Synthesis of triethoxysilane imidazolium based ionic liquids and their application in the preparation of mesoporous ZSM-5

Article abstract of DOI:10.1016/j.catcom.2011.11.012

Triethoxysilane containing imidazole based ionic liquids were synthesized. These ionic liquids were utilized as a structure directing agent for the synthesis of mesoporous ZSM-5. ZSM-5 was characterized by a complementary combination of X-ray diffraction, N₂ adsorption/desorption, Scanning electron microscopy, Transmission electron microscopy, Temperature programmed desorption techniques, Infrared spectroscopy and Nuclear Magnetic Resonance spectroscopy. The resultant zeolites were mesoporous and showed unique characteristics of a fully crystalline microporous MFI zeolite framework. Mesoporous ZSM-5 synthesized from imidazole based ionic liquids exhibited higher catalytic activity than conventional ZSM-5 in alkylation and cracking reactions. Significantly high catalytic activity of the mesoporous ZSM-5 suggests that large external surface area and accessible acid sites are beneficial for catalytic reactions involving large organic molecules.

Full text of DOI:10.1016/j.catcom.2011.11.012

Catalysis Communications 18 (2012) 11–15
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Synthesis of triethoxysilane imidazolium based ionic liquids and their application in
the preparation of mesoporous ZSM-5
Rajkumar Kore, Rajendra Srivastava ⁎
Indian Institute of Technology Ropar, Rupnagar, Punjab 140001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 21 July 2011
Received in revised form 3 October 2011
Accepted 11 November 2011
Available online 20 November 2011
Triethoxysilane containing imidazole based ionic liquids were synthesized. These ionic liquids were utilized
as a structure directing agent for the synthesis of mesoporous ZSM-5. ZSM-5 was characterized by a comple-
mentary combination of X-ray diffraction, N adsorption/desorption, Scanning electron microscopy, Trans-
2
mission electron microscopy, Temperature programmed desorption techniques, Infrared spectroscopy and
Nuclear Magnetic Resonance spectroscopy. The resultant zeolites were mesoporous and showed unique char-
acteristics of a fully crystalline microporous MFI zeolite framework. Mesoporous ZSM-5 synthesized from im-
idazole based ionic liquids exhibited higher catalytic activity than conventional ZSM-5 in alkylation and
cracking reactions. Significantly high catalytic activity of the mesoporous ZSM-5 suggests that large external
surface area and accessible acid sites are beneficial for catalytic reactions involving large organic molecules.
© 2011 Elsevier B.V. All rights reserved.
Keywords:
Ionic liquids
ZSM-5
Structure directing agents
Alkylation
Cracking
1
. Introduction
Ionic liquids (ILs) have been attracting considerable attention in
Among the templating methods, hard templating has been first
employed to create mesopores inside zeolites [12]. However, in this
method complicated multi steps are usually involved because of the
incompatibility between hard substrates and precursor species. In re-
cent years, a soft templating method including cationic or silylated
polymer and amphiphilic organosilane as the templates has been
drawing increasing attention for its ease of use and high efficiency
to create mesoporosity [13–16].
Very less effort has been made to prepare and utilize the silane
containing ionic liquids [18,19]. The objectives of this study is to pre-
pare a variety of silane containing imidazole based ionic liquids
(SIMILs) and explore their potential in the synthesis of mesoporous
ZSM-5. Further, the catalytic activities of mesoporous ZSM-5 were in-
vestigated and compared with conventional ZSM-5 in acid catalyzed
alkylation and cracking reactions.
recent time due to their unique physico-chemical properties [1].
Physical and chemical properties of ILs can be tailored by varying cat-
ions and anions. Most of the common ILs are formed by reacting a
nitrogen-containing heterocyclic compounds with an alkyl halide to
form quaternary ammonium salts. Efforts have been made to explore
the ability of ILs to serve as a catalyst [2,3]. A few efforts have been
made to utilize ILs as a structure directing agent for the synthesis of
nanostructured materials [4,5].
Zeolite has become an extremely successful catalyst for oil refin-
ing, petrochemistry, and organic synthesis in the production of fine
and specialty chemicals, particularly for molecules having kinetic di-
ameters below 1.5 nm [6–9]. Zeolites act as shape-selective catalysts
due to their strong acidity and uniform microporosity. The presence
of catalytic sites (Al) in micropores of zeolites imposes significant
limitations on a range of reactions involving large reactant and prod-
uct molecules. Diffusion limitation in the micropores can also lower
the overall reaction rate even for the reactions in which reactants
and products are smaller than the micropores [10], and potential of
the zeolite is not fully utilized. Various synthesis procedures have
been reported to improve the chemical and structural properties of
zeolite [11–17]. Non-templating method involves post-synthesis
etching of zeolites by acid/base/steam to achieve mesoporosity [17].
2. Experimental
2.1. Synthesis of structure directing agents
1-Methyl-3-(triethoxysilyl propyl) imidazolium chloride (SIMIL1)
was synthesized by following the reported procedure (Yield=89%)
[18]. First N-dodecylimidazole was prepared for the synthesis of 1-
dodecyl-3-(triethoxysilyl propyl) imidazolium chloride (SIMIL2).
Then SIMIL2 was prepared by following the reported procedure
(
Yield=71%) [19] (Scheme 1). Experimental details and spectroscop-
ic characteristic data of SIMILs are given in the supporting
information.
⁎
Corresponding author. Tel.: +91 1881 242175; fax: +91 1881 223395.
E-mail address: rajendra@iitrpr.ac.in (R. Srivastava).
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.11.012

