Facile Synthesis of Magnetic Hierarchical Core–Shell Structured Fe3O4@PDA-Pd@MOF Nanocomposites: Highly Integrated Multifunctional Catalysts

Article abstract of DOI:10.1002/cctc.201701693

A magnetic hierarchical core–shell structured Fe₃O₄@PDA-Pd@[Cu₃(btc)₂] nanocomposite has been fabricated via a facile layer-by-layer assembly method. It contains a core of polydopamine (PDA)-modified magnetic Fe₃O₄ nanoparticles (NPs), a transition layer of Pd NPs, and a porous outer shell of copper-based metal–organic framework (MOF) with controllable thickness. This novel nanocomposite was characterized by TEM, FTIR, XRD, XPS, N₂ adsorption–desorption isotherms, and VSM. The prepared Fe₃O₄@PDA-Pd@[Cu₃(btc)₂] (n=5) nanocomposite shows ultrahigh catalytic activity for the 4-nitrophenol reduction and Suzuki–Miyaura coupling reactions of aryl halides (Br, Cl) with arylboronic acids. Moreover, the nanocomposite also can be easily separated by an external magnet and reused at least 8 runs with excellent yields for both the reactions. The catalyst has outstanding catalytic performance, mainly because the integration of [Cu₃(btc)₂] with Fe₃O₄@PDA-Pd that combines the advantages of each component could exhibit a synergistic effect in the catalytic system.

Full text of DOI:10.1002/cctc.201701693

Accepted Article
Title: Facile synthesis of magnetic hierarchical core-shell structured
Fe3O4@PDA-Pd@MOF nanocomposites: highly integrated
multifunctional catalysts
Authors: Rong Ma, Pengbo Yang, Yao Ma, and Fengling Bian
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemCatChem 10.1002/cctc.201701693
Link to VoR: http://dx.doi.org/10.1002/cctc.201701693
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ChemCatChem
10.1002/cctc.201701693
FULL PAPER
Facile synthesis of magnetic hierarchical core-shell structured
Fe @PDA-Pd@MOF nanocomposites: highly integrated
multifunctional catalysts
3
O
4
Rong Ma, Pengbo Yang, Yao Ma, Fengling Bian*[a]
Abstract: In this report, a novel magnetic hierarchical core-shell
hierarchical core-shell structured nanocomposites use
inorganic oxides (SiO , CeO , TiO , etc.) as porous outer
structured Fe
fabricated via a facile layer-by-layer assembly method, which
contains a core of polydopamine (PDA) modified magnetic Fe O
4
3
O
4
@PDA-Pd@[Cu
3
2
(btc) ] nanocomposite has been
2
2
2
shells, which are generally prepared by templating
methods or selective etching, resulting in the
nanocomposites with troublesome and unfriendly post-
treatment process, low compositional diversity and limited
functionality.[11-13]
3
nanoparticles (NPs), a transition layer of Pd NPs, and a porous
outer shell of copper-based metal organic framework (MOF) with
controllable
characterized by TEM, FT-IR, XRD, XPS, N
isotherm and VSM. The prepared Fe
n=5) nanocomposite shows ultrahigh catalytic activity for the 4-
thickness.
This
novel
nanocomposite
adsorption-desorption
@PDA-Pd@[Cu (btc)
was
2
Metal Organic Framework (MOF), emerging as an
attractive class of crystalline porous materials with well-
assembled active functionalities from metallic ions and
organic linkers, would break the above-mentioned
limitation of inorganic porous shells due to its distinguished
properties such as facile preparation, diversified
components and tailorable porosity.[14,15] In addition, proper
design and construction of MOF nanocomposites that by
integrating functional materials (metal NPs, metal oxides,
polymers, etc.) is expected to present synergistically
boosted catalytic performances of improved catalytic
activity, enhanced stability, and prolonged lifetime,[16,17]
which would open new opportunity for fabricating highly
efficient hierarchical core-shell catalysts.[18] Recently, Ke et
al. fabricated multifunctional core-shell structured Au-
3
O
4
3
2
]
(
nitrophenol reduction and Suzuki-Miyaura coupling reactions of aryl
halides (Br, Cl) with arylboronic acids. Moreover, the
nanocomposite also can be easily separated by an external magnet
and reused at least 8 runs with excellent yields for both the
reactions. The catalyst possesses outstanding catalytic
performances mainly because the integration of [Cu
3 2
(btc) ] with
Fe @PDA-Pd that combines the advantages of each component
3
O
4
could exhibit synergistic effect in our catalytic system. The facile
synthesis strategy and brilliant catalytic performances of the
3 4 3 2
Fe O @PDA-Pd@[Cu (btc) ] (n=5) nanocomposite make it a
promising catalyst for the practical application.
3 4
Fe O @MIL-100(Fe) nanocomposite, which exhibited high
Introduction
catalytic activity toward the reduction of 4-NP owing to the
synergistic effect between MIL-100(Fe) and Au.[19] Until
now, there are only a few examples about the fabrication
of magnetic MOF nanocomposites with core-shell
structure.[20] One of the difficult challenges is how to direct
assemble MOF on the surface of magnetic NPs without
any specific surface modification. Although some plausible
solutions have been reported.[20a,21] It should be noted that
Core-shell structured multifunctional nanocomposites possess
not only the unique physical and chemical properties of
nanomaterials but also excellent properties of cores and shells,
making them promising candidates for various applications in
adsorption, drug delivery and catalysis.[1] As a significant
member of multifunctional nanocomposites, magnetic
hierarchical core-shell structured nanocomposites, which
there still remains challenges to implement
a facile
approach for fabricating magnetic core-shell MOF
nanocomposites with excellent catalytic performances.
