Presentation on theme: "Ionic Liquids in Green Chemistry Dr. Nie Wanli Chemistry Department of NWU, Xi’an."— Presentation transcript:
Ionic Liquids in Green Chemistry Dr. Nie Wanli Chemistry Department of NWU, Xi’an
What are Ionic liquids (ILs)? Why consider of ILs? The characteristic properties of ionic liquids The synthetic methods Research with ILs Outlook Ionic Liquids in Green Chemistry
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 o C. saltssodium chloride
Ionic liquids are salts that are liquid at low temperature (<100 o C) which represent a new class of solvents with nonmolecular, ionic character. What are ionic liquids?
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 media: 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.
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. What is “green chemistry” ?
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. Why consider Ionic liquids ?
Important IL Properties High ionic conductivity Non-flammable Non-volatile High thermal stability Wide temperature range for liquid phase (- 40 to + 200°C) Highly solvating, yet non-coordinating Good solvents for many organic and inorganic materials
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.acidsbases ligands 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
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 Typical RTIL Cations
A wide range of anions is employed, from simple halides which inflect high melting points, to inorganic anions such as:anions halides Anions: Anions for RTIL
[PF 6 ] - for moisture stable, water immiscible IL [BF 4 ] - 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] CF 3 SO 2 - CF 3 (CF 2 ) 3 SO 2 - Bis(triflyl)amide [Tf 2 N] Trifluoroacetate [TA] (CF 3 SO 2 ) 2 N - CF 3 CO 2 - Heptafluorobutanoate [HB] CF 3 (CF 2 ) 3 CO 2 -
Historical Development Ethylammonium nitrate, which is liquids at RT was first described in 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.
Ionic liquid synthesis Direct quaternization to form cation Alkylation reagents Indirect quaternization to form cation Direct quaternization to form cation Alkylation reagents Indirect quaternization to form cation
Ionic liquid synthesis General procedures:
organoaluminates air- and water-stable ionic liquids The types of RTILS
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: Al 2 Cl Cl - = 2 AlCl 4 - Acidic basic neutral 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. x (p) Mp ( o C)PH basic neutral acidic Table 1. Melting Point (Mp) and Viscosity ( ) 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 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.
“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.organic chemistryelectrochemistryphysical chemistryengineering Applications 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 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
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, emimBF 4 is reportedly stable up to 300 °C and emim- (CF 3 SO 2 ) 2 N 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. H 2, CO and O 2, 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. BF 4 2 and PF 6 2 and, hence, have the potential to be highly 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. H 2, CO and O 2, is generally good which makes them attractive solvents for catalytic hydrogenations, carbonylations, hydroformylations, and aerobic oxidations. Catalysis in ionic liquids general considerations potentially attractive media for homogeneous catalysis:
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’.
They are often composed of weakly coordinating anions, e.g. BF 4 - and PF 6 - 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 emimAlCl 4 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.
Stille reaction Y. (%) 1st use82 2nd use78 3rd use72 * Simple workup extraction Ionic liquid/catalyst phase can be reused Air and moisture stable vinyltributyltin iodocyclohexenone
Stabilize catalysts Simple workup Atom economy Other reactions Suzuki-Miyaura coupling reaction Trost-Tsuji coupling Hydroformylation (biphase)
Friedel-Crafts alkylations and acylations Arene exchange reactions The ionic liquids acts as both a solvent and catalyst for a acid catalysed processes involve cationic intermediates, e,g. carbenium and acylium ions Carbocation Chemistry IL containing chloroaluminate anions are strong Lewis acids and if protons are present they are superacidic.
Friedel-Crafts reaction--- acylation anisole quantitatively regioselective methoxybenzophenone 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.
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 [Al 2 Cl 7 ] - is the active Lewis acid. Conventional problems with these reactions (e.g., lower yields with solid arenes) are eliminated. ReactantAreneProductYield (%) BenzeneFe(C 5 H 5 )(C 6 H 6 ) + 53 tolueneFe(C 5 H 5 )(C 6 H 5 Me) + 64 napthaleneFe(C 5 H 5 )(C 10 H 8 ) + 53 Arene exchange reactions
Separations Witting reaction Others
Witting reaction Y. (%)E/Z (%) 1st use8297/3 2nd use83 6th use91 The separation of the product and triphenylphosphine oxide Extractions Reuse IL
Reduction ionic liquidtemp. ( o C)time (h)Y. (%) bmimBF emimBF emimPF emimPF 6 r.t.4894 Lower temperature
Fluorination entrysolventcosolventTemp.( o C)Time (h)1 (%)2 (%) 1MeCN H 2 Or.t. over night 71 small amount 2bmimBF 4 MeOH Short reaction time High yield N-fluoro-N’-(chloromethyl)triethylenediamine bis(tetrafluoroborate)3-fluorinated 2-oxoindoles
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 systems
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 BF 4 - and PF 6 - ionic liquids have been developed as moisture stable electrolytes
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 ‘designer’ligands prepared for special applications. Other types of ionic liquids: Other types of ionic liquids
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
Further information regarding physical properties, chemistry, and uses of ionic liquids:  Welton T. Chem. Rev., 1999, 99:  Wasserscheid P, Keim W. Angew Chem..Int. Ed. Engl., 2000, 39:  Freemantle M. (a) Chem. Eng. News, 2000, 78 (May)15: 37-39; (b) Chem. Eng. News, 2001, 79 (Jan)1:  Earle M J, Seddon K R. Pure Appl, Chem., 2000, 72 (7):  Chum H L, Koch V N et al. J. Am. Chem, Soc., 1975, 97:  Wilkes JS et al. Inorg. Chem., 1982, 21:  a) Blanchard L A et al. Nature, 1999, 399: 28; b) Blanchard L A et al. Ind. Egn. Chem. Res., 2001, 40: 287.  Chauvin Y, Mu mann L, Olivier H. Angew. Chem. Int. Engl., 1995, 34:  Monteiro A L et al. Tetrahedron Asymmetry, 1997, 2:  Song C E, Roh E J. Chem. Commun., 2000:  Dullins J E L et al. Organometallics, 1998, 17: 815.  Kakfman D E et al. Synlett., 1996: REFERENCES
 Mathews C J, Smith P J, Welton T. Chem. Commun., 2000:  Bellefon C de et al. J. Mol. Catal., 1999, 145: 121.  Adam C J et al. Chem. Commun., 1998:  Boon J A et al. J. Org. Chem., 1986, 51: 48.  Kun Qian, Yonquan Deng. J. Mol. Catal. A: Chem., 2001, 171:  Surretle J K D, Green L, Singer R D. Chem. Commun., 1996:  Wheeler C et al. Chem. Commun., 2001: 887.  Earle M J, McCormac P B, Seddon K R. Chem. Commun, 1998:  Hagiwara R, Ito Y J. Fluorine Chem., 2000, 105: 221.  Boularre V L, Gree R. Chem. Commun., 2000:  Gordone L M, McClusky A. Chem. Commun., 1999:  Kanalka G W, Maladi R R. Chem. Commun., 2000:  Fischer F, Sethi A, Welton T et al. Tetranedron letters, 1999, 40:  Earle M J, McCormac P B, Seddon K R. Gree. Chem., 1999, 1:  Visser A E, Swatloski R P, Reichert W M et al. Chem. Commun., 2001: 135.  Lall S I, Mancheno D, Castro S. Chem. Commun., 2000: 2413.