Gold-catalyzed aerobic epoxidation of trans-stilbene in methylcyclohexane. Part I: Design of a reference catalyst

Article abstract of DOI:10.1016/j.apcata.2011.11.005

The kinetics of the heterogeneous gold-catalyzed aerobic epoxidation of stilbene in the liquid phase has been shown to be hindered by diffusion limitations, due to the use of supports which are unsuitable to apolar reaction media. The choice of these supports is generally dictated by the ability of standard methods of preparation to stabilize highly dispersed gold nanoparticles on them. Hence, new methods need to be designed in order to produce catalytically active gold nanoparticles on hydrophobic supports in general and on passivated silicas in particular. By investigating Tsukuda's method to produce colloidal solutions of gold nanoparticles upon reduction of the triphenylphosphine gold chloride complex in solution, we found that direct reduction of AuPPh₃Cl in the presence of a commercially available silica support functionalized with dimethylsiloxane, Aerosil R972, leads, in a highly reproducible and potentially scalable way, to the best catalyst ever reported for this reaction.

Full text of DOI:10.1016/j.apcata.2011.11.005

Applied Catalysis A: General 415–416 (2012) 1–9
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Gold-catalyzed aerobic epoxidation of trans-stilbene in methylcyclohexane.
Part I: Design of a reference catalyst
∗
Kevin Guillois, Laurence Burel, Alain Tuel, Valérie Caps
Institut de Recherches sur la Catalyse et l‘Environnement de Lyon (IRCELYON, UMR 5256 CNRS – University of Lyon), 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
The kinetics of the heterogeneous gold-catalyzed aerobic epoxidation of stilbene in the liquid phase has
been shown to be hindered by diffusion limitations, due to the use of supports which are unsuitable to
apolar reaction media. The choice of these supports is generally dictated by the ability of standard methods
of preparation to stabilize highly dispersed gold nanoparticles on them. Hence, new methods need to be
designed in order to produce catalytically active gold nanoparticles on hydrophobic supports in general
and on passivated silicas in particular. By investigating Tsukuda’s method to produce colloidal solutions of
gold nanoparticles upon reduction of the triphenylphosphine gold chloride complex in solution, we found
that direct reduction of AuPPh3Cl in the presence of a commercially available silica support functionalized
with dimethylsiloxane, Aerosil R972, leads, in a highly reproducible and potentially scalable way, to the
best catalyst ever reported for this reaction.
Received 19 September 2011
Received in revised form 2 November 2011
Accepted 3 November 2011
Available online 12 November 2011
Keywords:
Gold
Hydrophobic
Reference catalyst
Aerobic oxidation
Alkene
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
The oxidative transformation of alkane and alkene into useful
unsupported, soluble gold colloids [17]. This indicated that gold
nanoparticles were the active sites and that the support affected
apparent kinetics essentially via mass-transfer limitations. In order
to use macro-kinetics in the liquid phase to study the mechanisms
involved, it is essential to prepare a significant amount of cata-
lyst which exhibits high affinity with the apolar solvent, e.g. using
passivated silica supports. The current synthesis leading to the
state-of-the-art catalyst actually relies on a sophisticated, multi-
step protocol based on reaction between {Au[N(SiMe ) ]} and an
oxygenated intermediates, such as alcohols and epoxides, is a key-
step in the synthesis of fine chemicals. Such reactions are typically
achieved in the presence of a metal salt under pure oxygen pres-
sure or using excess peracid, respectively. Given the hazardousness
of the processes and of the compounds involved, there was a
need for safer catalytic ways. In this respect, the development of
microporous titanium silicalite-1 (TS-1) [1,2], which can efficiently
catalyze the partial oxidation of certain alkanes and alkenes, using
hydrogen peroxide as the oxygen source [3], was an improvement.
However, the confinement of the active site in microporous cavi-
ties makes the process unsuitable to the oxidation of large substrate
molecules [4], such as stilbene for example [5–7]. Furthermore, the
process would be even greener if using air as the primary oxidant.
It recently appeared that supported gold nanoparticles (NP)
could catalyze the aerobic epoxidation of larger alkenes [8,9] and
especially the aerobic co-oxidation of stilbene and methylcyclo-
hexane [10–13]. Apparent reaction rates can be improved using
supports with non-oxygenated surfaces, such as graphite [14,15]
or a hydrophobic Au/SiO2 catalyst which exhibited enhanced
wettability in the poorly polar reaction medium [16], or using
3
2
4
aerosil silica support partially dehydroxylated upon stringent vac-
uum treatments. Passivation of the silica surface is subsequently
achieved upon degradation of the grafted complex. The main issue
with this method is the synthesis of the {Au[N(SiMe ) ]} precursor
3
2
4
which suffers from very low yields (ca. 5%), preventing any upscale
of the catalyst preparation.
