Preparation of metallophthalocyanine functionalized magnetic silica nanotubes and its application in ultrasound-assisted oxidative desulfurization of benzothiophene

Article abstract of DOI:10.1016/j.materresbull.2014.12.019

In this study, the preparation and catalytic application of tetra-substituted carboxyl iron phthalocyanine (FeC₄Pc)-loading magnetic silica nanotubes (MSNTs) in oxidative desulfurization (ODS) of benzothiophene was investigated. The resulting materials FeC₄Pc-MSNTs integrate the advantages of silica nanotubes and superparamagnetic characteristics, which exhibited excellent catalytic activity and reusability. Under the best operating condition for the catalytic oxidative desulfurization, the sulfur content in model oil was reduced from 600 ppm to 35 ppm with 94% of total sulfur. Our work herein reveals that the surface functionalized MSNTs will be a promising platform as catalyst carriers.

Full text of DOI:10.1016/j.materresbull.2014.12.019

Materials Research Bulletin 63 (2015) 181–186
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Preparation of metallophthalocyanine functionalized magnetic silica
nanotubes and its application in ultrasound-assisted oxidative
desulfurization of benzothiophene
*
Fang Wang, Guangjian Wang , Huijie Cui, Wantang Sun, Tangbo Wang
College of Chemical Engineering, Qingdao University of Science and Technology, Qingdao 266042, Shandong, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 26 July 2014
Received in revised form 12 November 2014
Accepted 5 December 2014
Available online 6 December 2014
In this study, the preparation and catalytic application of tetra-substituted carboxyl iron phthalocyanine
(FeC4Pc)-loading magnetic silica nanotubes (MSNTs) in oxidative desulfurization (ODS) of benzothio-
phene was investigated. The resulting materials FeC4Pc-MSNTs integrate the advantages of silica
nanotubes and superparamagnetic characteristics, which exhibited excellent catalytic activity and
reusability. Under the best operating condition for the catalytic oxidative desulfurization, the sulfur
content in model oil was reduced from 600 ppm to 35 ppm with 94% of total sulfur. Our work herein
reveals that the surface functionalized MSNTs will be a promising platform as catalyst carriers.
ã 2014 Elsevier Ltd. All rights reserved.
Keywords:
A. Nanostructures
A. Magnetic materials
B. Sol–gel chemistry
D. Catalytic properties
1. Introduction
SNTs have attracted special interest because of their easy
surface functionalization, colloidal suspension formation, and
Nowadays, increasing attention has been paid to the
environment issue caused by sulfur compounds in transporta-
tion fuels which is the main source of SOx in air [1]. Oxidative
desulfurization (ODS), an alternative or complementary technol-
ogy to hydrodesulfurization for deep desulfurization, has been
widely studied in recent years [2]. Different supported catalysts
compound with H2O2 have been used in the oxidation of the
hydrophilic nature [6]. The unique, hollow inner voids of
nanotubes allow for filling with species ranging from large
polymers to small molecules to meet different requirement
[7,8]. And by combining the attractive tubular structure of silica
nanotubes with magnetic properties, magnetic silica nanotubes
(MSNTs) not only have all the advantages of silica nanotubes,
but also have the potential to be separated from solution by
external magnetic field [9,10]. It is well known that metal-
lophthalocyanines have been used as an efficient biomimetic
catalysts for the oxidation, reduction and other reactions
because of their chemical and thermal stabilities and also their
rather cheap and facile preparation in large scale [11–14].
According to our previous work [15], the multifunctional
nanotubes supported metallophthalocyanine material was
expected to be a useful catalyst for ODS process. However,
the reports on the applications of these MSNTs in catalytic
chemistry are rare.
In this work, the direct preparation of surface
amino-functionalized MSNTs, the covalent attachment of FeC4Pc
on the surface of amino-functionalized MSNTs and its application
as a new catalyst for the ultrasound-assisted ODS with the
viewpoint of green chemistry is reported. The physicochemical
properties, catalytic performance and reusability of FeC4Pc-
MSNTs were studied. It reveals that FeC4Pc supported on MSNTs
organosulfur compounds, such as H2O2/Mo/g-Al2O3 [3], H2O2/Ti-
HMS [4], and H2O2/Fe/activated carbon [5]. Due to the bulky size
of some refractory sulfur compounds (eg.: benzothiophene,
dibenzothiophene and their alkyl-derivatives), materials with
relatively lager porous structure as supports are thought to be
best suited for ODS than conventional microporous solids. With
the development of nanotechnology, nanomaterials as promising
supported materials have found increasing applications in
modern industry applications. Among them, silica nanotubes
(SNTs) are attracting a great deal of attention due to their
fundamental significance and potential applications in various
areas.
