Developing heterogeneous catalysis for homogeneous reactions may not only enable the recyclability of the catalyst but can also lead to different catalytic reactivity and selectivity.  A direct correlation between the metallic nanoclusters size, oxidation state and catalytic reactivity was previously demonstrated.  In this study, 40 metal atoms clusters were encapsulatd in a poly(amido-amine) dendrimer, loaded on mesoporous silica support and activated toward a variety of p-activation reactions in solution phase. The metallic nanoclusters were activated by the addition of a PhICl2 oxidizer to the solution, which dispersed the nanoparticles and enabled the formation of highly oxidized metallic ions.  Dendrimer encapsulated Pt and Pd catalysts showed higher catalytic rate toward electrophilic reactions than their homogeneous counterpart.  Moreover, high stereoselectivity was measured with dendrimer encapsulated Au nanocluster catalysts for cyclopropanation reactions.  Employing the heterogeneous Au catalysts in flow reactors enabled full control over product selectivity in secondary reactions.  By increasing the reactants' residence time, the selectivity was tuned from 100% formation of primary products to 95% selectivity toward the secondary product.  Leaching of metal ions to the solution phase was prevented by using toluene as a solvent, due to repulsion forces between the hydrophilic catalyst and the hydrophobic solvent.  The catalysts were in-situ characterized by X-ray Absorption Spectroscopy (XAS), which indicated the formation of highly oxidized metal ions as the catalytically active species.  The reversibility between the ionic and metallic state of the dendrimer encapsulated nanoparticles was verified by XAS techniques.  These results demonstrate the unique ways in which the properties of heterogeneous catalysts can be utilized for tuning the catalytic reactivity and selectivity or organic reactions.

Bridging heterogeneous and homogeneous catalysis using small (<40 atoms) metallic nanoparticles

247th National Meeting (2014)

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