Nanosized gold-catalyzed selective oxidation of alkyl-substituted benzenes and n-alkanes

Article abstract of DOI:10.1016/j.apcata.2012.05.029

We report the synthesis of nanoporous silica-supported gold nanoparticle catalysts and their selective and efficient catalytic properties toward oxidation reactions of various substituted alkylbenzenes and linear alkanes. The Au nanoparticles were synthesized by reducing Au(III) ions in situ within the nanopores of hemiaminal-functionalized mesoporous silica by using the supported hemiaminal groups as reducing agents. The resulting mesoporous silica-supported gold nanoparticles efficiently catalyzed the oxidation reactions of different alkyl-substituted benzenes and linear alkanes with t-butyl hydroperoxide (TBHP) as an oxidant. The catalytic reactions gave up to ～99% reactant conversion and up to ～100% selectivity toward ketone products in some cases. This high selectivity toward ketone products by the catalysts was unprecedented, especially considering the fact that only mild reaction conditions and no additives were employed during the reactions.

Full text of DOI:10.1016/j.apcata.2012.05.029

Applied Catalysis A: General 435–436 (2012) 19–26
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Nanosized gold-catalyzed selective oxidation of alkyl-substituted benzenes and
n-alkanes
Ankush V. Biradara,b, Tewodros Asefaa,b,∗
a Department of Chemistry and Chemical Biology, Rutgers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854, USA
b Department of Chemical and Biochemical Engineering, Rutgers, The State University of New Jersey, 98 Brett Road, Piscataway, NJ 08854, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We report the synthesis of nanoporous silica-supported gold nanoparticle catalysts and their selective and
efficient catalytic properties toward oxidation reactions of various substituted alkylbenzenes and linear
alkanes. The Au nanoparticles were synthesized by reducing Au(III) ions in situ within the nanopores
of hemiaminal-functionalized mesoporous silica by using the supported hemiaminal groups as reducing
agents. The resulting mesoporous silica-supported gold nanoparticles efficiently catalyzed the oxidation
reactions of different alkyl-substituted benzenes and linear alkanes with t-butyl hydroperoxide (TBHP)
as an oxidant. The catalytic reactions gave up to ∼99% reactant conversion and up to ∼100% selectivity
toward ketone products in some cases. This high selectivity toward ketone products by the catalysts was
unprecedented, especially considering the fact that only mild reaction conditions and no additives were
employed during the reactions.
Received 3 March 2012
Received in revised form 15 May 2012
Accepted 20 May 2012
Available online 26 May 2012
Keywords:
Au catalysis
Oxidation
Nanocatalyst
Mesoporous catalyst
Au nanoparticles
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
(chalcones) [6], and hydrazones [7]. Despite its wide range of uses,
current industrial practice of ethylbenzene oxidation unfortunately
Oxidative catalysis is an important route for the synthesis of
many commodity chemicals as well as perfumes, drugs and phar-
maceuticals [1]. In particular, alkane oxidation allows the synthesis
of a number of value added chemicals from relatively more abun-
dant starting materials. However, alkane oxidation still remains
to be one of the most difficult reactions to perform [2] because it
requires harsh reaction conditions and corrosive chemical reagents
such as potassium permanganate, potassium dichromate or ammo-
nium cerium nitrate [3,4]. Thus, the developmentof catalysts that
can efficiently catalyze the oxidation of alkanes through activation
of C H bonds, that can operate under mild reaction conditions and
that can generate selectively the desired products currently remain
to be among major research efforts in catalysis.
Among the various substrates used in alkane oxidation reac-
tions, linear and phenyl-substituted alkanes such as ethylbenzene
stand out because their oxidation products are essential precur-
sors for a broad range of pharmaceuticals and synthetic materials.
For instance, the oxidation products of ethylebenzene such as
acetophenone and 1-phenylethanol serve as precursors in the
synthesis of optically active alcohols [5], benzalacetophenones
involves high temperature thermal autoxidation in the absence of
catalyst. Furthermore, to date only very few catalysts have been
explored for oxidation of ethylbenzene, many of which are found
to be inefficient catalysts for the reaction [8–12]. For instance,
cobalt(II) oxide-immobilized mesoporous silica (Co/SBA-15) was
reported to catalyze the oxidation of ethylbenzene; however, the
catalytic reaction was shown to work only at relatively high tem-
peratures (120–150 ◦C) and give only moderate % conversion of
ethylbenzene, with the highest reported value being 70.1% in 9 h
at 150 ◦C. Furthermore, the catalyst was reported to form mixed
uncontrolled oxidation products such as 1-phenylethyl hydroper-
oxide, benzoic acid, acetophenone and 1-phenylethanol [8].
The first demonstration on the use of Au (noble metal) nanopar-
ticles as a heterogeneous catalyst for gas phase oxidation reactions
was a fascinating research development in the field of oxidation
catalysis about two decades ago [9]. Since then, many other papers
have also appeared on the use of Au nanoparticles as catalysts for
various oxidation reactions [10–24]. For instance, graphite-, SiO2-,
or TiO2-supported Au nanoparticles syntesized by a method called
deposition-precipitation were reported to catalyze the epoxida-
tion reaction of alkenes [18]. The deposition–precipitation method
involved a step-wise process of deposition of Au(III) ions onto the
graphite, SiO2, or TiO2 support materials, followed by the reduc-
tion of the Au(III) ions into Au(0) [18]. More recently, a nanoporous
Au catalyst prepared by etching away Ag from an AuAg alloy was
shown to efficiently catalyze the oxidation of methanol to methyl
∗
Corresponding author at: Department of Chemistry and Chemical Biology, Rut-
gers, The State University of New Jersey, 610 Taylor Road, Piscataway, NJ 08854,
USA. Tel.: +1 848 445 2970; fax: +1 732 445 5312.
