Thermally stable gold/alumina aerogel catalysts prepared by a simultaneous synthesis process for solvent-free aerobic benzyl alcohol oxidation

Article abstract of DOI:10.1039/c3cy01064c

Gold nanoparticles supported on an alumina aerogel have been prepared through a simultaneous synthesis process in which the formation of the gold nanoclusters capped with (3-aminopropyl)triethoxysilane (APTES) and the hydrolyzation/co-condensation of the

Full text of DOI:10.1039/c3cy01064c

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Thermally stable gold/alumina aerogel catalysts
prepared by a simultaneous synthesis process
for solvent-free aerobic benzyl alcohol oxidation
Cite this: DOI: 10.1039/c3cy01064c
abc
a
a
a
Z. Li,
Y. Ji, C. Cadigan and R. M. Richards*
Gold nanoparticles supported on an alumina aerogel have been prepared through a simultaneous synthesis
process in which the formation of the gold nanoclusters capped with (3-aminopropyl)triethoxysilane
(APTES) and the hydrolyzation/co-condensation of the APTES and aluminum alkoxide occur concurrently.
Received 16th December 2013,
Accepted 1st May 2014
The obtained gold nanoparticles are trapped in and physically protected by the framework of the alumina
aerogel providing excellent thermal stability with no observable sintering at 650 °C. In the catalysis test
for the solvent-free aerobic benzyl alcohol oxidation, this catalyst exhibits significantly better activity than
reference gold/alumina aerogel catalysts prepared through a deposition precipitation and conventional wet
impregnation methods, especially at high temperature.
DOI: 10.1039/c3cy01064c
www.rsc.org/catalysis
suffer from severe sintering and loss of activity, greatly limiting
1
. Introduction
12
the practical application of gold catalysts. In fact, the sintering
of gold catalysts also happens during the moderate calcination
(250–350 °C) process of catalyst preparations. The thermal sta-
bility of gold catalysts can be somewhat improved by removal
of the chloride ions in the catalysts or by using expensive
The catalytic performance of nanoparticles can be finely
tuned either by their composition, which mediates electronic
structure, or by their shape, which determines surface atomic
arrangement and coordination. Beyond the exciting potential
for tailoring catalysts, nanostructured materials introduce
additional challenges to catalyst design. By being small and
containing surface atoms of varying unsaturated valencies,
nanoparticles of specific shape are more susceptible to mor-
13
chloride-free gold precursors. Numerous efforts have been
made to address this problem by modifying the surface of gold
nanoparticles or the surface of the supporting materials. One
promising approach is to physically trap the gold catalysts in
1
14
phological change in harsh chemical reaction environments.
the rigid structure of metal oxides. Very recently, atomic layer
Given the low melting points of nanoparticles as compared
to their bulk counterparts, imparting stability to nanoscale
catalysts (particularly gold) remains a challenge.
deposition (ALD) has been used to stabilize nanoporous gold
2 3
with a thin layer of Al O , greatly improving the thermal stabil-
2
15
ity of the gold. Furthermore, in our previous work, we have
reported gold catalysts intercalated in the walls of mesoporous
Gold has long been considered an ‘inert’ metal. However,
with the development of nanotechnology, it has been found
that nano-scale gold is highly active for CO oxidation with
11,16
silica with improved thermal stability.
In this work, we have improved the thermal stability of
Au/Al catalysts by simultaneously hydrolyzing/condensing
3
great selectivity even at room temperature. Thereafter, the
2 3
O
field of gold nanocatalysts has been thoroughly studied and
aluminum alkoxide and reducing the gold precursor in the
presence of (3-aminopropyl)triethoxysilane (APTES) as a
linking agent. The obtained catalyst can endure calcination
temperatures up to 650 °C while maintaining excellent cata-
lytic activity at 160 °C in the solvent-free aerobic benzyl alco-
4
has rapidly become a promising system for hydrogenation
5
,6
7,8
9
and epoxidation of alcohols and aldehydes as well as oxi-
8
,10,11
dation and even aerobic oxidation of alkanes.
However,
most applications of gold catalysts are performed under moderate
conditions with relatively low temperatures (normally 80–120 °C).
