Development of High Performance Heterogeneous Catalysts for Selective Cleavage of C-O and C-C Bonds of Biomass-Derived Oxygenates.

The environmental impact of CO2 emissions via the use of fossil resources as chemical feedstock and fuels has stimulated research to utilize renewable biomass feedstock. The biogenic compounds such as polyols are highly oxygenated and their valorization requires the new methods to control the oxygen to carbon ratio of the chemicals. The catalytic cleavage of C-O bonds and C-C bonds is promising methods, but the conventional catalyst systems encounter the difficulty to obtain the high yields of the desired products. This review describes our recent development of the high performance heterogeneous catalysts for the valorization of the biogenic chemicals such as glycerol, furfural, and levulinic acid via selective cleavage of C-O bonds and C-C bonds in the liquid-phase. Selective C-O bond cleavage by hydrogenolysis enables production of various diols useful as engineering plastics, antifreeze, and cosmetics in high yields. The success of the selective C-C bond scission of levulinic acid can be applied to a wide range of the biogenic oxygenates such as carboxylic acids, esters, lactones, and primary alcohols, in which the selective C-C bond scission at adjacent to the oxygen functional groups are achieved. Furthermore, valorization of glycerol by selective acetylation and acetalization, and of levulinic acid by hydrogenation is described. Our catalysts show excellent performance compared to the reported catalysts in the aforementioned valorization.

New Routes for Refinery of Biogenic Platform Chemicals Catalyzed by Cerium Oxide-supported Ruthenium Nanoparticles in Water.

Highly selective hydrogenative carbon-carbon bond scission of biomass-derived platform oxygenates was achieved with a cerium oxide-supported ruthenium nanoparticle catalyst in water. The present catalyst enabled the selective cleavage of carbon-carbon σ bonds adjacent to carboxyl, ester, and hydroxymethyl groups, opening new eight synthetic routes to valuable chemicals from biomass derivatives. The high selectivity for such carbon-carbon bond scission over carbon-oxygen bonds was attributed to the multiple catalytic roles of the Ru nanoparticles assisted by the in situ formed Ce(OH)3.

Mild Hydrogenation of Amides to Amines over a Platinum-Vanadium Bimetallic Catalyst.

Hydrogenation of amides to amines is an important reaction, but the need for high temperatures and H2 pressures is a problem. Catalysts that are effective under mild reaction conditions, that is, lower than 30 bar H2 and 70 °C, have not yet been reported. Here, the mild hydrogenation of amides was achieved for the first time by using a Pt-V bimetallic catalyst. Amide hydrogenation, at either 1 bar H2 at 70 °C or 5 bar H2 at room temperature was achieved using the bimetallic catalyst. The mild reaction conditions enable highly selective hydrogenation of various amides to the corresponding amines, while inhibiting arene hydrogenation. Catalyst characterization showed that the origin of the catalytic activity for the bimetallic catalyst is the oxophilic V-decorated Pt nanoparticles, which are 2 nm in diameter.

Metal-Support Cooperative Catalysts for Environmentally Benign Molecular Transformations.

Metal-support cooperative catalysts have been developed for sustainable and environmentally benign molecular transformations. The active metal centers and supports in these catalysts could cooperatively activate substrates, resulting in high catalytic performance for liquid-phase reactions under mild conditions. These catalysts involved hydrotalcite-supported gold and silver nanoparticles with high catalytic activity for organic reactions such as aerobic oxidation, oxidative carbonylation, and chemoselective reduction of epoxides to alkenes and nitrostyrenes to aminostyrenes using alcohols and CO/H2 O as reducing reagents. This high catalytic performance was due to cooperative catalysis between the metal nanoparticles and basic sites of the hydrotalcite support. To increase the metal-support cooperative effect, core-shell nanostructured catalysts consisting of gold or silver nanoparticles in the core and ceria supports in the shell were designed. These core-shell nanocomposite catalysts were effective for the chemoselective hydrogenation of nitrostyrenes to aminostyrenes, unsaturated aldehydes to allyl alcohols, and alkynes to alkenes using H2 as a clean reductant. In addition, these solid catalysts could be recovered easily from the reaction mixture by simple filtration, and were reusable with high catalytic activity.

On-demand Hydrogen Production from Organosilanes at Ambient Temperature Using Heterogeneous Gold Catalysts.

An environmentally friendly ("green"), H2-generation system was developed that involved hydrolytic oxidation of inexpensive organosilanes as hydrogen storage materials with newly developed heterogeneous gold nanoparticle catalysts. The gold catalyst functioned well at ambient temperature under aerobic conditions, providing efficient production of pure H2. The newly developed size-selective gold nanoparticle catalysts could be separated easily from the reaction mixture containing organosilanes, allowing an on/off-switchable H2-production by the introduction and removal of the catalyst. This is the first report of an on/off-switchable H2-production system employing hydrolytic oxidation of inexpensive organosilanes without requiring additional energy.

Green, Multi-Gram One-Step Synthesis of Core-Shell Nanocomposites in Water and Their Catalytic Application to Chemoselective Hydrogenations.

We devise a new and green route for the multi-gram synthesis of core-shell nanoparticles (NPs) in one step under organic-free and pH-neutral conditions. Simply mixing core and shell metal precursors in the presence of solid metal oxides in water allowed for the facile fabrication of small CeO2 -covered Au and Ag nanoparticles dispersed on metal oxides in one step. The CeO2 -covered Au nanoparticles acted as a highly efficient and reusable catalyst for a series of chemoselective hydrogenations, while retaining C=C bonds in diverse substrates. Consequently, higher environmental compatibility and more efficient energy savings were achieved across the entire process, including catalyst preparation, reaction, separation, and reuse.

