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

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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

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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.

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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
character.
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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

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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
  media
• greener processes
• recycling homogeneous catalysts

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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.

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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
processes.
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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
  processes.

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Important IL Properties

• High ionic conductivity
• Non-flammable
• Non-volatile
• High thermal stability
• Wide temperature range for liquid phase (- 40 to
  200C)
• Highly solvating, yet non-coordinating
• Good solvents for many organic and inorganic
  materials

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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.

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• 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.

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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

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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
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Anions for RTIL

• A wide range of anions is employed, from simple
  halides which inflect high melting points, to
  inorganic anions such as

Anions
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• 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
  cation.
• Less common anions include
• Triflate TfO Nonaflate
  NfO
• CF3SO2-
  CF3(CF2)3SO2-
• Bis(triflyl)amide Tf2N
  Trifluoroacetate TA
• (CF3SO2)2N-
  CF3CO2-
• 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

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Ionic liquid synthesis
General procedures
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The types of RTILS

• organoaluminates
• air- and water-stable ionic liquids

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Organoaluminates

• 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
  neutral
• Basic haloaluminates preclude solvation and
  solvolysis of metal ion species

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• 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

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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
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Ambient-Temperature, Air- and Water- stable Ionic
liquids

• 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
  anion.
• Can be used for the immobilization of
  transition-metal catalyst precursors in biphase
  catalysis.
• 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.

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The melting point is influenced by the nature
of cation and anion
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Applications

• 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
catalysis

• 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
  oxidations.

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• 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
  catalysts.
• 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.

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• 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.

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• 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.

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Applications

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

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Solvent Properties

• Diels-Alder reaction
• Aldol condensation
• Others

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

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Aldol Condensation

• Solubility

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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.
  seddon)
• Organometallic synthesis (T. welton)
• This list is not exhaustive

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Transition Metal Catalyzed Reaction

• Hydrogenation
• Heck reaction
• Stille reaction
• Other reactions

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Hydrogenation reaction
Dupont et al.

• Two phase system
• Simple workup -------decantation
• Ionic liquid/catalyst phase can be reused

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IL in Two-Phase Catalytic Reactions
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Heck Reaction (1)
stilbenes
styrene

• Polar solvent
• Expensive
• Phosphine ligand

• Less expensive
• High yields
• Without phosphine

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Heck reaction (2)
enol ethers

• High regioselectivity
• Simple workup -------distillation

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Stille reaction
vinyltributyltin
iodocyclohexenone

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

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Other reactions

• Suzuki-Miyaura coupling reaction
• Trost-Tsuji coupling
• Hydroformylation (biphase)

• Stabilize catalysts
• Simple workup
• Atom economy

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Carbocation Chemistry

• IL containing chloroaluminate anions are strong
  Lewis acids and if protons are present they are
  superacidic.

• 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

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Friedel-Crafts reaction---acylation
methoxybenzophenone
anisole

• quantitatively
• regioselective

• Y 64 in acetonitrile
• p-/o- ratio of 93/7

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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
  acid.
• Conventional problems with these reactions (e.g.,
  lower yields with solid arenes) are eliminated.

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Separations

• Witting reaction
• Others

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Witting reaction

• The separation of the product and
  triphenylphosphine oxide

• Extractions
• Reuse IL

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Reduction

• Lower temperature

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Fluorination
N-fluoro-N-(chloromethyl)triethylenediamine
bis(tetrafluoroborate)
3-fluorinated 2-oxoindoles

• Short reaction time
• High yield

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Ring opening reaction
epoxide

• room temperature, economic

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• 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.

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Enzymatic reaction

• similar yields to those of organic solvent
  systems

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Others
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Electrochemistry

• 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

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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,
  etc.
• Comparators for direct comparison of IL and
  traditional solvents
• Industrial input into a research Agenda
• Economic synthetic pathways
• Wider availability

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L et al. Organometallics, 1998, 17 815.12
Kakfman D E et al. Synlett., 1996 1091.
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Thanks