E-mail address: tasefa@rci.rutgers.edu (T. Asefa).
0926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.05.029

20
A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
formate at low temperatures [19]. The material’s efficient catalytic
activity was attributed to the effective dissociation of O2 over the
nanoporous gold surface. In another work, the immobilization of Au
nanoparticles within magnetic nanomaterials was demonstrated
to produce easily separable supported Au nanocatalysts for alcohol
oxidation reactions [20]. In addition, in a very recent report by Kid-
wai et al. [21], Au nanoparticles were successfully used as catalysts
for the oxidative transformation of secondary alcohols to ketones.
In another most recent paper published by Hu et al. [22], the syn-
thesis of thin gold nanowires (GNWs) and their use for oxidation
of alkenes and alkenesat 1 atm O2 were reported [22]. Although
the GNWs showed excellent selective catalytic activity to ketone
products, they gave relatively poor % conversions. Au nanoparti-
cles on titania (Au/TiO2) were also shown to catalyze ethylbenzene
oxidation into the corresponding ketone under 1 atm O2 at 90 ◦C;
however, the reaction gave only 26% conversion with 85% selectiv-
ity of ketone [23]. Most recently, the synthesis of gold nanoparticles
on pyrrolidinone-modified SBA-15 and their use for oxidation of
cyclohexene and styrene in molecular O2 was reported; however,
the % conversion of the reactants was low, although the selectivity
of the catalytic reaction was good in some case [24].
the external surface of the mesostructured material with methyl
groups, 4.0 g of this as-synthesized SBA-15 was suspended in a
solution containing 30 mL of hexamethyldisilazane (HMDS) and
300 mL of toluene. The solution was then mildly stirred for 8 h at
room temperature. This allowed the functionalization of the exter-
nal Si OH groups of the as-synthesized SBA-15 with-SiMe3 groups.
These external SiMe3 groups, in turn, prevented possible growth of
bigger Au nanoparticles on the outer surface of the SBA-15 material
during the reduction–deposition of Au(III) ions in subsequent steps
(see below). The solid sample was recovered by filtration, washed
with toluene and ethanol (2× 10 mL in each case), and then let to dry
under ambient condition. The sample was then calcined in tube fur-
nace at 350 ◦C for 5 h under a flow of air to remove the Pluronic 123
templates from the mesopores without affecting the grafted SiMe3
groups, as reported before [29,30]. The resulting mesoporous SBA-
15, with SiMe3 groups on its external surfaces and with free silanol
groups in its mesopores, was denoted as Me-SBA-15.
2.3. Synthesis of hemiaminal-functionalized SBA-15
(Hemiaminal-SBA-15)
Here we report the synthesis and efficient catalytic activity of
mesoporous silica-supported nanosized Au particles to the selec-
tive oxidation of alkyl-substituted benzenes and n-alkanes to
ketone products using TBHP as an oxidant. Ketones are versatile
functional groups in organic chemistry and key intermediates for
a number of products [6,7]. Thus their selective synthesis in high
yields from oxidation of alkanes would be tremendously important
in various chemical processes. Furthermore, most of the previous
reports on oxidative catalysis by Au nanoparticles have focused on
alkene and alcohol substrates, which are relatively easier to oxidize
than alkanes [19–21,24]. In few cases, where alkanes were used as
substrates, especially with molecular oxygen, which is a better oxi-
dant than TBHP [23,25–27], the Au nanoparticle catalyzed reactions
gave less selective products [26] or less yields [23,25–27] compared
to the ones obtained in our case here.
The Me-SBA-15 synthesized above was dried in an oven for 12 h
at 80 ◦C before being grafted with amine groups. The well-dried
Me-SBA-15 (1.5 g) was stirred in a solution of 4.5 mL of APTS in
120 mL of toluene for 6 h at 80 ◦C to graft its mesoporous channel
walls with primary amine groups. The solution was filtered, and
the resulting solid material was washed with anhydrous ethanol
(2× 10 mL) and left to dry under ambient condition. This pro-
duced amine-functionalized mesoporous silica (or NH2-SBA-15).
The NH2-SBA-15 (1 g) material was then stirred in a mixture of
20 mL of ethanol and 10 mL of 37.2% formaldehyde solution at 40 ◦C
for 1 h. This produced a white colored, hemiaminal-functionalized
mesoporous silica sample, denoted here as Hemiaminal-SBA-15.
2.4. In situ synthesis of Au nanoparticles within the pores of
SBA-15 (Au/SBA-15)
2. Experimental
2.1. Materials and reagents
For the in situ synthesis of Au nanoparticles within the meso-
porous silica material, 50 mg of Hemiaminal-SBA-15 was dispersed
in 10 mL of three different concentrations (0.01, 0.1 and 1.0 mM) of
HAuCl4 solutions that were prepared in ethanol/water (1:4 ratio).
After stirring for 30 min at 80 ◦C, the solutions were filtered. The
solid materials were washed with 20 mL water and then 10 mL
ethanol, and let to dry under ambient conditions. This produced
Au/SBA-15 samples, labeled as A, B, and C, respectively. For compar-
ison purposes, reference Au/SBA-15’ materials from SBA-15, whose
external surface was not capped with SiMe3 groups, were also pre-
pared (see Supporting Materials for details). These materials were
labeled as A’, B’, and C’, respectively.
