High performance magnetic separation of gold nanoparticles for catalytic oxidation of alcohols

Article abstract of DOI:10.1039/b916825g

We present the magnetic separation approach to facilitate the recovery of gold nanoparticle (AuNP) catalysts. The use of magnetically recoverable supports for the immobilization of AuNPs instead of traditional oxides, polymers or carbon based solids guara

Full text of DOI:10.1039/b916825g

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PAPER
www.rsc.org/greenchem | Green Chemistry
High performance magnetic separation of gold nanoparticles for catalytic
oxidation of alcohols†
a
b
a
Rafael L. Oliveira, Pedro K. Kiyohara and Liane M. Rossi*
Received 14th August 2009, Accepted 30th September 2009
First published as an Advance Article on the web 29th October 2009
DOI: 10.1039/b916825g
We present the magnetic separation approach to facilitate the recovery of gold nanoparticle
AuNP) catalysts. The use of magnetically recoverable supports for the immobilization of AuNPs
(
instead of traditional oxides, polymers or carbon based solids guarantees facile, clean, fast and
efficient separation of the catalyst at the end of the reaction cycle. Magnetic separation can be
considered an environmentally benign separation approach, since it minimizes the use of auxiliary
substances and energy for achieving catalyst recovery. The catalyst preparation is based on the
3
+
immobilization of Au on the surface of core–shell silica-coated magnetite nanoparticles,
followed by metal reduction using two different methods. AuNPs were prepared by thermal
reduction in air and by hydrogen reduction at mild temperature. Interestingly, the mean particle
size of the supported AuNPs was similar (ca. 5.9 nm), but the polydispersity of the samples is
quite different. The catalytic activity of both catalysts in the aerobic oxidation of alcohols was
investigated and a distinct selectivity for benzyl alcohol oxidation was observed.
Introduction
Simple filtration is inefficient to accomplish product isolation
in systems comprised of nanoparticles stabilized in solution or
very thin solids. In these circumstances, an alternative that has
drawn attention lately is the use of magnetically recoverable
solid supports. Magnetic separation is easy, fast, clean and a
very promising recovery tool for isolation of thin solids from
the reaction medium. It can be considered an environmentally
benign separation approach, since it minimizes the use of
auxiliary substances and energy for achieving catalyst recovery.
Magnetic supports, mainly composed of superparamagnetic
nanoparticles, can be attracted to relatively low static magnetic
field strengths and are easily recovered, since no residual
magnetization is observed when the magnetic field is removed.
The immobilization of homogeneous and NP-based catalysts on
magnetically separable solid supports has been shown to be a
The development of new strategies for the recovery and recycling
of catalysts, which minimizes the consumption of auxiliary
substances, energy and time used in achieving separations can
result in significant economical and environmental benefits.
Gold in the bulk form has been regarded as being chemically
inert towards chemisorption of reactive molecules such as
oxygen and hydrogen. Consequently, bulk gold was considered
to be an uninteresting metal from the point of view of catalysis.
However, the catalytic properties of gold are revealed when the
size is reduced to few nanometers, particularly with dimension
1
less than 10 nm. The overall performance of gold nanoparticle
(
AuNP) catalysts highly depends on the size and shape of
the nanoparticles, the structure and properties of the catalyst
2
support and the gold–support interface interactions. A variety
4-7
powerful tool for catalyst recovery and recycling.
of interesting catalytic properties of AuNPs in both oxidation
We report herein the preparation of AuNPs supported on
a magnetic solid that can be easily recovered from solution
by applying an external magnetic field, while keeping good
control over different parameters, such as particle size, size
distribution and catalytic activity. The oxidation of alcohols
was used to probe the catalytic activity of the catalyst. Catalytic
oxidations are green alternatives to stoichiometric oxidations
with permanganate, manganese dioxide and chromium(VI)
reagents, major sources of waste, particularly in fine chemicals
1-3
and reduction reactions has been reported. Despite this, little
attention has been paid to possible difficulties arising from
size reduction, and the challenges of separating the very small
particles, most of the time in a colloidal equilibrium, from
the products. In this regard, AuNPs have been dispersed on
solid supports to increase the catalyst stability and to facilitate
3
the catalyst recovery. As the size of the support decreases,
separation using physical methods, such as filtration or cen-
trifugation, becomes a difficult and time-consuming procedure.
