In Situ-Induced Synthesis of Magnetic Cu-CuFe2O4@HKUST-1 Heterostructures with Enhanced Catalytic Performance for Selective Aerobic Benzylic C-H Oxidation

Article abstract of DOI:10.1021/acscatal.6b02614

The multifunctional design of heterogeneous catalysts which can realize the selective oxidation of C-H bonds with oxygen at low temperature is of crucial importance in the catalysis community. Here, we report the designed synthesis of Cu-CuFe₂O₄@HKUST-1 heterostructures that is highly active for the selective oxidation of benzylic C-H bonds under mild conditions (60 °C). In the synthetic process, the Cu(0) component of Cu-CuFe₂O₄ nanoparticles gradually released Cu²⁺ ions which were in situ transformed into the HKUST-1 shell. The high activity can be attributed to a rationally multifunctional design of Cu-CuFe₂O₄@HKUST-1 heterostructures, in which Cu-CuFe₂O₄ can provide multiple active sites while HKUST-1 attracts and preconcentrates molecular oxygen. Notably, the catalytic performance under optimized reaction conditions (60 °C) was achieved in the oxidation of fluorene to fluorenone (conversion: > 99%, selectivity: > 99%). Furthermore, the Cu-CuFe₂O₄@HKUST-1 catalyst could be easily recovered by magnetic separation and reused for 10 times without significant loss of catalytic activity.

Full text of DOI:10.1021/acscatal.6b02614

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Article
In-situ Induced Synthesis of Magnetic Cu-CuFe2O4@HKUST-1
Heterostructures with Enhanced Catalytic Performance
for Selective Aerobic Benzylic C-H Oxidation
Shuang Fan, Wenjun Dong, Xiubing Huang, Hongyi Gao, Jingjing Wang, Zhaokui Jin, Jia Tang, and Ge Wang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b02614 • Publication Date (Web): 23 Nov 2016
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InꢀSitu Induced Synthesis of Magnetic Cuꢀ
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CuFe2O4@HKUSTꢀ1
Heterostructures
with
Enhanced Catalytic Performance for Selective
Aerobic Benzylic CꢀH Oxidation
Shuang Fana, Wenjun Donga, Xiubing Huanga, Hongyi Gaoa*, Jingjing Wanga, Zhaokui Jinb,
Jia Tanga and Ge Wanga*
a
State Key Laboratory for Advanced Metals and Materials, University of Science and
Technology Beijing, Beijing 100083, PR China, Eꢀmail: gewang33@126.com;
hygao2009@163.com
b School of Biomedical Engineering, Health Science Center, Shenzhen University
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ABSTRACT: The multifunctional design of heterogeneous catalysts which can realize the
selective oxidation of CꢀH bonds with oxygen at low temperature is of crucial importance in the
catalysis community. Here, we report the designed synthesis of CuꢀCuFe2O4@HKUSTꢀ1
heterostructures that is highly active for the selective oxidation of benzylic CꢀH bonds under
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mild conditions (60 C). In the synthetic process, the Cu(0) component of CuꢀCuFe2O4
nanoparticles gradually released Cu2+ ions which were inꢀsitu transformed into the HKUSTꢀ1
shell. The high activity can be attributed to a rationally multifunctional design of Cuꢀ
CuFe2O4@HKUSTꢀ1 heterostructures, in which CuꢀCuFe2O4 can provide multiple active sites
while HKUSTꢀ1 attract and preconcentrate molecular oxygen. Notably, the catalytic
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performance under optimized reaction conditions (60 C) was achieved in the oxidation of
fluorene to fluorenone (conversion: >99 %, selectivity: >99 %). Further, the Cuꢀ
CuFe2O4@HKUSTꢀ1 catalyst could be easily recovered by magnetic separation and reused for
ten times without significant loss of catalytic activity.
KEYWORDS: Benzylic CꢀH oxidation, Mild reaction conditions, CuꢀCuFe2O4@HKUSTꢀ1, Inꢀ
situ transformation, Multifunctional design.
