From aluminosilicate solutions

Ellis and Wilson also examined cement formation from aluminosilicate glasses and concentrated solutions of PVPA (Wilson Ellis, 1989 Ellis, 1989 Ellis Wilson, 1990). These cements, like the glass polyalkenoate cements, are a type of glass-ionomer cement. 

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... 

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. 

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. 

Alkali and alkaline-earth aluminosilicates are insoluble, and Muller et al. (138) resorted to tetramethylammonium (TMA) aluminosilicates to measure 27A1 chemical shifts in aluminosilicate solutions. Solutions with different Si/Al ratios and the pure TMA aluminate solution were studied. The molar ratio TMAOH Si Al varied from 3 0 2 to 9 6 2. Theoretically there are 15 distinct Q"(mSi) units with Q = Al (n 0-4 m 0-n). However, dimeric aluminate anions are found only in very concentrated solutions and even then in very small quantities, which led the authors to suggest that the Loewenstein rule is obeyed in aluminate and aluminosilicate anions. The exclusion of Al—O—Al linkages limits the number of possibilities to five... 

The primary minerals of igneous rocks are all mildly basic compounds. When they react in excess with acids such as HC1 and CO2, they produce neutral or mildly alkaline solutions plus a set of altered aluminosilicate and carbonate reaction products. It is improbable that ocean water has changed through time from a solution approximately in equilibrium with these reaction products, which are clay minerals and carbonates. 

The clear aluminosilicate solutions from which Ueda et al (4 ) studied crystallisation of zeolites Y, S and P were based on the composition range 1ONaO.(0.35-0.55)A1 0 (22-28)SiO. (250-300)HO. Figure 1... 

Figure 1. Crystallisation fields of zeolites Y, S and P at 100°C from clear aluminosilicate solutions. In the cross-hatched area gel and solution co-exist. (Reproduced with permission from Ref. 4. Copyright 1984 Butterworths.)...

Figure 2 is a three-dimensional representation of the results and shows the dependence of the gel time on composition for potassium aluminosilicate solutions. Gel times do not depend simply on the concentration of any one component. Rather, a valley of shortest gel times extends almost diagonally across the plot from low-silica, low-alkali, to high-silica, high-alkali compositions. Values to either side of the "valley" are higher. The longest gel times - up to two weeks - occur in solutions of high-silica, low-alkali content. 

Figure 2 Log(gel time) for potassium aluminosilicate solutions. Each dot represents a solution composition from Figure 1.

The silicate species discussed in the preceding section can react with aluminate anions, Al(OH)4 to produce aluminosilicate anions. Si NMR spectra of solid silicates and aluminosilicates indicate that the replacement of Si by A1 in the second coordination sphere of a give Si causes a low-field shift of about 5 ppm. Since each Si atom can have up to four metal atoms in its second coordination spere, fifteen possible Qn(mAl) structural units can be envisioned. The estimated chemical shift ranges for these units are given in Table 3. It is apparent from this table that the 29si spectrum of an aluminosilicate solution in which A1 and Si atoms were statistically distributed would be much more complex than that of an analogous solution containing only silicate species. 

Because of the quadrupolar nature of Al, 27Al NMR spectra of aluminosilicate solutions exhibit broad lines from which it is possible to determine the coordination of Al and its connectivity with Si, but the precise environment of each Al atom cannot be defined. A peak for Al(OH>4 is observed at 75 - 79 ppm, and up to three peaks in the range 58 - 72 ppm which have been assigned to various Al(0Si)n(O")4 n building units. 

Alkaline aluminosilicate solutions aith a Si/Al ratio close to one, exist only at much loner concentrations than do silicate solutions for a given pH. This results directly from the general expression giving the activity of an oligomer. Indeed, as pH increases, the Sit OH) 4 concentration becomes very Ion nhereas the A1(0H)4 concentration remains practically constant and at a much higher level. 

There is a lot of evidence that the crystallization of zeolites from aluminosilicate gels is a solution-mediated transformation process in which the amorphous phase is a precursor for silicate, aluminate and/or aluminosilicate species needed for the growth of the crystalline phase (1-9). Generally, it is well known that the kinetics of most gel-zeolite and zeolite-zeolite transformations can be expressed mathematically by the simple kinetic equation (1,2, 10-12),... 

The heating of the reaction mixture induces the growth of nuclei-I of both zeolites from the solution supersaturated with soluble aluminosilicate species. Since the growth rate of zeolite X is considerably greater than the growth rate of zeolite Na-Pc (see Figure 6), zeolite X appears as the first crystalline phase. The... 

The observations made here are not sufficient to prove any particular synthesis mechanism. However we may speculate as to mechanisms that are consistent with the observations. The synthesis of LTA probably proceeds by formation of sodalite units from D4R. This mechanism has been postulated before(14,15). D4R are present in silicate solutions containing sodium(16), potassium(17), and TMA(18) and in aluminosilicate solutions containing TMA(19). In TMA silicate solutions the fraction of Si in D4R decreases with dilution(20) and in TMA aluminosilicate solutions D4R s decrease with decreasing Si/Al(21). D4R with strict alternation of Si and Al, as required for zeolite A, can join in only one way and this is apparently facile as no template... 

The manufacture of the industrially important zeolite types A, X and Y is generally carried out by mixing sodium aluminate and sodium silicate solutions, whereupon a sodium aluminosilicate get is formed. In this gel Si02- and Al203-containing compounds pass into the liquid phase, from which the zeolites are formed by cry.stallization. As the zeolite growth components are removed from the solution more gel dissolves. The reaction mechanism for zeolite formation is presently not yet fully understood. There is experimental evidence that, depending upon the reaction conditions, different mechanisms are possible. 

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