Selective partial oxidation in supercritical water: the continuous generation of terephthalic acid from para-xylene in high yield

Abstract

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.

Is it Possible to Achieve Highly Selective Oxidations in Supercritical Water? Aerobic Oxidation of Methylaromatic Compounds

“…[1,15] In supercritical water oxidation, the same products have been seen for p-xylene. [11,16] Similarly, in subcritical batch aerobic oxidation of toluene, m-xylene, oxylene, mesitylene (1,3,5-trimethylbenzene), and ethylbenzene, the same intermediates and products have been reported which have also been seen in metal/ bromide-catalysed autoxidation in acetic acid. [1,17] The response to increases in catalyst concentration, temperature, and reaction time reported in scH 2 O are similar to …”

“…The continuous oxidation was carried out in scH 2 O with MnBr 2 as the catalyst, using the optimised conditions previously reported by us for the oxidation of p-xylene to terephthalic acid. [11] The expected products and reactivity of methylaromatic compounds in the metal/bromide-catalysed autoxidation of methylaromatics in acetic acid have been detailed in the literature. [1] To a first approximation, the methyl groups are consecutively oxidised to their aromatic acids.…”

“…[10] We have recently reported the continuous partial selective oxidation of p-xylene to terephthalic acid using manganese(II) bromide as the catalytic system. [11] Our process generates almost no 4-carboxybenzylaldehyde, thought to be due to the faster reaction rates experienced at higher temperature. 4-Carboxybenzaldehyde is one of the most problematic impurities in current commercial processes for manufacturing terephthalic acid since it co-precipitates with the terephthalic acid and requires an additional plant to hydrogenate it back to p-toluic acid, which is then separated by fractional crystallisation in water.…”

Liquid phase oxidation of p-xylene to terephthalic acid at medium-high temperatures: multiple benefits of CO2-expanded liquids

“…(3) The combination of corrosive bromide as a catalyst component and a high operating temperature requires the use of highly resistant but expensive titanium reactors. A recent breakthrough made by Poliakoff et al, 4 Savage et al 5 and Fulton et al 6 has shown that this reaction can be effectively catalyzed by MnBr 2 with pure O 2 in a new reaction mediumhigh temperature and supercritical water, thus circumventing the use of acetic acid. However, the scope of such reaction systems is limited by the even harsher reaction conditions (T = 300-400…”

“…Under certain conditions, the Co/Zr(acac) 4 catalyzed oxidation did not work at all with N 2 /O 2 but proceeded effectively with CO 2 /O 2 , affording high yields of benzoic acid. In this work, we extend the CO 2 effect to this important industrial TPA process, which is extremely challenging because of much higher temperatures and the complexity of intermediates.…”