Fabrication of Fe3O4- l -dopa-CuII/SnIV@Micro-Mesoporous-SiO2 Catalyst Applied to Baeyer-Villiger Oxidation Reaction

Article abstract of DOI:10.1002/cctc.201501107

A Magnetic mFe₃O₄-l-dopa-Cu^II/Sn^IV@micro-mesoporous-SiO₂ catalyst was successfully prepared. The catalyst exhibits high and stable catalytic activity for the Baeyer-Villiger oxidation reaction with air as oxidant. Furthermore, the selectivity can reach nearly 100 %. Meanwhile the catalyst can be easily separated by an external magnet and reused at least up to five cycles without any notable loss in catalytic activity. In addition, the effect of Sn and Cu on the oxidation of cyclohexanone is discussed.

Full text of DOI:10.1002/cctc.201501107

DOI: 10.1002/cctc.201501107
Full Papers
Fabrication of Fe3O4-l-dopa-CuII/SnIV@Micro-Mesoporous-
SiO2 Catalyst Applied to Baeyer–Villiger Oxidation
Reaction
Hongfei Huo, Li Wu, Jianxin Ma, Honglei Yang, Le Zhang, Yuanyuan Yang, Shuwen Li, and
Rong Li*[a]
A
Magnetic mFe3O4-l-dopa-CuII/SnIV@micro-mesoporous-SiO2
rated by an external magnet and reused at least up to five
cycles without any notable loss in catalytic activity. In addition,
the effect of Sn and Cu on the oxidation of cyclohexanone is
discussed.
catalyst was successfully prepared. The catalyst exhibits high
and stable catalytic activity for the Baeyer–Villiger oxidation re-
action with air as oxidant. Furthermore, the selectivity can
reach nearly 100%. Meanwhile the catalyst can be easily sepa-
1. Introduction
Lactones have been applied widely as key molecules for the
synthesis of important bioactive compounds, natural products
and polymers; and represent a valuable family of synthons for
various organic transformations. Lactones or esters can be syn-
thesized from their corresponding ketones by employing the
Baeyer–Villiger (B-V) oxidation.[1–3] The B-V oxidation is one of
the most significant and practical oxidation reactions and has
been extensively used for the synthesis of antibiotics, steroids,
pheromones, monomers for polymerization, and various fine
chemicals.[4–6] In general, B-V oxidations have usually been con-
ducted by highly expensive, hazardous, shock sensitive, and
environmentally malignant organic peracids as oxidants,[5]
therefore, developing greener alternatives as oxidants for B-V
reaction is a technologically crucial issue. According to many
reports, two main protocols have been used for investigating
greener oxidants for the B-V reaction.[7] One is the use of aque-
ous hydrogen peroxide as oxidant,[4,8–11] this, however carries
some shortcomings. For example, hydrogen peroxide is kineti-
cally inert compared to peracids and the presence of water in
the reaction mixture leads to the hydrolysis of the product.[12]
Moreover, it is dangerous to use high concentrations of hydro-
gen peroxide in organic solvents.[12] The other important proto-
col has been demonstrated in the presence of aldehydes as
dehyde oxidation system is fascinating, because it has the po-
tential to lower the cost of current industrial processes[1] and
avoid using organic peracids. Nevertheless, O2/aldehyde sys-
tems typically require pure oxygen and large amounts of sacri-
ficing agents over the stoichiometric ratio (e.g., 3 equiv). If
pure oxygen can be replaced with air and the amounts of sac-
rificing agents over the stoichiometric ratio can be lower, the
system will become more fascinating and environmentally
friendly. Recently, B-V oxidation reaction has been reported to
be catalyzed by different catalysts (graphite, carbon materials,
m-ZrP) with no more than 2 molar equiv of benzaldehyde as
sacrificial agents. However, these systems suffered from the
limitations of much consumption of metal, relatively low activi-
ty or longer reaction time.[1,3,8]
Previous studies showed that Sn as Lewis acid centers incor-
porated into a zeolite framework (zeolite Beta) have been
found to be very active and highly selective catalysts for the
B-V oxidation of ketones with hydrogen peroxide.[16,17] For in-
stance, tin-containing beta zeolite could oxidize adamanta-
none and cyclohexanone to the corresponding lactones with
excellent selectivity, which was developed by Corma et al.[17] In
addition, introducing Sn into the siliceous framework of meso-
porous molecular sieves of different structural types resulted in
the formation of highly active and selective catalysts for oxida-
tive reactions.[18]
a
sacrificing agent, which is known as the Mukaiyama
method.[13] There are some reports in which heterogeneous
catalysts have been developed for effective B-V oxidation with
peracids and H2O2.[10,14,15]
Copper is a fascinating metal catalyst especially for the oxi-
dation reactions such as B-V oxidation, benzylic oxidation, alde-
hydes to acids, CÀH oxidation, reaction of carbanions and carb-
anion equivalents and alkene oxidation etc.[19–25] Inspired by
the efficiency of tin-containing zeolite and CuII in oxidation re-
actions, we envisaged that the efficiency in the B-V oxidations
of ketones could be improved by CuII and SnO2 bimetallic cata-
lysts.
