Yoshiyuki Kuroda

One-Step Exfoliation of Kaolinites and Their Transformation into Nanoscrolls

Kaolinite nanoscrolls, rolled kaolinite sheets with a tubular form, were prepared by a one-step route in which intercalation of guest species and swelling with solvent proceed at the same time. A methoxy-modified kaolinite was exfoliated by the intercalation of hexadecyltrimethylammonium chloride. The formation of nanoscrolls by the one-step route proceeded only by several alkyltrimethylammonium salts and 1-hexadecyl-3-methylimidazolium chloride. Intercalation of primary amines caused the formation of nanoscrolls by a two-step route in which the intercalation and swelling proceed separately. The successful one-step route is ascribed to the relatively weak interactions between the head groups of guest species and the interlayer surface of methoxy-modified kaolinite, and the interaction is thought to allow the formation of a flexible array of interlayer guest species for swelling. The tubular structure was mostly retained after the heat treatment at 600 °C to form hierarchically porous aluminosilicates with amorphous frameworks. The nanoscrolls intercalated organic guests species, which are not directly intercalated into methoxy-modified kaolinite, between the scrolled layers. The formation route to nanoscrolls is quite dependent not only on the surface modification of kaolinite but also on the structure of guest species.

Integrated structural control of cage-type mesoporous platinum possessing both tunable large mesopores and variable surface structures by block copolymer-assisted Pt deposition in a hard-template

Cage-type mesoporous Pt with tunable large mesopores possessing smooth and rough pore surfaces were prepared selectively by the deposition of Pt in the absence and presence of a block copolymer in a hard-template, respectively.

Heterogeneously Catalyzed Aerobic Cross-Dehydrogenative Coupling of Terminal Alkynes and Monohydrosilanes by Gold Supported on OMS-2†

Alkynylsilane derivatives are an important class of compounds and have been used not only as versatile alkynyl nucleophiles for common organic synthesis but also as siliconprotected intermediates for olygo-yne polymers, silylene– acetylene copolymers, and acetylene-bridged organometallic complexes. Up to the present, the vast majority have commonly used metal–acetylides and halosilanes (or pseudohalosilanes) for synthesis of alkynylsilanes; that is, silylation has been performed by pre-functionalization of terminal alkynes with alkyl lithium or Grignard reagents to form metal–acetylide species, followed by coupling with halosilanes. However, this antiquated silylation has shortcomings of 1) multi-step procedures with stoichiometric reagents, 2) inevitable formation of large amounts of byproducts during the pre-functionalization step as well as the coupling, and 3) use of toxic halosilanes and moisture-sensitive reagents. Thus, the development of efficient catalytic procedures instead of the above-mentioned stoichiometric ones, that is, the direct use of terminal alkynes without preactivation and the replacement of halosilanes with hydrosilanes, is an important subject. If the coupling could be performed with recoverable and reusable heterogeneous catalysts, it would be more desirable from the standpoint of green chemistry. Metal-catalyzed cross-dehydrogenative coupling represents the next generation of cross-coupling and is now emerging as an important reaction in organic synthesis. Cross-dehydrogenative coupling can construct C C, C X, and X X bonds (X = heteroatoms) through direct activation of C H and/or X H bonds, which can avoid pre-functionalization of substrates and formation of vast amounts of byproducts. With regard to cross-dehydrogenative coupling of terminal alkynes and hydrosilanes, to date there have been several homogeneously catalyzed procedures using metal salts and complexes such as H2PtCl6/I2, [5a] CuCl/TMEDA (TMEDA = N,N,N’,N’-tetraethylenediamine), M(hPh2CNPh)(hmpa)3 (M = Yb [5c] or Ca, hmpa = hexamethylphosphoramide), LiAlH4, [5e] and Zn(OTf)2/pyridine (Tf = SO2CF3). [5f] These procedures typically require additives to attain high yields of desired alkynylsilanes, and the recovery and reuse of the catalysts are very difficult. Although MgO and KNH2/Al2O3 [6b] can catalyze the cross-dehydrogenative coupling, the scope of both terminal alkynes and hydrosilanes is quite limited; for example, the reaction of ethynylbenzene hardly proceeds with KNH2/Al2O3. [6b] Therefore, efficient catalytic systems with widely usable and reusable heterogeneous catalysts have never been reported to date, to the best of our knowledge. Herein, we demonstrate for the first time that gold supported on a cryptomelane-type manganese oxide-based octahedral molecular sieve OMS-2 (Au/OMS-2, average particle size of gold: 4.5 nm, Figure S1, see the Supporting Information for the preparation) can act as an efficient reusable heterogeneous catalyst for cross-dehydrogenative coupling of terminal alkynes and monohydrosilanes using O2 as a terminal oxidant [Eq. (1)]. Various kinds of structurally

Selective cleavage of periodic mesoscale structures: two-dimensional replication of binary colloidal crystals into dimpled gold nanoplates.

Specific crystallographic planes of binary colloidal crystals consisting of silica nanoparticles are two-dimensionally replicated on the surface of gold nanoplates. The selectivity of the surface patterns is explained by the geometrical characteristics of the binary colloidal crystals as templates. The binary colloidal crystals with the AlB(2)- and NaZn(13)-type structures are fabricated from aqueous dispersions of stoichiometrically mixed silica nanoparticles with different sizes. The stoichiometry is precisely controlled on the basis of a seed growth of silica nanoparticles. Dimpled gold nanoplates are formed by the two-dimensional growth of gold between partially cleaved surfaces of templates. The selectivity of the surface patterns is explained using the AlB(2)-type binary colloidal crystal as a template. The surface pattern is determined by the preferential cleavage of the plane with the lowest density of particle-particle connections. The tendency to form well-defined cleavage in binary colloidal crystals is crucial to formation of dimpled gold nanoplates, which is explained using the NaZn(13)-type binary colloidal crystal as a template. Its complex structure does not show well-defined cleavage, and only distorted nanoplates are obtained. Therefore, the mechanism of the two-dimensional replication of binary colloidal crystals is reasonably explained on the basis of their periodic mesoscale structures and crystal-like properties.

