Selective Aerobic Oxidation of Alcohols over Gold-Palladium Alloy Catalysts Using Air at Atmospheric Pressure in Water

Article abstract of DOI:10.1002/cctc.201900015

A series of bimetallic Au?Pd alloy nanoparticles (NPs) with varied Au/Pd molar ratios was supported on Gamma-alumina (γ-Al₂O₃) surface through the co-impregnation-reduction method using HAuCl₄ and PdCl₂ as metal precursors. Characterization results suggested the high dispersion of Au?Pd alloy NPs, as well as the interplay between the two compositions. Benefitting from the synergistic effects of Au?Pd alloying, the bimetallic Au-Pd_x@γ-Al₂O₃ catalysts were highly efficient for selective aerobic oxidation of various alcohols either aromatic or aliphatic in water using air at atmospheric pressure as a sole oxidant without the need for any cocatalyst. Quantitative yields of corresponding aldehydes or ketones were achieved over Au-Pd_1.2@γ-Al₂O₃ within 2 h, whereas pure Au@γ-Al₂O₃ or Pd@γ-Al₂O₃ was far less efficient. More importantly, the “alloying effect” appeared to stabilize the bimetallic NPs against aggregation and poisoning, which made the Au-Pd_x@γ-Al₂O₃ more stable and reusable in the aerobic oxidation of alcohols.

Full text of DOI:10.1002/cctc.201900015

Accepted Article
Title: Selective aerobic oxidation of alcohols over gold-palladium alloy
catalysts using air at atmospheric pressure in water
Authors: Rong Tan, Wei Zhang, Ziqiang Xiao, Jiajun Wang, Wenqin
Fu, and Donghong Yin
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Selective aerobic oxidation of alcohols over gold-palladium alloy
catalysts using air at atmospheric pressure in water
Wei Zhang, Ziqiang Xiao, Jiajun Wang, Wenqin Fu, Rong Tan,* and Donghong Yin
Abstract: A series of bimetallic Au-Pd alloy nanoparticles (NPs) with
varied Au/Pd molar ratios was supported on Gamma-alumina (γ-
Al2O3) surface through the coimpregnation-reduction method using
HAuCl4 and PdCl2 as metal precursors. Characterization results
suggested the high dispersion of Au-Pd alloy NPs, as well as the
interplay between the two compositions. Benefitting from the
synergistic effects of Au-Pd alloying, the bimetallic Au-Pdx@γ-Al2O3
catalysts were highly efficient for selective aerobic oxidation of
various alcohols either aromatic or aliphatic in water using air at
atmospheric pressure as a sole oxidant without the need for any
cocatalyst. Quantitative yields of corresponding aldehydes or
ketones were achieved over Au-Pd1.2@γ-Al2O3 within 2 h, whereas
pure Au@γ-Al2O3 or Pd@γ-Al2O3 was far less efficient. More
importantly, the “alloying effect” appeared to stabilize the bimetallic
NPs against aggregation and poisoning, which made the Au-Pdx@γ-
Al2O3 more stable and reusable in the aerobic oxidation of alcohols.
limited in most of these systems where pure oxygen or a high
pressure of air is generally required. And, external alkalis (e.g.,
KOH, K2CO3 or NaOAc) were necessary over palladium catalyst
for the aerobic oxidation, which inevitably resulted in the
5d]
aggregation of palladium NPs during reactions.[5b,
It was
highly desirable to develop stable, efficient, and “green”
selective oxidation process using air (1 atm) as the oxidant in
water without any external alkali.
Lately, alloy NPs, which were composed of two or more kinds
of metals, have attracted significant interest in the field of
catalytic oxidations by O2.[6] Since electron-rich Au NPs were
beneficial for the adsorption and activation of O2,[7] they were
incorporated into Pd NPs to boost various oxidations with O2,[8]
such as direct synthesis of H2O2 from H2 and O2,[8a, 8f] selective
oxidation of alkene,[8b] glycerol oxidation,[8c] and so on. Apart
from activation of O2, interplay between the two compositions
also resulted in electronic heterogeneity, since the
electronegativity of Pd (2.20) is lower than that of Au (2.54).[9] As
a consequence, both electron-rich and slightly positively charged
sites were present on the alloy NP surface. It meaned that both
electrophilic or nucleophilic reactant molecules could be
enhanced,[7a, 9b] which led to superior catalytic reactivity. These
salient features made it possible to perform selective oxidation
of alcohols with air under atmospheric pressure with the use of
Au-Pd alloy NPs. However, dispersion of the bimetallic NPs on a
solid support surface was difficult, because metals with similar
physical and chemical properties tended to separate and
aggregate from other metals.[6c] γ-Al2O3 with large surface area
and high thermal stability has gained great attention as supports
in heterogeneous catalysis.[10] Especially, the abundant cation
vacancies on γ-Al2O3 surface favored stabilizing metal NPs,[11]
and in turn enhanced the stability of metal NPs-based catalyst.
Therefore, loading Au-Pd alloy on γ-Al2O3 might provide stable
and efficient bimetallic catalyst to realize the aerobic oxidation of
alcohols with atmospheric air in water.
Herein, we have prepared γ-Al2O3 supported Au-Pd alloy
catalysts (Au-Pd@γ-Al2O3) through coimpregnation-reduction
method, in which both Au and Pd precursors are coadsorbed
onto the γ-Al2O3 support followed by reduction with NaBH4.
