Hydrothermal bomb

Kea.tlte, Keatite has been prepared (65) by the crystallisation of amorphous precipitated silica ia a hydrothermal bomb from dilute alkah hydroxide or carbonate solutions at 380—585°C and 35—120 MPa (345—1180 atm). The stmcture (66) is tetragonal. There are 12 Si02 units ia the unit cell ttg = 745 pm and Cg = 8604 pm the space group is P42. Keatite has a negative volumetric expansion coefficient from 20—550°C. It is unchanged by beating at 1100°C, but is transformed completely to cristobahte ia three hours at 1620°C. 

Hydrothermal bombs containing aqueous mixtures of a metal compound and phosphoric acid are typically heated to temperatures of 100-350 °C, generating autogenous pressures up to 300 bar. The pressure rises more rapidly above the critical temperature of water, 373 °C, and hydrothermal pressures of 3000bar are typically generated at 600-1000 °C in sealed metal tubes surrounded by a supporting pressure of an inert gas. 

The products of such reactions depend upon the pressure, temperature, pH, phosphate, and cation concentrations, and may be difficult to predict or rationalize. For example, the equation (3) produces an acid phosphate and a phosphate hydroxide. Microporous aluminophosphates and related phases (see Section 5.1.2) are prepared in hydrothermal bombs using hydrated cations or molecular templates such as organic amines or ammonium cations to direct the porous framework. Many new structures with metal phosphate chains, layers, or three-dimensional networks have been prepared hydrothermally in recent years, for example, templated vanadium phosphates and iron phosphates. ... 

The use of a hydrothermal bomb for preparation of a metal organic framework is a well-known technique in inorganic chemistry [31], However, the use of a hydro-thermal bomb for the preparation of scaffold is very rare. The final mixmre with the appropriate composition for scaffold preparation is sealed in a PTFE-lined acid digestion bomb and heated at 40°C for 8 h under autogeneous pressure. After that, the bomb is kept at room temperature to cool the product, which is then frozen at —20°C, Finally, the product is vacuum dried to obtain the desired scaffolds [32-34] (Dutta PK et al, unpublished results). 

FIG. 2 Mass spectrometer temperature-programmed dehydration (MSTPD) spectra showing the evolution of water as a function of temperature from (1) hydroxyapatite precipitated from an aqueous solution via an amorphous precursor phase, (2) hydroxyapatite prepared in a hydrothermal bomb, and (3) apatite from a mineral source. (After Ref. 28.)... 

The reaction is carried out in water, but the reagents are solids. A solid is precipitated and by reaction with other components of the solid mixture or with dissolved species it is transformed into a new solid. The reaction takes place in a closed vessel (a hydrothermal bomb) usually built in stainless steel lined with Teflon. The reaction takes place at 150-500 °C (depending on the liner used) and high autogenous pressures. Water acts as a pressure transmitter and as a solvent. Seed crystals and a temperature gradient are sometimes used for crystal growth. Under these conditions, solubilization of very insoluble species (e.g., silica) talces place, and the reaction proceeds for instance ... 

Solvo- and hydrothermal syntheses are commonly employed routes to nanomaterials. These involve dispersion of starting materials in a solvent of choice which are placed in a teflon liner and sealed in a hydrothermal bomb. The elevated temperatures and subsequent pressures drive product formation. There are many examples of these reactions in the literature and some recent reports are detailed below. 

Hydrothermal transformation of ferrihydrite in a teflon bomb at 180 °C for several days yields platy crystals up to several pm in size (Schwertmann and Cornell, 2000). 

Hydrothermal Growth. Hydrothermal growth is used in the form of solution transport for the growth of synthetic quartz. Cmshed natural quartz is placed into the lower part of a high pressure steel vessel, called a bomb, and thin seed plates are located in the upper region, as seen in Figure 5. The vessel is filled, for example, to 80% capacity with a 4% NaOH [1310-73-2] solution the NaOH acts as a mineralizer to increase the solubility of Si02. 

The second in situ technique is NMR. An autoclave fitting with the NMR cavity was designed by Gerardin et al. [59] and allows to follow the evolution of many parameters of the synthesis via the NMR characteristics of the different nuclei versus temperature and reaction time. The first measurement that can be reached now is the absolute value of the pH in hydrothermal conditions and the quantitative evolution of the concentration of protons in the bomb with the parameters of the synthesis [60], They proved that 14N NMR chemical shifts of well chosen amine compounds (imidazole and DABCO which possess complementary pKas) are precise pH indicators in aqueous solutions from room temperature to 475 K. Use of both amines permit to cover a wide range of about 9 pH units, with a precision of 0.1 pH unit. 

Hydrothermal synthesis does not require the water to be above its critical point. Huan, et al. published a synthesis of VOC6H5PO3XH2O prepared from phenylphosphonic acid, CeH5PO(OH)2 and vanadium(III) oxide, V2O3 (Huan et al., 1990). The two reagents were added to water, sealed in a Teflon acid digestion bomb, and heated to 200°C. Pure water has a vapor pressure of 225 PSI at 200°C, well within the bursting pressure of the bomb (1800 PSI). Unlike the quartz example, in this case, the solvent became incorporated into the final product. 

Morey bomb and Tuttle-Roy test tube bomb (made by Tem-Press), which are shown in Figure 1.1 and Figure 1.2. Hydrothermal synthesis involves water both as a catalyst and occasionally as a component of solid phases in synthesis at elevated temperatures (greater than 100°C) and pressures (more than a few atmospheres). At present, one can get many kinds of autoclaves to cover different pressure-temperature ranges and volumes. In the U.S., there are three companies ... 

Hydrothermal transformation of various Fe oxides. Ferrihydrite (2-line), lepidocrocite, akaganeite and goethite (if poorly crystalline) can be converted to large (1-3 am) hexagonal plates of hematite if kept under water in a teflon bomb at 180 °C for 10 days. 

Various metal complexes such as metal phthalocyanines, metal salenes or Ru pyridyl complexes have been incorporated in molecular sieves such as cavity-structured zeolites (faujasites, supercages with 1.3-nm diameter), channel-structured aluminium phosphates (AIPO4-5, channel diameter 0.73 nm) and channel-structured silicates MCM-41 (channel diameter 3.2 nm) [51-53]. Different strategies were applied for the inclusion of the phthalocyanines. For example, whereas the zeolite-encaged phthalocyanines (1 R = -FI M = Co(II), Ru(II), etc.) are synthesized by the reaction of a transition metal ion-exchanged zeolite with phthalonitrile in a closed-bomb vessel [54], in the cases of AIPO4-5 and MCM-41 substituted derivatives of phthalocyanines were added to the mixture during the hydrothermal synthesis of the molecular sieve [55,56]. 

Most of the above described coordination polymers have been obtained thanks to hydrothermal methods. Typically, they consists in synthesizing the coordination polymers in a Teflon-lined Parr bomb under relatively high temperatures. The reaction occurs at high temperature (typically between 150 °C and 250 °C) and elevated pressure (autogenous pressure). As far as lanthanide coordination polymers are concerned, this synthetic method seems to lead to materials exhibiting high dimensionality and low hydration rate (Cao et al., 2002b). That is why... 

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