Catalytic cycles Wacker process

The free HCl and Cl generated in the catalytic cycle produce environmentally harmful chlorinated by-products to the extent that more than 3 kg of HCl need to be added to the reactor per tonne of acetaldehyde produced to keep the catalytic cycle going. Modified catalysts such as ones based on palladium/ phosphomolybdovanadates have been suggested as a way of reducing byproduct formation to less than 1% of that of the conventional Wacker process. These catalysts have yet to make an impact on commercial acetic production, however. 

Anodic oxidation is used to promote the recycling of palladium(il) in the Wacker process for the conversion terminal alkenes to methyl ketones. Completion of the catalytic cycle requires the oxidation of palladium(O) back to the palla-dium(li) state and this step can be achieved using an organic mediator such as tri(4-bromophenyljamine. The mediator is oxidised at the anode to a radical-cation and... 

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 ... 

Another example is the palladium-catalyzed oxidation of ethylene to acetaldehyde in the presence of oxygen and cupric salts, the so-called Wacker reaction. This catalytic cycle combines two stoichiometric processes, which involve first the reduction of Pd11 to Pd°, followed by reoxidation with Cu11. The understanding of the first step of this process came from the earlier work of Kharasch et al., who showed that the stoichiometric dinuclear complex shown in Figure 2.14 decomposed in the presence of water to acetaldehyde (ethanal), Pd° and HC1 [38]. 

The industrially important acetoxylation consists of the aerobic oxidation of ethylene into vinyl acetate in the presence of acetic acid and acetate. The catalytic cycle can be closed in the same way as with the homogeneous Wacker acetaldehyde catalyst, at least in the older liquid-phase processes (320). Current gas-phase processes invariably use promoted supported palladium particles. Related fundamental work describes the use of palladium with additional activators on a wide variety of supports, such as silica, alumina, aluminosilicates, or activated carbon (321-324). In the presence of promotors, the catalysts are stable for several years (320), but they deactivate when the palladium particles sinter and gradually lose their metal surface area. To compensate for the loss of acetate, it is continuously added to the feed. The commercially used catalysts are Pd/Cd on acid-treated bentonite (montmorillonite) and Pd/Au on silica (320). 

The electrophilic activation of a C—C multiple bond as a result of coordination to an electron-deficient metal ion is fundamental to much of organometallic chemistry, both conceptually and in synthetic applications (11). The Wacker process, a classic example of an efficient catalytic oxidation, is an important industrial reaction, used for the conversion of ethylene into acetaldehyde. The catalytic reaction begins with the coordination of ethylene to a Pd(ll) center, leading to activation of the ethylene moiety. The key step is the reaction of the metal-olefin complex with a nucleophile to give substituted metal-alkyl species (12). The integration of this reaction into a productive catalytic cycle requires the eventual cleavage of the newly generated M—C bond. 

The oxidation of ethylene to acetaldehyde using PdCb and CuCb as catalysts undo- an oxygm atmosphere is well known as the Wacker process (Scheme 1), and is one of the most important industrial processes employing transition metal catalysts.This industrial oxidation reaction of ethylene involves the following three stoichiometric reactions. These sequential oxidation and reduction reactions constitute a catalytic cycle. 

Several important nomadical catalytic oxidations go via organometalhc mechanisms. The commercially useful Wacker process converts ethylene to acetaldehyde with air as oxidant, using Pd(II) and Cu(II) catalysts. The Pd(II) binds to the ethylene to give an organometalhc intermediate, the alkene complex. This complex subsequently uses water as the O source to oxidize the ethylene to acetaldehyde, the Pd being reduced in the process. The resulting Pd(0) is reoxidized to Pd(II) with two equivalents of Cu(n) and the Cu(I) so formed is then reoxidized by air to close the cycle. 

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. ... 

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 palladium is released as HPdCI - still Pd(II)- but this decomposes rapidly to Pd(0) to end the cycle. In practice Pd(0) is reoxidised to Pd(II) with catalytic Cu(II) and the stoichiometric oxidant, 02, oxygen itself, regenerates the Cu(II). The Wacker process is run on a large scale in industry to make simple carbonyl compounds but does find some use in the laboratory. 

We now study one cycle in detail to illustrate the notations. Figure 26.2 shows a simplified catalytic cycle for the Wacker process which converts ethene to acetaldehyde (equation... 

Fig. 26.2 Catalytic cycle for the Wacker process for simplicity, we have ignored the role of coordinated H2O, which replaces Cl...

Electron withdrawal from the coordinated alkene to an electrophilic metal center makes the coordinated alkene susceptible to attack by an external nucleophilic agent or by a ligand coordinated to the metal. A classic example using modification of the chemical nature of ethylene coordinated to a cationic metal center can be seen in palladium-catalyzed Hoechst-Wacker process [111]. The catalytic cycle can be represented by Scheme 1.37, which is comprised of the main cycle to convert the ethylene coordinated to Pd(II) into acetaldehyde and auxiliary cycles to re-oxidize the Pd(0) species to Pd(II) with Cu(I). The Cu(I) produced in the process is oxidized in turn to Cu(II) with oxygen. 

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. 

In the foregoing examples, rate equations were developed on the basis of a single rate-determining step. It is possible that many steps of a cycle are simultaneously controlling, as in the Wacker process. The rate equation for such a reaction tends to be more complicated but can be developed by the methods discussed in Chapter 7. Thus for the oxidation of triphenylphosphine with a Pt complex, a rate equation can be developed based on the catalytic cycle shown in Figure 8.9 (Halpern and Pickard, 1970 Birk et al., 1968a,b) ... 

Liquid-phase oxidation of gaseous substrates with O2, such as the oxidation of ethylene to acetaldehyde (Wacker process) is another example of this class of reactions. A mathematical model for a bubble column reactor for this reaction, assuming plug flow of gas and mixed flow of liquid, was developed (Rode et al., 1994). It was shown that a critical oxygen concentration in the inlet is necessary to sustain the catalytic cycle, and a model for predicting this was proposed (Bhattacharya and Chaudhari, 1990). 

In 1960, quickly after the introduction of the Celanese process, Wacker-Chemie commercialized a liquid phase vinyl acetate process which represented and extension of its earlier acetaldehyde process wherein acetic acid was simply substituted for water. (See equation [19]. This chemical transformation is also referred to as oxidative acetoxylation.) As shown in Figure 2, wherein R=Ac, the liquid phase oxidative acetoxylation of ethylene utilized the same catalytic cycle as the Wacker-Chemie acetaldehyde 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. 

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