An Efficient and Reusable Multifunctional Composite Magnetic Nanocatalyst for Knoevenagel Condensation

Article abstract of DOI:10.1055/s-0037-1612076

A range of multifunctional magnetic metal-organic framework nanomaterials consisting of various mass ratios of the metal-organic framework MIL-53(Fe) and magnetic SiO ₂ @NiFe ₂ O ₄ nanoparticles were designed, prepared, ch

Full text of DOI:10.1055/s-0037-1612076

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© Georg Thieme Verlag Stuttgart · New York
2019, 30, A–D
letter
A
en
N. Yao et al.
Letter
Synlett
An Efficient and Reusable Multifunctional Composite Magnetic
Nanocatalyst for Knoevenagel Condensation
Nan Yao
Jin Tan
multifunctional green catalysis
Yang Liu
Yu Lin Hu*
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9-5
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2
SiO2
R1
College of Materials and Chemical Engineering, Key Laboratory
of Inorganic Nonmetallic Crystalline and Energy Conversion Ma-
terials, China Three Gorges University, Yichang 443002, P. R. of
China
O
NC
CN
NiFe2O4
CN
R2
R1
R2
room temperature
60–120 min
CN
huyulin@ctgu.edu.cn
huyulin1982@163.com
R1 = aryl, heteroaryl, alkyl
R2 = aryl, alkyl, H
17 examples
90–99% yield
MIL-53(Fe)@SiO2@NiFe2O4
cooperative catalysis
Received: 25.11.2018
Accepted after revision: 21.12.2018
Published online: 06.03.2019
trile fiber,8 rhodium–platinum nanoparticles supported on
thiocarbamide-functionalized graphene oxide,9 a core–shell
silica-layered double hydroxide,10 ionic liquids,11–13 and
others.14–18 Most of these catalysts are efficient but, never-
theless, suffer from various limitations, such as a relative
lack of availability or environmental contamination. There-
fore, the development of more-sustainable and more-effi-
cient catalysts for the Knoevenagel condensation continues
to be a challenging goal.
DOI: 10.1055/s-0037-1612076; Art ID: st-2018-k0766-l
Abstract A range of multifunctional magnetic metal–organic frame-
work nanomaterials consisting of various mass ratios of the metal–or-
ganic framework MIL-53(Fe) and magnetic SiO2@NiFe2O4 nanoparticles
were designed, prepared, characterized, and evaluated as heterogeneous
catalysts for the Knoevenagel condensation. The as-fabricated nanoma-
terials, especially the nanocatalyst MIL-53(Fe)@SiO2@NiFe2O4(1.0),
showed good catalytic performance in the Knoevenagel condensation
at room temperature as a result of synergistic interaction between the
Lewis acid iron sites of MIL-53(Fe) and the active sites of the magnetic
SiO2@NiFe2O4 nanoparticles. In addition, the heterogeneous catalyst
was readily recovered and a recycling test showed that it could be re-
used for five times without significant loss of its catalytic activity, mak-
ing it economical and environmentally friendly.
Metal–organic frameworks (MOFs), as a class of fasci-
nating functional materials, have attracted considerable at-
tention due to their intriguing structural diversity, versatile
physical and chemical properties, and potential extensive
applications in the fields of catalysis and chemical transfor-
mation.19–21 Although some MOFs exhibit good catalytic ac-
tivities for the Knoevenagel condensation,22–25 most have
drawbacks in terms of costly separation and a lack of recy-
clability. Consequently, there is a considerable demand for
the development of MOF-based heterogeneous catalysts
that are more environmentally benign and more efficient.
