Three-phase reactions triphase catalysis

As stated, the solid PTC is suitable for the industrial processes concerning the removal of the catalyst from the reaction mixture and its economic recycle. The real mechanism of reaction in a triphase catalysis is not completely understood. However, the reaction rate and the conversion of reactant in a triphase catalysis (TC) is highly dependent on the organiphilicity (hydrophilicity or hydroprobicity) of the polymer support of the catalyst and the polarity of the organic solvent. Not only the partition of the organic to the aqueous solutions is affected by the organophilicity of the polymer-supported catalyst, but also the concentration distribution of the catalyst between two phases is influenced by the organophilicity of the polymer-supported catalyst. 

Ohtani et al. used polystyrene-supported ammonium fluoride as a phase transfer catalyst (triphase catalysis) for several base-catalyzed reactions, such as cyanoethylation, Knoevenage reaction, Claisen condensation and Michael addition. The catalytic activity of the polystyrene-supported ammonium fluid was comparable to that of tetrabutylammonium fluoride (TBAF). The ionic loading and the ammonium structure of the fluoride polymers hardly affected the catalytic efficiency. The reaction was fast in a non-polar solvent (e.g., octane or toluene) from which the rate-determining step of the base-catalyzed reaction is very similar to that of the nucleophilic substitution reactions. 

1 The interaction between soiid poiymer (hydrophilicity) and the organic soivents 

In triphase catalysis, solvated resin supports are important carriers for solid-phase organic synthesis in combinatorial chemistry. The physical properties of resin, resin swelling, dynamic solvation, and solvated supports are important factors in affecting the synthesis. However, these factors are also affected by solvent. Selective solvation of resin alters the local reactivity and accessibility of the bound substrate and the mobility of the entrapped re- 

The basie steps involved in reactions with resin-supported PTC catalysts differ from ordinary two-phase PTC reactions in one important respect ordinary PTC reactions require only one reagent to be transferred from their normal phase to the phase of the second reactant. Use of resin-supported catalysts requires that both reagents diffuse to active PTC sites on the eatalyst surface, or for reactions with slow intrinsic rates, both reagents must also diffuse to the active sites inside the resin bulk phase. The need for diffusion processes with solid catalysts also means that both reagents are required to diffuse to and penetrate the stagnant outer layer of liquid(s) (the Nemst layer), coating the catalyst particle. 

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Table 17.1 Classification of three-phase reactions and reactors. G = gas, L = liquid, S = solid, TPC = triphase catalysis...

A different example of triphasic catalysis for the Heck, Stille and Suzuki reactions relied on a three-phase microemulsion/sol-gel transport system. Gelation of an z-octyl(triethoxy)silane, tetramethoxysilane and Pd(OAc)2 mixture in a H2O/CH2CI2 system led to a hydrophobicitized sol-gel matrix that entrapped a phosphine-free Pd(ii) precatalyst. The immobilized precatalyst was added to a preformed microemulsion obtained by mixing the hydrophobic components of a cross coupling reaction with water, sodium dodecyl sulfate and a co-surfactant, typically zz-propanol or butanol. This immobilized palladium catalyst was leach proof and easily recyclable. 

Insoluble catalysts offer an important advantage of simple catalyst removal by filtration or centrifugation after the completion of a PTC reaction. Regen [82] demonstrated that quaternary onium cations chemically bound to insoluble resins could act as PTC catalysts and suggested the term triphase catalysis to describe the related PTC reactions. Insoluble PTC catalysts can be grouped into three categories, namely, the resin bound, the inorganic solid bound, and the third-liquid-phase catalysts as described in Section... 

The reaction of triphase catalysis is carried out in a three-phase liquid (organic)-solid (catalyst)-liquid (aqueous) medium. In general, the reaction mechanism of the triphase... 

Multi-phase catalysis performed in ILs can lead to various phase systems where the catalyst should reside in the IL. Prior to the reaction, and in cases where there are no gaseous reactants, two systems can usually be formed a monophase, that is, the substrates are soluble in the IL and biphasic systems where one or all the substrates reside preferentially in an organic phase. If a gas reactant is involved, biphasic and triphasic systems can be formed. At the end of the reaction, three systems can be formed a monophasic system a biphasic system where the residual substrates are soluble in the ionic catalytic solution and the products reside preferentially in the organic phase and triphasic systems, formed, for example, by ionic catalytic solutions, with an organic phase containing the desired product and a third phase containing the byproducts. In most cases, catalysis performed in ILs involves two-phase systems (before and after catalysis). 

In liquid-liquid-solid (three phase) systems such as triphase catalysis, mass transfer in the aqueous and organic phases occurs serially in a single PTC cycle, that is, ion exchange in the aqueous phase followed by the main reaction in the organic phase. The design of a contactor for getting these values simultaneously has to be quite different from that for two-phase systems. 

The technique of "triphase catalysis", where liquid-liquid reactions are catalyzed by "phase transfer catalysts" chemically fixed on inert polymer supports, is an extremely interesting example of three-phase systems. They may be potentially of great interest technically since the catalysts can be recovered or used continously (56). 

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