Superparamagnetic Core–Shell Metal–Organic Framework Fe3O4/Cu3(btc)2 Microspheres and Their Catalytic Activity in the Aerobic Oxidation of Alcohols and Olefins

Article abstract of DOI:10.1002/ejic.201600761

Core–shell structured Fe₃O₄/Cu₃(btc)₂ (btc = 1,3,5-benzenetricarboxylate) microspheres have been successfully synthesized by coating the metal–organic frameworks on poly(acrylic acid) (PAA) functionalized Fe₃O₄ microspheres by the layer-by-layer assembly method. The MOF composite exhibited excellent catalytic activity in the aerobic oxidation of alcohols and the epoxidation of olefins due to the porosity of MOFs and large amount of readily accessible Cu²⁺. Furthermore, the catalyst could be easily segregated from the catalytic system by using an external magnetic field due to its magnetic properties. The heterogeneous catalyst displayed good reusability and broad reagent scope in the aerobic oxidation of alcohols and epoxidation of olefins.

Full text of DOI:10.1002/ejic.201600761

A Journal of
Accepted Article
Title: Superparamagnetic core-shell metal-organic framework Fe3O4/Cu3(BTC)2
microspheres and their catalytic activities for the aerobic oxidation of alcohols
and olefins
Authors: Jie Li; Hongyi Gao; Li Tan; Yi Luan; Mu Yang
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201600761
Link to VoR: http://dx.doi.org/10.1002/ejic.201600761

European Journal of Inorganic Chemistry
10.1002/ejic.201600761
FULL PAPER
Superparamagnetic
core-shell
metal-organic
framework
Fe3O4/Cu3(BTC)2 microspheres and their catalytic activities for the
aerobic oxidation of alcohols and olefins
Jie Li, Hongyi Gao, Li Tan, Yi Luan and Mu Yang*
Abstract: Core-shell structured Fe3O4/Cu3(BTC)2 (BTC = 1, 3, 5-
benzenetricarboxylate) microspheres were successfully synthesized
by coating the metal-organic frameworks on polyacrylic acid (PAA)
functionalized Fe3O4 microspheres with layer-by-layer assembly
method. The MOF composite exhibits excellent catalytic activities in
the aerobic oxidation of alcohols and epoxidation of olefins due to
the porosity of MOFs and large amount of readily accessible Cu2+.
Furthermore, the catalyst can be easily segregated from the catalytic
system using an external magnetic field due to its high
magnetization. The heterogeneous catalyst displays good reusability
and shows extensive applicability in the aerobic oxidation of alcohols
and epoxidation of olefins.
ordered nanoporous structure provides high porosity for reactant
to access the metal-ion active sites.[17, 18] Various kinds of MOFs,
19-21]
23]
such as copper-based,[5,
iron-based,[22,
zinc-based,[24]
26]
cobalt-based MOFs,[25,
and zirconium-based MOFs[27] have
dispalyed as efficient catalysts for the aerobic oxidation of
20-22, 24]
19, 27]
phenols,[26] alkenes,[17,
benzylic alcohols,[5,
and
cycloalkanes,[25] etc. Recently, nanoscaled MOFs as catalyst for
aerobic oxidation have attracted tremendous attention due to
fast mass transport in contrast to its bulk counterpart, especially
in liquid-phase catalysis.[4, 28-30] The abundant accessible active
sites in the framework of the nanoscaled MOFs enhance the
utilization efficiency of catalysts, and various strategies have
been explored for the construction of conveniently applied
nanoscaled MOFs catalyst, such as magnetic encapsulated
32]
33, 34]
nanoparticles,[31,
core-shell microspheres,[5,
hollow
and yolk-shell
Introduction
microspheres,[35] hollow nanocubes,[35,
36]
microspheres,[37-39] etc. It is especially worth-noting that the
multifunctional magnetic MOFs catalyst possesses obvious
superiority in convenient separation and recovery due to the
magenetic properties. Recently, we reported the magnetic
functionality to the nanoscaled Fe-MIL-101 via Fe3O4
nanoparticle encapsulation. The recoverable Fe3O4/Fe-MIL-101
