Nano–silica functionalized with thiol–based dendrimer as a host for gold nanoparticles: An efficient and reusable catalyst for chemoselective oxidation of alcohols

Article abstract of DOI:10.1002/aoc.4440

In this paper, we present the synthesis of Au nanoparticles supported on nanosilica thiol based dendrimer, nSTDP. The catalyst was prepared by reduction of HAuCl₄ with NaBH₄ in the presence of nSTDP. The resulting Au_np–nSTDP materials were characterized by FT–IR and UV–vis spectroscopic methods, SEM, TEM, TGA, XPS and ICP analyses. The characterization of the catalyst showed that Au nanoparticles with the size of 2–6?nm are homogeneously distributed on the nSTDP dendrimer with a catalyst loading of about 0.19?mmol/g of catalyst. The Au_np–nSTDP catalyst was used in the oxidation of alcohols with tert–butyl hydroperoxide (TBHP) as oxidant. The influence of vital reaction parameters such as solvent, oxidant and amount of catalyst on the oxidation of alcohols was investigated. These reactions were best performed in an acetonitrile/water mixture (3:2) in the presence of 0.76?mol% of the catalyst on the basis of the Au content at 80?°C under atmospheric pressure of air to afford the desired products in high yields (80–93% for benzyl alcohols). The Au_np–nSTDP catalyst exhibited a high selectivity toward the corresponding aldehyde and ketone (up to 100%). Reusabiliy and stability tests demonstrated that the Au_np–nSTDP catalyst can be recycled with a negligible loss of its activity. Also this catalytic exhibited a good chemoselectivity in the oxidation of alcohols.

Full text of DOI:10.1002/aoc.4440

Received: 3 March 2018
DOI: 10.1002/aoc.4440
Revised: 22 April 2018
Accepted: 25 April 2018
F U L L P A P E R
Nano–silica functionalized with thiol–based dendrimer as a
host for gold nanoparticles: An efficient and reusable
catalyst for chemoselective oxidation of alcohols
Sara Haghshenas Kashani | Amir Landarani‐Isfahani | Majid Moghadam
|
Shahram Tangestaninejad
| Valiollah Mirkhani | Iraj Mohammadpoor‐Baltork
Department of Chemistry, Catalysis
Division, University of Isfahan, Isfahan
81746–73441, Iran
In this paper, we present the synthesis of Au nanoparticles supported on
nanosilica thiol based dendrimer, nSTDP. The catalyst was prepared by reduc-
tion of HAuCl4 with NaBH4 in the presence of nSTDP. The resulting
Aunp–nSTDP materials were characterized by FT–IR and UV–vis spectroscopic
methods, SEM, TEM, TGA, XPS and ICP analyses. The characterization of the
catalyst showed that Au nanoparticles with the size of 2–6 nm are homoge-
neously distributed on the nSTDP dendrimer with a catalyst loading of about
0.19 mmol/g of catalyst. The Aunp–nSTDP catalyst was used in the oxidation of
alcohols with tert–butyl hydroperoxide (TBHP) as oxidant. The influence of vital
reaction parameters such as solvent, oxidant and amount of catalyst on the oxi-
dation of alcohols was investigated. These reactions were best performed in an
acetonitrile/water mixture (3:2) in the presence of 0.76 mol% of the catalyst on
the basis of the Au content at 80 °C under atmospheric pressure of air to afford
the desired products in high yields (80–93% for benzyl alcohols). The
Aunp–nSTDP catalyst exhibited a high selectivity toward the corresponding alde-
hyde and ketone (up to 100%). Reusabiliy and stability tests demonstrated that
the Aunp–nSTDP catalyst can be recycled with a negligible loss of its activity. Also
this catalytic exhibited a good chemoselectivity in the oxidation of alcohols.
Correspondence
Majid Moghadam; Shahram
Tangestaninejad; Valiollah Mirkhani,
Department of Chemistry, Catalysis
Division, University of Isfahan, Isfahan
81746–73441, Iran.
