Comparison of the catalytic activity of gold nanoparticles supported in ceria and incarcerated in styrene copolymer

Article abstract of DOI:10.1007/s10562-009-0243-1

The catalytic activity of gold nanoparticles supported on nanoparticulated ceria or incarcerated in a styrene copolymer has been compared for three general gold catalyzed reactions, namely the aerobic benzylamine and aniline oxidations and for phenylacetylene-aminebenzaldehyde coupling. Remarkable differences between the activity of both catalysts were observed.

Full text of DOI:10.1007/s10562-009-0243-1

Catal Lett (2010) 134:204–209
DOI 10.1007/s10562-009-0243-1
Comparison of the Catalytic Activity of Gold Nanoparticles
Supported in Ceria and Incarcerated in Styrene Copolymer
•
Yolanda Perez Carmela Aprile Avelino Corma
•
•
´
Hermenegildo Garcia
Received: 18 October 2009 / Accepted: 29 November 2009 / Published online: 22 December 2009
Ó Springer Science+Business Media, LLC 2009
Abstract The catalytic activity of gold nanoparticles
supported on nanoparticulated ceria or incarcerated in a
styrene copolymer has been compared for three general
gold catalyzed reactions, namely the aerobic benzylamine
and aniline oxidations and for phenylacetylene-amine-
benzaldehyde coupling. Remarkable differences between
the activity of both catalysts were observed.
intriguing facts in gold catalysis is the strong dependence of
the activity on the nature of the support [3, 4]. This is the case
of the low-temperature CO oxidation for which nanopar-
ticulated ceria renders gold nanoparticles more active than
titania P-25 while deposition on silica makes gold nano-
particles inert for this reaction [4, 5]. Also, we have found
that aniline coupling to form azobenzenes is strongly
dependent on the nature of the support, titania being a con-
venient support while gold on active carbon was inactive [6].
Two of the most widely used types of supports for gold
nanoparticles are metal oxides and organic polymers. Con-
sideringtheinfluenceofthesupport,animportantissueingold
catalysis is to compare the activity among different supports.
Typically comparison of the catalytic activity of gold nano-
particles depending on the supports is routinely performed in
each work min the field of gold catalysis. However, upto now,
no information about the relative catalytic activity of gold
supported on nanoparticulate ceria or polymeric support has
been provided from data of the same laboratory carrying out
the experiments under the sameconditions. This manuscript is
aimed at providing this information.
Keywords Gold catalysts ꢀ Aerobic oxidation ꢀ
Alkyne coupling
1 Introduction
Gold catalysis is currently a topic of much interest due to the
relatively unexplored catalytic activity of this element
compared to other noble metals [1–4]. There is a consider-
able number of examples showing that the catalytic activity
of supported gold can be unique for some processes,
exceeding that of other noble metals [2]. One of the most
In this context, it has recently reported that gold nano-
particles of small size and narrow size distribution entan-
gled in the polymeric network of a styrene copolymer
exhibit an exceedingly high catalytic activity for the room
temperature aerobic oxidation of alcohols [7]. This poly-
mer-entangled gold catalyst has also been found highly
active for the aerobic oxidation of hydroquinones and their
derivatives [8, 9]. Considering the remarkable performance
of this catalyst based on gold nanoparticles entangled in
styrene copolymer and that other metal oxide supports such
as ceria nanoparticles are also highly active, it would be
desirable to determine the relative performance of both
types of gold supported catalysts under the same conditions
for other gold-catalyzed reactions different from alcohol
C. Aprile ꢀ A. Corma (&) ꢀ H. Garcia (&)
´
Instituto Universitario de Tecnologıa Quımica CSIC-UPV,
Universidad Politecnica de Valencia, Av. De los Naranjos s/n,
´
´
46022 Valencia, Spain
e-mail: acorma@itq.upv.es
H. Garcia
e-mail: hgarcia@qim.upv.es
´
Y. Perez
Universidad Rey Juan Carlos, Tulipan s/n, 28933 Mostoles,
´
´
Madrid, Spain
e-mail: yolanda.cortes@urjc.es
123

