Patent ID: 7012157

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 which comprises the at least one saturated hydrocarbon precursor compound, molecular oxygen and at least one inert gas and has a starting pressure P through a catalyst charge which is disposed in a reactor unit and whose catalysts are such that their active composition is at least one multimetal oxide M which contains the elements Mo and V, at least one of the elements Te, Sb and Bi, 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 which has reflections h, i and k whose peak locations are at the reflections (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 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 catalyst charge in its entirety consists of two catalyst beds I, II which comprise different catalysts and are spatially successive in the flow direction of the reaction gas mixture, the catalyst bed I being such that if the overall catalyst charge of the same reactor unit were to consist only of catalyst bed of the type I, the selectivity S I of (meth)acrylic acid formation with otherwise identical conditions of the gas phase partial oxidation would, depending on the conversion, be increased by increasing the reaction temperature, of the saturated hydrocarbon precursor compound based on single pass of the reaction gas mixture through the reactor unit, pass through a maximum value S I max , and the catalyst bed II being such that if the overall catalyst charge of the same reactor unit were to consist only of catalyst bed of the type II, the selectivity S II of (meth)acrylic acid formation with otherwise identical conditions of the gas phase partial oxidation would, depending on the conversion, be increased by increasing the reaction temperature, of the saturated hydrocarbon precursor compound based on single pass of the reaction gas mixture through the reactor unit, pass through a maximum value S II max , with the proviso that the value S I max is at a lower conversion of the saturated hydrocarbon precursor compound than the value S II max and, with increasing conversion of the saturated hydrocarbon precursor compound, S I <S II , whereas S I max >S II max .