Solution silicate species

Figure 2 Distribution of silicate species present in supernatant solution of silicalite-1 in NH4OFI at 190°C as a function of their size according to ESI-mass spectrometry.

Fig. 9 Schematic representation of three approaches to generate nanoporous and meso-porous materials with block copolymers, a Block copolymer micelle templating for mesoporous inorganic materials. Block copolymer micelles form a hexagonal array. Silicate species then occupy the spaces between the cylinders. The final removal of micelle template leaves hollow cylinders, b Block copolymer matrix for nanoporous materials. Block copolymers form hexagonal cylinder phase in bulk or thin film state. Subsequent crosslinking fixes the matrix hollow channels are generated by removing the minor phase, c Rod-coil block copolymer for microporous materials. Solution-cast micellar films consisted of multilayers of hexagonally ordered arrays of spherical holes. (Adapted from [33])...

The TISAB also slops complexation between fluoride and other ions in solution, particularly Al . Fe and silicate species. 

I. Hasegawa and S. Sakka, Silicate Species with Cagelike Structure in Solutions and Rapid Solidification with Organic Quaternary Ammonium Ions, Zeolite Synthesis (ACS Symposium Series 398) Am, Chem. Soc., Washington, DC 1989, p.140/51... 

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

It is found in this study that an adjustment of pH value of solution by acid (HF or HC1) to 10.5 is very important for the effective formation of uniform mesopores. However, the acid should be added into the mixture solution after the addition of surfactant otherwise, the formation of the ordered mesoporous structure would be affected. The explanation is that when acid is added to a mixture solution without surfactant, the pH value of system will reduce and subsequently influence the interaction between cationic surfactant and anionic silicate species in the mixture, leading to the poor polymerization of inorganic silicate species. In addition, when HF is used prior to the addition of surfactant, the formation of stable NajSiFg can deactivate the polymerization of silicate species, further terminating the growth of mesoporous framework. 

The extraction procedure of silicate species from the aqueous solutions was explained earlier [7]. An amount of 5 ml of solution was quickly poured into 15 ml of a stirred 0.5 N HC1 solution. An amount of 20 ml tetrahydrofuran (THF) was added and stirring continued for 30 min. Addition of 10.7 g NaCl resulted in a phase separation. The THF layer containing the silicate species was separated from the aqueous layer. The extraction with THF of the aqueous layer was repeated and the THF solutions combined to maximize extraction efficiency. Finally THF was evaporated in a rotavap at 20°C. The final products from the concentrated solutions were gel like. The extracts from the diluted systems were powdery. The experimental details on the gel permeation chromatography (GPC) and IR spectroscopy can be found in refs. [2] and [3], respectively. 

These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of free bromide anion increasing at pH around 11 when the polymerization of silica starts. 

Silicate solution species are controlled by the pH of the medium and silicon concentration.14 Figure 1.5 displays the aqueous silicate species as a function of pH for concentrations of 0.01 and 105 M of silicon. 

Figure 2 depicts the compositions of the different solutions. For the polymeric species the absolute amounts are shown for the other, smaller silicates the relative amounts. It is evident that, especially at low OH/Si ratios (i.e., < 0.5, which is a normal value for Si-rich zeolite synthesis mixtures) the larger part of the silicate species present in solution consists of uncharacterized, polymeric silicates. The values obtained in the absence of DMSO (lower part of Table III and Figures 2a and 2b are in good agreement with literature findings (14). 

Basic Silicate Solutions Dynamics. Exchange reactions between silicates as well as zeolite formation involve condensation and hydrolysis reactions between dissolved silicate species. Therefore, we have extensively studied the dynamics of basic silicate solutions in order to obtain better knowledge of the properties of possible zeolite precursor species. Our first results were published earlier (11). Here we have again used selective excitation Si-NMR experiments, applying DANTE-type (13) pulse sequences to saturate a particular Si resonance belonging to a particular Si site. The rate of transfer of magnetization from this saturated site to other sites is then a measure of the chemical exchange rate between the two sites. 

All excess water and alkali were added to the aluminate solution, which was then mixed with the silicate solution. This ensured that the range of silicate species was identical at the start of every reaction. 

All or some of the excess alkali ( 22, 44, 66 and 88% of total alkali) was added to the silicate solution, which was allowed to equilibriate and then mixed with the aluminate solution. Thus the silicate species were progressively depolymerised in response to the extra alkali at the beginning of the reaction. 

The second method of mixing (for potassium solutions only) involved depolymerising the silicate species of the initial silicate solution, by the addition of alkali, throughout a series of experiments, and produced veiy different results, which do not lend themselves to the type of plot used in Figure 2. This is partly because, in many cases, the gel time is dramatically decreased. 

The depolymerization of silicate species under the influence of dilution or increased alkali seems to be unaffected by the presence of aluminate. However, even with enriched 29Si, the spectra take time to measure and so information about kinetics is lost and the spectra relate to the solutions some time after mixing. 

However, aluminium appears to complex preferentially with any larger silicate species that are present, and these complexes, once formed, polymerise further only slowly. In Method 1, where the initial silicate solution is quite highly polymerized, the long gel times observed in the presence of excess silica are the result of a lack of nutrient, all available aluminium being already bound to the larger silicate units. The 27AI NMR spectra confirm this interpretation. 

The silicate species discussed in the preceding section can react with aluminate anions, Al(OH)4 to produce aluminosilicate anions. Si NMR spectra of solid silicates and aluminosilicates indicate that the replacement of Si by A1 in the second coordination sphere of a give Si causes a low-field shift of about 5 ppm. Since each Si atom can have up to four metal atoms in its second coordination spere, fifteen possible Qn(mAl) structural units can be envisioned. The estimated chemical shift ranges for these units are given in Table 3. It is apparent from this table that the 29si spectrum of an aluminosilicate solution in which A1 and Si atoms were statistically distributed would be much more complex than that of an analogous solution containing only silicate species. 

---