Tuning the selectivity of a supported gold catalyst in solvent- and radical initiator-free aerobic oxidation of cyclohexene

Article abstract of DOI:10.1039/c3cy01011b

The selectivity of supported gold catalysts in aerobic oxidation of cyclohexene under solvent-free conditions without addition of a radical initiator was tuned by either WO₃ or the metal-organic framework MIL-101, used as a support/co-catalyst. WO₃ was found to promote the formation of cyclohexene oxide via reaction of cyclohexenyl hydroperoxide with cyclohexene, while MIL-101 catalysed conversion of cyclohexenyl hydroperoxide to 2-cyclohexen-1-one.

Full text of DOI:10.1039/c3cy01011b

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Tuning the selectivity of a supported gold catalyst
in solvent- and radical initiator-free aerobic
oxidation of cyclohexene†
Cite this: Catal. Sci. Technol., 2014,
4, 752
Daniil S. Ovoshchnikov,‡a Baira G. Donoeva,‡ab Bryce E. Williamsona
ab
*
and Vladimir B. Golovko
Received 4th December 2013,
Accepted 14th December 2013
The selectivity of supported gold catalysts in aerobic oxidation of cyclohexene under solvent-free condi-
tions without addition of a radical initiator was tuned by either WO3 or the metal–organic framework
MIL-101, used as a support/co-catalyst. WO3 was found to promote the formation of cyclohexene oxide
via reaction of cyclohexenyl hydroperoxide with cyclohexene, while MIL-101 catalysed conversion of
cyclohexenyl hydroperoxide to 2-cyclohexen-1-one.
DOI: 10.1039/c3cy01011b
www.rsc.org/catalysis
One of the approaches to improve the selectivity of the
catalysed reaction towards the desired product is to utilize
Introduction
The epoxidation of alkenes is an important industrial process
for production of fine chemicals.1 Traditionally, it involves
the use of stoichiometric oxygen donors,2 peroxycarboxylic
acids3 or hydrogen peroxide,4 which either produces environ-
mentally undesirable wastes or, in the case of H2O2, increases
the cost of the process. Utilization of molecular oxygen is
highly desirable to make epoxidation “green” and cost-effective.
Another important way to reduce the environmental impact
of a reaction is to conduct it under solvent-free conditions.5
Since the discovery of the exceptional catalytic activity of gold
in low-temperature oxidation of CO by Haruta6 and in hydro-
chlorination of ethylene by Hutchings,7 gold catalysts have
proven to be efficient in a number of different reactions:
cyclizations, rearrangements, selective hydrogenation, C–C
coupling reactions, and selective oxidation of alcohols and
olefins.8–10 Among other reactions, aerobic oxidation of
cyclohexene was shown to be catalysed by supported nano-
sized gold.11–14 However, the reaction required the use of
t-butyl hydroperoxide as a radical initiator,13 and the high yield
of cyclohexene oxide was achieved only by conducting the
reaction in a specific solvent.11 Under solvent- and radical
initiator-free conditions, mainly products of allylic oxidation
are formed.14,15
a bifunctional catalyst.16 An elegant example of such an
approach is the phenol hydrogenation process catalysed by
palladium deposited on Al2O3: while the palladium nano-
particles activate the hydrogen, the support, being a Lewis
acid, activates the substrate and stabilizes cyclohexanone,
preventing its over-hydrogenation to cyclohexanol.17
Recently, we showed that metallic gold nanoparticles
formed from triphenylphosphine stabilized Au clusters catal-
yse formation of cyclohexenyl hydroperoxide in the aerobic
oxidation of cyclohexene.15 In the present study, we investi-
gated the effects of the supports and co-catalysts on the activ-
ity and selectivity of the Au-based catalysts for aerobic
oxidation of cyclohexene under solvent-free conditions, with-
out the addition of a radical initiator. We showed how the
selectivity of this reaction can be tuned towards formation
of either cyclohexene oxide or 2-cyclohexen-1-one as major
products.
