Thiol-functionalized fructose-derived nanoporous carbon as a support for gold nanoparticles and its application for aerobic oxidation of alcohols in water

Article abstract of DOI:10.1002/aoc.3159

Gold nanoparticles supported on thiol-functionalized fructose-derived nanoporous carbon (AuNPs@thiol-Fru-d-NPS) were found to be a simple bench-top, biocompatible, recyclable and selective catalytic system for the aerobic oxidation of various types of alcohols into their corresponding aldehydes and ketones at room temperature under the environmentally friendly conditions with excellent yields. Copyright

Full text of DOI:10.1002/aoc.3159

Full Paper
Received: 10 February 2014
Revised: 15 March 2014
Accepted: 6 April 2014
Published online in Wiley Online Library: 5 June 2014
(wileyonlinelibrary.com) DOI 10.1002/aoc.3159
Thiol-functionalized fructose-derived nanoporous
carbon as a support for gold nanoparticles and
its application for aerobic oxidation of alcohols
in water
Mojtaba Mahyari, Ahmad Shaabani*, Mohammad Behbahani
and Akbar Bagheri
Gold nanoparticles supported on thiol-functionalized fructose-derived nanoporous carbon (AuNPs@thiol-Fru-d-NPS) were
found to be a simple bench-top, biocompatible, recyclable and selective catalytic system for the aerobic oxidation of various
types of alcohols into their corresponding aldehydes and ketones at room temperature under the environmentally friendly
conditions with excellent yields. Copyright © 2014 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: nanoporous carbon; gold nanoparticles; oxidation of alcohols
be utilized in the production of aldehydes, which is useful in food
processing or in the cosmetics industry. In addition, these reactions
Introduction
[
1]
Catalysis is a key factor in the acceleration of chemical reactions,
have attracted great interest as one of the key reactions for process-
[
21–24]
so the development of eco-friendly and efficient catalytic systems
continues to be a major challenge in the 21st century. Therefore,
scientists and industrial groups are researching to find new strate-
gies and solutions to make catalysts not only more ecologically
ing biomass.
The catalyst plays a vital role in achieving high
efficacy and selectivity in the oxidation process. Heterogeneous
catalysis has been a key to development of environmentally benign
and sustainable processes and a cornerstone in the concept of
[
2–5]
[25–27]
sound but also more efficient and cost-effective (i.e. recyclable).
‘green chemistry’.
The production of functionalized nanostructured carbonaceous
materials using environmentally friendly processes from inexpen-
sive natural precursors is a hot topic in modern materials science.
Since the first report 150 years ago, the number of publications
on heterogeneous oxidation of alcohols with noble metals have
[6,7]
[28,29]
increased exponentially.
Gold was thought to be an ineffec-
In particular, nanoporous carbon materials have attracted consider-
able interest in recent years as they are considered to be promising
candidates for many fields such as separation science, electro-
tive noble metal for the oxidation reaction owing to its chemical
[
30–33]
inertness.
Interaction of gold and support affects the size of
[8]
the encapsulated gold nanoparticles (AuNPs), which is very
important in the performance of the catalyst in achieving the
[
9]
[10]
[11]
[12]
chemistry, energy storage,
practical approach is the thermal dehydration/transformation of
carbohydrate-based biomass in H O under autoclave conditions at
relatively low temperatures (e.g. 180–200°C) and pressures (<10 bar),
fuel cells
and catalysis.
One
[
19,30,34–37]
best results in selective oxidation.
In continuation of our efforts to introduce new and efficient
2
[
38–41]
catalysts for various organic transformations,
in oxidation reactions,
in particular
[13]
[42–45]
termed hydrothermal carbonization (HTC).
spherical particles with functionally rich surfaces.
This process yields
herein we wish to report the synthe-
[
14]
HTC has
sis of AuNPs on thiol-functionalized fructose-derived nanoporous
carbon (AuNPs@thiol-Fru-d-NPS). Dispersion of the supported
catalyst in water allows the aqueous aerobic oxidation of various
primary and secondary aliphatic and aromatic alcohols to the
corresponding aldehyde and ketone derivatives at room temperature
(Fig. 1). To the best of our knowledge, this work is the first report on
applying the recently synthesized ordered fructose-derived
nanoporous carbons for catalytic purposes.
clear advantages, such as being totally green, economical, mild
and fast. Recently, the first report on the synthesis of ordered
porous carbonaceous materials by a direct sustainable HTC/soft
templating approach was presented by Kubo et al. They took
advantage of D-Fructose (Fru) as a carbohydrate resource for
[
15]
preparation of novel porous materials.
