Promoted catalysts for the oxidation of olefins

The process for the manufacture of unsaturated aldehydes which comprises the step of contacting in the vapor phase, at a temperature in the range of 500.degree. to 1100.degree.F, a mixture of oxygen and an olefin selected from the group consisting of propylene and isobutylene, said mixture having a molar ratio of oxygen to olefin of from about 0.5:1 to about 5:1 with a promoted catalyst composition consisting essentially of a base catalyst, a support and a promoter component, said base catalyst consisting essentially of the oxides of antimony and uranium, the Sb:U atomic ratio being within the range of about 1:50 to 99:1, said support being silica and said promoter component consisting essentially of an oxide of at least one element selected from the group consisting of iron, magnesium, bismuth, copper, zinc, arsenic, and cesium.