Preparation of bimetallic metal-organic framework microflowers by spray method

Article abstract of DOI:10.1246/bcsj.20180237

The NiCo-MOF microflowers are fabricated by a rapid spray method, which are assembled by 2D NiCo-MOF nanosheets with uniform crystal morphology and homogeneous dispersion of Ni and Co. Because of their large exposed active sites and nanoscale thickness, the NiCo-MOF microflowers exhibit good catalytic performance for the reduction of 4-nitrophenol.

Full text of DOI:10.1246/bcsj.20180237

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Preparation of Bimetallic Metal-Organic Framework Microflowers by Spray Method
Linlin Li, Liying Zhang,* Xiaojue Bai, Mingjian Xia, Lei Shao, Tieqiang Wang,
Xuemin Zhang, Yunong Li, Xuan Qi, and Yu Fu*
Advance Publication on the web November 3, 2018
doi:10.1246/bcsj.20180237
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2018 The Chemical Society of Japan
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Publication manuscripts.

Preparation of Bimetallic Metal-Organic Framework Microflowers by Spray
Method
Linlin Li, Liying Zhang*, Xiaojue Bai, Mingjian Xia, Lei Shao, Tieqiang Wang, Xuemin Zhang, Yunong Li, Xuan Qi
and Yu Fu*1
1College of Science,Northeastern University, Shenyang, 110819, P. R. China,
E-mail: fuyu@mail.neu.edu.cn, zhangliying@mail.neu.edu.cn
Linlin Li and Liying Zhang contributed equally to this work.
Yu Fu
Liying Zhang
Abstract
The NiCo-MOF microflowers are fabricated by a rapid
catalysis[2] and analytical science[3]. The microflower structure
is one of the typically hierarchical nanostructures which is
formed by the integration of nanobuilding blocks with the
spray method, which are assembled by 2D NiCo-MOF
nanosheets with uniform crystal morphology and homogeneous
dispersion of Ni and Co. Because of their large exposed active
sites and nanoscale thickness, the NiCo-MOF microflowers
exhibit good catalytic performance for the reduction of
[
4]
combination of appropriate order
.
At present, the
nanobuilding blocks of microflower materials are mainly
metal-organic frameworks (MOFs), metal oxides and inorganic
[
5]
salts . Among them, MOFs have attracted more and more
attentions, ascribing to their diverse chemical composition,
coordination structure and porous architectures. MOFs are a
class of organic-inorganic hybrid porous materials, which are
formed by metal ions and organic ligands through coordination
4
-nitrophenol.
Keywords: bimetallic metal-organic framework, spray
method, microflower
[
6,7]
bonds . Moreover, the MOF microflowers provide a feasible
means to combine the merits of both 2D and 3D materials with
2D MOF nanosheets as building blocks. Such kinds of
structures not only offer a stable framework for immobilizing
individual 2D nanosheets, but also inherit all the advantageous
1
. Introduction
Owing to large exposed surface area and convenient
charge transfer, the microflower materials are highly desirable
[
1]
for their good applied prospects in the fields of sensing
,
Scheme 1. The schematic illustration of the fabrication process of NiCo-MOF microflowers by spray method.