To the best of our knowledge, there is, to date, no reported
method to directly prepare gold nanoparticles on hydrophobic sil-
icas. This is attributed to two factors. First, in many gold-catalyzed
reactions, the support actually takes an active part in the mecha-
nism, whether through surface hydroxyl groups or surface oxygen
ions [18–22]. Second, when carried out in the liquid phase, reac-
tions catalyzed by gold nanoparticles usually involve polar solvents
and substrates in which supports bearing oxygen functions at the
surface (oxides [23–27], activated carbons [28–31], . . .) are prop-
erly dispersed. Until the discovery of gold-catalyzed alkane/alkene
oxidations, there was thus no real incentive to prepare Au NP on
hydrophobic supports. On the contrary, oxygenated surface groups
actually proved useful in immobilizing gold. Hydroxyls have been
shown to react with gold precursors in the deposition–precipitation
∗ Corresponding author. Present address: KAUST Catalysis Center (KCC), 4700
King Abdullah University of Science and Technology, Thuwal 23955 – 6900, Saudi
Arabia. Tel.: +966 2 808 0304.
E-mail address: valerie.caps@kaust.edu.sa (V. Caps).
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.11.005

2
K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
[
18,32–35], anion-exchange [36–42], cation adsorption [43–47]
redissolution in dichloromethane (10 mL), a triphenylphosphine
−
1
and gas phase grafting (chemical vapor deposition) [48–50] meth-
ods. As mentioned above, hydroxyls were recently used to graft
the {Au[N(SiMe ) ]} precursor on a silica support which was sub-
solution in dichloromethane (5 mL/0.65 mol L ) is slowly added
and stirred at ambient temperature for 2 h. The resulting white
triphenylphosphine gold (I) chloride complex powder is isolated
after solvent evaporation, washed twice with cold methanol
(10 mL) and once with cold ethanol (5 mL) and dried for 10 h under
vacuum at room temperature. The liquid phase 31P NMR (CDCl3,
101 MHz) spectrum shows the characteristic singlet at 33.1 ppm.
3
2
4
sequently passivated upon reduction of the grafted complex and
reaction between fragments of the decomposed ligands and resid-
ual silanols. When silanols are not reactive enough, they are used
to graft more “active” functional groups, such as amines and thiols
[
51–54] which have a fine affinity with gold. On the other hand,
hydroxyls can destabilize lightly protected gold sols (e.g. with PVA
55,56]), which adsorption can benefit more from the presence
2.2. Synthesis of Au:TPP [62,66]
[
of oxygen vacancies and structural defects in reducible, crystal-
lized supports for example [13,57–60]. Thiol-protected gold sols
are much less prone to destabilization, which allows the immo-
bilization of calibered Au NP on various kinds of surfaces [61].
However, the strong interaction between Au and S makes these col-
loids unsuitable for efficient catalytic oxidation of stilbene in MCH,
AuPPh Cl (100 mg/0.2 mmol) is added to absolute ethanol
5.5 mL). Dichloromethane (<0.1 mL) is added drop by drop until
3
(
complete dissolution of the white powder. The dark red Au:TPP
−1
colloidal solution (0.0364 mol L ) is obtained by chemical reduc-
tion of AuPPh Cl in ethanol/dichloromethane via slow addition of
3
sodium borohydride (40 mg).
as shown recently [17]. On the other hand, AuPPh Cl, which can be
3
synthesized in high yields, seems an interesting precursor to pre-
pare Au NP on hydrophobic silica surfaces. It was used to prepare
Au55 [9] and more recently Au11 [62] clusters efficiently deposited
on hydroxylated silica surfaces.
In Part I of this paper, we show first that the size of
the triphenylphosphine-protected gold particles (obtained by
2
.3. Syntheses of gold catalysts
2
−1
The hydrophilic Aerosil 200 (200 m g ), hydrophobic
Aerosil R812 (treated with hexamethyldisiloxane/Me SiOSiMe ;
2
dimethyldichlorosilane; 110 m g ; 0.6–1.2 wt.% C; total weight
loss at 1000 C < 2.5%, including < 0.5% H O) fumed silica supports,
as well as titania P25 (60 m g ; 1.6 wt.% OH) were purchased
from Evonik Industries and used without further purification.
They are respectively referred to as SiO2-Aerosil 200, SiO2-Aerosil R812
SiO2-Aerosil R972 and TiO in the following.
3
3
2
−1
60 m g ;
2.0–3.0 wt.% C) and Aerosil R972 (treated with
2
−1
reduction of AuPPh Cl) can be affected by their deposition on
◦
3
2
hydroxylated surfaces and second, that 3 nm Au NP (gold disper-
sion > 40%) can be obtained on a commercially available methylated
2
−1
silica by direct chemical reduction of a mixture of AuPPh Cl com-
3
,
plex and the silica support. We show that this latter material
matches the catalytic properties of the state-of-the-art gold catalyst
for the aerobic epoxidation of stilbene under standard conditions.
