Gold nanoparticles supported on cellulose aerogel as a new efficient catalyst for epoxidation of styrene

Article abstract of DOI:10.1007/s13738-017-1060-x

A new efficient heterogeneous catalyst was introduced for the epoxidation of styrene. The catalyst was obtained from deposition of gold nanoparticles on the cellulose aerogel. The catalyst was characterized with XRD, TGA, EDX, BET, FAAS and SEM. High yield and excellent selectivity were achieved for the epoxidation of styrene in solvent-free conditions at room temperature using H₂O₂ as a green oxidant during 1?h. The reaction has some advantages such as solvent-free and mild reaction conditions, low catalyst loading, high yield, excellent selectivity, green oxidant and short reaction duration. In addition, the catalyst is recyclable and applicable for six times without decrease in yield.

Full text of DOI:10.1007/s13738-017-1060-x

J IRAN CHEM SOC
DOI 10.1007/s13738-017-1060-x
ORIGINAL PAPER
Gold nanoparticles supported on cellulose aerogel as a new
efficient catalyst for epoxidation of styrene
Sajjad Keshipour · Masoumeh Khezerloo1
1
Received: 29 August 2016 / Accepted: 13 January 2017
©
Iranian Chemical Society 2017
Abstract A new efficient heterogeneous catalyst was
introduced for the epoxidation of styrene. The catalyst was
obtained from deposition of gold nanoparticles on the cel-
lulose aerogel. The catalyst was characterized with XRD,
TGA, EDX, BET, FAAS and SEM. High yield and excel-
lent selectivity were achieved for the epoxidation of sty-
rene in solvent-free conditions at room temperature using
H O as a green oxidant during 1 h. The reaction has some
advantages such as solvent-free and mild reaction condi-
tions, low catalyst loading, high yield, excellent selectivity,
green oxidant and short reaction duration. In addition, the
catalyst is recyclable and applicable for six times without
decrease in yield.
both high toughness and strength are needed. Organic aero-
gels based on cellulose are interesting due to their biologi-
cal and sustainable origins [4]. Cellulose is a carbohydrate
polymer made up of repeating β-d-glucopyranose units and
consists of many hydroxyl groups giving the cellulose mol-
ecule a high degree of functionality. This most abundant
natural biopolymer has the characteristic properties such
as hydrophilicity, chirality, biodegradability and high func-
tionality. Cellulose aerogels are generally less brittle, even
flexible and easily compressed without disintegration [4].
Epoxidation of olefins is an important process in a num-
ber of important organic transformation reactions. Epox-
ides are industrially interesting compounds since they have
applications in the synthesis of several perfume materials,
anthelmintic preparations, epoxy resins, plasticizers, drugs
and sweeteners [5]. One of the most interesting industrial
epoxides is the styrene oxide. Styrene oxide can be used for
producing epoxy resin diluting agent, flavoring agent, ultra-
violet absorbent, etc., and is also an important intermediate
in organic synthesis and pharmaceutical compounds syn-
thesis such as Flouxetine and Norflouxetine [6]. Therefore,
development of new routes for preparation of styrene oxide
by an easier and a low-cost approach is of great interest to
researchers working in this field.
