Molecular Design of Supported MoO<i><sub>x</sub></i>Catalysts with Surface TaO<i><sub>x</sub></i>Promotion for Olefin Metathesis
Bin Zhang, Shuting Xiang, Anatoly I. Frenkel, Israel E. Wachs
Abstract
A series of supported 3% MoOx catalysts were synthesized by incipient-wetness impregnation of a 5–15% TaOx 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 TaOx sites on the γ-Al2O3 support that capped the Al2O3 surface hydroxyls. The change in available surface hydroxyls caused the subsequent anchoring of MoOx species to occur at different surface hydroxyls. This shifted the anchoring of MoOx 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 TaOx sites. The TaOx surface-modified γ-Al2O3 support increased the number of activated surface MoOx 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 MoOx sites control the number of activated MoOx 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.