Aluminosilicate hydrogels

The present study deals with the application of RDF to the sodium aluminosilicate hydrogels, which were formed by mixing, aging and heating of sodium silicate solution with sodium aluminate solution under stirring, in order to produce NaA-zeolite crystal. 

Since the main peaks are 3.25A, 4.2-4.6A and 7.0A, the most reasonable unit structure of the sodium aluminosilicate hydrogel is the 4-member chain illustrated in Fig, 11. Many 4-member chains easily form sodalite cages as shown in Fig. 12. In the case of a simpler silica structure, the double 4-member ring (cube) was recognized by Sakka et al C5). 

Zeolite rho was prepared from aluminosilicate hydrogels containing sodium and cesium cations. The procedure is entirely comparable with the synthesis of faujasite except for substitution of CsOH for about 10% of the NaOH in the faujasite synthesis gel. Alumina trihydrate (Alcoa C-33 grade) was dissolved in 50% NaOH solution at 100°. After cooling to ambient temperature, the required amount of CsOH solution was added, and the resulting liquor was blended into 30% silica sol (duPont Ludox LS-30) with vigorous mixing. After 3-7 days incubation at 25°, the synthesis gel was held at constant temperature, 80, 90, or 100°, until crystals formed maximum crystallinity was usually achieved in 2-4 days. 

Figure 1. Crystallization profile for SSZ-13 synthesis from Cubic P zeolite and from conventional aluminosilicate hydrogel.

The synthesis of erionite was reported by Zhdanov (11) in 1965. The medium was described as a mixed sodium-potassium aluminosilicate hydrogel at 90°-100°C but further details are not given. Breck and... 

In 1968, Breck and Flanigen for the first time proposed the solid-phase mechanism based on their studies of the crystallization of aluminosilicate. They found that the formation and transformation of amorphous aluminosilicate hydrogel always happened during the crystallization process of zeolites and that the composition of the hydrogel was similar to that of the resultant zeolites. 

Figure 5.27 Nucleation and growth model in aluminosilicate hydrogel. Reproduced with permission from [44], Copyright (1998) Elsevier...

The zeolite ZSM-3 was prepared from aluminosilicate hydrogels containing sodium and lithium cations. The crystallization technique consists of first preparing a precursor solution of concentrated sodium aluminosilicate and then mixing it with aqueous sodium silicate and aluminum chloride solutions to form the starting hydrogel slurry. This slurry is filtered to remove excess soluble sodium silicate. Lithium is added to this filter cake as lithium hydroxide solution. This mixture is held at temperatures of 60° to 100 °C until ZSM-3 crystals form. At 60 °C, crystallization requires 5 days while at 100 °C, crystals are formed in 16 hours. In order to obtain the desired SiOo/Al203 ratio in the crystalline product, the aluminum chloride content is varied. 

Bodart, R, B. Nagy, J., Gabelica, Z. and Derouane, E.G. (1986) Rictors governing the synthesis of zeolites from aluminosilicate hydrogels a comparative study of the crystallization mechanisms of zeolites Y, mordenite and ZSM 5 ,/. Chim, Phys.-Chim. Biol 83(11-12), 777-790. 

The predominant process, whether employed semicontinuously or batchwise is the aluminosilicate hydrogel route, sometimes referred to as the silicate route. Sodium silicate and sodium aluminate solutions are mixed together in the desired ratio in an alkaline aqueous solution (see Equation 22.5). 

Thereafter, the sodium aluminosilicate hydrogel is hydrothermally transformed into the highly-ordered crystalline sodium zeolite 4A (see Equation 22.6) [103,109,119,122,142]. 

An increase in temperature leads to a decrease in crystallization time because the solubility of the aluminosilicate hydrogel is enhanced at higher temperatures. Thus, the aging is accelerated, incubation period shortened, and crystallization velocity increased. The latter increases steadily with time and drops after a relatively high crystallization degree has been obtained. The reaction is commonly monitored by means of x-ray powder diffraction and is stopped at this point [104,110,137]. 

The aluminosilicate hydrogel-containing solution may then be pumped to a separate crystallization vessel of a size of up to 100 m and above. The crystallization of zeolite A is usually carried out batchwise at temperatures of 90-95°C. It is accomplished after 40-60 min. Discontinuous processes may use the same reactor for precipitation and crystallization [15,99,104,108,110,116, 119,122,125-128,139,140,142]. 

Zeolite AX is prepared by the aluminosilicate hydrogel route. Therefore, a commercial sodium silicate solution (28.4% Si02 and 14.2% Na20) is fed to a mixture of sodium aluminate and potassium hydroxide at a temperature of 65°C. The amorphous alkali aluminosilicate hydrogel is maintained at the same temperature under agitation for 30 min and without agitation for 12 h. The product is filtered off, washed, and dried by the usual means [174,175]. 

Fig. 4 Al MAS NMR spectra for aluminosilicate hydrogels prepared from aluminum acetate in media of various pH...

Effect of the Duration and Temperature of Ageing on THE Size of Aluminosilicate Hydrogel Globules... 

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