Online version is available on http://research.guilan.ac.ir/csm
CSM
Chemistry of Solid Materials
Vol. 2 No. 1 2014
[Research]
Role of gel aging in template-free synthesis of micro and nano-
crystalline sodalites
S. M. Pourali
1
, A. Samadi-Maybodi
2
*
1
Ph.D Student, Analytical Division, Faculty of Chemistry, University of Mazandaran,
Babolsar, Iran
2
Professor, Analytical Division, Faculty of Chemistry, University of Mazandaran,
Babolsar, Iran
* Corresponding authour's Email: samadi@umz.ac.ir
Article history:
(Received: 17 Aug 2014, Revised: 16 Sep 2014, Accepted: 12 Oct 2014)
ABSTRACT
A facile effective stirring aging at room temperature prior to conventional hydrothermal
treatment was employed in the template-free synthesis of micro- and nano-crystalline sodalites
with two different initial gel compositions. The effect of initial Si/Al molar ratio, NaOH
concentration and stirring aging time were investigated on the morphology and particle size of
the synthesised sodalites. The results revealed that applying various stirring aging time can
change the proportion of the contaminant phase associated with sodalite and alter the
morphology of the sodalite crystals from wool ball-like consisting of nano-threads to cabbage-
like with nano-leaves, as well as size distribution of nanocrystalline sodalite. The size of the
nanocrystals was in the range of 50-80 nm as observed by FE-SEM and the yield was as high as
about 15%.
Keywords: Nanocrystalline Sodalite, Wool ball-like, Cabbage-like, Morphology, Argon
isotherm.
1. INTRODUCTION
Sodalites, a class of microporous
aluminosilicates with general formula
of M8[ABO4]6X2, are built of a six-
membered ring (6R) with a pore size of
0.28 nm, and the maximum diameter of
the void included in the framework of
0.63 nm [1]. So far, sodalite crystals
attracted considerable attention due to
their widespread application in
hydrogen storage [2-4], semiconductors
[5, 6] and pigment occlusion [7]. Nano-
sized and micro-sized sodalite crystals
could be easily synthesised in presence
of organic templates [8-12] or
structure-directing agents (SDAs) [13]
through hydrothermal treatment. Nano-
sized sodadlites have been obtained
using SDAs by solid-solid
transformation of Al2O3 pillared clay in
the alkaline solution [14, 15], direct
solid transformation of preformed
silicate nanocrystals [16], and washing
sodium aluminosilicate solution by
NaOH solution to remove the
amorphous phase [17]. However,
removal of SDAs often leads to
irreversible aggregation of the
nanocrystals and decrease in
crystallinity. The yield of zeolite
nanocrystals by this approach is S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
22
normally lower than ~ 10%, calculated
based on silicon source used [18].
Furthermore, SDAs tend to alter Si/Al
molar ratio of the final products, thus
remarkably affect their application
[19]. The organic additives are non-
recyclable and their application is
costly and requires calcination which
results in the production of CO2 and
NOx pollution problems [20]. Okubo T.
et al. reported the hydrothermal
synthesis of sodalite nanocrystals
without using any organic additives at
low temperature [17]. Undoubtedly, the
morphology of zeolites is originally
related to the framework type and also
closely related to the micropore size,
crystal size and shape, and directly
affects the physicochemical properties
of zeolites. Morphological properties of
zeolites are particularly important in
catalytic applications where the particle
shape can have a dramatic effect on the
product distribution due to the
differences in rates of transport/
diffusion and reaction. Recently, great
efforts have been directed toward
designing zeolites with desired
functions by applying nanotechnology.
Thus, there has been a great interest in
developing synthetic approaches to
control crystal size and morphology of
zeolites [21-23].
2. EXPERIMENTAL
2.1 Reagents and Materials
Sodium aluminate, sodium metasilicate
(H2O 43%, Na2O 29%, SiO2 28%) and
sodium hydroxide were purchased from
Merck (AR grade) and used without
further purification. Double distilled
water was used throughout the
experiments.