Research on B-V oxidation using molecular oxygen, benzal-
dehyde and a heterogeneous catalyst are rare.[1,23,7] The O2/al-
[a] H. Huo, L. Wu, J. Ma, H. Yang, L. Zhang, Y. Yang, S. Li, Prof. Dr. R. Li
College of Chemistry and Chemical Engineering
Lanzhou University
Tianshui South Road, Lanzhou, 730000 (P.R. China)
E-mail: liyirong@lzu.edu.cn
Herein, the aerobic B-V oxidation of ketones catalyzed by
CuII and SnIV using Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst
was developed. The fabrication procedure involves six main
Supporting Information for this article is available on the WWW under
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steps as shown in Scheme 1. The role of bimetallic catalysts in
the aerobic oxidation was discussed and a bimetallic mecha-
nism involving activated cyclohexanone for enhancing the effi-
ciency was also proposed. The present study provides an ex-
ample that the bimetallic catalyst could catalysis the reaction
of the B-V oxidation of cyclohexanone actively.
2.3. Production of Fe3O4-l-dopa-CuII/SnII@micro-mesoporous-
SiO2 (Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2)
To grow a mesoporous silica shell on the Fe3O4-l-dopa-CuII/SnII mi-
crospheres, the as-made Fe3O4-l-dopa-CuII/SnII nanospheres
(300 mg) are redispersed in a mixed solution containing CTAB
(0.45 g, 1.2 mmol), distilled water (100 mL), concentrated ammonia
solution (2.0 mL, 28 wt%), and ethanol (150 mL). The re-
sultant mixed solution is ultrasonic dispersed for 5 min
and then stirred for 30 min to form a uniform dispersion.
Subsequently, TEOS (0.8 mL) is added drop-wise to the
dispersion under continuous stirring. After magnetic stir-
ring for 6 h, the product is collected by centrifugation
(8000 rpm, 3 min) and washed with distilled water and
ethanol for several times. Then the collected nano-
spheres are dried at 408C in a vacuum drying oven over
night for further use. Finally, the product was calcined at
5008C (ramping rate, 38CminÀ1) in air atmosphere for
6 h to remove CTAB template and other organic species.
The amount of Cu, Sn, and Fe in the Fe3O4-l-dopa-CuII/
SnIV@m,m-SiO2 catalysts was found to be 1.0, 4.0, and
14 wt%, based on ICP analysis. Fe3O4-l-dopa-CuII/
SnIV@m,m-SiO2 has coated the CuO and SnO2 particles,
prevented aggregation of metallic nanoparticles, pro-
moted the mass diffusion and transported of reactants.
2.4. Characterization of the catalyst
The synthesized catalyst was confirmed by correspond-
ing characterization means. The morphology and micro-
structure of the catalyst were characterized by transmis-
sion electron microscopy (TEM). The TEM and EDX
images were obtained through Tecnai G2 F30 electron
microscope operating at 300 kV, and samples were obtained by
placing a drop of a colloidal solution onto a nickel grid and evapo-
rating the solvent in air at room temperature. The conversion was
estimated by GC (P.E. AutoSystem XL) or GC-MS (Agilent 6,890N/
5,973N). Inductive coupled plasma atomic emission spectrometer
(ICP-AES) analysis was conducted with PerkinElmer (Optima-
4300DV). Specific surface areas were calculated by the Brunauer–
Emmett–Teller (BET) method and pore sizes by the hybrid density
functional theory cylindrical pores in pillared clay model non-nega-
tive regularization method (Tristar II 3020). Inductive coupled
plasma atomic emission spectrometer (ICPAES) analysis was con-
ducted with PerkinElmer (Optima-4300DV). Magnetic measurement
of Fe3O4 and Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 was performed
using a Quantum Design vibrating sample magnetometer (VSM) at
room temperature in an applied magnetic field sweep from À15
to 15 kOe.
