Confined iron nanowires enhance the catalytic activity of carbon nanotubes in the aerobic oxidation of cyclohexane

Article abstract of DOI:10.1002/cssc.201100807

Inside job: New applications of carbon materials pave the way towards greener chemical syntheses. The encapsulation of metallic Fe within CNTs improves electron transfer between the metal and the CNTs. The resulting material offers a high catalytic activity and easy magnetic separation of catalyst in the heterogeneous selective oxidation of cyclohexane. Copyright

Full text of DOI:10.1002/cssc.201100807

DOI: 10.1002/cssc.201100807
Confined Iron Nanowires Enhance the Catalytic Activity of Carbon
Nanotubes in the Aerobic Oxidation of Cyclohexane
[
a]
Xixian Yang, Hao Yu,* Feng Peng,* and Hongjuan Wang
Among recently emerged novel catalysts, metal-free carbon-
hexoyl hydroperoxide radicals, thereby increasing the rate of
[
1]
[8]
based materials show promise in oxidative/direct dehydro-
the autocatalytic reaction. According to previous experimen-
[
2]
[3]
genations of alkanes, the oxygen reduction reaction, wet-air
tal observations, increasing the electron mobility of CNTs can
[
4]
[5]
[8]
oxidation of phenol water, methane decomposition, acrole-
be expected to improve their activity. We modified the elec-
[
6]
[7]
in oxidation, alcohol oxidation, and alkyne hydration. Re-
cently, we succeeded in using both pristine and nitrogen-
doped CNTs to catalyze the aerobic oxidation of cyclohexane
in the liquid phase; an extremely important reaction for pro-
tronic characteristics by filling the interior of CNTs, inspired by
the fact that confined catalysts may be tailored by the elec-
tron-deficient environment inside CNTs, caused by the p elec-
tron density shifting from the concave surface to the convex
[8]
[9b]
ducing cyclohexanol, cyclohexanone, and adipic acid (AA).
surface.
When CNTs are filled with metals, charge transfer
These new applications of carbon materials pave the way to-
wards greener chemical syntheses.
between graphite walls and encapsulated metals may result in
an increased electron density of the CNTs’ outside convex sur-
faces. Meanwhile, the catalytically active surface area does not
decrease because the surface area that contacts the reactants
is not occupied, and thus the oxidative activity may be
enhanced.
The excellent catalytic performances of carbons are linked to
their manifold structures and morphologies. One of the most
[
9]
interesting is the confinement effect of CNTs. Results from
[
10]
Bao and co-workers have shown that the electronic charac-
teristics of Fe O3 change dramatically if encapsulated inside
The above-mentioned strategy was realized by in situ filling
of Fe into CNT interiors during the growth of CNTs by chemical
vapor deposition, using ferrocene as catalyst and a mixture of
2
CNTs, allowing to tailor their catalytic performance in Fischer–
Tropsch synthesis. The confined catalysts also show improved
[
11]
[14]
activity and selectivity in CO hydrogenation, ammonia de-
xylene and dichlorobenzene (DCB) as carbon sources. Scan-
[
12]
[13]
composition, and cinnamaldehyde hydrogenation. Howev-
er, most of the studies on the confinement effect focus on
modifying the catalytic agent inside the CNTs. How the catalyt-
ic properties of CNTs are influenced by the filled interiors is
rarely reported.
ning electron microscopy (SEM) and transmission electron mi-
croscopy (TEM) images of as-synthesized CNTs are shown in
Figure 1. Without co-feeding DCB with xylene, a conventional
vertically aligned CNT array was obtained. Adding DCB into the
precursor resulted in dramatically increased filling rate of Fe.
Most nanotubes contained long nanowires, up to 1 mm in
length, inside the channels (see Supporting Information, Fig-
ure S1). The introduction of Cl-containing DCB resulted in
a random orientation of the CNTs and the formation of Fe par-
ticles coated by carbon shells, while the size distributions of
Fe-filled CNTs were very similar (see Figure S2). XRD patterns of
the Fe-filled CNTs (see Figure S3) indicate that the nanostruc-
tures located inside the CNTs are composed of a-Fe and g-Fe.
A drastic increase of Fe content, from 5.3 to 19.0 wt%, oc-
curred when the amount of DCB was increased from 0 to
Herein, we report a strategy to enhance the catalytic activity
of CNTs in cyclohexane oxidation through the interaction be-
tween graphite walls and confined metallic nanowires
(Scheme 1). CNTs have been demonstrated to stabilize cyclo-
6.25% (see Table 1). Further increasing the DCB concentration
did not further increase the Fe content. Even a sample pre-
pared with 25% DCB in xylene contained 20.5 wt% Fe.
