Fabrication of hierarchical composite microspheres of copper-doped Fe3O4@P4VP@ZIF-8 and their application in aerobic oxidation

Article abstract of DOI:10.1039/c6nj02239a

Novel hierarchical magnetic composite microspheres of copper-doped Fe₃O₄@poly(4-vinylpyridine-co-divinylbenzene)@ZIF-8 (Cu-FPZ, Cu stands for copper doped) were successfully fabricated. The core-shell Fe₃O₄(PAA)@P4VP magnetic microspheres were first synthesized by a polymerization approach, in which the 4-vinylpyridine (4-VP) monomer interacted with a-COOH group of poly(acrylic acid) modified Fe₃O₄ by means of a hydrogen-bond interaction. Then Zn²⁺ was adsorbed on the surface of the P4VP shell, followed by the formation of a ZIF-8 porous shell with 2-methylimidazolate. Finally, nearly 5.45 wt% of copper content was incorporated with the ZIF-8 framework to form a magnetic core-shell copper-doped Fe₃O₄@P4VP@ZIF-8. The obtained copper-doped Fe₃O₄@P4VP@ZIF-8 catalyst was applied in the selective oxidation of alcohols and epoxidation of olefins using molecular oxygen as the oxidant. The results demonstrated that the magnetic core-shell copper-doped Fe₃O₄@P4VP@ZIF-8 catalyst showed better catalytic activity with significantly improved turnover number and turnover frequency (up to 8.25 h^-1) than other copper MOFs. Furthermore, this heterogeneous catalyst could be cycled at least 15 times without significant loss of activity.

Full text of DOI:10.1039/c6nj02239a

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Fabrication of Hierarchical Composite Microspheres of Copper-doped
Fe3O4@P4VP@ZIF-8 and Their Aerobic Oxidation Application
Junying Hou, a Yi Luan, a Jie Yu, a Yue Qi, a Ge Wang*a and Yunfeng Lu *b
aaaReceived 00th January 20xx,
Accepted 00th January 20xx
A novel hierarchical magnetic composite microspheres copperꢀdoped Fe3O4@Poly(4ꢀvinylpyridineꢀcoꢀ
divinylbenzene)@ZIFꢀ8 (CuꢀFPZ, Cu stands for copper doped) was successfully fabricated. The coreꢀshell
Fe3O4(PAA)@P4VP magnetic microspheres were first synthesized by a polymerization approach, in which the
4ꢀvinylpyridine (4ꢀVP) monomer interacted with a ꢀCOOH group of poly(acrylic acid) modified Fe3O4 by
means of hydrogenꢀbond interaction. Then Zn2+ was adsorbed on the surface of the P4VP shell, followed by
the ZIFꢀ8 porous shell formation with 2ꢀmethylimidazolate. Finally, nearly 5.45 wt% copper content was
incorporated with the ZIFꢀ8 framework to form a magnetic coreꢀshell copperꢀdoped Fe3O4@P4VP@ZIFꢀ8.
The obtained copperꢀdoped Fe3O4@P4VP@ZIFꢀ8 catalyst was applied in the selective oxidation of alcohols
DOI: 10.1039/x0xx00000x
www.rsc.org/
and epoxidation of olefins using molecular oxygen as the oxidant. The results demonstrated that the magnetic
coreꢀshell copperꢀdoped Fe3O4@P4VP@ZIFꢀ8 catalyst had a better catalytic activity with significantly
improved turnꢀover number and turnꢀover frequency (up to 8.25 hꢀ1) than other copper MOFs. Furthermore,
the heterogeneous catalyst could be cycled at least 15 times without significant loss of activity.
the one hand, the magnetic core provides a facile catalyst recovery
approach after reaction; on the other hand, it can improve the
1. Introduction
stability and dispersity of catalyst materials.14ꢀ15 The magnetic coreꢀ
shell supports for metal salt immobilization can be broadly divided
into two types: inorganic and organic supports, such as
Fe3O4@SiO2,16 Fe3O4@C,17 Fe3O4@Polyaniline18 etc. Y. W. Li and
Z. Y. Tang prepared magnetic coreꢀshell structures as heterogeneous
catalyst.19ꢀ23 J. T. Ma synthesized Fe3O4@ppyꢀPd for benzylic
alcohol aerobic oxidation and the hydrogenation reduction of
nitroaromatic.24 B. Saha fabricated PSꢀAMP@Mo heterogeneous
catalyst for the epoxidation of alkenes.25 S. L. Jian synthesized
PANI@CoPc for the aerobic oxidation of alcohols in liquid phase.26
The coreꢀshell heterogeneous catalysts suffered from metal leaching
and reduction in catalytic activity after the catalyst was recycled due
to the instability of the metal immobilization. The development of a
stable and recyclable coreꢀshell heterogeneous catalyst is highly
desirable.
The selective aerobic oxidation of alcohols is one of the most
important organic reactions since only the aldehyde product is
obtained over several other side products.1 Alcohol oxidation can be
performed with a variety of oxidations such as chromium (VI)
reagents,2 iodosylbenzene3 and hydroperoxides,4 etc. Most of these
commercially available oxidants are highly toxic and hazardous.
