Enhancing the catalytic activity of carbon nanotubes by filled iron nanowires for selective oxidation of ethylbenzene

Article abstract of DOI:10.1016/j.catcom.2014.03.031

Iron nanowire filled carbon nanotubes (Fe@CNTs) were synthesized by chemical vapor deposition method and employed as heterogeneous catalysts for selective oxidation of ethylbenzene to acetophenone with molecular oxygen. The results showed that filled iron

Full text of DOI:10.1016/j.catcom.2014.03.031

Catalysis Communications 51 (2014) 77–81
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Catalysis Communications
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Short Communication
Enhancing the catalytic activity of carbon nanotubes by filled iron
nanowires for selective oxidation of ethylbenzene
Jin Luo a,b, Hao Yu b, Hongjuan Wang b, Feng Peng b,
⁎
a
School of Chemistry Science and Technology, Institute of Physical Chemistry, Zhanjiang Normal University, Zhanjiang 524048, PR China
School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510640, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Iron nanowire filled carbon nanotubes (Fe@CNTs) were synthesized by chemical vapor deposition method and
employed as heterogeneous catalysts for selective oxidation of ethylbenzene to acetophenone with molecular
oxygen. The results showed that filled iron nanowires efficiently enhanced the catalytic activity of CNTs, arising
from improving electron transfer. Moreover, Fe@CNTs could be easily separated from the reaction mixtures by
external magnet and displayed excellent stability with no significant loss of catalytic activity after six cycling
reactions. The result presented herein paves the way for the development of novel carbon catalysts for the liquid
phase oxidation of ethylbenzene.
Received 13 January 2014
Received in revised form 3 March 2014
Accepted 25 March 2014
Available online 2 April 2014
Keywords:
Heterogeneous catalysis
Carbon nanotubes
Ethylbenzene
© 2014 Elsevier B.V. All rights reserved.
Aerobic oxidation
1. Introduction
surface and an electron-enriched exterior surface [9,15]. In our previous
work [14], we succeeded in using iron filled CNTs (Fe@CNTs) to catalyze
Recently, the selective oxidation of ethylbenzene (EB) to higher-
value added product acetophenone (AcPO) has received continuous
growing interest, because AcPO is used as an important intermediate
for the synthesis of pharmaceuticals, perfumes and resins [1,2]. Com-
mercially, AcPO is produced by liquid phase cobalt-catalyzed oxidation
of EB using molecular oxygen with acetic acid as solvent and manganese
or bromide species as promoters [3,4]. However, not only is it unfriendly
to the environment but it is also difficult to separate and recycle the
catalysts from the reaction mixture. Fortunately, heterogeneous cataly-
sis can overcome these problems. Especially, ferromagnetic metal filled
carbon nanotubes (CNTs) have been widely studied in catalysis owing
to their unique structures and significant magnetic properties [5–7].
On the one hand, the ferromagnetic metallic encapsulates are effectively
protected by CNTs from oxidation and show long-term stability [8]. On
the other hand, the well-defined nanosized channels of CNTs formed by
graphene layers provide an intriguing confinement environment for
nanocatalysts and catalytic reactions [9]. More importantly, the con-
fined metal nanoparticles into CNTs have already proclaimed the ability
to improve the catalytic activity in several important reactions such as
syngas conversion [10,11], hydrogenation [12], ammonia decomposi-
tion [13] and oxidation [6,14]. The enhancement effect was caused by
electron-deficiency, that is, the curvature of CNT walls causes the π
electron density of the graphene layers to shift from the concave inner
to the convex outer surface, leading to an electron-deficient interior
the aerobic oxidation of cyclohexane in liquid phase, and demonstrated
that the enhancement of activity was caused by the improved electron
transfer from iron to CNTs.
Although the influence of the Fe-filling of CNTs on the catalytic prop-
erties has been described for the oxidation of cyclohexane (naphthenic
hydrocarbon) for the first time, there is no report on other oxidation
reaction systems [14]. Recently, we succeeded in using CNTs to catalyze
the aerobic oxidation of ethylbenzene in liquid phase, and demonstrat-
ed that the electron transfer in graphene skeletons of CNTs played an
important role [16]. Therefore, it is worth further proving our viewpoint
in the liquid-phase selective oxidation of ethylbenzene (aromatic
hydrocarbon) to acetophenone. Herein, we have further modified the
electronic characteristics by filling iron nanowires into CNTs. The results
showed that filled iron nanowires efficiently enhanced the catalytic
activity of CNTs for aerobic oxidation of EB, arising from improving elec-
tron transfer. More importantly, Fe@CNTs could be easily separated
from the reaction mixtures through applying an appropriate magnetic
field, and exhibited excellent potential for industrial application of EB
oxidation to AcPO. The present work clearly shows that it is generally
practical to tailor the catalytic performance by tuning electronic proper-
ty through filling the CNT channels with metallic iron.
