Facile fabrication of magnetically recyclable metal-organic framework nanocomposites for highly efficient and selective catalytic oxidation of benzylic C-H bonds

Article abstract of DOI:10.1039/c4cc03728f

HKUST-1@Fe₃O₄ chemically bonded core-shell nanoparticles have been prepared by growing HKUST-1 thin layers joined by carboxyl groups onto Fe₃O₄ nanospheres. These magnetic core-shell MOF nanostructures show exce

Full text of DOI:10.1039/c4cc03728f

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Facile fabrication of magnetically recyclable
metal–organic framework nanocomposites for
highly efficient and selective catalytic oxidation
of benzylic C–H bonds†
Cite this: Chem. Commun., 2014,
50, 8374
Received 16th May 2014,
Accepted 9th June 2014
Yifa Chen,‡a Xianqiang Huang,‡ab Xiao Feng,a Jikun Li,a Yingyu Huang,a
Jingshu Zhao,a Yuexin Guo,a Xinmei Dong,a Ruodan Han,a Pengfei Qi,a
Yuzhen Han,a Haiwei Li,a Changwen Hu*a and Bo Wang*a
DOI: 10.1039/c4cc03728f
www.rsc.org/chemcomm
HKUST-1@Fe3O4 chemically bonded core–shell nanoparticles have storage, separation, sensing, molecular recognition and hetero-
been prepared by growing HKUST-1 thin layers joined by carboxyl geneous catalysis.6 However, only a few MOFs have shown
groups onto Fe3O4 nanospheres. These magnetic core–shell MOF catalytic activity in the oxidation of benzylic C–H bonds with
nanostructures show exceptional catalytic activity for the oxidation unsatisfactory conversion rate and/or selectivity, needless to say
of benzylic C–H bonds and they can be recovered by magnetic that no rational design has been reported on highly efficient
separation and reused without losing any activity.
C–H activation using MOF-based heterogeneous catalysts with
proven recyclability.7
The oxidation of C–H bonds is one of the most important and
One of the methods to design easily recyclable catalysts is
fundamental reactions. Highly efficient catalysis of this trans- to introduce magnetism and test reusability by magnetic separation
formation remains a challenge and great efforts had been made in between cycles.8 Beyond directly incorporating magnetic
to explore new catalysts.1 The oxidation of benzylic C–H bonds, components in MOF building units, forming composites using
for instance, is a powerful tool to generate high value chemical simple yet catalytically active MOF nanostructures that are
feedstock from less expensive raw materials such as alkyl anchored on the surface of magnetic nanoparticles could serve
aromatics. In this regard, the means to convert benzylic hydro- as a simple alternative route for this purpose.
carbons into valuable compounds have attracted considerable
Recently core–shell structured nanocomposites have attracted
attention in recent years.2 Despite the high conversion rate, increasing attention due to their novel properties and potential
many benzylic C–H bond oxidation reactions reported to date rely on applications that cannot be achieved solely with a single component.9
homogeneous catalysts and thus recyclability and reusability of these We thus decided to directly grow MOF nanolayers onto Fe3O4
specially-designed catalysts can be problematic.3 Immobilization of nanospheres that are grafted with carboxylic functional groups.
the molecular catalysts and developing heterogeneous catalytic The chemical bonding of copper ions and carboxyl oxygen can
systems from scratch have proved to be two valuable strategies to help to tightly hold the active MOF layers and maintain the
solve this problem.4
intactness of the nanocomposites through catalytic cycles.
Herein, we report a facile yet powerful strategy to synthesize
Metal–organic frameworks (MOFs) are a class of porous
polymeric materials consisting of metal ions or metal clusters HKUST-1@Fe3O4 core–shell nanoparticles (50 to 100 nm) by
covalently joined by organic links.5 By virtue of their structural assembling HKUST-1 thin layers bonded through carboxyl
tunability, well-defined channels and rich functionalities, groups onto Fe3O4 nanospheres. We also successfully employed
MOFs found their way in potential applications including gas the magnetic HKUST-1@Fe3O4 core–shell nanocomposites as
highly efficient catalysts for the oxidation of benzylic hydrocarbons
up to 99% conv. and 99% sel. Furthermore, these MOF core–shell
nanocomposites can be easily recovered via simple magnetic
a Key Laboratory of Cluster Science, Ministry of Education School of Chemistry,
Beijing Institute of Technology, Beijing 100081, P. R. China.
separation and reused without losing activity through multiple
reaction cycles. This may serve as a start in designing MOF catalysts
for a continuous magnetic fluid bed.
The synthetic procedures of HKUST-1@Fe3O4 core–shell
nanocomposites are summarized schematically in Fig. 1. Firstly,
the carboxyl functionalized Fe3O4 cores with a diameter of about
20 nm (ESI,† Fig. S1) are capped with polyvinylpyrrolidone (PVP),
E-mail: bowang@bit.edu.cn, cwhu@bit.edu.cn
b Shandong Provincial Key Laboratory of Chemical Energy Storage and Novel Cell
Technology, School of Chemistry & Chemical Engineering, Liaocheng University,
Liaocheng 252059, P. R. China
† Electronic supplementary information (ESI) available: Experimental section;
synthesis, characterization, catalytic cycle details and additional figures and
tables. See DOI: 10.1039/c4cc03728f
‡ These authors contributed equally to this work.
