Catalysis of Organic..

316 Dendrimer TemplatesWe are developing a new route for preparing model catalysts that usesPolyamidoamine (PAMAM) Dendrimer Encapsulated Nanoparticles (DENs) as NPstemplates and stabilizers. PAMAM dendrimers, which bind transition metal cationsin defined stoichiometries, can be used to template mono- and bimetallic NPs insolution. DENs have been used as homogeneous catalysts for a variety of organicreactions, including hydrogenations (particularly size-selective hydrogenations), andSuzuki, Heck, and Stille coupling reactions.(5) Bimetallic DENs are also activehydrogenation catalysts in solution, and their catalytic activity can be tuned bycontrolling NP composition.(5)The potential to ultimately control NP properties makes DENs extremelyattractive as precursors to supported catalysts. The possibility of controlling particlesize and composition makes DENs uniquely suited to exploring the relativeimportance of these effects on catalytic reactions. Because the nanoparticles areprepared ex situ and can be deposited onto almost any substrate or support, DENsoffer the opportunity to examine tailored NPs using materials comparable to thoseemployed as industrial catalysts. However, before DENs can be utilized asheterogeneous catalyst precursors, appropriate methods must be developed toremove the organic template. If activation conditions are too harsh, particleagglomeration may suppress the potential advantages of the dendrimer method. Ifactivation conditions are too mild, incomplete removal of the dendrimer may leaveorganic residues on the particles and poison the catalyst.BackgroundPrevious work has focused largely on dendrimer removal from Pt-DENs supportedon silica. Our studies have shown the amide bonds that comprise the dendrimerbackbone are relatively unstable (they begin decomposing at mildly elevatedtemperatures, ca. 100 °C) and that the Pt nanoparticles help to catalyzed thedendrimer decomposition.(3,6,7) However, extended higher temperature oxidationand/or reduction treatments (several hours at 300 °C) are required to completelyremove organic material from Pt DENs. For Pt/SiO 2 catalysts, dendrimer oxidationappears to lead to the formation of surface carboxylates, which partially poisoncatalytic activity.(6,8) A recent surface science study by Chen and coworkerssupports these general findings and provides convincing evidence that Pt plays animportant role in catalyzing dendrimer decomposition.(9)Crooks and coworkers, who studied Pd and Au DENs immobilized in sol-geltitania, similarly reported the onset of dendrimer mass loss at relatively lowtemperatures (ca. 150 °C). Pd helped to catalyze dendrimer decomposition in theirsystem, as well. Temperatures of 500 °C or greater were required to completelyremove organic residues from their materials.(10) This treatment resulted in