Homogeneous catalysis process

Homogeneous Catalysis, S. Bhaduri and D. Mukesh, Wiley-Interscience 2000, 239 pp., ISBN 0-471-37221-8. This introductory textbook emphasizes the practical side, with a chapter dedicated to chemical engineering basics and process unit operations. The authors cover several examples of industrial homogeneous catalysis processes. Although the book is aimed at graduate students, it contains relatively few references to the primary literature. Each chapter is accompanied by a good selection of review questions and problems. 

As discussed later regarding the role of multiphase operations for homogeneous catalysis, research has been focused mainly on the issue of recyclability of homogeneous catalysts. Relevant progress has been made in this aspect, but the other problem, which could be indicated as intensification of homogeneous catalysis processes, is the issue that should be solved, at least for medium-large scale industrial productions. 

Because of the success experienced in organic synthesis by using phase transfer reaction conditions IS), it was expected that this would be an excellent method for hydrogenating organic soluble dienes by using the water soluble catalyst K3[Co(CN)5H]. Others recently have carried out homogeneous catalysis processes under phase-transfer reaction conditions 14,15,16). 

Methanol plays a central role in Cl chemistry. Research on the reactions of methanol continues to be very active. For example, the carbonylation of methanol to acetic acid using the Rh complex catafyst and the iodide promoter has some drawbacks, althou it was an important achievement. Successful development of a noniodide system would eliminate the corrosion problem and the need for using C)q)ensive zirconia as a material of construction. There is also active research on nonRh-based catalysts [70] and pofymer-supported Rh catalysts [71]. SRI International reported that the latter could be more economical than the present homogeneous catalysis process [72],... 

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. 

Metallacarboranes. These are used in homogeneous catalysis (222), including hydrogenation, hydrosilylation, isomerization, hydrosilanolysis, phase transfer, bum rate modifiers in gun and rocket propellants, neutron capture therapy (254), medical imaging (255), processing of radioactive waste (192), analytical reagents, and as ceramic precursors. 

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. 

Several L-amino acids are produced on a large scale by enzymatic resolution of N-acetyl-D,L-amino adds (Figure A8.4). Acylase immobilised on DEAE-Sephadex is for example employed in a continuous process while Degussa uses the free acylase retained in a membrane reactor. In the latter process the advantage of reuse of the enzyme and homogeneous catalysis are combined. 

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. 

The past fifteen years have seen evidence of great interest in homogeneous catalysis, particularly by transition metal complexes in solution predictions were made that many heterogeneous processes would be replaced by more efficient homogeneous ones. There are two motives in these changes—first, intellectual curiosity and the belief that we can define the active center with... 

Oxidation of carbon monoxide by metal ions and homogeneous catalysis of the water gas shift reaction and related processes. J. Halpern, Comments Inorg. Chem., 1981,1, 3-15 (42). 

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. 

For most applications, enzymes are purified after isolation from various types of organisms and microorganisms. Unfortunately, for process application, they are then usually quite unstable and highly sensitive to reaction conditions, which results in their short operational hfetimes. Moreover, while used in chemical transformations performed in water, most enzymes operate under homogeneous catalysis conditions and, as a rule, cannot be recovered in the active form from reaction mixtures for reuse. A common approach to overcome these limitations is based on immobilization of enzymes on solid supports. As a result of such an operation, heterogeneous biocatalysts, both for the aqueous and nonaqueous procedures, are obtained. 

Generally, the above transesterification reactions are catalyzed by strong acids or alkalis [1, 2]. In the homogeneous catalytic process by acids or alkalis, neutralization is required of the product. This post-treatment produces waste water, and increases equipment investment and production cost. Recently, more attention has been paid to the heterogeneous catalysis process [3] for an easier production process and to reduce pollution of the environment. 

Experiments showed that high methyl ester yields can be achieved with solid bases and super acids under moderate reaction conditions. The solid bases were more effective catalysts than the solid super acids. High stability can be achieved by an ordinary inexpensive preparation process, and the catalyst can be separated easily from the reaction products in the heterogeneous catalysis process. The costly catalyst removal process can be avoided compared with the homogeneous process. Therefore, the heterogeneous process using a solid catalyst should be more economical for biodiesel production. 

This chapter focuses on heterogeneous catalysis, which is most important in fine chemicals production. Table 3.1 presents a number of examples of catalysis in fine chemistry. These examples are divided in heterogeneously catalysed processes and homogeneously catalysed processes. A detailed treatment of heterogeneously catalysed processes for the production of fine chemicals is also given in the book edited by Sheldon and van Bekkum (2001). 

In homogeneous catalysis soluble catalysts are applied, usually in the liquid phase, in contrast to heterogeneous catalysis, where solid catalysts are used. Homogeneous catalysis is applied in many processes in both bulk and fine chemicals production. 

Homogeneous catalysis has an important role to play in enantioselective reactions. To improve product safety, the pharmaceutical industry is producing an increasing number of products in enantiomerically pure form. Other important (future) markets include agrochemicals, polymers and fine chemicals. Although the number of practised processes is quite small the potential is high. 

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