Comparison of the Catalytic Activity of Gold Nanoparticles
205
oxidation. It is the purpose of this report to compare the
catalytic activity of gold nanoparticles supported on
nanoparticulated ceria and in styrene copolymer for three
general types of gold catalyzed reactions including two
aerobic oxidations and one-three-component coupling.
mineral oil) in anhydrous THF (70 mL) and the mixture
was stirred at room temperature for 1 h. Then, 4-vinylb-
enzyl chloride (6.40 mL, 45.4 mmol) was added. The
reaction was further stirred for 12 h, then cooled at 0 °C,
diluted with diethyl ether and quenched by adding a satu-
rated solution of ammonium chloride. The mixture was
extracted with diethyl ether, the combined organic layers
were dried over sodium sulfate and the solvent removed
under reduced pressure. The crude was purified by
flash chromatography to affort 2-(2-(2-(2-(4-vinylbenzyl-
oxy)ethoxy)ethoxy)ethoxy)ethanol as pale yellow oil with
2 Experimental
2.1 Preparation of Au/npCeO2
1
First nanoparticulate ceria was produced. The preparation
of nanoparticulated ceria was carried out following a
reported procedure [10]. In short, an ammonia aqueous
solution (1.12 L, 0.8 M) was added under stirring at
ambient temperature over 375 mL of a Ce(NO3)4 (0.8 M).
The colloidal dispersion of CeO2 nanoparticles was heated
in a poly(ethylene terephthalate) vessel at 100 °C during
24 h. The resulting yellow precipitate was filtered and
dried under vacuum overnight. The cerium oxide synthe-
sized has, owing to the small size of the nanoparticles, a
very high surface area (180 m2 g-1).
a yield of 72%. H NMR (CDCl3) d: 3.45–3.55 (m, 2H),
3.59–3.70 (m, 14H), 4.53 (s, 2H), 5.27 (d, 1H,
J = 10.4 Hz), 5.80 (d, 1H, J = 18.0 Hz), 6.78 (dd, 1H,
J = 10.4 and 17.9 Hz), 7.35 (d, 2H, J = 8.4 Hz), 7.48 (d,
2H, J = 8.4 Hz). 13C NMR (CDCl3) d: 61.6, 69.4, 70.5,
70.6, 70.7, 72.1, 73.2, 113.8, 127.0, 128.2, 136.2, 137.0,
138.9.
2.2.2 Synthesis of 4-Vinylbenzyl Glycidyl Ether
To a stirred suspension of NaH (1.92 g, 48 mmol, 60% in
mineral oil) in anhydrous THF (40 mL), glycidol
(1.01 mL, 16 mmol) was added at 0 °C. The mixture was
stirred for 1 h at room temperature and 4-vinylbenzyl
chloride (4.50 mL, 32 mmol) was added. After 12 h, the
reaction was cooled at 0 °C, diluted with diethyl ether and
a saturated aqueous solution of ammonium chloride was
added as quencher. The aqueous layer was extracted three
times with diethyl ether, the combined layers were dried
over sodium sulfate and the solvent removed under reduced
pressure. The crude was purified by flash chromatography
to affort product 4-vinylbenzyl glycidyl ether as pale yel-
Au was deposited on the nanoparticulated ceria using
the deposition/precipitation procedure [11]. In brief, a
solution of HAuCl4ꢀ3H2O (90 mg) in 160 mL of deionised
water was brought to pH 10 by addition of a solution of
NaOH 0.2 M. Once the pH value was stable the solution
was added to a gel containing colloidal CeO2 (4.01 g) in
H2O (50 mL). After adjusting the pH of the slurry at a
value of 10 by addition of a 0.2 M solution of NaOH, the
slurry was stirred vigorously for 18 h at room temperature.
The Au/npCeO2 solid was then filtered and exhaustively
washed with several litres of distilled water until no traces
of chlorides were detected by the AgNO3 test. The catalyst
was dried under vacuum at room temperature for 1 h. Then
3.5 g of the supported catalyst were added over 30 g of
1-phenylethanol at 160 °C and the mixture was allowed to
react for 20 min. The catalyst was filtered, washed, with
acetone and water, and dried under vacuum at room
temperature. The total Au content of the final catalyst
Au/npCeO2 was 0.5 wt% as determined by inductively
coupled plasma (ICP) chemical analysis after digesting at
60 °C the Au/CeO2 in aqua regia (1:10 in weight). For the
preparation of 2.0 wt% Au/npCeO2 the method used was
the same as above starting with 350 mg of HAuCl4ꢀ3H2O.
1
low oil with a yield oh 60%. H NMR (CDCl3) d: 2.60–
2.67 (m, 1H), 2.80–2.88 (m, 1H), 3.18–3.25 (m, 1H), 3.39–
3.45 (m, 1H), 3.77–3.83 (d, 1H, J = 11.7 Hz), 4.59 (s, 2H),
5.27 (d, 1H, J = 10.7 Hz), 5.78 (d, 1 H, J = 17.4 Hz),
6.75 (dd, 1H, J = 10.7 and 17.4 Hz), 7.34 (d, 2H,
J = 8.4 Hz), 7.44 (d, 2H, J = 8.4 Hz). 13C NMR (CDCl3)
d: 35.8, 44.0, 50.8, 71.2, 113.0, 125.8, 129.3, 135.2, 136.4,
138.4.
2.2.3 Synthesis of Styrene Copolymer
The styrene copolymer was obtained by dissolving a
mixture of styrene (1.05 g), 2-(2-(2-(2-(4-vinylbenzyl-
oxy)ethoxy)ethoxy)ethoxy)ethanol (3.0 g), 4-vinylbenzyl
glycidyl ether (2.0 g), and 2,20-azobis(4-methoxy)-2,4-
dimethylvaleronitrile (92.0 mg) in chloroform (11.0 mL).
The solution was continuously purged with Ar and stirred
for 72 h at room temperature. The resulting polymer
solution was slowly poured into ether. The precipitated
polymer was separated by centrifugation, washed several
2.2 Preparation of Au/PSt
2.2.1 Synthesis of 2-(2-(2-(2-(4-vinylbenzyloxy)ethoxy)
ethoxy)ethoxy)ethanol
Tetraethyleneglycol (8.81 mL, 45.4 mmol) was carefully
added to a suspension of NaH (1.82 g, 45.5 mmol, 60% in
123