Toluene, tetraethoxysilane (TEOS), 3-aminopropyltriethox-
ysilane (APTS, 99%), formaldehyde (37.2%), hexamethyldisilazane
(HMDS), ethylbenzene, diphenylmethane, 1,3-diethylbenzene,
propylbenzene, n-hexane, acetonitrile, methanol, tetrahydrofuran
(THF), ethyl acetate, t-butyl hydroperoxide (TBHP) and hydrogen
peroxide (H2O2) (28%) were purchased from Sigma–Aldrich, and
they were all used as received without further purification. Anhy-
drous ethanol and hydrochloric acid were obtained from Fisher
Scientific. HAuCl4 was purchased from Strem Chemicals. Pluronics-
123 ((PEO)20(PPO)70(PEO)20) was obtained from BASF.
2.2. Synthesis of SBA-15 and Me-SBA-15
2.5. Catalytic oxidation reaction
SBA-15 was synthesized by following the original procedure
The catalytic oxidation reactions were carried out in a 50 mL
three-neck round-bottom flask. In a typical oxidation reaction, an
alkane substrate (1 mmol), Au/SBA-15 catalyst (15 mg), TBHP or
H2O2 oxidant (2 mmol), and chlorobenzene (0.5 mL) as an internal
standard were mixed with a solvent (10 mL). Acetonitrile, tetrahy-
drofuran, ethyl acetate or toluene were used as the solvents for the
reaction. The reaction mixture was stirred with a magnetic stirrer,
and samples were withdrawn from it in different intervals of time
using syringe filters and analyzed by gas chromatography (GC) and
gas chromatography–mass spectrometry (GC–MS).
reported by Stucky and co-workers with
a minor modifi-
cation [28,29]. In typical synthesis, 12 g of Pluronic 123
a
((PEO)20(PPO)70(PEO)20), 313 g Millipore water and 72 g HCl
(∼36 wt %) were mixed and stirred vigorously at 40 ◦C until all
the Pluronic 123 was dissolved. Into the solution, 25.6 g of TEOS
was added and stirred at 45 ◦C for 24 h. The solution was then kept
under static condition at 80 ◦C in oven for 24 h to age. The resulting
reaction mixture was filtered, and the precipitate was washed with
copious amount of water and dried under ambient condition. This
produced as-synthesized SBA-15 mesostructured material. To graft

A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
21
Fig. 1. Transmission electron microscope (TEM) images and Au nanoparticle size distribution of Au/SBA-15 catalysts (a) A, (b) B and (c) C, which were prepared with 0.01,
0.1 and 1.0 mM, respectively, of HAuCl4 solutions.
2.6. Characterization
3. Results and discussion
The BET gas adsorption–desorption measurements were carried
out on Micromeritics Tristar 3000 volumetric adsorption analyzer
after degassing the samples at 160 ◦C for 12 h. The TEM images
were acquired by using a FEI Tecnai T-12 S/TEM transmission elec-
tron microscope that was working at 120 keV. The samples for TEM
were prepared by sonicating the mesoporous samples in ethanol for
3 min, casting a drop of the solutions on a formvar-carbon coated
copper grid and letting them to dry under ambient conditions. The
reaction mixtures periodically collected from the catalytic reac-
tions were analyzed by GC (Agilent HP 6850 equipped with a flame
ionization detector and a HP-1, MS 30 m × 0.200 mm × 0.25 ␮m
capillary column). GC–MS was conducted with HP-5971 that has
an HP-5 MS 50 m × 0.200 mm × 0.33 ␮m capillary column in it.
3.1. Synthesis and characterization
Fig. 1 displays transmission electron microscope (TEM) images
of mesoporous silica-supported Au nanoparticles. The nanoparti-
cles were synthesized by reducing Au(III) ions with hemiaminal
groups that were tethered onto the mesopore channel surface of
SBA-15 mesoporous silica [29]. It is worth noting that before extrac-
tion of the Pluronic 123 templates, the outer surface of the SBA-15
mesostructured silica must have been functionalized with methyl
(
SiMe3) groups using hexamethyldisilazane (HMDS). This pro-
cedure produced mesostructured SBA-15 silica containing SiMe3
groups on the outer surface of the material. The Pluronic 123 tem-
plates were then removed by heating the material at moderate

22
A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
temperature of 350 ◦C. This resulted in a mesoporous material with
reactive silanol (Si OH) groups within the inner channel pores
and SiMe3 groups on the outer surface (Supporting Fig. S1) [28,29].
The presence of the SiMe3 groups (or conversely the absence
of silanol groups) on the outer surface of the SBA-15 particles
(or Hemiaminal-SBA-15) inhibited the uncontrollable growth of
Au nanoparticles on the outer surface of the SBA-15 particles. In
other words, the Au nanoparticles were able to grow only within
the channel pores of the Hemiaminal-SBA-15 material. Without
the SiMe3 groups, bigger Au nanoparticles on the outer surface of
the SBA-15 material formed, as shown in the TEM images in Fig. S2.
Thus, upon addition of Au(III) solution into the Hemiaminal-
SBA-15 material, large numbers of reasonably monodisperse Au
nanoparticles within the channel pores of the material were formed
from the reaction between the hemiaminal groups and the Au(III)
ions. The resulting samples were generally labeled as Au/SBA-15.
