PH-sensitive gold nanoparticle catalysts for the aerobic oxidation of alcohols

Article abstract of DOI:10.1021/ic201608j

Gold nanoparticles (NPs) stabilized by carboxylate modified polyvinylpyrrolidone have been prepared and fully characterized. The gold NPs efficiently catalyze the aerobic oxidation of benzyl alcohol in water at ambient temperature and are easily separated from the reaction mixture by lowering the pH of the solution, causing the NPs to precipitate. The mechanism of the precipitation process has been studied. Due to the efficiency of this process, the NPs may be reused as catalysts by readjusting their pH.

Full text of DOI:10.1021/ic201608j

ARTICLE
pubs.acs.org/IC
pH-Sensitive Gold Nanoparticle Catalysts for the Aerobic Oxidation
of Alcohols
Yuan Yuan, Ning Yan, and Paul J. Dyson*
Institut des Sciences et Ing ꢀe nierie Chimiques, Ecole Polytechnique F ꢀe d ꢀe rale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland
S Supporting Information
b
ABSTRACT: Gold nanoparticles (NPs) stabilized by carboxylate
modified polyvinylpyrrolidone have been prepared and fully char-
acterized. The gold NPs efficiently catalyze the aerobic oxidation of
benzyl alcohol in water at ambient temperature and are easily
separated from the reaction mixture by lowering the pH of the
solution, causing the NPs to precipitate. The mechanism of the
precipitation process has been studied. Due to the efficiency of this
process, the NPs may be reused as catalysts by readjusting their pH.
’
INTRODUCTION
Indeed, the catalytic activity of Au NPs is highly sensitive to their
4e
size and the surface modifiers used. Moreover, while the
mechanism of the pH-switch comprises a simple protonation/
deprotontion process, as far as we are aware, a detailed picture of
the aggregation process has yet to be reported.
Gold is chemically inert and catalytically inactive in the bulky
state but becomes highly catalytic for many reactions when1
formulated into nanoparticles (NPs) with diameters < 10 nm.
Since Haruta et al.’s discovery of CO oxidation, catalyzed by
supported Au NPs, the catalytic properties of Au NPs have been
extensively explored. In the domain of colloid chemistry (in
which NPs are homogenously dispersed in solvents), catalytically
2
Herein, we describe Au NPs stabilized by sodium poly(1-
vinylpirrolidin-2-one-3-carboxylate) (polymer 1 in Figure 1).
These NPs form pseudohomogenous aqueous solutions at pH
3
>
2.5 and spontaneously precipitate at pH < 2.4. Moreover, the
active Au NPs have been successfully prepared in the presence o4f
Au NPs are active, stable catalysts for the aerobic oxidation of
alcohols and operate under ambient conditions. By switching the
pH, the NPs could be easily separated from the reaction mixture,
enabling facile recycling with only a minor loss of the catalyst
various stabilizers, including poly(N-vinyl-2-pyrrolidone) (PVP),
7
5
6
dendrimers, amphiphilic copolymers, and polyelectrolytes.
These “soluble” NP systems allow reactions to be carried out
in a pseudohomogenous manner, generally leading to higher
activities and selectivities, and the application of milder reaction
(
0.0002% per cycle) and aggregation (from 6.7 to 7.9 nm) of
8
the NPs.
conditions compared to heterogeneous systems. One of the
limiting factors that hinders the practical use of soluble Au NP9
catalysts, however, is the difficulty in recycling and reusing them.