8
and pharmaceuticals manufacture.
a
Experimental
Instituto de Qu ´ı mica, Universidade de S a˜ o Paulo, 05508-000, S a˜ o Paulo,
SP, Brazil. E-mail: lrossi@iq.usp.br; Fax: +55 11 3815 5579; Tel: +55
Synthesis of silica-coated magnetite nanoparticles
1
1 30912181
b
Instituto de F ´ı sica, Universidade de S a˜ o Paulo, 05508-090, S a˜ o Paulo,
SP, Brazil
The synthesis of oleic acid-coated Fe
3
O
4
nanoparticles and the
7
core–shell Fe @SiO composite was reported elsewhere.
3
O
4
2
† Electronic supplementary information (ESI) available: TEM and EDS
analyses of the spent catalyst. See DOI: 10.1039/b916825g
The solid was modified with amino groups by reaction with
1
44 | Green Chem., 2010, 12, 144–149
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3
-aminopropyltriethoxysilane (APTES) in dry toluene
under to give the amino-functionalized support
Fe
N ,
@SiO
2
(
3
O
4
2
/NH
2
). The final solid was washed with toluene
◦
and dried at 100 C for 20 h.
Synthesis of supported Au(0) nanoparticles
Supported AuNPs were prepared as follows: 50 mg of
Fe
3
O
4
@SiO
2
/NH
2
was added to 10 mL of an aqueous HAuCl
4
-
1
◦
solution (1 g L , pH~6). The mixture was stirred at 25 C for
2
h. The solid was then magnetically collected from the solution
and washed twice with hot distilled water. The Au(0)NPs were
Scheme
AuNPs.
1
Step-by-step preparation of magnetically recoverable
obtained by reduction of the Au(III) precursor either by thermal
◦
treatment (oven, 150 C (constant), 8 h, in air) or by H
(
2
reduction
◦
3
+
glass reactor, 650 mg 2, 3 mL cyclohexane, 80 C, 4 atm H
2
,
approach to prepare AuNPs is based on the uptake of Au ions
3
h). The color of the solid changes from brown to dark purple
from HAuCl aqueous solution by the silica-coated magnetic
4
after formation of Au NPs.
nanoparticles, previously functionalized with –NH2 groups,
followed by metal reduction under mild conditions. The amino-
functionalized support (1) was loaded with 0.72 wt% of gold,
as determined by atomic absorption analysis (ICP OES). It is
interesting to notice that the non-functionalized support, when
submitted to a similar metal solution, do not adsorb gold ions.
Characterization methods
High-resolution transmission electron microscopy (HRTEM)
and X-ray energy dispersive spectroscopy (EDS) were performed
at the Laborat o´ rio Nacional de Luz S ´ı ncrotron (LNLS, Cam-
pinas, SP) using a Jeol-3010 ARP microscope, and conventional
TEM images were performed using a Philips CM 200 microscope
3
+
The Au -loaded magnetic solid (2) was used as precursor for
the preparation of supported AuNPs either by thermal treatment
(
catalyst 3) or by H
AuNPs could be evidenced by the SPR bands at 567 nm (catalyst
) and 522 nm (catalyst 4) observed in the UV-Vis spectra shown
2
reduction (catalyst 4). The formation of the
(
IF-USP). Samples for TEM observations were prepared by
placing a drop containing the nanoparticles in a carbon-coated
copper grid. The histograms of the gold nanoparticles size
distribution, assuming a spherical shape, were obtained from
the measurement of the diameter of about 300 particles found in
arbitrarily chosen areas of enlarged micrographs. The samples
were observed in different regions of the Cu grid.
3
in Fig. 1.
Catalytic experiments
All reactions were performed using a modified Fischer–Porter
glass reactor loaded under a positive pressure of dry N from
2
a Schlenk line. Liquids and solutions were transferred via a
syringe. In a typical reaction, the glass reactor was loaded
with the supported Au(0)NPs catalyst (50 mg, 1.8 mmol Au),
the substrate (1.5 mmol), and 2 mL of toluene. The reactor,
immersed in an oil bath, was loaded with O to the desired
2
pressure. The temperature was maintained by an oil bath placed
in a hot-stirring plate connected to a digital controller (ETS-D4
IKA). The reactions were conducted under magnetic stirring,
using a Teflon-coated magnetic stir bar, for the desired time.