Scheme 1. Preparation of the catalysts.
2. Experimental
2.1. Synthesis of the l-dopa modified magnetic nanoparti-
cles[26]
Typically, FeCl3 (0.973 g) was dissolved in ethylene glycol (30 mL)
to form a clear solution, followed by the addition of NaOAc
(1.970 g) and l-dopa (0.0547 g). The mixture was stirred vigorously
for 30 min, and then sealed in a Teflon-lined stainless-steel auto-
clave (50 mL capacity). Then, the autoclave was heated to and
maintained at 2008C for 12 h, and allowed to cool to room tem-
perature. The black products were separated by magnetic force
and washed several times with ethanol and dried at 508C for 24 h.
2.5. Catalytic activity and recyclability
2.2. Loading of SnII and CuII on l-dopa functionalized mag-
netic nanoparticles (Fe3O4-l-dopa-CuII/SnII)
The B-V oxidation of cyclohexanone was performed in a three-neck
bottle placed in a temperature-controlled water bath. Typically, di-
chloroethane (DCE, 10 mL), a certain amount of cyclohexanone,
benzaldehyde and Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst
(50 mg) were charged into the flask. The mixture was stirred uni-
formly throughout the liquid. Air (20 mLminÀ1) was continuously
introduced into the bottle. Then, the catalyst was collected by cen-
trifugation washed with distilled water and ethanol for several
times and calcined in air at 5008C for 6 h before being subjected
to the next catalytic cycle. The reaction system was analyzed by
GC (P.E. AutoSystem XL) or GC-MS (Agilent 6,890N/5, 973N).
Fe3O4-l-dopa-dopa (300 mg) was dispersed in distilled water
(120 mL) and stirred for 20 min as part A. SnCl2·2H2O (0.112 g) and
Cu(NO3)2·3H2O (0.120 g) was dissolved in HCl solution (25 mL
0.02m) as part B. Parts A and B were mixed together under ultra-
sonication for 1 h. Subsequently, the mixture was stirred vigorously
for 8 h. At last, the precipitate was separated by magnetic force,
washed three times with distilled water, and then dried at 508C for
12 h.
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Figure 1c is a TEM of Fe3O4-l-dopa-CuII/SnIV@SiO2. As can be
seen from the image, the average diameter of the as-synthe-
sized spherical particles was about 330 nm and they were
nearly monodisperse. As shown in Figure 1e, the morphologies
and structures of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 nano-
spheres remain after calcination at 5008C in air atmosphere for
6 h. Figure 1d and f are enlarged TEM image of Fe3O4-l-dopa-
CuII/SnIV@ SiO2 and Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2, and the
thickness between the two red line of the m,m-SiO2 shell is
about 15 nm.
3.0 Results and discussion
3.1. Characterizations of the Fe3O4-l-dopa-CuII/SnIV@m,m-
SiO2 catalyst
The morphologies and structures of the products at different
synthetic steps are observed by TEM. Figure 1a shows the TEM
image of Fe3O4 nanospheres, in which well-dispersed particles
with average diameter about 300 nm could be observed clear-
ly. Figure 1b is an enlarged TEM image of Fe3O4 nanospheres.
The elemental composition of the Fe3O4-l-dopa-CuII/
SnIV@m,m-SiO2 samples was determined by EDX analysis. The
result shown in Figure 2 reveals that the as-prepared products
Figure 2. EDX spectrum of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2.
contain Si, Cu, Sn, Fe Ni, C, and O. Among these elements, Ni,
C, and O are generally influenced by the nickel network sup-
port films and their degree of oxidation; Si, Cu, Sn, Fe, and O
signals result from the products. The EDX results imply that
the Cu and Sn nanoparticles have been successfully immobi-
lized on the surface of Fe3O4 nanoparticles. The content of Cu
and Sn in the Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalysts was
found to be 1.1 and 3.9 wt%, based on EDX analysis.
Shown in Figure 3 are representative nitrogen adsorption/
desorption isotherms and the corresponding pore size distribu-
tion of the Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2. The BET surface
area and single point total pore volume are 765 m2 gÀ1 and
0.45 cm3 gÀ1, respectively, which are considerably large values.
Figure 1. a) TEM of Fe3O4 nanospheres; b) enlarged TEM of Fe3O4 nano-
spheres; c) TEM of Fe3O4-l-dopa-CuII/SnIV@SiO2; d) enlarged TEM of Fe3O4-l-
dopa-CuII/SnIV@SiO2; e) TEM of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2; f) enlarged
TEM of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2.