The Fe-filled CNTs were used to catalyze cyclohexane oxida-
tion. The reaction was carried out in a mechanically stirred
300 mL autoclave in batch mode, operated at 1258C and
Scheme 1. Oxidation of cyclohexane over metal-filled CNTs.
1
.5 MPa O with cyclohexanone as initiator (which reacts with
2
cyclohexyl hydroperoxide and energetically assists in the cleav-
[15]
age of the RO-OH bond ) and acetone as solvent. Table 1
compares the catalytic performances of the different Fe-filled
CNTs. Varying the DCB content (and thus the Fe content) has
a significant impact on the catalytic behavior of the Fe-filled
CNTs. The conversion of cyclohexane at 8 h was elevated from
[
a] X. Yang, Dr. H. Yu, Prof. F. Peng, Dr. H. Wang
School of Chemistry and Chemical Engineering
South China University of Technology
Guangzhou, Guangdong, 510640 (PR China)
Fax: (+86)020-87114916
E-mail: cefpeng@scut.edu.cn
19.1 to 36.7%, corresponding to a twofold increase of the ini-
yuhao@scut.edu.cn
tial specific conversion rate when the Fe content increased
from 5.3 to 19.0 wt% (entries 1 and 3). Meanwhile, the AA se-
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cssc.201100807.
ChemSusChem 0000, 00, 1 – 5
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cause refluxing the Fe-filled CNTs in HCl to remove washable
Fe resulted in a decrease of the conversion (Figure S5). The
specific activity declined between entries 3 and 7 (DCB content
increased from 6.25 to 25%) although the Fe filling rate slight-
ly increased from 19.0 to 20.5 wt%, suggesting that further in-
creasing the Fe content cannot elevate the activity (see below
for more details). Compared to a control experiment using
commercial CNTs of >99% purity (entry 9), the sample pre-
pared with 19.0 wt% Fe provided higher conversion of cyclo-
hexane by 5.8%, while displaying a much higher specific activi-
ty normalized by surface area. At this stage, the synthesis of
Fe-filled CNTs has not been fully optimized, and the intrinsic
activity can be further increased by synthesizing Fe-filled CNTs
with high surface areas.
The Fe filling endows the CNTs with strong saturation mag-
netization, allowing for easy separation of catalyst from the
mixture of products and reactants. As shown in Figure 2, the
saturation magnetization of the sample prepared with 19 wt%
ꢀ
1
Fe (6.25% DCB) reached 28.62 emug , about 6 times that of
the CNTs produced with pure xylene. With the assistance of
a magnet, the Fe-filled CNTs were recycled and reused for 3
runs, offering stable catalytic performances (Table 1, entry 4)
demonstrating the excellent stability of Fe-filled CNTs.
To verify whether iron oxide contributed to the catalytic ac-
tivity, we deposited 4.2% FeO on CNTs that were thoroughly
x
washed with hydrochloric acid. There was no obvious change
in cyclohexane conversion before and after loading, indicating
that deposited FeO does not participate in the catalysis (see
x
Supporting Information for further discussion). No Fe and Cl
species were detected on the CNT surfaces by X-ray photoelec-
tron spectroscopy (XPS) and SEM–EDS (Figures S6 and S7), sug-
gesting that most Fe was encapsulated in the CNTs. However,
some Fe was accessible, probably through the opened ends of
the CNTs. This Fe can be more active than that deposited on
CNTs, and its oxidation/reduction behavior can be different be-
Figure 1. a,b,d,e,h) SEM, c,f,i) TEM, and g,j) HRTEM images of Fe-filled CNTs,
synthesized by DCB-assisted chemical vapor deposition. The DCB contents
of the carbon precursor were 0% (a–c), 3% (d–g), or 6.25% (h–j).
[10]
lectivity over the sample with 19.0 wt% Fe reached 60.5%
cause of the confinement effect. To rule out Fe activity, the
amount of accessible Fe was measured by refluxing Fe-filled
CNTs in HCl. It was found that the specific activity was unrelat-
ed to the amount of accessible
(entry 3), indicating a promising selective production of AA.
Filling with Fe indeed considerably improved the activity, be-
Fe, indicating that the oxidation
reaction is not directly catalyzed
Table 1. Properties and catalytic performances of Fe-filled CNTs prepared with precursors containing different
amounts of DCB.
[
a]
by confined Fe (Figure S8).
Homogenous catalysis, due to
Fe leaching in the presence of
adipic acid, is unlikely to play an
important role in our experi-
ments. Firstly, although there is
no adipic acid at the beginning
of reaction, the initial reaction
rate was always highest, demon-
strating that at least the initial
stage of the reaction was not
homogenously catalyzed by Fe.