2
2
Keywords Cellulose aerogel · Epoxidation · Styrene
oxide · Gold nanoparticles · Heterogeneous catalyst
Introduction
Aerogels are ultralightweight solid materials with an inter-
connected porous structure. The important characteristics
of aerogels are large specific surface area, high porosity
and low thermal conductivity [1–3]. Inorganic aerogels
such as silica aerogels have some commercial applications
in packaging, lightweight construction and structural insu-
lation. The intrinsic fragility of inorganic aerogels is the
drawback, which limits their practical applications where
Various styrene epoxidation approaches were
reported, and for most of them hydrogen peroxide is an
ideal oxidant from both environmental and economic
viewpoints. Hydrogen peroxide is a relatively less
expensive and environmentally safe and forms water
as the only by-product. The epoxidation of styrene can
be performed under strongly alkaline conditions with
H O as the oxidant [7]. However, the use of strong
*
Sajjad Keshipour
s.keshipour@urmia.ac.ir
2
2
bases is highly undesirable due to the production of
large amounts of industrial waste. Al O , MgO and CaO
are efficient metal oxide for the epoxidation of styrene
1
Department of Nanochemistry, Nanotechnology
Research Centre, Urmia University, G. C., Urmia,
P. O. Box 165-5715944931, Iran
2
3
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J IRAN CHEM SOC
with hydrogen peroxide as the oxidant [7–9]. Recently,
epoxidation of olefins with metals as the catalyst such as
gold nanoparticles has attracted a great deal of attentions
Experimental
Materials and methods
[
10]. A series of S-containing organic–inorganic hybrid
mesoporous silicas were used as supports for the fabri-
cation of supported gold nanoparticle [11]. The catalyst
showed high activity for epoxidation of styrene at 60 °C
using H O . Gold nanoparticles supported on Y O cata-
All reagents were purchased from Aldrich or Merck and
used without further purification. Scanning electron micros-
copy (SEM) and energy-dispersive X-ray spectroscopy
(EDS) were performed with scanning electron microscope
of TSCAN Company. X-ray powder diffraction (XRD) data
were collected on an XD-3A diffractometer using Cu Kα
radiation. Thermal gravimetric analysis (TGA) was per-
formed with Linseis STA PT1000. Au(0) determination
was carried out on an FAAS (Shimadzu model AA-680
atomic absorption spectrometer) with a hollow cathode
lamp at 242.8 nm, using an air-acetylene flame.
2
2
2
3
lyzed the epoxidation reaction of styrene selectively at
5–97 °C [12]. Gold nanoparticles deposited on amine-
7
functionalized nanoscale metal–organic framework
MIL-101(Cr) was introduced as an efficient recycla-
ble catalyst for epoxidation of styrene [13]. Numerous
methods for the epoxidation of olefins with gold nano-
particles were reported [10].
Cellulose and its derivatives can be used as a sup-
port since they are renewable, biodegradable and non-
toxic [14]. There are reports about cellulose derivatives
as the support for some important catalysts such as
Cu(0) [15], Pd(0) [16, 17], Cu(I)/Pd(0) [18] and Co(II)
Preparation of cellulose aerogel
A mixture of cellulose (4 g), urea (20 g), NaOH (3.8 g) and
H O (180 mL) was sonicated for 10 min. Then, cellulose
2
[
19, 20]. In continuation of our efforts to develop new
aerogel was obtained by freeze-drying the mixture for 12 h
catalytic systems with cellulose as the support [17–20],
herein a new efficient catalytic system was introduced
for the oxidation of styrene by gold nanoparticles sup-
ported on cellulose aerogel (Au@CA) as a heterogene-
ous recoverable catalyst with H O as a green oxidant
and washing with H O (3 × 10 mL).
2
Preparation of Au@CA
In a typical procedure, a mixture of H AuCl (1 mmol) and
2
2
3
4
in H O as a green solvent or solvent-free conditions
cellulose aerogel (2 g) in 10 mL of H O was stirred at room
2
2
where styrene oxide is obtained as the sole product.
Cellulose aerogel was used as the support due to high
porosity which can afford a high surface area for the
gold NPs.
temperature. After 24 h, a solution of NaBH (2 mmol,
5 mL) was added to the mixture during 1 h and stirring con-
tinued for 24 h. Finally, the catalyst was separated via fil-
4
tration, washed with H O (3 × 10 mL) and dried at 70 °C.
2
Fig. 1 XRD pattern of Au@CA
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J IRAN CHEM SOC
Typical procedure for the epoxidation of styrene
reaction vessel during 0.5 h. After 0.5 h, Au@CA was sep-
arated via filtration and washed with acetone (2 × 5 mL).
The filtrate solvent was evaporated under vacuum and the
product purified with column chromatography with n-hex-
ane–ethylacetate (5:1).
