Superparamagnetic mesoporous Mg-Fe bi-metal oxides as efficient magnetic solid-base catalysts for Knoevenagel condensations

Article abstract of DOI:10.1039/c0dt00710b

Superparamagnetic mesoporous Mg-Fe bi-metal oxides with varied Mg-Fe atomic ratios have been successfully synthesized as solid base catalysts. The M2F-400 catalyst with Mg/Fe atomic ratio = 2 showed extraordinarily high activities for Knoevenagel reaction

Full text of DOI:10.1039/c0dt00710b

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Superparamagnetic mesoporous Mg–Fe bi-metal oxides as efficient magnetic
solid-base catalysts for Knoevenagel condensations†
Zhe Gao, Jian Zhou, Fangming Cui, Yan Zhu, Zile Hua and Jianlin Shi*
Received 23rd June 2010, Accepted 7th October 2010
DOI: 10.1039/c0dt00710b
Superparamagnetic mesoporous Mg–Fe bi-metal oxides with
varied Mg–Fe atomic ratios have been successfully synthe-
sized as solid base catalysts. The M2F-400 catalyst with
Mg/Fe atomic ratio = 2 showed extraordinarily high activities
for Knoevenagel reactions even at room temperature. It could
be magnetically separated, recycled, and reused for at least
five cycles.
In recent years, magnetic materials have emerged as valuable
alternatives to conventional heterogeneous supports.12–15 The
magnetic separation function offers many advantages over con-
ventional filtration and other purification methods. For example,
in the recycling process, the catalysts can be simply and efficiently
recovered from reaction media with an external magnetic field.
This can be considered as a green technology that avoids the conse-
quences brought about by filtration steps. Magnetic materials have
been used in acid–base catalysis,16 and especially, magnetically
separable solid base catalysts have been reported in recent years17–19
However, the introduction of additional non-porous magnetic core
into alkaline substances usually leads to the lowered surface area of
the catalysts, which would affect the catalytic property by lowering
the TOF value, for example. In order to solve this problem, it is
necessary to greatly increase the active surface area of the magnetic
catalysts.
Recently, our group has developed a simple template-free
strategy to prepare a high surface area superparamagnetic meso-
porous spinel ferrite by controlled thermal decomposition of metal
oxalate precursor.20 Herein, such an oxalate precursor decom-
position method was extended to synthesize non-stoichiometric
mesoporous Mg–Fe bi-metal oxides. This high surface area
mesoporous material which possesses superparamagnetic prop-
erty can be directly used as a highly efficient, environmentally
friendly, magnetically separable and reusable solid base catalyst for
Knoevenagel condensations without further loading other species
or extra functionalization.
1. Introduction
Base-catalyzed condensation, alkylation, addition, cyclization and
isomerization reactions are important steps for building large
and complex molecules for the synthesis of many fine chemicals
and pharmaceutical products.1–5 In comparison to the broad
applications of acidic zeolites as solid catalysts in chemical tech-
nology, much less attention has been paid to solid basic catalysts.
Solid base is an important variety of catalysts offering excellent
opportunities for replacing homogeneous base. The use of solid
base catalysts rather than liquid bases would have advantages
of reducing operating costs associated with base neutralization
and product purification, diminishing corrosion and other related
environmental problems, and in the meantime allowing easier
separation and recovery of the catalysts.
The versatile Knoevenagel condensation reaction (Scheme 1)
is one of the most useful and widely employed methods for
carbon–carbon bond formation in organic synthesis.6,7 It is the
condensation between aldehydes/ketones and active methylene
compounds in the presence of organic bases like pyridine,
piperidine and ethylenediamine.7 However, such a procedure
usually results in a large amount of organic wastes due to the
polymerization and self-condensation of the organics. The use
of different types of solid base catalysts, such as alkali-ion-
exchanged zeolites, alkali-ion-exchanged sepiolite,8 reconstructed
hydrotalcite,9 amino-functionalized SBA-1510 and mesoporous
sodalite,11 for the condensations, has been reported.
