Wacker Hoechst

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 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-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 ... 

Acetaldehyde Ethylene o2 3 100-130 °C, 3-10bar, [PdCl2-CuCl2], C = 40%, S = 95% one or two steps Homogeneous, heterolytic Wacker-Hoechst Acetic acid, n-butanol 2-ethylhexanol... 

The first and foremost reaction of this type is the now classical Wacker-Hoechst oxidation [1-4] of ethene to ethanal (acetaldehyde) by means of a PdCl2/CuCl2 as catalyst and 02 as terminal oxidant (Scheme 1, a). 

Scheme 2. The Cu(l)/Cu(ll)-reoxidation system in the Wacker-Hoechst olefin oxidation reaction.

Figure 1. Schematic catalytic cycle of the Wacker-Hoechst ethene oxidation.

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%. 

The liquid phase processes resembled Wacker-Hoechst s acetaldehyde process, i.e., acetic acid solutions of PdCl2 and CuCl2 are used as catalysts. The water produced from the oxidation of Cu(I) to Cu(II) (Figure 27) forms acetaldehyde in a secondary reaction with ethylene. The ratio of acetaldehyde to vinyl acetate can be regulated by changing the operating conditions. The reaction takes place at 110-130°C and 30-40 bar. The vinyl acetate selectivity reaches 93% (based on acetic acid). The net selectivity to acetaldehyde and vinyl acetate is about 83% (based on ethylene), the by-products being CO2, formic acid, oxalic acid, butene and chlorinated compounds. The reaction solution is very corrosive, so that titanium must be used for many plant components. After a few years of operation, in 1969-1970 both ICI and Celanese shut down their plants due to corrosion and economic problems. 

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... 

Only a small minority of organometallic reactions have cleared the hurdle to become catalytic reality in other words, catalyst reactivation under process conditions is a relatively rare case. As a matter of fact, the famous Wacker/Hoechst ethylene oxidation achieved verification as an industrial process only because the problem of palladium reactivation, Pd° Pd", could be solved (cf. Section 2.4.1). Academic research has payed relatively little attention to this pivotal aspect of catalysis. However, a number of useful metal-mediated reactions wind up in thermodynamically stable bonding situations which are difficult to reactivate. Examples are the early transition metals when they extrude oxygen from ketones to form C-C-coupled products and stable metal oxides cf. the McMurry (Ti) and the Kagan (Sm) coupling reactions. Only co-reactants of similar oxophilicity (and price ) are suitable to establish catalytic cycles (cf. Section 3.2.12). In difficult cases, electrochemical procedures should receive more attention because expensive chemicals could thus be avoided. Without going into details here, it is the basic, often inorganic, chemistry of a catalytic metal, its redox and coordination chemistry, that warrant detailed study to help achieve catalytic versions. 

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. 

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