Three different samples were synthesized by mixing three different
concentrations, i.e., 0.01, 0.1 and 1.0 mM, of HAuCl4 with 50 mg
of Hemiaminal-SBA-15 for 30 min at 80 ◦C. The three Au/SBA-15
materials had different sized Au nanoparticles in them and they
were labeled as A, B and C, respectively (see section 2 for details).
The Au/SBA-15 samples and their parent materials, including
Me-SBA-15, NH2-SBA-15 and Hemiaminal-SBA-15, were charac-
terized by various spectroscopic and analytical methods. The N2
gas adsorption measurements showed a type-IV isotherm for all
the samples, indicating the presence of mesoporous structures in
all of the Au/SBA-15 samples as well as their parent materials
(Supporting Information, Fig. S3). For comparison purposes, the
N2 gas adsorption/desorption isotherms of the reference Au/SBA-
15’ materials prepared from SBA-15, whose external surface was
not capped with SiMe3 groups, were also included in Fig. S4. The
Brunauer–Emmett–Teller (BET) surface areas of Me-SBA-15, NH2-
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
-0.1
AuSBA-15 (A)
AuSBA-15 (B)
AuSBA-15 (C)
300
400
500
600
700
800
Wavelength (nm)
Fig. 2. Diffuse UV–Vis spectra of Au/SBA-15 samples A, B, and C. The spectra shows
the characteristic plasmon bands of Au nanoparticles in the ranges of 521 to 525 nm.
revealed that there were ∼0.97 mmol hemiaminal groups per gram
of Hemiaminal-SBA-15 sample.
Further analysis of the Au/SBA-15 samples by TEM (Fig. 1)
showed the presence of reasonably monodisperse Au nanoparticles
with average particle sizes of 5.4 ( 1.2), 6.9 ( 1.7) and 8.4 ( 2.3)
nm in samples A, B and C, respectively. More importantly, these dif-
ferent sized Au nanoparticles allowed us to investigate the effect
of size of Au nanoparticles on the catalytic activities of Au/SBA-
15 in alkane oxidation reactions (see below). The TEM images also
exhibited the presence of well-ordered mesoporous channels in the
Au/SBA-15 materials. Similarly, the TEM images of the parent mate-
rials (for instance, Me-SBA-15) also showed highly well-ordered
mesoporous structures in the material (Fig. S6).
SBA-15 and Hemiaminal-SBA-15 were 829, 449 and 348 m2g−1
,
The formation of Au nanoparticles in the Au/SBA-15 mate-
rials was further confirmed by diffuse UV–Vis spectroscopy
(Fig. 2). The characteristic plasmon bands corresponding
to Au nanoparticles were observed at ∼521 nm for sam-
respectively. This clearly indicates that there is a decrease in surface
area as more organic groups are immobilized within the pores of the
materials, as expected. The BET surface areas of the Au/SBA-15 sam-
ples A, B and C were 388, 381, and 373 m2g−1, respectively. The pore
diameter of SBA-15 material, before deposition of Au nanoparticles
in it, had monodisperse pore sizes with average BJH pore diame-
ter of ∼8.0 nm. The mesoporous samples obtained after deposition
of Au nanoparticles also had reasonably monodisperse mesopores
(Fig. S3); however, the average pore sizes decreased slightly to aver-
age BJH pore diameters around 6.0 nm, presumably because the
pores were filled with the Au nanoparticles.
ples
A
and
B
and at ∼525 nm for sample C. The slightly
more blue-shifted UV–Vis absorption maxima of samples
A
and B compared to that of C suggests that the size of Au nanopar-
ticles in the former were slightly smaller than that in the latter,
which is in agreement with the TEM results. The spectra did not
reveal any major absorption bands between 200 and 300 nm,
indicating the absence of any residual, possibly unreduced Au+3
ions or Au+ ions in the materials.
The thermogravimetric analysis (TGA) for the mesoporous sam-
ples Me-SBA-15, NH2-SBA-15 and Hemiaminal-SBA-15 showed
a weight loss below 100 ◦C, which was attributed to the loss
of physisorbed water (Fig. S5). The TGA traces showed weight
losses of 3.4, 7.0, and 9.9% in the temperature range of 100–550 ◦C
for samples Me-SBA-15, NH2-SBA-15, and Hemiaminal-SBA-15,
respectively. These weight reductions in the temperature range of
100–550 ◦C were mainly due to the loss of methyl, organoamine,
and/or hemiaminal groups. Furthermore, the weight loss in the
temperature range of 100–550 ◦C from NH2-SBA-15 was more than
twice that from Me-SBA-15. This clearly suggests the presence of
more organic groups in the former due to the presence of not
only SiMe3 groups but also 3-aminopropyl groups in it. Simi-
larly, significantly higher weight loss in the temperature range of
100–550 ◦C was obtained from Hemiaminal-SBA-15 compared to
that from NH2-SBA-15, which should be due to the presence of the
bulkier hemiaminal groups in the former. The presence of all the
different organic groups were further confirmed by elemental anal-
yses as well as by 13C CP MAS NMR spectra shown in Fig. S1. The
results were also consistent with those reported previously by us
[29] and by Sun et al. [30]. Careful calculations of the differences
in weight losses seen in the TGA traces for the different samples
In addition, powder X-ray diffraction (XRD) was used to char-
acterize the Au nanoparticles in Au/SBA-15 samples (Fig. S7). The
XRD patterns of all the Au/SBA-15 samples showed Bragg reflec-
tions at 2ꢀ values of 38◦, 44◦, 64◦ and 77◦. These Bragg reflections
were indexed as the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) diffracting
planes, respectively, of metallic Au [31]. Careful inspection of the
full-width-at-half-maxima (FWHM) of the Bragg reflections at 2ꢀ
of 38◦ on the XRD patterns indicated that the peaks were slightly
broader for A compared to those of B and C. This suggests that the Au
nanoparticles in A were slightly smaller in size than those in B and
C, which is also consistent with the results obtained by TEM and
UV–Vis analyses. Their low angle XRD patterns also showed that
the materials had highly ordered mesostructures with d-spacing
values between 11.6 and11.9 nm (Fig. S8).
