An excellent Au/meso-γ-Al2O3 catalyst for the aerobic selective oxidation of alcohols

Article abstract of DOI:10.1039/c1gc15827a

Alumina is traditionally considered to be inappropriate as a support for high-performance gold catalysts. Here, a highly active Au/meso-AlO catalyst for liquid-phase alcohol oxidations was synthesized using a home-made mesoporous γ-Al₂O₃ (meso-AlO) via a cation-anion double hydrolysis (CADH) method. The Royal Society of Chemistry.

Full text of DOI:10.1039/c1gc15827a

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Green Chemistry
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COMMUNICATION
An excellent Au/meso-c-Al2O3 catalyst for the aerobic selective oxidation
of alcohols†
Guofeng Zhao, Min Ling, Huanyun Hu, Miaomiao Deng, Qingsong Xue and Yong Lu*
Received 11th July 2011, Accepted 16th August 2011
DOI: 10.1039/c1gc15827a
Alumina is traditionally considered to be inappropriate
as a support for high-performance gold catalysts. Here, a
highly active Au/meso-AlO catalyst for liquid-phase alcohol
oxidations was synthesized using a home-made mesoporous
c-Al2O3 (meso-AlO) via a cation–anion double hydrolysis
(CADH) method.
energy levels of Au NPs or indirectly modify their particle size.5
For example, Comotti et al. prepared a series of model gold
catalysts on different supports, and it was clearly demonstrated
that nature of the support strongly influenced the catalyst
activity.6 Furthermore, very active catalysts can be prepared by
using microcrystalline supports rather than non-nanocrystalline
ones.7 Generally, active oxides (e.g., TiO2, CeO2 and MnOx) are
found to endow Au NPs with a better performance than inert
ones (e.g., Al2O3 and SiO2).6–8
For the alcohol selective oxidation, several alumina-based,
mixed oxide-supported gold catalysts have been reported. For
example, a series of gold catalysts supported on different binary
Cu, Al and Mg mixed oxide supports have been developed.9
Besides, the Ga–Al mixed oxide can be used as a surprising sup-
port for the fabrication of highly active gold catalysts.4 However,
these described supports have preparation difficulties and cost
inhibitions. From the practical point of view, Al2O3 is the most
frequently used support because of its favorable bulk density,
price, porous structure, stable properties and good preparation
reproducibility. Unfortunately, Al2O3 is traditionally considered
to be an “inert” support, giving Au NPs of poor catalytic
activity.10 Alumina-supported gold catalysts for the oxidation of
CO11a and glucose,11b and the epoxidation of styrene11c have been
developed, but none have been reported for alcohol selective
oxidation until now.
Herein, using a home-made mesoporous g-Al2O3 (denoted
as meso-AlO) derived from a cation–anion double hydrolysis
(CADH) method,12 we have successfully synthesized high-
performance Au/meso-Al2O3 (denoted as Au/meso-AlO) cata-
lysts with an Au loading of 1 wt% via a deposition–precipitation–
urea (DPU) method.3,4 Such catalysts exhibit excellent activity
for the selective oxidation of alcohols.
The home-made support prepared by the CADH method was
a characteristic mesoporous structure with a BET surface area of
260 m2 g-1 and a pore volume of 0.53 cm3 g-1 (Fig. S1, ESI†). The
type H-2 hysteresis loops suggest that “ink-bottle-like” pores
may be present in the sample, and the average pore size is 7.1 nm
(Fig. S1, ESI†). The X-ray diffraction (XRD) pattern of the
as-prepared sample shows an g-Al2O3 phase (Fig. S2, ESI†).12a
When Au NPs were deposited onto the meso-AlO, the pattern
of the Au/meso-AlO catalyst was similar to that of the support
alone, indicating the high dispersion of the Au NPs and the
Gold had for many years been considered to be catalytically
inactive and was frozen out of mainstream catalysis interests.
