Synthesis, characterization and catalytic properties of monodispersed nano-sphere silica sulfuric acid

Article abstract of DOI:10.1016/j.molcata.2013.01.010

In this research, the first report on synthesis of nano-sphere silica sulfuric acid (NS-SSA) as new catalyst was reported. The catalyst was prepared by the reaction of nano-sphere silica with chlorosulfonic acid at room temperature. The catalyst has been identified using various techniques (XRD, SEM, TEM, EDX, TGA, FT-IR) and results were shown that it was a spherical shape and its particle size was between 60 and 90 nm. The catalyst can be easily recovered and reused for seventeen reaction cycles for protection of amines without considerable loss of activity. It was found that the catalyst can be efficiently in large scale and we examined some reactions in scales of 50 and 100 mmol. Also, this method has some advantages such as high TOF of catalyst, chemoselectivity, easy work-up and short reaction time.

Full text of DOI:10.1016/j.molcata.2013.01.010

Journal of Molecular Catalysis A: Chemical 370 (2013) 111–116
Contents lists available at SciVerse ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Synthesis, characterization and catalytic properties of monodispersed
nano-sphere silica sulfuric acid
M.A. Zolfigol , A. Khazaei , M. Mokhlesi , F. Derakhshan-Panaha
a,∗
a,∗
b,∗
a
Faculty of Chemistry, Bu-Ali Sina University, P.O. Box 6517838683, Hamedan, Iran
Fateh Sanat group Company, Nitelpars Co., Fateh Bldg., No32, Aftab St., Khodami St., Vanak Sq., PC: 1994964743, Tehran, Iran
b
a r t i c l e i n f o
a b s t r a c t
Article history:
In this research, the first report on synthesis of nano-sphere silica sulfuric acid (NS-SSA) as new catalyst
was reported. The catalyst was prepared by the reaction of nano-sphere silica with chlorosulfonic acid
at room temperature. The catalyst has been identified using various techniques (XRD, SEM, TEM, EDX,
TGA, FT-IR) and results were shown that it was a spherical shape and its particle size was between 60
and 90 nm. The catalyst can be easily recovered and reused for seventeen reaction cycles for protection of
amines without considerable loss of activity. It was found that the catalyst can be efficiently in large scale
and we examined some reactions in scales of 50 and 100 mmol. Also, this method has some advantages
such as high TOF of catalyst, chemoselectivity, easy work-up and short reaction time.
Received 5 November 2012
Received in revised form 14 January 2013
Accepted 15 January 2013
Available online 24 January 2013
Keywords:
Heterogeneous
Reusable
©
2013 Elsevier B.V. All rights reserved.
Large scale
Nano-sphere silica sulfuric acid
Protection of amines
1
. Introduction
found that this compound was shown a greater activity as com-
pared to other homogeneous. They used heterogeneous sulfonated
catalysts in acylation of aromatic compounds in the presence of
acetic anhydride as acylating agent [14]. Suzuki and his cowork-
ers prepared core/shell monodispersed mesoporous silica spheres
containing sulfonic acid groups in the core portion and adsorption
sites (hydrophobic organic functional groups) in the shell portion.
Mesoporous silicas with uniform size and shape have gained
considerable attention in scientific research due to their unique
properties such as high specific surface area, narrow pore size
distribution, large pore volumes, low density, good thermal and
chemical stability, and low toxicity. These kinds of materials are
widely applied in controlled drug delivery, catalysis, thermal insu-
lators, biomolecular separation and so on [1–7]. In the two last
decades, studies found that functionalization and modification of
mesopores silicas, made them suitable for future technical appli-
cations. Meanwhile, the functionalized mesoporous silicas that
incorporate organic and inorganic functional groups within their
mesopores are of great interest for use as novel heterogeneous cat-
alysts. For example, various mesoporous silica compounds such
as spheres, SBA, MCM and FSM that have been functionalized by
sulfonic acid groups offer simpler, less costly, more reactive and
more environmentally benign alternatives than their homogeneous
counterparts [8–12]. Shanks supported propylsulfonic and arene-
sulfonic groups on mesoporous silica in a one-step approach of
co-condensing inorganic–organic reagents. The resulting materi-
als were tested for their catalytic performance in the esterification
of fatty acid with methanol to produce methyl esters [13]. Melero
and co-workers prepared arenesulfonic functionalized SBA-15 and
In the obtained material, SO H-core/hydrophobic-shell has radi-
3
ally aligned hexagonal mesopores and display different chemical
properties in the core and shell portions. They investigated the
efficiency of heterogeneous sulfonated catalysts in condensation
reaction of 2-methylfuran and acetone [15]. In addition, other acidic
group such as perfluorosulfonic acids [16–18] has also been incor-
porated into mesoporous silicas to achieve higher catalytic activity.
However, there are many reports for the synthesis of heteroge-
neous catalysts for the chemical reactions; but most of them are
associated with some disadvantages. For example, these catalysts
contain a hydrocarbon tail between heterogeneous point and sul-
fonic acid functional group. Entering the hydrocarbon part into
bioenvironmental systems causes a lot of pollution which sepa-
ration or elimination of them are not economically at all [19,20].
Therefore, study for synthesis of new heterogeneous catalyst with
high recovery and low pollution is of interest to chemists. Based
on such a strategy and along the course of our investigations
into the development of new catalysts [21–23], we describe here
synthesis, characterization and application of nano-sphere silica
sulfuric acid (NS-SSA) as new heterogeneous catalyst for the first
time.
∗ Corresponding authors. Tel.: +98 811 8381601; fax: +98 811 8278207.
E-mail address: mmokhlesi313@gmail.com (M. Mokhlesi).
1
381-1169/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2013.01.010

