Enhanced shape selective catalysis of mixed cyclic ketones in aerobic Baeyer-Villiger oxidation with magnetic Cu-Fe3O4 supported mesoporous silica microspheres

Article abstract of DOI:10.1016/j.tet.2018.04.009

Various strategies have been developed to improve the conversion for the Baeyer-Villiger oxidation. However, the catalytic effects of the Baeyer-Villiger oxidation for the mixed ketones are rarely reported, though it is also important for the natural and industrial separation processes. In this report, magnetite Cu modified Fe₃O₄ supported mesoporous silica microspheres (Cu-Fe₃O₄@mSiO₂) have been successfully synthesized by two step direct hydrothermal method (DHT). Over 99% of cyclohexanone conversion was obtained with mild air oxidation and benzaldehyde as sacrificing agent over Cu-Fe₃O₄@mSiO₂. The catalytic system also shows higher conversion rates for small molecular ketones in the mixed ketone reactants, which was attributed to the enhanced mass transfer effect and Fe-Cu composite active sites in the magnetite mesoporous silica microspheres. The catalyst could be recycled for four times with similar catalytic performance, which shows enhanced shape selectivity in aerobic Baeyer-Villiger oxidations for mixed cyclic ketones.

Full text of DOI:10.1016/j.tet.2018.04.009

Accepted Manuscript
Enhanced shape selective catalysis of mixed cyclic ketones in aerobic Baeyer-Villiger
oxidation with magnetic Cu-Fe O supported mesoporous silica microspheres
3 4
Chunming Zheng, Shubin Chang, Chuanwu Yang, Dongying Lian, Chao Ma,
Chunrong Zhang, Xiangrui Fan, Shichao Xu, Xiaohong Sun
PII:
S0040-4020(18)30377-6
10.1016/j.tet.2018.04.009
TET 29424
DOI:
Reference:
To appear in: Tetrahedron
Received Date: 23 November 2017
Revised Date: 31 March 2018
Accepted Date: 3 April 2018
Please cite this article as: Zheng C, Chang S, Yang C, Lian D, Ma C, Zhang C, Fan X, Xu S, Sun X,
Enhanced shape selective catalysis of mixed cyclic ketones in aerobic Baeyer-Villiger oxidation with
magnetic Cu-Fe O supported mesoporous silica microspheres, Tetrahedron (2018), doi: 10.1016/
3 4
j.tet.2018.04.009.
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ACCEPTED MANUSCRIPT
Enhanced shape selective catalysis of mixed cyclic ketones in
aerobic Baeyer-Villiger oxidation with magnetic Cu-Fe3O4
supported mesoporous silica microspheres
Chunming Zhenga,*, Shubin Changa, Chuanwu Yanga, Dongying Liana, Chao Maa, Chunrong
Zhanga, Xiangrui Fana, Shichao Xua, Xiaohong Sunb,
ChunMing Zheng, State Key Laboratory of Separation Membranes and Membrane
Processes, School of Environmental and Chemical Engineering, Tianjin Polytechnic
University, Tianjin 300387, P.R. China, zhengchunming@tjpu.edu.cn Tel.: +86 022
83955661
ShuBin Chang, School of Environmental and Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300387, P.R. China, 1472451963@qq.com Tel.: +86
022 83955661
ChuanWu Yang, School of Environmental and Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300387, P.R. China, 1558862045@qq.com Tel.: +86
022 83955661
DongYing Lian, School of Environmental and Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300387, P.R. China, 1761713372@qq.com Tel.: +86
022 83955661
Chaos Ma, School of Environmental and Chemical Engineering, Tianjin Polytechnic
University, Tianjin 300387, P.R. China, 1325249973@qq.com Tel.: +86 022
83955661
Chunrong Zhang, School of Environmental and Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300387, P.R. China, 2770158793@qq.com Tel.: +86
022 83955661
Xiangrui Fan, School of Environmental and Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300387, P.R. China, 1223220075@qq.com Tel.: +86
022 83955661
ShiChao Xu, School of Environmental and Chemical Engineering, Tianjin
Polytechnic University, Tianjin 300387, P.R. China, xushichao_tj@126.com Tel.: +86
022 83955661
Xiaohong Sun, Key Laboratory of Advanced Ceramics and Machining Technology,
Ministry of Education, School of Materials Science and Engineering, Tianjin
University, Tianjin 300072, P.R. China, sunxh@tju.edu.cn Tel.: +86 022 27406114

ACCEPTED MANUSCRIPT
Graphical Abstract
Cu nanoparticles(NPs)
Silica
CTAB+NH4OH
Cu NPs+TEOS@H2O
Fe3O4
particles
Cu-Fe3O4@mSiO2
meso-microspheres

ACCEPTED MANUSCRIPT
Enhanced shape selective catalysis of mixed cyclic ketones in
aerobic Baeyer-Villiger oxidation with magnetic Cu-Fe3O4
supported mesoporous silica microspheres
Chunming Zhenga,*, Shubin Changa, Chuanwu Yanga, Dongying Liana, Chao Maa, Chunrong
Zhanga, Xiangrui Fana, Shichao Xua, Xiaohong Sunb,
a State Key Laboratory of Separation Membranes and Membrane Processes, School of
Environmental and Chemical Engineering, Tianjin Polytechnic University, Tianjin 300387,
China.
b Key Laboratory of Advanced Ceramics and Machining Technology, Ministry of Education,
School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China.
