6
Q. Ma et al. / Catalysis Communications 53 (2014) 5–8
Table 1
The differences in the catalytic activities of the studied oxides are ob-
viously related to the differences in their catalytic mechanisms. When
aqueous hydrogen peroxide is mixed with TiO2, Fe2O3, Co3O4, ZnO or
ZrO2, we find the rapid decomposition of hydrogen peroxide, while for
MoO3 and WO3, the decomposition of hydrogen peroxide is not found.
Presumably, TiO2, Fe2O3, Co3O4, ZnO and ZrO2 are associated with the
possible reactions (R1, R2). H2O2 will first adsorb molecularly onto the
surface of the transition metal oxides according to the reaction R1 and
then undergo decomposition according to the reaction R2.
Catalytic performance of transition metal oxides for oxidation of 2-heptylcyclopentanone
to δ-dodecalactone with aqueous hydrogen peroxide.
Entry
Catalyst
Conversion (%)a
Yield (%)a
1
–
Trace
3
4
2
2
Trace
1
6
21
2
4
Trace
3
4
2
2
Trace
1
5
19
2
3
2
TiO2
TiO2
3b
4
Fe2O3
Fe2O3
Co3O4
Co3O4
ZnO
5b
6
7b
8
9b
10
11b
12
13b
14
15b
ZnO
H2O2ðaqÞ þ MxOy→H2O2−MxOy
ðR1Þ
ðR2Þ
ZrO2
ZrO2
MoO3
MoO3
WO3
WO3
53
98
11
87
41
78c
9
H2O2−MxOy→1=2O2 þ H2O þ MxOy:
72c
Reaction conditions: 2-heptylcyclopentanone (2.5 mmol), catalysts (50 mg), experiments
performed at 40 °C for 12 h, and aqueous hydrogen peroxide (30 wt.%) (6.25 mmol).
Along with the decomposition of hydrogen peroxide in the presence
of TiO2, Fe2O3, Co3O4, ZnO or ZrO2, the concentration of aqueous hydro-
gen peroxide is decreased, and then the difficulty of interaction between
hydrogen peroxide and 2-heptylcyclopentanone is gradually increased.
The different catalytic activities of TiO2, Fe2O3, Co3O4, ZnO and ZrO2 in-
dicate that the catalytic capabilities of these oxides toward decomposi-
tion of H2O2 are very different. Phenomena such as hydrogen bonding
between H2O2 and the hydroxylated surfaces of the transition metal
oxides; the degree of interaction between the O atoms of H2O2 and
the surface exposed on the transition metal oxides; the adsorption
energies of the products formed—all these parameters will determine
the pathway that the catalytic process for the H2O2 decomposition
will follow [30]. The conversion of 2-heptylcyclopentanone and the
yield of δ-dodecalactone are higher on the ZnO than on the TiO2,
Fe2O3, Co3O4 and ZrO2, it is because the decomposition of H2O2 on
ZnO is slower than on the TiO2, Fe2O3, Co3O4 and ZrO2. The rapid decom-
position of H2O2 on the TiO2, Fe2O3, Co3O4 and ZrO2 might explain why
the catalytic activities of these transition metal oxides in different
concentration of aqueous hydrogen peroxide are low and similar. The
Baeyer–Villiger reaction mechanism proceeding on TiO2, Fe2O3, Co2O3,
ZnO and ZrO2 may be similar to which Paul et al. have supposed [22].
The mechanism of Baeyer–Villiger oxidation proceeding on MoO3 is
of considerable interest in view of its high catalytic activity. It is worth-
while mentioning that the solubility of MoO3 in water and aqueous hy-
drogen peroxide. When mixed MoO3 (50 mg) with water (0.6 mL) and
stirred for 12 h at 40 °C, MoO3 is separated from the mixture without
loss by filtration, the water is still in the normal color; when mixed
MoO3 (50 mg) with aqueous hydrogen peroxide (30 wt.%, 0.6 mL)
with stirring at 40 °C, 2 h later, the powder of MoO3 disappears and a
a
Calculated by GC.
Aqueous hydrogen peroxide (50 wt.%) (6.25 mmol).
Isolated yield.
b
c
and further purified by column chromatography over silica gel using 1:5
ethyl acetate–hexane as eluent to furnish the pure δ-dodecalactone.
3. Results and discussion
As Table 1 shows, the absence of transition metal oxides effect in the
Baeyer–Villiger oxidation of 2-heptylcyclopentanone indicates that the
formation of δ-dodecalactone is not predominantly caused by aqueous
hydrogen peroxide. Among the transition metal oxides used for the
Baeyer–Villiger oxidation of 2-heptylcyclopentanone with different
concentration of aqueous hydrogen peroxide, MoO3 and WO3 were sig-
nificantly more active than other transition metal oxides. We repeated
the experiments (entries 13 and 15) for three times, the conversion of
2-heptylcyclopentanone and yield of δ-dodecalactone was changed a
little. The differences in the conversion of 2-heptylcyclopentanone
and yield of δ-dodecalactone for entries of 12–13 and 14–15 in
Table 1 can be explained by the different amount of water in the reac-
tion system. Water in the reaction system hinders hydrogen peroxide/
2-heptylcyclopentanone interaction. The catalytic activity of the other
transition metal oxides is considerably lower than MoO3 and WO3,
they show only slightly different reactivity in the conversion
of 2-heptylcyclopentanone and yield of δ-dodecalactone except ZnO.
a
b
1.0
1.0
0.8
0.8
2-heptylcyclopentanone
2-heptylcyclopentanone + MoO3
0.6
0.6
H2O + MoO3
H2O2 + MoO3
H2O2
0.4
0.2
0.4
0.2
0.0
-0.2
-0.4
0.0
-0.2
-0.4
100
200
300
400
500
600
700
800
100
200
300
400
500
600
700
800
λ (nm)
λ (nm)
Fig. 1. UV/visible absorption spectra of the mixture of MoO3 and aqueous hydrogen peroxide (a), the mixture of MoO3 and 2-heptylcyclopentanone (b).