Synthesis and Characterization of Dendrimer-Derived Supported Iridium Catalysts

Abstract

The synthesis of Ir/γ-Al 2 O 3 using the dendrimer metal nanocomposites (DMN) approach is reported. Fourth generation hydroxyl-terminated polyamidoamine dendrimer was complexed with Ir 3+ in aqueous solution and the process monitored using ultraviolet-visible and X-ray absorption spectroscopy. No discernible reduction of Ir 3+ to form zerovalent nanoparticles was observed after bubbling hydrogen or adding NaBH 4 into the complex solution. Standard wet impregnation of the DMN precursors were used to prepare Ir/γ-Al 2 O 3 , which were compared with conventionally prepared samples. In situ transmission Fourier transform infrared spectroscopy during dendrimer thermal decomposition in different atmospheres and CO adsorption allowed for identification of catalyst activation treatments that expose the maximum metal surface area. The particle size distributions of these catalysts were investigated using high resolution transmission electron microscopy, revealing that all of the catalysts have small particle sizes (0.4-3 nm) with narrow distributions. An optimized oxidation/reduction treatment produced a DMN-derived supported catalyst with higher metallic dispersion. DMN-derived catalysts were tested for liquid-phase hydrogenation of benzonitrile, and show an increase in TOF with increasing dispersion. The selectivity toward dibenzylamine is affected by the catalyst preparation method, with the oxidation/reduction treatment resulting in lower selectivity. IntroductionTransition metal-based heterogeneous catalysts are frequently employed in numerous industrial reactions, as well as environmental applications such as automotive catalytic converters and fuel cell technologies. Among transition metals, noble metals are widely used since they possess unique properties that enhance activity and selectivity in chemical reactions. Supported iridium catalysts are outstanding candidates for a variety of catalytic reactions due to their stability, activity and selectivity under reaction conditions. 1-6 For example, Ir catalysts possess unique characteristics for effective and selective synthesis of substituted N-aryl-hydroxylamines, chloro-substituted anilines and both symmetric and asymmetric azoxybenzenes. 6 Furthermore, supported Ir has been successfully employed for hydrogenation of R, -unsaturated aldehydes to selectively produce unsaturated alcohols. 4,5,7-11 Supported Ir catalysts are commonly prepared via wet impregnation 4,5,12-16 and incipient wetness impregnation. 11,[17][18][19][20] In supported metal catalysts, it is often desired to minimize the amount of noble metal needed (i.e., cost) by maximizing the metal surface area per unit volume. Consequently, the ability to produce small and highly dispersed metal nanoparticles is of great interest. However, conventional synthetic techniques (e.g., impregnation, ion-exchange) followed by calcination/reduction steps often result in wide particle size distributions. In addition to less than optimal performance, such nonuniform materials can also be difficult to characterize. 2...

PAMAM Dendrimer-Derived Ir/Al2O3 Catalysts: An EXAFS Characterization

“…The Ir local structure, and its chemical and electronic state were investigated for: (1) IrCl 3 Á3H 2 O salt in water; (2) Ir 3? -dendrimer complex in solution both before and after exposure to reducing agents (NaBH 4 and H 2 ); (3) after support impregnation; and (4) after dendrimer removal and catalyst activation via oxidation and/or reduction treatments explained in detail previously [18]. The results show conclusively that Ir nanoclusters/particles form only after the Ir-dendrimer precursor has been deposited and dried onto the support.…”

“…Ir-dendrimer nanocomposites were prepared as explained elsewhere [18]. Essentially, hydroxyl-terminated generation four (G4OH) poly(amidoamine) dendrimer was allowed to complex with IrCl 3 Á3H 2 O (AlfaAesar) in aqueous solution under inert atmosphere.…”

“…It has been reported that these metal cations can be reduced with sodium boron hydride (NaBH 4 ) to a zero valence state once they are complexed inside the dendrimer, resulting in the formation and stabilization of metal nanoparticles or clusters [13,14]. Particularly, hydroxyl-terminated poly(amidoamine) (PAMAM-OH) dendrimers have been used as a chelating agent of metal ions in solution to subsequently prepare supported Pt [1,5,7,9,11,[14][15][16], Pd [16], Au [16,17], Ru [2,3], Ir [18], Rh [5], and bimetallic Pt-Au [17] and Pt-Cu [10].…”

“…Previously, we reported that Ir/Al 2 O 3 catalysts could be successfully prepared by using Ir-DMN precursors [18]. Moreover, dendrimer-derived (DD) Ir/Al 2 O 3 exhibited higher catalytic activity than the conventional counterparts for benzonitrile hydrogenation.…”

Metal ion remediation by polyamidoamine dendrimers: a comparison of metal ion, oxidation state, and titania immobilization

“…Titania was selected for several reasons including its common use in wastewater processing for photocatalysis so it is already proven as stable under relevant and similar conditions. Adsorption of metal laden dendrimers is commonly examined for loading of nanoparticles onto metal oxide supports (e.g., silica (Castillo and Kuhn 2012;Huang et al 2008), alumina (Alexeev et al 2006;Deutsch et al 2007;Lopez-De Jesus et al 2008), titania (Crump et al 2008;Scott et al 2005), and zirconia (Deutsch et al 2007)) for the synthesis of heterogeneous catalysts. More in-depth studies also exist (Cahill et al 2008).…”