Recently, molecular oxygen has attracted much more attention due
to the advantages of its being inexpensive, green and abundant.5ꢀ6
Furthermore, water, which is its only byꢀproduct, is environmentally
friendly in the catalysis of the selective oxidation of alcohols.7
However，due to the dynamic inertia of molecular oxygen in the
reaction, it is essential to develop catalysts for the efficient activation
of molecular oxygen.8 The Stahl group pioneered the development of
a Cu/bipyridine/TEMPO system for the efficient promotion of the
oxidation of alcohols using molecular oxygen as oxidant.9 From an
industrial point of view, it would be ideal to utilize a recyclable and
efficient heterogeneous Cu/TEMPO system.10ꢀ11
Metal organic frameworks (MOFs) are
a class of porous
polymeric materials building from metal centres and organic
linkers.27ꢀ28 In recent years, a variety of MOFs with desired
properties have been built by careful design of the secondary
building units or organic ligands.29 MOFs have been widely studied
for their application in gas storage,30 molecular sensing,31 dury
delivery,32 biomedicine33 and so on. Furthermore, they have received
a great deal of attention as heterogeneous catalysts for their high
surface areas,34 unique topology35 and tunable properties.36 Using the
covalently linked metal centres as the nodes of the MOFs, the
topology structure can effectively prevent the leaching of the active
sites.37ꢀ38 Therefore, the introduction of copper as the node of the
metalꢀorganic framework structure can solve the leaching problem of
the metal salt, and furnish a chemical stable heterogeneous catalyst
Recently, the design and synthesis of magnetic coreꢀshell structure
has been developed dramatically in catalysis research.12
Immobilizing the metal salt on magnetic composites is considered as
one of the effective ways of achieving heterogeneous catalyst.13 On
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of the magnetic core.
In this paper, a novel magnetic coreꢀshell CuꢀFPZ catalyst
was fabricated employing a stepꢀbyꢀstep assembly strategy. The
magnetic composite microspheres were composed by a uniform
Fe3O4(PAA) core, providing high magnetic response. In
addition, a P4VP middle layer assisted the adsorption of Zn2+,
and a MOFs shell with copper active sites contributed to the
catalytic reaction. The synthesized copperꢀdoped ZIFꢀ8 shell on
the surface of the magnetic coreꢀshell structure formed a stable
MOF structure taking advantage of the strong covalent bonds.
The Znꢀbased ZIFꢀ8 structure was employed as the framework
for the copper exchange in order to achieve copper modified
ZIFꢀ8 shell. The wellꢀdispersed copper active sites were linked
by covalent bonds in the copperꢀdoped ZIFꢀ8 structure, which
prevented the leaching of the copper ions. Furthermore, our
synthesized magnetic coreꢀshell CuꢀFPZ catalyst was evaluated
for catalytic efficiency by means of the selective oxidation of
alcohols and aerobic olefins epoxidation. The magnetic coreꢀ
shell CuꢀFPZ catalyst exhibited a high catalytic activity with
high turnꢀover number and turnꢀover frequency (up to 8.25 hꢀ1)
and the yield was maintained 99% after 15 cycles.
Scheme 1. Schematic illustration of the magnetic coreꢀshell CuꢀFPZ catalyst
preparation.
2.2.2. Preparation of ZIF-8, copper-doped ZIF-8 and the
magnetic core-shell Cu-FPZ catalyst. The pure ZIFꢀ8 was
produced under solvothermal conditions in the manner as previously
reported.41 Copper salts (0.5 mmol) were dissolved in 15 mL
absolute methanol, and then the prepared ZIFꢀ8 (1 mmol, 0.227g)
was added to the solution and heated to 90 oC for 1 day. Finally, the
product was separated, washed with absolute ethanol and dried under
vacuum at 80 oC for 12 h. The samples were named as CuNO3ꢀZIFꢀ
8, CH3COOCuꢀZIFꢀ8 and CuSO4ꢀZIFꢀ8 according to their copper
sources. The magnetic coreꢀshell CuꢀFPZ catalyst was achieved
using the same method by changing the ZIFꢀ8 to FPZ composite
microspheres.
2. Experimental
2.1. Materials
Ferric chloride hexahydrate [FeCl3ꢀ6H2O, 97%], 2ꢀmethylimidazole
[2ꢀMeIM, 97%], 4ꢀvinylpyridine [4ꢀVP, 96%], divinylꢀbenzene
[DVB, 80%], polyvinyl pyrrolidone [PVP, Mw=58000], potassium
2.3. Characterization.
Scanning electron microscope (SEM) images of samples were
persulfate
[KPS,
AR],
2,2,6,6ꢀtetramethylꢀ1ꢀpiperidinyloxy
obtained with
a ZEISS SUPRA 55. Transmission electron
[TEMPO, 98%] were purchased from Alfa Aesar. Poly acrylic acid
[PAA, Mw=1800] was purchased from SigmaꢀAldrich. Sodium
acetate [CH3COONaꢀH2O, 99%], ethylene glycol [EG, AR], zinc
nitrate hexahydrate [Zn(NO3)2ꢀ6H2O, 99%], copper nitrate trihydrate
microscopy (TEM) images of coreꢀshell catalysts were revealed by a
JEOL JSMꢀ6700. The high resolution transmission electron
microscopy (HRTEM) analysis was performed on a TEI Tecnai F20
transmission electron microscope operating at an accelerating
voltage of 200 kV. The phase composition of the products was
characterized by Xꢀray powder diffraction (XRD, Cu Kα radiation,
λ=0.1542 nm) via a M21X diffractometer. The BET surface area,
pore volume and pore diameter were measured by an AUTOSORBꢀ
1C analyzer. Magnetic measurements for the magnetic composites
were carried out with a MPMSꢀXL superconducting quantum
interference device (SQUID) at room temperature. Fourier transform
[Cu(NO3)2ꢀ3H2O,
[Cu(CH3COO)2ꢀH2O,
99%],
99%],
cupric
copper
acetate
sulfate
monohydrate
pentahydrate
[CuSO4ꢀ5H2O, 99%], were purchased from Beijing Chemical
Reagent Company. 4ꢀVP and DVB were distilled under reduced
pressure before use. All other chemicals above were used without
further purification.
