Silicate condensation reactions

Figure 4.7 Silicate condensation reactions simulated computationally by Lewis et Free energy calculations, taking the effect of solvent...

When a sodium silicate solution is acidified condensation reactions occur. Firstly, silanol... 

The sol-gel route for silica fabrication is based on the strong tendency of silicic acid in solution to take part in condensation reactions [8,18,58]. It can be presented in the general form as ... 

The reaction results in the formation of a Si O—Si siloxane bond in which two silicon atoms are bridged by oxygen. The remaining three Si—OH sylanol groups can also be involved in condensation reactions. This will lead to the polymerization of silicic acid with the formation of branched inorganic polymers. 

The use of alkoxides for fabrication of silica materials has some advantages that will be discussed later. They serve as a precursor a silicic acid is generated in the course of their hydrolysis that thereafter enters into condensation reactions (1). The main processes may be presented by Scheme 3.1. 

Two procedures have been used to prepare phenyl-functionalized silicate mesophase under acidic conditions. The only difference between them, is the pre-hydrolysis step, which was added in preparation B to initiate the hydrolysis and condensation reactions of the two precursors, prior to reaction with the CTAB solution. This step may cause the formation of co-condensed species between PTES and TEOS, and thus prevent phase separation [25],... 

The inner surface of the silicalemma, i.e., the limiting membrane within which silica deposition occurs549 consists of a protein template enriched in serine and threonine. This protein will present a layer of hydroxyl groups which can undergo condensation reaction with silicic acid molecules with a consequent loss of water (Fig. 48). As a result, the initial layer of condensed silicic acid will be held fixed to the protein template of silicic acid (Fig. 49). Such a situation is kinetically more favorable than simply allowing the silicic acid molecules to come together by random collision. 

Fig. 48. Proposed condensation reaction between silicic acid and serine on the protein template of the silicalemma (Hecky et a/.548 )...

Venuto, P. B., Landis, P. S. Organic reactions catalyzed by crystalline alumino-silicates. III. Condensation reactions of carbonyl compounds, J. Catal., 1966, 6, 237-244. 

The approach of Qide et al. to produce thin films of TiQ2 via a sol-gel strategy uses a mixture of titanium isopropoxide, ethanol, water, and triethanolamine. The chemistry is similar to what is used in the silicate gel where a hydrolysis/condensation reaction is exploited (Wright and Sommerdijk, 2001) ... 

The reaction leading to the SIO2 polymers consists of two steps. In the first one an organic silicone compound, such as silicone tetraethoxlde. Is hydrolyzed to some form of silicic acid. This step Is Immediately followed by a condensation reaction giving polymers with Sl-O-Sl bonds. The total reaction Is... 

The concept of a fractal dimension enables the structures of silicate species to be divided into two classes (1) those with a dimension less than 3, which may be considered to be true polymers, and (2) those with a dimension of 3, which are perhaps best described as colloidal particles (5). The colloidal class may be further subdivided into particles with fractally rough surfaces (D > 2) and particles with smooth surfaces (D = 2). In this chapter, the term polymer will be used for any product of the condensation reaction, regardless of fractal dimension, although the term polymeric will be used for structures with dimensions less than 3, and the term colloidal will be used for structures with dimensions equal to 3, in keeping with the terminology used in the literature (9). 

Further work is needed to clarify the nature of the differences between the water- and alcohol-producing condensations of silicate species. But one conclusion is clear The two condensation reactions proceed by different reaction pathways (or at least have different transition states) and do not necessarily follow the same reactivity trends. 

Minerals with Layer Structures. By a condensation reaction involving three of the four hydroxyl groups of each silicic acid molecule, a condensed silicic acid can be made, rvith composition (H.SisOs), which has the form of an infinite layer, as shown in Figure 31-4. The... 

The kinetics of the silicic acid condensation was investigated using the molybdate method introduced by Alexander [14]. Upon addition of a molybdate solution to a silicate solution, monomeric and dimeric silicic acid react to form the yellow molybdosilicate [SiMoi204o] [5]. The change in the absorption of this solution reflects the decrease in monomeric and dimeric silicic acid due to the condensation reaction. Because of the reaction equilibrium between silicic acid and the molybdate as well as between monomeric, dimeric, and oligomeric silicic acids, it is important to adhere to a well-defmed, constant, and reproducible time protocol for the formation of the molybdosilicate. 

The pH value of the reaction mixture shows a relation to the condensation rate the higher the pH value, the faster the condensation reaction. Of course, this relation is well known from simpler silica chemical systems. It thus appears that polyamines do not directly influence the first steps of the condensation reaction, for example by catalytic action or by coordinative interaction between the amine and the silicic acid anions, but that the accelerating effect the polyamines exert is related to the increase of the pH value, i.e. to their basicity. As the molybdate method probes only the first steps of the condensation reaction, significant interactions may, however, occur between larger silicate species and the amines. 

Very good linear dependence for hydrodealkylation was found except for zeolite samples with high Si/AI ratio. The higher values of the initial reaction rates obsen/ed on these samples are presumably due to another type of catalytic centers (for example one-electron acceptor centers) on very high siliceous zeolites (Si/AI >1000). The dependence of turnover number (TON) for the particular toluene transformation reactions as a function of the strong acid sites concentration has shown [24] that hydrodealkylation occured on strong acid sites and didn t depend on the concentration of these sites. However, TON dependence for hydrocracking and condensation reactions was non-linear and influenced by the concentration of active sites. 

Aqueous samples are treated similarly beginning with the acidification step. The entire sample is then put through the hydrophobic resin, and the fulvic acids are eluted at pH 7. The humic acids are removed with 0.1 M NaOH (2). After extraction, purification of the samples can be accomplished by freeze-drying and dialysis. The use of strong acids and bases has been criticized for several reasons. They can promote degradation, decarboxylation, oxidation, and condensation reactions. Strong acids and bases can also dissolve siliceous materials and lyse cells, resulting in contamination of the sample. Other extractants have been proposed, such as sodium pyrophosphate or sodium fluoride however, the classical procedure offers the most complete dissolution of humic material from solid samples and is still most often used (72). 

Many zeolites were crystallized from strongly basic solution containing silicate and aluminate. In strongly basic solution, silicate exists as polymerized-state polysilicates with a variable distribution of molecular weight. Aluminate reacted with polysilicate to form various zeolite structures via condensation reaction under hydrothermal or solvothermal conditions. Therefore, in the study of the synthetic chemistry of zeolites, one of the key issues is to understand the existing state and reactivity of the polymerized silicate. 

Many productive methods have been developed for the preparation of silica sol including acidification/121 electrolysation-electrodialysis,[13] ion-exchange,[14] peptization/111 and hydrolysis of silicon compounds/101 which can be grouped into two main types. One is called the aggregation method that contains two steps the polymerization of silicate ions and the aggregation of these polysilicate anions via condensation reaction between the hydroxy groups of the particles. The other one is called the peptization method, i.e., dispersal of a precipitate of Si02 to form colloid. The acidification method will be discussed in detail below. 

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