PDA, an industry-available product, possesses
abundant catechol and amine functional groups, which can
not only coat on virtually all types of material surfaces but
also serve as extremely versatile platform for secondary
reactions to fabricate a variety of ad-layers.[22,23] In this
3 4
consist of a core of inner modified Fe O NPs, transition layer of
noble-metal active sites, and a porous outer shell, have
attracted considerable attention in heterogeneous catalysis.[2,3]
This is because they can combine the fascinating features of
each component, exhibiting good magnetic responsiveness,
excellent catalytic activity and structural stability.[4-8] Recently,
Zhao et al. prepared multifunctional Fe
nanocomposite and applied it to the reduction of 4-nitrophenol
4-NP) and epoxidation of styrene.[9] Ye et al. synthesized
hierarchical core-shell structured Fe @C-Pd@mCeO
3 4 2 2
O @SiO -Au@mSiO
work, we report the synthesis of
hierarchical core-shell structured
Pd@[Cu (btc) ] nanocomposite by a versatile and simple
layer-by-layer assembly method. The use of PDA as an
intermediate can not only protect the Fe O NPs from
a
novel magnetic
Fe O @PDA-
3
4
(
O
3 4
2
3
2
nanosphere, and used it for the Suzuki reaction and the
reduction of 4-NP.[10] These catalysts all displayed good
magnetic separability and reusability. However, most of
3
4
corrosion in harsh environments and immobilize Pd NPs,
but also induce the growth of [Cu (btc) ] on the surface of
the Fe @PDA-Pd. [Cu (btc) (btc=1,3,5-
benzenetricarboxylate) was chosen as a porous outer shell,
because [Cu (btc) ] with controllable thickness can be
3
2
3
O
4
3
2
]
[a]
R. Ma, P. Yang, Y. Ma, Dr. F. Bian
College of Chemistry and Chemical Engineering
Lanzhou University
3
2
conveniently prepared by tuning the assembly cycle
number, meanwhile it possesses outstanding adsorption
ability for specific reactants.[24,25] The as-synthesized
nanocomposites show outstanding catalytic activity and
Lanzhou 730000(P.R. China)
E-mail: bianfl@lzu.edu.cn
Supporting information for this article can be found under
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ChemCatChem
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FULL PAPER
reusability for the reduction of 4-NP in H
2
O and Suzuki
The morphology and structure of obtained nanocomposites
coupling reaction in EtOH/H
this is the first report on the integration of [Cu
Fe @PDA-Pd into one nanostructure to significantly
2
O. To our best knowledge,
were analyzed by TEM. As shown in Fig. 1a, Fe
nanocomposites possess well-defined
3
O
4
@PDA
3
(btc) ] with
2
a
core-shell
3
O
4
configuration with an average diameter of 530 nm, and the
thickness of PDA is estimated to be 65 nm. TEM (Fig. 1b) and
improve not only the intrinsic catalytic performances of Pd
active species, but also the recyclability of catalysts.
3 4
HRTEM (Fig. 1c) images of Fe O @PDA-Pd nanocomposites
show that Pd NPs with average size of 8 nm are both dispersed
on PDA surface and embedded in PDA layer. The lattice fringes
are observed, a discerned lattice spacing of 0.22 nm are
assigned to the (111) crystalline of Pd.[26] TEM (Fig. 1d and e)
Results and Discussion
images of Fe
boundary between the [Cu
which is attributed to the slight mass difference of the two
components.[21a,27] However, compared with Fe
@PDA-Pd,
Fe @PDA-Pd@[Cu (btc) ] (n=5) nanocomposites have
3
O
4
@PDA-Pd@[Cu
3 2
(btc) ] (n=5) indicate that the
3 2
(btc) ] and the PDA is obscure,
3 4
O
3
O
4
3
2
a
rough surface and a larger size of 540 nm, indicating that the
outer shells of MOF have been successfully covered on
Fe
mapping were also performed to reveal the elemental
distribution in Fe @PDA-Pd@[Cu (btc) (n=5)
3 4
O @PDA-Pd. The scanning TEM (STEM) and elemental
3
O
4
3
2
]
nanocomposites. The STEM image in Fig. 1g shows that the
nanocomposites are typical core-shell structures. The
corresponding elemental mappings display that the element Fe
of Fe
PDA, Pd, and Cu of [Cu
with differently sized spherical shapes, demonstrating that
Fe @PDA-Pd@[Cu (btc) (n=5) nanocomposites have
3
O
4
is mainly located in the core, while the elements N of
Scheme
1
3 4 3 2
Synthetic procedure of the Fe O @PDA-Pd@[Cu (btc) ]
3
(btc) ] are homogenously dispersed
2
nanocomposites.
A
novel magnetic hierarchical core-shell structured
@PDA-Pd@[Cu (btc) ] nanocomposite was synthesized
layer-by-layer assembly method and its synthetic
3
O
4
3
2
]
Fe
3
O
4
3
2
hierarchical core-shell structures. Moreover, the size of the
whole nanocomposites obviously increases when the number
of assembly cycles increases from 5 to 15 (Fig. 1f), implying
that the thickness of the shell increases with increasing the
number of assembly cycles.
by
a
procedure was demonstrated in Scheme 1. Initially, Fe
3
O
4
NPs
was
was prepared by a solvothermal method. Then, Fe
3
O
4
modified with PDA by self-polymerization of dopamine under
alkaline conditions. Next, the Fe @PDA-Pd nanocomposites
were formed by adding Fe @PDA to a PdCl solution without
thermal treatment. Finally, using Cu(CH COO) ·H O and H btc
as precursors, the [Cu (btc) ] shell with controllable thickness
was coated on the surface of Fe @PDA-Pd nanocomposites
by tuning the assembly cycle number.
3 4
O
Fig. 2 presents the FT-IR spectra of (a) Fe
@PDA, (c) Fe @PDA-Pd and (d) Fe
Pd@[Cu (btc) ] (n=5). In Fig. 2a, the adsorption peaks at 589
O
3 4
, (b)
O
3 4
2
Fe
3
O
4
O
3 4
O @PDA-
3 4
3
2
2
3
3
2
3
2
cm-1 and 3421 cm-1 are corresponding to the Fe-O stretching
vibration and O-H stretching vibration respectively.[28] In Fig. 2b,
besides the characteristic adsorption peaks of Fe-O bond, the
absorption peaks at 1610 cm-1 and 1513 cm-1 are related to the
3 4
O
Fig.
Fe
n=15). HR-TEM image of (c) Fe
Fe @PDA-Pd@[Cu (btc) (n=5) and the corresponding elemental
mappings for Fe, N, Pd and Cu elements.
1
TEM images of (a) Fe
@PDA-Pd@[Cu (btc) (n=5), and (f) Fe
@PDA-Pd. STEM image of (g)
3
O
4
@PDA, (b) Fe
3
O
4
@PDA-Pd, (d,e)
3
O
4
3
2
]
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
]
(
3 4
O
Fig. 2 FT-IR spectra of (a) Fe , (b) Fe
3
O
4
3 4 3 4
O @PDA, (c) Fe O @PDA-Pd and
3
O
4
3
2
]
(
d) Fe @PDA-Pd@[Cu (btc) ] (n=5).
