Preparation of Pt@Fe2O3 nanowires and their catalysis of selective oxidation of olefins and alcohols

Article abstract of DOI:10.1002/chem.201003429

Iron oxide coated platinum nanowires (Pt@Fe₂O₃ NWs) with a diameter of 2.8 nm have been prepared by the oxygen oxidation of FePt NWs in oleylamine. These "cable"-like NWs were characterised by transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy and X-ray absorption fine structure analysis. These Pt@Fe ₂O₃ NWs were used as "non-support" heterogeneous catalysts in oxidation of olefins and alcohols. The results revealed that it is an active and highly selective catalyst. Styrene derivatives were tested with molecular oxygen as the sole oxidant, with benzaldehyde successfully obtained from styrene in an absolute yield of 31 %, whereas the use of tert-butyl hydroperoxide as the sole oxidant in the oxidation of alcohols led to yields of more than 80 % of the corresponding ketone or aldehyde. This unsupported catalyst was found to be more active (TOF=96.5 h^-1) than other reported Fe₂O₃ nanoparticle catalysts and could be recycled multiple times without any notable decrease in activity. Our findings will extend the use of such nanomaterial catalysts to new catalytic systems.

Full text of DOI:10.1002/chem.201003429

DOI: 10.1002/chem.201003429
Preparation of Pt@Fe2O3 Nanowires and their Catalysis of Selective
Oxidation of Olefins and Alcohols
Haiyan Hong,[a] Lei Hu,[a] Min Li,[a] Junwei Zheng,[b] Xuhui Sun,[c] Xinhua Lu,[a]
Xueqin Cao,*[a] Jianmei Lu,*[a] and Hongwei Gu*[a]
Abstract: Iron oxide coated platinum
nanowires (Pt@Fe2O3 NWs) with a di-
ameter of 2.8 nm have been prepared
by the oxygen oxidation of FePt NWs
in oleylamine. These “cable”-like NWs
were characterised by transmission
electron microscopy, X-ray diffraction,
X-ray photoelectron spectroscopy and
X-ray absorption fine structure analy-
sis. These Pt@Fe2O3 NWs were used as
“non-support” heterogeneous catalysts
in oxidation of olefins and alcohols.
The results revealed that it is an active
and highly selective catalyst. Styrene
derivatives were tested with molecular
oxygen as the sole oxidant, with ben-
zaldehyde successfully obtained from
styrene in an absolute yield of 31%,
whereas the use of tert-butyl hydroper-
oxide as the sole oxidant in the oxida-
tion of alcohols led to yields of more
than 80% of the corresponding ketone
or aldehyde. This unsupported catalyst
was found to be more active (TOF=
96.5 hÀ1) than other reported Fe2O3
nanoparticle catalysts and could be re-
cycled multiple times without any nota-
ble decrease in activity. Our findings
will extend the use of such nanomateri-
al catalysts to new catalytic systems.
Keywords: heterogeneous catalysis ·
iron · nanowires · oxidation · plati-
num
Introduction
and -particles can display novel characteristics in oxidation
reactions.[4] Unfortunately, most metal catalysts fail the
other green requirements, as they are toxic, difficult to
handle and require high activation temperatures.
Selective oxidation is fundamentally important in many
food processing, pharmaceutical and fine chemical process-
es.[1] It is also widely used in chemical laboratories and in-
dustries, which makes research in this topic pertinent. In
recent studies, the focus has been on economic metal cata-
lysts that are environmentally friendly (green catalysts).[2]
One of the requirements of green catalysts is that they must
be able to be separated from the target material easily once
the reaction is completed. Some proposed catalysts in this
category are heterogeneous materials such as nanoscale
metals.[3] Studies have demonstrated that metal nanocrystals
Iron oxide has been established as a green catalyst or sup-
port because of its low toxicity, mild reaction conditions and
high catalytic activity and has recently become the focal
point of intensive scientific research.[5] In 2007, Shi et al.[6]
demonstrated the use of “free” g-Fe2O3 nanoparticles to
achieve 19% conversion in the oxidation of styrene with
99% selectivity. They also found that this catalyst could be
used for the oxidation of alcohols. Supported iron oxide
nanoparticles have been reported to be an active and reusa-
ble catalyst in the microwave-assisted selective N-alkylation
of anilines with benzyl alcohols mediated by hydrogen auto-
transfer.[7] Hutchings and co-workers[8] reported that gold
nanocrystals adsorbed on iron oxide have exceptional prop-
erties in oxidation catalysis, including the oxidation of
carbon monoxide at ambient temperature. Volpe and co-
workers[9] used iron oxide as the support for gold catalysts
in the hydrogenation of unsaturated aldehydes with higher
selectivity.
