Aerobic oxidation of α,β-unsaturated alcohols using sequentially-grown AuPd nanoparticles in water and tetraalkylphosphonium ionic liquids

Article abstract of DOI:10.1016/j.cattod.2012.04.053

Metallic and bimetallic nanoparticles (NPs) (Au, Pd, and AuPd) were synthesized in water with poly(vinylpyrroldine) (PVP) as a polymer stabilizer, as well as in tetraalkylphosphonium chloride ionic liquids (ILs). A borohydride reduction technique was used for the preparation of the monometallic NPs, while a sequential reduction strategy was seen to generate the putative core-shell varieties. Pd and sequentially grown Au-Pd NPs were seen to serve as efficient catalysts for the oxidation of α,β-unsaturated alcohols, while the Au NPs themselves showed minimal activity. However, a system with both Au NPs and a Pd(II) salt (K₂PdCl₄) also efficiently catalyzed the oxidation reaction in water. A number of aliphatic and aromatic unsaturated alcohols were oxidized using oxygen as an oxidant with these as-prepared catalysts at 60 °C in the absence of base. TEM and EXAFS studies of the nanoparticle catalysts before and after the oxidation reaction revealed that the Pd(II) salt was reduced in situ during catalytic conditions, and that there was large changes in particle size and morphology after catalysis, which suggests Ostwald ripening and the presence of a redox mechanism. In the IL system, the ease of Pd reduction led to high activities for many α,β-unsaturated alcohols in the absence of the Au promoter.

Full text of DOI:10.1016/j.cattod.2012.04.053

Catalysis Today 207 (2013) 170–179
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Aerobic oxidation of ␣,␤-unsaturated alcohols using sequentially-grown AuPd
nanoparticles in water and tetraalkylphosphonium ionic liquids
1
1
∗
Aimee Maclennan , Abhinandan Banerjee , Robert W.J. Scott
Department of Chemistry, University of Saskatchewan, 110 Science Place, Saskatoon, SK S7N 5C9, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Metallic and bimetallic nanoparticles (NPs) (Au, Pd, and AuPd) were synthesized in water with
poly(vinylpyrroldine) (PVP) as a polymer stabilizer, as well as in tetraalkylphosphonium chloride ionic liq-
uids (ILs). A borohydride reduction technique was used for the preparation of the monometallic NPs, while
a sequential reduction strategy was seen to generate the putative core–shell varieties. Pd and sequen-
tially grown Au–Pd NPs were seen to serve as efficient catalysts for the oxidation of ␣,␤-unsaturated
alcohols, while the Au NPs themselves showed minimal activity. However, a system with both Au NPs
and a Pd(II) salt (K2PdCl4) also efficiently catalyzed the oxidation reaction in water. A number of aliphatic
and aromatic unsaturated alcohols were oxidized using oxygen as an oxidant with these as-prepared
Received 1 February 2012
Received in revised form 24 April 2012
Accepted 25 April 2012
Available online 2 June 2012
Keywords:
Nanoparticles
Alcohol oxidation
Ionic liquids
◦
catalysts at 60 C in the absence of base. TEM and EXAFS studies of the nanoparticle catalysts before and
Quasi-homogeneous catalysis
EXAFS
after the oxidation reaction revealed that the Pd(II) salt was reduced in situ during catalytic conditions,
and that there was large changes in particle size and morphology after catalysis, which suggests Ostwald
ripening and the presence of a redox mechanism. In the IL system, the ease of Pd reduction led to high
activities for many ␣,␤-unsaturated alcohols in the absence of the Au promoter.
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
of choice if they can be used safely, simply for the sake of accessi-
bility and cost-effectiveness [4].
The oxidation of alcohols to their respective carbonyl com-
pounds, although known to the scientific community since the
9th century [1], continues to represent a major challenge as far
Our group and many others are examining the “greening” of
important chemical reactions by developing methods by which
reactions of industrial importance (such as oxidations, hydrogena-
tions, and so on) can be carried out in solvents ear-marked as
“green”, and with catalysts that are readily recyclable, with high
yields and minimal waste products (e.g. high selectivities). Stable
metal nanoparticles (NPs) in solvents such as water, supercriti-
cal carbon dioxide, ionic liquids, or eutectic mixtures provide an
alternative route to traditional homogeneous or heterogeneous
catalysis [5]. In fact, such “quasi-homogeneous” nanocatalysis
involving suspended nanoparticle catalysts in liquid media strad-
dles the boundaries between homogeneous and heterogeneous
catalysis, combining some of the advantages of both approaches.
While the catalytic versatility of metal NPs is well-known, their
tendency to agglomerate and precipitate in the absence of stabi-
lizers can restrict their catalytic uses: however, several strategies
have been successfully applied to minimize such catalyst deactiva-
tion, and many different reactions have been screened for potential
application of “quasi-homogeneous” nanocatalysis.
1
as the industrial implementation of such processes is concerned.
Traditionally, in the laboratory, such conversions have been per-
formed using highly toxic transition metal compounds (such as
chromium salts) in volatile and flammable organic solvents [2].
