Gold nanoparticles supported on three-dimensional nitrogen-doped graphene: An efficient catalyst for selective aerobic oxidation of hydrocarbons under mild conditions

Article abstract of DOI:10.1002/aoc.3315

The development of efficient and selective aerobic oxidation of alkylarenes to form more functional compounds by heterogeneously catalysed routes still presents a great challenge in the fine chemical industry and is a major research topic. In this work, gold nanoparticles supported on three-dimensional nitrogen-doped graphene-based frameworks (Au NPs@3D-(N)GFs) were successfully synthesized and found to have an impressive performance as bifunctional catalysts (nitrogen dopant as base and gold nanoparticles as active site) in the controlled oxidation of alkylarenes. The catalyst was found to be a simple bench top, stable, recyclable and selective catalytic system for the aerobic oxidation of various types of alkylarenes into their corresponding ketones at room temperature under environmentally friendly conditions with good yields and high selectivity.

Full text of DOI:10.1002/aoc.3315

Full Paper
Received: 4 February 2015
Accepted: 25 February 2015
Published online in Wiley Online Library: 4 May 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3315
Gold nanoparticles supported on three-
dimensional nitrogen-doped graphene: an
efficient catalyst for selective aerobic oxidation
of hydrocarbons under mild conditions
Mojtaba Mahyari, Mohammad Sadegh Laeini, Ahmad Shaabani*
and Hanif Kazerooni
The development of efficient and selective aerobic oxidation of alkylarenes to form more functional compounds by heteroge-
neously catalysed routes still presents a great challenge in the fine chemical industry and is a major research topic. In this work, gold
nanoparticles supported on three-dimensional nitrogen-doped graphene-based frameworks (Au NPs@3D-(N)GFs) were successfully
synthesized and found to have an impressive performance as bifunctional catalysts (nitrogen dopant as base and gold nanoparti-
cles as active site) in the controlled oxidation of alkylarenes. The catalyst was found to be a simple bench top, stable, recyclable and
selective catalytic system for the aerobic oxidation of various types of alkylarenes into their corresponding ketones at room tem-
perature under environmentally friendly conditions with good yields and high selectivity. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: three-dimensional graphene; nitrogen-doped; gold nanoparticles; selective aerobic oxidation; alkylarene
high surface area and excellent chemical and physical stability are
three-dimensional graphene-based frameworks (3D-GFs). 3D-GFs
Introduction
Over the past decade, the development of many chemical transfor-
mations such as oxidation of hydrocarbons as the cheapest route
to synthesize useful molecules from natural materials has
been gaining increasing attention.[1–6] The selective oxidation of
alkylarenes (C–H oxidation), the particular binding of oxygen atoms
as an essential transformation, is vital in industrial petroleum-based
chemical processes. So far, various oxidants, such as toxic metal
oxides,[7] peroxides,[8–10] halides,[11] etc., have been employed in
numerous studies in this field.[12–15] The ideal oxidation conditions
from the viewpoint of green chemistry would be air or O2 as
oxidant, avoiding the use of toxic and/or hazardous reagents and
solvents, removing the need for any activators such as inorganic
bases and carrying out the reaction at room temperature with a
recoverable catalyst.[16–18]
The catalyst plays an indispensable role in achieving high efficacy
and selectivity in the oxidation process. Heterogeneous catalysis, a
cornerstone in the concept of ‘green chemistry’, is important for the
growth of sustainable, environmentally benign and safe chemical
processes.[19–21] The effect of the supports and active sites is very
important in the performance of these catalyst to achieve the best
results in selective aerobic oxidation.[22,23] In the past decade,
graphene has been of particular interest to researchers.[24] In fact,
the extraordinary thermal and mechanical properties, high stability
in various conditions and high specific surface area are ideal for
a support in heterogeneous catalysis. Two important issues
concerning the catalytic support that must be considered are in-
creasing the surface area and raising the catalytic performance with
easy modification of the surface. One of the important structures of
graphene that have unique properties such as low mass density,
with continuously interconnected macroporous structures have
gained a lot of attention in the last few years as a new generation
of porous carbon materials.[25–29] These materials can serve as
robust matrices for accommodating metals, metal oxides and
active polymers for various applications, especially catalytic
systems.[30–33] One of the main challenges for improvement in the
performance of catalysts in oxidation reactions, is the interaction
of oxygen as oxidizing agent with the support and substrate. To
overcome this challenge, chemical modification of the surface with
nitrogen doping[34] and transition metals is a promising method.
