Exceptional performance of sulfonic acid-incorporated-MCM-41 mesoporous materials prepared using a silane containing polysulfide linkages in the acetylation of anisole

Article abstract of DOI:10.1039/b708770e

Sulfonic acid-incorporated-MCM-41 mesoporous materials prepared using a silane containing tetrasulfide linkages showed exceptional yields of the acetylated product in the acetylation of anisole due to their high content of strong acid sites. The Royal Soc

Full text of DOI:10.1039/b708770e

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Exceptional performance of sulfonic acid-incorporated-MCM-41
mesoporous materials prepared using a silane containing polysulfide
linkages in the acetylation of anisole{
OZoon Kwon, SeMin Park and Gon Seo*
Received (in Cambridge, UK) 11th June 2007, Accepted 5th July 2007
First published as an Advance Article on the web 2nd August 2007
DOI: 10.1039/b708770e
Sulfonic acid-incorporated-MCM-41 mesoporous materials
prepared using a silane containing tetrasulfide linkages showed
exceptional yields of the acetylated product in the acetylation of
anisole due to their high content of strong acid sites.
the mole percentage of silicon atoms from TESPT in the mixture.
We prepared various synthetic mixtures having values of x in the
range of 0–80 to examine the effect of TESPT content on the
regularity of the obtained mesoporous materials. MPTES was also
employed as a sulfur-containing silane to compare its role as a
reagent for introducing sulfonic acid groups to that of TESPT.
The hydrothermal reaction of the synthetic mixtures was carried
out at 90 uC for 48 h, followed by washing and filtering. The
treatment of the filtered cakes with a mixture of hydrochloric acid
(2 N) and methanol removed the surfactant molecules from the
mesopores. The oxidation of the sulfide groups exposed on the
Sulfonic acid has been widely employed as a homogeneous catalyst
in various organic syntheses because of its high activity and
selectivity, in spite of the severe difficulties involved in separating it
from the products and disposing of it after use. The addition of
mercaptopropyltriethoxysilane (MPTES) to synthetic mixtures of
mesoporous materials is an ordinarily used method of introducing
mercapto groups on their pore walls, which can then be converted
pore walls with Br –HCl, followed by washing with methanol,
2
into sulfonic acid (–SO H) groups through oxidation. The
3
produced MCM-41 mesoporous materials incorporating sulfonic
incorporation of sulfonic acid groups on porous silica supports
produces highly convenient solid acid catalysts exhibiting the
acid groups. The mesoporous materials that were prepared are
named as TESPT-x-MCM and MPTES-x-MCM: TESPT and
MPTES denoted the sulfur containing silanes used in the
preparation.
1–6
advantages of homogeneous catalysts.
However, too large a
content of MPTES in the synthetic mixtures lowers the regularity
of the mesopores, and thus there is a strict limitation on the
Fig. 1 shows the XRD patterns of the MCM-41 mesoporous
materials incorporating sulfonic acid groups. The MCM-41
prepared from the synthetic mixture without a sulfur-containing
silane showed a sharp diffraction peak which was attributed to the
presence of highly ordered mesopores. The addition of TESPT up
to x = 10 did not lower the intensity of the diffraction peak, but
further increasing the content of TESPT caused a gradual decrease
in the intensity of the diffraction peak. The TESPT-50-MCM
catalyst obtained from the synthetic mixture with a large amount
of TESPT showed a very small diffraction peak, indicating that
too high a content of TESPT deteriorated the regularity of the
mesopores. However, the diffraction peaks of TESPT-x-MCM
catalysts were considerably higher than those of the corresponding
MPTES-x-MCM catalysts prepared from the synthetic mixtures
containing the same amount of MPTES. The TESPT-10-MCM
1,3
concentration of acid sites on mesoporous materials. Otherwise,
the addition of fluorocarbon surfactant to the synthetic mixture is
essential for the preparation of ordered mesoporous materials
6
containing a high content of sulfonic acid groups.