In recent years, magnetic nanoparticles have attracted
broad attention owing to their unique high surface area, or-
dered pore arrangement, and superparamagnetic proper-
ties.26,27 An inherent advantage of magnetic nanoparticles is
the ease with which they can be recovered and separated
for subsequent reuse in chemical transformations, simply
by using an external magnet and decantation. Furthermore,
it has been found that the stability, dispersibility, and cata-
lytic properties of MOFs can be enhanced by imparting
magnetic properties to the framework.28,29 Therefore, MOF
catalysts containing magnetic nanoparticles might exhibit
an outstanding catalytic performance as a result the combi-
nation of the advantageous properties of the two materials.
On the basis of the aforementioned considerations, we
report on the use of a combination of nickel ferrite
(NiFe2O4) magnetic nanoparticles with an MIL-53(Fe) MOF
Key words metal-organic frameworks, magnetic nanocatalyst,
Knoevenagel condensation, cooperative catalysis, heterogeneous catal-
ysis, iron catalysis
Knoevenagel condensation is one of the most important
and challenging C–C bond-forming reactions, as the ,-un-
saturated products have extensive applications in the pro-
duction of fine chemicals and intermediates.1,2 Knoevenagel
condensations are normally catalyzed by base catalysts in
liquid-phase systems under homogeneous conditions.1,3
Unfortunately, the separation and handling of waste mate-
rials generated under these conditions creates undesirable
economic and environmental problems. Consequently,
there is an urgent need to develop new and efficient hetero-
geneous catalytic systems to perform these transforma-
tions. In this regard, a number of representative heteroge-
neous catalysts have been developed for the Knoevenagel
condensation, including phosphotungstic acid immobilized
on mesoporous graphitic carbon nitride (g-C3N4),4 Ni/SiO2,5
g-C3N4,6 aziridine-functionalized multiwalled carbon nano-
tubes,7 triethylenetetramine-functionalized polyacryloni-
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D

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N. Yao et al.
Letter
Synlett
to form a new multifunctional magnetic MOF catalysts.
Magnetic nanomaterials MIL-53(Fe)@SiO2@NiFe2O4contain-
ing various quantities of MIL-53(Fe) encapsulated on the
surface of SiO2@NiFe2O4were examined as efficient and en-
vironmentally friendly catalysts for Knoevenagel condensa-
tions under mild conditions. The recyclability and reusabil-
ity of the nanocatalysts were also examined.
The as-synthesized nanomaterials were characterized
by means of FTIR, XRD, SEM, UV/vis, N2 physisorption, and
XPS analyses. FTIR spectra of the nanomaterials are shown
in Figure S1 of the Supplementary Information (SI). The
characteristic peaks at about 1631 and 762 cm–1 were at-
tributed to the C=C and C–H stretching vibrations of the
benzene ring of MIL-53(Fe), whereas those at about 1542
and 1397 cm–1 were attributed to O–C–O stretching vibra-
tions of the carboxyl groups of MIL-53(Fe).29–33 The charac-
teristic bands at about 3446 and 952 cm−1 were ascribed to
the stretching vibration of OH and Si–OH, respectively,
whereas the band at about 1085 cm−1 was ascribed to the
stretching vibration of Si–O–Si.30,31 The peak at 596 cm−1 is
related to typical stretching vibrational modes of Fe−O.34,35
The results for the characteristic vibrational peaks of the
as-synthesized products therefore confirmed the successful
formation of the magnetic nanomaterials.
Figure S2 in the SI shows the XRD patterns of the pre-
pared nanomaterials. The diffraction peaks observed at 9.4,
12.9, and 17.8° belonged to typical patterns of crystalline
MIL-53(Fe).36,37 The six typical diffraction peaks of nano-
materials were observed at 2θ = 30.4, 35.2, 42.7, 53.8, 57.4,
and 52.3°; these can be assigned, respectively, to the (220),
(311), (400), (422), (511), and (440) reflections of magnetic
NiFe2O4nanoparticles [JCPDS No. 10-0325].34,35 These re-
sults indicated the presence of a highly ordered meso-
porous structure, and confirmed that the magnetic nano-
materials had been successfully formed. The characteristic
peaks of NiFe2O4 were retained after encapsulation with
MIL-53(Fe), indicating that the crystal structure of NiFe2O4
was maintained and intact during the synthesis process.