Email: moghadamm@sci.ui.ac.ir;
stanges@sci.ui.ac.ir; mirkhani@sci.ui.ac.ir
Funding information
Research Council of the University of
Isfahan; Iranian National Science Foun-
dation (INSF), Grant/Award Number:
94013580
KEYWORDS
alcohol oxidation, gold nanoparticles, reusable catalyst, thiolated dendrimer
1 | INTRODUCTION
selective oxidation of organic compounds.[8,9] Gold have
found multiple uses in fine chemical synthesis, for exam-
Noble metals such as palladium, platinum or ruthenium
have been widely used as efficient catalytic species in the
different organic transformations and they are more
efficient than gold catalysts. In addition, the gold in the
bulk form has poor chemical reactivity, but its catalytic
activity is dramatically enhanced when converted to nano-
particles.[1–3] Gold have found different applications in
organic synthesis such as propylene epoxidation[4–7] and
ple, selective hydrogenations, carbon–carbon bonds for-
mation, oxidation of organic compounds,[10–13] Friedel
Crafts alkylation of aromatics with benzyl alcohol,[14]
hydroamination of alkynes,[15] fixation of CO2 to cyclic car-
bonates,[16] cyclization of functionalized carboxylic
acids.[17] Au particles of less than 5 nm diameter have been
found to be active for water gas shift reaction,[18–22] NO
reduction/dissociation,[23,24] hydrogenation[25,26] and SO2
Appl Organometal Chem. 2018;e4440.
wileyonlinelibrary.com/journal/aoc
© 2018 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.4440

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HAGHSHENAS KASHANI ET AL.
dissociation.[27] In this respect, catalysts based on gold
nanoparticles have shown excellent performance, stability
and reusability in oxidation of alcohols and sugars under
mild conditions, and are superior catalysts compared to
other metals such as Pd and Pt.[28]
compared to their conventional noble metal counter-
parts.[34,35] In the alcohol oxidation catalysed by gold
nanoparticles, the size of gold particles[36] and the sup-
port nature play key roles in determining the catalytic
activity and selectivity of the reaction.[37]
The recent reversal of the market prices of Au (US $
43/g) with respect to Pd (US $ 34/g) and Pt (US $ 31/g)
can drive Au catalysts to commercialization with an eco-
nomical advantage.[29] Au has d‐states so low in energy
that the interaction with oxygen 2p states is net repulsive.
It is therefore unlikely that Au should be a good catalyst
for an oxidation reaction.[30] On the other hands, thou-
sands of tons of gold are recycled from stoichiometric
technical applications, and in addition, thousands of tons
are produced in gold mines every year. This, in compari-
son with rhodium, palladium, and platinum, leads to a
higher stability of the price, which is an advantage for
possible industrial uses. Furthermore, one should keep
in mind that the price of a catalyst is often dominated
by the ligand rather than by the metal.[31]
Heterogeneous catalysts are superior over homoge-
neous counterparts in oxidation reactions due to favor-
able structural properties, high chemical stability, and
prolonged reusability.[32,33] Among the supported cata-
lysts, immobilized gold nanoparticles have attracted con-
siderable attention in the oxidation of alcohols owing to
their higher selectivity and less proneness to leaching
Dendrimers with their tree‐like structure are used
as a template for the synthesis of NPs. They have a
uniform structure and therefore can yield well–defined
NPs[38–41] and prevent the agglomeration of NPs by
encapsulated within their pores.[38,42,43] Also, due to
unique structure of these materials, a substantial frac-
tion of the NPs is available on the surface which par-
ticipates in catalytic reactions.[39,40] On the other
hand, the dendrimer branches can be used as selective
gates to control access of small molecules to the encap-
sulated NPs.[39,44]
Recently, some applications of dendritic materials in
organic syntheses have been reported by our research
group.[45–53] By combination the advantages of gold
nanoparticles as catalyst in the oxidation of alcohols
and the ability of dendrimers in the stabilization of nano-
particles, here we wish to present an efficient and envi-
ronmentally friendly method for preparation of gold
nanoparticles immobilized on nano–silica functionalized
thiolated dendritic polymer (Aunp–nSTDP). This hetero-
geneous catalyst was applied as catalyst in the oxidation
of alcohols (Scheme 1).
SCHEME 1 Alcohol oxidation reaction
with Aunp–nSTDP

HAGHSHENAS KASHANI ET AL.
