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 WOx/SiO2 is an important industrial process and has been intensely studied over alternative catalysts with supported MoO 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 MoOx/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 Mo Ox species, with CaO having resulted in the highest increase. The strength of the effect was established to depend on the kind of Mo Ox 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 MoOx-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 Mod to MoOlv. 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.