1
12
M.A. Zolfigol et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 111–116
◦
2
. Experimental
the sample was degassed at 300 C for 3 h. The BET surface area
was calculated from a multipoint BET analysis of the nitrogen
adsorption isotherms. The X-ray powder diffraction (XRD) patterns
were recorded on a Philips X’Pert MPD Pro X-ray diffractometer
with graphite-monochromatized Cu Ka radiation (k = 0.1541 nm).
The TEM images were taken over a JEOL JEM-2000EX instrument
operated at 80 kV. The SEM analyses were done with a TES-
CAN/VEGA with a maximum acceleration voltage of the primary
electrons between 10 and 15 kV. Thermogravimetric analyses using
a Perkin-Elmer TGA were performed on both support materials
2
.1. Chemicals and catalyst synthesis
All chemicals such as amines, di-Boc, TEOS, ClSO H and ammo-
3
nium hydroxide were purchased from Merck Chemical Company.
All the other solvents and chemicals were obtained from commer-
cial sources and purified using standard methods. All compounds
were identified by comparison of their melting points and spectral
data with those reported in the literature.
(nano-sphere silica and nano-sphere silica sulfuric acid). To eval-
2
.1.1. Synthesis of nano-sphere silica
Different silica nanoparticles were prepared by adjusting the
uate the overall amount of surface hydroxyl groups available for
◦
anchoring reactions, the weight loss between 300 and 600 C was
◦
concentrations of ethanol:water:ammonium hydroxide. Ethanol,
water, TEOS, and ammonium hydroxide were mixed in molar ratio
7
milky with time. The silica particles were collected by centrifuga-
tion and washed with distillated water for three times [1].
determined. A heating rate of 10 C/min under argon was applied to
purge off-gasses from the TGA electronics and sample region. Semi-
quantitative EDX (Röntec, Quantax/QX2) analysis was used for the
characterization of element concentration and vanadium distribu-
tion within prepared catalysts. Fourier transform-infrared spectra
of the samples were recorded on a Perkin-Elmer FT-IR spectrometer
5:31:1:4 respectively and stirred for 3 h till the solution turned
1
7259 using KBr disks.
2.1.2. Synthesis of nano-sphere silica sulfuric acid (NS-SSA)
Nano-sphere silica (1.0 g) was dispersed in CH C12 (10 mL)
2
in a flask. Chlorosulfonic acid (0.638 g, 5.5 mmol) was dissolved
in CH C1 (10 mL) and added to the nano-sphere silica suspen-
3. Results and discussion
2
2
sion through a constant-pressure dropping funnel under stirring,
over a period of 30 min at suspension through a constant-pressure
dropping funnel under stirring, over a period of 30 min at room
temperature. After the addition was completed, the mixture was
stirred for another 30 min at room temperature. The pale brown
solid was collected by filtration and washed with methyl t-butyl
ether 50 mL. Finally it was dried at room temperature.
At first, nano-sphere silica was prepared from Si(OEt) , and then
4
was reacted with chlorosulfonic acid in CH Cl2 at room tempera-
2
ture to afford nano-sphere silica sulfuric acid (NS-SSA) (Scheme 1).
3
.1. Morphologies and physical properties of nano-sphere silica