* Corresponding author. Tel.: +86 53285889106; fax: +86 53285889106.
E-mail address: guangjianwangnet@126.com (G. Wang).
http://dx.doi.org/10.1016/j.materresbull.2014.12.019
0025-5408/ã 2014 Elsevier Ltd. All rights reserved.

182
F. Wang et al. / Materials Research Bulletin 63 (2015) 181–186
exhibits a high ODS activity because the unique silica surface
chemistry of MSNTs provided a benign microenvironment for
catalytic reaction.
2.2.2. Preparation of FeC4Pc-MSNTs
To covalently bind FeC4Pc to the NH2-MSNTs, 3 mg FeC4Pc was
firstly added to 2 mL anhydrous dimethylformamide by stirring for
10 min. After appropriate amount of EDC and NHS were added into
the FeC4Pc solution, stirring was continued for another 20 min.
0.2 g NH2-MSNTs was then added. The sample solution was stirred
at room temperature for 2 h to allow the immobilization
equilibrium. Thereafter, the immobilized FeC4Pc was separated
from the solution by centrifugation and washing with water
several times to remove the desorbed FeC4Pc completely.
2. Experimental
2.1. Materials
A commercially available AAO membrane with a diameter of
47 mm and a quoted pore diameter of 200 nm was purchased from
Whatman. Tetraethoxysilane (TEOS), 3-aminopropyltriethoxysi-
lane (APTEOS) and ethyl-[3-(dimethylamino) propyl]–carbodii-
mide hydrochloride(EDC) were purchased from Aldrich.
N-hydroxysuccinimide (NHS) was purchased from Sigma. FeC4Pc
were obtained from Sigma. Hydrogen peroxide solution was
prepared by dilution of 30% aqueous solution and standardized by
titration with the standard solution of potassium permanganate.
All other chemicals were of analytical-reagent grade and used
without further purification.
2.3. Characterization
Scanning electron microscope (SEM) and X-ray energy disper-
sion spectroscope (EDS) images of the MSNTs were taken on a LEO
1530 SEM equipped with ISIS 300 INCA energy and INCA crystal
system (Oxford Instrumental Pte. Ltd.). A Tecnai F30 transmission
electron microscope (TEM) was used to obtain TEM micrographs of
MSNTs. Magnetic characterization of MSNTs was performed by a
superconducting quantum interference device (Magnetic Property
2.2. Synthesis
Measurement System XL-7, Quantum Design). A Hitachi F-
4500 fluorescence spectrofluorimeter was adopted to record the
steady-state fluorescence spectra and make the fluorescence
measurements. A Beckman DU-7400 UV–vis diode array spectro-
photometer was used to record absorption spectra.
2.2.1. Preparation of amino modified MSNTs
The surface amino modified MSNTs(NH2-MSNTs) were pre-
pared according to the literature [16–18]. Firstly, the AAO
membrane was immersed in a mixture solution composed of
APTEOS, ethanol, sodium acetate and water for 10 min and heated
in vacuum at 130 ꢀC for 2 h. After a brief mechanical polishing and
rinsing, the membrane was modified with magnetite nanoparticles
by coprecipitating Fe3+ and Fe2+ under an N2 stream. Finally, the
magnetite-modified AAO membrane was immersed in a pre-made
hydrolyzed silica solution for 1 min at 4 ꢀC. This pre-made solution
was prepared by mixing TEOS, APTEOS, ethanol, H2O and HCl and
stirred at room temperature. The AAO membrane was then dried at
90 ꢀC under vacuum for 2 h. After dissolving completely the
alumina membrane in NaOH (0.1 M), the resulting NH2-MSNTs
were collected by centrifugation, washed with deionized water
and ethanol several times.
