Yutaka Ikushima

A novel type of high-pressure membrane reactor has been developed for hydrogenation in supercritical carbon dioxide (scCO₂). The main objectives of the design of the reactor are the separate feeding of hydrogen and substrate in scCO₂ 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 °C in 10 MPa (H₂: 1 MPa, CO₂: 9 MPa) with a flow rate of substrate ranging from 0.05 to 1.0 mL/min.

Supercritical water (scH2O) 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 scH2O for acid- or base-catalyzed organic synthesis, because the ion product of scH2O 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 scH2O. Contrary to the conventional wisdom that acid- or base-catalyzed reactions will not take place in scH2O, these results indicated the possibilities of scH2O 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 scH2O.

A simple, efficient and environmentally benign approach for the synthesis of titania nanowire is achieved by using natural fibres as templates and ionic liquids as solvent.

Gold nanoparticles are deposited into the channels of MCM‐48 through a simple H₂‐assisted reduction of HAuCl₄ (aqueous solution) in supercritical carbon dioxide medium at 70 °C within 2–4 h. The nanoparticles were characterized by powder X‐ray diffraction (PXRD), N₂ adsorption‐desorption, transmission electron microscopy (TEM), and UV‐Vis spectroscopy. The particle size of the synthesized material is tunable with the pressure (density) of the supercritical carbon dioxide medium. At the fixed temperature (70 °C) and hydrogen pressure [P(H₂)=2 MPa], the Au particle size varies from ca. 25 nm to ca. 2 nm with the change in CO₂ pressure from 7 MPa to 17 MPa. At the low solvent density conditions, larger particles of ∼25 nm were obtained. On the contrary, a high solvent density of CO₂ slows down particle aggregation, resulting in the small particle size within the range of 2–5 nm. This change in particle size with CO₂ pressure and the interaction of the particles with the silica support were correlated well with long‐range van der Waals interactions and consequently the Hamaker constant for the gold nanoparticle‐CO₂ (A₁₃₁) and silica‐gold core‐CO₂ (A₁₃₂), respectively. Supercritical carbon dioxide alone can provide a unique environment for stabilizing gold nanoparticles in the channels of the cubic mesoporous MCM‐48 support and exquisite control of the particle size without perturbing the support structure. The synthesized material is highly stable, recyclable and no metal nanoparticle leaching was observed. The selective hydrogenation of crotonaldehyde with the synthesized material provides convincing evidence that the particles are inside the pores and available to the reactant molecules.

Ni(II)-catalyzed hydrogenation of citral in supercritical carbon dioxide (scCO₂) results in the selective hydrogenation of CO over CC. The catalyst is completely inactive in conventional organic solvents. However, in presence of small amounts of CO₂ in the organic solvent, the catalyst become active to the citral hydrogenation. Theoretical calculations and spectroscopic studies suggest the probability of the formation of a complex between CO₂ and Ni(II) during the reaction. This, along with unique physical properties of scCO₂, may be an important factor in directing the activity and selectivity of the reaction.

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 β‐D‐glucose and by simply adjusting the pH environment in aqueous medium. The β‐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 β‐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.

Water is an environmentally friendly chemical species that can be employed either as catalyst or as reactant. The use of water has attracted much interest for many acid-catalyzed organic syntheses. These reactions have been studied under hot water conditions with maximum formation of ions. Under these circumstances, long reaction times are required to obtain suitable yields however with poor selectivities given the influence of side reactions. The microreaction system of the present study can heat up a substrate solution very quickly from ambient conditions to the supercritical water state (scH2O) and can then quench rapidly the same solution to sufficiently low temperatures. This allows epsilon-caprolactam production with close to 100% selectivity and 80% yield in a acid free reaction system in less than 1-second reaction time. To justify these results, first principle molecular dynamics calculations and in situ IR measurements are employed as they can be applied to the structural and electronic properties of water in the near critical point region. Close to the critical point at low fluid densities, small clusters, mainly dimers and trimers, are the dominant features with no H-bonds expected. The significant reduction of H-bond strength in H2O, peculiar to near the critical point, leads to the activation of protons or H3O+. Hence, in the scH2O microreaction system, the acidic properties of H2O at the supercritical state are considered as the likely pathway promoter which prevents the hydrolysis and the pyrolysis of cyclohexanone-oxime as well as the pyrolysis of epsilon-caprolactam. It is also observed that dilute acids at supercritical conditions further enhance the epsilon-caprolactam process with close to 100% yield and 100% selectivity.

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

Silver and silver iodide nanocrystals have been synthesized in the water‐in‐CO₂ 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.