Such systems are rife with disadvantages: the reaction mixtures
are inherently toxic, often corrosive, and the process can pose a
serious threat to the environment if performed regularly on a large
scale [3]. Another point worth noting is the probability of “over-
oxidation” of primary alcohols in such systems, where the reaction
proceeds to give the carboxylic acid rather than halt at the alde-
hyde stage. Moreover, from the standpoint of green chemistry,
catalytic processes are preferred over stoichiometric ones. While
the application of novel sources of oxygen such as ozone, PhOI,
N-morpholine-N-oxide, or urea–hydrogen peroxide have notable
utility, for industrial processes either air or oxygen are the oxidants
After the pioneering work by Haruta, it is now well-known that
Au NPs below a certain critical size are capable of efficient catalysis
of oxidation reactions at low temperatures [6]. In 2005, Tsunoyama
et al. showed that PVP-stabilized Au NPs in water selectively
catalyze the oxidation of benzyl alcohol to benzaldehyde in the
∗ Corresponding author. Tel.: +1 306 966 2017.
E-mail address: robert.scott@usask.ca (R.W.J. Scott).
1
These authors contributed equally to this work.
0
920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.04.053

A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
171
presence of base [7–9], and many other groups have also reported
alcohol oxidations with Au NPs [3,10–15], as well as enhanced
activities for AuPd NPs for mild alcohol oxidations [16–26]. We
previously studied alcohol oxidations in aqueous solutions using
Au, Pd, and bimetallic AuPd NP catalysts in aqueous solutions, and
determined that AuPd systems were much more reactive than their
monometallic Au and Pd counterparts [18]. Recently, both our-
selves [25], and others [21,24], have shown that core–shell AuPd
NPs (either as-synthesized or synthesized in situ during reaction
conditions) are particularly active for alcohol oxidations, with sub-
stantial activities and selectivities for ␣,␤-unsaturated alcohols
such as crotyl alcohol at room temperature in the absence of bases.
However, it was not apparent how generic such results were to
other alcohol systems, and what the dominant mechanism was in
the reaction. In addition, little to no studies are available detailing
the recyclability of such nanoparticle systems. Catalytic oxidations
in ionic liquids (ILs) are rather less-well-known than those in other
media: even in cases where such studies have been carried out,
the actual catalytic species are often transition metal complexes
potassium tetrachloropalladate, K PdCl , (both 99.9%, metals basis)
2 4
were all obtained from Alfa Aesar, and the metal salts were stored
under vacuum and flushed with nitrogen after every use. Com-
mercial samples of the trihexyl(tetradecyl)phosphonium chloride
(P[6,6,6,14]Cl) room-temperature ionic liquid (IL) were generously
donated by Cytec Industries Ltd. Commercial samples of ILs were
◦
dried under vacuum at 70 C for 10–12 h with stirring before use.
Deuterated solvents were purchased from Cambridge Isotope Labo-
ratories. 18 Mꢀ cm Milli-Q water (Millipore, Bedford, MA) was used
throughout.
2.2. Synthesis of PVP stabilized Pd, Au and AuPd NPs in water
PVP-stabilized metallic and sequentially grown bimetallic NPs
were synthesized by previously reported procedures [18,25];
metallic NPs were synthesized using sodium borohydride while
sequentially grown AuPd NPs were synthesized using ascorbic
acid as the selective reducing agent for the Pd shell. To pre-
pare the Au NPs (also used as “seed” particles), 50 mL of 0.35 mM
−
5
(
such as methyltrioxorhenium [27], OsO4 [28], RuCl3 [29], and so
PVP (1.75 × 10 mol) was added to 5 mL of 10 mM HAuCl •3H O
4
2
−
5
on), and/or the oxygen is harvested from novel oxidants rather
than using molecular oxygen or air. Heteropolyacids have been
immobilized on imidazolium IL modified SBA-15 for alcohol oxi-
dations [30], and ILs such as choline hydroxide have also been used
to stabilize Ru NPs on MgO supports for oxidations [31]. How-
ever, AuPd systems have not been well explored in IL systems
to date.
(5 × 10 mol) and stirred at 800 rpm for 30 min. The solution was
then placed on ice, stirring set to 1200 rpm, and 5 mL of 100 mM
−
4
NaBH4 (5 × 10 mol) was added quickly to the solution. The reac-
tion was left to stir for 30 min on ice, then stirred for another 30 min
at room temperature. To quench the reaction, 5 mL of 100 mM HCl
−
4
(5 × 10 mol) was added to the solution and stirred for 30 min.
