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Preparation and characterization of nano-NaX zeolite by microwave assisted

hydrothermal method

Article in Advanced Powder Technology · January 2013

DOI: 10.1016/j.apt.2013.10.021

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Original Research Paper

Preparation and characterization of nano-NaX zeolite by microwave

assisted hydrothermal method
Mahdi Ansari a, Abdolreza Aroujalian a,b,⇑, Ahmadreza Raisi a,b, Bahram Dabir a, Mahdi Fathizadeh a
a
Department of Chemical Engineering, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran
b
Food Process Engineering and Biotechnology Research Center, Amirkabir University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, Tehran, Iran

a r t i c l e i n f o a b s t r a c t

Article history: In the present work, nano-NaX zeolite crystals were synthesized via microwave and conventional hydro-
Received 4 August 2013 thermal methods. The effects of reaction time, temperature and heating method on the characteristics of
Received in revised form 25 September 2013 zeolite nanoparticles such as particle size, crystallinity, morphology, size distribution and surface area
Accepted 31 October 2013
were investigated. The prepared NaX zeolite nanoparticles were characterized by the XRD, SEM, DLS,
Available online xxxx
BET and XRF analysis. The results showed that both heating methods produced the NaX zeolite crystals
with nano-in size. The microwave heating produced smaller zeolite nanoparticles with relatively nar-
Keywords:
rower particle size distribution, required much shorter heating times and did not significantly change
Nano-NaX zeolite
Microwave heating
composition or crystallinity, compared with the conventional heating method. It was also observed that
Conventional hydrothermal the time and temperature of microwave heating had significant effects on the prepared zeolite particles
Crystallization time and the pure zeolite X nanoparticles or a mixture of zeolite X and A can be synthesized using the micro-
Crystallization temperature wave heating method by control of the crystallization time and temperature.
Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder
Technology Japan. All rights reserved.

1. Introduction hydrothermal method with the assistance of tetramethylammo-

nium (TMA) as an organic template. The organic templates are usu-
Zeolites are aluminosilicate molecular sieves crystals that have ally used to reduce the crystal size [21,23]. However, these
uniform pores of molecular dimensions [1]. They are widely used templates are expensive and nonrecyclable. The removal of the
as commercial adsorbents in separation and purification processes templates also leads to an irreversible aggregation of nanocrystals
[2–4.], as catalysts for the preparation of advanced materials [5–7] into higher values of solid particle sizes [24]. Therefore, efforts
and as membrane materials and additives in the membrane pro- have been made to synthesize template-free nano-zeolite. Zhan
cesses [8–10]. They are also used in medicine, agriculture and et al. [14] developed an organic-template-free approach using
detergents as well as in nuclear and biogas industries. Recently, appropriate silicate sources and control of hydrothermal crystalli-
special attention has been paid to nanometer-sized zeolites due zation conditions to synthesize various NaX zeolitic nano-crystals.
to their exclusive properties, such as large external surface area Also, Wang et al. [12] prepared nano-crystals of zeolite X and A
and activities, more accessible active sites as well as shorter diffu- using thermoreversible polymer hydrogel to control the zeolite
sion pathways, compared with micrometer-sized zeolite crystals growth rate. Although, this polymer can be readily removed by
[11]. simple washing, it is expensive yet. Aly et al. [25] synthesized
There are a large number of researches studying the synthesis mordenite zeolite in the absence of organic template by hydrother-
methods of nano-zeolites, as reviewed by Tosheva and Valtchev mal method.
[11]. Various nano-zeolites like NaA [12,13], faujasite-X Furthermore, careful adjustment of the operating variables such
[12,14,15], faujasite-Y [16,17], ZSM-5 [18,19] and silicalite-1 as gel composition, temperature, crystallization time and aging
[20,21] can be synthesized via a hydrothermal method using clear time play an important role in control of the particle size, morphol-
aluminosilicate solutions in the presence of organic templates. For ogy and characteristics of the zeolite crystals. Zhan et al. [14] pre-
example, Schoeman et al. [21] as well as Mintova and Valtchev [22] pared ultrafine NaX zeolite by control of hydrothermal conditions,
synthesized the nanocrystalline faujasite zeolites using the such as agitation rate, time and temperature of crystallization.
Generally, the hydrothermal synthesis method of zeolites must
⇑ Corresponding author at: Department of Chemical Engineering, Amirkabir be performed at low temperature to avoid the formation of unde-
University of Technology (Tehran Polytechnic), Hafez Ave., P.O. Box 15875-4413, sired phases. Therefore, long crystallization time, from many hours
Tehran, Iran. Tel.: +98 (21) 64543163; fax: +98 (21) 66405847. to a few days is required to obtain the desired nano-zeolite
E-mail address: aroujali@aut.ac.ir (A. Aroujalian).

