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

Effect of Support and Promoter on Activity and Selectivity of Gold Nanoparticles in Propanol Synthesis from CO₂, C₂H₄, and H₂

Direct propanol synthesis from CO₂, H₂, and C₂H₄ was investigated over TiO₂- and SiO₂-based catalysts doped with K and possessing Au nanoparticles (NPs). The catalysts were characterized by scanning transmission electron microscopy and temperature-programmed reduction of adsorbed CO₂. Mechanistic aspects of CO₂ and C₂H₄ interaction with the catalysts were elucidated by means of temporal analysis of products with microsecond time resolution. CO₂, which is activated on the support, is reduced to CO by hydrogen surface species formed from gas-phase H₂ on Au NPs. C₂H₄ adsorption also occurs on these sites. In comparison with TiO₂-based catalysts, the promoter in the K–Au/SiO₂ catalysts was found to increase CO₂ conversion and propanol production, whereas Au-related turnover frequency of C₂H₄ hydrogenation to C₂H₆ decreased with rising K loading. The latter reason was linked to the effect of the support on the ability of Au NPs for activation of C₂H₄ and H₂. The positive effect of K on CO₂ conversion was explained by partial dissolution of potassium in silica with formation of surface potassium silicate layer thus inhibiting formation of potassium carbonate, which binds CO₂ stronger and therefore hinders its reduction to CO