Boehmite silica sulfuric acid: as a new acidic material and reusable heterogeneous nanocatalyst for the various organic oxidation reactions

Article abstract of DOI:10.1007/s13738-016-0937-4

In this research, project boehmite silica sulfuric acid (Boehmite-SSA) has been applied as new acidic porous catalyst for the selective oxidation of sulfides to sulfoxides and oxidative coupling of thiols to corresponding disulfides using hydrogen peroxide as oxidizing agent. The products have been obtained in short reaction times and high yields. Boehmite nanoparticles was prepared, coated by silica and then reacted with chlorosulfuric acid to obtain Boehmite-SSA. This catalyst was characterized by FT-IR, TGA/DTA, XRD, TEM, SEM, EDS and BET techniques. BET curve of boehmite nanoparticles identified as a typical type IV isotherm (definition by IUPAC), which are the characteristics of mesoporous material. Nitrogen adsorption/desorption measurement indicated that boehmite nanoparticles had BET surface area of about 122.8?m²/g. The catalyst was easily separated and reused for the several runs without significant loss of its catalytic efficiency.

Full text of DOI:10.1007/s13738-016-0937-4

J IRAN CHEM SOC
DOI 10.1007/s13738-016-0937-4
ORIGINAL PAPER
Boehmite silica sulfuric acid: as a new acidic material
and reusable heterogeneous nanocatalyst for the various organic
oxidation reactions
Arash Ghorbani‑Choghamarani1 · Maryam Hajjami1 · Bahman Tahmasbi1 ·
Nourolah Noori1
Received: 3 April 2016 / Accepted: 30 June 2016
© Iranian Chemical Society 2016
Abstract In this research, project boehmite silica sulfu-
ric acid (Boehmite-SSA) has been applied as new acidic
porous catalyst for the selective oxidation of sulfides to
sulfoxides and oxidative coupling of thiols to correspond-
ing disulfides using hydrogen peroxide as oxidizing agent.
The products have been obtained in short reaction times
and high yields. Boehmite nanoparticles was prepared,
coated by silica and then reacted with chlorosulfuric acid
to obtain Boehmite-SSA. This catalyst was characterized
by FT-IR, TGA/DTA, XRD, TEM, SEM, EDS and BET
techniques. BET curve of boehmite nanoparticles identi-
fied as a typical type IV isotherm (definition by IUPAC),
which are the characteristics of mesoporous material.
Nitrogen adsorption/desorption measurement indicated
that boehmite nanoparticles had BET surface area of about
122.8 m2/g. The catalyst was easily separated and reused
for the several runs without significant loss of its catalytic
efficiency.
structure of boehmite consists of double sheets of octa-
hedral with aluminum ions, and the sheets themselves
are composed of octahedral chains [2]. Recently, boe-
hmite nanoparticles have been used as an optical mate-
rial, absorbent, coatings, cosmetic products, composite
reinforcement material in ceramics, vaccine adjuvants,
pillared clays and sweep flocculation for fresh water
treatment [1, 3, 4]. Also boehmite was used as the pre-
cursor material for the synthesis of alumoxanes [4]. Boe-
hmite nanoparticles have several attractive features such
as non-toxic, easily and readily available, high surface
area (>120 m2/g), ease of surface modification, easy and
inexpensive procedure of preparation, favorable biocom-
patibility and thermal, chemical and mechanical stabil-
ity [5, 6]. Also the surface of boehmite nanoparticles
covered with hydroxyl groups, which existence of many
hydroxyl groups on the nanoboehmite surface provide
valuable sites that can be grafted with various functional
groups to produce expensive supported-catalysts [7, 8].
However, alumina has been used as catalyst support [9–
12] but modification of boehmite nanoparticles has been
rarely reported as heterogeneous support in the recover-
ing and recycling of expensive homogeneous catalysts
[13]. Therefore, owing to the context of reuse of cata-
lyst and green chemistry, herein boehmite-SSA as a new
nano heterogeneous catalyst has been reported for the
selective oxidation of sulfides to sulfoxides and oxida-
tive coupling of thiols to corresponding disulfides using
hydrogen peroxide (with only H2O as a by-product) as
an inexpensive and environmental oxidant. The oxidation
of sulfides to sulfoxides and oxidative coupling of thiols
into corresponding disulfides are useful in the synthesis
of new molecules, as well as for various medical, chemi-
cal, biological, materials and nanotechnological applica-
tions [14–23].