Since the first report 150 years ago, the heterogeneous oxidation
of organic compounds with noble metals has been much
improved. Due to its chemical inertness, gold was thought to be
an ineffective noble metal particularly in catalysis for oxidation
reactions.[22,23,35,36] Because of the precious and limited world
supply of gold metal, improving the efficiency and reducing the
cost for the practicality and commercialization of oxidation reac-
tions are essential. So, various investigations have been carried
out for increasing the catalytic efficiency through tailoring the size
and dispersion of gold nanoparticles (Au NPs) on solid supports.[37]
The interactions between gold and the support affect the size
of the encapsulated Au NPs and the dispersion of the catalyst in
*
Correspondence to: Ahmad Shaabani, Department of Chemistry, Shahid Beheshti
University, GC, PO Box 19396-4716, Tehran, Iran. E-mail: a-shaabani@sbu.ac.ir
Department of Chemistry, Shahid Beheshti University, GC, PO Box 19396-4716,
Tehran, Iran
Appl. Organometal. Chem. 2015, 29, 456–461
Copyright © 2015 John Wiley & Sons, Ltd.

Au NPs@3D-(N)GFs for aerobic oxidation of hydrocarbons
reaction media, which are very important for overcoming the prob-
lems for gold metal mentioned above and for enhancing the
performance of the catalyst.[38–42] To achieve the best results in
oxidation reactions, the best approach presented here is doping
the surface of a three-dimensional framework with nitrogen.
In continuation of our efforts to introduce new and efficient
catalysts based on graphene and other heterogeneous catalysts
for oxidation reactions,[13,32,33,43–48] and also the development of
an efficient, economical, stable, long-lived and reusable catalyst to
further improve the kinetic and thermodynamic properties of the
oxidation of alkylarenes, herein we report the successful prepara-
tion and excellent catalytic activity of Au NPs immobilized on
three-dimensional nitrogen-doped graphene-based frameworks
(Au NPs@3D-(N)GFs) as a bifunctional catalyst (N dopant as base
and Au NPs as active sites). It should be noted that this is the first
example of the synthesis and use of 3D-(N)GFs-supported Au NPs
as a novel catalyst with unique properties for the selective oxidation
of alkylarenes to the corresponding carbonyl componds. Selective
aerobic oxidation of various alkylarenes to the corresponding alde-
hyde and ketone derivatives was carried out under environmentally
friendly conditions with good yields and high selectivity in short
reaction time at room temperature (Fig. 1). Moreover, the effect of
nitrogen doping on the catalytic activity of Au NPs immobilized
on 3D-(N)GFs was investigated.
spectroscopy (XPS) was performed using a VG Multilab 2000 spec-
trometer (ThermoVG Scientific) in an ultrahigh vacuum.
Synthesis of Graphene Oxide
Graphene oxide (GO) was synthesized using the Hummer
method.[49] Graphite powder (1.00 g, 325mesh) and concentrated
H2SO4 (23 ml) were added to a 250 ml conical flask and the mixture
was stirred. Sodium nitrate (0.50 g) was added and the resulting
mixture was cooled to 0 °C. Under vigorous agitation, KMnO4
(3.00 g) was added slowly and the mixture was stirred for 1 h while
the temperature was kept below 35°C. Then, water (45 ml) was
added slowly to the reaction mixture and the solution was stirred
for 30min at 90 °C. Next, H2O2 (10 ml of a 30% solution) and deion-
ized water (140 ml) were added to the mixture. The resulting
precipitate was centrifuged and washed repeatedly with HCl
(5%, 3 × 15ml) and EtOH and dried under vacuum at 60 °C. The
GO was obtained as a brown powder.
