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Ionic Liquids in Green Chemistry Dr. Nie Wanli Chemistry


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Title: Ionic Liquids in Green Chemistry Dr. Nie Wanli Chemistry

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Ionic Liquids in Green Chemistry
  • Dr. Nie Wanli
  • Chemistry Department of NWU, Xian

Ionic Liquids in Green Chemistry
  • What are Ionic liquids (ILs)?
  • Why consider of ILs?
  • The characteristic properties of ionic liquids
  • The synthetic methods
  • Research with ILs
  • Outlook

What are ionic liquids?
  • Definition
  • ------ Quite simply, they are liquids that are
    composed entirely of ions.
  • In the broad sense, this term includes all the
    molten salts, for instance, sodium chloride at
    temperatures higher than 800 oC.

What are ionic liquids?
------ Ionic liquids are salts that are liquid at
low temperature (lt100 oC) which represent a new
class of solvents with nonmolecular, ionic
Room temperature Ionic liquids
  • Room temperature ionic liquids (RTIL) are salts
    that are liquid over a wide temperature range,
    including room temperature.
  • Variations in cations and anions can produce
    literally millions of ionic liquids, including
    chiral, fluorinated, and antibacterial IL.
  • Large number of possibilities allows for
    fine-tuning the ionic liquid properties for
    specific applications

The driving forces
  • The problems in the chemical industry with the
    volatile organic compounds (VOCs)
  • toxic and/or hazardous
  • serious environmental issues, such as
    atmospheric emissions and contamination of
    aqueous effluents
  • The driving force in the quest for novel reaction
  • greener processes
  • recycling homogeneous catalysts

The key to waste minimization
  • The key to waste minimization in chemicals
    manufacture is the substitution of classical
    stoichiometric syntheses by atom efficient,
    catalytic alternatives.

What is green chemistry ?
Recently ionic liquids have often been
discussed as promising solvents for clean
processes and green chemistry. These two
catchwords means to reduce drastically the
amounts of side and coupling products and the
solvent and catalyst consumption in chemical
Why consider Ionic liquids ?
  • ILs are environmentally-friendly alternatives to
    organic solvents for liquid/liquid extractions.
    Catalysis, separations, and electrochemistry.
  • ILs will reduce or eliminate the related costs,
    disposal requirements, and hazards associated
    with volatile organic compounds (VOCs).
  • The ability to fine-tune the properties of the IL
    medium will allow selection of IL to replace
    specific solvents in a variety of different

Important IL Properties
  • High ionic conductivity
  • Non-flammable
  • Non-volatile
  • High thermal stability
  • Wide temperature range for liquid phase (- 40 to
  • Highly solvating, yet non-coordinating
  • Good solvents for many organic and inorganic

Great promise
  • Designability. By combining different anions with
    cations, it is possible to generate a huge number
    of different ionic liquids, each with their own
    specific solvent properties. Some ionic liquids
    are water soluble, others are not. Some dissolve
    typical organic solvents, other are not.
  • They can be functionalized to act as acids, bases
    or ligands and have the potential to catalyze
    certain reactions in certain systems.
  • Ionic liquids are non-volatile, hence they may be
    used in high vacuum systems and high temperature
    reactions without the requirement of a pressure
    vessel to contain the vapors.

  • They are good solvents for a wide range of both
    inorganic, organic and polymeric materials and
    unusual combinations of reagents can be brought
    into same phase. However they do not dissolve
    glass, polyethylene, or Teflon. High solubility
    usually implies small reactor volumes in the
    final process.
  • They are immiscible with a number of organic
    solvents and provide a non-aqueous, polar
    alternative for two phase systems, this has been
    used to effect total catalyst recovery in a
    number of transition metal catalyzed reactions.
    Hydrophobic ionic liquids can also be used as
    immiscible polar phase with water.
  • They are often composed of poorly coordinating
    ions, so they have the potential to be highly
    polar non-coordinating solvents, this is
    particularly important when using
    transition-metal based catalysts.

Characteristics of RTIL
  • Choice of cation and anion determine physical
    properties (e.g. melting point, viscosity,
    density, water solubility, etc.)
  • Cations are typically big, bulky, and asymmetric
    accounting for the low melting points
  • The anion contributes more to the overall
    characteristics of the IL and determines the air
    and water stability
  • Melting point can be easily changed by structural
    variation of one of the ions or combining
    different ions

Typical RTIL Cations
  • Room temperature ionic liquids consist of bulky
    and asymmetric organic cations such as

Imidazolium ion Pyridium ion
Ammonium ion Phosphonium ion
Scheme 1. Important types of cation
Anions for RTIL
  • A wide range of anions is employed, from simple
    halides which inflect high melting points, to
    inorganic anions such as

