Surface chemistry represents one of the most central aspects of chemistry. It is crucial for many different areas, for example, catalysis when using surface-bound metal complexes. These immobilized catalysts are of enormous interest because they can be recovered easily by settling from reaction mixtures and reused, while remaining as active and selective as their homogeneous analogs. Immobilized catalysts are typically tethered to an insoluble solid support like silica via bifunctional phosphine linkers such as (EtO)₃Si(CH₂)₂PPh₂ that incorporate ethoxysilane groups for their anchoring on the surface and phosphine groups to coordinate the metal.
The Bluemel group has obtained, characterized, and studied many superior immobilized catalysts with unprecedented lifetimes and activities. Based on a deeper understanding of fundamental processes taking place at the surface, numerous crucial improvements could already be implemented. However, many applications of immobilized catalysts are still hampered by their decomposition, leaching, agglomeration, and uncontrolled nanoparticle formation, all of which change their activities and selectivities. In general, the success of an immobilized catalyst is crucially dependent on the linker and its attachment to the oxide support. In particular, a well-defined reaction of the ethoxysilyl groups with the surface of diverse oxide supports is indispensable.
In this presentation, an improved, straightforward method for covalently binding phosphine linkers to silica via ethoxysilane groups will be demonstrated. This method leads to well-defined sub-monolayers of the phosphines on silica surfaces without cross-linking of the ethoxysilyl groups, which typically entails clogged pores and metal agglomeration during catalysis, thus leading to less active and selective catalysts. The novel immobilization method has been supported by multinuclear CP/MAS solid-state NMR spectroscopy of the dry materials, as well as suspension NMR and HRMAS NMR of slurries.

Ehsan Shakeri

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