Surface ReOx Sites on Al2O3 and Their Molecular Structure–Reactivity Relationships for Olefin Metathesis

Abstract

Supported ReOx/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 ReOx 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 AlIV and μ1 AlVI sites and are difficult to activate for propylene metathesis. The surface ReO4–II species are formed at more acidic μ2 AlVI and μ3 AlVI 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 AlIV sites with sacrificial surface TaOx 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.