These ordered carbo-
hydrate-derived porous carbons are very good candidates for
stabilizing noble metals and heterogeneous catalysis because
of their high surface area and convenient method of synthesis,
as well as their being inexpensive, biocompatible, thermally
stable and recyclable.
*
Correspondence to: Ahmad Shaabani, Department of Chemistry, Shahid Beheshti
University, GC, PO Box 19396–4716, Tehran, Iran. E-mail: a-shaabani@sbu.ac.ir
Selective oxidation reactions are one of the most important
classes of transformations in organic chemistry. The catalytic oxida-
Department of Chemistry, Shahid Beheshti University, GC, PO Box 19396-4716,
Tehran, Iran
[
16–20]
tion of alcohols is especially interesting
since this reaction can
Appl. Organometal. Chem. 2014, 28, 576–583
Copyright © 2014 John Wiley & Sons, Ltd.

Au NPs@nanoporous carbon for aerobic oxidation of alcohols in water
(
XRD) data were collected on an XD-3A diffractometer using
Cu-K radiation using silicon as a standard material. Elemental
analysis (CHN) was performed on a Thermo Finnigan Flash EA112
elemental analyzer (Okehampton, UK). N adsorption surface areas
α
2
were measured by Brunauer-Emmett-Teller (BET) technique using a
Micrometrics ASPS 2010 analyzer (Fig. 2).
Figure 1. Selective aerobic oxidation of alcohols by NPs@Fru-d-NPS.
Preparation of Fructose-Derived Nanoporous Carbons
Fructose-derived nanoporous carbon was synthesized according
to the recently reported procedure using Fru as a carbon
Experimental
[
15]
source.
In this approach, 0.25 g Plorunic F127 and 1.20 g
Materials
fructose were dissolved in 10 ml deionized water; the solution
was then transferred to a Teflon-lined sealed steel autoclave
and maintained at 130°C for 3 days. A dark-brown solid precipi-
tate was obtained, washed with water and ethanol and dried at
room temperature. Calcination was performed under nitrogen
atmosphere at 550°C for 6 h (Fig. 2). The synthesis of fructose-
derived nanoporous carbon was confirmed by XRD and nitrogen
adsorption analysis.
Pluronic® F127 triblock (M = 12 600, EO106-PO70-EO106) copolymer
w
was purchased from Aldrich Co. All other materials, including solvents,
acids, 3-thiol-propyltriethoxysilane and D-fructose, were of analytical
grade and purchased from Merck Co. (Darmstadt, Germany).
Double-distilled water from a Milli-Q purification system (Millipore,
Bedford, MA, USA) was used for the preparation of solutions.
Apparatus
Preparation of Thiol-Functionalized Fructose-Derived
Nanoporous Carbon
Chemicals purchased from Merck and Aldrich companies were
used without further purification. IR spectra were recorded on a
Bomem MB-Series FT-IR spectrophotometer. X-ray photoelectron
spectroscopy (XPS) analysis was performed using a VG Multilab
In this approach, in a typical reaction, 1.0 g fructose-derived
nanoporous carbon was suspended in 50 ml toluene, and
3-thiol-propyltriethoxysilane (2.0 ml) was added to the mixture,
2
000 spectrometer (ThermoVG Scientific) in an ultra-high vac-
which was then refluxed for 48 h under nitrogen atmosphere.
The black solid was removed from the solvent by filtration,
washed with methanol and acetone and then dried at room
temperature. The synthesis of thiol-functionalized fructose-
derived nanoporous carbon was confirmed by IR, low-angle
XRD, TEM, and elemental and thermal analysis.
