Multiphase catalytic systems

The present chapter targets multiphasic catalytic systems that can be represented in general as L-L-S and L-L-L-S systems (Figure 6.3). The liquid phases are two or three, and separate at ambient conditions. One of the Ls is a catalyst-phihc liquid phase that can be either ionic or hydrophilic, the equivalent to the supported liquid film described in the previous section. Figure 6.3 shows the two different arrangements of the multiphasic systems that is considered here. 

More recently imidazoiium ILs have been used as a template for the synthesis of a plethora of stable transition-metal NPs with small size and narrow size distributions [23, 24]. These particles immobilized in ILs constitute highly active multiphase catalytic systems for various reactions. The main goal of this chapter is to disclose the mechanistic and structural aspects of the formation and stabilization of transition-metal NPs in imidazoiium ILs, especially those that have been used in catalytic reactions. 

Molecular catalysis. The term molecular catalysis is used for catalytic systems where identical molecular species are the catalytic entity, like the molybdenum complex in Figure 8.1, and also large molecules such as enzymes. Many molecular catalysts are used as homogeneous catalysts (see (5) below), but can also be used in multiphase (heterogeneous) systems, such as those involving attachment of molecular entities to polymers. 

This section describes catalytic systems made by a heterogeneous catalyst (e.g., a supported metal, dispersed metals, immobilized organometaUic complexes, supported acid-base catalysts, modified zeolites) that is immobilized in a hydrophilic or ionic liquid catalyst-philic phase, and in the presence of a second liquid phase—immiscible in the first phase—made, for example, by an organic solvent. The rationale for this multiphasic system is usually ease in product separation, since it can be removed with the organic phase, and ease in catalyst recovery and reuse because the latter remains immobilized in the catalyst-philic phase, it can be filtered away, and it does not contaminate the product. These systems often show improved rates as well as selectivities, along with catalyst stabilization. 

Analogously, over the years, Arai and co-workers have investigated silica-supported ethylene glycol as a catalyst-philic phase, which contained a metal precursor, for C-C bond-forming reactions, such as the Heck reaction. They describe a multiphasic system with an organic phase (solvent) that contains only reactants and products without any catalyst. The products could be recovered by simple filtration, and the catalyst recycled many times without deactivation, since it did not precipitate, thus making the catalytic system stable and reusable (Figure 6.7). °... 

Another approach to isolate the catalyst from the products is the application of perfluorinated catalytic systems, dissolved in fluorinated media [63], which are not non-miscible with the products and some commonly used solvents for catalysis like THE or toluene at ambient temperature. Typical fluorinated media include perfluorinated alkanes, trialkylamines and dialkylethers. These systems are able to switch their solubility properties for organic and organometallic compounds based on changes of the solvation ability of the solvent by moving to higher temperatures. This behavior is similar to the above-mentioned thermomorphic multiphasic PEG-modified systems [65-67]. 

In the new edition, the material on Chemical Reactor Design has been re-arranged into four chapters. The first covers General Principles (as in the earlier editions) and the second deals with Flow Characteristics and Modelling in Reactors. Chapter 3 now includes material on Catalytic Reactions (from the former Chapter 2) together with non-catalytic gas-solids reactions, and Chapter 4 covers other multiphase reactor systems. Dr J. C. Lee has contributed the material in Chapters 1, 2 and 4 and that on non-catalytic reactions in Chapter 3, and Professor W. J. Thomas has covered catalytic reactions in that Chapter. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Contemporary commercial homogeneous nickel based catalytic systems utilize high concentrations of olefin. We therefore investigated the Ni(sacsac)(PBu3)Cl derived system in toluene (1 255 w/w solvent). The solution was apparently homogeneous before activation, but formed a multiphase system after activation. 

We first present general criteria for the rational use of MSRs on the basis of fundamentals of chemical reaction engineering [21-24], The main characteristics of MSRs are discussed, and the potential gain in reactor performance relative to that of conventional chemical reactors is quantified (Section 2). Subsequently, the most important designs of fluid-solid and multiphase reaction systems are described and evaluated (Sections 3 and 4). Because microstructured multichannel reactors with catalytically active walls are by far the most extensively investigated MSRs for heterogeneous catalytic reactions, we present their principal design and recent synthetic methods separately in Section 5. 

This chemical and physico-chemical behavior of the binary H2O-CO2 mixture [38] suggests that water is an attractive liquid to be combined with supercritical carbon dioxide in multiphase catalysis. CO2/H2O systems have adequate mass-transfer properties, especially if emulsions or micro-emulsions can be formed ([39] and refs, therein). The low pH of aqueous phases in the presence of compressed CO2 (pH ca. 3-3.5 [40]) must be considered and the use of buffered solutions can be beneficial in the design of suitable catalytic systems, as demonstrated for colloid-catalyzed arene hydrogenation in water-scC02 [41]. 

The only rigorous way to scale-up multiphase catalytic reactors is the use of reactor models, or, more precisely, the use of mathematical models. Because these models in their most accurate versions are more complex than for other reactor types> this results in considerable expenditure of time and money usually not available in the design stages of a process. Therefore, these mathematical models should be simplified reasonably on the rational basis of knowledge about the system as e.g. presented in the preceding chapter. 

This review on concurrently operated multiphase packed bed reactors shows that much information on the behavior of these reactor types has been accumulated in the past, but we are still far from a complete elucidation. The difficulty still exists that not enough information is available on systems different from air/water nonporous packings to safely scale-up multiphase reactors using a sophisticated mathematical model. The fact that fluid-dynamics and thermal effects may be different in laboratory units from those in technical reactors restricts the usefulness of simplified, i.e. lumped, models in reactor scale-up. On the contrary, the different mechanisms acting in multiphase catalytic reactions have to be kept separated to a certain extent, thus enabling the correct inclusion of their probably changing amount of influence during scale-up. 

We start our discussion by emphasizing how flow behavior is related to the transport of molecules and chemical reactions in micrometer- and submicrometer-sized channel networks. We discuss measurement of flow and transport properties and demonstrate how these characteristics translate to a range of diflerent microfluidic applications multiphase flow through porous media [1], human airways [2], miniature cell-biological systems [3, 4], flow in microfluidic catalytic monoUths [5] and the use of interfacial forces as a means for actuation in microdevices [6]. The discussion of multiphase microfluidic systems in this chapter complements several recent reviews on general aspects of transport phenomena in microfluidic systems [37, 174-179]. 

E. Cao, W. B. Motherwell, A. Gavriilidis, Single and multiphase catalytic oxidation of benzyl alcohol by tetrapropylammo-nium perruthenate in a mobile microreactor system, Chem. Eng. Technol. 2006, 29, 1372-1375. 

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