A process for preparing methacrylic acid by vapor phase oxidation of methacrolein

a. A process for preparing methacrylic acid by catalytic vapor phase oxidation of methacrolein with molecular oxygen or a gas containing molecular oxygen. b. This oxidation reaction is carried out in the presence of a catalyst of the formula: Moa Pb Asc Cud Cre Xf O g wherein X represents at least one element selected from Ce, Nd, W, and Fe; a, b, c, d, e, f and g respectively represent atomic ratios and a = 12, b = 0.5 to 3.0, c = 0.01 to 1.2, d = 0.01 to 2 , 0, e = 0.05 to 2.0, f = 0 to 1.0; and g is given by the valences of the other components and is usually between 37 and 58, which catalyst is prepared with or without reduction by using a dibasic carboxylic acid, an oxycarboxylic acid, mannitol or pyrogallol. c. Manufacture of methacrylic acid