Figure 3. a) Nitrogen adsorption-desorption isotherm of the Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2, b) the corresponding mesoporous diameter distribution, and
c) the corresponding micropore diameter distribution.
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The pore size was calculated by using the non-negative regula-
rization method, as shown in the inset of Figure 3b. As
a result, average pore size was about 2.3 nm for Fe3O4-l-dopa-
CuII/SnIV@m,m-SiO2. On the basis of above analysis, it can be
concluded that these data further identified the hollow micro-
mesoporous structure of the synthesized Fe3O4-l-dopa-CuII/
SnIV@m,m-SiO2
Magnetization curves revealed the superparamagnetic be-
havior of the magnetic NPs. The hysteresis loops of Fe3O4 and
Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 are shown in Figure 4. It can
Figure 5. Effect of reaction time on the conversion of the B-V oxidation of
cyclohexanone with Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 as catalyst. Reaction
conditions: cyclohexanone (2 mmol), benzaldehyde (4 mmol), the catalyst
(50 mg), 1,2-dichloroethane (10 mL), 323.15 K, air (20 mLminÀ1). Legend:
conversion (%). Determined by GC-MS measurement based on the internal
standard method (dodecane).
Figure 4. Room temperature magnetization curves of a) Fe3O4 and b) Fe3O4-
l-dopa-CuII/SnIV@m,m-SiO2.
be seen that the magnetic saturation values of these are 21.0
and 7.9 emugÀ1, respectively. The decrease of the saturation
magnetization suggests the presence of the mesoporous SiO2
shell and some CuO and SnO2 particles inside the uniformly
pore of hollow mesoporous sphere.
Figure 6. Effect of reaction temperature on the conversion of the B-V oxida-
tion of cyclohexanone with Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 as catalyst. Re-
action conditions: cyclohexanone (2 mmol), benzaldehyde (4 mmol), the cat-
alyst (50 mg), 1,2-dichloroethane (10 mL), air (20 mLminÀ1), 6 h. Legend:
conversion (%). Determined by GC-MS measurement based on the internal
standard method (dodecane).
3.2. Catalyst testing for B-V oxidation
The B-V oxidation of cyclohexanone was performed using air
and various aldehyde over Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 in
various solvent to find the optimum reaction conditions. Firstly,
with cyclohexanone as the model substrate, the optimum reac-
tion time, temperature, solvent, and the molar ratio of benzal-
dehyde/cyclohexanone were tested.
To investigate the effect of time on product distribution, re-
action times of 1, 2, 3, 4, 5, 6, and 7 h were examined in the
presence of the catalyst. The results of the influence of the re-
action time on the exploration are presented in Figure 5. With
increasing of the reaction time, the conversion of the reactants
was increased as expected, while little change was observed
after 6 h. The optimum reaction time was determined as 6 h.
Effects of the reaction temperature were shown in Figure 6.
As we can see, with increasing the temperature, the conver-
sion of cyclohexanone to e-caprolactone firstly increased but
then reduced. At temperature from 303.15 to 343.15 K, the
conversion of e-caprolactone firstly increased remarkably then
decreased sharply. The optimized temperature was found at
323.15 K.
Table 1. The effect of solvent on the conversion of the B-V oxidation of
cyclohexanone with Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 as catalyst.[a]
Entry
Solvent
Conversion of
cyclohexanone
[%][b]
Yield of
e-caprolactone
[%][b]
Selectivity of
e-caprolactone
[%]
1
2
3
4
5
6
7
DCE
>99.9
82.5
22.3
81.2
45.5
>99.9
82.5
22.3
81.2
45.5
>99.9
>99.9
>99.9
>99.9
>99.9
water
CHX
AN
DIOX
DMSO
THF
traces
traces
traces
traces
[a] Reaction conditions: cyclohexanone (2 mmol), benzaldehyde (4 mmol),
the catalyst (50 mg), solvent (10 mL), air (20 mLminÀ1), 6 h, 323.15 K;
[b] Conversion (%) is based on GC-MS analysis measurement based on
the internal standard method (dodecane).