Experimental
Materials
Gold (99.99%), sodium borohydride (>97%), hydrochloric
acid (37%), nitric acid (65%), dichloromethane (99.8%),
n-decane (99%), n-hexane (98%), triphenylphosphine (98%),
cyclohexene (99%, inhibitor-free), 2-cyclohexen-1-ol (95%),
2-cyclohexen-1-one (98%) and cyclohexene oxide (98%) were
purchased from Sigma-Aldrich and Merck. Oxygen (99.7%)
was obtained from BOC gases. The titanium(IV) oxide (P25, TiO2),
silicon dioxide (Aerosil OX 50, SiO2) and tungsten(VI) oxide
(nanopowder, <100 nm particle size, WO3) were purchased
from Evonik and Sigma-Aldrich. All materials were used
without further purification.
a Department of Chemistry, University of Canterbury, Christchurch 8140,
New Zealand. E-mail: vladimir.golovko@canterbury.ac.nz;
Fax: +64 3 364 2110; Tel: +64 3 364 2442
b The MacDiarmid Institute for Advanced Materials and Nanotechnology,
New Zealand
† Electronic supplementary information (ESI) available: TGA of gold clusters.
Characterization of MIL-101, including PXRD patterns, TGA and nitrogen
physisorption analysis data. See DOI: 10.1039/c3cy01011b
‡ These authors contributed equally to this work.
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Catalyst preparation
chromatography-mass spectrometry (GC-MS) using a Shimadzu
GCMS-QP2010. Quantitative analysis of reaction mixtures
was performed via GC-FID using calibration solutions of
commercially available products. The concentration of cyclo-
hexenyl hydroperoxide was determined using iodometric
titration in 80% acetic acid.21 Pre-treatment of the reaction
mixture with PPh3 (ref. 22) did not change the concentration
of 2-cyclohexen-1-one compared to an untreated mixture,
indicating that cyclohexenyl hydroperoxide was not decom-
posing during GC analysis and therefore allowing us to plot a
calibration curve for cyclohexenyl hydroperoxide on the basis
of comparison of GC data and iodometric titration results.
Triphenylphosphine-stabilized (PPh3-stabilized) gold clusters,
[Au9(PPh3)8](NO3)3 (Au9) and Au101(PPh3)21Cl5 (Au101, approx-
imate composition), were synthesized following previously
described methods.18,19 Successful preparation and purity of
clusters were confirmed by 1H and 31P NMR, transmission
electron microscopy (TEM) and thermogravimetric analysis (TGA).
As-made gold clusters were deposited onto the oxide sup-
ports from CH2Cl2 solution. Typically, a calculated amount of
gold cluster dissolved in CH2Cl2 (10 mL, 1–2 mg mL−1) was
added dropwise to a vigorously stirred slurry of SiO2, TiO2 or
WO3 (500 mg) in CH2Cl2 (15 mL). The mixture was stirred for
30 min and the solid was collected by centrifugation. A
colourless supernatant solution confirmed complete cluster Results and discussion
deposition. The catalysts were washed with CH2Cl2 (20 mL)
and dried under vacuum at room temperature.
Preparation of Au/WO3, Au/SiO2 and Au/TiO2
The NMR spectra of synthesized clusters closely match previ-
In the case of deposition of Au9 on WO3, for loadings
greater than 0.1 wt%, n-hexane (50 mL) was slowly added to
the slurry to ensure cluster deposition. Solids were collected
by centrifugation, washed with n-hexane and dried under
vacuum at room temperature.
The metal–organic framework MIL-101 was synthesized
following a previously described method20 and characterized
using PXRD, TGA and surface area measurements (see the ESI†).
ously published spectra: the 31P spectrum of Au9 clusters in
CD2Cl2 has a singlet at δ = 56.9 ppm; the 1H spectrum of
Au101 in CD2Cl2 shows a broad phenyl resonance, centred at
7.1 ppm, with minimal signal (7.52 ppm) due to unbound
AuClPPh3.17,18 TGA showed 52% (Au9) or 26% (Au101) weight
loss of the starting mass in comparison to calculated
values of 52% and 21%, respectively, based on the loss of
triphenylphosphine ligands (see the ESI† for details). The
diameters of gold cores for Au101 and Au9 were 1.5 nm and
0.9 nm, respectively, as determined by TEM, which is in
accordance with previously published data.17,18 We have
chosen non-porous nanopowders of TiO2, SiO2 and WO3 as
supports with similar morphologies – particle sizes below
100 nm and surface areas ≤50 m2 g−1.
