Oxidation of naphthalene to 1,4-naphthoquinone and phthalic anhydride

In the production of 1,4-naphthoquinone along with phthalic anhydride by the catalytic gas phase oxidation of naphthalene, the improvement which comprises maintaining in the reaction space a sulphur concentration of about 0.002 to about 0.1% by weight based on the naphthalene used. Advantageously the sulphur content is about 0.01 to 0.02%, the sulphur being in elementary form or conbined organic or inorganic form, e.g. sulphur dioxide. The preferred catalyst is vanadium pentoxide, alkali metal sulphate and alkali pyrosulphate and it is pretreated by conducting over it nitrogen containing 0.1 to 1% by volume of oxygen along with naphthalene and then a sulphur dioxide - air mixture. The yield of naphthoquinone is increased markedly compared with process effected in the presence of far more sulphur.