Hydrothermal alkaline conditions

Solvent isotope effect studies with H-NMR, H-NMR, LC-MS and gas-MS analyses were used to study formation of H2 and lactic acid from glycerol under hydrothermal alkaline conditions in order to identify different intermediates and elucidate the reaction mechanism." ... 

The products are thermodynamically favored under the oxic alkaline conditions that are characteristic of most of the ocean. Reaction rates are slow, so metal oxides tend to precipitate onto detritus or preexisting nodules because of the catalytic effect of the surfaces. The Fe and Mn are supplied by both river and hydrothermal sources. For Mn, these two sources are about equal. 

ASSEMBLY CONDITIONS Hydrothermal Alkaline or Acidic Room temperature Neutral pH... 

Silicalite (samples 3-9) and dealumina ted ZSM-5 (samples 1-2) were subjected to hydrothermal treatment under mild alkaline conditions at various boron-to-zeolite ratios, temperatures and times of treatment (see Table I). To consider the question of the status of boron in the boronated samples we have used nB MAS NMR, a technique capable of providing direct information on the type of short-range environment of the nucleus. 

Among the rare earth elements, cerium is an exception. The impurity of CeV04 NPs prepared under high alkaline conditions was Ce02, rather than cerium hydroxides, because Ce(OH)3 was prone for dehydration and oxidation. Chen s)mthesized CeV04 NPs from NH4VO3 and Ce(N03)3 in hydrothermal reactions with different pH (ranging from 9 to 14). The pH value of 12 was shown to be the upper threshold for pure vanadate NPs (Chen 2006). 

The prime requirement for a molecule to act as an SDA in a porosil synthesis is stability under the hydrothermal synthesis conditions. Although these are very mild compared to conventional high-temperature solid-state syntheses, they are harsh enough to destroy labile organic compounds. In the usual procedure, a reaction mixture consists of a reactive silica source (amorphous silica, e.g, fumed or precipitated silica, silica gel), a mineralizer (e g. NaOH), the SDA and water. Alkaline mineralizers such as NaOH raise the pH to 12 or higher These strongly alkaline solutions are treated at elevated temperatures (up to 200°C) for several days. 

Il in, Turutina, and co-workers (Institute of Physical Chemistry, the Ukrainian S.S.R. Academy of Sciences, Kiev) (113-115) investigated the cation processes for obtaining crystalline porous silicas. The nature of the cation and the composition of the systems M20-Si02-H20 (where M is Li+, Na+, or K+) affect the rate of crystallization, the structure, and the adsorption properties of silica sorbents of a new class of microporous hydrated polysilicates (Siolit). These polysilicates are intermediate metastable products of the transformation of amorphous silica into a dense crystalline modification. The ion-exchange adsorption of alkali and alkaline earth metals by these polysilicates under acidic conditions increases with an increase in the crystallographic radius and the basicity of the cations under alkaline conditions, the selectivity has a reverse order. The polysilicates exhibit preferential sorption of alkali cations in the presence of which the hydrothermal synthesis of silica was carried out. This phenomenon is known as the memory effect. 

Investigations on the hydrothermal crystallisation of nitrate enclathrated cancrinite were performed using the alkaline transformation of zeolites A and X at a temperature of 353 K in 2-molar and 16-molar NaOH-solutions. The conversion of the zeolites was followed in the early stage of the reactions for times up to 48 hours by XRD and IR- spectroscopy. A fast and total transformation of zeolite X into cancrinite could be stated in most of the experiments, independent of the alkalinity. In contrast the conversion of zeolite A under low alkaline conditions was slower and accompanied by a sodalite-cancrinite cocrystallisation as well as the formation of an intermediate phase between both structure types. The results indicate a more kinetically controlled reaction mechanism for zeolite A transformation. 

Solid-state NMR has been used extensively to identify the environment of cations such as aluminium that are introduced into the silicate walls. When the mesoporous silicate is synthesised under alkaline conditions, for example, aluminium is readily incorporated into tetrahedral sites in the silicate network. Subsequent calcination and hydrothermal treatment can result in some of this aluminium changing coordination from tetrahedral to five- and six-fold, with chemical shift vales of 30 and 0ppm. Similar studies have been performed on other substituting elements. 

As the twelfth most abundant element in the earth s crust (0.106% in crustal rocks), manganese is found in over 250 different minerals of which some 10-15 are of commercial importance. In primary hydrothermal deposits the metal occurs as silicates, but as it is readily depleted from igneous and metamorphic rocks by weathering, particularly under acid conditions, it is normally found in commercially useful ores as oxides or carbonates, deposited, as are iron and aluminum, under alkaline conditions [9]. 

Synthesis may also be carried out at a lower pH using fluoride-containing media, wherein F ions are thought to act as structure directors via strong interactions with framework Si atoms. Consequently, the nucleation rate is decreased, which yields larger crystals relative to standard alkaline hydrothermal routes.The fluoride route under neutral/acidic pH conditions is also extremely useful to synthesize zeolite-like materials called zeotypes, which contain elements other than silicon and aluminum (e.g., titanosilicates, zirconosilicates, etc.). Under alkaline conditions, the precursors would be preferentially precipitated as hydroxide species rather than ordered arrays. 

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