Microgel-stabilized gold nanoclusters: Powerful "quasi-homogeneous" catalysts for the aerobic oxidation of alcohols in water

Article abstract of DOI:10.1016/j.jcat.2007.07.024

Gold nanoclusters of small size (2.5 nm) and narrow size distribution were synthesized in solution using tailor-made soluble cross-linked polymers (microgels) as exotemplates and stabilizers. The resulting microgel-stabilized nanoclusters could be conveniently isolated by precipitation, stored in the solid state, and redispersed in water and polar organic solvents. They were found to exhibit remarkable catalytic activity (average TOF up to 960 h^-1) in the aerobic oxidation of benzylic and aliphatic alcohols and also of polyols in water under mild conditions (50-70 °C, 1-3 atm O₂).

Full text of DOI:10.1016/j.jcat.2007.07.024

Journal of Catalysis 251 (2007) 1–6
www.elsevier.com/locate/jcat
Microgel-stabilized gold nanoclusters: Powerful “quasi-homogeneous”
catalysts for the aerobic oxidation of alcohols in water
Andrea Biffis a,∗, Sara Cunial a, Paolo Spontoni b, Laura Prati b,∗
a
Dipartimento di Scienze Chimiche, Università di Padova, via Marzolo 1, I-35131 Padova, Italy
b
Dipartimento di Chimica Inorganica, Metallorganica ed Analitica, Università di Milano, via Venezian 21, I-20133 Milano, Italy
Received 7 June 2007; revised 17 July 2007; accepted 20 July 2007
Available online 28 August 2007
Dedicated to Professor Benedetto Corain in occasion of his 65th birthday
Abstract
Gold nanoclusters of small size (2.5 nm) and narrow size distribution were synthesized in solution using tailor-made soluble cross-linked poly-
mers (microgels) as exotemplates and stabilizers. The resulting microgel-stabilized nanoclusters could be conveniently isolated by precipitation,
stored in the solid state, and redispersed in water and polar organic solvents. They were found to exhibit remarkable catalytic activity (average
−1
◦
TOF up to 960 h ) in the aerobic oxidation of benzylic and aliphatic alcohols and also of polyols in water under mild conditions (50–70 C,
1–3 atm O ).
2
© 2007 Elsevier Inc. All rights reserved.
Keywords: Alcohol; Microgel; Oxidation; Dioxygen; Water; Gold
1. Introduction
activity and/or selectivity has been demonstrated for other reac-
tions as well, most notably in the field of liquid-phase selective
oxidations, particularly in the oxidation of carbohydrates and of
polyols [4].
Gold can be arguably defined as the catalytic metal of the
beginning of the twenty-first century. Indeed, research on gold
metal, gold(III) compounds (most notably AuCl3), and gold(I)
complexes has permeated nearly all aspects of catalytic chem-
istry in the course of the last 10 years, with applications includ-
ing a plethora of different reactions, such as hydrogenations,
oxidations, and C–C and C–heteroatom bond-forming reac-
tions [1]. In particular, research on nanosized gold metal cata-
lysts has been actively pursued, because it has been recognized
that size- and shape-controlled gold nanoparticles (“gold nano-
clusters”) [2] exhibit superior catalytic performance compared
with randomly sized and shaped particles, at least for certain
reactions. A powerful demonstration of this principle is the re-
cently developed process for CO oxidation by supported gold
nanoclusters, where the best catalytic activities are achieved
with nanoparticles around 3 nm in size [3]. More recently, the
importance of the size of gold nanoclusters in determining good
Among the synthetic strategies which have been proposed
for the preparation of gold nanoclusters [5], the oldest and most
commonly used is arguably their generation in solution by re-
duction of suitable precursors in the presence of a stabilizer
(sometimes the reducing agent itself) able to interact with the
metal surface and thus prevent agglomeration [6]. By control-
ling the nature and amount of stabilizer as well as the reduction
conditions, it is sometimes possible to direct the growth of gold
nanoparticles to a definite size [4b,7,8]. The resulting stabilized
gold nanoclusters are dispersed in solution and can be used as
catalysts as such [7,8] or subsequently heterogenized on solid
supports by different means (e.g., surface adsorption, covalent
anchoring, embedding by sol–gel techniques) to prevent their
aggregation under reaction conditions [9]. However, immobi-
lization often results in a decrease of the active surface area of
the nanoclusters and, in turn, of their specific catalytic activity.