The catalyst was magnetically recovered by placing a permanent
magnet in the reactor wall and the products were collected with
a syringe and analysed by gas chromatography (GC) and GC-
MS. The isolated catalyst could be reused when a new amount
of substrate was added.
3 4 2 2
Fig. 1 UV-Vis spectra of ethanolic suspensions of Fe O @SiO -NH 1
(
a), Au(0) catalyst 4 (b) and Au(0) catalyst 3 (c).
The morphology of the supported AuNPs were examined by
transmission electron microscopy (TEM), shown in Fig. 2a and
b. The support is comprised of magnetic cores (Fe ~ 10 nm)
3
O
4
spherically coated with silica (spheres of ~ 40 nm in diameter).
The mean diameter of the supported AuNPs in both catalysts
was estimated at 5.9 nm with polydispersity of 18% for catalyst
4
and 32% for catalyst 3. The AuNPs are fairly well-distributed
Results and discussion
over the support in both catalysts; however, the particle size
distribution of catalyst 4 is sharper if compared to catalyst 3,
as can be seen in the size distribution histograms adjusted to
lognormal functions presented in Fig. 2c–d. The SPR band of
catalyst 4 appears at lmax ca. 520 nm, which agrees with the higher
monodispersity of the AuNPs. Moreover, the lmax red shift of
about 45 nm and the broadened optical spectrum of catalyst 3
A schematic illustration of the step-by-step approach used for
the preparation of gold catalysts composed of supported AuNPs
on a magnetically recoverable solid is shown in Scheme 1. The
magnetically recoverable support used in this study consists of
core–shell silica-coated magnetite nanoparticles (Fe
prepared following the procedure described recently. The
3
O
4
@SiO
2
)
6
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Fig. 2 TEM images of catalyst 3 (a) and catalyst 4 (b). Histogram
showing particle size distribution and lognormal fitting of the AuNPs
of catalyst 3 (c) and the AuNPs of catalyst 4 (d).
can be realized by aggregation of small particles, light scattering
9
and lower overall quality of the supported nanoparticles.
The distribution and composition of each nanoparticle of the
nanocomposite was revealed by energy dispersive spectroscopy
(
EDS) analysis of the image shown in Fig. 3. The core
nanoparticles were found to be exclusively composed of Fe
atoms (Fig. 3b), and the darker nanoparticles of Au atoms
(
Fig. 3c), through a detailed EDS analysis of the image areas
shown in box (b) and box (c), respectively. The EDS analysis
shown in Fig. 3a corresponds to the whole nanocomposite which
contains Fe, Au and Si (Cu from sample holder).
Fig. 3 HRTEM image and EDS analysis of the AuNP magnetically
recoverable catalyst 4 in different areas of the image: (a) corresponds to
the whole area, (b) corresponds to the iron oxide particle shown in box
b and (c) corresponds to the gold particle shown in box c.
The magnetic properties of the Fe
3
O
4
@SiO nanocomposite
2
7
were investigated elsewhere. It displays excellent magnetic
properties characterized by negligible coercivity and remanence,
and very high saturation magnetization M
S
at room temperature
-
1
(
M
S
~ 69 emu g at 70 kOe). These magnetic properties
describe a superparamagnetic material that responds to an
external magnetic field but does not remain magnetized when
the magnetic field is removed. Such materials can be easily
concentrated from the solution with a magnet and immediately
redispersed after removing the magnetic field, since the absence
of remanence prevents aggregation of particles.