and nano-sphere silica sulfuric acid
In the next step, nano-size and spherical structure of NS-SSA
was confirmed by transmission electron microscopy (TEM) (Fig. 1).
As Fig. 1 indicates, the sphere size of NS-SSA was 60–90 nm.
Moreover, the XRD peaks of the prepared samples of NSS and NS-
SSA showed the same positions with different intensities (Fig. 2).
The peak intensity of nano-sphere silica was decreased after func-
tionalization.
2
2
.2. Catalytic reactions
.2.1. General procedure for the protection of amines
An amine (5 mmol) was added to a magnetically stirred mixture
of NS-SSA (5 mg) and di-tert-butyl dicarbonate (5.5 mmol, 1.19 g)
at room temperature. After completion of the reaction (as moni-
tored by TLC), the reaction mixture was diluted with EtOH (5 mL)
and centrifuged for 3 minutes. Then, the clear liquid was separated,
and the residue containing the catalyst was kept for recovery. EtOH
was distillated off under vacuum to yield the highly pure N-Boc
derivative.
The nitrogen adsorption–desorption isotherms were then mea-
sured and the pore-size distributions, specific surface area and pore
volume were analyzed by the BJH method [15]. The pore sizes, the
specific surface areas, and the pore volumes of the samples that
were obtained are also listed in Table 1. Also, Fig. 3 shows the
isotherm and the pore size distribution for nano-sphere silica (S1)
and nano-sphere silica sulfuric acid (S3). The results shown, the
N2 adsorption–desorption isotherm of nano-sphere silica sample
2
.2.2. General procedure for the protection of amines in large
scale
A mixture of di-tert-butyl dicarbonate (100 mmol, 21.8 g), nano-
2
exhibited high surface area (1411 m /g). As is predictable, after the
sphere silica sulfuric acid (0.5 g) was stirred at room temperature
for 2 min. Then, an amine (100 mmol) was added. After completion
of reaction (monitored by TLC), the reaction mixture was diluted
with EtOH (100 mL) and centrifuged for 7 min. EtOH was removed
under vacuum to yield the corresponding N-Boc derivative.
loading of SO H group on nano-sphere silica caused a significant
decrease in surface area (65.55 m /g).
3
2
Thermal analysis of the samples gave information on the stabil-
ity of the SO H groups. In the TG/DTG curves of nano-sphere silica
3
◦
and nano-sphere silica sulfuric acid, the weight loss below 150 C is
due to the desorption of water. Decomposition of the sulfonic acid
◦
2.3. Characterization
groups took place at about 380 C (Fig. 4).
In the EDX spectrum of the as-synthesized sample, the peaks of
Si and S were obviously observed and no other impurities occurred
(Fig. 5). Finally, the acidity (the number of H ) of nano-sphere silica
N2 sorption measurement was performed on a NOVA-2020
material physical structure determinator. Before measurement,
+
OH
3
OSO H
HO
OH
HO
HO SO NS-SiO2 OSO
3
HO SO OSO H
3
SO
3
OSO H
H
2
O, EtOH
ClSO
CH Cl
3
H
Si(OEt)
4
HO NS-SiO2 OH
3
3
H
NH OH, r.t.
4
2
2
, r.t.
30 min
HO
OH
3
OH
Nano-sphere silica
NSS)
3
OSO H
Nano-sphere silica sulfuric acid
(NS-SSA)
(
Scheme 1. Synthesis of nano-sphere silica sulfuric acid (NS-SSA).