Keywords Boehmite nanoparticles · Sulfoxide · Disulfide ·
Silica sulfuric acid · Oxidation
Introduction
Boehmite nanoparticle is a cubic orthorhombic struc-
ture of aluminum oxide hydroxide which can prepared
in water using commercial available materials [1]. The
* Arash Ghorbani-Choghamarani
arashghch58@yahoo.com
1
Department of Chemistry, Faculty of Science, Ilam
University, P.O. Box 69315516, Ilam, Iran
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J IRAN CHEM SOC
The obtained boehmite-silica (0.5 g) was dispersed in
dry n-hexane (5 mL) by ultrasonic bath for 30 min. Subse-
quently, chlorosulfuric acid (0.75 mL) was added drop wise
over a period of 30 min and the mixture was stirred for 4 h
at room temperature. Then, the final product was filtered
and washed by dry ethanol and n-hexane, respectively, to
remove the unattached substrates. The product (boehmite-
SSA) dried at room temperature and stored in a refrigerator.
Experimental
Materials
Chemicals and solvents used in this work were obtained
from Sigma-Aldrich, Fluka or Merck chemical companies
and used without further purification. Boehmite-SSA was
characterized using a Holland Philips X’pert X-ray powder
diffraction (XRD) diffractometer, at a scanning speed of
2°/min from 2° to 80°. The particle size and morphology
were investigated by a FESEM-TESCAN MIRA3 scanning
electron microscopy (SEM), on an accelerating voltage of
25.0 kV and also using Zeiss-EM10C transmission electron
microscope (TEM). IR spectra were recorded as KBr pel-
lets on a VRTEX 70 model BRUKER FT-IR spectropho-
tometer. Thermogravimetric analyses (TGA) of the samples
were recorded using a Shimadzu PL-STA 1500 device in
the temperature range 30–800 °C. Nitrogen adsorption/
desorption isotherms were recorded at 120 °C with a
Brunauer–Emmett–Teller Analysis Adsorption BELSORP
Mini II.
General procedure for the oxidation of sulfides
A mixture of sulfide (1 mmol), H2O2 (0.4 mL) and boe-
hmite-SSA (0.003 g) in ethyl acetate (2 mL) was stirred at
room temperature (the progress of the reaction was moni-
tored by TLC). After completion of the reaction, catalyst
was separated using filtration and washed with ethyl ace-
tate, and the product was extracted with ethyl acetate. The
organic layer was dried over anhydrous Na2SO4 (1.5 g).
Finally, ethyl acetate was evaporated, and product obtained
in good to high yield.
General procedure for the oxidative coupling of thiols
Preparation of catalyst
Boehmite-SSA (0.002 g) was added to a mixture of thiol
(1 mmol) and H2O2 (0.4 mL) in ethyl acetate (2 mL). Then,
the mixture was stirred for the appropriate time at room
temperature. The progress of reaction was monitored by
TLC. After completion of the reaction, the catalyst was
separated by filtration and the mixture was washed with
ethyl acetate. The product was extracted with ethyl ace-
tate. The organic layer was dried over anhydrous Na2SO4
(1.5 g). The products were obtained in good to high yield.
The solutions of NaOH (6.490 g) in 50 mL distilled water
were added to the solutions of Al(NO3)3·9H2O (20 g) in
30 mL distilled water drop wise under vigorous stirring.
The resulting milky mixture was subjected to mixing in the
ultrasonic bath for 3 h at 25 °C. The resulted nanoboehmite
was filtered and washed by distilled water and was kept in
the oven at 220 °C for 4 h.
The obtained boehmite nanoparticles (1 g) were dis-
persed in water (10 mL) and ethanol (50 mL) by sonica-
tion for 30 min. Under continuous stirring, PEG (5.36 g),
ammonia solution (10 mL) and TEOS (2 mL) were, respec-
tively, added into the suspension and continuously reacted
for 38 h at room temperature. Then, the product (boehmite-
silica) was filtered and washed with ethanol and distilled
water; the obtained boehmite-silica was dried at room
temperature.
