Magnetic NiO nanoparticles confined within open ends MWCNTs: A novel and highly active catalyst for hydrogenation and synthesis of imines

Article abstract of DOI:10.1039/c6ra05561c

A new nanocatalyst has been synthesized by confining magnetic nickel nanoparticles within carbon nanotubes (CNTs) and characterized by XRD, TEM, Raman and VSM. The character of the nanocatalyst passivated with a gas mixture was that it can be stored safely in air below 150 °C and needs no activation prior to use. In the catalytic test, a nickel oxide nanocatalyst confined inside the Multi-walled carbon nanotubes nanochannels (NiO/MWCNTs-in) was found to be a highly efficient and reusable catalyst for the reduction of various aromatic nitro compounds to various aromatic amines, the conversion and selectivity of which were almost up to 100% and exceed 80%. The prominent merit of the catalyst is that the overall formation rate of product inside the nanotubes exceeds that outside. Moreover, it is inexpensive, and could be prepared and scaled up easily. Besides, it can be simply separated from the reaction mixtures by an external magnetic field As a result of the possible confinement effect of CNTs, the employment of the CNTs channels as nanoreactors for catalysis may provide opportunities for the development of new heterogeneous catalysts.

Full text of DOI:10.1039/c6ra05561c

RSC Advances
View Article Online
View Journal | View Issue
PAPER
Magnetic NiO nanoparticles confined within open
ends MWCNTs: a novel and highly active catalyst
for hydrogenation and synthesis of imines†
Cite this: RSC Adv., 2016, 6, 58805
Gangquan Chen,a Wenbin Gao,b Xuejun Wang,c Hongfei Huo,b Wenzhu Li,b
b
b
a
*
*
Le Zhang, Rong Li and Zuixiong Li
A new nanocatalyst has been synthesized by confining magnetic nickel nanoparticles within carbon
nanotubes (CNTs) and characterized by XRD, TEM, Raman and VSM. The character of the nanocatalyst
passivated with a gas mixture was that it can be stored safely in air below 150 ꢀC and needs no activation
prior to use. In the catalytic test, a nickel oxide nanocatalyst confined inside the Multi-walled carbon
nanotubes nanochannels (NiO/MWCNTs-in) was found to be a highly efficient and reusable catalyst for
the reduction of various aromatic nitro compounds to various aromatic amines, the conversion and
selectivity of which were almost up to 100% and exceed 80%. The prominent merit of the catalyst is that
the overall formation rate of product inside the nanotubes exceeds that outside. Moreover, it is
inexpensive, and could be prepared and scaled up easily. Besides, it can be simply separated from the
reaction mixtures by an external magnetic field As a result of the possible confinement effect of CNTs,
the employment of the CNTs channels as nanoreactors for catalysis may provide opportunities for the
development of new heterogeneous catalysts.
Received 2nd March 2016
Accepted 23rd May 2016
DOI: 10.1039/c6ra05561c
www.rsc.org/advances
amination as well as by polymer-supported. But the reported
catalytic synthesis routes depend strongly on the reaction
Introduction
conditions (pH, temperature, pressure and metal ratio) and the
nature of catalyst (metal, particle size and support). Moreover,
these methodologies oen require complex procedures, long
reaction time, and large quantities of aromatic solvents.17 With
the increasing interest in human health and environmental
protection, more attention is paid to “green chemistry”.
Therefore, it is necessary to develop an efficient, mild and
practical method for the reduction of aromatic nitro
compounds for synthesis of imines.18–23 Therefore, the paper
reports a new catalyst that was found to be a highly efficient
catalysis for the reduction of various aromatic nitro compounds
to various aromatic amines.
Multi-walled carbon nanotubes (MWCNTs) have been widely
studied as supports for metal nanoparticles in catalytic hydro-
genation and oxidation reactions because of its excellent elec-
tronic properties, good physical and chemical stability, and
large surface area.24–26 In addition, MWCNTs have well-dened
hollow interiors. This opens interesting possibilities to intro-
duce other matters into the cavities.27–30 Nevertheless, as far as
we know, the applications of nickel oxide nanoparticles
conned within MWCNTs nanochannels in reduced catalytic
reaction have not been reported.