Presented in Table 1 is the influence of solvent on the B-V
oxidation reaction of cyclohexanone over Fe3O4-l-dopa-CuII/
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SnIV@m-SiO2. It demonstrated that the solvent played an im-
portant role in the B-V oxidation system. The conversion of cy-
clohexanone was closely related with the polarity of solvent
(Table 1, entries 1–4). The nonpolar solvent like 1,2-dichloro-
ethane (DCE) was favorable to the oxidation of cyclohexanone,
which gave higher conversion of cyclohexanone than that of
polar solvent such as acetonitrile (AN) water and 1,4-dioxane
(DIOX). Furthermore, the use of cyclohexane (CHX), N,N-di-
methyl formamide (DMF) or tetrahydrofuran (THF) as solvent
would result in poor performance. The water could also be
used as solvent to the oxidation of cyclohexanone (Table 1,
entry 2), but the conversion of cyclohexanone was lower than
DCE. So the optimal solvent for cyclohexanone oxidation was
DCE. In connection with the distinct results, it was supposed
firstly that the solubility of in various solvents could be a possi-
ble reason. However, there was no substantial difference
among them through our estimation. Therefore, the enhanced
cyclohexanone conversion with nonpolar solvent is due to the
solvation effect, rather than the Fe3O4-l-dopa-CuII/SnIV@m,m-
SiO2 catalyst solubility effect.
The result of the B-V oxidation of cyclohexanone with vari-
ous aldehyde as sacrificing agents are summarized in Table 3.
It can be seen clearly that 1) benzaldehyde was favorable to
the oxidation of cyclohexanone, which gave higher conversion
of cyclohexanone than 4-bromine benzaldehyde and p-tolual-
dehyde. 2) The effect of substituent in various benzaldehyde
also influenced the conversion of cyclohexanone in B-V oxida-
tion (Table 3, entries 2–4). The benzaldehydes with an electron-
withdrawing group can perform as a better sacrificing agents
than the benzaldehydes with electron donating group for B-V
oxidation (Table 3, entries 2–4). The benzaldehydes with an
electron-withdrawing group or an electron donating group
perform worse than the benzaldehyde (Table 3, entries 2–4, 1).
3) Furthermore, most aromatic aldehydes perform better than
fatty aldehydes (Table 3, entries 1–3, 5–7). Therefore, aldehydes
played an important role in the B-V oxidation system. It was
supposed that the mechanism of the B-V oxidation involved
free radical species.
The present oxidation system could be attractive if it can be
applied to various ketones. To evaluate the scope of catalytic
system, the catalytic activities towards B-V oxidation of various
ketones were also explored and the results were shown in
Table 4. Cyclohexanone and cyclopentanone are more easily
than other ketones to convert into corresponding ester, which
was consistent with results of other reports,[2,3] and p-methyl-
acetophenone is more easily than acetophenone to convert
into corresponding ester. Because methyl is a group which is
able to stabilize the positive charge.[16] The fatty ketones is dif-
ficult to transform into corresponding ester through B-V oxida-
tion reaction.
To investigate the effect of the molar ratio of benzaldehyde/
cyclohexanone, the molar ratio of 1:1, 1.5:1, 2:1, 2.5:1, and 3:1
were examined in the presence of the catalyst. The effects of
the molar ratio of benzaldehyde/cyclohexanone were shown in
Table 2. It demonstrated that the molar ratio of benzaldehyde/
cyclohexanone also played an important role.
Table 2. The effect of the molar ratio of benzaldehyde/cyclohexanone on
the conversion of the B-V oxidation of cyclohexanone with Fe3O4-l-dopa-
CuII/SnIV@m,m-SiO2 as catalyst.[a]
Table 5 showed the comparative analysis of catalytic activity
of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst with other report-
ed solid catalysts for B-V oxidation of cyclohexanone to e-cap-
rolactone in O2/benzaldehyde system. It clearly exhibited that
the Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst performed better
catalytic activity over the reported catalytic systems: Graphite,
FeTPPCl/SnO2, Ketjen Black and MnALPO-36 with respect to
the yield of e-caprolactone. The other reported catalytic sys-
tems suffered from the limitations of relatively low activity,
higher amounts of benzaldehyde or longer reaction time.
Though the reaction time of graphite catalytic system is short-
er and the amounts of benzaldehyde (benzaldehyde/cyclohex-
Entry Benzaldehyde/ Conversion of Yield of
Selectivity of
cyclohexanone cyclohexanone e-caprolactone e-caprolactone
[mmolmmolÀ1
]
[%]
[%]
[%]
1
2
3
4
5
1:1
1.5:1
2:1
2.5:1
3:1
75.2
75.2
>99.9
>99.9
>99.9
>99.9
>99.9
98.1
>99.9
>99.9
>99.9
98.1
>99.9
>99.9
>99.9
[a] Reaction conditions: cyclohexanone 2 (mmol), the catalyst (50 mg),
DCE (10 mL), air (20 mLminÀ1), 6 h, 323.15 K; [b] Conversion (%) is based
on GC-MS analysis measurement based on the internal standard method
(dodecane).
