Hydrothermal method of synthesis

A hydrothermal method of synthesis refers to a heterogeneous reaction carried out in a closed system in an aqueous solvent with T > 298 K and P > 1 bar. Such reaction conditions permit the dissolution of reactants and the isolation of products that are poorly soluble under ambient conditions. 

Several high-temperature procedures have been described in the literature for the preparation of the transition-metal dioxides. Direct oxidation of the metals, lower oxides, chlorides, or nitrate precursors provides a convenient route to the dioxides of several metals Ti, Mn, Ru, Rh, Os, Ir, and Pt.1,3-5 (Syntheses of the rutile forms of rhodium and platinum dioxides by direct oxidation requires application of high pressures.5) Reduction of higher oxides is the most common method of synthesis for these dioxides V02, Nb02, Mo02, W02, and /3-Re02.4,6-8 Stoichiometry in these reactions is most readily controlled by use of the respective metal or a lower oxide as reductant. Chromium dioxide is normally synthesized by hydrothermal reduction of the trioxide.9... 

Recently, renewed attention has been given to so-called soft chemistry methods of synthesis of new metastable materials [9]. The synthesis of new microporous materials containing transition metals in the framework is of growing interest due to the expected catalytic redox properties [10]. The microporous titanium(IV) silicates [11] discovered have already proven the concept by showing very good catalytic activities and are widely used nowadays [12]. Similarly, hydrothermally synthesized titanium phosphates with open-finmework or layered structures are attracting attention as potential materials with similar properties [13]. 

Precursor formation is the initial formation of the active solid material. This can be done by precipitation or coprecipitation of the required chemical species, decomposition, hydrothermal synthesis, adsorption or impregnation of active species onto a support, and other methods of synthesis. 

The catalysts used in the aforementioned studies were always titanium silicates of MFI structure prepared by hydrothermal synthesis. Ti can, however, be inserted in the silica lattice by post-synthesis treatment of a dealuminated H-ZSM-5 with TiCl4 vapor [11]. Titanium silicalite-2 (TS-2), with the MEL structure of ZSM-11, was prepared shortly after the first synthesis of TS-1 [15]. Both catalysts have been used for the hydroxylation of phenol. Kraushaar-Czarnetzki and van Hooff showed that no major catalytic differences resulted from the method of synthesis of TS-1 [11]. The slow rate of reaction they observed was probably the result of large crystal size and low titanium content [7]. Tuel and Ben Taarit demonstrated there was no perceptible difference between the catalytic activity of TS-2 and TS-1 [8]. This was predictable, because of the close similarity of the Ti-site structure, chemical composition, and pore dimensions of the two titanium silicates. 

The solvothermal reaction between metal halides and polysulfide anions is also a useful method for the synthesis of metal-polysulfide clusters. Hydrothermal reaction of K2PtCl4 with K2S4 (5 eq) at 130 °C in a sealed tube... 

Various methods are applied to the synthesis of titania particles including sol-gel method, hydrothermal method [2], citrate gel method, flame processing and spray pyrolysis [1]. To utilize titania as a photocatalyst, the formation of ultrafme anatase titania particles with large crystallite size and large surface area by various ways has been studied [4]. 

The mechanical incorporation of active nanoparticles into the silica pore structure is very promising for the general synthesis of supported catalysts, although particles larger than the support s pore diameter cannot be incorporated into the mesopore structure. To overcome this limitation, pre-defined Pt particles were mixed with silica precursors, and the mesoporous silica structures were grown by a hydrothermal method. This process is referred to as nanoparticle encapsulation (NE) (Scheme 2) [16] because the resulting silica encapsulates metal nanoparticles inside the pore structure. 

Moreira, M.L., Andres, J., Varela, J.A. and Longo, E. (2009) Synthesis of fine microsized BaZr03 powders based on a decaoctahedron shape by the microwave-assisted hydrothermal method. Crystal Growth and Design, 9, 833-839. 

A preformed chitosan-silica composite with 60% weight inorganic part [7] is used as the source of silica for the zeolite synthesis. An alkaline solution of sodium aluminate (Na 2.1 M, Al 1 M) was used in three methods of preparation (A) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted and submitted to a hydrothermal treatment at 80 °C during 48h (B) beads of the chitosan-silica composite were immersed in the aluminate solution and the system underwent a hydrothermal treatment at 80 °C for 48h (C) beads of the chitosan-silica composite were stirred overnight in the aluminate solution, extracted, dried at 80 °C and exposed to water vapour at 80°C during 48h. 

The production of Ag Carbon nanocables was reported by Yu et al. [20]. In this case the authors used the hydrothermal carbonization of starch in the presence of Ag N03 leading to the one-step formation of carbon/Ag hybrid nanocables. Such silver-carbon nanocables can have a length as long as 10 mm and overall diameters of 1 micron with a 200-250 nm silver lining. When made at higher concentrations, they tend to fuse with each other (Fig. 7.5(a) and (b)). This method was extended to a polyvinyl alcohol(PVA)-assisted synthesis of flexible noble metal (Ag, Cu) carbon composite microcables [21]. 

The method developed by Milton in the late 1940s, involves the hydrothermal crystallization of reactive alkali metal aluminosilicate gels at high pH and low temperatures and pressures, typically 100°C and ambient pressure. Milton, Breck and coworkers synthesis work led to over 20 zeolitic materials with low to intermediate Si/Al ratios (1-5) [86]. Chapter 3 and references [1] and [25] provide more detailed discussion of synthesis. 

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