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(189.03 mg, 0.48 mmol) was stirred at room temperature
for 12 h. At the end of reaction, the mixture was evapo-
rated to dryness in a rotary evaporator. The obtained
powder, was suspended in 50 ml of ethanol and 10 ml
of water, sodium borohydride (182 mg, 4.8 mmol) was
added and stirred for 12 h at 0 °C. At the end of the reac-
tion, the solvent was evaporated by a rotary evaporator,
and washed with methanol, water and acetone continu-
ously, to remove the unreacted precursors, and finally
dried in vacuum at 60 °C for 24 h.
2 | EXPERIMENTAL SECTION
2.1 | General remarks
The chemicals used were purchased from Fluka and
Merck chemical companies. FT–IR spectra were
recorded on a Jasco 6300D spectrophotometer. Diffuse
reflectance UV–vis (DR UV–vis) spectra were obtained
on a JASCO V–670 spectrophotometer. Thermogravi-
metric analysis was determined with TG 50 Mettler
thermogravimetric analyzer, under nitrogen flow at a
– 1
uniform heating rate of 20 °C min
in the range of
2.3 | General procedure for oxidation of
alcohols catalyzed by Aunp–nSTDP
30 – 900 °C. The Au content of the catalyst was deter-
mined by a Jarrell–Ash 1100 ICP analysis. The X–ray
photo–electron spectroscopy (XPS) measurements were
In a round–bottom flask equipped with a magnetic stir-
rer bar, a solution of K2CO3 (1 mmol) in H2O (2 ml)
was added to a solution of alcohol (0.5 mmol) and
tert‐butyl hydroperoxide (1 mmol) in acetonitrile
(3 ml). Afterward, the adequate amount of catalyst
(40 mg of catalyst, 0.76 mol% Au) was added to the
solution and the mixture was refluxed under stirring
(1000 rpm) at 80 °C under atmospheric pressure of
air. The progress of the reaction was monitored by
GC. At the end of the reaction, the catalyst was
removed by simple filtration and washed with the
performed using
a Gamma data–scienta ESCA200
hemispherical analyzer equipped with an Al
(Kα = 1486.6 eV) X–ray source. The SEM and TEM
images were taken by a Hitachi S–4700 field emis-
sion–scanning electron microscope (FE–SEM). The sam-
ples were coated with gold for preparing of SEM–EDX
images and a Philips CM10 Transmission Electron
Microscope operating at 100 kV, respectively. X‐Ray dif-
fraction (XRD) images were obtained with a Bruker
XRD D8 Advance instrument with Co Kα radiation at
40 KV. Gas chromatography (GC) experiments were
performed with
a
Shimadzu GC–16A instrument
equipped with a FID detector using a 2 m column
packed with silicon DC–200 or Carbowax 20 M.
2.2 | Preparation of Nano–Silica Thiolated
Dendritic Polymer Supported Au
Nanoparticles (Aunp–nSTDP)
The nanosilica thiolated dendritic polymer, nSTDP, was
prepared according to our recently reported method.[45]
A suspension of nano–silica supported dendrimer
(800 mg) in distilled water (100 ml) and HAuCl4
FIGURE 1 DR UV–vis spectrum of a) nSTDP, b) Aunp– nSTDP
SCHEME 2 Synthesis of the Aunp–nSTDP catalyst

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HAGHSHENAS KASHANI ET AL.
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and Characterization of
Gold Nanoparticles Immobilized on Nano–
Silica Thiol–based Dendritic Polymer
Recently, we reported the preparation of the nano–silica
thiol–based dendritic polymer (nSTDP) and application
of its nano palladium containing catalyst in the C–C cou-
pling reactions.[45] After synthesis and characterization of
nSTDP, the dendritic polymer was used as host for carry-
ing the gold nanoparticles. The supported Au nanoparti-
cles were obtained by the reaction of HAuCl4 with
nSTDP in the presence of sodium borohydride as reduc-
ing agent (Scheme 2).
