Wacker—Hoechst process

Acetaldehyde, first used extensively during World War I as a starting material for making acetone [67-64-1] from acetic acid [64-19-7] is currendy an important intermediate in the production of acetic acid, acetic anhydride [108-24-7] ethyl acetate [141-78-6] peracetic acid [79-21 -0] pentaerythritol [115-77-5] chloral [302-17-0], glyoxal [107-22-2], aLkylamines, and pyridines. Commercial processes for acetaldehyde production include the oxidation or dehydrogenation of ethanol, the addition of water to acetylene, the partial oxidation of hydrocarbons, and the direct oxidation of ethylene [74-85-1]. In 1989, it was estimated that 28 companies having more than 98% of the wodd s 2.5 megaton per year plant capacity used the Wacker-Hoechst processes for the direct oxidation of ethylene. 

The direct oxidation of ethylene is used to produce acetaldehyde (qv) ia the Wacker-Hoechst process. The catalyst system is an aqueous solution of palladium chloride and cupric chloride. Under appropriate conditions an olefin can be oxidized to form an unsaturated aldehyde such as the production of acroleia [107-02-8] from propjiene (see Acrolein and derivatives). 

Another attractive commercial route to MEK is via direct oxidation of / -butenes (34—39) in a reaction analogous to the Wacker-Hoechst process for acetaldehyde production via ethylene oxidation. In the Wacker-Hoechst process the oxidation of olefins is conducted in an aqueous solution containing palladium and copper chlorides. However, unlike acetaldehyde production, / -butene oxidation has not proved commercially successflil because chlorinated butanones and butyraldehyde by-products form which both reduce yields and compHcate product purification, and also because titanium-lined equipment is required to withstand chloride corrosion. 

A variation of the Pd/Cu Wacker-Hoechst process, termed OK Technology, has been proposed by Catalytica Associates (40—46). This process avoids the use of chlorides and uses a Pd/Cu catalyst system which incorporates a polyoxoanion and a nitrile ligand. 

The Wacker-Hoechst process has been studied in great detail and in all textbooks it occurs as the example of a homogeneous catalyst system illustrating nucleophilic addition to alkenes. Divalent palladium is the oxidising agent and water is the oxygen donor according to the equation ... 

The large scale manufacture of acetaldehyde with the Wacker-Hoechst process takes place in a two-phase gas/liquid system. Ethylene and air (or O2) react with the acidic (pH 0.8-3) aqueous catalyst solution in a corrosion-resistant titanium or lined reactor. 

The formation of chlorinated by-products and the processing of aqueous chloride solutions are putting heavy ecological constraints on this technology nowadays. Additionally corrosion problems related to the use of highly acidic solutions have always been a major drawback for the Wacker-Hoechst process. 

The Wacker-Hoechst process has been practised commercially since 1964. In this liquid phase process propylene is oxidized to acetone with air at 110-120°C and 10-14 bar in the presence of a catalyst system containing PdCl2. As in the oxidation of ethylene, Pd(II) oxidizes propylene to acetone and is reduced to Pd(0) in a stoichiometric reaction, and is then reoxidized with the CuCl2/CuCl redox system. The selectivity to acetone is 92% propionaldehyde is also formed with a selectivity of 2-4%. The conversion of propylene is more than 99%. 

Co saturated hydrocarbons are used extensively in the United States, whereas the acetylene process was used almost exclusively in Europe until recently. These processes were extended by the late 1950 s and early 1960 s by a new approach called the Wacker process or the Wacker-Hoechst process, consisting of the liquid phase catalytic oxidation of ethylene to acetaldehyde, as outlined in Table II. 

This development began to reduce steadily the capacities of acetaldehyde which previously had been made by oxidation of ethylene (Wacker-Hoechst process cf. Section 2.4.1) and converted to acetic acid (cf. Section 2.4.4). Moreover, the Monsanto process, the second-generation process for methanol carbonylation is now being followed by the third generation of highly efficient carbonylation processes, enabling acetic anhydride as well as acetic acid to be produced (cf Scheme 2 Tennessee-Eastman [36] and BP [37] processes). The most advanced process (Hoechst [40]) has so far not been implemented industrially because of neglects... 

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). 

Technical Applications (Wacker-Hoechst Processes) 2.4.1.4.1 Acetaldehyde from Ethylene... 

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 . 

On the other hand, process steps which are known in principle (and thus may be verified industrially in due course) but have not yet been applied are referred to as applied processes as well. Examples are special variants of hydroformylation or carbonylation for the manufacture of special chemicals, modifications of oxacyla-tions (in the context of the Wacker-Hoechst process), the copolymerization of ethylene with carbon monoxide (Shell), and several other processes. 

Acetaldehyde synthesis by liquid phase oxidation of ethylene (Wacker-Hoechst processes)... 

Direct oxidation of n-butenes by the Wacker/Hoechst process, in the presence of palladium and copper chlorides, around 110°C, at 13.10 Pa absolute, with a yield of 85 to 88 molar per cent (see Section 103.3). 

The Wacker-type oxidation of olefins is one of the oldest homogeneous transition metal-catalyzed reactions [1], The most prominent example of this type of reaction is the oxidation of ethylene to acetaldehyde by a PdCl2/CuCl2/02 system (Wacker-Hoechst process). In this industrial process, oxidation of ethylene by Pd(ll) leads to Pd(0), which is reoxidized to Pd(ll) via reduction of Cu(ll) to Cu(l). To complete the oxidation-reduction catalytic cycle, Cu(l) is classically reoxidized to Cu(ll) by O2 [2, 3], The use of bidentate ligands [4], bicomponent systems constituted of benzoquinone and iron(ll) phfhalocyanine [5] or chlorine-free oxidants such as ferric sulfate [6], heteropoly acid [7], and benzoquinone [8], make it possible to increase the selectivity reaction by avoiding the formation of chlorinated products. 

Fig. 3-2 Acetaldehyde production in the two-stage Wacker-Hoechst process...

Wacker-Hoechst process oxidation of ethylene to acetaldehyde with Pd/Cu catalysts followed by oxidation to acetic acid. 

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). 

Scheme 5.3.5 Ethylene oxidation according to the Wacker-Hoechst process.

Economic studies by Aquilo et al of the Celanese Chemical Co. have shown that acetaldehyde production via reductive methanol carbonylation is superior to the Wacker—Hoechst process, if reinvestment is considered... 

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