Oxidation of benzyl alcohol using supported gold-palladium nanoparticles

Article abstract of DOI:10.1016/j.cattod.2010.10.028

Gold-palladium nanoparticles either in the form of colloids or supported nanoparticles have been extensively used as redox catalysts in recent years. These materials are very effective for the transformation of organic compounds to highly useful chemical

Full text of DOI:10.1016/j.cattod.2010.10.028

Catalysis Today 164 (2011) 315–319
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Catalysis Today
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Oxidation of benzyl alcohol using supported gold–palladium nanoparticles
Peter Miedziaka, Meenakshisundaram Sankara, Nikolaos Dimitratosa, Jose A. Lopez-Sancheza,
Albert F. Carleya, David W. Knighta, Stuart H. Taylora, Christopher J. Kielyb, Graham J. Hutchingsa,∗
a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff CF10 3AT, UK
b Center for Advanced Materials and Nanotechnology, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015-3195, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 3 November 2010
Gold–palladium nanoparticles either in the form of colloids or supported nanoparticles have been exten-
sively used as redox catalysts in recent years. These materials are very effective for the transformation
of organic compounds to highly useful chemical products. The catalytic materials are usually prepared
either using deposition-precipitation or impregnation techniques, but recently significant attention has
been focused on the use of colloidal methods. Here we compare and contrast the preparation and catalytic
reactivity of Au–Pd supported nanoparticles synthesised by impregnation, deposition-precipitation and
colloidal methods. The catalyst materials have been evaluated for the oxidation of benzyl alcohol, as a
model reaction. In addition, we have focused our attention on the utilisation of different types of reac-
tors (autoclave versus glass reactors) and we now emphasise the possibility of using simplified types of
reactors which have the advantage of significant cost savings and ease of application.
Keywords:
Gold and palladium supported catalysts
Benzyl alcohol oxidation
Sol-immobilisation
Impregnation
Deposition-precipitation
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
a second metal component, activity, selectivity and catalyst life-
time can often be improved, thus minimising deactivation [11,13].
The utilisation of molecular oxygen in combination with the
uses of heterogeneous catalytic systems has been identified as an
attractive green methodology for the production of fine chemicals
in the fragrance and pharmaceutical industries. The current com-
mercial processes are based on the use of stoichiometric inorganic
oxidants such as potassium dichromate. However, these processes
typically produce large amounts of waste and often result in poor
atom efficiency, making them both environmentally unfriendly and
ultimately non-economic [1–3]. In response, many homogeneous
systems able to catalyse liquid phase oxidation have been used
successfully, resulting in high product yields [4–7]. However, the
disadvantage of the use of homogeneous systems is widely known
to be associated with separation and recycling. Thus, interest has
shifted towards the development of heterogeneous catalytic sys-
tems, which can be used in both batch and fixed-bed reactors. A
variety of heterogeneous catalysts has been developed in the last
decade, based on multicomponent oxides, heteropolyacids, vana-
dium phosphorus oxides and platinum group metals deposited on a
support [8–11]. In fact, the most promising heterogeneous catalysts
are currently based on platinum group metals [11–13]. However,
the drawback of utilising platinum group metals individually as
catalysts is that frequently poor selectivity and deactivation are
often observed [12]. Using a bimetallic catalyst, thus incorporating
Recent studies have demonstrated the efficacy of Au, Pt and Pd
derived bimetallic catalysts for the aerobic partial oxidation of alco-
hols and polyols [11,14–29]. In our previous papers we have shown
that catalysts based on supported Au–Pd nanoparticles are effective
catalysts for the oxidation of polyols and alcohols [30–34], as well
as for the synthesis of hydrogen peroxide [35–39]. In this paper
we examine the oxidation of benzyl alcohol using mild conditions
and contrast the use of three reactor systems based either on the
use of an autoclave reactor or the use of glass reactors operating
at lower reaction pressures and we show that use of glass reac-
tors can be effective. In addition, we use three methods for the
preparation of Au–Pd supported catalysts namely: impregnation,
sol-immobilisation and deposition-precipitation methods, which
are the major methods used for the synthesis of gold based cata-
lysts [22,40–43]. We have used the oxidation of benzyl alcohol as
a model reaction and we have demonstrated previously that the
catalysts can be used for the selective oxidation of a broad range of
alcohols [31].
