The influence of pH in metal/bromide catalyzed homogeneous aerobic autoxidation. Part 2. Precipitation of metals by aromatic acids

• Aromatic acids formed during alkylaromatic oxidation can precipitate the catalyst metals.
• The amount of precipitation can be decreased by decreased pH or increasing water content in the solvent.
• The presence of bromide reduces the amount of precipitation.
• Hemimellitic acid has an unusual change in precipitation as a function of pH.

Two of the most common homogeneous aerobic oxidation catalysts are mixtures of Co/Mn/Br and Co/Mn/Zr/Br. They are especially effective in converting polyalkylbenzenes and naphthalenes to their respective aromatic acids using acetic acid as the solvent. Precipitation of the catalyst metals can occur when the product or by-product of these reactions is trimellitic (TMLA), pyromellitic (PMA), or hemimellitic acid (HMLA) (the 1,2,4-, 1,2,4,5, and 1,2,3- isomers respectively) by reaction of the metal(II) acetates to form insoluble metal carboxylic acid complexes. This can cause a decrease in rate of reaction and loss of selectivity to the aromatic acid. It is shown that this precipitation can be reduced or eliminated by reducing the pH, i.e. the acetate concentration, of the solvent. At a PMA concentration at which Co(II) and Mn(II) perchlorates or tetrafluoroborates do not react with PMA, the addition of increments of ammonium acetate results in increasing precipitation of the metals. The degree of precipitation is decreased by reducing the acetate concentration by using HBr rather than NaBr as the bromide source – the one exception being Co with HMLA. The presence of bromide itself decreases the degree of precipitation. In all cases, except one, the solubility of Co(II), Mn(II), and Zr(IV) increases with increasing water content in acetic acid. The exception is Co(II) with HMLA where the solubility first decreases with water content, up to ∼ 15 wt%, and increases thereafter. The solvent system definition of acids and bases is used to rationalize the results in acetic acid/water mixtures to that observed in water.

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