Synthesis of 3D-(N)GFs
3D-(N)GFs were synthesized with a combined hydrothermal and
freeze-drying process.[50] Typically, the as-prepared GO was firstly
dispersed in water with ultrasonication reaching a concentration
up to 1.5mg mlÀ1. An amount of 10 ml of this aqueous dispersion
of GO (1.5mgmlÀ1) and 1.2 mmol of dicyandiamide were sonicated
for 5 min, and then the mixture was transferred to a Teflon-lined au-
toclave and hydrothermally treated at 180 °C for 12 h. Finally, the
resulting sample was freeze-dried overnight to obtain 3D-(N)GFs.
Experimental
Materials and Methods
All chemicals were purchased from Aldrich or Merck and used with-
out further purification. IR spectra were recorded with a Bomem
MB-Series FT-IR spectrophotometer. Transmission electron micros-
copy (TEM) was performed with a LEO 912AB electron microscope.
Field emission scanning electron microscopy (SEM; Hitachi, model
S-4160) was used for surface imaging. Identification and quantifica-
tion were carried out with a Varian model 3600 gas chromatograph
(Varian Iberica, Madrid, Spain) equipped with a split/splitless capil-
lary injection port and flame ionization detector. A CP-Sil-8 fused sil-
ica capillary column (25m, 0.32 mm inner diameter and 0.52 mm
film thickness) from Chrompack was employed. Elemental analyses
(carbon, hydrogen, nitrogen, sulfur) were done using a conven-
tional combustion method based on the burn off of samples, and
the gases were detected with a thermoconductivity detector.
Thermogravimetric analysis was carried out using an STA 1500
instrument at a heating rate of 10°CminÀ1 in air. An ultrasonic bath
(EUROSONIC® 4D ultrasound cleaner with a frequency of 50 kHz
and an output power of 350 W) was used to disperse materials
in solvent. Powder X-ray diffraction data were collected with an
XD-3A diffractometer using Cu Kα radiation. X-ray photoelectron
Preparation of Au NPs@3D-(N)GFs
An aqueous solution of HAuCl4 (0.2g in 3.0ml) and 3D-(N)GFs (0.2g
in 10.0 ml) were mixed and placed in an ultrasonic bath (50 kHz) for
30min to well disperse metal ions in the functionalized GO sheets.
Then 10.5ml of a 1.0M NaBH4 solution in 0.3M aqueous NaOH was
added to the mixture to reduce it and form Au NPs. After stirring for
4 h, it was filtered under vacuum, washed well with ethanol and
water and dried under vacuum at 50 °C for 12 h.
General Procedure for Catalytic Aerobic Oxidation of
Hydrocarbons
A mixture of hydrocarbon (1.0mmol) and Au NPs@3D-(N)GFs
(0.15 mol% Au, 0.002 g) was added in portions to a two-necked vial
containing water (4.0 ml) as solvent with bubbling of air at room
temperature. The progress of the reaction was followed using gas
chromatography (GC). GC conversions and yields were obtained
using n-decane as an internal standard based on the amount of or-
ganic compound employed relative to authentic standard product.
After the reaction, the catalyst was removed via centrifugation and
washed twice with CH2Cl2 (6.0ml) to separate the absorbed organic
compounds. Then the organic phase was combined and extracted
twice. Finally the solvent was removed under vacuum to afford the
pure product.
Results and Discussion
Our approach is guided by four imperatives: (i) the catalyst should
be easily synthesized and be safe, green, stable and recyclable; (ii)
the selected material should support Au NPs with controlled size
and uniform distribution; and (iii) the development of an efficient
Figure 1. Oxidation of hydrocarbons using Au NPs@3D-(N)GFs as catalyst.
Appl. Organometal. Chem. 2015, 29, 456–461
Copyright © 2015 John Wiley & Sons, Ltd.
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M. Mahyari et al.
and selective oxidation reaction under moderate conditions such as
ecocompatible solvent and oxidizing agent.
