Ullmann's Encyclopedia of Industrial Chemistry:
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Oxidation of Acetaldehyde
Acetic anhydride can be obtained directly by liquid-phase oxidaton of acetaldehyde. The peracetic acid formed from oxygen and acetaldehyde reacts under suitable conditions with a second molecule of acetaldehyde to form acetic anhydride and water [64]:
Rapid removal of the reaction water and the use of suitable catalysts are essential in this process. Mixtures of acetic acid and acetic anhydride are always obtained; their ratio can be varied within wide limits by changing the reaction conditions. Generally, the highest possible anhydride yield is sought.
Because of the rapid hydrolysis of acetic anhydride above 60 °C, the process is operated preferably between 40 °C and 60 °C [65]. Suitable catalysts are combinations of metal salts [66]. Particularly important are mixtures of manganese acetate and copper acetate [67], of cobalt acetate and nickel acetate, and of cobalt and copper salts of higher fatty acids [68]. Manganese acetate should hinder the formation of explosive amounts of peracetic acid during the oxidation of acetaldehyde. For increasing the rate of oxidation, the use of pure oxygen at a pressure of several hundred kilopascals instead of air has been proposed [69].
The strongly exothermic reaction requires efficient cooling. For this purpose, the addition of low-boiling solvents has been found to be of assistance. Methyl and ethyl acetates are favored because they form azeotropic mixtures with water (but not with acetic acid or acetic anhydride) and hence allow a rapid, continuous separation of the water formed in the reaction. The ratio of acetic anhydride to acetic acid in the product depends on the ratio of ethyl acetate to acetaldehyde in the initial mixture (Table (1)).
In practice, a 1 : 2 mixture of acetaldehyde and ethyl acetate is oxidized with the addition of 0.05 to 0.1 % cobalt acetate and copper acetate at 40 °C; the ratio of Co:Cu is 1:2. The ratio of acetic anhydride to acetic acid obtained is 56:44, whereas on oxidizing in the absence of ethyl acetate this ratio is only 20:80 [16]. The optimization of other reaction conditions can also lead to an increase in the acetic anhydride-acetic acid ratio. For example, at 55 °C and atmospheric pressure, a ratio of 80:20 was achieved[70]. At a higher temperature (62 – 90 °C, 200 – 300 kPa, acetaldehyde concentration in the final mixture of up to 40 %) a ratio of 75:25 was obtained at high aldehyde conversion [71].
Other suitable low-boiling solvents are methylene chloride, diisopropyl ether, cyclohexanone, or ethylidene diacetate. Nonvolatile esters also can be used as diluents, provided they do not have to be removed from the reaction zone. These include alkyl benzoates and alkyl phthalates [72].
The acetaldehyde oxidation is illustrated in Figure (5) by the process of Usines de Melle [73]. The gas mixture containing oxygen and acetaldehyde is pumped into the reactor (a). The oxidation takes place in the liquid phase and in the presence of catalysts. The reactor effluent is sent through a water-cooled condenser (b) constructed as a separator; non-condensable gases are sent to the packed column (c). Fresh acetaldehyde is introduced at the top of this column. The condensates from both the cooler (b) and the column (c) are distilled to obtain the product. Acetaldehyde is recovered from the branch stream (d) of the non-condensable gas. The other part of the gas flow is supplemented with air and returned to the reactor.
Both towers and vessels are suitable as reactors if the heat of reaction can be dissipated. The process of Distillers Co. [69] is shown in Figure (6) as an example. The off-gas contains combustible low-boiling products, such as acetaldehyde, and solvents, such as methyl acetate and ethyl acetate. These can be flared off.
[64] Wacker-Chemie, DE 867 689, 1940 (A. Krug, J. Sixt).
Table 1:
Initial Ethyl Acetate/Acetaldehyde 20:80 30:70 60:40 70:30
Acetaldehyde conv. % 80 80 80 80
% Acetic Anhydride based on Acetaldehyde 13.5 57 64 68.5
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