Multimetal oxide containing silver, vanadium and a promoter metal and use thereof

1. A multimetal oxide of the formula I,

Ag a-c Q b M c V 2 O d *e H 2 O   I

where

a is from 0.3 to 1.9,

Q is an element selected from among P, As, Sb and/or Bi,

b is from 0 to 0.3,

M is a metal selected from among Nb, Ce, W, Mn, Ta, Pd, Pt, Ru and Rh,

c is from 0.001 to 0.2, with the proviso that (a-c)≧0.1,

d is a number which is determined by the valence and abundance of the elements other than oxygen in the formula I and

e is from 0 to 20,

which has a crystal structure whose X-ray powder diffraction pattern displays reflections at lattice plane spacings d of 15.23±0.6, 12.16±0.4, 10.68±0.3, 3.41±0.04, 3.09±0.04, 3.02±0.04, 2.36±0.04 and 1.80±0.04 Å.

2. The multimetal oxide according to claim 1 , wherein

b is 0 and

c is from 0.01 to 0.1.

3. The multimetal oxide according to claim 1 , which has a specific surface area determined by the BET method of from 3 to 250 m 2 /g.

4. The multimetal oxide according to claim 1 , wherein M is Ce or Mn.

5. A precatalyst, which can be converted into a catalyst for the gas-phase partial oxidation of aromatic hydrocarbons, comprises an inert nonporous support and at least one layer comprising a multimetal oxide according to claim 1 applied thereto.

6. The precatalyst according to claim 5 , comprises from 5 to 25% by weight, based on the total weight of the precatalyst, of multimetal oxide.

7. The precatalyst according to claim 5 , whose inert nonporous support material comprises steatite.

8. A catalyst for the gas-phase partial oxidation of aromatic hydrocarbons, comprises an inert nonporous support and, applied thereto, at least one layer comprising, as catalytically active composition, a silver-vanadium oxide bronze which comprises at least one metal M selected from the group consisting of Nb, Ce, W, Mn, Ta, Pd, Pt, Ru and Rh, wherein the Ag:V atomic ratio is from 0.15 to 0.95 and the M:V atomic ratio is from 0.0005 to 0.25, which catalyst is produced from a multimetal oxide composition according to claim 1 .

9. The catalyst according to claim 8 , wherein the silver-vanadium bronze contains Ce or Mn.

10. The catalyst according to claim 8 , having a layer whose catalytically active composition has a BET surface area of from 2 to 100 m 2 /g.

11. A process for preparing aldehydes, carboxylic acids and/or carboxylic anhydrides, comprises bringing a gaseous stream into contact with an aromatic hydrocarbon and a gas comprising molecular oxygen in the presence of a catalyst according to claim 8 at elevated temperature.

12. The process according to claim 11 , wherein the catalyst is produced in situ from a precatalyst which can be converted into a catalyst, the catalyst comprises an inert nonporous support and at least one layer comprising a multimetal oxide of the formula I,

Ag a-c Q b M c V 2 O d *e H 2 O   I

where

a is from 0.3 to 1.9,

Q is an element selected from among P, As, Sb and/or Bi,

b is from 0 to 0.3,

M is a metal selected from among Nb, Ce, W, Mn, Ta, Pd, Pt, Ru and Rh,

c is from 0.001 to 0.2, with the proviso that (a-c)>0.1,

d is a number which is determined by the valence and abundance of the elements other than oxygen in the formula I and

e is from 0 to 20,

which has a crystal structure whose X-ray powder diffraction pattern displays reflections at lattice plane spacings d of 15.23+0.6, 12.16+0.4, 10.68+0.3, 3.41+0.04, 3.09+0.04, 3.02+0.04, 2.36+0.04 and 1.80+0.04 Å.

13. The process according to claim 11 , wherein the reaction mixture obtained or a fraction thereof is brought into contact with at least one further catalyst whose catalytically active composition comprises vanadium pentoxide and anatase, wherein said anatase is a form of titanium dioxide.

14. The process according to claim 13 , wherein the gaseous stream is passed successively over a bed of an upstream catalyst and a bed of a downstream catalyst, wherein the bed of upstream catalyst comprises a catalyst comprises an inert nonporous support and, applied thereto, at least one layer comprising, as catalytically active composition, a silver-vanadium oxide bronze which comprises at least one metal M selected from the group consisting of Nb, Ce, W, Mn, Ta, Pd, Pt, Ru and Rh, wherein the Ag:V atomic ratio is from 0.15 to 0.95 and the M:V atomic ratio is from 0.0005 to 0.25, which catalyst is produced from said multimetal oxide of formula (I) and the bed of downstream catalyst comprises at least one catalyst whose catalytically active composition comprises vanadium pentoxide and anatase.

15. The process according to claim 14 , wherein the catalytically active composition of the downstream catalyst comprises from 1 to 40% by weight of vanadium oxide, calculated as V 2 O 5 , from 60 to 99% by weight of titanium dioxide, calculated as TiO 2 , up to 1% by weight of a cesium compound, calculated as Cs, up to 1% by weight of a phosphorus compound, calculated as P, and up to 10% by weight or antimony oxide, calculated as Sb 2 O 3 .

16. The process according to claim 15 , wherein the bed of the downstream catalyst comprises at least two layers of catalysts whose catalytically active composition has a differing Cs content, with the Cs content decreasing in the flow direction of the gaseous stream.

17. The process according to claim 11 , wherein o-xylene or naphthalene or a mixture of o-xylene and naphthalene is used as aromatic hydrocarbon and is oxidized to phthalic anhydride.

18. The process according to claim 11 , wherein the reaction mixture obtained or a fraction thereof is brought into contact with at least one further catalyst whose catalytically active composition comprises vanadium pentoxide and anatase, wherein said anatase is a form of titanium dioxide.