It also exhibits superior catalytic performances in the aerobic oxi-
dation of cyclohexene. By leading to a state-of-the-art gold catalyst
which can be produced in a suitable quantity (a few grams), this
new, straight-forward, practical synthetic methodology provides
a reference catalyst for the methylcyclohexane-promoted aerobic
epoxidations of stilbene and cyclohexene, and potentially for other
reactions in apolar media.
2-P25
−1
2
MCM-48 (913 m g ; 3D cubic array of pores of 2.8 nm) and
2
−1
SBA-15 (863 m g ; 2D hexagonal array of pores of 7.2 nm) were
synthesized using procedures described in [54] and [67].
2
−1
TiO (250 m g ; 7–9 nm anatase crystallites) was obtained at
2
◦
1
00 C via a hydrolytic process described in [68].
Gold is then introduced onto these supports by one of the fol-
lowing protocols:
Protocol 1: cluster deposition (from Au:TPP) [62]
The Au:TPP colloidal solution (275 ␮l/0.01 mmol Au) is added to
a solution of dichloromethane/ethanol (80:20 v/v, 24 mL) contain-
2
. Experimental
ing 1 g of the support (SiO2-Aerosil 200, MCM-48, SBA-15, TiO ) in the
2
absence of light. The mixture, which was aiming at a gold loading
of 0.2 wt.% on the support, is kept under vigorous stirring at room
temperature for 5 h (15 h for Aerosil 200). The colored powder is
isolated by centrifugation, washed with water (twice 150 mL) and
Chloroauric acid (99.9%), dimethylsulfide (99+%), methanol
99.9%), ethanol (Purity 99.9%), pentane (>99%), dichloromethane
99.8%), triphenylphosphine (100%), hexane (>95%) were pur-
(
(
chased from Sigma–Aldrich and used without further purification.
◦
finally with hexane (150 mL). The powder is dried at 90 C under
◦
air, and then treated at 200 C under vacuum for 2 h, in order to
2.1. Synthesis of triphenylphosphine gold (I) chloride
remove the phosphine.
Protocol 2: direct precursor deposition (from AuPPh Cl) followed
3
All manipulations are carried out in the absence of light and
by surface reduction
air. The AuPPh Cl complex is synthesized from reduction of
AuPPh Cl (155 mg/0.32 mmol) is dissolved in absolute ethanol
3
3
chloroauric acid by dimethylsulfide, followed by ligand exchange
with triphenylphosphine (Scheme 1), as adapted from published
protocols [63–65]. An excess of dimethylsulfide (1.4 mL/20 mmol)
(60 mL), plus dichloromethane (3 mL) in order to facilitate the
dissolution of the complex. In separate flasks, 1 g of each support
(SiO2-Aerosil R812, SiO2-Aerosil R972, TiO2-P25) is mixed with 20 mL of
III
−3
−1
is slowly added to 15 mL of a yellow solution of HAu Cl4 in
ethanol and 21 mL of the AuPPh Cl solution (5.0 × 10 mol L ),
3
−1
◦
◦
methanol (0.2 mol L ) at 0 C under Ar. After 2 h at 0 C, the
white dimethylsulfide gold chloride complex precipitates out of a
colorless solution. It is washed twice with cold methanol (10 mL),
once with pentane (10 mL) and is dried under vacuum. After
in order to aim at a gold loading of 2 wt.%. The mixtures are
stirred for 1 h before a solution of sodium borohydride in ethanol
(20 mL/10 equiv. NaBH ) is added. The mixtures are subsequently
4
stirred for 15 h. The resulting colored powders are isolated by
PPh3
MeOH
°C
H C
3
S
HAuCl4
Ph P
+
H C
CH3
S
Au Cl
3
Au Cl
3
0
2
CH Cl2
Room temperature
excess
H C
3
2
hours
Scheme 1. Synthesis of triphenylphosphine gold (I) chloride.

K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
3
centrifugation and washed with a water/ethanol mixture (60:40
v/v, twice 150 mL) and once with hexane (150 mL). The powders
are dried at 90 C under air, and then treated at 200 C under
vacuum for 2 h, in order to remove the phosphine.
Protocol 3: scale-up of Protocol 2
50
4
4
3
5
0
5
◦
◦
5
g of SiO2-Aerosil R972 hydrophobic fumed silica is directly added
30
−
3
−1
)
to a solution of AuPPh Cl in ethanol (220 mL/2.3 × 10 mol L
3
2
2
5
0
and stirred for 1 h, in order to aim at a gold loading of 2 wt.%.
An excess of sodium borohydride (20 mL of a freshly prepared
−
1
0
.3 mol L solution in ethanol/10 equiv. NaBH ) is added. The mix-
15
4
◦
ture is stirred overnight (13 h) at 20 C. The dark brown powder is
then isolated by centrifugation, washed with a water/ethanol mix-
ture (60:40 v/v, twice 200 mL), then washed with hexane (150 mL)
and dried at 90 C for 10 h. Finally, it is heated at 200 C for 2 h under
vacuum.