12
R. Kore, R. Srivastava / Catalysis Communications 18 (2012) 11–15
Scheme 1. Schematic representation for the synthesis of SIMILs.
2
.2. Synthesis of mesoporous ZSM-5 and conventional ZSM-5
interpreted as capillary condensation in mesoporous void spaces.
ZSM-5-SIMIL1 shows N sorption isotherm similar to conventional
2
In a typical synthesis, required amount of SIMILs, 1.33 g of tetra-
propylammonium bromide (TPABr), and 0.40 g of NaOH were
completely dissolved in 18.2 g of H O and mixed with 27.8 g of dilut-
ed sodium silicate solution (Si/Na=1.75; 10 wt.% SiO ). A solution
containing 0.24 g of sodium aluminate (53 wt.% Al , 43 wt.% Na O;
Riedel–deHaën) and 13.3 g of H O was added drop wise under stirring
to the resultant mixture. Then, 13 g of 10 wt.% H SO solution was
added to the synthesis mixture under vigorous stirring. The final
molar composition of the mixture was 2.5 Al /40 Na O/93 SiO /10
TPABr/26 H SO /9000 H O/7 SIMILs. The mixture was heated under
stirring at 423 K for 3 days in a Teflon-coated stainless steel autoclave.
The precipitated product was filtered and washed with distilled water.
The product was dried in an oven at 373 K and subsequently calcined
in air at 823 K (Yield: ZSM-5-SIMIL1=73%; ZSM-5-SIMIL2=77%).
Conventional ZSM-5 was synthesized at 443 K using the synthesis
ZSM-5. However, its surface area and pore volume are higher than
the conventional ZSM-5. Specific surface area and pore volume of
ZSM-5-SIMIL2 were the highest, among the samples investigated in
this study (Table 1). SEM and TEM investigations confirmed that the
particle size of zeolite decreases in the order ZSM-5>ZSM-5-
SIMIL1>ZSM-5-SIMIL2 (Fig. 3). TEM images showed that the materi-
al has disordered mesopore structure and the sample looks like con-
sisting of unshaped inter grown crystals.
2
2
2
O
3
2
2
2
4
27
2
O
3
2
2
Al MAS NMR results show that ZSM-5 contains only tetrahedral
2
4
2
aluminum sites, whereas ZSM-5-SIMIL2 contains approximately 99%
tetrahedral aluminum sites and only approximately 1% of octahedral
Al (extra framework Al) (Fig. S1, supporting information). Pyridine
IR studies confirm that materials contain Brönsted as well as Lewis
acid sites. Pyridine chemisorbed on Brönsted acid sites results in IR
−
1
bands at 1642 and 1550 cm . Bands due to pyridine on Lewis acid
−
1
composition 2.5 Al
2
3 2 2
O /40 Na O/100 SiO /10 TPABr/26 H
SO
2 4
/9000
sites are observed at 1610, and 1446 cm . An intense band at
−
1
H
2
O
by following the synthesis procedure described above
1486 cm
and Lewis acid sites. Acidity of the sample was investigated using
NH -TPD (Fig. 4). The total acidity decreases in the order: ZSM-
5>ZSM-5-SIMIL1>ZSM-5-SIMIL2 (Table 1). In the TPD profiles of
was observed for pyridine chemisorbed on both Brönsted
+
(
Yield=64%). Samples were ion-exchanged into the NH
aqueous NH NO solution, followed by calcinations at 823 K for 6 h
to convert it to the H form.
4
using 1 M
4
3
3
+
2
.3. Material characterizations and catalytic measurements
4
0000
Material characterizations and catalytic measurements are given
30000
in the supporting information.
2
1
0000
0000
0
3
. Results and discussion
ZSM-5
50
In this study SIMILs were synthesized using one-step/two-step
1
1
1
0
0
0
20
20
20
30
30
30
40
synthetic routes (Scheme 1). SIMIL1 is oily liquid whereas SIMIL2 is
sticky solid at ambient condition. None of the SIMILs were soluble
in hexane but both of them were soluble in methanol. It may be
6
4
2
0000
0000
0000
0
2
noted that SIMIL1 was completely soluble in H O but SIMIL2 is spar-
ingly soluble in water. In this study, SIMILs were utilized in the syn-
thesis of mesoporous ZSM-5 in which they act as a structure
directing agent (SDA). Zeolites were designated as ZSM-5-SIMIL1
and ZSM-5-SIMIL2 when SIMIL1 and SIMIL2, respectively were used
as SDA.
Both SDA were successful in synthesizing mesoporous ZSM-5.
Mesoporous ZSM-5 had the MFI framework structure with high
phase purity, which was confirmed by using XRD (Fig. 1). The XRD
pattern of ZSM-5-SIMIL2 is broad in nature when compared with
the conventional ZSM-5 and ZSM-5-SIMIL1. This shows that the par-
ticle size of sample synthesized by using SIMIL2 is smaller in size than
ZSM-5-SIMIL1
40
50
60000
40000
2
0000
0
ZSM-5-SIMIL2
2
by using SIMIL1. The N -adsoption isotherm of ZSM-5-SIMIL2 showed
a typical type-IV isotherm with H1 hysteresis loop similar to the
mesoporous materials (Fig. 2). The major difference in the isotherms
of ZSM-5-SIMIL2 from that of conventional ZSM-5 is a distinct in-
40
50
crease of N
2
adsorption in the region 0.4bP/Pob0.85, which is
Fig. 1. XRD patterns of ZSM-5 samples synthesized in this study.