Catalyst characterization
NMR spectra of gold clusters were recorded using an Oxford/
Varian AS500 500 MHz NMR spectrometer. TGA was per-
formed using Alphatech SDT Q600. TEM images were taken
using a Philips CM200 instrument at 200 kV. Samples were
deposited on holey carbon coated copper grids (300 mesh)
from n-hexane. Typically, at least 100 particles were counted to
calculate the average particle diameter. The gold content of
the catalysts was determined quantitatively by atomic absorp-
tion spectroscopy (AAS) using a Varian SpectrAA 220 FS instru-
ment. Diffuse reflectance UV-vis (DR UV-vis) spectra were
recorded on a GBC Cintra 404 spectrophotometer.
Two series of catalysts with target gold loadings ranging
from 0.1 wt% to 0.5 wt% were prepared. The actual loadings,
established using AAS, are shown in Table 1, where the two-
digit prefixes in the sample designations are indicative of
the approximate loadings in wt%. It was found that clusters
readily adhere from CH2Cl2 solution to SiO2 and TiO2 at all
target loadings.
In the case of WO3, for target loadings greater than 0.1 wt%,
successful deposition of Au9 required addition of n-hexane,
and maximum actual loadings of only ca. 0.3 wt% could be
achieved. Deposition of Au101 on WO3 was readily achieved
without the use of n-hexane for all target loadings.
Catalytic testing
Cyclohexene oxidation was performed in a glass reactor
equipped with a reflux condenser. Except where specified
below, the system was flushed with oxygen three times and
remained connected to an O2-filled rubber balloon through-
out the reaction. Typically, a mixture of cyclohexene (5 mL),
catalyst (50 mg) and n-decane (0.2 M, as an internal stan-
dard) was magnetically stirred (500 rpm) at 65 °C. After 16
hours, the reactor was cooled down to room temperature, the
condenser was rinsed with acetone (5 mL) and the reaction
mixture was separated from the solid catalyst by centrifuga-
tion. The catalyst was washed with acetone and dried under
vacuum before recycling.
After deposition, the mean diameter of Au101 clusters, as
determined by TEM, slightly increases (Table 1). We were
unable to detect as-deposited Au9 clusters using bright-field
TEM, which indicates that the size of metal core remains
below 1 nm. However, during the course of catalytic cyclo-
hexene oxidation, both types of clusters sinter to form
particles with mean diameters ranging from ca. 4 to 10 nm
(Table 1 and Fig. 1). The mean diameter of the nanoparticles
formed from Au101 during the catalytic cycle is decreasing
with lower gold loading. In contrast, Au9 clusters form bigger
nanoparticles at lower loading. This could suggest different
sintering mechanisms for two clusters – Au9 acts as feedstock
for nanoparticle growth, while Au101 clusters collide and
The liquid samples were analysed via gas chromatography
(GC) using a Shimadzu GC-2010 equipped with an Rxi-5SilMS
capillary column (30 m × 0.25 mm × 0.25 μm) and a flame
ionization detector (FID). Products were identified by gas
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Table 1 Catalyst properties
with mean diameters of 4–6 nm, while at lower surface
concentrations, clusters initially form fewer nanoparticles that
act as nucleation sites that keep growing, consuming the
remaining Au9 clusters to eventually form 8–10 nm particles.
As agglomeration of Au101 proceeds via cluster collision, the
degree of sintering should decrease with the lower surface
concentration of clusters.
Nanoparticle diameter, TEM (nm)
Gold loading,
AAS (wt%)
Catalyst
Mean s.e.f
s.d.f
0.5Au101/WO3a
0.5Au101/WO3b
0.5Au101/WO3
0.1Au101/WO3a
0.1Au101/WO3
0.3Au9/WO3a
0.3Au9/WO3b
0.3Au9/WO3c
0.3Au9/WO3
0.52
0.47
0.47
0.097
0.070
0.31
0.28
0.28
0.28
0.093
0.087
0.44
0.29
2.24 0.08
5.08 0.27
5.39 0.17
2.25 0.05
4.40 0.17
<1d
0.7
2.1
1.8
0.5
1.6
—
5.13 0.11
5.79 0.25
6.13 0.21
<1d
1.5
2.1
2.0
—
Aerobic oxidation of cyclohexene under solvent-free
conditions
0.1Au9/WO3a
Table 2 summarizes the catalytic activity of Au9 and Au101
deposited on TiO2, SiO2 and WO3 at various gold loadings for
the aerobic oxidation of cyclohexene, which was conducted
under solvent-free conditions without the addition of a radical
initiator and using molecular oxygen as the only oxidant.