catalyst exhibit high efficiency in the aerobic oxidation of
alcohols and epoxidation of olefines.[32] Another efficient
functional approach is using sub-micrometer magnetic particles
as core and MOFs nanoparticles as active shell, the Fe3O4@Fe-
MIL-100 core-shell microspheres showed high efficient activities
in catalytic Claisen-Schmidt condensation of benzaldehyde with
Widespread attention on aerobic oxidation using molecular
oxygen as an oxidant is associated with the urgent demand for
green chemistry and environmentally friendly industry.[1-3] The
resulting products of aerobic oxidation such as ethers,
aldehydes, ketones and carboxylic acids are of fundamental
importance as chemical intermediates in fine chemical
4-6]
engineering.[2,
A wide variety of research have been
published to explore high efficient catalytic systems for the
aerobic oxidation, since the molecuar oxygen is relatively stable
and need appropriate catalysts for activation. Transition metal
catalysts, such as Cu(II),[7-9] Fe(III),[6, 10, 11] Mn(II),[12, 13] Co(II)[14-16]
and the corresponding metal-ion complex, display high catalytic
activities and good selectivities for aerobic oxidation reactions.
However, homogeneous catalysts are tough to recover from the
reaction system and will generate pollution after the catalytic
process. Heterogeneous catalysts are easy to recycle, but only
the catalytic active sites on the surface can interact with the
substrates, which affects the catalytic efficiency to some extent.
acetophenone.[33] Although
a
wide variety of core-shell
Fe3O4/MOFs have been reported and used as excellent
40-42]
catalysts in various reactions,[33,
research works about
multifunctional core-shell Fe3O4/MOFs as efficient catalyst for
aerobic oxidation of alcohols and olefins using molecular oxygen
as an oxidant under atmospheric pressure at moderate
temperature have not been reported, and the exploration of such
an efficient green catalyst with high utilization of oxygen atom
source is desirable.
Combining
the
advantage
of
homogeneous
and
heterogeneous catalysts, metal-organic framework (MOF) has
become a research hotspot as high efficient catalyst since its
In this paper, we report
a facile route to synthesize
Fe3O4/Cu3(BTC)2 core-shell microspheres and employ the
heterogeneous catalyst in the aerobic oxidation of alcohols
and olefins at moderate condition. Polyacrylic acid (PAA)
functionalized Fe3O4 microspheres were firstly obtained by
a one-step solvothermal method, and then the core-shell
Jie Li, Hongyi Gao, Li Tan, Yi Luan, Prof. Dr Mu Yang*
Beijing Key Laboratory of Function Materials for Molecule & Structure
Construction, School of Material Science and Engineering, University of
Science and Technology Beijing, Beijing, 30 Xueyuan Road, 100083, P. R.
China.
E-mail: yangmu@ustb.edu.cn
structured
Fe3O4/Cu3(BTC)2
microspheres
were
synthesized through layer-by-layer strategy under mild
condition. Cu3(BTC)2 is chosen as an effective heterogeneous
catalyst and can allow the selectively oxidation of alcohols to the
Supporting information for this article is given via a link at the end of the
document.
19,
43]
corresponding aldehydes.[4,
The multifunctional

European Journal of Inorganic Chemistry
10.1002/ejic.201600761
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Fe3O4/Cu3(BTC)2 core-shell catalyst with active Cu(II) species on
the shell and magnetic recovery capability exhibits efficient
activity and high selectivity in the aerobic oxidation of alcohols
and olefins using molecular oxygen as an oxidant under
atmospheric pressure at moderate temperature. What’s more,
the superparamagnetic Fe3O4 core can realize the efficient
separation and recovery of the magnetic MOFs
heterogeneous catalyst by applying an external magnetic
field.
