Heterogeneously catalyzed gas-phase partial oxidation of acrolein to acrylic acid

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 and Ti and whose X-ray diffraction pattern has no reflections with the peak position 20=50.0±0.30 but has reflections h, i and k whose peaks are at the diffraction angles (2θ) 22.2 t 0.5° (h), 27.3±0.50 (i) and 28.2+0.50 (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 0.50,
• 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
• R=Pi/(Pi±Pk)
• and
• the full width at half height of the reflection i and of the reflection k being in each case ≦10,
• wherein the catalytically active multimetal oxide material is one of the stoichiometry (I)
• Mo1VaM1bM2cM3dOn (I),
• 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).