  • PF6- for moisture stable, water immiscible IL
  • BF4- for moisture stable, but water miscible IL
    depending on the ratio of ionic liquid water,
    system temperature, and alkyl chain length in the
  • Less common anions include
  • Triflate TfO Nonaflate
  • CF3SO2-
  • Bis(triflyl)amide Tf2N
    Trifluoroacetate TA
  • (CF3SO2)2N-
  • Heptafluorobutanoate HB
  • CF3(CF2)3CO2-

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Historical Development
  • Ethylammonium nitrate, which is liquids at RT was
    first described in 1914.
  • In the later 1940s, n-alkylpyridinium
    chloroaluminates were studied as electrolytes for
    electroplating aluminum.
  • The first examples of ionic liquids based on
    dialkylimidazolium cations were reported in the
    early 1980s. They contain chloroaluminate anions
    and proved to be useful catalysts/solvents for
    Friedel-Crafts acylations.
  • The first example of the new ionic liquids, that
    currently are receiving so much attention as
    novel media for homogeneous catalysis,
    ethylmethylimidazolium tetrafluoroborate was
    reported in 1992.

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Ionic liquid synthesis
  • Direct quaternization to form cation
  • ------Alkylation reagents
  • Indirect quaternization to form cation

Ionic liquid synthesis
General procedures
The types of RTILS
  • organoaluminates
  • air- and water-stable ionic liquids

  • Since the organoaluminate ionic liquids have
    donor and acceptor patterns, The Lewise acidity
    can be modulated by the relative amount of the
    aluminum compound. Acidic or basic IL attainable
    through varying the concentration of the
    following species
  • Al2Cl7- Cl-
    2 AlCl4-
  • Acidic basic
  • Basic haloaluminates preclude solvation and
    solvolysis of metal ion species

  • Large electrochemical windows for both chloro and
    bromo ionic liquids.
  • The advantage of this controlled Lewis acid ionic
    liquids is their use in Ziegler-Natta Type
    catalytic reactions
  • BUT moisture sensitive

Table 1. Table 1. Table 1.
Table 1. Melting Point (Mp) and Viscosity (n )
of 1-Ethyl-3-methylimidazolium Chloride/Aluminum
Chloride Ionic liquid at different Molar
Fractions (x) of the Aluminum Compound
Ambient-Temperature, Air- and Water- stable Ionic
  • Can be obtained by the substitution of the halide
    anion of the 1,3- dialkylimidazolium cation by
    other weekly coordinating anions.
  • In order to be liquid at room temperature, the
    cation should preferably be unsymmetrical. The
    melting point is also influenced by the nature of
  • Can be used for the immobilization of
    transition-metal catalyst precursors in biphase
  • Due to their inherent ionic nature, ionic liquids
    can effectively stabilize cationic
    transition-metal special that are known to be
    more attractive than their neutral analogues.

The melting point is influenced by the nature
of cation and anion
  • Because of their properties, ionic liquids
    attract great attention in many fields, including
    organic chemistry, electrochemistry, physical
    chemistry, and engineering.

1. as reaction media for synthesis and
catalysts 2. in electrochemistry 3. in
separation processes 4. as electrolytes in solar
cells 5. as lubricants 6. as propellants in small
satellites 7. matrixes in MALDI mass
spectrometry 8. Applications in other areas
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Catalysis in ionic liquids general
considerations Room temperature ionic liquids
exhibit many properties which make them
potentially attractive media for homogeneous
catalysis 4 They have essentially no vapour
pressure, i.e. they do not evaporate and are easy
to contain. 4 They generally have reasonable
thermal stability. While tetraalkylammonium salts
have limited thermal stability, owing to
decomposition via the Hoffmann elimination,
emimBF4 is reportedly stable up to 300 C and
emim- (CF3SO2)2N up to 400 C.16a In other words
many ionic liquids have liquid ranges of more
than 300 C, compared to the 100 C liquid range
of water. 4 They are able to dissolve a wide
range of organic, inorganic and organometallic
compounds. 4 The solubility of gases, e.g. H2, CO
and O2, is generally good which makes them
attractive solvents for catalytic hydrogenations,
carbonylations, hydroformylations, and aerobic
oxidations. 4 They are immiscible with some
organic solvents, e.g. alkanes, and, hence, can
be used in two-phase systems. Similarly,
lipophilic ionic liquids can be used in aqueous
biphasic systems. 4 Polarity and
hydrophilicity/lipophilicity can be readily
adjusted by a suitable choice of cation/anion
(see earlier) and ionic liquids have been
referred to as designer solvents.7 4 They are
often composed of weakly coordinating anions,
e.g. BF42 and PF62 and, hence, have the potential
to be highly
Catalysis in ionic liquids
------general considerations
potentially attractive media for homogeneous
  • They have essentially no vapour pressure which
    facilitates product separation by distillation.
  • They are able to dissolve a wide range of
    organic, inorganic and organometallic compounds.
  • The solubility of gases, e.g. H2, CO and O2, is
    generally good which makes them attractive
    solvents for catalytic hydrogenations,
    carbonylations, hydroformylations, and aerobic

