Catalytic epoxidation of stilbene with FePt@Cu nanowires and molecular oxygen

Article abstract of DOI:10.1039/c0cc03204b

A novel FePt@Cu nanowire catalyst was prepared by the reduction of Cu(acac)₂ on the surface of FePt nanowires, in oleylamine (OAm). This nanowire catalyst efficiently epoxidised stilbene in the presence of molecular oxygen, and the conversion and selectivity were maintained with repeated use of the catalyst, compared with recycled catalyst.

Full text of DOI:10.1039/c0cc03204b

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Catalytic epoxidation of stilbene with FePt@Cu nanowires and
molecular oxygenw
Lei Hu,a Linyan Shi,a Haiyan Hong,a Min Li,a Qinye Bao,b Jianxin Tang,b Jianfeng Ge,a
Jianmei Lu,*a Xueqin Cao*a and Hongwei Gu*a
Received 12th August 2010, Accepted 9th September 2010
DOI: 10.1039/c0cc03204b
A novel FePt@Cu nanowire catalyst was prepared by the
reduction of Cu(acac)2 on the surface of FePt nanowires, in
oleylamine (OAm). This nanowire catalyst efficiently epoxidised
stilbene in the presence of molecular oxygen, and the conversion
and selectivity were maintained with repeated use of the catalyst,
compared with recycled catalyst.
stilbene in o-xylene using molecular oxygen as the sole oxidant.
The catalyst showed outstanding activity and stability.
The novel FePt@Cu NW catalyst was prepared as shown in
Fig. 1. The precursor (FePt NWs) was prepared following the
method reported by the Sun group,7 in which 200 mg of
Pt(acac)2 was added to 20 ml of oleylamine with the reaction
mixture degassed at 120 1C with Ar for 15 min. As the solution
turned brown, Fe(CO)5 (B0.15 ml) was injected. The tempera-
ture was raised to 160 1C and held there for 30 min. Cu(acac)2
(50 mg in 10 mL OAm) was injected, and the temperature was
then raised to 180 1C for 30 min before being reduced to room
temperature. The black product was collected by repeated
centrifugation from ethanol, and was easily dispersed in
hexane or toluene.
Alkene epoxidation is a key reaction in the fine chemical
industry, and the resultant epoxides are essential precursors
in the synthesis of various important plasticizers, perfumes
and epoxy resins.1 Traditional production of epoxides typically
used chemical oxidants like PhIO, NaClO, and peracids.2
These oxidants are often expensive and tend to be hazardous
to handle. Some epoxidation reactions show nonselectivity for
the epoxide formed, and also lead to undesirable products
resulting in chemical waste. Since the 1980s, microporous
titanium silicalite TS-1 has been used as a catalyst for alkene
epoxidation. However, steric restriction imposed by the micro-
porous structure on the reactant makes it unsuitable for the
epoxidation of bulky alkenes. Novel metallic nanomaterials
have more recently been found to be effective alkene epoxida-
tion catalysts. Quek et al.3 showed cobalt to be an efficient
epoxidation catalyst, as Co-TUD-1 catalysing the trans-stilbene
The FePt and FePt@Cu NWs were characterized by trans-
mission electron microscopy (TEM) and high-resolution TEM
(HRTEM). The diameter of the FePt NWs was about 2 nm,
and their length was over 200 nm (Fig. 1A and E). Cu was
evenly distributed on the surface of the FePt NWs, causing the
diameter to increase to about 4 nm (Fig. 1B and F). For FePt
NWs, the visible lattice fringes corresponded to a spacing of
0.216 nm, and were in agreement with the expected d-spacing
of the (111) plane of the FePt alloy (0.22 nm) (Fig. 1C).
epoxidation produced an 87.7% yield of epoxide. The Valerie
´
Caps4 group suggested Au/TiO2 was a useful catalyst for
trans-stilbene epoxidation in methylcyclohexane, in the
presence of TBHP. Christopher and Linic5 concluded that
Ag nanowire (NW) catalysts exhibited higher selectivity for
ethylene oxide (EO) epoxidation than conventional particle
catalysts. This was attributed to a higher concentration of Ag
(100) surface facets in the NW catalysts.
Single-crystal studies6 have demonstrated that oxygenated
copper is better than oxygenated silver as an epoxidizing agent
for lower alkenes like ethylene, styrene, and trans-methylstyrene
(all analogues of stilbene). In the present work, we have
deposited copper on FePt NW templates to form FePt@Cu
NWs, and then used them as catalysts for the epoxidation of
a 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, China 215123.
