Process for producing 2,6-naphthalene-dicarboxylic acid

A process for producing 2,6-naphthalenedicarboxylic acid which comprises oxidizing 2,6-diisopropylnaphthalene or its oxidation product as a starting material with molecular oxygen in a reaction medium containing at least 50% by weight of an aliphatic monocarboxylic acid having not more than 3 carbon atoms in the presence of an oxidation catalyst comprising (A) at least one heavy metal element selected from the group consisting of cobalt and manganese, and (B) bromine element, the 2,6-diisopropylnaphthalene and/or its oxidation product being used in a proportion of 0.1 to 5 moles per gram-atom of the heavy metal element of the oxidation catalyst.