Magnetic nanoparticles as a catalyst vehicle for simple and easy recycling

Article abstract of DOI:10.1039/b209391j

The surface of magnetic ferrite nanoparticles (CoFe₂O₄) was coated with a Rh-based cationic complex, [Rh(cod)(η⁶-benzoic acid)]BF₄, to make them homogeneously dispersable and thermodynamically stable without an

Full text of DOI:10.1039/b209391j

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Magnetic nanoparticles as a catalyst vehicle for simple and
easy recyclingy
Tae-Jong Yoon, Woo Lee, Yoon-Seuk Oh and Jin-Kyu Lee*
School of Chemistry and Molecular Engineering, Seoul National University, Seoul 151-747,
Korea. E-mail: Jinklee@snu.ac.kr; Fax: +82-2-882-1080; Tel: +82-2-880-8830
L e t t e r
Received (in Montpellier, France) 25th September 2002, Accepted 18th November 2002
First published as an Advance Article on the web 2nd January 2003
The surface of magnetic ferrite nanoparticles (CoFe2O4) was
coated with a Rh-based cationic complex, [Rh(cod)(g6-benzoic
acid)]BF4 , to make them homogeneously dispersable and ther-
modynamically stable without an excess amount of surface cap-
ping molecules in normal organic solvents. Since magnetic
nanoparticles themselves have strong magnetic properties and
could be considered as a nanometer-sized solid support for the
surface-anchored Rh-based catalyst, the ferrite nanoparticle-
supported Rh catalyst showed very effective catalytic activity
toward the hydroformylation reaction of olefins and could be
easily recovered from the reaction mixture by magnetic decanta-
tion to be used for subsequent reactions.
Immobilization of homogeneous catalysts on various support
materials such as organic polymers and inorganic silica and
other metal oxides has been investigated for many years to
Scheme 1 Schematic illustration of the synthesis of organic-soluble
magnetic nanoparticles and the recycling of the nanomagnet-supported
catalyst by magnetic decantation.
recover and recycle catalysts, especially when expensive and/
or toxic heavy metal complexes are employed.1 However,
immobilization of homogeneous catalysts usually decreases
the catalytic activity (or efficiency) due to the problem of diffu-
sion of reactants to the surface-anchored catalysts. In order to
increase the active surface area, porous support materials such
as zeolite and porous silica have been employed and have
shown some promising results.2,3 Another way to increase
the surface area is, of course, to utilize smaller sized support
materials with the same total volume. When the size of the sup-
port materials is decreased to the nanometer scale, the surface
area will increase dramatically and the nanometer-sized sup-
ports will even be dispersible in solution, forming an emulsion.
However, in this extreme case of immobilized systems, the
same problems of isolation and recycling of homogeneous cat-
alysts will also be encountered. If nanometer-sized support
materials have another characteristic property, such as ferro-
magnetism, which allows them to be dispersed or aggregated
by controlling the conditions (such as applying an external
magnetic field in the case of ferromagnetic materials), it seems
possible to separate and easily recycle immobilized catalysts on
the nanometer-sized supports. In this communication, we pro-
pose a new utilization of magnetic nanoparticles as a catalyst
support by modifying the surface of ferrite nanoparticles
(CoFe2O4) with an active Rh-based cationic catalyst, [Rh(cod)-
(Z6-benzoic acid)]BF4 , to make them homogeneously dispersed
and very stable in normal organic solvents without an excess
amount of surface capping molecules, and most importantly
easily isolated and recycled (Scheme 1).
Water-soluble Co-ferrite particles were prepared by a slight
modification of the known co-precipitation method,4 which
involves ionic sites on the surface leading to electrostatic repul-
sion between magnetic nanoparticles. The chemical composi-
tion of bare (water-soluble) Co-ferrite nanoparticles was
determined by both inductively coupled plasma-atomic emis-