2.2. Catalyst preparation
infrared spectra (FTꢀIR) were measured by
a Nicolet 6700
spectrometer. Thermogravimetric analysis (TGA) was recorded
using a STA 449F3 instrument under nitrogen flow. The metal ions
loading of the catalyst was analyzed using inductively coupled
plasmaꢀatomic emission spectrometry (ICPꢀAES) on a Vavian 715ꢀ
ES.
2.2.1.
Preparation
of
magnetic
core-shell
Fe3O4(PAA)@P4VP@ZIF-8 (FPZ) composite microspheres. The
Fe3O4(PAA) and the Fe3O4(PAA)@P4VP magnetic nanoparticles
were synthesized according to the procedure in the literature.39ꢀ40
The asꢀprepared Fe3O4(PAA)@P4VP (30 mg) microspheres were
dispersed into 5 mL (10 mM) Zn(NO3)2ꢀ6H2O methanol solution for
15 minutes and then 5 mL (10 mM) 2ꢀmethylimidazole (2ꢀMeIM)
methanol solution was added to the solution for 30 minutes at room
temperature. In between each step, the solution was separated by a
permanent magnet and washed with methanol three times. After 25
cycles, the samples were separated, washed with absolute ethanol
2.4. Catalytic performance evaluation.
2.4.1. Aerobic oxidation of alcohols. The catalytic properties were
examined under the following conditions: magnetic coreꢀshell Cuꢀ
FPZ catalyst (1 mol% copper), base (0.50 mmol), TEMPO (0.25
mmol), alcohols (0.50 mmol) and solvent (2.5 mL). The air was
evacuated and an oxygen balloon was supplied in the above reaction
system. The supernatant was analyzed via gas chromatographyꢀmass
o
and dried under vacuum at 80 C for 6 h. The sample was named as
Fe3O4(PAA)@P4VP@ZIFꢀ8 (FPZ).
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spectrometry (Agilent 7890/5975CꢀGC/MSD) using nitrobenzene as
the internal standard.
2.4.2. Typical catalytic procedure for aerobic epoxidation of
olefins. The catalytic properties of olefins were examined under the
following conditions: magnetic coreꢀshell CuꢀFPZ catalyst (1 mol%
copper), olefins (0.5 mmol), trimethylacetaldehyde (0.5 mmol) and
CH3CN (2.5 mL). The air was evacuated and an oxygen balloon was
set up for supplying oxygen to the system. The solution was also
analyzed via gas chromatographyꢀmass spectrometry (Agilent
7890/5975CꢀGC/MSD).
2.4.3. Catalyst recycling for aerobic oxidation of benzyl alcohol.
Under optimal reaction conditions, the selective aerobic oxidation of
benzyl alcohol was examined with recovered magnetic coreꢀshell
CuꢀFPZ catalyst. The catalyst was separated, washed with ethanol
and dried. Finally, it was reused for the next run.
Fig. 2 TEM images of (a) the magnetic coreꢀshell CuꢀFPZ catalyst and EDS
elemental maps of (b) Fe, (c) O, (d) C, (e) Zn and (f) Cu.
2.4.4. Leaching test for the aerobic oxidation of benzyl alcohol.
The magnetic coreꢀshell CuꢀFPZ catalyst was separated after 4 h
reaction, and the supernatant liquid was tested by GC/MS using
nitrobenzene as the internal standard. Then the mixture was stirred
for an additional 8 h. After the reaction, the product was analyzed by
GC/MS using nitrobenzene as the internal standard.
achieve the monodispersed coreꢀshell Fe3O4(PAA)@P4VP
microspheres.43 Following this, the pyridine moieties of P4VP
absorbed a large number of zinc(II) ions, and then initiated the
growth of ZIFꢀ8 shell on the surface of the coreꢀshell
Fe3O4(PAA)@P4VP. Next, a stable and sufficient thickness of ZIFꢀ8
shell was formed by stepꢀbyꢀstep strategy. Once coreꢀshell FPZ
composite microspheres were synthesized, partial Zn2+ ions were
substituted by Cu2+ ions on the framework of the ZIFꢀ8, which
furnished the magnetic coreꢀshell Cuꢀ FPZ catalyst (Scheme 1).