−
4
To further stabilize the NPs, 35 mL of 1.0 mM PVP (3.5 × 10 mol)
was added to the final solution and stirred for 30 min. The particles
were dialyzed overnight using cellulose dialysis membrane with
In this study, the oxidation of a variety of ␣,␤-unsaturated alco-
hols (aliphatic as well as aromatic) in the presence of molecular
oxygen have been investigated. The catalytic systems of choice
are Au, Pd, Au/Pd(II), and sequentially grown AuPd NPs, while the
reaction media include water in the presence of PVP stabilizer as
well as a tetraalkylphosphonium chloride IL capable of inherent
NP stabilization. Careful growth of Pd shells on Au seeds using
mild reductants such as ascorbic acid have been used to generate
sequentially grown AuPd NPs while minimizing secondary nucle-
ation of Pd NPs. In situ formation of Pd shells on Au cores may also
be achieved by the oxidation of the substrate itself, and concurrent
reduction of Pd(II) in the presence of Au NPs, while individually
each of these show very little catalytic activity. Similar catalytic NP
systems are generated in trihexyl(tetradecyl)phosphonium chlo-
ride ILs in the absence of any external stabilizers, and the resulting
Pd NPs show high activities even in the absence of the Au promoter.
Tentative mechanisms for ␣,␤-unsaturated alcohol oxidation in
water and in trihexyl(tetradecyl)phosphonium halide ionic liq-
uids have been suggested. Finally, TEM and EXAFS studies of the
core–shell catalyst before and after reaction, as well as of Au
NP/Pd(II) salt after reaction has been conducted to study the struc-
ture of these NP catalysts, and results suggest that the initial step
of the oxidation reaction with Au NPs in the presence of Pd(II)
salts leads to in situ formation of AuPd catalysts. Since allylic alde-
hydes are valuable fine chemicals finding extensive use in the
pharmaceutical, agrochemical and cosmetic industries, we believe
it is worthwhile to seek clean technologies for their synthesis
via atom-efficient oxidations that also address issues of catalyst
reusability, environmental impact, waste disposal, and renewable
feedstock.
a molecular cut off of 12,400 g/mol under N . Pd NPs were syn-
2
thesized using the same procedure, but K PdCl4 was used instead
2
of HAuCl •3H O. The sequentially-grown 1:3 AuPd NPs were syn-
4
2
thesized by mixing 100 mL of previously made Au “seed” particles
−
5
−3
(5 × 10 mol) with 15 mL of 100 mM ascorbic acid (1.5 × 10 mol)
on ice, stirring at 800 rpm. To this solution, 15 mL of 10 mM K PdCl
2
4
−
4
(1.5 × 10 mol) was added then left to stir at 800 rpm on ice for
1 h. Upon completion, the particles were dialyzed overnight in the
same manner as the Au seed particles. For the generation of the Au
NP/Pd(II) system, an identical procedure was followed, except for
the addition of ascorbic acid and the second dialysis step.
2.3. Synthesis of Pd, Au and AuPd NPs in IL
For the synthesis of Pd NPs in IL, 1.6 mg of K PdCl4
2
−
6
(5.0 × 10 mol) was added to
a 10 mL sample of the tri-
hexyl(tetradecyl)phosphonium chloride IL mixed with 2 mL
1,4-dioxane at room temperature, and vigorously stirred. To this
solution, a stoichiometric excess of LiBH4 reagent (1.0 mL, 2.0 M in
THF) was injected drop-wise over a period of five minutes [32].
A brisk effervescence followed, and the entire solution turned
brown, indicating nanoparticle formation. After the addition of
LiBH , volatile impurities including the dioxane were removed by
4
◦
vacuum-stripping the system at 70 C. The Pd NP solution thus
obtained was stored under nitrogen in capped vials until use. For
the synthesis of Au NPs, a similar procedure was followed. Tetra-
−
6
chloroauric acid (2.0 mg, 5.0 × 10 mol) was dissolved in 10 mL
P[6,6,6,14]Cl diluted with 2 mL 1,4-dioxane at room temperature
to give a golden yellow solution which turned colorless, violet
and then wine-red upon drop-wise addition of 1.0 mL of a 2.0 M
LiBH4 reagent. Excess LiBH4 was quenched with methanol and
volatiles were subsequently removed by vacuum-stripping. For
the synthesis of sequentially grown AuPd NPs, a sequential reduc-
tion procedure was followed using the Au NP seeds as synthesized
above, followed by the addition of 200 mg ascorbic acid to the reac-
tion medium. While stirring this mixture under ice, a 5 mL solution
2
. Experimental
2.1. Materials
All chemicals except for the ones listed below were purchased
from Sigma Aldrich and used as received. Poly(vinylpyrrolidone)
PVP) (M.W. 58,000 g/mol), tetrachloroauric acid, HAuCl ·4H O and
−
6
(
of K PdCl4 (15.0 × 10 mol) dissolved in methanol was added to
4
2
2

1
72
A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
Fig. 1. TEM images of PVP-stabilized (A) as-synthesized Au NP seeds and (B) as-synthesized sequentially-reduced 1:3 AuPd NPs and (C) Au NP/Pd(II) 1:3 mixture after 24 h
reaction with cinnamyl alcohol and (D) sequentially-grown 1:3 AuPd NPs after 24 h reaction with cinnamyl alcohol.
it all at once. The mixture was then stirred under ice for ∼1 h,
and volatiles were removed from it by vacuum-stripping. For the
generation of the Au NP/Pd(II) system, an identical procedure was
followed, except for the addition of ascorbic acid.
25 ␮L of a neat extract was mixed with 1 mL ethyl acetate in a
GC-vial, and subjected to analysis.