Aromatic imine compounds and imines are versatile interme-
diates in organic synthesis, especially for the preparation of
agrochemicals,
pharmaceuticals,1,2
heterocycles,
anti-
inammatory agents, non-natural aminoacids, and anticancer
agents.3–5 The oldest industrially practiced method for the
reduction of the aromatic nitro compounds is the Bechamp
reduction,6 which has a history of 150 years up to now. At the
same time, the rst preparation of imines was reported by Schiff
more than a century ago.7 Currently, numerous methods for the
reduction of aromatic nitro have been reported.8–13 However,
some questions still existed, such as (1) seriously environmental
pollution from residue, (2) strong acidic or alkaline media, (3)
the reaction conditions of most of these methods can destroy
many sensitive functional groups. Generally, several methods
for the synthesis of imines are also described in the litera-
ture;14–16 they can be obtained from aldehydes, gem-dibromo-
methylaryl derivates, formamides, and palladium catalyzed
aKey Laboratory of Mechanics on Western Disaster and Environment (Lanzhou
University), Ministry of Education, Lanzhou 730000, China. E-mail: lizuixiong@
sohu.com
bCollege of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou
730000, P. R. China. E-mail: liyirong@lzu.edu.cn
Herein we report the hydrogenation of aromatic nitro
compounds for synthesis of imines reactions using passivated
magnetic nickel oxide nanoparticles encapsulated within
MWCNTs channels. The activity and selectivity of a nickel oxide
cTraditional Chinese Medicine Undergraduate, Gansu University Traditional Chinese
Medicine, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c6ra05561c
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 58805–58812 | 58805

View Article Online
RSC Advances
Paper
Scheme 1 Hydrogenation and synthesis of imines on the nickel oxide nanoparticles encapsulated within the MWCNTs (NiO/MWCNTs-in) and
adsorbed outside MWCNTs (NiO/MWCNTs-out) with lower temperature and pressure.
nanocatalyst conned inside the MWCNTs nanochannels mixture was stirred for 48 hours at room temperature. Then the
(Scheme 1), compared with nickel oxide nanocatalyst immobi- mixture was heated to 110 ꢀC with a heating rate of 1 ꢀC minÀ1
lized outside the MWCNTs (Scheme 1), are greatly enhanced for and held at 110 ꢀC for 24 h. By this slow drying method, the
the catalysis using low temperature and low pressure as the catalyst precursor was introduced into the MWCNTs channels.
mild conditions. These results show the important and unique Finally, the sample was heated in a stream of nitrogen at a rate
features of the reduced catalyst within the nanotubes. As
a result of the possible connement effect of MWCNTs,
employment of the MWCNTs channels as nanoreactors31 for
hydrogenation and synthesis of imines may provide opportu-
nities for the development of new heterogeneous reduced
catalysts.
Experiment
Materials
Nickel nitrate hexahydrate (NiNO3$6H2O, >99%) as a single
nickel source, anhydrous sodium carbonate, nitric acid and
ethanol were received. All chemicals were of analytical grade
and used as received without further purication. Deionized
water was used throughout the experiments. MWCNTs (purity >
95 wt%, diameter: 20–40 nm, about 50 mm length) were
purchased from Shenzhen Nanotech Port Ltd. Co., China.
Fig. 1 FT-IR spectra of (a) open ends MWCNTs, (b) NiO/MWCNTs-in
and (c) NiO/MWCNTs-out.
Hydrogen with purity of 99.99% was used in the hydrogenation.
Preparation of multi-walled carbon nanotubes (MWCNTs)
with open ends
The MWCNTs used in this work were shortened, functionalized,
and then the ends were opened. Samples (1.0 g) of the MWCNTs
were suspended in 45 mL concentrated nitric acid (68%, anal-
ysis grade) and reuxed for 14 h in an oil bath maintained at 140
ꢀC. Aer washing with distilled water and drying at 100 ꢀC
overnight, the black insoluble product was sonicated in chlo-
roform and dried in vacuo.
Preparation of a nickel oxide nanocatalyst conned inside the
multi-walled carbon nanotubes nanochannels (NiO/
MWCNTs-in) catalyst
The MWCNTs (0.5 g) with open ends were immersed in an
aqueous solution of 1.36 g of hydrated nickel nitrate (55 wt%, 50
mL) at room temperature. Aer ultrasonic treatment for 3 h, the and NiO/MWCNTs-out.