The morphology and structure of the Fe3O4(PAA), the
Fe3O4(PAA)@P4VP, the FPZ and the magnetic coreꢀshell CuꢀFPZ
catalyst were initially characterized by SEM and TEM. Fig. 1a
shows that the average diameter of wellꢀdispersed Fe3O4(PAA) is
approximately 230 nm. The polymerization of 4ꢀVP and DVB
occurs on the surface of the Fe3O4(PAA), and the wellꢀdispersed
Fe3O4(PAA)@P4VP multiple nanoꢀspheres are uniform. The
average diameter of layer is ca. 60 nm (Fig. 1b). The ZIFꢀ8 shell is
assembled onto the surface of the Fe3O4(PAA)@P4VP successfully,
the overꢀall diameter on average is calculated to be approximately
310 nm (Fig. 1c) and the layer is ca. 100 nm (Fig. S2). The
morphology is retained after the exchange of copper metal ions into
the ZIFꢀ8 by ionꢀexchanged process (Fig. 1d).
3. Results and discussion
3.1. Characterization of the magnetic core-shell Cu-FPZ catalyst
The magnetic coreꢀshell CuꢀFPZ catalyst was successfully formed
through a multiꢀstep synthesis process. Initially, the magnetic PAA
modified Fe3O4 nanoparticles were synthesized by a solvothermal
approach.42 Then 4ꢀvinylpyridine (4ꢀVP) monomers, divinylꢀ
benzene (DVB) crosslinker and polyvinyl pyrrolidone (PVP)
stabilizer were copolymerized onto the surface of the Fe3O4(PAA) to
The Energyꢀdispersive spectroscopy (EDS) elemental mapping
measurements demonstrate the distribution of Cu and other elements
in the magnetic coreꢀshell CuꢀFPZ catalyst (Fig. 2). It is suggested
that Fe and O are mainly in the core of the whole microspheres,
while C, Zn and Cu are distributed on the surface of the
microspheres. This result strongly indicates that copperꢀdoped ZIFꢀ8
shell is being formed. Furthermore, the contents of Cu, Zn of the
magnetic coreꢀshell CuꢀFPZ are determined to be 5.45 wt%, 13.6 wt%
respectively using ICPꢀAES analysis (Table S1). However, the
TEMꢀEDS of CuꢀFPZ shows that the weight content of the Cu, Zn
elements are 8.83% and 23.87%, respectively (Figure S3, Table S3).
The difference in the percentage of copper and zinc contained in the
CuꢀFPZ is attributable to the use of TEMꢀEDS method for surface
elemental analysis. However, ICPꢀAES method is a quantitative
measurement for most elements by decomposition of sediment.44
The crystal structure of the Fe3O4(PAA), the Fe3O4(PAA)@P4VP,
the FPZ and the magnetic coreꢀshell CuꢀFPZ catalyst were
investigated by Xꢀray diffraction (XRD) (Fig. 3). The diffraction
peaks of the Fe3O4(PAA) are in agreement with the peaks of the
Fig. 1 (a) SEM images of the Fe3O4(PAA), (b) TEM images of the
Fe3O4(PAA)@P4VP particles, (c) SEM images of the coreꢀshell FPZ
composite microspheres and (d) TEM images of the magnetic coreꢀshell Cuꢀ
FPZ composite microspheres.
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62.5 o, which are assigned to (220), (311), (400), (422), (511) and
(440) crystal planes, respectively (Fig. 3a). The characteristic
absorption bands of the Fe3O4(PAA)@P4VP are similar to the
Fe3O4(PAA), as expected (Fig. 3b). There are several new peaks at
(011), (002), (112), (022), (013), (222) (114), (233) and (134) crystal
planes (Fig. 3c) which are consistent with the pure ZIFꢀ8 (Fig. S4),
indicating the formation of the ZIFꢀ8 shell. In addition, the metal ion
exchange process does not affect the structure of the FPZ composite
microspheres and there is also no peak of copper oxide (Fig. 3d). It
is clear that all the copper is in the MOF structure.
The porosity of the FPZ composite microsphere is evaluated by N2
adsorptionꢀdesorption isotherms (Fig. 4). The isotherms are typical
type I isotherms according to the International Union of pure and
applied chemistry (IUPAC) classification, which reveals the
microporosity of the material.46 The surface area of the samples is
calculated using the BET method. The microporous volume was
calculated by tꢀplot method, and the total pore volume is obtained
from the volume of nitrogen adsorption at P/P0=0.98753. The results
show that the BET surface area of the FPZ is 743 m2/g, the total
microporous volume is 0.23 cm3/g, and the total pore volume is 0.58
cm3/g (Fig. 4 and Table S2). The surface of the pure ZIFꢀ8 is 1560
m2/g, with a total microporous volume of 0.47 cm3/g and a total pore
volume of 0.72 cm3/g (Fig. S1 and Table S2). The surface area of the
FPZ is slightly lower than the theoretical surface area of the pure
ZIFꢀ8 on the coreꢀshell structure (1038 m2/g). It is postulated that the
nonporous Fe3O4@P4VP core makes no contribution to the
microporous structures.41,47 And the results also indicate that the
formation of the ZIFꢀ8 framework is located on the surface of the
Fe3O4@P4VP.
Fig. 3 PXRD patterns of the Fe3O4(PAA) (a), the Fe3O4(PAA)@P4VP (b),
the FPZ (c) and the magnetic coreꢀshell CuꢀFPZ catalyst (d).