0921-8831/$ - see front matter Ó 2013 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
http://dx.doi.org/10.1016/j.apt.2013.10.021

Please cite this article in press as: M. Ansari et al., Preparation and characterization of nano-NaX zeolite by microwave assisted hydrothermal method,
Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.10.021
2 M. Ansari et al. / Advanced Powder Technology xxx (2013) xxx–xxx

crystals. This is a significant disadvantage of the conventional 2.3. Nano-zeolite characterization

hydrothermal method, because of the increase in preparation costs
at large scale production. Microwave heating is a good alternative The powder’s X-ray diffraction (XRD) patterns were recorded at
to the conventional heating method. The microwave heating is 25 °C on a Philips instrument (X’pert diffractometer using Cu Ka
more rapid and uniform, which causes the homogeneity of nucle- radiation) with a scanning speed of 0.03° (2h) min1 to confirm
ation, reduction of synthesis time and polydispersity in the crystal the NaX zeolite structure. The crystallinity of the sample was
size distribution without significant changes in physical–chemical determined from the peak areas of 6° (1 1 1), 16° (3 3 1) and 27°
properties such as Si:Al ratio, surface charge, hydrophilicity, and (6 4 2) and the average crystal dimension was calculated using
crystallinity, and eventually, produces zeolite nanocrystals with Scherrer’s equation [32]:
excellent colloidal stability, sorption kinetics, and sorption capac-
ity in aqueous electrolytes [26]. There are many researches which Bk
d¼ ð1Þ
used the microwave heating in the synthesis of zeolites and related b cos h
materials [27–31]. The majority of these works focus on the prep-
where d is average crystal size (nm), B is the Scherrer’s constant, k is
aration of zeolite A [27,28,30], Y and ZSM-5 [29]. A search in the
the wavelength of the X-rays (nm) and h is the value of the Bragg
literature revealed that there are no researches about the synthesis
angle (i.e. the angle of the peak maximum). b is the full width at
of nano-NaX zeolite using microwave heating.
half-maximum (FWHM) of the broad peak after correction for
In this study, ultrafine nano-NaX zeolites were prepared by the
intrinsic instrumental line broadening and must be in radians.
microwave assisted hydrothermal method by controlling the crys-
Nano-NaX zeolite morphological characterization was obtained
tallization time and temperature. The effect of crystallization time
using scanning electron microscopy (SEM, TESCAN, VEGA 3SB). The
and temperature on the size, morphology and crystallinity of NaX
samples were coated under vacuum with a 10–20 nm thin layer
nano-crystals was investigated. The prepared zeolite nanoparticles
of gold by the sputtering system.
were characterized by X-ray powder diffraction (XRD), Scanning
The XRF (Philips instrument) was used to evaluate the elemen-
Electron Microscope (SEM), X-ray Fluorescence (XRF) and Dynamic
tal compositions of nanoparticles and determine Si/Al ratio.
Light Scattering (DLS) analysis.
The specific surface area was also measured for the zeolite
nanoparticles with nitrogen adsorption and BET method on a
2. Materials and methods Quantachrome Autosorb-1 instrument.
Besides, the hydrodynamic diameter and size distribution of the
2.1. Materials NaX zeolite nanoparticles were determined by dynamic light scat-
tering (DLS) using a Malvern Zetasizer Nano-(Malvern Instruments,
The chemical reagents used contained fumed silica (7 nm, Sig- Worcestershire) (Wavelength: 632.8 nm, Scattering Angle: 173°).
ma Aldrich, USA), NaOH (Merck, Darmstadt, Germany) and NaAlO2 The zeolite crystals were re-dispersed in water by 1 h ultrasonica-
(Sigma Aldrich, USA). tion at 20 °C for dynamic light scattering analysis.