However, this ice-like concept was broken down by several
unexpected discoveries of highly dispersed gold nanoparticles
(Au NPs) on oxide supports exhibiting an astonishing activity.1
Since then, this hunting ground of gold catalysis has been pio-
neered in many leading-edge catalysis studies, and Au NPs have
demonstrated an extraordinarily high catalytic performance,
making them not just an outstanding catalyst but also the best
catalyst for some reactions.2
The outstanding catalytic performance of Au NPs depends
on their size and shape, the degree of coordinative unsaturation
of the gold atoms, the nature of the support and the gold–
support interactions.2a–c Predictably, dispersing Au NPs of
optimum size on a suitable support may fabricate effective
gold catalysts for titled reactions, and such catalysts could be
determined and achieved from experiment-operation aspects
through elaborately selecting the most suitable support material
and preparation method. Therefore, the preparation method and
the thermal treatment conditions for gold catalysts have been
studied systematically.3 Nowadays, the deposition–precipitation
method with urea (DPU) is widely used to obtain excellent
gold catalysts.2e,3,4 Additionally, a reductive atmosphere, low
calcination temperature and slow heating rate preferably lead
to small gold particles.3c Despite the above advances, there
continues to be significant interest in the selection of the support,
because the support can directly change the relative electronic
Shanghai Key Laboratory of Green Chemistry and Chemical Processes,
Department of Chemistry, East China Normal University, 3663 North
Zhongshan Road, Shanghai, China, 200062.
E-mail: ylu@chem.ecnu.edu.cn; Fax: +86 21-62233424; Tel: +86
21-62233424
† Electronic supplementary information (ESI) available: Figs. S1–S8 and
Table S1. See DOI: 10.1039/c1gc15827a
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Table 1 The liquid-phase oxidation of alcohols catalyzed by Au/meso-AlO catalystsa
Entry
Catalyst
Substrate
Product
Reaction time/h
Conversion (%)
Selectivity (%)
1
2
3
4
5
6
7
8
CAT-1
CAT-2b
CAT-3c
CAT-4d
CAT-1e
CAT-1
CAT-1e
CAT-5f
CAT-6g
CAT-1h
CAT-7i
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzyl alcohol
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
Benzaldehyde
2
2
2
2
99
62
45
67
99
77
79
24
30
42
25
>99
96
95
98
2
>99
>99
>99
95
96
>99
93
0.5
0.5
2
2
0.5
2
9
10
11
a Catalyst preparation conditions: light-free DPU, DPU time 4.5 h, DPU temperature 90 ◦C, urea/Au molar ratio of 100 and calcination in air at
300 ◦C for 4 h if not specified otherwise. Catalyst-test conditions: catalyst 0.1 g, alcohol 1 mmol, reaction temperature 80 ◦C, O2 bubbling rate 20 ml
min-1. b CAT-2 was prepared in natural light with the other conditions identical. c The same sample as in Fig. 1B with a DPU temperature of 70 ◦C.
d The sample (same as in Fig. 1B with a DPU temperature of 70 ◦C) was calcined at 300 ◦C in H2 for 4.5 h. e CAT-1 was calcined in H2 (50 ml min-1)
for 4 h. f CAT-5 was prepared using a-Al2O3 as a support under the same conditions as in note a; gold loading of 1 wt%. g CAT-6 was prepared under
the same conditions as in note f; gold loading of 0.05 wt%. Dosage of CAT-6 was 2 g in the catalyst test. h Alcohol 2.25 mmol, reaction temperature
90 ◦C, 1 wt% Au/meso-AlO of 0.1 g, toluene as a solvent 10 ml. i CAT-7 was prepared in natural light with a DPU time of 3 h.
maintenance of the g-Al2O3 phase. Additionally, the surface area
of the catalyst slightly decreased from 260 to 220 m2 g-1, with
other textural properties (not shown) almost unchanged after
the loading of Au.
To measure the catalytic activity of the Au/meso-AlO cat-
alysts, our studies began with the aerobic oxidation of benzyl
alcohol with toluene as a solvent. Enlightened by the fact that
gold catalysts are not easy to handle and small variations in
the catalyst preparation conditions can make a great difference
between a good and a poor catalyst,3 great caution and detail was
focused on its preparation. Firstly, it is reported that Aud+ can
be photoreduced by natural light, and hence it is recommended
that preparations are performed in the absence of light.3a,3c,13
Indeed, the benzyl alcohol conversion decreased from 99%
over CAT-1 (Table 1, entry 1) to 62% over CAT-2 (Table 1,
entry 2). Therefore, all preparations were performed in the
absence of natural light. Subsequently, the detailed conditions
of catalyst preparation and post-treatment were investigated
Fig. 1 The influences of DPU time (A), temperature (B), urea/Au
molar ratio (C) and calcination temperature in air (D) on the catalyst
performance for the liquid-phase selective oxidation of benzyl alcohol.
