Silicate solutions formation

Silicate Grouts. Sodium silicate [1344-09-8] h.3.s been most commonly used in the United States. Its properties include specific gravity, 1.40 viscosity, 206 mPa-s(=cP) at 20°C Si02 Na20 = 3.22. Reaction of sodium silicate solutions with acids, polyvalent cations, such organic compounds as formamide, or their mixtures, can lead to gel formation at rates, which depend on the quantity of acid or other reagent(s) used. 

Andersson, K. R., Dent Glasser, L. S. Smith, D. (1982). Polymerization and colloid formation in silicate solutions. In Falcone, J. G. (ed.) Soluble Silicates. ACS Symposium Series No. 194, Chapter 8. Washington, DC American Chemical Society. 

Oyane, A., Kawashita, M., Nakanishi, K, Kokubo,T., Minoda, M., Miyamoto, T. and Nakamura, T. (2003) Bonelike apatite formation on ethylene-vinyl alcohol copolymer modified with silane coupling agent and calcium silicate solutions. Biomaterials, 24, 1729-1735. 

The initial transition of dissolved silicate molecules into solid nanoparticles is perhaps the least explored step in the synthesis of zeolites. One impediment to understanding this mysterious step is the poorly elucidated molecular composition of dissolved particles. The major mechanistic ideas for the formation of zeolites approach these structures differently i) many researchers believe that secondary building units (SBU) must be present to form initial nanoslabs [1,2] ii) some others prioritize the role of monomers to feed artificially introduced crystal nuclei or assume that even these nuclei form via appropriate aggregation of monomers [3] iii) silicate solutions are also frequently viewed as random mixtures of various siloxane polymers which condense first into an irregular gel configuration which can rearrange subsequently into a desired crystal nucleus at appropriate conditions [4,5],... 

By achieving random dispersion of the Kaolin in the sodium silicate solution prior to formation of the silica-alumina gel, it was possible to disperse the clay crystals. They condensed somewhat perpendicular to each other and were bound together by silica-alumina gel. I therefore speculated that spray drying, during which the gel system contracts, might create a dual structure. An analogy would be a house built of cards (Kaolin), cemented together with silica-alumina gel. 

Solutions of monosilicic acid may also be obtained by careful hydrolysis of tetrahalo-, tetraalkoxy-, or tetraacyloxysilanes by electrolysis or acidification of alkali silicate solutions or by ion exchange (qv). By operating under carefully controlled conditions at low temperature and pH, solutions may be obtained that remain supersaturated with respect to amorphous silica for hours at temperatures near 0°C. Eventually, however, polymerization reactions involving the formation of siloxane linkages occur, leading ultimately to the formation of colloidal particles and further aggregation or gel... 

Zeolite Y may be obtained either by using a sodium silicate solution or from silica sol (7, 8). The formation of zeolite Y depends on the time of action of NaOH on the sol before the introduction of aluminate (7). It has been believed that this phenomenon depends upon a depolymerization of the silica contained in the sol. The phenomena occurring during the synthesis of zeolite Y were also investigated by measuring changes in the liquid and solid phases of the hydrogel. 

In the first place, we learned more about the formation of nickel hydrosilicates under certain circumstances from an investigation of Van Eijk van Voorthuysen and Franzen (2). These investigators made a number of preparations by combining boiling dilute solutions of nickel nitrate and alkali silicate in various proportions. In order to find out to what extent co-precipitation is required for the formation of hydrosilicate structures, acid was added to a nickel hydroxide suspension in a silicate solution by which silica is precipitated, or conversely, alkali was added to a suspension of silica gel in a nickel nitrate solution. Some of the preparations were subjected to a hydrothermal treatment at 250° C. for 50 hrs. with a sufficient quantity of water for developing the best possible structure. 

Eq. (la) describes a solid solution formation with H20. A cation of valency n may be replaced by n H+, keeping the number of oxygen anions constant. For instance, if n 2, we have in the case of a silicate or oxide respectively ... 

The field of mesoporous materials has developed rapidly since the first reports on these materials in 1992, as these last examples show. The trend is to utilize inexpensive, multifunctional micelle- or aggregate-forming surfactants or templates which may adopt many different liquid crystal-like configurations in aqueous solution. Formation of a silicate structure with well-defined pore dimensions and connectivity may then be accomplished by the appropriate choice of the synthetic conditions. Additional microporous and macroporosity may be incorporated by using macroporous host materials, as in the case of Stucky of the work by and coworkers, who created mesophases with unprecedented architecture.[47]... 

