Active and stable ceria-zirconia supported molybdenum oxide catalysts for cyclooctene epoxidation: Effect of the preparation procedure

Ce0.75Zr0.25O2 supports were prepared by precipitation in the presence of either ammonia or urea as precipitating agents. Mo(VI) oxide was dispersed onto the supports by either co-precipitation or wet impregnation, using ammonium eptamolybdate tetrahydrate as the Mo precursor. In all the cases, the nominal Mo content was 6.6 wt.%, selected to be under the value of the reported monolayer capacity of molybdenum oxide. The catalysts prepared by using ammonia as precipitating agent showed a superior catalytic activity in the cyclooctene epoxidation, with cumene hydroperoxide as oxidizing agent. The stability of the catalysts was confirmed by multiple runs and the absence of leaching was verified according to the Sheldon method. Physico-chemical characterization of the catalysts revealed that urea promotes the formation of hydrated cerium(III) oxycarbonate, which likely hampers a homogeneous distribution of active MoOx species that, conversely, are fairly stabilized on the support when ammonia is used as precipitating agent.