uum. Transmission electron microscopy (TEM) analyses were
performed using a LEO 912AB electron microscope. TEM samples
were prepared by dipping a copper grid with Formvar film into
the freshly prepared catalyst solution. A few minutes after the
deposition, the aqueous solution was blotted away with a strip
of filter paper and then dried in air. Identification and quantifica-
tion were carried out on a Varian model 3600 gas chromatograph
Preparation of AuNPs@thiol-Fru-d-NPS
(Varian Iberica, Madrid, Spain) equipped with a split/splitless
capillary injection port and flame ionization detector (FID). A
CP-Sil-8 fused-silica capillary column (25 m × 0.32 mm i.d. and
Aqueous HAuCl4 (0.2 g in 3.0 ml water) and thiol-functionalized
fructose-derived nanoporous carbon (0.2 g in 10.0 ml) were
mixed and placed in an ultrasonic bath (50 kHz) for 30 min to well
disperse metal ions in the thiol-functionalized fructose-derived
0
.52 mm film thickness) from Chrompack was employed. GC
conversions and yields were obtained using n-decane as an internal
standard. 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. X-ray powder diffraction
4
nanoporous carbon. Then 10.5 ml of a 1.0 M NaBH solution in
0.3 M aqueous NaOH was added to the mixture as a reductant
for the formation of gold nanoparticles. 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. Figure 2 shows
the synthesis of AuNPs@thiol-Fru-d-NPS
catalyst.
General Procedure for the Catalytic
Aerobic Oxidation
of Alcohols
A mixture of an alcohol (1.0 mmol),
K2CO3 (0.5 mmol) and AuNPs@thiol-
Fru-d-NPS (0.1 mol% Au, 0.001 g) was
added in portions to a two-necked vial
2
containing H O (4.0 ml) as solvent with
bubbling oxygen at room temperature.
The progress of the reaction was
followed by GC. GC conversions were
obtained based on the amount of
remaining alcohol in the reaction
Figure 2. Synthesis of AuNPs@thiol-Fru-d-NPS.
Appl. Organometal. Chem. 2014, 28, 576–583
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Mahyari et al.
mixture relative to the standard sample containing 1.0 mmol al-
cohol. Moreover, the chromatograms of the reaction mixture
were compared with the standard sample containing 1.0 mmol
of related aldehyde product to obtain the yields. In all measure-
ments, n-decane was used as an internal standard to eliminate
the injection error. After completion of the reaction, the reaction
mixture was cooled to room temperature, and the catalyst was
separated by filtration and washed with EtOH (2 × 5 ml).
Result and Discussion
Figure 3. TEM image of AuNPs@thiol-Fru-d-NPS.
Our approach was guided by four imperatives: (i) the catalyst
should be easily synthesized from a natural precursor and be safe
and inexpensive; (ii) the synthesized system should be hydro-
philic to act as a high-dispersible catalyst in water for oxidation
reactions; (iii) the support should encapsulate gold nanoparticles
to control the size and uniform distribution of nanoparticles; (iv)
an efficient, inexpensive, environmentally friendly and benign
synthetic process for facile conversion and selective oxidation re-
action should be developed.
procedure was successful for functionalization. Formation of
thiol-functionalized fructose-derived nanoporous carbon was con-
firmed by IR spectroscopy, low-angle X-ray diffraction, elemental
and thermal analysis and N adsorption surface area measure-
2
ment. In the FT-IR spectrum of the thiol-functionalized nanoporous
ꢀ1
carbon, the band at 2972 cm is attributed to aliphatic CH2
groups of 3-thiol-propyltriethoxysilane, which confirmed the at-
tachment of thiol groups to the support. Since the SH and OH
peaks are broad, the IR band of SH is not clear. The number of thiol
groups on the surface of nanoporous carbon was evaluated by el-
emental analysis (CHNS). According to the results of elemental
analysis (C, 25.1%; H, 5.7%; S, 22.6%), the loading of thiol is approx-
It is known that the activity of gold NPs generally depends on
their size, the nature of the support and the preparation
[46–48]
method.
It has been found that the activity of supported
gold particles noticeably increases with decreasing size of
nanoparticles. Increasing oxygen chemisorption is one of the
important factors in increasing the activity for aerobic oxida-
tion reactions. Another factor is the electronic structure of
the gold atoms constituting the active center. Therefore,
controlling the size of nanoparticles is very important in
achieving the highest activity. For this goal, mercapto groups
ꢀ
1
imately 7 mmol g nanoporous carbon. The large number of thiol
groups can be attributed to the high surface area of the
nanoporous carbon. The specific surface area of the modified ma-
2
ꢀ1
terial was measured to be 178 m g by single-point nitrogen ad-
sorption. By comparing this surface area with that of the initial
2
ꢀ1
material (304 m g ), it can be concluded that the inside pores
were functionalized by thiol groups as well as the outer surface.
Figure 3 shows the TEM image of AuNPs@thiol-Fru-d-NPS,
indicating the size distribution and uniformly dispersed gold
nanoparticles. Moreover, TEM analysis presented the nanoparticle
size as 5–7 nm.
