Ionic Liquids for Catalysis

Annegret Stark, Bernd Ondruschka

Friedrich-Schiller-University, Institute for Technical Chemistry and Environmental Chemistry, Lessingstr. 12, 07743 Jena, Germany

Chemfiles, Volume 5, Article 6

Ionic Liquids have been thoroughly investigated as solvents in most types of catalytic reactions.1–4 Their merit lies in the ease with which their physical–chemical properties can be tuned by varying either the anion, the cation, or its substitution pattern. By this means, an Ionic Liquid with optimal properties (viscosity, solubility of substrates and products, etc.) for a given application can be designed (i.e, designer solvents).5 In catalysis, this feature can be favorably exploited by rendering a homogeneous reaction mixture bi-phasic, thus combining the advantages of homogeneous and heterogeneous catalysis. An alternative to phase separation is the removal of products by thermal means; because of the negligibly low vapor pressure of Ionic Liquids, the solvent and catalyst can be retained quantitatively during distillation processes.

The following examples illustrate that many catalysts possess a high activity in the Ionic Liquid solvent, and product separation is achieved by facile phase separation.

As early as 1995, Chauvin et al. demonstrated that high conversions and selectivities are obtained in the rhodium-catalyzed hydrogenation, isomerization, or hydroformylation of alkenes in the presence of Ionic Liquids. Specifically, Ionic Liquids based on anions with modest coordination ability, such as 1-butyl-3-methylimidazolium hexafluoroantimonate (BMIM SbF6, Product No. 51027), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIM PF6), and 1-butyl-3-methylimidazolium tetrafluoroborate (BMIM BF4), gave better results when compared to mono-phasic molecular solvents.6

Scandium- or platinum(II)-catalyzed Diels–Alder reactions in either BMIM PF6, BMIM SbF6, 1-butyl-3-methylimidazolium trifluoromethanesulfonate (BMIM CF3SO3), and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIM N(CF3SO2)2) were also investigated. Very high endo/exo ratios (>99/1) and improved reaction rates were achieved when compared to dichloromethane. Efficient recycling was demonstrated for ten cycles on the example of the reaction of methyl vinyl ketone and 2,3-dimethylbuta-1,3-diene.7,8

Ionic Liquids also have been studied as solvents in bio-catalysis. For example, high enantiomeric excesses were obtained in the lipase-catalyzed trans-esterification of various substrates4,9–11 in Ionic Liquids based on the 1-alkyl-3-methylimidazolium or 4-methyl-N-alkylpyridinium cation and anions such as tetrafluoroborate, hexafluorophosphate, trifluoromethanesulfonate, etc. This finding was attributed to interactions of the Ionic Liquid with the enzyme, altering the selectivity of a biocatalytic reaction. Additionally, both the Ionic Liquid and enzyme were recyclable after extraction of the products from the Ionic Liquid phase with diethyl ether.10

Several researchers have found that impurities in Ionic Liquids exhibit an influence on the reaction outcome in catalysis.4,6,12–22 Nevertheless, the quantitative specification of each batch of Ionic Liquid used is seldom reported, and the comparison of results from different laboratories is thus hampered.

Impurities, such as water, unreacted amine (e.g., 1-methylimidazole), and traces of halides most frequently stem from the preparation of Ionic Liquids. In the example of the metathesis of 1-octene catalyzed by Grubbs’ 1st generation catalyst, Sigma-Aldrich’s high purity 1-butyl- 3-methylimidazolium tetrafluoroborate (Product No. 39931)) or 1-ethyl-3-methylimidazolium tetrafluoroborate (Product No. 39736), were spiked with known amounts of either of these impurities (water <200 ppm, halogens (Cl-) <10 ppm) to demonstrate the importance of the quality of Ionic Liquids employed in catalysis.

Figure 1 shows that the catalyst is relatively resistant to the presence of water, and even a 100-fold excess of water over catalyst (2.04 mol % water in Ionic Liquid) does not inhibit the reaction significantly.

Influence of added increments of water to pure BMIM BF

Figure 1. Influence of added increments of water to pure BMIM BF4 on the conversion of the metathesis of 1-octene at room temperature, as a function of time. Reaction conditions: 0.02 mol % catalyst precursor (relative to 1-octene), Ionic Liquid:1-octene = 1:1 (vol.)

However, the presence of traces of chloride exhibits a much more pronounced effect, as shown in Figure 2. A ratio of chloride:catalyst of 2.3 (0.07 mol % chloride in Ionic Liquid, corresponding to only 100 ppm) reduced the turn-over frequency from 820 to 700 h-1.

Influence of added increments of 1-hexyl-3-methylimidazolium chloride (HMIM Cl) to pure BMIM BF

Figure 2. Influence of added increments of 1-hexyl-3-methylimidazolium chloride (HMIM Cl) to pure BMIM BF4 on the conversion of the metathesis of 1-octene at room temperature, as a function of time. Reaction conditions: 0.03 mol % catalyst precursor (relative to 1-octene), Ionic Liquid:1-octene = 1:1 (vol.).

Interestingly, the catalyst precursor is most sensitive to the presence of traces of 1-methylimidazole. Figure 3 shows that residual 1-methyl-imidazole in an Ionic Liquid deactivates the catalyst fully at a ratio of 6.3:1 1-methylimidazole:catalyst, which corresponds to traces of 0.18 mol % (600 ppm) of 1-methylimidazole in the Ionic Liquid.

Influence of added increments of 1-methylimidazole to pure BMIM BF

Figure 3. Influence of added increments of 1-methylimidazole to pure BMIM BF4 on the conversion of the metathesis of 1-octene at room temperature as a function of time. Reaction conditions: 0.03 mol % catalyst precursor (relative to 1-octene), Ionic Liquid:1-octene = 1:1 (vol.).

We are pleased to introduce a new set of highly purified Ionic Liquids for catalysis with water contents below 200 ppm and halogen content (Cl-) below 10 ppm.



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