Fig. 2 XRD patterns of MWCNTs, MWCNTs (open), NiO/MWCNTs-in
58806 | RSC Adv., 2016, 6, 58805–58812
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
À1
ꢀ
ꢀ
of 10 C min from room temperature to 100 C and kept at
this temperature for 1 h before ramping at 4.0 ꢀC minÀ1 to 450
ꢀC. The sample was then annealed for 3 h at 450 C.
ꢀ
Preparation of nickel oxide nanocatalyst immobilized outside
the multi-walled carbon nanotubes nanochannels (NiO/
MWCNTs-out) catalyst
The MWCNTs (0.5 g) were immersed in an aqueous solution of
1.36 g of hydrated nickel nitrate (55 wt%, 50 mL) at room
temperature. The catalyst precursor was reduced with sodium
carbonate and stirred for 48 h, and then ltered at room
temperature. Finally, the sample was heated in a stream of
nitrogen at a rate of 10 ꢀC minÀ1 from room temperature to 100
ꢀC and kept at this temperature for 1 h before ramping at 4.0 ꢀC
minÀ1 to 450 ꢀC. Then annealed for 3 h at 450 C.
ꢀ
Characterization
FT-IR spectra were recorded on a Nicolet NEXUS 670 FT-IR
spectrometer with
a DTGS detector, and samples were
measured with KBr pellets. XRD measurements were performed
on a Rigaku D/max-2400 diffractometer using Cu-Ka radiation
as the X-ray source in the 2q range of 10–100ꢀ. The size and
morphology of the magnetic nanoparticles were observed by
a Tecnai G2 F30 transmission electron microscopy and samples
were obtained by placing a drop of a colloidal solution onto
a copper grid and evaporating the solvent in air temperature.
Raman scattering spectrum was measured at room temperature
using an excitation wavelength of 514.5 nm (JY-HR800) under
the backscattering geometry. Magnetic measurements of NiO/
MWCNTs-in and NiO/MWCNTs-out were investigated with
a quantum design vibrating sample magnetometer (VSM) at
room temperature in an applied magnetic eld sweeping from
À15 to 15 kOe.
Fig. 3 TEM images of NiO/MWCNTs-in and NiO/MWCNTs-out.
General procedure for the catalytic test
The catalytic hydrogenations were carried out in a 50 mL
stainless steel reactor equipped with a magnetic stirrer. In
a typical reaction procedure, substrates (1.0 g) and 10 mL
ethanol (solvent) were mixed with passivated nanocatalyst (0.05
Fig. 4 Raman spectra of purified MWCNTs, NiO/MWCNTs-in and
NiO/MWCNTs-out.
Fig. 5 Magnetization curves of (a) NiO/MWCNTs-in and (b) NiO/MWCNTs-out.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 58805–58812 | 58807

View Article Online
RSC Advances
Paper
Table 1 Catalytic hydrogenation of various aromatic nitro compounds over NiO/MWCNTs-ina
Entry
Substrate
Product
Time (h)
Pres. (MPa)
Conv. (%)
97
Sel. (%)
100
1
5
1.5
2
3
5
5
1.5
1.5
96
60.1
99b
97c
4
5
5
5
1.5
1.5
98.5
95.8
100
100
6
5
1.5
95.6
100
7
8
5
5
1.5
1.5
93.2
72.4
100
100
9
5
5
5
5
1.5
1.5
1.5
1.5
88.4
96.1
98
100
100
100
100
10
11
12
98
13
14
5
5
1.8
1.8
99
100
100
83.9
15
5
1.8
95
100
58808 | RSC Adv., 2016, 6, 58805–58812
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
Table 1 (Contd.)
Entry
16
Substrate
Product
Time (h)
5
Pres. (MPa)
1.8
Conv. (%)
90.1
Sel. (%)
100
17
5
5
1.8
1.8
99
100
100
18
87.8
a
Reaction conditions: 50 mg NiO/MWCNTs-in; 4.0 mmol substrate; 10 mL ethanol. b Selectivity aer 5 runs. c Hydrogenation with 50 mg of NiO/
MWCNTs-out.
g). The reactor was ushed three times with hydrogen (0.5 MPa) indicates that the single phase fcc nickel oxide was successfully
and pressurized to the desired pressure (1.5 MPa). Then, it was synthesized by a wet-chemistry method.