[a] H. Hong, L. Hu, M. Li, X. Lu, Prof. Dr. X. Cao, Prof. Dr. J. Lu,
Prof. Dr. H. Gu
Key Laboratory of Organic Synthesis of Jiangsu Province
Key Laboratory of Absorbent Materials and
Techniques for Environment
College of Chemistry, Chemical Engineering and Materials Science
Soochow University, Suzhou, 215123 (P.R. China)
Fax : (+86)6588-0905
E-mail: hongwei@suda.edu.cn
lujm@suda.edu.cn
Recently, one-dimensional (1D) nanostructures such as
wires, rods, belts and tubes have sparked an explosive inter-
est in the development of novel materials.[10] Compared with
nanoparticles, one of their attractive characteristics is their
large surface area, which presents much higher catalytic ac-
tivity than nanocrystal catalysts commonly used in organic
or electrochemical catalysis.[11] Non-supported nanostruc-
tures as catalysts will also have a much larger surface, how-
ever, nanoparticle catalysts are usually unstable and coagu-
lation is frequently unavoidable in contrast to the nanowire
xqcao@suda.edu.cn
[b] J. Zheng
Institute of Chemical Power Sources, Soochow University
Suzhou, 215123 (China)
[c] Prof. Dr. X. Sun
Functional Nano & Soft Materials Laboratory (FUNSOM) &
Jiangsu Key Laboratory for Carbon-Based
Functional Materials and Devices
Suzhou, Jiangsu, 215123 (China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201003429.
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FULL PAPER
catalyst. Thus, the investigation of “free” unsupported cata-
lysts with tunable catalytic activity is of great significance
for both academia and industry.
ised PtFex NWs and Pt@Fe2O3 NWs. The diameters of the
PtFex NWs were ꢀ1.9 nm and their lengths were several mi-
crometers (Figure 1A and B). After oxidation, the diameters
increased to ꢀ2.8 nm (Figure 1C and D) and the iron in the
PtFex NWs was oxidised to Fe2O3 by the Kirkendall
effect.[12] An X-ray diffraction (XRD) pattern (Figure S1 of
the Supporting Information) of the Pt@Fe2O3 NWs showed
a small peak at 21.28, which indicates the formation of iron
oxide.[13] X-ray photoelectron spectroscopic (XPS) analysis
(Figure S2) was used to confirm the iron oxide structure.
Peaks corresponding to Fe 2p3/2 and Fe 2p1/2 were observed
at around 710.4 and 721.8 eV, respectively, and are in agree-
ment with literature values for Fe2O3.[14] The oxidation state
of the iron species was measured by X-ray adsorption fine
structure spectroscopy (XAFS) performed in the BL 14W1
beamline (Figure S3). Typical features of Fe2O3 can be ob-
served in the iron K-edge XAFS spectra of the Pt@Fe2O3
NWs.[15] These results further demonstrate the formation of
Fe2O3. Energy dispersive spectroscopy (EDS; Figure S4)
and inductive coupling plasma (ICP) were used to analyse
the components of the Pt@Fe2O3 NWs and the loading of
the Fe2O3 was found to be about 25 wt.%.
Following on from previously reported Fe2O3 catalysts
and one-dimensional nanocatalysts, we have investigated the
catalytic performance of Pt@Fe2O3 NWs in the selective oxi-
dation of olefins or alcohols under mild conditions. The re-
sults reveal that our Pt@Fe2O3 NWs show unique catalytic
activities in oxidation reactions. We were intrigued that such
a structure may enhance the constituent metal properties by
offering higher selectivity while maintaining the catalytic ac-
tivity during oxidation. This unsupported catalyst was found
to be more active (TOF=96.5 hÀ1) than other reported
Fe2O3 nanoparticle catalysts. Note that their tunable length
facilitates recycling of the catalyst. The effect of using a re-
cycled catalyst in styrene oxidation is demonstrated and our
Pt@Fe2O3 NW catalyst could be recycled multiple times
without any notable decrease in activity.
Results and Discussion
Ultra-thin Pt@Fe2O3 NWs were prepared by the oxidation
of PtFex NWs under a gentle flow of oxygen gas in oleyl-
ACHTUNGTRENNUNGamine (Scheme 1). Figure 1 shows representative transmis-
Our initial investigation focused on the selective oxidation
of styrene with Pt@Fe2O3 NWs used as an activated reusable
heterogeneous catalyst (Scheme 2). Table 1 shows the cata-
lytic activities under different reaction conditions. The sol-
sion electron microscopy (TEM) images of the as-synthes-
Scheme 1. Formation of Pt@Fe2O3 NWs.
