Method for preparing methacrylic acid

A method for preparing methacrylic acid by vapor phase oxidation of isobutane with molecular oxygen or a molecular oxygen-containing gas in the presence of a catalyst, characterized in that said catalyst comprises:

• (i) a sparingly water-soluble salt of a heteropolyacid, and (ii) a composite oxide containing phosphorus, molybdenum and vanadium, and that said catalyst has been prepared by a method which comprises mixing an aqueous solution of a heteropolyacid with an aqueous solution containing at least one element selected from potassium, rubidium, cesium and thallium to prepare a slurry containing a resulting sparingly water-soluble salt of the heteropolyacid, adding to this slurry an aqueous solution separately prepared by dissolving in water raw materials containing phosphorus, molybdenum and vanadium and, if necessary, optional components, concentrating the resulting slurry under heating and stirring to dryness, drying the resulting solid, and calcinating the solid in an oxygen-containing gas.