2.5. Characterization
2.4. General procedure for oxidation reactions
UV–Vis spectra were obtained using a Varian Cary 50 Bio
UV–Visible spectrophotometer with a scan range of 200–800 nm
and an optical path length of 1.0 cm. 1H and C NMR spectra
were obtained using a Bruker 500 MHz Advance NMR spectrom-
eter; chemical shifts were referenced to the residual protons of the
deuterated solvent. TEM analyses of the NPs in IL and water, both
before and after catalytic cycles, were conducted using a Philips
410 microscope operating at 100 kV. The samples in IL were pre-
pared by ultrasonication of a 1% solution of the NP/IL solution in
DCM followed by drop-wise addition onto a carbon-coated copper
TEM grid (Electron Microscopy Sciences, Hatfield, PA). To deter-
mine average particle diameters, a minimum of 100 particles from
each sample were manually measured from several TEM images
using the ImageJ program. The Hard X-ray MicroAnalysis beamline
13
Oxidation reactions were carried out in a round-bottomed flask,
sealed with a septum. In a general procedure, 10 mL of the NP solu-
◦
tion (in water or IL) was stirred and heated to 60 C under a flow of
oxygen for about 10 min for aqueous systems, and for about 1 h in IL
systems to compensate for the higher viscosity of the IL. After this,
00 or 5000 equivalents (based on one equivalent of metal cata-
5
lyst) of the alcohol substrate was injected into the flask. For aqueous
work, at incremental times 1.0 mL of the reaction mixture was sam-
pled and the products were then extracted by two, 0.5 mL aliquots
of ethyl acetate, shaking in 1 min intervals for 5 min per extraction.
For IL samples, after ∼12 h, the reaction vessel was removed from
the constant temperature bath, and subjected to vacuum-stripping
while being heated for extraction of the products and/or unreacted
substrate. Conversion and selectivity were obtained from GC using
a FID detector (Agilent Technologies 7890A) and a HP-Innowax cap-
illary column. Two reactions were typically run for each sample. The
neat organic liquids extracted were subsequently characterized by
4
(HXMA) 061D-1 (energy range, 5–30 keV; resolution, 1 × 10 ꢁE/E)
at the Canadian Light Source was used for recording X-ray absorp-
tion spectra at the Pd K-edge and the Au LIII-edge. The beamline
optics include water-cooled collimating KB mirrors (Rh for the Au
LIII-edge and Pt for Pd K-edge), and a liquid nitrogen cooled dou-
ble crystal monochromator housing two crystal pairs [Si(1 1 1) and
1
13
H NMR, C NMR, and GC-FID techniques. For GC-FID analysis,

A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
173
Table 1
Summary of nanoparticle sizes obtained by TEM.
Nanoparticle
Size (nm)
Water
Ionic liquid
Au NPs
Pd NPs
2.9 (1.4)
2.7 (1.0)
3.9 (1.7)
–a
2.2 (0.8)
4.3 (1.3)
3.7 (1.0)
8.0 (5.0)
8.7 (3.9)
9.0 (2.9)
1
1
1
:3 AuPd Seq. grown NPs
:3 AuPd Seq. grown NPs after reaction
:3 Au NPs/Pd(II) after reaction
b
6.4 (7.8)b
–
Pd NPs after reaction
a
Particles formed worm-like structures.
Bimodal distribution of particles.
b
Si(2 2 0)]. The X-ray measurements were conducted in ion cham-
ber filled with a helium and nitrogen mixture for the Au LIII-edge
and pure helium for the Pd K-edge. The energy scan range for the
measurement was between −200 eV and +1000 eV at each edge.
Pd and Au foils were used for the respective edges as references.
All EXAFS measurements were conducted in transmission mode
at room temperature using samples trapped in alumina at 2.5% by
weight metal and pressed into pellets [33]. The software package
IFEFFIT was used for data processing which included fitting the pre-
edge region to a straight line, and the background above the edge
was fit to a cubic spline function [34,35]. The EXAFS function, ꢂ, was
obtained by subtracting the post-edge background from the over-
all absorption and then normalizing with respect to the edge jump
step. The EXAFS fitting was performed in R-space between 1.4 and
Fig. 2. UV–Vis spectra of PVP-stabilized Au, Pd, and sequentially-grown 1:3 AuPd
NPs.
the previous study that Au NPs in the presence of Pd(II) salts (specif-
ically K PdCl ) also showed tremendous activity for crotyl alcohol
2
4
oxidation activation at room temperature while the individual Au
NPs and Pd(II) salts showed next to no activity, and hypothesized
that in situ reduction of Pd onto the Au NPs was responsible for
this activity [25]. Thus in this study we wished to test the general-
ity of both the high activity of sequentially-grown 1:3 AuPd NPs for
other allylic and benzylic alcohols (e.g. ␣,␤-unsaturated alcohols)
and whether the Au NP/Pd(II) salt mixtures showed strong activi-
ties for other substrates as well. In order to activate other allylic and
benzylic alcohols with moderate conversions, it was necessary to
3
.4 A˚ for the Au edge and 1.4–3.0 A˚ for the Pd edge using theoreti-
cal phase-shifts and amplitudes generated by FEFF. fcc bulk lattice
parameters (i.e., first shell coordination numbers of 12) were used
2
to determine the amplitude reduction factor, S , for Au and Pd by
0
analyzing Au and Pd reference foils [36].
