Multiphase homogeneous catalytic reactions

The versatility of ILs has driven increasing interest in using them in extraction and multiphasic homogeneous catalytic reactions [104] where one phase is chosen to dissolve the catalyst and be immiscible with the second phase which contains the reactant and products. Such processes occur at the interface between the IL and the overlying aqueous or organic phase, and are dependent on the access of the material to the surface and the transfer of material across the interface. A clearer understanding of the mechanisms behind these processes requires a more detailed examination of the surface properties of the ionic liquids. 

The situation may become even more complicated if a second liquid phase is present it may either serve as the reaction space containing the catalyst while the product as well as a part of the reactants exist in the first liquid phase, or it may act as a solvent into which a desired intermediate is extracted from the reacting liquid phase (cf. Section 3.1.1.1). To describe quantitatively the course of a homogeneous catalytic reaction in a multiphase chemical reactor it is necessary to combine the following information in a suitable reactor model ... 

It is the intention of this contribution to give an overview of the procedures to be applied to model a multiphase reactor and its performance for a homogeneous catalytic reaction for a detailed multiphase design the reader is referred to appropriate textbooks. 

Very often the rates of chemical transformations are affected by the rates of other processes, such as heat and mass transfer. The process should be treated as a part of kinetics. The gas/liquid mass transfer in multiphase heterogeneous and homogeneous catalytic reactions could be treated in a similar way. The mathematical framework for modelling diffusion inside solid catalyst particles of supported metal catalysts or immolisided enzymes does not differ that much, but proper care should be taken of the reaction kinetics. 

Different types of reactors are applied in practice (Figure 1.14). Stirred tank reactors (STR), very often applied for homogeneous, enzymatic and multiphase heterogeneous catalytic reactions, can be operated batchwise (batch reactor, BR), semi-batchwise (semibatch reactor, SBR) or continuously (continuous strirred tank reactor, CSTR)... 

The motivation for stating point (4) arises from the fact that many so-called homogeneous catalytic reactions are acmally present in multiphase slurries and these slurries arise, for example, and, in some cases, due to the addition of various insoluble additives or auxiliaries or the formation of precipitates. For the present chapter, point (4) explicitly excludes surface-mediated reaction steps in the homogeneous catalytic reaction mechanism. 

Potential advantages of perfluorinated solvents are their inertness against most of the employed reaction conditions, their low acute toxicity and a high solubility for gases making it interesting to evaluate their introduction in homogeneous catalytic multiphasic protocols. 

However, research on catalytic reactions in ionic liquids should not focus only on the question how to make some specific products more economical or ecological by using a new solvent and presumably a new multiphasic process. By bridging in a novel and highly attractive manner the gap between homogeneous and heterogeneous catalysis, the application of ionic liquids in catalysis gives rise to more fundamental questions. 

Fluorous catalysis is now a weU-established area and provides a complementary approach to aqueous- or ionic-biphase catalysis and the other possibilities of multiphase homogeneous catalysis (not to mention combinations of the different processes). Since each catalytic chemical reaction could have its own perfectly designed catalyst (e.g., a chemzyme), the possibility of selecting from biphase systems ranging from fluorous to aqueous systems provides a powerful portfolio for catalyst designers. 

On occasion, a reaction takes place in more than one phase of a multiphase reactor. An example is the so-called catalytic combustion. If the temperature is high enough, a hydrocarbon fuel such as propane can be oxidized catalyticaUy, on the surface of a heterogeneous catalyst, at the same time that a homogeneous oxidation reaction takes place in the gas phase. This situation calls for two separate definitions of the reaction rate, one for the gas phase and the other for the heterogeneous catalyst. 

Biphasic hydroformylation is a typical and complicated gas-liquid-liquid reaction. Although extensive studies on catalysts, ligands, and catalytic product distributions have appeared, the reaction mechanism has not been understood sufficiently and even contradictory concepts of the site of hydroformylation reaction were developed [11, 13, 20]. Studies on the kinetics of hydroformylation of olefins are not only instructive for improvement of the catalytic complexes and ligands but also provide the basic information for design and scale-up of novel commercial reactors. The kinetics of hydroformylation of different olefins, such as ethylene, propylene, 1-hexene, 1-octene, and 1-dodecene, using homogeneous or supported catalysts has been reported in the literature. However, the results on the kinetics of hydroformylation in aqueous biphasic systems are rather limited and up to now no universally accepted intrinsic biphasic kinetic model has been derived, because of the unelucidated reaction mechanism and complicated effects of multiphase mass transfer (see also Section 2.4.1.1.2). 

In situ spectroscopy is the only approach to obtain reliable information on mechanisms and the role of intermediates in chemical reactions as well as on structure-reactivity relationships in catalysis [1-5]. Especially, in heterogeneous catalysis, the application of a variety of in situ methods has gained a lively development during the recent two decades. But also in homogeneous catalysis, as well as in catalytic multiphase systems, in situ characterization methods are increasingly applied. A survey of techniques, which are most commonly applied in gas/soUd and multiphase systems and the method-specific information, is presented in Table 3.1. 

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