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ZEOLITES AND MOLECULAR SIEVES: SYNTHESIS AND MOLECULAR SIEVING PROPERTIES 165

species by a condensation–polymerization mechanism. Expressed in moles per
mole of A1 2 O 3 for Na 2 O/SiO 2 /H 2 O, typical compositions of the reactants are (at
pH >10):

1. Type 4A zeolite, 2/2/35
2. Type X zeolite, 3.6/3/144
3. Type Y zeolite, 8/20/320
The gels are crystallized in a closed hydrothermal system at temperatures
◦
◦
between 25 and 175 C. Temperatures as high as 300 C are used in some cases.
The time for crystallization ranges from a few hours to several days. A large
amount of work was directed at determining the reaction conditions (e.g., tem-
perature, time, and degree of agitation) and compositional parameters that yield
a single-phase, fully crystallized zeolite. Figure 7.5 shows one of the diagrams
(Breck, 1974; Kouwenhoven and de Kroes, 1991). Many “reactant composition”
diagrams for synthesizing a number of zeolites are given by Breck (1974).
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.

7.2.2. Organic Additives (Templates) in Synthesis of Zeolites and
Molecular Sieves
The basic building block for zeolite types A and X is the sodalite cage or beta
cage, as shown in Figure 7.1. The beta cages are connected by 4-prisms to form
A zeolite and are connected by 6-prisms to form type X zeolite. Beside beta
cage, 15 other cages are known (Gellens et al., 1982). All can be used as the
basic building blocks for zeolite structures.
The zeolites that are built with cages have the topology of cages and cavities.
These cavities are interconnected by windows, as shown in Figure 7.1. The other
common topology is the tubular form. There are 10 basic tubular building units,
with 4 shown in Figure 7.6. Gellens et al. (1982) showed the 10 tubular units as
well as 16-cage building units.
The use of organic amines (mainly quaternary amines) as additives in the
synthesis gel to influence the subsequent crystallization has been an exciting ele-
ment and powerful tool in zeolite synthesis. These additives are often referred
to as “templates” or “structure directing agents,” although their roles in crys-
tallization are far more complex than templating and are not well understood
(Barrer, 1978; Breck, 1974; Szostak, 1998; chapters in Karge and Weitkamp,
1998). They are usually added after the gel solution is prepared, and after syn-
thesis the residual hydrocarbons are removed from the crystals by air burning at
◦
500–600 C. In addition to the amine templates, HF or KF is frequently used to
increase the crystallization rates, apparently by increasing the concentration of
free silicate ions.