Silicate species in solution

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. 

The method of Lentz (35,36) was adopted for trimethylsilylation of the aqueous silicate solutions. The mixture of cone, hydrochloric acid, water, 2-propanol and hexamethyldisiloxane was used as the trimethylsilylating reagent. Trimethylsilylated derivatives obtained were adaptable to gas-phase analysis. The distribution of silicate species in solutions, which was analyzed quantitatively by the trimethylsilylation technique combined with gas-liquid chromatography, was expressed as the Si(>2 recovery, that is, the percentage of silica as a silicate species in total silica component in the solutions. 

Mesoporous materials of the M41S family with their regular arrays of uniform pore openings and high surface areas have attracted much attention since their first synthesis in 1992 (61), because their properties were expected to open new applications as catalysts and/or adsorbents. These materials are formed by condensation of an amorphous silicate phase in the presence of surfactant molecules (usually ammonium salts with long alkyl chains). However, the chemistry of the steps of the synthesis process is still not fully clear. Ideas put forward so far include (a) condensation of a silicate phase on the surface of a liquid crystalline phase preformed by the surfactant molecules (62) (b) assembly of layers of silicate species in solution followed by puckering of those layers to form hexagonal channels (63) and (c) formation of randomly disordered rod-like micelles with the silicate species... 

Silica Polymer—Metal Ion Interactions in Solution. The reaction of metal ions with polymeric silicate species in solution may be viewed as an ion-exchange process. Consequendy, it might be expected that silicate species acting as ligands would exhibit a range of reactivities toward cations in solution (59). Silica gel forms complexes with multivalent metal ions in a manner that indicates a correlation between the ligand properties of the surface Si—OH groups and metal ion hydrolysis (60,61). For Cu ", Fe ", Cd ", and Pb ", ... 

Sodium silicate, Na2Si03, yields a number of silicate species in solution as a function of pH the species composition in solution can vary also as a function of the ratio of the silicate in Na2Si03. The solution equilibria of Si02 itself are as follows ... 

Laser Raman spectroscopy and 29Si ft-nmr spectroscopy have been used to examine directly the structure of silicate species in solution (34—41). 

Up until the mid-1970 s, various indirected methods of studying the silicate species in solution, recently summarized by Dent Glasser and Lachowski(10), indicated that these solutions are indeed complex mixtures of silicate anions, with varying degrees of polymerization, in a dynamic equilibrium. 

The amount of Si ions dissolution is found to be dependent on surface modification, which was confirmed by induchvely coupled plasma-atomic emission spectrometer (ICP-AES) analysis. Table 2.2 shows the dissolution amount of Si ions with and without surface modification of fumed silica slurry. Without surface modification, the amount of Si dissoluhon was 1.370 0.002 mol/L, whereas surfaces modified with poly(vinylpyrrolidone) (PVP) polymer yielded a dissoluhon of 0.070 0.001 mol/L, almost 20 hmes less than the unmodified surface. Figure 2.6 represents the electro-kinetic behavior of silica characterized by electrosonic amplitude (ESA) with and without surface modification. When PVP polymer modified the silica surface, d5mamic mobility of silica particles showed a reduchon from -9 to -7 mobility units (10 m /Vxs). Dynamic mobility of silica particles lacking this passivation layer shows that silica suspensions exhibit negative surface potentials at pH values above 3.5, and reach a maximum potential at pH 9.0. However, beyond pH 9.0, the electrokinetic potential decreases with an increasing suspension pH. This effect is attributed to a compression of the electrical double layer due to the dissolution of Si ions, which resulted in an increase of ionic silicate species in solution and the presence of alkali ionic species. When the silica surface was modified by... 

Hydrolysis of silicate precursors, typically alkoxides, Si(OR)4, to give silicate species in solution. 

Dent-Glasser, L.S. and Lachowski, E.E. (1980) Silicate Species in Solution. Part 1. Experimental Observations, J. Chem. Soc. Dalton Trans, 393-398. 

Many minerals of known structure have been studied by solid state Si NMR [13] and have thus provided a partial basis for identification of silicate species in solution. After pioneering Si NMR work on silicate solutions at low fields [14] had shown that the Si atoms with different connectivities could be easily identified, rapid progress followed and the introduction of sophisticated NMR techniques revealed more detailed information on the nature of silicate solutions. Figure 2 illustrates the power of high-resolution Si NMR very sharp lines allow in principle the distinction of many species. The achievements of this technique or the combination of NMR spectroscopy with chemical trapping have been impressive and include the following ... 

The assignment of NMR peaks to at least 19 silicate species in solution. Table 1. 

Prolonged dissolution of the amorphous silica gel causes further oligomerization of silicate species in aqueous solution, eventually leading to a broad distribution of silicate species in solution. The final distribution of silicate anions present in aqueous solutions depends primarily on the silica to base ratio (Wijnen et al. 

To obtain this equation it was necessary to take into account the silicate species in solution at equilibrium. The following ionization constants for these species in 0.5 Af NaClO solution at 25 C were reported by Bilinski and Ingri (34a). 

Under these conditions the concentration of Si in solution is too small for observation by NMR, so only Al-NMR can give as much information about solution as solid phase. The results are shown in Fig. 2. In solution, only one sharp line belonging to monomeric tetrahedral aluminium exists (the same is true for Si not shown). The intensity of this aluminum is decreasing, following the patterns in Fig. 2. The very broad peak (59 ppm) belongs to amorphous tetrahedral Al-from the gel. In the course of the action, this peak became narrower and shifted to 58.3 ppm, which is typical for tetrahedral zeolite aluminium. During the whole process (except maybe immediately after Si increase in solution) alumino-silicate species in solution were not found. It confirms that the process is going within gel phase and if solution crystallization exists, it amounts to only 2—5% of total yield. 

The alkali silicates is one of the raw materials classically used in the formulation of new materials like geopolymers. Geopolymers are amorphous three-dimensional aluminosilicate binder materials which are synthesized at ambient temperature by the alkaline activation of silica solution and aluminosilicates derived from natural minerals, calcined clay or industrial byproducts. Previous study focused on the aluminosilicate sources have shown that the presence of impurities and the reactivity of the metakaolin (aluminosilicate source) can lead to the formation of one or several networks in geopolymers materials [1]. Indeed, the different sources of metakaolins conduct to the presences of various siliceous species in the solution which react with alumina. These multiple combinations lead to the formation of different networks (then various properties of geopolymers). To understand the formation of these various networks, studies relative to the neutralization of siliceous species in solution have been done. Parmentier [2] showed that the ammonium molybdate could react with silica to create silicomolybdic compounds. More recent studies demonstrated that ammonium molybdate could also react with these species in an alkaline environment [3]. These analyses showed that ammonium molybdate could react not only with monomers and dimers, but also with larger molecules. According to this, molybdate can permit to complex siliceous species and to modify polymerization reactions. The aim of the study is to study the influence of the ammonium molybdate addition on the kinetics of the polycondensation reaction as well as on the formation of several networks. 

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