Working at higher temperatures, gold nanoparticles generally
2 3
hol oxidation. Au/Al O catalysts prepared by a conventional
impregnation method and a deposition–precipitation (DP)
method have also been tested as controls.
a
Department of Chemistry and Geochemistry, Colorado School of Mines,
1
500 Illinois St., Golden, CO 80401, USA. E-mail: rrichard@mines.edu;
2
. Results and discussion
Fax: +1 303 273 3629; Tel: +1 303 273 3612
b
Chemical and Materials Engineering, University of Alberta, 9107 116 St.,
2.1 Synthesis of Au/Al O catalyst
2 3
Edmonton, AB, T6G 2V4, Canada
c
An ultrastable Au/Al O catalyst was prepared by a simultaneous
2 3
synthesis method, during which the reduction of Au and the
National Institute for Nanotechnology (NINT), NRC, 11421 Saskatchewan Dr.,
3+
Edmonton, AB, T6G 2M9, Edmonton, AB, T6G 2M9, Canada
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Fig.
1
The schematic illustration of the simultaneous synthesis
@Al . The arrow marked as ‘1’ signifies the
process of Au/SiO
2
2 3
O
reduction of gold and hydrolysis and condensation of aluminum
alkoxide and APTES; the arrow marked as ‘2’ signifies the 5 hour
calcination process at 500 °C.
hydrolyzation/condensation of the aluminum alkoxide took
place at the same time. APTES, well-known for its affinity
to gold nanoparticles, was used as a linking agent. As shown
in Fig. 1, the as-formed gold nanoclusters, bonded to the
2
(NH –) end of APTES, were further stabilized during the
co-condensation of ethoxy groups and concurrent sol–gel
processes occurring with aluminum alkoxide. The co-condensation
process prevents the gold clusters from further growth and
results in a uniform structure with tiny gold particles trapped
2 3 2
in the framework of Al O (with a small amount of SiO from
APTES) after supercritical drying and calcination at 500 °C.
Fig. 2 Nitrogen adsorption/desorption isotherms at −196 °C (A) and BJH
pore-size distribution (B) of Au/SiO @Al O , Au/Al O and Al O aerogel.
2 2 3 2 3 2 3
The nomenclature Au/SiO @Al O is used henceforth in
2
2 3
this manuscript for this catalyst system. As a control, a pure
Al aerogel has been prepared under the same condi-
tions without addition of APTES and gold precursor. The
obtained Al aerogel was used as supporting material
to prepare the reference catalyst by impregnation method
termed Au/Al O3 herein).
2 3
O
Table 1
2
N -adsorption data and Au content of different samples
2
O
3
a
b
c
d
Calcine
[°C]
S
BET
V
total
Au
Au size
2
−1
3
−1
Sample
[m g
]
[cm g ] [wt%] [nm]
(
2
Al
O
2 3
500
500
350
441
384
252
1.08
1.07
1.03
Au/SiO
2
@Al O
2 3
1.1
1.0
3–5 (spheres)
3–30 (irregular)
2
Au/Al O
3
2.2 Porous texture and chemical features of the catalyst systems
a
The surface area SBET is calculated by a multi point BET method
Surface area and porosity were analyzed by N -adsorption at
b
2
(7 points were selected in the range of P/P = 0.1–0.3). Total pore
volume. Weight content of Au in catalysts, determined by ICP-MS
analysis. The size of gold nanoparticles is determined by TEM.
0
c
−
196 °C. As shown in Fig. 2A, Al
2
O
3
aerogel, Au/SiO
2
@Al
2
O
3
d
and Au/Al O all exhibit type IV isotherms with H1-type hys-
2
3
teresis loops at P/P0 = 0.5–1.0. This is characteristic of a
combined pore system with mostly small mesopores with
some larger mesopores. Compared with the other two, the
hysteresis loops of Au/Al O are less developed at high
in the simultaneous deposition process did not influence the
formation of the sol–gel pore structure. The impregnation
and calcination processes resulted in a loss of more than
40% of the surface area of the aerogel. In fact, the effect of
solvent on the support materials is an unavoidable factor
in most post treatment methods to prepare heterogeneous
catalysts via deposition–precipitation (DP) or wetness impreg-
2
3
0
pressure (P/P = 0.75–1.0), indicating fewer large mesopores in
the sample. This difference is also observed in the pore size
distribution plots (Fig. 2B) calculated by the Barrett–Joyner–
Halenda (BJH) model with a sharp peak centered at 2.4 nm
(pore radius) in all three samples. The pores of this size are
formed by the packing of the Al O nanoparticles. There are
2 2 3 2 3
nation. The gold content of Au/SiO @Al O and Au/Al O
2
3
also broad peaks in the range of 5–50 nm, which correspond
are 1.1% and 1.0% respectively as measured by inductively
coupled plasma mass spectrometry (ICP-MS) analysis. The
to the large pores formed during formation of the Al O gel.
