Synthesis and characterization of Au nanocatalyst on modifed bentonite and silica and their applications for solvent free oxidation of cyclohexene with molecular oxygen

Article abstract of DOI:10.1016/j.molcata.2015.05.026

In the present work, the selective liquid phase oxidation of cyclohexene mainly to 2-cyclohexe-1-one has been investigated over gold nanoparticles (GNPs) with molecular oxygen in a solvent-free condition. Gold nanoparticles were synthesised on two modifie

Full text of DOI:10.1016/j.molcata.2015.05.026

Accepted Manuscript
Title: Synthesis and characterization of Au nanocatalyst on
Modifed Bentonite and Silica and Their Applications for
Solvent Free Oxidation of Cyclohexene with Molecular
Oxygen
Author: M.Shahabi nejad G. Ghasemi M.V. Mart ´ı nez-Huerta
M. Ghiaci
PII:
S1381-1169(15)00210-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2015.05.026
MOLCAA 9512
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
19-2-2015
30-5-2015
30-5-2015
´
Please cite this article as: M.Shahabi nejad, G.Ghasemi, M.V.Martinez-Huerta,
M.Ghiaci, Synthesis and characterization of Au nanocatalyst on Modifed
Bentonite and Silica and Their Applications for Solvent Free Oxidation of
Cyclohexene with Molecular Oxygen, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2015.05.026
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Synthesis and characterization of Au nanocatalyst on Modifed Bentonite
and Silica and Their Applications for Solvent Free Oxidation of
Cyclohexene with Molecular Oxygen
a
a
b
a
M. Shahabi nejad , G. Ghasemi , M. V. Martínez-Huerta , M. Ghiaci *mghiaci@cc.iut.ac.ir
a
Department of Chemistry, Isfahan University of technology, Isfahan, 8415683111, Iran.
Institute of Catalysts and Petroleochemistry, CSIC, Marie Curie 2, 28049 Madrid, Spain.
b
*Corresponding author.
1

Graphical Abstract
2

Highlights
►
gold nanoparticles supported on modified bentonite and modified silica.
►
Liquid phase oxidation of cyclohexene mainly to 2-cyclohexene-1-one and 2-
Cyclohexene-1-ol.
A high conversion (92%) and a high selectivity (97%) to 2-cyclohexene-1-one was
obtained.
►
►
TEM images show that the immobilized gold nanoparticles on modified silica have
diameter around 5-10 nm.
3

Abstract: In the present work, the selective liquid phase oxidation of cyclohexene mainly to
-cyclohexe-1-one has been investigated over gold nanoparticles (GNPs) with molecular
2
oxygen in a solvent-free condition. Gold nanoparticles were synthesised on two modified
supports of silica and bentonite. In this respect the surface of silica and bentonite was
modified with organic ligands consist of thiol and thioester groups. The catalysts were
2
characterized by TEM, XPS, N adsorption/desorption, FT-IR, and CHNS techniques. TEM
images show that the gold nanoparticles over modified bentonite and silica have diameters in
the range 0f 50 and less than 10 nm, respectively. The results show that the catalytic activity
of gold nanoparticles over modified silica, SiO
better than the gold nanoparticles immobilized on the modified bentonite, MEDPT@CP-
bentonite-Au (0). The catalytic activity over SiO -pA-Acrylate-Thioamide-Au (0) recycled
2
-pA-Acrylate-Thioamide-Au (0), is much
2
catalyst remained at a satisfactory state after at least 4 cycles. Activity tests were carried out
in an autoclave under solvent-free conditions. In order to obtain maximum conversion, the
reaction parameters such as reaction temperature and time were optimized. Under optimized
conditions, a maximum of 92% conversion and 97% selectivity was achieved with the SiO
pA-Acrylate-Thioamide-Au (0) catalyst.
2
-
Keywords : Nanogold catalyst; Cyclohexene oxidation; Molecular Oxygen; Heterogeneous
Catalysis.
4

