Synthesis of zeolites A, X, and

Many alkali metal hydroxides and raw materials containing silica and alumina can be used in low-temperature synthesis. The steps involving the Na20-Al20s-Si02-H20 system, which is used in synthesizing zeolites of types A, X, and Y, are as follows (Breck, 1974)  

Naj [(A102)x(Si02)y] mH20 - - solution (zeolite crystal) 

The first step involves gel formation between sodium hydroxide, sodium silicate, and sodium aluminate in aqueous solution at room temperature. The gel is probably formed by the copolymerization of the silicate and aluminate 

As the synthesis proceeds at elevated temperatures, zeolite crystals are formed by a nucleation step, followed by a crystal growth step involving assimilation of alumino-silicate from the solution. The amorphous gel phase continues to dissolve, thereby replenishing the solution with alumino-silicate species. This process results in the transformation of amorphous gel to crystalline zeolite. 

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Figure 1. Patented batch compositions for the synthesis of zeolites A, X, and Y. Coordinates expressed as mole percent.

A number of organic compounds were tested for their influence on the synthesis of zeolites A, X and Y [ 15]. Between 0.5 and 200 % by weight of each com-... 

Mid 1930s-1940s Pioneering work of Barrer in adsorption and synthesis 1949-1954 Discovery and synthesis of zeolites A, X, Y (Milton-Breck)... 

Synthesis of Zeolite A. Zeolite A was crystallized from a batch of overall composition 2.5 Na20-Al203-1.7 SiO2-150 H20 at 60, 75, and 90°C (10). The same system was seeded with zeolite A crystals of 0.5-5 /zmeter size at an initial conversion level of 25% and crystallized at 60°C. As with the crystallization of zeolite X from seeded systems, the data were treated by ignoring the presence of seed crystals. The crystallization curves are shown in Figure 8. During the induction time period in the unseeded system, crystallization was taking place at a slow rate in the seeded system, both at 60°C. After this slow crystallization period the crystallization rate reached 22% per hour at the 50% conversion level... 

In this study, we have shown that both alcohol and D20 have an Important effect on the nucleation and crystal growth of zeolites with Si/Al ratios between 1-2. In the case of alcohol, the formation of large pore zeolites such as zeolites X or Y is markedly accelerated at low alcohol levels. We attribute this to a stabilization of the cation-water complex and structured H20 which act as templates. However, at high alcohol levels, the structure of water disintegrates and leads to the formation of more condensed zeolites such as sodalite or cancrinite. Synthesis of zeolite A in D20 is slower than that in water, which primarily arises from the primary and secondary isotope effect during the condensation polymerization reactions necessary for zeolite growth. 

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. 

Figure 3. Effect of batch composition on the synthesis of zeolites A and X.

Four additional factors can also contribute to a limited shift in the time required for crystallization. These are the rate at which the sodium aluminate solution is added to the sodium silicate solution (13), anion activation (14, 15, 16), the source of Si02 (17), and the reaction temperature (10). It has been found that the effect of these variables is most pronounced when synthesizing higher Si02/Al203 ratio zeolites, such as X and Y. Their effect on the synthesis of zeolite A is considerably less and sometimes negligible. 

Zeolites. A large and growing industrial use of aluminum hydroxide and sodium aluminate is the manufacture of synthetic zeolites (see Molecular sieves). Zeolites are aluminosilicates with Si/Al ratios between 1 and infinity. There are 40 natural, and over 100 synthetic, zeoUtes. All the synthetic stmctures are made by relatively low (100—150°C) temperature, high pH hydrothermal synthesis. For example the manufacture of the industrially important zeolites A, X, and Y is generally carried out by mixing sodium aluminate and sodium silicate solutions to form a sodium aluminosilicate gel. Gel-aging under hydrothermal conditions crystallizes the final product. In special cases, a small amount of seed crystal is used to control the synthesis. 

Yu et al. [252] successfully employed UV Raman laser spectroscopy for the characterization of the framework vibration range of zeolites A,X, Y, MOR, L, and Beta. UV Raman laser spectroscopy proved to be advantageous in that it was much less disturbed by fluorescence than the conventional Raman laser technique. The authors claimed that x-membered rings (xMR) in the structures were manifested by absorptions in the wavenumber ranges (in cm 0 470-530 (4MR), 370-430 (5MR), 290-410 (6MR) and 220-280 (8MR). The method was also used for the characterization of TS-1, [Fe]ZSM-5, [V]MCM-41 and in synthesis studies (vide infra). 

Chamell used triethanolamine (2,2, 2"-nitrilotriethanol) as an additive in the synthesis of zeolites A and X and reported that crystals as large as 100 and 140 xm, for zeolites A and X, respectively, were formed (Chamell, 1971). Subsequent work showed that triethanolamine formed a complex with Al +, the presence of which would reduce the tendency for nucleation and crystal growth (Coker and Jansen, 1998). 

Figure 3. (lefthand) X-ray powder diffraction pattern and (righthand) scanning electron micrograph of zeolite A-chitosan composite resulting from in-situ synthesis of the zeolite... 

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