Hydrolysis reactions powder synthesis

The most common method for preparing BaTiOj powder is the conventional solid state reaction [3], alternative methods have been developed [12], such as alkoxide hydrolysis [4], hydrothermal synthesis [5], colloid method [6], and so on. 

The hydrolysis reaction usually occurs at room temperature and dehydration occurs below 600°C, resulting in the formation of very fine ceramic particles, that is, 2-5 nm [22]. This method has been successfully used to make high-purity submicrometer-sized oxides from several metal alkoxides [23,24]. Focus on multicomponent oxide powder synthesis through the two-step hydrolysis and dehydration of metal alkoxides constitutes the remainder of this chapter. 

Firstly, Che kinetics and mass transfer effects of catalyst synthesis via deposition precipitation onto pre-shaped and powder carriers have been studied under pseudo-stationary conditions. The precipitation of manganese hydroxide onto silica by urea hydrolysis has been used as a model reaction. The overall disappearance of manganese ions from the aqueous solution could be described as a first-order process. The rate-determining step for Mn deposition is related -as expected - to the urea hydrolysis. From the distribution of Mn over the silica granules after precipitation the rate constant for the surface deposition process has been determined. The latter process has a much higher rate constant than the rate—determining hydrolysis reactions. The surface reaction appears to determine the ultimate distribution by a combined process of reaction and diffusion. The consequences of this study for the viability of deposition precipitation onto pre-shaped carriers for practical application are addressed. 

The synthesis of substrates 88-96 for the IMDA reaction is depicted in Schemes 14 and 15. Treatment of 74 with I2, PPh3, and imidazole [91] provided iodide 75. The substitution of the iodo group in 75 by an anion generated from 2-methylpropionitrile [56-60] provided a hepta-nitrile derivative 76 in 95% yield. Then the secondary hydroxyl group in 76 was protected as an MOM ether to provide 77. Reduction of 77 with DIBALH followed by acidic hydrolysis gave aldehyde 78. The Horner-Emmons reaction of 78 with triethyl 4-phosphonocrotonate in the presence of LiOHH20 and molecular sieves 4A powder [68]... 

Hydrolysis and condensation rates depend on the molecular structure of metal alkoxides and alkoxide precursors have to be chosen as a function of the desired material final product. In the case of Ti02, for instance, monomeric precursors such as Ti(OPF)4, in which Ti is fourfold coordinated, react very quickly with water leading to the uncontrolled precipitation of polydispersed Ti02. The reaction is much slower with oligomeric precursors such as [Ti(OEt)4] in which Ti has a higher coordination number. Spherical monodispersed Ti02 powders can be produced via the controlled hydrolysis of diluted solutions of Ti(OEt)4 in EtOH. On the contrary, monomeric precursors are more convenient for the sol-gel synthesis of multicomponent oxides. The perovskite phase BaTiOs is formed upon heating around 800 °C when [Ti(OEt)4] is used as a precursor. This temperature decreases down to 600 °C with the monomeric precursor Ti(OPT)4 which favors the formation of Ti-O-Ba bonds. ... 

Additively, two control experiments were performed. The first experiment involved excluding either cadmium salt or selenium precursor from the reacting mixture. When no cadmium was added, the solution had a brownish colour but no precipitate formed. A white flake-like precipitate with fibre-like structure was observed when Se precursor was excluded from the reaction mixture. In the second control experiment, the pH value of Cd salt solution in 1) glycine and 2) water was raised until the hydrolysis of cadmium started (pH 10 and pH 7, correspondingly). In both cases a white precipitate formed. The precipitate formed as a result of the hydrolysis of Cd in water was powder-like, while the product formed in glycine had a flake-like appearance. TEM images of these precipitates are fibre-like for both samples. This confirms that such features previously seen in other samples were formed due to the hydrolysis of cadmium ions, which would result in the formation of Cd(OH)x. The formation of such nanoflakes was previously reported for the synthesis of cadmium hydroxide in an aqueous media by hydrothermal method [2]. 

Figure 2.9 Enzymatic synthesis of jj-nitrobenzyl-D-glucopyranoside (pNBG) by reverse hydrolysis in a monophasic dioxane-water system. The reaction was performed in 1.0 mL of dioxane-buffer medium (Na2HP04-KH2PO4, 70 mM, pH 6.0) by shaking a mixture of 0.25 mmol glucose, 1.0 mmol pNBA and 5 mg of enzyme powder at 50 °C and 160 rpm. Symbols ( ) initial rate ( ) final conversion.

The present applications of ceria-based ceramics impose strict requirements on the quality and purity of the powders used. Several studies have described the synthesis of ceria nanopowders of high quality and with a well-defined morphology. Typical methods of preparation include hydrothermal synthesis [263, 264], the hydrolysis of an alkoxide solution (sol-gel) ]265], chemical precipitation [266], mechanochemical processing ]267], and gas-phase reaction ]268]. Emulsion techniques can also be used, as these reduce not only the production costs of high-purity spherical powders but also the degree of aggregation. Thus, ceria powders with an average particle size <20 nm and a narrow particle size distribution can be... 

Hill (1898) showed the reversal of hydrolysis with a maltase extracted from a brewer s yeast. He dried the organism to a powder, which he then heated to 100 °C while dry. The maltase was finally extracted from the yeast with 10 volumes of 0.1% (w/v) sodium hydroxide. The extract, which was neutral to litmus, not only hydrolysed maltose [a-o-Glc-(l - 4)-Glc (9)] to glucose, but also would synthesize a product from glucose which Hill believed, on the basis of a careful analytical study of the osazone, to be maltose. At a total sugar concentration of 40% (w/v) the reaction would approach its equilibrium position from either side (Table 1.4, Figure 1.2), and the synthesis was less effective at lower concentrations. Emmerling... 

SevCTal types of reactions may be employed in hydrothermal synthesis (69). A common feature is that precipitation of the product generally involves forced hydrolysis undCT elevated temperature and pressure. TTie powders have several desirable characteristics, but they also suffer from a few disadvantages so that their benefits are normally not fully realized. In hydrothermal synthesis, the crystalline phase is commonly produced directly so that a calcination step is not required, as in the case of several other synthesis routes. The powders also have the characteristics of very fine size (10-100 nm), narrow size distribution, singlecrystal particles, high purity, and good chemical homogeneity. 

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