Patent ID: 7718561

Claim:
Catalysts for preparing phthalic anhydride by gas phase oxidation of o-xylene or naphthalene, comprising a first catalyst of a first catalyst zone disposed toward the gas inlet side, a second catalyst of a second catalyst zone disposed closer to the gas outlet side and a third catalyst of a third catalyst zone disposed even closer to or at the gas outlet side, the catalysts of the catalyst zones having different compositions and in each case a catalytically active composition comprising TiO 2 , wherein the catalytically active composition content decreases from the first catalyst of the first catalyst zone to the third catalyst of the third catalyst zone, with the proviso that a) the first catalyst of the first catalyst zone has an active composition content between about 7 and 12% by weight, b) the second catalyst of the second catalyst zone has an active composition content in the range between 6 and 11% by weight, the active composition content of the second catalyst of the second catalyst zone being less than or equal to the active composition content of the first catalyst of the first catalyst zone, and c) the third catalyst of the third catalyst zone has an active composition content in the range between 5 and 10% by weight, the active composition content of the third catalyst of the third catalyst zone being less than or equal to the active composition content of the second catalyst of the second catalyst zone.