The Gas-phase Ethylene to Vinyl Acetate Process

Ethylene acetoxylation was also developed as a gas phase process following the liquid phase process and has been in commercial use since 1968. There is a notable difference between the two processes in the liquid phase the presence of palladium salts and redox systems results in the formation of both vinyl acetate and acetaldehyde, whereas in the gas phase process, using palladium metal, 

One version of the gas phase process was developed by National Distillers Products (now Quantum Chemical) in the USA and another independently in Germany by Bayer together with Hoechst. In both versions, ethylene is reacted with acetic acid and oxygen on a palladium-containing fixed-bed catalyst at 5-10 bar and 175-200°C to form vinyl acetate and water. The explosion limit restricts the O2 content in the feed mixture so that the ethylene conversion is relatively small ( 10%). The acetic acid conversion is 20-35% with selectivi-ties to vinyl acetate of up to 94% (based on C2H4) and about 98-99% (based on AcOH). The most important side reaction of this process is the total oxidation of ethylene to carbon dioxide and water. Other by-products are acetaldehyde, ethyl acetate and heavy ends. After a multistep distillation the vinyl acetate purity is 99.9% with traces of methyl acetate and ethyl acetate that do not affect the subsequent use in polymerization. 

In addition to palladium, the catalysts used commercially always contain alkali salts, preferably potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium, while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts remain in operation for several years but undergo deactivation. The drop in activity is due to a gradual sintering of the palladium particles which causes the catalytically active area to decrease progressively. Under reaction conditions potassium acetate is slowly lost from the catalyst and must continuously be replaced. 

Box 6 How Catalyst Manufacture Can Cope with Reaction Conditions 

The role of gold in the Pd/Au/K acetate catalysts is to stabilize the size of Pd crystallites and avoid sintering. The role of potassium acetate is to maintain the catalyst activity and decrease CO2 selectivity. Potassium acetate favours a strong adsorption of acetic acid on palladium, lowering the barrier to vinyl acetate formation. Gold by itself is inactive in the catalysis of vinyl acetate. Pd only catalysts produce vinyl acetate at much lower rates than the Pd/Au/K catalyst system and their activity decays rapidly. 

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