R. Kore, R. Srivastava / Catalysis Communications 18 (2012) 11–15
13
1
1
1
0
0
0
0
0
.4
.2
.0
.8
.6
.4
.2
.0
(
b)
(
a)
4
3
3
2
2
1
1
00
50
00
50
00
50
00
ZSM-5-SIMIL2
ZSM-5
ZSM-5-SIMIL2
ZSM-5
ZSM-5-SIMIL1
5
0
0
ZSM-5-SIMIL1
0
10
20
30
40
50
0
.0
0.2
0.4
0.6
0.8
1.0
Pore size (nm)
Relative pressure (p/p0)
2
Fig. 2. (a) N -adsorption isotherms and (b) pore size distribution of ZSM-5 samples synthesized in this study.
ZSM-5 and ZSM-5-SIMIL1, two well-resolved symmetric peaks were
observed. They dominate with maximum at 480 K and 650 K for
ZSM-5 and 465 K and 670 K for ZSM-5-SIMIL1. These peaks demon-
strate the presence of weak acid sites and strong acid sites. In ZSM-
contain only three hydrolysable moieties (with one hydrophobic
alkyl imidazole group), which is disadvantage for the formation of ex-
tended tetrahedral SiO linkages. Consequently, the zeolite growth
2
might be significantly retarded at the organic and inorganic inter-
faces, resulting in the formation of intercrystalline mesoporous
ZSM-5. 13C MAS NMR of ZSM-5-SIMIL2 (Fig. 5) and FT-IR of as-
synthesized samples (Fig. S2, supporting information) confirm the in-
corporation of SIMILs in the as-synthesized form of zeolites.
5
-SIMIL2, the first peak becomes smaller and less well-resolved and
the maxima of second peak shifts to slightly higher temperature
690 K). This indicates that the acid sites in ZSM-5-SIMIL2 are stron-
(
ger than the acid sites present in ZSM-5-SIMIL1/ZSM-5.
It is true that when conventional mesoporous structure directing
agent such as cetyltrimethylammonium bromide (CTMABr) is used
along with TPABr, it is difficult for the mesopore structure director
to participate in this zeolite crystallization [14]. During the crystalli-
zation process using such soft templates, phase separation between
surfactant and zeolite crystals took place and resulted in the forma-
tion of amorphous mesoporous material, bulk zeolite without
mesoporosity, or their physical mixtures. Hence, a suitable SDA is re-
quired that is capable of having a strong interaction with the growing
crystal surface, which can effectively modulate the crystallization
process by participating in it. The present synthesis strategy utilizes
SIMILs for the generation of intercrystalline mesoporosity in ZSM-5.
In ZSM-5-SIMILs, zeolite crystallization was mediated by the nano-
scale segregation of organic-rich and organic-lean domains on the
growing zeolite particles and intercrystalline mesopores are formed
due to crystal packing of these growing zeolite particles. SIMILs
The catalytic activities of mesoporous ZSM-5 samples synthesized
in this study were compared with conventional ZSM-5 for some im-
portant reactions such as alkylation and cracking. In the present
work, zeolites were tested for the alkylation of toluene with benzyl
chloride to obtain diphenylmethanes, which are important chemicals
used in fine chemicals and pharmaceutical industries (Scheme 2). The
C–C chain branching makes the cracking reaction more difficult in
branch polyethylene. The cracking of polyethylene requires strong
acid sites [20,21]. Catalytic cracking of HDPE (MW≈125000) was
chosen to evaluate the efficiency of these catalysts. Catalytic cracking
of HDPE produces a large variety of products such as alkanes, isoalk-
anes, alkenes, aromatics such as benzene derivatives, naphthalene
derivatives, etc. [22]. Following order was obtained for the catalytic
activity of these catalysts in the alkylation and cracking reactions:
ZSM-5bZSM-5-SIMIL1bZSM-5-SIMIL2 (Tables
2 and 3). ZSM-5
exhibited the lowest activity whereas ZSM-5-SIMIL2 exhibited the
Table 1
Textural properties of ZSM-5 samples synthesized in this work.
Si/Ala
S
BET (m /g)
2
Ext. surface area
Total pore volume
(ml/g)
Mesopore volume (ml/g)
Total acidity
(mmol/g)
Ratio of strong/weak
acid sitesb
Catalyst
2
b
(m /g)
ZSM-5
ZSM-5-SIMIL1
ZSM-5-SIMIL2
19.5
24.3
26.4
330
462
565
63
202
307
0.18
0.40
0.64
0.08
0.27
0.50
0.27
0.22
0.20
1.06
1.17
1.67
a
Obtained from ICP analysis.
Obtained from NH -TPD.
3
b