◦
increase reaction temperatures to 60 C using 1 atm oxygen as the
3
. Results and discussion
oxidant (but no external base has been added to the system). Cataly-
sis results for the five chosen ␣,␤-unsaturated alcohols are shown in
Table 2, and the major and minor products are shown in Scheme 1.
For all the systems studied, Au NPs showed no or next to no activity
3.1. Oxidation reactions in water
PVP-stabilized Au, Pd, sequentially grown 1:3 AuPd NPs and Au
under these conditions. Results for the K PdCl4 salt by itself were
2
NP/Pd(II) 1:3 mixtures were synthesized. Fig. 1(A) and (B) shows
representative TEM images of samples of PVP-stabilized Au NPs
and sequentially grown 1:3 AuPd NPs. Average particle sizes were
found to be 2.9 nm ± 1.4 nm and 3.9 nm ± 1.7 nm for these systems,
respectively, which is in general agreement with previous work,
although the Au NP sample showed some larger particles present
and thus were not as monodisperse as could be desired. Pd NPs
were 2.7 nm ± 1.0 nm, which within error is similar to that of the
Au NPs; particle sizes of all samples by TEM are listed in Table 1. The
ca. 1 nm increase of size for the sequentially-grown particles is con-
sistent with, but does not in itself prove, core–shell type growth of
the particles. The UV–Vis spectra of Au, Pd, and sequentially grown
non-uniform; both cinnamyl alcohol and crotyl alcohol showed sig-
nificant conversions over 24 h in the presence of the Pd(II) salt
alone, while the other substrates did not react to any significant
extent. Similarly, Pd NPs by themselves did not show significant
activities for many substrates, although nearly 12% conversion was
seen for allyl alcohol. Interestingly, both the sequentially-grown
1:3 AuPd NPs and the 1:3 Au NP/Pd(II) mixtures showed moder-
ate activity for most substrates, typically with similar conversions
and selectivities towards the aldehyde (or ketone in the case of
1-hexen-3-ol) product. For 1-hexen-3-ol, the 1:3 Au NP/Pd(II) mix-
ture showed significantly higher reactivity, for reasons that are not
known at this time. High selectivities towards the aldehyde product
were seen for benzyl and cinnamyl alcohols, while moderate selec-
tivities were seen in the crotyl alcohol reaction, in which significant
hydrogenation and isomerization products were also seen at short
time periods. For 1-hexen-3-ol, the major product of the reaction
was actually the saturated ketone, 3-hexanone, which is an isomer-
ization/tautomerization product (or a hydrogenation + oxidation
product) [38,39]; while the expected oxidation product, 1-hexen-
3-one was formed in much lower amounts. Similarly, the major
product for the allyl alcohol oxidation reaction was the isomeriza-
tion/tautomerization product, 1-propanal. Similar reactivity of allyl
alcohol to give large amounts of 1-propanal has been previously
documented by our group using Pd catalysts under hydrogenation
conditions [39]. The formation of minor hydrogenation products
in many reactions can be attributed to a partial hydrogen coating
on the NP surface, since it has been shown by Schwabe that even
1
:3 AuPd NPs are shown in Fig. 2. There is a slight plasmon shoul-
der at ∼530 nm in the Au NP sample, which is no longer apparent in
the sequentially grown AuPd NP sample, which is similar to results
seen before in this system [25,33,37], and is in agreement with TEM
results which suggest Pd growth on the Au NP seeds. However, no
information about whether the final particles have specific struc-
tures (core–shell, cluster on cluster, etc.) can be obtained from this
TEM and UV–Vis information alone.
We have previously shown that sequentially-grown PVP-
stabilized 1:3 AuPd NPs were optimal catalysts for the room-
temperature oxidation of crotyl alcohol in water in the absence of
base [25]; however, it was noted in the previous study that crotyl
alcohol was an unusual substrate in this regard (e.g. room tem-
perature/base free activation) and other allyl and benzyl alcohols
showed much lower activity at these conditions. We also noted in

1
74
A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
Table 2
Summary of catalytic results for the oxidation of ␣,␤-unsaturated alcohols using PVP-stabilized NPs in water.
a
Substrate
NP catalyst
Conversionb
Selectivity (%)
Aldehyde or ketone
Other
Allyl alcohol
Au NPs
K2PdCl4
Pd NPs
0
2
11.6
19.9
0
66
29
14
9
0
34
d
71
86
d
1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
d
17.1
91
Benzyl alcohol
Cinnamyl alcohol
Crotyl alcohol
Au NPs
K2PdCl4
Pd NPs
0
2.3
2.7
4.3
3.1
0
34.3
6.8
34.2
29.3
–
14.0
0
0
0
0
0
0
0
4
0
11
10
–
66
–
100
100
100
100
0
96
100
89
90
–
1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
Au NPs
K2PdCl4
Pd NPs
1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
Au NPs
K2PdCl4
Pd NPs
34
–
–
1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
36.9
37.9
0
73
65
0
∼97
∼37
29
27
27
35
0
1
-Hexen-3-ol
Au NPs
K2PdCl4
Pd NPs
Trace amounts
Trace amounts
17.3c
∼3
∼63
e
1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
71
35.1c
73
e
a
b
c
◦
Reaction conditions: All reactions are carried out at 60 C with a substrate:catalyst ratio of 500:1.
