The results showed that there was no reactivity advantage for CO2 over n-hexane for simple hydrogenation
reactions. The conversion and selectivity were much higher for
hydrogenation of carbon dioxide in [bmim][PF6 ]–scCO2 biphasic medium starting with the di-n-propylformamide (80 ◦ C,
276 bar, 5 h), when they were compared to the conversion and
selectivity values obtained only in scCO2 for less bulky diethylamine and n-propylamine.
Bössmann et al. [162] also utilized the benefits of IL/CO2
biphasic system to immobilize the organometallic catalyst in
IL phase and recover the product without harming the catalyst. They investigated the continuous flow system for the
hydrovinylation of styrene with Wilke’s catalyst in IL/CO2
biphasic system. They initially identified the suitable ILs
that would allow the activation of the catalyst. It was
found that the activation strongly depended on the IL’s
anion type. The reaction conversion rates for the anions
BARF (BARF: tetrakis[3,5-bis(trifluoromethyl)phenyl]borate),
Al[OC(CF3 )2 Ph]4 , Tf2 N and BF4 were 100, 90.5, 69.9 and 39.6,
respectively for [emim] cation. It was also noted that the activation of the catalyst was not just a simple anion-exchange
reaction, and the specific environment of the ionic solvent sys-
tem seemed to activate the catalyst. When [emim] and [4-mbp]
(4-mbp: 1-n-butyl-4-methylpyridinium) were used as different
cations with the same anion, it was reported that at comparable conversions, higher enantioselectivity values were found
with [4-mbp][BF4 ] and [4-mbp][Tf2 N] than the corresponding [emim] salts. The continuous flow styrene hydrovinylation
was conducted in [emim][Tf2 N]–CO2 at 80 bar. The catalyst
was stable over a reaction time of 61 h and enantioslectivity
decreased slightly at that period, while products were extracted
with compressed CO2 . Tkatchenko et al. [163] studied the palladium catalyzed dimerization of methyl methacrylate at 83 ◦ C
and 200 bar in [bmim][BF4 ]–scCO2 biphasic system. The selectivity (>98%) was identical to that of the monobasic system. The
turnover frequency and turnover numbers were comparable for
both monophasic and biphasic systems, increasing with increasing substrate to palladium ratios in the IL phase. They concluded
that, as the CO2 -rich phase acted as a substrate and product reservoir, there was a possibility for the reaction to be conducted
under continuous feed and product recovery conditions with
greener solvents.
Hou et al. [164] investigated the oxidation of 1-hexene by
molecular oxygen in [bmim][BF6 ]–scCO2 biphasic system as
well as in [bmim][BF6 ], scCO2 , and in the absence of solvent
with catalysts PdCl2 and CuCl2 . The results showed that the
selectivity to the desired product 2-hexanone was much higher
when the reaction was done in [bmim][BF6 ]–scCO2 biphasic
system (125 bar, 333.2 K and 17 h of reaction time). Additionally, the catalyst was more stable in biphasic system than it was in
scCO2 only. Kawanami et al. [165] performed the chemical fixation of CO2 to cyclic carbonates in a IL–scCO2 biphasic system.
They reported that synthesis of propylene carbonate from propylene oxide and carbon dioxide in [omim][BF4 ]–scCO2 biphasic
system (14 MPa, 100 ◦ C) was achieved with nearly 100% yield
and selectivity within 5 min and TOF value was 77 times higher
than those so far reported. They also observed that the yield
was increased as the alkyl chain length of the IL’s cation was
also increased from C2 to C8 . Moreover, different epoxide substrates having phenyl substituted groups and alkyl side chain
groups were examined for the synthesis of the corresponding
carbonates in [omim][BF4 ]–scCO2 biphasic system at 14 MPa
and 100 ◦ C. Gao et al. [166] studied the transesterification
between isoamyl acetate and ethanol in scCO2 , [bmim][BF6 ]
and [bmim][BF6 ]–scCO2 system using p-xylenesulfonic acid
(p-TSA) as the catalyst. The results showed that the equilibrium
conversion in [bmim][BF6 ]–scCO2 media was lower than those
observed in scCO2 or [bmim][BF6 ]. An interesting application
of IL–scCO2 biphasic system in synthesis was electro-oxidation
of benzyl alcohol to benzaldehyde in an electrochemical cell
[167]. The reaction was carried out at 318.2 K and up to
10.3 MPa, and two ILs, 1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4 ]) and 1-butyl-3-methylimidazolium
hexafluorophosphate ([bmim][PF6 ]), were used as the solvents
and electrolytes. [bmim][BF4 ] was a better medium for the
electro-oxidation of benzyl alcohol. The Faradic efficiency
(FE) and selectivity of benzaldehyde increased as the pressure
increased up to 9.3 MPa, whereas the FE decreased as the pressure was increased further. The authors noted that the products
173
could be easily recovered from the IL by using scCO2 extraction after the electrolysis, and the IL could be reused. Yoon et
al. [168] studied Heck coupling of iodobenzene with olefins in
[bmim][PF6 ] catalyzed by PdCl2 /Et3 N. They did not incorporate
scCO2 in the reaction, it was used after the reaction for product
recovery.