2.2. Preparation of nano-NaX zeolite 3. Results and discussion

The nanometer-sized faujasite-X zeolite is synthesized using The X-ray diffraction pattern of a pure substance is like a finger-
both conventional and microwave heating techniques. In both print of the substance. The powder diffraction method is thus ide-
heating methods, a 250 mL polypropylene bottle was initially ally suited for characterization and identification of crystalline
washed with double de-ionized water and secondly, aluminosili- materials like zeolites. Powder XRD patterns of the prepared zeo-
cate gel was made by mixing freshly prepared aluminate and sili- lites taken after different hydrothermal crystallization periods at
cate solutions together in the molar ratio of 5.5 Na2O:1.0 Al2O3:4.0 various temperatures are presented in Figs. 2–4. It can be seen
SiO2:190 H2O. An aluminosilicate gel containing 5.34 g of NaOH, from Fig. 2 that at 90 °C broad and weak peaks appeared on the
2.42 g of NaAlO2, 3.43 g of SiO2, and 50.0 g of H2O was applied. XRD pattern for a 0.5 h crystallization time. The intensity and
The prepared reaction gel was continually stirred at room temper- sharpness of the diffraction peaks were significantly enhanced
ature for 24 h, then the hydrothermal crystallization was per- after 1 h crystallization, and also when it continued for the next
formed by the conventional or microwave technique. Since 3 h, but they did not change appreciably after 4 h of crystallization.
heating is known as a driving force of the crystallization process, All the XRD patterns at 90 °C match very well with the simulated
the fundamental step in the synthesis is the application of conven- XRD powder pattern for NaX zeolite [12,14,15] indicating that
tional or microwave heating. Conventional hydrothermal crystalli- the synthesized crystals are pure NaX zeolite. At a crystallization
zation was conducted at 60 °C for 4 days in a shaker with a rotation temperature of 110 °C (Fig. 3), after passing 0.5 h of crystallization
rate of 250 rpm [14], while microwave heating was applied at a the taken XRD pattern showed very weak peaks of zeolite X and A.
temperature range of 90–130 °C for 15–240 min. The products With an increase in time, the rate of crystal growth of NaX zeolite
were regained with centrifugation, washed with de-ionized water increased and finally after 1 h pure NaX zeolite was obtained, but a
until the pH value reached below 8, and then for further character- further increase in the crystallization time, i.e. after 2 h, led to re-
ization dried at room temperature for 24 h. growth of zeolite A crystals. A similar trend was observed for the
The microwave equipment used in this study was a commercial temperature of 130 °C as the crystallization time increased from
microwave oven (CE1110C, Samsung, Korea) with 900 W output 0.25 to 1 h. As shown in Fig. 4, at 0.25 h of crystallization, pure
power at a wavelength of 2.45 GHz. The oven was equipped with NaX zeolite was gained and then, by increasing the crystallization
an electronic system in order to accurately control the crystalliza- time to 0.5 h and then to 1 h the peaks of zeolite A appeared in the
tion temperature. The control system provided pulsed microwave XRD pattern. The starred peaks in Figs. 3 and 4 indicate the peaks
pumping by switching the magnetron, externally. The temperature of zeolite A. The production of zeolite A at crystallization temper-
inside the chamber was continuously measured by a Teflon coated atures of 110 and 130 °C may be related to the mechanism of zeo-
thermocouple and adjusted by a temperature controller (TC4Y, lite formation and building unit of A and X zeolites. In zeolite
Autonics, Korea) with a precision of ±0.01 °C. A schematic diagram synthesis, formation of building units and nucleation of zeolite
of the modified microwave equipment is shown in Fig. 1. using the building units occur, followed by crystal growth by

Please cite this article in press as: M. Ansari et al., Preparation and characterization of nano-NaX zeolite by microwave assisted hydrothermal method,
Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.10.021
 M. Ansari et al. / Advanced Powder Technology xxx (2013) xxx–xxx 3

Fig. 1. Microwave scheme.

Fig. 2. The XRD patterns of NaX zeolites synthesized by the microwave heating at Fig. 4. The XRD patterns of NaX zeolites synthesized by the microwave heating at
90 °C after (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 3 h and (e) 4 h crystallization time. 130 °C after (a) 0.25 h, (b) 0.5 h and (c) 1 h crystallization time.