2.2 Synthesis
In a typical procedure, sodium
aluminate, sodium hydroxide and water
were placed in a plastic beaker and
heated to 70 °C while stirred to ensure
achievement of a homogeneous
mixture, and then cooled to room
temperature. Sodium metasilicate was
dissolved in water, heated to 70 °C and
stirred to obtain a homogeneous
mixture, then cooled to room
temperature. Afterwards, silicate
solution was added slowly to the
aluminate solution under vigorous
stirring. Subsequently, the gel was aged
at room temperature for certain time,
according to Table 1, with vigorous
stirring prior to conventional
hydrothermal treatment to get a
homogenous gel-mix. Finally, the gel
was transferred into a Teflon-lined
stainless-steel autoclave,placed in an
air-oven maintained at the required
temperature. At the end, the
precipitates were recovered by
centrifugation, washed with double
distilled water and dried in air. The gel
composition (molar ratio of the used
chemicals), synthesis conditions, and
resulting solid phases are presented in
Table 1.For FE-SEM and argon
adsorption-desorption isotherm studies
on the samples, the synthesized
products were calcined at 600 °C for
2h.
Table 1. Synthesis conditions of the samples
Duration Temperature
Stirring aging (samples
labeled as)
H2O Na2O SiO2 Al2O3
Gel
Composition
48 h 100 °C
0, 20 min, 72 h (S1, S2,
S3)
267 6 10 1 a
48 h 100 °C
0, 1 h, 2.5 h (S4, S5,
S6)
50 2.1 3.8 1 b S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
23
2.3 Characterization
Powder X-ray diffraction (XRD)
patterns of the as-synthesized samples
were recorded on a GBC MMA X-ray
difractometer using Cu Kα radiation of
wavelength 0.154178 nm at 35.4 KV
and 28 mA. Diffraction data were
recorded between 5 and 50º2θwith a
scanning speed of 5º/min.The FT-IR
spectrum was recorded on a Bruker
Tensor 27 Spectrometer.
Field-emission scanning electron
microscopy (FE-SEM) images were
provided using a HITACHI, S-4160
field-emission electron microscope
operating at 15 kV for indicating the
morphology of the samples.
The specific surface area was evaluated
using the Brunauer–Emmett–Teller
(BET) method, and the pore size
distribution was calculated from
desorption branches of argonisotherms
applying the Barrett–Joyner–Halenda
(BJH) method (Quantachrom Nova
2000e, USA).
3. RESULTS AND DISCUSSION
Figure 1 shows the XRD patterns of the
zeolites S1 and S4prepared by two
different starting gel compositions (a
and b, Table 1) under same synthetic
condition. As it can be seen, Figure 1a
and 1b shows mixed phases of
analcime and NaP-1 zeolites prepared
under no stirring aging. With gel
composition a, having higher Si/Al
molar ratio (5), zeolite NaP-1 was the
dominant phase while for gel
composition b, having lower Si/Al
molar ratio (1.9), both phases showed
high degree of crystallinity.
Fig. 1. The XRD patterns of samples S1 and S4 prepared with gel compositions a and b under
no stirring aging.
To further investigate the phase
transformation and morphology
changes of both gel compositions,
different stirring aging times were
employed prior to conventional
hydrothermal reaction and the results
were followed by XRD and FE-SEM. S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
24
First, evolution of gel composition a
was studied under two different stirring
aging time of 20 min and 72 h (S2 and
S3, respectively). Fig. 2 illustrates the
XRD patterns of the samples S2 and S3
with S2 indicating the formation of a
new phase of sodalite octahydrate
zeolite. It also showed a significant
decrease in analcime phase comparing
to S1 with no stirring aging and a
moderate decrease in NaP-1 phase.
Fig. 2. The XRD patterns of the samples S2 and S3 prepared under 20 min and 72 h stirring
aging, from gel composition a.
On the other hand, under prolonged
stirring aging time of 72 h, the XRD
pattern of S3 (Figure 2) showed a
dramatic change in phase dominancy
with sodalite octahydrate being the
dominant phase and NaP-1 zeolite as a
remaining impurity phase and no
analcime peak was observed. This
decrease in NaP-1 and analcime phases
after adding to aging step is the result
of their consumption toward the
formation of a more pure and stable
phase of sodalite octahydrate. After
confirming the phase formation by
XRD analysis, the morphology and
particle size distribution were observed
by FE-SEM (Figure 3).