  • They are immiscible with some organic solvents,
    e.g. alkanes, and, hence, can be used in
    two-phase systems. This gives rise to the
    possibility of a multiphase reaction procedure
    with easy isolation and recovery of homogeneous
  • Polarity and hydrophilicity / lipophilicity can
    be readily adjusted by a suitable choice of
    cation/anion and ionic liquids have been referred
    to as designer solvents.

  • They are often composed of weakly coordinating
    anions, e.g. BF4- and PF6- and, hence, have the
    potential to be highly polar yet non-coordinating
    solvents. They can be expected, therefore, to
    have a strong rate-enhancing effect on reactions
    involving cationic intermediates.
  • Ionic liquids containing chloroaluminate ions are
    strong Lewis, Franklin and Brønsted acids.
    Protons present in emimAlCl4 have been shown to
    be superacidic. Such highly acidic ionic liquids
    are, nonetheless, easily handled and offer
    potential as non-volatile replacements for
    hazardous acids such as HF in several
    acid-catalysed reactions.

  • Publications to date show that replacing an
    organic solvent by an ionic liquid can lead to
    remarkable improvements in well-known processes.
  • There are also indications that switching from a
    normal organic solvent to an ionic liquid can
    lead to novel and unusual chemical reactivity.
  • This opens up a wide field for future
    investigations into this new class of solvents in
    catalytic application.

  • Solvent Properties
  • Transition Metal Catalysed Reaction
  • Carbocation Chemistry
  • Separations
  • Electrochemistry
  • Photochemistry

Solvent Properties
  • Diels-Alder reaction
  • Aldol condensation
  • Others

Diels-Alder reaction
methyl acrylate ester
  • Endo selectivity ----highly polar solvents
  • Increases in the reaction rate
  • Allows water sensitive reagents to be used
  • Simple workup
  • Ionic liquid can be reused

Aldol Condensation
  • Solubility

Recent activity with RTIL as solvent
  • sc-CO2 Stripping after Extraction (J. Brennecke)
  • Conductive RTIL (P. Bonhote)
  • Ionic liquid-polymer gel electrolytes (R. Carlin)
  • Catalytic hydrogenation reaction (J. Dupont)
  • Electrochemistry in RTIL (C. Hussey)
  • Butene dimerization (H. Olivier)
  • Benzene polymerization (B. Osteryong)
  • Two-phase separations (R. D. Rogers)
  • Friedel-Crafts regioselectivie alkyl. (K.
  • Organometallic synthesis (T. welton)
  • This list is not exhaustive

Transition Metal Catalyzed Reaction
  • Hydrogenation
  • Heck reaction
  • Stille reaction
  • Other reactions

Hydrogenation reaction
Dupont et al.
  • Two phase system
  • Simple workup -------decantation
  • Ionic liquid/catalyst phase can be reused

IL in Two-Phase Catalytic Reactions
Heck Reaction (1)
  • Polar solvent
  • Expensive
  • Phosphine ligand
  • Less expensive
  • High yields
  • Without phosphine

Heck reaction (2)
enol ethers
  • High regioselectivity
  • Simple workup -------distillation

Stille reaction
  • Simple workup -------extraction
  • Ionic liquid/catalyst phase can be reused
  • Air and moisture stable

Other reactions
  • Suzuki-Miyaura coupling reaction
  • Trost-Tsuji coupling
  • Hydroformylation (biphase)
  • Stabilize catalysts
  • Simple workup
  • Atom economy

Carbocation Chemistry
  • IL containing chloroaluminate anions are strong
    Lewis acids and if protons are present they are
  • The ionic liquids acts as both a solvent and
    catalyst for a acid catalysed processes involve
    cationic intermediates, e,g. carbenium and
    acylium ions
  • Friedel-Crafts alkylations and acylations
  • Arene exchange reactions

Friedel-Crafts reaction---acylation
  • quantitatively
  • regioselective
  • Y 64 in acetonitrile
  • p-/o- ratio of 93/7

Friedel-Crafts reaction---akylations
The Friedel-Crafts alkylation of benzene with
long chain ?olefin catalyzed by
chloroaluminate ionic liquids modified by HCl
which was attributed to the superacidities of
these media, were shown to give higher rates and
more favorable product distributions.
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Arene exchange reactions
  • IL can function as both catalyst and solvent
  • In a series of arene exchange reactions on
    ferrocene, an acidic bmim chloroaluminate IL
    was used where Al2Cl7- is the active Lewis
  • Conventional problems with these reactions (e.g.,
    lower yields with solid arenes) are eliminated.