2
3
However, in the case of Au/Al O , this peak is significantly
2 2 3 2
Au/SiO @Al O catalyst also contains around 5% SiO , which
2
3
smaller than that of other samples, suggesting part of the
pore structure formed in gelation was destroyed in the con-
ventional wetness impregnation preparation of gold catalysts.
is from APTES.
2.3 Structure and morphology of gold nanoparticles
As listed in Table 1, Au/SiO
2 2 3
@Al O has a large surface area
2
−1
−1
(
(
384 m g ), slightly lower than that of pure Al O aerogel
441 m g ), indicating that the gold nanoparticles formed
Considering that the Au/SiO @Al O catalyst was calcined at
500 °C, we are extremely interested in the morphology of the
2
3
2
2 3
2
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gold nanoparticles: whether or not they agglomerated like the
1
7,18
typical gold nanoparticles supported on Al O .
In order
2
3
to increase the contrast between gold nanoparticles and
the alumina, Scanning Transmission Electronic Microscopy
(STEM-Dark field mode) was utilized to study the morphology
and distribution of gold nanoparticles supported on alumina
aerogel. Fig. 3A shows an overview of the Au/SiO @Al O cata-
2
2 3
lyst, where the uniform gold nanoparticles are evenly distrib-
uted on the alumina support. Those gold nanoparticles show
a sphere-like structure and are 3–5 nm in diameter (Fig. 3B).
No agglomeration of gold nanoparticles was observed after
calcination at 500 °C. The selected area dispersive X-ray
Fig. 4 HAADF-STEM images of Au/Al
B) magnification.
2
O
3
at low (A) and high
(
(SAEDX) analysis shown in Fig. 3C indicates the existence
of Si (from APTES), Au, Al, O and C (from the carbon
film on TEM grids). To evaluate the thermal stabilities of
the Au/SiO @Al O catalyst, the sample was also calcined at
2 3
For comparison, Fig. 4 shows the Au/Al O catalyst pre-
pared by the wet impregnation method. The alumina support
shows a similar morphology with that of Au/SiO @Al O and
2
2 3
6
50 °C and 750 °C. As seen in Fig. 3D, the gold nanoparticles
2
2 3
retain uniform size and are well dispersed on the support
after calcination at 650 °C. Furthermore, the high magnifica-
tion image shown in the inset reveals that the particles
are only slightly larger at 5–10 nm. Even after calcination at
is typical of an alumina aerogel. However, the supported gold
shows various shapes and sizes with some 3–5 nm spheres as
well as significant agglomerations (some reaching 30 nm).
Given the reference catalyst was calcined at a much lower
temperature (350 °C), it is inferred that the simultaneous syn-
thesis method yields a gold catalyst with significantly
improved thermal stability than conventional gold catalysts.
Fig. 5A shows the X-ray diffraction pattern of the Au/SiO @Al O
7
50 °C, about half of the gold particles are still around 5 nm
in size, but some larger agglomerations begin to appear at
this calcination condition (Fig. 3E). From the inset, larger par-
ticles around 20–30 nm can be observed.
2
2 3
2 3 2 3
and Au/Al O samples. The Al O supports of both samples
exhibit very broad diffraction peaks indicating that they are
composed of very fine nanoparticles. However, the gold peak
is not observable in any of the samples. That is mainly
because the two strongest diffraction peaks of Au, namely
Fig. 3 HAADF-STEM images of Au/SiO
the EDX analysis at the selected area (C); HAADF-STEM images of
Au/SiO @Al catalyst calcined at 650 °C (D) and 750 °C (E).
2 2 3
@Al O catalyst (A, B) and
Fig. 5 XRD spectra (A) and diffuse-reflectance UV-vis spectra (B) of
Au/SiO @Al O and Au/Al O catalysts.