sion spectroscopy (ICP-AES, SHIMADZU/ICPS-1000IV)
analysis and electron probe micro analysis (EPMA, WDS
mode, JEOL/JXA-8900R), giving reproducibly the composi-
tion in the form of (CoFe2O4)core(Fe0.19Ox)shell . Such a non-
stoichiometric deviation from the nominal composition of
typical spinel ferrites (i.e., MFe2O4) might originate from the
contribution of a stable amorphous ferric hydroxide [i.e.,
FeO(OH)] layer existing at the surface of the magnetic nano-
particles as suggested recently by Sousa et al.5
The powder XRD (Philips/PW 3710) data of the synthe-
sized nanoparticles exhibit the typical diffraction pattern of
the spinel structure, confirming the core structure of the mag-
netic Co-ferrite nanoparticle. The magnetic properties of the
Co-ferrite nanoparticles were determined with a vibrating sam-
ple magnetometer (VSM; Lake Shore Model 7304) and a value
of Ms, 293 K ꢀ 60 emu gꢁ1 was obtained.
In order to check the idea of utilizing magnetic nanoparticles
as a catalyst support, the known Rh-based hydroformylation
catalyst system was selected.6 The cationic rhodium complex
having a carboxylic acid functional group was prepared, which
could be attached onto the magnetic Co-ferrite nanoparticle as
suggested in the literature; carboxylic acid is a suitable binding
group for the surface of ferrite nanoparticles.7 [Rh(cod)(Z6-
benzoic acid)]BF4 (cod ¼ cyclooctadiene, compound 1) was
y Electronic supplementary information (ESI) available: XRD and
FT-IR data, as well as the detailed experimental conditions for the cat-
alytic hydroformylation reactions. See http://www.rsc.org/suppdata/
nj/b2/b209391j/.
DOI: 10.1039/b209391j
New J. Chem., 2003, 27, 227–229
227
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Table 1 Hydroformylation of 4-vinylanisole by the nanomagnet-
supported catalyst 2 (R ¼ C6H5OCH3)
easily prepared in high isolated yield by treating an ethanolic
solution of [Rh(cod)Cl]2 with 2 equiv. of benzoic acid and
AgBF4 ; this method is slightly modified from the known
synthetic method for {[Rh(cod)(Z6-arene)]BF4} in order to
introduce the carboxylic acid site.6 Compound 1 is soluble in
acetone, DMF, and methylene chloride, but insoluble in hex-
ane and diethyl ether. Organic-soluble [Rh(cod)(Z6-benzoic
acid)]BF4 coated Co-ferrite nanoparticles were easily obtained
by using a modified surface-capping reaction.
From the electron microscopic analyses of the organic-solu-
ble Co-ferrite magnetic nanoparticles derivatized with the Rh
complex, it was confirmed that well separated nanoparticles
with a size distribution ranging from 8 to 20 nm were homoge-
neously dispersed over the entire area (Fig. 1). The selected
area electron diffraction (SAED) pattern of the sample
revealed that the core spinel structure of the Co-ferrite nano-
particle was not altered during the surface modification pro-
cess. The chemical composition of the Rh-catalyst-capped
magnetic nanoparticle (nanomagnet-supported catalyst, 2)
was established by ICP-AES and EPMA (WDS mode), inde-
pendently, to be (CoFe2O4)core(Fe0.19Ox)shell–{[Rh(cod)(Z6-
Entry
Time/h
l/ba
Yield (%)b
1
3
3
3
3
3
1
9/91
> 99
> 99
> 99
> 99
> 99
> 99
2 (reused)
3 (reused)
4 (reused)
5 (reused)
6c
10/90
10/90
9/91
10/90
10/90
a
Structural ratio of aldehyde products was measured by 1H-NMR
spectroscopy (b: 9.26 ppm, l: 9.78 ppm); b stands for the branched
b
form and l for the linear form. Determined by 1H-NMR spectro-
c
scopy and gas chromatography.
compound 1.
Homogeneous reaction with
benzoic acid)]BF4}0.013
.
Nanomagnet-supported catalyst 2 can also be recovered with
the solubility control method by adding a large amount of a
poor solvent such as alcohol to the reaction mixture followed by
ultra-centrifugation, if preferred. The recovered nanomagnet-
supported catalyst (2) could be completely re-dispersed
in methylene chloride solvent again and could be reused with-
out any further unusual treatment. Reaction results from
consecutive usages (5 times) of catalyst 2 did not show any
differences in catalytic activities (entries 2–5 in Table 1).
In summary, an organic-soluble, processible and very stable