3
O
4
3
2
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FULL PAPER
Fig. 3 XRD patterns of (a) Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
] (n=5), (b)
(btc) ].
Fe @PDA-Pd @[Cu (btc) ] (n=15) and (c) simulated [Cu
3
3
O
4
2
3
2
Fig.
Pd@[Cu
Fe @PDA-Pd and (d) Fe
4
XPS spectra of (a) Fe
(btc) (n=5); the high-resolution spectra for Pd 3d of (b)
@PDA-Pd@[Cu (btc) ] (n=5).
3 4 3 4
O @PDA-Pd and (c) Fe O @PDA-
3
2
]
C=C stretching vibrations of aromatic ring,[28] the broad peak at
390 cm-1 is attributed to the O-H and N-H stretching
3
O
4
3
O
4
3
2
3
vibration,[29] the peak appearing at 1294 cm-1 can be assigned
to the C-OH stretching vibration of phenol compounds,[30] all the
XPS elemental survey scan of Fe
3 4
O @PDA-Pd (Fig. 4a)
illustrates that the surface of Fe @PDA-Pd nanocomposite
3
O
4
above characteristic peaks demonstrate that the Fe
nanocomposites were successfully prepared. After immobilizing
Pd on the Fe @PDA nanocomposites (Fig. 2c), the intensity
of the peak at 1294 cm-1 becomes weaker than that of
Fe @PDA due to the interaction between Pd NPs and
PDA.[31] Compared with Fig. 2c, there are some new peaks
appearing in the spectrum of Fe @PDA-Pd@[Cu (btc) ] (n=5)
Fig. 2d). The peaks at 730 cm-1 and 759 cm-1 can be
associated with Cu-O stretching vibration,[32] the peaks at 1373
3
O
4
@PDA
contains C, N, O, Pd elements, while the signal of Fe element
was not detected due to the effective PDA coverage on the
3
O
4
3 4
Fe O NPs. In the high-resolution XPS spectrum of Pd 3d (Fig.
4b), the binding energies of 337.2 eV and 342.5 eV are
assigned to PdII 3d3/2 and PdII 3d5/2, the binding energies of
3
O
4
0
0
335.5 eV and 340.3 eV are attributed to Pd 3d3/2 and Pd 3d5/2
,
revealing that both PdII and Pd0 exist in the Fe
@PDA-Pd
3
O
4
3
2
3 4
O
(
nanocomposites. Compared to Fig. 4a, the peak intensities of N,
Pd elements decrease and a new peak of Cu element appears
-1
-1
-1
cm , 1444 cm , 1577 cm can be attributed to the C-O
stretching vibration, C=O symmetric stretching vibrations, and
in Fig. 4c, confirming that the [Cu
successfully coated onto the surface of Fe
high-resolution XPS spectrum of Pd 3d (Fig. 4d) shows only
Pd0 existed in the Fe
@PDA-Pd@[Cu (btc) (n=5),
indicating that PdII were reduced to metallic Pd0 during the
processes of coating MOF shell onto the Fe @PDA-Pd. The
content of Pd0 in the Fe
@PDA-Pd@[Cu (btc) ] (n=5) was
3
(btc)
2
]
shells were
3
O @PDA-Pd. The
4
the C=O asymmetric stretching vibrations in the
respectively,[33] demonstrating the [Cu
(btc) has been
successfully coated on the Fe @PDA-Pd nanocomposites.
The crystallographic structure of the Fe @PDA-
Pd@[Cu (btc) ] (n=5) nanocomposites was detected by XRD.
In Fig. 3a, the diffraction peaks located at 30.2 , 35.6 , 43.3 ,
3
H btc
3
2
]
3
O
4
3
2
]
3 4
O
3
O
4
3 4
O
3
2
O
3 4
3
2
o
o
o
determined to be 2.27 wt% using Atomic absorption
spectroscopy (AAS).
o
o
5
3.6 , 57.2 and 62.8o correspond to the crystal faces (220),
(
311), (400), (422), (511) and (440) of Fe
3
O
4
(JCPDS no. 89-
The
N
2
adsorption-desorption isotherms of obtained
o
4319). The diffraction peak located at 40.0 can be well
indexed to (111) crystal plane of Pd NPs. Besides these
nanocomposites are shown in Fig. 5. The BET surface area is
calculated to be 51.50, 90.33 and 126.28 m2g-1 for the
o
o
o
o
peaks, there are additional peaks at 6.5 , 9.5 , 11.5 , 13.4
3 4 3 2
Fe O @PDA-Pd@[Cu (btc) ] nanocomposites with assembly
o
and 17.5 that match with the simulated [Cu
in Fig. 3c, only the intensities of the characteristic
diffraction peaks of [Cu (btc) ] are very weak, because of
MOF shell not being thick enough. Furthermore, the XRD
pattern analysis of Fe @PDA-Pd@[Cu (btc) ] (n=15)
nanocomposite was performed (Fig. 3b). Compared with
Fig. 3a, the intensities of the peaks assigned to [Cu (btc)
obviously become strong, implying that the amount of
Cu (btc) ] coated on the Fe @PDA-Pd increases along
3
(btc)
2
] pattern
cycles of 5, 10 and 15, respectively, indicating the BET surface
area increases with increasing the number of assembly cycles.
