Method for the heterogeneously catalyzed partial gas phase oxidation of propene into acrylic acid

A process for partially oxidizing propene to acrylic acid in the gas phase under heterogeneous catalysis by initially conducting a starting reaction gas mixture 1 comprising propene, molecular oxygen and at least one inert gas and containing the molecular oxygen and the propene in a molar O2:C3H6 ratio of ≧1 in a first reaction stage at elevated temperature over a first fixed catalyst bed

• whose shaped catalyst bodies are annular,
• whose active composition is at least one multimetal oxide I of the general formula
• Mo12X1aX2bX3cX4dX5eOn (I),
• where the variables are defined as follows:
• X1=at least one element from the group comprising Bi, Co and Ni,
• X2=at least one of the two elements W and Nb,
• X3=at least one of the two elements Fe and Cr,
• X4=at least one element from the group comprising K, Cs and Sr,
• X5=at least one element from the group comprising Si, Al and Zr,
• a=from 5 to 10,
• b=from 0 to 4,
• c=from 2 to 5,
• d=from 0.02 to 0.15,
• e=from 0.5 to 4, and
• n=a number which is determined by the valency and frequency of the elements in I other than oxygen,
• whose volume-specific activity in the flow direction of the reaction gas mixture over the first fixed catalyst bed is either constant or increases at least once, and
• whose active composition does not change over the first fixed catalyst bed,
• in such a way that the propene conversion on single pass is ≧90 mol % and the accompanying selectivity of acrolein formation and also of acrylic acid by-production together are ≧90 mol %, the temperature of the product gas mixture leaving the first reaction stage is optionally reduced by direct cooling, or by indirect cooling, or by direct and indirect cooling, secondary gas is optionally added to the product gas mixture in the form of molecular oxygen, or inert gas, or molecular oxygen and inert gas, and the product gas mixture is afterwards conducted as the starting reaction gas mixture 2 comprising acrolein, molecular oxygen and at least one inert gas and containing the molecular oxygen and acrolein in a molar O2:C3H4O ratio of ≧0.5 in a second reaction stage at elevated temperature over a second fixed catalyst bed,
• whose shaped catalyst bodies are annular,
• whose active composition comprises at least one multimetal oxide II of the general formula
• Mo12VfX6gX7hOm (II)
• where the variables are defined as follows:
• X6=at least one of the two elements W and Nb,
• X7=at least one of the elements from the group comprising Sb, Cu, Ni and Fe,
• f=from 1 to 5,
• g=from 1 to 2,
• h=from 1 to 4, and
• m=a number which is determined by the valency and frequency of the elements in II other than oxygen,
• whose volume-specific activity in the flow direction of the reaction gas mixture over the second fixed catalyst bed increases at least once, and
• whose active composition does not change over the second fixed catalyst bed,
• in such a way that the acrolein conversion on single pass is ≧90 mol % and the selectivity SAA of the acrylic acid formation assessed over both reaction stages, based on converted propene, is ≧80 mol %,
• wherein the starting gas reaction mixture 1 is composed of
• from 6 to 15% by volume of propene,
• from 4 to 20% by volume of water,
• from ≧0 to 10% by volume of constituents other than propene, water, oxygen and nitrogen,
• sufficient molecular oxygen that the molar ratio of molecular oxygen present to molecular propene present is from 1.5 to 2.5,
• and the remainder up to 100% by volume of the total amount of molecular nitrogen.