The effect of crystallization temperature can also be realized by

comparing the XRD patterns for the 1 h crystallization time at var-
ious operating temperatures. As observed from Figs. 2–4, increas-
ing temperature at constant time, not only makes the crystal
growth faster and increases the intensity and sharpness of the dif-
fraction peaks, but also causes the zeolite A crystals to grow with
the NaX zeolite. Thus, low temperature is more favorable for pure
synthesis of the NaX zeolite.
In order to compare the NaX zeolites synthesized by the micro-
wave heating with those prepared by the conventional hydrother-
mal method, the XRD patterns are shown in Fig. 5. The X-ray
diffraction pattern of a commercial NaX zeolite (ZEOCHEMÒ
Molecular Sieves 13X, Zeochem Co., Ltd.) is also presented in this
figure. It can be observed that all three XRD patterns match to-
gether very well.
Furthermore, the average crystal size of the prepared zeolites at
various synthesis conditions was estimated using Scherrer’s equa-
tion (Eq. (1)) from the peak width of (1 1 1) reflection plane and the
Fig. 3. The XRD patterns of NaX zeolites synthesized by the microwave heating at
results are presented in Table 1. It was observed that as the average
110 °C after (a) 0.25 h, (b) 0.5 h, (c) 1 h and (d) 2 h crystallization time.
crystal size increased with increasing crystallization temperature.
adsorption of building units on the surface of zeolite nuclei and These behaviors may be related to this fact that the nucleation en-
crystal. Moreover, the building units of X (FAU) zeolite are SOD ergy is much less than the crystal growth energy [35]. As men-
(Sodalite) and D6R (Double 6-ring), while the building units of A tioned above, crystal formation of zeolites are usually composed
(LTA) zeolite are SOD [33,34]. Impurity phase of A zeolite is formed of three steps, formation of building units, nucleation, and crystal
at high temperature. This means that the formation of the building growth. Usually, a nucleation rate is lower than the growth rate.
units of SOD and D6R depended on the temperature. Sang et al. In other words, the crystal growth easily occurs. As a synthesis
[16] also observed that by increasing temperature during the syn- temperature increases, number of zeolite nuclei formed in solution
thesis of Y zeolite, the P zeolite produced instead of the Y zeolite. increases, leading to preparation of nano-sized zeolite. Moreover,

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4 M. Ansari et al. / Advanced Powder Technology xxx (2013) xxx–xxx

Table 3
The XRF analysis results for the nano-NaX zeolite synthesized by the microwave
heating at 90 °C for 3 h (composition in wt.%).

Na Al Si O Si/Al
85.938 85.594 106 383.360 1.25

Fig. 5. The XRD patterns of NaX zeolites synthesized by the microwave heating at
90 °C and 3 h (a), the conventional heating at 90 °C and 96 h (b) and the commercial
NaX zeolite (c).

crystal formation and resolution of zeolite crystal simultaneously Fig. 6. The ratio of the actual yield to the theoretical yield with crystallization time.
progress, leading to formation of larger zeolite particles. Previ-
ously, it has been reported that leaving zeolite LTA for extended
periods of time at room temperature arrives at a narrow particle Table 3 presents the XRF analysis results. The Si/Al ratio of 1.25
size distribution with small crystal sizes up to 0.4 lm [36]. There- was calculated for the nano-NaX zeolite prepared by microwave
fore, it can be concluded that temperature and time of crystalliza- heating through the XRF analysis. The unit cell mass of the NaX
tion needs to be balanced to find the best crystal size. According to zeolite calculated using the composition provided by XRF test is
these results, the best temperature for the pure synthesis of nano- Na86[Al86Si106O384].
NaX zeolite is 90 °C. To determine the efficiency of the microwave heating, the ratio
For finding maximum specific area and optimum crystallization of the actual yield to the theoretical yield was calculated. Fig. 6
time, the BET analysis was performed and the results are presented represents the variation of the ratio of the actual yield to the the-
in Table 2. For comparison, the BET results of a NaX zeolite that is oretical yield with the crystallization time for 90 °C. Actual yield
synthesized by the conventional hydrothermal method, and the was calculated by measuring the zeolite weight after the drying
commercial NaX zeolite are listed in Table 2. Increasing crystalliza- step and the theoretical yield was calculated by unit cell composi-
tion time caused to enhancing crystallinity and crystal size of zeo- tion provided by the XRF test and amount of precursors presented
lites. Low crystallinity will reduce the specific surface area due to in the gel. As observed in this figure, the highest yield was obtained
this fact that the amorphous aluminosilicate will block outside at the crystallization time of 3 h. The ratio of the actual yield to the
pores of the zeolite crystals [37]. As can be seen from this table, theoretical yield has also been depicted in Fig. 6 and can be seen to
for 1 h crystallization time due to low crystallinity, the specific sur- be almost constant.
face area has the lowest amount but by increasing time to 3 h, the The SEM images of the nano-zeolites were synthesized by the
specific surface area reaches 536 m2/g. A further increase in the microwave heating at 90 °C for 3 h and by the conventional heating
crystallization time leads to the growth of crystals and larger par- at 60 °C for 96 h and the commercial NaX zeolite are presented in
ticles without any change in the crystallinity and thus the specific Fig. 7. The SEM images obviously proved that the zeolite crystals
surface decreases. were morphologically similar and the sample prepared by the

