Low Temperature Oxidation of p-xylene to Terephthalic Acid

Abstract

The oxidation of p-xylene by air to give terephthalic acid is almost universally carried out with cobalt and manganese salts as catalysts. In the Amoco process, 95% acetic acid is used as a solvent and bromide ion as a promotor. The conditions are drastic (195–205°C, 15–30 bar of air) in order to oxidize the intermediates p-toluic acid and terephthalic aldehyde, giving a selectivity for terephthalic acid better than 90%. We reinvestigated this reaction at low temperature with an oxygen flow (1 bar) in an attempt to achieve a similar selectivity for terephthalic acid. The reactions were carried out in 50 ml of acetic acid using 56 mmol of p-xylene and 5 mmol of cobalt and manganese acetate (9:1) under an oxygen flow (60 ml/min) for 12 h. Preliminary experiments showed that it was necessary to initially add acetic anhydride (120 mmol), which eliminates the reaction water formed, and a promotor to maintain the radical chain process. Hydrobromic acid was found to be the best promotor. A quantity of 2.5 mmol optimized the selectivity without producing brominated products. A co-oxidant (such as paraldehyde) was not necessary under these conditions.