Process for producing phthalic anhydride and catalyst therefor

A process for producing phthalic anhydride, which comprises packaging a first-stage catalyst into a multitube fixed bed reactor to a height of 30 to 70% of the total height of the catalyst layer from the gas inlet side and a second-stage catalyst, which is the same as the first-stage catalyst except that its phosphorus content is 0.4 to 1.2% by weight as P.sub.2 O.sub.5, to a height of 70 to 30% of the total height of the catalyst layer of the converter from the gas outlet side, said first-stage catalyst comprising a catalytically active material composed of 1 to 20 parts by weight as V.sub.2 O.sub.5 of vanadium oxide, 99 to 80 parts by weight of anatase-type titanium oxide being porous and having a particle diameter substantially of 0.4 to 0.7 micron and a specific surface area of 10 to 60 m.sup.2 /g, and per 100 parts by weight of the sum of said two components, 0.01 to 1 part by weight as Nb.sub.2 O.sub.5 of niobium oxide, 0.05 to 1.2 parts by weight as an oxide of at least one ingredient selected from the group consisting of potassium, cesium, rubidium and thallium and 0.2 to 0.4 parts by weight as P.sub.2 O.sub.5 of phosphorus, and a porous carrier having an alumina content of not more than 10% by weight, a silicon carbide content of at least 80% by weight and an apparent porosity of at least 10% supporting said catalytically active material thereon in an amount of about 3 to 15 grams per 100 cubic centimeters of said carrier, wherein the total volume of pores having a diameter of 0.15 to 0.45 micron present in the layer of the catalytically active material on the carrier is at least 50% of that of pores having a diameter of not more than 10 microns present in said layer of the catalytically active material;

• and passing a starting gas comprising o-xylene or naphthalene and oxygen or a molecular oxygen-containing gas, said starting gas having a temperature of not more than 150.degree. C. at the inlet of the converter and a concentration of oxygen of not more than 12% by volume, through the stacked catalyst layers at a temperature of 300.degree. to 450.degree. C., thereby catalytically oxidizing the o-xylene or naphthalene.