3
2
These values are lower than that of pure [Cu
3
(btc)
(SBET=1200-1507m g ), which is mainly ascribed to the Fe
core exited in the nanocomposites and the low content of
Cu (btc)
in nanocomposites.[34] However, the BET surface area
of Fe @PDA-Pd@[Cu (btc) ](n=5) is 10 times higher than
that of Fe @PDA-Pd (SBET=5.07 m g ). Therefore, the
enhanced surface area of Fe @PDA-Pd@[Cu (btc) ] (n=5) is
2
]
2
-1
3 4
O
3
O
4
3
2
3
2
3
2
]
3
O
4
3
2
2
-1
3
O
4
[
3
2
3
O
4
3
O
4
3
2
with increasing the number of assembly cycles. These
results are consistent with the above TEM results.
beneficial for the efficient adsorption and fast diffusion of
reactants.[35] Moreover, the BJH pore size distribution analysis
The XPS was conducted to investigate the surface
elements and chemical oxidation state of palladium in
of Fe
shown in the Fig. S1. The average pore diameter of
Fe @PDA-Pd is 11.78 nm, while that of Fe @PDA-Pd
3 4 3 4 3 2
O @PDA-Pd and Fe O @PDA-Pd@[Cu (btc) ] (n=5) are
Fe
3
4
O @PDA-Pd and Fe
O
3 4
@PDA-Pd@[Cu
3
(btc)
2
] (n=5). The
3
O
4
3 4
O
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FULL PAPER
(
n=5) to the 4-NP and NaBH
4
solution, as shown in Fig. 7a, the
4
-NP absorption peak at 400 nm gradually decreases,
new absorption peak at 295 nm appears
corresponding to the formation of 4-AP. The absorption peak at
00 nm completely disappears within 8 min (Fig. 7a), indicating
that the reaction was finished. The plot of In(C /C ) versus
reaction time for Fe @PDA-Pd@[Cu (btc) ] (n=5) presents
meanwhile
a
4
t
o
O
3 4
3
2
linear relationship (Fig. 7b), revealing that the reduction
followed the pseudo-first-order reaction kinetic model. The
apparent rate constant (kapp) was calculated to be 0.72 min-1
using the rate equation of In(C
For comparison, the catalytic performances of
@PDA, Fe @PDA-Pd and Fe @PDA@[Cu (btc)
n=5) were also explored. As shown in Table 1, Fe @PDA
exhibits no catalytic activity in terms of kapp (entry 1), thus the
catalytic activities of Fe @PDA-Pd and
Fe @PDA@[Cu (btc) ] (n=5) are derived from Pd active sites
and [Cu (btc) ] respectively (entries 2-3). Moreover, the
t o
/C )=-kappt.
Fe
3
O
4
3
O
4
3
O
4
3
2
]
(
3 4
O
3 4
O
3
O
4
3
2
Fig.
Fe
and (d) Fe
5
4
N
2
adsorption-desorption isotherms of (a) Fe
@PDA-Pd@[Cu (btc) ] (n=5), (c) Fe @PDA-Pd@[Cu
@PDA-Pd@[Cu (btc) ](n=15).
3
O
4
@PDA-Pd (b)
3
O
3
2
3
O
4
3
(btc) ] (n=10)
2
3
2
3
O
4
3
2
precursor Cu(OAc)
reaction system, and Cu(OAc)
the reduction of 4-NP(Fig. S3), confirming that the Cu2+ in
Cu (btc) ] can act as catalytical active sites to promote the
catalytic activities.[34,36] Among all these nanocomposites, the
Fe @PDA-Pd@[Cu (btc) ] (n=5) shows the highest catalytic
activity (Table1, entries 1-4), indicating that the coating of
Cu (btc) ] on the Fe @PDA-Pd could achieve enhancement
of catalytic activity. It is mainly because the [Cu (btc) ] could not
only endow nanocomposite with intrinsic properties of MOF but
also integrate with Fe @PDA-Pd to provide synergistic effect
in catalytic application. On the one hand, the porous outer shell
of [Cu (btc) ], which possesses high specific surface area and
2
in [Cu
3
2
(btc) ] was introduced into the
2
shows high catalytic activity for
@
[Cu
Fe
nm) leaching.
The magnetic properties of magnetic samples were
measured by a VSM at room temperature. In Fig. 6, the
magnetic hysteresis curves of (a) Fe , (b) Fe @PDA-Pd
] (n=5) show that they
and their saturation
3
(btc)
2
] (n=5) is 4.79 nm, which is lower than that of
3
O @PDA-Pd in favor of the prevention of Pd active sites (~8
4
[
3
2
3
O
4
3
2
O
3 4
3 4
O
[
3
2
3 4
O
and (c) Fe
3
O
4
@PDA-Pd@[Cu
3 2
(btc)
3
2
possess
superparamagnetism,
magnetization values are 73.54, 25.15 and 23.29 emu/g,
respectively, indicating that these magnetic samples can be
conveniently separated by an external magnet. In addition, the
photograph of magnetic separation and redispersion of the
3 4
O
3
2
abundant accessible metallic ions and organic linkers, can
absorb and enrich 4-NP (Fig. S4) via van der Waals,
electrostatic, and π-π stacking interactions,[24] leading to the
improvement of efficient contact between the reactants and
3 4 3 2
Fe O @PDA-Pd@[Cu (btc) ] (n=5) in water are shown in the
inset image of Fig. 6.
2
+
active species (Pd NPs and Cu ); on the other hand, both Pd
NPs in Fe
@PDA-Pd and the Cu2+ in MOF, as active species,
3
O
4
could cooperatively catalyze the reduction of 4-NP, resulting in
enhanced catalytic activity. Therefore, it can be concluded that
the introduction of [Cu
role in synergistic enhancement of catalytic activity of
Fe @PDA-Pd@[Cu (btc) ] (n=5) nanocomposites.
Moreover, the effect of MOF shell thickness on the
catalytic activity was further explored. As shown in table 1, the
app was calculated to be 0.72, 0.48 and 0.42 min-1 for the
reduction of 4-NP using Fe @PDA-Pd@[Cu (btc)
nanocomposites with assembly cycles of 5, 10 and 15,
3 2
(btc) ] shell plays an extremely crucial
3
O
4
3
2
k
O
3 4
3
2
]
Fig.
Fe
photograph of magnetic separation and redispersion of the Fe
Pd@[Cu (btc) ] (n=5) in water).
6
Room temperature magnetization curves of (a) Fe
@PDA-Pd and (c) Fe @PDA-Pd@[Cu (btc) ] (n=5) (inset image: the
@PDA-
3 4
O , (b)
3
O
4
3
O
4
3
2
3
O
4
3
2
Following the successful preparation of Fe
3 4
O @PDA-
Pd@[Cu (btc) ] catalyst, its practicability was studied by the
3
2
Fig. 7 (a) Time-dependent UV-vis spectra of the 4-NP and NaBH
the presence of Fe @PDA-Pd@[Cu (btc) (n=5), (b) plot of In(C
versus reaction time for the reduction of 4-NP using 25 µL (1mg/mL) of
Fe @PDA@[Cu (btc) ], Fe @PDA-Pd and Fe @PDA-Pd@[Cu (btc)
n=5, 10 and 15), where C and C are the concentration of 4-NP at time t and
, respectively.
