Preparation of phthalic anhydride

A method for preparing phthalic anhydride by catalytic vapor phase oxidation of ortho-xylene or naphthalene, which comprises passing a gas mixture comprising molecular oxygen, an inert gas and a member selected from the group consisting of ortho-xylene and naphthalene through a catalyst A comprising a first catalytically active substance on a first porous inert carrier, said first catalytically active substance consisting essentially of 1 to 20 parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by weight of anatase-type TiO.sub.2, and, based on the total weight of the V.sub.2 O.sub.5 and TiO.sub.2, 0.01 to 1.0% by weight of Nb.sub.2 O.sub.5, 0.5 to 1.1% by weight of P.sub.2 O.sub.5, 0 to 0.25% by weight of K.sub.2 O and 0 to 0.5% by weight of Cs.sub.2 O, the total weight of the K.sub.2 O and Cs.sub.2 O being at least 0.05% by weight, 0.05 to 0.3% by weight of Na.sub.2 O and 0 to 0.5% by weight of Rb.sub.2 O, said first porous inert carrier containing not more than 10% by weight of aluminum, calculated as Al.sub.2 O.sub.3, and containing at least 50% by weight of SiC, and having an apparent porosity of at least 10%, at least 50% of the total pore volume of the pores of diameters less than 10 microns in the supported catalytically active substance being pores of diameters between 0.15 and 0.45 micron, and

• passing the thus treated gas mixture through a catalyst B comprising a second catalytically active substance on a second porous inert carrier, said second catalytically active substance consisting essentially of 1 to 20 parts by weight of V.sub.2 O.sub.5, 99 to 80 parts by weight of anatase-type TiO.sub.2, and, based on the total weight of V.sub.2 O.sub.5 and TiO.sub.2, 0.01 to 1.0% by weight of Nb.sub.2 O.sub.5, 1.2 to 2.5% by weight of P.sub.2 O.sub.5, 0 to 0.25% by weight of K.sub.2 O and 0 to 0.5% by weight of Cs.sub.2 O, the total weight of the K.sub.2 O and Cs.sub.2 O being at least 0.05% by weight, 0.05 to 0.3% by weight of Na.sub.2 O and 0 to 0.5% by weight of Rb.sub.2 O, said second porous inert carrier containing not more than 10% by weight of aluminum, calculated as Al.sub.2 O.sub.3, and containing at least 50% by weight of SiC, and having an apparent porosity of at least 10%, at least 50% of the total pore volume of the pores of diameters less than 10 microns in the supported catalytically active substance being pores of diameters between 0.15 and 0.45 micron.