1. Introduction
The catalytic aerobic transformation of alkenes into value-added oxygenated
derivatives is still a challenge in the modern chemistry and industry world [1-4]. In particular,
the oxidation of cyclohexene is often inefficient as there is a C=C bond and four állylic-H
atoms in the cyclohexene molecule. Oxidation of cyclohexene is an important organic
reaction for the synthesis of chemical intermediates like 2-cyclohexene-1-ol and 2-
cyclohexene-1-one used in the manufacture of high-value pharmaceuticals [5]. A greater
demand for these oxidation products and increased environmental concerns warrant the
introduction of catalytic systems using heterogeneous catalysts and the environmentally
friendly oxidants such as molecular oxygen or hydrogen peroxide [6-8]. The use of H
2
O
2
is
really appreciated because the only by-product is water, but the relatively high cost of H
2
O
2
severely hinders its wide application in catalytic oxidation [9]. On the other hand, catalytic
systems using heterogeneous catalysts and molecular oxygen as the oxidant resulted in three
important advantages: the facility to separate the catalyst after the reaction, lower energy
costs and a higher stability of the irreversible reaction over oxidation products [10,11]. In this
research, supported gold nanoparticles over modified bentonite and modified silica have been
used. Bentonite has long been used as catalyst [8] and as support for many catalysts, because
it is a green support, chemically inert and thermally stable. The silica-supported metal
catalysts have advantages over the other heterogeneous catalysts, since they show excellent
properties such as high stability (chemically and thermally), good accessibility, and also due
to the fact that organic groups can be literally anchored to the surface to provide catalytic
centers [9-12]. Herein, and in continuation of our previous works [13,14], we wish to report
the activity of two new catalysts and compare their preference in this important reaction.
In recent years, an increasing interest has been directed to the catalytic potential of
gold catalysts. Gold nanoparticles are excellent catalyst for oxidation of hydrocarbons and
when its particles are small enough they are so hectic that can perform the oxidation process
by using molecular oxygen. According to many different studies, the catalytic activity and
selectivity of supported gold nanoparticles depend upon the support and preparation method.
Among the various techniques for obtaining supported gold catalysts, co-precipitation,
deposition-precipitation, vapor-phase deposition, co-sputtering are the most important and
applicable ones. In this work, the gold nanoparticles were synthesized in an efficient and
gentle condition. In the first stage, the surface of bentonite and silica was modified by a new
pincer-type ligand; this organic ligand consists of thiol and thioester groups. Clearly, we have
5

modified the surface of bentonite and silica by different methods but the pincer! parts of the
two ligands are identical, so that one would expect that the gold nanoparticles are trapped in
comparable environment. Advantages of this method are: (1) a very mild condition (pressure
and temperature) has applied during the synthesis of nanoparticles. (2) By increasing or
decreasing the molar ratio of the ligand on the surface of the support, it is possible to control
the size of provided nanoparticles. (3) Bonds between thiol and thioester groups with gold
nanoparticles are probably coordination bonds which causes gold nanoparticles to have a very
stable attachment to the support. In this work, we have investigated the activity of the
synthesized gold nanoparticles in the oxidation of cyclohexene. The reactions were conducted
in the solvent-free conditions by using molecular oxygen as a cheap and green oxidant. One
of the catalysts has shown a desirable activity and selectivity that will be discussed in the
following sections.
2. Experimental Section
2.1. Materials
The parent bentonite had the following chemical composition (in wt%): SiO
(1.67), MgO (1.87), Al (13.61), CaO (2.01), TiO (0.19), Na O (2.26), K
It was obtained from Salafchegan mine (Salafchegan, Iran). SiO (400 mesh), n-cetyl
pyridinium bromide (CP), methyl acrylate, dodecylamine, and ethanedithiol were purchased
from Merck and used as received. The metal precursor AuCl (99%) was obtained from
Aldrich. All other chemicals used this study were of analytical grade.
2
(65.04),
Fe
2
O
3
2
O
3
2
2
2
O (0.75).
2
3
2.2. Characterization techniques
Transmission electron microscopy (TEM) was carried out on the powder samples
with a Tecnai F30TEM operating at an accelerating voltage of 300 kV.
BET surface area and pore size distribution were measured on a Micromeritics
Digisorb 2600 system at −196 °C using N
were degassed at 450 °C.
2
as adsorbate. Before measurements, the samples
FTIR spectra of the catalyst were recorded on a JASCO FTIR 680 plus spectrometer
with the KBr pellet method.
BEIFEN 3420 gas chromatograph equipped with a FID detector was used to identify
the reaction products. The column was a 30 m HP-FFAP with 0.32 mm i.d. and 0.5µm film
6