According to the findings reported in the literature, the size
and the nature of the support and the preparation method
can play a vital role in the activity of Au NPs. The important fact
is that with decreasing size of Au NPs, their activity will signifi-
cantly increase. The size of the NPs affects oxygen chemisorption
and the electronic structure of the gold atoms. So, controlling
the size is very important to achieve the highest activity, and ni-
trogen groups are good candidates for this goal. In recent years,
nitrogen-doped groups have attracted increasing attention for
synthesis of Au NPs for the following reasons: (i) acting as fixa-
tives and stabilizers which limit the mobility and aggregation
of Au NPs on surfaces; (ii) controlling the size of Au NPs that
depends on the presence and percentage of nitrogen groups
in the framework; (iii) achieving a high loading of Au NPs on
surfaces; and (iv) functioning as base to avoid the heed to use
additive base in oxidation reactions.
Figure 3. SEM images of GO.
Characterization of Au NPs@3D-(N)GFs
The designed catalyst (Au NPs@3D-(N)GFs) was synthesized in
three steps: (i) GO was prepared using the Hummer method;[49]
(ii) the hydrothermal method was used for the synthesis of
3D-(N)GFs as support from GO sheets; and (iii) Au NPs as active sites
for the oxidation reaction were loaded on the 3D-(N)GFs support.
All steps are shown schematically in Fig. 2.
First, the morphology and sample surface of the suspended GO
sheets were investigated using SEM (Fig. 3). The SEM images show
the morphology of ensembles of GO sheets with relatively large
surface and lateral dimensions of several micrometres. Also, a
TEM image (Fig. 4) was obtained of GO sheets, drop-cast onto a lacy
carbon grid. The high quality and the few layers of GO sheets are
clearly discernible.
After hydrothermal reaction on GO, SEM images confirm the
three-dimensional morphology and the size of as-prepared
Figure 4. TEM image of synthesized GO.
3D-(N)GFs. The images (Fig. 5) show a porous structure formed
by interconnected framework of ultrathin graphene nanosheets. Ac-
cording to the images, the pore size of the as-prepared 3D-(N)GFs
changes from several micrometres to a few hundred nanometres.
Knowing that the loading amount of catalytic active sites is depen-
dent on the surface area, estimating this parameter using the
Brunauer–Emmett–Teller (BET) method was accomplished. Data
obtained with this technique, using nitrogen adsorption–
desorption analysis, reveal a typical BET surface area of up to
266.0m2 gÀ1 for 3D-(N)GFs. Therefore, the results indicate that the
as-prepared support has a high surface area to embed Au NPs as
active catalytic sites.
To investigate the morphology of Au NPs@3D-(N)GFs and to
study the distribution of Au NPs on the support surface, SEM and
TEM images were obtained (Fig. 6). A uniform dispersion of NPs
on the surface of 3D-(N)GFs is observed in TEM images. According
to TEM images, the size of Au NPs is 3–5 nm.
The chemical compositions of 3D-(N)GFs and Au NPs supported
on the 3D-(N)GFs were confirmed using elemental analysis (CHN),
FT-IR spectroscopy, powder X-ray diffraction and XPS.
XPS analysis as a powerful and most accurate technique for
surface characterization was used to investigate the composition
Figure 2. Synthesis of Au NPs@3D-(N)GFs.
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Appl. Organometal. Chem. 2015, 29, 456–461

Au NPs@3D-(N)GFs for aerobic oxidation of hydrocarbons
Figure 7. XPS spectrum of Au NPs@3D-(N)GFs.
Figure 5. SEM images of 3D-(N)GFs.
Table 1. Optimization of reaction conditions for oxidation reaction of
2,3-dihydro-1H-indenea
Entry
Amount
of catalyst
(Au content)
(mmol%)
Solvent
Oxidant Conversion Selectivity
(%)b
(%)b
1
None
0.10
0.15
0.2
H2O
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
Air
O2
5
25
90
90
58
89
75
92
91
80
89
92
95
97
2
H2O
3
H2O
>99
>99
95
4
H2O
5
0.15
0.15
0.15
0.15
0.15
0.15
0.15
0.15
Solvent free
THF
6
97
7
CH2Cl2
DMF
95
8
>99
>99
95
9
Xylenr
CH3CN
Toluene
H2O
10
11
12
Figure 6. (A, B) SEM images and (C, D) TEM images of Au NPs@3D-(N)GFs.