The saturation magnetic value of the Fe3O4(PAA), the
Fe3O4(PAA)@P4VP, the FPZ and the magnetic coreꢀshell CuꢀFPZ
catalyst are 67.7, 36.1, 25.5 and 23.3 emu/g (Fig. 5). The results
indicate that the effective magnetic contents are ca. 53.3%, 37.6%,
34.4% in the Fe3O4(PAA)@P4VP, the FPZ and the magnetic coreꢀ
shell CuꢀFPZ catalyst, respectively. The saturation magnetization of
the Fe3O4(PAA)@P4VP (Fig. 5b) is lower than that of the
Fe3O4(PAA) particles (Fig. 5a), which can mainly be attributed to the
encapsulation of the nonmagnetic P4VP shell.48 The saturation
Fig. 4 Nitrogen adsorption–desorption isotherms and pore size distribution of
the coreꢀshell FPZ composite microspheres.
Fig. 5 VSM curves of the Fe3O4(PAA) (a), the Fe3O4(PAA)@P4VP (b), the
FPZ (c) and the magnetic coreꢀshell CuꢀFPZ catalyst (d) at room
temperature (inset: photographs of the collection of the magnetic coreꢀshell
CuꢀFPZ catalyst by applying a magnetic field (after) to its suspension
(before).
Fig. 6 FTIR spectra of the Fe3O4 (PAA) (a), the Fe3O4(PAA)@P4VP (b), the
crystalline Fe3O4 (JCPDS file 19ꢀ0629).45 The typical characteristic
peaks of the Fe3O4 appear at 30.1o, 35.4 o, 43.1 o, 53.4 o, 56.9 o and
FPZ (c) and the magnetic coreꢀshell CuꢀFPZ catalyst (d).
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Table 1 Optimization of selective aerobic oxidation of benzyl
alcohol a
Catalyst, TEMPO, O2
CH3CN, base, 60 oC
OH
O
Entry
Catalyst
Solvent
NaHCO3
Conv. b
(%)
Sel. b
(%)
ꢀ
(mol%)
ꢀ
CuꢀZIFꢀ8
CuꢀZIFꢀ8
CuꢀFPZ c
CuꢀFPZ
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
<5
1
2
300
ꢀ
99
45
99
99
<5
<5
<5
65
73
16
99
99
99
99
99
99
ꢀ
3
300
100
100
100
100
100
100
100
4
CuꢀFPZ
5
Fig. 7 TGA curves of the Fe3O4 (PAA) (a), the Fe3O4(PAA)@P4VP (b), the
ꢀ
FPZ (c) and the magnetic coreꢀshell CuꢀFPZ catalyst (d).
6
Fe3O4@P4VP
FPZ
7
magnetization decreases slightly after the introduction of
nonmagnetic MOF shell (Fig. 5c and 5d). It is worth mentioning
that the magnetic coreꢀshell CuꢀFPZ catalyst exhibits strong
magnetic responsivity and can be separated within 60s (Fig. 5
inset).
8
CuꢀFe3O4
CuꢀFe3O4@P4VP
Cu3(BTC)2
99
99
99
9
10
11 d
12 e
13 f
14
15
16
17
18
The characteristic peaks of the Fe3O4(PAA), the
Fe3O4(PAA)@P4VP, the FPZ and the magnetic coreꢀshell CuꢀFPZ
catalyst were evaluated by Fourier transform infrared (FTIR)
spectroscopy (Fig. 6). The band at 592 cmꢀ1 is the characteristic
peak for FeꢀO vibrations.49 The bands at 1621, 1409 cmꢀ1
correspond to CꢀO stretching of the carboxylic acid groups,50 which
indicates that the ꢀCOOH functional groups of the PAA were
capped on the surface of the Fe3O4 (Fig. 6a). The peak at 2927 cmꢀ1
is attributable to the stretching vibration of ꢀCH2ꢀ in the vinyl and
the characteristic absorptions at 1598, 1414 cmꢀ1 are attributable to
the stretching of the pyridine,51 which proves that the P4VP is
encapsulated on the surface of the Fe3O4(PAA) (Fig. 6b). The
spectra of the FPZ composite microspheres show the additional
adsorption bands (Fig. 6c). The peak at 1580 cm−1 is attributable to
the C=N stretching vibration, whereas the intense bands at 1300ꢀ
1500 cm−1 are ascribed to the entire imidazole ring stretching.52 The
peaks at 1140 cm−1 and 998 cm−1 are attributable to the CꢀN
stretch.53 The spectra of the magnetic coreꢀshell CuꢀFPZ catalyst are
CuꢀBDC
CH3CN
CH3CN
100
100
18
12
99
99
CuꢀTDPAT
CuꢀZIFꢀ8
CuꢀFPZ
CH3CN
PhCH3
100
100
79
42
99
99
CuꢀFPZ
CuꢀFPZ
CuꢀFPZ
CHCl3
CH3OH
THF
100
100
100
88
87
15
99
99
99
a Reaction conditions: benzyl alcohol (0.5 mmol), 1 mol% catalyst based on
copper, solvent (2.5 mL), base, TEMPO (0.25 mmol), 60 oC for 12 h under 1
c
atm O2. Copper doped Fe3O4@P4VP@ZIFꢀ8. d 1.8 mg (17.6 wt% copper
based on ICPꢀAES) catalyst. e 2.1 mg (15.2 wt% copper based on ICPꢀAES)
catalyst. f 2.3 mg (14.1 wt% copper based on ICPꢀAES) catalyst.
b Determined by GCꢀMS using nitrobenzene as the internal standard.
almost identical to the spectra of the FPZ (Fig. 6d), suggesting that catalyst are ca. 17 wt%, 56 wt%, 69 wt%, 72 wt%, respectively. The
the framework of the ZIFꢀ8 is maintained despite the copper metal results are consistent with the effective magnetic content observed
ions exchange. This observation is also in agreement with the PXRD by the saturation magnetization.
result of the magnetic coreꢀshell CuꢀFPZ catalyst.
