Hydroxysodalite

Figure 4.9 Crystallization field in the Na20/Al203/Si02 system at 100°C and a water content of (a) 90 to 98 mol% H20 and (b) 60 to 85 mol% H20 (A,X,Y, Zeolite A, X, Y P, Zeolite P1 (GIS) and HS, Hydroxysodalite). (Reprinted from D. Kern, N. Schall, W. Schmidt, R. Schmoll, J. Schurtz, Winnacker-Kuchler Chemische Technik, Prozesse und Produkte, Vol. 3, Siliciumverbindungen, pp. 849-850. Copyright 2005. With permission from Wiley-VCH.)...

At alcohol levels of 50 volume percent or higher, hydroxysodalite crystals are formed at rapid rates (-200 minutes). The role of alcohol on the zeolitization process was examined with ethanol at temperatures of 90-95 C. 

Zeolites Y (FAU) and HS (hydroxysodalite, SOD) both contain sodalite units in their framework. Both can be synthesized easily without TMA but can also be synthesized in the presence of TMA(9,10). All sodalite cages in HS were found to contain a TMA(J3) but quantitative results were not reported for Y(10). The present work describes synthesis of the three zeolites, ZK-4, HS, and Y, that contain the sodalite unit. These syntheses were carried out to prepare samples for NMR and neutron scattering studies, results of which will be reported elsewhere. Some results of these syntheses are presented here these results may help to elucidate the role that TMA plays in the synthesis of the three zeolites. 

A series of preliminary test syntheses of ZSM-20 from reference gel compositions using variable TEA+/Si02 values (TEA+ introduced as bromide as to keep the alkalinity constant) revealed that ZSM-20 is formed in optimal conditions for ratios close to 0.9, value which was recommended by Valyocsik (341. For values lower than 0.5, dense phases such as Analcime, zeolite P or compact Na silicates are formed. For higher TEA+ concentrations ( TEA+/Si02 higher than 1.5), thermodynamically more stable Hydroxysodalite or zeolite Omega (ZSM-4) cocrystallize. 

The synthesis of zeolite A, mixtures of A and X, and zeolite X using batch compositions not previously reported are described. The synthesis regions defined by triangular coordinates demonstrate that any of these materials may be made in the same area. The results are described in terms of the time required to initiate crystallization at a given reaction temperature. Control of the factors which can influence the crystallization time are discussed in terms of "time table selectors" and "species selectors . Once a metastable species has preferentially crystallized, it can transform to a more stable phase. For example, when synthesis conditions are chosen to produce zeolite A, the rate of hydroxysodalite formation is dependent on five variables. These variables and their effect on the conversion of zeolite A to hydroxysodalite are described mathematically. 

By combining published results with the present work, it would appear that within the range of synthesis conditions tested there is a continuum between the species hydroxysodalite, zeolite A, zeolite X, and zeolite Y. This concept is depicted on Figure 5,... 

Zeolite A remaining in its mother liquor and reacted for extended periods of time will convert into hydroxysodalite (12). 

Figures 6, 7 and 8 demonstrate how the species detected can change with time. Both the conversion of zeolite A to hydroxysodalite and the conversion of zeolite X to zeolite A can be easily seen in this three hour time sequence. 

Lower values dampen the reaction Reaction Time - Longer times produce more hydroxysodalite Shorter times produce less hydroxysodalite Rate of addition, source of Si02 and low levels of anion activators had little influence within the synthesis conditions tested. 

Starting with any given set of zeolite A synthesis conditions, one may control the amount of hydroxysodalite formed by properly manipulating these variables. It should be noted that more than one factor can vary at the same time. If their variation produces compensating effects, i.e. lower Na20/Si02 ratio and lower H2O/... 

Examples of how these variables affect the conversion of zeolite A to hydroxysodalite are shown on Table III. Regression analysis of data relating these five variables to the amount of hydroxysodalite found in zeolite A batches gives the following equation ... 

Given a set of synthesis conditions and sufficient reaction time to produce hydroxysodalite, pure zeolite A can be obtained by decreasing the reaction time until the % HS approaches 0. If the reaction time is fixed, any of the other variables can be varied and achieve the same result. An example of how reaction time affects the conversion of zeolite A to hydroxysodalite, when all other variables are held constant, appears on Figure 9. 

Figure 9. Effect of time on the conversion of zeolite A to hydroxysodalite. Values are calculated using % HS equation.

Powerful evidence for the liquid-phase mechanism comes from the direct crystallization of zeolite from clear solution. In the early 1980s, Koizumi and coworkers carried out an extensive study on this topic. They directly synthesized analcime, hydroxysodalite, zeolite B, mordenite, zeolite P, faujasite,[30] erionite, and potassium chabazite from clear solution. Pang et al. directly crystallized zeolite A[31] and FAPO-5[32] from clear solution as well. The study on the direct synthesis of faujasite [30, 33] from clear solution will be elaborated below. 

Figure 7.5. Compositional synthesis windows for the Na20-Al203-Si02-H20 system at 100°C and 90-98 mol% H2O. Source of Si02 is (a) sodium silicate and (b) colloidal silica. The area enclosing a letter represents the composition that yields the corresponding phase, while the + marks the typical composition of the product. A, X, and Y = zeolites types A, X, and Y B = zeolite P R = chabazite S = gmelinite and HS = hydroxysodalite (from Breck and Flanigen, 1968, with permission).

Fig. 7. The effect of varying the concentration of triethanolamine on the size and purity of zeolite A. Synthesis at 95°C, composition 0.84 Si02 Al203 1.94 Na20 194 H20 y TEA, a y = 2.12 b y = 4.23 c y = 6.34 d y = 30.0. At high triethanolamine concentrations, the products are predominantly zeolite X and hydroxysodalite. All four electron micrographs are shown at the same magnification. Reprinted with permission from Stud Surf Sci Catal, vol. 49A, Scott G, Dixon AG, Sacco A, Thompson RW, Synthesis of zeolite NaA in the presence of triethanolamine, (1989), pp. 363-72, Elsevier Science Inc.

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