Results and discussion
In continuation of our studies on the application of modi-
fied boehmite nanoparticles in organic reactions [3, 6],
herein, we have prepare report the boehmite silica sulfuric
acid (boehmite-SSA) as a high efficient, nanoparticles and
Scheme 1 Synthesis of
boehmite-SSA
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Fig. 1 SEM image of boehmite (a), boehmite-silica (b) and boehmite-SSA (c)
reusable heterogeneous catalyst for some organic oxida-
tion reactions such as oxidation of sulfides into sulfoxides
and oxidative coupling of thiols into their corresponding
disulfides using H2O2 as oxidant at room temperature.
The Boehmite-SSA was prepared by the concise route
that has been outlined in Scheme 1.
analysis (TGA), transmission electron microscope (TEM),
powder X-ray diffraction (XRD), scanning electron micros-
copy (SEM), energy-dispersive X-ray spectroscopy (EDS)
and N2 adsorption–desorption isotherms.
The morphological and size of the catalyst was evalu-
ated by scanning electron microscopy and transmission
electron microscope. The unit cells of boehmite were gen-
erated in regular geometric shape (Fig. 1a). Also, the SEM
Boehmite nanoparticles and boehmite-SSA have been
characterized by FT-IR spectroscopy, thermogravimetric
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In order to extend the scope of characterization of this
catalyst, a typical EDS spectrum prepared from boehmite-
SSA that is shown in Fig. 3. As it can be seen from this
figure, EDS spectrum shows the presence of O, Al, Si and S
species in the catalyst (Fig. 3).
The TGA was used to determine the percentage of chem-
isorbed silica sulfuric acid onto boehmite nanoparticles.
The TGA curve of boehmite nanoparticles (black curve),
boehmite-silica (pink curve) and boehmite-SSA (green
curve) is shown in Fig. 4a. In the TGA curves, physically
adsorbed water mass loss occurred at temperatures below
250 °C as well as the endothermic weight loss at 300 °C
is attributed to water loss from structural hydroxyl groups
in the precursor [24]. In TGA/DTA curve of the boehmite-
SSA, a weight loss of about 41 % from 200 to 800 °C is
observed (Fig. 4b). Meanwhile, weight loss about 16 and
20 % from 200 to 800 °C is occurred for boehmite nanopar-
ticles and boehmite-silica, respectively. These results were
shown that silica sulfuric acid groups have been grafted on
the surface of boehmite nanoparticles.
Fig. 2 TEM image of boehmite-SSA
The XRD patterns of boehmite nanoparticles, boehmite-
silica and boehmite-SSA are shown in Fig. 5. As it can be
seen from Fig. 5, the boehmite phase was characterized by
the peak positions at 14.40 (0 2 0), 28.41 (1 2 0), 38.55
(0 3 1), 46.45 (1 3 1), 49.55 (0 5 1), 51.94 (2 0 0), 56.02
(1 5 1), 59.35 (0 8 0), 65.04 (2 3 1), 65.56 (0 0 2), 68.09
(1 7 1) and 72.38 (2 5 1) from the XRD patterns, which
all the peaks can be confirmed the crystallization of boe-
hmite with an orthorhombic unit cell [7, 13]. The results of
XRD patterns of boehmite-silica and boehmite-SSA reveal
that the surface modification of the boehmite nanoparticles
does not lead to their phase change. Also, the broad peaks
around 2θ = 20–30 in the XRD patterns of boehmite-silica
and boehmite-SSA are typical for silica and can’t be seen
in boehmite itself [28].
Fig. 3 EDS spectrum of boehmite-SSA
The FT-IR spectrum of boehmite-SSA shows several
peaks that are characteristic of a functionalized silica sul-
furic acid, which clearly differs from that bare boehmite
nanoparticles and boehmite-silica (Fig. 6). The FT-IR spec-
trum of the boehmite nanoparticles (Spectrum a) shows
two strong bands at 3086 and 3308 cm−1, which are due
to the both symmetrical and asymmetrical modes of the
O–H bonds on the surface of boehmite nanoparticles [25,
26]. In the FT-IR spectrum of boehmite nanoparticles
(Spectrum a–c), several peaks that appear at 480, 605 and
image of the boehmite-silica showed that particles have a
regular geometric shape in comparison with nanoboehmite
(Fig. 1b). As shown in Fig. 1c, the catalyst was formed
of nanometer-sized particles (20–50 nm). Also, the size
and morphology of boehmite-SSA was studied by TEM
technique. As shown in Fig. 2, the unit cells of boehmite-
SSA were obtained in regular geometric shape with about
20–40 nm.