Therefore, the stabilization of dispersions of gold nanoclusters
under reaction conditions represents a way to finalize their high
activity.
*
Corresponding authors. Fax: +39 049 8275223.
E-mail address: andrea.biffis@unipd.it (A. Biffis).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.07.024

2
A. Biffis et al. / Journal of Catalysis 251 (2007) 1–6
We [10] and others [11–15] have recently introduced micro-
solution almost immediately changed in color from pale yellow
to reddish-brown or reddish-purple, depending on the microgel
used. The solution was concentrated to about half of its original
volume, and the nanocluster-containing microgel was subse-
quently precipitated by pouring the solution into the fivefold
volume of diethylether under efficient stirring. Isolated yields
were about 90% in all cases.
gels as stabilizers for metal nanoclusters. Microgels [16] are
nanoscopic objects in themselves, in that they are cross-linked
macromolecules with a globular shape, 101–102 nm in size.
Such macromolecules can be prepared by slight modification of
standard polymerization techniques and resemble in their struc-
ture and behavior soluble cross-linked biological macromole-
cules, such as proteins. Microgels build up low-viscosity, stable
solutions in appropriate solvents and can be easily isolated from
these by precipitation, ultracentrifugation, or ultrafiltration. Re-
markably, they can be tailored to bear chemical functionalities
that can interact with metal ions or complexes, which are subse-
quently reduced inside the microgel to yield metal nanoclusters.
The resulting nanoparticles grow to a final size that is influ-
enced by the morphology of the microgel molecule, which thus
acts as an exotemplate for the nanoclusters [10b].
Microgel-stabilized, size-controlled metal nanoclusters have
recently found promising applications in different fields ranging
from catalysis to drug delivery and materials science [10–15].
In particular, we have previously demonstrated the usefulness
of Pd nanoclusters as catalysts in C–C coupling reactions, such
as Heck and Suzuki couplings [10b,10c], as well as in the se-
lective oxidation of secondary alcohols to the corresponding
carbonyl compounds with molecular oxygen in water [10e].
In this contribution, we extend our investigation to microgel-
stabilized Au nanoclusters and demonstrate their potential as
catalysts for the aerobic oxidation of alcohols.
2.3. Preparation of Au nanoclusters immobilized on active
carbon
1% Au on AC catalyst was prepared as described previously
with slight modifications [9b]. A polyvinyl-alcohol-stabilized
sol was prepared by adding solid NaAuCl4·2H2O (0.043 mmol)
and PVA (410 µL 2% w/w solution in water) to 130 mL of
H2O. After 3 min, NaBH4 (1.3 mL 0.1 M solution in water)
was added to the yellow solution under vigorous magnetic stir-
ring. The brownish gold(0) sol was immediately formed. Within
a few minutes of its generation, the colloid (acidified at pH 2,
by sulphuric acid) was immobilized by adding activated car-
bon (X40S from Camel) under vigorous stirring. The amount
of support was calculated to have a final metal loading of 1%
w/w.
2.4. TEM measurements
Samples for TEM measurements were prepared by placing
a drop of a solution of microgel-stabilized metal nanoclusters
in dichloromethane or N,N-dimethylformamide on a carbon-
coated copper grid, followed by solvent evaporation at room
temperature. TEM micrographs were obtained at CIGS Univer-
sity of Modena, Italy, with a JEOL 2010 microscope with GIF
operated at an accelerating voltage of 200 keV. Average metal
nanocluster sizes and size distributions were computed as the
average of at least 100 particles taken from different fields.
2. Experimental
Solvents and chemicals were of reagent grade and were used
as received, apart from the monomers for microgel synthesis,
which were freshly distilled to free them from inhibitors before
use.