S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
25
Fig. 3. FE-SEM images of the samples S2 and S3 prepared under 20 min and 72 h stirring
aging, using gel composition a.
As it can be seen in Figure 3, sample
S2 has a wool ball-like morphology
and spheres of sodalite aggregate
together. The average size of each wool
ball is ~ 3 μm with threads having the
width range of 80-120 nm. Also, the
presence of diamond-like NaP-1
crystals is observable which is in
agreement with its XRD pattern (Figure
2, S2). The size of each diamond is
about 2μm, which are marked with
arrows. However, under prolonged
aging of 72 h (S3), the morphology of
the wool ball-like sodalite changed to
well-shaped cabbage-like structures
with an average size of 3 μm and leaves
in the range of 50-80 nm. Figure 4
illustrates another schematic image of
cabbage-like (S3) crystals at higher
magnifications showing mainly nano-
leaves which are the nano-features of
the microcrystals of sodalites.
Furthermore, it illustrates that the nano-
leaves are connected to each other
through an axis, which results in the
hierarchical cabbage-like morphology.
S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
26
Fig. 4. Nano-leaves of cabbage-like microcrystalline sodalites at 1000X magnification.
Secondly, evolution of gel composition
b was studied under two different
stirring aging of 1 h and 2.5 h (labeled
as S5 and S6, respectively). Figure 5
shows the XRD patterns of mentioned
samples with S5 representing pure
sodalite octahydrate phase. The lack of
analcime and NaP-1 phases in the XRD
patterns implied the transformation of
these unstable phases to a more stable
phase of sodalite which is also well-
known to be preferentially formed at
high NaOH concentrations (Table 1)
[24]. The obtained products showed
high sodalite crystallinity. With
increase in aging time to 2.5 h, not only
the peaks associated to sodalite
octahydrate were observed but also a
new phase emerged which is marked
with asterisk (Figure 5, S6). The newly
appeared peaks can be well-identified
as cancrinite. Due to the structural
similarity between sodalite and
cancrinite, they can be synthesised
under similar reaction conditions [25].
Also, the higher Na2O/H2O molar ratio
of the gel composition b (0.04) than gel
composition a (0.02) favors the
formation of cancrinite phase in the
former mixture after enough aging time
(Table 1) [26]. In addition, a slight
decrease in peak intensity and a bit
broadening can be seen in the XRD
pattern (Figure 5, S6) suggesting the
consumption of sodalite phase in favor
of cancrinite and a change in size
distribution of the crystals. These XRD
results can clearly demonstrate that 1 h
of stirring aging time is enough to
obtain pure sodalite octahydrate phase
and further aging time can lead to
formation of disfavored new phase of
cancrinite. Also, Debye-Scherrer
formula was applied to calculate the
particle size of this pure sodalite phase
which resulted in~ 30-40 nm (S5).
S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
27
Fig.5. The XRD patterns of sodalite octahydrate samplesof S5 and S6, prepared under 1 h and
2.5 h stirring aging from gel composition b, respectively. The cancrinite phase is denoted with
asterisk.
The Fourier transform infrared (FT-IR)
spectrum of pure nano-sodalite (S5) is
shown in Fig. 6. The absorption band at
~ 3500 cm-1
can be assigned to the
stretching vibration of structural
hydroxyl group (OH-
) of silicate lattice
and the strongest vibration at ~990 cm-1
is due to the asymmetric stretching
mode of the tetrahedrally coordinated
Si [14, 27]. The other ʻfingerprintʼ
absorptions were also observed at 524
and 465 cm-1
which correspond to the
symmetric stretching mode of internal
SiO2 tetrahedra, and the structure-
intensitive T-O bending mode of
tetrahedral TO4 units (T= Al and Si),
respectively. A diagnostic feature of
the sodalite formation is the appearance
of a new absorption peak at 434cm-1
due to the formation of single four-
membered ring (S4R) of sodalite unit
[28].
The symmetric stretching vibrations (υs
(T-O)) in the 670-730 cm-1
region are
due to the symmetric stretch of T-O-T.
