Process and catalyst for preparing unsaturated carboxylic acid

Claim 1

In a process for preparing acrylic acid or methacrylic acid by the vapor-phase catalytic oxidation of the corresponding unsaturated aldehyde with molecular oxygen at a temperature of from 240° C. - 450° C., the improvement comprising: reacting said unsaturated aldehyde over a catalyst consisting essentially of molybdenum, phosphorus, magnesium, element Q, element R, element T and oxygen represented by the formula: Mo.sub.12 P.sub.a Mg.sub.b Q.sub.b.sub.' R.sub.c T.sub.d O.sub.e wherein Mo is molybdenum, P is phosphorus, Mg is magnesium, O is oxygen, Q is at least one metal selected from the group consisting of calcium, strontium and barium, R is at least one metal selected from the group consisting of potassium, rubidium, cesium and thallium, T is at least one metal selected from the group consisting of vanadium, tungsten and nickel, a, b, b', c, d and e are atomic ratios of the respective elements, wherein a = 0.5-6, b=0.2-6, b + b' = 0.2-6, b' ranges from 0 to a value determined by the magnesium content of the catalyst and the limits of b + b', c = 0.2-6, d = 0-6, and e is a value dependent upon the oxidation state of the catalyst.