Preparation of Ga2O3 doped sulfonated Tin oxides as a highly active and recyclable heterogeneous solid acid catalyst for aldol reactions

Article abstract of DOI:10.1166/jnn.2019.16101

Ga₂O₃ doped sulfonated tin oxide catalysts were prepared via co-condensation method in ethanol solvent, followed by sulfonation and calcination. The samples were characterized by isothermal nitrogen adsorption/desorption, powder X-ray diffraction (XRD), thermal gravimetric analysis (TG), Raman spectra and DRIFT spectra. The number of acid sites on the catalysts was measured with the potentiometric titration of butyl amine. The results showed that the addition of small amounts of Ga₂O₃ to sulfonated tin oxide resulted in an enhanced acid site density, which makes Ga₂O₃ doped sulfonated tin oxide catalysts highly active for aldol reactions. The catalyst containing 1.5% Ga₂O₃ exhibited much higher activity than those of SO₂-4 /SnO₂, SO₂-4 /ZrO₂ and H₃PO₄ in aldol condensation of prenal and prenol for citral precursor, which is a important in fragrance industry. Besides the high activity, the catalyst also exhibited good recyclability, making 1.5% GST an efficient and promising solid catalyst for aldol reactions.

Full text of DOI:10.1166/jnn.2019.16101

Article
Journal of
Nanoscience and Nanotechnology
Vol. 19, 3658–3662, 2019
Copyright © 2019 American Scientific Publishers
All rights reserved
www.aspbs.com/jnn
Printed in the United States of America
Preparation of Ga2O3 Doped Sulfonated Tin Oxides as a
Highly Active and Recyclable Heterogeneous Solid
Acid Catalyst for Aldol Reactions
Dewei Zhai1ꢀ2ꢀ3, Changyan Cao1ꢀ2ꢀ∗, Bin Shao3, Dong Liu3, and Weiguo Song1ꢀ2ꢀ∗
1Beijing National Laboratory for Molecular Sciences, Laboratory of Molecular Nanostructures and Nanotechnology,
Chinese Academy of Sciences Research/Education Center for Excellence in Molecular Sciences Institute of Chemistry,
Chinese Academy of Sciences, Beijing 100190, China
2University of Chinese Academy of Sciences, Beijing 100049, China
3Xinchang Pharmaceutical Plant, Zhejiang Medicine Co., Ltd., Xinchang 312500, China
Ga2O3 doped sulfonated tin oxide catalysts were prepared via co-condensation method in ethanol
solvent, followed by sulfonation and calcination. The samples were characterized by isothermal
nitrogen adsorption/desorption, powder X-ray diffraction (XRD), thermal gravimetric analysis (TG),
Raman spectra and DRIFT spectra. The number of acid sites on the catalysts was measured with
the potentiometric titration of butyl amine. The results showed that the addition of small amounts
of Ga2O3 to sulfonated tin oxide resulted in an enhanced acid site density, which makes Ga2O3
IP: 84.54.57.236 On: Tue, 19 Mar 2019 14:23:44
doped sulfonated tin oxide catalysts highly active for aldol reactions. The catalyst containing 1.5%
Copyright: American Scientific Publishers
Ga2O3 exhibited much higher activity than those of SO2−/SnO2, SO42−/ZrO2 and H3PO4 in aldol
Delivered by Inge4nta
condensation of prenal and prenol for citral precursor, which is a important in fragrance industry.
Besides the high activity, the catalyst also exhibited good recyclability, making 1.5% GST an efficient
and promising solid catalyst for aldol reactions.
Keywords: Sulfated Tin Oxides, Ga2O3, Doped, Aldol Reaction, Citral Precursor.
1. INTRODUCTION
(SO24−/ZrO2ꢁ is one of the most studied acid catalysts.
Compared to SO42−/ZrO2, sulfated tin oxides (SO42−/SnO2ꢁ
usually have stronger acidity and exhibit higher catalytic
activity in some acid catalyzed reactions.11ꢀ12 However,
studies related to SO24−/SnO2 are very rare because of the
difficulty in preparation of this type of catalysts.10 Dur-
ing the preparation of SO24−/SnO2, stannic hydroxide gel
precursor was usually obtained as fine particles, result-
ing in difficulty in the filtration of precipitates. In addi-
tion, the acidity was relative low due to the low surface
area of SO24−/SnO2. Therefore, it is highly desirable to
develop an efficient route to synthesis of SO24−/SnO2 with
high surface area and abundant acid sites as well as facile
filtration.