Detailed characterization techniques suggested the high
dispersion of Au-Pd alloy NPs on γ-Al2O3 surface, as well as the
interplay between the two compositions. The alloy NPs realized
highly efficient selective oxidation of various alcohols with
atmospheric air in water, giving almost quantitative yield
(>99%) of carbonyl compounds even with 2 h under base-free
condition, while, pure palladium NPs or gold NPs alone was far
less efficient. Furthermore, Au/Pd molar ratio of the alloy NPs
had an important impact on performance of the catalysts since
it determined the interplay between alloy NPs. More importantly,
as the supported heterogeneous catalysts, the alloy NPs could
be facilely recovered, and be reused for at least five times
without loss of their activity and selectivity.
Introduction
Selective oxidation of alcohols in water is one of the most
important reactions in organic synthesis, not only due to its
environmentally benign catalysis process, but also because the
obtained aldehydes or ketones are versatile chemical
intermediate in fine chemicals.[1] Some pioneering contributions
are known in this emerging field. For example, Neumann et al.
reported the efficient oxidation of secondary alcohols with
aqueous H2O2 catalyzed by polyoxometalates.[2] Wang et al.
employed C3N4 as a photocatalyst to activate O2 for the selective
oxidation of benzyl alcohols under visible light irradiation.[3] Air is
undoubtedly the cheapest and ideal oxidant for this
transformation from the standpoints of atom efficiency and green
chemistry.[4] Palladium-based catalysts, especially palladium
nanoparticles (NPs), have been developed for the aerobic
oxidation of alcohols in aqueous media.[5] For example,
Pd@A20E10 (Pd@amphiphilic hybrid carbon nanotubes) was
used to catalyze the oxidation of alcohols at 100 °C using air (5
bar pressure) as a sole oxidant.[5f] Pd@U-E15 (Pd@periodic
mesoporous organosilica) was used for the oxidation of benzylic
alcohols with O2 (1 atm) in water at 90 °C in the presence of
alkali (e.g., K2CO3).[5d] Although great successes have been
achieved, the utilities of atmospheric pressure of air are still
Dr. Wei Zhang, Dr. Ziqiang Xiao, Dr. Jiajun Wang, Dr. Wenqin Fu, Prof.
Rong Tan and Prof. Donghong Yin
National & Local Joint Engineering Laboratory for New Petro-chemical
Materials and Fine Utilization of Resources; Key Laboratory of Chemical
Biology and Traditional Chinese Medicine Research (Ministry of
Education) , Hunan Normal University, Changsha, Hunan, 410081, China
E-mail: yiyangtanrong@126.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201######.
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10.1002/cctc.201900015
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Results and Discussion
NPs was 0.233 nm, which was just between the values of
Au(111) (0.235 nm) and Pd(111) (0.225 nm) and consistent with
the values reported in the reference.[13] This could be a strong
evidence for the formation of Au–Pd alloy NPs. In addition, the
elemental mapping analysis (Figure 1d) showed that the
distribution of Au and Pd in the same region almost overlapped
with each other, indicating that there were no specific core–shell
or other heterostructures for the as-prepared Au–Pd alloyed NPs.
As a result, gold component could separate the contiguous Pd
sites in the bimetallic Au-Pd alloy NPs, providing sufficient Pd–
Preparation of Catalysts
Asymmetric selective oxidation of alcohols with air in water is
undoubtedly the most safe and “green” process to give the
valuable carbonyl compounds. Although Pd NPs are recognized
to be catalytically active for the aerobic oxidation, the utilities of
atmospheric pressure of air are still limited in most of these
systems where pure oxygen or a high pressure of air is generally
required.[5] Alloying with other metal provided an effective
strategy to boost the activity for the oxidations by O2.[6, 7] Given
that electron-rich Au NPs were beneficial for the adsorption and
activation of O2,[7, 12] we decided to incorporate Au into Pd NPs
for realizing selective oxidation of alcohols with air in water.
Higher electronegativity of Au (2.54) than that of Pd (2.20)
resulted in electronic heterogeneity at the alloy NP surface,[9]
which might lead to the improved catalytic activity of the alloy
structure. Hydrophilic γ-Al2O3 with high surface area, together
with abundant cation vacancies on surface, was a desirable
support for the bimetallic alloy NPs and rendered them water-
dispersive. The Au-Pdx@γ-Al2O3 catalysts with different Au/Pd
molar ratios were prepared by coimpregnation-reduction method
14]
Au interface sites favorable for this aerobic oxidation.[6a,
Alloy of Pd and Au could be achieved even for Pd-rich catalysts,
such as Au-Pd3.0@γ-Al2O3, as evidence from the elemental
mapping analysis (Figure S2-B). Furthermore, we noticed that
the particle size gradually decreased with increasing palladium
content in the Au-Pd alloy NPs (Figure S1). The observation
provided further evidence for the chemical combination of Au
and Pd to form bimetallic alloy rather than physical combination.
[9b]
using HAuCl4 and PdCl2 as the metal precursors. The
synthesis procedure involved the simultaneous deposition of
HAuCl4 and PdCl2 on γ-Al2O3, and followed by reduction with
NaBH4. Careful tuning of the initial molar ratio of Au and Pd
metal ions provided control of the chemical composition of the
Au-Pd alloy NPs. The metal loading was determined by ICP-
OES. As shown in Table 1, for all the catalysts with various
Au/Pd atomic ratios, the total metal loading was fixed at ca. 3.0
wt. %.