M.A. Zolfigol et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 111–116
113
Fig. 1. (A) TEM of nano-sphere silica, (B) TEM of nano-sphere silica sulfuric acid.
Table 1
Properties of nano-sphere silica before and after functionalization.
ClSO3H (mmol)a
Pore size (nm)
Pore volume (cm /g)
3
Specific surface area (m /g)
2
mmol H /g
+
Sample
S1
S2
S3
S4
–
3.98
8.54
3.37
4.47
1.406
0.077
0.061
0.109
1411
36.49
65.55
98.21
–
11
9.4
7.4
8.25
5.5
4.25
a
1
g of silica nano-sphere with certain mmol of ClSO3H.
The band at 800 cm 1 is assigned to symmetric vibrations (Si
−
O
Si)
and the band at 1011 cm is assigned to the asymmetric vibrations
of (Si Si) of nano-sphere silica sulfuric acid (Fig. 6). The band of
−
1
O
−1
the S O vibrations appears about 675 cm .The band at 1173 and
1
S
286 cm 1 are assigned to symmetric and asymmetric stretching of
−
O respectively. In the hydroxyl region, a very broad IR absorption
band centered at 3458 cm is observed, this is usually ascribed to
the OH groups (Fig. 6).
−
1
3.2. Acid catalytic activity of NS-SSA
After characterizations of nano-sphere silica sulfuric acid,
experiments were performed to optimize the reaction conditions
for protection of amines as a substrate in solvent-less medium
(Scheme 2).
Firstly, we tested the protection of 4-methoxyaniline using dif-
Fig. 2. XRD of nano-sphere silica (up) and XRD of nano-sphere silica sulfuric acid
down). (For interpretation of the references to color in this figure legend, the reader
is referred to the web version of the article.)
(
ferent samples of NS-SSA as catalyst at room temperature. As it is
shown in Table 2, the samples of S2 and S3 were shown the simi-
lar results and in the case of S4 the yield of product was reduced.
We also calculated turn over frequencies (TOF) states on the effec-
tiveness of the catalyst for the protection of amines (Table 2). The
sulfuric acid was determined by titration with NaOH. The amount
+
+
of H for various samples of NS-SSA was shown in Table 1.
TOF values showed that sample of S3 (0.01 mol% of H ) was more
The IR Spectra of nano-sphere silica sulfuric acid showed some
effective than S2 and S4. Based on data obtained, sample S3 was
chosen as the catalyst to be used in next studies of the protection
of amines.
−
1
1
peaks in region of 474, 675, 800, 1011, 1173, 1286 and 3458 cm
The band of the Si O rocking vibrations appears about 474 cm
.
.
−
Fig. 3. N2 adsorption–desorption isotherms (A) nano-sphere silica (S1) and (B) nano-sphere silica sulfuric acid (S3).