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1. INTRODUCTION
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Selective oxidation of saturated CꢀH bonds is one of the most fundamental processes in the
fine chemical synthesis and industrial chemistry due to the wide applications of oxidized
products in the agricultural, domestic, and industrial sectors of society.1,2 Nowadays, the highly
selective oxidation of sp3 CꢀH bonds at low temperatures represents a critical challenge because
of the high activation energy of sp3 CꢀH bonds and the more active of oxidized products than the
initial CꢀH bonds themselves.3ꢀ8 One important class of initial reactants is benzylic CꢀH bonds,
which can be oxidized to corresponding alcohols, carbonyl compounds or aldehydes, which are
commercially versatile intermediates in the manufacture of various products, such as dyes,
solvents, dyestuffs, preservatives, and flame retardants.9ꢀ11 In the industry, expensive inorganic
oxidants are usually used for the selective aerobic oxidation of benzylic CꢀH bonds, such as
manganese reagents, chromium reagents or sodium chlorite, however, a large amount of toxic
waste is produced which is not environmentallyꢀfriendly.12,13 Therefore, tremendous efforts have
been devoted to achieve the highly selective aerobic oxidation of benzylic CꢀH bonds at low
temperatures using cheap and clean oxidants.
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In the past decades, peroxideꢀoxidants such as tertꢀbutylhydroperoxide (TBHP) or H2O2 were
reported to realize the highly selective oxidation of benzyl CꢀH bonds, but these oxidants still
have some problems, such as corrosive to equipment, toxic, and environmentally unfriendly.14ꢀ17
In addition, the formation of tertꢀbutanol from the consumption of TBHP will be left in the
reaction solution and further separation is needed. Therefore, using cheap and clean molecular
oxygen (e.g., pure O2, air) as oxidant in the selective oxidation of benzyl CꢀH bonds is highly
desired. Even though some homogenous catalysts have been reported to achieve high conversion
and selectivity for benzyl CꢀH oxidation using O2 as oxidant at low temperature, the difficulty in
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catalyst recycling limits their practical applications.18 Some solid heterogeneous catalysts, such
as mesoporous MnCeOx, NHPI/Fe(BTC), CuFe/rꢀAl2O3, AuꢀPd/C and MILꢀ101(Cr) have been
reported to be catalytically active for aerobic oxidation of saturated CꢀH bonds using oxygen
molecular as oxidant.19ꢀ23 For example, Lu et al. reported the selective oxidation of cyclohexane
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process operated at 100 C and under 10 atm of O2 in the presence of MnCeOx catalyst (12 h,
conversion: 17.0%, selectivity: 92%).19 Hutchings et al. reported the oxidation of toluene process
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operated at 160 C and under 10 atm of O2 in the presence of AuꢀPd/C catalyst (7h, conversion:
4.4 %, selectivity: 76.2%).22 Garcia et al. used MILꢀ101(Cr) as reusable solid catalyst for
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autoxidation of benzyl CꢀH bonds using O2 as oxidant at 120 C, and the catalyst exhibited 87%
selectivity at 30% conversion for indane oxidation.23 However, these catalytic systems were
usually accompanied by the need of high temperature, elevated O2 pressure, special strategies to
increase dissolved O2 concentration or long reaction time, these excessive energy input often
results in uncontrolled conversion rate and/or selectivity. In the view of chemical kinetics, the
development of a solid catalyst that operates at lower temperature, may avoid deep radical
oxidation to a large degree, expectedly obtaining a higher saturated CꢀH bonds selectivity.19
Nowadays, realizing the selective oxidation of benzylic CꢀH bonds at low temperature under 1
atm pressure of O2 or air using recycled heterogeneous catalysts with high conversion and high
selectivity represents a critical challenge.
Rational design of desirable catalysts has become a fascinating and significant object in
nanotechnology and catalytic science. For selective oxidation of benzylic CꢀH bonds by oxygen
under low temperature, the integration of the active species and high oxygen storage capacity is a
key factor.24 Recently, CuFe2O4 magnetic nanoparticles (MNPs) has emerged as one of the most
attractive catalysts, which can exhibited good performance in several different types of catalytic
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oxidation, such as alcohols oxidation and CO oxidation due to its cation that occupy the
tetrahedral and octahedral holes in a dense cubic packing of oxygen anions.25ꢀ26 Thus, CuFe2O4
MNPs may act as an active catalyst for selective aerobic oxidation of benzylic CꢀH bonds. In
addition, a more efficient catalyst could be obtained if a high local concentration of O2 was to be
created in the vicinity of active species. Previously, Tuci et al. reported that the MOFs materials
are capable of concentrating molecular oxygen in alkene oxidation by interaction of open metal
ions and oxygen.27 Thus, we chose metalꢀorganic frameworks (MOFs) as another desirable
structural feature in the design of solid catalyst, because MOFs with their high surface area,
porosity and metalꢀbinding sites are capable of absorbing significant amount of molecular
oxygen.28ꢀ29 Accordingly, we introduce a novel catalyst design protocol to integrate active
CuFe2O4 and MOFs to afford high performance for selective oxidation of benzylic CꢀH bonds by
molecular oxygen under mild reaction conditions.