2
D MOF nanosheets as building blocks. Such kinds of
macroflowers were fabricated by spray method under mild
conditions. Bimetallic salts and H BDC were respectively
dissolved in ternary mixed solvent of
N,N-dimethylformamide (DMF), ethanol and water
(V:V:V=16:1:1), the concentrations of Ni(OAc) ·4H O,
2 2 2
Co(OAc) ·4H O and H BDC were 0.028 mol·L , 0.056
mol·L , 0.083 mol·L , respectively. The NiCo-MOF
microflowers were obtained by sparying 0.6 mL bimetallic
2
solutions onto the 4.5 mL H BDC solution in a glass culture
structures not only offer a stable framework for immobilizing
individual 2D nanosheets, but also inherit all the advantageous
features of 2D materials, such as high surface-to-volume ratios,
highly exposed active sites and excellent charge transport
capability . For example, Yuan, Chen and co-workers reported
the flower-like ferrocene confined MOFs show high sensitivity
in an electrochemical immunsensor for amyloid-β detection,
which is fabricated by solvothermal method . Zou, Zhao and
co-workers reported the hybrid flower-like boron nitride
nanosheets/MOFs show an enhanced catalytic activity in the
acetalization of benzaldehyde owing to facilitated diffusion
2
a
2
2
[
8]
-1
-
1
-1
[
9]
dish at the flow rate of 36 μL/min. The purple precipitate was
washed with DMF and ethanol for three times, then heated
o
under vacuum at 120 C for 10 h.
[
10]
process in the hierarchical microflower structure
Benefiting from the synergistic effects of compsite
materials, bimetallic MOFs with uniform dispersion of
.
The NiCo-MOF bulk crystals were fabricated according
to literature[15]. The bimetallic salts and H
BDC dissolved in a
2
ternary mixed solvent of DMF, ethanol and water
(V:V:V=8:1:1), respectively. The two solutions were mixed and
[
11]
bimetallic active sites have gained considerable attention
.
o
Two species of metal are incorpotated during the crystallization
process to synthesize a homogeneous bimetallic MOF, which
provides excellent synergistic effects due to the uniform
distribution of bimetallic nodes in integral MOF structures.
According to the research of Wang's group, the NiCo-MOF
catalyst showed a much better catalytic hydrogenation of
furfuryl alcohol to tetrahydrofurfuryl alcohol comparing with
reacted at 140 C for 48 h. After being washed with DMF and
ethanol for three times, the products were heated under vacuum
o
at 120 C for 10 h.
Catalytic Test. The catalytic performance of NiCo-MOFs
in the reduction 4-NP was tested. The NaBH aqueous solution
4
(2.5 mL, 0.1 M), 4-NP (20 μL, 5 mM) and NiCo-MOFs
aqueous solution (20 μL, 0.125 mg/mL) were mixed. The
reaction progress was monitored by UV-Vis spectrometry.
[
12]
individual Ni or Co MOF-derived catalysts . However, it is
still great challenge to construct bimetallic MOF
a
microflowers by a facile method.
3. Results and Discussion
In this paper, a rapid and feasible spray method is
presented to fabricate bimetallic MOF (NiCo-MOF)
The morphology of NiCo-MOF microflowers prepared by
spray method is characterized by scanning electron microscopy
(SEM). The SEM image shows the morphology of NiCo-MOF
is uniform flower-like microspheres (Figure 1a). 2D nanosheets
are observed in NiCo-MOF microflowers, which are staggered
and superimposed to form flower-like structure (Figure 1b).
Moreover, the thickness of NiCo-MOF nanosheets is 50-80 nm
and the length is a few microns (Figure 1c), which is identical
to the typical 2D structures. The average diameter of
NiCo-MOF microflowers is 5.6 ± 0.7 μm by measuring one
