Zeolite sodium aluminosilicates, amorphous

This paper is aimed at clarification of the change of concentration with time in the liquid phase before crystallization starts. To find optimum conditions for the commercial production of pure zeolites of the types A and faujasite, the reaction of fine-particle amorphous silica with sodium alu-minate solution was studied at 20°, 40°, and 75°C. The liquid phase separated by filtration nucleates the zeolite types Ay sodalite, phillipsite, and faujasite, depending on stirring time before liquid-solid separation. Quite similar conditions are observed in precipitated sodium aluminosilicate gels and mother liquor. 

At that time there were synthetic zeolites used for softening water by base exchange. These were amorphous sodium aluminosilicates. Active cracking catalysts were made from them by exchanging the sodium ions with other ions. 

Zeolites are hydrous aluminosilicates that are widely used as catalysts in the chemical process industry. Zeolite A is usually synthesized in the sodium form from aqueous solutions of sodium silicate and sodium aluminate. Kerr (7) and Liu (8) studied an alternative method of synthesis from amorphous sodium aluminosilicate substrate and aqueous sodium hydroxide solution. The reaction (essentially a crystallization or recrystaUizadon process) can be viewed as... 

Synonyms Aluminosilicic acid, sodium salt Aluminum sodium silicate Silicic acid, aluminum sodium salt Sodium aluminosilicate Sodium aluminum silicate Sodium feldspar Zeolite Zeolites Definition Series of hydrated sodium aluminum silicates produced by reaction of sodium silicate and kaolinite clay Formula NajO AI2O3 SiOa with mole ratio = 1 1 13.2 Properties Wh. fine amorphous powd. or beads, odorless and tasteless insol. in water, alcohol, org. soivs. partly sol. in strong acids and alkali hydroxides 80-100 C pH 6.5-10.5 (20% slurry)... 

Further, the residue also gets enriched in the amorphous ingredients, as quantified by Rietveld method. The Rietveld result contradicts the findings from the JCPDS [1], which reveals several zeolitic phases in the residues. In such a scenario, the amorphous phase may commensurate with some sodium aluminosilicates (i.e., zeolites, detected by JCPDS) [1] in the residues, which may be associated with low intensity XRD peaks that cannot be detected by the Rietveld software. 

The method generally used for the synthesis of amorphous [Si-Al] is by the appropriate combination of sodium silicate and sodium aluminate, as explained for the synthesis of aluminosilicate zeolites in Section 3.4.1. But, in this case, the gel is not hydrothermally treated in an autoclave in order to crystallize a zeolite. It is instead thermally dried (see Section 2.7.1) to obtain an amorphous [Si-Al], The produced amorphous material is then exchanged with NH4, using a method similar to that previously explained for zeolites ... 

Influence of Ethanol. Three different amorphous aluminosilicate solids of Si/Al ratios 1.33, 1.48 and 4.28 were synthesized by mixing sodium silicate and aluminate solutions of various concentrations. These solids were extensively ion-exchanged with LiCl and NaCl solutions. The lithium and sodium containing solids (2g) were then mixed with 50 mL of 1JJ LiOH and NaOH, respectively. The hydroxide solutions contained 0%, 10%, 25%, 50% and 75% ethanol (volume by volume). These samples were then heated to 90-95 C, and formation of zeolites was monitored by powder diffraction. In one experiment, the lithium aluminosilicate solid was reacted in the NaOH system. 

The fraction of carbon deposit introduced to the raw material mixture apparently influences the character of zeolite-carbon adsorbents. It mainly increases the specific volume of mesopores and, to a lesser extent, the pores with dimension of 0.4 - 2 nm. On the other hand, the increasing fiction of mineral phase resulted in increase of ultramicropores specific volume. It is a result of growing yield of forming zeolite phase in the adsorbents. The presence of transport pores leads to the higher reaction extent of amorphous aluminosilicate with sodium hydroxide and frtcilitates crystallization of zeolite. 

The conclusions made here concern the crystallization of aluminosilica gels, but Kerr (24) came to analogous conclusions in studying the formation of zeolite A from the amorphous aluminosilicate, treating it with sodium hydroxide solution—i.e., under conditions different from those of the usual crystallization of aluminosilica gels. 

The role of the microemulsion environment in this zeolite synthesis process is undoubtedly complex. Presumably, addition of the sodium aluminate solution to the silicate-containing microemulsion first results in the formation of an amorphous aluminosilicate precipitate. Apparently this material does not remain as dispersed particles in the microemulsion fluid phase. One possible reason is that the stability field of the one-phase microemulsion regions shrinks with an increase in temperature. The ability of NaOH to promote the hydrolytic decomposition of AOT [150,151] will also contribute to the destabilization of the surfactant aggregates. It is likely that the adsorbed AOT molecules associated with the amorphous precipitate play some role in the subsequent transformation to crystalline zeolite. However, the details are yet to be determined. 

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]. 

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