The liquid phase oxidation of para-, meta- and ortho-xylenes to their corresponding benzene dicarboxylic acids in the presence of bromine and a catalyst containing manganese and cobalt components has been disclosed in U.S. Pat. No. 2,833,816 and has been practiced worldwide. Additionally, the liquid phase oxidation of a dimethylnaphthalene feed material to a naphthalenedicarboxylic acid can also be accomplished in the presence of bromine and a catalyst containing manganese and cobalt components. See for example, U.S. Pat. No. 5,103,933. These benzene dicarboxylic acids and naphthalenedicarboxylic acids are useful for preparing polyester materials. Such polyesters are used to manufacture synthetic fibers and films for textile and packaging applications, respectively.
Although the use of bromine is advantageous for conducting such liquid phase oxidation reactions, its use does have some drawbacks. For example, bromine contributes to the corrosion of the reactor vessel used for the oxidation reaction as well as the equipment used to process the reaction mixture subsequent to the oxidation reaction. The corrosion metals produced thereby contaminate the crude aromatic carboxylic acid product, and these corrosion metals are detrimental to the hydrogenation catalyst used in a subsequent purification step where the aromatic carboxylic acids are treated with hydrogen gas in the presence of the hydrogenation catalyst. Additionally, the bromine in the oxidation reaction mixture contributes to the production of methyl bromide, a gaseous, hazardous compound. Thus, for environmental reasons, it would be desirable to reduce the amounts of methyl bromide produced during the oxidation of a xylene or a dimethylnaphthalene compound to the corresponding aromatic dicarboxylic acid. By reducing the amount of bromine in the oxidation reaction mixture, corrosion is reduced as well as the amount of methyl bromide formed. However, we determined that a reduction in the amount of bromine, i.e. lower molar ratios of bromine to the total amount of cobalt and manganese, causes an unacceptable precipitation of the manganese component of the catalyst. This precipitation is easily identified because it produces a grey colored rather than white aromatic carboxylic acid product. We believe at least part of the manganese precipitates as manganese dioxide.
In commercial scale operation, a major portion of the oxidation catalyst metals are recycled to the oxidation reaction mixture. The catalyst metal recycle is accomplished by separating the oxidation reaction mother liquor from the solid aromatic carboxylic acid product, and part of mother liquor containing dissolved catalyst metals is recycled back to the oxidation reaction mixture. However, the precipitation of the manganese which occurs at low molar ratios of bromine to catalyst metals prevents efficient recycle of the catalyst metals. Additionally, since the precipitation of the manganese occurs in an uneven manner, the mother liquor will have varying levels of catalyst metals making it difficult to recycle the proper amount of catalyst to the oxidation reaction mixture. This results in unacceptable variability in the aromatic carboxylic acid product.
The art, therefore, needs a method to reduce the ratio of bromine to cobalt and manganese catalyst metals used in the liquid phase oxidation of dimethyl aromatic compounds without causing the precipitation of the manganese portion of the catalyst. The instant invention provides such a method.