Investigation of the Enhancing Effect of Solid Cocatalysts on Propene Formation in Ethene/<i>trans</i>-2-Butene Metathesis over MoO_<i>x</i>/SiO₂–Al₂O₃

The metathesis of ethene and 2-butenes to propene over WO_<i>x</i>/SiO₂ is an important industrial process and has been intensely studied over alternative catalysts with supported MoO_<i>x</i> species. Such studies have, however, not analyzed the effect of cocatalysts on propene production. Here, we utilized CaO, Al₂O₃, or SiO₂–Al₂O₃ located upstream to MoO_<i>x</i>/SiO₂–Al₂O₃ (0–100 wt % SiO₂) to probe their influence on the rate of propene formation and on stream stability. All three prebed materials did not produce propene but significantly enhanced the metathesis activity of supported MoO_<i>x</i> species, with CaO having resulted in the highest increase. The strength of the effect was established to depend on the kind of MoO_<i>x</i> species and support material. From a mechanistic viewpoint, prebeds generate a gas-phase promoter from <i>trans</i>-2-butene but not from ethene, which further reacts with MoO_<i>x</i>-containing catalysts and increases their activity. The promoter is responsible for the stabilization and/or formation of catalytically active Mo–carbene species but not for the reduction of MoO^VI to MoO^IV. The obtained results provide new insights for the design of metathesis catalysts and open the possibility for tuning their activity and on-stream stability through the operation of prebed materials and metathesis catalysts in individual reactors at different temperatures