1
0
5
0
◦
◦
0
- 0.5 0.5 - 1 1 - 1.5 1.5 - 2 2 - 2.5 2.5 - 3 3 - 3.5 3.5 - 4 4 - 4.5
Au particle diameter (nm)
Au/TiO2-WGC (Sample A #84) was purchased from the World
Gold Council [69] and used after heating in air for 2 h at 250 C.
This treatment did not affect its physico-chemical properties. Gold
◦
Fig. 1. Transmission electron micrograph (left) and gold particle size distributions
(
right) of Au:TPP.
particles have an average diameter of 3.5 ± 1.5 nm. Gold loading is
1
(
.5 wt.%. The titania support is Degussa P25, a mixture of anatase
Trans-stilbene conversion and trans-stilbene oxide yield are
80%) and rutile (20%). It is used as a reference in the catalytic
ratios of the number of mole of stilbene converted and the number
of mole of epoxide formed both over the initial number of mole of
stilbene introduced at the beginning of the reaction. Selectivity is
defined as the ratio of yield over conversion. Mass-specific epox-
idation rates are defined as the number of mole of trans-stilbene
oxide formed per gram of gold per hour. Turnover frequencies (TOF)
are defined as the number of mole of partial oxidation products
formed per mole of surface gold per hour. They are calculated from
the epoxidation rates by taking into account the gold dispersion,
which is determined from the mean particle diameter (as derived
by TEM analysis) by assuming typical cuboctahedron geometry of
the gold particle. Dispersion is e.g. 37% in the Au/TiO2-WGC reference
catalyst.
reaction.
2.4. Characterization
Gold loading is determined by inductively coupled plasma opti-
cal emission spectroscopy (HORIBA Jobin Yvon Activa ICP-OES).
Transmission electron microscopy (TEM) is carried out on a JEOL
2
010 LaB6 microscope operating at 200 kV. Size distributions are
obtained by direct observation of more than 200 gold particles
(
typically between 300 and 500) on bright field images.
X-ray photoelectron spectroscopy (XPS) experiments are car-
ried out on
monochromated Al K␣ X-rays (1486.6 eV, 150 W), a pass energy
of 20 eV, a hybrid lens mode and an indium sample holder in
a Kratos Axis Ultra DLD spectrometer using
Superiority of the optimized catalyst is confirmed in the aerobic
oxidation of cyclohexene, performed under the same conditions
as the oxidation of trans-stilbene (1 mmol substrate, 2 ␮mol Au,
−9
ultra-high vacuum (P < 10 mbar). The analyzed surface area is
7
00 ␮m × 300 ␮m. Charge neutralization is required for all sam-
2
0 mL methylcyclohexane, and 0.05 mmol TBHP initiator). The only
ples. The peaks (O 1s, Au 4f, Si 2p, S 2s, P 2s, Cl 2p, Na 1s)
are referenced to the C–(C, H) components of the C 1s band at
difference is the type of TBHP used (9.1 ␮L of a 5–6 M solution in
decane from Aldrich). Products are analyzed by GC, using a solu-
tion of cyclohexene (99% Aldrich), cyclohexene oxide (98% Fluka),
cyclohexen-1-ol (95% Aldrich), cyclohexen-1-one (98% Fluka) in
acetonitrile for external calibration.
2
84.6 eV. Shirley background subtraction and peak decomposition
using Gaussian–Lorentzian products are performed with the Vision
.2.6 Kratos processing program. A degree of asymmetry is added
to fit the Au 4f peaks.
2
2.5. Catalytic evaluation
3. Results and discussion
Catalytic evaluation is carried out in round-bottom flasks under
The triphenylphosphine-stabilized gold clusters (Au:TPP),
obtained under our conditions from chemical reduction of
conditions previously optimized for the aerobic epoxidation of
trans-stilbene. Trans-stilbene (substrate, 1 mmol), the gold cata-
lyst (Au: 2 ␮mol), methylcyclohexane (solvent, 20 mL/155 mmol)
and tert-butyl hydroperoxide (initiator, 0.05 mmol/7 ␮L of a 70%
TBHP in water Alfa Aesar solution) are stirred together at 500 rpm
AuPPh Cl by sodium borohydride in ethanol (containing small
3
amounts of dichloromethane), exhibit a distribution of particle
diameters comprised between 0.8 nm and 3 nm. It is centered at
about 1.8 nm with a standard deviation of 0.5 nm (Fig. 1). The pop-
ulation of clusters between 1.6 and 2.0 nm is the largest (40%),
the populations below 1.2 nm and above 2.3 nm representing each
about 15% of the total number of particles. The larger sizes obtained,
as compared with the main population of 0.8 nm gold clusters
◦
and held at 80 C for 72–100 h. The reaction is carried out in
air at atmospheric pressure; the mixture is simply exposed to
ambient air throughout the reaction. Products are analyzed by
gas chromatography (Shimadzu GC-2014), using an Equity TMS
◦
3
1
0 m × 0.25 mm × 0.25 ␮m column programmed from 60 C to
obtained by Tsukuba et al. from NaBH4 reduction of AuPPh Cl in
3
◦
◦
◦
80 C, an injector and FID detector set at 280 C and 200 C respec-
absolute ethanol, associated with a larger standard deviation, is
attributed to the slow addition of the reductant and the possibly
extended time of reduction. The presence of DCM could neverthe-
less also have an impact on the formation of the gold particles and
thus on the particle sizes. DCM, which was necessary in our case to
−1
tively, and He as carrier gas (26.6 mL min ) and high performance
liquid chromatography (Perkin-Elmer HPLC Series 200), using a
Spheri-5 RP-18 220 mm × 4.6 mm × 3 ␮m C18 reverse-phase col-
umn, an acetonitrile/water mobile phase mixture at a constant flow
rate of 1.0 mL min and a Series 200 UV detector set at 250 nm.
