Fluorination effects on rotational correlation times of tris(ß-diketonato)aluminum(III) in CO2 by 27Al NMR relaxation measurements.

(27)Al NMR longitudinal relaxation times, T(1,obs)((27)Al), of [Al(acac)(3)] and [Al(hfa)(3)] (Hacac = acetylacetone, Hhfa = hexafluoroacetylacetone) in CH(3)CN and CO(2) were measured over a wide range of temperature and pressure. The rotational correlation times, τ(r), of the tris(ß-diketonato)aluminum(III) complexes were determined from T(1,obs)((27)Al) using (27)Al quadrupole coupling constants, eQq/h((27)Al), which were also obtained to be 3.11 and 3.22 MHz for [Al(acac)(3)] and [Al(hfa)(3)], respectively, in CD(3)CN by the dual spin probe technique in the present study. At each temperature, τ(r) increased almost linearly with increasing viscosity, η, in both CH(3)CN and CO(2); however, τ(r) in CO(2) at near critical densities deviated appreciably upward, as shown in a similar analogue of bis(acetylacetonato)beryllium(II), [Be(acac)(2)] (Umecky; et al. J. Phys. Chem. B 2002, 106, 11114). The η/T dependence of τ(r) was examined to discuss intermolecular interactions between the complexes and solvent molecules in terms of the fluorination and geometrical effects. The degree of solute-solvent interactions increases in the order [Be(acac)(2)] < [Al(hfa)(3)] < [Al(acac)(3)] in CH(3)CN and [Al(acac)(3)] < [Be(acac)(2)] < [Al(hfa)(3)] in CO(2). The results suggest that dipolar CH(3)CN molecules interact with negatively charged oxygen atoms in the complexes, whereas nonpolar CO(2) prefers fluorinated substituents as well as quasi-aromatic rings in the ligands. Moreover, the relationship between the rotational and translational motions of tris(acetylacetonato)metal(III), [M(III)(acac)(3)], in CO(2) was investigated.

Preparation of silica sphere with porous structure in supercritical carbon dioxide.

Silica sphere with porous structure has been synthesized in supercritical carbon dioxide. The structure originates from a delicate CO(2) trapping phenomenon intended for void formation in the inorganic framework. Silicate polymerization and subsequent removal of CO(2) by depressurization leaves the porous architecture. The key factor to obtain stable porous spherical structure was CO(2) pressure. Different characterization techniques such as X-ray diffraction, scanning and transmission electron microscopy and N(2) adsorption-desorption isotherm were used to determine the framework structure, morphology and porosity of the material. Microscopic visualization of calcined material suggested that the spherical structure was consisted of macroporous windows of diameter approximately 100 nm and the space between macropores presents a wormhole like mesoporous/microporous structure. The pore diameter of the mesoporous structure has been calculated as approximately 3 nm. X-ray diffraction and N(2) adsorption isotherm analysis confirmed the presence of micropores and also the macropores. In addition, the resulting material possess high thermal and hydrothermal stability associated with fully SiO(4) cross-linking. The spherical structure with different types of porosity was successfully obtained without using any molding agent.

Development of a novel catalytic membrane reactor for heterogeneous catalysis in supercritical CO2.

A novel type of high-pressure membrane reactor has been developed for hydrogenation in supercritical carbon dioxide (scCO(2)). The main objectives of the design of the reactor are the separate feeding of hydrogen and substrate in scCO(2) for safe reactions in a continuous flow process, and to reduce the reaction time. By using this new reactor, hydrogenation of cinnamaldehyde into hydrocinnamaldehyde has been successfully carried out with 100% selectivity at 50 degrees C in 10 MPa (H(2): 1 MPa, CO(2): 9 MPa) with a flow rate of substrate ranging from 0.05 to 1.0 mL/min.

Amphiphilic block copolymer-stabilized gold nanoparticles for aerobic oxidation of alcohols in aqueous solution.

Stable Au nanoparticles in P123 aqueous solution using a simple method have been developed and the colloidal Au nanoparticles were successfully applied for the effective oxidation of various alcohols using molecular O2 as oxidant at 30 degrees C in aqueous solution.

Applications of environmentally benign supercritical water to organic syntheses.

