The Complex Synergy of Water in the Metal/Bromide Autoxidation of Hydrocarbons Caused by Benzylic Bromide Formation

Abstract

One of the most active and selective catalysts in homogeneous liquid phase oxidation using molecular oxygen (O 2 ) is a mixture of cobalt, manganese and bromide salts in acetic acid. It has been used to produce hundreds of different carboxylic acids in high yield and purity including the commercial production of terephthalic acid from pxylene. Water is normally a by-product in these reactions and it is shown here that its concentration is an important reaction variable. In anhydrous acetic acid, with reagents with sufficiently strong electronwithdrawing substitutents (toluene, 4-carboxytoluene, 4-chlorotoluene), all of the active bromide becomes inactive via benzylic bromide formation. The Co/Mn/ Br catalyst is therefore converted to a Co/Mn catalyst which is dubbed −catalyst failure× because of its undesirable characteristics of lower activity, decreased selectivity especially towards over-oxidation and color formation. For 4-chlorotoluene, increasing the water concentration to 5 weight % initially decreases the rate of reaction but eventually is more active and selective because the oxidation and hydrolysis of the benzylic bromide allows for sufficient active catalytic bromide. It is shown that benzylic bromides do not −promote× the reaction and that both oxidation and solvolysis of the benzylic bromide occurs during autoxidation. During polymethylbenzene oxidation, benzylic bromide formation occurs only with the most reactive methyl group. The complex factors during metal/bromide autoxidation ± some favored by increased water concentration and others detrimental ± are outlined.

Comments on the paper “Low-bromide containing MC catalyst for the autoxidation of para-xylene” by B. Saha and J.H. Espenson [J. Mol. Catal. A 271 (2007) 1–5]

“…Identical phenomena and very similar experiments were reported earlier [2] for p-toluic acid and 4-tolylaldehyde (intermediates in p-xylene oxidation) as well as for toluene and 4-chlorotoluene. In this paper, the formation of benzylic bromides were not assumed, but measured during the reactions.…”

Progress in the research and development of p-xylene liquid phase oxidation process

“…(1) is quite different from the previous first-order kinetics in literatures [9,10]. However, it is in accordance with the Co-Mn-Br catalytic cycle model suggested by Partenheimer [5][6][7]. The model considered that, during the course of oxidation, a large amount of peroxides is generated, which can oxidize the divalent metal cations Co(II) and Mn(II) to the trivalent ones Co(III) and Mn(III).…”

“…Although the PTA process has been commercialized for decades, the studies on the mechanism and engineering aspects are still inadequate. References [3][4][5][6][7][8][9][10][11][12] reviewed the research and development (R&D) works concerning PX oxidation in different periods. Since 1998, a research group in Zhejiang University (ZJU) has concentrated the attention on the research and development of PTA process, and carries out a comprehensive research program following the methodology of chemical engineering.…”

A spray reactor concept for catalytic oxidation of p-xylene to produce high-purity terephthalic acid

“…Partenheimer [106,107], Saha and Espenson [108] also found that a decrease in Co/Mn/Br catalytic activity is attributed to the benzylic bromide formation and benzylic bromide has virtually no activity or promotional effect in Co/Mn/Br autoxidations whereas the inorganic ionic bromide is active. The mechanism of BPTA oxidation has not been well studied.…”

“…Simultaneously, Co(III) is reduced to Co(II) by hydrogen bromide (faster) and also by benzyl bromide, albeit at a much slower rate. Partenheimer [106] reported that the competing mechanisms of oxidation and solvolysis for the disappearance of benzylic bromide occur at approximately the same rates under the studied experimental conditions. Table 2.3) Table 2.3) were carried out to study the effect of substrate concentration on the regressed rate constants.…”