2. Results and discussion
2.1. Catalyst characterization
The metal contents of all precursors obtained from the ICP-AES
analysis are presented in Table 1. As expected, the magnesium
content decreased in the order M2F > MF > MF2 and the
iron content increased in the same order. However, the actual
MgO contents were lower than the designed values, implying
the significant Mg losses during precipitation/filtration processes.
Nevertheless, we still obtained two different types of bi-metal
oxide materials respectively with Mg-rich (M2F and MF) and Mg-
deficient (MF2) compositions relative to the formula of MgFe2O4.
Scheme 1 The Knoevenagel reaction between aldehydes and active
methylene compounds.
Table 1 ICP-AES data for all three precursors
State Key Lab of High Performance Ceramics and Superfine Microstructure,
Shanghai Institute of Ceramics Chinese Academy of Sciences, 1295 Ding-
xi Road, Shanghai, 200050, P. R. China. E-mail: jlshi@sunm.shcnc.ac.cn;
Fax: (+86)-21-52413122; Tel: (+86)-21-52412714
† Electronic supplementary information (ESI) available: Additional data.
See DOI: 10.1039/c0dt00710b
Sample
Mg/Fe (atom)
Mg (wt%)
Fe (wt%)
M2F
MF
MF2
1.69
0.62
0.44
29.36
15.57
11.15
39.89
56.67
58.83
11132 | Dalton Trans., 2010, 39, 11132–11135
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Table 2 Pore structural parameters of all synthesized mesoporous bi-
The powder X-ray diffraction patterns of MgO, Fe2O3,
MgFe2O4 standards and the calcined Mg–Fe bi-metal products
at different temperatures are shown in Fig. 1. The diffraction
lines become slightly stronger and sharper after calcined at higher
temperature (500 ◦C). The diffraction peaks of MF2-400 and
MF2-500 samples can be well-indexed to spinel MgFe2O4 (JCPDS
17-0464) in terms of both peak positions and intensities, where
the diffraction peaks at 2q values of 30.1, 35.5, 43.1, 53.5, 57.0
and 62.6◦ can be ascribed to the reflections of (220), (311),
(400), (422), (511) and (440) planes of MgFe2O4, respectively.
As for M2F-400/500 and MF-400/500 materials, the relative
peak intensity has changed remarkably, so the patterns of the
materials could not be simply indexed to a single crystalline
structure. Compared to MF2-400/500 samples, the diffraction
lines at 43◦ and 63◦, which can be well indexed to the (200) and
(220) reflections of MgO (JCPDS 87-0653), become significantly
stronger in samples at higher Mg contents, indicative of the
formation of crystalline MgO in addition to spinel MgFe2O4. The
formation of additional crystalline MgO phase in M2F-400/500
and MF-400/500 materials matches well with their higher MgO
content as indicated in Table 1. Therefore, both the composition
and phase structure analysis demonstrated that M2F-400/500 and
MF-400/500 were actually the mixtures of MgO and MgFe2O4.
The crystallite size of MgO in M2F-400 and M2F-500 could be
estimated, very roughly due to the peak overlap with those of
MgFe2O4, to be 3.4 and 13.0 nm by Scherrer’s equation using the
(200) reflection. Therefore, such a MgO-rich material can also be
regarded as a composite with nano-sized MgO crystallites in situ
loaded in the mesoporous structure of spinel MgFe2O4.
metal oxide samples
Sample
S
BET/m2 g-1
d
BJH/nm
Pore volume/cm3 g-1
M2F-400
MF-400
MF2-400
M2F-500
MF-500
MF2-500
173.6
131.2
73.4
34.0
39.1
46.0
4.5
3.4
4.5
16.8
14.4
7.2
0.371
0.280
0.195
0.276
0.232
0.174
species formed homogeneously within the mesoporous structure
of the non-stoichiometric materials.
N2 adsorption–desorption isotherms and the BJH desorption
pore size distribution (PSD) plots of all samples are shown in ESI
Fig. S2,† and the pore structure parameters are summarized in
Table 2. The curves of all samples are typical Langmuir IV type
containing capillary condensation steps, which clearly suggests
the existence of mesoporous structure. With the increase of
temperature, BET surface area decreases due to Otswald ripening.
