Monodisperse iron oxide nanoparticles embedded in Mg-Al hydrotalcite as a highly active, magnetically separable, and recyclable solid base catalyst

Article abstract of DOI:10.1246/bcsj.20100059

Magnetically separable Mg-Al hydrotalcite was prepared by titration method in various molar ratios of (Mg + Al) to Fe, and we compared their catalytic behavior for epoxidation of 2-cyclohexen-1-one using hydrogen peroxide. The catalyst was characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), superconducting quantum interference device (SQUID), and inductively coupled plasma (ICP). The FeHT100 (ratio of (Mg + Al) to Fe = 100) showed high activity and selectivity in epoxidation of 2-cyclohexen-1-one with hydrogen peroxide. After magnetic separation, FeHT100 kept superior properties and could be reused for four reactions without loss of activity.

Full text of DOI:10.1246/bcsj.20100059

8
46
Bull. Chem. Soc. Jpn. Vol. 83, No. 7, 846–851 (2010)
© 2010 The Chemical Society of Japan
Monodisperse Iron Oxide Nanoparticles Embedded in Mg­Al
Hydrotalcite as a Highly Active, Magnetically Separable,
and Recyclable Solid Base Catalyst
Shun Nishimura, Atsushi Takagaki, and Kohki Ebitani*
School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi 923-1292
Received March 3, 2010; E-mail: ebitani@jaist.ac.jp
Magnetically separable Mg­Al hydrotalcite was prepared by titration method in various molar ratios of (Mg + Al)
to Fe, and we compared their catalytic behavior for epoxidation of 2-cyclohexen-1-one using hydrogen peroxide. The
catalyst was characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), superconducting
quantum interference device (SQUID), and inductively coupled plasma (ICP). The FeHT100 (ratio of (Mg + Al) to
Fe = 100) showed high activity and selectivity in epoxidation of 2-cyclohexen-1-one with hydrogen peroxide. After
magnetic separation, FeHT100 kept superior properties and could be reused for four reactions without loss of activity.
3
1,32
adsorbents,33­35
Progress in nanomaterials over the last few decades has
tions as anion or cation exchangers,
3
6
afforded plenty of opportunities for the development of novel
classes of heterogeneous catalysts.1 Magnetic nanoparticles
have attractive features including magnetic moments, coerciv-
ity, remanent magnetization, and spontaneous magnetism, and
these properties have been applied in high-density storage
media, magnetic semiconductors, magnetic fluids, hyperther-
mia treatment, separation and purification of biomolecules, and
stabilizers of pigment, and reservoirs for drugs and bio-
In particular, hydrotalcite, which is the most
well-known LDH, has been frequently used as a solid base
transesterification,
Recently, Duan and co-workers reported
that magnetic Mg­Al­OH­LDH stabilized on CoFe O or
MgFe2O4 acted as solid base catalyst for self-condensation of
acetone, where magnetic oxide was synthesized by conven-
­4
37,38
molecules.
3
9­41
42,43
catalyst for aldol condensation,
epoxidation.
and
4
4,45
2
4
5­9
drug delivery.
From the viewpoint of catalysis, magnetic
4
6,47
properties can serve in effective separation protocols. In order
to attempt separation and recovery of catalysts from reaction
products, a large amount of energy and costs are consumed for
reactor maintenance. Creating the catalysts with inherent
magnetic properties could lead to simpler and more environ-
mentally benign separation.
tional coprecipitation method.
However, Duan et al. did
not discuss the effect of magnetic properties on the catalysis
of hydrotalcite. It is important for developing an effective
magnetically separable base catalyst to clarify the influence of
the additive on hydrotalcite.
In this report, we have designed a magnetic hydrotalcite
catalyst by using monodisperse magnetic Fe oxide nano-
particles (mono-FeNPs), and examined its usefulness as a
separable and reusable solid base catalyst. The use of mono-
disperse magnetic nanoparticles, which have controllable
Magnetic separation is a fascinating methodology for
1
0
11­13
catalysts such as heteropoly acid, metal complex,
basic
1
4,15
ion liquid,
and sulfonic- or amine-functionalized nano-
Although these highly active catalysts are
1
6,17
catalysts.
9
,48
generally homogeneous or nanosized heterogeneous, which
are difficult to separate, they become easy to collect when
magnetic. Moreover, separation of solid and solid catalyst can
be achieved if one possesses magnetic properties, which is
beneficial for one-pot synthesis using two kinds of solid
magnetism, size, and solvent affinity,
affords clear observa-
tion of morphological change and the influence of the addition
of magnetic properties to the prepared nanocomposite catalyst.
Experimental
1
8,19
catalysts.
There are currently many examples of magne-
Materials.
Sodium oleate was purchased from Tokyo
Chemical Industry Co. Tetramethylammonium hydroxide,
Mg(NO ) ¢6H O, Al(NO ) ¢9H O, Na CO ¢10H O, FeCl ¢
20­22
23­25
tized solid catalysts for oxidation,
hydrogenation,
2
6,27
28
29
coupling,
hydrolysis, esterification, and photocatalytic
3
2
2
3 2
2
2
3
2
2
3
0
reaction. The importance of magnetic separation is rising
along with extending the utilization and development of
catalysts. Especially, magnetically separable acid and base
catalysts are not well understood in spite of many reports of
magnetically separable metal catalysts.
4H O, FeCl ¢6H O, and NH aqua were supplied by Wako
2 3 2 4
Pure Chemicals. Ethanol, hexane, and NaOH were obtained
from Kanto Chem. Co. Oleic acid and Amberlyst-15 were
supplied by Aldrich Inc.
Preparation of Monodispersed Fe3O4 Nanoparticles.
Mono-FeNPs were synthesized by thermal decomposition
of Fe­oleate complex according to a previous report with
some modification. First, Fe­oleate complex was prepared by
refluxing FeCl ¢6H O (20 mmol) and sodium oleate (60 mmol)
Layered double hydroxide clays (LDHs), which are repre-
2
+
3+
x+
n¹
49
sented by a general formula of [M 1¹xM x(OH)2]
A
x/n¢
2+
3+
mH O where M and M are di- and trivalent metal ions and
2
n¹
A
indicates the interlayer anions, show extensive applica-
3
2
Published on the web June 30, 2010; doi:10.1246/bcsj.20100059

