PURPOSE: To produce trimellitic acid anhydride in a high yield by a semi- continuous liquid-phase oxidation of pseudocumene in the presence of an oxidation catalyst of a heavy metal compound and a bromine source.

CONSTITUTION: Pseudocumene is allowed to be oxidized, in the presence of a heavy metal oxide catalyst incorporating Ce, Zr, Co, Mn in a liquid phase (e.g. acetic acid), using molecular oxygen at 175 to 275°C. At the time of this oxidation, bromine is added in at least two stages. In the first stage, 0 to 35% of the total bromine is added, and the rest is added in the last stage. All of Ce is added in the last stage. Oxidation is carried out at 175 to275°C in the last stage, and at 125 to 165°C in the preceding stages. 3 to 10 mg atoms of the catalyst metals in total are used in respect to 1 g mole (pseudo-cumene + bromine source), the total amount of bromine applied is 1.4 to 10 mg per 1 g mole pseudocumene. The catalyst incorporates 9-10 wt.% Ce, 2-5 wt.% Zr, 25-40 wt.% Mn, and 30-70 wt.% Co.