One-Step Dual Template Mediated Synthesis of Nanocrystalline Zeolites of Different Framework Structures

Article abstract of DOI:10.1021/acs.cgd.6b00299

A novel, dual template mediated, one-step direct synthesis route is reported here for the preparation of nanocrystalline zeolites of different framework structures (such as ZSM-5, mordenite, and sodalite). In this synthesis strategy, a suitably designed dicationic/tetra-cationic soft template was used along with a conventional zeolite structure director to obtain nanocrystalline zeolites with nanosheet morphology. Nanocrystalline zeolites exhibited large surface area, pore volume, and intercrystalline mesopores. The long hydrophobic chain containing a multiammonium template cooperatively participates in the zeolite crystallization process along with a conventional microporous zeolite structure director to form the ultrathin microporous zeolite framework, while the hydrophobic interaction between the long chains restricted the excessive growth of zeolites and induced the formation of intercrystalline mesopores. Nanocrystalline zeolites exhibited exceptionally high activity in the acid-catalyzed reactions involving large molecules when compared with conventional zeolites. This synthesis strategy can be extended for the preparation of zeolites of different framework structures or other porous materials in the future.

Full text of DOI:10.1021/acs.cgd.6b00299

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Article
One-Step Dual Template Mediated Synthesis of
Nanocrystalline Zeolites of Different Framework Structure
Poonam Rani, Rajendra Srivastava, and Biswarup Satpati
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00299 • Publication Date (Web): 06 May 2016
Downloaded from http://pubs.acs.org on May 11, 2016
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One-Step Dual Template Mediated Synthesis of Nanocrystalline Zeolites of
Different Framework Structure
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Poonam Rani,a Rajendra Srivastava,*a and Biswarup Satpatib
aDepartment of Chemistry, Indian Institute of Technology Ropar, Rupnagar-140001, Punjab,
India
bSurface Physics and Material Science Division, Saha Institute of Nuclear Physics, 1/AF,
Bidhannagar, Kolkata 700 064, India
______________________________________________________________________________
E-mail: rajendra@iitrpr.ac.in
Phone: +91-1881-242175; Fax: +91-1881-223395
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_____________________________________________________________________________
Abstract
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A novel, dual template mediated, one-step direct synthesis route is reported here for the preparation
of nanocrystalline zeolites of different framework structure (such as ZSM-5, Mordenite and
Sodalite). In this synthesis strategy, suitably designed di-cationic/tetra-cationic soft template was
used along with conventional zeolite structure director to obtain nanocrystalline zeolites with
nanosheet morphology. Nanocrystalline zeolites exhibited large surface area, pore volume, and
inter-crystalline mesopores. The long hydrophobic chain containing multi-ammonium template
cooperatively participate in the zeolite crystallization process along with conventional
microporous zeolite structure director to form the ultrathin microporous zeolite framework, while
the hydrophobic interaction between the long chains restricted the excessive growth of zeolites
and induced the formation of inter-crystalline mesopores. Nanocrystalline zeolites exhibited
exceptionally high activity in the acid-catalyzed reactions involving large molecules when
compared with conventional zeolites. This synthesis strategy can be extended for the preparation
of zeolites of different framework structure or other porous materials in the future.
Keywords: Nanocrystalline zeolites; soft template; dual templating pathways, mesoporous
zeolites; multi-cationic surfactants; acid catalysts.
_______________________________________________________________
1. INTRODUCTION
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Zeolites are industrially important microporous aluminosilicates that are widely used in
petrochemical and fine chemical industries.1-8 Ion-exchange property of the zeolites makes them
applicable in water purification and electrochemical applications.1-7 Ultra-microporous zeolites are
used in the air separation technology.1-7 Due to their applications in the wide range of
interdisciplinary research areas, tremendous efforts are being made to prepare zeolites of different
framework structure.8, 9 Microporous zeolites are prepared in the presence of pore generating
species such as alkylammounium halide/hydroxide as structure directing agents (SDA).1-9
Micropores are generated by the removal of SDA.1-9 However, applications of zeolites are limited
to small molecules that can diffuse through the microporous channels.10-19 Although zeolites play
significant role in the petrochemical industry due to their shape selectivity but pore diffusion
limitation imposed by the microporous zeolites reduces the life time of catalyst due to the coke
formation.20-22 To overcome the diffusion limitation, nanocrystalline or hierarchical zeolites with
inter/intra-crystalline mesopores are being developed.23-29 Nanocrystalline zeolites with 10-15 nm
size are expected to be very promising for catalysis, because they would significantly reduce the
diffusion path length and enhance the catalytic activity and life time of zeolite catalysts.
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Soft templates and hard templates both have been used in the synthesis of nanocrystalline
zeolites.23-41 Mesoporous carbon, carbon nanotubes and polymers have been used as hard
templates.30-33 Soft template method is preferred over hard template method due to versatility, cost-
effectiveness, and large scale production of the materials. Conventional mesopore generating
single quaternary ammonium group failed to function as an effective structure-directing agent for
the synthesis of nanocrystalline zeolites. Initially, organosilanes have been developed for the
synthesis of nanocrystalline ZSM-5 and Beta.34-38 In recent time, efforts are being made to develop
di-cationic and multi-cationic SDA for the synthesis of disordered and ordered nanocrystalline
zeolites of different framework structure.39-48 In general, the surfactants are composed of a long-
chain alkyl group (C16-C22) and two/more quaternary ammonium groups spaced by an alkyl/aryl
linkage. The multi-ammonium head groups act as effective SDA for the formation of ultrathin
microporous zeolite framework, while the hydrophobic interaction between the long chain tails
restricts the excessive growth of zeolites and induce the formation of inter/intra-crystalline
mesopores.
