Gold-palladium supported on porous steel fiber matrix: Structured catalyst for benzyl alcohol oxidation and benzyl amine oxidation

Article abstract of DOI:10.1016/j.catcom.2011.04.025

Au, Pd and Au-Pd alloy are deposited on porous steel fiber matrices via sputtering technique. By this technique the preparation of the heterogeneous catalysts is clean, simple and fast as noble metal complexes, solvents or reducing agents are not needed.

Full text of DOI:10.1016/j.catcom.2011.04.025

Catalysis Communications 12 (2011) 1260–1264
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Gold–palladium supported on porous steel fiber matrix: Structured catalyst for
benzyl alcohol oxidation and benzyl amine oxidation
1
Hongfan Guo , Marianna Kemell, Afnan Al-Hunaiti, Sari Rautiainen, Markku Leskelä, Timo Repo ⁎
Laboratory of Inorganic Chemistry, Department of Chemistry, University of Helsinki, P.O. Box 55, Helsinki FI-00014, Finland
a r t i c l e i n f o
a b s t r a c t
Article history:
Au, Pd and Au-Pd alloy are deposited on porous steel fiber matrices via sputtering technique. By this technique
the preparation of the heterogeneous catalysts is clean, simple and fast as noble metal complexes, solvents or
reducing agents are not needed. The studied Au-Pd catalyst exhibits good activity in the aerobic oxidation of
benzyl alcohol and benzyl amines. Especially in the oxidation of benzyl amines, the Au-Pd catalyst shows a
great synergistic enhancement in the activity.
Received 11 March 2011
Received in revised form 19 April 2011
Accepted 22 April 2011
Available online 29 April 2011
©
2011 Elsevier B.V. All rights reserved.
Keywords:
Au-Pd
Sputtering
Benzyl alcohol
Benzyl amines
Oxidation
1
. Introduction
high specific surface area [8]. Among the Au, Au-Pd and Pd catalysts,
the Au-Pd catalyst exhibits most promising activity in the oxidation of
The oxidation of alcohols to corresponding carbonyl compounds
2
benzyl alcohol and benzyl amines by O . To the best of our knowledge,
and the oxidative dehydrogenation of amines to imines are of
significance in organic synthesis [1–5]. Traditionally, these oxidations
are performed using stoichiometric amounts of oxidants, such as
chromate or permanganate, but these reagents have serious toxicity
this is the first report on the sputtering method applied to preparation
of bimetallic Au-Pd catalyst on such special support for catalytic
oxidation.
issues [2]. Therefore, being one of the greenest oxidants O
2
has
received much attention [2]. On the other hand, promising catalytic
2. Experimental
2
activity of Au based catalysts in these oxidations [1–5] by O makes
them attractive.
The deposition of Au by sputtering is described elsewhere [8]. Pd
and Au-Pd were deposited by the same technique. The matrix (NV
Bekaert SA, Belgium) used as the catalyst support consisted of
stainless steel 316 L fibers and was ~0.5 mm thick. Before use, it
was annealed in air at 650 °C for 4 h. Noble metals were deposited on
the matrices in a high resolution sputter coater (Cressington 208HR,
UK; 0.02 mbar under argon; sputtering current: 80 mA). The target
materials used were pure Au (99.99%, Cressington), pure Pd (99.95%,
Ted Pella. INC, USA) and a Au-Pd alloy (99.99%, 80/20 by weight,
Cressington).
The sputter coater is fitted with a MTM-20 high-resolution terminat-
ing thickness controller to determine the nominal deposition thickness
(NDT) of deposited noble metals. NDT is quantified as a flat metallic film
deposited on a flat substrate. The amount of deposited metals can be
calculated by the following formula: m=V×ρ=2×(A×NDT)×ρ, where
m=mass of the metal, ρ=density of the metal, A=footprint area of the
fiber matrix which is multiplied by 2 because metal was deposited on both
sides.
The common Au based catalysts are supported on metal oxide
powders [6]. They are usually prepared by various solution synthesis
methods. Physical methods, such as thermal evaporation and
sputtering, are simple and clean methods, which directly deposit
metal to various surfaces without production of waste stream, and
without impurities introduced to the catalyst. In comparison, solution
synthesis requires solvents, soluble metal precursors and reductants,
such as sodium borohydride, hydrazine and hydroxylamine [7].
Here, a new concept for preparation of heterogeneous noble metal
catalyst is developed. We employ sputtering method to deposit Au,
Au-Pd and Pd directly onto porous steel fiber matrices. This deposition
of the noble metals is rather simple, and takes a few minutes. The steel
fiber matrix (Fig. 1) is a type of structural and functional material with
⁎
Corresponding author. Tel.:+358 919150194; fax: +358 919150198.
E-mail address: Timo.Repo@helsinki.fi (T. Repo).
Current address: College of Chemical Engineering, Shenyang University of Chemical
1
The area of the fiber matrix used per catalytic measurement was
Technology, Shenyang 110142, PR China.
4.48 cm2 in alcohol oxidation and 8.96 cm2 in amine oxidation.
1566-7367/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2011.04.025

