Complex Catalysis in the Liquid Phase

While the term homogeneous catalysis pertains to all those reaction systems where the catalyst and the substrate are in the same phase, many chemists identify it with liquid-phase reactions catalyzed by organometallic complexes. Indeed, this fastgrowing field attracts most of the research effort in homogeneous catalysis. New 

Catalysis Concepts and Green Applications. Gadi Rothenberg Copyright 2008 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 978-3-527-31824-7 

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The coordination complex chemistry in zeolites provides a very useful conceptual bridge to coordination-chemistry controlled catalysis in the liquid phase. This is discussed in this chapter for the oxidation of ethylene to produce vinyl acetate from acetic acid and ethylene. The catalytic system appears to consist of dimeric or trimeric Pd complexes. The elementary reaction steps can take place in the direct contact with the metal centers, the so-called inner-sphere mechanism. The reaction can also proceed through an outer-sphere mechanism in which proton transfer between reactants and acetate plays an essential role. 

Because homogeneous catalysis is usually carried out in the liquid phase, temperature control is relatively easy. However, the temperature must not be too high to prevent degradation of the catalyst complexes. 

A new solid-support strategy - the sequential column asymmetric catalysis - has been developed for the synthesis of enantiopure p-lactams <02CEJ4115>. In this strategy, reagents and catalysts are attached to a solid-phase support and loaded onto sequentially-linked colums. The substrates are present in the liquid phase that flows through the column. As a substrate encounters each successive column, it grows in complexity. 

Semeluk and Bernstein (130) have shown that the rate of exchange between pure CDCI3 and CHBra is catalyzed by amines in the liquid phase. When all components are present in the gas phase, there is no catalysis by amines. The results are interpreted as indicating an ionic mechanism. Additional evidence pointing to an ionic mechanism in alkylation and isomerization reactions is furnished by the fact that, although aluminum halides do not form complexes directly with aromatic hydrocarbons (Kablulsom and Ssadanow, 131 Menschutkin, 132), conductance studies have shown the existence of ions on the addition of aluminum halide to aliphatic halides (Wertzporock and Kowalski, 133). Addition of aromatic hydrocarbons to the above solutions increases the conductivity (Wertzporock, 134). 

Adsorption on a solid catalyst surface, complex formation in homogeneous catalysis with metallo-organic complexes and in biocatalysis with enzymes share the same principle, i.e. the total number of sites is constant. Therefore, the rate expressions for reactions on heterogeneous, homogeneous and biocatalysts have a similar form. The constant number of active sites results in rate expressions that differ from homogeneous gas phase kinetics. Partial pressures are usually used in rate expressions for gas-phase reactions, while concentrations are used when the reactions take place in the liquid phase. It appears that definitions and nomenclature of particular kinetics constants in the different sub-communities differ sometimes. In the following sections the expressions used by the different subdisciplines will be compared and their conceptual basis outlined. 

The transition metal needed for the homogeneous catalysis will probably be introduced to the system as a organometallic complex, that is the complex RhCUPCCeHslsls an inorganic complex, that is lanthanum chlorides (7) or a finely divided metal, that is Pd. Such species will be called catalyst precursors (8). In many situations, the catalyst precursor may be insoluble, or only sparingly soluble in the liquid phase Pl-... 

Catalysis by metal complexes in the liquid phase is presently a very important area in chemistry. Intense development of this field is due to several evident advantages of such catalysts. They are characterized by high catalytic activity, capability of reacting only with specific substrates (specificity) and in a specific position (selectivity). 

Finally, due to relative simplicity of kinetic and physicochemical studies in the liquid phase and potentialities for wide varying of the structure and properties of the catalyst and medium, metal complex systems are convenient objects for the solution of basically novel problems of catalysis as a whole. 

Transition metal catalysis in liquid/liquid biphasic systems principally requires sufficient solubility and immobilization of the catalysts in the IL phase relative to the extraction phase. Solubilization of metal ions in ILs can be separated into processes, involving the dissolution of simple metal salts (often through coordination with anions from the ionic liquid) and the dissolution of metal coordination complexes, in which the metal coordination sphere remains intact. 

Biphasic catalysis in a liquid-liquid system is an ideal approach through which to combine the advantages of both homogeneous and heterogeneous catalysis. The reaction mixture consists of two immiscible solvents. Only one phase contains the catalyst, allowing easy product separation by simple decantation. The catalyst phase can be recycled without any further treatment. However, the right combination of catalyst, catalyst solvent, and product is crucial for the success of biphasic catalysis [22]. The catalyst solvent has to provide excellent solubility for the catalyst complex without competing with the reaction substrate for the free coordination sites at the catalytic center. 

In comparison with traditional biphasic catalysis using water, fluorous phases, or polar organic solvents, transition metal catalysis in ionic liquids represents a new and advanced way to combine the specific advantages of homogeneous and heterogeneous catalysis. In many applications, the use of a defined transition metal complex immobilized on a ionic liquid support has already shown its unique potential. Many more successful examples - mainly in fine chemical synthesis - can be expected in the future as our loiowledge of ionic liquids and their interactions with transition metal complexes increases. 

For instance, catalysis in liquid/liquid two phases is generally referred to as biphasic catalysis and has widened the practical scope of homogeneous catalysis the catalyst is present in one liquid phase, while reactants and products are present in the other liquid phase. Thus, the catalyst can be separated by simple phase separation. Celanese is operating a 300 000 t/a plant for propylene hydroformylation using a water-soluble rhodium phosphine complex in a biphasic mode of operation at the Ruhrchemie site in Oberhausen [142],... 

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