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Our group has developed wide range of mono-cationic and multi-cationic surfactants for
the synthesis of nanocrystalline zeolites of different framework structure.44-47 It is important to
note that for one framework type zeolite, one special type of soft template is required. This led to
develop wide range of soft templates for the synthesis of nanocrystalline zeolites. We did not come
across any synthesis strategy in which one soft template is used to produce nanocrystalline zeolites
of different framework structure. In this manuscript, we report for the first time that one suitable
soft template can be used to prepare nanocrystalline zeolites of different framework structure with
large surface area and pore volume just by adding the designed SDA in the synthesis gel
composition of conventional zeolites. In this study, the details about the synthesis of
nanocrystalline ZSM-5 is provided. Furthermore, results for the formation of nanocrystalline
mordenite and sodalite are also presented using the same structure directing agent.
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2. EXPERIMENTAL SECTION
Synthesis of structure directing agents (SDA)
Schematic representation for the synthesis pathways adopted for the preparation of SDA is given
+
in Scheme 1. For the synthesis of SDA1, first [C18H37-N (CH3)2-C6H12-N(CH3)2]Br (designated
as C18-N-C6-N) was prepared. In a typical synthesis, 1-bromooctadecane (10 mmol) and N,N,N′,N′-
tetramethyl-1,6-diaminohexane (100 mmol) were dissolved in acetonitrile (50 mL)/toluene (50
mL) mixture (1:1 by volume) and stirred at 363 K for 24 h. After completion of the reaction, the
precipitated product was washed with cold diethyl ether several times and dried under vacuum at
323 K for 12 h to obtain C18-N-C6-N. In the second step, C18-N-C6-N (10 mmol) and 1-
bromohexane (20 mmol) were dissolved in 30 mL of acetonitrile and refluxed for 12 h. After
completion of the reaction, the precipitated product was washed with cold diethyl ether several
times and dried under vacuum at 323 K for 12 h to obtain SDA1.
SDA1: 1H NMR (DMSO): δ = 3.21 (m, 8H), 2.98 (s, 12H), 1.58-1.70 (m, 8H), 1.12-1.34 (m, 40H),
0.82-0.86 ppm (t, 6H). 13C NMR: δ =62.98, 62.70, 49.94, 31.23, 28.86, 28.65, 28.48, 25.79, 25.29,
22.05, 21.66, 21.44, 13.79 ppm.
For the synthesis of SDA2, 1-bromooctadecane (20 mmol) and N,N,N′,N′-tetramethyl-1,6-
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diaminohexane (10 mmol) were dissolved in acetonitrile (50 mL)/toluene (50 mL) mixture and
stirred at 363 K for 24 h. After completion of the reaction, the precipitated product was washed
with cold diethyl ether several times and dried under vacuum at 323 K for 12 h to obtain SDA2.
9
SDA2: 1H NMR (DMSO): δ = 3.21 (m, 8H), 2.98 (s, 12H), 1.58-1.70 (m, 8H), 1.12-1.34 (m, 64H),
0.82-0.86 ppm (t, 6H). 13C NMR: δ =62.98, 62.70, 49.94, 31.23, 28.86, 28.65, 28.48, 25.79, 25.29,
22.05, 21.66, 21.44, 13.79 ppm.
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For the synthesis of SDA3, first [C18H37-DABCO]Br (hereafter represented as C18-D) was
prepared. 1,4-diazabicyclo[2.2.2]octane (40 mmol) and 1-bromooctadecane (10 mmol) were
dissolved in acetone (50 mL) and stirred at ambient condition for 12 h. After completion of the
reaction, the white precipitate was washed with acetone several times and dried under vacuum at
353 K for 12 h to obtain C18-D. C18-D (10 mmol) and 1,6-dibromohexane (5 mmol) were reacted
in toluene (50 mL) under reflux condition for 2 days. After completion of the reaction, the white
precipitate was washed with acetone several times and dried under vacuum at 353 K for 12 h to
obtain SDA3.
SDA3: 1H NMR (DMSO): δ = 3.9 (m, 24H), 3.5 (t, 8H), 1.7 (m, 8H), 1.1-1.4 (m, 64H), 0.82-0.85
13
ppm (t, 6H). C NMR: δ =63.4, 63.1, 50.4, 31.3, 29.33, 29.3, 29.1, 29.0, 28.8, 25.6, 25.0, 22.4,
21.6, 21.4, 14.0 ppm.
Syntheses of zeolites
In a typical synthesis of ZSM-5 zeolites, sodium hydroxide (half amount) and
tetrapropylammonium bromide (TPABr) were dissolved in distilled water (solution A). In a
separate vessel, sodium aluminate (53 wt % Al2O3, 43 wt % Na2O) was dissolved in aqueous
sodium hydroxide solution containing remaining half amount of NaOH (solution B). Then solution
B was added into solution A and the resultant solution was stirred at ambient condition for 20 min,
until it became clear solution. Subsequently, SDA was added to the resultant solution and reaction
mixture was stirred at ambient condition until SDA was completely dissolved.