4
mixture in
reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) with
3
O
4
3
2
]
t
/C
o
)
o
NaBH
4
at 20 C. The UV-vis spectra indicate that the reduction
3
O
4
3
2
3
O
4
3
O
4
3
2
]
does not occur in the absence of the catalyst even for 10h (Fig.
S2). However, the addition of Fe @PDA-Pd@[Cu (btc)
(
t
o
3
O
4
3
2
]
0
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ChemCatChem
10.1002/cctc.201701693
FULL PAPER
Table 1 Comparison of the apparent rate constant (kapp) of 4-NP reduction by
Table 3 Suzuki-Miyaura reactions between aryl halides and arylboronic acids
Fe
3
O
4
@PDA, Fe
O
3 4
@PDA-Pd, Fe
3
O
4
@PDA@[Cu
3
(btc)
2
] and Fe
3
O
4
@PDA-
] (n=5)[a]
catalyzed by Fe
3
O
4
@PDA-Pd@[Cu
(btc)
3 2
Pd@[Cu
3
(btc)
2
] (n=5,10 and 15)
Yield [b]
(%)
[c]
TOF
Entry
1
Aryl halide
Arylboronic acid
-1
(h )
-
1
Entry
Catalyst
Time(min)
20
Kapp(min )
97
82
32333
1
2
3
4
5
6
Fe
Fe
@PDA@[Cu
3
O
4
@PDA
@PDA-Pd
(btc)
0
3
O
4
12.5
0.41
0.32
1a
23429
-
Fe
3
O
4
3
2
] (n=5)
18
1b
Trace
93
Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
] (n=5)
8
0.72
0.48
0.42
2
31000
32000
Fe
Fe
3
O
O
4
@PDA-Pd@[Cu
@PDA-Pd@[Cu
3
3
(btc)
(btc)
2
2
] (n=10)
] (n=15)
11.5
12
3
4
96
3
4
95
93
31667
31000
respectively (entries 4,5,6). These results reveal that over-thick
MOF shells could decrease the synergistic catalytic activity of
Fe
longer diffusion distance and thus inferior mass transfer.[9,17]
The catalytic performances of the Fe @PDA-
Pd@[Cu (btc) ] (n=5) nanocomposites were also explored by
the Suzuki coupling reaction. In model reaction of
3
4
O @PDA-Pd and MOF, which is mainly ascribed to the
5
3
O
4
6
7
94
98
31333
32667
32667
32667
2583
3
2
a
bromobenzene and phenylboronic acid, different reaction
parameters including base, the solvent, and the amount of
8
98
catalyst were optimized (Table 2). For the bases, K
the most appropriate for the Suzuki reactions among other
bases including NaOH, Et N and n-Bu N (entries 1-4). Next, the
effect of different solvents on the model reaction was examined
using K CO as a base (entries 4-6). EtOH/H O (v/v=1:1) was
2 3
CO was
9
98
3
3
10
93 [d]
2
3
2
the best solvent for the reaction with high yield of 97%. Finally,
the effect of catalyst amount was also explored (entries 4,7,8),
and the best result was acquired by using 0.006 mol% catalyst.
11
12
89 [d]
2472
2305
83 [d]
Table 2 Optimization of the Suzuki-Miyaura cross-coupling of bromobenzene
[a] Reaction conditions: aryl halides (4.0 mmol), arylboronic acids (6.0 mmol),
with phenylboronic acid using Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
] (n=5) catalyst [a]
K CO (12.0 mmol), solvent (8 mL, EtOH:H O=1:1), catalyst (1.0 mg), at 75
2
3
2
Yield [c]
o
3 4
C for 30 min. The catalyst for entry 1a: Fe O @PDA-Pd (the content of Pd in
Entry
Base
Et
n-Bu
NaOH
Solvent
Catalyst [b] (mol%)
(%)
the Fe
3
O
4
@PDA-Pd was 3.01 wt%), 1b: Fe
3 4 3 2
O @PDA@[Cu (btc) ] (n=5),
others: Fe
3
O
4
@PDA-Pd@[Cu (btc) ] (n=5). [b] Isolated yield. [c] TOF: [mol
3
2
1
2
3
4
5
3
N
H
H
H
H
2
2
O:EtOH=1:1
O:EtOH=1:1
O:EtOH=1:1
O:EtOH=1:1
0.006
23
Trace
79
product] [mol Pd]-1 h-1. [d] at 75 oC for 6 h.
3
N
0.006
3 4
activity (entry 1b), demonstrating that combining Fe O @PDA-
Pd with porous MOF could synergistically contribute to the
improvement of catalytic activity for Suzuki reaction.
2
2
0.006
The expansion of Fe
3 4
O @PDA-Pd@[Cu
3
(btc)
2
]
(n=5)
K
2
2
CO
CO
3
3
0.006
97
catalyst in Suzuki reactions of various aryl halides with
arylboronic acids was also explored, and the results were
showed in Table 3. The reactions of substituted
K
H
2
O
0.006
23
6
7
8
K
2
K
2
K
2
CO
CO
CO
3
3
3
EtOH
0.006
0.003
0.012
85
82
97
bromobenzenes containing electron-donating group (-OH, -CH
and electron-withdrawing group (-NO2, -COCH were
proceeded extremely well and afforded satisfactory yields (93-
8%) in 30 min (entries 2-7). Even for the meta- and ortho-
3
)
3
)
H
H
2
2
O:EtOH=1:1
O:EtOH=1:1
9
substituted aryl halides, the catalysts performed efficiently to
give the coupling products in yields of 95% and 93% (entries 4-
[
a] Reaction conditions: bromobenzene (4.0 mmol), phenylboronic acid (6.0
mmol), base (12.0 mmol), solvent (8.0 mL), at 75 oC for 30 min. [b] Catalyst
5) respectively. Furthermore, different arylboronic acids like 4-
(
based on bromobenzene). [c] Isolated yield.
methylphenylboronic acid and 4-chlorophenylboronic acid were
also coupled with 4-bromoacetophenones efficiently and
satisfactory yields of 98% were obtained (entries 8-9).