Layer-by-layer assembly of imogolite nanotubes and polyelectrolytes into core-shell particles and their conversion to hierarchically porous spheres

Abstract Core-shell particles were prepared by the layer-by-layer (LbL) assembly of imogolite (IMO) nanotubes and poly(sodium 4-styrenesulfonate) (PSS) on polystyrene particles (diameter: 800 nm) coated preliminarily with poly(diallyldimethylammonium chloride) (PDDA). PSS and imogolite were alternately adsorbed on the particles to form core-shell particles with one to three bilayers of PSS/IMO. Macroporous hollow spheres were formed by removing polystyrene cores via heat treatment or extraction when the number of bilayers was 2 or 3. The sample formed by extraction (the number of bilayer was 3) showed only macroporosity and PSS remained in the shell, whereas the heat-treated sample showed hierarchical micro- and macroporosities. When the diameter of polystyrene particles decreased from 800 nm to 300 or 100 nm, hollow spheres were deformed because of the increase in the relative length of imogolite nanotubes against the size of polystyrene particles. Imogolite is a promising building block of hierarchically porous materials with core-shell morphologies using LbL assembly.

Facile patterning of assembled silica nanoparticles with a closely packed arrangement through guided growth

A facile patterning of assembled silica nanoparticles with a closely packed arrangement is demonstrated over a wide area through a guided growth approach utilizing a micro-mold.

Preparation of Mesoporous Basic Oxides through Assembly of Monodispersed Mg–Al Layered Double Hydroxide Nanoparticles

Mesoporous basic Mg-Al mixed metal oxides (MMOs) with a high surface area and large pore size have been prepared through the assembly of monodispersed layered double hydroxide nanoparticles (LDHNPs) with block copolymer templates. The particle sizes of the LDHNPs were mainly controlled by varying the concentration of tris(hydroxymethyl)aminomethane (THAM), which was used as a surface stabilizing agent. LDHNPs and micelles of a block copolymer (Pluronic F127) were assembled to form a composite. The composites were calcined to transform them into mesoporous MMOs and to remove the templates. The Brunauer-Emmett-Teller surface areas, mesopore sizes, and pore volumes increased as a result of using the templates. Moreover, the pore sizes of the mesoporous MMOs could be controlled by using LDHNPs of different sizes. The mesoporous MMOs prepared from the LDHNPs showed much higher catalytic activity than a conventional MMO catalyst for the Knövenagel condensation of ethyl cyanoacetate with benzaldehyde. The mesoporous MMO catalyst prepared using the smallest LDHNPs, about 12 nm in size, showed the highest activity. Therefore, the use of monodispersed LDHNPs and templates is effective for preparing highly active mesoporous solid base catalysts.

The Critical Effect of Niobium Doping on the Formation of Mesostructured TiO2: Single‐Crystalline Ordered Mesoporous Nb‐TiO2 and Plate‐like Nb‐TiO2 with Ordered Mesoscale Dimples

Highly ordered mesoporous niobium-doped TiO2 with a single-crystalline framework was prepared by using silica colloidal crystals with ca. 30 nm in diameter as templates. The preparation of colloidal crystals composed of uniform silica nanoparticles is a key to obtain highly ordered mesoporous Nb-doped TiO2 . The XPS measurements of Nb-doped TiO2 showed the presence of Nb(5+) and correspondingly Ti(3+) . With the increase in the amount of doped Nb, the crystalline phase of the product was converted from rutile into anatase, and the lattice spacings of both rutile and anatase phases increased. Surprisingly, the increase in the amount of Nb led to the formation of plate-like TiO2 with dimpled surfaces on one side, which was directly replicated from the surfaces of the colloidal silica crystals.

Synthesis of ultrasmall Li–Mn spinel oxides exhibiting unusual ion exchange, electrochemical, and catalytic properties

The efficient surface reaction and rapid ion diffusion of nanocrystalline metal oxides have prompted considerable research interest for the development of high functional materials. Herein, we present a novel low-temperature method to synthesize ultrasmall nanocrystalline spinel oxides by controlling the hydration of coexisting metal cations in an organic solvent. This method selectively led to Li–Mn spinel oxides by tuning the hydration of Li+ ions under mild reaction conditions (i.e., low temperature and short reaction time). These particles exhibited an ultrasmall crystallite size of 2.3 nm and a large specific surface area of 371 ± 15 m2 g−1. They exhibited unique properties such as unusual topotactic Li+/H+ ion exchange, high-rate discharge ability, and high catalytic performance for several aerobic oxidation reactions, by creating surface phenomena throughout the particles. These properties differed significantly from those of Li–Mn spinel oxides obtained by conventional solid-state methods.

A Single‐Crystalline Mesoporous Quartz Superlattice

There has been significant interest in the crystallization of nanostructured silica into α-quartz because of its physicochemical properties. We demonstrate a single-crystalline mesoporous quartz superlattice, a silica polymorph with unprecedentedly ordered hierarchical structures on both the several tens of nanometers scale and the atomic one. The mesoporous quartz superlattice consists of periodically arranged α-quartz nanospheres whose crystalline axes are mostly oriented in an assembly. The superlattice is prepared by thermal crystallization of amorphous silica nanospheres constituting a colloidal crystal. We found that the deposition of a strong flux of Li(+) only on the surface of silica nanospheres is effective for crystallization.