Patent ID: 7371893

Claim:
A process for preparing phthalic anhydride, in which a gaseous stream comprising an aromatic hydrocarbon, selected from o-xylene and/or naphthalene and a gas comprising molecular oxygen is passed at a temperature of from 300 to 450° C. over a bed of a first catalyst and a bed which is made up of a second catalyst having a higher activity than the first catalyst and is located downstream of the first catalyst in the flow direction of the gaseous stream, wherein the catalytically active composition of the first catalyst comprises at least vanadium oxide, titanium dioxide and antimony oxide and the weight ratio of vanadium, calculated as V 2 O 5 , to antimony, calculated as Sb 2 O 3 , in the first catalyst is from 3.5:1 to 4.5:1 and wherein the catalytically active composition of the second catalyst comprises at least vanadium oxide, titanium dioxide and antimony oxide and the weight ratio of vanadium to antimony in the second catalyst is less than or equal to the corresponding weight ratio in the first catalyst.