S. Nishimura et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
847
in a mixed solvent composed of hexane (70 mL), distilled water
(30 mL), and ethanol (40 mL). Next, the formed Fe­oleate
complex (6 mmol) was mixed with oleic acid (3 mmol) and
tri-n-octylamine (82 mmol) at room temperature. Finally, the
mixture was decomposed by heating at 643 K, and mono-
FeNPs stabilized by oleic acid were formed. After the
procedure, the hydrophobic mono-FeNPs capped with oleic
acid were dispersed in tetramethylammonium hydroxide solu-
tion to make the nanoparticles hydrophilic.
2
.0
HT
Preparation of Magnetic Mg­Al Hydrotalcite. Magnetic
Mg­Al hydrotalcite with Fe O was prepared by coprecipita-
FeHT200
3
4
tion method. Mg(NO ) ¢6H O (12.5 mmol), Al(NO ) ¢9H O
3
2
2
3 2
2
(
1
2.5 mmol), and synthesized mono-FeNPs were dispersed in
00 mL of water. Water solution containing both NaOH and
FeHT100
FeHT50
FeHT25
Na2CO3 (NaOH/Na2CO3 = 2) was slowly added dropwise into
the above solution until pH ca. 10 under vigorous stirring. The
resulting solution was aged at 333 K for 2 h, then filtered,
washed with water, and dried at 373 K overnight. Synthesized
magnetic Mg­Al hydrotalcite with Fe O4 is described as
3
FeHTX, where X denotes the (Mg + Al)/Fe molar ratio of
each sample used. As a comparison, pure hydrotalcite (HT) was
prepared using a similar method.
FeNPs × 5
Preparation of Coprecipitated Fe3O4. Both FeCl2¢4H2O
(
3.7 mmol) and FeCl ¢6H O (7.4 mmol) were dissolved in
3 2
1
80 mL water, and 4 M NH4 water solution was added
10
20
30
40
50
60
70
80
dropwise into the above solution until pH ca. 10. After the
resulting precipitate was aged for 2 h at 333 K, it was filtered,
washed, and dried at room temperature.
Preparation of HT Containing Mono-FeNPs by Adsorp-
tion Method. The prepared HT and the synthesized mono-
FeNPs were stirred together in 50 mL H2O for 8 h at room
temperature. Then, the mixture was filtered, washed, and dried
at room temperature.
2
θ / degree
Figure 1. XRD patterns for HT, FeHTXs, and mono-
FeNPs. ( ) Hydrotalcite phase, ( ) magnetite phase.
HT, FeHTXs, and mono-FeNPs are shown in Figure 1. Mono-
FeNPs shows magnetite (Fe O ) phase, and the crystal size was
3
4
D311 = 7.3 nm calculated by Scherrer’s equation. Prepared HT
exhibited clear peaks of hydrotalcite-like LDHs at 2ª angles of
11.2, 22.6, 34.5, 38.0, 45.7, 60.3, and 61.5° corresponding to
(003), (006), (012), (015), (018), (110), and (113) reflections,
respectively, and basal spacing d003 is 0.79 nm. All diffraction
patterns and basal spacing of FeHTXs were similar to those of
HT. For FeHTXs, small peaks around at 2ª angles of 35.5,
42.9, and 62.7 attributed to a magnetite phase were observed,
depending on the amount of mono-FeNPs. These results
indicated that HT was formed even in the presence of mono-
FeNPs, and the mono-FeNPs had little effect on the structure
of HT.
Characterizations.
Powder X-ray diffraction (XRD)
patterns were obtained with a Rigaku RINT2000 X-ray
diffractometer using Cu K¡ radiation (­ = 0.154 nm) and a
1
power of 40 kV and 20 mA. The morphologies of samples
were observed by transmission electron microscopy (TEM,
HITACHI H-7100). Magnetic properties were measured by
superconducting quantum interference device (SQUID, Quan-
tum Design MPMS-XL) ranging from ¹5.0 to 5.0 T. The
concentrations of Fe for each FeHTX catalysts were determined
by an inductively coupled plasma (ICP, Shimadzu ICPS-7000).
Epoxidation of 2-Cyclohexen-1-one Using Hydrogen
Peroxide. The reaction was carried out in a Schlenk-flask
Structural Morphology.
Transmission electron micro-
in methanol solvent at 313 K for 4 h under a flow of N . After
scope (TEM) images of mono-FeNPs, FeHT25, FeHT100, and
HT are shown in Figure 2. Mono-FeNPs are highly mono-
disperse nanoparticles, with about 20.0 nm average diameter
(Figure 2a) and narrow size distributions (inset Figure 2).
These morphologies were similar to those reported in the
2
the reaction, the catalyst was washed with 30 mL acetone twice,
dried at 353 K overnight, and reused for further reaction. A
gas chromatograph (GC) with a capillary column (TC-FFAP,
0
.25 mm © 30 m), was used to analyze the products.
4
9
A One-Pot Synthesis of 5-Hydroxymethylfurfural (HMF)
literature. The TEM image of HT (Figure 2d) shows the
plate-like morphology which is characteristic of LDH mate-
from Glucose. The reaction was performed at 373 K for 3 h in
mL of N,N-dimethylformamide using 0.05 g of magnetic
5
0­52
3
rials.
Moreover, for both FeHT25 (Figure 2b) and
hydrotalcite and 0.1 g of Amberlyst-15. High-performance
liquid chromatography (HPLC) using a Bio-rad Aminex HPX-
FeHT100 (Figure 2c), the formation of nanocomposite where
mono-FeNPs were embedded in plate-like hydrotalcite was
observed, indicating that FeHTXs could be prepared without
changing the morphologies of hydrotalcite or mono-FeNPs.
8
7H column was used to analyze the products.
Results and Discussion
Magnetic Properties.
In order to evaluate magnetic
Crystal Structure. X-ray diffraction (XRD) patterns for
properties, magnetization curves were measured by a super-