While, PVP-stabilized Au NPs are highly active catalysts for
aerobic alcohol oxidation, for example, recycling these NPs is
’
RESULTS AND DISCUSSION
Au NPs coated with stabilizer 1 (Figure 1), denoted as 1-Au
NPs, were prepared from an aqueous solution of HAuCl and the
stabilizer by NaBH reduction (see Experimental Section for
4
4
problematic. Indeed, only a few examples describing the recov-
4
10
ery of Au NP catalysts from water have been reported, and
consequently the development of new Au NP systems that can be
used in a facile fashion is desirable.
details). TEM of the 1-Au NPs shows that they are spherical with
a diameter of 6.7 ( 1.8 nm (Figure 2). The high-resolution TEM
image (Figure 2c) reveals that the Au NPs have well-defined
crystalline planes; the marked interplanar d spacings are 0.235
and 0.203 nm, which correspond to the d111 (0.2355 nm) and the
d200 (0.2039 nm) planes for face-centered cubic (fcc) gold. The
powder XRD pattern of the 1-Au NPs, shown in Figure 3,
contains information regarding both the stabilizer and the Au
NP. The three broad diffraction peaks positioned at a Bragg angle
The main approaches used to recycle “soluble” NP catalysts
11
are based on the use of biphasic reaction conditions, the appli-
1
2
cation of membrane separation methods and ultra filtration,
13
and magnetic separation. In addition to these more traditional
1
4
approaches, a protocol based on “smart” NPs has been proposed,
in which the NPs are coated with a stimuli-sensitive polymer that
undergoes physical changes as environmental parameters, such
(
2θ) between 10 and 30° correspond to the stabilizer and
1
5
16
as pH or temperature, are modified. Au NPs that are
17
pH-sensitive have recently attracted attention; however, their
catalytic activity and recyclability have been scarcely explored.
Received: July 27, 2011
Published: October 10, 2011
r 2011 American Chemical Society
11069
dx.doi.org/10.1021/ic201608j
|Inorg. Chem. 2011, 50, 11069–11074

Inorganic Chemistry
ARTICLE
Figure 1. Sodium poly(1-vinylpirrolidin-2-one-3-carboxylate), 1.
Figure 4. UVÀvis spectra of the 1-Au NP solution as a function of pH.
the 1-Au NPs did not change when the pH changed from 9.6 to
2
.5, as shown in Figure 4, indicating that the NPs remain
dispersed in the solution phase. The onset of precipitation
commences at ca. pH 2.4 and is complete (based on UV/vis
spectroscopy—see Figure 4) at pH 2.0, resulting in a colorless
solution and a dark purple solid. The precipitate may be redissolved
(
quantitatively—see below) by raising the pH of the solution.
Following redissolution, the plasmon band maximum is observed
at 520 nm, the value observed prior to precipitation, suggesting
that the size and shape of the Au NPs remain unaltered during
pH switching. TEM analysis of the Au NPs after redissolution
revealed a slight increase in NP size, essentially within the error of
TEM analysis (see Figure S1 in the Supporting Information).
The response of the 1-Au NPs to pH was characterized by
TEM at pH values of 2.4, 2.0, and 1.5 (Figure 5). Since rapid
precipitation of the NPs occurs, especially at pH values e 2.0, the
samples for TEM analysis were prepared immediately after the
desired pH was reached. Compared to the 1-Au NPs obtained
from solution at high pH, which are homogenously dispersed on
the copper grid (Figure 2a), the 1-Au NPs self-assemble into
spheres with diameters of several hundred nanometers at pH 2.4
Figure 2. (a) Representative TEM image, (b) size distribution, and (c)
HRTEM image of the 1-Au NPs at pH 9.2.
(
Figure 5a, d), although significant numbers of free Au NPs are
present. At pH 2.0, 1-Au NPs further aggregate, with only a few
Au NPs observed outside the sphere (Figure 5b,e). At pH 1.5, the
sphere structure is much denser, and free Au NPs outside the
spheres are not observed (Figure 5c,f). These TEM images
provide insight into the dynamic changes of the 1-Au NPs during
the precipitation process, and importantly, the Au NPs appear to
remain separated from each other within the spherical aggregates
at low pH, indicating that the individual NPs are stable at the low
pH values.