thickness. The initial temperature was 170 °C for 5 min. The GC was then ramped at 10
C/min to 280 °C, and hold for 1 min at that temperature. The products of the oxidation of
cyclohexene were confirmed by GC/MS (Fisons Instruments 8060, USA).
°
2
2
.3. General procedures
.3.1. Purification of the raw bentonite
Since SEM and XRD analysis revealed the presence of significant amounts of quartz
and feldspar, the raw material (Salafchegan bentonite) was purified using the following
procedure. Bentonite slurry (5 wt %) in water was prepared and swollen at room temperature
under continuous stirring. After 5 h, the suspension was diluted to 2 wt % by the addition of 1
3
mol/dm NaCl solution. The suspension was stirred overnight and then decanted. This ion-
exchange procedure was repeated three times with fresh NaCl solution. Finally, the solid was
washed free of chloride ions. In the last washing, stirring was stopped and the particles
contained in the suspension were separated from the quartz and feldspar precipitates and
centrifuged. The resulting cake was dried at room temperature. The cation-exchange capacity
(
CEC) of the Na-bentonite was 0.7 mequiv/g, as measured using the Ming and Dixon method
15]. The specific surface area of the unmodified Na-bentonite, as determined by N
[
2
2
2
adsorption (single point method), was 63 m /g. Purification increased this value to 124 m /g.
.3.2. Preparation of monolayer bentonite
To obtain bentonite with monolayer surfactant coverage (organobentonite), the
purified Na-bentonite was modified with n-cetyl pyridinium bromide (CPB) solution (cmc;
2
−
3
3
−4
critical micelle concentration = 10 mol/dm ) with a concentration below the cmc (5 × 10
3
mol/dm ). Thus, 2 g of Na-bentonite was dispersed in 100 mL of CPB solution (20% w/w
ethanol−water, pH 6) and the resulting dispersion was shaken for 48 h and centrifuged. The
solid was washed with water to remove excess of the surfactant and surfactant loosely
attached to the bentonite particles, then dried at room temperature. The specific surface area
2
of the monolayer bentonite, as determined by N adsorption (single point method) was 26
2
m /g.
2
.3.3. Synthesis of s,s′-bis(2-mercaptoethyl)-3,3′-(dodecylazanediyl) dipropanethioate
MEDPT)
For modification of the monolayer bentonite we have used MEDPT. This compound
(
was prepared as mentioned below. In the first step, 5.4 g (29.1 mmol) dodecyl amine and 15
g (174.2 mmol) methyl acrylate were refluxed in 30 mL methanol under a nitrogen
atmosphere for 24 h. Subsequently, 2.2 mL (26.2 mmol) ethanedithiol was added to the
7