Corresponding author. Tel.: +86 022 83955661; fax: +86 022 83955140.
E-mail address: zhengchunming@tjpu.edu.cn (C.M. Zheng).
Corresponding author. Tel.: +86 022 27406141; fax: +86 022 27406114.
E-mail address: sunxh@tju.edu.cn (X.H. Sun).
1

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Abstract
Various strategies have been developed to improve the conversion for the
Baeyer-Villiger oxidation. However, the catalytic effects of the Baeyer-Villiger
oxidation for the mixed ketones are rarely reported, though it is also important for the
natural and industrial separation processes. In this report, magnetite Cu modified
Fe3O4 supported mesoporous silica microspheres (Cu-Fe3O4@mSiO2) have been
successfully synthesized by two step direct hydrothermal method (DHT). Over 99 %
of cyclohexanone conversion was obtained with mild air oxidation and benzaldehyde
as sacrificing agent over Cu-Fe3O4@mSiO2. The catalytic system also shows higher
conversion rates for small molecular ketones in the mixed ketone reactants, which
was attributed to the enhanced mass transfer effect and Fe-Cu composite active sites
in the magnetite mesoporous silica microspheres. The catalyst could be recycled for
four times with similar catalytic performance, which shows enhanced shape
selectivity in aerobic Baeyer-Villiger oxidations for mixed cyclic ketones.
Key words: magnetite mesoporous silica microspheres; Baeyer-Villiger oxidation;
copper; enhanced shape selectivity
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1. Introduction
Baeyer-Villiger (B-V) oxidation is a valuable organic reaction, which could provide
a direct pathway to convert ketones to corresponding lactones or esters 1-4. However,
organic peracids as traditional oxidants has many disadvantages such as high cost,
low selectivity and environmental hazards 5-10. Therefore, developing high efficient
and green oxidants is an important issue. According to these reports, molecular
oxygen and H2O2 as environmental-friendly oxidants have been successfully carried
13
out for B-V oxidation using different catalysts like graphite 11,12, Cu-MCM-41
,
Ketjen Black 14, Sn-MFI nanosheets and Sn-Beta zeolites 15,16. The O2/aldehydes
system for B-V oxidation is an important method, which could avoid using explosive
peracetic acid, corrosive hydroperoxides and expensive protection. However, in the
O2/aldehydes oxidation system, large amounts of aldehydes over stoichiometric ratio
(e.g., 3 equiv.) were required as sacrificing agents for the oxidation of cyclohexanone,
resulting in large amounts of benzoic acid. Moreover, in practical applications, the
oxidation of ketones often has some complicated mixed systems, which also reduced
the oxidation and selectivity of ketones. Meanwhile, the system suffered from the
limitation of complicated synthesis of catalysts 17, 18, relatively low activity, longer
reaction time and much consumption of aldehydes 19, 20. If the efficiency of sacrificing
agent becomes higher and the selectivity of the catalyst was better, the catalytic
system would be better applied to practical applications, which plays important shape
selective catalytic effect and become more environmental friendly and fascinating.
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In recent years, core-shell magnetic mesoporous silica nanoparticles have been
subjected to extensive research for the combined functionalities of magnetism and
mesoporous nanostructures. With their unique and strong magnetic responsivity, high
chemical stability, enhanced metal oxides dispersion and recoverability, magnetic
microspheres have shown great application potentials in many research fields, such as
21
22
23
bioseparation , enrichment , enzyme immobilization , adsorption and catalysis
24-26. What is more, because of its high reduction potential and redox property, copper
is also widely used as high efficient metal catalytic component for the oxidation
28
reactions of hydrocarbon compounds, such as alcohol 27, aniline compounds
,
especially for B-V oxidation of various ketones 13, 29, 30. The redox reaction of iron
ions with copper was speed up for the increased rate of transferring the electrons on
the silicon-based catalysts. Meanwhile, the catalysts activities were also promoted by
the excellent electron transfer ability of catalyst. Therefore, it motivated us to explore
and design the catalysts consisting of Fe-Cu composite metal oxides for the aerobic
B-V oxidation.
Herein, in this work, magnetic Cu modified Fe3O4 supported mesoporous silica
microspheres (Cu-Fe3O4@mSiO2, core-shell) was fabricated for aerobic
Baeyer-Villiger oxidation of cyclic ketones with benzaldehyde. Cu-Fe3O4@mSiO2
converts mixed cyclic ketones to corresponding lactone under relatively mild
conditions with high activity and differential selectivity. The mechanism for different
catalytic activity and selectivity of composite Cu-Fe3O4 metallic center was also
investigated with N2 sorption, XRD, UV-vis, FT-IR, TEM and VSM characterizations.
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2. Results and discussion
2.1. Catalyst characterizations
Fig. 1. Large angle XRD patterns of synthesized Fe3O4 (a), Fe3O4@mSiO2 (b) and
Cu-Fe3O4@mSiO2 (c).