Scanning electron microscopy (SEM) images of the as-
synthesized nanomaterials are shown in Figure S3 of the SI.
Micrograph (d) shows the presence of well-ordered arrays
of SiO2@NiFe2O4nanoparticles, typical of the morphology of
magnetic particles.34,35 It could be clearly observed that the
morphologies of the magnetic MIL-53(Fe) nanomaterials
consisted of agglomerations of small spheres of uniform
nanometer-sized particles.
Figure S4 of the SI shows the diffuse reflectance UV–vis
spectra of the prepared nanomaterials. The broad distinct
absorption peak at 210–230 nm was attributed to the char-
acteristic absorption properties of SiO2@NiFe2O4.30,34,35 No
other typical absorption peaks were found on the patterns
of the nanomaterials MIL-53(Fe)@SiO2@NiFe2O4, indicating
the absence of extra-crystal framework and a highly uni-
form distribution of species in these mesoporous frame-
works.
N2 adsorption–desorption measurements are a power-
ful tool for the characterization of nanomaterials. Figure S5
of the SI shows the isotherms and pore-size distribution of
the prepared nanomaterial MIL-53(Fe)@SiO2@NiFe2O4(1.0).
It is clear that MIL-53(Fe)@SiO2@NiFe2O4(1.0) has a BET sur-
face area of 102.86 m2·g–1 and a total pore volume of 0.5329
cm3·g–1. The pore-size distribution confirms that well de-
fined pores with a pore-size distribution of 2.12 nm are
present in this nanomaterial. These results show that this
nanomaterial exhibits a Type I isotherm, characteristic of
highly ordered mesoporous materials.28,29
To obtain additional information about the elemental
composition and the chemical nature of the active species
in the nanocatalyst MIL-53(Fe)@SiO2@NiFe2O4(1.0), we per-
formed XPS studies. As shown in Figure S6 of the SI, the
nanocatalyst consisted of C, O, Si, Fe and Ni, and the peaks
corresponding to C1s, O1s, Si2p, Fe2p, and Ni2p appeared at
284.6, 531.1, 102.6, 711.6, and 856.5 eV, respectively.27,30,31
The XPS results provided further confirmation of the struc-
ture of the nanocatalyst MIL-53(Fe)@SiO2@NiFe2O4(1.0),
and were consistent with the XRD and FTIR analyses.
To examine the catalytic activity of the nanomaterials
MIL-53(Fe)@SiO2@NiFe2O4, we chose the Knoevenagel con-
densation of benzaldehyde with malononitrile as a model
reaction. Initially, we examined the catalytic activities of
MIL-53(Fe)@SiO2@NiFe2O4 nanocatalysts with various mass
ratios of MIL-53(Fe) and SiO2@NiFe2O4: MIL-53(Fe)@SiO2@
NiFe2O4(0.5), MIL-53(Fe)@SiO2@NiFe2O4(1.0), and MIL-
53(Fe) @SiO2@NiFe2O4(1.5) (Table 1, entries 1–3). Among
these nanocatalysts, MIL-53(Fe)@SiO2@NiFe2O4(1.0) was
the most effective catalyst, affording a conversion of >99%
and a product yield of 98% (Table 1, entry 2). These results
indicated that the heterogeneous nanocatalyst MIL-
53(Fe)@SiO2@NiFe2O4(1.0) has excellent catalytic activity
for the condensation. This catalyst has sufficiently active
sites for the reaction, probably as a result of synergy be-
tween the active sites of MIL-53(Fe) and those of the mag-
netic SiO2@NiFe2O4 particles. The amount of catalyst also
influenced the outcome of the condensation reaction. On
increasing the amount of nanocatalyst to 0.1 g, an increase
in the conversion and the product yield were observed (Ta-
ble 1, entries 2 and 6–8), whereas a further increase to 0.15
g did not significantly improve the conversion and yield
(Table 1, entry 9). It is a promising result that MIL-
53(Fe)@SiO2@NiFe2O4(1.0) exhibited a high catalytic activi-
ty at a low catalyst loading 0.1 g in the catalytic reaction.