Tetraethylorthosilicate (TEOS, 98%) was added under vigorous stirring to the above reaction
mixture. The molar gel composition was 30 SiO2: 0.375 Al2O3: 6.6 Na2O: 1066 H2O: x TPABr: y
SDA, where x:y = 1:1, 2:0, 0:2. The resultant gel mixture was aged at 333 K overnight with
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vigorous stirring. Finally, the reaction mixture was transferred to a Teflon-coated stainless-steel
autoclave and hydrothermally treated at 423 K for 5 d under stirring condition. After the reaction,
the product was isolated by filtration followed by washing with distilled water. The product was
dried at 373 K and subsequently calcined in air at 873 K for 4 h to remove the organic SDA. Bulk
ZSM-5 was prepared only in the presence of TPABr and it is represented as ZSM-5. Material
prepared with 2SDA1 is represented as ZSM-5(2SDA1). Material prepared with 1:1 equivalent of
SDA1 and TPABr is represented as ZSM-5(SDA1-TPABr). ZSM-5 materials were also prepared
with 1:1 equivalent of TPABr and SDA2/SDA3/SDA4 and they are represented as ZSM-5(SDA2-
TPABr), ZSM-5(SDA3-TPABr), and ZSM-5(SDA4-TPABr), respectively.
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For the synthesis of mordenite zeolites, TEOS was mixed with deionized water. Required
amount of HCl was added drop-wise to the TEOS solution to reach a pH value of 1.0 (solution A).
For complete hydrolysis of silica source, solution A was stirred at ambient condition for 24 h. In
a separate beaker, aluminum sulfate was dissolved in aqueous sodium hydroxide solution and
stirred at ambient condition for 10 min. Subsequently, tetraethylammonium bromide (TEABr) and
SDA were added to the above solution to get solution B. Then the solution A was added into
solution B and the resultant mixture was stirred at ambient condition for 20 min. The resultant gel
was aged at ambient condition for 24 h. Finally, the synthesis mixture was transferred to a Teflon-
lined stainless steel autoclave and hydrothermally treated at 443 K for 4 d. The final gel
composition was 15SiO2: 8Na2O: 900H2O: 1Al2O3: x TEABr: y SDA, where x:y = 1.75:1.75 or
3.5:0. After the reaction, the obtained solid was filtered and washed with distilled water. The
product was dried at 373 K and subsequently calcined in air at 873 K for 4 h to remove the organic
SDA. Bulk mordenite was prepared only in the presence of TEABr and it is represented as MOR.
Material prepared with 1:1 equivalent of SDA1 and TEABr is represented as MOR(SDA1-
TEABr).
In a typical synthesis of sodalite (SOD) zeolite, sodium silicate (26.5% SiO2, 10.6 % Na2O in
aqueous solution), sodium hydroxide and tetrapropylammonium bromide (TPABr) were dissolved
in distilled water (solution A). In a separate vessel sodium aluminate was dissolved in aqueous
sodium hydroxide solution (solution B). Then the solution B was added into solution A and the
resultant solution was aged at ambient condition for 6 h. The resultant gel had a molar composition
of 2SiO2: 20 Na2O: 1 Al2O3: 160 H2O: x TPABr: y SDA1, where x:y= 0.02:0.02 or 0.04:0. The
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resultant gel was transferred to a Teflon-lined stainless steel autoclave and hydrothermally treated
at 423 K for 24 h. After the reaction, the obtained solid was filtered and washed with distilled
water. The product was dried at 373 K and subsequently calcined in air at 873 K for 4 h to remove
the organic SDA.
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Materials characterization and catalytic experiments
Details of instruments used for material characterization and procedures adopted for the
catalytic reactions are provided in the supporting information section.
3. RESULTS AND DISCUSSION
Synthesis and characterization of structure directing agents
In this study two di-cationic (SDA1 and SDA2), one tetra-cationic (SDA3) and one
commercial mono-cationic cetyltrimethylammonium bromide (SDA4) SDA were investigated as
mesopore directing agents (Scheme 1). SDA were characterized by NMR and data are provided in
experimental section. In-depth NMR characterization data are provided here for the SDA1, which
is found to be the best suited SDA investigated in this study. 1H, 13C, COSY and HETCOR NMR
spectra are shown here to illustrate the formation of SDA1 (Figure S1-S4 in the Supporting
Information). NMR spectra shown here also contain DMSO and H2O signals arising from the
deuterated solvent in which these organic compounds were dissolved. The triplet signal in the 1H
NMR spectrum (Figure S1 in the Supporting Information) is due to the terminal –CH3 protons
(designated as “a”). –CH3 protons attached to quaternary ammonium group (designated as “d”) are
observed downfield at 2.98 ppm, whereas the protons “e, e1” (N-CH2) are observed at 3.2 ppm
with triplet splitting. Similarly, protons “b” and “c” are observed at 1.2 and 1.6 ppm respectively
with multiplet. 13C NMR spectrum confirms the occurrence of three different types of carbon
associated with quaternary ammonium nitrogen. 4 α-carbons attached to quaternary ammonium
nitrogen designated as “d” are observed at 49.9 ppm (Figure S2 in the Supporting Information).
Whereas 4 α-carbons attached to quaternary ammonium nitrogen designated as “e, e1” are
observed downfield at 62.7 and 62.9 ppm. The terminal carbons (“a”) are observed at 13.7 ppm.
