Tomoo Mizugaki

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).

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

Development of concerto metal catalysts using apatite compounds for green organic syntheses

Catalytic chemistry and catalyst design play key roles in achieving green sustainable chemistry, which strives for reductions in waste, energy, materials, risks and hazards, and costs. Our approach to green sustainable chemistry described here is the development of highly functionalized heterogeneous metal catalysts based on the unique characteristics of natural inorganic crystallites, such as hydroxyapatite, as macro-ligands for active metal species. These catalysts show the concerto effect between metal active species and surface properties, and so are considered concerto catalysts. This review emphasizes the creation of well-defined active metal sites on apatite compounds that exhibit novel catalytic performance in selective oxidations using molecular oxygen as a clean oxidant, highly efficient carbon–carbon bond formations including asymmetric reactions, and chemical fixation of carbon dioxide.

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.

Development of Heterogeneous Olympic Medal Metal Nanoparticle Catalysts for Environmentally Benign Molecular Transformations Based on the Surface Properties of Hydrotalcite

In this review, we describe the development by our research group of highly functionalized heterogeneous Olympic medal metal (gold, silver, and copper) nanoparticle catalysts using hydrotalcite as a support, aimed towards Green and Sustainable Chemistry. Olympic medal metal nanoparticles can cooperate with the basic sites on the hydrotalcite surface, providing unique and high performance catalysis in environmentally-benign organic transformations such as aerobic oxidation of alcohols, lactonization of diols and selective deoxygenation of epoxides and nitro aromatic compounds.

Creation of monomeric La complexes on apatite surfaces and their application as heterogeneous catalysts for Michael reactions

Using a cation-exchange method, an equimolar substitution of La3+ for Ca2+ occurred by the treatment of stoichiometric hydroxyapatite (HAP: Ca10(PO4)6(OH)2) with an aqueous solution of La(OTf)3, affording a monomeric hydroxyapatite-bound La complex (LaHAP). Physicochemical characterization by means of XRD, XPS, IR, and La K-edge XAFS analyses proved that a monomeric La3+ phosphate complex was generated on its surface. Such monomeric La3+ species function as an efficient heterogeneous catalyst for the Michael reaction of 1,3-dicarbonyls with enones under aqueous or solvent-free conditions. The work-up procedure is straightforward and the spent catalyst could be recycled without any loss of the catalytic activity. Further application to an asymmetric version was also investigated using various apatite catalysts modified with chiral organic ligands. Enantioselectivity was found to depend on the chiral ligand, solvent, and rare earth metal triflate precursor (RE(OTf)3) for the reaction of methyl 1-oxoindan-2-carboxylate with methyl vinyl ketone. Under optimized reaction conditions, a monomeric fluoroapatite-bound La complex catalyst modified with (R,R)-tartaric acid (TA-LaFAP) provided the Michael adduct quantitatively in up to 60% ee.

Catalytic investigations of carbon–carbon bond-forming reactions by a hydroxyapatite-bound palladium complex

A new type of hydroxyapatite-bound palladium complex (PdHAP-1) was synthesized by treatment of a nonstoichiometric Ca-deficient hydroxyapatite, Ca9(HPO4)(PO4)5(OH), with PdCl2(PhCN)2 in acetone solution. Characterization by means of physicochemical methods revealed that a monomeric PdII phosphate complex could be generated at a Ca-deficient site, which displayed outstanding catalytic activities for the Mizoroki–Heck reaction and Suzuki–Miyaura coupling reaction of aryl bromides. The remarkably high catalytic activity of the hydroxyapatite catalyst is ascribed to the exceptionally robust monomeric Pd structure, in which Pd is surrounded by anionic phosphate ligands, as confirmed by XAFS analysis. It is also proven that, upon adjustment of the solvent system, the PdHAP-1 was able to catalyze the Suzuki–Miyaura coupling of activated aryl chlorides in the presence of TBAB. Under such conditions, the in situ generated Pd nanocluster on the surface of hydroxyapatite was effective as a catalytically active species.

Core-shell AgNP@CeO2 nanocomposite catalyst for highly chemoselective reductions of unsaturated aldehydes.

Selective silver: A core-shell AgNP-CeO2 nanocomposite (AgNP@CeO2) acted as an effective catalyst for the chemoselective reductions of unsaturated aldehydes to unsaturated alcohols with H2 (see figure). Maximizing the AgNP-CeO2 interaction successfully induced the heterolytic cleavage of H2, resulting in highly chemoselective reductions. Furthermore, a highly dispersed AgNP@CeO2 system was also developed that exhibited a higher activity than the original AgNP@CeO2.

A single-site hydroxyapatite-bound zinc catalyst for highly efficient chemical fixation of carbon dioxide with epoxides

A zinc-based hydroxyapatite catalyst in conjunction with a Lewis base proved to be efficient for the coupling of CO2 and epoxide in the absence of additional organic solvents under an atmospheric CO2 pressure; the work-up procedure is straightforward and the catalyst could be reused without loss of catalytic activity and selectivity.