Production of acrylic acid

In a process for producing acrylic acid by a two-stage, vapor-phase catalytic oxidation which comprises subjecting a mixture gas of propylene, steam, and air to a first-stage reaction thereby to convert the propylene into, principally, acrolein and subjecting the gaseous product formed in the first-stage reaction to a second-stage reaction thereby to convert the acrolein into, principally, acrylic acid, in the presence of a multicomponent catalyst which comprises molybdenum and/or vanadium capable of providing a conversion of acrolein of 90% or more at a reaction temperature of about 220 to 340 C with a contact time of about 0.5 to 6 seconds, the improvement which comprises carrying out said oxidation under the conditions:

• A. that a composite oxide catalyst expressed by the formula
• Mo.sub.a Bi.sub.b Ni.sub.c Co.sub.d Fe.sub.e Na.sub.f Mn.sub.g B.sub.h K.sub.i Si.sub.j O.sub.x,
• where a through x represent atomic ratios of the respective elements, and, when a is 12, b is 4 through 7, c is 0.05 through 5, d is 0.05 through 5, e is 0.05 through 2, f is 0 through 1, g is 0 through 1, (f+g) is 0.01 through 1, h is 0.02 through 2, i is 0 through 1, j is 6 through 48, and x is a number satisfying the valence of an element other than oxygen,
• is used in the first-stage reaction;
• B. that (1) the composition of said mixture gas supplied to a bed of the catalyst in the first-stage reaction is characterized by an oxygen/propylene mol ratio of (1.1 to 2.0)/1, a propylene concentration of 7 to 13 percent, and a steam concentration of 2 to 30 percent, the temperature of the mixture gas prior to its introduction into the catalyst bed being 260.degree. C. or lower, and (2) the reaction conditions on the first-stage catalyst bed are a reaction temperature of 260.degree. to 370.degree. C. and a contact time of 1 to 8 seconds;
• and
• C. that a second mixture gas of the gaseous product formed in the first-stage reaction and a gas resulting from addition of air or oxygen gas to a waste gas separated from gaseous product formed in the second-stage reaction is supplied to a bed of the catalyst in the second-stage reaction, (1) the oxygen content of the second mixture gas being such that the mol ratio X/Y, wherein X is the sum of the quantity of oxygen in the mixture gas subjected to the first-stage reaction and the quantity of oxygen thus added to the waste gas, and Y is the quantity of propylene in the mixture gas subjected to the first-stage reaction, will be (1.6 to 2.8)/1, and (2) the temperature of the second mixture gas prior to its introduction into the second-stage catalyst bed being 280.degree. C. or lower, said waste gas being a portion of a waste gas which results from cooling of the gaseous product formed in the second-stage reaction and removal therefrom of a greater part of the acrylic acid therein by condensation.