TEOS method

TEOS method - TEOS is hydrolized in an aqueous solution of TEAOH with stirring, then TEOT is also hydrolized. Finally, a solution of aluminium nitrate in TEAOH and water is added and the mixture is left, while stirring, until all the ethanol formed in the hydrolisis is evaporated, (ref. 11)... 

To further illustrate the differences between the methods, Fig. 2 and 3 show the yield and the Si/Al ratio, respectively, of the zeolite as a function of time of crystallization for synthesis runned by the four methods. Obviously, it is not possible to compare synthesis with the same chemical compositions because of the differences of the methods. Accordingly, in Fig. 2 and 3 we compare synthesis runs that gave, for every method, high Si/Al ratios and high zeolite yields. In this figures it is seen how our "classical" syntheses of Ti-Beta zeolite (amorphous silica and TEOS methods) have been surpassed by the new, previously unpublished methods (TEOS/seed and cogel methods), if the Si/Al ratio of the zeolite and its yield are compared. Furthermore, the TEOS/seed method is the most versatile one in terms of varying the chemical composition of the zeolite, and thus we have prepared materials wich are essentially pure silica (A1 plus Ti contents below 0.2 atoms per unit cell of 64 tetrahedra). 

Fig. 4. Selectivity to diphenols (in mol % relative to H202) vs catechol to hydroquinone ratio in the hydroxylation of phenol with H202 using several Ti-Beta catalysts. Phenol / acetone / H20 = 50 / 65 / 6.5 (mol) Ti-Beta / phenol = 4.5 g/mol H202/phenol = 5 % (mol). Reaction temperature 80°C. 3h Reaction time. Ti-Beta materials prepared by TEOS / Seed (o), cogel ( ), aerosil (+) and TEOS ( ) methods.

Fig. 14. Soladasli gel techniques for overcladding where TEOS = (C21150)481 (a) wet gel method and (b) rod-in-tube method.

Mixing. In method 1, a suspension of colloidal powders, or sol, is formed by mechanical mixing of colloidal particles in water at a pH that prevents precipitation (step A in Fig. 1) (8). In method 2 or 3, a Hquid alkoxide precursor such as Si(OR)4, where R is CH (TMOS), C2H (TEOS), or C Hy, is hydrolyzed by mixing with water (eq. 2). 

At present time the use of oxide single erystals sueh as bismuth germanate (Bi Ge O, ) and pai atellurite (TeO,) as deteetors in opto-eleetronies stimulate produetion of high purity Bi, Te, Ge and their oxides Bi O, GeO, TeO,. This requires development of analytieal teehniques for purity eontrol of these materials. For survey traee analysis atomie emission speetrometry (AES) and mass speetrometry (MS) with induetively eoupled plasma (ICP) is widely used. However, the deteetion limits of impurities aehievable by these methods for the analysis of high purity solids are limited by neeessity of sample dissolution in pure aeids and dilution up to 5 10 times for ICP-MS and 50-100 for ICP-AES. One of the most effeetive ways to improve the analytieal performanees of these methods is pre-eoneentration of miero-elements. 

Setchell, Teo Kuhn (1985) observed that glass-ionomer cements prepared from poly(acrylic acid) were more resistant to erosion than such cements prepared from maleic acid copolymers. This has been confirmed by Wilson et al. (1986) and by Billington (1986), even when, as in the latter case, the same glass was used in both cements. The method has been reviewed recently by Billington, Williams Pearson (1992). 

Later, Thangaraj et al. (275,281) developed a novel, improved route (prehydrolysis method) for the preparation of good quality TS-1 samples. In this method the silica source (tetraethyl orthosilicate TEOS) in Ao-propanol was first hydrolyzed with 20% aqueous TPAOH solution prior to the (dropwise) addition of titanium butoxide in dry iso-propanol under vigorous stirring. Crystallization was done statically at 443 K for 1-5 days and the solid was calcined at 823 K for 10 h. The TS-1 samples thus obtained exhibited high catalytic activity in hydroxylation reactions. 

