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Caged Rhodium Makes A Better Catalyst

Catalysis: By encapsulating rhodium nanoparticles in an organic cage, chemists transform a heterogeneous catalyst into a homogenous one

by Bethany Halford
June 8, 2015 | A version of this story appeared in Volume 93, Issue 23

Credit: Qiang Xu

Rhodium nanoparticles (purple) are solubilized by placing them in organic cages (C = gray, pink; N = blue).
Rh nanoparticles (purple) in soluble organic cages (C = gray, pink; N = blue).
Credit: Qiang Xu

Rhodium nanoparticles (purple) are solubilized by placing them in organic cages (C = gray, pink; N = blue).

Metal nanoparticles are useful catalysts in many reactions, but keeping them stable in solution over many catalytic cycles has proven challenging. Chemists have taken a number of approaches to the problem, such as using dendrimer supports and metal-organic frameworks to protect the nanoparticles. Using a different tack, researchers in Japan have shown that building a soluble organic cage around rhodium nanoparticles can make the heterogeneous catalysts slip easily into solution, effectively converting them into a homogeneous catalyst (J. Am. Chem. Soc. 2015, DOI: 10.1021/jacs.5b04029). Qiang Xu and his colleagues at Japan’s National Institute of Advanced Industrial Science & Technology created the caged nanoparticles by reducing rhodium acetate in the presence of a porous cyclo - imine cage molecule. The resulting catalyst consists of rhodium nanoparticles about 1 nm across trapped within a cage that has reactant-permeable windows. The caged catalyst is stable and has proved to be superior to all other catalysts in the meth­anolysis of ammonia borane—a reaction that breaks down that hydrogen storage material. And the catalyst can be recovered simply by drying and washing it.


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