Ethylene oxidation to acetaldehyde

Catalysts used to convert ethylene to vinyl acetate are closely related to those used to produce acetaldehyde from ethylene. Acetaldehyde was first produced industrially by the hydration of acetylene, but novel catalytic systems developed cooperatively by Farbwerke Hoechst and Wacker-Chemie have been used successfully to oxidize ethylene to acetaldehyde, and this process is now well established (7). However, since the largest use for acetaldehyde is as an intermediate in the production of acetic acid, the recent announcement of new processes for producing acetic acid from methanol and carbon monoxide leads one to speculate as to whether ethylene will continue to be the preferred raw material for acetaldehyde (and acetic acid). 

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 ... 

The process proceeds by homogeneous catalysis on PdCl2. It had been known much eaher that solutions of Pd complexes stoichiometrically oxidize ethylene to acetaldehyde, but the crucial discovery was the exploitation of this reaction in a catalytic cycle. A closed-cycle process was developed in which an excess of the oxidizing agent Cu " re-oxidizes the palladium formed in the process without its depositing on die reactor walls. The Cu" formed in the redox process is re-oxidized to Cu " by oxygen. The reaction steps are described by Equations 3-6 to 3-8. 

An example which illustrates a different aspect of catalysis is the Wacker process for oxidizing ethylene to acetaldehyde. The earlier technique involved two steps and the isolation of an intermediate. However, consideration of the aqueous reactions ... 

Multiply bonded O ligands in the model organometallic complex [Pt4(l,5-COD)4 (P3-0)2C12][BF4]2 have been shown to oxidize ethylene to acetaldehyde and norbor-nene to a platinaoxetane complex. Such a reaction might suggest an unexpected role of 0x0 complexes in oxidation chemistry (e.g. Wacker chemistry). ... 

The Wacker process, an important industrial procedure now used to make 6 million tons a year of aldehydes, relies on nucleophilic attack on an alkene complex. The fact that aqueous PdCb oxidizes ethylene to acetaldehyde had been known—although not understood—since the nineteenth century the reaction consumes the PdCb as oxidant and deposits metallic Pd(0). It took considerable imagination to see that such a reaction could ever be useful on an industrial scale because PdCla is obviously far too expensive to use stoichiometrically. It is often useful to find a way to convert a stoichiometric to a catalytic process. J. Smidt of Wacker Chemie realized in the late 1950s that it is possible... 

Ca.ta.lysis, The most important iadustrial use of a palladium catalyst is the Wacker process. The overall reaction, shown ia equations 7—9, iavolves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-cataly2ed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or ia a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chloriaated by-products. 

The Wacker process for the oxidation of ethylene to acetaldehyde with PdCb/CuCb at 100°C (212°F) with 95 percent yield and 95 to 99 percent conversion per pass. 

Dehydrogenation processes in particular have been studied, with conversions in most cases well beyond thermodynamic equihbrium Ethane to ethylene, propane to propylene, water-gas shirt reaction CO -I- H9O CO9 + H9, ethylbenzene to styrene, cyclohexane to benzene, and others. Some hydrogenations and oxidations also show improvement in yields in the presence of catalytic membranes, although it is not obvious why the yields should be better since no separation is involved hydrogenation of nitrobenzene to aniline, of cyclopentadiene to cyclopentene, of furfural to furfuryl alcohol, and so on oxidation of ethylene to acetaldehyde, of methanol to formaldehyde, and so on. 

The process is similar to the catalytic liquid-phase oxidation of ethylene to acetaldehyde. The difference hetween the two processes is the presence of acetic acid. In practice, acetaldehyde is a major coproduct. The mole ratio of acetaldehyde to vinyl acetate can he varied from 0.3 1 to 2.5 1. The liquid-phase process is not used extensively due to corrosion problems and the formation of a fairly wide variety of by-products. 

The oxidation of ethylene to acetaldehyde by palladium chloride in water has been known since the nineteenth century.80 However, the reaction requires the use of a stoichiometric amount of PdCl2, resulting in Pd(0) deposit. Anderson, in 1934, observed a similar reaction (but... 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

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] ... 

Palladium-catalyzed oxidation of hydrocarbons has been a matter of intense research for about four decades. The field was initiated by the development of the aerobic oxidation of ethylene to acetaldehyde catalyzed by palladium chloride and co-catalyzed by cupric chloride (the Wacker process, equation l)1. 

The complex ion catalyzes various types of organic reactions including oxidation of ethylene to acetaldehyde in aqueous solution (the 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-... 

Palladium chloride or the chloropalladite ion catalyze the oxidation of olefins to aldehydes or ketones, presumably by forming unstable palladium-olefin complex intermediates 196). A reaction of great industrial importance is the palladium chloride/cupric chloride catalyzed oxidation of ethylene to acetaldehyde 195). The first stage is presumably the oxidative hydrolysis of ethylene,... 

The previous examples involve reduction (hydrogenation) of organic molecules, but transition metal complexes can also catalyze oxidation. For example, the Wacker process, which has been widely used to convert ethylene to acetaldehyde, depends on catalysis by palladium(II) in the presence of copper(II) in aqueous HC1. The role of the copper chloride is to provide a means of using air to reoxidize the palladium to palladium(II). Once again, Zeise-type coordination of the ethylene to the metal center is believed to be involved ... 

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 ... 

Selective oxidation of ethylene to acetaldehyde was carried out over carbon-supported Pd and Pt membrane catalysts.1322 The concept of supported liquid-phase catalysis was also successfully applied in the Wacker oxidation.1323 The Wacker reaction can be performed in alcohol-supercritical C02.1324 C02 as cosolvent accelerates reaction rates and remarkably affects the selectivity towards methyl ketone in the presence of an alcohol. 

The oxidation of ethylene to acetaldehyde in the gas phase, carried out at rather low temperatures (100—200°C), is very similar to the Wacker liquid phase process. One of the main steps in this process is... 

The first reaction is stoichiometric and would be of little value since palladium is expensive except that, in the presence of Cu2+, palladium metal is oxidized back to Pd2+ before it precipitates (Eq. 15.178). The Cu1 produced is reoxidized by molecular oxygen. The mechanism for conversion of ethylene to acetaldehyde has been extensively studied and the intermediates shown in the above cycle are now accepted by most chemists.183... 

WACKER REACTION. The oxidation of ethylene to acetaldehyde in the presence of palladium chloride and cupric chloride. 

We do not know if the vinylic alcohol is actually an intermediate or whether a hydride-71 complex of it rearranges directly to the aldehyde as probably happens in the palladium-catalyzed oxidation of ethylene to acetaldehyde. The formation of 4% 2-methyl-2-phenylpropanal is unexpected. This product must arise from a reversed addition of the phenylpalladium group followed by a hydrogen transfer from the hydroxyl-bearing carbon to the palladium, followed by reductive elimination of a hydridopalladium group. An alkyoxypalladium intermediate has been proposed (39). 

Fig. 64. Redox cycle for the oxidation of ethylene to acetaldehyde. V5+ (oxidized heteropolyanion) represents vanadium in the oxidized heteropolyanion. (From Ref. 368.)...

Heterolytic liquid-phase oxidation processes are more recent than homolytic ones. The two major applications are the Wacker process for oxidation of ethylene to acetaldehyde by air, catalyzed by PdCl2-CuCl2 systems,98 and the Arco oxirane" or Shell process100 for epoxidation of propylene by f-butyl or ethylbenzene hydroperoxide catalyzed by molybdenum or titanium complexes. These heterolytic reactions require less drastic conditions than the homolytic ones... 

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). 

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