>99
>99
of the as-prepared catalyst (Au NPs@3D-(N)GFs) surface (Fig. 7).
The XPS spectrum of the Au NPs@3D-(N)GFs reveals C 1 s, N
1 s and O 1 s peaks at 285.2, 400.2 and 534.2 eV, respectively,
which is in good accordance with the designation of the exper-
iment. Identification of the oxidation state of the metals is an-
other ability of XPS. The absorption bands at 84.0 and 87.8eV
which can be readily assigned to Au (4f7/2) and Au (4f5/2), respec-
tively, are consistent with those reported for the Au(0) oxidation
state.
To investigate the catalytic properties of the as-prepared catalyst,
the oxidation reaction of 2,3-dihydro-1H-indene was employed as
a model reaction. Various amounts of the catalyst were examined
for the model reaction (Table 1, entries 1–4). The optimized amount
of the catalyst is 0.15 mol% Au (with respect to 2,3-dihydro-1H-
indene), 0.002 g. To determine the best solvent, several solvents,
THF, DMF, CH2Cl2, CH3CN, xylene, toluene and water, and solvent-
free conditions were studied (Table 1, entries 3, 5–11). The results
show that the efficiency of catalyst in terms of yield in water is
more than in other solvents which is due to the high dispersion
of the catalyst. Moreover, choosing water as a suitable and
green reaction medium leads to easy work-up. Then, the model
reaction was carried out in the presence of O2 or air as oxidant
aReaction conditions: 2,3-dihydro-1H-indene (1.0 mmol), solvent (5.0 ml),
room temperature, 5 h.
bConversion and selectivity determined by GC analysis.
and the results show that the reaction times and yields are
slightly better for O2 under the same conditions (Table 1, entries
3 and 12). But air, due to availability and lower cost, is the pre-
ferred choice. The optimum conditions for aerobic oxidation of
2,3-dihydro-1H-indene are 0.15 mol% of catalyst in the presence
of air at atmospheric pressure and room temperature (Table 1,
entry 3).
One of the main advantages of the as-prepared catalyst is the in-
fluence of 3D-(N)GFs as support on increasing the efficiency of the
catalyst in the aerobic oxidation reaction. For this reason, the cata-
lytic oxidation reaction of 2,3-dihydro-1H-indene as model reaction
was carried out using GO, 3D-(N)GFs and Au NPs under the same,
optimal, reaction conditions. The results are summarized in Table 2.
To make an accurate and logical comparison, the metal contents of
Au NPs@3D-(N)GFs and Au NPs were the same for the model
Appl. Organometal. Chem. 2015, 29, 456–461
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M. Mahyari et al.
Table 2. Comparison of results obtained using Au NPs@3D-(N)GFs, GO and 3D-(N)GFs for oxidation reactionsa
Entry
1
Catalyst
Product
Time (h)
Yield (%)b
Selectivity (%)b
AuNPs@3D-(N)GFs
2,3-Dihydro-1H-indene
Acetophenone
5
5
5
5
5
5
5
5
90
91
25
15
35
32
40
42
>99
>99
>99
>99
>99
>99
97
2
3
4
GO
2,3-Dihydro-1H-indene
Acetophenone
3D-(N)GFs
Au NPs
2,3-Dihydro-1H-indene
Acetophenone
2,3-Dihydro-1H-indene
Acetophenone
95
aReaction conditions: 2,3-dihydro-1H-indene (1.0 mmol), catalyst, H2O (4.0 ml), air (1 atm), room temperature.
bYield and selectivity determined by GC analysis.
reactions. When GO (0.005 g) is used as catalyst, a low yield is ob-
tained under our mild reaction conditions (Table 2, entry 2). Be-
cause of the occurrence of π–π interactions between benzene
skeleton of the reactants and graphene, employing 3D-(N)GFs as
support can enhance the adsorption of aromatic reactants. More-
over, oxygen adsorption on the bridge sites of graphene is pro-
moted by 3D-(N)GFs which results in activation of the oxygen
molecules because of their interaction with the nitrogen-doped
support. The presence of mobile oxygen on graphene can enhance
the removal of hydrogen and thus promote the aerobic oxidation
of organic compounds.[34] Actually the presence of basic sites on
the solid, π–π interactions of the aromatic groups with the support
favouring the close proximity of reagents to catalytic sites, and the
active role of the support in aerobic oxidations lead to the improve-
ment of the results, in comparison with the homogeneous catalyst
(Table 2, entries 1 and 4), as shown by the results described in
Table 1.
