Hydrothermal conditions: amorphous silica

Scheme 2. Encapsulation of size- and shape-controlled Pt nanoparticles under neutral hydrothermal synthesis conditions of SBA-15. Silica templating block copolymers and silica precursors were added to PVP-protected Pt nanoparticle solutions and subjected to the standard SBA-15 silica synthesis conditions. Neutral, rather than acidic pH conditions were employed to prevent particle aggregation and amorphous silica formation [16j. (Reprinted from Ref. [16], 2006, with permission from American Chemical Society.)...

The distribution of A1 spedes of varying coordination (tetrahedral, pentacoordinated and octahedral) can be influenced by changing the conditions of hydrothermal pretreatment of amorphous silica-alumina catalysts. However, for a given composition, activity per unit surface area and selectivity were independent of pretreatment conditions. Thus, gas oil cracking activity and selectivity in amorphous silica-alumina cannot be... 

The formation of crystalline quartz from a gel at low temperatures has not been established experimentally. Direct crystallization of quartz from sea water has been observed (Mackenzie and Gees, 1971). In the opinion of Harder and Fleming (1970), quartz is formed only by adsorption of Si02 by hydroxides of Fe, Al, Mg etc. from undersaturated solutions, while amorphous silica arises in supersaturated solutions. The mechanism of crystallization of gels to quartz was studied in detail in our laboratory by Mitsyuk (1974). It was established that in amorphous silica the process of quartz formation goes on at an appreciable rate in hydrothermal conditions —T— 150-250°C and F = 0.5-4.0 kbar—and is characterized by the following features ... 

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. 

The highest content was found in formation waters at depths of 2-3 km, and in hydrothermal waters from 200-400 to 500-700 ppm. The content of silicic acid in some sodium carbonate-bicarbonate brines can be as high as 2 700 ppm at pH 10 (Jones et al. 1969). However, natural waters usually show a concentration of silicic acid considerably lower than the solubility limit of amorphous silica under the same conditions (Fournier and Rowe 1962 Table 3.5). Consequently, modern subsurface waters are undersaturated with respect to amorphous silica, while marine water is unsaturated also with respect to quartz (Fournier and Rowe 1977). 

Nonetheless in a number of studies on water lubricated wear of SiC the loss of silica was reported and attributed at least partially to the solution of it in the water [5, 7, 15]. The solution to this problem could be hydrothermal conditions. At T > 100°C increased pressure is required to keep the water in a fluid state. From geoscientific evidence and experiments it is well known that the solubility and solution kinetics of silica rises with increasing pressure quite strongly [13, 16, 17]. Amorphous silica has here also a higher solubility relative to crystalline species [18]. 

It is worth noting that both Pd-aUoy and sUica-based membranes present some problem about material instability in the WGS environment. The Pd-aUoy membranes can be negatively affected by surface carbonization, sulfur poisoning, and hydrothermal embrittlement, whereas the amorphous silica-based membranes can show some degradation caused by the condensation reaction of sUanol in hydrothermal conditions (Tang et al., 2010). In particular, the siliceous MFI-type zeolite membranes, constituted by a crystalline microporous zeolite membrane, in recent years have been seen as attractive candidates for the WGS reaction because of the high-temperature hydrogen separation and for their intrinsic sulfur tolerance and hydrothermal stability. 

Microamorphous silica is not easily crystallized. When an ionic material such as a salt is rapidly precipitated from a highly supersaturated solution, it may initially be amorphous, but it rapidly rearranges to the ordered crystalline state. However, in the case oF silica in which bonds are largely covalent, such rearrangement can occur only at elevated temperature or in the presence of a solvent such as water under hydrothermal conditions. Silica, in effect, is a polymeric material. Walton (106) has pointed out why an intermediate amorphous phase is likely to be precipitated if the material is of high molecular weight or polymeric. 

Weisz and Miale compared the activity for hexane cracking of a number of zeolites (Table 4.25) with a highly active silica — alumina (10% alumina). The zeolites are at least 10 times as active as amorphous silica —alumina. The catalytic process, however, cannot utilize the activity from a pure zeolite catalyst. The catalyst must be modified to decrease the acid-strength to avoid excessive formation of coke and low molecular weight gases, at the expense of gasoline. Moreover, the catalyst must be able to withstand the thermal and hydrothermal conditions experienced in regeneration. It must also withstand breakup in the mechanical circulation systems. A detailed description of the preparation of industrial catalysts is found in the literature. 

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