Patent ID: 8232425

Claim:
A process for heterogeneously catalyzed partial gas phase oxidation of propylene to acrylic acid, in which, in a first reaction zone, a starting reaction gas mixture 1 which comprises propylene, molecular oxygen and at least one inert diluent gas and comprises the molecular oxygen and the propylene in a molar O 2 :C 3 H 6 ratio of ≧1, in a first reaction stage at elevated temperature, is first conducted through at least one first catalyst bed whose catalysts have at least one multimetal oxide comprising Mo, Fe and Bi as the active composition, such that the propylene conversion in single pass through the catalyst bed is ≧90 mol % and the accompanying selectivity of acrolein formation and of acrylic acid by-product formation together is ≧80 mol %, the temperature of the product gas mixture 1 leaving the first reaction stage optionally is reduced by direct cooling or by indirect cooling or by direct and indirect cooling, and optionally a secondary gas in the form of molecular oxygen or inert gas or molecular oxygen and inert gas is added to product gas mixture 1, and then product gas mixture 1, as a starting reaction gas mixture 2 which comprises acrolein, molecular oxygen and at least one inert diluent gas and comprises the molecular oxygen and the acrolein in a molar O 2 :C 3 H 4 ratio of ≧0.5, in a second reaction stage at elevated temperature, is conducted through at least one second catalyst bed whose catalysts have at least one multimetal oxide comprising Mo and V as the active composition, such that the acrolein conversion in single pass through the catalyst bed is ≧90 mol % and the selectivity of acrylic acid formation assessed over both reaction stages, based on propylene converted, is ≧70 mol %, then the acrylic acid present in the product gas mixture 2 formed in the second reaction stage, in a first separation zone, is converted therefrom to the condensed phase and then, in a second separation zone, the acrylic acid is removed from the condensed phase by use of at least one thermal separation process, wherein starting reaction gas mixture 1, based on the molar amount of propylene present therein, comprises from 200 mol ppm up to 3 mol % of cyclopropane and the at least one thermal separation process in the second separation zone comprises at least one crystallizative removal of acrylic acid.