Remaining aliphatic carbons are observed in the range of 20-32 ppm. Designated protons and
carbons correlate well with each other and this correlation is shown in H-1H (Figure S3 in the
1
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Supporting Information) and 13C-1H correlation spectra (Figure S4 in the Supporting Information),
which further confirms the formation of SDA1.
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Synthesis, characterization, and catalytic activity of nanocrystalline zeolites
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First gel composition was optimized for the synthesis of conventional ZSM-5. Conventional
ZSM-5 was prepared using tetrapropyl ammonium bromide (TPABr) with the molar composition
(30 SiO2/0.375 Al2O3/6.6 Na2O/1066 H2O/2.0 TPABr). Under the similar synthesis gel
composition, when SDA1 was used in place of TPABr (material is designated as ZSM-5(2SDA1),
ZSM-11 was obtained (Figure 1a). XRD pattern of ZSM-5(2SDA1) shows that the third intense
peak in the region around 25 is almost absent. Likewise, the peak at 45 is doublet in ZSM-5
zeolite, whereas it appears to be a singlet for ZSM-5(2SDA1). These observations confirm that the
material is very similar to that of ZSM-11.49 Later zeolites were synthesized using the molar
composition where equimolar amount of TPABr and SDA were used (30 SiO2/0.375 Al2O3/6.6
Na2O/1066 H2O/1.0 TPABr/1.0 SDA). Following this strategy and using various SDA and TPABr,
ZSM-5(SDA1-TPABr), ZSM-5(SDA2-TPABr), ZSM-5(SDA3-TPABr), and ZSM-5(SDA4-
TPABr) materials were obtained (Figure 1). On careful inspection of XRD, one can say that ZSM-
5(SDA1-TPABr)exhibits broad XRD pattern when compared to ZSM-5 (Figure 1). Following the
synthesis strategy adopted in this work, ZSM-5 materials obtained with other SDA do not exhibit
broad XRD pattern (Figure 1b). It is worthy to note that SDA3 is known to produce ZSM-5
nanosheet but fails to provide broad XRD pattern under the present synthesis strategy.44
The difference in the textural properties of the ZSM-5 materials is evident from their
adsorption properties. N2-adsorption isotherm of ZSM-5(SDA1-TPABr) shows three increasing
steps (Figure 2). The steep increase observed in the region of 0.0 < P/P0 < 0.1 (where P is the
actual adsorption pressure and P0 is the equilibrium vapour pressure of N2) is a result of micropore
filling. The second increase within 0.4 <P/P0 < 0.7 region indicates condensation of N2 in
mesopores. The volume increase above P/P0 = 0.8 is due to N2 condensation in the void volume
between particles. The adsorption branch of the capillary condensation region was used to analyze
the mesopore size distribution according to the Barrett-Joyner-Halenda algorithm. The result
shows a bimodal distribution of mesopore diameters centred at 5 nm and 9 nm. The micropore size
distribution was analyzed with nonlocal density functional theory (NLDFT), using the adsorption
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region below P/P0 = 0.1, revealing bimodal pore size distributions with maxima at 0.55 and 1.2
nm. For ZSM-5, the 0.55 nm maximum is attributed to micropores associated with 10-membered
oxygen rings (10-MR).50 The 1.2 nm peak is attributed to a transition in the adsorbate packing
density or an orthorhombic-monoclinic transition in the crystal symmetry caused by the gas
adsorption, which is known to be unique for MFI-type zeolites.51 ZSM-5 exhibits type II isotherm
with two well-resolved increasing steps (Figure 2). N2-adsorption isotherm of ZSM-5(2SDA1) is
found to be similar to that of ZSM-5(SDA1-TPABr). N2-adsorption isotherms for other ZSM-5
materials prepared in this study are provided in Figure S5 in the Supporting Information. Textural
properties obtained using N2-adsorption study is summarized in Table 1. Surface area, external
surface area, and pore volume for ZSM-5(SDA1-TPABr) are the highest among the ZSM-5
materials obtained using various other SDA.
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SEM is used to investigate the morphology of zeolite materials (Figure 3). Spheroid
morphology with average particle size of 5 m is obtained for Bulk ZSM-5. ZSM-5(2SDA1)
exhibits irregular morphology which is built with zeolite nanocrystals. ZSM-5(SDA1-TPABr) and
ZSM-5(SDA2-TPABr) exhibit uniform crystal morphology with particle size in the range of 400-
700 nm. ZSM-5(SDA3-TPABr) exhibits irregular crystal morphology which is built with nanowire
like nanocrystals. Large zeolite crystals that are built with micrometer size zeolite sheets are
obtained for ZSM-5(SDA4-TPABr). The morphology of ZSM-5(SDA4-TPABr) is different from
that of conventional ZSM-5. Transmission electron microscopy (TEM) images are shown here for
some selected samples. TEM images are shown for ZSM-5(SDA1-TPABr) that exhibits excellent
textural properties when compared to other ZSM-5 materials. TEM image confirms that each
irregular zeolite particles (average size = 500-600 nm) are built with 10-20 nm zeolite nanocrystals
(Figure 4a). Selected area diffraction pattern indicates that material is highly ordered in the
micropores length scale (Figure 4a, inset). High resolution TEM (HRTEM) investigation shows
that these zeolite nanocrystals exhibit nanosheet like crystal morphology that are oriented in
different direction (Figure 4b, c). Selected area diffraction pattern obtained from many such
nanosheets like structure clearly shows the highly crystalline nature of the material (Figure 4b,
inset). Chemical investigation was carried out by energy dispersive X-ray spectroscopy (EDS)
using STEM-HAADF technique (Figure 4d, e). EDX spectrum from a region marked by area in
Figure 4e confirms the presence of O, Al, and Si atoms. Long chain containing di-cationic SDA1
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seems to trigger the growth of zeolite nanosheets during the crystallization process. TEM images
show that ZSM-5(2SDA1) contains randomly oriented zeolite nanocrystals (Figure 5a,b). HRTEM
image shows that the particles are built with small domain of nanosheet architecture along with
large domain of randomly oriented zeolite nanocrystals that are highly ordered in micropores
length scale (Figure 5c). EDX spectrum (Figure 5d) from a region marked by area in STEM-
HAADF image in Figure 5e confirms the presence of O, Al, and Si atoms.