2. Experimental
2.1. Materials
HAuCl4·3H2O (99.99% purity) and PdCl2 (99.99% purity) were
supplied by Johnson Matthey, Pd(NO3)2.was supplied by Aldrich,
Titania (Degussa P25) was used as the support. Na2CO3 (99.0%
of purity), NaBH4 of purity > 96% (Aldrich), polyvinylalcohol (PVA)
∗
Corresponding author. Tel.: +44 29 2087 4059; fax: +44 29 2087 4059.
E-mail addresses: hutch@cardiff.ac.uk, hutch@cf.ac.uk (G.J. Hutchings).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.10.028

316
P. Miedziak et al. / Catalysis Today 164 (2011) 315–319
(Aldrich, MW = 10,000, 80% hydrolyzed) were used in the prepa-
ration. Fresh stock aqueous solutions of PdCl2 (Johnson Matthey)
(acidified with HCl), HAuCl4·3H2O of the desired concentration,
NaBH4 (0.1 M) and PVA (1%, w/w) were prepared prior to the prepa-
ration of the catalytic materials.
a 30 m CP-Wax 52 CB column) were employed. The products were
identified by comparison with known standards. For the quantifica-
tion of the amounts of reactants consumed and products generated,
an external standard method was used.
2.4. Characterisation of catalysts
2.2. Catalyst preparation
The materials prepared by impregnation and immobilisation
have been extensively characterised in our previous studies using
STEM-HAADF microscopy and XPS [31,33,38,39,44,46,47]. Briefly
we describe here details related to their structures. Catalysts pre-
pared by the impregnation method generate a broader particle size
distribution and greater median size than the sol-immobilisation
method. Specifically, catalysts synthesised by the impregnation
method show that the Au–Pd/TiO2 catalyst has particles mainly in
the 2–5 nm range with ca. 8% of larger particles (>20 nm). STEM-
XEDS [31,37,38,46,47] shows that the TiO2-supported material has
a core–shell structure with a palladium-rich shell and a gold-rich
core, an observation that is infrequently observed with other oxide-
supported Au–Pd catalysts prepared by wet impregnation. The
reason for the formation of core–shell structures is due to the cal-
cination procedure used with the impregnated catalysts. We have
noted earlier [31] that calcination at 400 ◦C is required to form sta-
ble re-usable catalysts with this preparation method. During the
calcinations process the Pd migrates to the surface of the nanopar-
ticles when an oxide is used as the support.
Characterisation of catalysts prepared via the sol-
immobilisation method has also been reported previously [33,39].
The median diameter of the sol-immobilised pure-Au are about
4.0 nm, while for pure-Pd the median size on the TiO2 support
was 3.8 nm. The median particle sizes for the sol-immobilised
Au–Pd alloy were 3.8 nm on the TiO2 support. It is also important
to emphasise that the major advantage of the sol-immobilisation
preparation method is that it significantly minimises the formation
of larger particles (>10 nm) that are commonly found in the cata-
lysts prepared by impregnation. Therefore the particle size distri-
bution is significantly narrower for the sol-immobilisation method
as compared to those made by the impregnation method. STEM-
HAADF images of typical alloy particles have shown that the Au–Pd
nanoparticles prepared by sol-immobilisation are homogeneous
alloys [33,39]. For the bimetallic Au–Pd supported catalysts syn-
thesised by the deposition-precipitation method using sodium car-
bonate it was found that the metal nanoparticles are in the 2–10 nm
particle size range and consist of Au–Pd alloys with gold core–Pd
shell morphology formed during the calcination process [44].
Bimetallic Au–Pd supported catalysts were prepared using the
following three preparation methods: impregnation, deposition-
precipitation using sodium carbonate and sol-immobilisation. The
detailed procedures for the three preparation methods have been
described in detail elsewhere [32,33,44]. Briefly, for the synthesis of
Au–Pd/TiO2 catalyst synthesised by the sol-immobilisation method
the following procedure was used. To an aqueous fresh PdCl2 and
HAuCl4 solution of the desired concentration, the required amount
of a PVA solution (1 wt.%) was added (PVA/(Au + Pd) (wt./wt.) = 1.2);
a freshly prepared solution of NaBH4 (0.1 M, NaBH4/(Au + Pd)
(mol./mol.) = 5) was then added to form a dark-brown sol. After
30 min of sol generation, the colloid was immobilised by adding
titania (acidified at pH 1 by sulfuric acid) under vigorous stirring
conditions. The amount of support material required was calcu-
lated so as to have a total final metal loading of 1 wt.%. After 2 h the
slurry was filtered, the catalyst washed thoroughly with distilled
water (neutral mother liquors) and dried at 120 ◦C overnight.