hundred of individual microflowers, and the low relative
standard deviation of 12.4% suggests the good monodispersity
of NiCo-MOF microflowers (Figure 1d). The SEM image and
its corrsponding energy-dispersive X-ray (EDX) elemental
mapping images of microflowers (Figure 1e and 1f, Figure S1b,
S1c and S1d) show that the ratio of Co/Ni in NiCo-MOF
microflowers is approximately 1:1, and Ni and Co elements are
[
13,14]
microflowers under mild conditions
. The fabrication
process is illustrated in Scheme 1, metal ion solution of two
2
+
2+
kinds of metal ions (Ni and Co ) are atomized to small
droplets, then uniformly sprayed on surface of the organic
ligand (1,4-benzenedicarboxylic acid,
2
H BDC) solution.
Bimetallic metal ions and the organic ligand were formed
crystal nucleus, with the process of anisotropic growth led to
the morphology of 2D MOF nanosheets and ultima flower-like
structure. Owing to their large exposed surface areas with
bimetallic active sites, the NiCo-MOF microflowers showed
good catalytic performance in the reduction of 4-nitrophenol
(
4-NP).
2
. Experimental
Materials. Cobalt acetate tetrahydrate [Co(OAc)
nickel acetate tetrahydrate [Ni(OAc)
,4-benzenedicarboxylic acid (H
2
·4H
·4H
BDC),
2
O],
O],
2
2
uniformly
dispersed
throughout
entire
NiCo-MOF
1
2
microflowers.
N,N-dimethylformamide (DMF) and ethanol were purchased
from Sinopharm Chemical Reagent Co. Ltd. Sodium
The coordination interaction in NiCo-MOF is confirmed
by Fourier transform infrared spectroscopy (FT-IR) (Figure 2a).
-
1
borohydride (NaBH
4
)
and 4-nitrophenol (4-NP) were
The characteristic peak of 1508 cm ascribes to the vibration
absorption of benzene ring in organic ligand H BDC. There are
purchased from Alfa Aesar. Pure water was purchased from
Shenyang Wahaha Group Co. Ltd. All reagents and solvents
were used without further purification.
2
-
1
two characteristic absorption peaks at 1388 and 1588 cm ,
which are corresponded to the symmetrical stretching vibration
and anti-symmetrical stretching vibration absorption peak of
carboxylate groups, respectively. The characteristic absorption
Characterization. The scanning electron microscopy
(SEM) images and the energy-dispersive X-ray (EDX)
elemental mapping images were obtained by Hitachi SU8010.
X-ray diffraction (XRD) patterns were recorded by Shimadzu
XRD-6000 with Cu Kα1 (λ = 1.54 Å) radiation. Fourier
transform infrared spectra (FT-IR) were recorded by a Bruker
VERTEX 70. Thermogravimetric analysis (TGA) was carried
out with a thermobalance (TGA-2050, TA Instruments) under
peaks C=O and O-H of carboxyl groups in H
2
BDC are not
-
1
found at 1710 and 1245 cm , which indicate H
2
2+
BDC has been
2
+
deprotonated and coordinates with Ni and Co successfully.
All these indicate the existing of coordination bonds between
metal ions and BDC. The thermal stability of NiCo-MOF
microflowers is confirmed by thermogravimetric analysis
N
2
from room temperature to 800 ℃ with a temperature ramp
of 10 ℃/min. The ultrasonic generator (DP30) and the nozzle
ZPQ-S-95) were manufactured by Siansonic Technology Ltd.
(TGA), which is performed under a N
2
atmosphere with
heating rate of 10 ℃·min from ambient temperature up to
800 (Figure 2b). The TGA curve of NiCo-MOF
-
1
(
℃
The UV-Vis spectra were obtained by Maya2000 pro optic
spectrometer.
microflowers shows a distinct weight loss at 410 ℃, ascribing
to the decomposition of NiCo-MOF, which indicates good
thermal stability of NiCo-MOF microflowers.
Preparation
of
NiCo-MOF.
The
NiCo-MOF