(
RSH) are the best candidates. In recent years, mercapto
groups have been used for synthesis of gold nanoparticles for
the following reasons: (i) as stabilizers which limit the mobility and
[27]
aggregation of AuNPs on surfaces;
(ii) controlling the size of
AuNPs, which depends on the presence and percentage of
mercapto groups in the framework; and (iii) to achieve a high
Figure 4 shows the XPS survey data of AuNPs@thiol-Fru-d-NPS,
indicating the binding energy (BE) of different atoms (Au, Si, S, C
[49,50]
loading of AuNPs on the surface.
Functional carbonaceous materials are produced by the trans-
formation of carbohydrates by an inexpensive, aqueous-based,
mild and easy hydrothermal carbonization (HTC) route. We used
fructose to produce functional carbonaceous materials for the
following reasons: inexpensive precursors, development of uni-
formly sized, organized, shaped pore texture and consequently
controlled pore wall chemistry. Moreover, HTC of Fru occurs at
a significantly lower temperature (130°C) compared to other
hexoses (e.g. D-glucose; 180°C) and therefore soft templates can
be used for synthesis of fructose-derived nanoporous carbon.
The synthesized AuNPs@thiol-Fru-d-NPS was characterized by
atomic absorption spectroscopy (AAS), CHN analysis, TGA, TEM,
FT-IR, XPS and XRD.
Fructose-derived nanoporous carbon has been synthesized by
hydrothermal condensation of fructose according to a previously
reported study. The formation of fructose-derived nanoporous
carbon was confirmed by low-angle X-ray powder diffraction
2
and nitrogen adsorption analysis. The N adsorption analysis
2
ꢀ1
showed a surface area of 304 m g for this non-porous mate-
rial. Functionalization of the fructose-derived nanoporous carbon
with thiol groups has been performed like functionalization of
fructose with silane agents, which is a well-established method
[49,50]
in the literature.
As the surfaces of fructose and nanoporous
carbon are the same and both have hydroxyl groups, this
Figure 4. Wide XPS scan of AuNPs@thiol-Fru-d-NPS.
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Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 576–583

Au NPs@nanoporous carbon for aerobic oxidation of alcohols in water
nanoporous carbon was confirmed by
low-angle XRD and nitrogen adsorp-
tion analysis (Fig. 5a). The existence of
a sharp peak in the XRD pattern of fruc-
tose-derived nanoporous carbon at 2θ
=
0.41° confirmed its formation. Com-
parison of the reported XRD pattern
[
15]
of initial nanoporous carbon
with
the modified one showed that the
pores remained intact after modifica-
tion. In Fig. 5(b), well-defined peaks at
2
7
θ values of 38.22°, 44.34°, 64.60 and
7.64° are related to (111), (200), (220)
and (311) Bragg’s reflection based on
the fcc structure of gold nanoparticles.
The broadening of Bragg’s peaks con-
firms the ormation of nanoparticles.
The Scherrer equation τ = Kλ/B cos θ
was used to calculate the mean size
of gold nanoparticles, where τ is the
average particle size, K is the shape
factor (taken as 0.9), λ is the X-ray
wavelength (0.153), β is the full width
at half maximum (FWHM) intensity and
θ is the Bragg angle. The size of
nanoparticles was thus obtained to be
about 7 nm for gold nanoparticles,
which is in good agreement with TEM
results.
The thermal stability of nanoporous
carbon after introducing the thiol
groups has been investigated by TGA
and differential thermal analysis
(
DTA), shown in Fig. 6 (curves A and
B, respectively). The thermal analysis
was measured under air atmosphere
ꢀ
1
and with a ramp rate of 10°C min
up to 800°C. According to these
results, this catalyst is stable up to
Figure 5. XRD patterns of (a) thiol-Fru-d-NPS and (b) AuNPs@thiol-Fru-d-NPS.
1
80°C and losing about 53% of its weight, which confirms the
and O). The BEs at around 84.1, 103.4, 162.3, 286.3 and 532 eV can
[51,52]
[53]
[54]
elemental analysis results.
To evaluate the catalytic properties of this catalyst, oxidation
reaction of benzyl alcohol was performed as a model reaction.
be assigned to Au(4f),
elements, respectively. The narrow range spectra of C(1s) and O
1s) are given in Figs S1 and S2 (supporting information). The photo-
electron spectrum of Au(4f) of gold nanoparticles corresponding to
Si(2p),
S(2p), C(1s) and O(1s)
(
[51,52]
metallic Au (85.0 and 87.4 eV) was seen (see ESI, Fig. S3).
