Simple metal oxides as efficient heterogeneous catalysts for epoxidation of alkenes by molecular oxygen

Article abstract of DOI:10.1246/cl.2004.1140

Magnetite iron oxide (Fe₃O₄) has been found to be an efficient heterogeneous catalyst for the epoxidation of alkenes by molecular oxygen in the absence of a sacrificial reductant among various transition metal oxides. The reaction probably proceeds via a radical mechanism.

Full text of DOI:10.1246/cl.2004.1140

1140
Chemistry Letters Vol.33, No.9 (2004)
Simple Metal Oxides as Efficient Heterogeneous Catalysts for Epoxidation of Alkenes
by Molecular Oxygen
Jun Liang, Qinghu Tang, Guanyu Meng, Hongli Wu, Qinghong Zhang,ꢀ and Ye Wangꢀ
State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University,
Xiamen 361005, P. R. China
(Received June 24, 2004; CL-040733)
Magnetite iron oxide (Fe3O4) has been found to be an effi-
shown in Figure 1, many transition metal oxides could catalyze
the epoxidation of styrene with O2, forming styrene oxide as the
main product and benzaldehyde as the main byproduct. Among
the transition metal oxides examined, Fe3O4 exhibited the high-
est styrene conversion. Styrene oxide selectivity was also the
highest over Fe3O4.
cient heterogeneous catalyst for the epoxidation of alkenes by
molecular oxygen in the absence of a sacrificial reductant among
various transition metal oxides. The reaction probably proceeds
via a radical mechanism.
The epoxidation of alkenes is of highly importance because
epoxides are versatile synthetic intermediates in chemical indus-
try. Currently, epoxidation is mainly carried out with organic
peracid as an oxidant or by a chlorohydrin process, and a large
amount of byproducts is formed in either case.1 Although hydro-
gen peroxide is an environmentally benign oxidant and several
heterogeneous catalysts such as TS-1 have been developed for
the epoxidation with H2O2,2 there are still high incentives to ex-
ploit molecular oxygen for the epoxidation of alkenes from the
environmental, safe and economic considerations. However, on-
ly very few successes have been achieved with O2 as the oxidant
except for the epoxidation of ethylene. Although the autooxida-
tion of alkanes such as cyclohexane with O2 in the presence of
transition metal compounds such as cobalt acetate provided use-
ful oxygenated products, the autooxidation of alkenes with the
first-row transition metal compounds typically produced a high
degree of oxidized polymers as well as cleaved products rather
than epoxides.3 Some transition metal salts and complexes and
even scattering heterogeneous catalysts were reported to show
activity in the epoxidation reactions with O2, but in most cases,
a co-reductant was indispensable.4,5 Several homogeneous cata-
lysts such as ruthenium complexes, ruthenium- and iron-substi-
tuted polyoxometalates can utilize O2 for the epoxidation of al-
kenes without need of the co-reductant,6 but very scarce
heterogeneous catalysts are known.7 Recently, we found that a
very simple and cheap iron oxide, Fe3O4, could function as an
efficient epoxidation catalyst using O2 alone. In this communica-
tion, we report the superior catalytic performances of Fe3O4 for
the epoxidation of unfunctionalized alkenes.
Figure 1. Catalytic performances of various transition metal
oxides in the epoxidation of styrene with O2. Reaction condi-
tions: catalyst, 2.8 mg; temperature, 373 K; styrene, 10 mmol;
DMF, 20 mL; flow rate of O2, 3 mL minꢁ1; reaction time, 4 h.
It is of significance to note that three iron oxides, i.e., FeO,
Fe3O4, and Fe2O3, show different catalytic performances. Fur-
ther comparisons of these samples are shown in Table 1. FeO
and Fe3O4 exhibited similar styrene oxide selectivity, but sty-
rene conversion and turnover frequency (TOF, moles of alkene
converted per mole of Fe per hour) over FeO were lower than
those over Fe3O4. The lower activity of FeO may result from
its lower specific surface area (<0:1 m2 gꢁ1). On the other hand,
hematite, Fe2O3, possessing similar specific surface area with
Fe3O4, was almost inactive for the reaction. It is thus reasonable
to speculate that the Fe2þ existing in the crystalline structure of
The epoxidation of styrene was performed using a batch-
type reactor operated under atmospheric pressure. Typically,
N,N-dimethylformamide (DMF) was used as a solvent, and O2
was bubbling into the liquid mixture containing styrene and
the solvent. For the epoxidation of other alkenes, the reaction
mixture was loaded into a Teflon bottle placed in a stainless-
steel, high pressure catalytic reactor. O2 is pressured into the
reactor and typically the reaction was allowed to proceed for
4–24 h. After each reaction, the catalyst was filtered off, and
the liquid organic products were quantified by a gas chromato-
graph using toluene as an internal standard.
Table 1. Epoxidation of styrene with O2 catalyzed by iron-con-
taining compoundsa
Surface
Area
/m2 gꢁ1
Styrene Epoxide
Conv.
/%
Entry
No.
TOF
/hꢁ1
Catalyst
Select.
/%
1
2
3
4b
5b
FeO
Fe3O4
Fe2O3
Fe2þ
<0:1
4.7
3.5
—
14.8
38.0
0.4
0.3
0.7
53.3
56.5
21.5
0
9.5
26.3
0.3
0.05
0.1
Fe3þ
—
0
b
aReaction conditions are the same as in Figure 1. Fe2þ and
Fe3þ were added by dissolving FeSO4 and Fe2(SO4)3
(0.2 mmol) into the solvent and the reactant mixture.
Figure 1 shows the catalytic performances of various transi-
tion metal oxides in the epoxidation of styrene by O2. Without
catalyst, almost no conversion of styrene was observed. As
Copyright ꢀ 2004 The Chemical Society of Japan

