Zeolite Synthesis: A Comparative Analysis of Conventional Hydrothermal and Microwave-Assisted Methods
Abhijit Anil Joshi
PRHSS Arts Commerce and Science College, Dharangaon
Abstract: ZSM-5 zeolite was synthesized
using both conventional hydrothermal and microwave-assisted hydrothermal
techniques to investigate the effect of the synthesis approach on its
structural and physicochemical characteristics. The precursor gel was subjected
either to control aging or to microwave irradiation prior to hydrothermal
crystallization at 160 °C under autogeneous pressure. X-ray diffraction (XRD)
patterns confirmed the successful formation of highly crystalline MFI-type
ZSM-5 in both cases, with comparatively higher peak intensity observed for the
microwave-treated sample, indicating improved crystallinity. FTIR spectroscopy
revealed characteristic framework vibrations, including bands associated with
double five-membered ring (D5R) units, confirming the development of the ZSM-5
structure. Thermo gravimetric and differential thermal (TG–DTA) analyses showed
sequential removal of adsorbed water and organic template species,
demonstrating good thermal stability of the synthesized materials. Scanning
electron microscopy (SEM) images displayed well-defined crystalline morphology,
with the microwave-assisted method yielding relatively uniform and elongated
nanocrystals as a result of rapid and homogeneous nucleation.
Keywords: Zeolite, microwave,
hydrothermal XRD, SEM
Introduction: Zeolites
are three-dimensional crystalline aluminosilicate materials that occur both
naturally and as laboratory-synthesized compounds. Owing to their well-defined
microporous framework, high surface area, and tuneable physicochemical
properties, zeolites have found extensive applications in environmental
remediation, heterogeneous catalysis, biotechnology, gas sensing, and medicinal
fields [1].
Microwave-assisted synthesis has
gained considerable attention as an effective approach for zeolite preparation
due to its rapid, homogeneous, and volumetric heating characteristics. In
contrast to conventional heating methods, which rely on heat transfer through
conduction and convection from the reactor walls, microwave irradiation
directly interacts with polar molecules and ionic species in the reaction
medium, thereby enhancing nucleation rates and accelerating crystal growth
[2-3]. As a result, this technique significantly reduces synthesis time and
overall energy consumption while frequently producing zeolites with high
crystallinity, narrow particle size distribution, and well-defined morphology.
Moreover, microwave-assisted
hydrothermal synthesis provides improved control over reaction conditions,
rendering it a promising and sustainable route for the large-scale synthesis of
zeolites intended for catalytic, adsorption, and separation applications. In
the present study, a comparative analysis of zeolite synthesis using
conventional hydrothermal and microwave-assisted hydrothermal methods is
carried out to evaluate the effects of the synthesis route on the resulting
material properties.
Materials methods: In the
synthesis of ZSM-5 zeolite, four precursor solutions designated as A, B, C, and
D were prepared separately. Solution A was prepared by dissolving 1 g of
aluminium sulfate in 50 mL of double-distilled water taken in a 100 mL beaker.
Subsequently, 16 g of tetraethyl ammonium bromide (TEA-Br) was added drop wise
to the solution under continuous stirring using a magnetic stirrer for 15
minutes to ensure complete dissolution and homogenization. Solution B was
prepared by adding 96 g of sodium silicate solution (composition: 27.2 wt%
SiO₂, 8.4 wt% Na₂O, and 64.4 wt% H₂O) to 45 mL of distilled water, followed by
thorough mixing to obtain a uniform solution. Solution C was prepared by
dissolving 6.6 g of Sulfuric acid (H₂SO₄) in 50 mL of distilled water in a 100
mL glass beaker. Perchloric acid was used as Solution D without further
modification.
Solution A was then added drop
wise to Solution B under continuous stirring to form a homogeneous precursor
mixture. The pH of the resulting gel was carefully adjusted to 10.3 by the
controlled addition of Solutions C and D. The reaction mixture was subsequently
stirred vigorously for 1 hour to ensure uniform gel formation. The final oxide
molar composition of the synthesis gel was: 78 Na₂O : Al₂O₃ : 270 SiO₂ : 6.9 (TEA)₂O : 7550 H₂O.