The presence of Au in the samples was further corroborated by
ICP-AES, which showed the presence of 1.08, 3.86 and 4.56 wt%
Au in samples A, B and C, respectively. This corresponds to 55,
196, and 232 ␮mol Au per gram of Au/SBA-15, respectively. This
clearly shows that the mol % of Au increases in the order of A < B < C,
which is consistent with the amount of Au(III) used in the synthe-
ses. However, the difference in wt% of Au between samples B and
C was relatively smaller (i.e., 3.86–1.08 wt% Au = 2.78 wt% Au) than

A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
23
O
TBHP as oxidant are necessary for the oxidation reaction of ethyl-
benzene to proceed. It is worth noting, however, that TBHP itself
can undergo some Au nanoparticle-catalyzed decomposition into t-
BuOH. In fact, in our control experiment, in which only Au/SBA-15
catalyst B and TBHP were present, 11% of the TBHP was decom-
posed into t-BuOH. Thus, two equivalent of TBHP was used in
our catalytic reactions in order to ensure that enough TBHP was
present in the reaction mixture or to make up for any possibly
decomposed TBHP.
OH
Au/SBA15
Oxidant
1
2
Scheme 1. Au/SBA-15 catalyzed oxidation of ethylbenzene.
that between A and B (4.56–3.86 wt% Au = 0.7 wt% Au), although
the corresponding ratio in the mol of Au(III) used for the synthe-
sis of the Au nanoparticles was the same. These discrepancy was
most likely because the large fraction of Au(III) ions remained unre-
acted in case of C due to the limited number of hemiaminals (or
the reducing agents) present in the Hemiaminal-SBA-15 sample
and the higher amount of Au(III) ions used for the synthesis of this
particular sample. Indeed, the filtrate collected from the synthe-
sis of sample C contained much more unreacted Au(III) ions when
analyzed by ICP-AES than those obtained from samples A and B.
3.3. Effect of the size of gold nanoparticle and the type of solvents
on catalytic efficiency
Because TBHP served successfully as an oxidant for ethylben-
zene oxidation in the presence of our Au/SBA-15 catalyst, it was
used as an oxidant in the additional experiments we performed to
determine the effects of nanoparticle sizes on the catalytic activities
of Au/SBA-15. In the presence of two equivalent of TBHP, cata-
lyst A, which contained 5.4 1.2 nm Au nanoparticles, gave 68%
conversion of ethylbenzene and high selectivity (94%) toward ace-
tophenone product, with a minor (11%) 1-phenylethanol byproduct
(Table 1 and Fig. S9). Au/SBA-15 catalysts B and C, which con-
sisted of 6.9 1.7 and 8.4 2.3 Au nanoparticles, respectively,
generated 79 and 89% conversions of ethylbenzene, with 93 and
94% selectivities, respectively, toward acetophenone product. The
remaining 6–7% byproduct in both cases was the secondary alcohol
1-phenylethanol.
Based on these results, one could conclude that catalyst C had
a better catalytic activity and selectivity toward a ketone prod-
uct than catalysts A and B, and catalyst B, in turn, had better
catalytic efficiency than catalyst A. However, when comparing
the results based catalytic turn-over-numbers (TONs) and turn-
over-frequencies (TOFs) (Table 1), an opposite trend in catalytic
efficiencies could be observed. Catalyst A gave the highest TON and
TOF (764 and 23 h−1) than catalyst B (274 and 8 h−1) and catalyst
C (256 and 7 h−1). This suggested that among the three different
Au/SBA-15 catalysts we studied for oxidation of ethylbenzene, the
catalytic efficiency of A was actually greater than that of B or C when
they were compared based on their catalytic activities per total
mol of Au. However, because not all the supported Au nanoparti-
cles such as those in the middle of the mesopores and not all the
Au atoms in the nanoparticles are exposed well enough to partici-
pate in the catalytic reactions, the reported catalytic TONs and TOFs
per total mol of Au might also be underestimated TON and TOF or
they do not precisely reflect the relative catalytic efficiency of the
materials. However, for lack of other better methods, such results
and calculations are often used to compare catalytic activities of
different materials [25,33–35], as we have done here.
3.2. Catalytic propreties
The catalytic activities of the synthesized Au/SBA-15 materials
A, B, and C were then investigated in alkane oxidation reac-
tions (Scheme 1) under similar conditions. The substrates included
ethylbenzene, other alkyl-substituted benzenes, n-hexane and
n-hexadecane. Ethylbenzene was chosen as a model substrate
because its oxidation products are important precursors for a
number of useful products [5–7]. Other alkyl-substituted ben-
zenes as well as n-hexane and n-hexadecane were used as
substrates in order to demonstrate the versatility of the cata-
lysts and the scope of the catalytic reaction, and also to evaluate
the relative catalytic activity/selectivity of our Au/SBA-15 cata-
lysts with respect to other similar catalysts that were reported
previously [26].
