Wacker Process Operation

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. 

Two versions of the process were developed the single-step process, in which the reaction and regeneration are conducted simultaneously in a single reactor, and O2 is used as the oxidizing agent and the two-step process, in which the reaction and regeneration take place separately in two reactors. Air can be used for the oxidation. 

In the single-step process ethylene and O2 are fed into the catalyst solution at 3 bar and 120-130°C, where 35-45% of ethylene is converted. The heat of reaction is used to distil off acetaldehyde and water from the catalyst solution. Incompletely converted ethylene must be recycled. This requires the use of pure O2 and ethylene (99.9%) that must be free of inert gases. Inert gas accumulation upon recycling would require venting with consequent ethylene losses. 

In both processes the aqueous crude aldehyde is concentrated and byproducts are removed in a two-step distillation. Both processes give 94% yields of aldehyde, along with small amounts of 2-chloroethanol, ethyl chloride, acetic acid, chloroacetaldehydes and acetaldehyde condensation products. The Wack-er-Hoechst process currently accounts for 85% of the worldwide production capacity for acetaldehyde. 

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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. 

Early mechanistic studies have indicated that the oxypalladation step in the Wacker process proceeds through an <37z/z-pathway,399 although recent deuterium-labeling experiments have shown the viability of a yy/z-mechanism involving insertion of a metal-coordinated oxygen into the alkene.400,401 For example, with excess chloride ion present, the Wacker-type cyclization of a deuterated phenol system occurred in a primarily //-pathway, whereas the oxypalladation step favored a yy/z-mode in the absence of excess chloride ion (Scheme 16). Thus, either mechanism may be operative under a given set of experimental conditions. 

With reaction conditions of 200-225°F, 150—225 psi, and a palladium chloride-cupric chloride catalyst, MEK yields are 80-90%. The operating costs of the Wacker process for MEK (and acetone and several other petrochemicals as well) are relatively low. But the plant Is made of more expensive materials. Because of the corrosive nature of the catalyst solution, critical vessels, and the piping are titanium-based.(chats expensive ), and the reactor is rubber-lined, acid-resistant brick. ... 

Comparing the reaction rate, i.e. ethylene consumption with the pH value, both plotted against the mole ratio ([Cu"]/[Cu ] -i- [Cu ]), i.e., the degree of oxidation of the catalyst solution, it has been shown that a reduction in the reaction rate and the pH value occurs at the same degree of oxidation, depending on the Cl/Cu ratio (Figure 1). This point is in fact consistent with the neutralization of the copper oxychloride while the reaction according to eq. (7) proceeds [6, 12, 13, 46], and is very important for the operation of the Wacker process (see Section 2.4.1.4.1). 

Oxidative carbonvlation of ethylene this process, developed by Union Oil operates in the liquid phase, between 135 and 150°C. at 7.5.106 Pa absolute, with a high ethylene and low carbon monoxide partial pressure, and in the presence of a catalyst system similar to the one used in the Wacker process for manufacturing acetaldehyde. The main reactions are the following ... 

When media other than water are used, different but related processes operate. Thus, the oxidation of ethylene in acetic acid can be directed to give vinyl acetate, ethylene glycol acetate, or 2-chloroethyl acetate [9]. Similarly, the synthesis of acetals or ketals can be achieved in an alcoholic medium [10]. Although the oxidation of alkenes in such a medium is closely parallel to the Wacker process, the chemistry of these reactions is far beyond the scope of this section, which is limited to Wacker-type reactions in aqueous media, and will not be discussed here. 

The commercial Wacker process for the manufacture of acetaldehyde operates in a single-stage... 

A combination of j6-H elimination reactions seems to be operating in the last steps of the Wacker process, once the coordinated ethylene has been transformed into a hydroxyethyl group (Scheme 6.26) [89]. 

However, more significant was the introduction of a new one step process for the oxidation of ethylene to acetaldehyde using a PdCl2-CuCl2 catalyst (equation [16]) sometime between 1957-1959. The process, now known as the Wacker Process, is operated at ca. 10 atm and 100-110°C, requires a large excess of chloride, and produces AcH in about 95% yield. The mechanism for this reaction is shown in figure 2. 

