Controlled Synthesis of Subnano Pd Cluster Catalysts by Fine Tuning of Dendrimers

T. Mizugaki, T. Kibata, Z. Maeno, T. Mitsudome, K. Jitsukawa, K. Kaneda
Osaka University, JP

Keywords: dendrimer, subnanocluster, palladium, hydrogenation

Summary:

We recently succeeded in the fine control of the Pd cluster sizes in wide range using the fifth generation poly(propylene imine) (PPI) dendrimers. Preorganization of Pd ions within the PPI dendrimer with butanediamine core(G5-C4) and successive reduction lead to the formation of precisely defined Pd clusters of Pd4, Pd8, and Pd16, respectively, as determined by Pd K-edge XAFS analysis. Furthermore, the maximum sizes of the Pd clusters could be tuned by changing the core of the PPI dendrimers using ethylendiamine(G5-C2) and 1,6-hexanediamine(G5-C6) to give Pd12 and Pd18 clusters, respectively. These Pd clusters showed the size dependent catalytic activity in hydrogenation of olefins, where the TOF increased with increasing the cluster size. The keys of this success in the size control of the subnano Pd clusters are 1) complexation of Pd2+ ions with the internal amino groups, 2) sterically confined nanovoids within the fifth generation of PPI dendrimers, and 3) tunable molecular structures of the PPI dendrimers as nanocapsules by changing the core and the surface units.