In order to characterize the porous structure of synthesized catalysts, large-angle
XRD patterns of synthesized Fe3O4, Fe3O4@mSiO2 and Cu-Fe3O4@mSiO2 were
investigated and shown in Fig. 1. The typical characteristic diffraction peaks matched
with the standard Fe3O4 reflections (JCPDS 89-0688, green standard peak), which
suggests the Fe3O4 particles are well retained in all prepared processes with well
crystalline and pure phase. From Fig. 1, there is a broad peak around 22º, which is due
to the presence of amorphous porous SiO2 (JCPDS 82-1571, main standard peak),
implying the porous silica has been successfully coated on the surface of Fe3O4
particles. With the modification of Cu on Fe3O4@mSiO2 and the subsequent
calcination of the catalyst at 550 °C for 6 h, the diffraction peaks of Cu-Fe3O4@mSiO2
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at 30.175◦, 35.543◦, 43.199◦, 57.135◦, 62.744◦ could be indexed to the (111), (220),
(311), (400), (511) and (440) planes of CuFe2O4 crystal (JCPDS 77-0010), which
implied the iron and copper species react strongly and combine together to formed a
new phase. Meanwhile, the diffraction peak of SiO2 increases which indicates the
31
high SiO2 content and purities in the sample . This conclusion could be further
evidenced by the following TEM characterization. The small XRD characterization
was not shown since there was no obvious peak in 0~5º, which might due to the
mesoporous structure of SiO2 was not highly ordered.
Fig. 2 shows the TEM images of synthesized Fe3O4 particles, Fe3O4@mSiO2
microspheres and Cu-Fe3O4@mSiO2 microspheres. From Fig.2a, the magnetite Fe3O4
particles are uniformly synthesized both in morphology and size, which have a mean
diameter of about 300 nm. In Fig. 2b, the Fe3O4@mSiO2 microspheres have a
core-cell nanoparticle texture with thin silica layer of ~20 nm in thickness according
to the difference between core and shell, demonstrating the Fe3O4 nanoparticles have
been successfully coated by porous silica shell. As shown in Fig.2c, the TEM images
exhibits Cu modified Fe3O4 supported mesoporous silica microspheres core-shell
structure. The porous silica layer in the outer layer was about ~20 nm in thickness25.
Notably, the shape and texture of Cu-Fe3O4@mSiO2 microspheres nearly remain
unchanged after calcination in tube furnace at 550 °C in air for 6 h. The surface
roughness of the material slightly increased, which suggested the isolated introduction
of Cu2+ into the material did not change the structure of Cu-Fe3O4@mSiO2
microspheres.
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Fig. 2. TEM images of synthesized Fe3O4 particles (a), Fe3O4@mSiO2 (b) and
Cu-Fe3O4@mSiO2 microspheres (c).
Fig. 3 shows N2 adsorption-desorption isotherms and the corresponding
mesoporous size distribution of the synthesized Cu-Fe3O4@mSiO2 (20) microspheres.
From Fig. 3a, N2 adsorption isotherms of IV-type curves could be observed with a H2
hysteresis loop, which is typical for mesoporous materials. Fig. 3b shows the catalyst
has a sharp peak centered at average pore size of 2.5 nm, indicating the mesoporous
structure of the catalyst. The BET surface area and the total pore volume are 405 m2/g
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and 0.30 cm3/g, respectively. According to above analysis, it reflects the incorporation
of copper into the framework cannot change the uniform hollow micro-mesoporous
structure of the Fe3O4@mSiO2, which could benefit the selectivity for the small
molecular ketones in the mixed cyclic ketones 26.
Fig. 3. N2 adsorption-desorption isotherms of Cu-Fe3O4@mSiO2 (20)
microspheres (a) and the corresponding mesoporous diameter distribution (b).
Fig. 4 shows the FT-IR spectra of Cu-Fe3O4@mSiO2 with different Si/Cu ratio in
the range of 500 ~ 1500 cm-1. The bands at around 1080 cm-1 and 800 cm-1 were
distinct for all the catalysts and attributed to the asymmetric stretching vibration and
symmetric stretching vibration mode of Si-O-Si bridges, respectively 30. The band at
around 960 cm-1 is related with Si-O-Cu vibration and has been interpreted according
to the literature 32. Meanwhile, the peak is also widely regarded as the characterization
peak for the transition metal ions in the silica framework. With the Cu content
increasing, the intensity of the band at around 960 cm-1 also increased. These results
imply the Cu element has been incorporated in the Fe3O4@mSiO2 framework and the
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catalysts activities for cyclic ketones could also be promoted with the redox reaction
and the cooperated electron transfer ability of Fe-Cu composite metal oxides.
Fig. 4. FT-IR spectra of Fe3O4@mSiO2 (e) and Cu-Fe3O4@mSiO2 with different
Si/Cu ratio at 20:1 (a); 50:1 (b); 100:1 (c); 200:1 (d), responding to Fe/Cu ratio (20:1,
40:1, 80:1; 160:1).
