Page 216 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 4 CONTROL OF NANOSTRUCTURE OF MATERIALS
understand the nucleation and the crystal growth,
and the schemes in molecular levels are getting
clearer. Here, synthesis of Si-ZSM-5 (Zeolite
Socony Mobil #5) is introduced since several
groups provide a unified view [9].
29
1 H- Si CP Magic Angle Spinning Nuclear
Magnetic Resonance (MAS NMR) was applied
to understand the interaction between an SDA

((C H ) N :tetrapropyl ammonium ion, TPA ) and
3
7 4
100 silicate at the early stage of the synthesis, and behav-
ior for the SDA to organize silicate was investigated.
Crystallinity % Nucleation Rate have weak intermolecular interaction of non-covalent
H in the SDA and Si in the precursor of framework
bonding. Based on the analysis, it was revealed that

the TPA –silicate complex is formed prior to the
onset of the long-range ordering in – 10 nm scale that
corresponds with crystal lattice of the zeolite as illus-
0 trated in Fig. 4.3.3(a–c). TPA and silicate are soluble

Time
in water, but not very hydrophilic; therefore, water
molecules are ordered around them by hydrophobic
Figure 4.3.2
Outline of zeolite synthesis. hydration. When both of them approach each other

and the hydrated layers are overlapped, TPA is cov-
ered with silicate and ordered water molecules are
released. Thus, this complex formation is thermody-
frameworks. Highly reactive, amorphous hydrogels namically reasonable.
are formed by mixing these reactants, which is encap- Continued nucleation and crystal growth processes
sulated in an autoclave and heated at prescribed tem- were investigated with in situ combined small angle
perature ( 250°C) to result in the crystallization of X-ray scattering (SAXS)–wide angle X-ray scattering
zeolites. Apparent changes are shown in Fig. 4.3.2. (WAXS) [9] using a clear solution instead of hydro-
Several investigations have been carried out in gel. The schemes proposed from the results are
order to understand the formation mechanism of Fig. 4.3.3(d–f). Throughout the process, 2.8 nm clus-
framework structure with the void spaces. Since it is ters are observed, and their aggregation proceeds
difficult to perform in-situ observation in dense prior to the onset of the long-range ordering (crystal-
hydrogel under high temperature, high pressure, and lization). WAXS can detect the formation of crys-
high pH conditions, the details of the formation have talline nuclei and the particle size distribution can be
been only speculated based on the crystallization con- determined from SAXS. From these analyses, it is
dition of the zeolites. Thanks to the recent develop- proposed that the nuclei are converted from aggre-
ment of analytical tools, formation mechanism of gates of the clusters, and the crystal growth proceeded
zeolites has been getting clearer, especially for high- through the transportation of the cluster decomposed
silica zeolites. from the aggregates. Also for hydrogel systems and
on other pure-silica zeolites (Si-beta, Si-ZSM-12),
(3) Formation of microporous structure of zeolite experimental results to support a similar scheme have
Zeolite synthesis route consists of nucleation and been reported.
crystal growth. As for both of the steps, As shown in the case of Si-TPA-ZSM-5, the role of
SDA is to organize silicate or aluminosilicate around
– solution-mediated transport mechanism, it to form precursor complex [8]. Hydrophobic hydra-
– solid-hydrogel transformation crystallization tion provides the driving force for the organization.
mechanism Thus, SDA should

have been proposed. There have been several experi- – be soluble in water, but less hydrophilic,
mental results to support each of them [8]. In some – be stable under hydrothermal synthesis condition,
cases, the same product can be obtained via different and
routes depending on the synthesis condition and – have suitable size and shape.
resources. It should be noted that water is highly con-
cerned in solid–hydrogel transformation since a lot of Thus, some quaternary alkylammonium cations and
water is included in the gels. organic amines have been employed as SDAs.
The system that has been studied most intensively As mentioned above, the initial stages of zeolite syn-
is the synthesis of high- or pure-silica zeolites with thesis with SDAs is getting clearer although this syn-
SDAs. Several tools have been employed to thesis method was developed after inorganic synthesis

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