Takato Mitsudome

Design of a Silver–Cerium Dioxide Core–Shell Nanocomposite Catalyst for Chemoselective Reduction Reactions

The interaction of metals with ligands is the key factor in the design of catalysts and much effort has been devoted to the rational control of metal–ligand interactions in order to exploit catalytic properties. Quite sophisticated heterogeneous catalysts have been produced by controlling the size and shape of active metal species, and by screening and altering the composition of the supports. The supports can be considered as “macro ligands” for supported active metals, and the fine-tuning of the interactions between active metal species and supports is the most important factor through which high catalytic performance can be attained. Despite many intrinsic advantages of heterogeneous catalysts over homogeneous ones, such as their durability at high temperatures and reusability, the fine-tuning of metal–ligand interactions in heterogeneous catalysts is more difficult than in homogeneous catalysts, and remains a challenging objective. Our research group has recently reported that silver nanoparticles (AgNPs) on a basic support of hydrotalcite (Ag/HT) catalyzed the chemoselective reductions of nitrostyrenes and epoxides to the corresponding anilines and alkenes when using alcohols or CO/H2O as a reducing reagent while retaining the reducible C=C bonds. During the reductions, polar species of hydrides and protons were formed in situ at the interface of AgNPs/HT through a cooperative effect between the AgNPs and basic sites (BS) of HT, which were then exclusively active for the reduction of the polar functional groups (Figure 1). However, the use of H2 instead of alcohols or CO/H2O in our Ag catalyst system caused reductions of both the polar groups (nitro and epoxide) and the nonpolar C=C bonds. This nonselective reduction was due to the formation of nonpolar hydrogen species through the homolytic cleavage of H2 at the AgNPs surface, which is active for C=C bond reduction (Figure 2a).

Gold nanoparticle catalysts for selective hydrogenations

With increasing demand for efficient organic transformations based on the concept of Green Sustainable Chemistry, the development of highly selective reaction systems using heterogeneous catalysts is greatly desired. In selective hydrogenation using heterogeneous catalysts, gold nanoparticle catalysts have recently attracted enormous attention due to their unique catalysis compared with that of conventional hydrogenation catalysts based on Pt, Pd, and Ru nanoparticles. This review describes recent progress in state-of-the-art gold nanoparticle catalysts for chemoselective hydrogenations under liquid-phase conditions. The high and unique catalytic activity of these gold nanoparticles for the hydrogenation of unsaturated carbonyl compounds to unsaturated alcohols, functionalized nitroaromatics to the corresponding anilines, alkynes to alkenes, epoxides to alkenes, amides to amines, etc., is described. The dependence of the high catalytic performance of gold nanoparticles on the metal–support interactions, the morphology (size and shape), and oxidation states is also discussed.

Magnetically recoverable heterogeneous catalyst: Palladium nanocluster supported on hydroxyapatite-encapsulated γ-Fe2O3 nanocrystallites for highly efficient dehalogenation with molecular hydrogen

The dechlorination of various organochlorides using atmospheric molecular hydrogen (H2) can be efficiently catalysed by magnetically separable Pd nanoclusters supported on the surface of hydroxyapatite-encapsulated γ-Fe2O3 (PdHAP-γ-Fe2O3). For instance, dechlorination of chlorobenzene gave benzene with an excellent turnover frequency (TOF) of 2500 h−1 in the presence of 1 atm of H2. This PdHAP-γ-Fe2O3 catalyst can be recovered briefly using an external magnetic field and reused at least three times without loss of its high catalytic activity.

Supported Gold and Silver Nanoparticles for Catalytic Deoxygenation of Epoxides into Alkenes