The catalytic oxidation of benzyl alcohol was used to probe the
activity and recovery of the supported AuNPs in catalysts 3 and
4
. The first attempt to catalyze the oxidation of benzyl alcohol
using our supported AuNPs was performed under atmospheric
pressure of O and no products (aldehyde or ester) were observed
Table 1, entry 1). It was also observed that in the absence of
2
(
base, the catalyst was not efficient in catalyzing the selective
◦
oxidation of alcohols, even at 100 C and 5 atm O
2
(Table 1,
Fig. 4 Pressure dependence of the catalytic performance of supported
AuNPs: catalyst 3 (circle) and catalyst 4 (square). Reaction conditions:
1.5 mmol of benzyl alcohol, 50 mg catalyst, 70 mg K CO , 2 mL toluene,
entry 2). The conversion increased to 75% (95% selectivity) as
long as a small amount of K CO was present (Table 1, entry 3).
As it can be seen in Fig. 4, there is a strong dependence of the
2
3
2
3
◦
100 C, 6 h.
1
46 | Green Chem., 2010, 12, 144–149
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a
Table 1 Oxidation of alcohols by magnetically recoverable Au NP catalyst
b
Selectivity (%)
b
Entry
1
Catalyst
Substrate
P O
2
(atm)
Conversion (%)
Aldehyde or ketone
Ester (homocoupling)
c
3
3
3
3
4
3
4
3
4
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
1-Phenyl ethanol
1-Phenyl ethanol
Cyclohexanol
Cyclohexanol
—
0.0
2.2
75
84.3
100
100
100
31
—
99
95
95
—
1
5
5
53
0
0
0
0
d
2
5
5
6
3
6
3
6
6
3
4
5
6
7
8
9
47
100
100
100
100
42
a
◦
b
Reaction conditions: 1.5 mmol of substrate, 2 mL of toluene, 50 mg of catalyst (1.8 mmol Au), 70 mg K
2
CO
3
, 100 C, reaction time 6 h. Determined
c
d
2
by GC-MS. Bubbling O . Without base.
oxidation reaction with the O
2
pressure for both catalysts
3
and 4. Catalyst 3 is less active and requires higher pressure
for the substrate conversion, but the selectivity was maintained
in the pressure range studied. On the other hand, catalyst 4
is more active, reaching 100% conversion at 3 atm of O , but
2
the high activity is accompained by the drop of selectivity to
benzaldehyde to ca. 50%. The oxidation reaction catalyzed by
catalyst 3 is highly selective to benzaldehyde, found as the main
product. It is also known that benzaldehyde can be further
oxidized into benzoic acid, and, finally, benzyl alcohol and
10
benzoic acid would interact and produce benzyl benzoate. In
fact, benzyl benzoate was found as the only by-product in the
oxidation of benzyl alcohol by catalysts 3 and 4.
Secondary alcohols such as 1-phenyl ethanol and cyclohex-
anol (Table 1, entries 6–9) were also oxidized to the correspond-
ing ketones with 100% selectivity. In all experiments, the catalyst
was recovered by using a permanent magnet and the products
were collected with a syringe and analysed by GC-MS.
Fig. 5 Conversion and selectivity of the catalytic oxidation of benzyl al-
cohol to benzaldehyde using the magnetically recovered AuNP catalysts
3 and 4 (conditions similar to entry 3, Table 1).
Recycling experiments were examined for the catalytic ox-
atomic absorption analysis (ICP OES). Results have shown that
negligible Au leaching of our catalysts occurs (less than the limit
of quantitation of 2.5 ppb of Au). This demonstrates the high
affinity of gold and our magnetic support, the high performance
of the magnetic separation in the present catalytic system and,
as a first conclusion, indicates that the catalyst deactivation is
not related to metal loss. Another key factor to be considered
is the stability of the catalyst support and nanoparticles when
subjected to the reaction conditions. TEM images and EDS
analysis of the spent catalysts (see ESI†, Fig. S1) and TEM image
of the catalyst support subjected to basic reaction conditions
(see ESI†, Fig. S2) provided supporting evidence for the catalyst
deactivation under reuse. First, the morphology of the catalyst
◦
idation of benzyl alcohol at 5 atm of O
2
and 100 C using
catalysts 3 and 4. After the first reaction, the catalyst was
recovered in the reactor wall by using a permanent magnet,
the products were collected with a syringe, and the recovered
catalyst was washed with ethanol (2 mL) inside the reactor,
and dried under vaccum. The catalyst was reused in successive
oxidation reactions by adding new portions of solvent, base and
substrate. This procedure was repeated four times, however the
supported AuNP catalyst 3 lost 50% of the initial conversion,
considering the same reaction time. After the 4th cycle, the
conversion of benzyl alcohol was 35% but the selectivity to
aldehyde was maintained as shown in Fig. 5. The deactivation
process of catalyst 4 is more severe than catalyst 3 under similar
experimental conditions. The activity of 100% in the 1st run
drops to less than 10% in the 4th run.
