DOI: 10.1002/chem.201002251
On the Stabilization of Gold Nanoparticles over Silica-Based Magnetic
Supports Modified with Organosilanes
Rafael L. Oliveira,[a] Daniela Zanchet,[b, c] Pedro K. Kiyohara,[d] and Liane M. Rossi*[a]
Abstract: The immobilization of gold
nanoparticles (Au NPs) on silica is
made possible by the functionalization
of the silica surfaces with organosi-
lanes. Au NPs could only be stabilized
and firmly attached to silica-support
surfaces that were previously modified
with amino groups. Au NPs could not
be stabilized on bare silica surfaces and
most of the NPs were then found in
the solution. The metal–support inter-
actions before and after the Au NP for-
mation, observed by X-ray absorption
fine structure spectroscopy (XAFS), in-
dicate a stronger interaction of gold-
ACHTUNGTRENN(UNG III) ions with amino-modified silica
surfaces than with the silanol groups in
bare silica. An amino-modified, silica-
based, magnetic support was used to
prepare an active Au NP catalyst for
the chemoselective oxidation of alco-
hols, a reaction of great interest for the
fine chemical industry.
Keywords: gold
·
nanoparticles
·
organosilanes
·
silica X-ray
·
absorption spectroscopy
Introduction
functional alcohol oxidation than other metals and
alloys.[14–16] Pioneers in this field, Corma et al.[17] performed
very interesting mechanistic studies on the selective oxida-
tion of alcohols by Au NPs. The systematic study of the in-
fluence of the Au NP size, the nature of the support, and
the catalyst preparation procedure has contributed to the
design of more active gold catalysts. Much progress in this
field has led to improved catalytic activities;[18–31] however,
the use of traditional inorganic supports still requires
workup procedures, such as filtration or centrifugation, to
collect the supported catalysts. Multifunctional nanomateri-
als containing magnetic NPs have emerged as a new class of
catalyst supports with the great advantage of being readily
collected and recovered from the reaction mixture by using
an external magnetic field. Catalyst separation and recycling
is achieved by simple magnetic separation without the need
of filtration, centrifugation, or any other time and energy-
consuming workup procedure.[32–39] We have successfully ap-
plied magnetic separation to the easy recovery, recycle, and
reuse of Pd,[32,33] Rh,[34] Ru,[35] and Pt[36] NPs for application
in hydrogenation and oxidation reactions. In these examples,
a silica-coated, magnetic NP catalyst support was used.
In general, supported Au NPs are prepared by the deposi-
tion–precipitation or co-precipitation procedure and the NP
sizes are tentatively adjusted by varying experimental pa-
rameters, such as the pH, reducing agent, concentration of
precursors in solution, and temperature of calcination. How-
ever, the surfaces of acid or hydrophobic supports, such as
SiO2, WO3, and active carbon, are not suitable for the depo-
sition of anionic species and usually lead to large >20 nm
supported Au NPs. In such acid supports, other synthetic
methods are required to produce small Au NPs, such as dep-
osition of pre-synthesized colloidal gold,[40–44] chemical vapor
deposition,[45] or atomic beam deposition.[46] In general,
these methods involve many steps, the use of expensive sta-
The selective oxidation of alcohols to yield aldehydes, ke-
tones, carboxylic acids, and their derivatives is one of the
most important reactions in organic chemistry.[1] The chemo-
selective oxidation, by discrimination of oxygen-sensitive
functionalities such as carbon–carbon double-bond function-
al groups and hydroxyl groups—for example, the oxidation
of allylic alcohols to a,b-unsaturated carbonyl compounds—
is very interesting for the fine chemical industry. In general,
the use of metal-based oxidants containing osmium, chromi-
um, and manganese, which form stoichiometric amounts of
residues, is no longer tolerable.[2] The use of molecular
oxygen in oxidation reactions has been encouraged, because
water is theoretically the only co-product. Many catalysts
based on metal nanoparticles (NPs), such as palladium,
silver, and platinum, have been developed to make the use
of this “green oxidant” possible.[3–13] More recently, Au NPs,
though less active, have shown better selectivity in multi-
[a] Dipl.-Chem. R. L. Oliveira, Prof. Dr. L. M. Rossi
Instituto de Quꢀmica, Universidade de S¼o Paulo
Av. Prof. Lineu Prestes 748 S¼o Paulo
05508-000 S¼o Paulo (Brazil)
Fax : (+55)1138155579
[b] Prof. Dr. D. Zanchet
Laboratꢁrio Nacional de Luz Sꢀncrotron, CP 6192
Campinas, 13083-970 S¼o Paulo (Brazil)
[c] Prof. Dr. D. Zanchet
Instituto de Quꢀmica, Universidade Estadual de Campinas
CP 6154 Campinas, 13083-970, S¼o Paulo (Brazil)
[d] Prof. Dr. P. K. Kiyohara
Instituto de Fꢀsica, Universidade de S¼o Paulo, CP 66318
05315-970 S¼o Paulo (Brazil)
Supporting information for this article is available on the WWW
4626
ꢂ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2011, 17, 4626 – 4631