The following notation is used for the catalyst samples used in
this study: I denotes impregnation; SI denotes sol-immobilisation;
DPC denotes deposition-precipitation using sodium carbonate; w
denotes Au and Pd are present in a 1:1 ratio by weight; and m
denotes Au and Pd are combined in a 1:1 molar ratio.
2.3. Alcohol oxidation
Autoclave studies: Benzyl alcohol oxidation was carried out in
a stirred autoclave reactor (100 mL, Autoclave Engineers Inline
MagneDrive III). The vessel was charged with alcohol (40 mL) and
catalyst (0.1 g). The autoclave was then purged 5 times with oxygen
leaving the vessel at 10 bar gauge. The stirrer was set at 1500 rpm
and the reaction mixture was raised to the required temperature.
The reaction time was measured from the time that the mixture
reached the reaction temperature and samples from the reactor
were taken periodically, via a sampling system.
Glass reactor studies: Experiments were also performed using a
glass round bottom flask fitted with a balloon pressurised with oxy-
gen and a modified version of the commercially available carousel
multi reaction system manufactured by Radleys [45]. For both of
these reactors the pressure of the oxygen was 1 bar (relative). Both
of the reactors were equipped with a heater, a thermometer and a
magnetic stirrer. The stirrer was set at 1000 rpm (maximum stir-
rer speed). In a typical reaction, the requisite amount of catalyst
and benzyl alcohol were charged into the reactor and reactor was
purged 5 times with O2 and the reactor was fitted directly to the
O2 line in the case of the Radleys reactor and the O2 balloon in
the case of round bottom flask. After this step the reactors were
quickly mounted to the base that was preheated to the reaction
temperature. In the case of reactions with the Radleys reactors, the
data points in the time online graph were obtained from differ-
ent reactions that were stopped at different time intervals. In the
case of round bottom flask reaction, constant volume of the sam-
ples was withdrawn at regular time intervals. The samples were
cooled immediately in an icebath and centrifuged to remove the
solid catalyst. An aliquot of the clear supernatant reaction mixture
(0.5 mL) was diluted with mesitylene (0.5 mL, external standard for
GC analysis).
3. Results and discussion
The bimetallic Au–Pd supported catalysts were initially tested
in the liquid phase oxidation of benzyl alcohol at 100 ◦C with
10 bar of oxygen using an autoclave reactor. For these reaction
conditions we have demonstrated previously that there are no
diffusion limitations [33,34]. Results are shown in Fig. 1 and
Tables 1 and 2. The most active catalyst was the one prepared by the
sol-immobilisation method, followed by deposition-precipitation
and impregnation. Taking into account the particle size with the
sol-immobilisation method, which produces the smaller particle
size (around 4 nm), and a narrower particle size distribution, fol-
lowed by the deposition-precipitation and impregnation methods,
this was somewhat expected in terms of activity. Hence we con-
sider that the major factor determining catalyst activity under these
reaction conditions is the particle size distribution. However, the
surface composition also varies with the preparation method with
the Au–Pd/TiO2 catalysts prepared by impregnation and calcination
having a Pd-rich surface, but we consider this effect to be sec-
Samples were removed periodically and analysed by GC. For the
analysis of the products a GC–MS and GC (a Varian star 3400 cx with

P. Miedziak et al. / Catalysis Today 164 (2011) 315–319
317
30
25
20
15
10
5
100
90
100
90
80
80
70
70
20
10
20
10
0
0
0
1
2
3
4
5
0
Reaction time (h)
0
50
100
150
200
250
Time (mins)
Fig. 1. Selective oxidation of benzyl alcohol using an autoclave reactor: Reaction con-
ditions: benzyl alcohol, substrate/metal molar ratio = 58,580, T = 100 ◦C, pO2 = 10 bar,
reaction time = 4 h, stirring rate 1500 rpm. Key: ꢀ, benzyl alcohol conversion − 1 wt.%
(Au–Pd)/TiO2SIm; ꢁ, benzyl alcohol conversion − 5 wt.% (Au–Pd)/TiO2DPCw; ꢂ, benzyl
Fig. 2. Comparison of two different types of reactors: Radleys reactor versus round
bottom flask reactor for the oxidation of benzyl alcohol oxidation using 1 wt.%
(Au–Pd)/TiO2SIm catalyst prepared by sol-immobilisation method. Reaction condi-
tions: benzyl alcohol, substrate/metal molar ratio = 56,100, T = 100 ◦C, pO2 = 1 bar,
reaction time = 4 h, stirring rate 1000 rpm. Key: ꢁ, benzyl alcohol conversion in the
Radleys reactor; ꢀ, benzyl alcohol conversion in the round bottom flask; ᭹, ben-
zaldehyde selectivity in the Radleys reactor; ꢀ, Benzaldehyde selectivity in the round
bottom flask.