2.4. Catalytic activities
The catalytic oxidative desulfurization experiments were
conducted in a flask using a model oil(MO) consisting benzothio-
phene (BT) and octane with initial sulfur contents of 600 mg/L. A
typical experiment was performed as follows: In a round bottom
flask with a heated circulating bath, 10 mL of MO was mixed with
FeC4Pc-MSNTs (or FeC4Pc), and hydrogen peroxide. The mixture
was irradiated using ultrasound for a specific time at a given
temperature. After the reaction, the oxidized MO was extracted by
extraction solvent and the oil layers were collected and analyzed
for sulfur content by gas chromatography with flame photometric.
Scheme 1. Schematic illustration of the synthesis of FeC4Pc-MSNTs.

F. Wang et al. / Materials Research Bulletin 63 (2015) 181–186
183
3. Results and discussions
signal of the system decreased markedly (2, 20). The results
demonstrated the presence of amino groups on the surface of
MSNTs and a high coupling efficiency between FeC4Pc and the
NH2-MSNTs. Moreover, according to the absorption values of the
NH2-MSNTs, FeC4Pc-MSNTs (Fig. 3(b)), and series of free FeC4Pc in
which the calibration curve was made, the amount of bound
FeC4Pc on the MSNTs can be roughly determined, and the results
show that the amount of FeC4Pc conjugated into the magnetic
silica nanotubes was about 1.1 wt%. The high loading of FeC4Pc on
the MSNTs can be attributed to the large surface area offered by the
nanostructure and sufficient amino groups on the surface of
MSNTs. The high-capacity FeC4Pc immobilization ability of MSNTs
together with the attractive magnetic properties provided by the
Fe3O4 nanoparticles makes them promising for applications in
various fields.
3.1. Synthesis of FeC4Pc-MSNTs and characterization
Template synthesis within the pores of nanoporous alumina
membrane is a common method for the preparation of nanotubes
and nanowires [7,19], which provides a particularly easy route to
accomplish various functionalization. The synthetic procedure for
FePcC4-MSNTs was shown in Scheme 1. Usually, the surface
functionalization was made after liberating the nanotubes from the
template; however, it might cause the inner surface functionalized
simultaneously or plugs up both ends opening. In this study, as an
alternative, we directly carried out the surface functionalization on
the AAO membrane. This is to show that the AAO membrane was
first silanized with 3-aminopropyltriethoxysilane, then the inner
surface of aminopropyl group modified AAO membrane was coated
with a layer of magnetite and another amino silica layer one after
the other, while nanotubes were still embedded in the pore of AAO
template. After releasing the MSNTs from the AAO membrane,
some amino groups were directly exposed on the surface of MSNTs,
which can then be conjugated with the free carboxyl of FeC4Pc via
EDC/NHS conjugate chemistry.
Fig. 1(a) shows the TEM images of the prepared MSNTs in the
AAO membrane. It can be seen clearly that the magnetite layer on
the inner surface of the nanotube (the inner tube wall gets black
and thick), and the nanotube has an outside diameter of about
200 nm which was similar to the diameter of the membrane pores.
The SEM image from Fig. 1(b) indicates that the nanostructures are
really hollow nanotubes. EDS results obtained from Fig. 2(a) reveal
these MSNTs consist mainly of Fe and Si, which further confirm
that the magnetite distributed in the tubes. According to the room
temperature magnetization curves (Fig. 2(b)), the MSNTs are
superparamagnetic with the saturation magnetization of ꢁ1.0
emu/g, comparable to reported superparamagnetic silica nano-
tubes [9,10].
3.2. Optimization conditions for the ODS based on the catalysis of
FeC4Pc
It is well known that metallophthalocyanines have been used as
an efficient biomimetic catalysts for the oxidation [11,13]. In this
work, the sulfur removal of MO was used as probe reaction to study
the catalytic activity of FeC4Pc. In order to optimally utilize the
catalytic characterization of FeC4Pc in ODS process, we first tested
the effect of reaction temperature, ultrasound reaction time and
H2O2/S mole ratio on sulfur removal. Fig. 4 shows the efficiency of
desulfurization at different temperatures. The sulfur removal
increased at first then decreased with increasing reaction
temperature in the range of 40–80 ꢀC. Oxidation at higher
temperature was unfavorable due to the decomposition of
hydrogen peroxide to undesirable side products (H2O and O2)
other than hydroxyl radicals, which decreases the efficiency of the
desulfurization process and affects the quality of oil [21].
Moreover, reaction at temperature higher than 80 ꢀC may lead
to the oxidation of useful components in the fuel. In view of these
reasons, the reaction temperature was set at 60 ꢀC.