Table 3. The effect of various aldehyde on the conversion of the B-V oxidation of cyclohexanone with Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 as catalyst.[a]
Entry
Aldehyde
Aldehyde/
cyclohexanone
[mmolmmolÀ1
Conversion of
cyclohexanone
[%][b]
Yield of
e-caprolactone
[%]
Selectivity of
e-caprolactone
[%]
[b]
]
1
2
3
4
5
6
7
benzaldehyde
4-bromine benzaldehyde
p-tolualdehyde
p-anisaldehyde
isobutyraldehyde
aldehyde
2:1
2:1
2:1
2:1
2:1
2:1
2:1
>99.9
74.0
63.3
14.7
36.4
3.3
>99.9
74.0
63.3
14.7
36.4
3.3
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
>99.9
methanal[b]
0.1
0.1
[a] Reaction conditions: cyclohexanone 2 mmol, aldehyde 4 mmol, the catalyst 50 mg, 1, DCE 10 mL, air 20 mLminÀ1, 6 h, 323.15 K; [b] Conversion (%) is
based on GC-MS measurement based on the internal standard method (dodecane).
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Table 4. The B-V oxidation reaction of various ketones.[a]
Entry
Substrate
Product
Conversion
[%][b]
Yield
[%][b]
Selectivity
[%]
1
2
>99.9
>99.9
>99.9
>99.9
68.9
68.9
3
0.1
0.1
>99.9
>99.9
Figure 7. Reuse of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 for the BV oxidation
using air and benzaldehyde. Reaction conditions: cyclohexanone (2 mmol),
benzaldehyde (4 mmol), the catalyst (50 mg), 1,2-dichloroethane (10 mL),
323.15 K, 6 h, air (20 mLminÀ1). Legend: conversion (%). Determined by GC-
MS measurement based on the internal standard method (dodecane).
4
5
60.8
60.8
3.3. Plausible mechanism for the cocatalytic cyclohexanone
oxidation
traces
[a] Reaction conditions: ketones 2 mmol, aldehyde 4 mmol, the catalyst
50 mg, DEC 10 mL, air 20 mLminÀ1, 6 h, 323.15 K; [b] Conversion (%) is
based on GC-MS.
The catalysis of Fe3O4-l-dopa-SnIV@m,m-SiO2, Fe3O4-l-dopa-
CuII@m,m-SiO2, and Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 for the
Baeyer–Villiger type oxidation were examined by performing
the oxidation of cyclohexanone to e-caprolactone in the pres-
ence of benzaldehye. Table 6 summarizes the results of the cy-
clohexanone oxidation under air atmosphere at 508C. The
three kinds of catalysts showed high selectivity, over 99.9%,
and Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 showed the highest con-
version, over 99.9%, under this experimental condition. Where-
as the blank run without catalyst resulted in quite poor conver-
sion, just 16.0%. It indicated that catalyst played an important
role for the cyclohexanone oxidation. In addition, in the Fe3O4-
l-dopa-CuII@m,m-SiO2 or Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2
system, there was formic acid phenyl ester which was the by-
product of benzaldehyde, and both the generation rate of
formic acid phenyl ester were about 10%. However, in the
Fe3O4-l-dopa-SnIV@m,m-SiO2 or no catalyst system, there was
no formic acid phenyl ester. It was proposed that CuII as Lewis
acid might activate benzaldehyde which could improve the
generation rate of peroxybenzoic acid. When the concentration
anone=1:1) of the Ketjen Black catalytic system is lower than
the Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst catalytic system,
they suffered from the same limitations of relatively low activi-
ty.
The Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst could be easily
separated by using an external magnet and reused after being
calcined in air at 5008C for 6 h. Figure 7 shows the per-
formance of Fe3O4-l-dopa-CuII/SnIV@m-SiO2 in cycles of cyclo-
hexanone oxidation at 323.15 K in 6 h. It is clear that the
Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst shows steady per-
formance across the five runs, indicating that Fe3O4-l-dopa-
CuII/SnIV@m,m-SiO2 is a stable catalyst reusable for the B-V oxi-
dation reaction.