As discussed above, Au/SBA-15 containing different sized Au
nanoparticles (or the samples labeled here as A, B and C) were
formed from different concentrations of HAuCl4 solution. By using
the three different Au/SBA-15 samples as catalysts, the effect
of the size of the Au nanoparticles on the catalytic activities
of Au/SBA-15 in oxidation of alkanes, specifically alkylbenzene,
in the presence of chlorobenzene as an internal standard was
investigated [32]. Furthermore, (attempted) catalytic oxidation of
ethylbenzene by Au/SBA-15 was investigated by using air (O2),
H2O2 and TBHP as oxidant (Table S1). Attempted oxidation of
ethylbenzene with Au/SBA-15 catalyst B in air or with oxygen
bubbled into the reaction mixture, even at 300 Psig pressure
of air in a Parr reactor, at 70 or 110 ◦C gave no oxidation
product. When H2O2 was used as an oxidant with the cat-
alyst, again no oxidation of ethylbenzene took place. Finally,
when TBHP was used as an oxidant along with Au/SBA-
15, ethylbenzene underwent oxidation efficiently. However,
TBHP did not result in reaction or oxidation product for
36 h in the presence of the control sample SBA-15 (which
contained no Au nanoparticles). Thus, the Au/SBA-15 as catalyst and
In any case, the results in Table 1 clearly suggest that the
catalytic activities of Au nanoparticles of Au/SBA-15 catalysts in
ethylbenzene oxidation could vary with the size of the nanopar-
ticles. This is also consistent with previous reports for other
reactions involving oxidation of various organic substrates, which
also show that the catalytic reactions depend on the size of Au
Table 1
Au/SBA-15-catalyzed oxidation of ethylbenzene. Three Au/SBA-15 catalysts A, B, and C having different size Au nanoparticles were used as catalysts.a
Entry
Catalyst/Sample (Au average size)
Wt. % (mmol Au/g)
% Conversion
% Selectivity
TON
TOF (h−1
)
1
2
1
2
3
4
SBA-15
–
∼0
68
79
89
∼0
94
93
94
∼0
6
7
∼0
764
274
256
∼0
Au/SBA-15 catalyst A (5.4 1.2 nm)
Au/SBA-15 catalyst B (6.9 1.7 nm)
Au/SBA-15 catalyst C (8.4 2.3 nm)
1.08% (54.8 ␮mol/g)
3.86% (196.0 ␮mol/g)
4.56% (231.5 ␮mol/g)
23
8
6
7
a
Reaction condition: substrate, ethylbenzene, 1 mmol; oxidant: 80% TBHP (aq.), 2 mmol; solvent: acetonitrile, 10 mL; catalyst: Au/SBA-15 sample A, B or C, with 15 mg
overall mass; reaction temperature: 70 ◦C; internal standard: chlorobenzene (0.5 mL); reaction time: 36 h; and reaction atmosphere: air.

24
A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
Table 2
Table 3
Au/SBA-15 (B) catalyzed oxidation of ethylbenzene in different solvents.a
Oxidation of various alkyl-substituted benzenes catalyzed by Au/SBA-15 catalyst,
B.a
Entry
Solventb
% Conversion
Ketone
Alcohol
Entry
1
% Reactant Conversion
% Selectivity
Alcohol
1
2
3
4
Acetonitrile
THF
Ethyl acetate
Toluene
79
70
64
45
93
87
85
84
7
13
15
16
Ketone
79
93 c
88 e
7 d
a
Reaction condition: substrate, ethylbenzene, 1 mmol; oxidant: 80% TBHP (aq.),
2 mmol; solvent: different solvents, 10 mL; catalyst (Au/SBA-15, B), 15 mg; internal
standard: chlorobenzene, 0.5 mL; reaction temperature: 70 ◦C; reaction time: 36 h;
and reaction atmosphere: air.
2
3
4
76
99
12 f
0
b
All the solvents (except toluene at 110 ◦C) remained stable at 70 ◦C in the reaction
g
mixture, and they thus were suitable as solvents for oxidation of alkyl-substituted
benzenes, n-hexane and n-hexadecane. However, toluene at 110 ◦C reaction with
catalyst B underwent 16% conversion into benzaldehyde.
∼ 10
h
75
9
5 i
nanoparticles [25,33–35]. For instance, in the work by Haider et al.,
the rate as well the selectivity of the Au nanoparticles in alcohol
oxidation reactions were reported to be affected by the size of Au
nanoparticles, with 6.9 nm-sized Au particles yielding the highest
catalytic efficiency [33]. On the other hand, in the work by Good-
man and co-workers, smaller size (3.5–4.0 nm) Au nanoparticles
were found to be the most effective catalysts in gas phase oxidation
reactions, particularly CO oxidation [34,35].
5
95 b
92 j
8 k
a
Reaction conditions: substrate, shown above, 1 mmol; oxidant: TBHP, 2 mmol;
catalyst: Au/SBA-15, B, 15 mg; solvent: acetonitrile, 10 mL; internal standard:
chlorobenzene, 0.5 mL; reaction time: 36 h; and temperature: 70 ◦C.
b
Similar condition as in (a) except the product was collected after 8 h reaction
time.
c
Acetophenone.
1-Phenylethanol.
3-Ethylacetophenone.
3-Ethyl-1-phenylethanol.
Benzophenone.
Propiophenone.
1-Phenyl-2-propanol.