Table 1. Optimization of reaction conditions on oxidation reaction
XRD patterns of thiol-Fru-d-NPS and AuNPs@thiol-Fru-d-NPS
are shown in Fig. 5(a, b). The formation of fructose-derived
a
of benzyl alcohol
b
Entry
Amount of catalyst
Solvent
Yield (%)
(
Au content, mol%)
1
1
20
00
15
1
2
3
4
5
6
7
8
9
None
0.02
0.05
0.10
0.10
0.10
0.10
0.10
0.10
0.10
H
H
H
H
2
2
2
2
O
O
O
O
12
35
1
5
0
0
75
8
6
4
2
0
0
0
0
0
>99
72
B
-
-
5
DMF
THF
10
90
A
None
25
-15
CH
3
CN
87
-
-
20
25
toluene
xylene
96
1
0
98
-30
a
0
200
400
600
800
Reaction conditions: benzyl alcohol (1mmol), K
solvent (5 ml), at 25°C, air (1 atm), 1 h.
2 3
CO (0.5 mmol),
Temperature (°C)
b
Yield determined by GC analysis.
Figure 6. TGA (A) and DTA (B) graphs of thiol-Fru-d-NPS.
Appl. Organometal. Chem. 2014, 28, 576–583
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Mahyari et al.
a
Table 2. Optimization and amount of base
2 2 3 2
such as THF, DMF, CH Cl , CH CN, xylene, toluene and H O, and
solvent-free conditions, were tested. The efficiency of catalyst in
terms of yield in water is more than other solvents, which is due to
the hydrophilic nature of the catalyst (Table 1, entries 4–10). It is
notable that the data correspond to total selectivity at incomplete
conversion. Moreover, the influence of base on the reaction was also
b
Entry
Base
Amount of base (%)
Yield (%)
1
2
3
4
5
6
7
8
9
KOH
Et
Pyridine
50
50
50
50
40
30
20
10
60
86
71
67
97
74
60
46
23
97
3
N
K
2
K
2
K
2
K
2
K
2
K
2
CO
CO
CO
CO
CO
CO
3
3
3
3
3
3
2 3
studied. As shown in Table 2, K CO is the best-suited base (entries
1–4). After selecting K CO as the best base in our conditions, the
2
3
reaction was carried out in the presence of various amounts of
K CO . According to Table 2 (entry 4), the optimal amount of
2
3
K CO to achieve maximum efficiency is 50 mmol%.
2
3
To show the efficiency of the prepared catalyst for the oxida-
tion reaction, a large variety of primary, secondary cyclic and
aromatic alcohols to the corresponding carbonyl compounds
were subjected to the optimized conditions. All products were
obtained in high yields (Table 3). To evaluate the efficiency of
catalyst in the presence of oxygen, some derivatives were
examined and the results showed that good yields were
obtained, which were comparable with the yields of those
reactions in the presence of air but in a relatively shorter
reaction time. In particular, various types of allylic alcohols were
also oxidized to the corresponding α,β-unsaturated carbonyl with
a
Reaction conditions: benzyl alcohol (1mmol), H
air (1 atm), 2 h.
2
O (5ml), at 25°C,
b
Yield determined by GC analysis.