The PAA-modified Fe3O4 particles were prepared by a one-
step solvothermal method in the presence of polyacrylic acid.[44]
These particles are uniform in size and their average diameter is
230 nm (Figure 1a). After 10 cycles of repeated coatings, the
surface of the composited microspheres becomes rough and the
Cu3(BTC)2 shell is uniformly deposited on the surfaces of Fe3O4
particles (Figure 1b, c, d). The obtained Fe3O4/Cu3(BTC)2
microspheres is 350 nm in diameter, and the thickness of the
Cu3(BTC)2 shell is 60 nm on average.
Results and Discussion
+
+
*
Fe3O4(PAA)
Cu (BTC)
+
3
2
+
+
+
Synthesis of Fe3O4/Cu3(BTC)2 core-shell microspheres
+
+
a
*
*
*
*
*
*
*
b
*
*
*
simulated Cu3(BTC)2
50 60 70 80
10
20
30
40
2 Theta (degree)
Scheme 1. The synthetic procedure of Fe3O4/Cu3(BTC)2 core-shell
microspheres.
Figure 2. XRD patterns of (a) Fe3O4 and (b) Fe3O4/Cu3(BTC)2.
The core-shell structured Fe3O4/Cu3(BTC)2 microspheres were
prepared by combining the improved solvothermal method and
Layer-by-Layer assembly method. Firstly, monodispersed PAA
modified Fe3O4 particles were synthesized via solvothermal
method.[44] Then the Fe3O4 particles were immersed in
Cu(CH3COO)2·H2O solution and then in H3BTC solution
alternatively. PAA adhered on the surface of Fe3O4
nanoparticles favor not only the dispersion of the nanoparticles,
but also the coating of MOFs layer onto the magnetite core via
the carboxylic acid groups coordinate with Cu2+ ions, so the
Cu3(BTC)2 nanocrystals were subsequently grown layer by layer
on the surface of PAA modified core (Scheme 1). Finally, core-
XRD patterns confirm the composition and crystal structure of
the as-synthesized microspheres (Figure 2). The peaks at 2θ
values of 30.09o, 35.42o, 43.05o, 53.45o, 56.95o, 62.53o and
73.9o (Figure 2a) are assigned to the (220), (311), (400), (422),
(511), (440) and (533) planes of the face-centered cubic Fe3O4
(JCPDS card No. 19-0629), respectively.[45] The reflection peaks
at the 2θ values of 9.5o, 11.6o, 13.3o, 16.5o, 17.5o and 19.0o are
identical to the peaks of the Cu3(BTC)2 crystals, which
correspond to the (220), (222), (400), (422), (333) and (440)
46, 47]
planes of the Cu3(BTC)2 shell, respectively.[5,
The Cu
shell
structured
Fe3O4/Cu3(BTC)2
microspheres
were
content of the Fe3O4/Cu3(BTC)2 microspheres determined by
ICP-AES analysis is 5.6 wt.%, and the calculated Cu3(BTC)2
content is 17.8 wt.%.
successfully prepared after repeated cycles.
Fe3O4(PAA)
3423
Fe3O4/Cu3(BTC)2
600
1615
1367
1440
Cu3(BTC)2
4000
3000
2000
1000
Wave number (cm-1)
Figure 3. FTIR spectra of Fe3O4 and Fe3O4/Cu3(BTC)2.
Figure 1. TEM images of (a) Fe3O4 microspheres and (b, c) Fe3O4/Cu3(BTC)2
microspheres, (d) SEM images of Fe3O4/Cu3(BTC)2 microspheres.

European Journal of Inorganic Chemistry
10.1002/ejic.201600761
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recovery process of Fe3O4/Cu3(BTC)2 microspheres by applying an external
magnetic field.