H. Guo et al. / Catalysis Communications 12 (2011) 1260–1264
1261
Fig. 1. SEM images with different magnifications of the steel fiber matrix used.
Calculated amounts of noble metals in alcohol oxidation are given in
Table 1.
SEM images were obtained by Hitachi S-4800 Field emission
scanning electron microscope.
The aerobic oxidation of the alcohols was performed in a stainless
steel autoclave loaded with small glass liners and magnetic stir bars.
The stirring speed was 1000 rpm. Water and appropriate amounts of
NDTs as in Fig. 2a nor a discontinuous surface is obtained at high NDTs
as in Fig. 2b (two typical discontinuous sites are denoted in Fig. 2b).
This indicates that the Au-Pd alloy disperses better than Au alone on
the steel fiber matrix. The morphology of Pd on the fiber matrix is
similar with that of Au-Pd (pictures not shown).
To study the catalytic properties, aerobic oxidation of benzyl
alcohol in water was first chosen as a model reaction. In the absence of
a catalyst or with the pure fiber matrix without noble metals no
oxidation occurred. However, small amount of Au or Au-Pd (NDT:
0.5 nm) deposited on the fiber matrices gave activity (Fig. 3). Both Au
and Au-Pd catalysts were highly selective to benzoic acid when NaOH
2 3
alcohol and K CO or NaOH were added to the glass liners. The catalyst
matrix was cut into small pieces (~2 mm×2 mm) with scissors before
introduced into the reaction mixture. The reactor was then heated up
after oxygen pressure was introduced. After reaction, the unreacted
alcohol, aldehyde and ester were extracted with toluene. Benzoate
2 3
is used as the promoter (Fig. 3a). On the other hand, when K CO is
(
benzoic acid reacted with the base promoter) was transformed to
used as the promoter (Fig. 3b) selectivity of Au catalyst turns towards
ester formation. Benzoic acid is the main product with Au-Pd catalyst,
but its selectivity is reduced. Standard deviations in the measurement
benzoic acid by reacting with HCl, and then the benzoic acid was also
extracted with toluene. The combined toluene solutions were
analyzed by GC (Agilent 6890 N Gas Chromatograph, HP-Innowax
column, FID detector) using acetophenone as the external standard.
Every catalytic reaction under the same reaction conditions was
measured at least 3 times, and the average values are reported. The
catalytic reactions in comparison are measured at the same time. The
aerobic dehydrogenation of amines was carried out with the similar
reaction setup as used for the oxidation of alcohol. Benzylamine was
distilled under reduced pressure before use. Turnover frequency
of yield in Fig. 3 are given in Table 2. After reaction under O
2
atmosphere, the dispersion form of Au-Pd observed by SEM is the
same, and no new Au-Pd nanoparticles formed. Pd alone on the fiber
matrix is inert.
The reactivity increases as the NDT increases (Fig. 3a). When NDT
is 6 nm and NaOH is used as a base, Au-Pd gives 91% yield of benzoic
acid. The relative TOFs are given in Fig. 4. High TOF values towards
benzoic acid are noteworthy when comparing to earlier reported
catalysts for benzyl alcohol oxidation in water. For example, TOF is up
(
TOF) was calculated based on the total amount of noble metals
−
1
applied in the reaction [9–11], because the fraction of surface metal
atoms was not available.
to 400 h
benzoic acid: N90%; H
in Prati's Au catalyst (conversion: 57%; selectivity to benzoic acid:
in Tsukuda's Au catalyst (conversion: ~99%; selectivity to
−
1
2 2
O as the oxidant) [12], and is up to 1140 h
−
1
4
1%; selectivity to benzaldehyde: 32%) [11]. TOF is ~16 h in Baiker's
Au-Pd catalyst [13], and is up to 57 h
3
. Results and discussion
−
1
in Scott's Au-Pd catalyst [1].
Nearly full selectivity to benzaldehyde was reported in these two
cases. Incidentally, the oxidation from benzaldehyde to benzoic acid
The microstructure of the Au on the steel fiber matrices depends
on the amount of the deposited Au. The amount of Au is quantified as a
NDT (NDT=thickness of a flat metallic film deposited on a flat
surface; see Experimental section). SEM studies show that Au exists as
isolated nano-islands at low NDTs. The nano-islands are seen as
irregularly shaped bright features (Fig. 2a). When the NDT increases,
the nano-islands become larger, merge and form a more continuous
film (Fig. 2b). The structures of the supported Au-Pd alloys differ from
those of Au (Fig. 2c and d). Nano-islands are not formed even at low
was investigated by some efficient catalytic systems, such as Bi
t-BuOOH as the oxidant [14], β-cyclodextrin and NaClO as the oxidant
15], porphyrin in CH CN [16], etc.
Generally, alcohol oxidations over Au based catalysts need a base
2 3
O and
[
3
as a promoter. It is reasonable to assume that base is needed to
deprotonate alcohol to initiate the catalytic reaction [17]. Here both
Au and Au-Pd catalysts give neglectable conversions without a base.
The optimal base to alcohol ratio is ~1:1 (Fig. 5).
To investigate whether the ester is produced via the condensation
of benzyl alcohol and the generated benzoic acid, equimolar amounts
of alcohol and benzoic acid were introduced to the reaction and K CO
2 3
was selected as a base. Compared to the results in Figs. 3b and 5b,
neither the selectivity to ester nor the amount of produced ester is
increased. No ester is formed in the absence of a catalyst. Therefore,
the ester is most probably produced via a hemiacetal pathway
Table 1
Amount of noble metals in the catalysts with different NDTs per measurement in the
oxidation of alcohols.
NDT (nm)
0.5
1.5
3.0
6.0
4.39×10−
4.75×10−
5
5
1.32×10
1.42×10
−4
−4
2.63×10
2.85×10
−4
−4
5.27×10
5.71×10
−4
−4
Au (mmol)
Au-Pd (mmol)
(
Scheme 1) [18,19]. Strong base as NaOH may prevent the formation