Sulfur reduction of BT under varying ultrasonication times
(10–60 min) is shown in Fig. 5. Assisted by ultrasound, emulsions
produced by sonication are finer and more stable, which further
enhances the interfacial area available for reaction. Application of
ultrasonic irradiation in the system causes an increase in sulfur
reduction rate, which is due to an increase in effective local
concentration of reactive species and an improvement in mass
transfer in the interfacial region. Hence, high sulfur removal rate
was achieved at ultrasonication time of 30 min. On the other hand,
further increasing ultrasonication time from 30 to 60 min, no
The presence of amino groups on the inner/outer surface of
MSNTs and the efficiency of FeC4Pc immobilized can be identified
by fluorescamine method [20]. Fluorescamine itself is a colorless,
non-fluorescent reagent that reacts readily under mild conditions
with primary amines to form stable, highly fluorescent com-
pounds. Fig. 3(a) shows the fluorescence excitation and emission
spectra of the NH2-MSNTs/fluorescamine and FeC4Pc-MSNTs/
fluorescamine systems. It can be seen that before FeC4Pc reacting
with the aminos of NH2-MSNTs, the fluorescence of the system was
very high with the fluorescence emission maximum at 475 nm (1,
10). In contrast, after the conjugation reaction, the fluorescence
Fig. 1. TEM image (a) and SEM image (b) of the MSNTs.

184
F. Wang et al. / Materials Research Bulletin 63 (2015) 181–186
Fig. 2. EDS spectrum (a), and magnetization curve (b) of the MSNTs at room temperature.
obvious changes in desulfurization rate was observed. It is well
known that application of ultrasound causes cavitation where
extreme local conditions of high temperature and pressure are
created [22]. So we deduced that the decrease in oxidation activity
maybe due to the low thermal stability of H2O2 and FeC4Pc caused
by prolonging ultrasonication time. And perhaps another reason
for this phenomenon is that longer reaction duration will yield
more complete oxidation of organosulfur compounds to their
corresponding sulfones.
free FeC4Pc is also shown in Fig 7 for comparison. It can be seen
from Fig 7 that the bound FeC4Pc exhibits higher catalytic activity
than the free FeC4Pc which showed the relative lower sulfur
removal of 76% after 0.5 h. Under the same conditions, the catalyst
FeC4Pc-MSNTs showed the highest sulfur removal of 94%, being
higher than that of the free FeC4Pc by about 21%, and being roughly
equal to some latest research results [24,25]. According to our
previous experiment results [15], we think that the enhanced
catalytic activity can be attributed to the benign surrounding
microenvironment provided by the silica matrix, which can protect
FeC4Pc molecules from aggregation and increase the accessibilities
of active sites to BT. It has been reported that the amino-
functionalized carrier structures have important effects on the
dynamics, stability, especially activity of the immobilized catalysts
[26]. Furthermore, the strong interaction between FeC4Pc and
amino groups on MSNTs would likely lead to the even immobili-
zation of FeC4Pc on the surface of MSNTs and therefore allows
FeC4Pc to adopt conformations effective for BT molecules to access
the active sites.
Under reaction temperature 60 ꢀC, ultrasonication time 30 min,
effect of H2O2/S molar ratios on sulfur removal was investigated
(Fig. 6). It can be seen from Fig. 6 that the sulfur removal rate
increased from 39% at H2O2/S = 5 to 76% at H2O2/S = 15. With the
further increase of the H2O2/S ratio, the sulfur removal rate was
reduced. This decrease could be explained probably by the
excessive nonproductive decomposition of hydrogen peroxide to
oxygen and water when H2O2 concentration was higher than a
certain value. This is in agreement with the results of Duarte
previous study [23], who reported that higher values of molar
proportion led to the reduction of sulfur removal rate. Therefore,
H2O2/S = 15 was chosen as the best molar ratio.
3.4. Stability of FeC4Pc-MSNTs
3.3. Catalytic performance of FeC4Pc-MSNTs for BT removal
In order to investigate the stability of FeC4Pc-MSNTs, the sulfur
removal of MO over the fresh and regenerated FeC4Pc-MSNTs are
shown in Fig. 8. The reusability was examined by conducting the
activity measurements of the FeC4Pc-MSNTs for three cycles. After
The catalytic performance of FeC4Pc-MSNTs as a function of
reaction time is depicted in Fig 7, and the catalytic performance of
Fig. 3. (a) Fluorescence after reacting with fluorescamine. (b) UV–vis absorption spectra of MSNTs, FeC4Pc-MSNTs and FeC4Pc.