Table 1. Chemical composition of different catalysts
Au loading[a]
Pd loading[a]
Au/Pd
Entry
Catalysts
(wt. %)
(wt. %)
(mol ratio)
Figure 1. TEM analyses of Au-Pd1.2@γ-Al2O3. TEM image (a), size
distribution (b), HR-TEM image (c) and element mapping (d)
1
2
3
4
5
6
7
Au@γ-Al2O3
2.94
2.43
1.74
1.72
1.13
0.69
0
0
/
Au-Pd0.4@γ-Al2O3
Au-Pd1.2@γ-Al2O3
0.50
1.16
1.13
1.82
2.23
2.92
1.0: 0.4
1.0: 1.2
1.0: 1.2
1.0: 3.0
1.0: 6.0
/
XPS. X-ray photoelectron spectroscopy (XPS) gave further
evidence for the formation of Au-Pd alloy NPs on γ-Al2O3
surface, as well as the existence of interaction between the two
compositions. Figure 2 showed the XPS spectra of Au-Pdx@γ-
Al2O3, Pd@γ-Al2O3 and Au@γ-Al2O3. As expected, XPS survey
spectrum of Au-Pd1.2@γ-Al2O3 exhibited not only Al 2p (75.5 ev)
and O 1s (530.7 ev) singles, but also the Au 4f (84.7 ev) and Pd
3d (335.8 ev) peaks (Figure 2A).[15] It confirmed the coexistence
of Au-Pd alloy NPs on γ-Al2O3 surface. High-resolution spectra
of electron core-level binding energies (BEs) for Au 4f and Pd 3d
of Au-Pd alloy NPs revealed the presence of interactions
between the two metals. In comparison to the reported value for
BEs of bulk gold (84.00eV),[16] the Au 4f7/2 peak of Au@γ-Al2O3
was shifted to a higher BE by about 0.98 eV. This shift might be
ascribed to the interaction between Au NPs and the γ-Al2O3
support.[17] Gold was mainly in the Au+ state on γ-Al2O3 as a
result of the interaction between metal and the support. Indeed,
Au@γ-Al2O3 exhibited intense peaks at 84.98 and 88.02 eV in
XPS spectrum (Figure 2B), which were assigned to Au+ 4f7/2 and
[b]
Au-Pd1.2@γ-Al2O3
Au-Pd3.0@γ-Al2O3
Au-Pd6.0@γ-Al2O3
Pd@γ-Al2O3
[a] Determined by ICP-OES. [b] Au-Pd1.2@γ-Al2O3 after the 5th reuse.
Characterization of Samples
TEM. Transmission electron microscopy (TEM) images of typical
Au-Pd1.2@γ-Al2O3 was shown in Figure 1. Obviously, the
prepared bimetallic Au–Pd alloy NPs were highly dispersed on
the surface of γ-Al2O3 with average sizes of ca. 3.35 nm (Figure
1a and 1b). Figure 1c presented the HR-TEM image of the Au-
Pd1.2@γ-Al2O3. The measured lattice spacing of Au–Pd alloy
4f5/2.
[7e] Similar cationic Au+ species on Al2O3 has been reported
in ref.[18] The Au 4f BEs of Au-Pd alloys were slightly shifted to
negative values as compared with that of Au@γ-Al2O3 (Figure
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2B). According to the previous literature, the lower BEs of Au 4f
might be due to a gain of charge density in the d band,
concomitant with a loss in the sp band,[19] suggested the
formation of Au-Pd bond in the alloy. Given that the
electronegativity of Pd (2.20) is lower than that of Au (2.54),[9]
the electron donation from Pd to Au was responsible for the
negative shift of the Au 4f BEs.[20] Indeed, the Au 4f BE values
gradually decreased with increasing palladium content (Figure
2B), due to increased Au-Pd alloying interactions. For Au-
Obviously, a diffraction peak, characteristic of Au(111), was
observed at 2θ = 38.3 (ICDD PDF-2; Au: no. 00-004-0784) in the
PXRD pattern of Au@γ-Al2O3 (Figure 3A-a). This peak shifted to
a lower angle (2θ = 38.46) when Pd was incorporated with Au
(Figure 3A-b). This was believed to be associated with the
formation of Au-Pd alloy NPs.[21] Careful examination of this
peak showed that the peak position further shifted toward a
higher 2θ degree with increasing x value, indicating the Au-Pd
alloying interactions (Figure 3Ab-e).[21] Moreover, the broadening
Pd6.0@γ-Al2O3, the electron core-level BEs for Au 4f7/2 (83.78 eV) of this peak became more obvious with increasing x value
and 4f5/2 (87.48 eV) was approximate to those for Au0 species
(83.5 eV for Au 4f7/2 and 87.1 eV for Au 4f5/2
[7e]. Charge transfer
(Figure 3Ab-e). According to the Debye–Scherrer equation, D =
kλ/β cos θ, which gives the relationship between peak
broadening in XRD and the particle size, we deduced that the
particle sizes became smaller with increasing the Pd content in
the bimetallic samples. The inference was in agreement with the
TEM image of Au-Pdx@γ-Al2O3 shown in Figure S1. While, for
pure Pd@γ-Al2O3, no distinct reflection associated with Pd(111)
(2θ = 40.1, ICDD PDF-2; Pd: no. 00-005-0681) was observed in
the XRD pattern (Figure 3A-f). It was probably due to that the Pd
NPs were too small to be detected by XRD. Figure 3B showed a
plot of the lattice parameters of Au-Pd alloy NPs estimated from
the XRD diffraction pattern of (111) phases as a function of the
amount of Pd in Pd-Au NPs as determined by ICP. The dashed
line in Figure 3B showed the theoretical relationship between the
lattice constant and the Pd/Au ratio given by Vegard’s law.[22]
The lattice parameter decreased as Pd content in the alloy NPs
increased. And the plot closely followed the dashed line. These
results indicated that the two different metals in NPs were mixed
to form random alloys,[21b] as confirmed by elemental mapping
analysis (Figure 1d). The characteristic reflection of γ-Al2O3
remained in the XRD pattern of γ-Al2O3 supported catalyst. It
suggested the structural integrity of γ-Al2O3 when metal NPs
were loaded on the surface.
