Use of a multi-layer catalyst for producing phthalic anhydride

1. A catalyst for the preparation of phthalic anhydride by gas-phase oxidation of o-xylene and/or naphthalene, comprising a first catalyst zone located towards the gas inlet, a second catalyst zone located closer to the gas outlet and a third catalyst zone located even closer to the gas outlet with the catalyst zones containing an active composition comprising TiO 2 , which TiO 2 has a BET surface area of at least 15 m 2 /g, characterized in that the catalyst activity of the first catalyst zone is higher than the catalyst activity of the second catalyst zone.

2. The catalyst according to claim 1 , characterized in that the catalyst activity increases from the second to the third catalyst zone.

3. The catalyst according to claim 1 further comprising a fourth catalyst zone located closer to the gas outlet than the third catalyst zone, characterized in that the catalyst activity increases from the third to the fourth catalyst zone.

4. The catalyst according to claim 1 , characterized in that a total of four catalyst zones, is present.

5. The catalyst according to claim 1 , characterized in that the length of the first catalyst zone is from about 5 to 30% of the total length of all of the catalyst zones.

6. The catalyst according to claim 1 , characterized in that, when comparing the first catalyst zone with the second catalyst zone, the first catalyst zone exhibits at least one of the characteristics selected from the group consisting of

a. a higher active composition content,

b. a higher vanadium content,

c. a TiO 2 having a higher BET surface area,

d. a lower Sb content,

e. a lower Cs content,

f. a higher content of promoters which increases activity,

g. a higher bulk density, and

h. a lower content of promoters which lower the activity of the first catalyst zone.

7. The catalyst according to claim 1 , characterized in that the first catalyst zone comprises a higher content of promoters which increase its activity in comparison to the activity of the second catalyst zone.

8. The catalyst according to claim 1 , characterized in that the first catalyst zone has a higher bulk density than the second catalyst zone, as a result of the use of a different geometry of shaped catalyst bodies within the catalyst zones.

9. The catalyst according to claim 1 , characterized in that the individual catalyst zones comprise coated catalysts in which the active composition has been applied to an inert support.

10. The catalyst according to claim 1 , characterized in that the individual catalyst zones comprise as a percentage of the active composition the following:

Composition

Range

V 2 O 5 /% by wt.

 1-25

Sb 2 O 3 /% by wt.

0-4

Cs/% by wt.

0-1

P/% by wt.

0-2

with the remainder of the active composition comprising at least 90% by weight, of TiO 2 , the BET surface area of the TiO 2 used being in the range from about 15 to 45 m 2 /g and the active composition making up from about 4 to 20% by weight of the total weight of the catalyst.

11. The catalyst according to claim 1 , characterized in that the first catalyst zone has an active composition content of from about 6 to 20% by weight, and the active composition preferably comprises from 5 to 16% by weight of V 2 O 5 , from 0 to 5% by weight of Sb 2 O 3 , from 0.2 to 0.75% by weight of Cs, from 0 to 3% by weight of Nb 2 O 5 , from 0 to 1% by weight of P and TiO 2 as the balance.

12. The catalyst according to claim 1 , characterized in that the second catalyst zone has an active composition content of from about 6 to 12% by weight, with the active composition preferably comprising from 5 to 15% by weight of V 2 O 5 , from 0 to 5% by weight of Sb 2 O 3 , from 0.2 to 0.75% by weight of Cs, from 0 to 2% by weight of Nb 2 O 5 , from 0 to 1% by weight of P and TiO 2 as the balance.

13. The catalyst according to claim 1 , characterized in that the third catalyst zone has an active composition content of from about 5 to 11% by weight, with the active composition preferably comprising from 5 to 15% by weight of V 2 O 5 , from 0 to 4% by weight of Sb 2 O 3 , from 0.05 to 0.5% by weight of Cs, from 0 to 2% by weight of Nb 2 O 5 , from 0 to 1% by weight of P and TiO 2 as the balance.

14. The catalyst according to claim 3 , characterized in that the fourth catalyst zone has an active composition content of from 5 to 25% by weight of V 2 O 5 , from 0 to 5% by weight of Sb 2 O 3 , from 0 to 0.2% by weight of Cs, from 0 to 1% by weight of Nb 2 O 5 , from 0 to 2% by weight of P and TiO 2 as the balance.

15. The catalyst according to claim 3 , characterized in that the first catalyst zone has an active composition content of from about 7 to 20% by weight,

the second catalyst zone has an active composition content of from about 7 to 12% by weight, with the active composition content of the second catalyst zone being less than or equal to the active composition content of the first catalyst zone;

the third catalyst zone has an active composition content in the range from about 6 to 11% by weight, with the active composition content of the third catalyst zone being less than or equal to the active composition content of the second catalyst zone, and

the fourth catalyst zone has an active composition content in the range from 5 to 10% by weight, with the active composition content of the fourth catalyst zone being less than or equal to the active composition content of the third catalyst zone.

16. The catalyst according to claim 1 , characterized in that the BET surface area of the catalyst zone nearest the gas outlet is higher than the BET surface area of the preceding catalyst zones.

17. The catalyst according to claim 1 , characterized in that at least about 40%, of the total pore volume of the TiO 2 is formed by pores having a radius in the range from 60 to 400 nm.

18. The catalyst according to claim 1 , characterized in that the active composition is applied in a moving bed or fluidized bed.

19. The catalyst according to claim 1 , characterized in that at least 0.05% by weight of the active composition of at least one catalyst zone comprises at least one alkali metal, calculated as alkali metal(s).

20. The catalyst according to claim 1 , characterized in that an organic polymer or copolymer comprising a vinyl acetate copolymer is used as an adhesive for the catalytically active composition.

21. The catalyst according to claim 1 , characterized in that the catalyst is calcined or conditioned for at least 24 hours at >390° C., in an O 2 -containing gas.

22. The catalyst according to claim 1 , characterized in that niobium is present in an amount of from 0.1 to 2% by weight, of the active composition in at least one catalyst zone.

23. The catalyst according to claim 1 , characterized in that phosphorus is present in the active composition of at least the catalyst zone closest to the gas outlet.

24. The catalyst according to claim 1 , characterized in that the first catalyst zone has an activity which is at least 5%, higher than that of the subsequent second catalyst zone.

25. A process for the preparation of phthalic anhydride, comprising passing a gaseous stream comprising o-xylene and/or naphthalene and also molecular oxygen at elevated temperature over a catalyst comprising a first catalyst zone located towards the gas inlet, a second catalyst zone located closer to the gas outlet and a third catalyst zone located even closer to the gas outlet with the catalyst zones preferably each containing an active composition comprising TiO 2 , which TiO 2 has a BET surface area of at least 15 m 2 /g, characterized in that the catalyst activity of the first catalyst zone is higher than the catalyst activity of the second catalyst zone.