Identifying the Catalytic Active Site for Propylene Metathesis by Supported ReOx Catalysts

A series of supported ReOx catalysts were investigated that allowed identifying the unique surface anchoring sites on oxide supports responsible for activating the surface ReO4 sites for propylene metathesis (the catalytic active site). The catalysts were synthesized by incipient-wetness impregnation of aqueous HReO4 onto the oxide supports (Al2O3, ZrO2, TiO2, SiO2, and CeO2), characterized under dehydrated and propylene metathesis reaction conditions with in situ spectroscopy (Raman, DRIFTS, UV–vis and NAP-XPS), and chemically probed (CH3CH═CH2–TPSR, CH2═CH2/CH3CH═CHCH3 titration and steady-state self-metathesis of propylene to ethylene and 2-butene). The initially calcined supported rhenia species anchor as isolated surface Re7+O4 sites on the oxide supports by reacting with the surface hydroxyls (terminal S–OH, bridged S–OH–S, and tricoordinated S3–OH) of the oxide supports. The specific oxide support was found to control the number of activated sites (Al2O3 ≫ ZrO2 > CeO2 > TiO2 > SiO2) and propylene metathesis activity (Al2O3 ≫ ZrO2 ≫ TiO2 ∼ CeO2 ∼ SiO2), revealing that the oxide support action is a potent ligand for the surface ReOx sites. The activation and specific activity of the surface ReOx sites depend on several factors (nature of surface hydroxyls (S3–OH > S–OH–S > S–OH), coordination of the oxide support surface cation (ZrO7, AlO6, CeO4) and electronegativity of the oxide support cation (SiO2 > Al2O3 > TiO2 > ZrO2 > CeO2). No relationships exist between olefin metathesis activity and acid strength of surface Lewis and Brønsted sites. Prior studies primarily focused on supported ReOx/Al2O3, and the lack of examination of non-Al2O3 supported rhenia catalysts precluded comparison between efficient and inefficient olefin metathesis catalysts, which prevented identifying the catalytic active site for olefin metathesis by supported ReOx catalysts.