In this study, we solve the above problems with
small amounts of Ga2O3 modified SO24−/SnO2, which
was prepared through a two-step chemical co-precipitation
method. The addition of a certain amount of Ga2O3
increased the specific surface of SO24−/SnO2 catalyst and
SO24− loading, thereby improved the catalytic activity.
Acid catalyzing reactions have widely used in producing
various important chemicals in industry. The most tra-
ditional used acid catalysts are H2SO4, H3PO4, HF and
AlCl3.1 However, the equipment corrosion and the need
for post neutralization are undesired with these catalysts,
as it will increase the operation units, additional waste and
cost. To avoid these problems, using solid heterogeneous
acid catalysts to replace these liquid acid catalysts have
been favored. Many different kinds of solid acid catalysts,
such as sulfonated carbonized sugar,2 Nafion acid resins,3ꢀ4
sulfated metal oxides,5ꢀ6 tungstated zirconia,7 have been
reported with high catalytic activities in quite a lot of
acid catalyzing reactions. Particularly, sulfonated metal
oxides have received wide attention due to their high sur-
face acidity and catalytic activity in many acid reactions
such as isomerization, esterification and alkylation.8–10
Among these sulfonated metal oxides, sulfonated zirconia
∗Authors to whom correspondence should be addressed.
3658
J. Nanosci. Nanotechnol. 2019, Vol. 19, No. 6
1533-4880/2019/19/3658/005
doi:10.1166/jnn.2019.16101

Zhai et al.
Preparation of Ga2O3 Doped Sulfonated Tin Oxides
When used as the solid acid catalyst for aldol condensation
of prenal and prenol, 1.5% Ga2O3-doping SO24−/SnO2
showed the highest catalytic performance, and much
higher than those of SO24−/SnO2, SO42−/ZrO2 and H3PO4.
After the reaction, the filtered catalyst can be used repeat-
edly over 5 times without obvious deactivation. Further-
more, the reaction solution without further processing can
be directly used on the next reaction run. It not only greatly
improved the production efficiency, but also reduced envi-
ronmental pollution.
LabRam micro-spectrometer equipped with a 8 mW Ar
ion laser as excitation source (ꢃ = 514ꢄ5 nm). The spec-
tral range 0–1200 cm−1 was studied. GRAMS/32 software
(Galactic Inc. Salem NH) was used to analyze all spec-
tra. Diffuse reflectance FTIR spectra characterization is
recorded on Bruker Vector 22 infrared spectrometer.
2.3. Reaction Studies
Aldol condensation for preparing citral precursor was car-
ried out at 75 ꢀC in a three necked round bottom flask fitted
with a magnetic stirrer, a thermometer, a reflux condenser
and a distillation column (with the number of theoretical
plates of 8–12). A mixture of 100 g of prenal, 250 g of
prenol, 40 g toluene and 1.5 g of catalyst was added to the
flask. The concentrations of the reaction species were peri-
odically determined by a gas chromatography equipped
with a 30 m HP-5 capillary column and a FID on Agilent
Technologies 7890A.
2. EXPERIMENTAL DETAILS
2.1. Preparation of Catalysts
The SO24−/SnO2 catalyst was prepared by a two-step chem-
ical co-precipitation method. In the first step, 35.06 g
SnCl4 · 5H2O was dissolved in 200 mL water at room
temperature; Then the solution was dispersed in 200 mL
ethanol. Then 25–28% aqueous ammonia was added drop-
wise slowly with stirring and the final pH of the solution
was adjusted to 8. The precipitate was filtered and sub-
sequently washed with 50 wt% aqueous ethanol until all
chloride was eliminated. The precipitate was collected by
filtration and dried at 393 K for 12 h. In the second step,
the obtained tin oxide powder (2.0 g) was placed in a glass
flask with 25 mL of 3 mol · L−1 H2SO4; After stirred for
0.5 h, the precipitate was filtered by suction, dried at 393 K
for 12 h, and finally calcined at 775 K in air for 3 h for
obtaining SO24−/SnO (denoted as ST).