Figure 3. X-ray diffraction pattern of a 1-Au NP. The arrows indicate
the diffraction pattern due to the polymer.
indicate that 1 is highly disordered. All of the other peaks appear
at higher angles that are sharper and can be attributed to the
diffraction from the different gold lattice planes. Following a
According to DerjaguinÀLandauÀVerweyÀOverbeek (DLVO)
2
2
theory, NPs are stabilized by a combination of van der Waals
interactions and double-layer forces. In the system described
herein, van der Waals interactions presumably originate from the
polymer chain of 1 with the carboxylate group providing
electrostatic stabilization. Chloride ions originating from the
chloroauric acid NP precursor may also play a role in the overall
stabilization mechanism, although control experiments in which
18
Rietveld refinement, the Au (111) peak gives a full-width at
half-maximum (fwhm) of 1.10° at a 2θ value of 38.32°, and from
this, the average crystallite size of the Au NPs was calculated to be
19
8
.3 nm using the Scherrer equation. Although this value is
slightly larger than the average size estimated from the TEM
images, the two values match satisfactorily—TEM provides a
number-average distribution which is more sensitive to small
+
À
TBA Cl was added to the Au NP solution did not change the
pH-sensitive nature of the Au NPs. The pH-induced aggregation/
redispersion process originates from the protonation/deproto-
nation of carboxyl groups on 1, which modify the double layer. At
higher pH, a majority of carboxyl groups are deprotonated, giving
an ionic stabilizer, which generates a strong negative electrostatic
repulsion between the NPs. At low pH, the carboxyl groups are
protonated, which in turn decreases the negative charges on Au
NPs, and as a result the attractive forces dominate and aggregation
2
0
NPs. Finally, the UV/vis spectrum of the 1-Au NP solution
exhibited a strong absorption at 520 nm which was attributed to
the surface plasmon resonance characteristic of Au NPs (Figure 4),
21
further confirming the formation of Au NPs.
The 1-Au NPs are pH-sensitive, precipitating spontaneously
from solution at pH < 2.4, with the solution remaining transpar-
ent and homogeneous at pH > 2.5. The surface plasmon bands of
1
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Inorganic Chemistry
ARTICLE
Figure 5. TEM images of the 1-Au NPs at pH 2.4 (a and d), 2.0 (b and e), and 1.5 (c and f), showing the stepwise aggregation process as a function of
the pH.
and ketones, respectively (Table 1). For activated alcohols, the
reactions take place under ambient conditions (Table 1, entries
1
À5), whereas nonactivated alcohols require slightly harsher
conditions (Table 1, entries 6 and 7). The use of pure O only
2
slightly increases the reaction rate, suggesting the uptake of O2
into water is not rate-determining. Control experiments showed
that neither HAuCl nor 1 catalyze the oxidation of benzyl
4
alcohol oxidation, indicating that the Au NPs are the active
catalytic species.
Au NP catalysts exhibit strong, size-dependent activity; as the
size of Au NPs increases, a dramatic decrease in catalytic activity
4c,24
is observed.
The results in this paper, however, indicate that
Figure 6. Variation of ζ potential of the 1-Au NP solution (9) and the
Au NPs with a relatively large particle size may be highly active.
The turnover frequency (TOF) of benzyl alcohol was calculated
%
formation of the monoprotic acid of 1 (HA) and its conjugate base
À
(A ; —) as a function of pH.
À1
to be 21 h on the basis of the total number of Au atoms or 105
À1
25
h
on the basis of the surface Au atoms, which is higher or at
takes place, as shown by the ζ potentials (mV) of 1-Au NP
solutions at different pH values (Figure 6). The ζ value represents
the charge on the NPs, either positive or negative, with the surface
charge density indicated by the absolute value of the ζ potential.
