Ethylene palladium catalysts

Ca.ta.lysis, The most important iadustrial use of a palladium catalyst is the Wacker process. The overall reaction, shown ia equations 7—9, iavolves oxidation of ethylene to acetaldehyde by Pd(II) followed by Cu(II)-cataly2ed reoxidation of the Pd(0) by oxygen (204). Regeneration of the catalyst can be carried out in situ or ia a separate reactor after removing acetaldehyde. The acetaldehyde must be distilled to remove chloriaated by-products. 

A one-stage process for producing vinyl acetate directly from ethylene has also been disclosed. In this process ethylene is passed through a substantially anhydrous suspension or solution of acetic acid containing cupric chloride and copper or sodium acetate together with a palladium catalyst to yield vinyl acetate. 

A similar reaction was studied by Kowaka Jfi) who investigated the catalytic activity of palladium and its alloys with silver in the hydrogenation of ethylene. The author alluded to the poisoning effect of hydrogen pretreatment of the palladium catalyst. 

Quite recently Yasumori el al. (43) have reported the results of their studies on the effect that adsorbed acetylene had on the reaction of ethylene hydrogenation on a palladium catalyst. The catalyst was in the form of foil, and the reaction was carried out at 0°C with a hydrogen pressure of 10 mm Hg. The velocity of the reaction studied was high and no poisoning effect was observed, though under the conditions of the experiment the hydride formation could not be excluded. The obstacles for this reaction to proceed could be particularly great, especially where the catalyst is a metal present in a massive form (as foil, wire etc.). The internal strains... 

Keywords Acrylate comonomers, Ethylene, Mechanism, Palladium catalysts, Polar groups, Polymerization catalysis, Random copolymers... 

Bayer-Hoechst A gas-phase process for making vinyl acetate from ethylene and acetic acid, using a supported palladium catalyst. Developed jointly by Bayer and Hoechst. In 1991, nearly 2 million tonnes per year of vinyl acetate were made by this process. 

Silylstannanes add to ethylene in the presence of palladium catalysts (Equation (111)). A trialkylphosphine with moderate steric bulkiness is suitable as the ligand for the reaction.276... 

The demethanizer, deethanizer, and debutanizer are fractionating columns that separate the lighter and heavier compounds from each other. Traces of triple bonds are removed by catalytic hydrogenation with a palladium catalyst in both the C2 and C3 stream. Cumulated double bonds are also hydrogenated in the C3 fraction. These are more reactive in hydrogenation than ethylene or propylene. The C2 and C3 splitters (Fig. 8.4) are distillation columns that can be as high as 200 ft. The mechanism of cracking was previously discussed in Chapter 7, Section 6. 

The initiation of polymerizations by metal-containing catalysts broadens the synthetic possibilities significantly. In many cases it is the only useful method to polymerize certain kinds of monomers or to polymerize them in a stereospecific way. Examples for metal-containing catalysts are chromium oxide-containing catalysts (Phillips-Catalysts) for ethylene polymerization, metal organic coordination catalysts (Ziegler-Natta catalysts) for the polymerization of ethylene, a-olefins and dienes (see Sect. 3.3.1), palladium catalysts and the metallocene catalysts (see Sect. 3.3.2) that initiate not only the polymerization of (cyclo)olefins and dienes but also of some polar monomers. 

Ethylene is also employed as an olefmic component of the Mizoroki-Heck polymerization. Organic dihalides thus couple with ethylene in the presence of a palladium catalyst to afford PAV-type polymers 161 and 162, as shown in Equations (77) and (78). ... 

A stereospecific synthesis for cw-3-hexen-l-ol starts with the ethylation of sodium acetylide to 1 -butyne, which is reacted with ethylene oxide to give 3-hexyn-l-ol. Selective hydrogenation of the triple bond in the presence of palladium catalysts yields cw-3-hexen-l-ol. Biotechnological processes have been developed for its synthesis as a natural flavor compound, e.g., [12]. 

Two selective processes are important in the oxidation of ethylene the production of ethylene oxide and acetaldehyde. The first process is specifically catalyzed by silver, the second one by palladium-based catalysts. Silver catalysts are unique and selective for the oxidation of ethylene. No similar situation exists for higher olefins. The effect of palladium catalysts shows a resemblance to the liquid phase oxidation of ethylene in the Wacker process, in which Pd—C2H4 coordination complexes are involved. The high selectivity of the liquid phase process (95%), however, is not matched by the gas phase route at present. 

There have been used essentially only three catalysts foi the hydrogenation of ethylene oxides nickel, palladium on charcoal, and platinum black. Solvent normally employed include ethanol wait nickel, and ethanol, ethyl acetate, or acetic acid with the other. Reduction over platinum or palladium catalysts is usually conducted at room temperature and low pressure, whereas nickel catalysth Imvi-been employed in autoclaves at temperatures ranging from 3fT to nearly 200° and high pressures. For excellent general discussions ol catalytic redaction any of several outstanding sources14" 11-ltni m.i> be consulted. 

The gas-phase process, successfully commercialized independently by Bayer and USI,417 involves passing a mixture of ethylene, acetic acid and oxygen over a supported palladium catalyst contained in a multitubular reactor at 150 °C and about 5-10 atm pressure. The overall yield in vinyl acetate is about 92%, and the major by-product is C02. The catalyst consists of a palladium salt (e.g. Na2PdCl4) deposited on silica (or alumina) in the presence of a cocatalyst (e.g. HAuC14), reduced and impregnated with potassium acetate before use.384,418 The lifetime of the catalyst is about 2... 

Such a stabilization of the palladium catalyst can also be achieved in homogeneous liquid phase by the use of appropriate ligands. Thus, it has recently been shown that palladium(II) hydroxamates are effective catalysts for the acetoxylation of ethylene with high selectivity and a high turnover (>200) (equation (162), whereas Pd(OAc)2 rapidly becomes deactivated and precipitates in the form of metallic palladium.419 It is probable that the bidentate hydroxamate ligand stabilizes the hydride Pd—H species and prevents palladium from precipitating. 

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