magnetic nanoparticle was successfully synthesized by deliber-
ately choosing an ionic organometallic compound as a surface-
stabilizing molecule. The novel stability of this magnetic
nanoparticle in organic solution is most likely to be due to
the ionic repulsive forces between the surface-anchored ionic
organometallic compounds, which can overcome the magnetic
dipolar interaction that leads to aggregation of magnetic ferrite
particles. The prepared organic-soluble magnetic nanoparticle
could be used as an immobilized homogeneous catalyst, and
the unique properties of the nanosized magnetic support
revealed several novel features. An excellent catalytic activity,
comparable to that of the homogeneous catalyst, could be
obtained and the simple and efficient recycling of the catalyst
by magnetic decantation was clearly demonstrated. This find-
ing is expected to be very important in connection with the
processing of magnetic nanoparticles with various hybridizing
materials, such as polymeric and inorganic matrices, and
suggests that it is also very attractive for application as a
new support material for precious organometallic catalysts.
The nanomagnet-supported catalyst (2) showed excellent
catalytic activity with a good regioselectivity toward the
hydroformylation reaction of 4-vinylanisole, which is compar-
able to that of its homogeneous counterpart (i.e., compound
1), as presented in Table 1. Although the catalytic activity of
2 is one-third that of its homogeneous counterpart, probably
due to steric and diffusion rate differences (entries 1–5 vs. entry
6), this relative activity of 2 seems to be extraordinarily high
based on the general trend of decreasing activity in immobi-
lized catalysts [e.g., inorganic-supported, biphasic (water–
organic phase), and organic polymer-supported catalysts].8–10
The high catalytic activity of 2, even though the actual catalyst
molecules are immobilized on the support, can be largely
attributed to the size effect of the ferrite nanoparticle support,
which provides a very large effective surface for the catalytic
reactions. Therefore, the present nanomagnet-supported cata-
lyst 2 can be regarded as a ‘‘quasi-homogeneous’’ catalyst: it
has a very large effective surface for the catalytic reactions that
are comparable to those of the homogeneous molecular cata-
lyst. Furthermore, the most profitable feature of 2 lies in its
unique recycling process, where catalyst recovery can be
achieved by simple magnetic decantation after applying a
magnetic field on the surface of the reaction vessel.
Experimental
The mixed solution of bare Co-ferrite nanoparticles and an
excess amount of compound 1 in DMF was stirred for 2 h
at room temperature. The product was precipitated from
the mixture by adding a large amount of chloroform and hex-
ane (DMF:chloroform:hexane ¼ 1:3:3) and was collected by
centrifugation. After the collected crude solid product had
been washed several times with a mixed solution of chloro-
form and hexane (v/v ¼ 1:1), the product was re-dispersed
in acetone and any insoluble materials were removed by cen-
trifugation. From the clear supernatant, a dark brown pow-
dery product was obtained after evaporating the solvent. It
showed excellent solubility in various organic solvents such
as CH2Cl2 , THF, acetone, and DMF with long-term stability
(more than a few months). The successful modification of the
surface of Co-ferrite nanoparticles with Rh complex was
Fig. 1 TEM micrograph of (CoFe2O4)core(Fe0.19Ox)shell–{[Rh(cod)(Z6-
benzoic acid)]BF4}0.013 . The insets are the electron diffraction
pattern at the selected area (below left) and the micrograph at high
resolution (above left).
228
New J. Chem., 2003, 27, 227–229

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Acknowledgements
This work was supported by the Korean Science and Engineer-
ing Foundation through the Center for Molecular Catalysis at
Seoul National University. T.-J. Yoon, W. Lee and Y.-S. Oh
are grateful for the award of a BK21.
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