Conversions and selectivities listed over 24 h.
Conversions and selectivities listed after 4 h.
d
e
Major product for allyl alcohol reaction 1-propanal (isomerization product), ␣,␤-unsaturated ketone (prop-2-en-1-al) was minor product.
Major product for 1-hexen-3-ol reaction is 3-hexanone (isomerization product), ␣,␤-unsaturated ketone (1-hexen-3-one) was minor product.
during oxidation of alcohols, the potential of Pt-group metals lie in
the “hydrogen region” [40].
lead to lower overall conversions for both the AuPd NPs and Au
NP/Pd(II) systems with moderate selectivities to crotonaldehyde;
also, moderate decompositions to CO and propene were seen after
8 h [21,41]. The reaction has a TOF of 120 moles product/moles
Interestingly, for crotyl alcohol, which we have previously
shown can be activated at room temperature in the absence of base
with excellent conversions and a high selectivity to crotonalde-
−
1
catalyst h
over the first hour, which is comparable to earlier
◦
hyde [25], the harsher reaction conditions (60 C) here generally
room-temperature work [18,25], followed by a drastic reduction
Scheme 1.

A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
175
in the reaction rate after 1 h. We believe that the decrease in TOF
after 1 h and thus the lower overall conversions after 24 h seen at
these higher temperature conditions are due to significant Ostwald
ripening of the particles during the catalyst experiment. To further
explore this, the nanoparticle catalysts were examined after the
catalytic reaction. Fig. 1(C) and (D) shows the final TEM images of
the Au NP/Pd(II) and sequentially-grown AuPd NP systems, respec-
tively, after 24 h reaction. Significant growth in the NPs are seen in
both systems; the Au NP/Pd(II) system showed a bimodal distribu-
tion with the presence of extremely large particles and an average
particle size of 6.4 nm ± 7.8 nm, while the sequentially-grown AuPd
NP system showed wormlike particles after the 24 h reaction (no
particle size is reported due to the change in morphology). In the
◦
absence of a catalytic reaction (e.g. just heating to 60 C) there is
typically no change in average particle sizes in these systems. Both
of these results suggest significant ripening of particles during the
catalytic reaction, likely due to an Ostwald ripening mechanism.
Previously we indicated that the mechanism for this reaction may
be a redox mechanism in which the Pd is oxidized to Pd(II) and
reformed upon reaction with the alcohol substrate [25]; and oth-
ers have also supported an Pd(II) oxidation mechanism [24]. Such a
mechanism is likely to lead to homogeneous Pd(II) species in solu-
tion during the catalytic reaction which would greatly enhance
Ostwald ripening. Indeed, others have shown that Pd(II) species
can react stoichiometrically with many ␣,␤-unsaturated alcohols
◦
[
42,43]. Thus the lower activities seen for crotyl alcohol at 60 C
seems mostly due to the large changes seen in particle size during
the reaction; at lower temperatures the Ostwald ripening is much
slower, thus allowing for higher-surface area NPs to remain active
for longer periods of time. We believe the role of the Au in this reac-
tion is to destabilize the Pd to oxidation by oxygen, thus allowing
the reaction to turnover.
Finally, in order to further examine both the structure of the
sequentially grown AuPd NPs and the reaction product in the Au
NP/Pd(II) system, these systems were examined via EXAFS analy-
sis. Fig. 3 shows the Au LIII-edge and Pd K-edge EXAFS spectra in
k-space for the pure Au and Pd NPs, sequentially grown AuPd NPs,
and the final nanoparticles in the Au NP/Pd(II) system (after 24 h
reaction with crotyl alcohol); high quality data was collected over a
k-range from 0 to 14 for the Au edge and 0 to 10 for the Pd edge. Both
the Pd and Au edge k-space data show that the reaction product in
the Au NP/Pd(II) system has similar chemical environments around
the Au and Pd absorber atoms as the as-synthesized sequentially
grown AuPd NPs. The shift in periodicity on the Au k-space data
suggests some alloying of Au and Pd in both the bimetallic sam-
ples [33,44,45]. Fig. 4 shows the experimentally obtained EXAFS
data in R-space with the single-shell theoretical fits for sequentially
grown AuPd NPs and the final nanoparticles in the Au NP/Pd(II) sys-
tem. For the bimetallic NPs, the Au and Pd EXAFS data of the same
sample were simultaneously fit using the IFEFFIT software package
Fig. 3. EXAFS spectra in k-space for monometallic Au, Pd and sequentially-grown
1:3 AuPd NPs and Au NP/Pd(II) 1:3 mixture after 24 h reaction with crotyl alcohol.
