Hydrothermal/solvothermal synthesis

Liquid-phase synthesis takes place in the following synthetic approaches (1) coprecipitation, (2) sol-gel, (3) microemulsion, (4) hydrothermal/solvothermal synthesis, (5) microwave synthesis, (6) sonochemical synthesis, (7) template synthesis, and (8) biometric synthesis. 

In another type of anion-induced fluorination, -F anions are incorporated (coordinated to the metal centers) into MOF structure by means of inorganic acids. Recently, Ferey and co-workers and other research groups focused on the hydrothermal/solvothermal synthesis... 

Traditional zeoUte synthesis, for example, hydrothermal/solvothermal synthesis, necessarily is performed in the presence of solvents. The addition of solvents raises the cost of industrial production and also leads to waste solvent, which is hard to purify and recycle. Researchers should ask if they need a solvent at all in the green synthesis of... 

As opposed to traditional hydrothermal/solvothermal synthesis, where the solvents are predominantly molecular, ionothermal synthesis is based on the use of ionic liquids as solvents, and in many cases also as templates. Ionic Uquids are a class of molten salts, composed of organic cations and matching anions. At room temperature or near room temperature, ionic Uquids are traditionally defined as Uquids and can safely be used as solvents. In 2004, Morris and coworkers Urst developed the ionothermal synthesis approach to prepare zeoUte materials [54]. Since then, ionothermal synthesis has received great attention due to its potential advantages over traditional hydrothermal synthesis. 

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. 

Hydrothermal/solvethermal s)mthesis of RMO3 is also extensively adopted, by virtue of the low reaction temperature and well-crystallized products. Vazquez-Vazquez and Lopez-Quintela (2006) reported the solvothermal synthesis of Lai j-A MnOs (A = Ca, Sr, Ba) NPs in benzyl alcohol and acetophenone. The obtained precipitate was annealed to form crystalline products and acetophenone was found to be more suited to obtain clean perovskite phase. Zhu et al. (2008a) prepared single-crystalline YbMnOs and LuMnOa nanoplates via hydrothermal method. The products were found to be hexagonal phases. A possible formation mechanism was proposed, which involves the formation of ROOH phase as intermediate. 

With the wide use of hydrothermal and solvothermal synthesis, many polyoxogermanates with unique stmctures have been prepared. PolyoxoaUcoxovanadium germanate, (NH4)2[H2V9Ge8O26(L)6]-0.65H2O (H2T = HOCH2CH2-... 

Besides water for hydrothermal reactions, liquid ammonia (bp, 78°C Tc, 132°C Pc, 113 atm) is also used for the solvothermal synthesis of nitrides. Metastable or otherwise unobtainable nitride materials were reported to be formed by this rnethod. " Ammonium and amide (NH2) ions are the strongest acid and base, respectively, for the liquid ammonia system, and therefore ammonium salt acts as the acid mineralizer, while amide ion can be prepared by addition of alkali metals to the solvent. Since ammonia has a low boiling point, the reaction pressure is usually quite high. 

To increase the crystallization rate and to alter the product phase, an alkaline mineralizer is sometimes added to the solvothermal reaction. Some researchers believe that, compared with the hydrothermal process, solvothermal synthesis allows the product to be free from foreign ions because the organic solution, having a low relative permittivity, is free from ionic species. When precursor gels are prepared from alkoxide, one can prepare products free of foreign ions. However, when the precursor gel is prepared by precipitation from salt solutions, or when alkali/acid mineralizer or ionic surfactant is added to the solvothermal crystallization system, the above statement is a myth. In fact, ions are easily adsorbed or occluded in the product particles because of the low dielectric constant of the organic solvent. 

Another feature of hydro(solvo)thermal synthesis is the operability and tunability of hydrothermal and solvothermal chemistry, which bridges the synthetic chemistry and physical properties of synthesized materials. With deepening studies on hydrothermal and solvothermal synthesis chemistry, more and more reaction types have been discovered. Compared with other synthesis and preparation techniques, hydro(solvo)thermal synthesis methodology and techniques have irreplaceable advantages. So far, a variety of materials and crystals used in many fields could be hydrothermally or solvothermally synthesized, and the quality and properties of the resulting products are often much better than those prepared by other methods. 

Hydro(solvo)thermal synthesis chemistry focuses on the chemistry in preparation, synthesis, and assembly of special compounds or materials under hydro(solvo)thermal conditions. More importantly, hydrothermal or solvothermal synthesis routes can be used to prepare materials with special structures and properties, or phases, types, and morphologies which cannot be obtained by using solid-state reactions. In some cases, the materials can be obtained under mild conditions by using hydrothermal and solvothermal synthesis instead of under critical conditions by using a solid-phase reaction synthesis route. 

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. 

So far, hydrothermal and solvothermal synthesis have been widely used in the 1) modification, 2) crystal growth and morphology control, 3) phase-transition study, and 4) discovery of new species of zeolites and porous materials. 

So far, a large number of nondense aluminophosphates have been successfully synthesized via the solvothermal synthesis system. Most of them have anionic frameworks except for those crystallized from hydrothermal synthesis systems, such as AlP04-5, A1P04-11, and A1P04-21. The stoichiometries of these aluminophosphates include... 

Hydrothermal synthesis of microporous compounds in the presence of fluoride source refers to the hydrothermal or solvothermal crystallization of aluminosilicate zeolites or microporous aluminophosphate such as AlP04-n series in the presence of a fluoride source. The successful introduction of fluoride ion into the hydrothermal or solvothermal synthesis of microporous materials paves the way for the introduction of other complex-ion or chelation agent s to the hydrothermal crystallization of microporous compounds. 

Molecular design and rational synthesis of inorganic microporous crystalline materials are frontier subjects in the fields of zeolites science and molecular engineering. Zeolite synthesis is an active field of research because zeolites with uniform micropores are important in many industrial processes in catalysis, adsorption, and separation, and are finding new applications in electronics, magnetism, chemical sensors, and medicine, etc.12 91 Synthesis of such materials typically involves crystallization from a gel medium under hydrothermal/solvothermal conditions in the presence of organic amines as... 

In the synthesis of MOFs, varions methods/strategies have been employed, including solvent evaporation or diffusion, hydrothermal, solvothermal, using ionic liquids, electrochemistry, mechanochemistry, and ultrasonic methods [28]. And new ones are being explored with the development of MOFs field. Particularly, high-throughput screen has also been used in finding new MOFs materials [36, 37]. 

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