Preparation of Silica Gel

2 PREPARATION OF SILICA GEL CATALYST SUPPORTS 8.2.1 Preparation of Silica Gel 

Soluble silicates are commonly used as starting material. The key variables in these solutions are  

the ratio of SiC 2 to alkali metal oxide expressed as a molar ratio  

A powerful technique for investigating these different species is 29Si-NMR (see Fig. 8.1). Different lines can be observed in the 29Si-NMR spectrum of a silicate solution corresponding to the differently positioned 29Si nuclei in the (poly)sili-cate ions. The highest values for the chemical shift (8 = -71.5 ppm with respect to tetra methyl silane) are found for the monomeric silicate units while the resonances of fully condensed silicon atoms (i.e. Si-(0-Si)4 tetrasiloxy silane) are to be found at the lowest values for the chemical shift (8 = -110 ppm). 

The formation of silica gel from these silicate solutions proceeds according to the following steps  

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Laser dye/Si02 gel CT AB/decanol/decane/ formamide (nonaqueous jjlE) TEOS/HzO (pH 1. HNO, coumarin 120 or coumarin 311) Preparation of silica gels doped with coumarin 120 and coumarin 311 (50)... 

Enomoto, N., Furukawa, S., Ogasawara, Y., Akano, H., Kawamura, Y., Yashima, E., and Okamoto, Y. (1996) Preparation of silica gel-bonded amylose through enzyme-catalyzed polymerization and chiral recognition ability of its phenylcarbamate derivative in HPLC, Anal. Chem. 68, 2798-2804. 

From the above mentioned silica preparation paths, the sol-gel route is the most studied and consequently most documented. Sol-gel processing is used not only for the preparation of silica gels, but also for the synthesis of ceramic products, ranging from thin films and coatings over porous membranes to composite bodies.4... 

Silica gel is a polymer of silicic acid and has the general formula Si02 x H20. There are many ways to prepare silica gel. Let us consider only one preparation of silica gel to demonstrate how and where differences or variations in the silica may contribute to a different bonded (final) phase. A common way of making silica gel particles for LC is to use acidic hydrolysis of sodium silicate, followed by polycondensation dehydration of orthosilicic acid and grinding of the silica to make granular particles (e.g., 10 jam). If the reaction is not controlled (buffer, pH, temperature, etc.) the hydrogel will... 

Typical substances that find wide use as high-area supports include silica gel and y-alumina, which can be obtained with surface areas in the range 100-800 m2/g. Materials used as low-area supports ( 1 m2/g) include a-alumina and mullite (alumina-silica). It is not easy to make general statements about the preparation of industrial catalysts because of the great variety of forms they take, but in many cases one can distinguish between the chemical operations in which the various components are assembled in the desired form, and the fabrication step in which they are made into the desired shape. The first step will be illustrated by a description of the method of preparing of silica gel and y-alumina support material [1]. 

A.A.Chuiko, G.E.Pavlik, G.B.Budkevich and I.E.Neimark, A Method of Preparation of Silica Gels Containing Aminoalkyl Group, USSR Patent No. 182 719 (1966). 

The preparation of silica gel in the form of irregular grains, used in preparative chromatography, proceeds in a different fashion. First, orthosilicic acid is formed which... 

Mr. William S. HeinamanJ has called attention to the fact that the sulfuric acid added in the preparation of silica gel, synthesis 26 of Volume II,1 should be 6 M rather than 6 N to be consistent with the submitter s statement that "twice the theoretical amount of acid is employed. ... 

Research of Soviet scientists is surveyed. Contributions to various aspects of the colloid chemistry of silica are examined preparation and stabilization of silica hydrosols preparation of silica gels structural characterization of silicas surface chemistry elucidation adsorption and ion-exchange property examination and geometric and chemical modification of silicas, silica coatings, and so forth. 

Preparation of Silica Gels and Powders, Geometric Modification of Silicas, and Studies of Structural Characteristics... 

Belotserkovsky, G. M. Kolosentsev, S. D. Preparation of Silica Gel Technological Institute Leningrad, 1975. 

Practical examples have also been reported in the use of specific adsorbents (e g. preparation of silica gel with specific affinity for methyl orange). Such an approach involves preparation of the adsorbent in the presence of the concerned specific substance. Chromatography of a mixture of related substances has shown that there is a certain specificity for the substance in the presence of which the adsorbent was prepared. 

Haloalkenes, CH2=CH-(CH2)nX (n = 3-7 X = Cl, Br), are interesting synthons for use in the preparation of long chain alkenols and alkenoic acids 2 in cyclization reactions,3 9 as intermediates in the synthesis of pheromones 10 11 in the preparation of silica gel having a functional surface 12 and for the synthesis of oo-iodochloroalkanes,13 and organogermanium dendrimers.14 They are usually prepared from the corresponding alkenols. 

The preparation of silica gel and other adsorbents and the subsequent grading—Brockmann Numbers—is presented in greater detail in Chapter 4. 

Scheme I. Preparation of silica gel-bound crown compounds.

The preparation of silica gel plates for the cultivation of certain types of bacteria has been described by Muller and Holm (391). A silicic acid sol is made by adding a solution of sodium silicate to hydrochloric acid buffered at about pH 4.5. The gel is permitted to set in Petri dishes, washed, soaked in nutrient solution, and then autoclaved. For a purer medium, the intermediate sol may be dialyzed before it is permitted to set to gel (392). An improved procedure for incorporating nutrients directly into the acid, forming the gel under sterile conditions and eliminating dialysis, was developed by Sterges (393). A simplified method (394) is described as follows. Seven hundred milliliters of 0.5 N HCl is mixed with 300 ml of the mineral nutrient solution to be employed. To I volume of 0.5 N sodium silicate is added 1 volume of lime water, and to this 1 volume of the acid nutrient is added. The mixture is poured into Petri dishes, aged 2 hr, and placed in an oven at lOO C for 1 hr. The use of tetramethyl ester of orthosilicic acid is advocated by Ingelman and co-... 

Formal Derivation of the Silica Gel Framework MonosiUcic acid is not a starting material for preparation of silica gel. The silica gel framework may, however, be derived formally from it. Greatly simplified, there are five stages to this ... 

The final stages in the preparation of silica gels for TLO consist of ... 

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