Surface ReO_x sites on Al₂O₃ and their molecular structure-reactivity relationships for olefin metathesis

Supported ReO_x/Al₂O₃ catalysts were investigated for propylene metathesis as a function of surface rhenia loading and extensively characterized with in situ UV-vis, Raman, IR, XANES/EXAFS, and isotopic ¹⁸O-¹⁶O exchange studies.The experimental studies were complemented with DFT calculations using realistic models of the alumina surface.The surface ReO_x sites were found to be isolated surface dioxo (O=)₂ReO₂ species, which represent the most stable surface rhenia structures on alumina as shown by DFT.Two distinct surface ReO₄ species, however, were found to be present and only slightly differ in their bridging Re-O-Al bond lengths brought about by anchoring at different sites of the Al₂O₃ support.The deformed surface ReO₄-I species preferentially anchor at more basic μ₁ Al_IV and μ₁ Al_VI sites and are difficult to activate for propylene metathesis.The surface ReO₄-II species are formed at more acidic μ₂ Al_VI and μ₃ Al_VI sites and are the catalytic active sites for propylene metathesis.The surface ReO₄-II sites were readily activated by propylene while the deformed surface ReO₄-I sites were almost not affected by propylene, with only a few sites being activated.The steady-state propylene metathesis reaction rates are much higher for the surface ReO₄-II sites than the deformed surface ReO₄-I sites.The formation of the less reactive deformed surface ReO₄-I species could be blocked by occupation of the μ₁ Al_IV sites with sacrificial surface TaO_x species that resulted in catalysts exclusively containing the more active surface ReO₄-II sites on alumina.This is the first study to demonstrate that the surface ReO₄-II sites are the precursors for the catalytic active sites for propylene metathesis by supported ReO₄/Al₂O₃ catalysts and to molecularly design olefin metathesis catalysts that exclusively contain isolated surface ReO₄-II sites.