FTIR spectra of Fe3O4 and Fe3O4/Cu3(BTC)2 were tested to
detect the functional groups of the composite microspheres
(Figure 3). The relatively strong band at 600 cm-1 and 3423 cm-1
in the PAA modified Fe3O4 micropheres correspond to Fe−O
vibrations and −OH stretching vibrations, respectively.[44] The
peaks at 1615 cm-1 and 1440 cm-1 are attributable to the
asymmetric and symmetric stretching vibrations of C=O in
carboxylic acid group. The sharp peak at 1367 cm-1 ascribes to
C−O bond of the carboxylic acid group in Cu3(BTC)2 shell,
indicating that Cu3(BTC)2 is successfully coated onto the
magnetic core. The Cu3(BTC)2 shell is easily loaded on the
surface of PAA-modified Fe3O4 microspheres through bonding of
Cu2+ to carboxylic acid groups, and then the btc-units bond to
the Cu2+ ions to form the metal-organic framework.
The magnetic behavior of the Fe3O4/Cu3(BTC)2 microspheres
was studied by the magnetic hysteresis loops at room
temperature. The microspheres show superparamagnetic
properties with unperceivable remanence or coercivity, and the
specific saturation magnetization for the Fe3O4 and
Fe3O4/Cu3(BTC)2 microspheres are 55.2 emu/g and 33.2 emu/g.
The magnetic separation process of the Fe3O4/Cu3(BTC)2
microspheres was carried out by placing a magnet beside the
suspension of the microspheres, and the microspheres
response rapidly to the external magnetic field. The
superparamagnetic characteristic endows the conveniently
isolation of the catalyst from the liquid-phase catalytic reaction
system just by appling an external magnetic field.
0.45
0.40
300
0.35
Catalytic performance of Fe3O4/Cu3(BTC)2 core-shell
microspheres
0.30
250
0.25
0.20
200
0.15
4
6
8
10 12 14 16
Catalytic activity of the catalyst for the oxidation of alcohols
150
100
50
Pore diameter (nm)
Table 1. Aerobic oxidation of benzyl alcohol using Fe3O4/Cu3(BTC)2 as
catalyst under various reaction conditions a
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (P/P0)
Entry
1
Catalyst
Additive
Conv. b
>99%
Select. b
>99%
Fe3O4/Cu3(BTC)2
Na2CO3
Figure 4. Nitrogen adsorption-desorption isotherms of Fe3O4/Cu3(BTC)2.
2
3
4
5
6
7
8
-
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
-
0%
0%
-
Fe3O4
-
Cu3(BTC)2
>99%
89%
>99%
8%
>99%
>99%
>99%
>99%
>99%
The porous nature of the Fe3O4/Cu3(BTC)2 microspheres was
demonstrated by the nitrogen adsorption-desorption isotherms.
The surface area is 258.62 m2g-1 and the total pore volume is
0.50 cm3g-1, respectively. The microspheres contain wide range
of mesopores centered at 3.7 nm, 5.5 nm and 9.0 nm using to
the BJH method from the adsorption branch. The surface area of
Fe3O4/Cu3(BTC)2 microspheres is much lower than that of pure
Cu3(BTC)2 (1391 m2g-1),[4] which is due to the Fe3O4
nanoparticles with low surface area.
Cu(NO3)2∙3H2O
Cu(CH3COO)2∙H2O
Fe3O4/Cu3(BTC)2
Fe3O4/Cu3(BTC)2
NaHCO3
84%
a
Reaction conditions: 1mmol benzyl alcohol, 4.4 mol% catalyst, 1 equiv
additive, 0.5 equiv TEMPO, 2.3 mL acetonitrile, 75 oC, 1 atm O2, 6 h. the
b
results were analyzed by GC-MS using nitrobenzene as the internal
standard.