Fig. 5 shows the magnified hysteresis loops of Fe3O4 (a), Fe3O4@mSiO2 (b) and
Cu-Fe3O4@mSiO2 (c), which also clearly indicates that no remanence are detected,
reflecting the superparamagnetic property. The saturation magnetization value of
Fe3O4 is 73.88 emu/g. After coated with a layer of porous silica, the magnetization
value of Fe3O4 is measured to 58.49 emu/g, then further decreases to 17.31 emu/g
after coated with the layer of mesoporous silica and the introduction of Cu2+ ions. The
decrease in magnetizing saturation is attributed to the presence of nonmagnetic SiO2,
mesoporous silica and some copper nanoparticles inside the uniformly pore of hollow
mesoporous spheres. The mesoporous silica microspheres have high magnetite
contents and good magnetic responsivity, beneficial to a fast and efficient separation
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with the aid of an external magnetic field and purification processes. As shown in the
internal figure of Fig. 5., the mixed particles could be completely separated by
external magnet, suggesting the magnetic and easy separation properties of mixed
particles.
Fig. 5. Magnified hysteresis loops of different samples at room temperature: Fe3O4
(a), Fe3O4@mSiO2 (b), Cu-Fe3O4@mSiO2 (c).
In order to investigate the coordination form of Cu species, the UV-Vis spectra of
Cu-Fe3O4@mSiO2 with different Si/Cu ratio were measured spanning from 200 nm ~
800 nm (Fig. 6). All the catalysts showed a shoulder adsorption peak centering around
at 250 nm, which could be assigned to the ligand-to-metal charge transfer between
isolated Cu2+ and the oxygen of SiO2 28,33. From Fig. 6, the shoulder adsorption peak
also indicates the retention of Cu2+ in octahedral coordination. And the intensity of
this band increases proportionally with the addition of Cu2+ loading, especially for
Cu-Fe3O4@mSiO2 (20) and Cu-Fe3O4@mSiO2 (50), which suggests more
mononuclear Cu2+ were supported into mesoporous silica matrix with magnetic
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nucleus. In addition, the weak and broad bands between 400 and 600 nm in the
catalysts with different Si/Cu ratio suggests linear oligonuclear clusters inserted into
mesoporous channel. Furthermore, there is a shoulder peak between 600 and 800 nm,
33
which might indicate the formation of bulky CuO particles . Recently, the catalytic
efficiency of several iron- and copper-based BV catalysts could be improved since the
redox properties of dissolved transition metal cations (Fe2+, Cu2+) allow generating
highly active hydroxyl radicals in the aerobic oxidative condition. Rahman et al.
synthesized high surface area mesoporous M-Co-HMS-X (metal = Ni, Fe, Cu).
Efficient catalyst with high catalytic activity (>99% cyclohexanone conversion
and >99% epsilon-caprolactone selectivity) for the oxidation of cyclohexanone to
epsilon-caprolactone could be observed, which might due to the cooperative role of
cobalt and iron towards catalytic activity 30. Wang et al. also synthesized Fe-Cu
bimetal oxides with benzaldehyde and air as complex oxidant. The addition of Fe
could not only improve the catalytic activity, but also increase the recycling batch
performance. But the selectivity issue was not mainly discussed in the report 35.
Fig. 6. UV-vis spectra of Cu-Fe3O4@mSiO2 with different Si/Cu ratio (20:1; 50:1;
100:1; 200:1).
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2.2. Catalytic performance of materials
Table 1 shows the results of catalytic performance towards B-V oxidation of
different ketones. Based on above results, all cyclic ketones could be converted to the
corresponding lactones with excellent selectivity (>99.0 %) under mild reaction
conditions. Cyclohexanone is easier than cyclopentanone to be converted to
12
corresponding lactone in consistent with other reports (entries 1 ~ 2) . In addition,
acetophenones with a methyl group have higher conversion than acetophenone
without it (entries 3 ~ 5), which is related to the methyl group fixed the positive
charge 16. Meanwhile, the steric-hindrance effect of substituent in six-membered
cyclic ketones has an important influence on the conversion of cyclic ketones (entries
4 ~ 5). The B-V conversion of p-methyl acetophenone is higher than o-methyl
acetophenone. For the mesoporous structure of Cu-Fe3O4@mSiO2, it can be due to the
36
chemical structure of ketones which gives rise to the steric hindrance . In addition,
same mole of cyclohexanone, cyclopentanone, acetophenone, 4-methylacetophenone
and 2-methylacetophenone mixed together were oxidized according to the same
reaction conditions (entry ~ 6), the conversion of cyclohexanone is higher than that of
the other four substances, which could be deduced the mesoporous structure of
Cu-Fe3O4@mSiO2 has certain shape-selectivity for mixed cyclic ketones.
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Table 1 The BV oxidation reaction of various ketones with Cu-Fe3O4@mSiO2 as
catalyst.a
Entry
1
Substrate
Product
Conv. b(%)
Sel. c(%)
O
O
O
>99.0
>99.0
O
O
O
2
3
75.1
10.0
>99.0
>99.0
O
O
O
O
O
O
4
5
55.7
50.3
>99.0
>99.0
O
O
O
1
80.0
59.7
33.0
34.4
30.6
>99.0
>99.0
>99.0
>99.0
>99.0
2
3
4
5
Mixed
Ketones
1 ~ 5
Mixed
Esters
1 ~ 5
6d
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a
Reaction conditions: the catalysts (50 mg), DCE (10 mL), ketones (2 mmol, 206 uL),
benzaldehyde (4 mmol, 405 uL), air (20 mL/min), 50 °C, reaction time 6 h. b Conversion of
c
d
ketones based on GC analysis. Selectivity of various esters. Entries 1 ~ 5 mixed ketones
(identical molar quantity, 2 mmol). The dodecane was used as an internal standard.