Encouraged by the outstanding results of the Suzuki reactions
of aryl bromides with phenylboronic acids, the availability of this
catalyst to the challenging substrates of aryl chlorides was
investigated. To our delight, the reactions of 4-
The effect of different nanocomposites on catalytic activity
of the Suzuki coupling reaction between bromobenzene and
phenylboronic acid was also investigated under the optimal
reaction parameters. As illustrated in Table 3, the Fe
3 4
O @PDA-
Pd@[Cu (btc) ] (n=5) possesses superior catalytic performance
3
2
as
compared
to
Fe
3
4
O @PDA-Pd
(entries
1,1a),
Fe
3
4
O @PDA@[Cu
3
(btc)
2
] (n=5) displays negligible catalytic
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ChemCatChem
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Table 4 Performance of different hierarchical core-shell Pd-based catalysts for the reduction of 4-NP
Entry
catalyst
c(4-NP)[a] (mM)
Mcat [b] (mg)
Kapp [c] (min-1)
Knor [d] (min-1 g-1)
Ref
1
2
3
4
Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
](n=5)
3.306
0.144
0.058
0.074
0.025
0.02
0.1
0.72
0.122
0.096
0.65
28800
6100
960
This work
[42]
Fe
Fe
@CeO
Fe
h-Pd-CeO
Pd@TiO /Pd@TiO
cat ： the mass of catalyst. [c]
3 4
O /Pd@PPy
x
O
y
/Pd@mSiO
2
[43]
2
/Pd@Pd/TiO
2
2
325
[44]
5
6
7
3
O
4
/Pd@NC
0.029
0.074
0.074
0.058
1.5
2
0.33
2.36
5690
1573
154
[45]
[46]
[47]
2
2
2
0.307
[
a] c: concentration. [b]
m
Kapp: apparent rate constant. [d] Knor: normalized rate constant. Data were given or calculated in the
respective papers.
Table 5 The performance of different hierarchical core-shell Pd-based catalysts for the Suzuki-Miyaura reaction of bromobenzene with phenylboronic acid
-
1
Entry
Pd catalyst (mol%)
@PDA-Pd@[Cu (btc) ](n=5) (0.006)
Fe @C-Pd@mCeO (0.286)
@Pd/meso-TiO /Pd@meso-SiO
Pd@TiO
NC/Pd@mSiO
Pd/Fe @PMAA-Met (0.01)
Pd/Fe @SiO @KCC-1(0.2)
Solvent/temperature/time
O/75 oC/30min
O/80 oC/3h
Yield (%)
TOF (h )
32333
116.3
Ref
This work
[10]
1
2
3
4
Fe
3
O
4
3
2
EtOH/H
2
97
95
99
72
3
O
4
2
EtOH/H
2
2
2
(0.025)
EtOH/80 oC/60 min
3961
[38]
EtOH/80 oC/60min
1600
[47]
2
/Pd@TiO
2
(0.045)
o
5
6
7
2
(0.432)
EtOH/H
EtOH/H
2
O/80 C/5min
99
96
2748
19200
76
[48]
[49]
[50]
o
3
O
4
2
O/70 C/30 min
3
O
4
2
EtOH/80 oC/6h
90.8
nitrochlorobenzene with arylboronic acids proceeded well,
giving the desired products in 83%-93% yields (entries 10-12).
only enhance dispersibility of catalyst in reaction solvents of
water and ethanol,[39] but also make Pd active sites readily
contactable with the reactants due to its excellent affinity toward
To further assess the catalytic efficiency of Fe
3 4
O @PDA-
organic
molecules.[40]
Therefore,
the
integration
of
Pd@[Cu (btc) ] (n=5), the comparisons were performed with
3
2
different hierarchical core-shell Pd-based catalysts for the
reduction of 4-NP and Suzuki coupling reaction of
bromobenzene with phenylboronic acid. For the reduction of 4-
NP, the normalized rate constant knor (knor=kapp/mcat, where mcat
is the mass of catalyst) was introduced to evaluate the catalytic
activity of different catalysts.[37] As shown in Table 4,
Fe @PDA-Pd with [Cu
3
O
4
3
(btc) that combines excellent
2
]
features of each component could achieve synergistically
boosted catalytic performances. [1,41]
Except the excellent catalytic activity, the recyclability of
the supported catalyst is an important factor for any practical
application. Both the 4-NP reduction and Suzuki coupling
reaction of 4-bromoacetophenone with phenylboronic acid were
Fe
catalyst, with the highest Knor of 28800 min g . For the Suzuki
reaction, Fe @PDA-Pd@[Cu (btc) ] (n=5) catalyst is superior
to others in terms of its high TOF value (32333 h ) (Table 5).
The outstanding catalytic activity of Fe @PDA-
Pd@[Cu (btc) ] (n=5) may be mainly attributed to its unique
structure features.[35b,38] Firstly, the outer shell of [Cu
(btc) ],
3 4 3 2
O @PDA-Pd@[Cu (btc) ] (n=5) is the most efficacious
-1 -1
employed to explore the recyclability of Fe
Pd@[Cu (btc) ] (n=5) catalyst. As shown in Fig. 8, the catalyst
could be used at least 8 times with excellent catalytic activity for
both the reactions. The loss of Pd in Fe @PDA-
Pd@[Cu (btc) ] (n=5) after 8 successive runs was determined
3 4
O @PDA-
3
O
4
3
2
3
2
-1
3
O
4
3 4
O
3
2
3
2
3
2
by AAS, which gave the value of 0.15 wt% in the reduction of 4-
NP and 0.26 wt% in the Suzuki reaction respectively, indicating
that the magnetic hierarchical core-shell structured
possessing porous structure and plenty of Cu2+ and organic
units, can not only provide specific adsorption for the
enrichment of substrates from reaction liquid, but also possess
catalytic activity for certain reactions.[24,35a] Secondly, the
Fe
excellent stability. In contrast, the catalytic activity of
Fe @PDA-Pd obviously decreases after 8 successive runs of
catalytic Suzuki reaction (Fig. S5), and the loss of Pd in
Fe @PDA-Pd was 0.83 wt%. This finding clarified the
3
4
O @PDA-Pd@[Cu
3
(btc)
2
]
(n=5)
catalyst
possesses
[
3 2
Cu (btc) ] shell thickness can be well controlled and optimized,
3 4
O
which is favorable for the fast diffusion of reactants and thus
superior mass transfer, rendering the reactants readily
contactable with catalytically active sites. Finally, PDA layer
with abundant catechol and amine functional groups can not
3
O
4
3 2
reliable role of the [Cu (btc) ] shell as a barrier to prevent the
leaching of Pd NPs.
