Catalytic processes Wacker process

Smidt combined this reaction with a redox system, which led to a catalytic process ( Wacker process ) ... 

The free HCl and Cl generated in the catalytic cycle produce environmentally harmful chlorinated by-products to the extent that more than 3 kg of HCl need to be added to the reactor per tonne of acetaldehyde produced to keep the catalytic cycle going. Modified catalysts such as ones based on palladium/ phosphomolybdovanadates have been suggested as a way of reducing byproduct formation to less than 1% of that of the conventional Wacker process. These catalysts have yet to make an impact on commercial acetic production, however. 

Acetaldehyde is the product of the Wacker process. At the end of the fifties oxidation of ethene to ethanal replaced the addition of water to acetylene, because the acetylene/coal-based chemistry became obsolete, and the ethene/petrochemistry entered the commercial organic chemicals scene. The acetylene route involved one of the oldest organometallics-mediated catalytic routes started up in the 1920s the catalyst system comprised mercury in sulfuric acid. Coordination of acetylene to mercury(II) activates it toward nucleophilic attack of water, but the reaction is slow and large reactor volumes of this toxic catalyst were needed. An equally slow related catalytic process, the zinc catalysed addition of carboxylic acids to acetylene, is still in use in paint manufacture. 

The formation of carbon-carbon bonds by palladium-promoted reactions has been widely used in organic synthesis [114-116]. A major advantage is that most of these coupling reactions can be performed with catalytic amounts of palladium. Palladium(II)-catalyzed reactions, e.g., the Wacker process, are distinguished from palladium(O)-catalyzed reactions, e.g., the Heck reaction, since they require oxidative regeneration of the catalytically active palladium(II) species in a separate step [117]. Several groups have applied palladium-mediated and -catalyzed coupling reactions to the construction of the carbazole framework. 

The more expedient, direct catalytic oxidation route to acetone was developed in Germany in the 1960s. If you had been in charge of building the acetone business from scratch, you d probably not have built any IPA-to-acetone plants if you had known about the Wacker process. It s a catalytic oxidation of propylene at 200—250°F and 125—200 psi over palladium chloride with a cupric (copper) chloride promoter. The yields are 91-94%. The hardware for the Wacker process is probably less than for the combined IPA/acetone plants. But once the latter plants were built, the economies of the Wacker process were not sufficient to shut them down and start all over. So the new technology never took hold in the United States. 

Taken together, these initial findings may eventually lead to other recyclable DECS having either modulated activities or selectivities arising from steric effects. Moreover, other catalytic processes requiring a co-catalyst, such as the Wacker process, may be particularly amenable to dendrimer-based catalytic systems because (as discussed in Sect. 2.4.2) the dendrimer interior can accommodate two or more catalytic moieties for example, a metal particle (e. g., Pd) and a metal ion (e. g., Cu +), two different metal ions, or two different zero-valent metal. 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

This chapter has focused on inorganic and heterogeneous catalysts, because historically these are the major systems with which chemical engineers have been concerned. There are number of important homogeneous catalytic processes such as the Wacker process to make vinyl acetate from ethylene and acetic acid, and there are many acid and base homogeneous catalyst systems. 

Palladium-catalyzed addition of oxygen nucleophiles to alkenes dates back to the Wacker process and acetoxylation of ethylene (Sects. 1 and 2). In contrast, catalytic methods for intermolecular oxidative amination of alkenes (i.e., aza-Wacker reactions) have been identified only recently. Both O2 and BQ have been used as oxidants in these reactions. 

Although the oxidation of ethylene to acetaldehyde was known for a number of years,506 its utility depended on the catalytic regeneration of Pd(0) in situ with cop-per(II) chloride discovered by Smidt and coworkers.507 508 Air oxidation of Cu(I) to Cu(n) makes a complete catalytic cycle. This coupled three-step transformation is known as the Wacker process [Eqs. (9.97)-(9.99)]. The overall reaction [Eq. (9.100)] is the indirect oxidation with oxygen of alkenes to carbonyl compounds ... 