F. Wang et al. / Materials Research Bulletin 63 (2015) 181–186
185
Fig. 6. Effect of different H2O2/S molar ratios on the sulfur reduction of BT. Reaction
conditions: Temperature = 60 ꢀC, Time = 30 min.
Fig. 4. Effect of temperatures on the sulfur reduction of BT. Reaction conditions: n
(H2O2)/n(S) = 10, Time = 20 min.
each experiment the FeC4Pc-MSNTs were recovered, regenerated
by solvent extraction in order to remove the adsorbed species, and
then fresh MO, hydrogen peroxide were added for the next run. For
comparison, the repeated use performance using catalysts without
any treating was also conducted. As can be seen from Fig. 8, sulfur
removal decreased from 94% to 79% after triple use of catalysts
without solvent treatment, and sulfur removal still reached 88%
using the catalysts regenerated by methanol. The results show that
deactivation of catalysts can be inhibited by methanol extraction to
remove the adsorbed species (BT/BTO2) on the catalyst surface,
suggesting that the catalysts regenerated by methanol have a
better repeated use performance. Moreover, the catalysts FeC4Pc-
MSNTs can be easily recovered after completion of the reaction by
means of physical centrifugation or magnetic separation, which
provides a good precondition for the repeated use of immobilized
FeC4Pc.
Fig. 7. Catalytic performance of FeC4Pc and FeC4Pc-MSNTs as a function of reaction
time. Reaction conditions: n(H2O2)/n(S) = 15, Temperature = 60ꢀC, Time = 30 min.
3.5. The mechanism of BT oxidation in the FeC4Pc-H2O2 system
Oxidative desulfurization is considered to be one of the most
promising technologies for deep desulfurization of fuel oil. The
method is composed of oxidizing the sulfur compounds to form
sulfones and/or sulfoxides, which are highly polar and could be
easily removed via adsorption, solvent extraction or distillation
[27]. As one of the key steps for ODS process, the reactions of iron
(II) porphyrin complexes with various oxidants such as
peroxyacids and hydroperoxides have been extensively studied
[28,29]. It is reported that a monomeric porphyrin complex is
catalytically active in the porphyrin-H2O2 system [30]. Scheme 2
shows the mechanism of BT oxidation catalyzed by FeC4Pc-MSNTs.
A non-stacked monomeric FeC4Pc reacts with hydrogen peroxide
to generate iron(III) hydroperoxide porphyrin intermediate, then
the hydroperoxide O—O bond cleave to form a ferryl-oxo complex
Fig. 5. Effect of ultrasonication time on the sulfur reduction of BT. Reaction
conditions: n(H2O2)/n(S) = 10, Temperature = 60ꢀC.
Fig. 8. Reusability of the FeC4Pc-MSNTs. Reaction conditions: n(H2O2)/n(S) = 15,
Temperature = 60ꢀC, Time = 30 min.

186
F. Wang et al. / Materials Research Bulletin 63 (2015) 181–186
Scheme 2. The mechanism of BT oxidation catalyzed by FeC4Pc-MSNTs.
and a hydroxyl radical [29]. The hydroxyl radical was proposed as
the active species in the oxidative reduction of BT. Bonding FeC4Pc
on MSNTs can suppress the dimerization of FeC4Pc to a certain
extent, which would be favorable for the improvement of the
activity and the stability of the catalyst.
[2] C. Komintarachat, W. Trakarnpruk, Ind. Eng. Chem. 45 (2006) 1853.
[3] Y.H. Jia, G. Li, G.L. Nin, Fuel Process. Technol. 92 (2011) 106.
[4] Y. Wang, G. Li, X.S. Wang, C.Z. Jin, Energy Fuels 21 (2007) 1415.
[5] G.X. Yu, S.X. Lu, H. Chen, Z.N. Zhu, Carbon 43 (2005) 2285.
[6] X. Yang, H. Tang, K.H. Cao Song, W. Sheng, Q. Wu, J. Mater. Chem. 21 (2011)
6122.