For comparison, the condensation was also performed by
using SiO2@NiFe2O4 alone or bulk MIL-53(Fe) as the catalyst
(Table 1, entries 4 and 5, respectively). When SiO2@NiFe2O4
was used alone, only 80% product yield and 82% conversion
were obtained (Table 1, entry 4), whereas the use of bulk
MIL-53(Fe) as the catalyst afforded
a much more
lower product yield (72%) and conversion (74%) (Table 1,
entry 5). Therefore, MIL-53(Fe)@SiO2@NiFe2O4(1.0) is a
suitable nanocatalyst for the condensation.
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D

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Table 1 Catalyst Screening for the Knoevenagel Condensation of Benzaldehyde and Malononitrilea
Entry
Catalyst
Amount (g)
Time (min)
Conversionb (%)
Yieldc (%)
1
2
3
4
5
6
7
8
9
MIL-53(Fe)@SiO2@NiFe2O4(0.5)
MIL-53(Fe)@SiO2@NiFe2O4(1.0)
MIL-53(Fe)@SiO2@NiFe2O4(1.5)
SiO2@NiFe2O4
0.10
0.10
0.10
0.10
0.10
–
60
91
89
98
99
80
72
–
60
>99
>99
82
60
120
120
240
90
MIL-53(Fe)
74
–
–
MIL-53(Fe)@SiO2@NiFe2O4(1.0)
MIL-53(Fe)@SiO2@NiFe2O4(1.0)
MIL-53(Fe) @SiO2@NiFe2O4(1.0)
0.05
0.08
0.15
82
81
93
98
60
94
60
>99
a Reaction conditions: PhCHO (5 mmol), malononitrile (5 mmol), EtOH (15 mL), catalyst, r.t.
b Determined by HPLC analysis of the crude reaction mixture.
c Isolated yield.
The stability and reusability of a heterogeneous catalyst
are important factors in terms of its practical applications.
The thermal stability of nanocatalyst MIL-53(Fe)@SiO2@
NiFe2O4(1.0) was examined by thermogravimetry (SI; Fig-
ure S7). The small weight loss below 100 °C was the result
of desorption of water adsorbed on the surface of the sam-
ple. The marked mass loss of about 12% between 200 and
500 °C was attributed to breakdown of organic moieties.
These results showed that the nanocatalyst is thermally
stable below about 200 °C, which is beneficial in terms of
its reusability. The magnetization curve of the nanocatalyst
MIL-53(Fe)@SiO2@NiFe2O4(1.0), shown in Figure S8 of the
SI, confirmed that the material has typical superparamag-
To confirm the efficiency and capabilities of our MIL-
53(Fe)@SiO2@NiFe2O4(1.0) catalyst, we compared its cata-
lytic activity in the heterogeneous condensation of benzal-
dehyde and malononitrile with that of other reported het-
erogeneous catalysts (SI; Table S1). Our catalytic system
showed promising features in terms of its high activity and
efficiency, producing high to excellent yields under mild re-
action conditions. These results therefore confirm that MIL-
53(Fe)@SiO2@NiFe2O4(1.0) is a practical and suitable mag-
netic MOF nanocatalyst for the Knoevenagel condensation.