ꢀ
heated to the desired temperature (120 C) in an oil bath with
stirring. Aer reaction, the reactor was cooled and then slowly
depressurized. Finally, the catalyst was separated, and the
Morphological characterization (HRTEM)
reactants and products were analyzed by GC (P.E. AutoSystem Fig. 3 shows the HRTEM images of NiO/MWCNTs-in and NiO/
XL) or GC-MS (Agilent 6890N/5973N). For the synthesis of MWCNTs-out. It can be seen from the Fig. 3(a) and (b), the
imines, the process of reaction was the same as above. The nickel oxide nanoparticles were in the channel of MWCNTs.
catalyst dosage (0.015 g), reaction temperature (100 ꢀC) and Statistics show that there are more than 30% of nickel oxide
pressure (1.0 MPa) were different from the above.
particles inside MWCNTs. TEM image show that the nickel
oxide nanoparticles are fairly uniform and the particles are in
the size range of 10–20 nm. In addition, as a result of the
process of concentrated nitric acid treatment, most of the ends
Result and discussion
Fig. 1 shows the FT-IR spectra of the pure MWCNTs, NiO/ of MWCNTs have been opened from Fig. 3(a) and (b). As an
MWCNTs-in and NiO/MWCNTs-out catalysts in the wave example, Fig. 3(c) and (d) show the morphologies of the samples
length range from 4000 to 400 cmÀ1. The strong and wide with nickel oxide nanoparticles deposited on the MWCNTs.
absorption band at 3428 cmÀ1 of the MWCNTs sample indicates Although such a few of nickel oxide nanoparticles lled in the
that there were plenty of –OH groups on the surface of the MWCNTs, the efficiency of the reaction have much higher than
sample. By comparing the spectrum of the NiO/MWCNTs-in the NiO/MWCNTs-out catalyst. Firstly, this was possibly due to
and NiO/MWCNTs-out with that of the open ends MWCNTs, the connement effect of MWCNTs, employment of the
we nd there was not noticeable changes in Fig. 1, indicating MWCNTs channels as nanoreactors for catalysis may provide
that the introduction of nickel oxide nanoparticles in MWCNTs a benecial conditions. Secondly, the size of nanoparticles
did not affect the nanotubes structure.
increased with the increase of the reaction time might affect the
Fig. 2 illustrates the wide-angle powder XRD patterns (in the activity of catalyst. However, internal nanoparticles in the
2q range of 10–90ꢀ) of the MWCNTs, open ends MWCNTs and nanotubes nearly did not change. Thirdly, multi-walled carbon
the two catalysts. The peaks at 26ꢀ, 44ꢀ and 51ꢀ observed can be nanotubes can be regarded as an ideal pore structure t for
indexed as (002), (101) and (102) diffractions of graphite (JCPDS hydrogen storage, which may promote hydrogenation of
75-1621). By comparing the intensity of peaks, no apparent aromatic nitro compounds.34
difference was observed, which indicates that the graphene
Obviously the signicantly different catalytic activities of
sheets of the MWCNTs supports have not been destroyed aer NiO/MWCNTs-in and NiO/MWCNTs-out cannot be related to
acid treatment and the loading of nickel oxide species. The the number of particles because the loadings of active compo-
diffraction peaks at 44.5ꢀ, 51.8ꢀ, 76.8ꢀ and 92.9ꢀ in both Ni-lled nents and the size of the catalytic particles in both samples were
MWCNTs and Ni-deposited-MWCNTs correspond to Ni (JCPDS similar. Furthermore, the exclusive contribution of the
04-0850); suggesting the nickel oxide species in both samples geometrical effect of the channel structure on the unique
existed steadily in the form of Ni. The result was basically catalytic property inside the MWCNTs was ruled out by
consistent with the related reports in characterization.32,33 This a comparison experiment with mesoporous silica as a support,
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 58805–58812 | 58809

View Article Online
RSC Advances
Paper
Table 2 Imines formation from different nitrobenzene and different aldehydes over NiO/MWCNTs-ina
Entry
Substrate
Product
Conv. (%)
96.6
Sel. (%)
100
1
2
3
50
70
95.6
100
4
5
96.8
91
97.1
97.9
6
7
8
9
96
98.9
100
90
98
95.9
86.2
88.7
10
86
100
a
Reaction conditions: 15 mg NiO/MWCNTs-in; 1.0 mmol substrate; 10 mL ethanol for solvent; temperature with 100 ꢀC; pressure with 1.0 MPa and
time with 5 h.