Since these vibrations are sensitive to
the mass of the anion and cation
included in sodalite cage, the peak
positions give an invaluable diagnostic
for encapsulated molecules such as
NaOH and H2O [17, 29]. A sharp peak
corresponding to the water deformation
mode at ~ 1650 cm-1
was appeared in
the spectrum. Due to the strong
hydrogen bonding, the central O…H
stretching vibration is expected to be
broader and shift to lower numbers.
Therefore, the broad peak centered at
absorption 1400 cm-1
can tentatively be
assigned to this stretching vibration of
O…H [17].
S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
28
Fig.6. The FT-IR spectrum of sodalite octahydate sample S5, prepared from gel composition b
under 1 h of stirring aging.
Figure 7 illustrates the morphology and
particle size of the samples S5 and S6.
The FE-SEM image of S5 shows well-
shaped spherical crystals of sodalite
octahydrate with size distribution range
of 50-80 nm which is in good
agreement with Debye-Scherrer
calculation.
The yield of the product based on SiO2
was calculated to be about 15%. Also,
the FE-SEM image of the mixed phases
of sodalite and cancrinite shows
nanosphere crystals in the range of 40-
60 nm (Fig. 7, S6). The insets show the
higher magnification images.
S. M. Pourali, A. Samadi-maybodi /CSM Vol.2No.1, 2014 pp.21-31
29
Fig.7. The FE-SEM images of samples S5 and S6, prepared from gel composition b under 1 h
and 2.5 h of stirring aging, respectively.
The surface texture of the synthesised
nanocrystalline sodalite (S5) was
studied by argon adsorption-desorption
isotherm. It should be noticed that the
maximum window in the sodalite
structure is 6R with a size of 0.28 nm.
Therefore, N2 adsorption-desorption
isotherm is not useful to characterise
the internal micropore [30] and only
can roughly show the BET surface area
coming from external surface of SOD
particles [8]. Figure 8 shows the argon
adsorption-desorption isotherm (a) and
BJH method (b) analysis of sample S5.
Figure 8a displays type II adsorption-
desorption isotherm, being almost
reversible. This type of isotherm
represents unrestricted monolayer–
multilayer adsorption [31]. The results
showed that the BET surface area
(SBET) of nanozeolite sodalite is 79.78
m2
g-1
. The t-plot analysis showed that
the obtained sample had very low
micropore volume (< 0.002 cm3
g-1
)
and external surface area very close to
the SBET value which indicated that the
formation of microporous impurities in
the sodalite sample was negligible.
This type of isotherm represents
unrestricted monolayer–multilayer
adsorption [32-34]. Furthermore, the
average pore diameter of 1.14 nm was
obtained by BJH method (Figure 8b).
S. M. Pourali, A. Samadi-maybodi /CSM Vol.2 No.1, 2014 pp.21-31
30
Fig.8. Argon adsorption-desorption isotherm (a) and BJH method diagram (b) of
nanozeolite sodalite sample (S5)
4. CONCLUSION
In this work, a simple effective aging
process was employed to control
morphology and size distribution of
sodalite by using two different initial
gel compositions. The results indicated
that stirring aging at room temperature
prior to hydrothermal treatment can
result in different morphologies in each
gel mixture as well as reduction in
particle size. With a Si/Al molar ratio
of 5and 20 min of aging time,
microcrystals of sodalite with wool
ball-like morphology was observed
which had nano-threads (80-120 nm)
and altered to cabbage-like micro-
crystals with 50-80 nm leaves by
increasing in aging time to 72 h. On the
other hand, sodalite nanocrystals of 50-
80 nm were observed in high yield (~
15%) with Si/Al molar ratio of 1.9 and
1 h aging time while prolonged aging
time of 2.5 h led to formation of
cancrinite as a new phase. The
synthesized nanosodalite exhibited
small particles which were evidenced
by the broadening of XRD peaks,
Debye-Scherrer calculation, FE-SEM
images and argon adsorption-
desorption isotherm. Therefore, the
stirring aging process is promising for
simple synthesis of micro- and nano-
crystalline sodalites.
Acknowledgements
The authors would like to thank Mrs.
Mehrnoosh Sadeghi-Pari from Tehran
University, Nano-electronic Laboratory
for her infinite patience in FE-SEM
measurements.
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