Patent ID: 7795470

Claim:
A method for the heterogeneously catalyzed partial direct oxidation of n-propane to acrylic acid wherein the n-propane is present in a reaction gas inlet mixture comprising from 100 molppb to 1 mol% of cyclopropane, based on the molar amount of n-propane present in the reaction gas inlet mixture, said method comprising: feeding to a reaction stage the reaction gas inlet mixture which further comprises molecular oxygen and at least one inert diluent gas, directly partially oxidizing the n-propane present in the reaction gas inlet mixture to acrylic acid in the reaction stage by passing the reaction gas inlet mixture at elevated temperature through a catalyst bed comprising an oxidizing catalyst present in the solid state of aggregation, the conversion of n-propane during a single pass through the catalyst bed being ≧10 mol % and the selectivity of the acrylic acid formation, based on converted n-propane, being ≧40 mol %, feeding the reaction gas mixture, as product gas mixture comprising acrylic acid, out of the reaction stage into a first separation zone, wherein acrylic acid present in the product gas mixture of the reaction stage is converted into the liquid phase in the first separation zone and remaining gaseous residual product gas mixture comprising n-propane and depleted in acrylic acid is removed from the first separation zone and wherein at least a portion of the n-propane present in the residual product gas mixture is recycled to the reaction stage and acrylic acid is separated from the liquid phase in a second separation zone by using at least one thermal separation method, wherein the reaction gas inlet mixture comprises, based on its total volume, from 2 to 50% by volume of n-propane, from 0.1 to 60% by volume of CO 2 , from 1 to 50% by volume of O 2 , from 1 to 50% by volume of H 2 O and, from 100 molppb to 1 mol % of cyclopropane, based on the molar amount of n-propane present in the reaction gas inlet mixture; and the at least one thermal separation method in the second separation zone comprises at least one isolation of acrylic acid by crystallization.