Science 327, 68–72 (2010)

When a catalyst is in the same phase as the reacting molecules, separating it from the products can be a considerable challenge. Mixtures of two immiscible solvents, such as water and a hydrophobic organic liquid, can be used to help recover catalysts by exploiting the differing solubilities of the reactants, products and catalyst in the two liquid phases. As well as simplifying separation, such emulsions can also allow products formed in one solvent to transfer to the other so as to avoid further reaction. Surfactant molecules can be used in these mixtures to increase the interfacial surface area and aid the transfer of molecules between the two phases. However, these surfactants can also be difficult to separate from the final product mixtures. Daniel Resasco and colleagues at the University of Oklahoma have now developed solid nanoparticles that can simultaneously stabilize water–oil emulsions and catalyse significant organic reactions.

The researchers synthesized inorganic oxide nanoparticles that were decorated with carbon nanotubes. By combining hydrophobic nanotubes and hydrophilic oxides, the solid particles can seek out the interface between organic and aqueous phases, and by depositing palladium clusters onto the hybrid material, the nanoparticles can also catalyse reactions in both liquid phases. As the catalyst is a solid, it can be recovered simply by filtration.

The Oklahoma team use their nanoparticles to catalyse reactions important in the upgrading of biomass to fuels, in which small organic molecules are added together and oxygenated groups are removed.