Room-Temperature Metathesis of Ethylene with 2-Butene to Propene Over MoOx-Based Catalysts: Mixed Oxides as Perspective Support Materials

Abstract

We investigated the effect of supports based on ZrO2, TiO2, Al2O3, and SiO2 on the rate of propene formation in the metathesis of ethylene with 2-butene at 50 °C over Mo-containing catalysts possessing highly dispersed MoOx. Large improvements in this rate were achieved when using supports composed of mixed oxides (ZrO2–SiO2, ZrO2–PO4, TiO2–SiO2; Al2O3–SiO2) rather than of individual oxides (ZrO2, TiO2, Al2O3, SiO2). Although previous literature studies dealing with the metathesis reaction over Al2O3- or SiO2-suppported catalysts at higher temperatures suggest the importance of redox or acidic properties of supported MoOx species for catalyst activity, we were not able to establish any general direct correlation in this regard. Contrarily, the rate of propene formation can be significantly enhanced when promoting supports with an oxide promoter. We suggest that the created support lattice defects may facilitate the transformation of MoOx to Mo carbenes under reaction conditions or improve the intrinsic activity of the latter. Graphic Abstract

Fundamentals and application potential of the synergy effect between ZnO and Mo/SiO2 for propene production in the metathesis of ethylene and trans-2-butene

“…The temperature of the maximal hydrogen consumption rate assigned to the reduction of supported Mo 6+ O x species is around 600 °C. 8 An NH 3 -TPD test was performed to investigate the acidic properties of Mo/ SiO 2 (Fig. S5 †).…”

“…4,5 Supported Mo-based catalysts attract a lot of attention nowadays because they show high activity below 200 °C. [6][7][8][9][10][11][12][13][14] MoO x /SiO 2 , MoO x /Al 2 O 3 , and MoO x /SiO 2 -Al 2 O 3 systems are considered as promising alternatives to the commercially applied WO x / SiO 2 . 3,8,15 The metathesis activity of MoO x /SiO 2 at low temperature can be significantly improved by treating the catalyst in methanol, 10 CO, 16 or hydrocarbons (methane, ethane, propane, ethene and propene) at high temperatures to create Mo-carbenes.…”

“…[6][7][8][9][10][11][12][13][14] MoO x /SiO 2 , MoO x /Al 2 O 3 , and MoO x /SiO 2 -Al 2 O 3 systems are considered as promising alternatives to the commercially applied WO x / SiO 2 . 3,8,15 The metathesis activity of MoO x /SiO 2 at low temperature can be significantly improved by treating the catalyst in methanol, 10 CO, 16 or hydrocarbons (methane, ethane, propane, ethene and propene) at high temperatures to create Mo-carbenes. 12,16,17 Similar to WO x /SiO 2 , the enhancing effect of Al 2 O 3 , AlSiO x or CaO cocatalysts was established by Goelden et al 5 for the above-mentioned Mo-containing catalysts in the metathesis of ethylene with 2-butene to propene above 150 °C.…”

Molecular Design of Supported MoOxCatalysts with Surface TaOxPromotion for Olefin Metathesis

“…10 No general relationships between olefin metathesis activity and strength/amount of Lewis/Brønsted sites could be established. 12 A general conclusion about the influence of surface acid sites on olefin metathesis cannot be made from the above literature findings since these studies are clearly not consistent with each other. Possible reasons for these different observations are the use of oxide supports from different sources and the presence of surface impurities.…”

“…Although heterogeneous supported MoO x catalysts are easily prepared, there are multiple surface MoO x structures due to the nonuniform nature of the surfaces of oxide supports, which requires the use of advanced molecular-level characterization techniques to determine the catalytic active sites. For example, three distinct surface MoO x structures are present on the Al 2 O 3 support (isolated di-oxo MoO 4 on basic Al-OH, oligomeric MoO 5/6 on neutral Al 2 -OH and acidic Al 3 -OH, and crystalline MoO 3 nanoparticles). , The catalytic activity of supported MoO x catalysts is significantly influenced by the selection of the oxide support. − The SiO 2 -Al 2 O 3 mixed oxide support is a highly effective support for promoting olefin metathesis in comparison to the one component Al 2 O 3 or SiO 2 support. Anchoring active sites at acidic surface hydroxyls of the mixed oxide support results in a greater number of activated sites .…”