The mass transfer of the ketones with large molecular weight was reduced to the
active group on the catalysts surface, which might be due to the increased transfer
resistance for the molecular size and finite mesoporous structures of
Cu-Fe3O4@mSiO2. Hence, their activity is relatively lower for these ketones under the
same conditions 27, 29. With further modification of outer surface Fe-Cu composite
active sites, the shape-selectivity of Cu-Fe3O4@mSiO2 might be further precisely
controlled 30, 31
.
Table 2 shows the catalytic activities of silica materials with different molar ratio of
Si to Cu for the Baeyer-Villiger oxidation, which was tested in the presence of
benzaldehyde as sacrificing agent and air (20 mL/min). For all the cases, the
selectivity of ε-caprolactone reached 99.0 % based on the internal standard method.
For Fe3O4@mSiO2, the conversion of cyclohexanone and benzaldehyde were 27.3 %
and 29.6 %, separately, which was similar to the effect of the blank reaction though it
has got quite high surface area. However, copper doped Fe3O4@mSiO2 showed
increased catalytic performance towards B-V oxidation than that of Fe3O4@mSiO2
with the increasing content of Cu to the catalysts. When the molar ratio of Si to Cu
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was 20, the cyclohexanone and the benzaldehyde conversion reached 99.0 % and
92.0 %, respectively (table 2, entries 3 ~ 6).
Table 2 Catalytic results of Baeyer-Villiger oxidation of cyclohexanone using
Cu-Fe3O4@mSiO2 catalysts with different molar ratio of Si to Cu.a
Entry
Catalysts
Conv.b(%)
Sel.c(%)
Conv.d(%)
1
2
3
4
5
6
7
Blank
33.0
27.3
>99.0
96.8
82.8
63.7
90.0
>99.0
>99.0
>99.0
>99.0
>99.0
>99.0
>99.0
20.8
29.6
92.0
82.3
74.6
60.0
89.7
Fe3O4@mSiO2
Cu-Fe3O4@mSiO2(20)
Cu-Fe3O4@mSiO2(50)
Cu-Fe3O4@mSiO2(100)
Cu-Fe3O4@mSiO2(200)
I-Cu-Fe3O4@mSiO2(20)
a Reaction conditions: the catalysts (50 mg), DCE (10 mL), cyclohexanone (2 mmol, 206 uL),
benzaldehyde (4 mmol, 405 uL), air (20 mL/min), 50 °C, reaction time 6 h. b Conversion of
c
d
cyclohexanone based on GC analysis.
Selectivity of ε-caprolactone.
Conversion of
benzaldehyde. The dodecane used as an internal standard.
According to the characterizations of UV-Vis and FT-IR, the increasing of catalytic
activities was matched with the rising of isolated Cu2+ content in the catalysts. Since
the Cu element had varied valences, the synergetic effect of Cu and Fe redox
properties both existed on the surface of catalyst. And the redox reaction of iron ions
with copper was speed up for the increased rate of transferring the electrons on the
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37
silicon-based catalysts . Meanwhile, the catalysts activities were also promoted by
38
the excellent electron transfer ability of Cu-Fe3O4@mSiO2 catalyst . Therefore, the
isolated Cu2+ as Lewis acid could be the active sites in the aerobic B-V oxidation and
could activate benzaldehyde and cyclohexanone 13, 34. Furthermore, the oxidant
efficiency of benzaldehyde gradually increased with the content of copper in the
Fe3O4@mSiO2 (Table 2, entries 3 ~ 6). The reason may rely to the influence of
isolated Cu2+ species and the improved oxidant efficiency with the addition of
benzaldehyde and air. On the basis of above experiment results and analysis,
Cu-Fe3O4@mSiO2 (20) was chosen as the catalyst to explore the B-V catalytic
oxidation in the following experiments.
Table 3 Effect of different solvents on the conversion of cyclohexanone with
Cu-Fe3O4@mSiO2 as catalyst.a
Entry
Solvent
DCE
Conv.b(%)
>99.0
72.58
Sel.c(%)
>99.0
>99.0
>99.0
>99.0
/
Conv.d(%)
92.0
1
2
3
4
5
MeCN
DIOX
THF
88.98
5.29
11.05
7.35
8.85
DMSO
Traces
Traces
a Reaction conditions: the catalysts (50 mg), solvent (10 mL), cyclohexanone (2 mmol, 206 uL),
benzaldehyde (4 mmol, 405 uL), air (20 mL/min), 50 °C, reaction time 6 h. b Conversion of
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c
d
cyclohexanone based on GC analysis.
Selectivity of ε-caprolactone.
Conversion of
benzaldehyde. The dodecane as an internal standard.
