Solids precursor method

More successful are solid-phase methods in which the linear precursor is attached through a labile ester bond (e.g. o-nitrophenyl) to a polymer. 

In the via precursor method, however, it is difficult to prepare the ji-conjugated polymers with ideally developed -conjugation system the -conjugated polymer chains contain many conformational defects because the jc-conjugated chains are caused to develop from disordered precursor polymer, which form random coil conformation, in solid state. For the preparation of polymers with well-developed jc-conjugation system by the via precursor method, accordingly, it is necessary to introduce orientational and conformational orderliness of the precursor polymers in the films. 

The precursor method achieves mixing at the atomic level by forming a solid compound, the precursor, in which the metals of the desired compound are present in the correct stoichiometry. For example if an oxide MM 204 is required, a mixed salt of an oxyacid such as an acetate containing M and M in the ratio of 1 2 is formed. The precursor is then heated to decompose it to the required product. Homogeneous products... 

The products of the precursor method are usually crystalline solids, often containing small particles of large surface area. For some applications, such as catalysis and barium titanate capacitors, this is an advantage. 

The mixed hydroxide precursor (MH) was treated at 500, 900 and 1200 °C. Again a progressive increase in crystallinity was observed with the increase of the temperature. FT-IR spectra showed a band around 3400 cm 1 for the sample treated at 500 °C, which was absent in samples treated at 900 and 1200 °C. The explanation follows that given for the LDH system. As observed for the spinel oxide prepared by the solid-reaction method the treatment with mineral acid did not affect the PXRD pattern. 

Definitions. Colloids are solid particles with diameters of 1 100 nanometers, A sol is a dispersion of colloidal particles in a liquid. A gel is an interconnected rigid network of sub-micrometer dimensions. A gel can be formed from an array of discrete colloidal particles (Method I) or the 3-D network can be formed from the hydrolysis and condensation of liquid meial alkoxide precursors (Methods 2 and 3). shown in Fig. 11. The metal alkoxide precursors used in Methods 2 and 3 are usually Si(OR)4 where R is CHj. C-Hj. or C3H7. The metal ions can be Si, Ti. Sn. Al, and so on,... 

Synthesis of octreotide and derivative thereof can be carried out by two methods. The first method is synthesized initially by fragment condensation solution phase procedures. The synthetic process of octreotide has been described by Bauer et al. (1). The second method is the synthese by solid-phase procedures. Edward et al. (2) isolated side chain protected octreotide with a total yield of 14% by cleaving the protected peptide from the resin with threoninol. Arano et al. (3) carried out another solid phase method for octreotide, and produced it in overall 31.8% yield based on the starting Fmoc-Thr(tBu)-ol-resin. The basic difference from the other procedures already described is that the introduction of the threoninol is carried out upon the protected peptidic structure (resin-free), which, when appropriately activated, leads quantitatively and without needing to make temporary protections upon the threoninol, to the protected precursor of octreotide, which in turn, with a simple acid treatment leads to octreotide with very high yields. 

Depending on the way the solid precursor is volatized, three PVD methods are discerned ... 

Roberts, G. et al. (2001) Synthesis of lead nickel niobate-lead zirconate titanate solid solutions by a B-site precursor method, J. Am. Ceram. Soc., 84, 2869-72. 

The methods most commonly used for preparing catalysts are precipitation (Section A 2 1 3) and impregnation (Section A 2 2 1 1) In both of them, the catalyti cally active material is transferred from a liquid phase, usually an aqueous solution, to a solid By contrast, other catalysts are obtained from solid precursors Solid state reactions, namely solid-to-solid reactions in which both the starting material (the catalyst precursor) and the catalyst are solids, offer convenient methods to prepare several industrial catalysts, especially those con taming two or more metallic elements or their oxides The reason of the conspicuous efficiency of these methods to prepare phases containing two or several metallic elements is due to special features of solid-state reactions, compared to liquid-to-solid or gas-to-solid reactions This section briefly outlines these peculiarities and presents the most frequent types of solid-state processes used in preparing catalysts... 

The previous part of this section makes clear that, unless a method is found to directly prepare a catalytic phase (usually an oxide) containing all the wanted metallic elements, and in the required proportions, some more indirect method should be selected. In general, however, direct preparation from the gas or liquid phase will be impossible for the reasons given above. It becomes necessary to resort to a two-step or multiple-step approach using at least one solid precursor. 

In very rare cases catalysts can be prepared by dry methods, namely reactions in the solid state. However, the most commonly used methods for preparation of mixed oxide catalysts start from a liquid (Fig. 6). Since complete mixing at the molecular level is possible in solution, a solution of all the concerned salts is often taken as the common starting material. One can essentially distinguish two steps in the procedure the preparation of a homogeneous solid precursor and the careful decomposition of this precursor to the oxide. For the first step, it is crucial to avoid the segregation of the oxide species into separate solid phases. This is usually achieved by ... 

As mentioned earlier, diffusion distances for the reacting cations are rather large in the ceramic method. Diffusion distances are markedly reduced by incorporating the cations in the same solid precursor. Synthesis... 

Few reports have discussed the structures of Mo V Te Nb oxide catalysts in relation to propane oxidation and ammoxidation. Some reports indicate that not only the elemental composition but also preparative variables greatly affect the structure and performance of Mo-V-Te Nb oxide catalysts. Among the preparative variables, methods for precursor preparation appear to be critical. One example is Mo V Te-Nb oxide, which when prepared by a sohd-state method from corresponding oxides of each element is a mixture of M0O3 and (Mo-X)50i4 (X is other cations) and is inactive for the propane ammoxidation. However, Mo-V-Te-Nb oxide prepared by a hydrothermal reaction method from the same oxide by the solid-state method is a mono-phasic oxide with an orthorhombic layered structure, which selectively catalyzes propane to acrylonitrile. ... 

One of the difficulties with the classical solid-state reaction is that mechanical mixing methods are relatively ineffective in bringing the solid reactants in contact with one another. Diffusion lengths, on an atomic scale, are still enormous and the temperatures required may preclude the formation of phases that might be stable at intermediate temperatures. One method, called a precursor method, involves the formation of a mixed-metal salt of a volatile organic oxyanion such as oxalate by wet chemical methods, which result in mixing essentially on the atomic level. The salt is then ignited at relatively low temperatures to form the mixed-metal oxide. The method has been applied successffilly to the preparation of a number of ternary transition metal oxides with the spinel structure. ... 

Production of net-shape silica (qv) components serves as an example of sol—gel processing methods. A silica gel may be formed by network growth from an array of discrete colloidal particles (method 1) or by formation of an interconnected three-dimensional network by the simultaneous hydrolysis and polycondensation of a chemical precursor (methods 2 and 3). When the pore liquid is removed as a gas phase from the interconnected solid gel network under supercritical conditions (critical-point drying, method 2), the solid network does not collapse and a low density aerogel is produced. Aerogels can have pore volumes as large as 98% and densities as low as 80 kg/m (12,19). 

The solid-state method requires high temperature heating or high mechanical energy for diffusion of reactants. Synthesis in liquid phase is useful to prepare solid compounds that contain the different cations in an ideally atomic dispersion. There are two methods concerning this concept one is a precursor method and the other is a co-precipitation method. 

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