Table 3. Aerobic oxidation of alkylarenes using Au NPs@3D-(N)GFsa
Entry
Reagent
Yield (%)b Selectivity (%)b
To investigate the performance and efficiency of Au NPs@3D-(N)
GFs, several alkylarenes were subjected to the aerobic oxidation re-
action under the optimal conditions. All products were obtained in
good yields and high selectivity (Table 3).
One of the key challenges in the efficiency of heterogeneous
catalysts that needs to be monitored is the leaching of active sites
into the reaction medium. The leaching of Au NPs supported on
3D-(N)GFs was investigated using the following method. The initial
investigation reveals that Au NPs@3D-(N)GFs is air and moisture
stable. To confirm the stability of Au NPs on the support, after
50% conversion under the reaction conditions, the catalyst was fil-
tered off. Afterwards, the reaction was allowed to continue under
1
2
3
4
5
6
7
2,3-Dihydro-1H-indene
Benzophenone
90
94
97
97
96
95
95
>99
>99
>99
>95
>99
>99
>99
Ethylbenzene
1,2,3,4-Tetrahydronaphthalene
Butylbenzene
Isobenzofuran-1(3H)-one
9,9a-Dihydro-4aH-xanthene
aReaction conditions: alkylarene (1.0 mmol), catalyst (0.15 mol% Au,
0.002 g), H2O (4.0 ml), air (1 atm), 5 h, room temperatre.
bYield and selectivity determined by GC analysis.
Figure 8. Recyclability of Au NPs@3D-(N)GFs.
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Au NPs@3D-(N)GFs for aerobic oxidation of hydrocarbons
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using GC is 2% showing that the reaction does not proceed signif-
icantly. The filtered solution was analysed using atomic absorption
spectrometry, indicating that the gold content in the solution is
below the detection limit (0.1ppm) and repeating this test shows
0.10 ppm leaching of the catalyst. Given these results, it can be
deduced that nitrogen doping on the support can increase the
stability of NPs due to the affinity of nitrogen to gold.
The study of catalyst recyclability in practical applications is vital.
So, the recyclability of the AuNPs@3D-(N)GFs for catalytic oxidation
of 2,3-dihydro-1H-indene as a model reaction was tested. Mostly
the stability and recyclability of supported homogeneous catalysts
are measured based on the reaction yield for a long time in each
run, but since catalysis is a totally kinetic phenomenon, only kinetic
data should be considered to determine recyclability, stability and
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tored using GC. After conducting the reaction for 0.5h, the
reaction mixture was filtered off and AuNPs@3D-(N)GFs separated
as a black solid, which was washed with EtOH (2×5 ml) and reused.
The conversion yield and selectivity for each run are presented in
Fig. 8. Since the conversion yields for six runs do not decrease sig-
nificantly, we may conclude that Au NPs@3D-(N)GFs has good sta-
bility and recyclability.
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Conclusions
Based on the results, we achieved significant gains: (i) providing a
suitable support for stabilizing and encapsulating Au NPs to control
their size and uniform distributions by nitrogen doping on the sup-
port; (ii) good dispersion of the as-prepared catalyst in water as the
reaction solvent; (iii) embedding base centre (nitrogen dopant) and
active catalyst (Au NPs) on the support, providing a new efficient
bifunctional heterogeneous catalyst which eliminates the need for
an additive base; and (iv) introducing an environmentally friendly
and benign synthetic process for the facile transformation of
selective aerobic oxidation reactions of various alkylarenes into
their corresponding carbonyl compounds in water at room temper-
ature with good yields and high selectivity. Moreover, the recycla-
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the reaction medium.
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Acknowledgment
We gratefully acknowledge financial support from the Research
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Supporting Information
Additional supporting information may be found in the online
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