Some metal nanoparticles (NPs) have been shown to have unprecedented catalytic performance which far exceeds those of conventional metal complex catalysts. Especially, gold and silver NPs are known to exhibit outstanding catalytic ability in the aerobic epoxidation of propene and styrene, and in the industrial epoxidation of ethylene, respectively. Our recent research has focused on the catalytic potential of coinage-metal NPs under liquid phase conditions, wherein we found that gold, silver, and copper NPs supported on inorganic materials had unique catalytic properties for versatile organic synthesis such as the aerobic oxidation of alcohols, 9] oxidation of silanes into silanols using water, and hydration of nitriles. In the course of our study on the coinage-metal NP catalysis for the aerobic oxidation of alcohols, we envisioned that if epoxides could act as hydrogen accepters in place of molecular oxygen, the metal NPs could act as effective catalysts for reverse epoxidation, namely, deoxygenation of epoxides using alcohols as oxygen accepters (Scheme 1). Deoxygenation of epoxides into alkenes is important reaction in both organic synthesis and biological chemistry; for example, in the deprotection of oxirane rings and in the reproduction of vitamin K in the vitamin K cycle. Stoichiometric deoxygenations of epoxides have been carried out using a variety of reagents including phosphines, silanes, iodides, and heavy metals. However, these reagents are often toxic or employed in large excess, resulting in the production of undesired waste. In addition, most of these methods require special handling and harsh reaction conditions, which may affect other sensitive functional groups in the parent molecules. Several successful catalyst systems have appeared to date such as Re complexes with triphenylphosphine, a Fe complex with NaBH4, [18] and a Co complex with Na, but these systems suffer from the need for hazardous reductants, inert conditions, and display low catalytic activities (TOFs < 13 h , TONs< 20; TOF = turnover frequency, TON = turnover number), and low atom efficiencies. Therefore, the development of an efficient catalytic system for deoxygenation of epoxides remains of great importance. Herein, we discovered the intrinsic ability of gold and silver NPs in catalyzing deoxygenation of epoxides; gold and silver NPs supported on an inorganic material of hydrotalcite (HT; Au/HT and Ag/HT) allow highly efficient catalytic deoxygenation of epoxides into alkenes using alcohols. The selectivity for all the alkene products were over 99%, and excellent turnover number was achieved. To the best of our knowledge, this is the first report on the catalytic deoxygenation of epoxides using gold and silver NPs. The Au/HT and Ag/HT catalyst systems described herein offer a green protocol for removing oxygen from epoxides with the following advantages: 1) high catalytic activity and selectivity; 2) the use of safe and easy-to-handle catalysts and reducing reagents; 3) applicability to a wide range of epoxides; 4) a simple purification procedure owing to easy separation of the solid catalysts from the reaction mixtures; and 5) recyclebility of the catalysts without any loss in their efficiency. The discovery of this unique catalysis of gold and silver NPs will open new routes to selective functional transformations in organic synthesis. Scheme 1. The oxidation of alcohols using O2 versus deoxygenation of epoxides using alcohols.

Selective deoxygenation of epoxides to alkenes with molecular hydrogen using a hydrotalcite-supported gold catalyst: a concerted effect between gold nanoparticles and basic sites on a support.