FIGURE 2 IR spectrum of a) nSTDP (dendrimer) and b) Aunp
nSTDP (nano catalyst)
–
This new catalyst was characterized by different ana-
lytical techniques. The UV–vis spectrum provided infor-
mative evidence for immobilization of the gold
nanoparticles on thiol–based dendrimer. The FT‐IR
spectrum was used to confirm the presence of func-
tional groups on the dendrimer. Scanning electron
microscopy technique (SEM) was used for studying the
surface morphology of the synthesized materials. Also,
the distribution of the elements on the catalyst texture
was studied by elemental mapping. Transmission elec-
tron microscopy (TEM) was applied for measuring of
the particle size. Also, The XPS analysis was used for
determination of the gold oxidation state before and
after reaction. The phase and crystalline nature of the
prepared catalyst was also characterized by X‐ray
FIGURE
b) Aunp–nSTDP (nano catalyst)
3
Thermograms of: a) nSTDP (dendrimer) and
adequate amount of acetonitrile and H2O. Later on, the
products were extracted with Et2O and purified on a
silica gel column.
FIGURE 4 FE–SEM image of: (a) nSTDP and (b) Aunp–nSTDP. SEM–EDX spectrum of: (c) nSTDP and (d) Aunp–nSTDP

HAGHSHENAS KASHANI ET AL.
5 of 13
diffraction (XRD) technique. Finally, TGA was used to
investigate the thermal behavior of the synthesized cat-
alyst and support.
and a strong Si–O–Si stretching band about 1000–
1100 cm−1. Also, the characteristic bands for C–Haliph
appear at 1450 and 2950 cm−1. The appearance of bands
at 1690–1710 cm−1 (C═O) is a good indication for the
presence of carbonyl groups on the nano‐silica.[45] All
these bands are also observed for Aunp–nSTDP. As can
be seen no further information is obtainable from the
FT–IR spectrum of Aunp–nSTDP compared to nSTDP.
The weight loss of the dendrimer in the range of 30–
900 °C as a function of temperature was determined using
TGA, which is an irreversible process because of the ther-
mal decomposition. The organic weight loss of nSTPD
and the supported catalyst were 75% and 68%, respectively.
This indicates that the amount of organic material
The absorption spectrum of thiolated dendrimer
shows one absorption peak at 203 nm. However, the
Aunp–nSTDP shows three peaks at 209, 379 and 536 nm
(Figure 1). Colloidal AuNPs are known to exhibit strong
absorption in the visible region (380–780 nm) and local-
ized bands at 520–560 nm due to surface plasmon reso-
nance, which is caused by collective oscillations of the
conduction electrons upon excitation with visible light.[54]
The FT–IR spectra of nSTDP and Aunp–nSTDP are
shown in Figure 2. The FT–IR spectrum of the nSTDP
showed a broad O–H stretching band at 3200–3400 cm−1
FIGURE 5 Mapping spectrum of Aunp– nSTDP

6 of 13
HAGHSHENAS KASHANI ET AL.
FIGURE 6 TEM image for Aunp–nSTDP catalyst
decreased upon attachment of nano gold to nSTPD
(Figure 3).
FIGURE 7 Particle size distribution results for Aunp–nSTDP
catalyst
Figure 4A and 4B present the field emission scanning
electron microscopy (FE–SEM) images of nSTDP and
Aunp–nSTDP. The size and surface morphologies of the
nSTDP and Aunp–nSTDP are uniform. As shown in the
images, the nSiO2 particles are spherical and the average
diameters are in the range of 40 to 70 nm.
The energy dispersive X–ray (EDX) results, collected
from SEM analysis, proved the presence of S and Si ele-
ments in the nSTDP (Figure 4C) and also Au element in
the Aunp–nSTDP (Figure 4D). The presence of the Au
peak in the EDX spectrum of nSTDP is corresponded to
coating the sample with gold during the sample prepara-
tion for FE–SEM analysis.
crystal faces at 2 theta values of 38.2, 44.4, 64.6, 77.6
and 81.7 were obviously discovered (JCPDS file: 04–
0784). The diffraction peak intensity of the (111) faces
for gold is higher than that of any other peak, indicating
Elemental mapping of Aunp–nSTDP, depicted in
Figure 5, shows the uniform and homogenous dispersion
of C, Si, O, S and Au elements in the catalyst texture.
Figure 6 shows the transmission electron microscopy
(TEM) image of the Aunp–nSTDP. The picture verifies
that the particles are well dispersed and almost no aggre-
gation was observed. Also, the catalyst composed of
spherical, nanosized Au particles range from 2 to 6 nm
(Figure 7).