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In this work, we reported a facile inꢀsitu synthesis method to obtain designable magnetic Cuꢀ
CuFe2O4@HKUSTꢀ1 coreꢀshell structure at room temperature. We introduced ingeniously
magnetic CuꢀCuFe2O4 MNPs into the HKUSTꢀ1 systems, then the Cu(0) component of the Cuꢀ
CuFe2O4 MNPs can serve as the copper source which could be inꢀsitu transformed into the
MOFs shell. In the catalytic process, the CuꢀCuFe2O4 MNPs acts directly as the active species,
meanwhile the MOF shell absorbs and preconcentrates molecular oxygen to increase the
availability of CuꢀCuFe2O4 core and molecular oxygen. Such CuꢀCuFe2O4@HKUSTꢀ1
heterogeneous structure with integrated advantages of CuꢀCuFe2O4 core and MOFs shell indeed
show outstanding activity at the low temperature, affording high conversion of fluorene and good
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selectivity of fluorenone (60 C, conversion: >99%, selectivity >99%) with molecular oxygen as
the oxidant. This process could be extended to the selective oxidation of various benzyl CꢀH
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bonds with the corresponding ketones as products. In addition, CuꢀCuFe2O4@HKUSTꢀ1 solid
catalyst can be easily recovered by magnetic separation and reused without significant loss of
catalytic efficiency after being used for ten times. This study could provide a simple and general
strategy to obtain hierarchical catalysts with high oxygen storage capacity and active species,
which could expand the design concept for selective, O2ꢀbased oxidation of saturated CꢀH bonds
at mild temperatures.
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2. EXPERIMENTAL SECTION
Materials
Copper (II) chloride dihydrate (CuCl2·2H2O), ferric chloride hexahydrate (FeCl3·6H2O), sodium
acetate (NaAC), trisodium citrate dihydrate (Na3Cit), ethylene glycol (EG), absolute ethanol, nꢀ
butyl alcohol, and N,Nꢀdimethylformamide (DMF) were purchased from the Beijing Chemical
Reagent Company. 1,3,5ꢀBenzenetricarboxylic acid (H3BTC), pure CuFe2O4 nanoparticles and
all substrates were supplied by Alfa Aesar. All reagents used in this work were used without
further purification.
Preparation of Cu-CuFe2O4@HKUST-1 nanoparticles
The CuꢀCuFe2O4 nanoparticles were prepared as reported previously by a solvothermal reaction
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at 200 C.30 The CuꢀCuFe2O4 MNPs (0.10 g) were added into 19 mL of nꢀbutyl alcohol under
stirring. Then another 6 mL of ethanol solution of H3BTC (0.05 mmol/L) was added into the
above solution. After the mixture was stirred at room temperature for 12 h, the solid was
separated by centrifugation and washed by ethanol for several times. After dried under vacuum
at 40 oC for 12 h, the final product was referred to CuꢀCuFe2O4@HKUSTꢀ1.
For catalysis comparison, pure HKUSTꢀ1 was synthesized according to a previously reported
procedures31 and its phase purity was confirmed by powder XRD (Figure S1).
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General procedure for Cu-CuFe2O4@HKUST-1 catalyzed oxidation of fluorene
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The oxidation reaction was carried out in a 25 mL twoꢀnecked roundꢀbottom flask fitted a water
condenser. Typically, a certain amount of catalyst, fluorene (1.0 mmol), and Nꢀ
Hydroxyphthalimide (NHPI) (0.1 mmol) were dissolved in 5 mL of acetonitrile. After
vacuumizing three times, then set up with an O2 balloon, this reaction mixture was heated at 60
oC for 9 h. At the end of the reaction, nitrobenzene (0.2 mmol) was added as an internal standard
and the resulting liquid samples were analyzed by Agilent 7890/5975CꢀGC/MSD to determine
the catalytic efficiency.
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Reuse experiments
On completion of each reaction cycle, the catalyst was easily separated by magnet from solution
mixture, then washed with acetonitrile (ca. 5.0ꢀ10.0 mL) and ethanol (ca. 5.0ꢀ10.0 mL) several
times, and dried in an oven at 40 ◦C under vacuum prior to being applied for the reuse.