Secondly, with the leaching hy-
pothesis the concentration of Fe
in the reaction mixture would in-
crease during the course of the
[
b]
[c]
[d]
Entry DCB con- Fe content Specific surface
I
D
/I
G
Specific activity Conver- Selectivity [%]
2
ꢀ1
ꢀ2 ꢀ1
[e]
[f]
[g]
tent [%] [wt%]
area [m g
]
(Raman) [mmolm
h
]
sion [%] Cy=O CyꢀOH AA
1
2
3
4
5
6
7
8
9
0
3
6.25
6.25
6.25
13
25
control
control
5.3
12.3
19.0
–
13.5
19.7
20.5
–
34.8
34.9
32.7
–
33.5
26.9
27.9
–
0.47
0.49
0.51
–
0.51
0.60
0.62
–
4.58
5.05
9.24
–
6.81
3.45
3.72
–
19.1
25.2
36.7
37.1
29.8
14.7
14.2
5.9
17.7
11.0
9.9
10.4
14.5
17.1
18.2
21.5
14.9
24.6
19.3
19.1
11.8
14.3
21.3
28.6
44.8
13.7
46.3
58.6
60.5
63.4
58.4
49.1
43.1
14.8
58.6
[
[
h]
i]
[
[
j]
k]
<1
187
0.83
1.79
30.9
[
2
a] Reaction conditions: 398 K, 1.5 MPa O , 8 h, 93.6 g cyclohexane, 2.6 g cyclohexanone, 64 g acetone, 16.2 g
butanone, 200 mg catalyst. [b] TGA data. [c] Initial rate of cyclohexane consumption per square meter of cata-
lyst surface. [d] The byproducts include cyclohexyl hydroperoxide, glutaric acid, succinic acid, and other unde-
tected acids. [e] Cyclohexanone. [f] Cyclohexanol. [g] Adipic acid. [h] Recycled 3 times. [i] Refluxed in 6 molL
HCl for 1 h. [j] Without catalyst. [k] With commercial CNTs (CNano Technology Limited), containing <1% Fe.
ꢀ
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Figure 2. Magnetization curves of Fe-filled CNTs synthesized with 0% and
.25% DCB concentrations in precursor. The right panel shows the digital
6
Figure 3. Dependence of the specific activity of cyclohexane oxidation over
Fe-filled CNTs on work function.
photos of the suspensions after 8 h cyclohexane oxidation reaction (entry 3
in Table 1) standing for 1 min with and without magnetic field.
reaction, because adipic acid is formed gradually, and thus the
reaction rate should increase with time. However, this was not
observed (Figure S5). Finally, leaching should be faster if Fe is
loaded on the outer surfaces of the CNTs, due to the huge dif-
ference in accessibility. However, the activities of the CNTs and
Fe-filled CNTs have a wide spectrum of Fe content, defective-
ness, and morphology, the dependence describes the struc-
ture–activity relation so well that it becomes an effective index
to define the CNTs activity in the aerobic cyclohexane oxida-
tion. Such a structure–activity correlation is very different from
the mechanisms of carbon catalysis proposed in literature,
which attribute the activity to the defects or the activated
FeO /CNT were comparable.
x
The enhanced specific activity when properly filling CNTs
with Fe nanowires can be attributed to modified electron
transfer between the two. Metallic catalysts are known to
[5b,6,20]
oxygen on them,
which usually increases the number of
defects. In this work, the relationship between defectiveness
and activity is weak, as shown in Table 1. For example, the HCl
refluxing introduced no more defects on the CNTs, character-
ized by unchanged I /I before and after the treatment, but an
[
16]
donate electrons toward graphite supports. In the case of
[
17]
CNTs, a theoretical study from Gao and Zhao indicates that
metal clusters located inside CNTs donate more electrons to
the CNT walls than the cluster on the outside surface of the
CNTs, due to the electrostatic potential caused by the curva-
ture effect. The improved electron transfer due to ferromag-
netic metal filling inside CNTs has been applied to fabricate
D
G
obvious decrease in specific activity was observed. Meanwhile,
the decreased activity can be described well by the increased
work function of the HCl-washed sample, as shown in Figure 3.
The implication of the dependence of activity on the work
function is that the oxidation is facilitated if electrons can be
removed more easily from CNT surfaces. It has been previously
demonstrated that the formation of a CNT–cyclohexoyl hydro-
peroxide-radical super-molecular complex is of crucial impor-
tance in the oxidation. The present work clearly shows that it
is possible to tailor the catalytic performance by tuning the en-
ergetic barrier of forming the complex through filling the CNT
channels with metallic Fe. It paves a new way to design novel
carbon catalysts combined with magnetism facilitating separa-
tion of the catalyst from suspension.
