Palladium chloride, Wacker process

Among the most significant developments in the field of catalysis in recent years have been the discovery and elucidation of various new, and often novel, catalytic reactions of transition metal ions and coordination compounds 13, 34). Examples of such reactions are the hydrogenation of olefins catalyzed by complexes of ruthenium (36), rhodium (61), cobalt (52), platinum (3, 26, 81), and other metals the hydroformylation of olefins catalyzed by complexes of cobalt or rhodium (Oxo process) (6, 46, 62) the dimerization of ethylene (i, 23) and polymerization of dienes (15, 64, 65) catalyzed by complexes of rhodium double-bond migration in olefins catalyzed by complexes of rhodium (24,42), palladium (42), cobalt (67), platinum (3, 5, 26, 81), and other metals (27) the oxidation of olefins to aldehydes, ketones, and vinyl esters, catalyzed by palladium chloride (Wacker process) (47, 48, 49,... 

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). 

The palladium chloride process for oxidizing olefins to aldehydes in aqueous solution (Wacker process) apparendy involves an intermediate anionic complex such as dichloro(ethylene)hydroxopalladate(II) or else a neutral aqua complex PdCl2 (CH2=CH2)(H2 0). The coordinated PdCl2 is reduced to Pd during the olefin oxidation and is reoxidized by the cupric—cuprous chloride couple, which in turn is reoxidized by oxygen, and the net reaction for any olefin (RCH=CH2) is then... 

The Wacker process uses an aqueous solution of palladium(II) chloride, copper(II) chloride catalyst system. 

With the growing prominence of the petrochemicals industry this technology was, in turn, replaced by direct air oxidation of naphtha or butane. Both these processes have low selectivities but the naphtha route is still used since it is a valuable source of the co-products, formic and propanoic acid. The Wacker process, which uses ethylene as a feedstock for palladium/copper chloride catalysed synthesis of acetaldehyde, for which it is still widely used (Box 9.1), competed with the direct oxidation routes for a number of years. This process, however, produced undesirable amounts of chlorinated and oxychlorinated by-products, which required separation and disposal. 

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. 

Hoechst-Uhde (2) A variation of the Wacker process, which makes vinyl acetate from ethylene and acetic acid. The catalyst is an aqueous solution of palladium and copper chlorides. 

The more expedient, direct catalytic oxidation route to acetone was developed in Germany in the 1960s. If you had been in charge of building the acetone business from scratch, you d probably not have built any IPA-to-acetone plants if you had known about the Wacker process. It s a catalytic oxidation of propylene at 200—250°F and 125—200 psi over palladium chloride with a cupric (copper) chloride promoter. The yields are 91-94%. The hardware for the Wacker process is probably less than for the combined IPA/acetone plants. But once the latter plants were built, the economies of the Wacker process were not sufficient to shut them down and start all over. So the new technology never took hold in the United States. 

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. ... 

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 ... 

One of the earliest uses of palladium(II) salts to activate alkenes towards additions with oxygen nucleophiles is the industrially important Wacker process, wherein ethylene is oxidized to acetaldehyde using a palladium(II) chloride catalyst system in aqueous solution under an oxygen atmosphere with cop-per(II) chloride as a co-oxidant.1,2 The key step in this process is nucleophilic addition of water to the palladium(II)-complexed ethylene. As expected from the regioselectivity of palladium(II)-assisted addition of nucleophiles to alkenes, simple terminal alkenes are efficiently converted to methyl ketones rather than aldehydes under Wacker conditions. 

The oxidation of olefins to aldehydes using a palladium chloride-copper(II) chloride catalyst, the Wacker Process, is a well-established industrial reaction. The mechanism of this reaction has not been established in detail, but it most probably involves a cr-7r rearrangement... 

The cycle approach for oxidation has been adopted at an industrial level for the Wacker-Chemie process for acetaldehyde production, in which ethylene is first put in contact with the oxidized catalyst solution, containing palladium chloride, and in the second step the solution containing the reduced catalyst is sent to a regeneration reactor containing cupric chloride and inside which also air is fed. The regenerated catalyst solution is returned to the first oxidation stage. Another industrial application is the Lummus process for the anaerobic ammoxidation of o-xylene to o-phthaloni-trile [68]. Du Pont has developed the oxidation of n-butane to maleic anhydride catalyzed by V/P/O, in a CFBR reactor, and built a demonstration unit in Spain [69] however, a few years ago the plant was shut down, due to the bad economics. 

Acetic acid is manufactured by three processes acetaldehyde oxidation, //-butane oxidation, and methanol carbonylation.Ethylene is the exclusive organic raw material for making acetaldehyde, 70 percent of which is further oxidized to acetic acid or acetic anhydride. The single-stage (Wacker) process for making acetaldehyde involves cupric chloride and a small amount of palladium chloride in aqueous solution as a catalyst. 