The 1-Au NPs are negatively charged in the pH range 1.4À9.2,
with a dramatic change in charge density between pH 2 and 4. The
least comparable to the performance of other polymer-protected
gold NPs under similar conditions (a comparison is given in
Table S1 in the Supporting Information). We speculate that the
carboxylate group on the stabilizer transfers sufficient electron
density to the Au NP surface, facilitating O activation, the key
2
step in alcohol oxidation. Both IR spectroscopy and gas-phase
À
pK of 1 was determined as 4.4, and based on this value, the
studies on Au clusters show that Au
n
systems exhibit higher
a
percentage of the protonated form of 1 (HA) versus pH was also
plotted in Figure 6, which shows that the ζ potential of 1-Au NPs
corresponds closely to the protonation states of 1. At pH > 4.5, the
ζ potential is > À40 mV. Thus, the NP surface is highly negatively
charged, and the electrostatic repulsion is sufficient to prevent
attraction between the NPs. As the pH decreases, the carboxyl
groups become increasingly protonated, which in turn compresses
the electrical double layer, resulting in a lowered ζ potential and
reduced NP stability. At pH < 2.6, when a majority of carboxyl
groups are protonated, the ζ potential is very low. Under these
conditions, the dispersion is no longer stable, and Au NPs “salt
out” of the solution.
reactivity toward oxygen than cationic clusters—rationalized in
À
terms of partial electron transfer from Au
to the LUMO (π*)
n
26
of oxygen. Indeed, Au NPs with a smaller size (5.4 nm) were
obtained by performing the synthesis at 0 °C (see Figure S2 in
the Supporting Information for the TEM image), but the
catalytic activity did not increase. We also evaluated the reaction
mechanism by adding CCl
9À10), and a dramatic decrease in activity was observed. The
detrimental effect of CCl to the reaction highlights the impor-
, a radical scavenger (Table 1, entries
4
23
4
tant role of radicals for alcohol oxidation in this system, although
the mechanism of Au-catalyzed oxidation reactions is not con-
27
sistent in literature.
The 1-Au NPs were evaluated as catalysts for the oxidation
of alcohols using air as the oxidant; primary and secondary
alcohols were oxidized to their corresponding carboxylic acids
It is noteworthy that the 1-Au NPs appear to be very stable in
these reactions, and no precipitation was observed. Moreover,
since the pH-induced precipitationÀredispersion process is highly
1
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Inorganic Chemistry
ARTICLE
a
Table 1. Aerobic Oxidation of Alcohols Catalyzed by the 1-Au NPs
a
All oxidations were performed as follows: alcohol (0.2 mmol), the 1-Au NP solution (2.0 mL, 1.0 mM), K
2
CO
3
(0.4 mmol) under air. The conversion
b
c
and selectivity were estimated from GC analysis. The other product is mainly benzyl benzoate (20À30%). Reaction conditions: alcohol (0.4 mmol),
d
1
-Au NP solution (4.0 mL, 1.0 mM), K CO (0.8 mmol) under an atmospheric pressure of O . CCl (0.1 mL) was added.
2 3 2 4
Figure 7. Images for the recycling of the PVP-Au and 1-Au NPs.
Figure 9. TEM image (a) and size distribution (b) of 1-Au NPs after six
batches.
then redispersing the NPs in water at pH 9. The activity of the
recycled NPs was equivalent to that observed in the original
batch. This recycling protocol is extremely facile to implement,
and images for the workup process for the third catalytic batch for
the oxidation of benzyl alcohol are shown in Figure 7. The
conversion of benzyl alcohol to benzoic acid showed no decrease
in activity over six batches (Figure 8), whereas the PVPÀAu NP
2
8
control rapidly deactivated, in agreement with other studies.
After the first oxidation reaction, the size of the 1-Au NPs increased
slightly to ca. 7.4 nm (see Figure S4 in the Supporting Information
for TEM images), and Figure 9 shows the TEM image of the 1-Au
NPs after six batches, in which the size of the 1-Au NPs has further
Figure 8. Recycling of PVP-Au and 1-Au NPs in the oxidation of benzyl
alcohol.
increased to 8.4 nm (from an average of Au
to Au
after
∼
9300
∼18300
2
9
six cycles, estimated using a literature method).