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Thermo gravimetric analysis (TGA) shows that the as-synthesized ZSM-5(SDA1-TPABr) and
conventional ZSM-5 samples contain organic components corresponding to 28% (w/w) and 10%
of its total mass (Figure S6 in the Supporting Information). Therefore, 23% (w/w) loss can be
assigned to the SDA1 surfactant in ZSM-5(SDA1-TPABr). Since the SDA1 molecule is too bulky
to enter the micropore aperture of ZSM-5 zeolite, the amount of surfactant is believed to remain
on the external surfaces through electrostatic interactions between the ammonium heads of the
surfactants and the zeolite framework. Therefore, the generation of the nanocrystals can be
attributed to the molecular effect of SDA1 (details are given in the following section). Based on
the textural properties and TGA analysis, one can conclude that zeolite microporous structure
director TPABr and di-cationic SDA1 cooperatively participate in the crystallization process of
nanocrystalline ZSM-5. Physico-chemical characterization shows that the textural properties of
ZSM-5(SDA4-TPABr) are very similar to that of conventional ZSM-5. From this observation, one
can say that the di-cationic SDA1 is more suited for the nanocrystalline ZSM-5 synthesis. Di-
cationic SDA1 could protect zeolite nanocrystal surfaces from further growth more effectively
than CTMABr, which is probably due to strong electrostatic interactions. Certainly, the exclusive
use of SDA1 did not make them nanocrystalline ZSM-5 structure-directing agent. Texture property
(Table 1) shows that SDA3 also provide good quality nanocrystalline ZSM-5. Other SDA such as
SDA2 and SDA4 do not produce materials with high surface area and pore volume as it is obtained
using SDA1. These observations clearly show that only a suitable type of SDA cooperatively
interact with the TPABr and participate in the nanocrystalline ZSM-5 growth process to give
materials with excellent textural properties.
In order to understand the influence of SDA in the synthesis of zeolites, a brief illustration
for ZSM-5 and ZSM-11 framework structures is provided (Scheme 2). Both ZSM-5 and ZSM-11
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type zeolites have three-dimensional ∼0.55 nm channels. ZSM-5 framework forms sinusoidal 10-
member rings channels along the a-axis, interconnected with 10-member rings straight channels
along the b-axis. A tortuous porous architecture is present along the c-axis. In contrast to ZSM-5
topology, ZSM-11 has only straight channels (Scheme 2). It is well documented that small TPA
cations are situated at the intersection of the straight and sinusoidal channels.52 Bulkier mono-
cationic or di-quaternary ammonium cations with n > 4 are known to better fit in the straight
channels than along the sinusoidal channels.53,54 One may argue that how ZSM-5 framework is
formed using only TPABr. When the synthesis is performed using only TPABr, then one TPA
could be located at the intersection of the straight and sinusoidal channels whereas two or more
TPA cations could fit into the straight channels along the b-axis. In the present synthesis strategy,
when 1 equivalent of di-quaternary SDA1 and one equivalent of TPABr are used, then TPA cations
would prefer to locate at the intersection of the straight and sinusoidal channels whereas di-
quaternary ammonium cations would prefer to occupy the straight channels along the b-axis and
form ZSM-5 framework structure. However, when 2 equivalents of SDA1 are used then SDA1
can fit into straight channels only. In this case, it will fit into straight channels along the a-axis and
b-axis, which resulted in the formation of ZSM-11 framework structure.
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Following mechanism is proposed for the synthesis of ZSM-5 nanosheets in the presence
of equivalent amount of SDA1 and TPABr. Ordered and disordered stacking of thin zeolite
nanosheet layers can be seen in the HRTEM images (Scheme 3). HRTEM investigation confirms
that nanosheet layers exhibit large coherent domains characterized by wide a-c planes while the
framework thickness along the b-axis is extremely small. During the initial stage of synthesis,
nucleation of ZSM-5 framework structure (I, Scheme 3) takes place in the presence of TPABr and
SDA1 cations. Growth process is preferred to take place along a-c plane due to the presence of
hydrophobic long tail surfactant molecules along b-axis and form wide ZSM-5 layer along a-c
plane (II, Scheme 3). These ZSM-5 layers are stacked along b-axis to provide multilamellar
architecture (III, Scheme 3). The multilamellar grown crystals act as pillar supporting to each
other, preventing complete collapse of mesoporous structure. During the calcinations process,
removal of surfactant layer leads to form the ZSM-5 nanosheets with inter-crystalline mesopores.
