ScienceWeek

CHEMISTRY: ON IONIC LIQUIDS

The following points are made by Peter Wasserscheid (Nature 2006 439:797):

1) The unique physical and chemical properties of ionic liquids -- molten salts characterized by melting points below 100 deg C --can be used to enhance the efficiency of a wide range of electrochemical, analytical, synthetic and engineering processes[1]. This has generated huge interest in these liquids, with their first applications in industrial organic synthesis now being reported[2,3]. New work[4], a collaboration of researchers from Portugal, Northern Ireland and the United States, reports that some ionic liquids are volatile, and so can be distilled. The findings shake up the received wisdom that ionic liquids are involatile as a class, and might extend their range of applications still further.

2) Exactly how an ionic liquid behaves varies sharply according to its specific ionic composition: for example, both highly hydrophobic and highly hydrophilic liquids can be produced. Certain ionic liquids are known to undergo thermal decomposition through mechanisms such as the transfer of an alkyl group[5] or, in the case of protic ionic liquids (whose cation is a proton-donating, or Bronsted, acid), through deprotonation. For some imidazolium-based ionic liquids, in which a weakly acidic proton is bound to a carbon atom, the formation of highly reactive organic molecules known as carbenes in the presence of a strong base has also been reported. The decomposition products are usually volatile, and, in some cases, re-formation of an ionic liquid in colder parts of the apparatus has been observed.

3) Involatility had been assumed to be a property common to all ionic liquids that do not undergo thermal decomposition. Earle et al[4] change that assumption, showing that certain ionic-liquid structures known for their very high thermal stability, in particular bis{(trifluoromethyl)sulphonyl}amide salts, can be evaporated and recondensed under relatively mild conditions. The distillation of 1-hexyl-3-methylimidazolium bis{(trifluoromethyl)-sulphonyl}amide, for instance, is reported to occur at 170 deg C and at a pressure of 0.07 millibar.

4) It should be noted that Earle et al[4] do not give direct proof for the presence of ions in the gas phase of their distillation processes. But they do describe several instances of ionic liquid evaporation and recondensation for which alkyl- and proton-transfer processes leading to neutral volatiles are both highly unlikely. Of most interesting, they report the separation by distillation of a mixture of two ionic liquids. This is also by itself no absolute proof that ions are present in the gas phase: the separation could, in at least one of the demonstrated cases, just reflect the different volatility of carbenes formed in a process of decomposition. Yet despite these reservations, the evidence for the volatility of at least some ionic liquids is convincing.

References (abridged):

1. Wasserscheid, P. & Welton, T. (eds) Ionic Liquids in Synthesis (Wiley-VCH, Weinheim, 2003)

2. Freemantle, M. Chem. Eng. News 81, 9 (2003)

3. Maase, M. in Multiphase Homogeneous Catalysis (eds Cornils, B. et al.) 560-566 (Wiley-VCH, Weinheim, 2005)

4. Earle, M. J. et al. Nature 439, 831-834 (2006)

5. Maase, M. World patent WO 05/068404 (2005)

Nature http://www.nature.com/nature

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Related Material:

ON THE DISSOLUTION OF CELLULOSE BY IONIC LIQUIDS

Notes by ScienceWeek:

In general, in this context, the term "derivatization" refers to the process of deriving a compound from some other compound while maintaining general structure: e.g., trichloromethane (chloroform) is a derivative of methane.

The following points are made by R.P. Swatloski et al (J. Am. Chem. Soc. 2002 124:4974):

1) Cellulose is the most abundant biorenewable material, with a long and well-established technological base.(1) Derivatized products have many important applications in the fiber, paper, membrane, polymer, and paints industries. Cellulose consists of polydisperse linear glucose polymer chains which form hydrogen-bonded supramolecular structures(2); cellulose is insoluble in water and most common organic liquids. The growing interest to develop new cellulosic materials results from the fact that cellulose is a renewable resource, although many of the technologies currently used in cellulose processing are decidedly nongreen.(3) For example, viscose rayon is prepared from cellulose xanthate (production over 3 million tons per year) utilizing carbon disulfide as both reagent and solvent. Most recently, processes using more environmentally acceptable nonderivatizing solvents (N-methylmorpholine-N-oxide and phosphoric acid) have been commercialized. Solvents are needed for dissolution that enable homogeneous phase reactions without prior derivatization.(4)

2) C. Graenacher (5) first suggested in 1934 that molten N-ethylpyridinium chloride, in the presence of nitrogen-containing bases, could be used to dissolve cellulose; however, this seems to have been treated as a novelty of little practical value, since the molten salt system was at the time somewhat esoteric and has a relatively high melting point (118 C).

3) The authors report they have examined whether other solvents that would now be described as ionic liquids would dissolve cellulose and, especially, whether the availability of a wide and varied range of ionic liquids, coupled with the current understanding of their solvent properties would allow flexibility and control in the processing methodology, with increased solution efficiency and reduction or elimination of undesirable solvents. The authors report that ionic liquids can be used as nonderivatizing solvents for cellulose. Ionic liquids incorporating anions which are strong hydrogen bond acceptors were most effective, especially when combined with microwave heating, whereas ionic liquids containing "non coordinating" anions were nonsolvents. Chloride containing ionic liquids appear to be the most effective solvents, presumably solubilizing cellulose through hydrogen-bonding from hydroxyl functions to the anions of the solvent.

References (abridged):

1. Kirk-Othmer Encyclopedia of Chemical Technology, 4th ed.; Wiley: New York, 1993; Vol. 5, p 476.

2. Finkenstadt, V. L.; Millane, R. P. Macromolecules 1998, 31, 7776-7783.

3. Johnson, D. C. In Cellulose Chemistry and its Application; Nevell, T. P., Zeronian, S. H., Eds.; E. Horwood: Chichester, 1985; p181.

4. (a) Augustine, A. V.; Hudson, S. M.; Cuculo, J. A. In Cellulose Sources and Exploitation; Kennedy, J. F., Philipps, G. O., Williams, P. A., Eds.; E. Horwood: New York, 1990; p59. (b) Dawsey, T. R. In Cellulosic Polymers, Blends and Composites; Gilbert, R. D., Ed.; Carl Hanser Verlag: New York, 1994; p157.

5. Graenacher, C. Cellulose Solution. U.S. Patent 1,943,176, 1934.

J. Am. Chem. Soc. http://pubs.acs.org/JACS

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