Patent ID: 7019169

Claim:
A process for preparing (meth)acrylic acid by heterogeneously catalyzed gas phase partial oxidation of at least one saturated hydrocarbon precursor compound at elevated temperature, by conducting a starting reaction gas mixture comprising the at least one saturated hydrocarbon precursor compound, molecular oxygen and at least one inert gas through at least one catalyst bed I whose catalysts I are such that their active composition is at least one multimetal oxide I 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 diffractogram is an X-ray diffractogram I which has reflections h, i and k whose peak locations are at the reflection angles (2Θ) of 22.2±0.5° (h), 27.3±0.5° (i) and 28.2±0.5° (k), the reflection h being the most intense within the X-ray diffractogram I and also having a half-height width of at most 0.5°, and the half-height width of the reflection i and of the reflection k each being ≦1°, wherein the gas phase partial oxidation of the reaction gas mixture over the catalysts I in the at least one catalyst bed I is interrupted at least once by continuing the gas phase partial oxidation in at least one catalyst bed II whose catalysts II are such that their active composition is at least one multimetal oxide II whose X-ray diffractogram is different to the X-ray diffractogram I and has a stoichiometry of the general formula A, Mo 12 Bi a Fe b X 1 c X 2 d X 3 e X 4 f O n (A) where X 1 =nickel and/or cobalt, X 2 =thallium, an alkali metal and/or an alkaline earth metal, X 3 =zinc, phosphorus, arsenic, boron, antimony, tin, cerium, lead and/or tungsten, X 4 =silicon, aluminum, titanium and/or zirconium, a=from 0.2 to 5, b=from 0.01 to 5, c=from 0 to 10, d=from 0 to 2, e=from 0 to 8, f=from 0 to 10 and n=a number which is determined by the valency and frequency of the elements in A other than oxygen, or a stoichiometry of the general formula B [Y 1 a′ Y 2 b′ O x′ ] p [Y 3 c′ Y 4 d′ Y 5 e′ Y 6 f′ Y 7 g′ Y 2 h′ O y′] q (B) where Y 1 =only bismuth or bismuth and at least one of the elements tellurium, antimony, tin and copper, Y 2 =molybdenum or molybdenum and tungsten, Y 3 =an alkali metal, thallium and/or samarium, Y 4 =an alkaline earth metal, nickel, cobalt, copper, manganese, zinc, tin, cadmium and/or mercury, Y 5 =iron or iron and at least one of the elements vanadium, chromium and cerium, Y 6 =phosphorus, arsenic, boron and/or antimony, Y 7 =a rare earth metal, titanium, zirconium, niobium, tantalum, rhenium, ruthenium, rhodium, silver, gold, aluminum, gallium, indium, silicon, germanium, lead, thorium and/or uranium, a′=from 0.01 to 8, b′=from 0.1 to 30, c′=from 0 to 4, d′=from 0 to 20, e′=from >0 to 20, f′=from 0 to 6, g′=from 0 to 15, h′=from 8 to 16, x′, y′=numbers which are determined by the valency and frequency of the elements in B other than oxygen and p, q=numbers whose p/q ratio is from 0.1 to 10.