In this study, we prepared sulfonic acid-incorporated-MCM-41
mesoporous materials using the silane bis(3-triethoxysilylpropyl)-
tetrasulfide (TESPT), which contains polysulfide linkages. The low
miscibility of the polysulfide linkages with silica sources expels
them to the outside of the mesopores, thereby suppressing the
negative contribution of the silane in the synthesis of mesoporous
materials. The conversion of polysulfide linkages to sulfonic acid
2
groups using Br –HCl inhibits the oxidation of organic chains,
producing highly convenient solid acid catalysts with a high
7
concentration of acid sites.
Sulfonic acid groups incorporated on MCM-41 mesoporous
materials were prepared following Scheme 1. A part of
tetraethylorthosilicate (TEOS) was substituted by TESPT.
Synthetic mixtures containing cetyltrimethylammonium bromide
(C16TABr) were prepared following the procedure described in the
8
literature. Their compositions were: C16TABr : TEOS : TESPT :
NH OH : H O = 120 : 100 2 x : x : 800 : 11 400, where x denotes
4
2
School of Applied Chemical Engineering, Chonnam National University,
YongBong 300, Gwangju 500-757, Republic of Korea.
E-mail: gseo@chonnam.ac.kr; Fax: +82 62 530 1899;
Tel: +82 62 530 1876
{ Electronic supplementary information (ESI) available: Nitrogen adsorp-
13 29
tion–desorption isotherms, C and Si NMR spectra. See DOI: 10.1039/
Scheme 1 Incorporation of –SO H into MCM-41 mesoporous materials
3
b708770e
using TESPT.
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Chem. Commun., 2007, 4113–4115 | 4113

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nitrogen adsorption isotherms when x did not exceed 30. The size
of mesopores on the TESPT-x-MCM and MPTES-x-MCM were
in the range of 2.5–3.2 nm, regardless of the content and species of
silane.
The chemical state of TESPT was not changed, even though it
was incorporated into the mesoporous materials. Fig. S2 (see
1
3
ESI{) shows C NMR spectra of TESPT-20-MCM recorded
before and after the oxidation of the polysulfide linkages with Br2–
HCl. These spectra very much resemble those obtained during the
incorporation of sulfonic acid groups on mesoporous materials
1
using disulfide linkages. The complete disappearance of the peak
at d = 28.72 ppm showed the generation of sulfonic acid groups
from polysulfide linkages.
The sulfonic acid groups incorporated on MCM-41 adsorbed
ammonia and produced ammonium ions on them. Fig. 3 shows
the IR spectra recorded during the desorption of ammonia from
the TESPT-20-MCM and MPTES-20-MCM catalysts. The strong
2
1
absorption band at 1446 cm was attributed to ammonium ions
9
produced on the sulfonic acid groups. The desorption of
ammonia at elevated temperatures regenerated the absorption
2
1
band at 1377 cm attributed to sulfonic acid groups. Ammonia
molecules adsorbed on sulfonic acid groups incorporated on the
TESPT-20-MCM catalyst were retained at 200 uC even under
evacuation, indicating the presence of strong acid sites. The
MPTES-20-MCM catalyst showed similar behavior regarding the
Fig. 1 XRD patterns of (A) TESPT-x-MCM and (B) MPTES-x-MCM
catalysts.
21
desorption of ammonia, while the band at 1446 cm
was
showed a very high diffraction peak, but even the MPTES-10-
MCM catalyst showed an extraordinarily small diffraction peak,
indicating the poor regularities of mesopores in MPTES-x-MCM
catalysts.