)
effects from Pd to Au was also evident from the fact that Pd 3d
BEs for Au-Pdx@γ-Al2O3 was shifted to higher energy compared
with that of Pd@γ-Al2O3 catalysts. And the Pd 3d BEs values
gradually increased with increasing palladium content (Figure
2C). The XPS results demonstrated that there was charge
heterogeneity in the alloy NP, with both electron-rich sites and
slightly positively charged sites present. The electronic
heterogeneity allowed both electrophilic and nucleophilic
reactant molecules to be boosted, which may enhance the
intrinsic catalytic reactivity.
Au@Al2O3
O 1s
(A)
C 1s
A4 4f
Al 2p
O 1s
Au-Pd1.2@Al2O3
C 1s
Pd 3d
A4 4f
Al 2p
Al 2p
O 1s
Pd@Al2O3
C 1s
Pd 3d
400
0
100
200
300
500
600
100
80
60
40
20
0
8
7
6
5
4
3
2
1
0
Binding Energy (eV)
a
B
A
a
b
Au+
Au 4f5/2 Au 4f7/2
Au+
Pd@Al O
340.73
(B)
84.98
Au@Al O
(C)
2
3
88.02
2
3
c
d
335.48
b
e
Au -Pd @Al O
1.0 0.4
87.63
Au -Pd @Al O
1.0 0.4
335.51
335.54
2
3
84.06
83.86
83.78
2
3
c
340.88
d
Au -Pd @Al O
1.0 1.2
Au -Pd @Al O3
1.0 1.2
87.52
87.48
2
3
2
f
e
340.98
f
0
20 40 60 80 100 120
Reaction time (min)
0
20 40 60 80 100 120
Reaction time (min)
Au -Pd @Al O
1.0 3.0
Au -Pd @Al O
1.0 3.0
2
3
336.42
2
3
341.15
Figure 3. PXRD patterns (A) of Au@γ-Al2O3 (a), Au-Pd0.4@γ-Al2O3 (b), Au-
Pd1.2@γ-Al2O3 (c), Au-Pd3.0@γ-Al2O3 (d), Au-Pd6.0@γ-Al2O3 (e), Pd@γ-Al2O3
(f), and γ-Al2O3 (g); dependence of the lattice parameters of Pd-Au alloys on
the relative amount of Pd (B).
96 94 92 90 88 86 84 82 80
Bingding energy (eV)
346 344 342 340 338 336 334 332 330
Bingding energy (eV)
Figure 2. XPS survey spectra of Au@γ-Al2O3, Au-Pdx@γ-Al2O3 and Pd@γ-
Al2O3 (A); Au 4f XPS spectra (B) and Pd 3d XPS spectra (C) of Au@γ-Al2O3,
Au-Pd0.4@γ-Al2O3, Au-Pd1.2@γ-Al2O3, Au-Pd3.0@γ-Al2O3 and Pd@γ-Al2O3.
Catalytic Performances
Since benzaldehyde is a very important carbonyl compound for
widespread applications, the catalytic behavior of bimetallic
Au/Pd catalysts was evaluated in aerobic oxidation of benzyl
alcohol in water using air as an oxidant at atmospheric pressure.
The results were summarized in Table 2. To demonstrate the
“alloying effect” on catalytic performance of Au-Pdx@γ-Al2O3,
PXRD. Powder X-ray diffraction (PXRD) was used to further
study the interaction between the two compositions in Au-Pd
alloy catalysts. The wide angle PXRD patterns of Au-Pdx@γ-
Al2O3, Pd@γ-Al2O3 and Au@γ-Al2O3 wre shown in Figure 3A.
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the monometallic counterparts of Au@γ-Al2O3 and Pd@γ-Al2O3
were prepared as the control catalysts for comparison.
