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A process for the heterogeneously catalyzed gas-phase partial oxidation of acrolein to acrylic acid over a multimetal oxide material having a specific structure, which contains the elements Mo and V, at least one of the elements Te and Sb and at least one of the elements from the group consisting of

A process for the heterogeneously catalyzed gas-phase partial oxidation of acrolein to acrylic acid over a multimetal oxide material having a specific structure, which contains the elements Mo and V, at least one of the elements Te and Sb and at least one of the elements from the group consisting of Nb, Ta, W and Ti and is doped with promoter elements, is described.

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We claim: 1. A process for the heterogeneously catalyzed gas-phase partial oxidation of acrolein to acrylic acid over a catalytically active multimetal oxide material which contains the elements Mo and V, at least one of the elements Te and Sb and at least one of the elements from the group consist

We claim: 1. A process for the heterogeneously catalyzed gas-phase partial oxidation of acrolein to acrylic acid over a catalytically active multimetal oxide material which contains the elements Mo and V, at least one of the elements Te and Sb and at least one of the elements from the group consisting of Nb, Ta, W, Ce and Ti and whose X-ray diffraction pattern has no reflections with the peak position 2θ=50.0짹0.3째 but has reflections h, i and k whose peaks are at the diffraction angles (2θ) 22.2짹0.5째 (h), 27.3짹0.5째 (i) and 28.2짹0.5째 (k), the reflection h being the one with the strongest intensity within the X-ray diffraction pattern and having a full width at half height of not more than 0.5째, the intensity Pi of the reflection i and the intensity Pk of the reflection k fulfilling the relationship 0. 65≦R≦0.85, where R is the intensity ratio defined by the formula description="In-line Formulae" end="lead"R=P i/(Pi+Pk)description="In-line Formulae" end="tail" and the full width at half height of the reflection i and of the reflection k being in each case ≦1째, wherein the catalytically active multimetal oxide material is one of the stoichiometry (I) description="In-line Formulae" end="lead"Mo 1VaM1bM2cM3 dOn (I),description="In-line Formulae" end="tail" where M1 is at least one of the elements from the group consisting of Te and Sb; M2 is at least one of the elements from the group consisting of Nb, Ti, W, Ta and Ce; M3 is at least one of the elements from the group consisting of Pb, Ni, Co, Bi, Pd, Ag, Pt, Cu, Au, Ga, Zn, Sn, In, Re, Ir, Sm, Sc, Y, Pr, Nd and Tb; a=from 0.01 to 1, b=from >0 to 1, c=from >0 to 1, d=from >0 to 0.5 and n is a number which is determined by the valency and frequency of the elements other than oxygen in (I). 2. The process as claimed in claim 1, wherein 0. 67≦R≦0.75. 3. The process as claimed in claim 1, wherein 0. 69≦R≦0.75. 4. The process as claimed in claim 1, wherein 0. 71≦R≦0.74. 5. The process as claimed in claim 1, wherein R=0.72. 6. The process as claimed in any of claims 1 to 5, wherein the specific surface area of the catalytically active multimetal oxide material (I) is from 11 to 40 m2/g. 7. The process as claimed in claim 1, wherein the X-ray diffraction pattern of the catalytically active multimetal oxide material (I) has further reflections having their peak position at the following diffraction angles 2θ: 9.0짹0.4째 (l), 6.7짹0.4째 (o) and 7.9짹0.4째 (p). 8. The process as claimed in claim 7, wherein the X-ray diffraction pattern of the catalytically active multimetal oxide material (I) has further reflections with their peak position at the following diffraction angles 2θ: 29.2짹0.4째 (m) and 35.4짹0.4째 (n). 9. The process as claimed in claim 8, wherein the reflections h, i, l, m, n, o, p and q have the following intensities on the same intensity scale: h=100, i=from 5 to 95, l=from 1 to 30, m=from 1 to 40, n=from 1 to 40, o=from 1 to 30, p=from 1 to 30 and q=from 5 to 60. 10. The process as claimed in claim 1, wherein a=from 0.05 to 0.6. 11. The process as claimed in claim 1, wherein b=from 0.01 to 1. 12. The process as claimed in claim 1, wherein c=from 0.01 to 1. 13. The process as claimed in claim 1, wherein d=from 0.0005 to 0.5. 14. The process as claimed in claim 1, wherein a=from 0.1 to 0.6, b=from 0.1 to 0.5, c=from 0.1 to 0.5 and d=from 0.001 to 0.5. 15. The process as claimed in claim 1, wherein at least 50 mol % of M2, based on its total amount, is Nb. 16. The process as claimed in claim 1, wherein at least 75 mol % of M2, based on its total amount, is Nb. 17. The process as claimed in claim 1, wherein M2 is exclusively Nb. 18. The process as claimed in claim 1, wherein M3 is at least one element from the group consisting of Ni, Co, Bi, Pd, Ag, Au, Pb and Ga. 19. The process as claimed in claim 1, wherein M3 is at least one element from the group consisting of Ni, Co, Pd and Bi. 20. The process as claimed in claim 1, wherein M1 is Te, M2 is Nb and M3 is at least one element from the group consisting of Ni, Co and Pd. 21. The process as claimed in claim 1, wherein the multimetal oxide material (I) is contained in a total multimetal oxide material whose X-ray diffraction pattern has no reflection with the peak position 2θ=50.0짹0.3째. 22. The process as claimed in claim 21, wherein the multimetal oxide material (I) is present in the total multimetal oxide material in a form diluted with at least one finely divided material from the group consisting of silica, titanium dioxide, alumina, zirconium oxide and niobium oxide. 23. The process as claimed in claim 22, wherein the total multimetal oxide material contains ≧80% by weight of multimetal oxide material (I) and the X-ray diffraction pattern of the total multimetal oxide material has a reflection with the peak position 2θ=50.0짹0.3째. 24. The process as claimed in claim 22, wherein R ≧0.65 and ≦0.90 is fulfilled for the X-ray diffraction pattern of the total multimetal oxide material. 25. The process as claimed in claim 1, wherein the heterogeneously catalyzed gas-phase partial oxidation of acrolein is effected in the presence of propane and/or propene. 26. The process as claimed in claim 1, which is carried out in a tube-bundle reactor. 27. The process as claimed in claim 1, wherein the catalytically active multimetal oxide material (I) is a component of a coated catalyst.