In order to investigate the effect of different solvents on the B-V oxidation reaction,
Table 3 summarized the results of several solvents on the effect of B-V oxidation of
cyclohexanone to ε-caprolactone. From Table 3, the solvent was not a neglected factor
in the B-V reaction. Among several solvents investigated, the nonpolar solvent like
1,2-dichloroethane (DCE) gave much higher conversion of cyclohexanone and
benzaldehyde than that of other polar solvents, e.g. acetonitrile (MeCN), 1,4-dioxane
(DIOX). Furthermore, tetrahydrofuran (THF) and dimethylsulfoxide (DMSO) as
solvents would give poor performance in cyclohexanone and benzaldehyde
conversion.
Table 4 The effect of different aldehydes on the catalytic activity of aerobic B-V
oxidation of cyclohexanone with Cu-Fe3O4@mSiO2 as catalyst.a
Entry
Aldehyde
None
Conv.b(%)
0
Yield.c(%)
0
Sel.d(%)
0
1
2
3
4
5
6
Benzaldehyde
Propanal
>99.0
94.4
>99.0
94.4
>99.9
>99.9
>99.9
>99.9
>99.9
2,3-Dichlorobenzaldehyde
p-Tolualdehyde
p-Anisaldehyde
59.3
59.3
50.2
50.2
11.6
11.6
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a Reaction conditions: the catalysts (50 mg), DCE (10 mL), cyclohexanone (2 mmol, 206 uL),
aldehyde (4 mmol), air (20 mL/min), 50 °C, reaction time 6 h. b Conversion of cyclohexanone
based on GC analysis. c Yield of ε-caprolactone. d Selectivity of ε-caprolactone. The dodecane used
as an internal standard.
Therefore, 1,2-dichloroethane was the most suitable solvent for B-V oxidation
reaction of cyclohexanone. And the solvents of the reaction system proved to have
great influence for the aerobic B-V oxidation of cyclic ketones.
Table 4 summarized the cyclohexanone oxidation over Cu-Fe3O4@mSiO2 with
different aldehydes as sacrificing agents. For benzaldehyde as sacrificing agent, the
B-V oxidation of cyclohexanone gave higher conversion (99.0 %) and selectivity
(99.0 %) in the tested aldehydes. Meanwhile, the straight-chain aldehydes could
perform as a better sacrificing agent than these aldehydes with branched-chain, which
showed better performance in the aerobic B-V oxidation (Table 4, entries 2 ~ 6). The
reason may lie to straight-chain aldehydes were favorable to generate more free
radicals as a result of the resonance effect 30. The benzaldehyde with an
electron-withdrawing group gave higher conversion than the benzaldehyde with
electron-donating group for aerobic B-V oxidation. Furthermore, when there is no
aldehyde in the reaction, the poor performance and no reaction occurred for the BV
oxidation. Therefore, aldehyde also played an important role in the aerobic B-V
oxidation system, which was mentioned by the former investigation 34.
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2.3. Recycling studies
In order to estimate the stability and reusability of Cu-Fe3O4@mSiO2 for practical
application, the recycling tests were also investigated (Fig. 7). To compare the
changes of the catalytic performance before and after the reaction, the catalyst amount
and reaction time of Cu-Fe3O4@mSiO2 were reduced to 20 mg and 4 h, which the
steady and low conversion rates (about 25 %) could be obtained and shown after 5
consecutive runs. Under the high reaction rate, the real catalytic activity of
mesoporous Cu-Fe3O4@mSiO2 could not be obtained since the rate determining
processes are not only depended on the catalysts performance but also on the reactant
supply. From Fig. 7, the similar conversion of cyclohexanone and benzaldehyde
remains after the five consecutive runs, which means mesoporous Cu-Fe3O4@mSiO2
catalyst could be reused without remarkable loss in the catalytic performance during
the B-V oxidation processes. Hence, Cu-Fe3O4@mSiO2 could be a high efficient and
recyclable catalyst in the cyclohexanone B-V oxidation.
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100
80
60
40
20
0
1
2
3
4
5
100
80
60
40
20
0
1
2
3
4
5
100
80
60
40
20
0
1
2
3
4
5
Number of Recycling
Fig. 7. Reusability of the recovered Cu-Fe3O4@mSiO2 in the B-V oxidation of
cyclohexanone. Reaction conditions: the catalysts (20 mg), DCE (10 mL),
cyclohexanone (2 mmol, 206 uL), benzaldehyde (4 mmol, 405 uL), air (10
mL/min), 50 °C, reaction time 4 h.
2.4. Reaction mechanism
Based on above investigation, the catalytic mechanism of Cu-Fe3O4@mSiO2 for the
oxidation of cyclohexanone could be proposed. According to literature, the main two
reaction steps of cyclohexanone oxidation with O2/aldehyde includes (a) peracid
14
formation from aldehyde and O2 and (b) peracid oxidation of the cyclohexanone .