Scheme 2. Oxidation of styrene using Pt@Fe2O3 NWs as catalyst.
vent, which is very important for reaction selectivity, was
first optimised. Acetonitrile is commonly used as the solvent
in alkene oxidation reactions[16] and provided the highest
yield of benzaldehyde (31.3%; Table 1, entry 2), which is
[6]
higher than those reported with Fe2O3 or a gold cata-
lyst.[17,11b] When toluene was used as the solvent in this reac-
tion, a yield of benzaldehyde of 20.1% was obtained, but
with a higher selectivity (90%; Table 1, entry 6). A higher
selectivity of the epoxide product (30.9%) was obtained in
DMF (Table 1, entry 1), which is a similar result to that ob-
tained in our previous work[11b] using gold nanowires as cata-
lysts. Water was also used as the solvent in this reaction and
9.2% benzaldehyde was obtained with a higher selectivity
(91%; Table 1, entry 8). The Pt@Fe2O3 NWs also showed
high activity (19.3% yield) in the absence of solvent
(Table 1, entry 9).
We also calculated the TOF for this reaction, which can
reach 96.5 and 69.4 hÀ1 with respect to styrene conversion
and benzaldehyde yield in acetonitrile, which are higher
than values determined in other solvents. The catalyst can
be easily recycled by simple centrifugation and washing with
ethanol (Figure 2). The conversion and yield of the second
Figure 1. TEM and high-resolution TEM images of A,B) PtFe NWs and
C,D) Pt@Fe2O3 NWs.
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X. Cao, J. Lu, H. Gu et al.
Table 1. Optimisation of the reaction conditions for the selective oxida-
tion of styrene with a Pt@Fe2O3 NW catalyst.[a]
selectivity of the styrene oxidation reaction (Table 1, en-
tries 2 and 10–13). At a lower temperature (508C), the selec-
tivity and conversion of benzaldehyde were 93.8% and
4.5%, respectively. When the temperature was increased to
608C, the conversion of styrene was 43.5% and the benzal-
dehyde selectivity was 71.9%. At 708C, the conversion of
styrene was 49.5% and the benzaldehyde selectivity was
77.1%. At 908C, the benzaldehyde selectivity was reduced
to 50.5% and 46.3% epoxide selectivity was achieved. The
oxygen pressure was also an important factor. When we per-
formed the reaction at an initial oxygen pressure of 4 atm,
the conversion of styrene was 64.8% and the selectivity of
benzaldehyde and benzoic acid were 48.1 and 12.6%
(Table 1, entry 18), respectively. Thus, at a higher pressure
of oxygen, although the styrene conversion increased, the
yield of benzaldehyde decreased (Table 1, entries 11 and
18).
The reaction was optimised by using four other oxidants
(tert-butyl hydroperoxide (TBHP), benzoyl peroxide (BPO),
H2O2 and air), with molecular oxygen giving the highest
yield of benzaldehyde (38.2%; Table 1, entries 11 and 14–
17). When H2O2 was used as the sole oxidant, the conver-
sion of styrene was low (12.3%) although benzaldehyde
(91.6% selectivity) was the major product. With TBHP as
the sole oxidant, a high selectivity of the epoxide product
(80.8%) was observed with a yield of 27.4%. When BPO
was used as the sole oxidant, a higher conversion of styrene
was obtained but with benzoic acid as the main product
(72.5% in yield).
