Synthetic studies, hydrothermal

Since the first synthesis of mesoporous materials MCM-41 at Mobile Coporation,1 most work carried out in this area has focused on the preparation, characterization and applications of silica-based compounds. Recently, the synthesis of metal oxide-based mesostructured materials has attracted research attention due to their catalytic, electric, magnetic and optical properties.2 5 Although metal sulfides have found widespread applications as semiconductors, electro-optical materials and catalysts, to just name a few, only a few attempts have been reported on the synthesis of metal sulfide-based mesostructured materials. Thus far, mesostructured tin sulfides have proven to be most synthetically accessible in aqueous solution at ambient temperatures.6-7 Physical property studies showed that such materials may have potential to be used as semiconducting liquid crystals in electro-optical displays and chemical sensing applications. In addition, mesostructured thiogermanates8-10 and zinc sulfide with textured mesoporosity after surfactant removal11 have been prepared under hydrothermal conditions. 

Beyerlein et al. (33) studied the catalytic properties of a series of ultrastable synthetic faujasites dealuminated by steaming and by acid extraction to determine catalytic acidity as a function of framework characteristics. They found that carbonium-ion activity in isobutane conversion is proportional to framework-Al content, and comparing results obtained by using hydrothermally and AHF-dealuminated synthetic faujasite, they found that the steamed material, which contains extra-framework Al, gave a large increase in carbonium-ion activity compared with the AHF-treated material, which had a relatively clean framework. This indicates that strong acidity exhibited by mildly steamed synthetic faujasite, while directly related to framework-Al content, depends on a balance between framework and extra-framework Al, and that this extraframework Al contributes greatly towards catalytic performance. 

Another feature of hydro(solvo)thermal synthesis is the operability and tunability of hydrothermal and solvothermal chemistry, which bridges the synthetic chemistry and physical properties of synthesized materials. With deepening studies on hydrothermal and solvothermal synthesis chemistry, more and more reaction types have been discovered. Compared with other synthesis and preparation techniques, hydro(solvo)thermal synthesis methodology and techniques have irreplaceable advantages. So far, a variety of materials and crystals used in many fields could be hydrothermally or solvothermally synthesized, and the quality and properties of the resulting products are often much better than those prepared by other methods. 

Many zeolites were crystallized from strongly basic solution containing silicate and aluminate. In strongly basic solution, silicate exists as polymerized-state polysilicates with a variable distribution of molecular weight. Aluminate reacted with polysilicate to form various zeolite structures via condensation reaction under hydrothermal or solvothermal conditions. Therefore, in the study of the synthetic chemistry of zeolites, one of the key issues is to understand the existing state and reactivity of the polymerized silicate. 

F was used for the first time as mineralizer in the synthesis of zeolites under neutral or acidic conditions by Flanigen and Patton1941 Subsequently, Guth and Kessler further studied this new synthetic method.[95] Many high- or all-silica zeolites such as ZSM-5, ZSM-23, Theta-1, ferrierite, / , MTN, AST, UTD-1, ITQ-3, ITQ-4, and heteroatom (B, Al, Fe, Ga, and Ti)-substituted high-silica zeolites could be synthesized from an F synthetic system. In addition, most of the open-framework gallophosphates could be hydrothermally synthesized under the influence of F. ... 

V 0S04(H20)4]-S04-[HN(C2H4)3NH]. These molecular compounds, which we have fnlly characterized by single crystal x-ray structnre analysis, TGA, manganometric titration and spectroscopic studies, are reproducible and readily prepared in decent yields by the following hydrothermal synthetic methods. 

Other early MAS-NMR studies avoided the paramagnetic issue by synthesizing clays hydrothermally (16-18). Their conclusions were similar to previous conclusions Si is a powerful means of studying short-range order around Si in the lattice, while Al NMR is somewhat useful, but plagued by broadening and thus quantitation problems. Samples studied were synthetic beidellite (16], synthetic mica montmorillonites 117], and a series of synthetic 2 1 trioctahedral clays (18]. 

Pure VSb04 has been prepared and shown to have a structure similar to rutile, with a random distribution of Sb and V atoms. Magnetic measurements rule out the Sb V 04 formulation but it is not possible to make a decision between the alternatives Sb V 04 and Sb V 04. The ferroelectric properties of synthetic SbNb04 prepared by a hydrothermal method have been studied, and a phase diagram for the Li(Taj,Sbi a )03 system has been constructed from A -ray data, ... 

Other technological aspects of carbon nanotube synthesis currently under scrutiny include study of the growth mechanism [67,71], attempts to control the diameter [72-74], processes which yield very long CNTs [70,75], optimization of the catalyst composition [76], and improvements in purity [77]. A major area of focus is the production of CNTs at selected sites on a substrate (micropatterning) [78-81]. Other synthetic methods investigated have been (i) a solvothermal route, in which reactants are heated in solution in a sealed autoclave [82,83] (ii) a solid-state metathesis process [84] (iii) a hydrothermal process which produces MWNTs from amorphous carbon [85] and (iv) low-temperature processes [59]. 

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