58810 | RSC Adv., 2016, 6, 58805–58812
This journal is © The Royal Society of Chemistry 2016

View Article Online
Paper
RSC Advances
which yields a much poorer catalytic performance than NiO/ similarity. It displays that the content of nickel oxide in NiO-
MWCNTs-in catalyst.35,36
lled-MWCNTs and NiO-deposited-MWCNTs are equal.
Fig. 4 shows the Raman spectra of NiO/MWCNTs-in, NiO/
MWCNTs-out and open ends MWCNTs. Two main strong
Raman bands, the G-band and D-band were detected in the
three samples. The G-band can be attributed to the C–C
stretching (E2g) mode for the graphite. The D-band was indi-
cated with the defects and disorders of structures in carbon
materials.37 The intensity ratio (ID/IG) of the D- and G-band
represents the degree of disorder in graphite layer. It was
noted that the ID/IG ratio of both open ends MWCNTs and NiO/
MWCNTs-out is 0.881 and 0.917, which indicated good crys-
tallinity of the graphite sheets. However, the ID/IG ratio of NiO/
MWCNTs-in increases to 1.151, telling the higher level of
defects or disorder structure in the case of nickel oxide species
lled in channel of MWCNTs. It is well known that transition
metals are interacting with graphite through hybridization of
their d orbitals with carbon pz orbitals.38 The deformation of sp2
hybridization in the graphene walls causes p electron density to
shi from the concave surface to the convex surface.39–41 The
sidewall of MWCNTs was easily damaged by external force
because there were more defects in the inexed site.42 Subse-
quently the nearby nickel oxide nanoparticles catalyzed the
cracked wall to decompose thus formed the broad gaps during
the treatment.
The study conrmed that the structure defect of MWCNTs
was an important inuencing factor on catalytic activity owing
to the large charge transfer and decient electron on the
defective MWCNTs. In fact, as described above, the NiO/
MWCNTs-in catalyst with more structure defects causes more
changes in the reducibility, thermal stability, and sensitivity of
reaction. This was due to the large charge transfer from metals
to defective MWCNTs along the nanotube wall, which acceler-
ated the redox inside MWCNTs. Additionally, the substance
lled in defected MWCNTs can run off through defected sites,
suggesting the defected sites can be regarded as the pathway of
reactant or product during the catalytic process.32,43–46
To further explain the interaction of nickel oxide nano-
particles and MWCNTs sidewall, the magnetic state of the as-
prepared samples has been checked at room temperature and
measured by VSM. One can see that spectra show a ferromag-
netic behavior. The hysteresis loops of NiO/MWCNTs-in and
NiO/MWCNTs-out were shown in Fig. 5. It can be seen that the
magnetic saturation values (ss) and the coercivity (Hc) of these
were (39.5 emu gÀ1, 157 Oe) and (39.8 emu gÀ1, 110 Oe),
respectively. By comparison with the results of the saturation
magnetization and coercivity in Fig. 5, it was found that the
NiO/MWCNTs-in coercivity was higher than that of NiO/
MWCNTs-out. It can be thought that the interaction between
nanoparticles would decrease the coercivity. The reason was
possible that due to that the increase of Hc with the decreasing
of the particles size correlates well with the fact that the mean
particle size approaches the critical size for a single-domain
behavior.46–48 From the inserted in Fig. 5(a) and (b), the cata-
lyst can be efficiently separated easily from the solution with the
help of an external magnetic force. On the other hand, the
magnetic saturation values of the two curves were of essential
Catalyst testing for hydrogenation reactions and synthesis of
imines
Table 1 shows the catalytic hydrogenation results of various
aromatic nitro compounds to their corresponding products
over NiO/MWCNTs-in catalyst. In most cases, the reaction
ꢀ
condition (100 C, 1.5 Mpa, 5.0 h) was sufficient to give satis-
factory conversion and selectivity except entry 13–18 which has
ꢀ
required a higher reaction conditions (100 C, 1.8 Mpa, 5.0 h).