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.

Selective C-C coupling reaction of dimethylphenol to tetramethyldiphenoquinone using molecular oxygen catalyzed by Cu complexes immobilized in nanospaces of structurally-ordered materials.

Two high-performance Cu catalysts were successfully developed by immobilization of Cu ions in the nanospaces of poly(propylene imine) (PPI) dendrimer and magadiite for the selective C-C coupling of 2,6-dimethylphenol (DMP) to 3,3',5,5'-tetramethyldiphenoquinone (DPQ) with O2 as a green oxidant. The PPI dendrimer encapsulated Cu ions in the internal nanovoids to form adjacent Cu species, which exhibited significantly high catalytic activity for the regioselective coupling reaction of DMP compared to previously reported enzyme and metal complex catalysts. The magadiite-immobilized Cu complex acted as a selective heterogeneous catalyst for the oxidative C-C coupling of DMP to DPQ. This heterogeneous catalyst was recoverable from the reaction mixture by simple filtration, reusable without loss of efficiency, and applicable to a continuous flow reactor system. Detailed characterization using ultraviolet-visible (UV-vis), Fourier transform infrared (FTIR), electronic spin resonance (ESR), and X-ray absorption fine structure (XAFS) spectroscopies and the reaction mechanism investigation revealed that the high catalytic performances of these Cu catalysts were ascribed to the adjacent Cu species generated within the nanospaces of the PPI dendrimer and magadiite.

Highly efficient dehydrogenative coupling of hydrosilanes with amines or amides using supported gold nanoparticles.

Hydroxyapatite-supported gold nanoparticles (Au/HAP) can act as a highly active and reusable catalyst for the coupling of hydrosilanes with amines under mild conditions. Various silylamines can be selectively obtained from diverse combinations of equimolar amounts of hydrosilanes with amines including less reactive bulky hydrosilanes. This study also highlights the applicability of Au/HAP to the selective synthesis of silylamides through the coupling of hydrosilanes with amides, demonstrating the first example of an efficient heterogeneous catalyst. Moreover, Au/HAP shows high reusability and applicability for gram-scale synthesis.

Hydrogenation of sulfoxides to sulfides under mild conditions using ruthenium nanoparticle catalysts.

The first demonstration of the hydrogenation of sulfoxides under atmospheric H2 pressure is reported. The highly efficient reaction is facilitated by a heterogeneous Ru nanoparticle catalyst. The mild reaction conditions enable the selective hydrogenation of a wide range of functionalized sulfoxides to the corresponding sulfides. The high redox ability of RuO(x) nanoparticles plays a key role in the hydrogenation.

Selective synthesis of Rh5 carbonyl clusters within a polyamine dendrimer for chemoselective reduction of nitro aromatics.

The selective synthesis of the [Rh5(CO)15](-) cluster within the PPI dendrimer was successfully demonstrated. The dendrimer-encapsulated [Rh5(CO)15](-) was resistant to decomposition under the catalytic reaction conditions and exhibited extremely high selectivity for the chemoselective reduction of nitro groups of various nitro aromatics with other reducible groups using CO/H2O as a reductant.

Highly efficient etherification of silanes by using a gold nanoparticle catalyst: remarkable effect of O(2).

O2 is acting! A nanosized hydroxylapatite-supported Au nanoparticle (NP) catalyst exhibited high activity under aerobic conditions, and various silyl ethers could be obtained from diverse combinations of silanes with alcohols. Moreover, O2 was found to act not as a stoichiometric oxidizing reagent, but as a non-consumed promoter, significantly boosting the catalytic activity of AuNPs.

Highly atom-efficient oxidation of electron-deficient internal olefins to ketones using a palladium catalyst.

A 100% atom-efficient synthesis of ketones from electron-deficient internal olefins was achieved using O2 as a "green" oxidant (see scheme, DMA=N,N-dimethylacetamide, EWG=electron-withdrawing group). Various electron-deficient olefins were oxidized to the corresponding ketones with over 99% selectivity and without the formation of olefin isomers or their oxidized products.

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.

Investigation of size-dependent properties of sub-nanometer palladium clusters encapsulated within a polyamine dendrimer.

This study describes the first example of the investigation of electronic, geometric, and catalytic properties of Pd subnano metal clusters of Pd(4), Pd(8), and Pd(16) stabilized by poly(propylene imine) (PPI) dendrimers using XAFS and IR analyses. The relations among these properties are also discussed.

Highly efficient double-carbonylation of amines to oxamides using gold nanoparticle catalysts.

Hydrotalcite-supported gold nanoparticles (Au/HT) catalyzed the highly efficient double-carbonylation of amines to oxamides under mild reaction conditions. Various amines were selectively converted to the corresponding oxamides. The cooperation between gold nanoparticles and basic sites of HT plays a key role in the reaction.

Unique catalysis of gold nanoparticles in the chemoselective hydrogenolysis with H2: cooperative effect between small gold nanoparticles and a basic support.

Gold nanoparticles on hydrotalcite act as a heterogeneous catalyst for the chemoselective hydrogenolysis of various allylic carbonates to the corresponding terminal alkenes using H(2) as a clean reductant. The combination of gold nanoparticles and basic supports elicited significantly unique and selective catalysis in the hydrogenolysis.