3. RESULTS AND DISCUSSION
The SO24−/SnO2 (denoted as ST) and Ga2O3-doped
SO24−/SnO2 (denoted as GST) catalysts were prepared
through a two-step chemical co-precipitation method. The
main difference of our method with traditional synthesis
route is that 50 wt% aqueous ethanol was used instead of
pure water in the preparation of precursors. It could effec-
IP: 84.54.57.236 On: Tue, 19 Mar 2019 14:23:44
tively avoid the formation of stannic hydroxide gel precur-
sor, which is usually obtained as fine particles, resulting in
hard filtration of precipitates. The possible reason is that
the presence of ethanol can prevent stannic hydroxide gel
dissociation and become fine particles.
The prepared catalysts were first characterized by XRD
to determine the crystal structures. Figure 1 shows the
XRD patterns of SO42−/SnO2 (ST) and x-GST (x = 1ꢄ0ꢀ
1ꢄ5ꢀ2ꢄ0 and 3.0%) samples. The diffraction peaks of all
samples can be ascribed to tetragonal phase of crystalline
SnO2.14 There was no characteristic peak of Ga2O3, imply-
ing that Ga2O3 were highly dispersed in sulfonated tin
Copyright: American Scientific Publishers
2
Ga2O3 (GST) doped SO24−/SnO2 were also prepared
by chemical co-precipitation method. In the first step,
Ga(OH)3–Sn(OH)4 was obtained by adding 25–28% aque-
ous ammonia slowly into an aqueous solution of SnCl4 ·
5H2O and Ga(NO3ꢁ3 ·6H2O. The second step was the same
as the SO24−/SnO2 (ST) catalyst. The obtained catalysts
marked X% GST, X% represented the mol content of
Ga2O3 in Ga2O3–SO24−/SnO2 (GST).
Delivered by Ingenta
2.2. Characterizations
X-ray diffraction (XRD) patterns were recorded on a
Rigaku D/MAX-II A (30 kV, 40 mA), using nickel filtered
Cu Kꢂ radiation with wavelength of ꢃ = 1ꢄ5406 Å. The
scan speed was 6ꢀ/min in the range from 10–80ꢀ. Thermal
gravimetrical analysis (TG) was performed using a Rigaku
Thermoflex System with a heating rate 10 K/min from
3.0%GST
2.0%GST
ꢀ
room temperature to 1050 C in the air atmosphere. Spe-
cific surface areas were determined through the Brunauer-
Emmet-Teller (BET) method. The samples were submitted
1.5%GST
1.0%GST
ꢀ
to physical N2 adsorption/desorption assays at −196 C
in the ASAP 2000 equipment. The catalysts were previ-
ꢀ
ously activated at 350 C under 1ꢄ3 × 10−2 Pa. Potentio-
ST
metric titration was adopted to measure the total acidity
of the catalysts.13 The sample (0.05 g) was suspended in
10 mL acetonitrile and agitated for 3 h. The suspension
was titrated at 0.05 mL/min with 0.1 N n-butyl amine
in acetonitrile. The Raman spectra were carried out in a
15
30
45
60
75
2θ/(º)
Figure 1. XRD patterns of the samples.
J. Nanosci. Nanotechnol. 19, 3658–3662, 2019
3659

Preparation of Ga2O3 Doped Sulfonated Tin Oxides
Zhai et al.
oxides. Furthermore, it was found that the width of ST
peaks became broader after doping Ga2O3, suggesting the
sizes of ST diminished.
The diffuse reflectance FTIR spectra characterization
was then carried out to confirm the existence of sulfonated
species on the catalysts. Taken ST and 1.5% GST cata-
lysts for comparison, their DRFTIR spectra were shown
in Figure 2. It can be clearly seen that four peaks at
1040 cm−1, 1160 cm−1, 1310 cm−1 and 1350 cm−1 at
the range of 1000–1400 cm−1, which can be ascribed to
the absorption peaks of the vibration bidentate ligands
of Sn4+ ions with SO24−. Furthermore, the peaks inten-
sity became stronger after doping Ga2O3 into ST, indicat-
ing that a small amount of Ga2O3 doping could help the
sulfonated tin oxides to increase the sulfur sites on the
surface.15 Raman spectrum also supported this conclusion.