Even so, the reaction conditions (pressure of hydrogen,
temperature and reaction time) was lower and the result was
better than previous work.35,49 In order to compare with the
efficiency of the catalyst, entry 3 used the NiO/MWCNTs-out
catalyst shows that the conversion of the nitrobenzene was
lower than the NiO/MWCNTs-in catalyst (entry 1) in the same
conditions. Besides, a satisfactory yield (96%) was still obtained
even aer the catalyst has been reused ve times (entry 2). The
recyclability of the catalyst can be attributed to the efficient
stabilisation of the active nickel oxide species on the MWCNTs
surface. Having demonstrated that the NiO/MWCNTs-in cata-
lyst was highly effective for the hydrogenation reaction, its
activity was also investigated for the synthesis of imines
(Table 2).
Conclusion
In summary, we rstly reported an efficient, mild and practical
catalyst for the reduction of aromatic nitro compounds for
synthesis of imines with the novel passivated nickel oxide
magnetic nanocatalysts. It was found that when a nickel oxide
nanocatalyst was encapsulated within MWCNTs, the catalyst
was very active and lead to the highly reaction efficiency. The
reaction conditions employed in our study were relatively mild
compared with other reported systems, which need high pres-
sure, high temperature and long reaction time. Moreover, this
catalytic system was simple, inexpensive, easily scaled up. The
high activity was probably attributed mainly to the unique
properties of the nanochannels of the MWCNTs. This paper
shows the unique effect of the nanochannels of MWCNTs as
nanoreactors for catalysis. The encapsulation of a nanocatalyst
within MWCNTs as described in this work opens an opportunity
for developing novel, highly active heterogeneous catalysts.
Acknowledgements
This work was supported by the Key projects in the National
Science and Technology Pillar Program of China (2014
4BAK16B01).
References
1 G. Booth, Ullmanns encyclopedia of industrial chemistry,
Wiley-VCH Verlag, Weinheim, 2002.
This journal is © The Royal Society of Chemistry 2016
RSC Adv., 2016, 6, 58805–58812 | 58811

View Article Online
RSC Advances
Paper
2 F. Figueras and B. Coq, J. Mol. Catal. A: Chem., 2001, 173, 26 A. Corma, H. Garcia and A. Leyva, J. Mol. Catal. A: Chem.,
223–230.
2005, 230, 97–105.
3 Z. R. Xu and X. Y. Lu, J. Org. Chem., 1998, 63, 5031.
4 M. P. Doyle, W. H. Hu and D. J. Timmons, Org. Lett., 2001, 3,
3741.
27 J. Wang, Z. Dong, J. Huang, J. Li, X. Jin, J. Niu, J. Sun, J. Jin
and J. Ma, Appl. Surf. Sci., 2013, 270, 128–132.
´
´
´
28 M. Blanco, P. Alvarez, C. Blanco, M. V. Jimenez, J. Fernandez-
´
´
5 P. Vicini, A. Geronikaki, M. Incerti, B. Busonera, G. Poni,
C. A. Cabras and P. La Colla, Bioorg. Med. Chem., 2003, 11,
4785–4789.
6 A. J. Bechamp, Anal. Chim. Phys., 1854, 42, 140.
7 H. Schiff, Annals, 1864, 131, 118.
Tornos, J. J. Perez-Torrente, L. A. Oro and R. Menendez, ACS
Catal., 2013, 3, 1307–1317.
29 G. F. Wei, C. Shang and Z. P. Liu, Phys. Chem. Chem. Phys.,
2015, 17, 2078–2087.
30 I. Saldan, Y. Semenyuk, I. Marchuk and O. Reshetnyak, J.
Mater. Sci., 2015, 50, 2337–2354.
8 R. Raja, V. B. Golovko, J. M. Thomas, A. Berenguer-Murcia,
W. Zhou, S. Xie and B. F. G. Johnson, Chem. Commun., 31 E. Dujardin, T. W. Ebbesen, H. Hiura and K. Tanigaki,
2005, 2026–2028.
Science, 1994, 265, 1850–1852.
9 A. Corma, Science, 2006, 313, 332–334.
32 H. Yang, S. Song, R. Rao, X. Wang, Q. Yu and A. Zhang, J.
Mol. Catal. A: Chem., 2010, 323, 33–39.
´
10 A. Corma, P. Concepcion and P. Serna, Angew. Chem., Int.
Ed., 2007, 46, 7266–7269.
11 S. K. Mohapatra, S. U. Sonavane, R. V. Jayaram and
P. Selvam, Org. Lett., 2002, 4, 4297–4300.
33 K. Gerasopoulos, X. Chen, J. Culver, C. Wang and
R. Ghodssi, Chem. Commun., 2010, 46, 7349.
34 H. Zhang, X. Fu, J. Yin, C. Zhou, Y. Chen, M. Li and A. Wei,
Phys. Lett. A, 2005, 339, 370–377.
