Abstract: The metathesis of propene has been carried out on the MoO₃/SiO₂ catalyst modifiedwith rare earth oxides. In is found that most of the Ln₂O₃ can prolong the life of thecatalysts and shorten the "break-in" period. Among them Tb₄O₇ and Pr₆O₁₁ exhibiteda more marked effect. The stable duration of catalyst modified by Tb₄O₇ can be prolongedto approximately 1.6 times as long as that of the unmodified one. The "break-in" periodwas reduced to about 40% to 75%. From the results of the Tb₄O₇. MoO₃/SiO₂ catalyst, itcan be concluded that the role of Tb₄O₇ is in facilitating the formation of the activesurface site, i.e. the distorted square pyramidal Mo⁵⁺. The reaction of active sitc formationis in first order with respect to the precurser of the active species.

Key words: Rare earth oxides, Metathesis, MoO₃ catalyst