The default conditions were light-free, DPU time 4.5 h, DPU tempera-
systematically. Fig. 1 clearly shows that these conditions had
a strong influence on catalyst performance. Among all the
preparation parameters, DPU time was found to be most crucial
ture 90 ◦C, urea/Au molar ratio of 100 and a calcination temperature
of 300 ◦C in air, unless investigated in detail otherwise. The gold
loading of all the catalysts was 1 wt% and all the preparations were
performed in the absence of natural light, which is known to decompose
the gold precursors. Catalyst test conditions: alcohol 1 mmol, reaction
temperature 80 ◦C, O2 bubbling rate 20 ml min-1.
in determining catalyst activity. Most reported DPU times are
shorter than 4 h.3b,4,9b Here, however, increasing the DPU time
from 3 to 4.5 h promoted the benzyl alcohol conversion from
36 to 99%, and the conversion maintained this level upon
further increasing the DPU time (Fig. 1A). Additionally, using
DPU temperatures of ≥ 90 ◦C, higher than the reported ones
◦
(£ 80 C),3b,4,9b and a moderate urea/Au molar ratio were
catalyst activity than heating in air.3c CAT-3 (DPU at 70 ◦C)
was treated firstly in air, and the conversion was just 45%. The
same sample was treated in H2 and the conversion could be
increased to 67% over CAT-4, much lower than CAT-1 (DPU
at 90 ◦C) treated in air (Table 1, entries 1, 3 and 4, and Fig.
S3A, ESI†). Interestingly, such a great difference in catalyst
activity and selectivity was not observable over the CAT-1
sample after treating in both air and H2 (Table 1, entries 1 and
5–7, and Fig. S3B, ESI†). It could thus be inferred that the DPU
preparation intrinsically determines catalyst performance, and
the consequent thermal treatment plays only an assisting role.
clearly favorable for improving catalyst activity (Fig. 1B–C).
Furthermore, the calcination temperature was investigated. The
results show that the catalytic activity of the catalyst could be
tuned by adjusting the calcination temperature. The optimum
calcination temperature was identified to be 300 ◦C, consistent
with previous reports.4 Therefore, it is clearly demonstrated
that Au/meso-AlO catalyst with an excellent activity could be
obtained though accurately controlling the DPU preparation
conditions.
Moreover, it has been reported previously that heating “as-
prepared” samples in H2 is more favorable to improve the
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In detail, accurately controlling DPU time, DPU temperature
and the Au/urea molar ratio is crucial for obtaining an excellent
catalytic activity.
For reference, CAT-5 (Table 1, entry 8) was prepared using
a non-porous a-Al2O3 support under the identical preparation
conditions used for Au/meso-AlO. The benzyl alcohol conver-
sion was 24% over the Au/a-Al2O3, confirming the effect of
mesoporosity on the catalytic activity. Additionally, in order
to obtain small gold NPs, another referred catalyst, CAT-6
(Table 1, entry 9), was also prepared with a much lower gold
loading of 0.05 wt% because of the much lower surface area of
a-Al2O3 (10 m2 g-1). However, CAT-6 of 2 g (equivalent gold to
CAT-1 of 0.1 g) delivered a benzyl alcohol conversion of just
30%, further confirming the effect of textural properties on the
catalytic activity.
We then extended the tests to several other typical alcohols
over the Au/meso-AlO catalyst (Table S1, ESI†). The catalyst
could efficiently oxidize 1-phenylethanol to acetophenone at a
high conversion of 99%, but only 30% for 2-phenylethanol, even
after a longer reaction time. For octanols, 1-octanol showed a
greater activity than 2-octanol, being consistent with a previous
report.14 Additionally, cyclohexanol and cyclopropyl carbinol
were very reactive among aliphatic alcohols. Interestingly, a
highly selective oxidation of the unsaturated alcohol crotyl
alcohol also proceeded with very a high conversion of 94% and
a selectivity of 99%.
Fig. 2 The particle size distributions of the Au/meso-AlO catalysts.
(A) CAT-1 with a benzyl alcohol conversion of 99%; (B) CAT-2 with
a benzyl alcohol conversion of 62%; (C) CAT-3 with a benzyl alcohol
conversion of 45%; (D) CAT-7 with a benzyl alcohol conversion of 25%.