FIGURE 8 X‐ray diffraction pattern of Aunp–nSTDP catalyst
From the TEM size distributions, one can obtain the
average core diameter (D) of the nanoparticles. The aver-
age number of gold atoms in a cluster can then be calcu-
lated using the equation (1), in which (d) is the density of
the gold (59 atoms nm−3).
ꢀ ꢁ
π
6
NAu ¼ d
D3
(1)
In this work, the average core diameter (D) is about
2–6 nm. Therefore, the numbers of NAu are 247–6670
atoms in a hypothetical cluster.
The gold content of catalyst, measured by ICP, showed
a value of about 0.19 mmol/g of heterogeneous catalyst.
In the XRD pattern of the catalyst (Figure 8), the
peaks indexed to Au (111), (200), (220), (311) and (222)
FIGURE 9 XPS spectrum of Aunp–nSTDP catalyst

HAGHSHENAS KASHANI ET AL.
7 of 13
that the reduction of Au (III) to Au occurred preferen-
tially on Au (111) faces, as expected.[55] It is clear that
the typical silica is observed at a broad peak centered at
2θ = 22.5°, which indicates that the sample is semi‐
crystalline phase.[56]
The XPS elemental survey scans of the surface of
Aunp–nano silica show the peak corresponding to gold
(Figure 9). The calibration was performed with the C1s
peak (E = 284.5 eV).
alcohols with TBHP. First, the reaction conditions such
as amount of catalyst, kinds of solvent and base, and tem-
perature were optimized in the oxidation of benzyl alco-
hol in the presence of Aunp–nSTDP under mechanical
stirring (1000 rpm). The results are summarized in
Table 1. In the optimization of the amount of Aunp–
nSTDP catalyst, the highest yield was obtained in the
presence of 0.0076 mmol of Aunp–nSTDP on the basis of
The B.E. values of Au 4f7/2 and 4f5/2 spectral lines are
varied over a narrow range from 83.3 to 83.7 eV, and 87.4
to 87.6, respectively. These values clearly indicate that the
gold in the catalyst is present mainly in metallic form, i.e.
Au (0), which clearly visible in Figure 9. This is in a good
agreement with previously reported results.[57,58]
3.2 | Oxidation of alcohols with TBHP
catalyzed by Aunp–nSTDP
After preparation and characterization of Aunp–nSTDP,
its catalytic activity was investigated in the oxidation of
FIGURE 10 The variation of yield with catalyst amount at 80 °C
TABLE 1 Optimization of reaction conditions in the oxidation of benzyl alcohol catalyzed by Aunp–nSTDP after 2 h under mechanical
stirringa
Entry
Catalyst (mol%)
Catalyst (mg)
Oxidant
Base (mmol)
Solvent
T (°C)
Yield (%)b
1
0.38
0.57
0.76
0.86
0.95
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
0.76
20
30
40
45
50
40
40
40
40
40
40
40
40
40
40
40
40
40
TBHP
TBHP
TBHP
TBHP
TBHP
H2O2
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (0.5)
K2CO3 (1.5)
NaOH (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
K2CO3 (1)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
CH3CN/H2O (3:2)
H2O
80
80
80
80
80
80
80
40
60
80
80
80
80
80
80
80
80
80
62
83
92
92
93
70
58
61
78
72
91
67
51
65
83
80
65
84
2
3
4
5
6
7
O2 flow
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
TBHP
8
9
10
11
12
13
14
15
16
17
18
EtOH
CH3CN/H2O (1:1)
EtOH/H2O (3:2)
acetone/H2O (3:2)
MeOH/H2O (3:2)
aReaction conditions: benzyl alcohol (0.5 mmol), oxidant (1 mmol), catalyst, solvent (5 ml), base.
bGC yield based on the starting benzyl alcohol.

8 of 13
HAGHSHENAS KASHANI ET AL.