The prepared reaction mixture
was divided into two equal portions for comparative study. The first portion
was allowed to undergo aging at ambient conditions for 17 hours prior to
hydrothermal treatment. The second portion was subjected to microwave irradiation
at 500 W and 2.45 GHz for 3 minutes in a microwave oven. Following microwave
treatment, the resulting hydrogel was immediately transferred to a
stainless-steel autoclave without any aging step.
The microwave-treated gel was
heated at 160 °C for 16 hours under autogeneous pressure. After completion of
the hydrothermal treatment, the autoclave was allowed to cool naturally to room
temperature. The solid product was recovered by filtration and thoroughly
washed three to four times with distilled water until the washings were free
from residual gel particles. The filtered sample was then dried and
subsequently calcined at 500 °C for 4 hours in air to remove and decompose the
organic template species.
The aged hydrogel (first
portion) was transferred separately into a stainless-steel autoclave and
subjected to hydrothermal crystallization at 160 °C for 36 hours under autogeneous
pressure. The crystallization time was calculated after the reaction mixture
reached the desired temperature of 160 °C. Post-autoclave treatment, including
cooling, filtration, washing, drying, and calcination, was carried out
following the same procedure as described above.
Characterization:
The
X-ray diffraction (XRD) pattern of the synthesized ZSM-5 zeolite was recorded
over a 2θ range of 5°–60°
using Cu Kα radiation (λ = 1.5406 Å).
The diffraction pattern exhibits well-defined and intense peaks characteristic
of the MFI-type crystalline framework,
confirming the successful formation of ZSM-5 zeolite [4].
|
|
|
Fig.1 XRD of
ZSM5 zeolite without microwave treatment |
The prominent diffraction peaks
observed at 2θ values of approximately 23.1°,
23.9°, and 24.4° correspond well with the standard diffraction
pattern of ZSM-5 zeolite. The strong and sharp reflections in the 22–25° region, which are
typical fingerprint peaks of ZSM-5, confirm the presence of a well-ordered
zeolitic structure.
|
|
|
Fig. 2 XRD
of ZSM5 zeolite with microwave treatment |
The most intense and well-resolved peaks
appear in the 2θ range of 22–25°,
with prominent reflections at approximately 23.1°, 23.9°, and 24.4°. These peaks are considered the
fingerprint peaks of ZSM-5 zeolite
and match well with the standard diffraction data, confirming the successful
formation of the ZSM-5 phase. Furthermore, the relatively narrow peak widths
imply well-developed crystallite domains. The enhanced peak intensity,
particularly in the 22–25° region, suggests effective nucleation and crystal growth
during synthesis. Such features are often associated with optimized
hydrothermal conditions and are further enhanced in microwave-assisted
synthesis due to rapid and uniform volumetric heating [5].
|
|
|
|
(a) |
(b) |
|
Fig.3 (a) IR
spectrogram of ZSM5 without microwave and (b) with microwave |
|
Figure 3(a) presents the FTIR
spectrum of ZSM-5 synthesized via the conventional hydrothermal method. The
characteristic structure-insensitive asymmetric stretching vibration of the
Si–O–Si framework is observed at 1087.5 cm⁻¹, while the corresponding symmetric
stretching vibration appears at 795.9 cm⁻¹. A structure-sensitive asymmetric
stretching band, indicative of the MFI framework, is detected at 1220.8 cm⁻¹.