Entry
Solvent
Oxidant
T
Conversion
Selectivity [%][b]
[oC] [%][b]
a
b
c
d
1
2
3
4
5
6
7
8
DMF
acetonitrile
dioxane
DMSO
CHCl3
toluene
THF
water
solventless
acetonitrile
acetonitrile
acetonitrile
acetonitrile
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
O2
60
60
60
60
60
60
60
60
60
50
70
80
90
45.2
43.5
38.7
37.7
35.9
22.3
18.2
10.1
28.7
4.5
49.5
76.8
63.6
33.9
81.3
12.3
28.1
64.8
49.3 19.7 30.9 0.1
71.9 4.5 23.6 NA
74.4 3.7 21.9 NA
80.6 2.8 16.6 NA
58.2 14.0 27.6 0.2
90.0 1.2 8.4 0.4
81.4 3.7 14.9 NA
91.0 0.2 8.8 NA
67.3 9.6 23.1 NA
93.8 NA 6.2 NA
77.1 4.1 18.8 NA
43.9 7.9 48.2 NA
50.5 3.1 46.3 0.1
19.2 NA 80.8 NA
6.4 89.2 4.4 NA
91.6 NA 8.4 NA
59.8 NA 40.2 NA
48.1 12.6 38.7 0.6
9[c]
10
11
12
13
14
15
16
17
18[d]
acetonitrile TBHP 70
acetonitrile
acetonitrile
acetonitrile
acetonitrile
BPO
H2O2 70
air
O2
70
70
70
[a] Reagents and conditions: 0.8 mg Pt@Fe2O3 nanowires (25 wt.% Fe2O3
in the Pt@Fe2O3 NWs), 4.5 mmol styrene, 2 mL solvent, 24 h, 1 atm
oxygen. [b] Calculated by GC-MS using tert-butylbenzene as an internal
standard. [c] Reagents and conditions: 0.8 mg Pt@Fe2O3 nanowires
(25 wt.% Fe2O3 in the Pt@Fe2O3 NWs), 2 mL styrene, 608C, 24 h, 1 atm.
[d] Initial oxygen pressure of 4 atm.
The versatility of the Pt@Fe2O3 NW catalyst was tested
with a series of substituted styrene derivatives. Table 2
shows the results of the oxidation of olefins to the corre-
sponding aldehydes on the surface of the Pt@Fe2O3 NWs.
The substituent has an important effect on the yield of the
reaction. Higher yields were obtained with electron-donat-
ing substituents (Table 2, entries 1–3). The highest catalyst
activity was exhibited with 1-methoxy-4-vinylbenzene as re-
Table 2. Oxidation of substituted olefin derivatives using Pt@Fe2O3 NWs
as catalyst.[a]
Figure 2. Recovery and reusability of the Pt@ Fe2O3 NWs as catalyst for
the oxidation of styrene to benzaldehyde (reaction conditions: 4.5 mmol
styrene, 2 mL acetonitrile, 24 h, 1 atm oxygen, 708C).
Entry
Substrate
Product
Yield [%][b]
1
24.0 (21.6)
run are lower than those of the first run and are then rela-
tively stable, which indicates that the catalytic activity of the
fresh catalyst is higher than that of the recycled catalyst.
The small quantity of benzoic acid produced in the first re-
action is the most important factor. After the first reaction,
the acid will etch the Fe2O3 and partially deactivate the
Fe2O3. ICP analysis showed 1.1% iron leakage. When the
catalyst was reused, the selectivity towards the benzoic acid
was below the detection limit and only 1.4% iron leakage
was detected by ICP after six cycles.
2
3
4
5
24.7 (22.1)
28.1 (28.0)
10.3 (9.9)
NA
[a] Reagents and conditions: 0.8 mg Pt@Fe2O3 nanowires (25 wt.%
Fe2O3 in the Pt@Fe2O3 NWs), 4.5 mL olefin, 2 mL acetonitrile, 708C,
24 h, 1 atm oxygen. [b] Calculated by GC-MS using tert-butylbenzene as
an internal standard. The values in parentheses are the yields of the iso-
lated products.
The activity of Pt@Fe2O3 at different temperatures was
also investigated with acetonitrile as the solvent. The tem-
perature had a tremendous influence on the conversion and
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Pt@Fe2O3 Nanowires
FULL PAPER
Table 3. Oxidation of alcohols by Pt@Fe2O3 NWs as catalyst.[a]
actant (28.1% yield). When 1-nitro-4-vinylbenzene was used
as the substrate, a yield of only 10.3% of aldehyde was ob-
tained. When 4-vinylbenzoic acid was used as the substrate,
no aldehyde was detected by GC.
Entry Substrate
Product
Oxidant Yield [%][b]
TBHP
O2
41.3
1.6
1
H2O2
TBHP
O2
12.5
93.3
3.7
To demonstrate the unique reactivity and selectivity of
the catalyst, three types of nanomaterials (FePt, Fe2O3 and
Pt@Fe2O3)[18] were synthesised for use in the oxidation of
styrene (Figures S5–S7). The FePt nanoparticles showed no
activity and a yield of only 1.8% of benzaldehyde was ob-
tained (albeit with a high selectivity of 91.4%). When sus-
pended Fe2O3 nanoparticles were used, the yield and selec-
tivity of benzaldehyde were 22.5 and 82.0%, respectively,
which is similar to the reported “free” nano-g-Fe2O3 cata-
lyst.[6] Pt@Fe2O3 nanoparticles were also used as the catalyst
in the oxidation of styrene, with only 6.5% of styrene con-
verted and a yield of benzaldehyde of only 6%. The
Pt@Fe2O3 NWs showed far higher activity than the Fe2O3
and Pt@Fe2O3 nanoparticles, with a yield of benzaldehyde
of 31.3%. The interface between platinum and nano-Fe2O3
has been identified as the active centre for the activation of
O2.[15] This interface activation effect was demonstrated by
the catalytic performances of Pt@Fe2O3 yolk-shell nanopar-
ticles (Figure 3, column 4) and Pt@Fe2O3 nanowires
(Figure 3, column 1). The yolk-shell nanoparticles have no
Pt/Fe2O3 interface and a low conversion of styrene was ob-
served.
