Non-aqueous ionic liquids

Olivier, H. Recent developments in the use of non-aqueous ionic liquids for two-phase catalysis, J. Mol. Catal. A Chem., 1999,146(1-2), 285-289. 

Ionic liquids are sometimes, especially in the older literature, also referred to as molten salts, non-aqueous ionic liquids or room temperature molten salts. While all of these names are entirely valid, their meaning has somewhat changed over the years. The term molten salt is now used less frequently in the field of ionic liquids and generally refers to salts with melting points greater than 100°C. The expression non-aqueous ionic liquid was originally coined to differentiate synthetic ionic liquids from water, since... 

Whatever metal is used, homogeneous processes suffer from high cost resulting from the consumption of the catalyst, whether recycled or not. This is why two-phase catalytic processes have been developed such as hydroformylation catalyzed by rhodium complexes, which are dissolved in water thanks to hydrophilic phosphines (cf. Section 3.1.1.1) [17]. Due to the sensitivity of most dimerization catalysts to proton-active or coordinating solvents, the use of non-aqueous ionic liquids (NAILs) as catalyst solvents has been proposed. These media are typically mixtures of quaternary ammonium or phosphonium salts, such as 1,3-dialkylimi-dazolium chloride, with aluminum trichloride (cf. Section 3.1.1.2.2). They prove to be superb solvents for cationic active species such as the cationic nickel complexes which are the active species of olefin dimerization [18, 19]. The dimers. 

Figure 2. Different approaches of the variation of the application phase of oxo catalysts. FBS = fluorous biphase [multiphase] system PEG = polyethylene glycol NAIL = non-aqueous ionic liquid.

Membrane technology is a recent development to separate (or concentrate) water-soluble catalysts (mainly hydroformylation catalysts) [147, 149], although a prior art is known [194, 195]. There are proposals for the use of immobilized or re-immobilized aqueous phases for large-scale processes (cf. Ref. [222] and Section 3.1.1.6). Carbon dioxide as a solvent for biphasic hydroformylations has been described by Rathke and Klinger [184], although the use of CO2 for hydroformylation purposes was described earlier [185]. For the use of supercritical CO2 cf. Section 3.1.13 with non-aqueous ionic liquids cf. Section 3.1.1.2.2. Investigations with supercritical water are in an early state (e. g., Ref. [223]). 

Other interesting systems have been employed, such as CO/HjO (water gas) or CO/Hj (syngas) as reducing mixtures [49, 50], phase transfer catalysis [37], and more recently, aqueous [46, 47] and non-aqueous ionic liquid [48] biphasic catalysis which offer more promise for practical uses. Some interesting examples of metal complexes grafted onto oxides [55, 56] or supported metals [38, 39] as arene hydrogenation catalysts have been provided. 

Magna, L. Olivier, B. H. Saussine, L. Kruger-Tissot, V. Hydroformylation process using a cobalt catalyst in a non-aqueous ionic liquid with... 

Okubo, H., Keisukei, S. and Yokoyama, C. (2002) Heck reactions in a non-aqueous ionic liquid using silica supported palladium complex catalysts. Tetrahedron Lett., 43, 7115-8. 

Recent study described an efficient approach for the preparation of Rh(0) nanoparticles by reduction of RhCb with NaBH4 (scheme 2) followed by the stabilized by different isomers of bipyridine (scheme 3), the most used was 2,2 -bipyridine as ligand (Leger et al., 2008). These colloidal suspensions have shown interesting activities and selectivity for the hydrogenation of aromatic compounds in several non-aqueous ionic liquids according to the nature of the anion and cation fragments (scheme 2). 

Different N-donor compounds have already proved their efficiency as protective agents in the case of colloidal suspensions in non-aqueous ionic liquids. As an example, the synthesis of phenanthroline stabilized palladium nanocatalysts and their use for olefin hydrogenation in l-butyl-3-methylimidazolium hexafluorophosphate media was reported (Huang et al., 2003). 

---