Styrene (0.10 g, 1.00 mmol) was added to a round-bot-
tomed flask containing colloidal of Au@CA (0.04 g) in
H O (5 mL). H O (3 mmol) was added dropwise to the
2
2
2
Results and discussion
For the preparation of the catalyst, H AuCl was stirred
3
4
with cellulose aerogel for 24 h, and then Au(I) was reduced
to Au NPs with NaBH . The catalyst (Au@CA) was pro-
4
duced as a purple powder.
The catalyst was characterized with Brunauer–Emmett–
Teller analysis (BET), X-ray diffraction pattern (XRD),
flame atomic absorption spectroscopy (FAAS), SEM,
energy-dispersive X-ray (EDX) and thermal gravimet-
ric analysis (TGA). The specific surface area of the aero-
gel (S ), calculated from the BET equation, is about
BET
2
−1
1
92 m g . The high surface area of cellulose aerogel can
be assisted to perform the oxidation reaction with low cata-
lyst loading.
The structure of Au@CA was analyzed by powder XRD
with characteristic diffraction peaks of (111), (200) and
(220) for gold NPs and (101), (021), (002) and (040) for
cellulose aerogel (Fig. 1). These peaks confirmed the depo-
sition of gold NPs on cellulose aerogel.
SEM image of the catalyst indicates deposition of rod-
like gold NPs on the support with maximum average size
Fig. 2 SEM image of Au@CA
Fig. 3 EDX microanalysis of
Au@CA
1
1
100
000
O Kα
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
AuMα
AuMα
C Kα
AuLα
AuLl
keV
0
10.00
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J IRAN CHEM SOC
Fig. 4 TGA spectra of Au@CA
Table 1 Optimization of
the reaction conditions for
oxidation of styrene
Yield (%)a
Entry
Catalyst amount (Pd mol%)
Solvent
Time (min)
Product
1
0.13
0.17
0.21
0.13
0.17
0.21
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
0.17
H O
60
60
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Styrene oxide
Acetophenone
Acetophenone
84
96
96
87
96
96
83
91
82
80
81
72
71
68
69
2
2
H O
2
3
H O
55
2
4
No solvent
No solvent
No solvent
60
5
60
6
60
7b
8b
9
H O
60
2
No solvent
60
EtOH–H O (1:1)
60
2
10
11
12
13
14
15
DMA
MeCN
EtOH
CH Cl
120
120
120
120
120
120
2
2
PhCH3
MeOH
Reaction conditions: styrene (1 mmol), catalyst, H O (3 mmol), solvent (5 mL)
2
2
a
Isolated yield
b
H O (2 mmol)
2
2
between 81 and 89 nm (Fig. 2). The color of gold NPs
depends on size and shapes of the NPs; for example, the
nanorods of gold with size about 80 nm have purple color
concentration obtained was 0.43 mmol Au per 1 g catalyst
using FAAS analysis with calibration curve prepared with
Au solution standards.
[
21]. The purple color of Au@CA approved the size and
TGA of the catalyst showed that the catalyst
has good thermal stability (dec > 232 °C) in air
(Fig. 4). Another mass loss for the catalyst happened
at 330 °C.
shape of the gold NPs obtained from SEM.
The catalyst surface was analyzed with EDX microanal-
yses, which indicates the presence of Au (Fig. 3). The Au
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J IRAN CHEM SOC
Fig. 5 FT-IR spectra for the
catalyst before and after the
reaction
The catalytic activity of Au@CA was evaluated in the
oxidation of styrene. So, the oxidation of styrene (1) with
Table 2 Successive trials by using recoverable Au@CA for epoxida-
tion of styreneª
H O was investigated for the optimization of reaction con-
Yield (%)b
2
2
Trial
Recovered catalyst (g)
ditions. It was found that 0.17 mol% Au (Table 1, entries
–6) in H O or solvent-free conditions with three equiva-
1
2
3
4
5
6
0.04
0.04
0.04
0.04
0.04
0.04
96
96
95
95
95
95
1
2
lents of H O at room temperature are the best reaction
2
2
conditions for the epoxidation of styrene. Oxidation of sty-
rene proceeded to give styrene oxide (2) as the sole product
with 96% yield in short reaction duration (1 h) without any
by-product such as acetophenone or acetaldehyde. This is a
controlled oxidation reaction of styrene since the formation
of acetophenone or acetaldehyde is probable in oxidation
a
Styrene (1 mmol), catalyst (0.04 g), H O (3 mmol), H O (5 mL),
2
2
2
r.t., 1 h
of styrene. After screening a variety of solvents, H O and
b
2
Isolated yield
solvent-free conditions were determined to be the best con-
ditions (Table 1, entries 9–15). The reaction needs 3 mmol
H O for obtaining high yields, and the yields decreased in
performed with an oxygen balloon where 86% yield was
obtained in long duration (24 h). In addition, the epoxi-
dation reaction for cyclohexene and hexene did not per-
form in the optimized reaction conditions for styrene
epoxidation.
