Activation of Surface ReO_x Sites on Al₂O₃ Catalysts for Olefin Metathesis

The nature of activated surface ReO_x sites and surface reaction intermediates for supported ReO_x/Al₂O₃ catalysts during propylene self-metathesis were systematically investigated for the first time using in situ spectroscopy (Raman, UV-vis, XAS (XANES/EXAFS) and IR). In situ Raman spectroscopy reveals that olefins selectively interact with the surface dioxo ReO₄ sites anchored at acidic alumina hydroxyls. In situ UV-vis indicates that surface Re⁵⁺ and some Re⁶⁺ sites form, and in situ XAS indicates a reduction in the number of Re=O bond character in the propylene self-metathesis reaction environment, especially as the temperature is increased. The appearance of oxygenated products during propylene activation supports the conclusion that catalyst activation involves removal of oxygen from the surface rhenia sites (pseudo-Wittig mechanism). Isotopic CD₃CD=CD₂ → CH₃CH=CH₂ switch experiments demonstrate the presence of surface Re=CD₂ and Re=CDCD₃ reaction intermediates, with the surface Re=CD₂ species being the most abundant reaction intermediate. In situ IR spectroscopy indicates the presence of significant surface propylene π complexes on alumina and rhenia sites of the catalyst, which complicates analysis of surface reaction intermediates during propylene self-metathesis.