IL–scCO2 biphasic systems were also used for enzyme
catalysis [173–179]. Lozano et al. [173–174] investigated the
synthesis of butyl butyrate from vinyl butyrate and 1-butanol,
and the kinetic resolution of rac-1-phenylethanol with vinyl
propionate by transesterification. They used both free and immobilized C. antarctica lipase B (CALB) in IL ([emim][Tf2 N]
and [bmim][Tf2 N]) as catalyst for continuous biphasic biocatalysis. CO2 was used as transport medium for reactants
and products. The synthetic activity of the enzyme in IL–CO2
system was tested through operation/storage cycles. Operation
period (4 h) was followed by a storage period (20 h) of the
enzyme–IL system under dry conditions at room temperature.
The continuous synthesis of butyl butyrate from vinyl butyrate
and 1-butanol by transesterification was studied at 12.5 and
15 MPa at 40, 50 and 100 ◦ C in [bmim][Tf2 N]–CO2 system
[173]. The specific activity and the selectivity were enhanced
as the temperature increased, the selectivity was high (>95%)
in all cases giving higher than 50% conversions. The activity
decay for the repetitive uses of the free enzyme–IL system was
the highest at high temperature. The continuous synthesis of
(R)-1-phenylethyl propionate from the kinetic resolution of rac1-phenylethanol with vinyl propionate catalyzed by free CALB
dissolved in [emim][Tf2 N] and [bmim][Tf2 N] was also investigated at 15 MPa, 50 and 100 ◦ C [173]. The results showed
that the selectivity of the reaction, the activity and the halflife of the enzyme–IL system were lower for this reaction than
those observed for butyl butyrate synthesis. However, high enantioselectivity (>99.9%) was exhibited by the enzyme. As the
reaction temperature increased, the selectivity was increased
but the specific activity decreased. In another publication of
Lozano et al. [174], the continuous kinetic resolution of rac1-phenylethanol in IL–CO2 biphasic system was investigated at
120 and 150 ◦ C and 10 MPa. Both free and immobilized CALB
dispersed in [emim][Tf2 N] and [bmim][Tf2 N] were used as catalyst. [emim][Tf2 N] was a better IL in terms of obtaining higher
initial synthetic activity and longer half-life time of the free
enzyme–IL system. Immobilized enzyme–IL system had longer
half-life time and higher synthetic activity compared to free
enzyme–IL system. No activity losses during successive operation cycles were observed for immobilized enzyme at 120 ◦ C and
10 MPa. Additionally, at the same operating conditions (120 ◦ C,
10 MPa), in immobilized CALB–[emim][Tf2 N] system, high
selectivity values (>98%) were always obtained for successive
operation cycles, whereas in free CALB–[emim][Tf2 N] system,
the selectivity increased from 36 to 98.5% as the number of
operation cycles increased.
Reetz et al. [175] studied the acylation of 1-octanol
by vinyl acetate (batch and continuous mode) and kinetic
resolution of 1-phenylethanol (batch mode), which were
catalyzed by CALB in [bmim][BTA]–CO2 system (BTA:
bis(trifluoromethanesulfonamide)). The acylation of 1-octanol
174
by vinyl acetate in continuous mode was performed at 10.5 MPa
and 45 ◦ C. A total yield of 93.9% was obtained after 24 h. Reetz
et al. [180] introduced a new method for enantiomer separation utilizing biocatalytic kinetic resolution and SCF extraction
using an IL–scCO2 system. Application of this new method for
separation of racemic secondary alcohols by CALB catalysis
and CO2 extraction was represented. They converted several alcohols (2-octanol, 1-phenyl-ethanol, 3-methyl-2-butanol,
1-(2-phenylethyl) ethanol, and 1-(2-napthyl) ethanol) enantioselectively to corresponding acetates and laureates by immobilized
CALB suspended in [bmim][BTA], and separate (S)-alcohol
from the product (R)-ester via CO2 . Vinyl laureate was found to
make the ester less soluble in CO2 than the alcohol, which allows
for efficient separation, therefore it was used as the acylation
agent. In that work, both batch and continuous modes of operation were studied. In batch mode, when vinyl laureate was used
as the acylation agent, in the early fractions of isobaric extraction, the alcohol was extracted with high selectivity, whereas the
lauryl ester was obtained with high purity in the later fractions.