playing key roles in the catalytic activity.[9b] Logically, altering the
palladium to gold ratio may control the isolated Pd concentration
at the alloy APs surface which had a substantial effect on activity
and stability of the alloy NPs. For the Au-rich samples, such as
Au-Pd0.4@γ-Al2O3 and Au-Pd1.2@γ-Al2O3, Pd atoms were
believed to be efficiently isolated by Au atoms, resulting in
desired Pd–Au interface sites for this aerobic oxidation (Table 2,
entries 3 and 4). Decreased activity for the Au-Pd0.4@γ-Al2O3
should be due to the less active Pd sites (Table 2, entry 3), and
Au being intrinsically poorly active in the aerobic oxidation (Table
2, entry 1). In case of Au-Pd1.2@γ-Al2O3, the isolated Pd sites
were sufficient to afford quantitative yield of benzaldehyde in
water (Table 2, entry 4). Furthermore, the higher Pd/Au ratio
made Au species on Au-Pd1.2@γ-Al2O3 surface more electron-
rich than Au-Pd0.4@γ-Al2O3 due to the charge transfer from Pd
to Au. The electronic heterogeneity benefited the adsorption and
activation of O2, and thus further accelerating the catalytic
oxidation (Table 2, entries 4 vs. 3).[9] While, for the Pd-rich
samples, such as Au-Pd3.0@γ-Al2O3 and Au-Pd6.0@γ-Al2O3, the
number of Au particles is not enough to fully isolate the total
contiguous Pd sites, and Pd segregation is expected (Table 2,
entries 5 and 6). Even for alloyed particles in these catalysts, the
surface should be Pd-rich or even contained Pd patchs.[13] Their
activity thus dropped correspondingly. Indeed, Au-Pd6.0@γ-
Al2O3, in which Pd/Au molar ratio was reached to 6/1, showed
activity similar to the pure Pd catalyst (Table 2, entry 6 vs. 2).
Therefore, the formation of isolated single Pd atoms in random
Au-Pd alloys with a low Pd/Au ratio, together with electronic
heterogeneity at Pd-Au interface sites, enabled efficient,
selective aerobic oxidation of alcohols using air at atmospheric
pressure in water. Notably, the admixture didn’t affect the
selectivity of the bimetallic catalysts.
As expected, monometallic Au@γ-Al2O3 or Pd@γ-Al2O3 was
sluggish in the aerobic oxidation of benzyl alcohol in water,
giving moderate conversion of benzyl alcohol, although the
selectivity to benzaldehyde was satisfied (Table 2, entries 1 and
2). Introducing Au to Pd, and vice versa, led to a significant
enhancement in activity for the aerobic oxidation (Table 2,
entries 3-6 vs.1 and 2). Especially, only 2.2 mol% of Au-
Pd1.2@γ-Al2O3 was sufficient to ensure quantitative oxidization
of benzyl alcohol to benzaldehyde (yield >99%) in water within 2
h using air as the oxidant at atmospheric pressure (Table 2,
entry 4). The promotion of catalytic properties suggested the
positive “alloying effect” on the catalytic oxidation. Actually,
alloying with Au endowed the Au-Pdx@γ-Al2O3 with salient
features as follows: 1) Pd–Au interface sites: the Pd atoms were
recognized to be surrounded by Au atoms in the random alloys,
forming Pd–Au interface sites through isolating Pd sites on the
alloy surface, which is a critical portion of the surface regarding
promising catalytic aerobic oxidation.[6a,
2) electronic
21]
heterogeneity: different electronegativity of Au (2.54) and Pd
(2.20) resulted in charge heterogeneity on the alloy NP surface,
which might effectively enhance interactions between the alloy
NPs and reactant molecules; consequently, leading to a high
oxidation catalytic efficiency.[9] Furthermore, electron-rich Au
NPs were also believed to be beneficial for the adsorption and
activation of O2, which was effective in boosting the activity of
aerobic oxidation.[7, 12]
Table 2. Aerobic oxidation of benzyl alcohol using air in water over various
catalysts.[a]
Conv. [b]
(%)
Sel. [b]
(%)
TOF [c]
(h-1)
Entry
Catalysts
Kinetics was used to further investigate the “alloying effect” on
catalytic performance. Time course for the selective aerobic
oxidation of benzyl alcohol over the Au-Pdx@γ-Al2O3, Au@γ-
Al2O3 and Pd@γ-Al2O3 was monitored periodically in water. The
corresponding kinetic curves and rate curves are shown in
Figure 4. It is observed from the profile that conversion of benzyl
alcohol increased linearly up to 40 min over all the tested
catalysts, after which a significant increase is not observed. The
corresponding initial rate constants (kobs) reflected the catalytic
activity of tested catalysts. Clearly, benefitting from the “alloying
effect”, Au-Pdx@γ-Al2O3 were more efficient than monometallic
counterparts of Au@γ-Al2O3 and Pd@γ-Al2O3, as evidenced by
the higher conversion of benzyl alcohol and enhanced initial rate
constants (kobs) (Figure 4a-d vs. 4e and 4f). Despite in similar
alloy NP systems, the catalytic rates varied with the Pd/Au molar
ratio in Au-Pd alloys. Au-Pd1.2@γ-Al2O3 afforded the highest
rate among the Au-Pdx@γ-Al2O3, viewing from the kinetics point
(Figure 4a vs. 4b-d). The results further demonstrated the
chemical combination of Au and Pd to form bimetallic alloy
rather than physical combination. Different Pd/Au molar ratio
determined the formation of isolated single Pd atoms in random
Au-Pd alloys, as well as the electronic heterogeneity at the
formed Pd-Au interface sites, which were the crucial factors for
the catalytic activity of Au-Pdx@γ-Al2O3 in selective aerobic
oxidation of alcohols using air at atmospheric pressure in water.
1
2
3
4
5
6
Au@γ-Al2O3
20
59
>99
>99
>99
>99
>99
>99
4.51
13.3
15.8
22.3
19.1
15.1
Pd@γ-Al2O3
Au-Pd0.4@γ-Al2O3
Au-Pd1.2@γ-Al2O3
Au-Pd3.0@γ-Al2O3
Au-Pd6.0@γ-Al2O3
70
>99
85
67
[a] Catalyst (2.2 mol% substrate, based on total metal loading), benzyl alcohol
[b]
(0.2 mmol), water (5.0 mL), air, atmospheric pressure, 80 °C,
2 h.