1262
H. Guo et al. / Catalysis Communications 12 (2011) 1260–1264
Fig. 2. SEM images of (a, b) Au and (c, d) Au-Pd on the steel fiber matrix. NDT: (a, c)1.5 nm; (b)3.0 nm; (d)6.0 nm.
of the hemiacetal intermediate. This reaction pathway could explain
ester formation when K
used (Figs. 3 and 5).
When NaOH is used as a base the catalysts are also very active
towards secondary benzylic alcohol. For example, 1-phenyl-1-propanol
is oxidized into its relative ketone, propiophenone (Scheme 2),
with ~80% yield over Au catalyst and ~90% yield over Au-Pd catalyst,
2 3
CO or very low NaOH:alcohol molar ratio are
O
O
O
O
OH O2
H
OH
+
+
Aldehyde
Acid
Ester
1
00
Au-Pd
Acid
Aldehyde
(a)
Au
2 3
together with excellent selectivity. When K CO is used as a base, Au and
Au-Pd catalysts give only ~45% yield and ~55% yield, respectively.
8
6
4
2
0
0
0
0
0
Table 2
Standard deviations in the measurement of yield in Fig. 3.
NDT (nm)
0.5
1.5
3.0
6.0
Au in Fig.3a
Au-Pd in Fig.3a
Au in Fig.3b
5.0
6.9
3.9
5.5
4.9
6.4
4.7
2.4
4.7
6.9
3.1
4.5
4.4
6.4
2.7
5.1
3
6
3
6
0
.5
1.5
0.5
1.5
Nominal depostion thickness (nm)
Au-Pd in Fig.3b
1
00
acid
ester
aldehyde
Au-Pd
(
b)
9000
80
60
40
20
0
7
6
4
3
500
000
500
000
Au
Au-Pd
Au
3
6
3
6
0.5
1.5
0.5
1.5
1500
0
Nominal deposition thickness (nm)
3
6
3
6
0.5
1.5
0.5
1.5
Fig. 3. Yield and selectivity ofbenzyl alcohol oxidation as a functionof NDT when (a) NaOH
and (b) K CO used as the promoter. Reaction conditions: (a) 1.5 mmol and (b) 0.75 mmol
alcohol, 1.5 ml water, 3 h, 80 °C, 5 bar O ; basemol:alcoholmol=1:1. Amounts of Au and Au-
Nominal depostion thickness (nm)
2
3
2
Pd used in the alcohol oxidation are given in Table 1. Standard deviations in the
measurement of yield are given in Table 2.
Fig. 4. TOFs at benzyl alcohol oxidation as a function of NDT when NaOH as the
promoter (based on Fig.3a and Table 1).