The
thermal
stability
of
the
Fe3O4(PAA),
the 3.2. Catalytic activity of the magnetic core-shell Cu-FPZ for the
Fe3O4(PAA)@P4VP, the FPZ and the magnetic coreꢀshell CuꢀFPZ selective aerobic oxidation of alcohols
catalyst were evaluated by Thermogravimetric analysis (TGA) (Fig.
Fig. S9 and S10 show the application of magnetic coreꢀshell CuꢀFPZ
catalyst to study the yield versus time and temperature in benzyl
alcohol oxidation. The alcohol conversion reaches 78% in 6 h and
goes up to 99% in 12 h, together with 99% selectivity of
benzaldehyde (Fig. S9). In addition, the yields of benzaldehyde go
up from 33% to 99% as the temperature increases, indicating that a
temperature of 60 oC is necessary to achieve satisfactory catalytic
activity (Fig. S10). Meanwhile, different copper salts were tested as
precursors for the copperꢀdoped ZIFꢀ8 (Table S4). The results
clearly indicate that the CuSO4 modified ZIFꢀ8 performs better than
other copper salts doped ZIFꢀ8 structures. As a result, CuSO4 was
7
and S7). Two steps of weight loss of Fe3O4(PAA) and
Fe3O4(PAA)@P4VP are observed (Fig. 7 and Fig. S7). The first step
o
before 450 C is attributable to the loss of adsorbed surface water
o
and guest species,54 the second step before 800 C is attributable to
the degradation of the PAA chains or the polymers. The sample FPZ
and the magnetic coreꢀshell CuꢀFPZ catalyst show three major
o
weight loss steps. The first two steps before 500 C are attributable
to the loss of guest species and polymers, and the decomposition of
the MOF structure takes place at about 650 oC. The TG curves
demonstrate that the weight loss of the Fe3O4(PAA), the
Fe3O4(PAA)@P4VP, the FPZ and the magnetic coreꢀshell CuꢀFPZ
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Journal Name
employed as the copper source in further studies (Table S4, entries Table 2 Selective alcohols oxidation using the magnetic coreꢀshell
1ꢀ3).
A series of reaction parameters, such as choice of base,
CuꢀFPZ catalyst a
heterogeneous copper catalyst and solvents, were investigated to
determine the optimal reaction condition. Firstly, we aimed to reveal
the role of the base in the aerobic oxidation of alcohol using our Cuꢀ
FPZ catalyst. No benzyl alcohol was consumed in the absence of the
base (Table 1, entry 1) while the CuSO4 doped ZIFꢀ8 (CuꢀZIFꢀ8)
gave 99% conversion with 3 eq. of NaHCO3 (Table 1, entry 2). This
indicated that the base was crucial for the high conversion of benzyl
alcohol, as in agreement with our catalytic oxidation system. We
postulated that the P4VP could serve as the organic base in the
oxidation reaction, leading to the higher conversion of benzyl
alcohol during the deprotonation step. To further confirm our
proposal, the magnetic coreꢀshell CuꢀFPZ catalyst was tested in the
absence of the base and a benzyl alcohol conversion of 45% was
achieved (Table 1, entry 3). Using the P4VP that intergraded in the
magnetic coreꢀshell CuꢀFPZ catalyst, only 100 mol% NaHCO3 was
required to achieve 99% conversion of benzyl alcohol (Table 1,
entries 4ꢀ5). Therefore, it can be concluded that the P4VP shell and
ZIFꢀ8 framework of the magnetic coreꢀshell CuꢀFPZ catalyst
structure possess a large amount of amino basic sites,55 which can
function as the organic base for the efficient oxidation reaction.
Furthermore, no conversion of benzyl alcohol was observed in the
absence of the catalyst (Table 1, entry 6). The Fe3O4@P4VP and the
FPZ catalyst showed no catalytic conversion of benzyl alcohol
(Table 1, entries 7ꢀ8), indicating that the copper content was the
major catalytic active component for the aerobic oxidation reaction.
The obtained alcohol conversion of CuꢀFe3O4 and CuꢀFe3O4@P4VP
were 65% and 73%, respectively (Table 1, entries 9ꢀ10), which were
lower than our CuꢀFPZ catalyst (Table 1, entry 5). Several
heterogeneous copper catalysts were also investigated in order to
compare their catalytic activities to our CuꢀFPZ catalyst. The
turnover frequencies (TOFs) of pure Cu3(BTC)2, CuꢀBDC, Cuꢀ
TDPAT and CuꢀZIFꢀ8 were calculated to be 0.29 hꢀ1, 1.50 hꢀ1, 1.00
hꢀ1 and 6.58 hꢀ1 for aerobic oxidation of benzyl alcohol (Table 1,
entries 11ꢀ14 and Table S4, entries 4ꢀ6 and entry 3). By comparison,
the TOF of our magnetic coreꢀshell CuꢀFPZ catalyst was calculated
to be 8.25 hꢀ1 (Table 1, entry 5 and Table S4, entry 7), which
represents a significant improvement on other CuꢀMOF catalysts.