8
48
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
Magnetically Separable Hydrotalcite Catalyst
(
a)
(b)
1
6
18
20
22
24
26
Particle diameter / nm
2
0nm
100nm
(
c)
(d)
100nm
100nm
Figure 2. TEM images of (a) mono-FeNPs, (b) FeHT25, (c) FeHT100, and (d) HT. Inset shows size distribution of the mono-FeNPs.
conducting quantum interference device (SQUID). Figure 3
shows magnetization curves for mono-FeNPs at 10 and 300 K.
The magnetization of mono-FeNPs drastically increased under
a low external magnetic field, and was saturated under 5 T
without hysteresis at 300 K. At 10 K, the same behavior was
obtained with hysteresis, corresponding to strong soft magnetic
behavior. These magnetization curves of mono-FeNPs exhib-
ited superparamagnetic behavior, and the saturation magnet-
Table 1. Epoxidation of 2-Cyclohexen-1-one Using Hydro-
gen Peroxide in Methanol
a)
O
O
MeOH
O
+
H O
H O
2
2
2
+
3
13 K, 4 h
Fe concentrationb)
Yieldc)
/%
Selectivity
/%
¹
1
¹1
Catalyst
ization was 46.0 emu g at 10 K and 44.6 emu g at 300 K.
The magnetization curves for FeHTXs and HT at 10 and 300 K
are shown in Figure 4. The case of HT shows slight negative
magnetization, and intensity decreased with increasing external
magnetic field regardless of measurement temperature. All the
FeHTXs displayed similar magnetic curves to that of mono-
FeNPs, and they exhibited superparamagnetic behavior with
hysteresis at 10 K and without at 300 K. All magnetizations of
FeHTXs were saturated at approximately 0.5 T at 300 K. These
magnetizations of FeHTXs under 0.5 T, which is a similar
condition to catalyst separation using a Nd­Fe­B magnet (vide
infra), were 2.0 (FeHT25), 0.95 (FeHT50), 0.46 (FeHT100),
/wt %
HT
®
95
81
80
77
52
4
97
89
91
92
77
®
0
FeHT200
FeHT100
FeHT50
FeHT25
Fe3O4
Blank
0.28
0.53
1.29
2.99
®
d)
®
0
a) Reaction conditions: 2-cyclohexen-1-one (2 mmol), catalyst
0.15 g), 30% aq. H2O2 (8 mmol), methanol (5 mL), 313 K,
h. b) Calculated by ICP analyzer using a calibration curve.
(
4
c) Determined by GC using naphthalene (0.05 mmol) as an
internal standard. d) Prepared by titration method.
¹
1
and 0.20 (FeHT200) emu g , respectively. Compared with
mono-FeNPs, FeHTXs had a smaller amount of magnetiza-
tion. These tendencies show the saturated magnetizations of
FeHTXs were directly related to the amounts of mono-FeNPs.
Meanwhile, at fields above 0.5 T, their magnetizations gradual-
ly diminished. These tendencies suggest that the decrease of
magnetization for FeHTXs after saturation was due to the
diamagnetic properties of HT. These results indicate that
FeHTXs present magnetic properties of both superparamag-
netism (derived from mono-FeNPs), and diamagnetism (from
hydrotalcite).
The Amount of Fe on the FeXHT Catalysts. The Fe
concentrations of each FeHTX catalysts are shown in Table 1.
FeHT200 contains 0.28 wt % Fe, and the concentrations
gradually increased as the amounts of additive mono-FeNPs,
0.53 (FeHT100), 1.29 (FeHT50), and 2.99 (FeHT25) wt %.
Using mono-FeNPs as a magnetic agent was an effective
method to control the magnetization of catalyst. Additionally, it
is obvious that the saturated magnetizations were related

