VINYL ACETATE MONOMER FROM ETHYLENE, ACETIC ACID AND OXYGEN

Vinyl Acetate Monomer (also known as acetic acid ethenyl ester, acetic acid ethylene ether, acetoxyethene, 1-acetoxyethylene, ethenyl ethanoate, ethenyl acetate, and acetic acid vinyl ester) is a colourless mobile liquid with a pungent odour. It has the formula CH3COOCH=CH2 and is a key raw material in the production of chemicals which are then used to manufacture a wide variety of consumer and industrial products.

The main production method for vinyl acetate monomer is the reaction of ethylene and acetic acid with oxygen, in the presence of a palladium catalyst. The VAM is recovered by condensation and scrubbing and is then purified by distillation. A new manufacturing process, dubbed Leap, could offer large capital cost savings as a more efficient fluidised bed system replaces the fixed bed reactors currently in use.

The oldest means of manufacturing VAM is the addition of acetic acid to acetylene and this process is still used but not on a large scale.

Vinyl acetate monomer is stored in mild steel storage tanks and/or new or reconditioned steel drums and can be transported by bulk vessels or tank trucks. It has a specific gravity of 0.933 and a flash point of -8° C (closed cup) and is highly flammable. It should therefore be stored in a cool, dry, well-ventilated area that is free from the risk of ignition. For transportation purposes, it is classified as packing group II and hazard class 3 and it is an irritant.

Vinyl acetate is a colorless, flammable liquid that also has a characteristic smell that can quickly become irritating. This monomer is used principally in the production of polyvinyl acetate (PVAc) and other vinyl acetate co-polymers. Polyvinyl acetate is a precursor of polyvinilyc alcohol and polyvinyl acetate resins (PVA). Vinyl acetate is also copolymerized as a minor raw material for vinyl chloride and ethylene to form commercial polymers and acrylic fibers.

Vinyl acetate is completely soluble in organic liquids but not in water. At 20ºC a saturated solution of the monomer in water can contain between 2-2.4% of vinyl acetate, while a saturated water solution in vinyl acetate contains 1% of water. At 50ºC the solubility of the monomer in water increases in 0.1% in regards to 20ºC, while the solubility of water in vinyl acetate doubles at 50ºC.

The first available process for the synthesis of vinyl acetate was the acetoxilation of acetylene in gaseous form over a zinc acetate catalyst supported on carbon. This process consisted in the reaction of acetylene with acetic anhydride in a catalyzed medium and high temperature to form dietilene diacetate. This product passed through a cracking tower which in result gave as products acetic acid and vinyl acetate. 2 This type of reaction had a high production percentage (92-98%), but due to the increasing value of acetylene, new technologies and methodologies needed to be research in the 60’s.

The process was modified until acetoxilation in a gaseous phase of ethylene over palladium and gold catalyst supported on silica gel was capable. The catalyst used potassium acetate to help the reaction in a temperature range of 423 ñ 463 K and a pressure range of 600 to 1,000 kPa. This is how ethylene acetoxilation with oxygen, acetic acid and Pd as a catalyst was developed. This process consists of two parts: a process in a homogeneous liquid phase that is used to produce 25% of the production of vinyl acetate, and another heterogeneous gaseous phase process used to produce the last 75% of the product.

The synthesis of vinyl acetate from ethylene, acetic acid and oxygen over a Palladium catalyst is a very important industrial process, but its selectivity is affected by the production of CO2 due to the combustion of the ethylene. Various factors affect the synthesis of vinyl acetate, like for example, the dispersion of Pd, partial pressures from the reactives that are the catalyst additives (near 80% if it is Pd/SiO2 and greater to 94% if it is Pd-Au/SiO2) and contact time. The investigation done by Han, H. F., et al. shows that the ethylene is the principal cause for the production of CO2 due to the fact that combustion kinetic does not change if the acetic acid is taken out of the medium.

The process that involves ethylene, acetic acid, oxygen over a Pd catalyst medium proves to be the best option to develop

Currently, the manufacturing process most widely used to produce vinyl acetate is the vapor phase ethylene process, an oxidative reaction in which ethylene is bubbled through acetic acid at 120°C in the presence of palladium chloride catalyst. Impurities found in the reaction have been reported at less than 1% (for one manufacturer) and have included the following: acetaldehyde, ethyl acetate, and methyl acetate. The vapor phase ethylene process was developed in 1967 to take advantage of ethylene as a cheaper feedstock than acetylene, and came into widespread use in the 1970s. By 1981, the vapor phase ethylene process accounted for 92% of U.S. production and the vapor phase acetylene process accounted for the remainder. Various firms in the United States, Japan, West Germany, and the United Kingdom have independently and/or jointly modified the vapor phase ethylene process by using different types of catalysts in the reaction. The catalyst is usually palladium or its salt, although salts of rhodium, gold, pla
tinum, ruthenium, vanadium, and iridium have also been used. The advantage of these processes is that the catalyst lasts longer and undergoes less corrosion.

A less important commercial manufacturing process for vinyl acetate involves the reaction between acetaldehyde and acetic anhydride. The intermediate species, ethylidene diacetate, undergoes pyrolytic cleavage to vinyl acetate and acetic acid. This process was used in the United States until the 1960s, and may still be in use at small plants in China, India, and Mexico. Vinyl. acetate can also be synthesized in high yields by reacting vinyl chloride with sodium acetate in solution at 50°-75°C, using palladium chloride as a catalyst.