Encouraged by the above results, we evaluated the cata-
lytic activities of MIL-53(Fe)@SiO2@NiFe2O4(1.0) for the
heterogeneous condensations of various aldehydes and ke-
tones with malononitrile under the optimized conditions,
and the results are listed in Table S2 of the SI. Various aryl
aldehydes and ketones were smoothly converted into the
desired products in high to excellent yields within short re-
action times (60–120 min).38 To our delight, both electron-
deficient and electron-rich aryl aldehydes delivered the cor-
responding products in an isolated yield of 93–99% (SI; Ta-
ble S2, entries 1–10). Even aryl aldehydes bearing strongly
electron-withdrawing nitro or carboxy groups reacted
readily and efficiently to afford the desired products in ex-
cellent yields (SI; Table S2, entries 6 and 7). Note that the
reaction rates of ketones were slower than those of alde-
hydes and that longer reaction times were needed to obtain
good to excellent yields (SI; Table S2, entries 11–17). It is
also interesting to note that the reaction of malononitrile
with hexane-2,5-dione (SI; Table S2, entries 14 and 15)
gave the dicondensation product 2,5-dimethylhexa-1,5-di-
ene-1,1,6,6-tetracarbonitrile exclusively, whereas pentane-
2,4-dione gave the monocondensation product (1-methyl-
3-oxobutylidene)malononitrile (Table S2, entry 16). These
results confirm the high efficiency and versatility of our de-
signed nanocatalyst in all cases.
netic properties with
a saturation magnetization of
24.3 emu·g–1, which is sufficient to permit magnetic sepa-
ration of the solid nanocatalyst.
Next, we examined the recyclability of the nanocatalyst
MIL-53(Fe)@SiO2@NiFe2O4(1.0) in the Knoevenagel conden-
sation of benzaldehyde and malononitrile under the opti-
mized catalytic reaction conditions (SI; Figure S9). The cat-
alyst was easily separated by using an external magnet with
simple decantation and, when washed several times with
EtOH, could be reused directly for successive experimental
runs. We found that the recycled catalyst could be reused in
five reaction cycles with negligible loss of its catalytic activ-
ity, thereby confirming the excellent catalytic activity, sta-
bility, and reusability of this nanocatalyst. SEM of the re-
used MIL-53(Fe)@SiO2@NiFe2O4(1.0) after five runs indicat-
ed that the morphology and highly ordered nanostructure
of the catalyst were maintained during the course of the
condensation (SI; Figure S3e). XRD spectra of the reused
catalyst after five runs were similar to those of the fresh
catalysts, indicating that no obvious change in the crystal
structure of the catalyst occurred during the condensation
(SI; Figure S10).
© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D

D
N. Yao et al.
Letter
Synlett
In conclusion, a range of multifunctional magnetic MOF
nanomaterials with various mass ratios of MIL-53(Fe) to
magnetic SiO2@NiFe2O4 nanoparticles were synthesized,
characterized, and used as novel heterogeneous catalysts
for the Knoevenagel condensation. The presence of syner-
gistic effects between the Lewis acid iron sites of MIL-
53(Fe) and the basic sites of magnetic SiO2@NiFe2O4
nanoparticles in the nanomaterials makes them suitable as
active catalysts for the condensation. Our studies showed
that the multifunctional cooperative nanocatalyst MIL-
53(Fe)@SiO2@NiFe2O4(1.0) has a higher activity than other
catalysts in this reaction. The present method provides an
operationally simple, highly efficient, and environmentally
benign alternative for the Knoevenagel condensation. Fur-
thermore, our heterogeneous catalyst shows excellent reus-
ability and can be reused for five runs with negligible loss
of its activity. The advantages of the present method are the
high catalytic activity, easy workup, excellent reusability,
and green and environmentally benign nature.
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Funding Information
This work was supported by the National Natural Science Foundation
of China (21506115) and Excellent Dissertation of China Three Gorges
University (2018SSPY055).
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A mixture of PhCHO (5 mmol), malononitrile (5 mmol), EtOH
(15 mL), and MIL-53(Fe)@SiO2@NiFe2O4(1.0) (0.1 g) was mag-
netically stirred at r.t. until the reaction was complete (TLC).
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© Georg Thieme Verlag Stuttgart · New York — Synlett 2019, 30, A–D