H. Guo et al. / Catalysis Communications 12 (2011) 1260–1264
1263
1
00
Au-Pd
Raction A: oxidation of dibenzylamine
Acid
(
a)
Ester
Aldehyde
O2
N
H
N
8
6
4
2
0
0
0
0
0
Au
Raction B: oxidation of benzylamine
NH
2
O2
N
1
2
1
2
0.5
0.5
0
.25
0.25
Molar ratio of base to alcohol
Scheme 3. Oxidative dehydrogenation of dibenzylamine and benzylamine.
1
00
(
b) Au
Au-Pd
acid
ester
aldehyde
8
6
4
2
0
0
0
0
0
Table 3
Dehydrogenation of dibenzylamine and benzylamine.
Entry
Substrate
Catalyst
Conversion (%)
1
2
3
4
5
6
7
8
9
10
Dibenzylamine
Dibenzylamine
Dibenzylamine
Dibenzylamine
Dibenzylamine
Benzylamine
Benzylamine
Benzylamine
Benzylamine
Benzylamine
No catalyst
Support
Pd
Trace
3
4.5
9
59
15
25
26
29
67
1
2
4
1
2
4
0
.5
0.5
Au
0
.25
0.25
Au-Pd
No catalyst
Support
Pd
Au
Au-Pd
Molar ratio of base to alcohol
Fig. 5. Yield and selectivity of the benzyl alcohol oxidation as a function of (a) NaOHmol
:
alcoholmol and (b) K
and NDT is 6 nm.
2
CO3mol:alcoholmol. Reaction conditions are the same as in Fig. 3
Reaction condition: 100 °C, 0.15 mmol amine in 3.0 ml toluene, 5 bar O
nm for Au, Pd and Au-Pd catalysts; 1.142 μmol Au-Pd (~0.75 mol% Au-Pd related to
the amine).
2
, 24 h; NDT:
6
The dehydrogenation of dibenzylamine (secondary amine) and
coupling of benzylamine (primary amine) (Scheme 3) were also chosen
as the model reactions to study the catalytic properties. These two
reactions are very slow. For example, the TOF in dibenzylamine
that Pd acts as a diluent to improve Au dispersion. Small particle size
favors the oxidation of benzyl amines on Au catalyst [4]. Another
possibility is related to the stronger ability of Pd catalysts to
dehydrogenate C-H bonds than of Au catalysts [20]. As the dehydroge-
nation of amines to imines involves the cleavage of both C―H and N―H
bonds [21], the improved ability of Pd to cleave C―H bonds, together
with the fact that Au catalyst is in general active in amine oxidation,
might explain the observed synergistic effect.
−
1
oxidation is 1 h in Au/C (reaction time of 17 h and 5 mol% Au related
−
1
to amine) [2]. The TOF in benzylamine oxidation is 0.001 h
in Au
−
1
−1
powder [5], 0.3 h in Au/Al
OAc) /CeO (reaction time of 16 h and 1.74 mol% Au related to amine)
3].
2 3
O (reaction time of 24 h) [5], ~3 h in Au
(
3
2
[
As in alcohol oxidation also in the dehydrogenation of dibenzy-
lamine the Au-Pd catalyst has the highest activity in the series (entries
–5 in Table 3). The Au and Pd catalysts are significantly less active
than the Au-Pd catalyst which gives 60% conversion with a TOF of
1
4. Conclusions
−
1
3
.2 h . High selectivity of all catalysts towards imine is noteworthy
as benzaldehyde or benzaldoxime as possible by-products [4] is not
detected. Also in the coupling reaction of benzylamine, Au-Pd catalyst
has a much higher activity than Au and Pd catalysts (entries 8–10 in
Table 3). In applied conditions Au-Pd catalyst gives practically 100%
As shown here, a very clean and simple process of preparing
heterogeneous noble metal based catalysts is achieved via sputtering
technique. Au, Pd and Au-Pd were directly deposited from the
corresponding metal targets onto the steel fiber matrix with high
specific surface area. The Au-Pd structured catalyst exhibits good
activities in the oxidation of benzyl alcohol and benzyl amines and
shows a great synergistic enhancement in the reactivity especially in
the oxidation of benzyl amines. This green route deserves further
development. For example, choosing a matrix with an even larger
surface area as the support, optimizing the Au/Pd ratio as well as
examining different solvents may further improve the catalytic
activity.
−
1
selectivity to imine with a TOF of 3.7 h
.
Since Au-Pd gives much higher activity than Au and Pd in the
dehydrogenation of amines, synergetic effects in the former catalyst
must play an important role. One possibility to the increased activity is
O
OH
O
RCH
2
OH+ H+
[O]
[O]
2
RCH OH
R
C
H
R
CH OCH
Hemiacetal
2
R
R
C OCH
Ester
2
R
Acknowledgment
Scheme 1. Plausible route for the ester formation via hemiacetal oxidation [18,19].
Financial support by the Academy of Finland (Project No. 124187)
is gratefully acknowledged.
OH
O
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