Preparation of (meth)acrylic acid

A process for preparing (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation of at least one saturated hydrocarbon precursor compound, by charging a catalyst bed which is disposed in a reactor and whose catalytically active composition is a multimetal oxide which contains the elements Mo and V, one or both of the elements Te and Sb, and one or more elements selected from the group consisting of Nb, Ta, W, Ce and Ti, and whose x-ray diffractogram has reflections h, i and k whose peak locations are at the reflection angles (2Θ) 22.2±0.5° (h), 27.3±0.50° (i) and 28.2±0.5° (k), where

• the reflection h is the most intense within the x-ray diffractogram and has a half-height width of at most 0.50°,
• the intensity Pi of the reflection i and the intensity Pk of the reflection k satisfy the relationship 0.55≦R≦0.85 in which R is the intensity ratio defined by the formula
• R=Pi/(Pi+Pk)
• and
• the half-height width of the reflection i and of the reflection k are each ≦1°, at elevated temperature with a charging gas mixture which, in addition to the at least one saturated hydrocarbon precursor compound, molecular oxygen as an oxidant and steam as a promoter, also comprises at least one diluent gas which is substantially inert under the conditions of the heterogeneously catalyzed gas phase partial oxidation,
• wherein
• the steam content of the charging gas mixture is from 5 to 25% by volume and the molar ratio V of molecular oxygen present in the charging gas mixture to the at least one saturated hydrocarbon precursor compound present in the charging gas mixture is from 1.5:1 to 2.5:1.