Though multilayer form predominates the growth process but one cannot rule out the possibility
of disordered stacking of nanosheets. In this case, zeolite sheet is composed of single ZSM-5 layer
having a very narrow a-c plane (IV, Scheme 3). This architecture is considered to be composed of
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essentially the same building units as the multilamellar form but without long range stacking along
the b-axis. ZSM-5 layers with narrow a-c plane randomly stack to form disordered unilamellar
ZSM-5 (V, Scheme 3). The growth mechanism proposed here also explains the inability of
CTMABr to form nanosheet architecture as it is obtained using SDA1. CTMABr also prefers to
fit along b-axis (Scheme 2). It fits well in the channel along the b-axis with no hydrophobic tail
outside the ZSM-5 layer as explained in the case of SDA1. Therefore, multilamellar regular
stacking along b-axis or random stacking does not take place and nanosheet architecture is not
obtained. Instead highly packed ZSM-5 crystals are obtained as shown in the SEM images.
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The synthesis strategy adopted for the preparation of nanocrystalline ZSM-5 was extended for
the preparation of mordenite (designated as MOR). Conventional MOR was prepared using
TEABr as SDA with the molar composition of 15 SiO2/1.0 Al2O3/8 Na2O/900 H2O/3.5 TEABr.
Later zeolite was synthesized using the molar composition (15 SiO2/1.0 Al2O3/8 Na2O/900
H2O/1.75 TEABr/1.75 SDA1) where equimolar amount of TEABr and SDA1 were used and the
obtained material is designated as MOR(SDA1-TEABr). Using this strategy a good quality
nanocrystalline MOR is obtained (Figure 6). On careful inspection of XRD one can say that
MOR(SDA1-TEABr) exhibits somewhat broad XRD pattern when compared to conventional
MOR. N2-adsorption study shows that MOR(SDA1-TEABr) exhibits type IV isotherm with steep
increase in the range of 0.8 to 1.0 (P/P0). This corresponds to inter particle porosity (Figure S7 in
the Supporting Information). Nanocrystalline MOR(SDA1-TEABr) exhibits large external surface
area and pore volume when compared to MOR (Table 1). Conventional MOR exhibits large zeolite
particles built with micrometer size sheet-like morphology (Figure 7a,b). MOR(SDA1-TEABr)
exhibits nano-leaf like morphology which provides flower like architecture (Figure 7c,d). TEM
investigation shows that the flower-like architecture is actually built with nanosheets randomly
grown in different directions (Figure 7e-g). To the best of our knowledge, this is the first report
for the synthesis of nanocrystalline MOR in which dual surfactant strategy is used. Furthermore,
it is worthy to note that synthesis of nanocrystalline MOR is rarely reported in the literature. Our
attempts to synthesize nanocrystalline MOR using organosilanes and other SDA reported in our
previous studies have been failed.
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The similar strategy was adopted for the synthesis of sodalite (designated as SOD). XRD pattern
confirms the formation of SOD using TPABr (Figure S7 in the Supporting Information). A good
quality nanocrystalline SOD is obtained when the material was synthesized using equimolar
amount of TPABr and SDA1 (represented as SOD(SDA1-TPABr)). SOD(SDA1-TPABr) exhibits
agglomerated zeolite nanocrystals (Figure S8 in the Supporting Information). Morphology of SOD
is found to be different than that of SOD(SDA1-TPABr). Closure inspection of SEM images
clearly shows that zeolite particles are built with small nanocrystals of different shapes in these
two materials. TEM investigation further confirms that SOD(SDA1-TPABr) particles are built
with zeolite nanocrystals (Figure 8). Selected area diffraction pattern from one grain shows that
the material is highly crystalline in nature (Figure 8b). N2-adsoption study shows that the external
surface area of SOD(SDA1-TPABr) is 91 m2g-1, which is much larger than that of bulk SOD (2
m2g-1) (Table 1).
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Acidity of ZSM-5(SDA1-TPABr) and MOR(SDA1-TEABr) was determined by ammonia-TPD
(Temperature programmed desorption). For comparative study, acidity was also determined for
conventional ZSM-5 and MOR samples. In the TPD profiles of ZSM-5(SDA1-TPABr) and ZSM-
5 materials, two well-resolved symmetric peaks dominate with maximum at 473 K and 683 K were
observed. Whereas, two well-resolved peaks dominate with maximum at 505 K and 900 K were
observed for MOR(SDA1-TPABr) and MOR samples. Total acidity of various samples obtained
from NH3-TPD is provided in Table S1. Acidity measurements confirm that dual template
mediated synthesis of nanocrystalline ZSM-5 and MOR lead to form materials with similar acidity
as that of conventional ZSM-5 and MOR zeolites. Excellent acidity and enhanced textural
properties are expected to impart significant role in the catalytic activity.
An acid catalyst is believed to be a good catalyst if it exhibits good activity in the acylation and
alkylation reactions. Zeolite particles having nanometer dimension is expected to overcome the
diffusion limitations. In this study large reactant molecules are chosen in which conventional
zeolites are expected to exhibit less activity due to diffusion constraints. ZSM-5 samples
synthesized in this study were investigated in the acylation reactions of indole and 1,3-dimethoxy
benzene. MOR samples were investigated in the alkylation of benzene with benzyl alcohol.