2
H2O2
10.8
TBHP
O2
H2O2
TBHP
O2
H2O2
TBHP
O2
80.7
0.6
7.8
72.0
0.8
4.5
13.9
–
3
4
5
H2O2
–
[a] Reagents and conditions: 0.8 mg Pt@Fe2O3 nanowires (25 wt.%
Fe2O3 in the Pt@Fe2O3 NWs), 1 mmol alcohol, 2.5 mmol oxidant (TBHP
or 30% H2O2), 2 mL acetonitrile, 708C, 24 h. [b] Calculated by GC-MS
using tert-butylbenzene as an internal standard.
ceeded rapidly and gave the corresponding aldehyde or
ketone in excellent yields. When hexan-1-ol was used as the
reactant, hexanal was obtained as the main product in a
yield of 13.9%, hexan-2-one was obtained as the main prod-
uct in a yield of 80.7% when hexan-2-ol was used as the re-
actant, and cyclohexanone was obtained in a yield of 72%
from cyclohexanol.
Conclusion
We have presented a facile, reproducible procedure for the
preparation of Pt@Fe2O3 nanowires. Their use as an effec-
tive catalyst in the oxidation of olefins and alcohols in aceto-
nitrile has been demonstrated. This unsupported catalyst
was found to be more active (TOF=96.5 hÀ1) than other re-
ported Fe2O3 nanoparticle catalysts and could be recycled
multiple times without any notable decrease in activity. Our
findings will extend the use of such nanomaterial catalysts
to new catalytic systems.
Figure 3. Activity of Pt@Fe2O3 NWs (in the conversion of styrene and
benzaldehyde selectivity) compared with other catalysts. 1) Pt@Fe2O3
NWs, 2) Fe2O3 NPs, 3) FePt NPs and 4) Pt@Fe2O3 NPs (reaction condi-
tions: 4.5 mmol styrene, 2 mL acetonitrile, 24 h, 1 atm Oxygen, 608C).
Experimental Section
Synthesis of Pt@Fe2O3 nanowires: PtACHTNUGTRENUNG(acac)2 and oleylamine (OAm) were
Notably, the Pt@Fe2O3 NW catalyst is not only active in
the selective oxidation of olefins. Alcohols can also be
smoothly oxidised to the corresponding aldehydes or ke-
tones in the presence of Pt@Fe2O3 NWs (Table 3) by using
TBHP or other oxidants. When benzyl alcohol was used as
the reactant, 41.3% of benzaldehyde was obtained with a
high selectivity of 82%. The highest catalyst activity in the
oxidation of alcohols was obtained by using 1-phenylethanol
as the reactant; a conversion of 93.3% to acetophenone was
observed. Regardless of whether aliphatic alcohols used in
the reaction were linear or cyclic, the oxidation always pro-
mixed at room temperature under an atmosphere of argon and heated at
1208C. The solution was kept at this temperature for 20 min until it
turned brown. [Fe(CO)5] was injected into the hot solution and then the
temperature was raised to 1608C for 30 min without stirring; the solution
turned to black. It was then cooled to room temperature under a gentle
flow of oxygen gas. The black sediment was separated by centrifugation.
The nanowires were washed three times with ethanol and then dispersed
in hexane.
General procedure for catalytic oxidation of olefins over Pt@Fe2O3 NWs:
The catalyst was tested in a sealed tube. Pt@Fe2O3 NWs in hexane, olefin
or alcohol, tert-butylbenzene and a certain solvent were added to the
glass reactor. The glass reactor was sealed and evacuated and flushed
with oxygen three times . The reactions were took place at a certain tem-
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X. Cao, J. Lu, H. Gu et al.
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Received: November 27, 2010
Revised: April 21, 2011
Published online: June 15, 2011
8730
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