Potential Au leaching into the reaction mixture was stud-
ied with FAAS analysis. For this purpose, sample was taken
through a syringe filter during the oxidation reaction of sty-
rene, the solvent was evaporated, and the residue was dis-
solved in HNO3. The analysis of these samples with FAAS
showed that the Au concentrations in the reaction solution
were less than the detection limit. This result indicates that
virtually no Au leached from the surface into the solution.
2
2
low amounts of H O in both H O and solvent-free condi-
2
2
2
tions (Table 1, entries 7 and 8). The reaction in EtOH gave
acetophenone as the by-product in 11% yield and deter-
mined with GC. For the reaction with toluene and metha-
nol, acetophenone was obtained as the product, interest-
ingly. Styrene oxide was obtained as the by-product of the
reaction in toluene and methanol solvents with 4 and 21%
GC yield, respectively. Room temperature is an important
factor for this reaction, which gave more interest to the
reaction.
The reaction was examined using oxygen as an oxi-
dant instead of H O . For this purpose, the reaction was
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J IRAN CHEM SOC
Table 3 Comparison of the results of Au@CA-catalyzed epoxidation of styrene with pervious reports
Entry Catalytic system Recyclable Solvent Time (h) Temp. (°C) Isolated yield (%) (I)
Selectivity
or GC yield (%) (G)
1
2
3
4
Fe O -CuO@meso-SiO
Yes
No
CH CN
7.5
14
80
100 (G)
98.5 (G)
86.4 (G)
96 (I)
93
3
4
2
3
Au@silica
CH CN
80
82.8
95.1
100
3
Ag@Fe O4
Yes
Yes
Toluene
0.25
1
Ref.
r.t.
3
Au@CA
H O or solvent-free
2
Also, we did not observe any change in IR spectrum for the
catalyst recovered from the reaction (Fig. 5).
Acknowledgements We gratefully acknowledge financial support
from the Research Council of Urmia University.
Recyclability of the Au@CA was examined in the epox-
idation of styrene in H O. After carrying out the reaction,
2
References
the catalyst was separated via filtration as a violet solid,
washed with EtOH (2 × 5 mL) and reused. Only minor
decreases in the reaction yield were observed after six
repetitive cycles for this reaction (Table 2).
1
2
.
.
H.D. Gesser, P.C. Goswami, Chem. Rev. 89, 765–788 (1989)
N. Husing, U. Schubert, Angew. Chem. Int. Ed. 37, 22–45 (1998)
3. A.C. Pierre, G.M. Pajonk, Chem. Rev. 102, 4243–4265 (2002)
4. Q.Y. Mi, S.R. Ma, J. Yu, J.S. He, J. Zhang, ACS Sustain. Chem.
Eng. 4, 656–660 (2016)
The results of our catalyst are compared with previous
reports about epoxidation of styrene with respect to their
recyclability, solvent, reaction duration, temperature, yield
and selectivity (Table 3). The catalytic activity of Au@CA
as the epoxidation catalyst was compared with Fe O -CuO
5
6
.
.
S.C. Laha, R. Kumar, J. Catal. 204, 64–70 (2001)
D. Mitchell, T.M. Koenig, Synth. Commun. 25, 1231–1238
(1995)
7. Q. Gu, D. Han, L. Shi, Q. Sun, J. Nat. Gas Chem. 21, 452–459
2012)
3
4
(
supported on mesoporous silica [22], gold nanoparticles
supported on silica [23] and Ag supported on Fe O [24].