[
18]
CNT-based field emitters. The electron transfer from confined
metals to CNT walls may be maximized by intimate Ohmic con-
tact between the metals and internal CNT channels, leading to
a high electron density on CNT surfaces. On the other hand,
the increased defects in graphene sheets with the increased
DCB content, as indicated by the ID/IG ratio obtained from
Raman spectra (Table 1), result in a decrease of conductivity
[
19]
and electron mobility. This factor may be responsible for the
decrease of specific activity between samples prepared with
DCB contents of 6.25 to 25%. Within this range, although the
Fe content changed little, the high level of defects increased
the I /I ratio from 0.5 to 0.6, suppressing charge transfer.
In conclusion, Fe-filled CNTs prove suitable catalysts for the
aerobic oxidation of cyclohexane. An unprecedented activity is
obtained by filling CNTs with about 20 wt% metallic Fe, main-
taining the low-defect nature of the CNTs. The encapsulated Fe
allows for an easy magnetic separation of catalyst. The en-
hancement is due to improved electron transfer from Fe to the
CNTs, suggesting a promising method to enhance the catalysis
of external surfaces of CNTs from within.
D
G
A quantitative analysis helps to understand the dependen-
ces of activity on Fe content and defectiveness, shown in
Table 1. We used the work function (measured by UV photo-
electron spectroscopy, Figure S9) of Fe-filled CNTs as a measure
of the ability to donate electrons. Figure 3 shows the relation
between work function and specific activity. The specific activi-
ty of cyclohexane oxidation exponentially decreases with work
function, according to the equation
Experimental Section
1
0
ꢀ4:8F
r ¼ 2:5 ꢁ 10 e
Fe-filled CNTs were produced by chemical vapor deposition (CVD),
using ferrocene as catalyst and a mixture of xylene and dichloro-
benzene as carbon source in a horizontal tubular quartz furnace of
4 cm inner diameter. The concentration of DCB in the mixture was
where r is the specific conversion rate of cyclohexane
ꢀ
2
ꢀ1
(
mmolm h ) and F is the work function (eV). Although the
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www.chemsuschem.org
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varied from 0 to 3, 6.25, 13, and 25% (v/v). Ferrocene was dis-
Acknowledgements
solved in the DCB/xylene mixture to form solutions with concentra-
ꢀ1
tion of 0.1 gmL . A quartz slide was placed in the middle of the
furnace to collect CNTs. When the temperature of the reaction
region reached 8008C, the solutions were injected by a syringe
This work was supported by the National Science Foundation of
China (21133010, 20806027) and the Guangdong Provincial Na-
tional Science Foundation (9251064101000020). The authors
thank Dr. Rong Xiang at Sun Yat-Sen University for UPS measure-
ments and helpful discussion.
ꢀ
1
pump at a rate of 0.05 mLmin
for 2 h, accompanied with
6
0 sccm H and 400 sccm Ar. To test the role of Fe, as-grown CNTs
2
ꢀ
1
were refluxed in 6 molL HCl for 1 h to remove filled Fe.
The Fe content was measured by performing thermogravimetric
analysis (Netzsch, STA449C) in air from 30 to 8008C at a ramping
rate of 108Cmin and atomic absorption spectroscopy (TRACE
Keywords: carbon · electron transfer · iron · nanotubes ·
oxidation
ꢀ1
S4AA). BET specific surface areas were measured by N adsorption
2
at liquid N temperature in an ASAP 2010 analyzer. Raman spectra
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Received: December 10, 2011
Revised: January 30, 2012
Published online on && &&, 0000
ChemSusChem 0000, 00, 1 – 5
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemsuschem.org
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5
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These are not the final page numbers!

COMMUNICATIONS
X. Yang, H. Yu,* F. Peng,* H. Wang
&& – &&
Confined Iron Nanowires Enhance the
Catalytic Activity of Carbon
Nanotubes in the Aerobic Oxidation of
Cyclohexane
Inside job: New applications of carbon
materials pave the way towards greener
chemical syntheses. The encapsulation
of metallic Fe within CNTs improves
electron transfer between the metal
and the CNTs. The resulting material
offers a high catalytic activity and easy
magnetic separation of catalyst in the
heterogeneous selective oxidation of cy-
clohexane.
&6
&
www.chemsuschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemSusChem 0000, 00, 1 – 5
ÝÝ
These are not the final page numbers!