The catalytic activities of the Fe3O4/Cu3(BTC)2 microspheres
were tested by catalyzing the aerobic oxidation of benzyl
alcohol assisted by TEMPO as the co-catalyst and Na2CO3 as
19, 43, 48, 49]
a base additive.[7,
The results are represented in
Table 1. The reaction system catalyzed by Fe3O4/Cu3(BTC)2
achieved ~ 100% yield in 6 h (Entry 1). When catalyzed with
no catalyst or using Fe3O4 as catalyst, there were no
conversion detected, indicating that the active catalytic site is
MOFs material (Entry 2, 3). When catalyzed with pure
Cu3(BTC)2 catalyst, the reaction system showed high
conversion owing to the high-efficient catalytic activities of
MOFs catallyst (Entry 4). Homogeneous copper catalyst such
as copper nitrate trihydrate (Cu(NO3)2∙H2O) and copper
acetate monohydrate (Cu(CH3COO)2∙H2O) also exhibit high
activities (Entry 5, 6), confirming that the Cu(II) ions act as the
catalytical active sites to promote the catalytic activities. The
catalytic system with Cu(NO3)2∙H2O as catalyst has free Cu(II)
Figure 5. The magnetic hysteresis loops of (a) Fe3O4 and (b)
Fe3O4/Cu3(BTC)2 microspheres at 298K. The inset shows the magnetic

European Journal of Inorganic Chemistry
10.1002/ejic.201600761
FULL PAPER
in the solution, and the benzyl alcohol yielded up to 89% of
benzaldehyde. While the Cu(CH3COO)2∙H2O exhibit better
catalytic performance with ~ 100% yield. These imply that the
Cu(II) species are the catalytic sites and the carboxylate
ligand favors to enhance the catalytic activities through
33]
coordination.[4,
Furthermore, the porous Cu3(BTC)2
framework facilitates the contact of the substrate to more
abundant Cu(II) active sites. Therefore, the Fe3O4/Cu3(BTC)2
displays high efficiency in the aerobic oxidation of benzyl
alcohols. So as to get insight into the effect of alkaline
additive, the black experiments with no additive were
performed, and the catalytic reaction results in little amount of
benzaldehyde (Entry 7). The yield of benzaldehyde was high
as 84% when using NaHCO3 as the additive (Entry 8),
indicating that alkaline additive is indispensable for the
catalytic system and Na2CO3 is the optimal additive for this
catalytic oxidation at the reaction condition. The thickness of
the Cu3(BTC)2 shell can be tuned by different repeated coating
cycles, and the influence of repeated coating cycles on the
catalyst magnetic properties and the activity of aerobic
oxidation of alcohols have been studied and discussed (Figure
S1-S3).
Table 2. Aerobic oxidation of different alcohols using Fe3O4/Cu3(BTC)2 as
catalyst a
a Reaction conditions: 1mmol alcohol, 4.4 mol% catalyst, 1 equiv Na2CO3,
b
0.5 equiv TEMPO, 2.3 mL acetonitrile, 75 oC, 1 atm O2. the results were
analyzed by GC-MS using nitrobenzene as the internal standard.
The aerobic oxidation of various substituent alcohols was
performed to extend the species of the substrate alcohols under
identical condition. The electron-donating substituents such as
methyl and methoxy groups on the phenyl ring display excellent
yields (> 99%) and selectivities (> 99%) to their corresponding
aldehydes (Entry 2, 3). The electron-withdrawing substituents
like 4-fluorobenzyl alcohol (Entry 4) and 4-cyanobenzyl alcohol
(Entry 5) also result in good yield (> 99%) even with the
presence of negative influence electron-withdrawing substituent,
possibly because of the para-position of the substituent. The
heterocyclic alcohol such as 2-pyridinemethanol is oxidized to
pyridine-2-carboxaldehyde with a high yield (> 99%) under
identical condition (Entry 6). The allylic alcohols such as 3-
methyl-2-buten-1-ol (Entry 7) and cinnamyl alcohol (Entry 8) are
oxidized to the corrosponding 3-methyl-2-butenal and
cinnamaldehyde with moderate yields, and the C=C double
bonds are not affected during the conversion. The performance

European Journal of Inorganic Chemistry
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of catalytic oxidation of secondary alcohols such as 1-
phenylethanol (Entry 9), cyclopentanol (Entry 10) and 2-
cyclohexen-1-ol (Entry 11) are also tested, however, the
Fe3O4/Cu3(BTC)2 catalyst exhibited little conversion even after
12 h reaction, the low conversion can be explained by the steric
hindrance and electronic effects of the secondary alcohols.[5, 7, 43,
The recyclability of Fe3O4/Cu3(BTC)2 catalyst was also tested
in the aerobic oxidation of benzyl alcohol. The
superparamagnetic catalyst is conveniently isolated and
recovered from the catalyst system just through an external
magnetic field, and then washed with alcohol several times and
dried for the next run. The conversion maintains above 96% and
the selectivity remains at a high level of 99% after five
consecutive usage, indicating that the Fe3O4/Cu3(BTC)2 catalyst
own excellent cycle performance. The TEM (Figure S4) and
XRD results (Figure S5) show that the Fe3O4/Cu3(BTC)2 catalyst
maintains its morphology and composition after five catalytic
runs.
