Aqueous biphasic catalysis

Chaudhari, R. V. Bhanage, B. M. Kinetics of biphasic catalysis. In Aqueous-Phase Organomet. Catal. Cornils, B. Herrmann, W. A., Eds., Wiley-VCH Verlag GmbH, Weinheim, Germany, 1998, pp 283-294. 

Only the biphasic method, specially of aqueous-biphasic catalysis, has provided a fundamental remedy to the problem of stress-free and economical recovery and recycle of homogeneous oxo catalysts [12]. The fact that the catalyst, which still acts homogeneously, is dissolved in water, thus in a polar solvent, and remains dissolved, enables it to be separated from the nonpolar products without problems and with minimal effort after reaction. 

Aqueous biphasic catalysis is a special case of the two-phase processes of homogeneous catalysis. Despite the academic literature s provocative question "Why water " [18a, 18b], the advantages of water as the second phase and the "liquid support" are numerous. On the one hand, the search for the necessary solubility gap is much easier with water than with various organic-phase liquids (Figure 5.2). Additionally, water has many properties which predestine it as a ideal liquid support in homogeneous catalysis (see T able 5.1)[18c,18d]. 

In the case of ionic liquids, these general aspects for all fluid-fluid reactions are of particular importance, since mass transfer into an ionic liquid layer is generally slower than into an organic or aqueous medium. This is because ionic liquids usually have much higher viscosities than organic solvents. The least viscous ionic liquids are somewhat similar to ethylene glycol as demonstrated in Table 7.2. However, many ionic liquids used in liquid-liquid biphasic catalysis are significantly more viscous. 

Nevertheless, the application of ionic liquids in the liquid-liquid, biphasic Rh-catalysed hydroformylation offers technically interesting advantages vs. the traditional aqueous biphasic catalysis e.g. much higher solubility for longer chain olefins and the compatibility of the ionic liquid with phosphite ligands [51]. 

The first example of biphasic catalysis was actually described for an ionic liquid system. In 1972, one year before Manassen proposed aqueous-organic biphasic catalysis [1], Par shall reported that the hydrogenation and alkoxycarbonylation of alkenes could be catalysed by PtCh when dissolved in tetraalkylammonium chloride/tin dichloride at temperatures of less than 100 °C [2], It was even noted that the product could be separated by decantation or distillation. Since this nascent study, synthetic chemistry in ionic liquids has developed at an incredible rate. In this chapter, we explore the different types of ionic liquids available and assess the factors that give rise to their low melting points. This is followed by an evaluation of synthetic methods used to prepare ionic liquids and the problems associated with these methods. The physical properties of ionic liquids are then described and a summary of the properties of ionic liquids that are attractive to clean synthesis is then given. The techniques that have been developed to improve catalyst solubility in ionic liquids to prevent leaching into the organic phase are also covered. 

Figure 5.6 Some examples of sulfonated phosphine ligands widely used in aqueous-organic biphasic catalysis [19]...

Horvath recognized that SAPC solved the problem posed by the solubility of lypophilic substrates in aqueous biphasic catalysis with water-soluble homogeneous catalysts. He compared biphasic aqueous-organic catalysis with SAPC, in order to clarify whether in SAPC the catalyst remained dissolved in the... 

The first results of the batch hydrogenation of prenal and citral to geran-iol and nerol provided evidence for the use of aqueous biphasic catalysis to increase the selectivity of the conversion of the substrates the accumiilation of byproducts can be nearly suppressed by the fast extraction of the product from the catalyst phase. For this reason, the distribution of the product between extraction and catalyst phase had been studied in detail. 

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