The reaction was carried out in the presence of various
amounts of catalyst (Table 1, entries 1–4). The optimized
amount of the catalyst was 0.1 mol% Au (with respect to benzyl
alcohol), 0.001 g. In order to obtain the best solvent, various solvents,
a
Table 3. Aerobic oxidation of alcohols using AuNPs@ thiol-Fru-d-NPS
b
c
Entry
Product
Time (h)
Yield (%)
Selectivity (%)
R
1
R
2
Air
O
2
Air
O
2
1
2
3
4
5
6
7
8
9
Ph
H
H
H
H
H
H
H
H
H
2
1.1
1.4
1.3
1.3
1.4
1.5
1
97
95
95
96
96
95
97
94
95
95
93
94
97
97
95
97
96
94
95
96
97
94
>99
>99
>99
>99
>99
>99
>99
>99
98
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
98
p-F–Ph
p-Cl–Ph
p-Br–Ph
3
3.2
3
p-Me–Ph
p-MeO–Ph
p-NO –Ph
(CH2)
(CH
2.1
3.2
2
2
CH
CH
3
8
CO
CO
3.6
3.5
1.8
1.5
1.2
1.5
1
1.2
1.5
1
3
2 7
)
10
11
12
13
14
15
16
17
18
19
20
21
22
9H-Fluoren-9-one
97
2,3-Dihydro-1H-inden-1-one
1
98
Ph
Ph
Ph
Me
Me
H
0.5
1
98
>99
>99
97
p-NO
2
–Ph
0.5
1
p-MeO–Ph
2.3
3
CH
CH
Ph
CH
CH
3
(CH
2
)
8
CO
Me
1.5
1.8
1
96
3
CH
2
CH
2
CH
3
CH
2
CH
2
3
98
COPh
2
93
3
3
CH
CH
2
CH¼CH
CH
CH¼CH
H
H
H
H
2.1
2.3
2.6
2.4
1
98
2
2
1.1
1
98
>99
>99
>99
PhCHd¼CH
CH3CH¼CH
97
1.3
98
a
Reaction conditions: benzyl alcohol (1mmol), K
Yield determined by GC analysis.
2
CO
3
2
(0.5 mmol), H O (5ml), at 25°C, air (1 atm), 1 h.
b
c
Selectivity determined by GC analysis.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 576–583

Au NPs@nanoporous carbon for aerobic oxidation of alcohols in water
high activity and selectivity (Table 3, entries 10–12). High catalytic
activity and selectivity for the oxidation of alcohols was observed
and afforded the corresponding carbonyl compounds in excellent
yields.
results showed that the reaction yields were very low (Table 4,
entries 7 and 8).
The leaching of active metal species into solution is a very im-
portant issue to be considered when heterogeneous catalysts are
used. In order to investigate leaching, we carried out a series of
tests as described below. Our preliminary investigations showed
that the catalyst AuNPs@thiol-Fru-d-NPS was very stable to air
and moisture. In addition, in a separate test, the catalyst was
filtered off after 50% conversion at the reaction temperature.
Subsequently, the reaction was allowed to continue under similar
conditions, and the results showed that the reaction did not proceed
significantly. The yield of reaction obtained by GC was 2%. Moreover,
the results of AAS of the filtrate also confirmed that the Au content in
the solution was below the detection limit (0.1 ppm); repeating this
experiment with AuNPs@Fru-d-NPS, showed 0.12 ppm leaching.
Thus we conclude that, as expected, due to the high affinity of
Au and S, introducing the thiol groups to the support can
enhance Au loading and eliminate metal leaching.
To show the importance of functionalizing fructose-derived
non-porous carbon with thiol groups, gold nanoparticles
supported on fructose-derived non-porous carbon were also
investigated in the oxidation reaction. As indicated in Table 4, the
results are comparable in reaction yields and times, but the metal
loading of thiol-functionalized non-porous carbon and nanoporous
carbon is different. Actually, the weight percent of Au in the
AuNPs@thiol-Fru-d-NPS is approximately 19.7% w/w, while that of
AuNPs@Fru-d-NPS is just 7%. So, to have a correct comparison we
used different amounts of supported catalysts to give the same
amounts of Au nanoparticles; for example, the amount of supported
catalyst for AuNPs@thiol-Fru-d-NPS was 0.001 g, but for AuNPs@Fru-
d-NPS 0.012 g was used. Therefore, the Au loading on thiol-Fru-d-
NPS is substantially more than Fru-d-NPS and consequently it has a
cost limitation. Moreover, increasing the stability of the catalyst and
eliminating leaching of the Au nanoparticles are the other advantages
of functionalizing Fru-d-NPS with thiol groups in this catalyst. Also,
fructose-derived nanoporous carbon and thiol-functionalized
fructose-derived nanoporous carbon were investigated and the
To understand the unique promotional effect of AuNPs@thiol-
Fru-d-NPS, its catalytic activity was compared to other Au-supported
catalysts used in the oxidation reaction of benzyl alcohol with respect
to their yields, temperature and time required for the reaction
(Table 5). It is evident from Table 5 (entries 3 and 4) that high
a
Table 4. Comparison of results obtained from AuNPs@thiol-Fru-d-NPS with AuNPs@Fru-d-NPS
b
Entry
Catalyst
Product
Time
Selectivity
%)
Yield
(%)
(
R
1
R
2
1
2
3
4
5
6
7
8
AuNPs@thiol-Fru-d-NPS
AuNPs@thiol-Fru-d-NPS
AuNPs@thiol-Fru-d-NPS
AuNPs@Fru-d-NPS
AuNPs@Fru-d-NPS
AuNPs@Fru-d-NPS
Thiol-Fru-d-NPS
Ph
9H-Fluoren-9-one
H
1.8
H
2
>99
2.1
2
>99
95
98
92
91
91
52
50
97
CH
Ph
3
CH
2
CH¼CH
95
90
H
9H-Fluoren-9-one
1.8
H
97
2.1
2
CH
Ph
Ph
3
CH
2
CH¼CH
94
10
12
H
Fru-d-NPS
H
2
a
Reaction conditions: benzyl alcohol (1mmol), K
2
CO
3
(0.5 mmol), H
2
O (5ml), at 25°C, air (1 atm).