A Wacker catalyst is used in this process, similar to that for the manufacture of acetic acid. Since the acetic acid can also be made from ethylene, the basic raw material is solely ethylene. A liquid-phase process has been replaced by a vapor-phase reaction run at 70 to 140 psi and 175 to 200°C. Catalysts may be (1) carbon-palladium chloride-cupric chloride (C-PdCl2-CuCl2), (2) palladium chloride-alumina (PdCl2-Al203), or (3) palladium-carbon-potassium acetate (Pd-C-KOAc). The product is distilled into water, acetaldehyde that can be recycled to acetic acid, and the pure colorless liquid, which is collected at 72°C. The yield is 95percent. 

The aqueous palladium chloride oxidation of ethylene to acetaldehyde has been developed into an important commercial process. The discovery of how to make the reaction catalytic with respect to palladium chloride was, perhaps, as important to the process as the discovery of the oxidation reaction itself. This process known as the Wacker-Process, employs cupric chloride as a catalyst for the oxygen (air) reoxidation of... 

Wacker Process The Wacker process is primarily used to produce acetaldehyde from the oxidation of ethylene by palladium(II)Copper(II) chloride solution. 

To overcome the problems encountered in the homogeneous Wacker oxidation of higher alkenes several attempts have been undertaken to develop a gas-phase version of the process. The first heterogeneous catalysts were prepared by the deposition of palladium chloride and copper chloride on support materials, such as zeolite Y [2,3] or active carbon [4]. However, these catalysts all suffered from rapid deactivation. Other authors applied other redox components such as vanadium pentoxide [5,6] or p-benzoquinone [7]. The best results have been achieved with catalysts based on palladium salts deposited on a monolayer of vanadium oxide spread out over a high surface area support material, such as y-alumina [8]. Van der Heide showed that with catalysts consisting of H2PdCU deposited on a monolayer vanadium oxide supported on y-alumina, ethene as well as 1-butene and styrene... 

The invention of the Wacker process was a triumph of common sense. It had been known since 1894 that ethylene is oxidized to acetaldehyde by palladium chloride in a stoichiometric reaction (Figure 27). However, it was not until 1956 that this reaction was combined with the known reoxidation reactions of palladium by copper and, in turn of copper by oxygen. The total process developed by Wacker and Hoechst between 1957 and 1959 can be depicted as an exothermic catalytic direct oxidation to yield acetaldehyde. 

Since water is formed in this reaction, acetaldehyde is also a by-product. It can arise (1) directly according to olefin oxidaton (Wacker process), (2) through hydrolysis of vinyl acetate which occurs very easily in the presence of PdCl2, or - rather unlikely - (3) through an interaction of vinyl acetate with acetic acid in the presence of palladium chloride, a reaction according to eq. (5) and published by Clement and Selwitz [21] forming acetaldehyde and acetic anhydride. 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

Proposed mechanism for the oxidation of ethylene to acetaldehyde in the Wacker process. Chloride ligands have been omitted. The oxidation number of palladium is + 2 at all stages of this cycle except the upper left where reductive elimination of acetaldehyde gives Pd (0), which is oxidized by Cu (II). The complete cycle for the reoxidation of Cu (I) is not shown. 

The first step is catalysed by palladium chloride and is similar to the Wacker process for manufacturing acetaldehyde from ethylene. 

Lead tetraacetate initiates a similar type of oxidation with terminal alkenes, in the presence of acid, to give an aldehyde hy selective oxidation of the terminal carhon. l Ajj example is the conversion of styrene to phenylacetaldehyde in 98% yield. Palladium chloride (PdCl2) reacts with terminal alkenes, in the presence of oxygen and copper salts, to give a methyl ketone (this reaction is called the Wacker process and is discussed in sec. 12.6.A). It is more useful than the LTA oxidation. Oxidation of terminal alkenes with LTA leads to the aldehyde, whereas oxidation with PdCl2 leads to the methyl ketone. The PdCl2 oxidation is illustrated hy conversion of 402 to 403 in 77% yield, in Ikegami s synthesis of coriolin. ... 

The first process to be discussed is a traditional one that probably does not occur via a Jt-complex. Nonetheless, it sets the stage for the organopalladium chemistry to be discussed in this section. The Wacker process (or the Wacker oxidation) is used in industry to convert ethylene to acetaldehyde using soluble palladium catalysts.207 in this reaction, ethylene reacts with cupric chloride (CuCl2) and palladium chloride (PdCl2) to... 

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