Characterization
The morphologies, structure and chemical compositions of the asꢀprepared products were
investigated using a scanning electron microscope (SEM, ZEISS SUPRA55) (FEꢀSEM) at 200
kV and a transmission electron microscope (TEM, JEOL JEMꢀ100CX II) operating at 200 kV,
respectively. Nitrogen adsorptionꢀdesorption isotherms were measured at 77 K with a
Micromeritics ASAP 2420 adsorption analyzer. The specific surface areas of the catalysts were
calculated by the BET method and the poreꢀsize distributions of the catalysts were observed from
the adsorption branches of isotherms by using the BarrettꢀoynerꢀHalenda (BJH) model. The
magnetization curves of the asꢀprepared products were collected at room temperature via a
varying magnetic field range from ꢀ10 to 10 kOe using a BHVꢀ55 vibrating sample
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magnetometer (VSM). The structure and phase of the samples were investigated by Xꢀray
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powder diffraction (XRD, M21X) with Cu Ka radiation (40 kV, 150 mA, λ=1.5406 Å ). Copper
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contents and iron contents of the samples were obtained using inductively coupled plasmaꢀ
atomic emission spectrometry (ICPꢀAES). The Fourier transform infrared spectra (FTIR) was
obtained on a Nicolet 6700 Fourier spectrometer using potassium bromide (KBr) pellets
technique. The catalytic efficiency was analyzed using a Gas ChromaꢀtographyꢀMass Spectrum
(GCꢀMS, Agilent7890/597 5CꢀGC/M SD), and nitrobenzene was applied as an internal standard.
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3. RESULTS AND DISCUSSION
3.1 Morphology and composition of the Cu-CuFe2O4@HKUST-1 nanoparticles The in situ
conversion process of the CuꢀCuFe2O4@HKUSTꢀ1 can be described in Scheme 1. Initially, the
CuꢀCuFe2O4 MNPs were prepared using a simple solvothermal route.30 When H3BTC solution
was added into the dispersed solution of the CuꢀCuFe2O4 MNPs, the Cu(0) component in the Cuꢀ
CuFe2O4 MNPs produced Cu2+ source to induce the MOF nucleation on the CuꢀCuFe2O4 MNPs
surface. With the consumption of the Cu(0) component and the crystallization of HKUSTꢀ1, the
CuꢀCuFe2O4 core was gradually covered by HKUSTꢀ1 octahedral crystal shell. Then the Cuꢀ
CuFe2O4@HKUSTꢀ1 structure was constructed. During the formation process of the Cuꢀ
CuFe2O4@HKUSTꢀ1, the Cu(0) component of the CuꢀCuFe2O4 MNPs served as Cu2+ source,
which was in situ converted to HKUSTꢀ1 shell, while the relatively stable core retained its own
nature.
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Scheme 1. Schematic illustration of the formation of CuꢀCuFe2O4@HKUSTꢀ1 nanoparticles.
The morphology and chemical composition of the reaction products were characterized by
TEM and STEM. As shown in Figure 1a, the CuꢀCuFe2O4 MNPs were nearly spherical in shape
with a narrow size distribution (~200 nm). Then, the CuꢀCuFe2O4 MNPs were added into the
mixed solution of nꢀbutyl alcohol and ethanol containing H3BTC ligand at room temperature.
After 12 h, the CuꢀCuFe2O4 MNPs were covered with the HKUSTꢀ1 shell. Such a heterostructure
can be obviously observed by the TEM characterization, in which the CuꢀCuFe2O4 nanoparticles
as the core appears as brighter contrast due to the larger atomic mass (Figure 1b). As shown, the
size of CuꢀCuFe2O4@HKUSTꢀ1 nanoparticles with thick shells was larger than that of pristine
CuꢀCuFe2O4 MNPs. The element mapping images (Figures 1cꢀe) further confirmed the
heterostructure of the CuꢀCuFe2O4@HKUSTꢀ1 nanoparticles, in which the Fe element only
locates densely in the core region, while Cu element distributes throughout the region of this
nanoparticle, with relatively higher concentration in the shell region, indicating that the shell was
formed by the generation of the HKUSTꢀ1, which was further proved by the XRD results.
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Figure 1. TEM images of (a) CuꢀCuFe2O4 nanoparticle, (b) CuꢀCuFe2O4@HKUSTꢀ1
nanoparticle, HAADFꢀSTEM image of (c) CuꢀCuFe2O4@HKUSTꢀ1 nanoparticle and EDS
mapping images of Fe (d) and Cu (e) in the CuꢀCuFe2O4@HKUSTꢀ1 nanoparticle.