The UV-vis spectra of the catalysts could be divided into
three ranges: 200–250, 300–400 and 500–600 nm (Fig. S6,
ESI†), ascribed to anchored Au3+ complexes, Aun clusters
d+
Gold catalysts are well known to be hard to store, even in
with bridging hetero-ions and metallic Au NPs, respectively.15
With a similar adsorption spectra at around 200–450 nm, the
surface plasmon resonance of these four catalysts showed great
differences at around 460–600 nm (Fig. S6, ESI†). Notable
here is the blue shift in this region, observed to increase in
the order CAT-1 > CAT-2 > CAT-3 > CAT-7, indicating an
increasing Au NP size in the reverse order according to ref. 15a.
This observation is consistent with the TEM results (Fig. 2;
Fig. S5, ESI†), further confirming the above-mentioned Au NP
size-dependent relationship of the catalyst activity on benzyl
alcohol oxidation. In comparison, our Au/meso-AlO catalyst
(CAT-1) provides a smaller gold nanoparticle size (~2 nm) than
related previously reported catalysts4,8,9b (gold particle size is
larger than 4 nm; Fig. S7, ESI†). This is why CAT-1 can deliver
a much higher TOF than the reported catalysts mentioned
above.
◦
the dark at below 0 C.3c Fig. S4, ESI† indicates that our
catalysts showed an ideal storage ability, and could deliver a
high benzyl alcohol conversion of 94% even after storing for
125 days at -18 ◦C in a refrigerator. Additionally, compared
with previously reported catalysts, Au/meso-AlO exhibited a
higher intrinsic activity for alcohol oxidation. For example, the
turnover frequency (TOF) of the Au/meso-AlO catalyst for
benzyl alcohol oxidation was 295 and 360 h-1 at 80 and 90
◦C, respectively (Table 1, entries 1 and 10), higher than that
◦
of the catalysts Au/Cu–Mg–Al (TOF of 130 h-1 at 90 C),9b
◦
Au/Ga3Al3O9 (TOF of 237 h-1 at 80 C)4 and Au/MnO2-R
(TOF of 320 h-1 at 120 ◦C).8
As noted previously, g-Al2O3 is traditionally considered to be
an “inert” support for gold catalysts. However, our Au/meso-
AlO catalyst is very effective for the liquid-phase oxidation
of alcohols. It is predicted that this significant promotion of
catalytic activity might be related to the Au NPs themselves.
To understand the origin of this excellent activity, four catalysts
(CAT-1, CAT-2, CAT-3 and CAT-7, with benzyl alcohol con-
versions of 99, 62, 45 and 25%, respectively (Table 1, entries 1–3
and 11)) were chosen and characterized.
XRD patterns of the four catalysts indicate that the crystalline
structure and diffraction intensity look unchanged after the
loading of Au. No gold diffraction lines are observed, possibly
because the gold particle size is too small (<3 nm) and/or the
gold loading is too low (Fig. S2, ESI†). To overcome the XRD
limitation, these catalysts were characterized by TEM (Fig. 2;
Fig. S5, ESI†). CAT-1 provided the smallest average particle
size of ~2.0 nm, while the other three samples provided larger
Au NPs size ranging from 4.0–6.5 nm. Interestingly, the order
of Au NP size (CAT-1 < CAT-2 < CAT-3 < CAT-7) is in good
agreement with that of benzyl alcohol conversion.
In recent studies, the mechanism of the DPU method has
been investigated.2e,3 Urea begins to hydrolyze above 70 ◦C and
the pH value of the suspension increases. As a result, gold
species deposit preferably on the gold nucleation center on the
surface of alumina. An important “mature” period introduces
a redispersion process, resulting in the required particle size
distribution by controlling the DPU time. Indeed, increasing
the DPU time induces a decrease of the Au NP size (Fig. 2A),
thereby leading to a drastic increase in the conversion of benzyl
alcohol (Fig. 1A). Additionally, the DPU temperature and the
final pH value are considered to be important for the kinetics of
the redispersion of the Au particles.3 Clearly, a suitable pH value
at the different stages of catalyst preparation is favorable for
nucleation, growth and redispersion of the Au NPs, while lower
or higher pH values are not.3 Calcining the catalyst plays an
assisting role in transforming the ionic gold species into Au NPs,
affecting the Au particle size. It is clearly demonstrated that a
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fresh sample is dominated by ionic gold species. After calcination
at 300 ◦C, most of the ionic gold species are transformed
into metallic Au NPs, as evidenced by the increase of the UV
adsorption band at 500–600 nm (Au NPs), accompanied by a
decrease of the band at 200–250 nm (ionic gold) (Fig. S6, ESI†).