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electron penetrability for the Fe3O4 and HKUST-1 components
(Fig. 2b–c). Energy dispersive X-ray spectroscopy (ESI,† Fig. S2)
and elemental mapping (Fig. 2e) indicate the presence of Fe
and Cu ions in the composites. Analysis of the shells by TEM
illustrates that the spacing between the adjacent planes is measured
to be 0.21 nm (Fig. 2d). This value is consistent with the spacing of
(044) spacing planes of HKUST-1, revealing that the shells with a
thickness of 5–18 nm in the composites are indeed HKUST-1.10 It is
validated by the analysis in selected-area electron diffraction of the
shell that exhibits a clear crystalline spot ring of crystalline HKUST-1
(ESI,† Fig. S3). Powder X-ray diffraction (PXRD) patterns for the core–
shell composites unambiguously indicate the formation of the
HKUST-1 phase and the retaining of the Fe3O4 phase (ESI,† Fig. S4).
Fourier transform infrared (FT-IR) spectra further demonstrate
the formation of HKUST-1@Fe3O4 nanocomposites. As shown in
Fig. 3, the FT-IR spectrum of HKUST-1@Fe3O4 displays characteristic
bands of HKUST-1. The peaks at 667, 729 and 765 cmÀ1 are
attributed to the deformation modes of C–C–C in the benzene ring;
the peaks at 1059, 1109 and 1165 cmÀ1 are associated with the
C–O–C bending vibration of the ethylenedioxy moiety; the peaks
at 1374, 1446 and 1566 cmÀ1 are assigned to the C–H stretching of
the benzene ring. Concomitant with the encapsulation of Fe3O4 by
the HKUST-1 shell, the 1616, 1646 and 1674 cmÀ1 transmission
bands can be ascribed to CQO stretching in various structural
environments. Compared with Fe3O4, Fe3O4/Cu(NO3)2Á3H2O and
HKUST-1, CQO stretching vibrations in the composites are
obviously shifted, suggesting that HKUST-1 has been coated on
the surface of the Fe3O4 core through coordination interactions.
In order to identify the chemical composition of HKUST-1@
Fe3O4, N2 sorption isotherm measurement and elemental analysis
were performed. From the adsorption isotherms (Fig. 4), the
Brunauer–Emmett–Teller (BET) surface areas of 10, 1220 and
738 m2 gÀ1 are achieved for Fe3O4, HKUST-1 and HKUST-1@
Fe3O4 (pore size distributions displayed in Fig. S5, ESI†), respec-
tively. On the basis of the contributions of each component to
the total surface area of HKUST-1@Fe3O4, the mass percentage
of HKUST-1 is approximately evaluated to be 60.50% of the
Fig. 1 The schematic representation of the synthetic route of HKUST-1@
Fe3O4 and its potential applications in magnetic fluid beds.
which acts as a stabilizing agent and thus allows the magnetic
cores to uniformly disperse in a mixed solution of DMF–
C2H5OH–H2O. Then copper ions are added and they directly
coordinate with carboxyl groups anchored to the surface of Fe3O4
nanospheres. Finally, the nanoparticles are encapsulated by a
HKUST-1 shell via in situ generation of HKUST-1 frameworks
along with the introduction of trimesic acid and additional
Cu(OAc)2 into the mixture.
Scanning electron microscopy (SEM) (Fig. 2a) and high-
resolution transmission electron microscopy (HR-TEM) inves-
tigations (Fig. 2b–c) show that the obtained HKUST-1@Fe3O4
nanoparticles have a spherical morphology with dimensions
ranging from 50 to 100 nm. The nano-scaled composites are
confirmed as core–shell structures by TEM. The rough interface
between core and shell is clearly observed due to the different
Fig. 2 (a) SEM image of HKUST-1@Fe3O4. (b) and (c) TEM images of
HKUST-1@Fe3O4. (d) HRTEM image of the circled site from (c), the
distance of the adjacent planes is 0.21 nm. (e) EDS mapping images of
HKUST-1@Fe3O4.
Fig. 3 IR spectra of Fe3O4 (black), Fe3O4/Cu(NO3)2Á3H2O (cyan), HKUST-1
(red) and HKUST-1@Fe3O4 (blue).
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Table 1 The oxidation of benzylic hydrocarbons catalyzed by HKUST-1@
a
Fe3O4
Entry Substrates
1
Products
Conv. (%) Sele.b (%)
94.7
95.2
2
3
4
499
499
499
499
499
499
98.3
Fig. 4 Nitrogen sorption isotherms of HKUST-1 (green) and HKUST-1@
Fe3O4 (pink).