b
Yield determined by GC analysis.
Table 5. Comparison of results obtained from AuNPs@thiol-Fru-d-NPS with other supported Au catalysts for the oxidation of benzyl alcohol
Entry
Catalyst
Solvent
Temp. (°C)
Time (h)
Yield (%)
Selectivity (%)
Ref.
1
2
3
4
5
AuNPs@ thiol-Fru-d-NPS
H
2
O
25
25
2
16
6
99
99
>99
—
—
a
PS-PAMAM=AuNPs
DCM/H
Toluene
2
O
[55]
[37]
[56]
[57]
Fe
Fe
3
O
O
4
/Au
100
25
100
94
100
5
b
3
4
:PVP /Au
H
2
O
6
c
Au/MgCr-HT
Toluene
100
1
96
99
a
Polystyrene-Polyamidoamine = Gold nanoparticles.
b
Poly(N-vinyl-2-pyrrolidone).
Hydrotalcite.
c
Appl. Organometal. Chem. 2014, 28, 576–583
Copyright © 2014 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

M. Mahyari et al.
gold NPs is still not fully under-
stood. According to our experi-
mental results, as well as the
literature reports, we propose a
possible mechanism for the cata-
lytic oxidation of alcohols over a
catalytic system, as illustrated in
Fig. 7. Deprotonation of alcohol
was promoted with K CO3 as a
2
base to from alkoxide on the Au
surfaces. Gold catalyzes the β-hy-
dride elimination to produce the
corresponding aldehyde, along
with the formation of O2 and
H2O.
Many reports of stable and recy-
clable heterogeneous catalysts are
based on a single data point in
each run – often reaction yield
over a long time – to prove
catalyst stability and recyclability.
But catalysis is a totally kinetic phe-
nomenon, so one can argue that
only kinetic data should be consid-
ered to prove recyclability, stability
and deactivation. Measurement of
initial rates obtained from kinetic
plots is a good approach to
investigate recyclability and deac-
tivation, since if a single data point
was taken in each run after a long
reaction time, one might incor-
rectly conclude that the catalyst is
completely stable and recyclable.
Thus we investigated the oxida-
tion reaction of benzyl alcohol as a
model reaction after 0.5 h (the reac-
tion proceed was monitored by
GC). After carrying out the
reaction for 0.5 h, the reaction
mixture was filtered off and
AuNPs@thiol-Fru-d-NPS separated
as a black solid; this was washed
Figure 7. Plausible pathway for the oxidation of alcohols over AuNPs@thiol-Fru-d-NPS.
reaction temperatures are needed. Long reaction time is another
drawback of these reports.
Gold NPs have attracted much interest in the aerobic oxidation
of alcohols; however, the mechanism of oxidation reaction with
with EtOH (2 × 5 ml) and reused. The conversion yields and selec-
tivity for every run are shown in Fig. 8(a). AuNPs@ thiol-Fru-d-NPS
showed good stability and recyclability as the conversion yields
for six runs did not decrease significantly. Also, to investigate the
a
b
Figure 8. Reusability of the catalyst in the oxidation of benzyl alcohol.
wileyonlinelibrary.com/journal/aoc
Copyright © 2014 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2014, 28, 576–583

Au NPs@nanoporous carbon for aerobic oxidation of alcohols in water
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[
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We gratefully acknowledge financial support from the Research
Council of Shahid Beheshti University and the Catalyst Center of
Excellence (CCE).
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Appl. Organometal. Chem. 2014, 28, 576–583
Copyright © 2014 John Wiley & Sons, Ltd.
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