Alkaline aluminosilicate gels, zeolite

The crystallization of zeolites from alkaline aluminosilicate gels was studied by luminescence and Raman spectroscopy. Trace amounts of Fe3+ ions substituted for Al3+in the tetrahedral aluminosilicate gel framework exhibit characteristic phosphorescence spectra, which have been used to follow the buildup of the zeolite framework. Phosphorescence spectra of exchanged Eui+ cations and Raman spectra of (CH N+ cations present in the solid phase of the gel indicate that no zeolitic cages exist in this phase during the induction period. Raman spectra of the liquid phase of the gel system show only the presence of Si02-(0H)2 and Al(OH)a anions. Our results demonstrate that crystallization of zeolites occurs within the solid phase of the gel, which is believed to consist of amorphous tetrahedral alumino-... 

During this period, numerous synthetic zeolites were found, some of them with large adsorption capacities and adequate aperture for industrial applications.Two teams have carried out the main part of this work. While Milton and Breck used reactive alkaline aluminosilicate gels and temperatures under 1S0°C, Barter investigated the hydrothermal synthesis up to 450°C,with mixtures of composition aM2 0 AI2O3 nSi02 bH20 (M = Li, No, K, Ca, Sr, Ba...). In all these zeolites, however, die Si/Al rado remained between 1 and 5. 

As previously described, the hydrothermal synthesis of aluminosilicate zeolites is carried out with highly reactive aluminosilicate gels in autogenous conditions. This kind of synthesis may involve the structure-directing role of alkaline cations in solution-mediated crystallization of the amorphous gel [55],... 

Only scattered reports are available on the preparation of LSX even 1n an unbound form. Simply extrapolating the standard synthesis reagents and conditions used to produce typical X zeolite will not yield a S1/A1 ratio of less than 1.2. Kuhl and Sherry have published the most comprehensive Information on LSX synthesis.(10) In a British patent, they describe a preparation wherein aluminosilicate gels In highly alkaline mixed NaVK+ are subjected to a multiday aging period at 40°C, followed by... 

T Tnder suitably alkaline conditions and with a sufficient volume of water, it is possible to dissolve aluminosilicate gels to form clear liquids free of any residual solids. Such liquids are of interest because from them one may hope to examine the first stages of zeolite crystallization. The question of homogeneous vs. heterogeneous nucleation (7, 8) of zeolite crystals may be studied usefully when clear liquids provide the reaction mixture. For these reasons, we have examined the first solids to appear from such liquids and have followed their subsequent evolution. 

An alternative hypothesis, developed from studies of the synthesis of Linde A zeolite carried out by Kerr (5) and Ciric (6), pointed to growth occurring from solution. The gel was believed to be at least partially dissolved in solution, forming active aluminosilicate species as well as silicate and aluminate ions. These species linked to form the basic building blocks of the zeolite structure and returned to the solid phase. Aiello et al. (7) followed the synthesis from a highly alkaline clear aluminosilicate solution by electron microscopy, electron diffraction, and x-ray diffraction. These authors observed the formation of thin plates (lamellae) of amorphous aluminosilicates prior to actual crystal formation. 

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