(508c) Supported Dimeric Oxides from Organometallic Routes

Supported oxide catalysts can exhibit complex relationships between reactivity and surface coverage (loading) due to the prevalence of structure-sensitive catalyst mechanisms. In general, structures evolve from isolated cations at low surface coverage to crystallites at higher loadings. Methods exist for creating isolated sites via ligand-protected grafting or the creation of crystallite domains via nanoparticle deposition. Intermediate oligomeric structures may also be formed, but the catalytic relevance and even existence of these bridged oligomeric oxides are not always obvious, in part because of limitations in site titration and spectroscopic handles specific to such structures. Oligomeric structures could also be expected to co-exist with isolated cations or small crystallites at intermediate loadings, but this will also be case-specific.

Rather than teasing out the existence of such structures from energetically diverse populations on surfaces, we will present preliminary results on our route to the deliberate construction of dimers of catalytic oxides on high surface-area supports. We have synthesized mu-oxo bridged metal dimers stabilized by multidentate amine ligands. Suitable ligand modification creates covalent tethers for grafting to surfaces, but the charge of these complexes also allows for exchange and impregnation techniques. Initial syntheses have focused on known manganese triazacyclononane complexes grafted onto silica and other common oxide supports.

We will present physical characterization (TGA and nitrogen physisorption) and spectroscopic characterization (solid state NMR and UV-visible) to demonstrate that the metal oxide structures synthesized in solution translate into supported oxides of defined structure. It is expected that deliberate synthesis of supported, oligomeric catalytic oxides will lead to improved structure-function relationships and insight into how to target (or avoid) these structures in other catalyst syntheses.