2-Hexanone.
d
e
As shown in Table 2, the type of solvent used in ethylbenzene
oxidation in the presence of Au/SBA-15 catalyst B also affects both
the catalytic efficiency as well as the catalytic selectivity of the reac-
tion. The test was done by using acetonitrile, THF, ethyl acetate or
toluene as solvents in the catalytic reaction at 70 ◦C for 36 h. In the
case of acetonitrile, 79% conversion of ethylbenzene with 93% selec-
tivity toward acetophenone product was obtained (Table 2, entry
1). When THF was used as the solvent, a lower ethylbenzene con-
version of 70% and a lower selectivity of 87% toward acetophenone
product were obtained (Table 2, entry 2). In ethylacetate, the cata-
lyst showed further decrease in catalytic activity (64% conversion)
as well as selectivity (85%) toward acetophenone (Table 2, entry 3).
In the case of toluene (Table 2, entry 4), even lower catalytic activity
(45% conversion) and lower selectivity (84%) toward acetophenone
were observed. Generally, the dipolar, aprotic solvents such as THF
gave better results than the non-polar solvents such as toluene. This
decrease in the catalytic activity of Au/SBA-15 in oxidation reaction
in the order of acetonitrile > THF > ethylacetate ≥ toluene might be
because the nonpolar solvents have unfavorable polarity or dielec-
tric constant to the reaction’s intermediates [36]. Furthermore, the
reason that certain solvents work better than others seems also to
suggest that some of the solvents may undergo co-oxidation and
produce a more oxidizing agent in the reaction. It has been pre-
viously reported that solvents such as methylcyclohexene work
much better than any other solvent to form a peroxy radical, which
then epoxidzes alkene substrates more effectively under different
conditions [37]. Acetonitrile, which is one of the solvents we used
in our case and which worked well, is known to produce peroxy-
carboximidic acid—a powerful oxidation agent [38]. THF, which is
the second solvent that worked very well in our case, is also very
well known to undergo radical oxidation [37c]. Hence, the ‘good’
solvents probably facilitated the formation of radicals for the oxida-
tion of the substrate ethylbenzene. These somewhat hypothetical
but reasonable explanations for our reaction case obviously require
detailed further studies in the future to fully understand. Neverthe-
less, since among all the solvents we tried, acetonitrile resulted in
the highest % conversion of ethylbenzene while giving the highest
selectivity toward a ketone product (or acetophenone, 1), all the
other reactions for the subsequent studies (discussed below) were
performed in acetonitrile.
f
g
h
i
j
k
2-Hexanol.
used as substrates, and the results were compiled in Tables 3 and 4,
and in Fig. S9. Au/SBA-15 was found to be active, efficient and
more selective catalyst also for catalytic oxidation the series of
other alkyl-substituted benzenes listed in Table 3 as well as lin-
ear alkanes such as n-hexane and n-hexadecane (Tables 3 and 4).
For instance, Au/SBA-15 catalyst B catalyzed the oxidation of 1,3-
diethylbenzene with 80% conversion and 88% selectivity toward
3-ethylacetophenone product in 36 h. Interestingly, the Au/SBA-15
catalyzed oxidation of 1,3-diethylbenzene stopped after the oxi-
dation of only one of the substrate’s ethyl groups and gave only
4-ethylacetophenone product in 36 h (Table 3, entry 2). Au/SBA-
15 also catalyzed the oxidation of diphenylmethane with 99%
conversion, and a remarkably high selectivity of ∼100% toward
benzophenone product (Table 3, entry 3). Furthermore, Au/SBA-
15 catalyzed the oxidation of propylbenzene with 75% conversion
and 95% selectivity toward propiophenone, with 5% 1-phenyl-2-
propanol byproduct (Table 3, entry 4).
3.4. Catalytic properties on n-alkanes
When n-hexane was used as a substrate, Au/SBA-15 catalyzed
the substrate with 95% conversion in 8 h, giving 92% 2-hexanone
as a major product (Table 3, entry 5). However, when this reac-
tion was further continued for 30 h, all the 2-hexanone was further
converted into 2,4-di-hexanone product, without producing any
alcohol or acid byproducts.
To fully compare the relative catalytic activities of our Au/SBA-
15 materials with previous reports, an additional study of oxidation
n-hexadecane using Au/SBA-15 as catalyst was performed (Table 4).
The Au/SBA-15 also catalyzed the oxidation of n-hexadecane, and
more interestingly, it gave exclusively ketone products, with no
alcohol or other oxidized products. Furthermore, the catalytic
selectivity of our Au/SBA-15 catalyst toward particular ketone
products from among many other possible ones was found to be
much better compared to other catalysts. For instance, Au/SBA-
15 catalyst B gave only two or three ketone products, namely
In order to probe the scope of the catalytic reaction and the
potential of Au/SBA-15 materials as catalysts in alkane oxidation,
various other alkyl-substituted benzenes and linear alkanes were

A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
25
Table 4
Oxidation of n-hexadecane catalyzed by Au/SBA-15 (B) under various reaction conditions.a
Entry
Reaction solvent or condition
% Conversion
% Selectivity
2-hexadecanone
4-hexadecanone
3-hexadecanone
1
2
3
4
In Acetonitrile b
In Methanol b
Neat c
9
5
15
74
58
57
40
42
41
42
47
47
1
1
13
11
Neat d
a
This reaction is particularly chosen to show both the catalyst’s versatility as well as relatively high efficiency and selectivity compared to other closely related materi-
als/catalysts in similar reactions.
b
Reaction condition: substrate, n-hexadecane (1 mmol) in 10 mL solvent (acetonitrile or MeOH); oxidant: 80% TBHP (aq.), 2 mmol; catalyst: Au/SBA-15, B (15 mg); reaction
temperature: 70 ◦C; internal standard: chlorobenzene, 0.5 mL; and reaction time: 36 h.
c
Reaction condition: substrate, n-hexadecane (25 mmol), neat and with no solvent; oxidant: 80% TBHP (aq.), 2 mmol; catalyst: Au/SBA-15, B (15 mg); reaction temperature:
70 ◦C; and reaction time: 24 h.
d
Reaction condition: n-hexadecane (25 mmol), neat and with no solvent; oxidant: 80% TBHP (aq.), 50 mmol; catalyst: Au/SBA-15, B, (15 mg); reaction temperature: 150 ◦
C
in a Parr reactor; and reaction time: 6 h.