Table 5. Comparative chart of catalytic activity of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2with other reported solid catalysts for B-V oxidation of cyclohexanone
using molecular oxygen and benzaldehyde.[a]
Entry
Catalysts
t
Conversion of
cyclohexanone
[%][b]
Yield of
e-caprolactone
[%][b]
Selectivity of
e-caprolactone
[%][b]
Ref.
[h]
1
2
3
4
5
6
Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2
Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2
Ketjen Black[e]
3
6
6
8
3
6
>99.9
>99.9
91.0
96.0
99.6
>99.9
>99.9
91.0
96.0
99.0
>99.9
>99.9
>99.0
>99.0
99.4
this work
this work
[1]
[2]
[3]
FeTPPCl/SnO2
Graphite[d]
MnALPO-3627,[f]
78.0
76.4
98.0
[27]
[a] Reaction conditions: cyclohexanone (2 mmol), benzaldehyde (4 mmol), the catalyst (50 mg), DEC (10 mL), 323.15 K; [b] Conversion (%) is based on GC-
MS; [c] Using air and benzaldehyde as oxidant; [d] Cyclohexanone (4 mmol), benzaldehyde (4 mmol); [e] Cyclohexanone (10 mmol), benzaldehyde
(20 mmol), DEC (20 mL); [f] Cyclohexanone/benzaldehyde=1:3 (mol: mol).
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dopa-CuII@m-SiO2, or Fe3O4-l-dopa-CuII/SnIV@m,m-
SiO2 system the reaction rate was accelerated remark-
ably and over 80% benzaldehyde could be converted
to benzoic acid and about 10% benzaldehyde was
converted to formic acid phenyl ester. These results
could further prove that CuII could be used as Lewis
acid to activate benzaldehyde and SnIV as Lewis acid
could weakly activate benzaldehyde. Secondly, the
B-V oxidation reaction of cyclohexanone was con-
ducted by various catalysts with metachloroperben-
zoic acid as oxidant. The results were summarized in
Table 8. In the system of no catalyst, moderate yield
of e-caprolactone could be obtained by conducting
Table 6. The results of the cyclohexanone oxidation catalyzed by Fe3O4-l-dopa-
SnIV@m,m-SiO2, Fe3O4-l-dopa-CuII@m,m-SiO2 and Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2
under air atmosphere at 508C.[a]
Entry
Catalysts
t
Yield of
e-caprolactone
[%][b]
Yield of formic
acid phenyl ester
[%][b]
[h]
1
2
3
Fe3O4-l-dopa-SnIV@m,m-SiO2
Fe3O4-l-dopa-CuII@m,m-SiO2
Fe3O4-l-dopa-CuII/
SnIV@m,m-SiO2
6
6
6
32.8
81.8
>99.9
traces
10.9
11.3
4
none
6
16.0
traces
[a] Reaction conditions: cyclohexanone (2 mmol), benzaldehyde (4 mmol), the catalyst
(50 mg), DEC (10 mL), 323.15 K; [b] Conversion (%) is based on GC-MS.
Table 8. Further insights into the role of SnO2 and CuO for the oxidation
of peroxybenzoic acid reaches a certain value, benzaldehyde
could be oxidized into formic acid phenyl ester. Nevertheless,
SnIV could not or weakly activate benzaldehyde. The mecha-
nism involved free radical species was identified in the oxida-
tion of cyclohexanone by adding radical trap (BHT, 2,6-di-tert-
butyl-4-methylphenol), and the reaction was totally inhibited
and that the mechanism involved radical could be identified
the experimental results (Table 3, entries 2–4).
of cyclohexanone.[a]
Entry
Catalysts
t
[h]
Yield of
e-caprolactone [%][b]
1
2
3
no catalyst
0.5
0.5
0.5
35.5
84.3
41.3
Fe3O4-l-dopa-SnIV@m-SiO2
Fe3O4-l-dopa-CuII@m-SiO2
[a] Reaction conditions: 4 mmol benzaldehyde, the catalyst 50 mg, DEC
10 mL, 323.15 K; [b] Conversion (%) is based on GC-MS.