The presence of a band near 550 cm⁻¹ confirms the formation of the double
five-membered ring (D5R) structural units, which are characteristic of the
ZSM-5 framework. Additionally, a broad absorption band at 3465.8 cm⁻¹ is
attributed to the O–H stretching vibrations of adsorbed water molecules. Figure
3(b) shows the FTIR spectrum of ZSM-5 synthesized using microwave-assisted treatment
[6-7]. The structure-insensitive asymmetric and symmetric stretching vibrations
are observed at 1105.6 cm⁻¹ and 798.4 cm⁻¹, respectively. The
structure-sensitive asymmetric stretching band appears at 1227.3 cm⁻¹, confirming
the development of the MFI-type framework. The band corresponding to the double
five-membered ring structure is observed at 545.2 cm⁻¹, further validating the
formation of ZSM-5. A broad band at 3436.6 cm⁻¹ is assigned to the O–H
stretching vibrations of water molecules present in the zeolite structure. The
slight shifts in band positions between the conventionally synthesized and
microwave-treated samples may be attributed to differences in crystallinity,
framework ordering, or local structural environment induced by the synthesis
method.
A fig
4 is TG-DTA analysis of ZSM5 zeolite. The weight loss is present in three steps
for ZSM5 zeolite. In first step the weight loss is 4.4% (0.30mg) in the temperature
range 300K to 408K [8]. It is due to water desorption from zeolite cavities. In
the second step the weight loss is maximum and it is 18.87% (1.29mg) is due
decomposition of templeating species [9]. It is in the temperature
Fig. 4 TG-DTA
of ZSM5 zeolite without microwave
range
408K to 468K. In third step the water loss is 2.7% (0.18mg) in the temperature
range 468K to 718K. It is attributed to the removal
of occluded organics. [10] There is no observable weight loss above
718K. Two exotherm and one endotherm are observed in DTA analysis of ZSM5. The
first exotherm is observed at 363.4K corresponds to
oxidative decomposition of residual organic compounds. The second
exotherm is found at 483.3K is due to combustion of adsorbed species and the
endotherm is found to be present at 426.4K, indicates water loss [11].
Fig.5 TG-DTA of ZSM5 with microwave
The TG-DTA analysis for ZSM5 zeolite with
microwave treatment is shown in fig.5. In first step the weight loss is 4.7 %
(0.3mg) in the temperature range 300K to 390K due to the water loss. In
second step the weight loss is 12.7% (0.82mg) in the temperature range 510K to
750K due to water loss in pores. In the temperature range 750K to 823K the
weight loss is found to be 2.51% (0.16mg) in third step due the oxidative
decomposition of organic templates. Above 823K no weight loss is observed. The
total weight loss is about 19.1% due to dehydration of water present in zeolite
cavities and decomposition of templates (TEA-Br). In differential thermal
analysis, the exotherm observed at 455.6K is believed due to decomposition of
physically occluded TEA-Br template in the zeolite. The exotherm at 549.9K
corresponds to decomposition of TEA cations, which are occluded in the zeolite
framework [9-10]. The endotherm is observed at 820.9K due to some phase change
occurred.
The surface morphology of the synthesized
ZSM-5 zeolite was examined using scanning electron microscopy (SEM). The SEM
micrograph reveals the presence of well-defined,
highly crystalline particles with predominantly coffin-shaped and prismatic morphologies,
which are characteristic of ZSM-5 zeolite without microwave treatment
possessing an MFI-type framework.
|
|
|
|
Fig. 6 SEM
of ZSM5 zeolite (a) without microwave (b) with microwave treatment |
|
The particles
appear relatively uniform in shape and
size, with crystal dimensions in the sub-micron to micron range,
indicating controlled crystal growth during the synthesis process. The
well-faceted crystal edges and smooth surfaces suggest a high degree of
crystallinity, which is consistent with the sharp diffraction peaks observed in
the XRD analysis.
The crystals exhibit a
relatively uniform size distribution,
with individual particles predominantly in the nanometre range, as indicated by the 500 nm scale bar
is shown in fig.6 (b). The elongated morphology suggests preferential crystal
growth along specific crystallographic directions, which is typical for ZSM-5
and is often associated with well-ordered channel systems [12]. In
microwave-assisted hydrothermal synthesis, this type of uniform and elongated
morphology is often attributed to rapid
and homogeneous nucleation induced by volumetric microwave heating,
leading to enhanced crystallization kinetics compared to conventional
hydrothermal methods.