It is postulated that copper salt synthesized MOFs have many
more catalytic effective active sites than necessary. Most catalytic
active sites are not efficiently utilized, especially those deep inside
of the porous MOF structure. On the contrary, our CuꢀFPZ catalyst
aims to use a limited amount of copper atom in place of some zinc
ion. These copper sites can be utilized efficiently since they are
a Reaction conditions: benzyl alcohol (0.5 mmol), 1 mol% catalyst based on
copper, solvent (2.5 mL), base (0.5 mmol), TEMPO (0.25 mmol), 60 C for
12 h under 1 atm O2.
b Determined by GCꢀMS using nitrobenzene as the internal standard.
o
molecular oxygen as oxidant, 100 mol% (0.5 mmol) NaHCO3 as
base and 2.5 mL CH3CN as solvent. In addition, we conducted an
extreme experiment for the selective oxidation of benzyl alcohol. We
performed a 20 mmol scale reaction for benzyl alcohol and it was
clear that the yield could be achieved reaching 5.2% (99% selectivity)
in 1 h, and the TOF was 208 hꢀ1 (Table S4, entry 8). The result
verified that the proposed protocol for benzaldehyde from the
oxidation of benzyl alcohol is scalable under the investigated
conditions.
Having established these optimal reaction conditions, a collection
evenly distributed on the MOF shell. Moreover, copper active sites of alcohol substrates was further evaluated. The catalytic results are
are on the surface of the coreꢀshell structure, which allows the summarized in Table 2. The conversion of benzyl alcohol was 99%,
substrate molecules to enter and exit the porous MOF structure together with 99% selectivity of benzaldehyde under optimal
conditions (Table 2, entry 1). Electron rich benzylic alcohols, such
as 4ꢀmethylbenzyl alcohol and 4ꢀmethoxybenzyl alcohol gave 99%
yield to their corresponding aldehydes (Table 2, entries 2ꢀ3). As
expected, the 4ꢀfluorobenzyl alcohol with an electronꢀwithdrawing
group at the para position exhibited a compromised catalytic activity
(Table 2, entry 4). Furthermore, aerobic oxidation of 3ꢀphenylꢀ2ꢀ
propeneꢀ1ꢀol resulted in 99% conversion and 99% selectivity
without disturbing the C=C double bond (Table 2, entry 5). The
freely. In this way, our CuꢀFPZ catalyst demonstrates much
improved TON and TOF. The solvent evaluation indicates that the
acetonitrile is the most suitable solvent for the aerobic oxidation of
benzyl alcohol (Table 1, entry 5 and entries 15ꢀ18). In summary, the
optimal yield for primary aromatic alcohols is obtained at a
temperature of 60 oC using 1 mol% magnetic coreꢀshell CuꢀFPZ
catalyst based on copper, 0.25 mmol TEMPO as coꢀcatalyst,
6 | J. Name., 2016, 00, 1-3
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ARTICLE
Table 3 Aerobic olefin epoxidations using the magnetic coreꢀshell temperature and less time can avoid the formation of other products.
CuꢀFPZ catalyst a
In addition, the norbornene gave the corresponding epoxide with
99% yield (Table 3, entry 3). However, the styrene was consumed,
only 54% of the desired styrene oxide being formed due to the
significant amount of benzaldehyde that was formed (Table 3, entry
4). This is attributable to the kinetic instability of the styrene oxides,
which can be further oxidized to benzaldehyde.60 For 1,2ꢀ
disubstituted aromatic olefins, transꢀstilbene provided a higher yield
than its corresponding cisꢀstilbene due to the steric effect (Table 3,
entries 5ꢀ6).61
O
CuꢀFPZ catalyst, (CH3)3CCHO
O2, CH3CN,60 oC
R1
R2
R3
R4
R1
R3
R2 R4
Conv.b Sel.b
Entry
Substrate
Product
(%)
(%)
1
99
99
O
3.4. Recycling of the magnetic core-shell Cu-FPZ Catalyst.
2
3
99
99
99
99
99
54
O
The recycling of the asꢀprepared magnetic coreꢀshell CuꢀFPZ
catalyst was investigated (Fig. 8). The catalyst exhibits excellent
reusability for the selective aerobic oxidation of benzyl alcohol. 99%
conversion of benzyl alcohol and 99% selectivity of benzaldehyde
are achieved after the catalyst is recycled 10 times. Furthermore, this
coreꢀshell catalyst can be recycled 15 times without significantly
decreasing the yield of benzaldehyde.