The composition and phase structure of CuꢀCuFe2O4 and CuꢀCuFe2O4@HKUSTꢀ1
nanoparticles were further characterized by XRD measurements (Figure 2). The standard
diffraction peaks of CuFe2O4 (JCPDS file 77ꢀ0010) and copper (JCPDS file 85ꢀ1326) (Figure S3)
were observed in the XRD results of the CuꢀCuFe2O4 MNPs. The presence of Cu(0) component
was generated during the our solvothermal synthesis procedure, which has been proved in the
previous research.30, 32ꢀ34 After a new shell appeared on the CuꢀCuFe2O4 MNPs, new diffraction
peaks at 6.7°, 9.5°, 11.6°, 13.4°, 17.5° and 19.0° appeared, which matched well with the
simulated XRD patterns of HKUSTꢀ1 (Figure S4).35 It should be noted that, after the appearance
of the diffraction peaks of HKUSTꢀ1, the intensity of the Cu(0) decreased a lot, indicating that
the formation of the HKUSTꢀ1 shell was caused by the consumption of the Cu(0) component.
The intensity of the diffraction peaks of CuFe2O4 can be still kept well due to the stabilized
CuFe2O4.
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Figure 2. XRD patterns of the asꢀprepared CuꢀCuFe2O4 nanoparticle and the experimentally
acquired CuꢀCuFe2O4@HKUSTꢀ1 nanoparticle.
To gain more insight into the formation pathway of CuꢀCuFe2O4@HKUSTꢀ1
heterostructures, the morphologies and structures of the CuꢀCuFe2O4@HKUSTꢀ1 formed at
different reaction time were investigated in detail (Figure 3). As shown in Figure 3aꢀc, after the
reaction was conducted for 4 h, the surface of CuꢀCuFe2O4 became rough and was gradually
deposited with some HKUSTꢀ1 nanocrystals. With reaction time was extended to 8 h, those
HKUSTꢀ1 nanocrystals grew up and a continuous layer was formed on the surface of the Cuꢀ
CuFe2O4 nanoparticles. As the reaction proceeds, the HKUSTꢀ1 shell continued to grow onto the
outer surface of the nanoparticles. After reaction reached 12 h, the growth of HKUSTꢀ1 shells
was stopped for the reaction reaching its equilibrium.36 The growth of the HKUSTꢀ1 shells stops
since the Cu2+ ions and H3BTC ligands reached the diffusion equilibrium, the H3BTC ligands are
able to pass the pores of the HKUSTꢀ1 and reach the surface of CuꢀCuFe2O4 core where they
coordinate with the dissolved Cu2+ ions to form HKUSTꢀ1. Meanwhile, Cu2+ ions diffuse
outward and react with the H3BTC ligands at the outer surface of HKUSTꢀ1 layer. In addition,
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no freeꢀstanding HKUSTꢀ1 nanoparticles were found at any reaction time, suggesting that the
HKUSTꢀ1 growth was strictly limited to the surface of CuꢀCuFe2O4. The transformation of the
phase constitution of the product at corresponding reaction stage was also characterized by XRD.
As shown in Figure 3d, for the sample obtained at 4 h, besides those diffraction peaks attributed
to CuFe2O4 and the obvious diffraction peak for Cu(0), some very weak diffraction peaks of
HKUSTꢀ1 appeared. With the increase of the reaction time, the intensity of the diffraction peaks
belonging to HKUSTꢀ1 gradually increases, agreeing with the previous description.
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Figure 3. TEM images of CuꢀCuFe2O4@HKUSTꢀ1 heterostructures obtained after reaction time
of (a) 4 h, (b) 8 h, (c) 12 h; (d) XRD patterns corresponding to the samples obtained after
different reaction time.
To further investigate the conversion process from CuꢀCuFe2O4 MNPs to HKUSTꢀ1 and
confirm that Cu(0) components can serve as the source of metal ion, a series of experiments were
conducted. Initially, pure CuFe2O4 nanoparticles purchased from Alfa Aesar, nꢀbutyl alcohol and
H3BTC solution were in sequence added to 50 mL Glass bottles and stirred for 12 h. After 12 h
reaction time, the morphology of the product remained unchanged and there were no
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nanocrystals formed on its surface (Figure 4aꢀb). There were no XRD patterns of HKUSTꢀ1 and
only pure CuFe2O4 was observed (Figure 4c). These results indicated that the pure CuFe2O4
could not provide Cu ions to react with H3BTC ligands under the same synthetic conditions. In
our previous typical synthetic process, the Cu(0) component of our ingenious CuꢀCuFe2O4
MNPs played the critical role of providing Cu2+ ions for the information HKUSTꢀ1.