S. Nishimura et al.
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
849
5
4
3
2
1
0
0
0
0
0
0
20
3
2
1
0
1
2
3
2
1
(
a)
(a)
10
0
0
-
-
10
20
-1
-2
-
0.4
-0.2
0.0
0.2
0.4
-
0.4
-0.2
0.0
0.2
0.4
-
-
-
-
-
10
20
30
40
50
FeHT25
FeHT50
FeHT100
FeHT200
HT
-
-
-
-5
-4
-3
-2
-1
0
1
2
3
4
5
-
4
-2
0
2
4
Field / T
Field / T
5
0
0
0
0
0
0
20
3
2
1
0
2
1
0
(
b)
4
3
2
1
10
(b)
0
-
-
10
20
-
-
1
2
-0.4
-0.2
0.0
0.2
0.4
-0.4
-0.2
0.0
0.2
0.4
-
-
-
-
-
10
20
30
40
50
FeHT25
FeHT50
FeHT100
FeHT200
HT
-1
-
-
2
3
-5
-4
-3
-2
-1
0
1
2
3
4
5
-
4
-2
0
2
4
Field / T
Field / T
Figure 3. Magnetization curves for mono-FeNPs at (a) 10
and (b) 300 K. The insets emphasize details near the origin.
Figure 4. Magnetization curves for HT and FeHTXs at
a) 10 and (b) 300 K. The insets emphasize details near the
(
origin.
to the amounts of mono-FeNPs (see Supporting Information
Figure S1).
Catalytic Activity for Epoxidation of 2-Cyclohexen-1-
one.
The catalytic activities for magnetic FeHTXs were
evaluated for epoxidation of 2-cyclohexen-1-one using hydro-
gen peroxide as an oxidant. The epoxidation of ¡,¢-unsaturat-
ed ketones has a great impact, because the formed epoxy-
ketones can be converted to various organic materials. This
epoxidation of electro-deficient olefin using H O was tradi-
2
2
tionally performed in the presence of homogeneous base,
NaOH. The results for HT and FeHTX catalysts are shown in
Table 1. Without catalyst, the reaction did not proceed. In the
case of using HT, the yield and selectivity for 2,3-epoxy-
cyclohexanone were 95% and 97%, respectively. FeHT200,
FeHT100, and FeHT50 exhibited high catalytic activities (ca.
Figure 5. Magnetic separation after first reaction in the case
of FeHT100.
8
0% yield and ca. 90% selectivity) without significant loss,
compared with intrinsic hydrotalcite activity. Moreover, these
catalysts could be simply recovered from reaction solvent using
a magnetic field (Nd­Fe­B magnet of ca. 460 mT) after the
reaction (Figure 5 and see Supporting Information Figure S2).
To evaluate recyclability of the catalyst, the recovered HT and
FeHT100 were tested for further recycle reactions. HT and
FeHT100 maintained high catalytic performance and good

