Molecular Design of Supported MoO_xCatalysts with Surface TaO_xPromotion for Olefin Metathesis

Abstract

A series of supported 3% MoO x catalysts were synthesized by incipient-wetness impregnation of a 5–15% TaO x surface-modified γ-Al2O3 support. The catalysts were characterized by in situ spectroscopies (diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), Raman, UV–vis, X-ray absorption spectroscopy (XAS)) and multiple chemical probes (C2H4/C4H8 titration, C3H6-TPSR, steady-state propylene metathesis, NH3-IR adsorption). The supported tantalum oxide phase was present as surface TaO x sites on the γ-Al2O3 support that capped the Al2O3 surface hydroxyls. The change in available surface hydroxyls caused the subsequent anchoring of MoO x species to occur at different surface hydroxyls. This shifted the anchoring of MoO x species from basic (Al-OH) to neutral (Al2-OH) to more acidic (Al3-OH) surface hydroxyls as well as perturbation of the remaining alumina surface hydroxyls by the surface TaO x sites. The TaO x surface-modified γ-Al2O3 support increased the number of activated surface MoO x sites (Ns) by ∼6× and the turnover frequency (TOF) by ∼10×, resulting in an increased activity of ∼60×. It was found that the specific anchoring surface hydroxyls rather than the extent of oligomerization of the surface MoO x sites control the number of activated MoO x sites and TOF for propylene metathesis. No relationships between the nature of the surface Lewis/Brønsted acid sites and Ns and TOF were found to be present.

Introducing a Novel Method for Probing Accessibility, Local Environment, and Spatial Distribution of Oxidative Sites on Solid Catalysts Using Trimethylphosphine

“…56 However, most recently Zhang et al ascribed this contribution to oligomeric Mo species. 57 With regard to the SBA-15 supported catalysts, a similar species distribution becomes visible (see Table S2) with isolated Mo centers being the main component with around 50% (band at 5.5 eV). However, when the Mo content is increased from 0.1 to 2 wt %, a growth of Mo particles can be observed as indicated by the higher contribution of the band centered at 3.7 eV (increase from 12 to 19%), which agrees in trend with the increase of the average size of MoO x detected in the STEM micrographs.…”

“…The origin of the species centered at 4.6 eV was somewhat unclear as different studies made different assignments . However, most recently Zhang et al ascribed this contribution to oligomeric Mo species . With regard to the SBA-15 supported catalysts, a similar species distribution becomes visible (see Table S2) with isolated Mo centers being the main component with around 50% (band at 5.5 eV).…”

Olefin metathesis over supported MoOx catalysts: influence of the oxide support

“…3). 56 and a general conclusion about the influence of surface Lewis/Brønsted acidity on olefin metathesis could not be reached from the literature findings.…”

“…The surface MoO x sites anchored at the isolated hydroxyls could only be activated at high temperatures, while the surface MoO x sites anchored at the bridged or tri-coordinated hydroxyls permit activation at low temperatures. 10,56 Besides isolated surface hydroxyls, the Al 2 O 3 , TiO 2 and ZrO 2 contain either bridged (Al 2 O 3 , TiO 2 ) or tri-coordinated surface hydroxyls (Al 2 O 3 , ZrO 2 ) serving as the anchoring sites for MoO x . What is different between them is the different support cations with Al 2 O 3 having a higher electronegativity than the TiO 2 and ZrO 2 cations.…”

“…Furthermore, the presence of stable surface acetone and acetate surface intermediates further retards olefin metathesis for the more redox MoO x /TiO 2 and MoO x /ZrO 2 catalysts. The influence of surface Lewis/Brønsted acidity on propylene metathesis has recently been reviewed in detail 56 and a general conclusion about the influence of surface Lewis/Brønsted acidity on olefin metathesis could not be reached from the literature findings.…”