Homogeneous catalysis Wacker process

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-... 

The coordinative polymerization with soluble transition metal systems is part of the growing field of homogeneous catalysis on transition metal centers (Oxo-Process, Wacker-Process, Isomerization, Cyclooligomerization of olefis, etc.). The mechanisms of these reactions have not yet been completely elucidated. Any new knowledge could perhaps contribute to the detection of common trends and parallels, and would thus facilitate prediction and development of new processes. 

In spite of some declining industrial interest, the last 5 years have seen an unusual academic interest in the catalytic properties of the metal carbonyls. This has been part of a wider surge of interest in the organometallic chemistry of the transition metals and its application to homogeneous catalysis. Reactions such as Ziegler polymerization, the Oxo reaction, and the Wacker process are but a few of the many reactions of unsaturated molecules catalyzed in the coordination sphere of transition metal complexes (20). These coordination catalyses have much in common, and the study of one is often pertinent to the study of the others. 

This section is concerned with the activation of hydrocarbon molecules by coordination to noble metals, particularly palladium.504-513 An important landmark in the development of homogeneous oxidative catalysis by noble metal complexes was the discovery in 1959 of the Wacker process for the conversion of ethylene to acetaldehyde (see below). The success of the Wacker process provided a great stimulus for further studies of the reactions of noble metal complexes, which were found to be extremely versatile in their ability to catalyze homogeneous liquid phase reaction. The following reactions of olefins, for example, are catalyzed by noble metals hydrogenation, hydroformylation, oligomerization and polymerization, hydration, and oxidation. 

The field of homogeneous palladium catalysis traces its origin to the development of the Wacker process in the late 1950s (Eq. 7) [83]. Since this discovery, palladium-catalyzed reactions have evolved into some of the most versatile reactions for the synthesis of organic molecules [84,85]. Palladium-catalyzed Wacker-type oxidation of alkenes continues to be an active field of research [86-88], and several recent applications of NHC-coordinated Pd catalysts have been reported for such reactions. 

Conversion of ethylene to acetaldehyde with a soluble palladium complex was one of the early applications of homogeneous catalysis. Traditionally, acetaldehyde was manufactured either by the hydration of acetylene or by the oxidation of ethanol. As most of the acetic acid manufacturing processes were based on acetaldehyde oxidation, the easy conversion of ethylene to acetaldehyde by the Wacker process was historically a significant discovery. With the... 

The synthesis of acetaldehyde by oxidation of ethylene, generally known as the Wacker process, was a major landmark in the application of homogeneous catalysis to industrial organic chemistry. It was also a major step in the displacement of acetylene (made from calcium carbide) as the feedstock for the manufacture of organic chemicals. Acetylene-based acetaldehyde was a major intermediate for production of acetic acid and butyraldehyde. However the cost was high because a large energy input is required to produce acetylene. The acetylene process still survives in a few East European countries and in Switzerland, where low cost acetylene is available. 

The Wacker process was a major landmark and a great push towards the development of homogeneous catalysis. The mechanism of acetaldehyde formation differs fundamentally from the other oxidation processes as O2 itself is not directly involved. As is clear from Figure 28 the actual oxidant is Pd(II) which is reduced to Pd(0). The intimate pathway of the reaction involves nucleophilic attack and was the subject of much debate. 

To oxidize ethylene to acetaldehyde technically, two major approaches seem feasible (a) vapor-phase heterogeneous catalysis, and (b) liquid-phase homogeneous catalysis. The most pertinent references on the vapor-phase process are summarized in Table VI. However, neither this approach nor the electrolytic oxidation of ethylene (14) appears to have gained any commercial importance. Liquid-phase homogeneous catalysis is the approach practiced commercially, and this is understood when one talks about the Wacker process. The latter has been carried out in two principal ways ... 

William Christopher Zeise (1789-1847) was a Danish apothecary and professor in Copenhagen, Denmark. He synthesized the first metal-olefin complex by serendipity (this term is explained in Chapter 4), when he treated platinum(IV) chloride with ethanol and potassium chloride K[PtCl3( -C2H4)], sal kalico-platinicus inflammabilis , cf [73], TT-Complexation of olefins at transition metals nowadays comprises a key feature of homogeneous catalysis in terms of olefin activation, with the Wacker-Hoechst process being a prominent example (cf. Section 2.4.1). 

The term applied indicates the application-oriented objective of this work. It was an important criterion of selection not to supply merely a collection of unweighted facts and various practical examples of homogeneous catalysis. In this context applied means a selection of homogeneous catalyzed processes, which on the one hand have already arrived at industrial success (e. g., cai bonyla-tion of alcohols, hydroformylation, Wacker-Hoechst process). On the other hand, the book also includes homogeneously catalyzed reactions of which the state-of-the-art indicates commercialization in the near future. Moreover, for scientific reasons the inclusion of newer catalytic reactions or reaction principles is required, even when commercialization is not yet in sight. Both aspects are covered by the sections Applied Catalysis and Recent Developments . 

