ORGANIC SILICATES AND THEIR APPLICATIONS السيليكات العضوية وتطبيقاتها

Article The chemistry of ethyl silicate binders in refractory technology. Element Silicon – Si Electron Configuration: 1s2 2s2p6 3s2p23d

Shell model Electron Dot Model

Uses of Silicon: Used in glass as silicon dioxide (SiO2). It is used as a semiconductor to make microchips for electronics (like your computer). Silicon is also used in solar cells, tools, cement, grease and oils.

Tetraethyl orthosilicate ETHYL SILICATES TETRAETHOXYSILANE

SiC8H20O4

Tetraethyl orthosilicate Is the chemical compound with the formula Si(OC2H5)4. Often abbreviated TEOS, this molecule consists of four ethyl groups attached to SiO44- ion, which is called orthosilicate. As an ion in solution, orthosilicate does not exist. Alternatively TEOS can be considered to be the ethyl ester of orthosilicic acid, Si(OH)4. It is a prototypical alkoxidechemical compoundformula orthosilicateethyl esteralkoxide

TEOS SHAPE TEOS is a tetrahedral molecule. Many analogues exist, and most are prepared by alcoholysis of silicon tetrachloride:alcoholysis silicon tetrachloride SiCl4 + 4 ROH → Si(OR)4 + 4 HCl HCl where R = alkyl such as methyl, ethyl, propyl, etc.alkylmethyl ethylpropyl

Applications TEOS is mainly used as a crosslinking agent in silicone polymers. Other applications include coatings for carpets and other objects. TEOS is used in the production of aerogel. These applications exploit the reactivity of the Si-OR bonds.aerogel

Other reactions TEOS has the remarkable property of easily converting into silicon dioxide. This reaction occurs upon the addition of water: Si(OC2H5)4 + 2 H2O → SiO2 + 4 C2H5OH C2H5OH

Hydrolysis reaction This hydrolysis reaction is an example of a sol-gel process. The side product is ethanol. The reaction proceeds via a series of condensation reactions that convert the TEOS molecule into a mineral-like solid via the formation of Si-O-Si linkages. Rates of this conversion are sensitive to the presence of acids and bases, both of which serve as catalysts. sol-gelacids catalysts At elevated temperatures (>600 °C), TEOS converts to silicon dioxide:silicon dioxide Si(OC2H5)4 → SiO2 + 2O(C2H5)2 The volatile coproduct is diethylether.diethylether

Hydrolysis

COMPLETE HYDROLYSIS Figure. Equation that interprets the hydrolysis degree of alkyl silicates Scheme 1. Structure of commercial ethyl polysilicate (TES-40), n = 1–9.

Figure. Reaction of condensation through the protonated hydroxyl groups

ETHYL SILICATE AS precursor FOR (SiO2) Ethyl silicate, the common name for tetra ethyl ortho silicate (TEOS), has found worldwide acceptance in applications when a liquid precursor of silica (SiO2) is needed. When properly hydrolyzed, ethyl silicate produces very fine particles of silica which can act as a binder to adhere refractories into ceramic shapes or provide corrosion-resistant coatings in combination with zinc dust.

Manufacture TEOS, Si(OC2H5)4, is synthesized in one of two common ways: Firstly, directly from silicon metal and anhydrous ethyl alcohol: Si + 4C2H5OH ─catalyst→ Si(OC2H5)4 + 2H2 ↑ Secondly, from silicon tetrachloride and anhydrous ethyl alcohol: SiCl4 + 4C2H5OH → Si(OC2H5)4 + 4HCl ↑

Complete Hydrolysis Complete hydrolysis of ethyl silicate will produces silica and ethyl alcohol. acid or base Si(OC2H5)4 + 2H2O → SiO2 + 4C2H5OH TEOS WATER SILICA ETHANOL

pre-hydrolyzed TEOS T.E.S The most common measure of performance of pre-hydrolyzed TEOS is the gel time. Gel time is a measure of the velocity of a blend of TEOS polymers to achieve sufficient molecular size to render a solution of TEOS non-flowing, i.e. gelled. Under this condition, a solution of TEOS polymers forms large enough networks to entrap the remaining solvent alcohol.