8
.
D. Mandelli, M.C. Avan Vliet, R.A. Sheldon, U. Schuchardt,
3
4
Appl. Catal. A Gen. 219, 209–213 (2001)
Clean and mild reaction conditions, high yield and excel-
lent selectivity were obtained with Au@CA compared to
other catalysts represented in Table 3. The most important
factor for the epoxidation reaction is the reaction temper-
ature. The reaction was performed at room temperature
with Au@CA, successfully. The importance of this work
becomes clear when the results of styrene epoxidation with
Au@CA as the catalyst are compared with the results of
epoxidation reaction with Au@silica as the catalyst, which
has difference with Au@CA at the support. Mild and green
reaction conditions for Au@CA show that Au NPs sup-
ported on cellulose have high activity for the epoxidation
reaction of styrene. However, the epoxidation reaction of
styrene using Au@CA has advantages such as green sol-
vent, cheep oxidant, mild reaction conditions, high yield,
excellent selectivity and short reaction duration. Also, the
catalyst is recyclable for six times.
9. X.L. Bian, Q.M. Gu, L. Shi, Q. Sun, Chin. J. Catal. 32, 682–687
(2011)
1
0. C.D. Pina, E. Falletta, M. Rossi, Chem. Soc. Rev. 41, 350–369
2012)
(
1
1. Y. Jin, D. Zhuang, N. Yu, H. Zhao, Y. Ding, L. Qin, J. Liu, D.
Yin, H. Qiu, Z. Fu, D. Yin, Microporous Mesoporous Mater. 126,
159–165 (2009)
1
1
2. V.R. Choudhary, D.K. Dumbre, N.S. Patil, B.S. Uphade, S.K.
Bhargava, J. Catal. 300, 217–224 (2013)
3. M. Saikia, V. Kaichev, L. Saikia, RSC Adv. 6, 106856–106865
(2016)
14. Y. Habibi, L.A. Lucia, O.J. Rojas, Chem. Rev. 110, 3479–3500
2010)
(
1
1
5. K.R. Reddy, N.S. Kumar, Synlett 2246–2250 (2006)
6. C.M. Cirtiu, A.F. Dunlop-Brière, A. Moores, Green Chem. 13,
288–291 (2011)
17. S. Keshipour, S. Shojaei, A. Shaabani, Cellulose 20, 973–980
2013)
8. S. Keshipour, A. Shaabani, Appl. Organomet. Chem. 28, 116–
19 (2014)
(
1
1
19. A. Shaabani, S. Keshipour, M. Hamidzad, M. Seyyedhamzeh, J.
In conclusion, we have demonstrated that Au@CA is
an efficient catalyst for styrene epoxidation. High yield of
styrene oxide was selectively obtained in short reaction
duration in mild reaction conditions. Avoiding strong basic
moieties and high temperatures and application of a self-
controlling catalytic system which prevents from transfor-
mation of styrene oxide to acetophenone and acetaldehyde
Chem. Sci. 126, 111–115 (2014)
0. A. Shaabani, S. Keshipour, M. Hamidzad, S. Shaabani, J. Mol.
Catal. A Chem. 395, 494–499 (2014)
1. V. Sharma, K. Park, M. Srinivasarao, Mater. Sci. Eng. R Rep. 65,
1–38 (2009)
2
2
22. X. Zhang, G. Wang, M. Yang, Y. Luan, W. Dong, R. Dang, Catal.
Sci. Technol. 4, 3082–3089 (2014)
2
3. J. Liu, F. Wang, S. Qi, Z. Gu, G. Wu, New J. Chem. 37, 769–774
2013)
(
are characteristic features of this approach. H O as a green
24. D.H. Zhang, H.B. Li, G.D. Lia, J.S. Chen, Dalton Trans. 10527–
10533 (2009)
2
solvent and H O as a green oxidant are other advantages
2
2
of this report.
1
3