2 2 3 2 3
2
2
O
3
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(111) at 38.18° and (200) at 44.39° will be relatively small
but Au/SiO @Al O showed no significant advantage over
2
2 3
given the low Au concentration (around 1 wt%) and the truly
nanoscale size of Au particles. Additionally, the small Au
peaks can be concealed by the larger broad peaks of Al O at
Au/Al O at 120 °C and 140 °C indicating that the catalysts
2 3
work similarly under those moderate conditions. However,
Au/SiO @Al O gives a conversion of 21.90% at 160 °C,
2
3
2
2 3
similar positions (35.14° and 43.36°). However, the broad dif-
fraction peak of Al at 35.14° shows a small shoulder at
more than two times the conversion demonstrated by Au/Al O
(10.42%) with similar selectivity to benzaldehyde. Indeed,
2
3
2
O
3
around 38°, which might be attributed to the Au (111) diffrac-
higher conversion is always desirable as long as the catalyst
can handle the high temperature and still give reasonable
selectivity. The Au/Al O DP catalyst had the lowest activities
2 3
tion. It is well-known that the surface plasma resonance (SPR)
1
9
of gold nanoparticles is closely related to their particle size.
Diffuse-reflectance UV-vis spectra were used to investigate the
SPR of the two catalysts (Fig. 5B). Both catalysts show a sharp
SPR peak in the range of 500–600 nm as evidence that the
gold particles are nanoscale in size. Furthermore, the SPR
peak of Au/SiO @Al O (519 nm) was slightly blue shifted as
among the three Au catalysts at higher temperatures, but the
highest selectivity to benzaldehyde as reaction temperature ele-
vates to 160 °C.
To determine the Au leaching of our Au/Al O catalyst, the
2
3
solution after catalytic reaction was collected and Au compo-
sition determined by ICP. Results indicated that at 100 °C,
0.412% of Au was leached after reaction, the leaching of Au
slightly increased to 0.466% at 120 °C and finally reached
2
2 3
2 3
compared to Au/Al O , indicating that the Au particles in
Au/SiO @Al O are slightly smaller.
2
2 3
0
.523% for reaction at 160 °C. Au leaching is minor in all
2 2 3
2.4 Catalytic performance of ultrastable Au/SiO @Al O catalysts
catalytic reactions, even at a high temperature of 160 °C,
The oxidation of benzyl alcohol, as a model reaction for pri-
suggesting a good thermal stability of simultaneous synthe-
2
0
mary alcohol oxidation, was used in this work to study the
activity of the Au/SiO @Al O prepared by simultaneous syn-
sized Au/Al
2 3
O catalyst.
We attribute the good performance of Au/SiO @Al O to
2
2
3
2
2 3
thesis. All reactions were performed in the absence of solvent
with a 6 h reaction time. No detectable benzene or toluene
was produced in the reactions. The byproduct is mainly
benzyl benzoate rather than benzoic acid, indicating either
the immediate esterification between benzoic acid and benzyl
alcohol, or that oxidation occurs directly on the hemiacetal
its thermal stability. In fact, gold nanoparticles are generally
highly mobile on Al surfaces due to the Au–Al bond
energy being substantially weaker than that of Au–TiO2
2
O
3
2 3
O
(ref. 17) and small gold Au nanoparticles may be more
1
8
mobile than gold atoms. Thus, the agglomeration of gold
nanoparticles is problematic for the Au/Al O system, espe-
2
3
[
PhCH(OH)OCH Ph] formed from benzaldehyde and benzyl
cially when HAuCl
involved in the preparation, because the chloride ions can
significantly accelerate the agglomeration process. Fig. 4
clearly shows the aggregation of gold deposited on Al by a
4
(the most common gold precursor) is
2
5
,21
alcohol without ever forming benzoic acid.
performance of Au/SiO @Al O , Au/Al O , Au/Al O DP and pure
Al O (to evaluate the contribution of supporting material), it
Comparing the
2
2
2
2
3
2
3
2 3
2
3
2 3
O
can be inferred that the gold nanoparticles have minimal
contributions to the overall conversion at 100 °C (Table 2).