External calibration is carried out by injecting a standard solution
containing trans-stilbene (96%, Sigma–Aldrich) and trans-stilbene
oxide (99%, Aldrich) in acetonitrile.
−1
−2
−1
completely dissolve the high concentration (3.7 × 10 mol L ) of
gold complex, indeed has the ability to react with NaBH and could
4
thus modify the composition and equilibria of the Au:TPP synthetic
medium upon consumption of part of the reductant. Besides, it is

4
K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
noted that most of the few papers dealing with this synthesis also
report gold sizes of about 1.5 nm [70].
(
a) 25
The Au:TPP solution ([Au] = 3.7 × 10− mol L ), consisting
mainly of gold particles of about 200 atoms [71], is subsequently
impregnated onto Aerosil 200, MCM-48 and SBA-15 silicas using
Protocol 1. A low loading of 0.2 wt.% is intentionally aimed at in
order to limit aggregation and coalescence. This 0.2 wt.% maxi-
mum loading is reached on Aerosil 200 (Table 1) after the extended
deposition time of 15 h, instead of 5 h. This 100% deposition yield
is however achieved at the expense of the gold particle size. An
average particle diameter of 8.3 ± 3.8 nm is indeed found after
2
−1
2
1
1
0
5
0
5
0
◦
treatment at 200 C in vacuum. On the other hand, impregnation
of the high surface area MCM-48 and SBA-15 materials with the
1
.8 nm Au:TPP DCM/ethanol solution results in smaller particles of
0
-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9- 10- 11- 12- 13- 14- >15
11 12 13 14 15
1
0
about 3.8–3.9 ± 1.2 nm in diameter with loadings of 0.11–0.12 wt.%
in both cases. Furthermore, the average particle size observed just
after cluster deposition (i.e. before heat treatment) is measured
at 6.7 ± 3.5 nm in Au/SiO2 Aerosil200. This shows that growth of the
gold sol essentially occurs during adsorption onto the Aerosil silica
surface and that only minor growth occurs during the subsequent
vacuum treatment. Time of deposition is thus a critical synthesis
parameter in determining gold particle size upon adsorption of
the gold sol onto the silica surfaces, even at room temperature.
On the other hand, neither pore size, nor pore structure seems to
have a major impact on the size of the adsorbed gold particles. It
seems that the contact with hydroxylated silica surfaces destabi-
lizes the sol and that aggregation of the supported gold particles
increases with prolonged time of exposure, which can be aggra-
vated by the lower surface area of SiO2 Aerosil200 and thus higher
density of gold nanoparticles in Au/SiO2 Aerosil200 (Fig. 2). In general,
syntheses dealing with gold have to deal with the low melting point
Au particle diameter (nm)
(
b) 40
35
3
2
0
5
20
1
1
5
0
5
0
0
-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 >10
◦
of bulk gold (1064 C), hence with the low Tammann temperature
◦
Au particle diameter (nm)
of gold NP (∼130 C for 2.5 nm gold particles) [72]. Surface silanol
groups have previously been proposed to be involved in particle
mobility and growth [35].
(
c) 40
Nevertheless, “small” particle size can be achieved via cluster
deposition of Au:TPP on hydroxylated silicas by decreasing contact
time, which however negatively impacts deposition yields. Rea-
sonable 55–60% yields of deposition are still achieved on MCM-48
and SBA-15 respectively, despite the high ethanol/DCM ratio (0.25
vs. 0.20 in [62]), which is unfavorable to the sticking probability
of Au:TPP on hydroxylated silicas [62]. It is interesting that this
method does not allow loading high surface area titania with gold
nanoparticles. A deposition yield of less than 5% is obtained, indi-
cating a poor affinity between the triphenylphosphine gold clusters
and the poorly crystallized titania surface and suggesting that the
protected gold clusters interact much less with the titanol groups
than with the silanols.
