Process for producing methacrylic acid

A process for producing methacrylic acid, which comprises

• filling a plurality of catalysts of different activities into a plurality of reaction zones which have been formed in each of the reaction tubes of a fixed bed multi-tubular reactor by dividing said reaction tube into two or more portions in the direction of tubular axis, so that the activity of catalyst is higher as the reaction zone is closer to the outlet of the reaction tube, and
• introducing into the reaction zones containing the catalysts at least one compound selected from methacrolein, isobutyl aldehyde and isobutyric acid to oxidize the at least one compound with molecular oxygen or a molecular oxygen-containing gas, characterized in that said catalyst are compound oxides represented by the following general formula (I)
• Mo.sub.a P.sub.b A.sub.c B.sub.d C.sub.e O.sub.x (I)
• wherein Mo represents molybdenum;
• P represents phosphorus;
• A represents at least one element selected from arsenic, antimony, germanium, bismuth, zirconium, selenium, cerium, copper, iron, chromium, nickel, manganese, cobalt, tin, silver, zinc, palladium, rhodium and tellurium;
• B represents at least one element selected from vanadium, tungsten, and niobium;
• C represents at least one element selected from alkali metals, alkaline earth metals and thallium;
• O represents oxygen;
• a, b, c, d, e and x represent the atom number of Mo, P, A, B, C and O, respectively;
• when a is 12, b is 0.5-4, c is 0.001-5, d is 0.001-4 and e is 0.001-4;
• and x is a value determined by the oxidation state of each element, and in that said process comprises
• (i) preparing a plurality of catalysts of different activities by changing the types and/or amounts of the A group elements in the general formula (I) and then filling the catalysts into the reaction zones of each reaction tube so that the activity of catalyst is higher as the reaction zone is closer to the outlet of the reaction tube,
• (ii) preparing a plurality of catalysts of different activities by changing the types and/or amounts of the B group elements in the general formula (I) and then filling the catalysts into the reaction zones of each reaction tube so that the activity of catalyst is higher as the reaction zone is closer to the outlet of the reaction tube, or
• (iii) preparing a plurality of catalysts of different activities by changing the types and/or amounts of the elements of at least two groups selected from the A group, the B group and the C group in the general formula (I) and then filling the catalysts into the reaction zones of each reaction tube so that the activity of catalyst is higher as the reaction zone is closer to the outlet of the reaction tube.