Figure 1. (a-c) Different magnifications SEM images of NiCo-MOF microflowers. (d) The histogram of NiCo-MOF microflowers
diameter distribution. (e) SEM image of NiCo-MOF microflower and (f) its corresponding EDX elemental mapping images of Co, Ni,
O and C element distribution.
The crystalline structures of MOFs are also investigated
by X-ray diffraction (XRD). The XRD pattern (Figure 2c) has
Most aromatic nitro compounds are harmful to the
environment, but its hydrogenated aromatic amino compounds
are important industrial intermediates. Therefore, it is of great
practical significance to develop an effective and cheap catalyst
for the reduction of aromatic nitro compounds. NiCo-MOFs
could be used as catalysts for the reduction of 4-NP to produce
its hydrogenated compound 4-aminophenol (4-AP). The
UV-Vis absorption peak of the nitro group in 4-NP is at 317 nm.
four peaks at 9.08°, 14.30°, 15.82° and 17.98° corresponding to
—
the [200], [001], [201] and [201] crystallographic planes of the
NiCo-MOF structure. None of the additional Bragg diffractions
of the bulk crystals are detected due to the special 2D
nanosheet structures of NiCo-MOF. Moreover, the XRD pattern
of NiCo-MOF microflowers is similar to the 2D NiCo-MOF
[
15]
nanosheets as reported in Tang's work , which further proves
that the NiCo-MOF microflowers are consisted of 2D
4
After adding sodium borohydride (NaBH ) solution, the
absorption peak is red shifted to 400 nm due to the
[
15]
[16]
NiCo-MOF nanosheets. According to Tang's work , 2D
bimetallic layers separated by BDC molecules are formed by
both the Co and Ni atoms octahedrally coordinated by six O
atoms and these pseudo octahedra are further edge/corner
connected with each other along the [010]/[001] direction in the
deprotonation of 4-NP (Figure S2a) . In the absence of
NiCo-MOFs, the absorption peak of deprotonated 4-NP is
almost unchanged after 10 h (Figure S2b). When NiCo-MOFs
are added to the mixed solution, the absorption peak intensity
of the solution at 400 nm decreases with the increase of
reaction time. Simultaneously, the absorption peak of the amino
group appears near 300 nm. The reduction of 4-NP is
completed after 5 min with the addition of NiCo-MOF
microflowers (Figure 3a). For a better comparation, the
NiCo-MOF bulk crystals are also synthesized to catalyze the
reduction of 4-NP, whose structures are comfirmed by SEM
and XRD (Figure S3 and Figure S4). The reduction of 4-NP
takes longer for 11 min with the same amount of NiCo-MOF
bulk crystals (Figure 3c). Moreover, the rate constant (k) of the
(200) crystallographic plane (Figure 2d).
-
1
NiCo-MOF microflowers is 0.663 min and the turnover
-
1
frequency (TOF) value is 137.9 h , while the k of NiCo-MOF
-
1
-1
bulk crystals is 0.252 min and the TOF value is 67.5 h ,
which is calculated according to Figure 3b and Figure 3d. The
0 t
linear relationships between ln(c /c ) and reaction time t are
shown in Figure 3b and Figure 3d, while the rate constant (k) is
the fitting line slope. Comparing with the NiCo-MOF bulk
crystals, the NiCo-MOF microflowers are composed of 2D
nanosheets, which provides more exposed surface areas with
active metal sites. So the k and TOF of NiCo-MOF
microflowers are two times higher than NiCo-MOF bulk
Figure 2. (a) FT-IR spectrum of NiCo-MOF microflowers. (b)
The thermogravimetric profile of NiCo-MOF microflowers. (c)
XRD pattern of NiCo-MOF microflowers and the simulated
XRD pattern of NiCo-MOFs. (d) 3D crystalline structures of
NiCo-MOF microflowers. Nickel, cobalt, oxygen, carbon and
hydrogen are shown in green, purple, red, grey and white,
respectively.
1
crystals. The H NMR spectrum of 4-AP obtained by the
catalytic reduction from 4-NP confirms that the conversion rate
of 4-NP to 4-AP can reach to 100% (Figure S5). What is more,
the excellent synergistic effect arised from the homogeneous
distribution of Ni and Co of NiCo-MOF microflowers, leads to
the higher catalytic efficiency of NiCo-MOF microflowers than

the previously transition metal nanoparticles[17-19]
.
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microflowers is similar to the NiCo-MOFs (Figure S7a), which
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Figure 3. UV-Vis absorption spectra of the 4-NP reduction
versus reaction time under the catalysis of (a) NiCo-MOF
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4
. Conclusion
In summary, bimetallic NiCo-MOF microflowers with
uniform crystal structures have been fabricated by spray
method, which are consisted of 2D nanosheets. The Ni and Co
are homogeneous dispersed throughout the whole NiCo-MOF
microflowers. Owing to the larger exposed surface areas and
synergistic effect, the k and TOF value of NiCo-MOF
microflowers, as catalysts for the reduction of 4-NP, are 0.663
-
1
-1
min and 137.9 h , which are two times higher than
NiCo-MOF bulk crystals. The good catalytic performance of
NiCo-MOF microflowers will inspire the development of
bimetallic MOFs with hierarchical nanostructures for advanced
applications.
Acknowledgement
This work was supported by the National Natural Science
Foundation of China (21503037), Fundamental Research Funds
for the Central Universities (N160504002, N170503010) and
Open Project of State Key Laboratory of Supramolecular
Structure and Materials (sklssm201804, sklssm201822).