This review summarizes various advantages of supercritical hydrothermal methods for the synthesis of innovative materials. The merits of supercritical water, that is, higher miscibility with organic materials and gases, controllable oxidizing and reducing atmosphere, faster mass transport and reaction, and environmentally benign property, allow the synthesis of various innovative nanomaterials at an industrial scale. By using supercritical water, we have successfully synthesized organic-inorganic hybrid nanomaterials, where organic molecules attach on the surface of metal oxide nanocrystals. The surface chemical character of the synthesized hybrid nanomaterials can be finely tuned and allows us to hybridize them with organic materials including polymers and plastics.
Colloidal dispersions of noble metal nano-particles have been synthesized by the reduction of metallic ions in sub- and supercritical fluids of water, ethanol, 1-propanol, toluene and their mixtures. By using flow type reactor withstanding high-pressure and high-temperature, the reduction of metallic ions has been accomplished within a few seconds under the presence of protective polymer. Several kinds of metallic ions such as gold, platinum, rhodium, palladium, ruthenium, and their mixtures have been used to synthesize mono-metallic or bimetallic nano-particles successfully. The particle size and structure of particles have been controlled by the solvent species and temperature of the reaction.
The development of chemical recycling of waste plastics by decomposition reactions in sub- and supercritical fluids is reviewed. Decomposition reactions proceed rapidly and selectively using supercritical fluids compared to conventional processes. Condensation polymerization plastics are relatively easily depolymerized to their monomers in supercritical water or alcohols. Crosslinked polymer can be recycled by selective decrosslinking. Fiber reinforced plastics can also be recycled by depolymerization of resin part. Pilot scale or commercial scale plants have been developed and are operating with sub- and supercritical fluids.
In this article, recent advances in the utilization of waste biomass with subcritical water were reviewed. In the first application, the mixture of subcritical water and air was used for the incineration of waste biomass containing nitrogen atom completely and safely. In the second, subcritical water was used for the pretreatment of cellulose to accelerate the following cellulase saccharification. In the last, subcritical water was used for the production of powdery fuel from the mixture of waste biomass and plastics.
In this article, development of organic synthesis using high temperature and high pressure water as a reaction solvent was described. This process used a microreactor system. The microreactor system contributed to increase the reaction yield by the quick heating from room temperature to reaction temperature. Therefore, the side reactions during the heating process were inhibited. As an example, the corrosive and explosive organic synthesis, aromatic nitration reaction, was studied. The special microreactor system lined the inner wall by the corrosion resistant material was applied. Some results of noncatalyzed nitration reaction were discussed.
Chemical reactions in super- and subcritical water have been studied in our group over the decades, and recent advances are reviewed. The reaction mechanism in hydrothermal conditions is disclosed for ether and aldehyde in general form, and a new type of C-C bond formation is discovered in connection to the chemical evolution on primitive earth. Toward a new-generation hydrogen-fuel technology, it is proposed on the basis of physico-chemical reaction properties of formic acid that formic acid acts as a chemical tank for hydrogen storage and transportation. The hot-water chemistry is further discussed in the contexts of energy and environmental concerns, and its role in establishing green chemistry is stressed.
Recently, many research groups have been active at finding alternate media for performing material formation and chemical reaction. Liquid and supercritical CO2, and CO2-expanded liquids have received much attention as benign media satisfies several green chemistry and engineering principles. Here we describe recent research involving the use of CO2-based technique in the microencapsulation, nanoparticle composite formation and coating.
Thermophysical properties research on fluids of technical importance conducted over the last four decades by the present author's group at Keio University has been overviewed on the occasion of being honored as a recipient of the 2009 ASME Touloukian Award. Some experience obtained through long-term involvement in different international organizations has also been summarized so as to encourage further contribution in this field by next generation members of the present Japan Society of High Pressure Science and Technology.
This is a review paper presented as a “Jamieson Award Lecture” at the Joint AIRAPT-22 & HPCJ-50 held in Odaiba, Tokyo in July 2009. Recent technical development of ultra-high pressure and high temperature experiments using a laser-heated diamond anvil cell allows us to conduct in-situ x-ray diffraction experiments of Earth's core materials at multimegabar pressures. In this article, I briefly review the experimental results on the phase relations and crystal structures of iron alloys up to 300 GPa, which would serve as a basis for understanding the structure and the nature of the Earth's inner core.
A new hybrid-type anvil technique for high-pressure single-crystal magnetic neutron diffraction under 10 GPa is described. The hybrid anvil is composed of an opposed pair of two kinds of anvils. One is a large sapphire anvil or a supported SiC anvil and the other is a tungsten carbide (WC) anvil which has a hollow at the center of the culet. In a feasibility test of the hybrid anvil, we could generate pressures of up to 10 GPa at a load of 3.8 tons with high stability.