E-mail: hongwei@suda.edu.cn, lujm@suda.edu.cn,
xqcao@suda.edu.cn; Fax: +86-65880905; Tel: +86-65880905
b Functional Nano & Soft Materials Laboratory (FUNSOM) &
Jiangsu Key Laboratory for Carbon-Based Functional Materials
and Devices, Suzhou, Jiangsu, China 215123
w Electronic supplementary information (ESI) available: Experimental
procedures, TEM images, XPS results and product characterization
data. See DOI: 10.1039/c0cc03204b
Fig. 1 The FePt@Cu NWs formation from FePt NWs. FePt
NWs: (A) TEM; (C) HRTEM and (E) histograms of diameters.
FePt@Cu NWs: (B) TEM; (D) HRTEM and (F) histograms of
diameters. (G) EDS analysis of FePt@Cu NWs.
c
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For FePt@Cu NWs, the interfringe distance was 0.220 nm,
which was close to the lattice spacing of the (111) plane
(0.218 nm) of the CuPt alloy.8 The powder X-ray diffraction
(XRD) pattern (Fig. S1) showed an fcc structure and (111) was
the predominant plane.9 An energy dispersive spectrum (EDS)
(Fig. 1G) collected in different regions showed that the Cu
content of the FePt@Cu NWs was about 47%. The oxidation
states of Cu in the FePt@Cu NWs were examined by X-ray
photoelectron spectroscopy (XPS). The results (Fig. S2 and
respective discussion in the SI) suggest that the NWs consisted
primarily of metallic Cu on the surface of FePt NWs.10
trans-Stilbene was selected for the investigation of the
FePt@Cu NWs catalytic performance, as an activated and
reusable heterogeneous catalyst. Table 1 shows the cata-
lytic performance of the FePt@Cu NWs for trans-stilbene
epoxidation in different solvents at the same temperature.
NMR results showed that trans-stilbene oxide was the major
product. The solvent efficiency (in terms of epoxide yield) was
as follows: toluene, m-xylene o DMF o methylcyclohexane o
dioxane o p-xylene o o-xylene. The trend did not appear to
follow solvent polarity. Polar solvents (Table 1, entries 1 and 2)
were expected to hinder oxidation because of the low catalyst
and oxygen solubility. Aromatic hydrocarbons like o-xylene
and p-xylene were efficient solvents for the trans-stilbene oxide
formation (Table 1, entries 4 and 6).
Scheme 1 Proposed mechanism for the FePt@Cu-catalyzed aerobic
epoxidation of trans-stilbene in o-xylene (cycle 1).
intermediate formed 2,3-diphenyloxirane, and o-xylene was
oxidised to the corresponding alcohol, aldehyde or acid (cycles
1, 2 and 3). The alcohol or aldehyde could also react with the
activated oxygen to form peroxy compounds (cycles 2 and 3)
(Scheme S1). Table 1 shows neither toluene nor m-xylene was
an efficient solvent, despite structural similarities. No oxidised
products of toluene and m-xylene were detected by GC
(Table S1), which resulted in the catalyst becoming inactive
in the reaction for entries 3 and 5. Our observation was similar
to that of Valerie Caps, who reported that epoxidation
´
was accompanied by partial oxidation of the solvent, and
attributed it to a free radical mechanism.4 We used the same
conditions (methylcyclohexane solvent and TBHP oxidant) as
When using o-xylene as solvent, 2-methylbenzaldehyde,
o-tolylmethanol and 2-methylbenzoic acid were observed as
byproducts from the oxidation of the solvent (Fig. S3). An
aerobic epoxidation mechanism is proposed as follows:
molecular oxygen was first adsorbed on the surface of the
FePt@Cu NWs to form active oxygen. Both solvent and
stilbene could react with this active oxygen, thus two pathways
existed for the reaction: (1) if the solvent reacted with the
active oxygen, an unstable free radical intermediate was
formed, which could then form peroxide in the oxygenated
environment. Peroxide acted as an oxidant in the stilbene
oxidation with the epoxy compound as the major product;
(2) if the solvent was more stable than stilbene, stilbene could
react with the active oxygen and the epoxy compound was also
the major product. The former can explain the solvent effects.
When o-xylene was used as the solvent (Scheme 1, cycle 1),
o-xylene reacted with the active oxygen to form the free radical
intermediate. trans-Stilbene epoxidation by the free radical
Valerie Caps, and their selectivity and conversion were 99.9% and
´
87.3%, respectively. When oxygen was used as the sole oxidant,
the conversion was only 13.3% (Table 1, entries 7 and 8).
