Acetaldehyde palladium/copper catalysts

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

Acetaldehyde (CH3CH=0) boils close to room temperature (bp 20°C). It is manufactured mainly by the oxidation of ethylene over a palladium-copper catalyst, and about 1 billion pounds are produced worldwide each year. 

Meanwhile, Wacker Chemie developed the palladium-copper-catalyzed oxidative hydration of ethylene to acetaldehyde. In 1965 BASF described a high-pressure process for the carbonylation of methanol to acetic acid using an iodide-promoted cobalt catalyst (/, 2), and then in 1968, Paulik and Roth of Monsanto Company announced the discovery of a low-pressure carbonylation of methanol using an iodide-promoted rhodium or iridium catalyst (J). In 1970 Monsanto started up a large plant based on the rhodium catalyst. 

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 ... 

Acetaldehyde may be made (1) from ethylene by direct oxidation, with the Wacker-catalyst containing copper(II) and palladium(II) salts (2) from ethanol by vapor-phase oxidation or dehydrogenation or (3) from butane by vapor-phase oxidation. The direct oxidation of ethylene is the most commonly used process, accounting for 80% of acetaldehyde production. 

Ethylene can be oxidized to acetaldehyde in high yields similar to the Wacker-process if electrogenerated palladium(ll) is used as catalyst. In this way the copper(II) catalyzed air oxidation of palladium(O) is replaced by the electrooxidation according to Eq. (40). 

The Wacker process is based on three reactions oxidation of ethylene by Pd2+ in water, oxidation of Pd° to Pd2+ by Cu2+, and oxidation of Cu+ by dioxygen to Cu2+. These reactions are shown by 8.1 to 8.3. Note that if the three reactions are summed up, the net reaction becomes one mole of ethylene and half a mole of oxygen, giving one mole of acetaldehyde. Both palladium and copper ions shuttle between two oxidation states to act as the catalysts. 

Heteropolyacids can be very useful in oxidation reactions. In contrast with metal chelate catalysts that usually become oxidized and deactivated eventually, the heteropolyacids are extremely stable to oxidation. Acetaldehyde is produced commercially from ethylene by the Wacker reaction with a palladium(II) chloride catalyst, copper(II) chloride, oxygen, and water. The corrosive conditions are a disadvantage of the process. Catalytica Inc. has devised a process (6.55) that uses only 1% as much palladium and chloride as the usual process. It uses a small amount of palladium (II) chloride with a partial sodium salt of phosphomolybdovanadic acid.287... 

The Wacker oxidation is one of the longest known palladium-catalyzed organic reactions. It is the industrial process where ethylene is oxidized to acetaldehyde with oxygen in the presence of a catalytic amount of palladium and copper salt as the redox co-catalyst.1 There are plants that produce thousands of tons of acetaldehyde per year. Many reviews exist on this topic.2-6... 

Most of the catalysts employed in the chemical technologies are heterogeneous. The chemical reaction takes place on surfaces, and the reactants are introduced as gases or liquids. Homogeneous catalysts, which are frequently metalloorganic molecules or clusters of molecules, also find wide and important applications in the chemical technologies [24]. Some of the important homogeneously catalyzed processes are listed in Table 7.44. Carbonylation, which involves the addition of CO and H2 to a C olefin to produce a + 1 acid, aldehyde, or alcohol, uses rhodium and cobalt complexes. Cobalt, copper, and palladium ions are used for the oxidation of ethylene to acetaldehyde and to acetic acid. Cobalt(II) acetate is used mostly for alkane oxidation to acids, especially butane. The air oxidation of cyclohexane to cyclohexanone and cyclohexanol is also carried out mostly with cobalt salts. Further oxidation to adipic acid uses copper(II) and vanadium(V) salts as catalysts. The... 

Even with our modified definition of indifferent , we still require that the catalytic material should act indefinitely once introduced. This requirement is also fulfilled by a number of essential materials added to some catalytic processes, and often referred to as co-catalysts or promoters. For example, the copper (I)-copper (II) chloride redox system used in Wacker s palladium-catalysed oxidation of ethylene to acetaldehyde (section 11.7.7.3) behaves in a true catalytic manner in the single-reactor variant of the process (ethylene and O2 introduced into the same reaction vessel). 

Scheme 9.133. A representation of the palladium(II) chloride-catalyzed oxidation of ethene (ethylene, CH2=CH2) to ethanal (acetaldehyde, CH3CHO), the Wacker process. Generally, a copper(II) catalyst (not shown) is also employed to effect the reoxidation of the palladium. Interestingly, it is probable that a 1,2-hydride shift occurs as the palladium is lost (rather than elimination to the corresponding enol and then rearrangement to the aldehyde) since nse of H20 (denterinm oxide) in place of its proton analogue (water, HaO) yields aldehyde in which there is no denterinm ( ) incorporation.

Showa Denko has developed such a direct oxidation process and commercialized the technology in 1997 (100000ta ). The catalyst consists of three components (i) palladium supported on a carrier (0.1-2 wt%) (ii) a heteropoly add (e.g., phos-photungstic acid or silicotungstic acid) or its related lithium, sodium and copper salts (iii) selenium, tellurium, copper, silver, tin, lead, antimony, or bismuth. The process is operated in a fixed bed reactor at 150-160 °C and up to 8 bar. The gas stream entering the reactor consists of the reactants ethylene and oxygen, steam, and nitrogen as diluent. Water/steam is needed because it enhances the activity and selectivity of the reaction. The selectivity to acetic acid is 86%. The main byproducts are carbon dioxide and not fully converted acetaldehyde. 

Note that if these reactions are combined, as in the Wacker process, the net reaction is (8.2.1.4). In other words, in the Wacker process 1 mol of ethylene and Vi mol of dioxygen is reacted in the presence of palladium and copper as catalysts, to give 1 mol of acetaldehyde. 

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