alcohol conversion − 1 wt.% (Au–Pd)/TiO2Im
.
ondary in importance with respect to the main effect of particle
size. In terms of selectivity the catalysts were compared at iso-
conversion level (15%) and the results are presented in Table 2. The
highest selectivity to benzaldehyde was obtained with the bimetal-
lic Au–Pd supported catalysts synthesised by sol-immobilisation,
whereas the least selective was prepared by impregnation, with
a substantial level of toluene formed. One possible reason for this
trend could be the residual chlorine that remains on the titania
during the impregnation method, therefore the increase of surface
acidity could be responsible for the higher level of toluene forma-
tion. However, the catalysts prepared by impregnation also have a
Pd-rich surface. We have previously studied the origin of toluene
in benzyl alcohol oxidation [34] and we have shown that its forma-
tion is highly sensitive to the surface oxygen concentration and it
is also facilitated by acidic surfaces and hence the Pd-rich surface
may provide a more facile pathway than when more Au is present.
In the other two preparation methods extensive washing with dis-
tilled water typically occur during filtration, which minimises the
surface acidity.
tor, which would permit simplification of the process. The aim
was to determine if oxidation reactions with molecular oxygen
using the sol-immobilised catalysts could be carried out using stan-
dard laboratory equipment, as is the case for many hydrogenation
reactions using hydrogen. Therefore, we investigated the use of
glass reactors and selected two types of reactors, one based on a
round bottom flask fitted with a balloon filled with oxygen and
the other one based on a Radleys glass reactor fitted with a cap
and the ability to pressurise the reactor at low pressure (maximum
3 bar).
The comparison of the three different types of reactors is shown
in Tables 3 and 4. It is worth to note that in terms of TOF (turnover
frequency) similar catalytic activity was observed in all the cases
(Table 3). In Fig. 2, the reactions carried out using the Radleys
and round bottom flask reactors are shown. An increase of con-
version was accompanied by a slight decrease in the selectivity
to benzaldehyde, whereas the formation of consecutive products
such as benzoic acid and benzyl benzoate were below 1%. The
major by-product was toluene. In terms of selectivity, compar-
The results above clearly demonstrate that the most active cat-
alyst was the one synthesised by the sol-immobilisation method,
and hence we investigated the possibility of performing the liquid
phase oxidation of benzyl alcohol in a glass low pressure reac-
Table 1
Benzyl alcohol oxidation after 4 h reaction with Au–Pd catalysts prepared by impregnation, deposition-precipitation and sol-immobilisations methods.a
Entry
Catalyst
Conv. (%)
Selectivity (%)
Benzaldehyde
c
Benzoic acid
Benzyl benzoate
Toluene
DBEb
Benzene
TOF (h−1
)
1
2
3
1%(Au–Pd)/TiO2Im
5%(Au–Pd)/TiO2DPCw
1%(Au–Pd)/TiO2SIm
16.8
19.2
29.1
70.6
80.6
91.6
0.4
0.5
0.6
1.1
2.0
1.0
27.9
16.8
6.7
0.0
0.0
0.0
0.0
0.1
0.1
2940
4180
136,700
a
Reaction conditions: benzyl alcohol, substrate/metal molar ratio = 58,580, T = 100 ◦C, pO2 = 10 bar, reaction time = 4 h, stirring rate 1500 rpm.