The supported gold nanoparticles prepared by cluster deposi-
tion were then evaluated as catalysts in the aerobic epoxidation of
stilbene in methylcyclohexane initiated by TBHP. An appropriate
amount of catalyst is loaded in order to perform the reaction in the
presence of the same amount of gold (2 ␮mol, i.e. 0.2 mol% referring
to stilbene). Conversions (Fig. 3a) and epoxide yields (Fig. 3b) can
thus directly be compared. Au/SiO2-Aerosil200, which contains the
largest gold nanoparticles, turns out to be active in the reaction. Its
initial activity (first 10 h) in stilbene conversion and epoxide pro-
duction is even similar to that displayed by Au/SBA-15, in which
gold atoms are twice more dispersed (Table 2). However, the rate
of stilbene conversion then rapidly decreases, and while Au/SBA-
3
3
2
5
0
5
20
15
1
0
5
0
0
-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 >10
Au particle diameter (nm)
Fig. 2. Transmission electron micrograph and gold particle size distributions of
Au/SiO2-Aerosil200 (a), Au/MCM-48 (b) and Au/SBA-15 (c).
gold-catalyzed reaction but prevents the reaction to reach com-
pletion. It is possible that the larger facets favor recombination of
the bulky active radicals (RO , ROO ) proposed to be involved in
•
•
•
the reaction, including the non-selective ones (OH ) [13]. The other
1
5 leads to 100% conversion in 72 h, the conversion obtained over
factor that leads to incomplete reaction is the use of Au/MCM-48
as catalyst. Despite the 3D arrangement of the cubic porous struc-
ture, the pore openings are much smaller. This can lead to trapping
of the substrate inside the framework, which enhances thermal
and catalyzed degradation of stilbene. Selectivities to epoxide are
consistently low, estimated between 13 and 17% (Table 1). The
Au/SiO2-Aerosil200 stagnates at about 38% from 40 h onwards, yield-
ing about 30% epoxide. The selectivity of these 8 nm gold particles is
higher than that of the 3.8 nm particles on SBA-15 (Table 1). Facets,
which relative occurrence is higher in large particles (as compared
with corners and edges), thus seem to favor selectivity in this

K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
5
Table 1
Morphological and catalytic properties of gold catalysts prepared by Protocol 1.
Surface area (m2
g
−1
)
Gold loading (wt.%)
Deposition yield/deposition time
Gold particle size ± standard
tS conversion (selectivity
to epoxide) (%)
Support
deviation (nm)
24 h
72 h
Aerosil 200
MCM-48
SBA-15
TiO2
TiO2-P25 [69]
200
0.20
0.11
0.12
<90 ppm
1.5
100%/15 h
55%/5 h
60%/5 h
<5%/5 h
ca. 50% [32]
8.3 ± 3.8
3.9 ± 1.2
3.8 ± 1.2
n.d.
29 (66)
39 (13)
65 (54)
n.d.
38 (74)
60–70 (14–17)
100 (55)
n.d.
913
863
250
60
3.5 ± 1.5
40 (63)
>80
Table 2
Maximal epoxidation rates and turnover frequencies observed in the gold-catalyzed TBHP-induced aerobic epoxidation of trans-stilbene (1 mmol) in methylcyclohexane
◦
(
20 mL) at 80 C in air (1 atm).
−
1
−1
−1
TOF (h )
Catalyst
Gold dispersion (%)
Epoxidation rate (mol gAu
h
)
Au/SiO2-Aerosil 200
Au/MCM-48
Au/SBA-15
17
35
35
28
42
47
43
61
27
37
37
0.023
0.017
0.047
0.021
0.050
0.052
0.066
0.076
0.021
0.016
0.027
26
13
26
15
24
22
30
25
16
9
Au/SiO2-Aerosil R812
Au/SiO2-Aerosil R972
Au/SiO2-Aerosil R972 as dried (scale-up)
Au/SiO2-Aerosil R972 (scale-up)
Au/SiO2 [16]
Au/TiO2-P25
Au/TiO2-WGC (induction)
Au/TiO2-WGC
14
Table 3
Morphological and catalytic properties of gold catalysts prepared by Protocol 2 (unless otherwise stated).
Support
Surface area
Gold loading
(wt.%)
Deposition
yield (%)
Average gold particle
tS conversion (selectivity
to epoxide) (%)
2
−1
)
(
m
g
size ± standard deviation
(
nm)
2
4 h
72 h
>60
Aerosil R812
Aerosil R972
Aerosil R972 (scale-up, Protocol 3)
TiO2 (P25)
260
110
110
60
1.5
0.6
0.7
0.9
75
30
38
43
4.9 ± 2.3
3.1 ± 1.0
2.9 ± 1.2
5.2 ± 1.4
25 (60)
53 (74)
64 (70)
36 (56)
97 (73)
99 (79)
50 (82)
associated hectic activity pattern (Fig. 3a) further suggests irreg-
ular release of the substrate molecule from the silica network with
reaction time.