As shown in Figure 3, peaks near 998 and 1034 cm−1 were
the characteristics of S–O and S O stretching modes of
hydrated sulfate groups.16 The intensity of the S–O and
1.5%GST
ST
200
400
600
800
1000
1200
Raman shift (cm–1
)
Figure 3. Raman patterns of the samples.
list in Table I. It can be seen that 1.5% GST had the
highest amounts of sulfate of about 10.7 wt%, which was
higher than that in ST (8.1 wt%). It may be speculated
that the small amount of Ga2O3 could enhance the inter-
action between sulfate species and SnO2 and further raise
the amount of sulfate species. Moreover, the more sulfate
species may be associated with the higher catalytic activity
for sulfonated metal oxides.17
S
O bands vibration increased after doping Ga2O3 in sul-
fated tin oxide, suggesting higher sulfate content in the
GST compared with that in ST. All these results suggest
that sulfate was successfully immobilized on the surface
of ST and GST, and the amounts of sulfate increased after
Ga2O3 doping. In Raman spectra, other three peaks at 474,
684 and 776 cm−1 were corresponding to the Eg, A1g and
B2g vibration modes of tetragonal rutile SnO2 structure.
The surface acidity and acidic number of the catalysts
was further investigated in detail through Potentiometric
titration method with n-butyl amine.18ꢀ19 In general, E >
This result is agreement with XRDIP. I:n8o4t.h5e4r.5w7o.r2d3s,6GOanO: Tue, 19 Mar 2019 14:23:44
2
3
100 mV corresponding super acid sites, 0 < E < 100 mV
Copyright: American Scientific Publishers
doping did not change the structure of the bulk SnO2.
Delivered by Ingenta
correspond to strong acid sites, while the weak acid sites
Thermogravimetric analysis (TG) was further used to
determine the contents of sulfate in the catalysts. As shown
in Figure 4, two weight-loss stages were observed in the
TG curves: the first step below 200 ꢀC was due to the des-
orption of water, the second step between 500 and 900 ꢀC
was attributed to the decomposition of sulfate. Accord-
ing to the weight-loss curves at the second step, the con-
tent of sulfate species in all samples was calculated and
expressed with −100 < E < 0 mV and E < −100 mV
correspond to very weak sites. The potentiometric titra-
tion curves of the catalysts were depicted in Figure 5.
And the number of acid sites were determined accord-
ing to the curves, and were listed in Table I. The number
of super acid sites, strong acid sites and total acid sites
of GST series samples are 0.154–0.279, 0.027–0.051 and
0.203–0.415 mmol/g, respectively, which are obviously
higher than those of ST (0.094, 0.015 and 0.143 mmol/g,
respectively). As the doping content of Ga2O3 increased,
1040
1160
1310
1350
100
95
1620
1.5%GST
90
ST
0.5% GST
1.0% GST
1.5% GST
ST
85
2.0% GST
3.0% GST
SZ
80
1750
1500
1250
1000
0
400
800
Wavenumber (cm–1
)
Temperature (degree)
Figure 2. DRIFT spectra of ST and 1.5% GST catalysts.
Figure 4. TG curves of the catalysts calcined at 500 ꢀC.
J. Nanosci. Nanotechnol. 19, 3658–3662, 2019
3660

Zhai et al.
Preparation of Ga2O3 Doped Sulfonated Tin Oxides
Number of acid sites (mmol/g)
Table I. Structural parameters of various sulfated tin oxides.
Samples
SBET (m2/g)
W(SO24−) (%)b
E > 100 mV
0 mV< E < 100 mV
−100 mV< E < 0 mV
Total
SZa
ST
1.0% GST
1.5% GST
2.0% GST
3.0% GST
102
145
173
184
179
174
3ꢄ7
8ꢄ1
9ꢄ8
10ꢄ7
10ꢄ3
9ꢄ5
0ꢄ021
0ꢄ094
0ꢄ178
0ꢄ279
0ꢄ252
0ꢄ154
0ꢄ007
0ꢄ015
0ꢄ027
0ꢄ051
0ꢄ039
0ꢄ031
0ꢄ019
0ꢄ034
0ꢄ022
0ꢄ085
0ꢄ044
0ꢄ078
0ꢄ047
0ꢄ143
0ꢄ223
0ꢄ415
0ꢄ335
0ꢄ263
Notes: aSZ was calcined at 650 ꢀC for 3 h and all sulfated tin oxides samples were calcined at 500 ꢀC for 3 h. bSulfate content were calcined from TG curves.
the number of strong acid sites and total acid sites first
increased and then decreased. 1.5% GST possessed the
highest amount of acid sites, which was in line with the
fact this sample has the biggest surface area and sulfate
content (Table I). The surface areas of catalysts were ana-
lyzed by N2 adsorption–desorption method. As shown in
Table I, the BET surface areas of GST series catalysts are
162–184 m2/g, which were evidently higher than that of
ST (145 m2/g).