The usual result of mixing aluminate and silicate solutions is the formation of a gel, which may later separate into a clear supernatant liquid and a gel. One may ask whether nucleation is homogeneous (in solution) or heterogeneous (in gel). A partial answer may be provided if it can be demonstrated that zeolites can grow from clear solutions. Guth et al (2) and Ueda et al ( 3, ) have shown how such solutions can be prepared. 

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. 

Second, the amount of all small silicates present in solution seems to be constant throughout the duration of the synthesis. This finding is in line with the fast mutual exchange observed in silicate solutions (vide supra). Moreover, it indicates that if the zeolite is indeed grown from dissolved small silicates, the equilibrium between the polymeric and the smaller silicates is rapid enough to keep the amount of smaller silicates in solution at a constant level. The zeolite formation stops when the solution is no longer supersaturated with silica, which in this case means after about 10 days (see Table I). 

The order for the nucleation rste (see Figure 6 TEA < orga-nics-free < TPA), and the observation that when X - 5 NaOH + 3 wt % seed ZSM-5, ZSM-5 is formed about as fast as with X - 5 TPAOH (see Table II), do not support a precursor role for D5R silicates in all these synthesis reactions. This is because, on the basis of the D5R concentrations in analogous silicate solutions, the order TEA = TPA > organics-free is expected (cf. Table III) (4). If particular zeolite precursors are responsible for the formation of ZSM-5 then, clearly, TEA has a very retarding effect on their mutual condensation rate. 

The shortest gel times of all are observed (Figure 3) when the silicate solution is premixed with sufficient alkali to depolymerise it entirely to monomer. Figure 7 shows that increasing the amount of monomer increases the amount of aluminate/silicate association. This species seems to be critical for gel formation, being both the nutrient and the precursor to forming nuclei. When it is present in large quantities, gelation is almost instantaneous. 

In the first section of this paper, formation of silicate species with cage-like structures in organic quaternary ammonium silicate solutions are reviewed. In the last section, the process of the rapid selective formation of the silicate solids having the... 

Sakka, S., unpublished data.)- In the solutions, a number of methylsilsesquioxane species, formed by the hydrolysis of methyltriethoxysilane, with different structures are present even under the conditions where the cubic octamer is dominant in the aqueous silicate solutions. This indicates that the use of a silica source with tetra-functionality is required for the selective structure formation with the aid of organic quaternary ammonium ions. 

Effect of Temperature. The temperature of a silicate solution also affects the polymerization of silicate anions in the solution. The distribution of silicate anions in an organic quaternary ammonium silicate solution at a fixed N/Si ratio and SiC concentration varies with the temperature of the solution (7,8,13,14,16). Ray and Plaisted (8) reported the temperature dependence of the distribution of silicate anions in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 2/3 and a SiC>2 concentration of 1.0 mol dm. The amount of the cubic octamer in the solution decreases with increasing temperature, and the cubic octamer practically disappears above 50 °C, indicating that the cubic octamer is unstable at higher temperatures. However, Groenen et al. (14) found that the cubic octamer remained in a significant concentration even at 85 °C, which was close to the temperature of actual zeolite formation, in the tetramethylammonium silicate aqueous solution at a N/Si ratio of 1.0 and a Si02 concentration of 1.3 mol dm-. ... 

The observations made here are not sufficient to prove any particular synthesis mechanism. However we may speculate as to mechanisms that are consistent with the observations. The synthesis of LTA probably proceeds by formation of sodalite units from D4R. This mechanism has been postulated before(14,15). D4R are present in silicate solutions containing sodium(16), potassium(17), and TMA(18) and in aluminosilicate solutions containing TMA(19). In TMA silicate solutions the fraction of Si in D4R decreases with dilution(20) and in TMA aluminosilicate solutions D4R s decrease with decreasing Si/Al(21). D4R with strict alternation of Si and Al, as required for zeolite A, can join in only one way and this is apparently facile as no template... 

The formation of silica gel from these silicate solutions proceeds according to the following steps ... 

P.W.J.G. Wijnen, A spectroscopic study of silica gel formation from aqueous silicate solutions, Ph.D. Thesis, TU Eindhoven, 1990. 

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