2. Experimental
Fe@CNTs were prepared by chemical vapor deposition using ferro-
cene as catalyst and a mixture of xylene and orthodichlorobenzene
(DCB) as carbon sources in a horizontal tubular quartz furnace of 8 cm
⁎
Corresponding author. Tel./fax: +86 20 87114916.
E-mail address: cefpeng@scut.edu.cn (F. Peng).
http://dx.doi.org/10.1016/j.catcom.2014.03.031
1566-7367/© 2014 Elsevier B.V. All rights reserved.

78
J. Luo et al. / Catalysis Communications 51 (2014) 77–81
inner diameter [8,14]. Typically, the concentration of DCB in the mixture
was varied from 0, 2.5, 5, 10 to 20 vol.%. Ferrocene was dissolved in the
DCB/xylene mixture to form solutions with concentration of 0.1 g/mL. A
quartz slide was placed in the middle of the furnace to collect CNTs.
When the temperature of the reaction region reached 800 °C, the
solutions were injected by a syringe pump at a rate of 5 mL/h for 3 h,
accompanied with 150 sccm H2 and 1000 sccm Ar, respectively. Cooled
under Ar to room temperature prior to exposure to air, the correspond-
ing materials were denoted as Fe@CNTs-0, Fe@CNTs-2.5, Fe@CNTs-5,
Fe@CNTs-10 and Fe@CNTs-20, respectively.
To test the role of confined Fe, the unfilled CNTs were prepared by
chemical vapor deposition of xylene with Fe–Mo/Al2O3 as a catalyst
according to our previous work [17]. To remove residual Fe–Mo/Al2O3
catalyst, as-grown catalysts were stirred in concentrated HCl for 6 h
then washed with deionized water to pH = 6–7, and dried in air at
383 K overnight, denoted as CNTs-W. The iron content of CNTs–W
was 3.4% by thermal gravimetric analysis (TGA). We also loaded FeOx
on the CNTs–W to prepare Fe/CNTs–W with 7.8% Fe.
and anisole (1 mL) as an internal standard were added into the auto-
clave, and the reactor was flushed with argon to remove air. Then, the
reactor was heated to a stable operational temperature; subsequently,
pure O2 was fed into the reactor, and the pressure was kept constant
by supplying pure O2 during the reaction. The products were identified
by a Shimazdu GCMS-QP2010 detector and quantitated by Agilent
GC-6820 equipped with a 30 m × 0.25 mm × 0.25 μm HP-5 capillary
column and a flame ionization detector. It is worth noting that the
yield of 1-phenyl-ethyl-hydroperoxide (PEHP) could not be directly
measured by GC because of its thermal stability. According to literature
[18,19], PEHP can be converted quantitatively to 1-phenyl-ethyl alcohol
(PEA) with excessive Ph3P at room temperature. Furthermore, for the
test of reusability, the used catalyst was recovered from the reaction
mixture by an external magnetic force, washed orderly with deionized
water, ethanol and acetone, and dried in air. The experimental results
of oxidation reaction were repeated two times, the mean data were
shown. The range of experimental errors was about 5%.
The BET specific surface areas of the prepared samples were
measured by N2 adsorption at liquid N2 temperature in an ASAP 2010
analyzer. Fe content was measured by thermo gravimetric analysis
(Netzsch, STA449C) in air from 30 to 800 °C at a ramping rate of
10 °C/min. Raman spectra were obtained in a LabRAM Aramis micro
Raman spectrometer excitated at 633 nm with 2 μm spot size. XRD
patterns were recorded on a Bruker D8 ADVANCE diffractometer
which was equipped with a rotating anode using Cu Kα radiation
(40 kV, 40 mA). SEM images were obtained in a LEO 1530VP scanning
electron microscope. TEM images were obtained in a JEM-2010 micro-
scope operating at 200 kV. XPS analysis was performed with a Kratos
Axis ultra (DLD) spectrometer equipped with an Al Kα X-ray source,
the binding energies ( 0.2 eV) were referenced to the C1s peak at
284.6 eV.