Direct conversion of epoxides into the corresponding alkenes is an important reaction because it allows the use of oxirane rings as protecting groups for carbon–carbon double bonds. This transformation also occurs in the production of vitamin K in the human body and is useful for quantification of epoxide moieties in graphite epoxide or oxygenated carbon nanotubes. Traditionally, the deoxygenation of epoxides to alkenes has been conducted using stoichiometric amounts of reagents, which results in the production of large amounts of undesirable waste. To date, several catalytic deoxygenations using PPh3, Na/Hg, and NaBH4 as reductants have been reported. These catalysts, however, suffer from low activity, low atom efficiency, and tedious work-ups with moisturesensitive reaction conditions. An ideal “green” protocol for the catalytic deoxygenation of epoxides is the use of molecular hydrogen (H2) as a reducing reagent because, theoretically, water is the only by-product. However, the use of H2 often causes nonselective reduction of epoxides to yield alcohols and alkanes as by-products through hydrogenation of the epoxides and overhydrogenation of the desired alkenes, respectively. Although there are a few successful reports on the selective deoxygenation of epoxides using H2, selectivity for alkenes is restricted to low conversion levels and a limited range of substrates. 6] Therefore, the development of an efficient catalytic system for the selective deoxygenation of epoxides to the corresponding alkenes using H2 is highly desired. Recently, we discovered that heterogeneous gold and silver nanoparticle (NP) catalysts have high activities for the deoxygenation of various epoxides to alkenes with > 99% selectivity, using 2-propanol used as an environmentally friendly reductant. Furthermore, CO/H2O was found to work as an alternative reductant for the selective deoxygenation of epoxides to alkenes in water under mild reaction conditions. Herein, we demonstrate that gold NPs supported on hydrotalcite [HT: Mg6Al2(OH)16CO3·nH2O] (Au/HT) can act as a highly efficient heterogeneous catalyst for the deoxygenation of epoxides to alkenes with H2 used as an ideal reductant. Au/HT is applicable to various epoxides, and selectivities for alkenes are over 99% at high conversions. After the reaction, solid Au/HT can be easily recovered from the reaction mixture and reused with no decrease in its catalytic efficiency. The deoxygenation of styrene oxide (1a) using various inorganic-materials-supported Au NPs was carried out in toluene at 80 8C under 1 atm of H2 (Table 1). Among the Au NP catalysts tested, Au/HT exhibited the highest activity toward this deoxygenation to afford styrene (2a) in 95 % yield with a small amount of the overhydrogenated product ethylbenzene (3a ; Table 1, entry 1). Au/CeO2 and Au/Al2O3 also converted 1a, but selectivities for 2a were much lower than that of Au/HT (Table 1, entries 4 and 5). Interestingly, Au/TiO2 showed the highest selectivity for 2a, although the conversion of 1a was low (Table 1, entry 6). Au/SiO2 did not have any catalytic activity for this reaction (Table 1, entry 7). Notably, when the reaction temperature was lowered to 60 8C, Au/HT produced 2a as the sole product in quantitative yield with > 99% selectivity (Table 1, entry 2). Moreover, the carbon–carbon double bond of 2a was completely intact when the reaction time was prolonged (Table 1, entry 3). Next, various HT-supported metal NPs were examined in this reaction (Table 1, entries 8–13). Ag/HT, Ru/HT, Rh/ HT and Cu/HT did not function as catalysts (Table 1, entries 10–13). In the case of Pd/HT and Pt/HT, hydrogenation of 1a occurred to give 2-phenylethanol (4a), but no deoxygenated product was obtained (Table 1, entries 8 and 9). These results clearly revealed that the combination of Au NPs and HT had the best catalytic activity and selectivity toward the deoxygenation of epoxides to alkenes using H2. Scheme 1 shows the hydrogenation of 2a in the presence or absence of p-methylstyrene oxide (1b) using Au/HT or Au/ [*] A. Noujima, Dr. T. Mitsudome, Dr. T. Mizugaki, Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda Department of Materials Engineering Science Graduate School of Engineering Science Osaka University, 1-3, Machikaneyama Toyonaka, Osaka 560-8531 (Japan) Fax: (+ 81)6-6850-6260 E-mail: kaneda@cheng.es.osaka-u.ac.jp

Supported gold nanoparticles as a reusable catalyst for synthesis of lactones from diols using molecular oxygen as an oxidant under mild conditions

The oxidative lactonization of various diols using molecular oxygen as a primary oxidant can be efficiently catalyzed by hydrotalcite-supported Au nanoparticles (Au/HT). For instance, lactonization of 1,4-butanediol gave γ-butyrolactone with an excellent turnover number of 1400. After lactonization, the Au/HT can be recovered by simple filtration and reused without any loss of its activity and selectivity.

Metal–Ligand Core–Shell Nanocomposite Catalysts for the Selective Semihydrogenation of Alkynes†

