Autoclaves, hydrothermal synthesis

Figure 3.24 Multi-autoclave hydrothermal synthesis reactors. Reproduced with permission from [118], Copyright (1998) Wiley-VCH...

Figure 3.25 Multi-autoclave hydrothermal synthesis reactor equipped with autosampler (State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, jilin University)...

The same periodic structures can also be formed from alternating AIO4 and PO4 tetrahedra the resulting aluminophosphates are not called zeolites but AlPOs. Zeolites are made by hydrothermal synthesis under pressure in autoclaves, in the presence of template molecules such as tetramethylammonium, which act as structure directing agents. 

Hydrothermal Synthesis of Brookite. Typically, NaOH solution (2M) was added to 5 mL titanium tetrachloride with stirring to adjust the pH value of the solution to 10, producing a basic colloidal solution. The solution obtained was transferred into an autoclave and heated at 200°C for 24 h. The white precipitates were separated by centrifugation and washed three times with deionized water and once with ethanol. The product was dried at 70°C in air overnight and was ground to a fine powder before the deposition of gold precursor on its surface. 

Hydrothermal synthesis of ZnSe and CdSe, using powdered Se with either Zn or Cd heated to 180°C in a water filled autoclave, yields 70-100 nm particles without the need of any stabilizer or capping agent. While most hydrothermal synthesis routes of chalcogenides involve the preparation of binary systems, more complicated ternary compounds can also be prepared. For example, Znln2S4, a ternary chalogenide photocatalyst has been hydrothermally prepared and found to photocatalytically reduce water under visible... 

Hydrothermal Method. Iron [Fe(III)], barium, and the dopants are precipitated as their hydroxides and reacted with an excess of sodium hydroxide solution (up to 6 mol/L) at 250-350 °C in an autoclave. This is generally followed by an annealing treatment at 750-800°C to obtain products with the desired magnetic properties. Many variations of the process have been described [5.36]-[5.40], the earliest report being from 1969 [5.41], In later processes, hydrothermal synthesis is followed by coating with cubic ferrites, a process resembling the cobalt modification of iron oxides (see Section 5.1.2). The object is to increase the saturation magnetization of the material [5.42]-[5.44],... 

The hydrothermal synthesis system for secondary scale screening is based on multi-well-tumbled autoclaves modules [49]. Each module contains 16 PTFE vials each vial has an internal volume of 7 mL. The reactors can operate up to 200 °C. Many chemical and physical factors can affect the final hydrothermal synthesis outcome, making it a good application for high-throughput methods. In addition to the gel ingredients, these factors include the order of reagent addition, gel mix-... 

Simple silica matrices To further illustrate the limit of this chapter, we briefly discuss one type of materials that will not be systematically reviewed layered lanthanide silicates obtained by hydrothermal synthesis. An alkaline solution of sodium silicate and lanthanide chlorides was stirred to produce a gel which was subsequently put into an autoclave under pressure at 230 °C during seven days, a procedure during which lamellar photoluminescent silicates... 

The hydrothermal synthesis of the faujasites was carried out at 373 K in stainless steel autoclaves in static condition. The faujasites were synthesised using seeds of an aluminium ferri-silicate gel. 16 g of the homogeneous seed slurry (aged for 24 hrs at room temperature) was added to an aqueous gel mixture containing sodium silicate (86.8 g) sodium aluminate (5.6 g) aluminium sulphate (7.5 g) and ferric... 

As already mentioned, most of the syntheses use hydrothermal synthesis. The reaction takes place in the autoclave, which acts as a black box for the understanding of the mechanism of formation of the solids which are finally isolated. Nothing was known about the interaction between organic and inorganic species. This needed to choose appropriate systems for the study of the pertinent chemical parameters, to carefully characterize the compounds and use in situ measurements to collect informations able to shed some light on the steps of formation of these solids. Such an approach has been initiated only on oxyfluorinated species, and shall be developped in the following. 

As already mentioned at the end of Sec. 7.2, autoclaves in hydrothermal synthesis are black boxes , and all the attempts to explain the formation of the porous solids originated from ex situ experiments. The above hypothesis concerns reactions within the autoclave and requires for its analysis in situ measurements in hydrothermal conditions which were never performed so far. These experiments must give information both on the solution and on the solid. Moreover, in the systems chosen for such studies, the kinetics of formation must be adapted to the possibilities of observation of the chosen technique. Two of them are particularly useful for this purpose in situ diffraction studies using synchrotron radiation (SR) and in situ NMR (liquid and solid state). 