1
14
M.A. Zolfigol et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 111–116
Fig. 4. TG and DTG analysis of the nano-spheres silica (up) and nano-sphere sulfuric acid (down).
Table 2
Optimization of the reaction conditions.
a
Table 3
Entry
Sample
mmol of
amine
Catalyst
(mol%)
Time (min)/
(yield (%))b
TOF
(min
Conversion of the 4-OCH3-aniline to N-Boc derivative using various amount of sam-
ple S3.
−1
)
1
2
3
4
S1
S2
S3
S4
5
5
5
5
(0.005)c
0.011
0.009
100 (38)
10 (98)
10 (98)
15 (92)
–
Entry
Catalyst (g)
Time (min)
Yield (%)
890.9
1024
828.5
1
2
3
4
0.001
0.005
0.075
0.01
30
10
10
10
85
98
98
97
0.007
a
Reaction condition: 4-methoxyaniline (5 mmol), di-Boc (5.5 mmol), catalyst
.005 g, solvent-free.
0
b
Isolated yield.
c
0
.005 g.

M.A. Zolfigol et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 111–116
115
Fig. 5. EDS spectra of nano-sphere silica sulfuric acid (NS-SSA).
Fig. 6. FT-IR spectra of nano-sphere silica sulfuric acid (NS-SSA).
NS-SSA (0.005 g, 0.01 mol% of H+)
O
O
MeO
NH2
MeO
NH
di-Boc, Solvent free, R.T
Scheme 2. Protection of 4-methoxyaniline using NS-SSA under solvent free conditions.
Then, we optimized the amount of catalyst and the best conver-
sion of the amine to N-tert-butoxycarbonyl derivative was observed
using 0.005 g of the catalyst. Moreover, increasing the catalyst
amount did not improve the yield and the reaction time (Table 3,
entry 2).
Table 4
The protection of amines with di-Boc catalyzed by nano-sphere silica sulfuric acid
at room temperature.
a
To assess the generality and efficacy of the catalyst, various
amines were reacted with di-Boc to afford the corresponding N-
Boc derivatives in high to excellent yields within relatively short
reaction times (Table 4).
As it is shown in Table 4, NS-SSA was highly efficient for the
protection of all aromatic amines bearing electron-releasing and
electron-withdrawing substituent such as halogens on their aro-
matic rings. Also, to show the chemoselectivity of the described
system, we used the catalyst for the protection of 4-hydroxyaniline
Entry
Substrate
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
9
4-Methoxyaniline
3-Methylaniline
4-Methylaniline
2-Methylaniline
4-Hydroxyaniline
4-Chloroaniline
Azepane
1,4-Phenylene-diamine
Piperazine
14,10,13-Tetraoxa-7,16-
diazacyclooctadecane
5
20
30
50
5
10
5
45
5
98
91
96
80
98
80
85
88
96
93
(
Table 4 entry 5, Scheme 3); which produced the corresponding N-
10
10
Boc derivative in good yield and hydroxyl protection did not occur.
Aliphatic amines also, underwent protection reaction with high
yield (Table 4, entries 7–10).
a
Amine (5 mmol), di-Boc (5.5 mmol), NS-SSA 0.005 g, solvent-free.
Isolated yield.
b

1
16
M.A. Zolfigol et al. / Journal of Molecular Catalysis A: Chemical 370 (2013) 111–116
NS-SSA (0.005 g, 0.01 mol% of H+)
di-Boc, Solvent free, R.T
O
O
O
O
HO
NH2
HO
NH
+
O
0
NH2
1
00%
%
Scheme 3. Chemoselective conversion of 4-OH-aniline to N-Boc derivative.
Fig. 7. The catalytic activity of NS-SSA in 17 cycles for the protection of 4-methoxyaniline.
Table 5
The large scale protection of amines using NS-SSA/di-Boc.
Appendix A. Supplementary data
Substrate
mmol of substrate
Time (min)
Yield (%)a
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.molcata.2013.01.010.
4-OMe-aniline
4-OMe-aniline
4-Me-aniline
4-Me-aniline
3-Me-aniline
3-Me-aniline
50
100
50
100
50
15
15
40
40
30
30
90
87
89
90
91
86
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In conclusion, we have shown that nano-sphere silica sulfuric
acid could be easily prepared from commercially available mate-
rials. This catalyst efficiently promoted the protection of various
amines to N-Boc derivatives at room temperature with excellent
yields. This method has some advantages such as applicable at large
scales, chemoselectivity, easy work-up, short reaction time, and
high TON and TOF of the catalyst.
[
[