Cyclohexene conversion and product distribution depend
strongly on the nature of the support. TiO2-based catalysts
are inactive, showing conversions comparable to that of
the reaction in the absence of a catalyst (blank). Clusters
deposited on SiO2 show high conversions of cyclohexene,
with cyclohexenyl hydroperoxide (CyOOH) being the main
product. Other products were 2-cyclohexen-1-one (Cy-one),
2-cyclohexen-1-ol (Cy-ol) and cyclohexene oxide (Cy-oxide).
WO3-supported clusters have comparable activity, but the
main product is cyclohexene oxide.
0.1Au9/WO3
7.93 0.27
1.97 0.04
5.06 0.18
1.59 0.04
4.94 0.15
<1d
2.2
0.6
2.0
0.4
1.5
—
0.5Au101/SiO2a
0.5Au101/SiO2e
0.1Au101/SiO2a,e 0.12
0.1Au101/SiO2
0.3Au9/SiO2a
0.3Au9/SiO2b
0.3Au9/SiO2
0.1Au9/SiO2a,e
0.1Au9/SiO2
0.068
0.27
0.20
0.20
0.098
0.073
6.27 0.17
6.85 0.23
<1d
1.6
2.3
—
9.6 0.6
3.9
a
b
Recovered after the 1st catalytic cycle. Recovered after the 2nd
c
d
catalytic cycle. Recovered after the 3rd catalytic cycle. No gold
particles were detected by bright-field TEM. e Ref. 15. f s.e. – standard
error of the mean, s.d. – standard deviation of the distribution.
Unsupported gold nanoparticles have previously been
shown to have high catalytic activity in some liquid-phase
reactions;23 thus, it is important to investigate whether the
observed catalytic activity should be attributed to supported
or leached gold species. As seen from the AAS studies
(Table 1), gold does leach into solution from as-prepared
samples during the first catalytic cycle.
However, no such leaching was detected during subse-
quent recyclability tests. We suggest that as clusters agglom-
erate into bigger particles, leaching of gold species stops.
Table 2 Performance of catalysts in cyclohexene oxidationa
Selectivity, %
Catalyst
Conversion, % Cy-oxide Cy-ol Cy-one CyOOH
Blank
TiO2
SiO2
WO3
0.5Au9/TiO2
0.5Au101/TiO2
0.5Au101/WO3
0.1Au101/WO3
0.3Au9/WO3
0.1Au9/WO3
2
2
2
9
2
—
—
—
33
—
—
26
35
27
34
6
—
—
—
30
—
—
18
23
20
24
15
11
12
7
—
—
—
4
—
—
—
32
—
—
19
18
17
21
38
54
51
68
—
—
17
12
16
11
24
17
19
12
Fig. 1 Representative TEM images and particle size distributions of the
catalysts. (A) 0.5Au101/WO3 as deposited. Catalysts recovered after the
1st catalytic cycle: (B) 0.5Au101/WO3, (C) 0.3Au9/WO3, (D) 0.3Au9/SiO2.
2
50
36
50
33
b
0.5Au101/SiO2b 48
agglomerate through surface diffusion. We suggest that
agglomeration of Au9 clusters occurs similarly to the process
of crystal formation in which lower concentration of precursor
leads to the creation of smaller amount of nucleation sites
and thus to bigger crystals. Thus, at high gold loadings, the
majority of Au9 clusters rapidly sinter into stable nanoparticles
0.1Au101/SiO2
0.3Au9/SiO2b
0.1Au9/SiO2
39
43
25
7
7
5
a
Reaction conditions: cyclohexene (5 mL), n-decane (0.2 M) as an
internal standard, catalyst (0.05 g), O2 (~1 atm), 65 °C, 16 h, glass
reactor. b Ref. 15.
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This suggestion correlates with the fact that the degree of
leaching during the first cycle decreases with higher loading.