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ChemCatChem
10.1002/cctc.201701693
FULL PAPER
(
hydroxymethyl) aminomethane (Tris,
benzenetricarboxylic acid (H btc, ≥99%) were obtained from Beijing J &
K Scientific Company. Copper (II) acetate monohydrate was provided by
Tianjin fengyue Chemical Company. Palladium chloride (PdCl , ≥ 59.5%)
was supplied by Shenyang Keda Reagents Company. Sodium
hydroxide (NaBH ) was purchased from Sinopharm Chemical Reagent
≥
99.5%), and 1,3,5-
3
2
4
Co. Ltd. Other reagents used were of analytical grade and did not
require further purification. The water used was purified using a Millipore
system.
Characterization
Transmission electron microscopy (TEM) measurements were
performed on a Tecnai-G2-F30 (FEI, USA) at an accelerating voltage of
3
00 kV. Fourier transform infrared (FT-IR) spectra were obtained by a
NEXUS670 (Nicolet, USA) spectrometer over KBr disks. X-ray
diffraction (XRD) was performed on Shimadzu XRD-6000
diffractometer using Cu Kα radiation (λ=1.5418 Å). X-ray photoelectron
spectroscopy (XPS) was recorded on Thermon Scientific
ESCALAB250xi electron spectrometer with the C1s=284.8 eV as
binding energy standardization. The adsorption-desorption
a
a
N
2
experiments were carried out at 77 K, and the specific surface area was
determined using the Brunauer-Emmett-Teller (BET) method, the pore
size distributions were calculated using the Barrett-Joyner-Halenda
(
BJH) model. The magnetic properties of samples were measured with
3 4 3 2
Fig. 8 Recyclability of Fe O @PDA-Pd@[Cu (btc) ] (n=5) catalysts for the
a vibrating sample magnetometer (VSM) (Lake Shore 7304, USA). The
quantity of Pd in catalyst was obtained by Atomic absorption
spectrometer (AAS) with an AA240 (Varian Corporation, USA).
Ultraviolet-visible (UV-vis) spectra were recorded with a TU-1810PC
UV-Vis spectrophotometer. H NMR (400 MHz) and 13C NMR (100 MHz)
spectra were obtained using an AVANCE III 400 NMR spectrometer
reduction of 4-NP and Suzuki coupling reaction of 4-bromoacetophenone
with phenylboronic acid.
1
Conclusions
(
Bruker, Germany) with CDCl
3 6
/DMSO-d as solvents and TMS as the
In conclusion, we presented a facile and versatile strategy to
prepare a novel magnetic hierarchical core-shell structured
internal reference.
Synthesis of Fe
Fe
NPs were synthesized by a solvothermal method.[51] 2.70 g of
FeCl ·6H O was dissolved in 50 mL of ethylene glycol under vigorous
3 4
O NPs
Fe
sandwiched between the PDA modified Fe
structure and a porous outer shell of [Cu (btc)
3 4 3 2
O @PDA-Pd@[Cu (btc) ] nanocomposites with Pd NPs
3
O
4
3
O
4
core-shell
3
2
3
2
]. Besides
stirring at room temperature. After complete dissolution, 5.75 g of NaAc
possessing the common functions of magnetic hierarchical
core-shell structured nanocomposites like excellent magnetic
separation and the alleviation of Pd NPs leaching, the
was added and the solution was stirred for another 30 min. Then the
mixture was transferred to the autoclave and heated at 200 oC for 8 h,
3 4
and cooled to room temperature. The Fe O NPs were collected by an
introduction
of
PDA
and
[Cu
3
(btc)
2
]
endows
the
external magnet, washed three times with ethanol and deionized water,
and dried in a vacuum oven overnight.
nanocomposites with new features, such as good dispersibility
in water and ethanol, high porosity and efficient adsorbability
toward reactants. More importantly, there is a strong synergistic
Synthesis of Fe
3 4
O @PDA nanocomposites
0
.04 g Fe NPs were fully dispersed in 160 mL of 10 mM tris-HCl
3 4
O
effect between Fe
systems. The
3
O
4
@PDA-Pd and [Cu
3
(btc)
2
] in our catalytic
buffer (pH=8.5) by ultrasonication for 20 min. Subsequently, 0.16 g of
dopamine hydrochloride was dissolved into the aforementioned
suspension with continuous mechanical stirring at room temperature for
Fe @PDA-Pd@[Cu
3
O
4
3
(btc) (n=5)
2
]
nanocomposites with such unique features exhibit superior
catalytic activity for the reduction of 4-nitrophenol (a Knor of
3 4
24 h. The Fe O @PDA nanocomposites were separated and collected
-1 -1
2
8800 min g ) and the Suzuki cross coupling reactions (a TOF
with a magnet, followed by washing with deionized water three times,
and finally dried under vacuum overnight.
-1
reached up to 32667 h ). Meanwhile the nanocomposite could
be conveniently separated by magnet and reused at least 8
runs with high yields for both the reactions. Given its facile
preparation strategy and outstanding performance, it would be
Synthesis of Fe
.08 g of Fe @PDA was dispersed in 76.80 mL of PdCl
mM), and the solution was stirred at room temperature for 6 h. Then
Fe @PDA-Pd nanocomposites were harvested by an external
3 4
O @PDA-Pd nanocomposites
0
3
O
4
2
solution (0.5
of considerable interest to extend the Fe
3 4
O @PDA-
3 4
O
Pd@[Cu (btc) ] catalyst to other magnetic hierarchical core-
3
2
magnet, washed with deionized water for three times and dried under
vacuum overnight.