´
Y. Perez et al.
206
times with ether and dried for 24 h in vacuo to afford the
desired copolymer as a white solid with a yield of 50%.
other. The weight percentage of gold on the catalysts was
determined by ICP chemical analysis (Au/CeO2) and
quantitative X-ray fluorescence (Au/PSt). Au/PSt was
prepared as previously reported in the literature by cross-
linking a copolymer having three different styrene mono-
mers. The two monomers required for the synthesis of PSt,
namely 2-(2-(2-(2-(4-vinylbenzyloxy)ethoxy)ethoxy)eth-
oxy)ethanol and 4-vinylbenzyl glycidyl ether, in combi-
nation with styrene were previously synthesized and
characterized by 1H- and 13C-NMR spectroscopy. 1H-NMR
spectra of both monomers are represented in Fig. 1. During
preparation of Au/PSt we notice that the amount of Au
loaded in the polymer is difficult to control. This problem
could be associated to the way in which the gold nano-
particles interact with the polymer and remain entrapped by
the polymer chains. Therefore, to adjust the gold loading to
the target value, it is necessary to perform several trials
until a sample with the desired gold content is obtained. In
contrast, the deposition/precipitation method widely used
in the literature to obtain gold nanoparticles supported on
metal oxides appears reliable and reproducible with respect
to gold loading [11].
2.2.4 Synthesis of Au/PSt
This catalyst was prepared as reported [7]. Briefly, Au/PSt
was obtained by dissolving (AuClPPh3) in a small amount
of THF and adding this solution to a mixture of styrene
copolymer and NaBH4 in THF at room temperature. The
color of the solution immediately turned red. Hexane was
then added to form microcapsules, which were filtered,
washed, and dried under vacuum. The microcapsules were
next heated at 150 °C for 5 h to cross-link the side chains
and then washed with THF and water to afford Au/PSt as a
deep-purple solid.
2.3 Procedure for Oxidation of Benzylamine
to N-benzylidenebenzylamine
Benzylamine (107.16 mg, 1 mmol) dissolved in toluene
(2 mL) in the presence the solid catalyst (Au/benzylamine
mol ratio 1%), was stirred in a glass reactor (2.0 mL,
SUPELCO), pressurized with O2 (5 bars) and heated to
100 °C. The conversion was determined by GC analysis
and the products identified by GC–MS.
A series of Au/CeO2 and Au/PSt at two different gold
loadings about 0.5 and 2 wt% was used throughout this
comparative study. Figures 2 and 3 show representative
TEM images of catalysts Au/CeO2 and Au/PSt, respectively.
Particle size distribution for Au/CeO2 and Au/PSt at 0.5
wt% was determined from TEM images by counting a
statistically relevant number of particles in each case.
Figure 4 presents the results. As it can be seen there, both
catalysts show narrow size distribution with an average
gold particle size of 4.8 and 3.4 nm for Au/CeO2 and
Au/PSt, respectively. No significant differences were
observed when the loading was about 2 wt%, the average
2.4 Procedure for Aerobic Oxidation of Aniline
to Azobenzene
Aniline (93.13 mg, 1 mmol) dissolved in toluene (2 mL) in
the presence the solid catalyst (Au/aniline mol ratio 1%)
was stirred in a glass reactor (2.0 mL, SUPELCO), pres-
surized with O2 (5 bars) and heated to 100 °C. The con-
version was determined by GC analysis and the products
identified by GC–MS.
2.5 Procedure for Three-component Coupling of
Benzaldehyde, Piperidine and Phenylacetylene
Phenylacetylene (132.8 mg, 1.3 mmol), piperidine (102.2 mg,
1.2 mmol), benzaldehyde (106.1 mg, 1 mmol) and H2O
(1 mL) in the presence the solid catalyst (Au/benzaldehyde
mol ratio 0.12%) in a closed glass reactor (2.0 mL, SUPE-
LCO) were heated to 100 °C. The oil was separated from
water and analyzed by GC and identified by GC–MS.
3 Results and Discussion
Herein we have synthesised ceria supported gold nano-
particles (Au/CeO2) [5] and polystyrene entangled gold
nanoparticles (Au/PSt) [7] and compared them with each
Fig. 1 1H-NMR spectrum of 2-(2-(2-(2-(4-vinylbenzyloxy)eth-
oxy)ethoxy)ethoxy)ethanol (a) and 4-vinylbenzyl glycidyl ether (b)
123

