Patent ID: 7968491

Claim:
Catalysts for preparing phthalic anhydride by gas phase oxidation of o-xylene and/or naphthalene, comprising a first catalyst zone disposed toward a gas inlet side, a second catalyst zone disposed closer to a gas outlet side and a third catalyst zone disposed even closer to or at the gas outlet side, the catalyst zones having different compositions, wherein the composition of each catalyst zone comprises an active composition comprising TiO 2 with a content of Na of less than 0.3% by weight, wherein the active composition content decreasing from the first catalyst zone to the third catalyst zone, and wherein a) the first catalyst zone has an active composition content between about 7 and 12% by weight, b) the second catalyst zone has an active composition content between 6 and 11% by weight, wherein the active composition content of the second catalyst zone is less than or equal to the active composition content of the first catalyst zone, and c) the third catalyst zone has an active composition content in the range between 5 and 10% by weight, wherein the active composition content of the third catalyst zone is less than or equal to the active composition content of the second catalyst zone.