2.1. Preparation of microgel
2.5. Catalytic tests at atmospheric pressure
Monomers were mixed in the molar ratio N,N-dimethyl-
acrylamide:ethylene dimethacrylate:functional monomer 8:1:1
in a round-bottomed flask. The resulting mixtures (5 g) were
diluted with cyclopentanone (45 g). Azobis(isobutyronitrile)
(AIBN) (0.15 g, 3% w/w with respect to the monomer mix-
ture) was then added. The resulting solution was degassed, put
under nitrogen, and placed in a thermostatted oven preheated to
80 ◦C for 48 h. The polymerization solution was concentrated
to about half of its original volume and subsequently poured
into the fivefold volume of diethylether under efficient stirring.
The precipitated solid was filtered off and dried under vacuum
to constant weight. Isolated yields were >80% in all cases.
The catalytic tests were run in a three-necked, round-
bottomed flask equipped with a reflux condenser and a gas
inlet. In a typical procedure, a solution of microgel-stabilized
metal nanoclusters (50 mg) in distilled water (4 mL) was placed
into the flask, and 1-phenylethanol (0.13 mL, alcohol/metal ra-
tio = 100 mol/mol) was then added. Then the apparatus was
evacuated and flushed with dioxygen a couple of times, after
which it was connected to a balloon filled with dioxygen. The
reaction was started by placing the flask into an oil bath pre-
heated to 100 ◦C and starting efficient magnetic stirring. After
the given reaction time, the reaction solution was transferred
into a separation funnel and extracted with two 2-mL portions
of diethylether. The organic layers were combined, and their
content in reactant and product was determined by 1H NMR.
2.2. Preparation of microgel-stabilized Au nanoclusters
In the general procedure, 0.5 g of microgel was dissolved in
dichloromethane (80 mL) under an inert atmosphere. HAuCl4
(44 mg, 0.13 mmol) was then added, and the resulting solu-
tion was stirred at room temperature for 24 h. Subsequently,
NaHBEt3 (1.1 mL 1 M solution in THF) was added, and the re-
sulting solution was stirred at room temperature for 1 day. The
2.6. Catalytic tests under dioxygen pressure
The reactions were carried out in a thermostatted glass reac-
tor (30 mL) provided with an electronically controlled magnetic

A. Biffis et al. / Journal of Catalysis 251 (2007) 1–6
3
stirrer and connected to a large reservoir (5000 mL) contain-
ing oxygen at 1.5 atm. The oxygen uptake was followed by a
mass flow controller connected to a PC through an A/D board,
plotting a flow/time diagram. Alcohol (0.3 M) and the catalyst
(alcohol/metal = 500–3000 mol/mol) were mixed in distilled
water (total volume 10 mL), containing 1 equivalent of NaOH.
The reactor was pressurized at the desired pressure of O2 (1.5–
3 atm) and thermostatted at the appropriate temperature (50–
70 ◦C). The reaction was initiated by stirring. In the case of
partially soluble substrates, after the completion of the reaction,
the catalyst was filtered off and the product mixture extracted
with toluene. Catalyst recoveries were always 98 3% with this
procedure. In the case of homogeneous reactions, withdrawal of
samples was carried out periodically.
Tests at controlled pH (glucose) were performed by setting
the pH at 9.5 in a 718 STAT Titrino (Metrohom) equipped with
a 0.3 M NaOH reservoir, pressurized at 3 atm of O2. Glucose
and the catalyst (substrate/metal = 3000 mol/mol) were mixed
in distilled water (0.3 M). The temperature of the stirred mix-
ture was kept at 50 ◦C, and as soon as the reaction started, a
solution of 0.3 M NaOH was added automatically to maintain
the pH of the solution at a fixed value (9.5 0.1). The samples
were analyzed at various times by high-pressure liquid chro-
matography (HPLC).
In the oxidation of 1-octanol, benzyl alcohol, and 1-phenyl-
ethanol, identification and analysis of products were carried
out by gas chromatography using a Dani 86.10 HT gas chro-
matograph equipped with a capillary column (BP21, 30 m ×
0.53 mm, 0.5 µm film thickness). Product identification was
accomplished by comparison with authentic samples. The ex-
ternal calibration method was used to quantify the reagents and
products.