[c]
Determined by GC.
Turnover frequency (TOF) is calculated by the
expression of (product)/[(catalyst)×time].
We also found that the catalytic activity of Au-Pdx@γ-Al2O3
strongly depended on the Pd/Au molar ratio in alloy NPs. Au-
Pd1.2@γ-Al2O3 exhibited highest catalytic efficiency, giving
almost quantitative yield (>99%) of benzaldehyde within 2 h
(Table 2, entry 4). Further increasing or reducing the molar ratio
of palladium to gold in catalyst led to decreased activity in the
oxidation accordingly (Table 2, entries 3, 5, and 6). A possible
explanation for the different activity might be that the Pd/Au
molar ratio determined both the distribution of Pd sites at NP
surface and the electronic heterogeneity, with these two factors
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10.1002/cctc.201900015
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were smoothly oxidized to the corresponding aldehydes or
ketones in quantitative yields in water (Table 3, entries 1 and 2).
2-Hydroxybenzyl alcohol exhibited similar reactivity to the 4-
hydroxybenzyl alcohol in the aqueous aerobic oxidation, and the
yield of the corresponding aldehydes reached >99% in 2 h
(Table 3, entry 3 vs. 4). It indicated that steric hindrance had
negligible effect on the catalytic reaction. Polysubstituted benzyl
alcohols, such as vanillyl alcohol , could also quantitatively
converted into valuable isovanillin in water within 2 h (Table 3,
entry 5). As the secondary benzylic alcohols, benzhydrol and 1-
phenylethanol were also highly reactive in the Au-Pd1.2@γ-
Al2O3-catalyzed aerobic oxidation, although complete oxidation
of 1-phenylethanol was required for 4 h (Table 3, entry 6 vs. 7).
100
8
7
6
5
4
3
2
1
0
a
B
A
a
b
80
60
40
20
0
c
d
b
e
c
d
f
e
It is well-known that aliphatic alcohols are more reluctant to
f
23]
undergo oxidation.[18,
While, we noticed that Au-Pd1.2@γ-
Al2O3 also displayed a high activity for the oxidation of aliphatic
alcohols. For example, sec-butyl alcohol could be almost
completely oxidized to butanone within 4 h over the Au-Pd1.2@γ-
Al2O3 (Table 3, entry 8). For the oxidation of cyclohexanol, the
yield of cyclohexanone reached to 80% in 10 h (Table 3, entry 9).
Notably, for all the substrates, the selectivity of the
corresponding aldehydes or ketones was basically 100%. The
results indicated high universality of Au-Pd1.2@γ-Al2O3 in
aerobic oxidation of alcohols in water using air as an oxidant.
Upon consideration of our experimental results as well as the
literature reports,[7a, 24] we proposed possible reaction pathways
for the catalytic oxidation of alcohols over Au-Pd1.2@γ-Al2O3,
which were illustrated in Scheme 1. It illustrated that the
oxidation of benzyl alcohol proceeded through cooperation
between the bimetallic Au–Pd alloy NPs. First, the alcohol was
dissociatively adsorbed on the Pd surface, affording the alkoxide
and hydride (oxidative addition). And the oxygen molecule was
adsorbed onto the surface of the Au site, probably in a peroxo-
0
20 40 60 80 100 120
0
20 40 60 80 100 120
Reaction time (min)
Reaction time (min)
Figure 4. Fitted kinetic curves (A) and rate curves (B) of selective aerobic
oxidation of benzyl alcohol over the Au-Pd1.2@γ-Al2O3 (a), Au-Pd3.0@γ-Al2O3
(b), Au-Pd0.4@γ-Al2O3 (c), Au-Pd6.0@γ-Al2O3 (d), Pd@γ-Al2O3 (e) and Au@γ-
Al2O3 (f) using air at atmospheric pressure in water.
Encouraged by the superior activity of Au-Pd1.2@γ-Al2O3, we
further explored the aerobic oxidation of other alcohols under
this green condition. The results were summarized in Table 3.
Table 3. Aerobic oxidation of various alcohol using air in water over Au-
Pd1.2@γ-Al2O3.[a]
Time
(h)
Conv.[b]
(%)
Sel.[b]
(%)
TOF[c]
(h-1)
Entry
1
Substrates
Products
OH
O
O
H3CO
2
>99
>99
22.3
H3CO
-
like form (O2 ) that played a key role in oxidation of the
alcohol.[24c] Then, H atom on α-carbon of the adsorbed alkoxide
was transferred to the Pd surface to form the aldehyde and a
Pd-hydride species. Finally, superoxo-like molecular oxygen
species on Au site removed the hydride species from the Pd
surface, producing H2O2 which fast decomposed into H2O and
O2. Initial gold and palladium active species was thus recovered
for reuse. The reaction mixture at ca. 50% conversion was
subjected to the qualitative test for H2O2 production. The color of
KI-containing starch changed slightly to blue, thus suggesting
that H2O2 was formed along with the production of aldehyde.