Jira, R. Oxidation of olefins to carbonyl compounds (Wacker process). Applied Homogeneous Catalysis with Organometallic Compounds 1996, 1, 374-393. 

The Wacker-Smidt process—hereafter known simply as the Wacker oxidation, reaction, or process—enjoyed considerable success, yet its use has declined dramatically over the past 10 years for at least two reasons.49 First, manufacturing plants are expensive to build and maintain because they must be constructed to withstand a corrosive environment. Second, another procedure that yields acetic acid directly from synthesis gas was developed and now supplants the Wacker-Smidt process. This newer route also uses homogeneous catalysis involving Rh and Ir complexes and will be described in Section 9-5. 

Homogeneous catalysis by redox metals is also known for nonelectro-chemical processes. Thus, ethylene is oxidized to acetaldehyde in the Wacker process in aqueous solutions containing Pd " (504). Apart from complex formation and insertion (505), ionic oxidation and reduction may take place. It is noteworthy that palladium oxidation to form ions that act as homogeneous catalysts has been suggested as an important step in ethylene electrooxidation on solid palladium electrocatalysts 28, 29). 

R. lira, Oxidation of Olefins to Carbonyl Compounds (Wacker Process) , in Applied Homogeneous Catalysis with Ogranometallic Compounds, B. Comils, W. A. Herrmann, Eds., pp. 374—393, VCH, Weinheim, Germany, 1996. 

R] Jira, R. Oxidations Oxidation of olefins to carbonyl compounds (Wacker process). In Applied Homogeneous Catalysis with Organometallic Compounds (2nd Edition) Comils, B. Herrmann, W. A. (Eds.), Wiley-VCH Weinheim, Germany, 2002, /, pp386-405. 

Homogeneous catalysis with defined soluble transition metal complexes as catalysts has become one of the most effective means of transforming simple olefins into more valuable materials. The technically important hydroformylation of olefins to aldehydes or alcohols the Wacker process the dimerization of propylene to linear hexenes the oligomerization of ethylene to linear a-olefins are only a few examples. A feature common to all these processes is the insertion of a substrate olefin molecule, which is coordinatively bonded to the transition metal center M, into a metal-carbon or metal-hydrogen bond present at the same center ... 

The oxidation of olefins to carbonyl compoimds (the Wacker process in technical concerns, also called the Hoechst-Wacker process) was of great importance for the recognition of the usefulness of organometalhc homogeneous catalysis in the bulk chemicals industry [32]. The Wacker ethylene oxidation is one of the key steps in industrial homogeneous catalysis. Palladium catalysts are usually applied and have... 

The process of choice for acetaldehyde production is ethylene oxidation according to the so-called Wacker-Hoechst process [route (c) in Topic 5.3.2]. The reaction proceeds by homogeneous catalysis in an aqueous solution of HQ in the presence of palladium and copper chloride complexes. The oxidation of ethylene occurs in a stoichiometric reaction of PdQ2 with ethylene and water that affords acetaldehyde, metallic palladium (oxidation state 0), and HQ [step (a) in Scheme 5.3.5). The elemental Pd is reoxidized in the process by Cu(II) chloride that converts in this step into Cu(I) chloride [step (b) in Scheme 5.3.5). The Cu(II) chloride is regenerated by oxidation with air to finally close the catalytic cycle [step (c) in Scheme 5.3.5). 

The Wacker process is a paradigmatic example of homogeneous catalysis applied in the chemical industry [87]. The process, discovered in the 1950s, produces acetaldehyde from ethene and oxygen using a Pd catalyst. The reaction is proposed to proceed via the addition of Pd and a hydroxide group (originated from the... 

Linear low-molecular-weight a-olefins have become very important industrial intermediates during the past decades. This is partly due to recent developments in homogeneous catalysis such as the Oxo and Wacker processes, and partly to increased demand for biodegradable detergents and for plasticizers and additives for rubber or gasoline. 

Supported liquid-phase catalysts (SLPCs) combine the salient features of both homogeneous and heterogeneous catalysis for enhanced catalytic and/or process efficiency (337). SLPC catalysts, in which a liquid-phase (homogeneous) catalyst is dispersed within a porous support, have been used in Wacker-type ethylene oxidation for acetaldehyde and vinyl acetate production (337, 338). In the former case, a traditional homogeneous Wacker catalyst (vide supra) consisting of a chlorinated solution of Pd and Cu chlorides retained on a support with monomodal pore size distribution... 

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