The activity of gold nanoparticles increases with temperature,
typical wetness impregnation method calcined at 350 °C in
air. Though the oxidation of benzyl alcohol is run at relatively
low temperatures, the agglomeration may be even higher due
to the influence of the hot liquid. As shown in Fig. 1, in the
simultaneous synthesis process, the reduction of gold takes
place at the same time as the hydrolyzation and condensa-
tion of metal alkoxide and APTES. Three additional factors to
stabilize the gold nanoparticles can be expected from this
process, i) the amino head of APTES can promote even distri-
Table 2 Comparative data for solvent-free aerobic oxidation of benzyl
alcohol. The oxidation was carried out by bubbling oxygen into the
reaction mixture (5 ml benzyl alcohol and 5 mg catalysts) at different
a
temperatures. Results were obtained after 6 h of reaction
T (°C)
Catalysts
Al
Conversion
Selectivity (mol.%)
3
+
bution of Au by anchoring it; ii) when gold nanoparticles
form, the SiO condenses with the Al O support and due to
100
120
140
160
O
2 3
1.32
1.76
2.59
2.03
2.87
6.09
3.55
5.90
3.08
7.40
6.16
6.77
6.67
21.90
8.99
10.42
82.31
77.39
61.06
81.01
85.22
88.52
67.28
84.72
83.24
83.51
84.61
82.78
80.97
72.32
89.33
75.96
2
2 3
2 2 3
AuSiO @Al O
the well-known affinity between its amino head group and
gold surface, the APTES can increase the interface between
gold and Al O to stabilize gold nanoparticles; iii) although
Au/Al
Au/Al
2
O
O
3
DP
2
3
Al
2 3
O
2
3
2
Au/SiO @Al
2
O
O
O
3
the APTES functional group was damaged after calcinations
at 500 °C, the enhanced interfaces are still there to hamper
the mobility of gold nanoparticles. To confirm the stability of
the catalysts in the reaction, we have acquired bright-filed
Au/Al
Au/Al
2
O
O
3
DP
2
3
Al O
2 3
Au/SiO
2
@Al
2
3
3
Au/Al
Au/Al
2
O
O
3
DP
2 2 3
TEM images of Au/ SiO @Al O before and after reaction. As
2
3
shown in Fig. 6, no observable changes in particle morphol-
ogy or size can be found. Histogram indicated that size of Au
particles in the materials both before and after reaction
evenly distributed between the range of 2.5–5.5 nm, with
peaking at 4.5–5 nm.
Al
2 3
O
Au/SiO @Al
2 2
Au/Al
Au/Al
2
O
O
3
DP
2
3
a
The selectivity to benzaldehyde.
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of isopropanol and toluene (1: 1 by volume) and drying
by supercritical treatment at 265 °C in a Parr autoclave.
The obtained powder was calcined at 500 °C for 5 h under the
protection of argon. The Al O aerogel was prepared under the
2
3
same conditions without adding the gold precursor, APTES
and sodium borohydride.
3
.3 Synthesis of reference Au/Al
The Au/Al sample was prepared following a reported
method. In a typical experiment, 1.0 g of Al aerogel was
2 3
O catalyst
2 3
O
2
3
Fig. 6 Bright-field TEM images of Au/SiO
and after (B) reaction at 120 °C.
2 2 3
@Al O catalyst before (A)
2 3
O
stirred in 50 ml 9.7 mM chloroauric acid for 5 h at 70 °C.
Then the powder was filtered and re-suspended in 50 ml DI
water. After refluxing at 70 °C for 1 h, the powder was filtered
again and calcined at 350 °C for 5 h. The final product was a
light pink powder, denoted as Au/Al O .
3
. Experimental
2
3
3
.1 Materials and instruments
3.4 Synthesis of reference Au/Al
2 3
O DP catalyst
Hydrogen tetrachloroaurate (HAuCl
4
2
·3H O, 99.99%), cetyltri-
2
4
The preparation of Au/Al O DP follows literature reports.
