Ionic Liquid Applications

There now is a proven extensive organic chemistry in ionic liquids, of which a few key reactions are listed below:21,26,28

• Friedel-Crafts

• Heck and Suzuki coupling

• Chiral hydrogenation

• Oligomerization

• Polymerization

• Sulfonation

• Halogenation

• Diazotization

• N-Alkylation and O-alkylation

• Aldol condensation

It is clear that there are few restrictions to the possible reactions that can be studied. Of particular interest is an article demonstrating that the nature of the ionic liquid can dramatically influence the outcome of a reaction.69 In a study of the reactions between toluene and nitric acid, three completely different products were obtained by using three different ionic liquids (see Figure 5.8). Despite the thousands of articles demonstrating the synthetic utility of ionic liquids, however, there are only, as yet, a few on mechanism, the most notable being from Welton's

group.' In addition, three key articles, 9,7 published in 2000, were the motivation for the creation of the whole field of enzymatic catalysis in ionic liquids.38

Despite the large number of academic publications in this area, perhaps the most significant advance was the public announcement, in 2003, that BASF was operating the first commercial process employing ionic liquids.31,72 This new multiton process is called the BASIL™ (biphasic acid scavenging utilizing ionic liquids) process, and was developed by the BASF team (Matthias Maase, Rudiger Buttner, and Holger Ganz) under the leadership of Dr. Klemens Massonne, from

Matthias Maase
Figure 5.8 The reactions between toluene and nitric acid in (a) a halide-based ionic liquid, (b) a triflate-based ionic liquid, and (c) a mesylate-based ionic liquid.
Figure 5.9 The BASIL reactor showing the reaction mixture, the upper phase being the solvent-free pure product, and the lower the ionic liquid. (Photo: BASF.)

Ludwigshafen, Germany. BASF generated the ionic liquid in situ, to remove waste acidic hydrogen chloride. The key reaction involved is of a well-known generic type:

The product, an alkoxyphenylphosphine, is used as an important precursor for the synthesis of photoinitiators that are themselves used in the manufacture of printing inks, as well as glass fiber and wood coatings. Conventionally, the waste HCl is removed by adding a simple base (such as triethylamine), and filtering off the waste solid chloride salt that is formed. The ingenuity of the BASF team was to use the more expensive 1-methylimidazole, mim, as a base, to form the salt 1-methylimidazolium chloride, [Hmim]Cl, which melts above 75°C. Above this temperature, this salt is a colorless, dense ionic liquid, immiscible with the reaction mixture, which separates as a discrete layer (Figure 5.9). This is then removed by gravity separation (much easier and cheaper than filtration), and the 1-methylimidazole is regenerated from the ionic liquid, and recycled.73 The space-time yield for the formation of the product alkoxyphenylphosphine was thus increased from 8kgm_3h_1 using NEt3 to 690,000 kgm_3h_1 using 1-methylimidazole, a factor of 80,000 increased productivity! In early 2002, BASIL was established in regular production for the synthesis of alkoxyphenyl-phosphines and has been in regular operation since then.

Although BASIL was the first publicly recognized industrial ionic liquid process, Stephen Falling, of the Eastman Chemical Company, announced at the first International Congress on Ionic Liquids (COIL; Salzburg, June 2005)74 that they had, in fact, been running an ionic liquid process since 1996, for the production of 2,5-dihydrofuran (1400 tonnes per annum).75 At the same meeting, Dan Tempel, from Air Products, announced that they had developed a new subat-mospheric technology for storing (and delivering) Lewis basic gases (such as arsine and phosphine) and Lewis acidic gases (such as boron(III) fluoride) based on using Lewis acidic ionic liquids (such as [Cnmim][CuCl2]) and Lewis basic ionic liquids (such as [Cnmim][BF4]), respectively.75 This new technology appears to be safer and more efficient than the current technology (based on solids adsorption technology). Also announced at COIL was that Degussa is using ionic liquids as paint additives and is investigating them for use in lithium ion batteries, and that IoLiTec has developed a high-tech process for the cleaning of nozzles.74 Other companies with advanced research and/or pilot plants for ionic liquid applications include: IFP, Arkema, SASOL, Chevron, and Eli Lilly.74 A review of these industrial applications will appear elsewhere,76 and QUILL has published a map for the industrialization of ionic liquids,77 it is clear that ionic liquids are now well established in the industrial consciousness, and will not disappear.

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