3. Results and discussion
3.1. Characterization of catalysts
Fig. 1 shows the SEM and TEM images of the synthesized catalysts.
Without DCB, a well-aligned CNT array was obtained, while added
DCB resulted in random orientation of CNTs. TEM images reveal that
some CNTs were partially filled with long continuous Fe nanowires
and the filling ratio of Fe dramatically increased with the content of
DCB in the precursors increasing. Surprisingly, almost no Fe nanoparti-
cles were attached on the outer surface of CNTs.
Fig. 2(a) depicts the XRD patterns of the synthesized catalysts.
The main diffraction peaks in the 10° b θ b 70° can be assigned to
α-Fe, γ-Fe and graphite structure of CNTs [20,21]. It indicates that
Fe nanowires in synthesized samples are composed of α-Fe and γ-Fe
phases. Fig. 2(b) shows the TGA profiles of the synthesized catalysts.
No remarkable weight loss occurs before 500 °C in air, demonstrating
that they possess high thermal stability and the content of amorphous
The catalytic aerobic oxidation of EB was carried out according to our
previous work [16]. Typically, the oxidation reactions were conducted
in a Teflon-lined 100 mL stainless-steel autoclave equipped with a mag-
netic stirrer at 1100 rpm. EB (5 mL) and CH3CN (30 mL) as the solvent,
Fig. 1. SEM (a, b, d, f, h, j and l) and TEM (c, e, g, i, k and m) images of the Fe@CNTs (a, b and c), Fe@CNTs-2.5 (d and e), Fe@CNTs-5 (f and g), used Fe@CNTs-5 for 6th cycle (h and i), Fe@
CNTs-10 (j and k) and Fe@CNTs-20 (l and m).

J. Luo et al. / Catalysis Communications 51 (2014) 77–81
79
10000
8000
6000
4000
2000
0
a
a
Fe 2p
Graphite
-Fe
-Fe
C1s
a-Fe@CNTs-0
b-Fe@CNTs-2.5
c-Fe@CNTs-5
d-Fe@CNTs-10
e-Fe@CNTs-20
700 705 710 715 720 725 730 735 740
Cl 2p
e
d
c
b
a
194 196 198 200 202 204 206
0
200
400
600
800
1000
10
20
30
40
50
60
70
Binding Energy /eV
2 /degree
b
b
100
80
60
40
20
0
a-Fe@CNTs-0
b-Fe@CNTs-2.5
c-Fe@CNTs-5
d-Fe@CNTs-10
e-Fe@CNTs-20
e
d
c
b
a
0
100 200 300 400 500 600 700 800
Temperature /oC
Fig. 3. The XPS survey spectrum (a) and SEM–EDS result (b) of the Fe@CNTs-5.
c
Fe@CNTs-20
I /I =0.83
D
G
nanostructures [22]. The higher ID/IG ratio implies more defects in the
carbon-based nanostructures. It was noted that the ID/IG ratio increased
with increasing Fe content, indicating that the tubes present lattice
defects and disorders derived from Fe filled in the channel of CNTs.
Fig. 3(a) shows the XPS survey spectrum of the Fe@CNTs-5. It is
observed that there are no Fe and Cl on CNT surfaces, indicating that
most of the iron are encapsulated in CNTs. Moreover, Fig. 3(b) shows
the SEM–EDS result of the Fe@CNTs-5. It is confirmed that the nano-
tubes contain Fe and C, indicating that Cl is not part of the final CNT
structure.