O
O
4
5
O
99
60
99
99
O
6
a Reaction conditions: olefin (0.5 mmol), 1.0 mol% catalyst based on copper,
CH3CN (2.5 mL), trimethylacetaldehyde (1.0 mmol), 60 oC for 12 h, 1 atm
O2.
b Determined by GCꢀMS using nitrobenzene as the internal standard.
aerobic oxidation of heteroaromatic alcohols was also successful. 2ꢀ
pyridinemethanol provided an excellent yield of 2ꢀpyridinealdehyde
(99% conversion and 99% selectivity) (Table 2, entry 6). However,
2ꢀthiophenemethanol furnished the corresponding thiopheneꢀ2ꢀ
carboxaldehyde with a 72% yield (Table 2, entry 7). 1ꢀOctanol gave
a much lower yield of the desired aldehyde in the oxidation due to
the poor electron density of the aliphatic chain (Table 2, entry 8).56
Low activity also can be observed in secondary alcohols like 1ꢀ
phenylethanol and cyclohexanol (Table 2, entries 9ꢀ10). This may be
explained by the steric effects.57ꢀ58
Fig. 8 Recycling of the magnetic coreꢀshell CuꢀFPZ catalyst in the selective
aerobic oxidation of benzyl alcohol.
3.3. Aerobic Epoxidation of Olefins.
The yield versus time in the oxidation of cyclooctene was studied
using our magnetic coreꢀshell CuꢀFPZ catalyst under optimal
reaction conditions (Fig. S11). The cyclooctene conversion reached
73% in 6 h and it went up to 99% in 12 h, together with 99%
selectivity of cyclooctene oxide. Then the methodology was
extended to aerobic olefin epoxidation, which is one of the most
important organic reactions used in industry.59 The cyclic olefins,
such as cyclohexene and cyclooctene, showed great activity with
99% conversion and 99% selectivity under our optimal epoxidation
reaction condition (Table 3, entries 1ꢀ2). The high selectivity and
yield of epoxide were achieved under our mild conditions. Low
Fig. 9 Leaching test for the selective aerobic oxidation of benzyl alcohol over
the magnetic coreꢀshell CuꢀFPZ catalyst.
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A hot filtration test was carried out in order to confirm the
Journal Name
6
7
8
9
C. Parmeggiani and F. Cardona, Green Chem., 2012, 14, 4282ꢀ4285.
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coreꢀshell CuꢀFPZ catalyst (Fig. 9). The catalyst was separated from
the solution at 4 h, and the conversion of benzyl alcohol reached
62%. The supernatant of the acetonitrile suspension was then stirred
for another 8 h at 60 oC. No further conversion of benzyl alcohol was
detected after the heterogeneous catalyst was separated by magnet.
Furthermore, the inductively coupled plasma atomic absorption
spectroscopy (ICPꢀAAS) analysis of the filtrate after the removal of
the catalyst after the reaction showed that the copper content was 5.3
ppm, indicating that the leaching of copper into the solvent was
negligible during the reaction. These studies prove the high chemical
stability of the magnetic coreꢀshell CuꢀFPZ catalyst’s topological
structure.
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TGA. The TEM image for the catalyst showed that the morphology
was retained (Fig. S2). The PXRD and FTIR spectrum of the catalyst
after 15 cycles were similar to the fresh catalyst (Fig. S5 and S6),
suggesting that the crystalline structure and composition of the
magnetic coreꢀshell CuꢀFPZ catalyst were well maintained. The
TGA results showed that the fresh and recycled CuꢀFPZ catalysts
shared an almost identical thermal stability (Fig. S8).
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In summary, this research has developed an easy route to synthesize
a novel magnetic coreꢀshell copperꢀdoped Fe3O4@P4VP@ZIFꢀ8
catalyst. The magnetic composite microspheres were composed of a
magnetic Fe3O4(PAA)@P4VP core, a P4VP middle layer and a
copperꢀdoped MOF shell. The microspheres were applied in the
selective aerobic oxidation of alcohols and the aerobic epoxidation
of olefins. The results demonstrated that the magnetic coreꢀshell
copperꢀdoped Fe3O4@P4VP@ZIFꢀ8 catalyst had a significantly
improved turnꢀover number and turnꢀover frequency (up to 8.25 hꢀ1)
in comparison to many systems previously reported. A variety of
primary alcohols, secondary alcohols and olefins substrates were
tolerated in our optimal aerobic oxidation reaction condition.
Furthermore, large amounts of the basic sites on the shell P4VP and
ZIFꢀ8 significantly enhanced the activity of the reaction. The
magnetic coreꢀshell copperꢀdoped Fe3O4@P4VP@ZIFꢀ8 catalyst
also exhibited excellent recovery properties and maintained 96% of
its yield of benzaldehyde after recycling 15 times.
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Acknowledgements
We thank the Coꢀbuilding Special Project of Beijing Municipal
Education, 111 Project (No. B13004) and Fundamental
Research Funds for the Central Universities FRFꢀTPꢀ15ꢀ001A2
for financial support.
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DOI: 10.1039/C6NJ02239A
Fabrication
of
Hierarchical
of
Composite
Copper-doped
Microspheres
Fe3O4@P4VP@ZIF-8 and Their Aerobic Oxidation
Application
Junying Hou, a Yi Luan, a Jie Yu, a Yue Qi, a Ge Wang *a and Yunfeng Lu *b
Beijing Key Laboratory of Function Materials for Molecule & Structure Construction,
School of Materials Science and Engineering, University of Science and Technology
Beijing, Beijing 100083, People’s Republic of China
*E-mail: gewang@mater.ustb.edu.cn
The copper-doped Fe3O4@P4VP@ZIF-8 composite microspheres were fabricated for
highly efficient aerobic alcohol oxidation and olefin epoxidation.