Molecular Design of Supported MoO _xCatalysts with Surface TaO _xPromotion for Olefin Metathesis

A series of supported 3% MoO_xcatalysts were synthesized by incipient-wetness impregnation of a 5-15% TaO_xsurface-modified γ-Al₂O₃support. 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/C₄H₈titration, C₃H₆-TPSR, steady-state propylene metathesis, NH₃-IR adsorption). The supported tantalum oxide phase was present as surface TaO_xsites on the γ-Al₂O₃support that capped the Al₂O₃surface hydroxyls. The change in available surface hydroxyls caused the subsequent anchoring of MoO_xspecies to occur at different surface hydroxyls. This shifted the anchoring of MoO_xspecies 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_xsites. The TaO_xsurface-modified γ-Al₂O₃support increased the number of activated surface MoO_xsites (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_xsites control the number of activated MoO_xsites 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.