the Au content. Lower amounts of catalyst gave lower
yields (In the presence of 0.0019 mmol of Aunp–nSTDP
the yield was 35%), while when the catalyst amount was
increased to 0.0095 mmol, no significant increase was
observed in the yield (entries 1–5). It is noteworthy that
in the absence of Aunp–nSTDP, only 5% of the product
was detected in the reaction mixture. The variation of
yield with amount of the catalyst at 80 °C is depicted in
Figure 10. The nature of oxygen donor is another essen-
tial factor that needs to be optimized carefully. For this
TABLE 2 Oxidation of alcohols with TBHP catalyzed by Aunp–nSTDPa
Entry
Alcohol
Carbonyl compound
Yield (%)b
TOF (h−1 c
)
1
Benzyl alcohol
Benzaldehyde
92
93
89
85
84
80
91
83
81
85
55
63
30.3
30.6
29.3
28.0
27.6
26.3
29.9
27.3
26.6
28.0
18.1
20.7
2
4‐Methylbenzyl alcohol
4‐Chlorobenzyl alcohol
4‐Fluorobenzyl alcohol
4‐Bromobenzyl alcohol
4‐Nitrobenzyl alcohol
3‐Methoxybenzyl alcohol
3‐Nitrobenzyl alcohol
2‐Nitrobenzyl alcohol
2,4‐Dichlorobenzyl alcohol
1‐Hexanol
4‐Methylbenzaldehyde
4‐Chlorobenzaldehyde
4‐Fluorobenzaldehyde
4‐Bromobenzaldehyde
4‐Nitrobenzaldehyde
3‐Methoxybenzaldehyde
3‐Nitrobenzaldehyde
2‐Nitrobenzaldehyde
2,4‐Dichlorobenzaldehyde
1‐Hexanal
3
4
5
6
7
8
9
10
11
12
Cyclohexanol
Cyclohexanone
aReaction conditions: alcohol (0.5 mmol), TBHP (1 mmol), K2CO3 (1 mmol), catalyst (0.76 mol%, 40 mg), CH3CN/H2O (3:2 ml) at 80 °C after 2 h under mechan-
ical stirring.
bGC yield based on starting alcohol.
cTurn over frequency, defines as the mmole of produced aldehyde per mmole of gold per hour.
SCHEME 3 Investigation of
chemoselectivity and homoselectivity of
the presented method in the oxidation of
alcohols

HAGHSHENAS KASHANI ET AL.
9 of 13
purpose, the effect of various oxidants such as NaIO4,
H2O2, O2 and tert–butyl hydroperoxide was investigated.
The obtained results showed that TBHP can give highest
oxidation conversion (entries 3, 6). In the presence of
NaIO4 only 45% of the product was detected in the reaction
mixture. To optimize the solvent, different types of single
and mixed solvents were employed. Amongst, the mixture
of CH3CN/H2O (3:2) was proved to be the optimum reac-
tion medium (entries 3, 13–18). In CH3CN and CHCl3/
H2O only 43% and 48% of the product was detected, respec-
tively. In addition, the model reaction was carried out at
different temperatures; on the basis of the obtained results,
the optimum reaction temperature was found to be 80 °C
(entries 3, 8, 9). At room temperature 48% of the product
was obtained. Then, the same reaction was carried out in
the presence of different organic and inorganic bases and
K2CO3 (1 mmol) was found to be the most effective base
(entries 3 and 10–12). In the presence of NEt3 only 46% of
the benzaldehyde was produced. A reaction using pure
benzyl alcohol (10 mmol) in the absence of solvent was
checked and only 10% of the benzaldehyde was produced.
Therefore, it was concluded that the optimum reac-
tion conditions involved substrate (0.5 mmol), TBHP
(1 mmol), K2CO3 (1 mmol), and the Aunp–nSTDP catalyst
(0.0076 mmol) in CH3CN/H2O (3:2) at 80 °C under reflux
conditions. No carboxylic acid was produced in the oxida-
tion reactions and the only product is benzaldehyde.
TABLE 3 Comparison of the catalytic activity of Aunp–nSTDP
with some of previously reported systems in the oxidation of benzyl
alcohol
Entry
Catalyst
TOF (h−1
)
Ref.
1
2
3
4
5
6
7
8
Aunp–nSTDP
AuNPs@UiO–66
Au@U3O8
30.26
7.5
This work
[62]
[63]
[64]
[65]
[66]
[66]
[66]
0.13
2.25
11
Au@Zeolites (Y)
Au/zeolite–Y
Au/MgO
45.3
67
Au/NiO
FIGURE 11 The results of nano Au catalyst recovery in the
oxidation of benzyl alcohol
Au/La2O3
63
TABLE 4 Oxidation of benzyl alcohol to benzaldehyde by gold catalysts
Entry
Catalyst
BA:Au
Solvent
Conditions
Conversion (%)
Selectivity (%)a
Ref.