Table 1
The average crystal size and crystallinity of the prepared zeolites at various crystallization temperatures.

Heating method Temperature (°C) Time (h) Crystal size (nm) Crystallinity (%)
Microwave 90 1 26 72
Microwave 110 1 37 80
Microwave 130 1 44 89

Table 2
The specific surface area of NaX zeolites.

Heating method Temperature (°C) Time (h) Crystallinity (%) Crystal size (nm) Specific surface area (m2/g)
Microwave 90 1 72 26 411
Microwave 90 2 85 33 498
Microwave 90 3 93 37 536
Microwave 90 4 96 44 514
Conventional 60 96 91 40 530
Commercial – – – – 570

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Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.10.021
 M. Ansari et al. / Advanced Powder Technology xxx (2013) xxx–xxx 5

Fig. 8. The DLS results of the NaX zeolites synthesized by the microwave (at 90 °C
for 3 h) and conventional (at 60 °C for 96 h) heating methods.

Besides, the DLS analysis was performed to determine the

hydrodynamic diameter and particle size distribution of three
samples, the NaX zeolites synthesized by the microwave and con-
ventional heating methods, and the results are presented in Fig. 8.
The particle size of 95 and 112 nm were obtained for the micro-
wave and conventional heated NaX zeolites, respectively. In addi-
tion, the DLS results demonstrate that microwave heated crystals
had narrower size distribution than the conventionally heated zeo-
lites. The results of DLS analysis are in good agreement with the re-
sults of XRD and SEM analysis. Also crystal size of commercial
zeolite was about 2300 nm from SEM image (Fig. 7c).

4. Conclusions

The pure nano-NaX zeolite was successfully synthesized by the

microwave heating technique and the effects of crystallization
time and temperature and the heating method on the characteris-
tics of zeolite nanoparticles were investigated. The results showed
that at 90 °C, the pure nano-NaX zeolite was produced and the
crystallinity and surface area increased as the crystallization tem-
perature enhanced up to 3 h. While at 110 °C, the pure NaX zeolite
is just obtained at 1 h crystallization time, for times more or less
than 1 h, the crystals of zeolite X and A grow, simultaneously.
Increasing the crystallization time led to an increase in the growth
rate of zeolite A crystals. The XRD patterns revealed that increasing
temperature from 90 to 130 °C resulted in the growth of zeolite A
crystals beside the zeolite X. Also, the microwave-heated zeolite
nanoparticles had narrower particle size distribution with an aver-
age crystal size of 95 nm, in comparison with the conventionally
heated one, as confirmed by the XRD and DLS analysis results. In
addition, the surface area analysis showed that increasing crystal-
linity had a strong effect on the specific surface area. Finally, it can
be concluded that the pure zeolite X nanoparticles or a mixture of
zeolite X and A can be synthesized at relatively short processing
times using the microwave heating method by control of the crys-
tallization time and temperature.

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Please cite this article in press as: M. Ansari et al., Preparation and characterization of nano-NaX zeolite by microwave assisted hydrothermal method,
Advanced Powder Technology (2013), http://dx.doi.org/10.1016/j.apt.2013.10.021
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