Supercritical water materials synthesis

Hayashi H, Hakuta Y (2010) Hydrothermal synthesis of metal oxide nanoparticles in supercritical water. Materials 3 3794-3817... 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

The use of supercritical fluids, including SCW and NCW, in inorganic materials synthesis and the preparation of nanoparticles was recently reviewed. The hydrolysis and dehydration of metal nitrates and metal organic precursors in supercritical water is also known as hydrothermal synthesis (Figure 4.15). 

More polymerization reactions carried out at supercritical conditions, select biomass conversion supercritical fluid technologies for hydrogen production, wider use of supercritical water oxidation processes, portfolio of self-assembly applications, a spate of opportunities in process intensification, many supercritical fluid aided materials synthesis applications, and numerous reactions for synthesis of specialty chemicals are expected for years to come. 

A variety of chemical and biological reactions involving supercritical fluid technology are being explored and developed. They include polymerization reactions, biomass conversion, hydrogen production, applications of supercritical water oxidation, self-assembly applications, synthesis of specialty chemicals, manufacture of materials with tailored properties, and much more. These developments and new ones are expected to mature and be commercially deployed in years to come. 

The use of supercritical fluids as alternatives to organic solvents is revolutionising a huge number of important science areas (24). Scientific applications vary from established processes, such as the decaffeination of coffee and the extraction and synthesis of active compounds, to the destruction of toxic waste in supercritical water, the production of nanoparticles and new materials, to novel emerging clean technologies for chemical reactions and extraction. 

Supercritical fluid, especially supercritical water (SCW), that is above the thermodynamic critical point of water (374"C, 22.1 MPa), has attracted increasing attention in various applications, such as in supercritical water oxidation (SC WO), in supercritical water gasification (SCWG), and for the continuous synthesis of nanoparticles. The environment of reactors presents a big challenge for structural materials used in the components. Many kinds of materials including stainless steel, alloys, and ceramics have been studied for using in SCW atmosphere. However, the details of the corrosion mechanism of each ceramic in an SCW environment were not fully clarified. 

Adschiri, T., Y. W. Lee, M. Goto, and S. Takami. 2011. Green materials synthesis with supercritical water. Green Chemistry. 13,1380. 

The microemulsion process for material synthesis and other applications has sometimes been termed an emergent technology , and therefore, new developments are also taking place to increase the suitability of the process. One such important development that has taken place in microemulsion-mediated particle synthesis is the use of water-supercritical CO2 systems [225-227, 229, 425,445]. Though this system is also valid for macro- and miniemulsions, reports of particle synthesis (metals, sulfides, halides, as above) are so far essentially microemulsion-based. As the system is environment-friendly and follows some of the important requirements of green chemistry [452], this process is expected to be further matured into a technology for nanoparticle synthesis. The present scale of operation, however, is apparently very small. 

In this chapter, first the ionic reaction equilibrium, phase behavior, and solubility of metal oxides in supercritical water are discussed. Next, the specific features of hydrothermal synthesis under supercritical conditions are discussed based on the experimental results. The supercritical hydrothermal crystallization method was applied to the production of functional materials, barium hexaferrite (BaFei20i9), metal-doped oxide [Al5(Y- -Tb)30i2, YAG Tb], and Li ion battery cathode material (LiC02O4). The importance of understanding the chemical reaction equilibrium and phase behavior is discussed. 

D.A. Guzonas, W.G. Cook, Cycle chemistry and its effect on materials in a supercritical water-cooled reactor a synthesis of current uuderstaudiug, Corros. Sci. 65 (December 2012) 48-66. 

Lee JW, Lee JH, Tan-Viet T, Lee JY, Kim JS, Lee CH (2010) Synthesis of LiNii/3Mni Coi/302 cathode materials by using a supercritical water method in a bath reactor. Electrochim Acta 55 3015-3021... 

Ohde, H Hunt, F. and Wai, C.M. (2001) Synthesis of silver and copper nanopartides in a water-in-supercritical-carbon dioxide microemulsion. Chemistry of Materials, 13 (11), 4130-4135. 

Ionic liquids (ILs) are, together with water and supercritical fluids, one of the few alternative media for environmentally friendly processes, which seem to have more possibility of industrial application in the next 10 years. The range of demonstrated or proposed applications of ILs is extraordinary, going from their use as nonvolatile, non-flammable solvents in organic synthesis to catalysts, materials for aiding separations and gas capture, advanced heat transfer fluids, lubricants, antistatics, and so on [2 ]. Surpassing in magnitude the number of potential uses is the number of possible IL compositions, estimated to be in the billions [5]. The term ionic liquids includes all compounds composed exclusively by ions that are liquid... 

Bioreactions. The use of supercritical fluids, and in particular C02, as a reaction media for enzymatic catalysis is growing. High diffusivities, low surface tensions, solubility control, low toxicity, and minimal problems with solvent residues all make SCFs attractive. In addition, other advantages for using enzymes in SCFs instead of water include reactions where water is a product, which can be driven to completion increased solubilities of hydrophobic materials increased biomolecular thermostability and the potential to integrate both the reaction and separation bioprocesses into one step (98). There have been a number of biocatalysis reactions in SCFs reported (99—101). The use of lipases shows perhaps the most commercial promise, but there are a number of issues remaining unresolved, such as solvent—enzyme interactions and the influence of the reaction environment. A potential area for increased research is the synthesis of monodisperse biopolymers in supercritical fluids (102). 

Solvothermal process is now becoming a powerful technique for preparing nanomaterials. It is analogous to hydrothermal synthesis, except that non-aqueous solvents replace water as reaction medium. From the chemical reaction point of view, solvents in supercritical conditions play a significant role in reaction and crystallization. New materials, especially those having metastable phases and special nanostructures, can be obtained under mild conditions. By sealing the reaction system in an autoclave, the reactants and products prevent effectively from oxidation, hydrolysis and volatilization, and the reaction and crystallization can be realized synchronously. 

According to the reaction temperature, hydrothermal and solvothermal synthesis can be classified into subcritical and supercritical synthesis reactions. In subcritical synthesis, the temperature is in the range of 100 to 240 °C, while in supercritical synthesis, the temperature could reach 1000 °C and the pressure could reach 0.3 GPa. By using the special properties of solvent water and other reactants under supercritical high temperature and pressure, various syntheses with specific features could be conducted, resulting in the formation of numerous crystal materials with simple to very complex structures. In addition, it should be pointed out that some crystal materials cannot be obtained by using other preparation approaches except for using hydrothermal or solvothermal synthesis routes. 

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