Acylation of indole is more complex than other heterocyclic compounds due to the number of
possible products (Table 2). Influence of reaction parameters such as temperature, reactant/acetic
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anhydride ratio, amount of catalyst, and reaction time was investigated. Influence of acetic
anhydride/indole ratio show that with increase in the acetic anhydride/indole ratio, indole
conversion was increased. High indole conversion and good selectivity for 3-acetyl indole were
obtained when the reaction was carried out for 1.5 h using 1.25 equivalent of acetic anhydride with
respect to indole. With further increase in the acetic anhydride to indole ratio, 1, 3-diacetyl indole
selectivity was increased. Significant increase in the indole conversion was obtained when amount
of the catalyst was increased from 25 mg to 50 mg. With further increase in the catalyst amount,
no significant increase in the indole conversion was observed. Therefore, to achieve high
conversion, 50 mg was chosen as a catalyst amount for further study. With an increase in the
temperature from 313 to 393 K, indole conversion was found to increase from 8 to 90%. 50 mg
catalyst and 393 K were found to be the optimized catalyst amount and reaction temperature.
Catalytic activity data obtained using various ZSM-5 catalysts under the optimized reaction
condition are provided in the Table 2. Acylation of 1,3-dimethoxy benzene was found to be more
selective when compared to Indole. In this case highly selective mono-acylated product was
obtained (Table 2). Influence of reaction parameters such as temperature, 1,3-dimethoxy benzene
/acetic anhydride ratio, amount of catalyst, and reaction time was investigated and the reaction
condition was optimized. Under the optimized reaction condition, various ZSM-5 catalysts were
investigated and the results are summarized in Table 2. Alkylation of benzene with benzyl alcohol
leads to form two products, diphenyl methane and dibenzyl ether (Table 2). Reaction parameters
were optimized and the catalytic activity data obtained using MOR and MOR(SDA1-TEABr)
under the optimized reaction condition are provided in Table 2. Catalytic investigations suggest
that ZSM-5(SDA1-TPABr) exhibits exceptionally higher activity than conventional ZSM-5 and
other ZSM-5 materials prepared in this study. Similarly, higher activity is achieved in the
alkylation of benzene with benzyl alcohol using MOR(SDA1-TEABr) when compared to
conventional MOR. Based on the acidity measurements and catalytic activity results one can
conclude that the accessibility of reactant molecules to the acid sites located at the pore-mouth or
little bit farther inside the channels and enhanced diffusion of reactant/products through inter-
crystalline mesopores are responsible for the high catalytic activity. Furthermore, these catalysts
are easily separated from the reaction mixture, regenerated by simple calcinations process, and
recycled with no loss in the activity.
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4. CONCLUSIONS
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In conclusion, one-step, direct synthesis strategy using the combination of zeolite microporous
structure directing agent and di-cationic SDA1 allowed us to synthesize ZSM-5, MOR, and SOD
zeolite nanocrystals. The nanocrystal formation is attributed to the strong cooperative binding of
the SDA1 and micropore directing surfactant with the growing zeolite phase when compared with
the monovalent long chain surfactant. Long hydrophobic chains of SDA1 facilitate the ordered
stacking of microporous zeolite layers to produce nanosheet like morphology. Furthermore,
disordered stacking of zeolite layers are also observed in TEM images. During the crystallization
process, the hydrophobic interaction between the long chains of di-cationic SDA1 restricted the
excessive growth of zeolites and induced the formation of inter-crystalline mesopores.
Nanocrystalline ZSM-5 and MOR exhibited excellent activity in the acid-catalyzed reactions
involving large molecules that cannot enter the micropores of conventional zeolites. Large number
of acid sites on the external surface of these zeolites renders them highly active for the catalytic
conversion of large organic molecules, and the reduced crystal thickness facilitates diffusion and
thereby improved their performance in important catalytic applications. The present synthesis
strategy would be suitable for the synthesis of other zeolite nanocrystals that could improve
existing catalytic applications or even enable new applications. Furthermore, we believe that such
mixed surfactant strategy could be applicable for the synthesis of different class of nanomaterials.
Zeolites with nanosheet like architecture having the diameter of the unit cell dimension (10-15
nm) with inter/intra-crystalline mesoporous structure would be very promising for catalysis.
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ACKNOWLEDGEMENTS
Authors are thankful to CSIR, New Delhi for financial assistance through CSIR project 01
(2802)/14/EMR-II. We acknowledge Director, IIT Ropar for Instrumental facility and constant
encouragements. Authors thank D. Sakha, Chemical Engineering, IIT Kharagpur for the NH3-TPD
analysis.
Supporting Information
The Supporting Information is available free of charge on the ACS Publications website
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molecules. Chem. Eur. J. 2008, 14, 9507–9511.
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with enhanced textural and catalytic properties synthesized from organofunctionalized seeds.
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nanosheets of zeolite MFI as active and long-lived catalysts. Nature 2009, 461, 246–249.
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R. Directing zeolite structures into hierarchically nanoporous architectures. Science 2011,
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nanosheets of a single unit cell thickness. J. Am. Chem. Soc. 2010, 132, 4169–4177.
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Figure, Scheme, Table captions
Scheme
Schematic representation for the synthesis of structure directing agents used in
the synthesis of nanocrystalline zeolites.
Scheme 1
Schematic representation for the formation of ZSM-5 and ZSM-11 framework
structures in the presence of SDA1/SDA4 and TPABr.
Scheme 2
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Proposed mechanism for the formation of ZSM-5 nanosheets in the presence of
SDA1 and TPABr.