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J IRAN CHEM SOC
Fig. 4 a TGA diagram of boehmite (black line), boehmite-silica (pink line) and boehmite-SSA (green line). b TGA/DTA analysis of boehmite-
SSA
Fig. 5 XRD patterns of boehmite nanoparticles (black line), boe-
hmite-silica (red line) and boehmite-SSA (green line)
735 cm−1 can be related to the absorption of Al–O bonds
[2, 6]. Also, the nitrate impurity vibration at 1650 cm−1 and
the vibrations of hydrogen bands OH···OH by two strong
absorption bands at 1164 and 1069 cm−1 were observed in
FT-IR spectrum [2, 4, 6]. In the 1072 and 770 cm−1 spec-
tral region of the FT-IR spectra (Figure b and c), an overlap
of the stretching vibration of the Si–O–Si bonds with Al–O
and OH···OH stretching vibration leads to band broading
Fig. 6 FT-IR spectra of a boehmite, b boehmite-silica and c boe-
hmite-SSA
[27–29]. In the FT-IR spectra of boehmite-SSA (Spectrum
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J IRAN CHEM SOC
Table 1 Optimization of the reaction conditions for the oxidation of
sulfides (1 mmol) in the presence of different amounts of boehmite-
SSA in various solvents using H2O2 (0.4 mL)
Entry Sulfide
Solvent
Cat. (gr) Time (min) Yield (%)a
1
2
3
4
5
6
7
Methyl
phenyl
sulfide
Ethyl
acetate
–
90
90
60
50
60
60
60
Traceb
90
Methyl
phenyl
sulfide
Ethyl
acetate
0.002
0.003
0.005
0.003
0.003
Methyl
phenyl
sulfide
Ethyl
acetate
98
Methyl
phenyl
sulfide
Ethyl
acetate
98
Methyl
phenyl
sulfide
Acetone
Ethanol
97
Methyl
phenyl
sulfide
60
Fig. 7 Nitrogen adsorption–desorption isotherms of boehmite nano-
particles
Methyl
phenyl
sulfide
Solvent free 0.003
40
a
Isolated yields
b
The reaction in the absence of catalyst
Scheme 2 Boehmite-SSA catalyzed the oxidation of sulfides into
sulfoxides
the products were obtained in high to excellent yields. The
result of this study is summarized in Tables 1 and 2.
The possible mechanistic path for the oxidation of
sulfides is shown in Scheme 3 [31]; one explanation for this
transformation is the in situ formation of peroxy acid using
the reaction of boehmite-SSA with H2O2, followed by
the oxygen transfer to the organic substrate (Scheme 3a).
Another explanation is that boehmite-SSA acts as protic
acid, which polarizes the O–O bond in hydrogen peroxide
to produce the reactive oxygen transfer agent (Scheme 3b).
As a second part of our organic program toward the
development of new methods for the coupling of thiols,
herein we described the oxidative coupling of thiols into
corresponding disulfides in the presence of boehmite-SSA
using hydrogen peroxide (Scheme 4). In order to optimize
reaction conditions, we examined the oxidative coupling
of 2-mercaptobenzoxazole as a model compound using
H2O2 (0.4 mL) in various solvents and in the presence of
different amount of boehmite-SSA (Table 3). As shown in
c), the existence of the grafted silica sulfuric acid groups is
confirmed by SO3–H groups vibrations that appear at 938–
1220 and 3000–3400 cm−1 [30].
The nitrogen adsorption–desorption isotherms of boe-
hmite nanoparticles are shown in Fig. 7. Based on the
IUPAC classification, these materials display a typical type
IV isotherm, which is characteristic of mesoporous materi-
als. Based on Brunauer–Emmett–Teller (BET), surface area
for boehmite nanoparticles is 122.8 m2/g.
As a first part of our organic program, herein we
described the oxidation of sulfides into sulfoxides in
the present of boehmite-SSA using hydrogen peroxide
(Scheme 2).