12 X. Liu and S. Lu, J. Mol. Catal. A: Chem., 2004, 212, 127–130.
13 M. A. McLaughlin and D. M. Barnes, Tetrahedron Lett., 2006, 35 Y. Zheng, K. Ma, H. Wang, X. Sun, J. Jiang, C. Wang, R. Li and
47, 9095–9097. J. Ma, Catal. Lett., 2008, 124, 268–276.
14 J. S. M. Samec and J. E. Backvall, Chem.–Eur. J., 2002, 8, 2955. 36 Y. Zheng, K. Ma, H. Li, J. Li, J. He, X. Sun, R. Li and J. Ma,
15 A. K. Chakraborti, B. Srikant and S. Rudrawar, Tetrahedron
Lett., 2004, 45, 7641.
Catal. Lett., 2008, 128, 465–474.
37 A. Kudelski, Talanta, 2008, 76, 1–8.
16 H. Naeimi, F. Salimi and K. Rabiei, J. Mol. Catal. A: Chem., 38 M. Menon, A. N. Andriotis and G. E. Froudakis, Chem. Phys.
2006, 260, 100. Lett., 2000, 320, 425–434.
´
˜
17 K. P. Guzen, A. S. Guarezemini, A. T. G. Orfao, R. Cella, 39 R. C. Haddon, Science, 1993, 261, 1545–1550.
C. M. P. Pereira and H. A. Stefani, Tetrahedron Lett., 2007, 40 H. Dai, J. H. Hafner, A. G. Rinzler, D. T. Colbert and
48, 1845–1848.
18 Y. Tang, D. Yang, F. Qin, J. Hu, C. Wang and H. Xu, J. Solid 41 B. Shan and K. Cho, Phys. Rev. B: Condens. Matter Mater.
State Chem., 2009, 182, 2279–2284. Phys., 2006, 73, 0814011–0814014.
19 J. Xiang, X. Wen and F. Zhang, Ind. Eng. Chem. Res., 2014, 53, 42 M. Hulman, H. Kuzmany, P. M. F. J. Costa, S. Friedrichs and
15600–15610. M. L. H. Green, Appl. Phys. Lett., 2004, 85, 2068.
20 T. Zeng, X. Zhang, S. Wang, H. Niu and Y. Cai, Environ. Sci. 43 J. G. Wang, Y. A. Lv, X. N. Li and M. D. Dong, J. Phys. Chem. C,
Technol., 2015, 49, 2350–2357. 2009, 113, 890–893.
21 L. Wang, J. Zhang, X. Yi, A. Zheng, F. Deng, C. Chen, Y. Ji, 44 P. Kondratyuk and J. T. Yates, Acc. Chem. Res., 2007, 40, 995–
R. E. Smalley, Nature, 1996, 384, 147–150.
F. Liu, X. Meng and F.-S. Xiao, ACS Catal., 2015, 5, 2727–
1004.
2734.
45 C. H. Kiang, Carbon, 2000, 38, 1699–1701.
22 W. J. Yu, C. Liu, P. X. Hou, L. Zhang, X. Y. Shan, F. Li and 46 B. Burteaux, A. Claye, B. W. Smith, M. Monthioux, D. E. Luzzi
H. M. Cheng, ACS Nano, 2015, 9, 5063–5071. and J. E. Fischer, Chem. Phys. Lett., 1999, 310, 21–24.
23 Y. Du, J. Su, W. Luo and G. Cheng, ACS Appl. Mater. 47 X. C. Sun and X. L. Dong, Mater. Res. Bull., 2002, 37, 991.
Interfaces, 2015, 7, 1031–1034. 48 S. Wu, J. Colloid Interface Sci., 2003, 259, 282–286.
24 Y.-J. Gu and W.-T. Wong, Langmuir, 2006, 22, 11447–11452. 49 Y. Chen, J. Qiu, X. Wang and J. Xiu, J. Catal., 2006, 242, 227–
25 G. Guo, F. Qin, D. Yang, C. Wang, H. Xu and S. Yang, Chem.
Mater., 2008, 20, 2291–2297.
230.
58812 | RSC Adv., 2016, 6, 58805–58812
This journal is © The Royal Society of Chemistry 2016