8
50
Bull. Chem. Soc. Jpn. Vol. 83, No. 7 (2010)
Magnetically Separable Hydrotalcite Catalyst
HT
FeHT100
1
00
100
80
60
40
20
0
80
60
40
20
0
After 4 cycles
1
st 2 nd 3 rd 4 th
1 st 2 nd 3 rd 4 th
Figure 6. Recyclability of HT and FeHT100 catalysts. Photo shows case of FeHT100.
magnetic separability for at least 4 reactions for FeHT100
Figure 6). However, the reaction using FeHT25 showed 52%
(
yield and 77% selectivity, lower than that of other FeHTXs.
Fe O nanoparticles exhibited poor activity for this reaction
3
4
(4% yield). A control experiment revealed that the physical
mixture of HT and coprecipitated Fe O at similar composi-
3
4
tions as FeHT25 showed good catalytic activity (77% yield and
1% selectivity). The FeHT25 catalyst synthesized by an
9
adsorption method (where HT and mono-FeNPs were stirred
together in water solution and filtered) showed 50% yield
and 89% selectivity. Furthermore, the magnetization scarcely
effects on catalytic activity, because the relationships between
magnetization and catalytic activity was not collated (see
Supporting Information Figure S1), and the external magnetic
field from magnetic stirrer was very weak under reaction
condition (ca. 30 mT). Therefore, we suppose that the lower
activity of FeHT25 was attributable to excess of mono-FeNPs,
which acted as an inhibitor of the reaction.
FeHT100
Amberlyst-15
Figure 7. Magnetic separation after one-pot synthesis.
Further Catalytic Application for a One-Pot Synthesis of
5
-Hydroxymethylfurfural (HMF) from Glucose.
For
expanding catalytic application of magnetic HTs, FeHT100
was examined as a heterogeneous catalyst for one-pot synthesis
of 5-hydroxymethylfrufural (HMF) from glucose. HMF is a
material with good potential as an intermediate for various
chemical materials.53 HMF production from glucose is very
attractive from the viewpoint of effective utilization of biomass,
because glucose is the most abundant monosaccharide. We
have very recently demonstrated that HMF production from
glucose can be achieved in one-pot in the presence of solid acid
and solid base catalysts.55 In one-pot synthesis, base catalyst
isomerizes glucose to fructose, and acid catalyst dehydrates
fructose to HMF, resulting in efficient production of HMF with
high selectivity. Using 0.05 g HT as solid base catalyst and
Conclusion
In summary, we have succeeded in preparation of a highly
active and separable magnetic Mg­Al hydrotalcite catalyst
using mono-FeNPs. This hybrid material acted as a heteroge-
neous solid base and could be reused without significant loss of
activity, selectivity, and magnetic properties for epoxidation of
2-cyclohexen-1-one using hydrogen peroxide. The magnetic
Mg­Al hydrotalcite catalyst also showed activity and good
separability for the one-pot synthesis of 5-hydroxymethyl-
furfural from glucose.
5
4
0
.1 g Amberlyst-15 as solid acid catalyst showed 9% yield
and 30% HMF selectivity. When we used 0.05 g FeHT100 and
.1 g Amberlyst-15, 9% yield and 28% HMF selectivity were
We thank Ms. Mary Ann Mooradian (JAIST) for her careful
reading of, and useful comments about, the manuscript.
0
Supporting Information
obtained under the same conditions. After the reaction, the
FeHT100 catalyst was easily separated by using a magnet, and
Amberlyst-15 was quickly precipitated as shown in Figure 7.
This indicated that two different solid catalysts could be
separated using FeHT100, with activity comparable to HT.
Relationships of Fe concentration, catalytic activity, and
magnetization, and photographs of magnetic separation after
first reaction. This material is available free of charge on the
web at http://www.csj.jp/journals/bcsj/.

S. Nishimura et al.
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