[36]
1
Au:PVP
50
water
3 equiv. K2CO3, air, 27 °C, 6 h
1 equiv. NaOH, 5 bar O2, 80 °C, 3 h
1.5 bar O2, 130 °C, 5 h
10 bar O2, 160 °C, 6 h
10 bar O2, 160 °C, 6 h
O2 flow, 80 °C, 2 h
100
75
85b
90b
65
[68]
[37]
[69]
[69]
[70]
[71]
[72]
[65]
[73]
[74]
[75]
2
Au/steel fibre
Au/Al2O3
Au/C
5700
900
water
3
‐
69
4
76000
76000
200
‐
48
64
5
Au/TiO2
‐
67
64
6
Au/Ga3Al3O9
‐
98
>99
93
7
Au/TiO2
Au/HTd
1000
240
biphasicc
p‐xylene
toluene
toluene
‐
10 bar O2, 100 °C, 8 h
no oxidant, 120 °C, 6 h
1 equiv. K2CO3, air, 80 °C, 5 h
O2 flow, 80 °C, 1 h
83
8
>99
>95
>99
100
100
>99
>99
>99
69.5
85
9
Au/NaYe
Au/MIL‐101f
Au/MgO
Au/U3O8
110
10
11
12
100
9.6g
1.9g
TBHP, 94 °C, 0.5 h
‐
TBHP, 94 °C, 0.5 h
aSelectivity to benzaldehyde unless otherwise stated.
bSelectivity to benzoic acid.
cWater: p‐xylene = 7:1 (mol: mol).
dGold on hydrotalcite.
eGold on zeolite Y.
fGold on metal–organic framework.
gGram catalyst per mole of benzyl alcohol.

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HAGHSHENAS KASHANI ET AL.
To investigate the scope and generality of this method,
different alcohols were employed in this catalytic system
(Table 2). In this case, different linear, cyclic and phenyl–
substituted alcohols were oxidized with TBHP in the pres-
ence of 0.76 mol% of Aunp–nSTDP. The experimental data
showed that all reactions proceeded extremely well and the
electronic properties of the substituents and functional
groups on the phenyl ring had almost negligible effect on
the results. 1‐Hexanol and cyclohexanol as linear and
cyclic alcohols were successfully oxidized to their corre-
sponding carbonyl compounds.
TABLE 5 Investigation of catalyst leaching of the Aunp −nSTDP
catalyst in the oxidation of benzyl alcohol with TBHP after 2 ha
Run
Au leached (%)b
The chemoselectivity (using different kind of alcohols)
and homoselectivity (using a substrate with two similar
hydroxyl groups) were also examined.[59–61] To investigate
the chemeselectivity, an equimolar mixture of benzyl alco-
hol and cyclohexanol was subjected to oxidation with
TBHP under the optimized reaction conditions
(Scheme 3A). The results showed that benzyl alcohol was
selectively oxidized in the presence of n‐hexanol. On the
other hand, in the reaction of an equimolar mixture of ben-
zyl alcohol and 1‐hexanol with TBHP under the optimized
reaction conditions, benzyl alcohol was selectively oxi-
dized to benzaldehyde in 80% yield (Scheme 3B).
1
2
3
4
3
2
0
0
aReaction conditions: benzyl alcohol (0.5 mmol), TBHP (1 mmol), K2CO3
(1 mmol), catalyst (0.76 mol%, 40 mg), CH3CN/H2O (3:2 ml) at 80 °C after 2 h.
bDetermined by ICP analysis.
The oxidation of 1,4‐phenylenedimethanol with TBHP
was chosen for investigation of homoselectivity of this
method (Scheme 3C). Under the optimized reaction condi-
tions used for oxidation of one alcohol group, both alcohol
groups were oxidized and the terephthaldialdehyde was
produced in 47% yield. By doubling the amount of all reac-
tant, the yield of terephthaldialdehyde reached to 92%. The
efforts for obtaining the 4‐(hydroxymethyl) benzaldehyde
was failed. This shows that this method is not suitable for
homoselective oxidation of bifunctional alcohols.