50]
The catalytic performance of Fe3O4/Cu3(BTC)2 is in
accordance with that of reported homogeneous copper salts,[5, 48]
suggesting that the intrinsic active site of the Fe3O4/Cu3(BTC)2
catalyst is Cu(II) ions as well.
Recyclability of the catalyst
The PAA modified surface of the Fe3O4 core can easily
coordinate Cu2+ and form the good interface for firm anchoring
Cu3(BTC)2 MOFs shell, and the Cu3(BTC)2 shell is hard to be
coated onto the core with unmodified Fe3O4 nanospheres
(Figure S6). The abundant carboxylic acid groups of PAA can
coordinate with Cu2+ ions and favor the coating of MOFs layer
onto the magnetite core, besides, it can facilitate the dispersion
of the nanoparticles. The strong interface junction and stable
framework structure of MOFs guarantee the excellent stability
and recyclability of the Fe3O4/Cu3(BTC)2 catalyst. The high
porous MOFs material can adsorb effectively gas molecules and
organic substrates, increasing the contacts between the active
site Cu2+ with the reactants, which all improve the activities of
the heterogeous catalyst.
100
80
60
40
fresh catalyst
filtration of the catalyst
20
0
0
2
4
6
8
10
12
Time (h)
Figure 6. Leaching experiment of Fe3O4/Cu3(BTC)2 catalyst in the aerobic
oxidation of benzyl alcohol (Reaction conditions: 1mmol benzyl alcohol, 4.4
mol% catalyst, 1 equiv Na2CO3, 0.5 equiv TEMPO, 2.3 mL acetonitrile, 75 oC,
1 atm O2).
Catalytic activity of the catalyst for the epoxidation of
olefins
Table 3. Olefin epoxidation using Fe3O4/Cu3(BTC)2 as catalyst a
The leaching experiment of the Fe3O4/Cu3(BTC)2 catalyst was
studied in the aerobic oxidation of benzyl alcohol. The catalyst
was separated from the catalytic system when the conversion
was about 50% after 3 h reaction, and the reaction was allowed
to carry on. Negligible conversion is detected in comparing to
the system with Fe3O4/Cu3(BTC)2 catalyst (Figure 6). The
concentration of Cu(II) ions in the aqueous solution is 1 ppm
according to ICP-AES analysis, indicating the negligible leaching
for Fe3O4/Cu3(BTC)2 catalyst.
Conversion
Selectivity
100
80
60
40
20
0
a
Reaction conditions: 1mmol olefin, 2.6 mol% catalyst,
2 mmol
isobutyraldehyde, 2.3 mL acetonitrile, 40 oC, O2. b the results were analyzed
by GC-MS using nitrobenzene as the internal standard.
1
2
3
4
5
Cycles
We further explored the catalytic activities of the
Fe3O4/Cu3(BTC)2 for the aerobic epoxidation of olefins with
isobutyraldehyde as a sacrificial co-reductant,[4, 51] since it is also
Figure 7. The recyclability of Fe3O4/Cu3(BTC)2 catalyst in the aerobic
oxidation of benzyl alcohol (Reaction conditions: 1mmol benzyl alcohol, 4.4
mol% catalyst, 1 equiv Na2CO3, 0.5 equiv TEMPO, 2.3 mL acetonitrile, 75 oC,
1 atm O2).