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Figure 4. TEM image of (a) pure CuFe2O4 and (b) the corresponding products; (c) XRD patterns
of pure CuFe2O4 and the corresponding product.
3.2 The catalytic properties of the Cu-CuFe2O4@HKUST-1 nanoparticle The activated Cuꢀ
CuFe2O4@HKUSTꢀ1 has been preliminarily scrutinized as a heterogeneous catalyst for the
oxidation of selected oxidation of fluorene to fluorenone using molecular oxygen as oxidant. The
reaction was carried out under mild condition (60 oC) by using 15 mg of CuꢀCuFe2O4@HKUSTꢀ
1 (5 mol% Cu, determined by ICP) along with 1 atm of molecular oxygen as the oxidant and 5
o
mL acetonitrile as the solvent. After the reaction was conducted at 60 C for 9 h, fluorene was
converted to fluorenone with a good yield (>99%) and high selectivity (>99%) (Table 1, entry 1).
To identify active component of the CuꢀCuFe2O4@HKUSTꢀ1 nanoparticle, a series of control
experiments were conducted. The CuꢀCuFe2O4 MNPs, the homogeneous CuCl2·2H2O and
FeCl3·6H2O catalyst, pure HKUSTꢀ1 and a simple mixture of CuꢀCuFe2O4 and HKUSTꢀ1 were
used as catalyst to test their activity for the oxidation of fluorene. The results are listed in Table
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1. Using the homogeneous copper chloride or ferric chloride as catalyst, trace fluorene was
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converted to fluorenone (Table 1, entries 2ꢀ3). When CuꢀCuFe2O4 MNPs and pure HKUSTꢀ1
were employed as catalyst, the conversions of fluorenone were 76% and 39%, respectively
(Table 1, entries 4ꢀ5). The CuꢀCuFe2O4 MNPs showed higher conversion of fluorene (76%) and
good selectivity for fluorenone (>99%) than that of the pure HKUSTꢀ1, indicating the Cuꢀ
CuFe2O4 MNPs played the main role to catalyze the oxidation reaction. However, when a simple
mixture of CuꢀCuFe2O4 MNPs and HKUSTꢀ1 was used as catalyst, the catalytic activity was not
improved compared with CuꢀCuFe2O4 MNPs and pure HKUSTꢀ1 (Table 1, entry 6). Based on
the reaction results of the control experiments, the high performance of the Cuꢀ
CuFe2O4@HKUSTꢀ1 nanoparticle composed of CuꢀCuFe2O4 core and HKUSTꢀ1 shell may be
attributed to the designed structure. As shown in Figure 5, we have aimed to design Cuꢀ
CuFe2O4@HKUSTꢀ1 architecture which can integrate the advantages of the active CuFe2O4 and
MOFs to catalyze the benzylic CꢀH oxidation. As one part of structural feature, the CuFe2O4
with multiple redox states which have exhibited superior performance for alcohol oxidation was
chosen the catalytic active center.25 Thus, as the anther desirable structural feature, the HKUSTꢀ1
shell of CuꢀCuFe2O4@HKUSTꢀ1 nanoparticle can be used to attract and preconcentrate oxygen
via the open Cu accompanied by significant electron transfer, as well as the kinetic diameter of
O2 is slightly smaller than the pore limiting diameter of the MOF.28ꢀ29 The adsorption of oxygen
gives rise to a local high concentration in the vicinity of CuꢀCuFe2O4 and NHPI free radical sites,
CuꢀCuFe2O4@HKUSTꢀ1 nanoparticle indeed show the catalytic activity at the low temperature
with high conversion of fluorene and good selectivity of fluorenone (conversion: >99%,
selectivity: >99%) with molecular oxygen as the oxidant.
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Table 1 Benzylic oxidation with different catalystsa
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Entry
Cat.
Conv. ( % )b
Selec. ( % )b
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CuꢀCuFe2O4@ HKUSTꢀ1
CuCl2·2H2O
>99
4
>99
>99
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FeCl3·6H2O
CuꢀCuFe2O4
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>99
>99
>99
>99
HKUSTꢀ1
CuꢀCuFe2O4+ HKUSTꢀ1
a
Reaction conditions: fluorene (1.0 mmol), 15 mg of CuꢀCuFe2O4@HKUSTꢀ1 (other catalysts
with the same Cu or Fe content of CuꢀCuFe2O4@HKUSTꢀ1), NꢀHydroxyphthalimide (10 mol%),
o
b
acetonitrile (5.0 mL), at 60 C under 1 atm. of atmosphere stirring for 9 h. Conversion and
selectivity were determined by GC/MS using nitrobenzene as internal standard.
