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Activation of Surface ReOx Sites on Al2O3 Catalysts for Olefin Metathesis

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journal contribution
posted on 06.11.2015, 00:00 by Soe Lwin, Yuanyuan Li, Anatoly I. Frenkel, Israel E. Wachs
The nature of activated surface ReOx sites and surface reaction intermediates for supported ReOx/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.