Molecular nature of support effects in single-site heterogeneous catalysts: silica vs. alumina

Well-defined silica- and alumina-supported catalysts can display very different reactivity, which is often attributed to support effects. Advanced spectroscopic studies in combination with computational chemistry show that the origin of these differences is mainly molecular in nature. Silica partially dehydroxylated at 700 °C features mainly isolated silanols (SiOH) as surface functional groups, which can be exploited to generate well-defined and site isolated siloxy metal species. In contrast, alumina, a more complex support, exhibits both hydroxyl groups and highly reactive Al,O Lewis acid and base surface sites. Therefore, it is better understood as a multifunctional and adaptable ligand, which can lead to a variety of surface species, often more reactive (cationic) and/or stable than their known silica counterparts.