European Journal of Inorganic Chemistry
10.1002/ejic.201600761
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an important organic transformations in chemical industry,[4] and
the catalyst displays excellent performance even under mild
conditions (40 oC). Cyclic olefins such as cis-cyclooctene and
cyclohexene are completely converted to cyclooctene oxide and
cyclohexene oxide respectively within 8 h (Entry 1, 2), and the
complete epoxidation achieved at 6 h for norbornene with
outstanding selectivity (> 99%) (Entry 3). Trisubstituted olefins
such as α-pinene also achieved complete conversion in 10 h
without any by-product (Entry 4). The aromatic olefin such as
styrene is also fully oxidized within 10 h, and the major product
The magnetic Fe3O4 microspheres were prepared via an improved
solvothermal method published in our previous report.[44] FeCl3∙6H2O (4
mmol), PAA (0.6 mmol) and sodium acetate (0.1 mol) were dispersed in
ethylene glycol (40 mL) orderly under violent agitation at 60 oC. The
obtained yellow mixture was transferred to
a 50 mL Teflon-lined
stainless-steel autoclave and heated at 200 oC for 12 h. The obtained
powders were collected, washed with deionized water and ethanol, and
dried overnight.
Synthesis of Fe3O4/Cu3(BTC)2 microspheres.
is styrene oxide (84%) and there is
a
little amount of
5). Moreover,
benzaldehyde (16%) yielded (Entry
The freshly prepared PAA-modified Fe3O4 particles (0.05 g) were
dispersed alternately in Cu(CH3COO)2·H2O solution (20 mM, 4 mL) for
30 min and then in 1,3,5-benzenetricarboxylic acid (H3BTC) (20 mM, 4
cyclododecene and 1,2-dihydronaphthalene can also be
transformed to the their corresponding epoxides with high yield
and selectivity (Entry 6, 7). The high-efficient catalytic
performance reveals that the Fe3O4/Cu3(BTC)2 catalyst exhibits
wide applicability for the aerobic epoxidation of olefins and can
have potential applications in industrial areas.
o
mL) for 60 min at 70 C (the solvent is ethanol). Then the particles were
collected by magnetic separation and washed with ethanol. After ten
repeated
circulations,
core-shell
structured
Fe3O4/Cu3(BTC)2
microspheres were obtained.
Characterization
Conclusions
The morphology of the samples was characterized by transmission
electron microscopy (TEM, JEM-100CX II), high-resolution transmission
electron microscopy (HRTEM, Tecnai F20, 200 KV) and field emission
scanning electron microscopy (FESEM, ZEISS SUPRA-55). Powder X-
ray diffraction (XRD) patterns were examined on a M21X diffract meter
(Cu Kα, λ = 1.541 Ǻ) at 40 kV and 200 mA. Fourier transform infrared
spectra (FTIR) were detected using KBr pellet samples on a Nicolet 6700.
Nitrogen adsorption-desorption isotherms were tested using an Autosorb-
1-C analyzer (USA Quantachrome Instruments) at 77 K, and the pore-
size distributions were calculated using the Barrett-Joyner-Halenda (BJH)
method from the adsorption branch. The magnetic properties were
carried out using a vibrating sample magnetometer (Physical Property
Measurement System-9) at 298K. The catalytic properties were analyzed
by Gas Chromatography-Mass Spectrum (Agilent 7890/5975C-GC/MSD)
using nitrobenzene as the internal standard. The Cu content in the
Fe3O4/Cu3(BTC)2 microspheres and the concentration of Cu ions were
tested by inductively coupled plasma-atomic emission spectroscopy
(ICP-AES, Varian 715-ES).
In
summary,
we
synthesized
a
superparamagnetic
Fe3O4/Cu3(BTC)2 core-shell heterogenous catalyst by layer-by-
layer coating PAA-modified Fe3O4 microspheres. The
Fe3O4/Cu3(BTC)2 microspheres exhibit excellent catalytic
properties in the aerobic oxidation of alcohols and epoxidation of
olefins since that the active Cu(II) sites exist in the porous
Cu3(BTC)2 shell, and that the nanosized porous structutre
accelerates the transmission of organic substrates and
increases the contact between the active sites and the
substrates. The catalyst displays good reusability without loss of
catalytic activities after five consecutive usage, and the
functional Fe3O4 core facilitates the easily recovery from the
catalytic reaction system by using an external magnetic field.
