Spectroscopically distinct sites present in methyltrioxorhenium grafted onto silica-alumina, and their abilities to initiate olefin metathesis

Deposition of CH₃ReO₃ onto the surface of dehydrated, amorphous silica-alumina generates a highly active, supported catalyst for the metathesis of olefins. However, silica-alumina with a high (10 wt %) Re loading is no more active than silica-alumina with low (1 wt %)Joading, while CH ₃ReO₃ on silica is completely inactive. Catalysts prepared by grafting CH₃ReO₃ on silica-alumina contain two types of spectroscopically distinct sites. The more strongly bound sites are responsible for olefin metathesis activity and are formed preferentially at low Re loadings (≤1 wt %). They are created by two Lewis acid/base interactions: (1) the coordination of an oxo ligand to an Al center of the support and (2) interaction of one of the adjacent bridging oxygens (AlOSi) with the Re center. At higher Re loadings (1-10 wt%), CH₃ReO₃ also interacts with surface silanols by H-bonding. This gives rise to highly mobile sites, most of which can be observed by ¹³C solid-state NMR even without magic-angle spinning. Their formation can be prevented by capping the surface hydroxyl groups with hexamethyldisilazane prior to grafting CH ₃ReO₃, resulting in a metathesis catalyst that is more selective, more robust, and more efficient in terms of Re use.