Homogeneous catalysis A catalyst

Homogeneous Catalysis A catalyst is a substance, usually used in small amounts relative to the reactants, that increases the rate of a reaction without being consumed in the process. Liquid-phase reactions are often conducted in the presence of homogeneous catalysts. Typically, homogeneous catalysts are ions or metal coordination... 

In case of homogeneous catalysis a catalyst is in the same phase as the reactants. Three types of homogeneous catalysis are usually considered gas phase catalysis, acid-base and by transition metals. 

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],... 

Ultrafiltration has been used for the separation of dendritic polymeric supports in multi-step syntheses as well as for the separation of dendritic polymer-sup-ported reagents [4, 21]. However, this technique has most frequently been employed for the separation of polymer-supported catalysts (see Section 7.5) [18]. In the latter case, continuous flow UF-systems, so-called membrane reactors, were used for homogeneous catalysis, with catalysts complexed to dendritic ligands [23-27]. A critical issue for dendritic catalysts is the retention of the catalyst by the membrane (Fig. 7.2b, see also Section 7.5). 

In homogeneous catalysis, the catalyst is in the same phase as the reactants and products. Here we will concentrate on homogeneous catalysis in the liquid phase. In the classic case, the reactant (also called the substrate) molecules and the catalyst are reacted in a solvent. For example, the transesterification of fatty acid triglycerides with methanol (Figure 1.10) is catalyzed by hydroxide (OH-) ions. This is an important process for making fatty acid methyl esters which are then used as biodiesel. 

Heterogeneous catalysis is preferred over homogeneous catalysis. A critical issue is the catalyst design, which should ensure compatibility between the reaction and the separation. The temperature profile dominated by the vapor-liquid equilibrium, as well as the residence-time distribution controlled by the hydrodynamics of internals must comply with the achievable reaction rate and with the desired selectivity pattern. 

As in the original Monsanto process involving homogeneous catalysis and catalyst recycle, the product is removed as a liquid, because the gas phase of a stripping reactor would contain a low concentration of the high-boiling acetic acid. In the Acetica process, no catalyst recycle is needed, as the solid catalyst stays in the reactor. 

In the last few years, this technology has been extended to materials science for the rapid discovery and optimization of, e. g., polymers, catalysts, and electronic materials [1]. Homogeneous catalysis, where catalyst performance can be influenced through choices in ligand, metal, and reaction conditions, represents a very important subset of this broad field. 

Catalysis are classified into two types homogeneous and heterogeneous. In homogeneous catalysis the catalyst is present in the same phase as the reactants, as when a gas-phase catalyst speeds up a gas-phase reaction, or a species dissolved in solution speeds up a reaction in solution. Chlorofluorocarbons and oxides of nitrogen are homogeneous catalysts responsible for the destruction of ozone in the stratosphere. These reactions are examined in more detail in Section 20.5. A second example is the catalysis of the oxidation-reduction reaction... 

The search for new reactivity and new reactions is an important target in homogeneous catalysis. A declared goal is the selective activation of C-H bonds under mild conditions. Although there are numerous examples of stoichiometric C-H bond oxidative additions to transition metal centers, successful examples regarding catalytic functionalization of C-H bonds have been made only during the last five years. Notable advances have been achieved by Moore and coworkers who described in 1992 the ortAo-acylation of pyridine with olefins and carbon monoxide. The cluster compound triruthenium dodecacarbonyl has been used as catalyst (Scheme 10). 

In heterogeneous catalysis, which is of great industrial importance, the catalyst is a solid and the reactants are gases or liquids. In homogeneous catalysis, the catalyst and the reactants are in the same phase. Enzymes are catalysts in living systems. 

The use of liquids in homogeneous catalysis thus means not only a liquid support and from there a basic intervention in the handling and the operation of the catalyst, but also a modern separation technique for efficient work-up in organic synthesis [3], Figure 3 illustrates the enormous importance of the biphasic technique for homogeneous catalysis the catalyst solution is charged into the reactor together with the reactants A and B, which react to form the solvent-dissolved reaction products C and D. The products C and D have different polarities than the catalyst solution and are therefore simple to separate from the catalyst phase (which may be recycled in a suitable manner into the reactor) in the downstream phase separation unit. 

Hydroformylation is an industrially important carbonylation process that is used in the synthesis of aldehydes from olefins, CO and molecular hydrogen by homogeneous catalysis. Efficient catalyst recovery is crucial for the process to be economically viable. Pioneering work in fluorous chemistry was published in 1994 by Horvdth and Rabai, who studied the fluorous hydroformylation of olefins catalyzed by an Rh complex with a fluorous phosphine as a ligand. ... 

In homogeneous catalysis, the catalyst is usually dissolved in a liquid reaction mixture, though some or all of the reactants may be introduced as gases, or even as solids. A small number of examples exist in which the reactants and catalyst are all in the vapour or gaseous state one example is the cracking of acetic (ethanoic) acid to ketene (ethenone) and water at about 7WC, with diethyl phosphate vapour as an acid catalyst. 

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