The continuous selective oxidation of p-xylene (p-X) by O 2 (generated by thermal decomposition of aqueous H 2 O 2 ) catalyzed by MnBr 2 in supercritical H 2 O at ca. 400 °C is reported for the first time. The selectivity for terephthalic acid (TA) is > 90%. Compared to existing industrial processes, the reaction has the potential for a significant increase in energy efficiency and a substantial reduction in waste. This reaction is significant because the presence of H 2 O lowers the catalytic activity of MnBr 2 in the conventional route to TA via oxidation of p-X in CH 3 COOH.

Abstract:We have demonstrated that different methylaromatic compounds [1,4-dimethylbenzene (p-xylene), 1,3-dimethylbenzene (m-xylene), 1,2-dimethylbenzene (o-xylene), 1,3,5-trimethylbenzene (mesitylene) and 1,2,4-trimethylbenzene (pseudocumene)] can be aerobically oxidized in supercritical water (scH 2 O) using manganese(II) bromide as catalyst to give corresponding carboxylic acids in the continuous mode over a sustained period of time in good yield. No partially oxidized intermediates (i.e., toluic acid and benzaldehydes) were detected for the dimethylbenzenes and mesitylene reactions. By fine tuning pressure and temperature, scH 2 O becomes a solvent with physical properties suitable for single-phase oxidation since both organic substrate and oxygen are soluble in scH 2 O. There is a strong structural similarity of metal/bromide coordination compounds in the active oxidation solvents (acetic acid and scH 2 O) which does not exist in the much less active H 2 O at lower temperatures. This may account for the successful catalysis of the reactions reported herein. Aromatic acids produced by the loss of one methyl group occurred in all of these reactions, i.e., 3 ± 6% benzoic acid formed during the oxidation of the dimethylbenzenes. Part of this loss is thought to be due to thermal decarboxylation. The thermal decarboxylation process is monitored via Raman spectroscopy.

In this paper we show that a mixture of xylenes can be simultaneously oxidised in supercritical water (scH 2 O) in a continuous mode to a mixture of the corresponding carboxylic acids in high combined yield, despite the differences in reactivity of the xylene isomers in conventional oxidation. The single phase environment in scH 2 O together with the effect of higher temperatures should increase the reaction rate for each of these oxidation reactions and thus reduce the reactivity differences between the components of the C 8 refinery mixture. Such a process should lead to a considerable reduction in the overall energy input for the oxidation of xylenes. The process in scH 2 O described here could simplify the downstream purification processes to a simple crystallization process. This is commercially important, because the purification process can be as expensive as the reaction producing the product. Furthermore, the oxidation of mixed xylenes could avoid the need not only for downstream purification, but also for the upstream separation of the xylene isomers. The use of high temperature water also offers significant cost advantages through enhanced energy recovery, due to a higher process temperature. Finally, the process totally eliminates the use of organic solvents.

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