Investigation of the Enhancing Effect of Solid Cocatalysts on Propene Formation in Ethene/trans-2-Butene Metathesis over MoOx/SiO2–Al2O3

The metathesis of ethene and 2-butenes to propene over WO x /SiO 2 is an important industrial process and has been intensely studied over alternative catalysts with supported MoO x species. Such studies have, however, not analyzed the effect of cocatalysts on propene production. Here, we utilized CaO, Al 2 O 3, or SiO 2 –Al 2 O 3 located upstream to MoO x /SiO 2 –Al 2 O 3 (0–100 wt % SiO 2 ) 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 x species, with CaO having resulted in the highest increase. The strength of the effect was established to depend on the kind of MoO x species and support material. From a mechanistic viewpoint, prebeds generate a gas-phase promoter from trans -2-butene but not from ethene, which further reacts with MoO x -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.