Liquid Biphasic Reactions

Examples of applying biphasic systems to catalyzed reactions, such as phase-transfer catalysis, overpower the stoichiometric reactions. In a typical catalytic biphasic system, one phase contains the catalyst, while the other phase contains the substrate. In some systems, the catalyst and substrates are in the same phase, while the product produced is transferred to the second phase. In a typical reaction, when the two phases are mixed during the reaction and after completion, the catalyst remains in one phase ready for recycling while the product can be isolated from the second phase. The most common solvent combination consists of an organic solvent combined with another immiscible solvent that, in most applications, is water. However, there are few examples of suitable water-soluble and stable catalysts, and therefore various applications are limited to some extent [192]. Immiscible solvents other than water are recently becoming more applicable in biphasic catalysis because of the better solubility and stability of various catalysts in such solvents. For example, ionic liquids and fluorous solvents have many successful applications in liquid-liquid 

First cycle 96% Second cycle 97% Third cycle 94% 

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In contrast, we intend to demonstrate the principle aspects of catalyst recycling and regeneration using the ionic liquid methodology. These aspects will be explored in more detail for the example of Rh-catalysed hydroformylation (see Section 7.2). First, however, we will briefly introduce important general facts concerning transition metal catalysis in ionic liquids (see Section 7.1.2). This will be followed by a consideration of liquid-liquid biphasic reactions in these media from an engineering point of view (see Section 7.1.3). 

Figure 7.5. Concentration of gaseous feedstock 1 in a gas-ionic liquid biphasic reaction for slow (A) and fast (B) chemical reaction - mass transfer resistance on the gas side is neglected...

The few examples where SILP catalysis has been tested so far showed highly encouraging results. It is very likely that other applications where ionic catalyst solutions were tested in liquid-liquid biphasic reactions could be reinvestigated under SILP conditions. If very high catalyst stability over time can be realised or simple catalyst regeneration protocols can be developed than SILP catalysis can be expected to make its way into industrial processes. 

Liquid-liquid biphasic reaction was carried out in a 70-mL autoclave at a stirring speed of 1600 rpm with a catalyst charge of c(Rh) = 0.018x10"5 mol in 4mL ionic liquid. 

For the oxidation of alkanes Li et al. used iodobenzene diacetate [PhI(OAc)2] as oxygen source. An electron-deficient manganeseporphyrin catalyst was immobilized in [BMIM][PF6] and tested in liquid-liquid biphasic reaction mode with CH2CI2 as organic phase [154]. They found the catalyst more active in the [BMIM][PF6]/CH2Cl2 system than in neat CH2Q2- The increase in activity was attributed by them to the higher polarity of the ionic liquid. 

Key design elements influorous/organic liquid biphasic reactions... 

Detailed kinetic investigations of the reaction of cumene with propene in [C2mim]Cl—AICI3 (X(AlCl3) = 0.67) were conducted by Joni et al. in a liquid-liquid biphasic reaction mode [24]. Various products (di-, tri- and tetraisopropylbenzene) result from a series of consecutive alkylation reactions. It is necessary to take the solubility of these products into account to fit kinetic models to the data. A conductor-like screening model for real solvent (COSMO-RS) method was used to predict the relative solubilities of the products. Higher alkylated products are less soluble in the reactive ionic liquid phase, leading to an irtproved selectivity for the monoalkylated product. 

As illustrated in Figure 6.3. many liquid-liquid biphasic reactions using active catalysts dissolved in ionic liquids suffer from severe mass transport limitation issues. Thus, only a small part of the ionic liquid takes effectively part in the reaction, a fact that the reaction engineer calls Tow ionic liquid utilisation . 

Photograph of a homogeneously catalyzed, liquid-liquid biphasic reaction. 

An alternative process concept that realizes the task of catalyst/product separation under much milder conditions is the water-organic, liquid-liquid biphasic reaction... 

Catalyst Immobilization in Liquid-Liquid Biphasic Reaction Systems using Fluorous Phases, Supercritical CO2 or Ionic Liquids... 

Pentafluorophenoxy)ethyl-2-(piperidino)-ethanesulfonate (PFPES) as a new deriva-tization reagent has been used for the determination of cyanide in the presence of other anions using capillary gas chromatography-electron capture detector (CGC-ECD) [65]. The reactive cyanide and other anions such as iodide, nitrite, and thiocyanate in a liquid biphasic reaction system (PFPES does not react with fluoride and chloride anions) are derivatized. 

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