Ethylene vinyl acetate process limitation

A simplified process flow diagram of the ethylene-based vinyl acetate process is shown in Figure 20 [25]. Acetic acid feedstock is vaporized in the presence of fresh ethylene feed and unreacted ethylene recycle gas in the acetic acid vaporizer. The stream is preheated and high purity oxygen is added with a special mixing nozzle. The quantity of reactants and oxygen are carefully controlled to ensure that the mixture is outside of the explosive limits. 

The in situ regeneration of Pd(II) from Pd(0) should not be counted as being an easy process, and the appropriate solvents, reaction conditions, and oxidants should be selected to carry out smooth catalytic reactions. In many cases, an efficient catalytic cycle is not easy to achieve, and stoichiometric reactions are tolerable only for the synthesis of rather expensive organic compounds in limited quantities. This is a serious limitation of synthetic applications of oxidation reactions involving Pd(II). However it should be pointed out that some Pd(II)-promoted reactions have been developed as commercial processes, in which supported Pd catalysts are used. For example, vinyl acetate, allyl acetate and 1,4-diacetoxy-2-butene are commercially produced by oxidative acetoxylation of ethylene, propylene and butadiene in gas or liquid phases using Pd supported on silica. It is likely that Pd(OAc)2 is generated on the surface of the catalyst by the oxidation of Pd with AcOH and 02, and reacts with alkenes. 

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. 

When media other than water are used, different but related processes operate. Thus, the oxidation of ethylene in acetic acid can be directed to give vinyl acetate, ethylene glycol acetate, or 2-chloroethyl acetate [9]. Similarly, the synthesis of acetals or ketals can be achieved in an alcoholic medium [10]. Although the oxidation of alkenes in such a medium is closely parallel to the Wacker process, the chemistry of these reactions is far beyond the scope of this section, which is limited to Wacker-type reactions in aqueous media, and will not be discussed here. 

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