Understanding the Lewis Acidity of Co(II) Sites on a Silica Surface.

Heterogeneous catalysts consisting of isolated transition-metal sites dispersed on the surface of metal oxide supports are commonly used in the chemical industry. Often their reactivity relies on the Lewis acidity of the active sites on the surface of the catalyst. A recent report from our group showed that silica-supported Co(II) sites, prepared via surface organometallic chemistry, are active in both alkene hydrogenation and alkane dehydrogenation, possibly linked to the Lewis acidity of the Co(II) sites. Here we use molecular probes and analogues to both qualitatively and quantitatively model the Lewis acidity of the surface sites. Some sites do not bind probe molecules like carbon monoxide, tetrahydrofuran, and olefins, while others exhibit a continuum of Lewis acidities. This is consistent with variations in the coordination environment of Co. These results suggest that only the most Lewis acidic sites are involved in dehydrogenation and hydrogenation, consistent with catalyst poisoning studies.