In obtained optimum condition, the various sulfides were
converted to their corresponding sulfoxides in the presence
of boehmite-SSA as catalyst at room temperature. All of
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J IRAN CHEM SOC
Table 2 Oxidation of sulfides into sulfoxides using H2O2 in the presence of boehmite-SSA at room temperature in ethyl acetate
Entry
sulfide
Product
Time (min) Isolated yield (%)
1
50
98
2
3
10
20
97
96
4
5
25
35
97
98
6
7
75
95
95
180
8
9
130
120
96
94
Table 3, in the oxidative coupling of 2-mercaptobenzoxa-
zole, the best results were obtained in ethyl acetate in the
presence 0.002 g of boehmite-SSA at room temperature
(Table 3, entry 2).
On the basis of the literatures, a plausible reaction
mechanism for oxidative coupling of thiols in the pres-
ence of boehmite-SSA is shown in Scheme 5 [32]. Reac-
tion of H2O2 with boehmite-SSA leads to the intermediate
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J IRAN CHEM SOC
Scheme 3 Proposed mecha-
nism for the oxidation of
sulfides in the presence of
boehmite-SSA as catalyst
Scheme 4 Boehmite-SSA catalyzed the oxidative coupling of thiols
into disulfides
Table 3 Optimization of the reaction conditions for the oxidative
coupling of thiols (1 mmol) in the presence of different amounts of
boehmite-SSA in various solvents using H2O2 (0.4 mL)
Entry Sulfide
Solvent
Cat. (gr) Time (min) Yield (%)a
1
2
3
4
5
6
7
2-Mer-
captoben-
zoxazole
2-Mer-
captoben-
zoxazole
Ethyl
acetate
–
90
Traceb
97
Scheme 5 Proposed mechanism for oxidative coupling of thiols in
the presence of boehmite-SSA using H2O2
Ethyl
acetate
0.002
0.003
0.005
60
A which is active oxidant. Next, nucleophilic attack of the
thiol on this intermediate gives intermediate B which, in
turn, produces the corresponding products using reaction
with thiol (Table 4).
Because these heterogeneous oxidation systems were
performed in mild condition, overoxidation for oxidation
of sulfides or oxidative coupling of thiols was not observed
(Scheme 6).
The recyclability of boehmite-SSA was examined for
the oxidation of methyl phenyl sulfide and 4-methyl thio-
phenol in the presence of boehmite-SSA under described
conditions. The catalyst has been reused up to 5 runs with-
out any significant loss of its catalytic activity, and the
average isolated yield was obtained in 96 % and 95, for the
sulfoxide and disulfide products, respectively (Fig. 8).
In conclusion, boehmite silica sulfuric acid as new
porous material has been introduced and an efficient and
2-Mer-
captoben-
zoxazole
Ethyl
acetate
55
97
2-Mer-
captoben-
zoxazole
Ethyl
acetate
40
98
2-Mer-
captoben-
zoxazole
Acetonitrile 0.002
120
105
360
96
2-Mer-
captoben-
zoxazole
Ethanol
0.002
95
2-Mer-
captoben-
zoxazole
Solvent free 0.002
96
a
Isolated yields
b
The reaction in the absence of catalyst
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Table 4 Oxidative coupling of thiols into disulfides using hydrogen peroxide in the presence of boehmite-SSA at room temperature in ethyl
acetate
Entry
1
Thiol
Product
Time (min) Isolated yield (%)
50
10
98
96
2
3
160
25
96
98
4
5
75
45
97
94
6
7
8
9
20
90
40
95
96
97
environmentally friendly procedure has been developed for
the oxidation of sulfides to sulfoxides and oxidative cou-
pling of thiols into their corresponding disulfides in the
presence of this heterogeneous catalyst. This catalyst was
characterized by FT-IR, XRD, BET, TEM, SEM, EDS
and TGA. Furthermore, the catalyst could be isolated with
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Scheme 6 Oxidation of
sulfides and oxidative cou-
pling of thiols in the presence
of boehmite-SSA using H
without overoxidation
2O2
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Acknowledgments Financial support for this work by the research
affairs of Ilam University, Ilam, Iran is gratefully acknowledged.
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