Industrial processes Wacker process

One of the earliest uses of palladium(II) salts to activate alkenes towards additions with oxygen nucleophiles is the industrially important Wacker process, wherein ethylene is oxidized to acetaldehyde using a palladium(II) chloride catalyst system in aqueous solution under an oxygen atmosphere with cop-per(II) chloride as a co-oxidant.1,2 The key step in this process is nucleophilic addition of water to the palladium(II)-complexed ethylene. As expected from the regioselectivity of palladium(II)-assisted addition of nucleophiles to alkenes, simple terminal alkenes are efficiently converted to methyl ketones rather than aldehydes under Wacker conditions. 

With oxo synthesis, Wacker-type oxidations of alkenes is one of the older homogeneous transition-metal-catalyzed reactions [1], The most prominent example of this type of reaction is the manufacture of acetaldehyde from ethylene. This well-known reaction, which has been successfully developed on an industrial scale (Wacker process), combines the stoichiometric oxidation of ethylene by palladium ) in aqueous solution with the in situ reoxidation of palladium(O) by molecular oxygen in the presence of a copper salt (Eqs. 1 -4) [2]. 

The selective oxidation of ethylene to acetaldehyde with Pd VCu° chloride solutions has attained major industrial importance (Wacker process). This reaction can be regarded as an oxidative olefin substimtion (oxypalladation). Once again the in-... 

Formation of acetaldehyde and metallic Pd by passing ethylene into an aqueous solution of PdCl2 was reported by Phillips in 1894 15] and used for the quantitative analysis of Pd(II)[16], The reaction was highlighted after the industrial process for acetaldehyde production from ethylene based on this reaetion had been developed[l,17,18]. The Wacker process (or reaction) involves the three unit reactions shown. The unique feature in the Wacker process is the invention of the in situ redox system of PdCl2-CuCl2. 

Extensive studies on the Wacker process have been carried out in industrial laboratories. Also, many papers on mechanistic and kinetic studies have been published[17-22]. Several interesting observations have been made in the oxidation of ethylene. Most important, it has been established that no incorporation of deuterium takes place by the reaction carried out in D2O, indicating that the hydride shift takes place and vinyl alcohol is not an intermediate[l,17]. The reaction is explained by oxypailadation of ethylene, / -elimination to give the vinyl alcohol 6, which complexes to H-PdCl, reinsertion of the coordinated vinyl alcohol with opposite regiochemistry to give 7, and aldehyde formation by the elimination of Pd—H. 

Whereas this reaction was used to oxidize ethylene (qv) to acetaldehyde (qv), which in turn was oxidized to acetic acid, the direct carbonylation of methanol (qv) to acetic acid has largely replaced the Wacker process industrially (see Acetic acid and derivatives). A large number of other oxidation reactions of hydrocarbons by oxygen involve coordination compounds as detailed elsewhere (25). 

With the growing prominence of the petrochemicals industry this technology was, in turn, replaced by direct air oxidation of naphtha or butane. Both these processes have low selectivities but the naphtha route is still used since it is a valuable source of the co-products, formic and propanoic acid. The Wacker process, which uses ethylene as a feedstock for palladium/copper chloride catalysed synthesis of acetaldehyde, for which it is still widely used (Box 9.1), competed with the direct oxidation routes for a number of years. This process, however, produced undesirable amounts of chlorinated and oxychlorinated by-products, which required separation and disposal. 

The Wacker Reaction and Related Oxidations. An important industrial process based on Pd-alkene complexes is the Wacker reaction, a catalytic method for conversion of ethene to acetaldehyde. The first step is addition of water to the Pd(n)-activated alkene. The addition intermediate undergoes the characteristic elimination of Pd(0) and H+ to generate the enol of acetaldehyde. 

Consortium The Consortium fur Elektrochemische Industrie, founded by A. Wacker in Germany in 1903, is the corporate research laboratory of Wacker-Chemie. Many processes have been developed in this laboratory, but the one for which it is best known is the Wacker process for making acetaldehyde this has also been called the Consortium process. 

The naming of this process has been confused because of various corporate relationships. The basic invention was created in 1957 at the Consortium fur Elektrochemische Industrie, Munich, a wholly owned subsidiary of Wacker-Chemie. It has therefore been called both the Wacker process and the Consortium process. But for many years, Wacker-Chemie has had a close relationship with Farbwerke Hoechst and the latter company has participated in some of the development and licensing activities, so two other names have come to be used Wacker-Hoechst and Hoechst-Wacker. The live inventors (J. Schmidt, W. Hafner, J. Sedlmeier, R. Jira, and R. Riittinger) received the Dechema prize in 1962 for this invention. The acetaldehyde process was first operated commercially in 1960. In 1997, this process was used in making 85 percent of the world s production of acetaldehyde. Although Wacker-Chemie still makes vinyl acetate, it no longer uses the Wacker process to do so. 