Some interesting devices, both on classical (46) and miniamrized scale (47, 48) have been developed for the combinatorial hydrothermal synthesis of zeolites (49,50). Solutions of suitable precursors were dispensed into the wells of the multireactor autoclaves, which were then sealed and heated following appropriate protocols. X-ray characterization of the individuals allowed the determination of the properties of the library individuals. Both protocols and instruments allowed the synthesis of hundreds of composites and their automated (47) or semiautomated (46, 48) work-up and characterization. Structural information was derived from both reports, proving the applicability of combinatorial technologies to hydrothermal synthesis procedures. 

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. 

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... 

Hydro(solvo)thermal synthesis is a heterogeneous reaction in aqueous or non-aqueous media with temperatures above the boiling point of the solvent and pressures higher than 1 bar. Hydrothermal synthesis is believed to have been first introduced by Schafhautl in 1845 with water as the reacting media, and the device in the modern form of hydro(solvo)thermal synthesis, a sealed glass ampoule in an autoclave, was developed by de Senarmont in 1851 [11]. 

FIGURE 1.1 Autoclave with flat plate closure. (Morey, G.W., Hydrothermal synthesis, J. Am. Ceram. Soc., 36, 279, 1953.)... 

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 ... 

FIGURE 10.2 Left two different contact configurations between the support and the synthesis gel during hydrothermal synthesis. Right Commercial autoclave from Parr Instrument Company (http /www.parrinst.com/). 

The hydrothermal synthesis of the host compounds takes place in Teflon-lined steel autoclaves with 10 mL volume. The reaction mixture and parameters were optimized in order to obtain large... 

SAPO-5, SAPO-34 and SAPO-37 molecular sieves were prq>ared via hydrothermal synthesis. Typically, the synthesis gel was poured into a Teflon bottle which was then heated in an stainless steel autoclave for 24-48 h at 4S3K. The composition of the synthesis gel was as follows ... 

Hydrothermal synthesis is one of the important methods for producing fine powders of oxides. A hydrothermal system is usually maintained at a temperature beyond 100 °C and the autogenous pressure of water exceeds the ambient pressure, which is favorable for the crystallization of products. Recent research indicates that the hydrothermal method is also a practical means for preparing chal-cogenide and phosphide nanomaterials, and hydrothermal treatment is an effective method for passivating porous silicons. Similar to hydrothermal synthesis, in a solvothermal process, a non-aqueous solvent, which is sealed in an autoclave and maintained in its superheated state, is the reaction medium, where the reactants and products are prevented effectively from oxidation and volatilization and the reaction and crystallization can be realized simultaneously. Furthermore, organic solvents may be favorable for the dispersion of non-oxide nanocrystallites and may stabilize some metastable phases. 

Jia/Noble and coworkers [87,88] reported the successful synthesis of silicalite membranes on y-alumina composite supports using an interesting modification of the in situ crystallisation method. The support consisted of a short a-alumina tube coated on the inside with a 5 pm thick y-alumina film with an average pore diameter of 5 nm, commercially available from US Filter. The precursor solution was put into the support tube after plugging both ends with teflon and the filled tube was then placed in a teflon-lined autoclave. Hydrothermal treatment was carried out at 180°C for 12 h. After removal from the autoclave and washing the formed zeolite layer with water, the procedure was repeated with the tube inverted from its previous orientation to obtain a uniform coating. As reported by Vroon et al. [82,84,98], Jia/Noble [88] also concluded that at least two synthesis steps are necessary to obtain defect-free membranes. 

The Carbon Black Pearls labeled BP 2000 was supplied by Cabot Corp. 10 g of carbon black pearls (BP-2000) was dried in an oven at 110 °C overnight. In a 200-mL flask, 17.2 g of 40 % tetrabutylammonium hydroxide, 2.5 g of I BO and 15.1 g of ethanol was stirred to obtain a homogeneous solution. The dried carbon black was impregnated with this solution to incipient wetness. After evaporation of ethanol at room temperature for 12 h, carbon particles were impregnated with 19.3 g of tetraethyl orthosilicate. This mixture was left at ambient conditions to hydrolyze overnight. The hydrothermal synthesis was performed in a Teflon-lined autoclave at 180 °C for 72 h. Then the autoclave was cooled, a solid product was filtered and washed with distilled water. Calcination was carried out in a muffle oven at 550 °C for 18 h [86],... 

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