Homogeneous two-phase catalysis

This interplay of reaction, solution, and diffusion has already been discussed in Section 4.4 for fluid-fluid reactions. The equations derived there for gas-liquid systems (two-film theory) can be adapted directly for a homogeneously catalyzed liquid-hquid reaction if we use the partition coefficient for the reactant concentration in the reaction phase (here the solvent) instead of the Henry co dent for the concentration of the gaseous reactant in the liquid. In this case there is an excess of A, and Eq. (4.9.14) is then more appropriate. 

The basic assumption is a pre-equilibrium between reactant, catalyst, and complex. 

For Ca / mi the rate is maximized and zero order with respect to A. For Ca / Mi we have a first-order reaction with respect to the substrate A. Industrial biotechnological processes are run in or near the zero-order regime to maximize the rate. For biosensors, where the signal should be sensitive to the concentration, the first-order regime is desired. Biological processes in living systems (e.g., our body) also run in the first-order regime, as the control of a reaction is more important than the rate. 

Diffusional problems are negligible for homogeneously or enzyme catalyzed reactions, at least if the reactants, products, and catalyst form a single phase system. 

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This book is focused on the technique of aqueous, homogeneous two-phase catalysis, with the active catalyst for the reaction being (and - if not otherwise stated -remaining) dissolved in water. Thus, the reactants can be separated from the reaction products, which are typically organic in nature and relatively nonpolar, after the reaction is completed by simply separating the second phase from the catalyst solution. The catalyst can be recirculated without any problems (cf. Chapter 1 and Sections, 4.1 and 4.2). 

The concept and development of aqueous, homogeneous two-phase catalysis followed unconventional routes for chemical processes. After the idea was first expressed by Manassen [4] (not Bailar [5], as erroneously stated by Papadogianakis and Sheldon [6]), it was very quickly taken over by the university-based researcher Joo [7] (Debrecen, Hungary, whose inventive importance for the biphasic techniques has been partly underestimated [7b]) and in the industrial field by Kuntz [8], However, these studies remained curiosities and the far-sighted visions of... 

Exciting Results from the Field of Homogeneous Two-Phase Catalysis... 

Ionic Hquids (ILs) are low melting salts (<100 °C) and represent a promising solvent class, for example, for homogeneous two-phase catalysis [7-9] and extractions [10-12]. These and other appHcations of ILs have been reviewed in a number of papers [7, 13-16]. One of the main reasons that ILs have gained interest both in academia and in industry is that they have an extremely low vapor pressure [17, 18], which makes them attractive as alternative reaction media for homogeneous (two-phase) and heterogeneous catalysis [19, 20]. This paper focuses on the concept of a solid catalyst with ionic liquid layer (SCILL) as a novel method to improve the selectivity of heterogeneous catalysts. 

Figure4.8.4 Example of homogeneous two-phase catalysis propene dimerization with a homo Ni-catalyst and an ionic liquid as solvent. Adapted from Eichmann (1999).

One of the key factors controlling the reaction rate in multiphasic processes (for reactions talcing place in the bulk catalyst phase) is the reactant solubility in the catalyst phase. Thanks to their tunable solubility characteristics, the use of ionic liquids as catalyst solvents can be a solution to the extension of aqueous two-phase catalysis to organic substrates presenting a lack of solubility in water, and also to moisture-sensitive reactants and catalysts. With the different examples presented below, we show how ionic liquids can have advantageous effects on reaction rate and on the selectivity of homogeneous catalyzed reactions. 

Keywords. Homogeneous catalysis Organometalhc chemistry Two phase catalysis CC-coupling reactions Asymmetric catalysis... 

A comprehensive review [1] summarizes the environmental status of processes using catalytic conversion in water, and - more especially - several very recent comments highlight the main environmental features of Ruhrchemie/Rhone-Pou-lenc s (RCH/RP s) novel oxo process as a prototype of an aqueous biphasic technique [2]. Based on two-phase catalysis with water-soluble catalysts, this has now been used successfully for almost 20 years [3], Astonishingly, in the early days of academic research (following far behind the industrial utilization cf. Section 1) the importance of water as a liquid support of the thus immobilized homogeneous catalysts was underestimated and not undisputed. 

Soluble polymer-bound catalysts can be expected to receive continued attention as they offer specific advantages. By comparison to aqueous two-phase catalysis, a range of substrates much broader with respect to their solubility can be employed. By comparison to heterogenization on solid supports, the selectivity and activity of homogeneous complexes can be retained better. However, it must also be noted that to date no system has been unambiguously proven to meet the stability and recovery efficiency required for industrial applications. 

It was not until the work at Ruhrchemie AG (and thus the occupation of a skilled and experienced team in industry) that development led to the first large-scale utilization of the aqueous, homogeneous catalysis technique at the beginning of the 1980s, viz. in hydroformylation (the oxo process) [11]. The generally used embodiment of two-phase catalysis, for example as practised in Shell s SHOP method [12], was thus extended to aqueous two-phase catalysis. These (and the other industrial applications see Chapter 6) have led to the literature concerning these particularly attractive aqueous variants being dominated by publications from industry, particularly patent literature, rather than from academia, for virtually a decade. 

Aqueous, two-phase catalysis is also utilized industrially in a number of other processes apart from hydroformylation. The hydrodimerization of butadiene and water, a telomerization variant yielding 1-octanol or 1,9-nonanediol (cf. Section 6.9), is carried out at a capacity of 5000 tonnes per annum by the Kuraray Corporation in Japan. Rhone-Poulenc is operating two-phase, aqueous, catalytic C—C coupling processes (using TPPTS obtained from Ruhrchemie) for small-scale production of various vitamin precursors such as geranylacetones. Moreover, TPPTS-modified Ru catalysts have been proposed for the homogeneously catalyzed hydrogenation to convert unsaturated ketones into saturated ones. 

Figure 4.7-2 The three different types of metal-complex-catalyzed reactions in SCFs. Type I fully homogeneous single-phase reaction. Type II single-phase reaction medium with an insoluble metal complex as the catalyst. Type III two-phase catalysis with...

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