[7] J.H. Jung, M. Park, S. Shinkai, Chem. Soc. Rev. 39 (2010) p.4286.
[8] C.T. Sousa, C. Nunes, M.P. Proença, D.C. Leitão, J.L.F.C. Lima, S. Reis, J.P. Araújo, M.
Lúcio, Colloids Surf. B 94 (2012) 288.
4. Conclusion
[9] D.R. Bae, S.J. Lee, S.W. Han, J.M. Lim, D.M. Kang, J.H. Jung, Chem. Mater. 20
(2008) 3809.
In this work, we report a facile method to fabricate the
multifunctional surface amino-functionalized MSNTs via AAO
template membrane method together with modified sol–gel
method and FeC4Pc was immobilized on the surface of MSNTs
via EDC/NHS conjugate chemistry. It was found that the
immobilized FeC4Pc exhibited excellent catalytic activity and
reusability. The good performance of FeC4Pc-MSNTs in ODS
indicated that the organically functionalized MSNTs provided
high immobilization efficiency and a benign microenvironment for
catalytic reaction due to the unique silica surface chemistry. The
successful application of the FeC4Pc-MSNTs in ODS confirmed that
these MSNTs can act as novel supports for organic molecules, and
this will pave a new road for their application in many fields.
[10] S.J. Son, J. Reichel, B. He, M. Schuchman, S.B. Lee, J. Am. Chem. Soc. 127 (2005)
7316.
[11] C. Pérollier, A.B. Sorokin, Chem. Commun. 14 (2002) 1548.
[12] P. Modisha, T. Nyokong, J. Mol. Catal. A: Chem. 381 (2014) 132.
[13] S.L. Jain, B. Sain, J. Mol. Catal. A: Chem. 176 (2001) 101.
[14] N. Safari, P.R. Jamaat, M. Pirouzmand, A. Shaabani, J. Porphyr. Phthalocya. 8
(2004) 1209.
[15] J.C. Xing, F. Wang, J.L. Zou, X.L. Chen, J. Nanosci. Nanotechnol. 10 (2010)
5783.
[16] S.J. Son, J. Reichel, B. He, M. Schuchman, S.B. Lee, J. Am. Chem. Soc. 127 (2005)
7316.
[17] J. Yu, X. Bai, J. Suh, S. Lee, S. Son, J. Am. Chem. Soc. 131 (2009) 15574.
[18] C.C. Chen, Y.C. Liu, C.H. Wu, C.C. Yeh, M.T. Su, Y.C. Wu, Adv. Mater.17 (2005) 404.
[19] C.R. Martin, P. Kohli, Nat. Rev. Drug Discov. 2 (2003) 29.
[20] S. Udenfriend, S. Stein, P. Boehlen, W. Dairman, W. Leimgruber, M. Weigele,
Science 178 (1972) 871.
[21] Z. Wu, B. Ondruschka, Ultrason. Sonochem. 17 (2010) 1027.
[22] S.P. Tu, T.F. Yen, Energy Fuels 14 (2000) 1168.
Acknowledgements
[23] F.A. Duarte, P.A. Mello, C.A. Bizzi, Fuel 90 (2011) 2158.
[24] M.A.G. Nunes, P.A. Mello, C.A. Bizzi, L.O. Diehl, Fuel Process Technol.126 (2014)
521.
[25] J.K. Zhang, G.J. Wang, L.P. Zhang, X.T. Fu, Y. Liu, React. Kinet. Mech. Cat. (2014) ,
doi:http://dx.doi.org/10.1007/s11144-014-0750-y.
[26] W. Yang, H.Y. Qu, H.H. Yang, J.G. Xu, Anal. Lett. 37 (2004) 1793.
[27] S. Liu, B. Wang, B. Cui, L. Sun, Fuel 87 (2008) 422.
[28] G.-X. He, T.C.J. Bruice, Am. Chem. Soc. 113 (1991) 2747.
[29] W. Nam, H.J. Han, S.-Y. Oh, J. Am. Chem. Soc. 122 (2000) 8677.
[30] A. Hadasch, A. Sorokin, A. Rabion, B. Meunier, New J. Chem. 22 (1998)
45.
We thank professor Xiaolan Chen for helpful discussions and
comments on the manuscript. The work was supported by the
NSFC (21276130).
References
[1] A. Stanislaus, A. Marafi, M.S. Rana, Catal. Today 153 (2010) 1.