2-nitro-4-methylsulphonylbenzoic acid (NMSBA) is an important intermediate for the preparation of the highly effective herbicide mesotrione. NMSBA is produced from the oxidation of 2-nitro-4-methylsulphonyltoluene (NMST) by nitric acid commercially, which suffers from the serious problems of low yield and heavy pollution. In this paper, a homogeneous system consisting of Co/Mn/Br/ Á H 3 PMo x W 12Àx O 40 has been applied to catalyze the production of NMSBA by oxidizing NMST with oxygen in acetic acid. The experiments indicate that the best value of x in the phosphomolybdenum tungsten heteropoly acid is 3. The optimum composition of this catalyst system is 1163 ppm H 3 Pmo 3 W 9 O 40 Á 12.17H 2 O, 247.5 ppm Co, 453.5 ppm Mn, and 1075 ppm Br. Such a catalytic system is able to achieve a NMST conversion of 94.93% and a NMSBA selectivity of 99.84%. H 3 PMo 3 W 9 O 40 exceeds both H 3 PMo 12 O 40 and H 3 PW 12 O 40 as a catalyst in the production of NMSBA from the oxidation of NMST by oxygen. K E Y W O R D S 2-nitro-4-methylsulphonylbenzoic acid, 2-nitro-4-methylsulphonyltoluene, catalytic oxidation, phosphomolybdenum tungsten heteropoly acid

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BACKGROUND: Quinclorac (3,7-dichloro-8-quinolinecarboxylic acid) is commercially produced from the partial oxidation of 3,7-dichloro-8-(chloromethyl)quinolone by nitric acid in 98% sulfuric acid. Such technology suffers from some shortcomings, such as the discharge of NO x and wastewater, and the formation of nitro compounds. Oxygen is a clean, cheap oxidant. The key to realizing the production of quinclorac by oxidizing 3,7-dichloro-8-(chloromethyl)quinolone with oxygen is the development of an efficient catalyst.RESULTS: The transformation of 3,7-dichloro-8-(chloromethyl)quinolone to quinclorac oxidized by O 2 molecule in acetic acid solution can be accomplished under a homogeneous catalyst system composed of H 3 PMo 12 O 40 /Ce/Co/Br. Experiments show that the optimal catalytic system consists of 0.209 (wt)% phosphomolybdic acid, 0.022 (wt)% Ce(III), 0.027 (wt)% Co(II) and 0.0161 (wt)% Br − . CONCLUSION: Temperature is an important parameter affecting reaction rate. The influence of oxygen concentration in the gas phase on quinclorac yield and 3,7-dichloro-8-(chloromethyl)quinolone conversion is great. The activation energy of this catalytic reaction is 57.6 kJ mol −1 . The reaction orders for oxygen and 3,7-dichloro-8-(chloromethyl)quinolone are 0.26 and 0.93, respectively.