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. 

Catalytic Oxidation of Ethene to Acetaldehyde and Acetic Acid. -Evnin et al120 studied Pd-doped V2 Os catalysts for the vapor-phase oxidation of ethene to acetaldehyde in a heterogeneous type of Wacker process. From a mechanistic study they establish a redox mechanism with Pd both as the site of the ethene oxidation and of the reoxidation of the catalyst. On the basis of the role of the V4+ ions proposed by these authors, Forni and Gilardi121 substantiated this mechanism by adding tetra- and hexa-valent dopants to the V2 05 and studying the effects on the catalytic reaction. 

The aqueous palladium chloride oxidation of ethylene to acetaldehyde has been developed into an important commercial process. The discovery of how to make the reaction catalytic with respect to palladium chloride was, perhaps, as important to the process as the discovery of the oxidation reaction itself. This process known as the Wacker-Process, employs cupric chloride as a catalyst for the oxygen (air) reoxidation of... 

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... 

Nucleophilic attack by water on coordinated ethylene, as shown by Reaction 2.12, is the key step in the manufacture of acetaldehyde by the Wacker process (see Chapter 8). In Reaction 2.13 the high oxidation state of titanium makes the coordinated oxygen atom sufficiently electrophilic for it to be attacked by an alkene. As we will see in Chapter 8, this reaction is the basis for the homogeneous catalytic epoxidation of alkenes, using organic hydroperoxides as the oxygen atom donors. 

One of the mechanistic steps most often encountered and inferred from kinetic data is ligand dissociation, which leads to the generation of a catalytically active intermediate. If ligand is added to such a catalytic system, the rate of the reaction decreases. Examples of this in homogeneous catalytic reactions are many CO dissociation in cobalt-catalyzed hydroformylation, phosphine dissociation in RhCl(PPh3)3-catalyzed hydrogenation, Cl dissociation in the Wacker process, etc. The actual rate expressions of most of these processes are described in subsequent chapters. 

Catalytic oxidations with dioxygen can also proceed via heterolytic pathways which do not involve free radicals as intermediates. They generally involve a two-electron oxidation of a (coordinated) substrate by a metal ion. The oxidized form of the metal is subsequently regenerated by reaction of the reduced form with dioxygen. Typical examples are the palladium(II)-catalyzed oxidation of al-kenes (Wacker process) and oxidative dehydrogenation of alcohols (Fig. 4.6). 

In the group of Backvall a method was developed involving palladium and benzoquinone as cocatalyst (Fig. 4.42) [103]. The difficulty of the catalytic reaction lies in the problematic reoxidation of Pd(0) which cannot be achieved by dioxygen directly (see also Wacker process). To overcome this a number of electron mediators have been developed, such as benzoquinone in combination with metal macrocycles, heteropolyacids or other metal salts (see Fig. 4.42). Alternatively a bimetallic palladium(II) air oxidation system, involving bridging phosphines, can be used which does not require additional mediators [115]. This approach would also allow the development of asymmetric Pd-catalyzed allylic oxidation. 

Heating of the N,N-diarylamines with palladium(II) acetate in acetic acid at reflux results in smooth oxidative cyclization to the corresponding carbazole derivatives. A variety of substituents are tolerated in different positions. Thus, this procedure has found many applications in organic syntheses [30,55]. However, the drawback is that stoichiometric amounts of palladium(II) are required, as one equivalent of palla-dium(O) is formed in the final reductive elimination step. In the Wacker process, regeneration of the catalytically active palladium(II) species is achieved by oxidation of palladium(O) to palladium(II) with a copper(II) salt [57]. We were the first to demonstrate that oxidative regeneration of the catalytically active palladium(II)... 

The earliest large-scale catalytic use of palladium is the Wacker Process, which links together several well-known reactions ... 

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