Special Features of p-Xylene Oxidation

The most important mechanistic difference between cyclohexane and xylene oxidation is that in the former there is no direct electron transfer from the substrate to the metal ion. In other words, in cyclohexane oxidation the initiation step 8.18 has no role to play. In contrast for all methylbenzene derivatives, xylenes included, this pathway is of importance for chain initiation. As shown by 8.25, formation of the radical species takes place through a radical cation. 

The initial products are the p-methyl benzyl alcohol and p-methyl benzal-dehyde. Under the reaction conditions these are further oxidized, first to p-toluic acid, and ultimately to terephthalic acid. The alkoxy and peroxy radicals that take part in the oxidation of p-xylene to p-toluic acid are shown by 8.13 to 8.16. 

Commercially two main processes, that of Mid-century/Amoco and Dynamit Nobel/Hercules, are operated. In the former acetic acid is used as a solvent. Mixtures of cobalt and manganese bromide and acetate salts are used to catalyze the initiation step. The reaction conditions, a temperature of about 220°C and a pressure of 15 atm, are relatively severe. Under these conditions bromine and CH2C02H radicals are formed. These radicals can effect new initiation steps. In the overall process, though toluic acid is an intermediate, it is never isolated. The final isolated product is terephthalic acid (see reaction 8.10). 

In the Dynamit Nobel/Hercules process no solvent is used. A mixture of cobalt and manganese ethyl hexanoate is used as the catalyst under relatively mild conditions, about 160°C and 7 atm pressure. The product under these conditions is toluic acid, which is isolated and then converted into the methyl ester. The important point to note is that under the operating conditions toluic acid does not undergo any further oxidation. This means that toluic acid is more difficult to oxidize than p-xylene. The methyl ester of toluic acid is then co-oxidized with p-xylene. The product obtained is monomethyl terephthalate, which by reaction with methanol is then converted to dimethyl terephthalate. 

The advantage of the Amoco process is that high-purity terephthalic acid is produced in one step. The solubility of terephthalic acid in acetic acid is low its separation with high purity by crystallization is therefore relatively easy. However, the corrosive nature of the acids and the relatively drastic conditions make it necessary to use special material of construction for the reactors. 

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