Additionally, the XPS spectrum of the Au4f7/2 core level shows
three bands (Fig. S8, ESI†), which correspond to Au3+, Au+ and
Au0, and is dominated by Au0.5a The cationic gold is possibly
located at the gold–support interface, and more preferably at
the corners and steps of small Au NPs.2,16 Therefore, the longer
DPU time, higher DPU temperature, suitable urea/Au ratio and
optimal calcination temperature, synergistically work together
to produce an ideal catalyst in the present case using an “inert”
home-made meso-AlO oxide support.
A highly efficient Au/meso-AlO catalyst has been successfully
developed for the liquid-phase selective oxidation of alcohols by
depositing gold onto a home-made mesoporous g-Al2O3 using
the DPU method. The experimental testing results indicate
that the formation of small Au NPs of ~2.0 nm definitely
contributes to obtaining an excellent catalytic activity, which
can be realized by accurately controlling the DPU parameters
(including time, temperature and Au/urea molar ratio) and
calcination temperature. We anticipate that our results will make
the DPU method more useful for developing high-performance
gold catalysts, especially ones using an “inert” support and/or
others that are difficult to prepare by other methods.
spectra (XPS) were recorded on a Perkin-Elmer PHI 5000 C
spectrometer using Mg-Ka radiation (1253.6 eV) and an ana-
lyzer pass energy of 20 eV. Binding energies were referenced to
the adventitious C1s line at 284.6 eV. UV-vis DRS spectra were
recorded in the range 200–800 nm at room temperature using
a Shimadzu UV-2400PC spectrophotometer. The gold loading
of a typical Au/meso-AlO (1 wt%) sample was determined
to be 0.9 wt% by inductively coupled plasma atomic emission
spectrometry (ICP-AES) on a Thermo Scientific iCAP 6300 ICP
spectrometer.
Reactivity tests
The liquid-phase aerobic oxidation of alcohols was carried
out in a 25 ml three-necked flask equipped with a gas inlet
and outlet, reflux condenser and thermometer under ambient
pressure. Toluene (10 ml) and alcohol (1 mmol) were transferred
into the flask and heated to the required temperature. Then,
the catalyst Au/meso-AlO (0.1 g) was transferred to the flask.
The magnetic stirring speed was set at 900 rpm and oxygen
was bubbled through the vigorously stirred liquid at a rate
of 20 ml min-1. Samples were taken periodically during the
reaction. The products and catalyst were auto-separated by
gravity, and the products were analyzed using an HP 5890 gas
chromatography-flame ionization detector (GC-FID) with a 60
m HP-5 ms capillary column. The conversion and selectivity
were determined using undecane as an internal standard in all
reactions.
We thank the NSF of China (20973063, 21076083), the MOST
of China (2011CB201403), the Fundamental Research Funds
for the Central Universities, the Shanghai Rising-Star Program
(10HQ1400800), the Doctoral Fund of Ministry of Education
of China (20090076110006), the Shanghai Leading Academic
Discipline Project (B409) and the Electron Spectroscope Center
of the East China Normal University for their assistance with
TEM measurements.
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Experimental Section
Catalyst preparation
The mesostructured g-Al2O3 (denoted meso-AlO) sample was
prepared according to refs. 12, and the catalyst Au/meso-AlO
was prepared by the DPU method.3b,5 Following is the detailed
procedure: Meso-AlO (0.5 g) was added to a solution (50 ml)
of the desired amount of HAuCl4 (equally containing 0.005 g
Au) and urea. The suspension was then heated at 70–100 ◦C
and stirred for the appointed time. The suspension was filtered
and washed 5 times to remove residual Cl-1 (AgNO3 test). In
each washing, 100 ml of distilled water was used and stirred for
at least 10 min at room temperature. The obtained sample was
dried at 100 ◦C overnight and calcined for 4 h in air or H2 with
a flow rate of 50 ml min-1. The preparation was performed in a
dark room and the nominal Au loading was 1 wt%.
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