Conclusion: In the
present study, ZSM-5 zeolite was successfully synthesized using both
conventional hydrothermal and microwave-assisted hydrothermal methods, and a
detailed comparative analysis was carried out to evaluate the effect of
microwave treatment on crystallization behaviour, structural properties,
thermal stability, and morphology.
X-ray diffraction analysis confirmed the formation
of a highly crystalline ZSM-5 zeolite with an MFI-type framework in both
synthesis routes. However, the microwave-assisted sample exhibited sharper and
more intense diffraction peaks, particularly in the characteristic 22–25° (2θ)
region, indicating enhanced crystallinity and improved crystal growth kinetics
due to rapid and uniform volumetric heating.
FT-IR spectroscopy further supported the successful
formation of the ZSM-5 framework, as evidenced by the presence of
characteristic bands corresponding to asymmetric and symmetric T–O–T (T = Si,
Al) vibrations and the diagnostic double five-membered ring vibration near 550
cm⁻¹. Minor shifts in band positions and improved definition of framework
vibrations in the microwave-treated sample suggest better structural ordering
compared to the conventionally synthesized zeolite.
Thermo gravimetric and differential thermal
analyses revealed multi-step weight loss behaviour associated with desorption
of physically adsorbed water, decomposition of organic templates (TEA-Br), and
removal of occluded species. The microwave-assisted ZSM-5 zeolite showed a more
controlled and reduced template decomposition behaviour, indicating efficient
incorporation and removal of the organic structure-directing agent. The absence
of significant weight loss at higher temperatures confirms the good thermal
stability of the synthesized materials.
SEM analysis demonstrated clear differences in
morphology between the two synthesis methods. The conventionally synthesized
ZSM-5 exhibited well-defined coffin-shaped and prismatic crystals in the
sub-micron to micron range, while the microwave-assisted ZSM-5 showed smaller,
elongated, and more uniformly distributed nanocrystals. This morphological
refinement is attributed to rapid nucleation and accelerated crystallization
under microwave irradiation. Overall, the results clearly demonstrate that
microwave-assisted hydrothermal synthesis significantly enhances crystallinity,
reduces crystallization time, improves morphological uniformity, and lowers
energy consumption compared to the conventional hydrothermal method.
References
1]
Zhenhua Sun, Qingxiang Shu , Qikun Zhang , Shaopeng Li , Ganyu Zhu , Chenye
Wang, Jianbo Zhang, Huiquan Li and Zhaohui Huang Separations 2024, 11, 39
P.1-15.
2]
Sungtak Kim, Jochen Lauterbach, Microporous and Mesoporous
Materials, Volume 328, December 2021, 111446
[3] O.G.Somani
, Ph.D Thesis, Swami Ramanand Teerth Marathwada University, Nanded, 2002
[4]
Ch.Barlocher, in R.Von Ballomoos,(Ed), Collection of Simulated XRD Power
patterns for
Zeolites, Butterworths,
[5]
http://www.iza-structure.org/databases M. M. J. Treacy and J. B. Higgins, collection of simulated xrd powder patterns for zeolites, Fourth
revised edition, published on behalf of the structure commission of the international zeolite
association 2001
[6]
E.M.Flanigen, H.Khatami, and H.A. Szymanski,
Adv. Chem.Ser.101 (1971)p.201
[7]P.A.
Jacobs, H.K.Beyer, J.Valyon, zeolites 1(1981)161
[8]M.S.Joshi,
A.L.Choudhari, P.Mohan Rao, and R.G.Kanitkar, Thermochmica acta,
64(1983)39-45
[9]
D.M. Bibby, N.B. Milestone, L.P. Aldridge, Nature 285(1980) p.30.
[10] J.Cho, Proc. Natl.
Sci. Counc. Roc. 3(1979)233.
[11] M.R.S. Manton, J.C.Davidtz, J. Catal.
60(1979)156.
[12] Y K Krisnandi et al 2018
J. Phys.: Conf. Ser. 1095 012044
