Customized Nanoparticles

Catalysis with metal nanoparticles has been rendered more convenient and effective thanks to a new catalyst preparation method devised by a group in California. These dendrimer- and polymer-encapsulated metal nanoparticles can catalyze, with high efficiency, reactions that until now have been accessible exclusively via solution-phase chemistry.

The new work highlights a novel synthesis strategy for customizing the function of solid-phase catalysts that, compared with solution-phase catalysts, are more easily separated and recycled from solvents and dissolved products (Nat. Chem., DOI: 10.1038/nchem.468).

A central thrust in catalysis research focuses on combining advantages of solution-phase (homogeneous) and solid-phase (heterogeneous) catalysts. A common approach is to immobilize homogeneous catalysts on a solid support so they can be separated easily and inexpensively from reaction mixtures. Typically, such immobilized catalysts are limited to mediating the same reactions as their mobile counterparts.

Rather than following that strategy, chemists Cole A. Witham, F. Dean Toste, Gabor A. Somorjai, and coworkers at the University of California, Berkeley, have tailored the properties of platinum nanoparticles to drive reactions never before catalyzed by solids. Specifically, the team made the particles electrophilic by treating them with iodosobenzene dichloride. Then they encapsulated them in a polyamidoamine dendrimer or in polyvinylpyrrolidone and dispersed the composites on porous silica to render them thermally stable and to prevent them from aggregating.

Ring-closing test reactions show that in some cases the new catalysts are even more active than soluble platinum catalysts. For example, the encapsulated catalysts convert a phenylethynyl phenol to the corresponding benzofuran in roughly 100% yield—slightly higher than solution-phase platinum compounds.

David Milstein, a professor of chemistry at Weizmann Institute, in Rehovot, Israel, notes this “ground-breaking” strategy for tuning nanoparticle properties holds promise for discovering novel catalytic reactions beyond those offered by traditional heterogeneous systems or soluble metal complexes.