ORGANIC SILICATES AND THEIR APPLICATIONS

1. ORGANIC SILICATES AND THEIR APPLICATIONS السيليكات العضوية وتطبيقاتها

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

3. Shell model Electron Dot Model

4. 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.

5. Tetraethyl orthosilicate ETHYL SILICATES TETRAETHOXYSILANE

6. TETRAETHOXYSILANE SiC8H20O4

7. 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 ethylester of orthosilicic acid, Si(OH)4. It is a prototypical alkoxide

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

9. 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.

10. 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

11. 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. • At elevated temperatures (>600 °C), TEOS converts to silicon dioxide: • Si(OC2H5)4 → SiO2 + 2O(C2H5)2 • The volatile coproduct is diethylether.

12. Hydrolysis

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

14. Figure. Reaction of condensation through the protonated hydroxyl groups

15. 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.

16. 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 ↑

17. 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

18. pre-hydrolyzed TEOST.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.

19. 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. 40 50.00 • Water 14.52 • Ethanol 35.40 • HCl (37%) 00.08 • Table 1

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

22. 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 = ------------------------------- • 100 • For example, to produce a binder with 80% degree of hydrolysis, • X = 80 = 0.8 • 100 • and the formula becomes: • H⊕ • Si(OC2H5)4+ 1.6H2O → Si(OC2H5)0.8 (O)1.6 + 3.2 C2H5OH • 1 mole 1.6 mole 1 mole 3.2 mole • 208.33 lbs. 28.82 lbs 89.73 lbs. 147.42 lbs.

23. Degree of Hydrolysis with Various Water Additions to Ethyl

24. 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

25. 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⊕ ↓ I ⊕ I ↑ : : O O /⊕ \ / \ H H H H

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

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

28. 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. ↑ I I H2O OH - OH ↓ ROH + -OH

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

30. Production of Pure Silica • Gel products are also a source of pure silica either by burning or by • precipitation in water. • Si(OC2H5)4 + 12 O2 → SiO2 + 8CO2 + 10 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.

31. 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⊕ ≡Si I ⊕ ⊕ ↑ ↑ I ≡Si ⊕ ≡Si-OH H

32. 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.

33. 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

34. Production of Pure Silica • Silica products are also a source of pure silica either by burning or by • precipitation in water. • Si(OC2H5)4 + 12 O2 → SiO2 + 8CO2 + 10 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.

35. Gel formation

36. Gel formation

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

38. The analysis of the IR frequencies • Selected literature values in : • Si-O-Si antisymm. str. at 1100-1000 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 ~1100-1000 cm-1 • Si-O-C ~1250-1100 cm-1 (slightly higher)