Geoscience Reference
In-Depth Information
organic species randomly distribute in the gel. Then a self-assembly process occurs
among these reaction species driven by the nonbonding interactions (e.g., electrostatic
and H-bonding) between the inorganic and organic species. The arrangement of the
organic and inorganic species gradually becomes orderly, where several small organic
amines assemble together with their hydrophobic ends pointing inward and the hydro-
philic ends interacting with the inorganic species. The formation of such ordered inor-
ganic
organic composite species provides the fundamental structural units for
nucleation and crystal growth. However, the self-assembly process of the inorga-
nic
organic species needs to be further investigated
[58]
.
The cations influence greatly the type of zeolite that crystallizes from aqueous alka-
line magmas. Also, in the laboratory, synthesis of zeolite cations occupy an important
place, not only as structure- and composition-directing agents but also as agents influ-
encing the rate of zeolite synthesis
[28,40]
. The species such as Na
1
,Li
1
,Cs
1
,K
1
,
Rb
1
,Ca
2
1
,Sr
2
1
, tetra-alkylammonium cations (e.g., TMA
1
,TEA
1
,TPA
1
,dihy-
droxyethyl-dimethylammonium), dialkyl and trialkyl amines, and phosphonium
compounds are frequently used. Sometimes, neutral molecules, like crown-ethers, are
used to form charged species upon complexation of an alkali metal cation.
Structure-making cations are small cations, like Na
1
and Li
1
, which interact
strongly with water molecules because of their high charge density. As a result
of this, the original hydrogen bonds are broken and the water molecules will be
(reorganized) organized around the cations. This water
cation interaction is shown
schematically in
Figure 6.16
.
Self-
assembly
Reaction gel
Nucleation
Crystal growth
Figure 6.16 Scheme of the cooperative templating mechanism of multiple small amines in
the formation of extra-large micropore
[58]
.
