Direct crystalization method, synthesis

The direct high yield synthesis of oxaspiropentanes from almost any type of aldehyde or ketone represents a particularly useful transformation because of the high reactivity of such compounds. This approach proves to be exceptionally simple. The DMSO reaction mixture can be directly extracted with pentane or hexane, the hydrocarbon solvent removed and the product isolated by distillation or crystallization. Since diphenyl sulfide is the only by-product extracted with the oxaspiropentane, the mixture can normally be used for most further synthetic transformations. Table 2 summarizes some of the oxaspiropentanes prepared by this method. 

A direct and versatile synthesis of the thieno[2,3-6 ]pyrazine ring system was found by Schneller and coworkers and consists of the reaction of 2-chloro-3-cyanopyrazine (361) with ethyl a-mercaptoacetate in the presence of sodium carbonate (Scheme 106) (75JHC513, 76JHC273 cf. 79MI31703). The conversion of (362) to thieno[2,3- >]pyrazine (363) is accomplished by straightforward methods. The compound (363) forms colorless crystals with m.p. 44 °C. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

We prepared boron substituted mordenite by direct synthesis from gel precursors and by post- synthetic substitution into dealuminated mordenite. Direct substitution is favored in aluminum deficient gels, but exacting crystallization requirements for mordenite formation limit the amount of boron that can be incorporated into the framework structure. Higher substitution levels were achieved using a post-synthetic treatment. Boron substituted zeolite Y could not be prepared by a similar direct synthetic method, but post-synthetic methods were effective at providing low substitution levels. This demonstrates the more general utility of post-synthetic substitution methods. The hexane cracking activity of... 

M. Matsukata, K. Kizu, M. Ogura, and E. Kikuchi, Synthesis of EMT Zeolite by a Steam-assisted Crystallization Method using Crown Ether as a Structure-directing Agent. Cryst. Growth Des., 2001, 1, 509-516. 

In summary, it can be said that, in homogeneous alkaline solution, either methanolic or aqueous, an equilibrium is established between the aldose, nitromethane, and deoxynitroalditols, and that, for successful addition, this equilibrium must be displaced in favor of the alditols, either through precipitation of their sodium salts from methanolic solution, or by direct crystallization from aqueous solution. Because of these requirements, the method is not so widely applicable as the cyanohydrin synthesis, but, for selected sugars, such as n-gZgcero-D-ZaZo-heptose and o-erythro-i -manno-octose, its simplicity and the satisfactory yields obtained make it the method of choice. 

Catalyst preparations have been previously discussed (17-18). Thus, the rare earth phosphates were prepared by conventional methods. They were then modified by introducing an impregnating compound (alkaline earth salt) either directly into the synthesis mixture, or after filtration and drying. After impregnation, this catalyst precursor was c Icined at 500°C for two hours. The product crystals were hexagonal in structuro and the quantity of alkaline earth was measurable by elementary analysis. 

These materials are normally prepared by mechanical mixing of oxides, but the combustion method is also used to produce impurity-free materials with high specific surface area. A perovskite (SrTiFe03.5)-alloy (PtRu) cermet is obtained directly by combustion synthesis (fig.l). It presents a low crystallinity, low Pt crystal size ( 15nm) and a specific surface area of 48.5 mVg. 

Two main methods for synthesizing TS-1 by direct crystallization are described in the early literature (5a, 87). Both methods are variations of the general recipe for primary synthesis (as described in Section 2.1) and rely on the same template, namely, the tetra-fi-propylammonium (TPA) ion. The methods differ mainly in the details regarding the preparation of the precursor mixture for the crystallization of the titanium-containing silicalite. 

Nowadays, most methods belong to this synthesis type, the liquid medium playing practically always tte role of the solvent (solvothermal synthesis).On account of the complexity and the macromolecular character of the structures, but of the nature of the bonds, there are no known methods based on a direct crystallization of a liquid reagent as, for instance, in the case of the crystallization of NaG from a melt of NaQ. 

In this chapter, we will focus on the synthesis and characterization of lanthamun strontimn manganites for application as cathodes in SOFC. Firstly we will review some characteristics of perovskites which are important to imderstand their performance in SOFC crystal structure, electrical conductivity, thermal stability and chemical reactivity. Then, the review will be directed to the synthesis methods of LSM powders, focusing on those used by our group solid-state, combustion, and citrate methods. Finally, extensive results and discussions will be presented on the characterization of LSM synthesized by these three different methods. 

Cocoa butter substitutes and equivalents differ greatly with respect to their method of manufacture, source of fats, and functionaHty they are produced by several physical and chemical processes (17,18). Cocoa butter substitutes are produced from lauric acid fats such as coconut, palm, and palm kernel oils by fractionation and hydrogenation from domestic fats such as soy, com, and cotton seed oils by selective hydrogenation or from palm kernel stearines by fractionation. Cocoa butter equivalents can be produced from palm kernel oil and other specialty fats such as shea and ilHpe by fractional crystallization from glycerol and selected fatty acids by direct chemical synthesis or from edible beef tallow by acetone crystallization. 

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