In order to speed up the extraction for the ester-rich fraction, they
suggested increasing the CO2 density after most of the alcohol
has been isolated. This procedure was successful for the separation of several alcohols and lauryl esters. Additionally, they
include a separation chamber leading to a two-step extraction
procedure. First, extraction was done at 60 ◦ C and 105 bar, then
in the separation chamber the pressure was reduced to 80 bar and
then the CO2 was vented through cryo-traps. By this procedure
66% of the theoretical amount of the alcohol under investigation was isolated with an enantiomeric purity >99.9% and less
than 0.5% contamination with its corresponding lauryl esters.
Then the pressure of reactor and the extraction chamber was
increased to 200 bar and 89% of the theoretical amount of lauryl ester was isolated with an enantiomeric purity >99.9% and
less than 1% contamination with alcohol. Kinetic resolution of
rac-1-phenylethanol was used to demonstrate the continuous
process. The separation after extraction was provided through
two separation chambers with two steps of density reduction.
After 112 h operation, 81% of the theoretical amount of the (S)1-phenylethanol was isolated with an enantiomeric purity >97%
and less than 0.1% contamination with its corresponding lauryl
ester.
Lozano et al. [176] studied the synthesis of glycidyl esters
from rac-glycidol catalyzed with free and immobilized forms
of lipases from C. antarctica (CALA and CALB) and Mucor
miehei (MML) in toluene, IL and IL–scCO2 (40, 50 ◦ C and
100, 150 bar). Four different ILs were used: [emim][NTf2 ],
[bmim][PF6 ], [bmim][NTf2 ], and trioctylmethylammonium triflimide ([troma][Tf2 N]). Using ILs instead of a classical organic
solvent (toluene) and the increase in the alkyl chain length of
the acylation agent had both positive effect on the enzyme activity and when these effects combined, the synthetic activity can
be enhanced 95 times. CALA and MML favored the synthesis
of R-glycidyl esters, whereas CALB favored the synthesis of Sglycidyl esters. CALB showed the highest activity among other
enzymes. CO2 was used to transport the substrates and products. The synthetic activity of the free and immobilized lipases
decreased in IL–scCO2 biphasic system, but the enantioselec-
tivity remained unchanged with respect to the values obtained
in only IL reaction media.
CALB catalyzed ester synthesis in IL–scCO2 biphasic
systems with five different ILs ((3-hydroxypropyl)-trimethylammonium bis(trifluoromethylsulfonyl) imide [C3 OHtma]
[NTf2 ]; (3-cyanopropyl)-trimethylammonium bis(trifluoromethylsulfonyl) imide [C3 CNtma][NTf2 ]; butyl-trimethylammonium bis(trifluoromethylsulfonyl) imide, [C4 tma][NTf2 ]; (5cyanopentyl)-trimethylammonium bis(trifluoromethylsulfonyl)
imide [C5 CNtma][NTf2 ]; hexyl-trimethylammonium bis(trifluoromethylsulfonyl) imide, [C6 tma][NTf2 ]) were studied by
Lozano et al. [177]. The suitability of these ILs as enzymatic reaction media was tested for the kinetic resolution of
rac-phenylethanol by transesterification with vinyl propionate,
and all of the tested ILs were found to be suitable media for
enzyme catalysis. The synthetic activities and stabilities of the
enzyme were determined in these five ILs. Then, the performance of CALB catalysis in IL–scCO2 biphasic system in
continuous operation was tested for [C4 tma][NTf2 ]–scCO2 and
[C3 CNtma][NTf2 ]–scCO2 systems at 50 ◦ C and 10 MPa for
the synthesis of several short-chain alkyl esters (butyl acetate
(BA), butyl propionate (BP), butyl butyrate (BB), hexyl propionate (HP), hexyl butyrate (HB), and octyl propionate (OP)), by
transesterification from the respective vinyl alkyl esters with
alkyl-1-ols. The results showed that the synthetic activity of
CALB in [C4 tma][NTf2 ]–scCO2 system was higher than that in
[C3 CNtma][NTf2 ]–scCO2 , even though the opposite result was
obtained for activity values in pure IL. The authors concluded
that rate-limiting parameters (synthetic activity and mass transfer phenomena between IL and scCO2 phases) were related with
the solubility parameter of the IL’s alkyl chain and reagents.