In the oxidation of n-butanol, glycerol and glucose products
were analyzed and quantified by means of HPLC on a Varian
9010 HPLC equipped with a Varian 9050 UV (210 nm) and
a Waters RI detector in series. An Alltech OA-1000 column
(300 mm × 6.5 mm) was used with aqueous H3PO4 0.1% w/w
(0.5 mL/min) as the eluent. The procedures were as described
previously [4,17].
trolled, spontaneous reduction the rather large size of the re-
sulting Au nanoparticles (average diameter 10 4 nm), which,
as expected, exhibited poor activity in the aerobic oxidation
of alcohols [10e]. Therefore, our initial aim was to overcome
this problem by devising a way to control reduction of the
gold precursors more precisely. First, we changed the nature
of the gold(III) species and tried HAuCl4. However, despite the
fact that the spontaneous reduction process of microgel-bound
HAuCl4 was slower, it was still present. Therefore, we changed
the nature of the functional monomer in the microgels, choosing
to use the functional monomers methylthio-ethylmethacrylate
(MTEMA) or vinylpyridine (VP) instead of DMAEMA. Mi-
crogels containing the two novel functional monomers could
be prepared without deviating from the already-used synthetic
procedure. The molar composition of the monomer mixture
was DMAA:EDMA:functional monomer 8:1:1 in all cases;
the polymerization conditions were as reported in Section 2.
The microgels could be conveniently isolated by precipitation
as white powders that were readily redispersible in water and
in many different organic solvents, including alcohols, dialky-
lamides, nitriles, dichloromethane, acetone, and THF.
Microgel loading could be conveniently performed with
HAuCl4 in dichloromethane. The resulting solutions remained
golden yellow for hours with no observable color change, indi-
cating no metal reduction to Au nanoparticles. In contrast, an
almost instant color change to the characteristic reddish color
of Au nanoclusters occurred when NaHBEt3 was added to the
microgel solution.
TEM analyses were performed to assess the size of the
microgel-stabilized metal nanoclusters. Microgels based on
MTEMA as the nonfunctional monomer yielded Au nanoclus-
ters with an average size of 5.2 1.4 nm, whereas microgels
based on VP favored the formation of much smaller nanoclus-
ters (2.4 0.7 nm) with a narrow size distribution (Fig. 1).
These results confirm our previous observation that a more con-
trolled reduction of the microgel-bound metal precursors lead
to smaller metal nanoclusters with a narrower size distribution.
On the other hand, these results also point out that the functional
monomer may have a significant influence on the final size of
gold nanoclusters. We are currently investigating this aspect in
more detail.
3. Results and discussion
The dichloromethane solutions of microgel-stabilized Au
nanoclusters prepared as outlined above were clear and stable.
The nanocluster-containing microgels could be conveniently
isolated by precipitation and stored as powders. The powders
thus obtained could be readily redispersed in polar solvents
such as water, alcohols, or dialkylamides; in contrast, on stor-
age in the dry state, they quickly became largely insoluble in
less polar organic solvents, including dichloromethane (i.e., the
solvent in which they were prepared). We are currently attempt-
ing to rationalize this aging phenomenon.
As mentioned earlier, these microgels could be conveniently
used in solution for catalytic purposes. The advantage of using
dispersed nanoclusters instead of classical supported nanoclus-
ters lies in the higher exposed surface of the metal and also in
the resulting “quasi-homogeneous” system, which prevents dif-
fusional limitations.
We previously reported on the preparation of microgel-
stabilized Au nanoclusters using AuCl3 as metal precursor and
microgels prepared from N,N-dimethylacrylamide (DMAA)
as the main comonomer, ethylene dimethacrylate (EDMA) as
the cross-linker, and N,N-dimethylamino-ethylmethacrylate
(DMAEMA) as the functional, metal-binding comonomer
[10d,10e]. The microgels were loaded with gold(III) (5.5%
Au) by letting their trialkylamino functional groups coordi-
nate to AuCl3 in water or ethanol. However, we found that
the microgel-anchored gold(III) species prepared in this way
underwent spontaneous reduction already in the course of the
metal loading step. The actual reducing agents are probably
the polymer-bound trialkylamino groups themselves, which
are known to act in a similar way toward other, less easily
reducible metal centers [18]. We ascribed to such an uncon-

4
A. Biffis et al. / Journal of Catalysis 251 (2007) 1–6
Fig. 1. TEM micrographs of Au nanoclusters stabilized by microgels based on MTEMA (left) and VP (right).