  • Witting reaction
  • Others

Witting reaction
  • The separation of the product and
    triphenylphosphine oxide
  • Extractions
  • Reuse IL

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  • Lower temperature

3-fluorinated 2-oxoindoles
  • Short reaction time
  • High yield

Ring opening reaction
  • room temperature, economic

  • This reactions require a large excess of the
    amines at elevated temperatures. The high
    temperature reaction conditions are not only
    detrimental to certain functional groups but also
    to the control of regioselectivity.
  • Subsequently, a variety of activators or
    promoters such as metal amides, metal triflates
    and transition metal halides have been developed.
    However, many of these are often expensive or are
    needed in stoichiometric amounts, thus limiting
    their practicality.
  • In the system using ionic liquids, the reaction
    proceeds at room temperature to give
    -aminoalcohols in high yield. After the reaction,
    the product was extracted with ether.The ionic
    liquid was reused in five runs without any loss
    of activity.

Enzymatic reaction
  • similar yields to those of organic solvent

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  • Unique features of chloroaluminate ionic liquids
    include a large electrochemical window, although
    these anions are moisture sensitive
  • Possible applications include low cost and
    recyclable electrolytes for batteries,
    photoelectrochemical cells, and electroplating
  • BF4- and PF6- ionic liquids have been developed
    as moisture stable electrolytes

Other types of ionic liquids
As the range of application for ionic liquids
increase, the need for ionic liquids with special
chemical and physical properties also increases.
With this in mind, the term tast-specific ionic
liquid has been introduced to described
designerligands prepared for special
applications. Other types of ionic liquids
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Concluding remarks
Future IL research Needs
  • Comprehensive toxicity data
  • Combinatorial approach to IL development
  • Database of physical properties, chemistries,
  • Comparators for direct comparison of IL and
    traditional solvents
  • Industrial input into a research Agenda
  • Economic synthetic pathways
  • Wider availability

Further information regarding physical
properties, chemistry, and uses of ionic
liquids 1 Welton T. Chem . Rev., 1999, 99
2071.2 Wasserscheid P, Keim W. Angew Chem.
.Int. Ed. Engl., 2000, 39 3722.3 Freemantle
M. (a) Chem . Eng . News, 2000, 78 (May)15
37-39 (b) Chem . Eng . News, 2001, 79 (Jan)1
21-25.4 Earle M J, Seddon K R. Pure Appl,
Chem., 2000, 72 (7) 1391-1398.5 Chum H L,
Koch V N et al. J. Am. Chem, Soc., 1975, 97 3264
. 6 Wilkes JS et al . Inorg . Chem., 1982, 21
1236.7 a) Blanchard L A et al. Nature, 1999,
399 28 b) Blanchard L A et al. Ind. Egn. Chem.
Res., 2001, 40 287.8 Chauvin Y, Mubmann L,
Olivier H. Angew. Chem. Int. Engl., 1995, 34
2698.9 Monteiro A L et al. Tetrahedron
Asymmetry, 1997, 2 177-179.10 Song C E, Roh E
J. Chem. Commun., 2000 837-838.11 Dullins J E
L et al. Organometallics, 1998, 17 815.12
Kakfman D E et al. Synlett., 1996 1091.
13 Mathews C J, Smith P J, Welton T. Chem.
Commun., 2000 1249-1250.14 Bellefon C de et
al . J. Mol . Catal., 1999, 145 121.15 Adam C
J et al. Chem. Commun., 1998 2097-2098.16
Boon J A et al. J. Org. Chem., 1986, 51 48.17
Kun Qian, Yonquan Deng. J. Mol. Catal. A Chem.,
2001, 171 81-84.18 Surretle J K D, Green L,
Singer R D. Chem. Commun., 1996 2753-2754.19
Wheeler C et al . Chem. Commun., 2001 887.20
Earle M J, McCormac P B, Seddon K R. Chem.
Commun, 1998 2245.21 Hagiwara R, Ito Y J.
Fluorine Chem., 2000, 105 221.22 Boularre V
L, Gree R. Chem. Commun., 2000 2195-2196.23
Gordone L M, McClusky A. Chem. Commun., 1999
1431-1432.24 Kanalka G W, Maladi R R. Chem.
Commun., 2000 2191.25 Fischer F, Sethi A ,
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793-796.26 Earle M J, McCormac P B, Seddon K
R. Gree. Chem., 1999, 123-25.27 Visser A E,
Swatloski R P, Reichert W M et al. Chem. Commun.,
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