Comparison of the Catalytic Activity of Gold Nanoparticles
207
Fig. 2 Bright (a) and dark field
(b) TEM images of 0.5 wt%
Au/CeO2 and (c)
Fig. 3 Bright field TEM
images of 0.5 wt% Au/PSt
polymer
Fig. 4 Particle size distribution
od Au/CeO2 (a) and Au/PSt (b)
20
10
0
30
20
10
0
a
b
1
3
5
7
9
11
13
1
3
5
Size (nm)
Size (nm)
particle size being for both catalysts below 5 nm. From the
TEM study it can be concluded that regardless the support
and at the loading used, no influence of the particle size on
the catalytic performance should be expected.
particle size. The preferred catalyst for this aerobic oxi-
dation was gold supported on active carbon that exhibits an
improved performance over gold supported on titania [14].
Also, it was found that for benzyl amine oxidation the
smaller the particle size, the higher the activity of the
catalyst. Thus, considering the structural similarity
between active carbon and PSt since both are organic solids
constituted by aromatic six members rings and the small
particle size of gold nanoparticles in Au/PSt, it can be
anticipated that this Au/PSt will exhibit high activity.
The first reaction tested was the aerobic oxidation of
benzyl amines to N-benzylidene benzyl amine. This reac-
tion has been recently investigated by Angelici, who has
shown that even micrometric bulk gold nanoparticles can
be active for this reaction [12, 13]. This reaction has been
found to be influenced by the nature of the support and the
123