The catalytic tests were initially run under the reaction con-
Table 1
Oxidation of alcohols with dioxygen in water catalyzed by microgel-stabilized
Au nanoclusters or 1% Au-AC
ditions originally proposed by Uozumi and Nakao for Pd metal
catalysts [19] (water, neutral pH, 100 ◦C, 1 atm O2, 1 mol%
metal) with 1-phenylethanol as standard substrate. We previ-
ously successfully used these conditions with other microgel-
stabilized metal nanocluster catalysts [10e]. Au nanoclusters
protected by microgels based on the MTEMA monomer were
completely inactive as catalysts. Considering the high affin-
ity of gold for sulfur-containing compounds, it seems that the
amount of thioether groups present in the microgel is sufficient
to completely deactivate the surface of the gold nanoclusters
for the reaction. It must be noted, however, that gold nanoclus-
ters supported on insoluble functional resins bearing the same
thioether groups in lower concentration are active catalysts for
this kind of reaction [20]. On the other hand, Au nanoclusters
protected by microgels based on the VP monomer were very
active for the reaction, yielding the product in 100% yield after
24 h of reaction (not optimized). This catalytic efficiency ap-
pears to be somewhat lower than that of microgel-stabilized Pd
nanoclusters of similar size, where complete conversion was
reached within 6 h under identical reaction conditions [10e],
but we note that such a result was obtained at neutral pH. In
fact, gold metal catalysts usually require a basic pH to display
activity for the aerobic oxidation of alcohols in water [7,21],
although it was recently demonstrated that the oxidation of ben-
zyl alcohol can be carried out with gold metal catalysts under
solvent-less, base-free conditions [22].
Alcohol
Time Catalyst
(h)
Conversion TOF
Selectivity
(%)
−1
(%)
(h
)
1-Phenylethanol
1
1
Microgel
1% Au-AC 61
75
375 100 (ketone)
305 100 (ketone)
Benzyl alcohol
0.25 Microgel
48
960 49 (aldehyde);
36 (acid)
0.25 1% Au-AC 57
1140 32 (aldehyde);
41 (acid)
1-Octanol
4
4
8
Microgel
1% Au-AC 24
65
59
74 84 (acid)
30 77 (acid)
93 (acid)
a
n-Butanol
2
4
2
4
Microgel
56
76
280 88 (acid)
100 (acid)
1% Au-AC 32
54
160 81 (acid)
89 (acid)
◦
Note. Reaction conditions: 0.3 M alcohol, 0.3 M NaOH, 1.5 atm O , 60 C,
2
1/500 metal/substrate (mol/mol).
a
◦
Reaction performed at 3 atm O , 70 C, 1/1000 metal/substrate (mol/mol).
2
1–3 atm O2). Using these reaction conditions also allowed a
meaningful comparison with other previously tested gold-based
catalysts.
First, we verified that by using temperatures in the range 50–
70 ◦C, no precipitation of Au-VP microgel was observed even
using a strongly basic additive (NaOH). Subsequently, we ran
the catalytic tests using as substrates n-butanol, n-octanol, ben-
zyl alcohol, 1-phenylethanol, and also glycerol and glucose to
investigate the chemoselectivity of the reaction. The results are
shown in Tables 1 and 2, where data obtained by using gold
nanoparticles of similar dimension (3–4 nm) but supported on
activated carbon (1% Au-AC) are also reported for comparison.
Microgel-stabilized Au nanoclusters showed in general good
activity with simple alcohols and, apart from the case of ben-
zyl alcohol, behaved slightly better than carbon-supported nan-
oclusters of almost the same size. The recorded catalytic activ-
ity was remarkable with all substrates, reaching, as expected, its
We set out to evaluate the reactivity of our catalyst at basic
pH, but unfortunately our attempts were frustrated by catalyst
precipitation in the course of the reaction, which invariably
occurred above ca. 80 ◦C. At present, it is unclear whether pre-
cipitation must be ascribed to microgel decomposition and/or
nanoparticle aggregation, or else to a thermomorphic behav-
ior of the microgel; such a behavior actually has been already
observed with Pd nanoclusters stabilized by the same micro-
gel [23].