(
A) Pd K-edge, (B) Au LIII-edge.
problems will likely compromise data quality regardless. We note
that because of these errors one cannot make any definitive conclu-
sions regarding the final structures of the AuPd particles; however,
the EXAFS data unambiguously indicates that nearly all the Pd is in
the zerovalent state in both systems, and that core–shell morpholo-
gies are unlikely as the total first shell Au coordination number
(e.g. NAu–Au + NAu–Pd) is significantly below the bulk fcc value of 12
for both systems. The moderate NAu–Pd and NPd–Au values (albeit
with high errors on the latter) also indicates that significant Au/Pd
mixing is seen in both these systems.
[
35]. High-quality fits have been obtained for both the bimetallic
AuPd samples. We note that the Pd data shows significant Pd–Pd
and Pd–Au coordination environments but minimal Pd–O and/or
Pd–Cl contributions (a very small shoulder can be seen on the low
r-side of the Pd data); thus during the reaction, the Pd(II) is defini-
tively reduced to form bimetallic NPs in the Au NP/Pd(II) system.
The structural fit parameters for the sequentially grown AuPd NPs
and the reaction 0 to 10 product in the Au NP/Pd(II) system gener-
ated from the EXAFS fitting parameters are presented in Table 3. In
both cases, reasonable fits were obtained; however, higher errors
in coordination number for the Pd nearest neighbour were seen,
which is likely both due to the lower quality of Pd edge data and
inhomogeneities in the actual samples (which are particularly evi-
dent in the TEM of the reaction product in the Au NP/Pd(II) system).
Higher Pd loadings in sample may allow for quality data acquisi-
tion to higher k-space values in the future, though heterogeneity
3.2. Oxidation reactions in ILs
We have recently shown that the trihexyl(tetradecyl)
phosphonium chloride (P[6,6,6,14]Cl) IL is an excellent stabi-
lizer for both Au and Pd NPs for hydrogenation reactions [32]. In
this IL, the source of NP stabilization is thought to be twofold: the
halide ions that interact with the NP surface, and the long-chain
hydrocarbons that provide steric protection in the form of a
secondary coordination shell. Weaker coordinating anions such
as triflate were found to have much lower stabilizing effects in
previous work [32]. As AuPd NP recyclability is a major challenge
in the water system above (since the products need to be extracted
using ethyl acetate or a similar solvent), we were curious as to

1
76
A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
Fig. 4. EXAFS single-shell fits in R-space for sequentially-reduced 1:3 AuPd NPs at the (A) Au LIII and (B) Pd K-edges and Au NP/Pd(II) 1:3 mixture after 24 h reaction with
crotyl alcohol at the (C) Au LIII and (D) Pd K-edges.
whether the reaction could proceed in a more recyclable solvent
system such as an IL. This system is attractive as it would allow for
the products and un-reacted substrates to be removed from the
reaction mixture by vacuum extraction, allowing for a potential
re-use of the IL/NP catalyst system.
Pd and Au oxidation can occur [46]. Evidently an Ostwald ripening
mechanism in which Pd (and potentially Au as well) are oxidized
and re-reduced upon stoichiometric reactions with the alcohol sub-
strates is occurring to an even greater extent in the IL system as
compared to the aqueous system above, due to the extremely high
chloride environment in these ILs.
TEM images in Fig. 5 show the Au, Pd NPs and sequentially grown
1
:3 AuPd NPs before and after reaction. Before reaction, the Au,
Table 4 shows the product distributions for the catalytic
oxidations of four of the various ␣,␤-unsaturated alcohols in
trihexyl(tetradecyl)phosphonium chloride. We note that 5000:1
ratios of substrate to catalyst were used for the IL work, rather
than the 500:1 ratios in the aqueous work, as vacuum stripping
of the products and leftover substrates was more reliable at higher
substrate levels and IL-stabilized NPs were still quite stable in the
presence of larger amounts of substrate. While the Au NPs showed
almost no catalytic activity in the absence of Pd(II) salts, but were
active with Pd(II) salts present, Pd NPs and 1:3 AuPd NPs showed
variable degrees of conversion for most of the substrates under the
reaction conditions. Several things can be noted from the catalyst
data; first, the overall conversions over 12 h for reactions in the IL
are in most cases much higher than their respective aqueous sys-
tems. In particular, aromatic systems such as benzyl alcohol and
AuPd and Pd average NP sizes are 2.2 nm ± 0.8 nm, 3.7 nm ± 1.0 nm,
and 4.3 nm ± 1.3 nm, respectively (see Table 1). The Pd NPs are
larger than those formed in the aqueous system above, but in gen-
eral agreement with our previous work [32]. The larger size of the
NPs is confirmed by UV–Vis spectroscopy as shown in Fig. 6; a sig-
nificant plasmon band at 530 nm is seen for the Au NPs, which
dampens significantly upon sequential Pd deposition. However,
there is a significant growth of particles during a 12 h catalytic
cycle, as seen in Fig. 5(D–F); the Pd and Au NP/Pd(II) system grow
to 9.0 nm ± 2.9 nm and 8.7 nm ± 3.9 nm, respectively, while the
sequentially grown AuPd system shows a bimodal distribution with
a significant number of particles above 10 nm. Since halide ions are
also known to promote oxidative etching of Pd, NPs in halide ILs
in the presence of oxygen form an unique system in which both
Table 3
EXAFS fitting parameters for AuPd NP systems.