5
97.1
a
Reaction conditions: benzyl hydrocarbons (0.250 mmol), HKUST-1@
composites. The formula of HKUST-1@Fe3O4 is thus calculated
to be Cu3Fe5.2C18O18.9H12 (Fe3O4:HKUST-1 = 1.73). Elemental
analysis reveals that the composites are constituted of 19.34%
Cu and 28.63% Fe (ESI,† S6), which is in agreement with the
values Cu, 18.98% and Fe, 28.78% derived from the formula
based on adsorption analyses when taking the deviation into
account.
Given the fact that the oxidation of benzylic C–H bonds is
one of the key transformations in organic synthesis and nano-scaled
HKUST-1@Fe3O4 encompasses numerous potential catalytic centers,
we decided to evaluate the catalytic activity of the composites in
the context of the oxidation reaction (Fig. 5). The reactions were
performed using tert-butyl hydroperoxide (TBHP) as the oxidant.
The transformation of diphenylmethane to benzophenone was
selected as a model reaction. Heating a mixture of diphenyl-
methane (0.250 mmol) and TBHP (0.625 mmol) in benzonitrile
at 80 1C for 14 h in the presence of HKUST-1@Fe3O4 (25 mg) led
to the formation of benzophenone with 94.7% conversion and
95.2% selectivity.
Fe3O4 (25 mg), benzonitrile (0.5 mL), TBHP (0.625 mmol), 80 1C.
Selectivity to ketones.
b
Utilizing the optimized reaction conditions, the reaction
scope by employing a variety of benzylic C–H compounds was
subsequently explored. The corresponding ketones with aryl
side chain groups were successfully synthesized from corres-
ponding benzylic C–H compounds with a great conversion rate
(94.7–99%) and exceptional selectivity (95.2–99%) by using
HKUST-1@Fe3O4 as the catalyst (Table 1). These preliminary
results show that HKUST-1@Fe3O4 can facilitate the oxidation
of benzylic C–H bonds and serve as a highly efficient and
selective catalyst. And indeed, the yield of HKUST-1@Fe3O4
outperforms many effective heterogeneous catalysts reported to
date, i.e. nano-sized coordination cages [Cu4L4]4X (L = 1,3,5-
tris(1-benzylbenzimidazol-2-yl)benzene, X = ClO4, OTs, OTf),11
NHPI/Cu@PILC12 and even noble Pd@N-doped carbon,13 etc.
Control catalytic experiments of the catalytic oxidation of
diphenylmethane were performed on Fe3O4 and HKUST-1
under the same reaction conditions. The conversions were
56.4% and 86.5% when using Fe3O4 and HKUST-1, respectively.
Based on the above findings, we could confirm that the Fe3O4
not only act as a magnetic component for separation, but also
participates in the catalysis.14 The control experiments show
that the catalytic C–H activation can be obviously enhanced when
the core–shell nanostructures of HKUST-1@Fe3O4 are used.
This may suggest the synergy effect in the bi-component nano-
composites of HKUST-1@Fe3O4 when catalyzing the oxidation of
C–H bonds.
Recyclability is an important and essential feature for any
catalyst to be considered in large-scale catalytic reactions. A big
advantage of HKUST-1@Fe3O4 composites is that their saturation
magnetization is still high enough for efficient separation using
existing magnetic technology (ESI,† Fig. S7), which enables it to be
potentially used in magnetic fluid beds for continuous industrial
catalysis. As a proof-of-concept, the reusability of HKUST-1@Fe3O4
was thus tested.
Fig. 5 Schematic diagram of the oxidation of benzylic hydrocarbons
using HKUST-1@Fe3O4 as the catalyst.
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perspectives in terms of catalytic industrial processes and
sheds light on developing strategies aimed at rendering MOFs
with new functionalities.
We are grateful to the ‘‘1000 Plan (Youth)’’; 973 Program
2013CB834704; the National Natural Science Foundation of
China (Grant No. 21201018; 21231002; 21173021; 21276026);
the 111 Project (B07012).
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Fig. 7 Conversions and selectivities of HKUST-1@Fe3O4 in recycle experi-
ments. Reaction conditions: diphenylmethane (0.250 mmol); catalyst
(25 mg); 80 1C; TBHP (0.625 mmol), 14 h.
As shown in Fig. 6, after completion of the oxidation reaction, the
magnetic catalyst can easily be separated from the reaction mixture
with an external magnet. The recovered catalyst was re-activated by
washing with DMF four times (reclaimed with the same external
magnet after each rinse) and further reused directly in a subsequent
oxidation of benzylic C–H bond reactions. The catalytic activity did
not decrease after three catalytic cycles (Fig. 7) and the PXRD
patterns of the recovered HKUST-1@Fe3O4 samples are identical to
those of the freshly prepared HKUST-1@Fe3O4 (ESI,† Fig. S8). Atomic
absorption analysis of the liquid phase after separation of the
catalyst by an external magnet showed that there were no leaching
of the copper and iron ions from the catalyst to the reaction mixture
(ESI,† S9).
In summary, we successfully synthesized a chemically-
bonded, nano-sized and magnetic HKUST-1@Fe3O4 composite.
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successfully catalyze the oxidation of benzylic C–H bonds with
excellent conversion and selectivity. Moreover, this new MOF
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