Fe@CNTs-10
I /I =0.73
D
G
Fe@CNTs-5 used for 6th cycle
Fe@CNTs-5
I /I =0.69
D
G
I /I =0.67
D
G
3.2. Oxidation of ethylbenzene
Fe@CNTs-2.5
Fe@CNTs-0
I /I =0.60
D
G
In our previous work [16], we have reported that CNTs could
successfully catalyze the oxidation of EB with molecular oxygen, and
demonstrated that the supported Fe species on the surface of CNTs did
not contribute to the activity. The main products are AcPO, PEA, PEHP,
benzaldehyde (BA) and benzoic acid (BzA). Table 1 compares the
catalytic performances of the different carbon catalysts. The blank
experiment without carbon catalyst gives an EB conversion of 9.2%
with 57.2% selectivity for PEHP as the main product at 155 °C for 3 h,
as a result of the autoxidation reaction [23]. Surprisingly, the Fe@
CNTs-0 exhibited much higher activity than the CNTs-W, which was
not filled Fe nanoparticles according to our previous work [17]. In
particular, the initial specific surface activity of Fe@CNTs-0 was
2.4 times as high as that of the unfilled CNTs (Table 1, E2 and 4). More-
over, compared with the unfilled CNTs, Fe@CNTs-0 obtained much
higher selectivity for AcPO and lower selectivity for PEHP. It is indicated
that filled Fe nanowires can accelerate the PEHP decomposition to AcPO
and indeed contribute to the considerable improvement of activity due
to improving electron transfer from Fe nanowires to the CNTs. It was
I /I =0.49
D
G
800
1000
1200
1400
1600
1800
2000
Raman shift /cm-1
Fig. 2. XRD patterns (a), TGA patterns (b) and Raman spectra (c) of the synthesized
catalysts.
carbon soot can be neglected. It is noted that the Fe residue after TGA is
in the form of Fe2O3 and the content of Fe sharply increased with DCB
from 0% to 10%. However, further increasing the DCB concentration
did not contribute to the increase of Fe content. Fig. 2(c) shows two
characteristic Raman peaks at approximately 1570 cm−1 (G-band)
and 1320 cm−1 (D-band). The intensity ratio of D-band to G-band,
namely, ID/IG, is used to evaluate the defects of carbon-based

80
J. Luo et al. / Catalysis Communications 51 (2014) 77–81
Table 1
Properties and catalytic performances of the synthesized catalysts.
b
Entry
Catalyst
Fe
SBET
ID/IG
rw
Conv.
(%)
Selectivity (%)
AcPO
(wt.%)a
(m2 g−1
)
PEA
PEHP
BA
BzA
c
1
2
3
4
5
6
7
8
–
–
–
–
–
9.2
28.6
28.3
29.4
31.8
36.8
36.7
53
67
69
38.6
30.4
25.2
17.9
55.3
53.9
59.5
59.8
60.2
59.5
74
87
92
94
58.2
53.5
16.3
9.1
9.7
6.6
5.4
5.9
7.8
26
57.2
11.9
10.1
1.1
0.8
0.6
0.2
–
8.6
11.1
12.9
10.1
11.8
12.2
9.8
–
0
CNTs-W
3.4
7.8
6.4
12.8
17.3
16.9
–
45.7
49.5
32.9
30.1
25.9
–
1.01d
1.06d
0.49
0.60
0.67
0.69
–
0.05
0.05
0.12
0.16
0.25
–
12.6
13.4
22.7
22.2
21.1
22.7
–
Fe/CNTs-W
Fe@CNTs-0
Fe@CNTs-2.5
Fe@CNTs-5
Fe@CNTs-5e
SiO2/Al2O3–Mnf
SiO2/Al2O3–Mng
SiO2/Al2O3–Coh
TPFPPCoi
274
274
281
–
–
9
–
–
–
–
–
5
6
8
–
10
11
12
13
–
–
–
–
–
–
–
–
5.9
8.7
11.2
Fe@CNTs-10
Fe@CNTs-20
20.9
21.8
20.3
18.5
0.73
0.83
0.20
0.18
1.9
5.9
11.5
12.9
19.7
16.5
Reaction condition: 30 mL CH3CN, 5 mL EB, 100 mg catalyst, 1.5 MPa O2, 155 °C, 3 h, EB conversion and main product selectivity were determined by GC.
a
TGA data, refers to metal iron.
b
Initial rate of EB consumption per m2 of catalyst surface at 1 h, mmol·m−2·h−1
.
c
d
e
f
Without catalyst.
Ref. [17].
The used Fe@CNTs-5 for the 6th cycle.
Ref. [27]: 100 mg SiO2/Al2O3–Mn (SiO2/Al2O3-trimethoxysilylpropyl amine-bipyridyl ketone-Mn), 5 mL acetic acid, 2 mmol EB, 15 mol% NHPI, 100 °C, 8 h.
g
h
i
Ref. [28]: 50 mg SiO2/Al2O3–Mn (SiO2/Al2O3-trimethoxysilyl propyl amine-bipyridyl ketone-Mn), 2 mmol EB, EB/TBHP (80% aqueous solution) mole ratios of 1/5, 100 °C, 24 h.