Here are some procedures and charts that illustrate some of the possibilities: Starting formula for 100% hydrolyzed 20% silica solution: Component Weight % T.E.S Water Ethanol HCl (37%) Table 1

Gel Time Procedure If the solution in Table 1 is pH adjusted and the gel times plotted, the following curve is generated:

Partial Hydrolysis The stoichiometric equation for partial hydrolysis is as follows: acid or base Si(OC2H5)4+ 2XH2O → [Si(OC2H5)4(1-x) (O) 2x ] + 4X C2H5OH polymer degree (%) of hydrolysis Where X = For example, to produce a binder with 80% degree of hydrolysis, X = 80 = and the formula becomes: H ⊕ Si(OC2H5)4+ 1.6H2O → Si(OC2H5)0.8 (O) C2H5OH 1 mole 1.6 mole 1 mole 3.2 mole lbs lbs lbs lbs.

Degree of Hydrolysis with Various Water Additions to Ethyl

Important: It.s imperative that a small amount of acid or base be added to catalyze the hydrolysis. The following section outlines the reasons for this. The mechanism of hydrolysis of ethyl (or other alkyl = R) silicate is as follows: Acid Hydrolysis: H ⊕ ≡ Si - OR + H2O → ≡Si - OH + ROH Lewis Acid

Mechanism: ≡Si - OR → ≡Si - O - R → ≡Si + HOR↑→ ≡Si - OH + H ⊕ In this reaction, a silicic acid ester is generated, along with an alcohol, which leaves the reaction. A hydrogen (or Lewis acid) ion (H+) is consumed and regenerated with no net gain or loss, thus perpetuating the reaction. H⊕H⊕ ↓ H I ↑ :: O / \ H H I O /⊕\/⊕\ HH ⊕

BASE CATLYST A similar reaction can take place with a base: ≡ Si - OR + H2O → ≡ Si - OH + ROH Lewis Base OH -

Condensation: ≡Si - OH + ≡Si - OH → ≡Si - O - Si≡ + H2O Lewis Acid H ⊕

Mechanism: ≡Si - OR → ≡Si - OR → ≡Si - OH+ -OR In this reaction, two silicic acid esters react to form a dimer (or high polymer), generating H2O, which continues the hydrolysis reaction. Again, there is no net loss or gain of the H+ ion. ↑ - OH I OH I H2O ↓ ROH + -OH

Basic Condensation: ≡Si - OH + ≡Si - OH → ≡Si - O - Si ≡ + H2O Lewis Base OH-

Production of Pure Silica Gel products are also a source of pure silica either by burning or by precipitation in water. Si(OC2H5) O2 → SiO2 + 8CO H2O The burning process as shown above produces a product equivalent to fumed silica, a high surface area, nano-sized, particulate powder. Si(OC2H5)4 + 2H2O → SiO2 + 4C2H5OH Complete hydrolysis produces colloidal silica particles up to several hundred nanometers in diameter.

Acid Condensation: H ⊕ ≡Si - OH + ≡Si - OH → ≡Si - O - Si≡ + H2O Lewis Acid Mechanism ≡Si - OH → ≡Si - OH2 → O - H + H2O → ≡Si - O - Si≡ + H ⊕ ↑ ↑ H ≡Si-OH ⊕ I ≡Si ⊕ I ⊕

Ultimate Dimerization In this reaction, two silicic acid esters react to form a dimer (or high polymer), generating H2O, which continues the hydrolysis reaction. Again, there is no net loss or gain of the H+ ion.

Basic Condensation: OH- ≡Si - OH + ≡Si - OH → ≡Si - O - Si ≡ + H2O Lewis Base Mechanism: ≡Si - OH → ≡Si - O + H2O ↑ OH- ↓ Si-OH ↓ ≡Si - O - Si≡ + OH - -

Production of Pure Silica Silica products are also a source of pure silica either by burning or by precipitation in water. Si(OC2H5) O2 → SiO2 + 8CO H2O The burning process as shown above produces a product equivalent to fumed silica, a high surface area, nano-sized, particulate powder. Si(OC2H5)4 + 2H2O → SiO2 + 4C2H5OH Complete hydrolysis produces colloidal silica particles up to several hundred nanometers in diameter.

Gel formation

The IR spectra of isolated silica gels Features of the IR: intense bands in the Si-O-Si/Si-O-C region are observed

The analysis of the IR frequencies Selected literature values in : –Si-O-Si antisymm. str. at cm-1, the values depend on cyclic/open chain structure); Si-O-C antisymm. str. in the same region. –Si-O stretch at 950(cm-1) –Si-OH bend at 870 (cm-1) Tentative IR band assignments: –Si-O-Si ~ cm-1 –Si-O-C ~ cm-1 (slightly higher)