news of the w e e k tures offered by dendrimers in thefieldof Strategy facilitates catalyst recycling homogeneous catalysis," he says. "These features include their Product isolation precisely controlled Substrate — • Catalytic — • Product nanoscale container reaction Catalyst Catalyst properties, the acces— sibility of these fluoc = v rocarbon modified J Stirrer Catalyst is recycled | dendrimers based on Π Fluorous solvent D Organic solvent commercially available reagents, as well the catalysts can be recycled up to 12 as benefits related to 'green chemistry of times. They point out that the PAMAM these nanocomposites, such as facile sepdendrimer and the other materials used aration and recycling properties." Dennis P. Curran, a professor of Novel nanocomposite catalysts consist- for the hydrogénation are all commering of metal nanoparticles encapsulated cially available. "The method is very chemistry at the University of Pittsin the highly branched macromolecules general and could be applied to virtually burgh who is an expert in the chemistry known as dendrimers are promising any type of inorganic material that can of highly fluorinated substances, finds candidates for environmentally friendly be sequestered within a dendrimer inte- the work fascinating. "It is another example of the kinds of imaginative things catalysis applications, suggest the rior," they say. chemists who prepared them. Donald A. Tomalia, scientific direc- that can be done in fluorous chemistry," Chemistry professor Richard M. tor of the Center for Biologic Nanotech- he remarks. "Fluorous biphasic catalyCrooks at Texas A&M University, College nology at the University of Michigan, sis was introduced some years ago and Station, and Victor Chechik, now chemis- Ann Arbor, who reported the synthesis of there is now a small cottage industry of try lecturer at the University of York, En- PAMAM dendrimers in 1985, comments groups developing new methods based gland, encapsulated palladium nanoparti- that this is an excellent piece of work. "It on this idea." cles in a modified poly(amidoamine) truly demonstrates several important feaMichael Freemantle [PAMAM] dendrimer and showed that the nanocomposite is catalytically active in fluorous triphasic systems \J- Am. Giant crystals invade national lab Chem. Soc, 122,1243 (2000)]. In these systems, one phase is a solution of a cataThey're big, they're heavy, they're pure potassium dihydrogen phosphate. lyst in a perfluorinated organic solvent They're some of the world's largest fast-growth crystals, created at Lawrence Iivermore National Laboratory (LLNL) in Livermore, Calif. and the other is a reactant- or productIn the image at right, researcher Randy Floyd poses with an imposing arcontaining organic phase. ray of specimens. In fact, "We have developed a method for the crystal he's measuring making metal nanoparticle catalysts is the largest of its ilk ever that are structurally well defined and produced, grown in a mere sufficiently synthetically flexible for 5 2 days and weighing a whopping 701 lb. Its meatheir solubility to be easily controlled,'' surements are 2 6 χ 21 χ Crooks tells C&EN. "Importantly, we 2 3 inches. used simple noncovalent chemistry to The crystals have a pur control precisely the properties of the pose beyond looking pret dendrimer-encapsulated catalysts, inty—they'll be sliced into cluding their solubility, permeability, plates and used to convert light from the giant laser at and host properties. " LLNL's National Ignition Crooks and Chechik enhanced the Facility from infrared to solubility of the nanocomposites in fluoultraviolet. rous solvents by complexing the termiThe team of scientists nal amine groups of the dendrimer with who grew the crystals based perfluorinated carboxylic acids. "The their rapid-growth technique end groups of the dendrimers can be on one designed by Livermore researcher Natalia Zaitseva while she was a scientist in Russia in the late modified very easily, which renders the 1980s. The group, led by chemist Ruth Hawley-Fedder, immersed a thumbnaildendrimers soluble in essentially any sized seed crystal inside a 6-foot-tall tank of supersaturated potassium dihydro solvent, including water, organic solgen phosphate solution. The group maintained supersaturation by gradually re vents, fluorous solvents, and even suducing the 150 ° F starting temperature. percritical C02," Crooks explains. The group hopes to grow even bigger, better crystals in the future. In fact, The two chemists showed that the the current crystals would have kept on growing, except that they outgrew the tank, Hawley-Fedder notes. catalysts are highly active and selective Elizabeth Wilson for the biphasic hydrogénation of alkenes and conjugated dienes and that "We expect to achieve cost reductions in excess of $40 million per year over the next two years by combining these assets with our existing pigments business," says Luke R. Corbett, KerrMcGee's chairman and chief executive officer. Alex Tullo
Dendrimer-encapsulated catalysts are recyclable
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FEBRUARY 21,2000 C&EN