contact time used (15 h), and loadings of 0.6 and 1.5 wt.% are
obtained (Table 3). Hence both passivated silicas display similar
amount of gold per surface unit, suggesting a similar reactivity
of both dimethylsiloxane-rich and hexamethyldisiloxane-rich sur-
faces towards the gold precursor. Aerosil R812 (the silica with
a hexamethyldisiloxane-rich surface) however seems to facilitate
growth of gold particles during the process, leading to a lower activ-
ity of the gold catalyst in the TBHP-initiated aerobic epoxidation of
stilbene (Fig. 5). Nevertheless, the lower catalytic activity displayed
Other gold catalysts were prepared from AuPPh Cl by a differ-
3
ent method (Protocol 2), in which the complex solution is first
put into contact with a suspension of the support before being
chemically reduced in the presence of the support. In this case,
3
.1 nm and 4.9 nm gold particles are obtained on silica-based
Aerosil R972 and R812 respectively (Fig. 4), despite the lengthy
(
a) 100
(b) 100
Au/SiO2-Aerosil200
Au/MCM-48
Au/SiO2-Aerosil200
Au/MCM-48
80
80
60
40
20
0
Au/SBA-15
Au/SBA-15
Au/TiO2-WGC
Au/TiO2-WGC
60
40
20
0
0
20
40
60
80
0
20
40
60
80
Reacꢀon ꢀme (h)
Reacꢀon ꢀme (h)
Fig. 3. Trans-stilbene conversions (a) and epoxide yields (b) in the aerobic epoxidation of trans-stilbene in methylcyclohexane at 80 ◦C over Au/SiO2-Aerosil200 (ꢀ), Au/MCM-48
(ꢀ), Au/SBA-15 (ꢁ) and Au/TiO2-WGC (×).

6
K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
a) 25
by Au/SiO2-Aerosil R812, which is associated with a lower selectiv-
ity (Table 3), cannot exclusively be accounted for by the lower
dispersion of gold. It is possible that the larger carbon content of the
(
2
1
1
0
5
0
5
0
SiO2-Aerosil R812 support (2–3 wt.% vs. 0.6–1.2 wt.% in SiO2-Aerosil R972
somehow interferes in the reaction. Actually, the activity pattern
Fig. 5a), with a steady conversion rate which seems never to light
)
(
off and which is similar to that displayed by Au/TiO2-WGC (Table 2),
is typical of kinetics hindered by diffusion limitations. It has been
observed for example with thiol-protected gold nanoparticles [17].
It is possible that, in the case of Au/SiO2-Aerosil R812, the numerous
cumbersome, umbrella-like, surface methyl functions somehow
hinder access to the active gold particles.
0
-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 >10
On the other hand, the 3.1 nm gold particles supported on
SiO2-Aerosil R972 display an intrinsic activity similar to that of the
state-of-the-art catalyst (Table 2), which consists in 1.8 nm gold
particles dispersed on a partially dehydroxylated Aerosil (Degussa,
Au particle diameter (nm)
(b) 30
2
−1
2
00 m g ) silica surface passivated with trimethylsilane functions
16].
It is noted that Protocol 2 can also be applied to titania
2
2
1
1
5
0
5
0
5
0
[
P25. 5.2 nm gold particles (on average) are produced by this
method on this support, with a deposition yield of about 43%
(Table 3). The resulting catalyst has a similar intrinsic activity as the
reference Au/TiO catalyst in the aerobic epoxidation of stil-
2
-WGC
−
1
bene, of 16 h (Table 2). However, the conversion rates (Fig. 5)
seem to slow down much earlier than with the reference catalyst
(
Fig. 3), which is attributed to the larger gold particles in Au/TiO2.
0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 >10
The epoxide yield thus hardly reaches 40% in 60 h, instead of 60%
on the reference catalyst.
Au particle diameter (nm)
Given the satisfactory dispersion of gold (>40%) obtained on
SiO2-Aerosil R972 by Protocol 2 and the high catalytic performances
of the resulting material, the preparation method was applied to
produce 5 g of the Au/SiO2-Aerosil R972 catalyst (Protocol 3). In this
“scale-up” protocol, the silica powder support is directly added
(
c) 30
2
2
1
1
5
0
5
0
5
0
to a pure solution of Au(PPh )Cl in ethanol, instead of mixing
3
similar volumes of a dispersion of silica in ethanol and a solu-
tion of Au(PPh )Cl in ethanol. Hence, Protocol 3 is not the exact
3
reproduction of Protocol 2 but a slightly simplified version. The
−
3
−1
lower concentration of the Au(PPh )Cl solution (2.3 × 10 mol L
3
−
3
−1
instead of 5.1 × 10 mol L ) allows complete dissolution of the
gold complex; the use of DCM is thus avoided. Reduction of the
complex to gold nanoparticles on the support surface is, like in
0-1 1-2 2-3 3-4 4-5 5-6 6-7 7-8 8-9 9-10 >10
Protocol 2, ensured by NaBH . High concentration again allows
4
Au particle diameter (nm)
fast reduction, i.e. fast nucleation vs. growth. Hence, gold parti-
cles with a mean diameter of 2.7 nm (standard deviation of 1.2 nm)
Fig. 4. Transmission electron micrographs (left) and gold particle size distributions
right) of Au/SiO2-R812 (a), Au/SiO2-R972 (b) and Au/TiO2-P25 (c).
