Biphasic reaction engineering

BIphasic Reaction Engineering 591 Table 18.3 Examples of biphasic reactions of different classes... 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal-catalyzed reactions. Besides the engineering advantage of their nonvolatile natures, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations permits systematic optimization of the biphasic reaction (with regard, for example, to product selectivity). Attractive options to improve selectivity in multiphase reactions derive from the preferential solubility of only one reactant in the catalyst solvent or from the in situ extraction of reaction intermediates from the catalyst layer. Moreover, the application of an ionic liquid catalyst layer permits a biphasic reaction mode in many cases where this would not be possible with water or polar organic solvents (due to incompatibility with the catalyst or problems with substrate solubility, for example). 

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). 

In future, a complete quantitative analysis on the basis of chemical reaction engineering principles of homogeneously catalyzed gas-liquid-liquid reactions is needed to improve known aqueous biphasic reactions as well as to find new, highly active and selective homogeneous catalysts for organic synthesis. 

The various properties of water in different aspects (being important for the reactivity, reaction kinetics or mechanisms, reaction engineering, or other concerns) are discussed elsewhere. The procedures for tailoring the water-solubility of the catalysts are many-sided and may be generalized much more easily than the corresponding methods for SHOP (cf. Section 7.1), fluorous phase (Section 7.2), supercritical solvents (Section 7.4), water-soluble polymer-bound catalysts (Section 7.6), or NAIL utilization (Section 7.3) no wonder that all other biphasic applications remain singular or are still just proposals. Both the scientific and industrial com-... 

The reaction engineering (see Section 8.2.1) is advanced insofar that many alternative process concepts [lie, 15d] were tested during the development phase however, only a few were pursued further in the concept stage up to the pilot plant reactor. There remained the principle of the biphasic Ruhrchemie/Rhone-Poulenc oxo process which was developed in long-term tests and protected by a matched patent strategy, as described in Section 6.1.3. 

Obviously, there are many good reasons to study ionic liquids as alternative solvents in transition metal catalyzed reactions. Besides the engineering advantage of their exhemely low volatility, the investigation of new biphasic reactions with an ionic catalyst phase is of special interest. The possibility of adjusting solubility properties by different cation/anion combinations allows a systematic optimization of the... 

Fundamental Reaction Engineering of Aqueous Biphasic Catalysis... 

B. Ahmed-Omer, D. Barrow and T. Wirth, Effect of segmented fluid flow, sonication and phase transfer catalysis on biphasic reactions in capillary microreactors. Chemical Engineering Journal, 2007, 135, 280-283. 

Apart from the usually low activity and sometimes insufficient selectivity of P450s towards steroids (which can be improved by means of protein engineering), the low solubility of steroid compounds in water (1-100 pM [334]) represents a challenging problem for the establishment of whole-cell biocatalysis. Consequently, several promising reaction-engineering techruques that were applied for biotransformations of other hydrophobic compoimds have also been tested with steroid substrates. Among these are (1) biphasic reaction setups with an organic phase, which serves as substrate reservoir, (2) surfactant-... 

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