Experimental Section
Materials.
Catalytic performance evaluation
Aerobic oxidation of alcohols.
FeCl3∙6H2O, sodium acetate, sodium carbonate, sodium bicarbonate,
ethanol, ethylene glycol, acetonitrile, copper acetate monohydrate
(Cu(CH3COO)2·H2O) and copper(II) nitrate trihydrate (Cu(NO3)2·3H2O)
were obtained from Beijing Chemical Reagent Company. Poly(acrylic
acid) (PAA; Mw = 1800) and 1,3,5-benzenetricarboxylic acid (H3BTC)
were purchased from Sigma-Aldrich. Nitrobenzene (99%), 2,2,6,6-
tetramethyl-1-piperidinyloxy (TEMPO), benzyl alcohol (99%), 4-
methylbenzyl alcohol (99%), 4-methoxybenzyl alcohol (98%), 4-
fluorobenzyl alcohol (97%), 4-cyanobenzyl alcohol (97%), 2-
pyridinemethanol (98%), 3-methyl-2-buten-1-ol (99%), cinnamyl alcohol
(98%), 1-phenylethanol (97%), cyclopentanol (99%), 2-cyclohexen-1-ol
(95%), isobutyraldehyde (99%), cis-cyclooctene (95%), cyclohexene
(99%), norbornene (99%), α-pinene (97%), styrene (99.5%),
cyclododecene (97%), 1,2-dihydronaphthalene (96%) were purchased
from Alfa Aesar. All chemicals were used without further purification.
The aerobic oxidation of alcohols were carried out in a 25 mL round-
bottom flask fitted with a reflux condenser. In a typical process, benzyl
alcohol (1.0 mmol), the magnetic core-shell Fe3O4/Cu3(BTC)2 catalyst (50
mg, 4.4 mol%), Na2CO3 as as a base additive (1 equiv, 1.0 mmol), and
TEMPO as the co-catalyst (0.5 equiv, 0.5 mmol) were added into 2.3 mL
of acetonitrile. The air was evacuated and an oxygen balloon was
supplied to the reaction system, then the mixture was stirred at 75 oC.
Nitrobenzene (0.2 mmol) was added as the internal standard, and the
catalytic particles were removed from the solution by an external
magnetic field. The liquid organic products were analyzed by Gas
Chromatography-Mass Spectrum analysis.
Aerobic epoxidation of olefins.
Synthesis of Fe3O4 microspheres.

European Journal of Inorganic Chemistry
10.1002/ejic.201600761
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mol%), and isobutyraldehyde as a sacrificial co-reductant (2 equiv, 2.0
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Acknowledgements
We are grateful to thank the Co-building Special Project of
Beijing Municipal Education, Chinese Postdoctoral Science
Foundation (2015M580980) and the Fundamental
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European Journal of Inorganic Chemistry
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Entry for the Table of Contents (Please choose one layout)
FULL PAPER
Jie Li, Hongyi Gao, Li Tan, Yi Luan
and Mu Yang*
Page No. – Page No.
Superparamagnetic
metal-organic
core-shell
framework
Fe3O4/Cu3(BTC)2 microspheres and
their catalytic activities for the
aerobic oxidation of alcohols and
olefins
Core-shell metal-organic framework Fe3O4/Cu3(BTC)2 microspheres have
been successfully synthesized by a one-step solvothermal method and a
subsequent layer-by-layer strategy. The superparamagnetic heterogeneous
catalyst exhibits efficient activity and high selectivity in the aerobic oxidation
of alcohols and epoxidation of olefins. The magnetic core facilitates the
recovery of the MOFs heterogeneous catalyst by applying an external
magnetic field.
* Aerobic oxidation; magnetic recovery