Citral is one of the key intermediate chemical stock
material for a variety of high valued chemicals in phar-
maceutical and fragrance industries, including vitamin A,
ionone, methyl ionone, citronellol, nerol and geraniol
etc.20–23 The global annual output is around 40 thousand
tons. Synthesis of citral is mainly from the aldol conden-
51.3% and 48.4%, respectively. ST gave the conversion of
68.7% under the same conditions, which was higher than
those of SZ and H3PO4. Notably, GST exhibited higher
activity than ST, indicating that doping small amounts of
Ga2O3 into sulfated tin oxide could markedly improve the
catalytic activity for the aldol condensation. It was also
observed that the GST catalysts showed a volcano pattern
regarding the doping content of Ga2O3ꢀ i.e., the conver-
sion increased to a maximum then declined with increas-
ing doping content of Ga2O3. This transformation trend
agrees well with the trend of number of very strong and
strong acid sites with increasing doping content of Ga2O3.
Notably, 1.5% GST sample has the highest activity with
conversion of 86.7% after reaction for 10 h. The highest
activity of 1.5% GST can be ascribed to the high surface
sation reaction between prenal and prenol, followed by
IP: 84.54.57.236 On: Tue,a1r9eaM, saurlf2a0te19sp1e4ci:e2s3:c4o4ntent, very strong and strong acid
24–27Copyright: American Scientific Publishers
decomposition and rearrangement.
In the condensation
sites (Table I).
Delivered by Ingenta
reaction of prenal and prenol, the traditional method is
to use liquid acid catalysts with medium acidity, such as
phosphoric acid.27 Therefore, it is also highly desirable
to improve this process with solid acid catalyst. With the
above GST heterogeneous solid acid catalysts, we first test
their activity for aldol condensation in preparing citral pre-
cursor. Figuꢀre 6 shows the curves of conversion versus
time at 75 C on various catalysts. Only 1.8% conver-
sion after 10 h was observed from aldol condensation of
prenal with prenol in blank test without catalyst. Tradi-
tional SZ and homogeneous H3PO4 gave conversions of
The recycling property is important for GST heteroge-
neous catalyst. After each cycle of reaction, 1.5% GST
catalyst was separated by centrifugation to decant reaction
mixtures, washed several times with methanol to remove
the adsorbed reactants and products. The catalyst was then
ꢀ
dried at about 50 C and reused in a new reaction cycle
O
OH
+ 2H2O
CHO + 2
O
0.5% GST
1.0% GST
1.5% GST
2.0% GST
3.0% GST
ST
80
60
40
20
0
600
ST
400
SZ
H3PO4
1.0% GST
1.5% GST
200
0
2.0% GST
2.5% GST
0
2
4
6
8
10
–200
0.0
0.2
0.4
0.6
0.8
1.0
Time (h)
mmol n-Butylamine/g Catalyst
Figure 6. Prenal conversion depends on reaction time over various
Figure 5. Potentiometric titration curves of the samples.
J. Nanosci. Nanotechnol. 19, 3658–3662, 2019
catalysts.
3661

Preparation of Ga2O3 Doped Sulfonated Tin Oxides
Zhai et al.
100
Cooperation Team and the Youth Innovation Promotion
Association of CAS (2017049).
80
60
40
20
0
References and Notes
1. A. Corma, Chem. Rev. 95, 559 (1995).
2. K. Nakajima, M. Hara, and S. Hayashi, J. Am. Ceram. Soc. 90, 3725
(2007).
3. D. E. Lopez, J. G. Goodwin, D. A. Bruce, and E. Lotero, Appl.
Catal. A Gen. 295, 97 (2005).
4. D. E. Lopez, J. G. Goodwin, and D. A. Bruce, J. Catal. 245, 381
(2007).
5. K. Suwannakarn, E. Lotero, J. G. Goodwin, and C. Lu, J. Catal.
255, 279 (2008).
6. X. R. Chen, Y. H. Ju, and C. Y. Mou, J. Phys. Chem. C 111, 18731
(2007).
7. S. Furuta, H. Matsuhashi, and K. Arata, Catal. Comm. 5, 721
(2004).
1st
2nd
3rd
4th
5th
Figure 7. Catalyst reusability for the aldol condensation of prenal with
prenol at 60 ꢀC (reaction time = 10 h).
8. W. Stichert, F. Schuth, S. Kuba, and H. Knozinger, J. Catal. 198, 277
(2001).
without any further treatment. As shown in Figure 7, only
a slight drop of activity was observed after 5 cycles. The
total acid sites of catalyst after fifth used (0.411 mmol/g)
were nearly the same as the fresh catalyst (0.415 mmol/g).