2-hexadecanone and 4-hexadecanone (sometimes with a minor
3-hexadecanone product, depending on the reaction conditions)
(Table 4), whereas similar catalysts were reported to give five dif-
ferent ketone products in Ref. [26]. The products we obtained from
the catalytic oxidation of n-hexadecane were actually similar to
those reported by Pullabhotla and Jonnalagadda [39] for catalytic
ozonation of n-hexadecane using activated charcoal or 0.5% Pd-, Ni,
and V-loaded microporous ZSM-5 catalysts. In the latter case, only
the three ketones, that is, 4-hexadecanone, 3-hexadecanone and
2-hexadecanone, were also reported. However, 4-hexadecanone
was the major product while 3-hexadecanone and 2-hexadecanone
were produced in roughly the same, but less significant, amounts.
In our case, 3-hexadecanone was sometimes not observed at all,
depending on the reaction conditions while either 4-hexadecanone
or 2-hexadecanone was formed as a major product (see Table 4).
The products were fully confirmed by GC and GC–MS, as shown in
Figs. S10 and S11.
This significant catalytic selectivity exhibited by our Au/SBA-
15 in n-hexadecane oxidation compared to a similar materials
reported in Ref. [26] might be due to three reasons: (1) The
sizes of the Au nanoparticles in our Au/SBA-15 were bigger than
those reported in Ref [26]; (2) The supported Au nanoparticles in
our Au/SBA-15 do not possess strongly bound alkanethiol ligands
around them, or they are ‘naked’; and (3) difference oxidants were
employed in the two cases (although O2 – a greener or preferrable
oxidant – was successfully used in Ref. [26], it gave a mixture of
seven different ketones and six different alcohols; however TBHP
gave much more selective products in our case, albeit being a less
‘greener’ oxidant). It is worth noting again that our attempted cat-
alytic reaction using the greener oxidant, air or O2, with Au/SBA-15
was unsuccessful (Table S1).
3.5. Proposed reaction mechanism
Finally, the plausible mechanism for the Au/SBA-15 catalyzed
oxidation reaction was proposed (Scheme 2). TBHP, which suc-
cessfully served as an oxidant in our oxidation reaction, is well
known to undergo radical chemistry [40–45]. Indeed when we
added a radical scavenger TEMPO in our reaction mixture, the
alkane oxidative reaction stopped immediately. This suggested that
the Au/SBA-15-catalyzed reaction, not surprisingly, goes through
radical intermediates or via radical chain mechanism [45]. It is
also worth mentioning again that the catalytic reaction forms t-
BuOH byproduct. In addition, the catalyst (B) in the presence of two
equivalent of TBHP produces nearly similar selectivity (93%) toward
acetophenone product, along with a minor (7%) 1-phenylethanol
byproduct (Table 1), as the one with one equivalent TBHP, although
the latter gave a lower (51%) conversion of ethylbenzene com-
pared to the former which gave 79% conversion. The fact that both
reactions, at lower and higher TBHP, gave acetophenone (1) and
Scheme 2. Proposed mechanism for Au/SBA-15 catalyzed oxidation of alkane (ethylbenzene) into a predominantly ketone and a minor alcohol products along with some
t-BuOH byproduct.

26
A.V. Biradar, T. Asefa / Applied Catalysis A: General 435–436 (2012) 19–26
1-phenylethanol (2) in about similar proportions suggests that both
products, 1 and 2, predominantly form in parallel via two different
mechanisms, rather than through the possible transformation of
one of them (2) to the other (1).
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of our Au/SBA-15 catalyzed alkane oxidation probably starts with
Au nanoparticle-catalyzed decomposition of TBHP (t-BuOOH) into
t-BuOO. or t-BuO. radical species. This will be followed by two dif-
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4. Conclusions
In conclusion, we have synthesized mesoporous silica-
supported nanosized Au particles (Au/SBA-15) and demonstrated
their use as efficient and selective catalysts for oxidation of
ethylbenzene, various other alkyl-substituted benzenes, and dif-
ferent n-alkanes. The Au/SBA-15 catalysts catalyzed the reactions
with TBHP as oxidant and gave predominantly the corresponding
ketones under the reaction conditions we employed. Further-
more, the Au/SBA-15 catalysts generated the ketone products
selectively without requiring additives such as carboxylic acids.
The Au/SBA-15 materials were also proven to be versatile selec-
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of other alkyl-substituted benzenes such as propylbenzene and
diphenylmethane, giving their corresponding ketone products in
high yields and selectivity. In additionAu/SBA-15 materials cat-
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selectivities toward their corresponding ketone products. Finally,
the possible reaction mechanisms for these Au/SBA-15-catalyzed
alkane oxidation reactions into predominantly ketone product,
with minor alcohol product, were proposed.
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TA gratefully acknowledges the financial assistance provided by
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Nos: CAREER CHE-1004218, NSF DMR-0968937, and NSF NanoEHS-
1134289, through NSF-ACIF fellowiships in 2010, and NSF Special
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