relatively small. Furthermore, the desorption of ammonia from the
MPTES-20-MCM catalyst started at 70 uC, and this temperature
was considerably lower compared to the start temperature of
21
120 uC for the TESPT-20-MCM. The bands at 1446 cm in the
case of the TESPT-x-MCM, which appeared after the adsorption
of ammonia, were consistently larger than those on the
The regularities of mesopores on the TESPT-30-MCM and
MPTES-10-MCM catalysts were compared from their TEM
photos in Fig. 2. The TESPT-30-MCM with a large content of
TESPT still showed highly ordered mesopores, but ordered
mesopores were observed in the case of the MPTES-10-MCM
catalyst. The regular arrangement of mesopores was not observed
on the MPTES-30-MCM catalyst. Even though the addition of
TESPT to the synthetic mixtures lowered the regularity of the
mesopores, the mesoporous materials prepared using TESPT
showed considerably higher regularity than those prepared using
MPTES.
2
9
corresponding MPTES-x-MCM catalysts. Si MAS NMR
spectra of the TESPT-30-MCM and MPTES-30-MCM also
supported these results. As shown in Fig. S3 (see ESI{), the T
peak of the TESPT-30-MCM was higher than that of the MPTES-
30-MCM catalyst. This means that TESPT is more effective than
MPTES in introducing sulfonic acid groups to MCM-41.
A high content of mesopores was strongly required to achieve
high catalytic activities, because the large surface areas of the
sulfonic acid-incorporated-MCM-41 were indispensable to incor-
porate large numbers of sulfonic acid groups on them. The
amounts of sulfonic acid groups on the TESPT-x-MCM and
MPTES-x-MCM catalysts were also limited by the amounts of
TESPT and MPTES that were added, respectively. Therefore,
large amounts of acid sites could be obtained on these catalysts
The high regularity of the mesoporous materials resulted in
large amounts of nitrogen adsorption on them. As shown in Fig. S1
(see ESI{), the mesoporous materials prepared using either TESPT
or MPTES showed clearly the presence of mesopores on their
Fig. 2 TEM images of (A) TESPT-10-MCM, (B) TESPT-30-MCM, and
Fig. 3 Desorption of ammonia from (A) TESPT-20-MCM and (B)
(C) MPTES-10-MCM catalysts.
MPTES-20-MCM catalysts.
4
114 | Chem. Commun., 2007, 4113–4115
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Table 1 Surface areas and amounts of acid sites of the TESPT-
x-MCM and MPTES-x-MCM catalysts
Table 2 Acetylation of anisole with acetic anhydride over TESPT-
x-MCM and MPTES-x-MCM catalysts at 90 uC for 1 h
Catalyst TESPT-x-MCM
Catalyst MPTES-x-MCM
Catalyst TESPT-x-MCM
Conversion (%) Yield (%)
Catalyst MPTES-x-MCM
Conversion (%) Yield (%)
x
Surface
area/m g
Amount of acid
sites/mmol g
x
Surface
area/m g
Amount of acid
sites/mmol g
x
x
2
21
21
2
21
21
1
0
62
62
71
98
81
73
66
10 31
20 50
30 52
50 68
31
49
51
65
1
2
3
5
6
8
0
970
960
740
420
213
24
2.64
3.08
3.32
5.63
1.52
1.50
10 905
20 860
30 749
50 347
0.80
1.45
1.61
2.01
20 71
30 98
50 88
60 75
80 67
0
0
0
0
0
A large amount of sulfonic acid groups with strong strength is
essential to obtain high conversion and yield in the acetylation of
anisole. The TESPT-x-MCM catalysts prepared using TESPT
showed large surface areas and large amounts of acid sites
compared to the corresponding MPTES-x-MCM catalysts.
Although MPTES is a well-known material used for the
introduction of sulfonic acid groups into mesoporous materials,
TESPT, having tetrasulfide linkages, was more effective than
MPTES, without inducing a significant loss in the regularity of the
mesopores. The acid strength of the sulfonic acid groups in the
TESPT-30-MCM catalyst was suitable to achieve an exceptional
yield of the acetylation product. The 97.5% yield of the acetylated
product obtained from anisole on the TESPT-30-MCM catalyst
has not previously been reported even in the case of Nafion-
when the mesoporous materials prepared from the mixture of large
amounts of TESPT or MPTES showed high regularity in the
mesopores.
