Catalytic homogeneous catalysis

Catalysis in a single fluid phase (liquid, gas or supercritical fluid) is called homogeneous catalysis because the phase in which it occurs is relatively unifonn or homogeneous. The catalyst may be molecular or ionic. Catalysis at an interface (usually a solid surface) is called heterogeneous catalysis, an implication of this tenn is that more than one phase is present in the reactor, and the reactants are usually concentrated in a fluid phase in contact with the catalyst, e.g., a gas in contact with a solid. Most catalysts used in the largest teclmological processes are solids. The tenn catalytic site (or active site) describes the groups on the surface to which reactants bond for catalysis to occur the identities of the catalytic sites are often unknown because most solid surfaces are nonunifonn in stmcture and composition and difficult to characterize well, and the active sites often constitute a small minority of the surface sites. 

These appHcations are mosdy examples of homogeneous catalysis. Coordination catalysts that are attached to polymers via phosphine, siloxy, or other side chains have also shown promise. The catalytic specificity is often modified by such immobilization. Metal enzymes are, from this point of view, anchored coordination catalysts immobilized by the protein chains. Even multistep syntheses are possible using alternating catalysts along polymer chains. Other polynuclear coordination species, such as the homopoly and heteropoly ions, also have appHcations in reaction catalysis. 

Catalytic hydrogenation of tnfluoroacetic acid gives tnfluoroethanol in high yield [73], but higherperfluorocarboxybc acids and their anhydndes are reduced much more slowly over rhodium, iridium, platinum, or ruthenium catalysts [7J 74] (equation 61) Homogeneous catalysis efficiently produces tnfluoroethanol from tnfluoroacetate esters [75] (equation 61)... 

Hydrido complexes of all three elements, and covering a range of formal oxidation states, are important because of their roles in homogeneous catalysis either as the catalysts themselves or as intermediates in the catalytic cycles. 

Many transition metal complexes dissolve readily in ionic liquids, which enables their use as solvents for transition metal catalysis. Sufficient solubility for a wide range of catalyst complexes is an obvious, but not trivial, prerequisite for a versatile solvent for homogenous catalysis. Some of the other approaches to the replacement of traditional volatile organic solvents by greener alternatives in transition metal catalysis, namely the use of supercritical CO2 or perfluorinated solvents, very often suffer from low catalyst solubility. This limitation is usually overcome by use of special ligand systems, which have to be synthesized prior to the catalytic reaction. 

The study of catalytic polymerization of olefins performed up to the present time is certain to hold a particular influence over the progress of the concepts of the coordination mechanism of heterogeneous catalysis. With such an approach the elementary acts of catalytic reaction are considered to proceed in the coordination sphere of one ion of the transition element and, to a first approximation, the collective features of solids are not taken into account. It is not surprising that polymerization by Ziegler-Natta catalysts is often considered together with the processes of homogeneous catalysis. 

Homogeneous catalysis by transition metal clusters has been reviewed from the perspective of the specific transformations.Examples of very mixed-metal clusters catalyzing processes homogeneously are collected in Table IX. As is generally the case with homogeneous catalysis, the catalytic precursor is well defined, but the nature of the active catalyst is unclear. 

Much of the recent interest in insertion reactions undeniably stems from the emphasis placed on development of homogeneous catalysis as a rational discipline. One or more insertion is involved in such catalytic processes as the hydroformylation (31) or the polymerization of olefins 26, 75) and isocyanides 244). In addition, many insertion reactions have been successfully employed in organic and organometallic synthesis. The research in this general area has helped systematize a large body of previously unrelated facts and opened new areas of chemistry for investigation. Heck 114) and Lappert and Prokai 161) provide a comprehensive compilation and a systematic discussion of a wide variety of insertion reactions in two relatively recent (1965 and 1967) reviews. 

In contrast to heterogeneous catalysis, industrial applications of homogeneous catalysis are relatively scarce, largely being restricted to the speciality and pharmaceutical sectors. Homogeneous catalysts have been well researched, since their catalytic centres can be relatively easily... 

As explained in Chapter 1, catalytic reactions occur when the reacting species are associated with the catalyst. In heterogeneous catalysis this happens at a surface, in homogeneous catalysis in a complex formed with the catalyst molecule. In terms of kinetics, the catalyst must be included as a participating species that leaves the reaction unaltered, as indicated schematically in Fig. 2.7, which shows the simplest conceivable catalytic cycle. We will investigate the kinetics of this simple two-step mech-... 

In homogeneous catalysis, the catalytically active species is dissolved in the reaction medium and is present uniformly throughout the system. However, with resin catalysis, the catalytically active groups are anchored to the matrix and in the solvent-resin system are located at the surface of and within the body of the resin bead only. Ion exchangers are, in fact, particulate active. species and when used as catalysts combine with the physical and mechanical benefits of heterogeneous catalysts (Pitochelli, 1980). 

The second characteristic pertains to the fact that a very small amount of a catalyst may be able to maintain a high reaction rate over a long time. Although in practice some of the catalysts are extremely efficient for certain specific reactions, it is observed that in the majority of the cases of homogeneous catalysis, the enhancement in the reaction rate is proportional to the concentration of the catalyst used. In heterogeneous catalytic reactions involving reactant gases and solid surfaces, the total surface area of the solid may also affect the reaction rate. 

As can be seen, the catalytic process over a zeolite-supported cation, or an oxide-supported cation, can be considered as a supported homogeneous catalysis, as far as adsorbed reactants and products behave like reactive ligands. The model developed for lean DcNO. catalysts over supported cations (function 3), as well as this supported homogeneous catalysis approach, is also suitable for stoichiometric mixture (TWC) comprising CO and H2 as reductants over supported transition metal cations [20-22],... 

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