Therefore, we chose to change the reaction conditions, and
adopted those routinely used by Prati et al. with supported Au
colloids (1/500 to 1/3000 metal/substrate molar ratio, 50–70 ◦C,

A. Biffis et al. / Journal of Catalysis 251 (2007) 1–6
5
Table 2
When aliphatic alcohols were used, the beneficial effect of
using gold nanoclusters in microgel-stabilized, quasi-homo-
geneous form instead of having them heterogenized on carbon
was more apparent. The activity of the Au-VP catalyst was
ca. twice that of Au-AC, whereas selectivities at isoconversion
were comparable. Moreover, apparently the more oxidizable
the substrate becomes, the more 1% Au-AC becomes active
with respect to Au-microgel; indeed, the difference in activity
between the two catalysts decreases in the order n-octanol > n-
butanol > 1-phenylethanol > benzyl alcohol, with 1% Au-AC
being more active than Au-microgel in the latter case.
Using multifunctionalized substrates, such as glycerol or
glucose, we instead observed that 1% Au-AC was always more
active than Au-microgel, as shown in Table 2. We also observed
a difference in selectivity between Au-microgel and Au/C, with
former being more prone to C–C bond scission than the latter.
Thus, it seems that the catalyst activity also could be related to
the hydrophobicity or hydrophilicity of the substrate. In fact,
partial agglomeration of catalyst and substrate was observed
when n-octanol was used as the substrate and Au-AC as the
catalyst [27]. This behavior can be clearly ascribed to the hy-
drophobic nature of the carbon surface, which could be wetted
by octanol more readily than by water. Incidentally, catalyst ag-
glomeration, as well as preferential absorption of the reagent,
limit the accessibility of the catalyst surface by the water phase
in which the second reactant (O2) is dissolved, thereby limiting
the reaction rate. This limiting step was not observed using Au-
microgel that was rapidly dissolved in the aqueous medium. In
this case, 1-octanol was dispersed into the solution by stirring.
When more hydrophilic substrates were used instead, a “nor-
mal” triphasic system was obtained, and access to the active
surface was not limited by an organic layer even in the case of
Au-AC.
Oxidation of glycerol and glucose with dioxygen in water catalyzed by
microgel-stabilized Au nanoclusters or 1% Au-AC
Substrate Time Catalyst
(h)
Conversion Selectivity (%)
(%)
Glycerate Glycolate Tartronate
a
b
b
b
b
b
b
Glycerol 0.25 Microgel
10
15
29
47
50
52
60
60
61
48
46
46
26
26
23
1
1
1
7
8
0.5
1
0.25 1% Au-AC 26
0.5
1
55
83
11
c
d
d
d
d
d
d
Glucose 0.5
Microgel
6
15
35
>99
>99
>99
>99
>99
>99
1
4
0.25 1% Au-AC 28
0.5
1
56
81
a
◦
Reaction conditions: 0.3 M glycerol, 1.2 M NaOH, 3 atm O , 50 C, 1/1000
2
metal/substrate (mol/mol).
b
Selectivity to glycerate.
Reaction conditions: 0.3 M glucose, 3 atm O , 50 C, 1/3000 metal/
c
◦
2
substrate (mol/mol), maintaining pH at 9.5 by adding 0.3 M NaOH.
d
Selectivity to gluconate.
maximum with activated alcohols, such as benzyl alcohol and
1-phenylethanol.
The reaction gave full selectivity for ketone in the case of
secondary alcohols, whereas with primary alcohols (and espe-
cially with aliphatic alcohols), the carboxylic acid was formed,
sometimes together with a limited amount of the corresponding
ester. This tendency toward further oxidation of the intermedi-
ate aldehyde was also expected, because it is well known that
aldehydes are hydrated in water and in this form are oxidized
by gold metal catalysts more efficiently than the starting alco-
hol [1].
4. Conclusion
The obtained activities compare favorably with most other
reported catalytic systems based on Au metal nanoparticles.