This phenomenon is consistent with the TEM observations in ESI
Fig. S1.†
The magnetization curves of all samples were measured at room
temperature, as shown in Fig. 2. All of the curves do not reach
saturation. The magnetization values at 20 kOe are 2.02, 6.87,
7.24, 9.43, 12.67 and 14.04 emu g-1 for M2F-400, MF-400, M2F-
500, MF-500, MF2-400 and MF2-500, respectively. The magnetic
property is contributed by the spinel ferrite MgFe2O4 phase in the
samples. The enhancement of magnetization intensity at increased
calcination temperatures can be attributed to their enlarged
crystallites and improved crystallinity. At the same calcination
temperature, the magnetization intensity becomes higher at lower
MgO contents. The low coercive forces (4, 23, 14, 25, 16 and 62 Oe
for M2F-400, MF-400, M2F-500, MF-500, MF2-400 and MF2-
500, respectively) indicate that obtained mesoporous materials
possess a superparamagnetic feature. The magnetic properties of
these catalysts are useful for their efficient recovery and recycling
in liquid-phase reactions. As far as we know, it is the first
successful preparation of superparamagnetic mesoporous solid
base catalysts. Fig. 3 gives the CO2-TPD spectra for all samples.
Fig. 1 XRD patterns of samples M2F-400 (a), MF-400 (b), MF2-400 (c),
M2F-500 (d), MF-500 (e), MF2-500 (f), MgO (JCPDS 87-0653) (g), Fe2O3
(JCPDS 24-0072) (h), and MgFe2O4 (JCPDS 17-0464) (i).
TEM images demonstrate that all the samples are mesoporous-
structured. ESI Fig. S1 a, b and c show random but homogeneous
distributions of worm-like mesopore channels within the whole
particles.† With the increase of calcination temperature, the pore
sizes were enlarged accompanying the further crystallization and
crystal growth of the resultant nanoparticles (ESI, Fig. S1 d, e and
f).† No obvious separate MgO clusters/particles could be found
in all images. This indicates that the alkaline earth oxide MgO
Fig. 2 Magnetization curves of samples M2F-400 (a), MF-400 (b),
M2F-500 (c), MF-500(d), MF2-400 (e) and MF2-500 (f).
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Dalton Trans., 2010, 39, 11132–11135 | 11133
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Table 3 Yields and reaction time durations of typical Knoevenagel
reactions catalyzed by commercial MgO and all synthesized mesoporous
Mg–Fe bi-metal oxide catalystsa
Entry
Catalyst
R
Y
Yield (%)b
t/h
1
2
3
4
5
6
7
M2F-400
MF-400
MF2-400
M2F-500
MF-500
MF2-500
Commercial MgO
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
CN
CN
CN
CN
CN
CN
CN
99
93
72
81
64
61
99
0.5
0.5
0.5
0.5
0.5
0.5
0.3
a Reaction conditions: benzaldehyde (5 mmol), 0.1 g catalyst, malononitrile
(5.5 mmol), 10 mL ethanol, room temperature. b Isolated yield.
Table 4 Knoevenagel reactions of various aldehydes with active methy-
lene compounds at room temperature using M2F-400 as the catalysta
Fig. 3 Temperature-programmed desorption profiles of CO2 on samples
M2F-400 (a), MF-400 (b), MF2-400 (c), M2F-500 (d), MF-500 (e) and
MF2-500 (f).
Entry
R
Y
Yield (%)b
t/h
1
2
3
4
5
6
7
8
C6H5
C6H5
4-MeO-C6H4
4-MeO-C6H4
4-Cl-C6H4
4-Cl-C6H4
2-Furyl
CN
CO2Et
CN
CO2Et
CN
CO2Et
CN
CO2Et
CN
CO2Et
CN
99
71
78
65
98
81
99
96
98
92
99
89
0.5
2
0.5
2
0.5
2
0.25
0.5
0.25
1
All of them show broadened desorption peaks in the temperature
region of 50–160 ◦C, which can be assigned to bicarbonates formed
on relative weak Brønsted basic sites on the sample surface.21
The overall amounts of CO2 desorbed from M2F-400 and MF-
400 samples in this low temperature region are apparently larger
than those of others. Other CO2 desorption peaks were observed
above 400 ◦C in M2F-400, MF-400 and M2F-500 samples, which
can be attributed to the strong Lewis basic sites by isolated O2-
of MgO.21 Apparently, the CO2 desorption peak of M2F-400 is
located at the highest temperature (514 ◦C) among all samples,
indicating that M2F-400 possesses the highest basic strength. The
CO2 desorption peak of M2F-500 in this temperature region is
quite weak, most probably because of its relatively low surface
area. The overall amounts of CO2 desorbed from all samples
decreases at the elevated calcination temperature, and increases
at higher MgO contents of samples.