◦
(
are obtained after drying (Fig. 6a). Treatment in vacuum at 200 C
induces only a slight increase in the particle diameter (2.9 nm)
with no change in the standard deviation (Fig. 6b), indicating the
(
a)
(b) 1
00
1
00
Au/SiO2-Aerosil R812
Au/SiO2-Aerosil R972
Au/TiO2-P25
Au/SiO2-Aerosil R812
Au/SiO2-Aerosil R972
Au/TiO2-P25
80
8
6
4
2
0
0
0
0
0
60
40
20
0
0
20
40
60
80
0
20
40
60
80
Reacꢀon ꢀme (h)
Reacꢀon ꢀme (h)
Fig. 5. Trans-stilbene conversions (a) and epoxide yields (b) in the aerobic epoxidation of trans-stilbene in methylcyclohexane at 80 ◦C over Au/SiO2-R812 (ꢀ), Au/SiO2-R972
(ꢀ), Au/TiO2-P25 (ꢁ).

K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
7
◦
Fig. 6. Transmission electron micrographs (left) and gold particle size distributions (right) of Au/SiO2-Aerosil R972 before (a) and after (b) treatment at 200 C under vacuum.
further reduction of the formed nuclei (Au atoms from reduced
grafted complexes feeding the initial nuclei) rather than growth
by Ostwald ripening. This fast reduction on the hydrophobic sur-
face thus seems beneficial to get adequately small nanoparticles,
at a loading of 0.69 wt.% before heat treatment and 0.73 wt.%
after (Table 3). Furthermore, these characteristics are similar to
those displayed by the Au/SiO2-Aerosil R972 batch obtained by Pro-
tocol 2 on 1 g of silica. It is thus expected that Protocol 3 can be
further scaled-up in a very straight-forward manner to prepare
the new state-of-the-art catalyst for the aerobic epoxidation of
stilbene.
above 70% is consistent with that of the state-of-the-art catalyst
for this reaction [16] and is higher than that displayed by catalysts
with hydroxylated surfaces in particular. Hence, this “scaled-up”
Au/SiO2-Aerosil R972 catalyst is more efficient for TBHP-initiated aer-
obic epoxidation of stilbene than any other catalyst evaluated so far
in this reaction. It is noted that the silica support only marginally
contributes to the observed activity. In the absence of gold par-
ticles, the reaction carried out in the presence of SiO2-Aerosil R972
alone reaches a maximum of 22% stilbene conversion with 10%
yield of epoxide after 100 h (only 8% conversion and 4% yield in
24 h). On the other hand, the activation step (vacuum treatment at
◦
The superior activity of Au/SiO2-Aerosil R972 is indeed confirmed.
Stilbene conversion and epoxide production patterns observed on
200 C) is critical in achieving the superior catalytic performances.
The similarly high initial activity of the as-dried material (Fig. 7)
is indeed significantly reduced after a few hours of reaction. This
is attributed to the presence of S, Cl and P residues, as evidenced
◦
this “scaled-up” batch (Fig. 7) after vacuum treatment at 200 C
are similar than on the 1 g batch prepared by Protocol 2 (Fig. 5).
Its intrinsic activity is even slightly higher (Table 2). Its selectivity
1
00
100
80
60
40
20
0
80
60
40
20
0
%
0
20
40
60
80
0
20
40
60
80
Reacꢀon ꢀme (h)
Reacꢀon ꢀme (h)
Fig. 8. Cyclohexene conversion (ꢀ) and yields of cyclohexene oxide (ꢁ), cyclohexen-
1-ol (᭹) and cyclohexen-1-one (ꢀ) in the aerobic oxidation of cyclohexene over
Fig. 7. Trans-stilbene conversion (squares) and epoxide yield (triangles) in the
oxidation of trans-stilbene over Au/SiO2-Aerosil R972 (by Protocol 3) as dried (open
symbols) and after vacuum treatment at 200 C (plain symbols).
◦
Au/SiO2-Aerosil R972 (by Protocol 3, after vacuum treatment at 200 C) and over col-
◦
loidal Au–SiC8 [17] used a reference (empty symbols).

8
K. Guillois et al. / Applied Catalysis A: General 415–416 (2012) 1–9
by XPS, which is markedly reduced after the heat treatment in
vacuum at 200 C. This is consistent with the further reduction of
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[
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1
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Acknowledgements
8
Funding of K.G. PhD fellowship by the French National Research
Agency (ANR-08-JCJC-0090-01-ACTOGREEN project) is gratefully
acknowledged. This publication is based on work partly supported
by Collaborative Travel Funds, made by King Abdullah University of
Science and Technology (KAUST). The authors thank P. Mascunan
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