TS-1 (mesoporosity 20nm, 0.3-1.2 xm size), carbon black pearls 700 (Carbot Corp., average particle diameter =18 nm (ASTM D-3249)) were impregnated by the incipient wetness method with a clear solution of TPAOH, water, and ethanol. After evaporation of ethanol, the carbon particles were impregnated with 20% excess (relative to incipient wetness) of a mixture of TEOT and TEOS. Aging for a minimum of 3 h at room temperature and heating at 453 K for 72 h yielded the solid product, which was isolated, and the carbon black was removed by controlled combustion in air at 523 K for 8 h. 

Blasco et al. (12,13) developed a novel method for the synthesis of Al-free Ti-beta zeolite in a fluoride medium. The Ti-beta zeolite thus obtained (Ti-beta(F)) was free of connectivity defects and was hydrophobic. The typical unseeded synthesis of Al-free Ti-beta zeolite (Ti-beta(F)) involves hydrolysis of TEOS in aqueous solutions of TEAOH (35%) and H202, followed by hydrolysis of TEOT and evaporation of ethanol and water. The water lost in the evaporation and... 

Prehydrolysis method. The Si source (TEOS) in dry wo-propyl alcohol is hydrolyzed with 20% aqueous TPAOH prior to addition of Ti source, Ti(OBu)4. Gel composition SiO2 vTiO2 0.36-TPA 35H20 (x — 0-0.10) the synthesis time is reduced considerably (1-5 days at 433 K compared to 6-30 days at 448 K, as reported in the original patent (5))... 

OH ratio on the rate of crystallization and crystallite size investigated Prehydrolysis method. Synthesis using binary mixtures of tetrabutylphosphonium hydroxide and tetraethylphosphonium hydroxide instead of TPAOH as base and template TEOS and TBOT are sources of Si and Ti, respectively. Molar gel composition, SiO2 xTiO2 0.4 ( TEPOH + (1 — jd)TBPOH) 30H2O (x = 0-0.02) temperature = 443 K and synthesis time = 4 days Influence of nature of silicon and titanium alkoxides on the incorporation of Ti Wetness impregnation method... 

Synthesis using TiF4 (as the source of Ti), TEOS, TPAOH, and distilled water. Gel composition Si02 xTi02 0.4TPA 30H20, 0 < x < 0.05 Prehydrolysis method. Crystallization without evaporating the alcohol in the conventional synthesis (7,8)... 

Direct hydrothermal synthesis. Prepared using titanium isopropoxide (triethanolaminato) and TEOS as the sources of Ti and Si, respectively, and the Gemini-type surfactant 18-12-18 or cetyl-benzyl dimethylammonium chloride (CBDAC) as a template. In the grafting method, silicious MCM-48 first prepared and then the dry surface grafted with titanium isopropoxide... 

Ti-SBA-15 Grafting method. SBA-15 prepared first using the amphiphilic triblock copolymer poly(ethyleneox-ide) -poly (propyleneoxide) -poly (ethyleneoxide) (EO-PO-EO) as template and TEOS as Si source. The composition was 2 g copolymer 0.021 mol TEOS 0.12 mol HC1 3.33 mol H20. The solid was calcined at 600 K for 4 h to remove the copolymer. Ti in the form of titanium isopropoxide was grafted onto the dehydrated surface of SBA-15 Pore diameter = 6.3 nm, surface area = 518 m2/g, pore volume = 0.68 (25)... 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

The reverse microemulsion method can be used to manipulate the size of silica nanoparticles [25]. It was found that the concentration of alkoxide (TEOS) slightly affects the size of silica nanoparticles. The majority of excess TEOS remained unhydrolyzed, and did not participate in the polycondensation. The amount of basic catalyst, ammonia, is an important factor for controlling the size of nanoparticles. When the concentration of ammonium hydroxide increased from 0.5 (wt%) to 2.0%, the size of silica nanoparticles decreased from 82 to 50 nm. Most importantly, in a reverse microemulsion, the formation of silica nanoparticles is limited by the size of micelles. The sizes of micelles are related to the water to surfactant molar ratio. Therefore, this ratio plays an important role for manipulation of the size of nanoparticles. In a Triton X-100/n-hexanol/cyclohexane/water microemulsion, the sizes of obtained silica nanoparticles increased from 69 to 178 nm, as the water to Triton X-100 molar ratio decreased from 15 to 5. The cosurfactant, n-hexanol, slightly influences the curvature of the radius of the water droplets in the micelles, and the molar ratio of the cosurfactant to surfactant faintly affects the size of nanoparticles as well. 

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