The production of another important chemical and polymer intermediate, acetic acid, was revolutionized by the Wacker process that was introduced in 1960. It was a simple, high yield process for converting ethylene to acetaldehyde, which replaced the older process based on ethanol and acetylene. In the Wacker reaction, the palladium catalyst is reduced and then reoxidized. Ethylene reacts with water and palladium chloride to produce acetaldehyde and palladium metal. The palladium metal is reoxidized by reaction with cupric chloride, which is regenerated by reaction with o gen and hydrochloric acid. In 1968, BASF commercialized an acetic acid process based on the reaction of carbon monoxide and methanol, using carbonyl cobalt promoted with an iodide ion (74). Two years later, however, Monsanto scored a major success with its rhodium salt catalyst with methyl iodide promoter. Developed by James F. Roth, this new catalyst allowed operation at much milder conditions (180°C, 30-40 atm) and demonstrated high selectivity for acetic acid (75). 

Ab initio calculations on the trans effect at platinum(II) and rhodium(I) rationalize its operation in terms of the trigonal-bipyramidal reaction intermediates. Theoretical studies on some palladium(II) ethene compounds (all of them Wacker process intermediates) suggested a trans influence series OH >C1" >OH2 at this metal. ... 

The useful comparison between the Wacker process and alkane CH activation is that all the coordination steps identified in the Wacker reaction (activation, functionalization and reoxidation) have parallels in catalytic, alkane CH activation and functionalization systems that operate with electrophilic catalysts. Thus, the coordination of the double bond of the olefin to electrophilic Pd(II) followed by cleavage by nucleophilic attack of water can be compared to CH activation of CH4 by an electrophilic substitution (ES) pathway. 

Silicones, an important item of commerce, are widely available commercially (9,494). The principal manufacturers of silicone operate direct-process reactors to produce dimethyl dichi orosilane and, ultimately, polydimethyl siloxane. Typical plants produce more than 450 t per year. The siUcone industry is a global enterprise in the 1990s, with principal producers in the United States (Dow Coming, GE, and OSi), Europe (Wacker Chemie, Hbls, Rhc ne-Poulenc, and Bayer), and Southeast Asia (Shin-Etsu, Toshiba SiUcones, and Dow Coming, Japan). Table 15 Hsts the approximate sales of the principal producers for 1991. 

Wacker (1) A general process for oxidizing aliphatic hydrocarbons to aldehydes or ketones by the use of oxygen, catalyzed by an aqueous solution of mixed palladium and copper chlorides. Ethylene is thus oxidized to acetaldehyde. If the reaction is conducted in acetic acid, the product is vinyl acetate. The process can be operated with the catalyst in solution, or with the catalyst deposited on a support such as activated caibon. There has been a considerable amount of fundamental research on the reaction mechanism, which is believed to proceed by alternate oxidation and reduction of the palladium ... 

Other methods for the preparation of acetic acid are partial oxidation of butane, oxidation of ethanal -obtained from Wacker oxidation of ethene-, biooxidation of ethanol for food applications, and we may add the same carbonylation reaction carried out with a cobalt catalyst or an iridium catalyst. The rhodium and iridium catalysts have several distinct advantages over the cobalt catalyst they are much fester and fer more selective. In process terms the higher rate is translated into much lower pressures (the cobalt catalyst is operated by BASF at pressures of 700 bar). For years now the Monsanto process (now owned by BP) has been the most attractive route for the preparation of acetic acid, but in recent years the iridium-based CATTVA process, developed by BP, has come on stream. 

The BASF process1026 was developed to use o-xylene exclusively. Most processes, however, such as the Wacker-von Hey den process1027 1028 are capable of operating on both feedstocks or may use a mixture of the two compounds. Fluidized-bed operations were developed in the 1960s (Badger process1029 1030) but have been replaced by improved, more economical fixed-bed processes. The Alusuisse low-air-ratio (LAR) process, for instance, allows the use of a 9.5 1 air o-xylene mixture and achieves an increased catalyst productivity.1031 1032... 

An important concern in industrial processes is corrosion. Transition metal complexes under certain conditions can facilitate corrosion of the reaction vessels. The consequences are not only fouling of the surfaces but also loss of expensive catalyst. So the reactors are generally made of materials that are resistant to corrosion. Two such materials are stainless steel 316 (containing 16-18% Cr, 10-14% Ni, 1-3% Mo, <0.1% C, and the rest iron) and Hastelloy C (containing 14-19% Mo, 4-8% Fe, 12-16% Cr, 3-6% W, and the rest Ni). The latter is ideal for chlorides and acids but is 3-4 times more expensive than the former. Wacker s process is operated under highly corrosive conditions (high concentrations of H+ and Cl ) (see Section 8.2) hence it requires expensive, titanium-lined reactors. 

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. 

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