Hutchings’s Au/TiO2 or Au/zeolite catalysts yielded at low con-
version with benzyl alcohol a TOFt=0.5 h of 200–500 h−1 at
100 ◦C and 2 atm O2 under solvent-less conditions [22]. Baik-
er’s catalyst (Au on Cu–Mg–Al mixed oxides) gave at 90 ◦C
and 1 atm O2 in mesitylene a TOFt=0.5 h of 316 h−1 with ben-
zyl alcohol and a TOFt=3 h of 11 h−1 with 1-octanol; much
better results were obtained with 1-phenylethanol (TOFt=1 h of
1294 h−1) [24]. Oxidation of 1-phenylethanol with Kobayashi’s
Au on polystyrene gave a TOFt=1 h of 32 h−1 at room tempera-
ture and 1 atm O2 in benzotrifluoride:water 1:1 [25]. Oxidation
of 4-hydroxybenzyl alcohol using Tsukuba’s PVP-stabilized
Au nanoclusters in water gave an actual TOFt=1 h at 23 ◦C and
1 atm O2 of 15 h−1 and an estimated TOFt=1 h at 60 ◦C and
1.5 atm O2 of 33 h−1 [4b,7]. Finally, Corma’s Au on nanocrys-
talline CeO2 with benzylic alcohols exhibited a TOFt=2 h of
about 80 h−1 at 50 ◦C and 1 atm O2 in water [26], although
in this case alcohol oxidation actually was carried out by the
cerium(IV) centers in the support, with the Au metal merely
acting to regenerate these centers with concomitant O2 reduc-
tion.
In the present work, we have demonstrated the possibil-
ity of extending our method for the preparation of microgel-
stabilized, size-controlled metal nanoclusters to Au nanopar-
ticles. Through proper choice of the functional monomer in
the microgel (vinylpyridine), Au nanoclusters of small size and
with a narrow size distribution can be generated. The resulting
microgel-stabilized Au nanoclusters have been shown to be re-
markably active “quasi-homogeneous” catalysts for the aerobic
oxidation of primary and secondary alcohols in water. From a
catalytic standpoint, we note that the exposed surface plays a
determining role, and also that the relative affinity of the sub-
strate and the support for aqueous media should be taken into
account in forecasting the catalytic activity of gold particles in
liquid-phase oxidation.
References
[1] A.S.K. Hashmi, G.J. Hutchings, Angew. Chem. Int. Ed. 45 (2006) 7896,
and references cited therein.
[2] M.-C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293.
[3] (a) M. Haruta, M. Daté, Appl. Catal. A Gen. 222 (2001) 427;
(b) M. Valden, X. Lai, D.W. Goodman, Science 281 (1998) 1647.

6
A. Biffis et al. / Journal of Catalysis 251 (2007) 1–6
[4] (a) L. Prati, F. Porta, J. Catal. 224 (2004) 397;
[13] (a) D. Suzuki, H. Kawaguchi, Langmuir 21 (2005) 8175;
(b) D. Suzuki, H. Kawaguchi, Langmuir 21 (2005) 12106;
(c) D. Suzuki, H. Kawaguchi, Langmuir 22 (2006) 3818.
[14] (a) Y. Lu, Y. Mei, M. Drechsler, M. Ballauff, Angew. Chem. Int. Ed. 45
(2006) 813;
(b) H. Tsunoyama, H. Sakurai, T. Tsukuda, Chem. Phys. Lett. 429 (2006)
528;
(c) S. Carrettin, P. McMorn, P. Johnston, K. Griffin, C.J. Kiely, G.J. Hutch-
ings, Phys. Chem. Chem. Phys. 5 (2003) 1329.
[5] (a) G. Schmid (Ed.), Nanoparticles, Wiley–VCH, Weinheim, 2004;
(b) T. Sugimoto (Ed.), Fine Particles: Synthesis, Characterization and
Mechanisms of Growth, Marcel Dekker, New York, 2000;
(c) D.L. Feldheim, C.A. Foss (Eds.), Metal Nanoparticles: Synthesis,
Characterization and Applications, Marcel Dekker, New York, 2002.
[6] J. Turkevitch, P.C. Stevenson, J. Hilliers, Discuss. Faraday Soc. 11 (1951)
55.
[7] H. Tsunoyama, H. Sakurai, Y. Negishi, T. Tsukuda, J. Am. Chem. Soc. 127
(2005) 9374.
[8] C. Comotti, C. Della Pina, R. Matarrese, M. Rossi, Angew. Chem. Int.