Hernandez et al. [178] presented the synthesis of butyl
propionate in scCO2 and IL–scCO2 using a recirculating enzymatic membrane reactor, in which ␣-alumina membranes were
immobilized with CALB. The reactor was tested in only
scCO2 at 50 ◦ C and 80 bar. In the second part of the study,
the immobilized enzyme was coated with three different ILs,
i.e. [bmim][PF6 ], 1-butyl-2,3-dimethylimidazolium hexafluorophosphate [bdimim][PF6 ], [omim][PF6 ]. The selectivity was
95% when only scCO2 medium was utilized, and it increased to
greater than 99.5% when IL–scCO2 biphasic system was used.
Lozano et al. [179] investigated the importance of the
supporting material for the activity of immobilized CALB
in IL–hexane and IL–scCO2 biphasic systems at controlled
water activity. For this purpose they immobilized a commercial solution of free CALB onto twelve different silica supports,
modified with specific side chains (e.g. alkyl, amino, carboxylic,
nitrile, etc.) by adsorption. Both biphasic media was tested for
the kinetic resolution of rac-phenylethanol by transesterification with vinyl propionate. CALB activity and stability was
tested IL–hexane and IL–scCO2 biphasic systems with two
different ILs: [btma][NTf2 ] and [toma][NTf2 ] (btma: butyltrimethylammonium, toma: trioctyl-methylammonium). The
highest synthetic activity of immobilized enzyme was obtained
for butyl-silica derivative, however for all supports, the selectivity for the reaction was higher than 94% except for the quaternary
ammonium-Si support and the enantiomeric excess was greater
175
Fig. 13. A process suggested for cleaning of soils using IL and scCO2 [181].
than 99.9%. The immobilized activity decreased 10 times in
IL–hexane compared to that in only hexane, but half-life times
were enhanced by up to six times. The synthetic activity of
immobilized CALB increased by six times in IL–scCO2 compared to that in hexane.
Recently, the potential of ILs to dissolve soil contaminants at
ambient conditions and the ability of supercritical carbon dioxide (scCO2 ) to recover these contaminants from IL extracts were
mutually utilized to clean contaminated soils [181]. Naphthalene was used as the model component to represent a group of
soil contaminants, and 1-n-butyl 3-methylimidazolium hexafluorophosphate ([bmim][PF6 ]) was used as the IL. The results
demonstrated that soil contaminated with naphthalene was
cleaned using [bmim][PF6 ], and the amount of naphthalene
remaining in the soil was below the allowable contamination
limit. This study was the first in the literature which investigates
the soil/model-contaminant/IL/scCO2 system. On the basis of
the findings a process flowsheet for the IL extraction of contaminated soils and continuous scCO2 extraction of the contaminants
from IL extracts was suggested and is given in Fig. 13.15
8. Summary
This work aims to summarize and discuss the information
present in the literature about ILs and IL–CO2 systems. A
great number of researchers investigated the high-pressure phase
behavior of IL–CO2 systems and concluded that CO2 is highly
soluble in most ILs, and ILs are not measurably soluble in scCO2 .
The effects of pressure, temperature, nature of the anion and the
alkyl chain length of the cation on the solubility of CO2 reported
by various studies are discussed and summarized here. Volatile
and nonpolar scCO2 has become a good partner of nonvolatile
and polar IL and this new system with its unique properties
have been utilized to extract organic compounds from ILs using
scCO2 .
ILs are receiving more and more attention every day both in
academic research and commercial applications and they seem
as good replacements for volatile organic solvents. However,
there is a discussion about the greenness of the ILs due to their
incomplete physical, chemical and toxicological data. Although,
there are some question marks related to the specific character-
176
istics of ILs, it seems that most of the researchers will continue
to work with this new solvent, the developments of new applications utilizing ILs will increase rapidly and the number of
publications will rise exponentially in the future. The present
costs of the ILs are quite prohibitive in many probable commercial applications. However, there are hopes that in the near
future, the cost/benefit figures of the ILs will bring economic
viability to their more common use.
Acknowledgements
The financial supports provided by Boğaziçi University
Research Foundation via the Project No. 06A502 and TÜBİTAK
(The Scientific and Technical Research Council of Turkey) via
the Project No. 104M185 are gratefully acknowledged. The
authors thank the three anonymous reviewers for their detailed
and insightful comments and suggestions, which significantly
improved the manuscript.
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A review of ionic liquids towards supercritical fluid applications

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