reversible, the 1-Au NPs could be recycled and reused by
adjusting the pH to 1, removing the entire solution phase, and
ICP-MS was used to determine the extent of leaching of gold into
the product phase, revealing a gold content of only 38 ng mL ,
À1
1
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Inorganic Chemistry
ARTICLE
À6
corresponding to a molar concentration of 2 Â 10 mM, an
(added until the solution reached pH 1), causing the Au NP catalyst to
precipitate. The product was then extracted with ethyl acetate (2 Â
2 mL). The combined organic layers were analyzed on a Varian
Chrompack CP-3380 gas chromatogram (GC) equipped with an OV-
amount that is below the detection limit of inductively coupled
30
plasma optical emission spectrometry (ICP-OES). On the basis of
the original concentration of the catalytic solution (1 mM), the
leaching level is 0.0002% per cycle, indicting why reuse of the system
is so efficient.
1
01 capillary column (30 m  0.25 mm, using nitrogen as carrier gas).
After extraction of the product, the water phase was collected and diluted
000 times for analysis by inductively coupled plasma mass spectro-
1
metry (ICP-MS—see below). The solid was separated for recycling
experiments and mixed with fresh water (2 mL), and the pH was
adjusted to 9 by the addition of 1 M NaOH, resulting in complete
dispersion of the 1-Au NPs. The alcohol substrate and K CO were
2 3
added and the reaction started. Since the PVP-Au NPs are not pH-
sensitive, recycling was achieved by adjusting the pH to 9 and adding
’
EXPERIMENTAL SECTION
Materials and Methods. THF was dried with a solvent drying
system and was stored with 4 Å molecular sieves. Diisopropylamine was
refluxed with Na wire and distilled prior to use. N-vinyl-2-pyrrolidone
(NVP) was distilled under reduced pressure prior to use. Gold chloride
2 3
new substrate and K CO , following the product extraction process. An
trihydrate was purchased from Aldrich with a gold purity of 99.99%.
Other chemicals were obtained commercially and used without further
purification. The average molecular weight of PVP was 50 kDa (K-30).
Polymer 1 was prepared using a literature route and was freeze-dried
on a Christ Alpha 1-2 LD freeze-dryer (Germany).
Agilent 7500ce ICP-MS, interfaced to a CETAC CEI-100 microcon-
centric nebulizer, was used to determine the gold content in the aqueous
phase. Instrument control as well as data analysis was carried out using
the ChemStation B.03.03 software. The nebulizer was operated in self-
aspiration mode with the sheath liquid (50 mM formic acid, 20 ppb Ge)
closing the electrical circuit. Analyses were only started when a sufficiently
stable signal (RSD 72Ge < 5%) was obtained. An ICP-MS tuning solution
31
pK Measurements. The acid dissociation constant (K ) of 1 was
a
a
32
determined by back-titration since the acidic form of 1 is sparingly
soluble in water. Compound 1 (154 mg, 0.87 mmol based on repeating
unit) was dissolved in deionized water (1.2 mL) at 20 °C. HCl (2 M, 25 μL)
was added dropwise, and the pH value of the solution was recorded with a
containing lithium, yttrium, cerium, thallium, and cobalt in 2% HNO
3
(each
À1
10 mg L , Agilent Technologies) was used for calibration.
pH meter. The pK
a
was obtained by applying the equation pK
log[HA]/[A ]. The pK was found to be 4.4 ( 0.1, an average of 10 values.
Synthesis of Polymer-Protected Au NPs. The polymer-pro-
a
= pH +
À
a
’
CONCLUSIONS
Gold NPs stabilized by carboxylate-modified PVP display
33
tected Au NPs were prepared by NaBH reduction. An aqueous
4
excellent activity and stability in the aerobic oxidation of various
alcohols under mild conditions. Moreover, the carboxylate
groups give rise to a pH responsive precipitationÀredispersion
switch, visualized by TEM, that allows facile recycling of the
catalytic system. The excellent activity of the NP catalyst
observed in recycling experiments may be attributed to the high
stability of the 1-Au NPs, due to the stabilizer, combined with an
extremely low loss of the catalyst during recycling steps.