Nabae et al. found the Ketjen Black carbon could convert cyclohexanone to
ε-caprolactone without metal active sites in O2/aldehyde condition and proposed the
carbon materials is responsible for the formation of peracid from O2 and aldehyde
(step (a)) but not for the ketone oxidation (step (b)). It is also meaningful to know
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which reaction step is enhanced by the Cu-Fe3O4@mSiO2. Therefore,
m-cholorobenzaldehyde was used as catalytic performance test in the step (a) and
characterized with FT-IR to analyze the residue after the solvent was removed. From
FT-IR results shown in Fig.8, in the sample (a), the m-chlorobenzoic acid was
generated from m-cholorobenzaldehyde since characteristic peaks of m-chlorobenzoic
acid exists at 2100-3300 cm-1 (the broadband of O-H stretch and H-bonded) and 1719
cm-1 (C=O stretch). Compared with the sample (b), it could be confirmed that no
m-choloroperoxybenzoic acid generated without Cu-Fe3O4@mSiO2. For the
generated spectrum with Cu-Fe3O4@mSiO2 became complicated, sample (b) seemed
a kind of mixture. To confirm the presence of peroxybenzoic acid, sample (b) was
washed with sodium bicarbonate solution to remove the benzoic acid and then
recrystallized using dichloromethane and hexane solution 14,15. The spectrum of this
purified sample (c) showed several characteristic peaks of about 3500 cm-1 (O-H
stretch) and 1756 cm-1 (C=O stretch) which confirms the presence of
m-chloroperoxybenzoic acid. Therefore, the peracid generation was confirmed during
the O2/aldehyde B-V oxidation in Cu-Fe3O4@mSiO2 catalytic system. No
m-chloroperoxybenzoic acid was observed with the same purification procedure to the
sample (a) which suggested Cu-Fe3O4@mSiO2 microspheres could catalyze the
formation of peracid from O2 and aldehyde.
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(a) without catalyst
(b) with catalyst
(c) purified
(d) aldehyde
(e) peracid
(f) carboxylic acid
4000
3000
2000
1000
Wavenumber (cm-1)
Fig. 8 FT-IR spectra of (a) m-chlorobenzaldehyde treated without catalyst, (b)
m-chlorobenzaldehyde treated with Cu-Fe3O4@mSiO2, (c) purified sample from sample b,
(d) m-chlorobenzaldehyde, (e) m-chloroperoxybenzoic acid and (f) m-chlorobenzoic acid.
Reaction conditions: the catalysts (50 mg), DCE (10 mL), m-cholorbenzaldehyde (4 mmol),
air (20 mL/min), 50 °C, reaction time 6 h.
For the step (b), in order to confirm whether Cu-Fe3O4@mSiO2 could accelerate the
oxidation of cyclohexanone in the presence of peracid, the oxidation of
cyclohexanone by m-chloroperoxybenzoic acid was performed with and without the
Cu-Fe3O4@mSiO2 catalyst. From the results summarized in the Table 5, the
conversion and selectivity of the O2/aldehyde B-V oxidation remained similar with
and without the Cu-Fe3O4@mSiO2, which showed the Cu-Fe3O4@mSiO2 did not
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catalyze the cyclohexanone B-V oxidation once peracid generated in the reaction.
From above results, it clearly suggested the Cu-Fe3O4@mSiO2 catalyst were only
responsible for the generation of m-chloroperoxybenzoic acid and did not contribute
to the ketone oxidation for step (b).
Table 5 Cyclohexanone oxidation by m-chloroperoxybenzoic acid.a
Catalyst
Cu-Fe3O4@mSiO2
no catalyst
Conv.b(%)
Yield.c(%)
Sel.d(%)
>99
44
41
44
41
>99
a Reaction conditions: the catalysts (20 mg), DCE (10 mL), cyclohexanone (2 mmol, 206 uL),
m-chloroperoxybenzoic acid (4 mmol), air (10 mL/min), 50 °C, reaction time 4 h. b Conversion of
cyclohexanone based on GC analysis. c Yield of ε-caprolactone. d Selectivity of ε-caprolactone.
The dodecane used as an internal standard.
Scheme 1. Possible reaction pathways involved in the B-V oxidation of
cyclohexanone with Cu-Fe3O4@mSiO2
For mesoporous with Cu-Fe3O4@mSiO2 catalytic system, Cu and Fe based oxides
played important roles in the reaction. In the initial reaction, peroxybenzoic acid was
generated by benzaldehyde and oxygen in air. Then, cyclohexanone was attacked by
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perbenzoic acid to form Criegee adduct according to the common mechanism of B-V
oxidation. The reaction could be speed up since the porous structure of
Cu-Fe3O4@mSiO2 offered raised numbers of accessible active sites and enhanced
mass transfer of reactants and products35. This conclusion also could be evidenced
with the comparison of mesoporous Cu-Fe3O4/mSiO2 and nonporous
N-Cu-Fe3O4/SiO2 with the same copper loading (Fig. 9). From Fig. 9(a),
cyclohexanone could be consumed >99% after 6 h and the amounts of peracid species
were also higher than those in Fig. 9(b). The consumption of benzaldehyde remained
quite low in N-Cu-Fe3O4/SiO2 which also confirmed the low catalytic efficiency of
N-Cu-Fe3O4/SiO2. The main difference of mesoporous Cu-Fe3O4/mSiO2 and
nonporous N-Cu-Fe3O4/SiO2 catalytic system is the porous structure. The external and
internal mass transfer limitations of these two catalytic systems were different. And
this only structure difference for the copper and silica supported Fe3O4 microspheres
might be responsible for the enhanced mass transfer for the cyclohexanone and related
products. The followed scheme by the rearrangement to produce ε-caprolactone and
by- product benzoic acid was illustrated in Scheme 1.
