Alumino-silica gels

An important application of metal nanoparticles is their use as catalysts on solid supports or in confined media. When the solution containing metal ions is in contact with a solid support, the ions can diffuse in the pores and adsorb on the surface. Therefore, the penetration of the ionizing radiation enables the in situ reduction of metal ions and then the further coalescence of metal atoms inside the confined volumes of porous materials, such as zeolites, alumino-silica-gels, colloidal oxides such as TiOj or polymeric membranes. 

In order to avoid this limitation, the interest in synthesising mesoporous materials is nowadays increasing in catalysis as the high pore dimension (10-150 A) allows the access to bulky organic molecules. In 1991, the Mobil Oil Company synthesised a mesoporous alumino-silicate through a basic synthesis using alumino-silica gel and alkyl-... 

Colloidal support such as small Si02 particles restrict interparticle diffusion of silver atoms when formed by radiolysis of the ions at their surface. The silver oligomers absorbing at 290 and 330 nm are observed by pulse radiolysis. They are stable with respect to coalescence but they are oxidized by MV, O, Cu and RufNHj)/. Alumino-silica gels with silver ions give after irradiation optically clear xerogels containing silver clusters from 2.5 to 4.5 nm. 

From the time of the mixing of the starting materials to the moment of zeolite application, seve processes may occur all of which may contribute to the formation of the final product. In the majority of zeolite formation processes, the synthesis involves the formation of a gel phase. Investigations of the precrystalline gel phase have shown that the chemical composition of this phase resembles the composition of the final zeolite structure (Zhdanov 1971). As such, zeolite X could, for instance, be directly synthesized by treatment of the solid, precrystalline phase of the initid reaction mixture (Breck 1974). Formation of the alumino-silica gel thus must play a crucial role in zeolite synthesis. 

We have noted earlier that aluminium is unusual in forming alumino-phosphate complexes in phosphoric acid solution which may be of a polymeric nature. Bearing in mind the analogies between aluminium phosphate and silica structures, it may well be that during cement formation an aluminium phosphate hydrogel is formed. Its character may be analogous to that of silica gel, where a structure is built up by the... 

In the late 1940s zeolites were synthesized according to the procedure shown in Fig. 3.24. First an amorphous alumino-silicate gel is formed. This process is completely analogous to the production of alumina and silica gels described before. Subsequently this gel is crystallized into zeolite. The preparation of zeolites has drawn tremendous attention of the scientific and industrial community. A wide variety of zeolites have been synthesized, and reproducible synthesis procedures have been reported (often in the patent literature). Natural zeolites also exist massive deposits have been discovered in many places in the world. 

The adsorption of aliphatic acids (valeric, caproic, enanthic, caprylic, pelar-gonic) from 1% solution from cyclohexanone on an alumino-silicate catalyst, silica gel (KSK-2), and AI2O3 has been studied under dynamic conditions by Narmetova, Khashimova, Narimova, and Ryabova. The lower molecular weight acids were preferentially adsorbed. Both physical and chemisorption... 

The microporous alumino-silicate zeolites (Types A, X, and mordenite are frequently used) provide a variety of pore openings (3-10 A), cavity and channel sizes, and framework Si/Al ratios. They are also available in various cationic exchanged forms (Na, K, Li, Ag, Ca, Ba, Mg), which govern their pore openings and cationic adsorption site polarities. They are highly hydrophilic materials and must be dehydrated before use. The amorphous adsorbents contain an intricate network of micropores and mesopores of various shapes and sizes. The pore size distribution may vary over a wide range. The activated carbons and the polymeric sorbents are relatively hydrophobic in nature. The silica and alumina gels are more hydrophilic (less than zeolites) and they must also be dehydrated before use. 

Another bond used in basic castables and for producing acid resistant alumino-silicate castables involves the use of alkali silicates, either sodium silicate or potassium silicate. Alkali silicates will react with acids, salts, and metal hydroxides and stiffen or set by formation of a silica hydrogel. This gel will dewater continuously as temperature increases with complete dehydration at 350°C. Setting agents used to set alkali silicate bonded castables include sodium silicofluoride, aluminum polychloride, sodium phosphate, aluminum polyphosphate, magnesium polyphosphate, and calcium and magnesium hydroxides (13). 

These sol-gel preparations of cordierite clearly demonstrate the importance of the formation of heterometallic Si-OAl bonds in the solution. Starting from TEOS and two salts as precursors for Al l and Mg ions leads to the formation of a silica network, with no evidence for the presence of heterometallic bonds in the final gel. Aluminum alkoxides provide reactive species containing Al-OH groups which can easily react with Si-OEt groups to build an alumino-silicate network. 

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