Supercritical water (scH(2)O) is utilized for organic synthesis as an acid- or a base-catalyst as well as a useful replacement for organic solvents. There have been few works on the use of scH(2)O for acid- or base-catalyzed organic synthesis, because the ion product of scH(2)O is much lower than liquid water under certain conditions. Our recent studies by in situ Raman and IR spectroscopic measurements were allowed to expect the occurrence of acid- or base-catalysis of scH(2)O. Contrary to the conventional wisdom that acid- or base-catalyzed reactions will not take place in scH(2)O, these results indicated the possibilities of scH(2)O participating in Lewis acid- or base-catalysis, besides the Brønsted acid-base function. In this paper, we thus demonstrate that the new-generation syntheses of lactam, arylated olefin, and monoterpene alcohols can be successfully created in scH(2)O.

Facile "green" synthesis, characterization, and catalytic function of beta-D-glucose-stabilized Au nanocrystals.

We present a straightforward, economically viable, and "green" approach for the synthesis and stabilization of relatively monodisperse Au nanocrystals with an average diameter of 8.2 nm (standard deviation, SD=2.3 nm) by using nontoxic and renewable biochemical of beta-D-glucose and by simply adjusting the pH environment in aqueous medium. The beta-D-glucose acts both as reducing agent and capping agent for the synthesis and stabilization of Au nanocrystals in the system. The UV/Vis spectroscopy, Fourier transform infrared (FT-IR) spectroscopy, transmission electron microscopy (TEM), electron diffraction (ED), and X-ray diffraction (XRD) techniques were employed to systematically characterize Au nanocrystals synthesized. Additionally, it is shown that these beta-D-glucose-stabilized Au nanocrystals function as effective catalyst for the reduction of 4-nitrophenol in the presence of NaBH4 (otherwise unfeasible if only the strong reducing agent NaBH4 is employed), which was reflected by the UV/Vis spectra of the catalytic reaction kinetics.

Polar attributes of supercritical carbon dioxide.

Supercritical carbon dioxide (scCO(2)) is increasingly promoted as an environmentally benign alternative to conventional organic solvents. The supercritical state bridges the gap between liquid and gaseous states by offering gaslike diffusion rates and liquidlike solvent densities, thereby enabling potential opportunities as a reaction and separation medium in chemical industry. Understanding the solvent behavior of liquid and scCO(2) is of critical importance to enable the design of CO(2)-philic molecular systems and to expand the use of these solvent systems to a wider range of chemical processes. Historically CO(2) was treated as a nonpolar solvent, primarily because of its low dielectric constant and zero molecular dipole moment. CO(2) has also been described as a quadrupolar solvent because of its significant quadrupole moment. Recent studies suggest that, as far as the microscopic solvent behavior of CO(2) is concerned, CO(2) has the potential to act as both a weak Lewis acid and Lewis base. Also, strong theoretical and experimental evidence indicates that CO(2) can participate in conventional or nonconventional hydrogen-bonding interactions. All of these site-specific solute-solvent interactions are important to understand the fundamental nature of CO(2) as a solvent. In this Account, we discuss these polar attributes of CO(2) and their relation to solvation.

Self-assembly of beta-D glucose-stabilized Pt nanocrystals into nanowire-like structures.

In this work we demonstrate the self-assembly of beta-D glucose-protected Pt nanocrystals (average particle size = 4.1 nm) into nanowire-like assemblies under ambient conditions.

Synthesis of Ag and AgI quantum dots in AOT-stabilized water-in-CO2 microemulsions.

Silver and silver iodide nanocrystals have been synthesized in the water-in-CO(2) reverse microemulsions formed by the commonly used surfactant, sodium bis(2-ethylhexyl)sulfosuccinate (AOT), in the presence of 2,2,3,3,4,4,5,5-octafluoro-1-pentanol as cosurfactant. The nanometer-sized aqueous domains in the microemulsion cores not only act as nanoreactors, but the surfactant interfacial monolayer also helps the stabilization of the metal and semiconductor nanoparticles. The transmission electron microscopy results show that silver and silver iodide nanocrystals with average diameters of 6.0 nm (standard deviation, SD=1.3 nm) and 5.7 nm (SD=1.4 nm), respectively, were formed. The results indicate that the method can be utilized as a general and economically viable approach for the synthesis of metal and semiconductor quantum dots in environmentally benign supercritical carbon dioxide.