b
a
5
4
3
2
1
5
4
3
2
1
aldehyde
acid
peracid
cyclohexanone
e-caprolactone
aldehyde
acid
peracid
cyclohexanone
e-caprolactone
0
100
200
300
400
0
100
200
Reaction time / h
300
400
Reaction time / h
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Fig. 9. Comparison of mesoporous Cu-Fe3O4@mSiO2 (a) and nonporous
N-Cu-Fe3O4/SiO2 (b) for the B-V oxidation of cyclohexanone. Reaction conditions:
the catalysts (50 mg), DCE (10 mL), cyclohexanone (2 mmol, 206 uL),
benzaldehyde (4 mmol, 405 uL), air (20 mL/min), 50 °C, reaction time 6 h.
Furthermore, in the mixed ketones, the catalyst still exhibits higher conversion rate
for cyclohexanone (80%) with smaller molecular weight, and the conversion rate of
large molecules, such as acetophenone, is only 33% (Scheme 2). Hence, this catalysts
system could be used as shape selectivity catalysts in aerobic Baeyer-Villiger
oxidations for cyclic ketones with different molecular sizes.
Scheme 2. The shape-selectivity conversion for mixed ketones with
Cu-Fe3O4@mSiO2
3. Conclusion
In conclusion, magnetite Cu modified Fe3O4 supported mesoporous silica
microspheres (Cu-Fe3O4@mSiO2) have been prepared by two step hydrothermal
method. The microspheres possess high surface area, large pore volume, uniform
mesoporous and high magnetization. The catalytic activity of Cu-Fe3O4@mSiO2 was
investigated in aerobic Baeyer-Villiger reaction, which superb cyclohexanone
conversion (99.0 %) and selectivity of ε-caprolactone (99.0 %) could be obtained with
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benzaldehyde and in air condition. In the presence of mixed ketones, the catalyst still
exhibits higher conversion rate for cyclohexanone (80%) with smaller molecular
weight, indicating certain shape-selectivity of Cu-Fe3O4@mSiO2 for different cyclic
ketones. The catalyst could be reused after air calcination with slight loss in catalytic
performance for B-V oxidation. The Cu-Fe based oxides in the catalyst activates
cyclohexanone and the mesoporous structure could offer enhanced mass transfer for
the substrates and products. Additionally, the catalyst could be fast separated from
aqueous solution with external magnet after the catalytic reaction. Considering useful
magnetic properties, unique mesoporous structure and highly efficient catalytic
performance, superparamagnetic mesoporous Cu-Fe3O4@mSiO2 microspheres could
offer promising applications for aerobic Baeyer-Villiger oxidation of different and
mixed cyclic ketones.
4. Experimental section
4.1 Materials
The chemicals used in this work are ethylene glycol, ferric chloride, sodium acetate
(NaAC), tetraethyl orthosilicate (TEOS), ethanol, copper nitrate trihydrate (analytical
grade, Tianjin Chemical Corp.). Cetyltrimethylammonium bromide (CTAB) was
purchased from Sigma-Aldrich. The organic reagents used in the catalytic reaction
were supplied from Tianjin Chemical industry. All of the chemicals were used as
received without further purification.
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4.2 Synthesis
4.2.1 Preparation of Fe3O4 particles
Magnetic Fe3O4 nanoparticles were synthesized by an improved hydrothermal
39
reaction . Typically, 1.35 g of FeCl3·6H2O was dissolved into ethylene glycol (40
mL) and heated to 60 °C under stirring for 10 min, followed by the addition of sodium
acetate (3.6 g). After stirred vigorously for 30 min, the obtained yellow mixture was
transferred to a Teflon-lined stainless steel autoclave, sealed and crystallized at 180 °C
for 24 h. When cooled naturally to the ambient temperature, the obtained black
production was separated with magnet, washed completely with ethanol several times,
and then dried in a vacuum at 60 °C overnight.
4.2.2 Preparation of Cu-Fe3O4@mSiO2 and I-Cu-Fe3O4@mSiO2 microspheres
The Cu-Fe3O4@mSiO2 microspheres were prepared by a directly hydrothermal way.
Typically, Fe3O4 (100 mg) microspheres were redispersed into a mixed solution in
three-neck flask consisting of CTAB (0.3 g). Certain amount of Cu(NO3)2·3H2O were
dissolved into 80 mL distilled water, ethanol (60 mL) and aqueous ammonia solution
(1.1 mL, 28 %). After stirred mechanically for 30 min, TEOS (1.5 mL, 98 %) was
introduced dropwise into the mixture and kept stirring for 6 h at 50 °C. Subsequently,
the solution was transferred to a PTFE-lined stainless steel autoclave and heated to
100 °C for 12 h. The resulting production was filtered, washed with large amount of
water and ethanol, followed by dried at 40 °C for 12 h. Finally, the solid was calcined
at 823 K for 6 h with 3 K/min in tubed furnace to remove the template agent. The
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