The amounts of active acid sites on the sulfonic acid-
incorporated-MCM were determined from the decrease in pH of
their aqueous slurries caused by the addition NaCl solution. The
exchange of protons of the sulfonic acid groups with sodium ions
was responsible for the decrease in pH. Table 1 lists the surface
areas and amounts of acid sites in the sulfonic acid-incorporated-
MCM catalysts. Increasing the amount of TESPT in the synthetic
mixtures decreased the surface area of TESPT-x-MCM catalysts,
while increasing the amounts of acid sites up to the TESPT-50-
MCM catalyst. The same trends were also observed in MPTES-
x-MCM catalysts, but their amounts of acid sites were
considerably lower than those of TESPT-x-MCM catalysts.
The acetylation of anisole with acetic anhydride (eqn (1)) is a
typical acid-catalyzed reaction. Various homogeneous and hetero-
geneous acids were reported to be active for the acetylation, but
their activities and selectivities vary remarkably according to the
5
15
16
incorporated solid acid, sulfated zirconia, and zeolite H-Beta.
Notes and references
1
2
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3
4
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AlCl and FeCl showed a high yield of the acetylated product of
3
3
more than 90%. However, the generation of harmful chlorine gas
and a large amount of waste water lower the feasibility of these
1
2
5 M. Alvaro, A. Corma, D. Das, V. Fornes and H. Garcia, J. Catal.,
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2
1
6
Y.-F. Feng, X.-Y. Yang, Y. Di, Y.-C. Du, Y.-L. Zjang and F.-S. Xiao,
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mesoporous materials introduced extremely strong acid sites on
them. Too strong acid sites, however, accelerated the formation of
adducts between the ketone and anisole molecules, and thus, the
7
8
H. T. Clarke, Org. Synth., 1940, 20, 23, (Org. Synth. Coll., 1955, 3, 226).
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402, 867–871.
5
yield of the acetylated product did not exceed 65%.
9 Each self-supported wafer of the catalyst was made of 10 mg. The wafer
was evacuated at 200 uC for 2 h prior to exposing it to ammonia of
50 Torr at 50 uC. The desorption of ammonia was started after
ð1Þ
removing gaseous and physically adsorbed ammonia by evacuation.
0 S. Shylesh, S. Sharma, S. P. Mirajkar and A. P. Singh, J. Mol. Catal. A:
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1
The performance of TESPT-x-MCM and MPTES-x-MCM as
catalysts in the acetylation of anisole was strongly dependent on
the type of silane used for the incorporation of sulfonic acid
groups, as shown in Table 2. The conversion on the TESPT-30-
MCM catalyst, which was defined as the percentage of anisole
1
1 M. Eissen and J. O. Metzger, Chem.–Eur. J., 2002, 8, 3581–3585.
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1
3 The p-methoxyacetophenone was prepared by acetylation of 10 mmol of
anisole with 11 mmol of acetic anhydride over 0.2 g of catalyst at 90 uC
for 1 h in 30 ml three-neck round-bottomed flask.
13,14
consumed, was as high as 98%.
The yield of the acetylated
14 Anisole, o- and p-methoxyacetophenone in the products were analysed
using a high performance liquid chromatograph (Agilent 1100 series):
product was also the highest (97.5%) over the TESPT-30-MCM
catalyst, due to its exceptional selectivity for the acetylation.
MPTES-x-MCM catalysts showed similar variations in the
conversion and yield with the amount of MPTES added, but the
conversions and yields were considerably lower than those
obtained using the corresponding TESPT-x-MCM catalysts.
1
Waters Symmetry column (150 mm 6 4.6 mm), mobile phase
(acetonitrile : water = 7 : 3), detection wavelength = 230 nm.
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1
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1
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Chem. Commun., 2007, 4113–4115 | 4115