Aluminosilicate solutions

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 work of Muller et al. (138), who measured 27A1 chemical shifts in TMA-aluminosilicate solutions, has been discussed in Section 111,1. Briefly, they identified four kinds of Al- centered units, i.e., Q°, Ql(l Si), Q2(2Si), and Q3(3Si), and suggested that no Al—O—Al linkages are present in aluminosilicate anions. 

Equilibrium Data. The only laboratory studies of aluminosilicate-solution equilibria in which both solid phases and aqueous solutions have been well defined seem to be those of Hemley, who has studied the K system (11) and the Na system (12) and discussed the mixed Na-K system (13). To obtain reasonable equilibration times with well-defined phases, it was necessary to work at temperatures higher than 150°C. 

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.)...

Studies of Raman 2) and NMR (Jj) spectra of solutions of aluminates and silicates, and of their mixtures under conditions yielding clear aluminosilicate solutions, agree in showing at least partial suppression of the Al(OH) ion when silicate anions are present, supporting the view that aluminosilicate anions form around room temperature. 

A range of aluminosilicate solutions were investigated. The gelation behaviour, the species in solution (as observed by NMR) and the zeolite crystallization products are described. The effect of concentration and type of alkali metal cation present in solution gives information about the formation of aluminosilicate complexes and how they interact, under the influence of the cation, to form an aluminosilicate gel, the precursor to zeolite crystallization. 

Figure 1 Composition plot for the identification of aluminosilicate solutions. All solutions contain 0.1 M aluminate and variable silica and alkali. The compositions of the solutions indicated by the boxes are given in Table 1.

In these, any additional alkali and water were added to the aluminate solution, so that the initial silicate solution was the same through any series some gel time experiments for these aluminosilicate solutions have already been described M 8-201. 

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.

Figure 3 Log(gel time) of potassium aluminosilicate solutions 1e - 8e prepared by Method 2 of mixing. = normal mixing as in Method 1, = 22% of total alkali added to silicate solution before mixing with aluminate, 44% of total alkali, = 66% of total alkali and 0= 88% of total alkali.

Figure 4 shows the light-scattering curve for nine selected potassium aluminosilicate solutions. Two types of behaviour were apparent. For low-silica solutions, a low, steady amount of scattered light is followed by a sharp increase as the solution becomes cloudy prior to gelling, possibly suggesting a nucleation-and-growth mechanism. There is a direct relation between the persistence of the low value and the observed gel time - the... 

Figure 4 Light scattering curves for potassium aluminosilicate solutions prepared by Method 1 of mixing. All curves are scaled as for solution 1a.

This proved extremely informative. Aluminosilicate complexes were observed in every solution studied. Although the spectra measured are of solutions undergoing change, the time taken to collect the data was generally small compared with the gel time. Figure 5 shows spectra for the nine representative potassium aluminosilicate solutions. A summary of the shifts is given in Table II. 

TABLE II 27AI NMR CHEMICAL SHIFT RANGES (PPM) FOR ALUMINOSILICATE SOLUTIONS. 

Figure 6 Variation of the chemical shifts (ppm) of AI(OH)4 and AI(2Si) peaks with silica content for aluminosilicate solutions 1 e - 8e.

Figure 7 27Al NMR spectra of potassium aluminosilicate solution 6e prepared by Method 2 of mixing. Acquisition time was 0.067s and the number of scans averaged 6000. All data was collected in 4K and transformed in 32K. Spectrum 1 = normal mixing as in Method 1,2 = 22% of excess alkali added to the silicate soltuion before mixing with aluminate, 3 = 44% of excess alkali, 4 = 66% of excess alkali and 5 = 88% of excess alkali.

Figure 8 27AI NMR spectra of potassium aluminosilicate solution 4b over time. Acquisition time was 0.067s and the number of scans 5000 for each spectrum. All data was collected in 4K and transformed in 32K.

Figure 9 29Si NMR spectra of caesium silicate (0.8M Si, 4.98M CsOH) and aluminosilicate solution 8e plotted on an absolute intensity scale aluminosilicate relative to silicate. Acquisition time was 1.359s and the number of scans was 500 for each.

In this paper we shall summarize our recent findings concerning the nature and distribution of species present in silicate and aluminosilicate solutions and gels. Particular attention will be focused on establishing the effects of pH, cation composition, Si/Al ratio, and solvent composition. 

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. 

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