In recent years, hybrid nanocomposites with core–shell structures have increasingly attracted enormous attention in many important research areas such as quantum dots, optical, magnetic, and electronic devices, and catalysts. In the catalytic applications of core–shell materials, core-metals having magnetic properties enable easy separation of the catalysts from the reaction mixtures by a magnet. The core-metals can also affect the active shell-metals, delivering significant improvements in their activities and selectivities. However, it is difficult for core-metals to act directly as the catalytic active species because they are entirely covered by the shell. Thus, few successful designs of core–shell nanocomposite catalysts having active metal species in the core have appeared to date. Recently, we have demonstrated the design of a core–shell catalyst consisting of active metal nanoparticles (NPs) in the core and closely assembled oxides with nano-gaps in the shell, allowing the access of substrates to the core-metal. The shell acted as a macro ligand (shell ligand) for the core-metal and the core–shell structure maximized the metal–ligand interaction (ligand effect), promoting highly selective reactions. The design concept of core–shell catalysts having core-metal NPs with a shell ligand is highly useful for selective organic transformations owing to the ideal structure of these catalysts for maximizing the ligand effect, leading to superior catalytic performances compared to those of conventional supported metal NPs. Semihydrogenation of alkynes is a powerful tool to synthesize (Z)-alkenes which are important building blocks for fine chemicals, such as bioactive molecules, flavors, and natural products. In this context, the Lindlar catalyst (Pd/ CaCO3 treated with Pb(OAc)2) has been widely used. [13] Unfortunately, the Lindlar catalyst has serious drawbacks including the requirement of a toxic lead salt and the addition of large amounts of quinoline to suppress the over-hydrogenation of the product alkenes. Furthermore, the Lindlar catalyst has a limited substrate scope; terminal alkynes cannot be converted selectively into terminal alkenes because of the rapid over-hydrogenation of the resulting alkenes to alkanes. Aiming at the development of environmentally benign catalyst systems, a number of alternative lead-free catalysts have been reported. 15] Recently, we also developed a leadfree catalytic system for the selective semihydrogenation consisting of SiO2-supported Pd nanoparticles (PdNPs) and dimethylsulfoxide (DMSO), in which the addition of DMSO drastically suppressed the over-hydrogenation and isomerization of the alkene products even after complete consumption of the alkynes. This effect is due to the coordination of DMSO to the PdNPs. DMSO adsorbed on the surface of PdNPs inhibits the coordination of alkenes to the PdNPs, while alkynes can adsorb onto the PdNPs surface because they have a higher coordination ability than DMSO. This phenomenon inspired us to design PdNPs coordinated with a DMSO-like species in a solid matrix. If a core–shell structured nanocomposite involving PdNPs encapsulated by a shell having a DMSO-like species could be constructed, it would act as an efficient and functional solid catalyst for the selective semihydrogenation of alkynes. Herein, we successfully synthesized core–shell nanocomposites of PdNPs covered with a DMSO-like matrix on the surface of SiO2 (Pd@MPSO/SiO2). The shell, consisting of an alkyl sulfoxide network, acted as a macroligand and allowed the selective access of alkynes to the active center of the PdNPs, promoting the selective semihydrogenation of not only internal but also terminal alkynes without any additives. Moreover, these catalysts were reusable while maintaining high activity and selectivity. Pd@MPSO/SiO2 catalysts were synthesized as follows. Pd/ SiO2 prepared according to our procedure [16] was stirred in n-heptane with small amounts of 3,5-di-tert-butyl-4-hydroxytoluene (BHT) and water at room temperature. Next, methyl3-trimethoxysilylpropylsulfoxide (MPSO) was added to the mixture and the mixture was heated. The slurry obtained was collected by filtration, washed, and dried in vacuo, affording Pd@MPSO/SiO2 as a gray powder. Altering the molar ratios of MPSO to Pd gave two kinds of catalysts: Pd@MPSO/SiO21 (MPSO:Pd = 7:1), and Pd@MPSO/SiO2-2 (MPSO:Pd = 100:1). [*] Dr. T. Mitsudome, Y. Takahashi, Dr. T. Mizugaki, Prof. Dr. K. Jitsukawa, Prof. Dr. K. Kaneda Department of Materials Engineering Science Graduate School of Engineering Science, Osaka University 1–3, Machikaneyama, Toyonaka, Osaka 560-8531 (Japan) E-mail: kaneda@cheng.es.osaka-u.ac.jp

Highly Selective Hydrogenolysis of Glycerol to 1,3-Propanediol over a Boehmite-Supported Platinum/Tungsten Catalyst

Highly Selective Hydrogenolysis of Glycerol to 1,3-Propanediol over a Boehmite-Supported Platinum/ Tungsten Catalyst Business is boehming: Boehmite-supported platinum nanoparticles and tungsten oxides exhibit high catalytic activity towards the selective hydrogenolysis of glycerol to 1,3-propanediol. The reaction furthermore proceeds efficiently in aqueous solution without the requirement for any additives. This solid catalyst also demonstrates excellent durability, maintaining high catalytic activity and selectivity during recycling experiments.

One-step Synthesis of Core-Gold/Shell-Ceria Nanomaterial and Its Catalysis for Highly Selective Semihydrogenation of Alkynes.

We report a facile synthesis of new core-Au/shell-CeO2 nanoparticles (Au@CeO2) using a redox-coprecipitation method, where the Au nanoparticles and the nanoporous shell of CeO2 are simultaneously formed in one step. The Au@CeO2 catalyst enables the highly selective semihydrogenation of various alkynes at ambient temperature under additive-free conditions. The core-shell structure plays a crucial role in providing the excellent selectivity for alkenes through the selective dissociation of H2 in a heterolytic manner by maximizing interfacial sites between the core-Au and the shell-CeO2.

PAMAM dendron-stabilised palladium nanoparticles: effect of generation and peripheral groups on particle size and hydrogenation activity

Dendron stabilised Pd nanoparticles were prepared using the self-assembly of dendrons, which could catalyze a highly selective hydrogenation of dienes and acetylenes to monoenes.