Ji-Hyang Noh

Dendrimer-templated Pd nanoparticles and Pd nanoparticles synthesized by reverse microemulsions as efficient nanocatalysts for the Heck reaction: A comparative study.

Palladium nanoparticles (NPs) were prepared using a dendrimer-templated method using G4, G5 and G6 PAMAM-OH dendrimers as well as a reverse microemulsion method using the water/dioctyl sulfosuccinate sodium salt (aerosol-OT, AOT) surfactant/isooctane system with water to surfactant ratios (ω0) of 5, 10 and 13. These 6 catalysts were characterized by UV-Vis spectroscopy, TEM, EDX, and XRD. TEM micrographs showed that the average sizes of 2.74-3.32nm with narrower size distribution were achieved by using dendrimer-templated synthetic methods, whereas the reverse microemulsion method resulted in broad size distribution with an average size of 3.87-5.06nm. The influence of various reaction parameters such as base, catalyst dosing, alkene, aryl halide and temperature on the Heck C-C coupling reaction was evaluated. The activation parameters were derived from the reaction rate of each catalyst obtained at various temperatures. A correlation of catalytic activity, enthalpy of activation and particle size is discussed. Particle size changes of each catalyst were investigated after the catalytic reaction. Overall results indicated that dendrimer-templated Pd NP catalysts showed superior activity as compared to the Pd NPs synthesized by reverse microemulsions, with the dendrimer-templated G5-OH(Pd80) showing the best activity. These catalysts were also reusable for 3 cycles, retaining high yield and showing excellent yields under mild conditions. Therefore, the dendrimer-templated Pd NPs are efficient catalyst systems for the ligand-free Heck C-C coupling reaction.

Synergistic Effects of Gold–Palladium Nanoalloys and Reducible Supports on the Catalytic Reduction of 4-Nitrophenol

Herein we report on the catalytic activity of mesoporous nickel, iron, cerium, cobalt, and manganese oxides prepared using KIT-6 as a hard template via evaporation-assisted precipitation. The mesoporous metal oxides (MMOs) were characterized and used as heterogeneous catalysts in the reduction of 4-nitrophenol (4-Nip) by sodium borohydride (BH4-). Furthermore, polyamidoamide (PAMAM) dendrimers were used to synthesize gold-palladium nanoalloy particles. The size of AuPd/PAMAM was found to be 3.5 ± 0.8 nm in diameter before being immobilized on the aforementioned mesoporous metal oxides and used as catalysts in the reduction of 4-Nip. Prior to catalytic evaluation, the reduction profiles of the mesoporous metal oxides were investigated by hydrogen-temperature-programmed reduction (H2-TPR) and showed that mesoporous metal oxides can be easily reduced at lower temperatures and that the immobilization of gold-palladium nanoalloy particles lowers their reduction temperatures. Mesoporous cobalt and manganese oxides showed catalytic activity toward 4-Nip reduction, and the activity was enhanced after immobilization of the gold-palladium nanoalloys. Isolation of nanoparticles activity was achieved by immobilization of the gold-palladium nanoalloys on the inert silica support. From this we postulated an electron relay mechanism for the reduction of 4-nitrophenol. With the use of power rate law we showed that 4-Nip reduction follows pseudo-first-order kinetics.

Dendrimers as alternative templates and pore-directing agents for the synthesis of micro- and mesoporous materials

Dendrimers have been used to control the pore size and morphology of porous materials during their synthesis. Various characterization techniques have also been used to validate the formation of mesoporosity. Materials such as cetyltrimethylammonium bromide (CTAB) and other co-polymers are commonly used as templates for the synthesis of mesoporous materials. However, advantages of using dendrimers as templates for the synthesis of mesoporous materials include: (1) ease of control of the final pore size (depending on the dendrimer employed); (2) ease of removal of the dendrimer template by a simple extraction method or calcination process, which does not strongly interact with the inorganic species; (3) the monodispersed structure of the dendrimer leads to the formation of monodispersed pores with a narrow size distribution; and (4) the synthetic process require room (or relatively low) temperatures as opposed to elevated temperatures used for other surfactants. This mini-review is therefore focussed on the use of dendrimers as templating or pore-directing agents for the synthesis of micro- and mesoporous materials. The catalytic application of the mesoporous materials as heterogeneous supports is also discussed.