Numerous studies have been reported on the epoxidation of
alkenes using H2O2 or TBHP.12 Several inherent problems
11
were found with these two oxidants. Arends and Sheldon13
showed that H2O2 resulted in the active sites being susceptible
to leaching over several heterogeneous catalytic reactions.
TBHP always needs to be used with oxygen or else at a lower
temperature, and it needs a long reaction time as it quickly
decomposes above 75 1C. It is vastly more favourable to
employ molecular oxygen as a green environmentally friendly
oxidant. In our study, the effect of conventional oxidant was
tested and is summarized in Table 2. Our results show that
molecular oxygen had a high activity (Table 2 entry 4). When
TBHP was used exclusively, there was less epoxidation product
detected (Table 2 entry 2), and TBHP quickly decomposed.
Air also exhibited high activity with a yield of 70% epoxida-
tion product (Table 2 entry 3). When no oxidant was used, no
epoxidation products were detected (Table 2 entry 5).
Table 1 Oxidation of trans-stilbene in different solventsa
Entry
Solvent
Conv. (%)b
Select. (%)b
TONc
1
2
3
4
5
6
7
8
DMF
dioxane
toluene
o-xylene
m-xylene
p-xylene
methylcyclohexane
methylcyclohexaned
3.5
99
99
—
97.8
—
76.8
99
9
90
—
215
—
121
38
Table 2 Oxidation of trans-stilbene with different oxidantsa
36.7
Trace
87.0
Trace
49.2
13.3
87.3
Entry
Oxidant
Conv. (%)b
Select. (%)b
d
1c
2c
3
4
5
H2O2
TBHPd
air
15.5
1.1
74.4
87.0
—
99
99
94.0
97.8
—
99
218
oxygen
none
a
All reactions were carried out with 0.9 mg FePt@Cu NWs, 0.2 mmol
trans-stilbene and 2 ml solvent at 100 1C for 24 h under 1atm
a
All reactions were carried out with 0.9 mg FePt@Cu NWs, 0.2 mmol
b
b
c
c
oxygen. GC yield. TON stands for turnover number, i.e. the
number of mole of trans-stilbene converted per mole of copper in
d
24 h. 1 equiv. of TBHP added.
trans-stilbene, and 2 ml o-xylene at 100 1C for 24 h. GC yield. All
reactions had one equivalent of oxidant added. The concentration of
H2O2 was 30% and that of TBHP was 70% in water.
d
c
8592 Chem. Commun., 2010, 46, 8591–8593
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Table 3 Oxidation of trans-stilbene at different temperaturesa
In conclusion, we have shown that FePt@Cu NWs are an
effective catalyst for stilbene epoxidation in o-xylene with
oxygen as the sole oxidant. This catalytic reaction underwent
a free radical process, and the catalytic activity was com-
parable to that of a previously reported Au/TiO2 catalyst. Our
finding will broaden the application of nanomaterial catalysts
in organic reactions.
Entry
T/1C
Conv. (%)b
Select. (%)b
1
2
3
4
60
80
100
120
—
—
99
97.8
37.2
9.8
87.0
100
a
All reactions were carried out with 0.9 mg FePt@Cu NWs,
0.2 mmol trans-stilbene, and 2 ml o-xylene for 24 h under oxygen
This work was supported by the Natural Science Founda-
tion of China (NSFC) (20876101), NSF of Jiangsu Province
(BK2008158).
b
(1 atm). GC yield.
The activity of FePt@Cu NWs at different temperatures
was investigated using o-xylene as the solvent. Temperature
exerted a tremendous influence on the conversion and selecti-
vity of both o-xylene (Table S2) and trans-stilbene (Table 3).
At lower temperature (o60 1C), almost no reaction occurred
for the FePt@Cu catalyst. When the temperature was elevated
to 80 1C, the conversion efficiency increased to 9.8% with a
high selectivity (99%) (Table 3 entry 2). The optimal tempera-
ture for this reaction was 100 1C (Table 3 entry 3), and
the yield of the epoxidation product increased to 85%. When
the temperature was increased to 120 1C, trans-stilbene was
quantitatively converted, but the selectivity decreased to 37.2%.
Most of the trans-stilbene was converted to benzaldehyde and
benzoic acid.
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Fig. 2 Recovery and reuse of the FePt@Cu catalyst.
c
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Chem. Commun., 2010, 46, 8591–8593 8593