H2O + 1/2 O2
OH
2
3
4
2
2
2
>99
>99
>99
>99
>99
>99
22.3
22.3
22.3
H3C
H3C
OH
OH
OH
O
OH
OH
O
HO
HO
HO
HO
O
5
6
2
>99
>99
22.3
OCH3
OH
OCH3
2
4
>99
98
>99
>99
22.3
22.1
O
OH
O
OH
7
8
H2O2
O
R
1R2
H
R1
R2
OH
O
4
95
80
>99
>99
21.4
18.0
H
H
OH
O
-O
R2
R1
R2
R1
9
10
O
O
O
[a]
Au-Pd1.2@γ-Al2O3 (2.2 mol% substrate, based on total metal loading),
H
H
[b]
alcohol (0.2 mmol), water (5.0 mL), air, atmospheric pressure, 80 °C, 2 h.
Same as in Table 2. [c] Same as in Table 2.
To our delight, the Au-Pd1.2@γ-Al2O3 was also highly active
and extremely selective for aerobic oxidation of other alcohols
either aromatic or aliphatic in water. Primary benzylic alcohols,
such as4-methylbenzyl alcohol and 4-methyoxylbenzyl alcohol,
O2
Scheme 1. Plausible pathways for the selective aerobic oxidation of alcohols
over Au-Pd1.2@γ-Al2O3.
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Superiority of Au-Pd1.2@γ-Al2O3 over other reported catalysts
either monometallic or alloyed was shown by TOF values in
Table 4. Obviously, these reported monometallic catalysts, such
as Pd@U-E15 (Pd@periodic mesoporous organosilica),[5e]
Pd@MB-1500 (Pd@mesoporous carbon bead),[22] Pd@E10A20
(Pd@amphiphilic hybrid carbon nanotubes),[4] Pd@Cu(II)-
MOF,[25] Pd@MNP (Pd@magnetic nanoparticles),[26] Pd@pol
(Pd@polymer),[27] and AuPVA/TiO2 (polyvinyl alcohol-stabilized Au
@TiO2),[28] were far less efficient than Au-Pd1.2@γ-Al2O3 in
aerobic oxidation of benzyl alcohol, even though the oxidation
was performed in organic solvent using pure oxygen or a high
pressure of air as oxidant and external alkalis as cocatalyst
(Table 4, entries 1-7 vs. 10). When air (1 atm) was used as the
oxidant under based-free condition, only 0.75 h-1 of TOF value
was obtained over Pd@Cu(II)-MOF although the oxidation was
performed in xylene (Table 4, entry 4).[25] Higher catalytic
efficiency of the Au-Pd1.2@γ-Al2O3 should arise from the
correlation between nanostructure of the bimetallic catalysts and
synergistic effects. Furthermore, the TOF value of Au-Pd1.2@γ-
Al2O3 (22.3 h-1) with air (1 atm) was also higher than that of
various reported gold-palladium alloyed catalysts with pure O2 in
the aerobic oxidation of benzyl alcohol (Table 4, entries 8, 9 vs.
10).9b,12 Especially, the Au−Pd@PANI (Au−Pd@polymer support
polyaniline) gave only 16.0 h-1 of TOF even in the presence of
external NaOH in toluene (Table 4, entry 8).12 The results
demonstrated the advantages of Au-Pd1.2@γ-Al2O3 used for
aerobic oxidation of alcohols under mild reaction conditions.
unchanged at >99% (Figure 5A and 5C). The results suggested
the enhanced stability of Au-Pdx@γ-Al2O3 in the aerobic
oxidation. Aggregation of Pd NPs, a main reason for the
deactivation of metallic Pd catalysts,[5d, 30] was avoided by the
alloying with Au through “alloying effect”, as confirmed by TEM
images (Figure S3). The catalysts retained their nanoparticle
morphology without appreciable aggregation even after five
runs. Furthermore, addition of Au to Pd was reported to
suppress the undesirable carbon deposition on Pd NPs
surface,[13, 31] which also enhanced the stability of metallic Pd
catalysts. Leaching tests were performed by determining the
gold and palladium contents directly in the recovered Au-
Pd1.2@γ-Al2O3 by ICP-OES. The catalyst after five runs had gold
and palladium contents almost identical to that of the fresh
sample (Table 1, entry 4 vs. 3). After the workup, the aqueous
filtrate exhibited no catalytic activity for the oxidation. These
observations indicated that the γ-Al2O3 was an ideal support to
upload Au-Pd NPs, effectively preventing their leaching into the
aqueous phase under the reaction condition.
100
90
80
70
60
100
90
80
70
60
100
90
80
70
60
b
a
b
b
C
A
B
%
%
%
50
40
30
20
10
0
50
40
30
20
10
0
50
40
30
20
10
0
a
Table 4. Aerobic oxidation of benzyl alcohol in different Pd-based catalytic
systems.
Solvent and Temperature TOF
cocatalyst
a
Entry
Catalyst
Oxidant
Ref.
(oC)
(h-1)
1
2
3
Pd@U-E15
O2 (1 atm) water/K2CO3
90
80
80
10.8 5e
9.52 22
1
2
3
4
5
1
2
3
4
5
1 2 3 4 5
Run times
Run times
Run times
Pd@MB-1500 O2 (1 atm)
water
toluene/water
xylene
air (4.93
Pd@E10A20
atm)
Figure 5. The reuse of Au@γ-Al2O3 (A), Au-Pd1.2@γ-Al2O3 (B), and Pd@γ-
Al2O3 (C) in selective aerobic oxidation of benzyl alcohol with air (1 atm) in
water (a: conversion of benzyl alcohol; b: selectivity to benzaldehyde).