DBE = dibenzyl ether.
b
c
Calculation of TOF (h−1) after 0.5 h of reaction. TOF numbers were calculated on the basis of total loading of metals.
Table 2
Benzyl alcohol oxidation results at iso-conversion for Au–Pd catalysts prepared by impregnation, deposition-precipitation and sol-immobilisation methods.a
Entry
Catalyst
Conv. (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
Benzyl benzoate
Toluene
DBEb
Benzene
1
2
3
1%(Au–Pd)/TiO2Im
5%(Au–Pd)/TiO2DPCw
1%(Au–Pd)/TiO2SIm
15
15
15
70.6
81.3
88.1
0.4
0.6
0.4
1.2
2.5
1.0
27.8
15.5
10.4
0.0
0.0
0.0
0.0
0.1
0.1
a
Reaction conditions: benzyl alcohol, substrate/metal molar ratio = 58,580, T = 100 ◦C, pO2 = 10 bar, reaction time = 4 h, stirring rate 1500 rpm.
DBE = dibenzyl ether.
b

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P. Miedziak et al. / Catalysis Today 164 (2011) 315–319
Table 3
Benzyl alcohol oxidation performance after 4 h reaction for 1 wt.% Au–Pd/TiO2SIm catalysts prepared by sol-immobilisation method using three types of reactor systems.a
Entry
Reactor system
Conv. (%)
Selectivity (%)
Benzaldehyde
c
Benzoic acid
Benzyl benzoate
Toluene
DBEb
Benzene
TOF (h−1
)
1
2
3
Autoclave reactord
Radleys reactore
27.0
23.0
23.0
79.7
88.0
74.0
0.2
0.1
0.2
0.4
0.8
0.4
19.6
10.7
25.0
0.0
0.2
0.4
0.1
0.2
0.0
4900
5760
4830
Round bottom flaske
a
Reaction conditions: benzyl alcohol, T = 100 ◦C, pO2 = 1 bar, reaction time = 4 h.
b
c
DBE = dibenzyl ether.
Calculation of TOF (h-1) after 0.5 h of reaction. TOF numbers were calculated on the basis of total loading of metals.
Substrate/metal molar ratio = 58,580, stirring rate 1000 rpm.
d
e
Substrate/metal molar ratio = 56,100, stirring rate 1000 rpm.
Table 4
Benzyl alcohol oxidation results at iso-conversion for 1 wt.% Au–Pd/TiO2SIm catalysts prepared by sol-immobilisation method using three types of reactor systems.a
Entry
Reactor system
Conv. (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
Benzyl benzoate
Toluene
DBEb
Benzene
1
2
3
Autoclave reactorc
Radleys reactord
15
15
15
76.7
89.8
72.0
0.2
0.1
0.0
0.6
0.9
0.0
22.4
8.8
28.0
0.0
0.2
0.0
0.1
0.2
0.0
Round bottom flaskd
a
Reaction conditions: benzyl alcohol, T = 100 ◦C, pO2 = 1 bar, reaction time = 4 h.
DBE = dibenzyl ether.
Substrate/metal molar ratio = 58,580, stirring rate 1000 rpm.
Substrate/metal molar ratio = 56,100, stirring rate 1000 rpm.
b
c
d
ison at iso-conversion (15%) showed that the highest selectivity
to benzaldehyde was observed using the Radleys reactor followed
by the autoclave reactor system. The high selectivity to toluene
observed with the low pressure glass reactor system can be ascribed
to the lower availability of oxygen in the solution. We have shown
previously that at low oxygen pressure and hence limiting oxy-
gen concentration in the solution the toluene formation increased
[33,34]. Especially, at low oxygen pressure (1 bar of oxygen) the
formation of toluene and benzaldehyde are highly sensitive to
the availability of the surface oxygen [34]. Nevertheless, we have
demonstrated that it is possible to obtain high selectivity together
with high catalytic activity using a Radleys reactor, which is pres-
surised and continually fed with oxygen as in the case of an
autoclave reactor. It is the availability of oxygen rather than the
materials of construction of the reactors evaluated which is the con-
trolling parameter that secures high selectivity to benzaldehyde.
Taking into account the above results it is important to maintain
the pressure of oxygen at 1 bar continually to obtain high selectivity
to benzaldehyde under the corresponding reaction conditions.
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Acknowledgement
We thank the EPSRC for financial support.
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