The low reusability of the catalyst is a point of concern. One
of the key factors that must be considered is the possibility
that the active metal would leach into the reaction mixture,
especially in liquid phase oxidation reactions, because of the
possible dissolution of metals by the reactants and, particu-
larly, the carboxylic acid-type (by)products, thereby leading to
catalyst deactivation. To address this possibility, the isolated
products obtained after magnetic separation were analysed by
support (core–shell Fe
3
O
4
@SiO ) has changed; the silica layer
2
was strongly affected by the presence of base in the reaction
medium and the catalyst support lost the spherical morphology,
which caused aggregation. Second, large Au particles (~80 nm)
were revealed in the TEM image of spent catalyst, which suggests
the coalescence of the small, active gold particles into larger, less
active or inactive particles. The growth of the Au particles, above
1
the catalytic limit of about 10 nm, can also be a consequence
of the changes occurring in the catalyst support during the
reaction. Therefore, the use of base associated with a silica-
based catalyst support should be the main reason for the catalyst
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deactivation. Another point that might be relevant concerns the
presence of potassium as detected by EDS analysis. This is also
a consequence of the use of base that can cause metal surface
contamination.
It is also important to mention that some catalytic activity still
remains even after the 4th reuse (ca. 50% for catalyst 3), most
probably because small particles are still present, as indicated by
the TEM image.
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We successfuly prepared supported AuNP catalysts by the
reduction of gold salts previously immobilized on a magnetically
recoverable support. The method used in the reduction step
for metal nanoparticle formation can be crucial in determining
particle dispersion and distribution over the solid support and,
consequently, the catalytic activity of the metal nanoparticles.
In this work, two reduction methods were compared. Supported
AuNPs were prepared by thermal reduction in air and by
hydrogen reduction at mild temperature. Interestingly, the mean
particle size was similar ca. 5.9 nm, but the polydispersity of
the samples is quite different. The UV-Vis and TEM data of
the AuNPs prepared by hydrogen reduction, catalyst 4, suggest
that a higher control over the particle dispersion was achieved
by this reduction method. With respect to the catalytic activity,
catalyst 4 is more active in the aerobic oxidation reactions, but
less selective for the aldehyde product.
The catalysts reported here are very efficiently recovered by
magnetic separation, with negligible Au leaching to the solution.
This separation procedure is an efficient and green alternative to
the conventional separation techniques used for the recovery of
solids, such as filtration and centrifugation, since it is fast, clean,
easy to scale-up and constitutes a waste minimization and low
energy consumption procedure.
4
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but seems to be responsible for the catalyst deactivation under
reuse due to degradation of the catalyst support, which causes
metal particles to grow beyond the catalytically active limit.
The literature contains alternatives for the use of base in such
5
5.
3q,10
reactions, such as the use of basic and redox supports
4 For examples, see: (a) T.-J. Yoon, W. Lee, T.-S. Oh and J. K. Lee,
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2f
or bimetallic Au–Pd alloys. Additional experiments are in
progress in our laboratory in order to synthesize and charaterize
magnetic catalysts comprised of supports and/or metal particles
more resistant to base or, hopefully, active in the absence of
base.
(
2
g) D. K. Yi, S. S. Lee and J. Y. Ying, Chem. Mater., 2006, 18,
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The authors are grateful to the Brazilian agencies FAPESP and
CNPq for financial support and indebted to LNLS-Laborat o´ rio
de Microscopia Eletr oˆ nica (Brazil) for HRTEM images and
EDS analysis. The authors also acknowledge the Instituto
Nacional de Ci eˆ ncia e Tecnologia de Cat a´ lise em Sistemas
Moleculares e Nanoestruturados (INCT-CMN).
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Notes and references
1
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48 | Green Chem., 2010, 12, 144–149
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