14
R. Kore, R. Srivastava / Catalysis Communications 18 (2012) 11–15
Fig. 3. SEM (a–c) and TEM (d–f) images of ZSM-5 samples synthesized in this study.
8
7
4
5
(
iii) ZSM-5-SIMIL2
O
9
11
13
15
17
19
O
O
2
C22
C10-C19
C2,C8
(
(
ii) ZSM-5-SIMIL1
i) ZSM-5
Si
N
N
10
12
14
16
18
20
1
3
Cl
6
N
Br
2
2
21
2
3
(iii)
4
65
6
70
C7
60
C6
C9
40
(ii)
2
00 180 160 140 120 100
80
20
0
Fig. 5. 13C MAS NMR of the as-synthesized ZSM-5-SIMIL2.
(
i)
highest activity. Although, the activity of ZSM-5-SIMIL1 was found to
be low but its activity was much higher than the ZSM-5. The acidity of
mesoporous ZSM-5 samples was less than the acidity of ZSM-5
(Table 1), but they were found to be more active than the ZSM-5.
High activity of these materials is due to the presence of acid sites
located on the large external surface. Since the sizes of reactant
4
00
500
600
700
800
900
Temperature (K)
3
Fig. 4. NH -TPD profiles of ZSM-5 samples investigated in this study.
Scheme 2. Alkylation of toluene with benzyl chloride using ZSM-5 catalysts investigated in this study.

R. Kore, R. Srivastava / Catalysis Communications 18 (2012) 11–15
15
Table 2
catalytic activity of mesoporous ZSM-5 in these reactions is due to
the combined effect of accessible strong acid sites and large external
surface area.
Comparison of the catalytic activity of various catalysts in the alkylation of toluene with
benzyl chloride.
Catalyst
Conv. with respect
to benzyl chloride
Selectivity (%)
Mono-alkylation
Di-alkylation
(
mol.%)
Acknowledgments
ZSM-5
ZSM-5-SIMIL1
ZSM-5-SIMIL2
0.9
30.5
64.7
100
91
87
–
9
13
Authors thank the Department of Science and Technology, New
Delhi for financial assistance (SR/S1/PC/31/2009). Authors thank
SAIF IIT Bombay for TEM analysis and SIF, Indian Institute of Science
Bangalore for NMR analysis of solid samples.
Reaction conditions: benzyl chloride (5 mmol); toluene (50 mmol); catalyst (100 mg);
temperature (413 K); time (2 h).
Appendix A. Supplementary data
Table 3
Catalytic cracking of HDPE into volatile products using ZSM-5 samples synthesized in
this study.
Supplementary data to this article can be found online at doi:10.
016/j.catcom.2011.11.012.
1
Catalyst
Reaction time (min)
Conversion of HDPE (%)a
ZSM-5
ZSM-5
ZSM-5-SIMIL1
ZSM-5-SIMIL1
ZSM-5-SIMIL2
ZSM-5-SIMIL2
30
60
30
60
30
60
6.5
9.7
38.0
65.4
63.8
100
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