A higher surface density of gold clusters accelerates agglom-
eration and hence fewer non-sintered clusters have time to
leach into solution. Because the catalyst retains ca. 90% of its
catalytic activity during catalytic cycles 2–5 (Fig. 2), while
leaching becomes undetectable on these stages, we conclude
that the activity is very predominantly attributable to the sup-
ported, agglomerated particles and that gold species leached
during the first cycle are essentially catalytically inactive. The
methodology of hot filtering is typically used to determine
whether the catalyst is homogeneous or heterogeneous.24
In our case, the reaction did not slow down upon Au/WO3
removal using a 0.2 μm filter after 6 h, which, according to
the method, should indicate the homogeneous nature of the
catalysis. However, it is known that cyclohexenyl hydroperoxide
can catalyse the autoxidation of cyclohexene.25 Therefore, the
hot filtering test is not suitable for distinguishing between
heterogeneous and homogeneous catalysis for cyclohexene
oxidation when cyclohexenyl hydroperoxide is formed in a
sufficient amount.
To provide further evidence that leached gold species are
inactive in cyclohexene oxidation, we subjected the catalysts
to a pair of tests under reaction-like conditions in which gold
leaching would occur but cyclohexenyl hydroperoxide forma-
tion would be suppressed. In the first test, an initial reaction
was performed at 65 °C for 6 h using as-made catalyst, but
the reactor was filled with argon instead of oxygen. The solid
catalyst was then removed by hot filtration and the liquid
reaction mixture was subjected to typical reaction conditions
(65 °C or 16 h) with the reactor now filled with ca. 1 atm
oxygen. The existence of gold leachate in the reaction mixture
was confirmed using ICP-MS, but no detectable cyclohexene
conversion was observed. In the second test, cyclohexenyl
hydroperoxide formation was suppressed by addition of n-hexane.
An experiment was conducted under oxygen atmosphere
(ca. 1 atm) using the mixture of cyclohexene (50 μL) and
n-hexane (2 mL). After 6 h, the liquid phase was separated
by hot filtering and added to 5 mL of cyclohexene. When
subjected to typical reaction conditions, the mixture showed
no cyclohexene conversion over 16 h. In contrast, a solid cat-
alyst, isolated on the first stage of the second test, catalysed
the oxidation of cyclohexene in the mixture of n-hexane
(2 mL) and cyclohexene (5 mL), giving 11% conversion.
Investigation of the catalysts' bifunctionality
The similarity in morphology and size of the gold nano-
particles formed on SiO2 and WO3 supports during the cata-
lytic reaction is indicated by TEM and DR UV-vis spectroscopy.
In the latter, the bands attributed to the localized surface
plasmon resonance have similar shapes, positions and inten-
sities for both types of catalysts (Fig. 3).26–28 Interestingly,
SiO2- and WO3-based catalysts with similar gold loadings have
comparable turnover frequencies (TOFs) calculated from the
initial reaction rates (Fig. 4).
The product evolution profiles for both types of catalysts
have similar cyclohexenyl hydroperoxide accumulation stages.
Based on these data, we suggest that gold particles catalyse
cyclohexene conversion to cyclohexenyl hydroperoxide (reac-
tion I, Scheme 1), while WO3 catalyses reaction of cyclohexenyl
hydroperoxide with cyclohexene, producing cyclohexene oxide
and 2-cyclohexen-1-ol (reaction II, Scheme 1).
To support this hypothesis, we conducted a series of
experiments in which pure WO3 powder was mixed with
03Au9/SiO2 at different ratios (Fig. 5A). Interestingly, the
addition of just 2 wt% of WO3 (1 mg) changes the selectivity
of the reaction, with cyclohexene oxide becoming the main
product. With 10 wt% of WO3 (5 mg), the distribution of prod-
ucts is almost identical to that for 03Au9/WO3. It is possible,
however, that in this series of experiments, some gold species
leach from silica-based catalyst and adsorb on tungsten oxide,
thus forming a catalytic system with high selectivity towards
cyclohexene oxide.
To exclude this possibility, we performed a reaction with
the silica-based catalyst and hot filtered the reaction mixture
into a vial containing tungsten oxide (5 mg), which was then
Fig. 3 DR UV-Vis spectra of the catalysts. (a) 0.3Au9/WO3 as deposited.