Chemistry Letters Vol.33, No.9 (2004)
1141
Table 2. Epoxidation of various unfunctionalized alkenes by
a
Fe3O4 or FeO may contribute to its considerably higher activity
in the epoxidation of styrene by O2 with respect to Fe2O3. The
homogeneous catalytic reaction by dissolving FeSO4 or
Fe2(SO4)3 in the solvent (DMF) provided only very small
amount of benzaldehyde and no formation of styrene oxide
was observed. Thus, the epoxidation reaction observed over
Fe3O4 is heterogeneous in nature.
The influences of O2 partial pressure on catalytic performan-
ces have been investigated under atmospheric pressure by dilut-
ing O2 with N2. As shown in Figure 2, no reaction took place us-
ing pure N2 or pure argon (PðO2Þ ¼ 0) instead of O2. Styrene
conversion increased with O2 pressure from 0 to 0.1 MPa, fol-
lowing a Langmuir-type curve. Simultaneously, styrene oxide
selectivity slightly decreased, indicating some consecutive oxi-
dation of styrene oxide to benzaldehyde. Therefore, O2 played
key roles in the epoxidation reaction over Fe3O4. We speculate
that oxygen is chemisorbed and activated by the Fe2þ sites on
Fe3O4 forming active oxygen species for the epoxidation reac-
tions.
O2 over Fe3O4
Alkene
Conv.
/%
Epoxide
Select.
/%
Entry
No.
Time
/h
TOF
/h-1
Substrate
1
24
24
69.0
96.7
7.9
2.0
2b
35.4
63.4
83.3
75.1
3
8
24
4
21.8
2.9
4b
5
51.5
12.1
61.9
22.8
8.2
aReaction conditions: Fe3O4, 2.8 mg; temperature, 373 K; al-
kene, 10 mmol; DMF, 20 mL; O2 pressure, 2 MPa. bAlkene,
5 mmol.
may proceed through a radical mechanism. Further details on the
mechanism are still under investigations.
This work was supported by the National Basic Research
Program of China (No. 2003CB615803) and the NSF of China
(Nos. 20273054 and 20021002).
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Figure 2. Effect of O2 pressure on catalytic performances for
epoxidation of styrene over Fe3O4. The other reaction conditions
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Fe3O4 has also been utilized for the epoxidations of other al-
kenes with higher pressure of O2 because of their lower activities
in atmospheric pressure reactions, and the results are shown in
Table 2. It can be seen that the epoxidation of cyclooctene pro-
ceeds with high selectivity (>95%), while no reaction occurs
without the catalyst under the same reaction conditions. The ep-
oxidations of trans-stilbene, 1,1-diphenylstyrene and trans-ꢀ-
methylstyrene with O2 also resulted in reasonably high selectiv-
ity to epoxides over Fe3O4. It should be noted that only trans-ep-
oxides were obtained in the epoxidations of trans-stilbene and
trans-ꢀ-methylstyrene.
It has been verified that no change in catalytic performances
in the recycling use of Fe3O4 in the epoxidation of styrene by O2.
No change in the crystalline structure of Fe3O4 (spinal structure,
FeIIFeIII2O4) was observed after the reaction from the XRD
measurements. Thus, Fe3O4 is a very stable heterogeneous cata-
lyst for the epoxidation reactions.
5
6
The influences of the addition of a radical scavenger, tert-
butylated hydroxy toluene (BHT), on catalytic performances
for epoxidation of styrene over Fe3O4 have been investigated.
Styrene conversion decreased to almost zero after the addition
of ca. 10 mmol BHT to the reactant, suggesting that the reaction
7
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Chem. Commun., 2004, 440.
Published on the web (Advance View) August 7, 2004; DOI 10.1246/cl.2004.1140