mixture which was then refluxed for 2 days under N
2
atmosphere. The product s,s′-bis(2-
mercaptoethyl)-3,3′-(dodecylazanediyl) dipropanethioate (MEDPT) was separated and
-1
purified by column chromatography. IR (KBr): vmax/cm , 2852(s), 2559(m), 1638(m).
Elemental analysis % calculated for C22 , C = 54.89, H = 8.94, N = 2.91, S = 26.61
and observed % was C = 55.22, H = 9.17, N = 2.73 and S = 26.97. Mass (m/z): 481 (Scheme
H
4 2
43NS O
1).
2.3.4. Preparation of bilayer bentonite (MEDPT@CP-bentonite)
To obtain the bentonite with bilayer surfactant coverage, the monolayer bentonite was
−
3
3
modified using MEDPT solution (cmc = 10 mol/dm ) with a concentration below the cmc
−
4
3
(
5 × 10 mol/dm ). Thus, 0.1 g of the monolayer modified bentonite was dispersed in 20 mL
of MEDPT solution (20% w/w ethanol−water, pH 6). The resulting dispersion was shaken for
8 h and centrifuged. The solid was then washed with water to remove excess surfactant and
surfactant loosely attached to the bentonite particles, then dried at room temperature. The
4
2
specific surface area of the bilayer bentonite, as determined by N adsorption (single point
2
method) was found to be 25 m /g
2.3.5. Preparation of immobilized gold (0) on modified bentonite (MEDPT@CP-bentonite-
Au (0))
To prepare this catalyst, 0.05 g MEDPT@CP-bentonite was completely suspended in
-2
1
3
00 mL n-hexane for 24 hours on a magnetic stirrer, then 290 µl AuCl solution (1x10 M)
was added very slowly (10 µl each time) to the mixture during 12 hours. During this period,
the solution was shaken with constant speed. MEDPT-Bentonite that now contains golden
nanoparticles was isolated and dried at ambient temperature, and in the next step by passing
hydrogen gas over the catalyst in a fixed bed reactor set for 3 hours and at 230 °C, Au(III)
was reduced to Au (0).
2
.3.6. Preparation of immobilized gold (0) on modified silica catalyst (SiO
2
-pA-Acrylate-
Thioamide-Au (0)):
The silica (400 mesh) was refluxed in EtOH with trimethoxysilyl propylamine for 24
h and was soxhlted for removing the unreacted materials. Afterward the modification of
support was done by the refluxing it with methyl acrylate in MeOH for 24 h. Then
ethandithiol was added, and the mixture was refluxed for another 24 h. At the final step the
8

gold nanoparticles was immobilized on the modified silica using the procedure explained
above for preparation of Au nanoparticles on modified bentonite.
2.3.5. General procedure for oxidation of cyclohexene
In order to perform cyclohexene oxidation, a reaction was designed as follows: 10 mL
cyclohexene with 0.05 mg of the catalyst was transferred to a steel reactor and the substrate
was oxidized by oxygen by modifying the reaction parameters such as temperature, oxygen
pressure, reaction time and amount of the catalyst. It should be mentioned that after
separating the catalyst, the reaction mixture was diluted in methanol and injected to the GC.
The products were analyzed using GC-MS (Shimadzu QP5000; DB1 column). Identification
of products was done by comparing the GC retention times of expected products with those
of standard samples.
3. Results and Discussion
As a general view, we have modified two hydrophilic supports with organic moieties,
and investigated the cyclohexene oxidation on the prepared catalysts with molecular oxygen
in solvent-free conditions, or in other words, in non-polar conditions. Polanski et al. [16]
investigated oxidation of cyclohexene on Au nanoparticles supported on different supports in
water as media, and discussed the influence of support wettability on conversion and
selectivity of the reaction. In the present work, by modifying the two supports, i.e., silica and
bentonite, by organic moieties they would be able to suspend very well in solvent-free
conditions. Therefore, the observed activity of the catalysts should be largely due to
wettability of the catalysts by the substrate and molecular oxygen as oxidizing agent.
3.1. Modification of bentonite
s,s′-bis(2-mercaptoethyl)-3,3′-(dodecylazanediyl) dipropanethioate (MEDPT) was
immobilized on a modified bentonite starting from Na-bentonite with monolayer n-cetyl
pyridinium cation coverage (CP-bentonite). The scheme of the process is shown in Scheme 2.
Immobilization of MEDPT proceeded smoothly on the monolayer modified bentonite.
MEDPT surfactant consists of a 12 carbon chain and is able to put its alkyl chain part
between hydrophobic chains of the CP-surfactants. In fact, we could imagine that its
stabilizing factor is hydrophobic interactions of the alkyl chain of the MEDPT and
hydrophobic part of the CP-surfactants. From the CHNS analysis (Supplementary data), it
was estimated that the final modified bentonite, i.e., MEDPT@CP-bentonite, contained 0.57
9