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 Wacker Process Adding oxygen to compounds on an industrial scale requires a cheap source of... 

Catalysis. The most important industrial use of a palladium catalyst is the Wacker process. The overall reaction, shown in equations 7—9, involves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-catalyzed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or in a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chlorinated by-products. 

Industrial Applications. Several large scale industrial processes are based on some of the reactions listed above, and more are under development. Most notable among those currently in use is the already mentioned Wacker process for acetaldehyde production. Similarly, the production of vinyl acetate from ethylene and acetic acid has been commercialized. Major processes nearing commercialization are hydroformylations catalyzed by phosphine-cobalt or phosphine-rhodium complexes and the carbonylation of methanol to acetic acid catalyzed by (< 3P) 2RhCOCl. 

The oxidation of olefins to aldehydes using a palladium chloride-copper(II) chloride catalyst, the Wacker Process, is a well-established industrial reaction. The mechanism of this reaction has not been established in detail, but it most probably involves a cr-7r rearrangement... 

Terminal alkenes can be selectively oxidized to aldehydes by reaction with oxygen, using a palladium-copper catalyst in tertiary butanol (equation 35)160. This reaction is contrary to the normal oxidation process which yields a ketone as the major product. The palladium(II) oxidation of terminal alkenes to give methyl ketones is known as the Wacker process. It is a very well established reaction in both laboratory and industrial synthesis161162. The Wacker oxidation of alkenes has been used in the key step in the synthesis of the male sex pheromone of Hylotrupes bajulus (equation 36)163. 

Since 1961, the industrial importance of the hydroformylation reaction has been threatened by newer processes (19) such as the Ziegler polymerization of ethylene, the Wacker process, and the direct oxidation of petroleum (153). The industrial aspects of the Oxo reaction were reviewed in 1965 when the world production capacity for Oxo products was estimated at 0.5 million tons per year (39). 

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. 

Soon after the invention of the Wacker process the formation of vinyl acetate by the oxidative acetoxylation of ethylene using Pd(OAc)2 was discovered by Moiseev [16], and the industrial production of vinyl acetate based on this reation was developed. At present, vinyl acetate is produced commercially by a gas-phase reaction of ethylene, acetic acid and O using Pd catalyst supported on alumina or silica (eq. 1.11). 

The Wacker Oxidation is an industrial process, which allows the synthesis of ethanal from ethene by palladium-catalyzed oxidation with oxygen. Copper serves as redox cocatalyst. 

The Pd-catalyzed conversion of terminal alkenes to methyl ketones is a reaction that has found widespread use in organic chemistry [87,88]. These reactions, as well as the industrial Wacker process, typically employ CuCh as a co-catalyst or a stoichiometric oxidant. Recently Cu-free reaction conditions were identified for the Wacker-type oxidation of styrenes using fBuOOH as the oxidant. An NHC-coordinated Pd complex, in-situ-generated (I Pr)Pd(OTf)2, served as the catalyst (Table 5) [101]. These conditions min-... 

To the author s knowledge, there are at present no major industrial processes which could be strictly defined as nonacid catalysis that make use of zeolite-based catalysts. This is in contrast to acid catalysis where zeolites continue to make an impact. Technically, a number of zeolite-based catalysts for reactions, such as Wacker chemistry and olefin or diolefin oligomerization reactions, appear to be quite attractive, and it is almost certainly economic factors that have limited further development. 

An important concern in industrial processes is corrosion. Transition metal complexes under certain conditions can facilitate corrosion of the reaction vessels. The consequences are not only fouling of the surfaces but also loss of expensive catalyst. So the reactors are generally made of materials that are resistant to corrosion. Two such materials are stainless steel 316 (containing 16-18% Cr, 10-14% Ni, 1-3% Mo, <0.1% C, and the rest iron) and Hastelloy C (containing 14-19% Mo, 4-8% Fe, 12-16% Cr, 3-6% W, and the rest Ni). The latter is ideal for chlorides and acids but is 3-4 times more expensive than the former. Wacker s process is operated under highly corrosive conditions (high concentrations of H+ and Cl ) (see Section 8.2) hence it requires expensive, titanium-lined reactors. 

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