Nanoparticle sample
Shell
N
R (Å)
ꢁE0 (eV)
ꢃ2 (Å2)
R-factor
1
:3 AuPd sequentially grown NPs
Au–Au
Au–Pd
Pd–Pd
Pd–Au
Au–Au
Au–Pd
Pd–Pd
Pd–Au
5.9 (0.8)
2.4 (0.4)
6.0 (2.6)
3.8 (2.5)
6.4 (0.7)
3.4 (0.4)
7.0 (3.1)
2.9 (2.7)
2.812 (0.008)
2.768 (0.009)
2.73 (0.02)
2.768 (0.009)
2.814 (0.007)
2.777 (0.007)
2.76 (0.02)
4.4
0.010 (0.001)
0.008 (0.001)
0.008 (0.004)
0.008 (0.001)
0.009 (0.001)
0.007 (0.001)
0.008 (0.004)
0.007 (0.001)
0.020
(0.5)
−4.1
(2.6)
4.9
(0.4)
3.0
1
:3 Au NPs/Pd(II) after reaction
0.014
2.777 (0.007)
(1.2)

A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
177
Fig. 5. TEM images of as-synthesized P[6,6,6,14]Cl IL-stabilized (A) Pd NPs, (B) sequentially-reduced 1:3 AuPd NPs, and (C) Au NPs; and after 24 h reaction with cinnamyl
alcohol: (D) Pd NPs, (E) sequentially-reduced 1:3 AuPd NPs, and (F) Au NP/Pd(II) 1:3 mixture.
cinnamyl alcohol show extremely high conversions with high and
moderate selectivities towards the aldehyde, respectively. Again,
the major product for the allyl alcohol oxidation reaction is actu-
ally the isomerization/tautomerization product, 1-propanal (which
could also be formed as a hydrogenation + oxidation product). The
activities towards crotyl alcohol oxidation are somewhat simi-
lar to that of the aqueous system, albeit with significantly lower
selectivities. In order to ensure that the change in substrate:catalyst
ratio did not adversely affect the comparison of aqueous/IL sys-
tems, crotyl alcohol oxidation was examined in the IL system with
a 500:1 substrate:product ratio, and similar yields and selectivities
to crotonaldehyde were seen for both Pd NPs (62% conversion, 43%
selectivity) and sequentially grown 1:3 AuPd NPs (66% conversion,
56% selectivity) systems. The major finding is that AuPd NPs show

1
78
A. Maclennan et al. / Catalysis Today 207 (2013) 170–179
◦
tetraalkylphosphonium IL at 60 C. In water sequentially grown
AuPd NPs, either formed intentionally or in situ, were found to
be active for most ␣,␤-unsaturated alcohols; although particle size
growth due to Ostwald ripening was problematic. TEM and EXAFS
studies of sequentially grown AuPd NPs and Au NP/Pd(II) mix-
tures indicated that both systems led to the formation of AuPd
NPs with some mixing of Pd and Au in their structures. Conversely,
we found that Pd NPs alone have significant catalytic activity in
tetraalkylphosphonium chloride ILs, likely due to the ease of oxida-
tion of Pd in the high chloride environment. As such, Au promoters
have little to no effect on Pd catalytic activity in the IL system. The
concept of using a recyclable solvent in catalytic alcohol oxidations
still remains a problem worthy of intensive research.
Acknowledgements
The authors would like to thank Dr. Weifeng Chen and Ning
Chen at the Canadian Light Source (CLS) for the assistance with
EXAFS measurements, Al Robertson at Cytec for donation of
tetraalkylphosphonium chloride ionic liquids and NSERC and the
University of Saskatchewan for funding. Some of the research
described in this paper was performed at the Canadian Light
Source, which is supported by the Natural Sciences and Engineer-
ing Research Council of Canada, the National Research Council
Canada, the Canadian Institutes of Health Research, the Province
of Saskatchewan, Western Economic Diversification Canada, and
the University of Saskatchewan.
Fig. 6. UV–Vis spectra of P[6,6,6,14]Cl IL-stabilized Au, Pd, and sequentially-grown
:3 AuPd NPs.
1
Table 4
Summary of catalytic results for the oxidation of ␣,␤-unsaturated alcohols using
a
P[6,6,6,14]Cl-stablized NPs.
Substrate
NP catalyst
Conversionb
Selectivity (%)
Aldehyde
Other
83c
Allyl alcohol
Pd NPs
66
47
45
97
84
90
99
81
98
53
50
50
17
21
27
99
97
98
70
91
74
47
46
58
c
1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
79
73
1
c
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1
1
:3 AuPd Seq. NPs
:3 Au/Pd(II)
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