Figure 5. Design of CuꢀCuFe2O4@HKUSTꢀ1 catalyst for aerobic oxidation of benzylic CꢀH
bonds.
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Table 2 Benzylic oxidation of hydrocarbon derivatives a
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a
Reaction conditions: fluorene (1.0 mmol), 15 mg of HKUSTꢀ1@CuꢀCuFe2O4 catalyst, Nꢀ
Hydroxyphthalimide (10 mol%), solvent (5.0 mL), at 60 oC under 1 atm. of atmosphere stirring. b
Conversion and selectivity were determined by GC/MS using nitrobenzene as internal standard.
To investigate the general applicability of the magnetic CuꢀCuFe2O4@HKUSTꢀ1 catalyst, a
series of substrates were subsequently studied for benzylic oxidation and reaction results were
screened in Table 2. The optimized reaction condition was employed for the oxidation of other
substrates. It should be noted that 4,4’ꢀdifluorodiphenylmethane can be quantitatively converted
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with high conversion (95%) and high selectivity (99%) after a required reaction time (Table 2,
entry 1). The ethylbenzene and 1ꢀethylꢀ4ꢀmethoxybenzene showed 93% and 96% conversion
with high selectivity after 9 h (Table 2, entries 2ꢀ3), respectively. Moreover, ethylbenzene with
the paraꢀfluoro group, 1ꢀethylꢀ4ꢀfluorobenzene, was also evaluated, the corresponding
conversion was only 70% after a 24 h reaction time (Table 2, entry 4). This difference in terms of
reaction results was most likely due to the electronꢀdonating functional group at paraꢀposition of
ethylꢀbenzene favoring the oxidation. The oxidation reactions of benzylic CꢀH bonds bearing βꢀ
methoxy group or βꢀester group were also tested (Table 2, entries 5ꢀ6), and a surprising result
(>99% conversion; >99% selectivity) was shown due to the βꢀmethoxy group (electronꢀdonating
functional group) favoring this oxidation. It should be noted that a satisfying result was also
observed (91% conversion; 94% selectivity) when heteroaromatic compound such as 2ꢀ
ethylthiophene was employed as substrate (Table 2, entry 7). These preliminary results indicated
that CuꢀCuFe2O4@HKUSTꢀ1 catalyst with speciallyꢀdesigned displayed fantastic catalytic
activities about the aerobic oxidation of benzylic CꢀH bonds at mild reaction conditions.
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Figure 6. Leaching test of CuꢀCuFe2O4@ HKUSTꢀ1 catalyst for benzylic oxidation.
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To confirm the stability of the CuꢀCuFe2O4@HKUSTꢀ1 catalyst for benzylic oxidation, the
metal leaching tests were conducted. The solid catalyst was removed by centrifugation at 3 h,
and the reaction continued for another 6 h. The results showed that without Cuꢀ
CuFe2O4@HKUSTꢀ1 catalyst (Figure 6), the conversion of fluorene increased from 65% to 71%,
which is caused by Nꢀhydroxyphthalimide (Figure S8). In addition, there is no obvious presence
of the copper and iron in the solution detected by ICPꢀAES analysis of the filtrate.
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Figure 7. (a) Fieldꢀdependent magnetization curves of CuꢀCuFe2O4 (red) and Cuꢀ
CuFe2O4@HKUSTꢀ1 (black) at 293 K. The insets photographs of magnetic recovery of Cuꢀ
CuFe2O4@HKUSTꢀ1 after applying the external magnetic field; (b) The reusability of Cuꢀ
CuFe2O4@ HKUSTꢀ1 catalyst in the aerobic oxidation of benzyl oxidation.
As we known, recyclability for any catalyst is an important and essential feature in largeꢀscale
catalytic reactions. An outstanding advantage of the CuꢀCuFe2O4@HKUSTꢀ1 nanoparticles was
their controllable magnetic recyclability. The magnetization saturation values of CuꢀCuFe2O4
and CuꢀCuFe2O4@HKUSTꢀ1 samples were measured to be 41 emugꢀ1 and 32 emugꢀ1,
respectively (Figure 7a). The curves of CuꢀCuFe2O4@HKUSTꢀ1 samples depicted a magnetic
hysteresis loop and also presented the strong magnetic response via a varying magnetic field.