Ed. 43 (2004) 5812.
(b) Y. Lu, Y. Mei, M. Ballauff, M. Drechsler, J. Phys. Chem. B 110 (2006)
3930;
(c) Y. Mei, Y. Lu, F. Polzer, M. Ballauff, M. Drechsler, Chem. Mater. 19
(2007) 1062.
[15] (a) A. Pich, A. Karak, Y. Lu, A.K. Ghosh, H.-J.P. Adler, Macromol. Rapid
Commun. 27 (2006) 344;
(b) H. Xia, Y. Zhang, J. Peng, Y. Fang, Z. Gu, Colloid Polym. Sci. 284
(2006) 1221.
[16] (a) W. Funke, O. Okay, B. Joos-Müller, Adv. Polym. Sci. 136 (1998) 139;
(b) B.R. Saunders, B. Vincent, Adv. Colloid Interface Sci. 80 (1999) 1.
[17] S. Biella, L. Prati, M. Rossi, J. Catal. 206 (2002) 242.
[18] See, for example, R. McCrindle, G. Ferguson, G.J. Arsenault, A.J.
McAlees, J. Chem. Soc. Chem. Commun. (1983) 571.
[19] Y. Uozumi, R. Nakao, Angew. Chem. Int. Ed. 42 (2003) 194.
[20] C. Burato, P. Centomo, G. Pace, M. Favaro, L. Prati, B. Corain, J. Mol.
Catal. A Chem. 238 (2005) 26.
[21] See, for example, S. Biella, G.L. Castiglioni, C. Fumagalli, L. Prati, M.
Rossi, Catal. Today 72 (2002) 43.
[22] (a) D.I. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Car-
ley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings,
Science 311 (2006) 362;
[9] (a) J.-D. Grunwaldt, C. Kener, C. Woeggerbauer, A. Baiker, J. Catal. 181
(1999) 223;
(b) L. Prati, G. Martra, Gold Bull. 32 (1999).
[10] (a) A. Biffis, J. Mol. Catal. A Chem. 165 (2001) 303;
(b) A. Biffis, N. Orlandi, B. Corain, Adv. Mater. 15 (2003) 1551;
(c) A. Biffis, E. Sperotto, Langmuir 19 (2003) 9548;
(d) L. Minati, A. Biffis, Chem. Commun. (2005) 1034;
(e) A. Biffis, L. Minati, J. Catal. 236 (2005) 405.
[11] (a) M. Antonietti, F. Gröhn, J. Hartmann, L. Bronstein, Angew. Chem. Int.
Ed. 36 (1997) 2080;
(b) N.T. Whilton, B. Berton, L. Bronstein, H. Henze, M. Antonietti, Adv.
Mater. 11 (1999) 12;
(b) G. Li, D.I. Enache, J. Edwards, A.F. Carley, D.W. Knight, G.J. Hutch-
ings, Catal. Lett. 110 (2006) 7.
(c) L.M. Bronstein, D.M. Chernyshov, R. Karlinsey, J.M. Zwanziger, V.G.
Matveeva, E.S. Sulman, G.N. Demidenko, H.-P. Hentze, M. Antonietti,
Chem. Mater. 15 (2003) 2623.
[23] E. Sperotto, Tesi di Laurea, Università di Padova, 2003.
[24] P. Haider, A. Baiker, J. Catal. 248 (2007) 175.
[25] H. Miyamura, R. Matsubara, Y. Miyazaki, S. Kobayashi, Angew. Chem.
Int. Ed. 46 (2007) 4151.
[26] A. Abad, P. Concepción, A. Corma, H. Garcia, Angew. Chem. Int. Ed. 44
(2005) 4066.
[12] (a) S. Xu, J. Zhang, C. Paquet, Y. Lin, E. Kumacheva, Adv. Funct.
Mater. 13 (2003) 468;
(b) J. Zhang, S. Xu, E. Kumacheva, J. Am. Chem. Soc. 126 (2004) 7908;
(c) I. Gorelikov, L.M. Field, E. Kumacheva, J. Am. Chem. Soc. 126 (2004)
15938.
[27] N. Dimitratos, A. Villa, D. Wang, L. Prati, F. Porta, D. Su, J. Catal. 244
(2006) 113.