shell structured catalysts for various reactions, by replacing the
metal NPs and/or the MOF shell.[4,18]
Synthesis of Fe
A versatile layer-by-layer assembly strategy was utilized to fabricate the
Fe @PDA-Pd@[Cu (btc) nanocomposites. Briefly, The as-
synthesized Fe @PDA-Pd nanocomposites were added into 16 mL of
Cu(CH COO) ·H O ethanol solution (10 mM) and stirred for 15 min at
0 °C. The obtained nanocomposites were separated by magnetic field
and washed with ethanol. Subsequently, the nanocomposites were
dispersed in 16 mL of H btc ethanol solution (10 mM) under stirring for
0 min at 70 °C, collected by a magnet and cleaned with ethanol. After
3 4 3 2
O @PDA-Pd@[Cu (btc) ] nanocomposites
O
3 4
3
2
]
Experimental Section
3
O
4
3
2
2
Materials
7
Ferric chloride hexahydrate (FeCl
99%), 4-nitrophenol (4-NP), and ethylene glycol (EG) were purchased
from Tianjin Guangfu Fine Chemicals. Dopamine hydrochloride, tris
3 2
·6H O), sodium acetate (NaAc,
≥
3
3
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ChemCatChem
10.1002/cctc.201701693
FULL PAPER
a given number of cycles, a series of Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
]
[4]
[5]
[6]
[7]
Q. Wang, W. J. Jia, B. C. Liu, A. Dong, X. Gong, C. Y. Li, P. Jing, Y.
J. Li, G. R. Xu, J. Zhang, J. Mater. Chem. A 2013, 1, 12732.
J. Han, S. Lu, C. J. Jin, M. G. Wang, R. Guo, J. Mater. Chem. A
2014, 2, 13016-13023.
nanocomposites with different MOF shell thickness, which can be
denoted as Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
] (n=the number of assembly
o
cycles), were synthesized and dried under vacuum at 120 C for 6h.
Then, Fe
3
O
4
@PDA@[Cu
3
(btc)
2
] nanocomposites were synthesized
J. P. Ge, Q. Zhang, T. R. Zhang, Y. D. Yin, Angew. Chem., Int. Ed.
2008, 47, 8924-8928.
by the same synthetic procedure of the case of Fe
3
O
4
@PDA-
Pd@[Cu (btc)
3
2
] nanocomposites while using the Fe
3
O
4
@PDA instead of
Z. K. Sun, J. P. Yang, J. X. Wang, W. Li, S. Kaliaguine, X. F. Hou,
Y. H Deng, D. Y. Zhao, J. Mater. Chem. A 2014, 2, 6071-6074.
X. L. Liang, J. Li, J. B. Joo, A. Gutiérrez, A. Tillekaratne, I. Lee, Y.
D. Yin and F. Zaera, Angew. Chem., Int. Ed. 2012, 51, 8034-8036.
Y. H. Deng, Y. Cai, Z. K. Sun, J. Liu, C. Liu, J. Wei, W. Li, C. Liu, Y.
Wang, D. Y. Zhao, J. Am. Chem. Soc. 2010, 132, 8466-8473.
the Fe O @PDA-Pd.
3
4
[8]
General procedure for the reduction of 4-NP
Typically, 1.0 mL 4-NP solution (10.0 mM), 1.0 mL H
aqueous NaBH
oC, then 25 µL of Fe
2
O, and 1.0 mL of
[9]
4
solution (3.0 M) were added in a reaction vessel under
@PDA-Pd@[Cu (btc) (n=5)
20
3
O
4
3
2
]
[10] Y. L. Li, Z. Q. Zhang, J. F. Shen, M. X. Ye, Dalton Trans. 2015, 44,
16592-16601.
nanocomposites (1mg/mL) were added in above mixture. The color of
the mixture gradually changed from bright yellow to colorless. The
progress of reaction was recorded by UV-vis spectroscopy by taking out
a portion of 100 µL mixture solution and quickly diluted to 2 mL in a
certain period of time.[52]
[11] X. L. Fang, Z. H. Liu, M. F. Hsieh, M. Chen, P. X. Liu, C. Chen, N.
F. Zheng, ACS nano 2012, 6, 4434-4444.
[12] G. Z. Chen, F. Rosei and D. L. Ma, Nanoscale, 2015, 7, 5578-5591.
[13] S. H. Wang, Y. N. Fan, J. Teng, Y. Z. Fan, J. J. Jiang, H. P. Wang,
H. Grützmacher, D. W. Wang, C. Y. Su, Small 2016, 12, 5702-
5709.
In order to avoid the error caused by loss of the catalyst solid, we
amplify 20 times of the reaction system to investigate the reusability of
Fe
3
O
4
@PDA-Pd@[Cu
3
(btc)
2
] (n=5). After the reaction was complete, the
[14] L. C. He, Y. Liu, J. Z. Liu, Y. S. Xiong, J. Z. Zheng, Y. L. Liu, Z. Y.
Tang, Angew. Chem., Int. Ed. 2013, 52, 3741-3745.
[15] Y. Y. Liu, W. N. Zhang, S. Z. Li, C. L. Cui, J. Wu, H. Y. Chen, F. W.
Huo, Chem. Mater. 2014, 26, 1119-1125.
catalyst was harvested by an external magnet and washed with
deionized water for three times, then dried under vacuum at 120 oC for
6
h and used in the next run. The reaction process was performed within
min for each cycle.
8
[16] T. Zeng, X. L. Zhang, S. H. Wang, H. Y. Niu, Y. Q. Cai, Environ.
Sci. Technol. 2015, 49, 2350-2357.
General procedure for the Suzuki coupling reaction
Fe @PDA-Pd@[Cu (btc) ] (n=5) catalyst (0.006 mol% Pd based on
aryl halides), K CO (12.0 mmol), phenylboronic acid (6.0 mmol), Aryl
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5 mL reaction vessel. The reaction mixture was stirred at 75 oC for the
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ChemCatChem
10.1002/cctc.201701693
FULL PAPER
FULL PAPER
Rong Ma, Pengbo Yang, Yao Ma, Fengling
Bian*
Page No. – Page No.
Facile synthesis of magnetic hierarchical
core-shell
Pd@MOF
structured
Fe
3
O
4
@PDA-
a novel magnetic hierarchical core-shell structured Fe
nanocomposite were successfully fabricated by
3
O
4
@PDA-Pd@[Cu
3 2
(btc) ]
nanocomposites:
highly
a
facile method. The
integrated multifunctional catalysts
magnetically reusable catalysts show ultrahigh catalytic activity in the reduction
of 4-nitrophenol and Suzuki-Miyaura reactions with excellent reusability. The
3 4 3 2
synergistic effect between Fe O @PDA-Pd and [Cu (btc) ] was observed in this
catalytic systems.
This article is protected by copyright. All rights reserved.