´
Y. Perez et al.
208
Table 1 Oxidation of benzylamine to N-benzylidenebenzylaminea
In contrast, gold supported on active carbon was found
inactive to promote this oxidation [6]. Therefore, this ani-
line oxidation presents a different scenario than the previ-
ous oxidation of benzyl amine for which active carbon and
metal oxides were found to be suitable supports. Therefore
for the aerobic aniline oxidation, we anticipated based on
the similarity in structure and composition between active
carbon and PSt for which aromatic rings are present, that
active carbon and PSt could behave similarly. If this were
the case, then, Au/PSt should be notably inactive to pro-
mote aniline coupling. The results are shown in Table 2.
As it can be seen there, our expectations were correct and
Au/PSt was remarkably inactive as catalyst to promote the
aerobic oxidation of aniline. Azobenzene could only barely
be detected using 0.5 wt% Au/PSt. This catalytic behavior
sharply contrasts with the excellent performance of Au/
CeO2. Complete conversions with over 93% selectivity
were obtained for Au/CeO2.
NH2
N
Au Catalyst
O2 (5 bar)
Catalyst
Conversion (%)b
Yield/%c
Time/h
1
48
19
22
3
4
80
41
50
15
6
95
61
93
33
24
[99
97
0.5% Au/CeO2
2% Au/CeO2
0.5% Au/PSt
1.6% Au/PSt
a
99
95
97
73
[99
76
Reaction conditions: benzylamine (107.16 mg, 1 mmol) dissolved
in toluene (2 mL) in the presence the solid catalyst (Au/benzylamine
mol ratio 1%), stirred in a glass reactor (2.0 mL), pressurized with O2
(5 bars) and heated to 100 °C
b
The conversion was determined by GC analysis and the products
identified by GC–MS
The last reaction that was tested was the three compo-
nent coupling of benzaldehyde, piperidine and phenyl-
acetylene to form N-(1,3-diphenyl-2-propynyl)piperidine.
The reaction and the catalytic results obtained are indicated
in Table 3. Again, as in the previous two processes studied,
the catalytic performance of Au/PSt was notably inferior to
that of Au/CeO2. We have concluded in previous studies
that the cationic gold Au(III) species are the active sites for
this reaction [10] and that this positive gold species
becomes stabilized by using CeO2 as support [5]. Appar-
ently, the data from Table 3 suggest that the population of
positive Au(III) in Au/PSt is very minor.
c
The yield was determined by GC analysis at 24 h reaction time
The results obtained are indicated in Table 1. As it can be
seen there, we noticed a negative influence of the gold
loading on its catalytic activity, the best performing cata-
lysts being those with lower loading. This effect has been
previously also observed and attributed to the presence in
the catalyst of larger particle size as the loading increases
[7]. This somewhat larger particle size would reduce the
overall efficiency of gold with loading due to the smaller
surface area of the gold particles. In our case, we have not
observed significant particle size changes comparing
loading of 0.5 and 1.6 wt%, Thus, it could be that it is the
increase in the dispersion of gold nanoparticles on the
support, i.e. the percentage of surface area covered by gold,
the factor that is reducing the catalytic activity and favor-
ing low loadings due to higher dispersion. From the results
shown in Table 1, it can be concluded that for this aerobic
oxidation of benzyl amines, Au/CeO2 and Au/PSt samples
at the lowest gold loading (0.5%) exhibit similar catalytic
efficiency even if the first catalyst has a shorter induction
period. What appears clear is that an increase in the catalyst
gold loading has a detrimental effect on the performances
of both types of catalysts.
Table 2 Aerobic oxidation of aniline to azobenzenea
NH2
Au Catalyst
O2 (5 bar)
N
N
Catalyst
Conversion/%b
Yield (%)c Selectivity (%)
Time/h
6
10
100
31
2
10
93
23
–
10
93
74
–
0.5% Au/CeO2
2% Au/CeO2
0.5% Au/PSt
1.6% Au/PSt
a
98
29
1
The second reaction that was studied was the aerobic
oxidative coupling of aniline to azobenzene. In contrast to
the previous reaction for which gold supported on active
carbon was a highly active catalyst, for the aerobic oxi-
dation of aniline it has been determined that the nature of
the support plays an extremely important role [6]. Actually,
it has been reported that nanoparticles of CeO2 exhibit
already significant activity that is boosted further when
gold nanoparticles are deposited on it [6].
No reaction
Reaction conditions: Aniline (93.13 mg, 1 mmol), toluene (2 mL)
in the presence the solid catalyst (Au/aniline mol ratio 1%), stirred in
a glass reactor (2.0 mL), pressurized with O2 (5 bars) and heated to
100 °C
b
The conversion was determined by GC analysis and the products
identified by GC–MS
c
The yield was determined by GC analysis at 10 h reaction time
123

Comparison of the Catalytic Activity of Gold Nanoparticles
209
PSt is notably inefficient for the aerobic oxidative coupling
of aniline. The present data suggest a parallelism between
the catalytic behavior of gold supported on active carbon
and Au/PSt. One point of concern is the experimental
difficulty in anticipating the final gold loading on Au/PSt
based on the amount of AuClPPh3 present in the synthesis.
Considering the influence of gold loading on the catalytic
activity, this poor reliability can complicate catalyst
preparation.
Table 3 Three-component coupling of benzaldehyde, piperidine and
phenylacetylenea
CHO
Au Catalyst
H2O
N
+
+
N
H
Acknowledgments Financial support by the Spanish DGI (CTQ
2006-6857) is gratefully acknowledged. CA also thanks the Spanish
Ministry of Science and Education for a Juan de la Cierva Research
Associate Contract.
Conversion (%)b
Yield (%)c
1 h
4 h
6 h
98
0.5% Au/CeO2
53
60
15
14
90
95
56
23
97
99
83
30
2% Au/CeO2
0.5% Au/PSt
1.6% Au/PSt
a
[99
87
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4 Conclusions
Our study provides comparative data of the influence of the
support on the catalytic activity of gold nanoparticles for
three reactions. Based on these three reactions it can be
concluded that Au/CeO2 is the catalyst of choice and out-
performs Au/PSt. Thus, although the activity of Au/PSt for
the aerobic oxidation of benzyl amines is remarkable, Au/
´
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