solution of HAuCl
PVP (100 mM based on pyrolidone monomer, 2.0 mL) and water
6 mL). The mixture was adjusted to pH 9À10 with 1 M NaOH and
stirred for 1 h. NaBH (0.1 M aqueous solution, 0.3 mL) was rapidly
added in one portion with a stirring rate of 1000 rpm, affording a deep
red solution. The mixture was stirred for another 1 h and diluted to
4
(10 mM, 1.0 mL) was mixed with the solution of 1 or
(
4
10 mL with water.
Characterization of 1-Au NPs. The morphology and core size of
-Au NPs was established by transmission electron microscopy (TEM),
1
on a JEOL JEM-2010 microscope operating at 200 keV. One drop of 1-
Au NP solution at different pH values was placed on a copper grid coated
with carbon film. The grids were dried in a desiccator for 24 h at room
temperature before analysis. The size of Au NPs was estimated from the
average diameter of 180 particles. The composition and phase of the NPs
was determined by powder X-ray diffraction (XRD) analysis. The data
were measured in reflection mode on a Panalytical X-ray Powder
Diffraction System (MPD) using Cu Kα radiation and a PIX cell
detector. The data were collected in the 2θ range of 5À120° (step size
’
ASSOCIATED CONTENT
S
Supporting Information. Listings of change in size of the
b
Au NPs during the precipitationÀredispersion, synthesis of 1-Au
NPs with a smaller size, XPS analysis of 1-Au NPs, ICP-AES
analysis of the palladium concentration in HAuCl , size distribu-
4
tion after the oxidation reaction of benzyl alcohol, and a
comparison of the catalytic activity of 1-Au NPs with examples
from the literature for the oxidation of benzyl alcohol under
similar conditions. This material is available free of charge via the
Internet at http://pubs.acs.org.
0.01, 1 s per step, 20 scans). UV/vis absorption spectra were recorded on
a Lambda 850 UV/vis spectrometer (Perkin-Elmer) at 25 °C.
Measurement of ζ Potentials. ζ potentials of the 1-Au NPs at
different pH values were measured on a Malvern Zetasizer nano ZS
equipped with a microprocessor unit. The unit automatically calculates
the electrophoretic mobility of the particles and converts it to the ζ
’
AUTHOR INFORMATION
Corresponding Author
34
potential using the Smoluchowski equation. The samples were equili-
brated at 25 °C for 3 min, and an average of 15 measurements was taken
to calculate the potential. The concentration of the Au NPs used for the
ζ potential (and UVÀvis spectroscopy—see above) measurements was
*Tel.: +41 21 693 98 54. Fax: +41 21 693 98 85. E-mail: paul.
dyson@epfl.ch.
0.5 mM (based on Au atoms) with a polymer (repeating unit) to gold
’
ACKNOWLEDGMENT
ratio of 20:1. The solutions were adjusted to the desired pH with either
M HCl or 1 M NaOH (monitored with a pH meter), and after 24 h, the
This work was supported by the Sino-Swiss Science and
1
Technology Corporation Programme (No. 2010DFA42110).
N.Y. thanks the Marie-Curie Fellowship for financial support
(Number: 252125-TCPBRCBDP). We thank the Interdisciplinary
Centre for Electron Microscopy (CIME) at EPFL for access and
assistance with TEM. Dr. M. Groessl and C. Clavel are thanked for
ICP-MS measurements.
solution (or supernatant in cases where a precipitate had formed) of each
sample was analyzed.
Procedure for Alcohol Oxidation. In a typical experiment, the
1
(
-Au NP solution (2.0 mL, 1.0 mM), alcohol (0.2 mmol), and K
0.4 mmol) were stirred in the air at the desired temperature. After the
appropriate time, the reaction was quenched by the addition of 1 M HCl
2 3
CO
1
1073
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Inorganic Chemistry
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