This result suggested that the GST catalyst was very sta-
ble. Together with the superior activity, 1.5% GST is a
promising solid catalyst for aldol reaction.
9. G. D. Yadav and J. J. Nair, Micropor. Mesopor. Mater. 33, 1 (1999).
10. J. Zhao, Y. Y. Yue, D. W. Zhai, C. X. Miao, W. Shen, H. Y. He,
W. M. Hua, and Z. Gao, Catal. Lett. 133, 119 (2009).
11. S. Furata, H. Matsuhashi, and K. Arata, Appl. Catal. A Gen. 269, 187
(2004).
12. H. Matsuhashi, H. Miyazaki, Y. Kawamura, A. H. Nakamura, and
K. Arata, Chem. Mater. 13, 3038 (2001).
13. E. A. EI-Sharkawy, A. S. Khder, S. A. EI-Hakam, and A. I. Ahmed,
Catal. Comm. 9, 769 (2008).
14. Y. C. Du, S. Liu, Y. Y. Ji, Y. L. Zhang, S. Wei, F. J. Liu, and F. S.
Xiao, Catal. Lett. 124, 133 (2008).
4. CONCLUSIONS
We prepared a series of Ga2O3 doped sulfonated tin oxides
by co-precipitation method in ethanol solution, followed
by sulfonation and calcination. Compared with sulfonated
15. P. Salas, J. G. Hernhndez, and J. A. Montoya, J. Mol. Catal. A Chem.
123, 149 (1997).
IP: 84.54.57.236 On: Tue, 19 Mar 2019 14:23:44
16. X. Cui, H. Z. Ma, B. Wang, and H. Chen, J. Hazardous Materials
147, 800 (2007).
Copyright: American Scientific Publishers
tin oxide, a small addition of Ga2O3 to sulfonated tin
oxide resulted in a decreased tin oxide crystallite size and
increased surface area. In addition, Ga2O3 could help to
stabilize the surface sulfate complexes and increase the
effective acid sites on the catalysts. The Ga2O3 doped
sulfonated tin oxide catalysts exhibited much higher cat-
alytic activities than SO24−/SnO2, SO42−/ZrO2 and H3PO4 in
aldol condensation of prenal and prenol for citral precur-
sor synthesis. The Ga2O3-doped catalyst with 1.5% Ga2O3
showed the highest catalytic performance. Furthermore, no
obvious catalyst activity loss was observed after 5 cycles,
indicating the good recyclability.
Delivered by Ingenta
17. H. Saja, C. S. Deepa, K. S. Rani, and S. Sugunan, Appl. Catal. A
Gen. 230, 233 (2002).
18. R. Cid and G. Pecchi, Appl. Catal. 14, 15 (1985).
19. P. Sharma, S. Vyas, and A. Patel, J. Mol. Catal. A Chem. 214, 281
(2004).
20. W. Bonrath and T. Netscher, Appl. Catal. A Gen. 280, 55 (2005).
21. M. Bidaoui, C. Especel, N. Bouchenafa-Saib, D. Duprez,
O. Mohammedi, and S. Royer, Appl. Catal. A Gen. 445–446, 14
(2012).
22. X. X. Liu, Z. H. Zhang, Y. J. Yang, D. L. Yin, S. P. Su, D. C. Lei,
and J. Yang, Chem. Eng. J. 263, 290 (2015).
23. M. Bidaoui, C. Especel, S. Sabour, L. Benatallah, N. Saib-
Bouchenafa, S. Royer, and O. Mohammedi, J. Mol. Catal. A Chem.
399, 97 (2015).
24. A. Nissen, W. Rebafka, and W. Aquila, US 4288636 (1981).
25. P. Chabardes and J. Chazal, US 4933500 (1990).
26. C. Pierre, Tetrahedron Let. 29, 6253 (1988).
27. J. Therre, G. Kalbel, W. Aqulla, G. Wegner, and H. Fuchs,
US 6175044 (2001).
Acknowledgments: The authors thank the financial
supports from the National Natural Science Founda-
tion of China (NSFC 21333009, 21273244, 21573245),
Chinese Academy of Sciences-Peking University Pioneer
Received: 13 March 2018. Accepted: 13 April 2018.
3662
J. Nanosci. Nanotechnol. 19, 3658–3662, 2019