methylammonium bromide (CTAB, 99%), isopropanol (99.5%)
and 3-aminopropyl-triethoxysilane (APTES) were obtained from
Sigma-Aldrich. Freshly calcined (550 °C) molecular sieves (4 Å)
were added to the isopropanol to further remove any residual
water. All other reagents were procured from Aldrich and used
as received. Ultrapure deionized water (Continental Water Systems)
was used throughout the experiments. Transmission electron
microscopy (TEM) studies were performed on a FEI Tecnai
F20 S-Twin operated at 200 kV. The samples were prepared
by spreading an ethanolic suspension on carbon coated copper
grids. BET and isothermal measurements were performed on a
Quantachrome Nova 4000e. X-ray diffraction was acquired on
a Siemens D 5000 diffractometer (Cu Kα, λ = 1.5406 Å). The
diffuse reflectance UV-vis spectra were recorded on a UV4
2 3
Typically, a solution of HAuCl
water was heated to 80 °C and adjusted to pH = 8.5 with
NaOH. Al O aerogel (0.5 g) was then added to the solution
4 2
·3H O (10 mg) in 100 ml of DI
2
3
under vigorous stirring while the pH was still maintained at
.5 using NaOH and HCl. Once the pH stabilized, the mixture
was stirred at 80 °C for 2 h. The precipitation was collected,
washed several times and dried at 100 °C. The obtained pow-
8
der was calcined at 450 °C for 4 h and denoted as Au/Al O DP.
2
3
3.5 Benzyl alcohol oxidation
The liquid phase oxidation of benzyl alcohol over the
supported Au catalysts were carried out in a magnetically
stirred 25 ml three-neck flask with oil bath heating and reflux
condenser, under the following general reaction conditions:
reaction mixtures = 5 ml benzyl alcohol + 5 mg catalyst, reac-
tion time = 6 h, temperature varied from 100 °C to 160 °C in
different experiments, 99.9% oxygen was bubbled into the
(Unicom) spectrophotometer. Calcination was performed in a
tubular furnace with a ramping rate of 3 °C per minute.
3
2 2 3
.2 Synthesis of Au/SiO @Al O
−
1
reaction mixture at the rate of approximately 2 ml min .
After the reaction, the reaction mixture was filtered and the liquid
phase from was analyzed by a gas chromatograph equipped with
Hydrogen tetrachloroaurate was first dissolved in distilled
water and then transferred into toluene with the help of a
phase transfer agent (CTAB). Typically, an aqueous solution
of hydrogen tetrachloroaurate (5 ml, 0.02 M) was added to
the mixture of CTAB (50 mg) and toluene (10 ml) under vigor-
ous stirring. After 2 h, tetrachloroaurate was completely
transferred into the toluene phase to form a 0.01 M solution
a flame ionization detector, using N as a carrier gas. The reac-
2
tion products and unconverted reactants were identified by com-
parison with authentic standard samples. The conversion and
product selectivity were calculated as follows: conversion (%) =
moles of reactant converted)/(moles of reactant in feed) × 100
and product selectivity (%) = (moles of product formed)/(moles of
reactant converted).
(
(
solution A) and the water phase became colorless. In another
flask, 1.0 g of aluminium triisopropoxide was dissolved in
0 ml pretreated anhydrous isopropanol and then 50 ml
5
anhydrous toluene was added and stirring was maintained
for 5 h to form a transparent solution. 2 ml of solution A and
4. Conclusions
0
.05 ml APTES was added to this clear solution. After stirring
for one more hour, 15 ml isopropanol solution containing
.2 ml water and 5 mg sodium borohydride was added
In this work, we demonstrate a simultaneous synthesis pro-
cess which can significantly increase the thermal stability of
gold/alumina aerogel catalysts by increasing the Au–Al O
0
2
3
dropwise to the flask over 30 min. The gelation process was
performed by maintaining stirring for 24 h at room tempera-
ture. The Al O gel doped with gold nanoparticles was washed
interface and trapping gold nanoparticles in the framework
of an alumina aerogel. No agglomeration was found even
after 5 h calcination at 650 °C. Furthermore, the Au/Al O
2
3
2 3
three times by centrifuging, re-dispersing in a 100 ml solution
prepared by this process demonstrated superior performance
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at high temperatures than that of systems prepared by depo-
sition precipitation and wetness impregnation methods.
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0 J. M. Thomas, R. Raja, G. Sankar and R. G. Bell, Nature,
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for the Diffuse-reflective UV-vis spectra measurement. RR would
like to thank the National Science Foundation for support under
grant CHE-1214068.
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