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It introduces the production of vinyl acetate and polyvinyl alcohol

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CH3COOCH＝CH2 is abbreviated as vinyl acetate. A colorless, flammable liquid with a boiling point of 72.7°C. It is industrially produced mainly from ethylene and acetic acid. Due to the vinyl double bond present in the molecule, it readily undergoes polymerization reactions, making it suitable for the production of homopolymers or copolymers (see polymers). 　　Production methods There were once three production methods. The earliest industrial method was the addition of acetylene to acetic acid, followed by a two-step synthesis using acetaldehyde and acetic anhydride; the most important method in industry is the synthesis from ethylene. In 1985, the world production of vinyl acetate exceeded 2.85 Mt, the vast majority of which was produced by the ethylene synthesis method. 　　Acetylene-acetic acid addition method: This method was first industrialized by the American company DuPont in 1920. The reaction takes place in the gas phase, using zinc acetate supported on activated carbon as a catalyst, at temperatures of 170–250°C. C2H2 + CH3COOH → CH3COOCH=CH2; the molar ratio of acetylene to acetic acid vapor is 2.5–4. Fixed-bed or fluidized-bed reactors are used. The by-products formed in the reaction include acetaldehyde, butenal, propylene, benzene, etc. The acetylene conversion rate is 60%–70%, and the selectivity for vinyl acetate is 93% based on acetylene and 99% based on acetic acid. This method offers high product selectivity and the reaction is easy to control. 　　Two-step synthesis using acetaldehyde and acetic anhydride: Before 1970, the Serenus Company in the United States used this method to produce vinyl acetate; it has since been abandoned due to its lengthy production process. The first step is the reaction of acetaldehyde with acetic anhydride to produce ethyl diacetate: CH3CHO + (CH3CO)2O → CH3CH(OOCCH3)2. Then, using an acid catalyst and at 120°C, ethyl diacetate is decomposed to yield vinyl acetate: CH3CH(OOCCH3)2 → CH3COOCH=CH2 + CH3COOH. The ethylene method: Corning Incorporated in Japan and the American Industrial Chemicals Company established facilities for producing vinyl acetate via this ethylene-based method in 1968 and 1970 respectively. The reaction equation for this method is as follows: Industrially, there are two ways to carry out this reaction process: ① Liquid-phase method: A mixture of ethylene and oxygen is introduced into acetic acid containing a palladium chloride-copper chloride catalyst (see complex catalysts), under conditions of 100–130°C and 3–4 MPa. Alkali metal salts such as sodium acetate and potassium acetate are used as catalysts; the one-pass conversion rate is only about 5%. The resulting vinyl acetate reacts easily with water to form acetaldehyde, and must be separated from the catalyst solution promptly. The advantage of this method is that the by-produced acetaldehyde is reoxidized to acetic acid, which can meet the requirements of this process. However, due to the strong corrosiveness of the catalyst solution, equipment made of corrosion-resistant materials is required, and it has gradually been replaced by the gas-phase method. 　　②Vapor phase method: Ethylene, oxygen, and acetic acid vapors are passed through a catalyst bed; the commonly used catalyst is palladium supported on silica, with potassium acetate acting as a co-catalyst. The reaction temperature ranges from 165 to 180°C, and the pressure is between 0.6 and 0.8 MPa. Considering the explosion limit, thermal effects, and selectivity of the materials, it is necessary to limit the conversion rate of the reaction materials. The one-pass conversion rate of ethylene is maintained at around 10%, while the one-pass conversion rate of acetic acid is about 20%. The selectivity for vinyl acetate is 94% based on ethylene, and it can reach 98%–99% based on acetic acid. The main side reaction is the complete oxidation of ethylene. In addition, small amounts of acetaldehyde, ethyl acetate, and polymers are also produced. Compared with the liquid-phase method, this method produces less acetaldehyde as a by-product, and there is less severe equipment corrosion. 　　The reaction is carried out in a tubular reactor (see figure), with high-pressure water used outside the tubes to remove the heat of reaction. After being cooled, the reaction gas is fed into an absorption and separation tower, where vinyl acetate and by-products such as acetaldehyde are absorbed using acetic acid. Unreacted ethylene is discharged from the top of the tower; after washing, most of it can be recycled. In the purification system, the reaction mixture is subjected to the removal of light components (acetaldehyde), and then distillation is carried out in a distillation tower with the addition of a polymerization inhibitor to obtain the polymer-grade product. 　　The methanol carbonylation method was developed in the early 1980s by the American companies Hacon and Scientific Design. The reaction steps are as follows: 2CH3OH + 2CH3COOH → 2CH3COOCH3 + 2H2O; 2CH3COOCH3 + 2CO + H2 → CH3CH(OOCCH3)2 + CH3COOH; CH3CH(OOCCH3)2 → CH3COOCH=CH2 + CH3COOH. The overall reaction is: 2CH3OH + 2CO + H2 → CH3COOCH=CH2 + 2H2O. The key step in this process is the carbonylation of methyl acetate to produce ethyl diacetate. Various catalysts can be used for this reaction; for example, Rh-I systems. The reaction conditions are 150°C and 7 MPa, with a yield of around 63%. It has attracted widespread attention because both acetic acid and methanol can be produced from syngas, without the need for acetylene or ethylene. 　　Uses Vinyl acetate is mainly used to produce polyvinyl acetate and copolymers with other olefins. Polyvinyl acetate is further hydrolyzed to polyvinyl alcohol, which is used to manufacture polyvinyl alcohol fibers (i.e., vinylon) as well as adhesives. Ethylene-vinyl acetate resin (EVA), obtained by copolymerizing vinyl acetate and ethylene, is an important textile additive, adhesive, raw material for hot melt adhesives used in wire binding in the printing industry, and modifier for rigid polyvinyl chloride. The chloroacetic copolymer obtained by copolymerizing vinyl acetate and vinyl chloride is a raw material for manufacturing record discs. The aforementioned copolymers can also be used as coatings and colloidal protectants, among other things. Vinyl acetate can polymerize on its own, and the polymerization process is accelerated by light and heat. It should be stored in sealed stainless steel or enamel containers at room temperature, with a polymerization inhibitor added.

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