Catalysts recovered after the 1st catalytic cycle: (b) 0.3Au9/SiO2,
(c) 0.3Au9/WO3 and (d) 0.5Au101/WO3.
Fig. 2 Recyclability of 0.3Au9/WO3. Conversion of cyclohexene (black)
and selectivity towards cyclohexene oxide (grey).
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Fig. 5 Effect of the co-catalyst on the selectivity in cyclohexene
oxidation. Co-catalyst: WO3 (A) or MIL-101 (B).
Fig. 4 Reaction profile of cyclohexene oxidation catalysed by 0.3Au9/WO3
(A) and 0.3Au9/SiO2 (B). Cyclohexene conversion (■); left ordinate.
Yield of cyclohexene hydroperoxide (◊), cyclohexene oxide (○),
2-cyclohexen-1-one (×), 2-cyclohexen-1-ol (△); right ordinate.
formation of peroxo complexes of tungsten which are highly
active and selective in the epoxidation of olefins.31
We also found that the use of a different co-catalyst can
shift selectivity of Au/SiO2 towards allylic oxidation prod-
ucts. For such a co-catalyst, we have chosen the metal–
organic framework MIL-101, which was recently reported as
a catalyst for allylic oxidation of cyclohexene with molecular
oxygen, with 2-cyclohexen-1-one being the main product.32,33
Results of catalytic testing of the mixture consisting of pure
MOF and 0.3Au9/SiO2 in different ratios show the effect of
altering the selectivity of reaction similar to the one found
for WO3, but with 2-cyclohexen-1-one becoming the main
product (Fig. 5B). Reaction catalysed by the mixture of
MIL-101 : 0.3Au9/SiO2 (5 : 45, mg : mg) gives the maximum
2-cyclohexen-1-one yield of 16%, which is twice the yield we
have been able to achieve with the pure MOF.
Scheme 1 Proposed mechanism for cyclohexene oxidation.
Conclusions
collected by centrifugation. Inasmuch as potentially impreg-
nated WO3 was inactive in the conversion of the fresh
cyclohexene, we conclude that the observed change of selec-
tivity upon addition of WO3 to the Au/SiO2 catalyst should
be attributed to properties of pure WO3 acting as a co-catalyst
rather than to a synergistic effect between support and Au
nanoparticles. The discovered ability of WO3 to activate
cyclohexenyl hydroperoxide, promoting its reaction with
We have prepared a series of SiO2-, TiO2- and WO3-supported
gold catalysts by deposition of Au9 and Au101 clusters. Under
the conditions of solvent-free aerobic oxidation of cyclohexene,
clusters sinter into larger nanoparticles with mean diame-
ters ranging from ca. 4 to 10 nm, depending on the gold
loading. Leaching of gold species occurs during the first
catalytic cycle but becomes undetectable on further stages.
We have shown that leached gold species are inactive in
cyclohexene oxidation.
cyclohexene, is consistent with reports on activation of
29
H2O2
and alkyl hydroperoxides30 by WO3 through the
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We found that cyclohexenyl hydroperoxide, the formation
of which is catalysed by Au nanoparticles, can be converted to
other products in the presence of different heterogeneous
co-catalysts. Cyclohexene oxide is formed via reaction of cyclo-
hexenyl hydroperoxide with cyclohexene catalysed by WO3,
either present as a support or introduced to the reaction as a
co-catalyst physically admixed with silica-supported gold catalyst.
Selectivity towards formation of 2-cyclohexen-1-one is shifted
by using MIL-101 as a co-catalyst for gold supported on SiO2.
We have shown that careful choice of support or
co-catalyst for supported gold nanoparticles can tune the
selectivity of cyclohexene oxidation towards cyclohexene oxide
or 2-cyclohexen-1-one under solvent-free conditions without
addition of a radical initiator and using oxygen as the only
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Acknowledgements
We would like to thank Campbell McNicoll and Tim Kemmitt
from Callaghan Innovation NZ for their help with PXRD and
surface area measurements. This work was supported by the
University of Canterbury and the MacDiarmid Institute. B. D.
thanks the MacDiarmid Institute for the research scholarship
and D. O. thanks the University of Canterbury for UC Doctoral
scholarship.
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