Temperature dependence of local density augmentation for acetophenone N,N,N',N'-tetramethylbenzidine exciplex in supercritical water.

Local density augmentation around exciplex between acetophenone and N,N,N',N'-tetramethylbenzidine in supercritical water was measured by observing the peak shift of transient absorption spectrum at temperatures from 380 to 410 degrees C and at pressures from 6 to 37 MPa. Large local density augmentation was observed at lower solvent densities. Local density augmentation was evaluated by the excess density, which was defined as the difference between local density and bulk density, and the density enhancement factor, which was defined by the ratio of the local density to the bulk density. The number of solvating molecules was estimated with a Langmuir adsorption model. The excess density was found to exhibit a maximum at approximately 0.15 g cm(-3), which decreased with increasing temperature. The density enhancement factor was found to decrease with increasing temperature; however, its value was much greater than unity at 410 degrees C, which provides evidence that exciplex-water interactions still exist at these conditions. The temperature dependence of local density augmentation around the exciplex in supercritical water was comparable with that in supercritical carbon dioxide, which suggests that the ratios of the solute-solvent and solvent-solvent interactions are comparable between these two systems.

Synthesis of Ag2S quantum dots in water-in-CO2 microemulsions.

Ag2S nanocrystals with a mean diameter of 5.9 nm (sigma= 1.65 nm) and characteristic surface plasmon resonance absorption at 330 nm have been synthesized in water-in-supercritical CO2 reverse microemulsion using the commonly used AOT surfactant with 2,2,3,3,4,4,5,5-octafluoro-1-pentanol (F-pentanol) as cosurfactant.

Carbon dioxide-expanded liquid substrate phase: an effective medium for selective hydrogenation of cinnamaldehyde to cinnamyl alcohol.

It has been shown that CO(2)-expanded cinnamaldehyde liquid phase is a unique and effective medium for cinnamaldehyde hydrogenation to cinnamyl alcohol, due to interactions between the C[double bond, length as m-dash]O group of the substrate and CO(2) molecules and increased solubility of H(2).

Heck coupling reaction of iodobenzene and styrene using supercritical water in the absence of a catalyst.

Heck coupling reaction of iodobenzene and styrene proceeds rapidly and selectively in supercritical water even without any catalyst in the presence of base. Both the choice of base and the reaction conditions had a significant effect on the conversion and the selectivity of the coupling products. The addition of a relatively mild base such as potassium acetate facilitated the cross-coupling reaction, while the hydrolysis of phenyl halide was favored in the presence of a strong base. The conversion and the yields of coupling products increased with increasing temperature, reaching a maximum at 650 K near the critical temperature of water, and then decreased as the temperature was further increased. Water density had a significant influence on the reaction rate, showing nearly 30% augmentation with a slight increase in density from 0.45 to 0.56 g cm(-3), but had less effect on the product selectivity. Two possibilities of the role of water responsible for the noncatalytic Heck coupling reaction in supercritical water, that is, ion and water-catalyzed mechanisms have been considered.

Control of self-aldol condensation by pressure manipulation under compressed CO2.

Under supercritical CO2 conditions, simple adjustment of the pressure was found to successfully control the ratio of aldol to enal product in the self-aldol condensation of aldehyde, in which the enal product was obtained in a maximum selectivity of 94% at the critical pressure of 12MPa, whereas 85% selectivity to the aldol product was achieved at the subcritical region.

A rapid and effective synthesis of propylene carbonate using a supercritical CO2-ionic liquid system.

The synthesis of propylene carbonate from propylene oxide and carbon dioxide under supercritical conditions in the presence of 1-octyl-3-methylimidazolium tetrafluoroborate was achieved in nearly 100% yield and 100% selectivity within 5 minutes, whose TOF value is 77 times larger than those so far reported.

Innovation in a chemical reaction process using a supercritical water microreaction system: environmentally friendly production of epsilon-caprolactam.

Our microreaction system using supercritical water solutions achieves nearly 100% yield and 100% selectivity for epsilon-caprolactam production at reaction times shorter than 1 s.