15.6
4
4
5
Pd@Cu(II)-MOF air (1 atm)
130
85−90
100
0.75 25
10.7 26
5.5 27
8.3 28
10.2 9b
16.0 12
Pd@MNP
Pd@pol
air (1 atm) toluene/K2CO3
air (1 atm) water
O2 (6 atm) Water/NaOH
6
Conclusions
7
AuPVA/TiO2
100
8
Au−Pd@ZrO2 O2 (1 atm) trifluorotoluene
45
In summary, we have successfully carried out the efficient
aerobic oxidation of alcohols in water using atmospheric
pressure of air as a sole oxidant with the employment of Au-
Pdx@γ-Al2O3. Synergistic effect of alloying gold with palladium
significantly improved the activity, selectivity, and stability of the
gold–palladium alloy catalysts. A variety of alcohols either
aromatic or aliphatic were almost quantitatively transformed into
corresponding aldehydes or ketones over Au-Pd1.2@γ-Al2O3 in
water without the need for any cocatalyst. The TOF values were
higher that other reported monometallic or alloyed catalysts,
even though the reported oxidation was performed in organic
solvent using pure oxygen or a high pressure of air as oxidant
and external alkalis as cocatalyst. Moreover, the catalysts were
highly stabilized against metal agglomeration and leaching,
maintaining the high activities during several recycles. The
salient advantages made Au-Pdx@γ-Al2O3 promising for other
aerobic oxidations with atmospheric air in water, and also
inspired us to develop a wide variety of bimetallic alloy NPs for
other important sustainable catalysis.
9
Au−Pd@PANI O2 (1 atm) toluene/NaOH
100
Au-Pd1.2@γ-
this
22.3
10
air (1 atm)
water
80
Al2O3
work
Apart from improving catalytic efficiency, alloying with gold also
enhanced the stability of Au-Pdx@γ-Al2O3 catalysts. It is very
important for industrial applications, since a major bottleneck for
large-scale applications of metallic Pd catalyzed alcohol
oxidation is the rapid catalyst deactivation.[13, 29] Reusability of
typical Au-Pd1.2@γ-Al2O3, as well as monometallic Au@γ-Al2O3
or Pd@γ-Al2O3 for comparison, was investigated in selective
aerobic oxidation of benzyl alcohol with air (1 atm) in water. The
results were presented in Figure 5. Indeed, Au-Pd1.2@γ-Al2O3
exhibited high reusability during the aerobic oxidation (Figure
5B), while, Au@γ-Al2O3 and Pd@γ-Al2O3 gave significant
decrease in conversion of benzyl alcohol during the reuse for
five times, although the selectivity to benzaldehyde was kept
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10.1002/cctc.201900015
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Experimental Section
obtained carbonyl compounds have been identified by 1H NMR spectra,
which were available in Section S2 of ESI.
Materials and Reagents
Chloroauric acid (HAuCl4), palladium nitrate (PdCl2), L-lysine, sodium
borohydride, and γ-Al2O3 (20 nm of particle size) were purchased from
Sigma-Aldrich. Benzyl alcohol, α-phenylethanol, vanillyl alcohol, salicyl
Acknowledgements
alcohol,
α-phenylethanol,
cyclohexanol,
diphenylmethanol,
4-
This work was supported by National Natural Science
Foundation of China (21676078, 21476069), the Natural
Science Foundation of Hunan Province for Distinguished Young
Scholar (2016JJ1013), and the Key Laboratory of the Assembly
and Application of Organic Functional Molecules of Hunan
Province.
methoxybenzylalcohol,4-methylbenzyl alcohol and 2-butanol were pursed
from Aladdin. Other commercially available chemicals were laboratory
grade reagents from local suppliers. All of the solvents were purified by
standard procedures.
Method
A JEOL model JEM 2010 EX microscope at an accelerating voltage of
200 kV was used to obtain the transmission electron microscopy (TEM)
images, high-resolution TEM images and element mapping. Before TEM
observations, sample was ultrasonically dispersed in ethanol, and a drop
of the dispersion was placed on a microgrid carbon polymer supported
on a copper grid and allowed to dry at room temperature. X-ray
Photoelectron Spectroscopy (XPS) data were obtained with a K-Alpha+
electron spectrometer from Thermo fisher Scientific using 300 W mono Al
Kα radiation. The Au and Pd content of the prepared catalysts were
determined by inductively coupled plasma spectrometer (ICP-OES) on
an IRIS Intrepid II XSP instrument (Thermo Electron Corp.). X-ray
powder diffraction (XRD) analysis was carried out on a PANalytical X’pert
diffractometer using nickel-filtered Cu Kα radiation with a scanning angle
(2θ) of 10°-90°, operated at 40 kV and 40 mA. Thin layer
chromatography TLC) was conducted on glass plates coated with silica
gel GF254. The conversion and ee values were measured by a 6890N
gas hromatograph (Agilent Co.) equipped with a capillary column
HP19091G-B213, 30 m × 0.32 mm × 0.25 m).
Keywords: alloys • aerobic oxidation • cooperative effects •
water
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10.1002/cctc.201900015
ChemCatChem
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Synergistic effect in Au-Pd alloy
catalysts enhanced selective aerobic
oxidation of alcohols using air at
atmospheric pressure in water.
Wei Zhang, Ziqiang Xiao, Jiajun Wang,
Wenqin Fu, Rong Tan,* and Donghong
Yin
Page No. – Page No.
Selective
aerobic
oxidation
of
alcohols over gold-palladium alloy
catalysts using air at atmospheric
pressure in water.
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