Then, the magnetic separability of CuꢀCuFe2O4@HKUSTꢀ1 nanoparticles were also tested by
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applying a magnet near the glass bottle containing this nanoparticles, which were attracted
toward this magnet within a few seconds (inset of Figure 7a). The results directly demonstrated
the convenient separation of CuꢀCuFe2O4@HKUSTꢀ1 nanoparticles from the liquidꢀphase
system. In order to verify the reusability of CuꢀCuFe2O4@HKUSTꢀ1 catalyst, the oxidation of
fluorene was performed under the optimized catalytic condition. After completion of the each
reaction, the catalysts were easily separated from the reaction mixture by using an external
magnet, and then washed with ethanol three times and further reused directly. The Cuꢀ
CuFe2O4@HKUSTꢀ1 catalyst remained the catalytic activity over 10 cycles (Figure 7b). The
FTIR results (Figure S9) of the recovered catalyst match well with those of the freshly prepared
catalyst, which indicated that the crystal structure and composition of the magnetic catalyst
almost retained its inherent nature after ten cycles reaction. Furthermore, the SEM images
(Figure S9) showed that the both sizes and morphologies of the catalyst have no obvious change
compared to the fresh catalysts.
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4. CONCLUSIONS
In summary, we have shown the successful synthesis of magnetically recyclable Cuꢀ
CuFe2O4@HKUSTꢀ1 nanostructures via a facile inꢀsitu synthesis method. Along with the
gradual release of Cu2+ ions from Cu(0) component of CuꢀCuFe2O4 nanoparticles in the acid
ligand solution, HKUSTꢀ1 shell was formed and then covering the magnetical CuꢀCuFe2O4
nanoparticles. In this CuꢀCuFe2O4@HKUSTꢀ1 nanostructure, CuFe2O4 core offer the active sites,
while HKUSTꢀ1 shell enhanced the absorption of oxygen. The catalytic performance indicates
o
that the aerobic oxidation of benzylic CꢀH bonds can be proceeded under 60 C, offering >99%
conversion for fluorene and >99% selectivity for fluorenone. Furthermore, asꢀprepared
magnetical CuꢀCuFe2O4@HKUSTꢀ1 heterostructures can be easily separated from catalytic
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system and reused for ten times without losing its catalytic activity. Therefore, our strategy for
generation a rationally multifunctional designed heterogeneous catalyst opens up great
perspectives for the development of selective oxidation of saturated CꢀH bonds by molecular
oxygen at mild temperatures.
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ASSOCIATED CONTENT
Supporting Information
Additional characterization data, including SEM images, FTIR patterns, BET curves and XRD
patterns. This material is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Authors
*Eꢀmail: gewang@mater.ustb.edu.cn; hygao2009@163.com
Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval
to the final version of the manuscript.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENT
We thank National Key Research and Development Program of China (No. 2016 YFB0701100),
Chinese Postdoctoral Science Foundation (No. 2015M580980) and the Fundamental Research
Funds for the Central Universities (No. FRFꢀTPꢀ15ꢀ008A1) for financial support.
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ABBREVIATIONS
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MNPs, magnetic nanoparticles; H3BTC, 1,3,5ꢀBenzenetricarboxylic acid; MOFs: metalꢀorganic
frameworks
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1926ꢀ1933.
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Scheme 1.Schematic illustration of the formation of Cu-CuFe2O4@HKUST-1 nanoparticles.
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Figure 1. TEM images of (a) Cu-CuFe2O4 nanoparticle, (b) Cu-CuFe2O4@HKUST-1 nanoparticle, HAADF-
STEM image of (c) Cu-CuFe2O4@HKUST-1 nanoparticle and EDS mapping images of Fe (d) and Cu (e) in
the Cu-CuFe2O4@HKUST-1 nanoparticle.
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Figure 2. XRD patterns of the as-prepared Cu-CuFe2O4 nanoparticle and the experimentally acquired Cu-
CuFe2O4@HKUST-1 nanoparticle.
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Figure 3. TEM images of Cu-CuFe2O4@HKUST-1 heterostructures obtained after reaction time of (a) 4 h, (b)
8 h, (c) 12 h; (d) XRD patterns corresponding to the samples obtained after different reaction time.
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Figure 4. TEM image of (a) pure CuFe2O4 and (b) the corresponding products; (c) XRD patterns of pure
CuFe2O4 and the corresponding product.
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