Smidt process

The metals, as gauze or foil, and especially on supports such as charcoal or alumina onto which the metal salts are absorbed and reduced in situ under specified conditions, are widely used as catalysts for an extremely large range of reactions in the gas phase or in solution. One of the larger uses of Pt is for the reforming of hydrocarbons. Commercial uses in homogeneous reactions are fewer, but palladium is used in the Smidt process and Rh in hydroformylation and in acetic acid synthesis. 

The Wacker or Smidt process, used to synthesize acetaldehyde from ethylene, involves a catalytic cycle that uses PdCl4. A brief outline of a cycle proposed for this process is shown in Figure 14-17. The fourth step in this cycle is substantially more complex than that shown in the figure and has been the subject of much study. ... 

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. 

Smidt process (page 797) and rhodium chloride in acetic acid synthesis (page 793). Industrially, as well as in the laboratory, catalytic reductions are especially important.4... 

FIGURE 14.21 Wacker (Smidt) Wacker (Smidt) Process... 

The Wacker-Smidt process utilizes a Pd catalyst to convert ethylene gas into acetaldehyde, which is then oxidized to make acetic acid. The catalytic cycle is shown in Figure 19.32. 

Another family of catalytic reactions of olefins, typified by the Smidt process for oxidation of ethylene to acetaldehyde, proceeds by a different mechanism. In these reactions a nucleophile such as OH , Cl , or acetate ion attacks a coordinated olefin. A a-alkylpalladium compound may form as a short-lived intermediate, as discussed in a recent review by Aguilo 175). 

A common property of coordinated alkenes is their susceptibility to attack by nucleophiles such as OH , OMe , MeC02, and Cl , and it has long been known that Zeise s salt is slowly attacked by non-acidic water to give MeCHO and Pt metal, while corresponding Pd complexes are even more reactive. This forms the basis of the Wacker process (developed by J. Smidt and his colleagues at Wacker Chemie, 1959-60) for converting ethene (ethylene) into ethanal (acetaldehyde) — see Panel overleaf. 

The oxidation of ethylene to acetaldehyde by dioxygen catalyzed by palladium and cupric salts found important technological application. The systematic study of this process was started by Smidt [245] and Moiseev [246]. The process includes the following stoichiometric stages [247,248] ... 

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 ... 

Discovered by Phillips in 1894,382 the oxidation of ethylene to acetaldehyde by palladium(ll) salts in an aqueous solution was developed into a commercial process about 60 years later by Smidt and coworkers at Wacker Chemie.383,384 These researchers succeeded in transforming this stoichiometric oxidation by a precious metal (equation 150) into a catalytic reaction through the reoxidation of the resulting Pd° by molecular oxygen in the presence of copper salts (equations 151-152). 

The palladium-catalyzed oxidation of ethylene to acetaldehyde (the Wacker process) was discovered by Smidt and co-workers514-518 in 1959. This process combines the stoichiometric reduction of Pd(II) with reoxidation of metal in situ by molecular oxygen in the presence of copper salts. The overall reaction constitutes a palladium-catalyzed oxidation of ethylene to acetaldehyde by molecular oxygen ... 

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

The Wacker process is carried out in an aqueous medium containing hydrochloric acid. In addition to ethylene, Smidt and coworkers carried out the oxidation of other alkenes in an acidic aqueous solution of PdCh to prepare carbonyl compoimds. After this report, a few studies on the oxidation of higher alkenes were carried out in organic media. In general, terminal alkenes are converted to methyl ketones rather than aldehydes (equation 1). 

Research to convert ethylene directly to acetaldehyde was begun in 1956 at the Consortium fiir Elektrochemische Industrie G.m.b.H., a subsidiary of Wacker Chemie G.m.b.H., under the direction of J. Smidt. The results of this research were summarized (34, 35) in two fundamental publications and in numerous patents. Smidt and co-workers first surveyed the open literature to determine what approaches had been used to oxidize ethylene and what the resulting oxidation products were. Table III summarizes the pertinent literature findings up to 1956. None of the processes published offered much promise for converting ethylene to acetaldehyde directly. In their initial experiments, Smidt et al. (34) passed mixtures of ethylene, oxygen, and hydrogen over a catalyst of palladium deposited on activated carbon, obtaining traces of acetaldehyde. They also found that the acetaldehyde yield was increased when... 

For each mole of acetaldehyde formed, one mole of palladium chloride was reduced to metallic palladium. To make this process industrially attractive, it must be conducted so that palladium chloride acts as a catalyst rather than as an oxidant—i.e., so that the metallic palladium formed is reoxidized to palladium chloride and can be reused for the principal reaction. This was the second fundamental recognition, which helped make this process commercial. The search for proper oxidants for metallic palladium was facilitated by the observation of Smidt et al. (34) that if cupric or ferric chloride were added to palladium chloride in the vapor-phase oxidation of ethylene to acetaldehyde, the acetaldehyde yield was increased. Therefore, these compounds were also used in the liquid-phase oxidation. In such a system, the following reactions will occur in the presence of oxygen and hydrochloric acid, the latter being formed by the reaction above (34). 

Smidt, J. Seiber, R. Hafner, W. Jira, R Process for the Production of Carbonyl Groups in Organic Compounds 1964 US 3,153,083... 

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