Ref. [29]: 5 mg SiO2/Al2O3–Co (SiO2/Al2O3-trimethoxysilyl propyl amine-bipyridyl ketone-Co), 2 mmol EB, 2 mmol TBHP (80% aqueous solution), 80 °C, 24 h.
Ref. [30]: 2 mL EB, 2 mol catalyst, 1.5 MPa O2, 100 °C, 24 h.
reported that this electron transfer on the CNTs could lead to a high
electron density and a decreased local work function on the carbon
surface [14,24,25], which accelerated the decomposition of the most
important intermediate PEHP through the π–π interaction between
the PEHP and graphene sheets of the CNTs, consequently increasing
the overall conversion rate.
[28] or nano-Co(II) [29] and fluorinated metalloporphyrins [30]. The
results shown in Table 1 indicated that their activities were higher
than that of carbon-based catalysts but the Fe@CNTs-5 catalyst revealed
more favorable potential for industrial application because NHPI is a
difficult homogeneous catalyst to separate, a supported metal catalyst
is easy to leach in the presence of acidic medium while fluorinated
metalloporphyrins are expensive and deactivated by irreversible dimer-
ization and it is not easy to recover the catalyst at the end of the reaction
for reuse, and TBHP as oxidant is expensive and leads to new impurity.
However, it still should pay more endeavors to improve the catalytic
activity of carbon-based catalyst for EB oxidation to AcPO.
To further prove the role of confined Fe, Fe/CNTs–W catalyst
was prepared and compared. Although the Fe content of Fe/CNTs–W
was 1.22 times as high as that of Fe@CNTs-0, the initial specific surface
activity of Fe@CNTs-0 was 2.4 times as high as that of the Fe/CNTs–W
(Table 1, E3 and 4). The result clearly indicates that the enhancement
of catalytic activity was not caused by the supported Fe species on the
surface of CNTs but arose from the confined Fe nanowires into CNTs.
Interestingly, the filled Fe content significantly affects the catalytic
behavior of Fe@CNT catalysts. When Fe content increased from 6.4% to
17.3%, the conversion of EB at 3 h was elevated from 29.4% to 36.8%,
corresponding to an increase of initial specific surface activity by 2.1
folds, and the AcPO selectivity kept stable (Table 1, E4 and 6), strongly
implied that properly filling Fe nanowires into the CNTs has indeed
contributed to the considerable improvement of activity. However,
when Fe content further increased from 17.3% to 21.8%, the specific ac-
tivity was decreased from 0.25 to 0.18, indicating that further increasing
Fe content could not elevate the activity. It is probable that the increased
defects in graphene sheets with the increased Fe content resulted in a
decrease of conductivity and electron mobility [26] or an increase of
the work function of Fe@CNTs [14], which suppressed charge transfer.
The magnetic separation and recycling of the Fe@CNTs-5 catalyst
were investigated as shown in Fig. 4. Fe@CNTs-5 could be easily recov-
ered from the reaction mixture by an external magnetic force, and
showed outstanding recyclability. During six cycling tests, there was
almost no difference in both the conversion of EB and selectivity for
AcPO. As shown in Figs. 1 and 2(c), it is observed that the morphology
and ID/IG ratio of the used catalyst were unchanged obviously after
6 cycles. It indicates that Fe@CNTs-5 are promising heterogeneous
catalysts for industrial application of EB oxidation to AcPO.
a
50
b
EB
BzA
BA
PEHP
PEA
AcPO
100
80
60
40
20
0
40
30
20
10
0
1
2
3
4
5
6
Cycle
Moreover, we also compared with other reported catalysts such as
N-hydroxyphthalimide (NHPI) and SiO2/Al2O−3 supported manganese
catalyst [27], nano-sized SiO2/Al2O3 mixed oxides supported nano-Mn
Fig. 4. Magnetic separation (a) and recyclability (b) of the Fe@CNTs-5 in liquid-phase EB
oxidation.

J. Luo et al. / Catalysis Communications 51 (2014) 77–81
81
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This work was supported by the National Natural Science Founda-
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and Technology Project (No. BE2012127), the Program for New Century
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Product Technology (GC201205) and Zhanjiang Normal University
Natural Science Foundation for the Ph.D. Start-up Program (ZL1308).
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