Homogenous catalysis in solution

Homogeneous Catalysis in Solution with Tritiated Water... 

As a new strategy for the liquid-liquid biphasic reaction system, emulsion catalysis has not been well discussed in the past reviews. In this chapter, we will focus on the application of emulsion catalysis in organic reactions. The chapter is arranged as three parts based on the reaction types, and highlights the relation between homogeneous catalysis (in solution) and heterogeneous catalysis (in the heterogeneous dispersion). 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

Round hill, D. M., Organotransition-Metal Chemistry and Homogeneous Catalysis in Aqueous Solution. 38 155... 

Another example of homogeneous catalysis in aqueous solution is the dimerization of benzaldehyde catalyzed by cyanide ion, CN- (Wilkinson, 1980, p.28) ... 

Recovery ofthe soluble cattalysts presents the greatest difficulty in large scale appfications of homogeneous catalysis. In a way, aqueous biphasic catalysis itself provides a solution of this problem. It is not the aim of this book to discuss the various other methods of heterogenization of homogeneous catalysts. The only exception is the use of water-soluble... 

The Wacker process (Eq. 1) was developed nearly 50 years ago [1-3] and represents one of the most successful examples of homogeneous catalysis in industry [4-9]. This palladium-catalyzed method for the oxidation of ethylene to acetaldehyde in aqueous solution employs a copper cocatalyst to facilitate aerobic oxidation of Pd° (Scheme 1). Despite the success of this process, certain features of the reaction have Umited the development of related aerobic oxidation reactions. Many organic molecules are only sparingly sol-... 

Homogeneous catalysis is possible in liquid phase and can be performed for reactants that are only stable in solution. Heterogeneous catalysis in solution is difficult. 

Many important chemicals are produced commercially by reactions which are catalyzed by organometallic compounds and this feet provides one of the motivating forces for studying organometallic chemistry. Much of the focus in this section will be on homogeneous catalysis because solution reactions are better understood than are the surface reactions of heterogeneous systems. It is also easier to modify an organometallic compound and evaluate the effects of the modification than it is to alter and study a surface. 

Substances which increase the rate of a chemical reaction without themselves being used up or incorporated into the finished product are called catalysts. In heterogeneous catalysis the reaction takes place on the surface of a solid support. The activity of the catalyst in this case is determined by the structure and size of the surface area as well as the way the catalyst is produced. Catalysts are not limited to immobilization on solids, they can also be introduced as homogeneous catalysts in solution. Between heterogeneous and homogeneous catalysts there is the possibility to evenly distribute small particles (dispersions) of catalyst in a liquid phase. 

For all reactions involving vanadium phosphate phases and liquid-phase reactants, it is emphasized that leaching of vanadium into solution is possible and that these reactions are homogeneously catalyzed by vanadium ions. Indeed, this catalysis in solution has been reported in some of the investigations described above 266,303,304) and could have been important in other cases as well. 

Transition Metals and Homogeneous Catalysis in Aqueous Solution 157... 

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