To sum up, the in situ presence of excessive MgO species
in a nano-crystallite form within the mesoporous structure of
spinel ferrite MgFe2O4 by one-step oxalate co-precipitation and
calcination at a relatively low temperature (400 ◦C), leads to high
surface area, enhanced basic property, and at the same time,
lowered magnetization intensity. The superparamagnetic M2F-
400 material possesses the largest surface area and strongest
basic property among all samples, which endows the material
high catalytic activity for Knoevenagel condensation reactions as
presented below.
2-Furyl
9
4-NO2-C6H4
4-NO2-C6H4
3-NO2-C6H4
3-NO2-C6H4
10
11
12
0.25
1
CO2Et
a Reaction conditions: Aldehyde (5 mmol), 0.1 g catalyst, active methylene
compound (5.5 mmol), 10 mL ethanol, room temperature. b Isolated yield.
that of commercial pure MgO catalyst, though the later contains
100% MgO as the source of the catalytic activity. The catalytic
performances are consistent with CO2-TPD results, which reveal
that the catalytic activity is mostly determined by their relative
basic strength.
M2F-400 sample, which showed excellent activity toward the
model reaction as shown in Table 3, was then used as catalysts
for a series of reactions using two types of active methylene
compounds (malononitrile and ethyl cyanoacetate) and a variety
of aldehydes. The results are shown in Table 4. Malononitrile
was observed to be a better reagent than ethyl cyanoacetate
because of its higher electron-withdrawing ability. The reactions
between various aldehydes and malononitrile could reach the
complete conversion within 0.5 h, except for p-anisaldehyde, due
to the strong electron donating effect of its methoxyl group.
Ethyl cyanoacetate and various aldehydes were used in the
reactions under the optimized conditions, and high yields were
also obtained, though longer reaction periods were needed. All the
reactions proceeded selectively to the dehydrated products without
any side reaction. No self-condensation or hydrated products of
Knoevenagel adducts were detected.
2.2 Catalytic activities
Knoevenagel reactions between benzaldehyde and malononitrile
catalyzed by commercial MgO and all six synthesized catalysts
were studied as a model reaction to test their catalytic activities.
The results are summarized in Table 3. The reaction conditions,
namely the use of various solvents and the ratio between the
reactants, were tested to make sure that the reactions were carried
out under the optimized conditions. As can be seen in Table 3,
M2F-400 was the best catalyst among the six synthesized samples
in terms of yield within the same reaction period of 30 min.
The catalytic performance by M2F-400 is even comparable to
The recyclability of M2F-400 sample was checked on
five consecutive catalytic reaction cycles performed with p-
nitrobenzaldehyde and malononitrile in ethanol at room temper-
ature. The results are shown in Table 5. The M2F-400 sample
exhibited consistent but very slow deactivation due to ingress of
air moisture during recycling studies.
11134 | Dalton Trans., 2010, 39, 11132–11135
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Table 5 Recycling performance of M2F-400 catalyst for typical Knoeve-
nagel reaction
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In conclusion, superparamagnetic mesoporous Mg–Fe bi-metal
oxides have been successfully prepared by a simple controlled
oxalate decomposition strategy. They were used as active base
catalysts for Knoevenagel condensation without the need to load
additional basic species. The catalyst M2F-400 (Mg/Fe atomic
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The authors gratefully acknowledge the financial support from
National Natural Science Foundation of China (grant no.
20633090, 20703055 and 50872140), Shanghai Nano-Science
Projects (grant no. 0852nm03900) and Science Foundation for
Youth Scholar of State Key Laboratory of High Performance
Ceramics and Superfine Microstructures (grant no. SKL200901).
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 11132–11135 | 11135
©