Scheme 3
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Figure
XRD patterns of conventional ZSM-5, ZSM-5 (2SDA1) and nanocrystalline
ZSM-5 samples prepared using dual templating pathway.
Figure 1.
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N2-adsorption isotherms of ZSM-5, ZSM-5 (2SDA1), and ZSM-5(SDA1-TPABr)
(inset shows pore size distribution)
Figure 2.
SEM micrographs of various ZSM-5 materials prepared in this study.
(a) Low magnification TEM image (inset shows corresponding electron
diffraction pattern from a small grain showing single crystal nature), (b, c) High
magnification TEM image, (d) EDX spectrum of the area highlighted in HAADF
image (e, inset) of ZSM-5(SDA1-TPABr).
Figure 3.
Figure 4.
(a) Low magnification TEM image, (b, c) High magnification TEM images, (d)
EDX spectrum of the area highlighted in HAADF image (e, inset) of ZSM-
5(2SDA1).
Figure 5.
XRD patterns of conventional MOR and nanocrystalline MOR(SDA1-TEABr).
(a, b) SEM images of conventional MOR, (c, d) SEM images and (e-g) TEM
images of nanocrystalline MOR(SDA1-TEABr).
Figure 6.
Figure 7.
(a) TEM image (b) selected area diffraction pattern from one grain, (c) EDX
spectra of the area highlighted in HAADF image (d, inset) of SOD(SDA1-
TPABr).
Figure 8.
Table
Physico-chemical properties of zeolite materials prepared in this study.
Comparative catalytic activities of ZSM-5 and MOR zeolites investigated in
this study.
Table 1
Table 2
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Table 1
Material
ZSM-5
SDA
SBET (m2g-1) Ext. SA(m2g-1) VTotal (mLg-1)
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10
11
12
13
TPABr
SDA1 and TPABr
SDA2 and TPABr
SDA3 and TPABr
SDA4 and TPABr
SDA1
326
612
446
526
366
591
76
54
260
63
0.20
0.80
0.26
0.48
0.24
0.99
0.11
0.29
0.01
0.25
ZSM-5(SDA1-TPABr)
ZSM-5(SDA2-TPABr)
ZSM-5(SDA3-TPABr)
ZSM-5(SDA4-TPABr)
ZSM-11(2SDA1)
MOR
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181
66
315
8
TEABr
MOR(SDA1-TEABr)
SOD
SDA1:TEABr
TPABr
217
8
127
2
SOD(SDA1-TPABr)
SDA1:TPABr
182
91
SDA = Structure directing agents, SBET = Total surface area, SA = Surface area, VTotal = Total pore
volume. SBET was calculated from the adsorption data obtained in the region of 0.05<P/P0≤0.3.
Table 2
COCH3
COCH3
Ac2O
+
+
Catalyst
Neat, 393 K
N
H
N
H
N
N
COCH3
COCH3
1-Ac Indole
1,3-diAc Indole
3-Ac Indole
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OCH3
OCH3
4
5
Ac2O
6
7
8
Catalyst
Neat, 393 K
OCH3
OCH3
COCH3
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Catalyst
Neat, 353 K
OH
+
+
O
Catalyst
aAcylation of Indole
bAcylation of 1,3-dimethoxybenzene
Indole
Conv. (%)
3-Ac
Indole
Sel. (%)
35
1,3-dimethoxybenzene
Conv. (%)
Acylated
product
Sel. (%)
>99
ZSM-5
5
3
ZSM-5(SDA1-TPABr)
ZSM-5(SDA2-TPABr)
ZSM-5(SDA3-TPABr)
ZSM-5(SDA4-TPABr)
ZSM-2(2SDA1)
90
36
78
8
84
92
81
68
80
38
78
80
74
27
65
3.5
70
71
>99
>99
>99
>99
>99
>99
ZSM-5(SDA1-TPABr)d
cAlkylation reaction
Catalyst
Benzyl alcohol Conv. (%)
Diphenyl methane Sel. (%)
MOR
13.2
68.5
67.1
80
92
90
MOR(SDA1-TPABr)
MOR(SDA1-TPABr)d
Reaction condition: aIndole (2 mmol), acetic anhydride (2.5 mmol), catalyst (50 mg), temperature
(393 K), and time (1.5 h). b1,3-Dimethoxybenzene (5 mmol), acetic anhydride (5 mmol), catalyst
c
(50 mg), temperature (393 K), and time (4 h). Benzene (20 mmol), benzyl alcohol (2 mmol),
catalyst (50 mg), temperature (353 K), and time (2 h).
dCatalytic activity data obtained after the fifth recycles.
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(b)
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ZSM-5(SDA1-TPABr)
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ZSM-5(SDA3-TPABr)
ZSM-5(2SDA1)
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40
40
50
30
40
50
ZSM-5
ZSM-5(SDA2-TPABr)
50
30
40
50
2degree)
2degree)
Figure 1
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700
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100
0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
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0
5
10 15 20 25 30
Pore Diameter (nm)
ZSM-5
ZSM-5(2SDA1)
ZSM-5(SDA1-TPABr)
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Figure 2
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2 m
5 m
ZSM-5(2SDA1)
ZSM-5(SDA2-TPABr)
ZSM-5(2SDA1)
5 m
1 m
5 m
ZSM-5(SDA3-TPABr)
ZSM-5(SDA3-TPABr)
ZSM-5(SDA4-TPABr)
5 m
5 m
Figure 3
5 m
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