Hydrothermal reaction synthesis

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... 

Fujii and coworkers reported the synthesis and detailed structural analyses of alkylammonium/magnesium phyllosilicate hybrids [88], which were prepared by hydrothermal reaction from a mixture ofoctadecyldimethyl(3-trimethoxysilylpropyl)-ammonium chloride, silica sol, and magnesium hydroxide Mg(OH)2. The structure of the hybrid compound was studied by XRD, TEM, electron diffraction, high-resolution solid-state NMR, TG-DTA/MS, and elemental analysis. The resulting analytical information confirmed the unit structure, which consists of a 2 1... 

The hydrothermal method has been employed in recent years to synthesize a variety of solids that include aluminium phosphates (ALPOs) and other microporous transition-metal phosphates and transition-metal polychalcogenides (Davis Lobo, 1992 Haushalter Mundi, 1992 Liao Kanatzidis, 1990, 1992). Unlike zeolites, synthesis of ALPOs requires acidic or mildly basic conditions and no alkali metal cations. A typical synthetic mixture for making ALPO consists of alumina, H3PO4, water and an organic material such as a quaternary ammonium salt or an amine. The hydrothermal reaction occurs around 373-573 K. The use of fluoride ions, instead of hydroxide ions as mineralizer, allows synthesis of novel microporous materials under acidic conditions (Estermann et al, 1991 Ferey et ai, 1994). 

Boldyrev demonstrated that mechanochemical reactions of solids in the presence of water can be considered as hydrothermal ones. The products of mechanochemical synthesis for a series of reactions were found to be similar to the products of hydrothermal reactions. These reactions include the synthesis of CaO-FeO-2Si02 (gedenbergite) [31] and calcium hydrosilicates, eg., 5Ca0-6Si02-7H20 [32], as well as others. 

Boldyrev V.V., Khabibulhn A. Kh., Kosova N.V., Avvakumov E. G. Hydrothermal reactions under mechanochemical treatment. J. Mater. Synthesis and Processing 1996 4 377-81. 

The formation of water (as a reaction product) and local heating under mechanical activation as a result of collisions, friction and plastic friction of solid particles are the conditions for the reactions to occur via a dissolved state (hydrothermal-like synthesis). Therefore, it is reasonable to consider some features of these methods taking into account a large number of works in this direction. 

The possibility of mechanochemical reactions to occur via hydrothermal mechanism was demonstrated in [32] for the synthesis of calcinm hydrosilicates. Optimal water content values (humidity) of solid mixtures of correspondent hydrates were estimated for the achievement of local hydrothermal conditions (elevated temperature and pressure) in mechanochemical activators. It was shown experimentally that the main factor determining the kinetics and composition of the products was the molar fraction of the components, the same as for hydrothermal reactions, while the effect of temperatnre and pressnre was mnch weaker. 

Boldyrev V.V., Khabibullin A.Kh., Kosova N.V., Avvakumov E.G. Hydrothermal reactions under mechanical activation. J. Synthesis and Processing, 1996 4 371-81. Temuujin J., Okada K., Mackenzie K.l.D. Formation of layered magnesium silicate during the ageing of magnesium hydroxide-silica mixtures. J. Am. Ceram. Soc. 1996 81 574-76. 

Main factors which affect a hydrothermal reaction are the initial eomposition, reaction temperature and time. In mild hydrothermal synthesis, reaction temperatures lower than 240 °C are respected for both safety of high pressure in normal autoclaves and protection of softness of Teflon line. In our specific synthesis system, high temperature favorites the reaction and the most important factor was the base concentration in the initial reaction mixtures. The reaction time associated with reaction temperature affected the reaction. Crystallization kinetic experiment for a typical reaction showed that a reaction time more than lOh gave well-crystallized product and the further crystal growth needed additional time. Table 1 lists the starting reaction compositions and phase identification of products obtained at 240 °C for lOh. 

Microemulsion-Mediated Hydrothermal Synthesis Triton X-100 was served as the surfactant, n-hexanol as co-surfactant, cyclohexane as the continuous oil phase, and a solution of titanium -butoxide dissolved in an acid (HCl or HNO3) was employed as the dispersed aqueous phase. The concentration of hydrochloric acid or nitric acid ranged from 0.5 M to 2.0 M. A transparent feedstock of microemulsions was charged into a Teflon-lined stainless autoclave and hydrothermal reaction was... 

Hydrothermal Synthesis. Hydrothermal synthesis and reactive fluxes have been increasingly used as a method for synthetic actinide chemistry. Unique phases of actinide selenites, tellurites, iodates and periodates have all been synthesized under hydrothermal reaction conditions. New compounds and phases of oxyfluorides, phosphates, and sulfates using organic templates also have been synthesized with hydrothermal methods. " ... 

For the preparation of RE-POM complexes, traditional aqueous solution synthesis and hydrothermal synthesis [2] are the two main synthetic methods. In traditional aqueous solution synthesis, reactions are carried out in the temperature range from room temperature to the boiling point of water, and in general start from plenary or lacunary POMs and simple RE salts. Hydrothermal reactions typically proceed in the temperature range 120-200 C under autogenous pressure, and usually use simple metal salts of all the required elements as the starting materials. As different solubility problems are minimized under hydrothermal conditions, the... 

The synthesis of nanocrystalline Ti02, which is an important photocatalyst for the decomposition of toxic chemicals, is one of the more thoroughly investigated solvothermal/hydrothermal reactions. Approaches to this preparation have involved the decomposition of metal alkoxides, [158] a TOPO-capped autoclave synthesis of Ti02 by metathetic reachon [159] and decomposihon of a metal N-nitroso-N-phenylhydroxylamine complex [160,161]. In 1988, Oguri and coworkers reported the preparahon of anatase by hydrothermally processing hydrous htania prepared by the controlled hydrolysis of Ti(OEt)4 in ethanol [162]. The reachon condihons leading to monodispersed anatase nanoparhcles by this approach... 

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. 

Hydrothermal synthesis of a-alumina has been well studied. Since the hydro-thermal reaction of aluminum compound yields boehmite at relatively low temperatures (approximately 200°C), transformation of boehmite was examined and it was reported that more than 10 hours is required for complete conversion into a-alumina, even with a reaction at 445°C in a 0.1 N NaOH solution and in the presence of seed crystals. On the other hand, under glycothermal conditions, a-alumina is formed at 285°C for 4 h. The equilibrium point between diaspore (another polymorph of AlOOH) and a-alumina under the saturated vapor pressure of water was determined to be 360°C. However, near the equilibrium point, the transformation rate is very sluggish, and only a small conversion of diaspore is observed. Therefore complete conversion of diaspore into a-alumina requires a much higher temperature. Since boehmite is slightly less stable than diaspore, the hypothetical equilibrium point between boehmite and a-alumina would be lower than that for diaspore-alumina. However, a-alumina would not be formed by a hydrothermal reaction at such a low temperature as has been achieved in the glycothermal reaction. 

Idealized composition without vacancies F. Zhang, P.Y. Zavalij, and M.S. Whittingham, Synthesis and characterization of a pipe-structure manganese vanadium oxide by hydrothermal reaction, J. Mater. Chem., 9, 3137 (1999). 

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. 

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