In the mechanism, the reaction efficiency is usually related
with the generation rate of peroxybenzoic acid and Criegee
adduct-like intermediate. As previous research has shown,
SnO2 can be used as a Lewis acid catalyst owing to its acid
site.[28–31] Carbonyl compounds could be activated by the coor-
dination between the oxygen atom of the carbonyl group and
the Lewis acid site. For the bimetallic catalyst oxidation
system, the generation rate of peroxybenzoic acid could be im-
proved if there is the coordination between benzaldehyde and
Lewis acid site. On the other hand, the formation rate of Crie-
gee adduct-like intermediate should be accelerated in the
presence of the coordination between cyclohexanone and
SnO2 and CuII.
the reaction for 2 h. In the Fe3O4-l-dopa-SnIV@m,m-SiO2 or
Fe3O4-l-dopa-CuII@m,m-SiO2 system, the conversion of cyclo-
hexanone was higher, and the yield of e-caprolactone could be
reached to 92.1% only for 0.5 h. The above results proved that
both SnIV and CuII could activate cyclohexanone and the
former showed better activation capacity then the latter to cy-
clohexanone. Thirdly, the oxidation of benzaldehyde was con-
ducted by Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 catalyst with meta-
chloroperbenzoic acid as oxidant. The yield of benzoic acid
and formic acid phenyl ester were 60.0 and 28.0, respectively
by conducting the reaction for 2 h, which proved benzalde-
hyde could be oxidized into formic acid phenyl ester.
To obtain further insights into the role of SnO2 and CuO in
the observed acceleration, the following experiments were
conducted. Firstly, the catalysis of Fe3O4-l-dopa-SnIV@m,m-SiO2,
Fe3O4-l-dopa-CuII@m,m-SiO2, and Fe3O4-l-dopa-CuII/SnIV@m,m-
SiO2 for the oxidation of benzaldehyde (Table 7). In the Fe3O4-
l-dopa-SnIV@m-SiO2 system, moderate yield of benzoic acid
could be obtained by conducting the reaction for 6 h and
there was no formic acid phenyl ester. Whereas, in the Fe3O4-l-
Based on the discussion above, the catalytic mechanism of
Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 for the oxidation of cyclohex-
anone was proposed as shown in Scheme 2. In the catalytic
mechanism, CuII played important roles that could activate
benzaldehyde and cyclohexanone molecules in the aerobic ox-
idation of cyclohexanone, but the former was the main role.
SnIV mainly activates cyclohexanone molecules in the
aerobic oxidation of cyclohexanone. The Lewis acid
sites (SnIV and CuII) coordinated with the oxygen
Table 7. Further insights into the role of SnO2 and CuO for the oxidation of benzalde-
atom of the carbonyl group, which activated the cy-
clohexanone molecules in the reaction. An easier nu-
cleophilic attack of organic peracids on the activated
cyclohexanone favored the formation of a Criegee
adduct, followed by rearrangement to form e-capro-
lactone.
hyde.[a]
Entry
Catalysts
t
[h]
Yield of benzoic
acid [%][b]
Yield of formic acid
phenyl ester [%][b]
1
2
3
Fe3O4-l-dopa-SnIV@m-SiO2
Fe3O4-l-dopa-CuII@m-SiO2
Fe3O4-l-dopa-CuII/
6
6
6
32.8
81.8
86.5
traces
10.9
11.3
SnIV@m-SiO2
[a] Reaction conditions: 4 mmol benzaldehyde, the catalyst 50 mg, DEC 10 mL,
323.15 K; [b] Conversion (%) is based on GC-MS.
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In conclusion, we describe a method to form Fe3O4-l-dopa-
CuII/SnIV@m,m-SiO2 catalyst, and the catalyst shows high per-
formance for synthesis of e-caprolactone through B-V oxidation
reaction. The system of Fe3O4-l-dopa-CuII/SnIV@m,m-SiO2 bi-
metallic catalyst promoted the formation of e-caprolactone. In
the catalytic mechanism, CuII as Lewis acid played an impor-
tant role that could activate benzaldehyde and cyclohexanone
molecules in the aerobic oxidation of cyclohexanone, however,
the former was dominating. SnIV as Lewis acid, however, could
mainly activate cyclohexanone. Moreover, Fe3O4-l-dopa-CuII/
SnIV@m,m-SiO2 catalyst could be easily separated by an exter-
nal magnet. The catalyst showed stable catalytic activity for
the B-V oxidation reaction and could be reused without any
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This work was supported by the Key Projects in the National Sci-
ence and Technology Pillar Program of China (2014 4BAK16B01)
and the Fundamental Research Funds for the Central Universities
(lzujbky-2015-16 and lzujbky-2015-17).
Received: October 9, 2015
Revised: October 25, 2015
Keywords: Baeyer–Villiger · copper · oxidation · tin
Published online on January 15, 2016
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