The results of the present study were compared with
some results reported in the oxidation of benzyl alcohol
catalyzed by different Au containing catalysts. As can be
FIGURE 12 XPS spectrum of reused Aunp–nSTDP catalyst
SCHEME 4 The proposed mechanism
of alcohol oxidation by Aunp–nSTDP
catalyst

HAGHSHENAS KASHANI ET AL.
11 of 13
seen, this catalytic system is more efficient than the
others in the oxidation reaction by comparison of their
TOFs (Table 3).
which is difficult to be accomplished by gold alone.
Finally, the aldehyde releases from the catalyst surface
and the catalytic cycle continues.[67]
Consequently, dendrimer–encapsulated metallic par-
ticles (DEMs), due to the more activation of the surface
of metallic particles and isolation of catalytic active sites,
can exhibit higher catalytic activity compared to the other
catalytic systems.
High pH of the reaction system plays a crucial role in
enhancing the catalytic activity. The effect of basicity is to
provide an OH− anion and form an Au–OH− site, which
is essential for hydrogen abstraction from alcohol.[67]
Some literature results for oxidation of benzyl alcohol
to benzaldehyde catalyzed by gold catalysts are shown in
Table 4 together with reaction conditions. As can be seen,
our reported system is superior in terms reaction time,
reaction conditions, selectivity or catalyst amount
(Table 4).
Electrocatalytic oxidation studies using gold elec-
trodes strongly suggest that gold atoms having low coor-
dination number (CN) are more electropositive and are
therefore more easily oxidized than those of high CN,
and this suggests that peripheral gold atoms in small sup-
ported particles may readily transform into ions. The
simultaneous presence of both atoms and ions is there-
fore not ruled out.[78]
4 | CONCLUSIONS
This paper describes the synthesis and characterization of
a new thiol based dendritic material containing gold
nanoparticles. The new synthesized nano catalyst was
proved to be highly effective heterogeneous catalyst for
carbonyl compounds synthesis by the chemoselective oxi-
dation of alcohols. Moreover, this catalyst, Aunp–nSTDP,
can be well–dispersed in the reaction medium, conve-
niently separated from the reaction mixture, and reused
several times without significant loss of its activity.
3.3 | Catalyst reusability
Finally, we turned our attention to the recovery and reus-
ability of the catalyst which are very important for com-
mercial and industrial applications as well as green
process considerations. In this respect, the reusability of
the catalyst was investigated in the oxidation of benzyl
alcohol with TBHP under the optimized reaction condi-
tions at the end of the reaction, the catalyst was easily
separated by centrifugation, washed with EtOH and
H2O and acetonitrile, dried in an oven at 60 °C and
reused with fresh substrates. As can be seen, the catalyst
was reused several times without significant loss of its
catalytic activity (Figure 11). On the other hand, the
amount of Au in the reused catalyst after fourth run
showed a value of about 0.18 mmol/g. The filtrates after
each run were used for determination of the amount of
Au leaching. The results, which are summarized in
Table 5, showed that after second run, no Au was leached
in catalytic reactions.
ACKNOWLEDGEMENTS
The authors are grateful to the Iranian National Science
Foundation (INSF) (Grant No. 94013580) and Research
Council of the University of Isfahan for financial support
of this work.
ORCID
Majid Moghadam http://orcid.org/0000-0001-8984-1225
Shahram Tangestaninejad
5263-9795
http://orcid.org/0000-0001-
Iraj Mohammadpoor‐Baltork
7998-5401
http://orcid.org/0000-0001-
The nature of recovered catalyst was studied by XPS
spectroscopy (Figure 12). The BE of 85.0 eV, due to oxi-
dized surface gold species, can be distinguished in the
Au 4f7/2 energy region.[76] The binding energy at 85.5
0.1 eV for the reused catalyst indicated the presence of
Au+, probably at the gold/support interface. It is not clear
which oxidation state of gold is actually active in this
reaction.[77] The formation of Au+ is in accordance to
the proposed mechanism in Scheme 4.
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How to cite this article: Haghshenas Kashani S,
Landarani‐Isfahani A, Moghadam M,
Tangestaninejad S, Mirkhani V, Mohammadpoor‐
Baltork I. Nano–silica functionalized with thiol–
based dendrimer as a host for gold nanoparticles:
An efficient and reusable catalyst for
chemoselective oxidation of alcohols. Appl
Organometal Chem. 2018;e4440. https://doi.org/
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