mmol CP-surfactant and 0.24 mmol MEDPT surfactant per gram of the final solid. The final
modified bentonite was designated as MEDPT@CP-bentonite as mentioned before.
The specific surface area of the catalysts was investigated in different steps by BET
technique. According to the data of Table 1, by comparing the surface area of the parent
bentonite with the surface area of the modified bentonite, i.e., CP-bentonite, it clearly shows
that the surface area of the modified bentonite has decreased, as expected. After loading the
MEDPT surfactant on the CP-bentonite, and immobilizing gold nanoparticles on it, the
surface area has not been changed to a large extent. In fact, one would expect that when
bentonite becomes bilayer by the MEDPT surfactant molecules, they should not probably
occupy new spaces on the bentonite, and therefore we should not expect that the surface area
changes too much.
Fig. 1 represents the FTIR spectra of several samples starting from Na-bentonite to
bilayer modified bentonite. The peaks corresponding to the Si-O-Si vibration in the bentonite
-1
spectrum (Fig. 1a) appears as a strong band in the range of 1000-1100 cm ; the band at 920
-1
-1
-1
cm is due to Al-OH. The peak at 1637 cm and the broad band at 3446 cm were assigned
to the bending and stretching modes of adsorbed water, respectively. The strong bands at
-1
around 3619 cm are due to the lattice OH stretching modes [17]. The FTIR spectra of
modified bentonites as monolayer and bilayer (Fig. 1b and 1c) display new peaks at 1473-
−
1
1
488, 1577, 2854-2927, and a peak at 2567 cm was attributed to the thiol stretching
vibration [18–20] which confirms the presence of the thiol groups (Supplementary data).
3.2. Modification of silica
The preparation procedure to obtain the catalyst is outlined in Scheme 3. We
employed silica with mesh size 400 as the support, which was activated at 500 ºC for 3 h in
oven. The resulting silica was then functionalized with 3-aminopropyltrimethoxysilane,
followed by condensation with methyl acrylate and ethanedithiol, respectively, via
nucleophilic substitution. Upon completion, the resulting solid was filtered off, and to remove
unreacted reactants the obtained modified silica was washed with methanol for 24 h in a
continuous extraction apparatus (Soxhlet) to give SiO
modified silica was characterized by elemental analysis (CHNS) (C 12.45%, H 2.26%, N
.06%, S 10.28%).
2
-pA-Acrylate-Thioamide. The
1
1
0

The FTIR spectra of SiO
2
, SiO
2
-pA, SiO
2
-pA-Acrylate, and SiO
(Fig. 2a) exhibits strong
bands in the range of 1000-500 cm , due to the silica skeleton. The characteristic bands of Si-
2
-pA-Acrylate-
Thioamide are compared in Fig. 2. The FTIR spectrum of SiO
2
-1
-1
O and O-H are observed at 1087 and 3200-3500 cm , respectively. In the FTIR spectrum of
-1
SiO
aliphatic segments and NH
with SiO -pA-Acylate (Fig. 2c), the strong peak appears at 1557 cm is attributed to the
sulfamide group in SiO -pA-Acrylate-Thioamide.
Table 1 shows the specific surface area of the support, SiO
2
-pA (Fig. 2b) the peak at 2850-3000 and 3200-3400 cm show the existence of the
2
groups, respectively. Moreover, by condensation of ethanedithiol
-1
2
2
2
-pA-Acylate, and SiO
2
2
2
-
-
-
pA-Acrylate-Thioamide. Modification of silica to the stage that we have synthesized SiO
pA-Acylate, decreased the specific surface area. Interestingly, by adding MEDPT to SiO
pA-Acylate the surface area increased to some extent that might be reasonable, because if we
accept that MEDPT does not occupy any new space on the silica and only decreases the
hydrophilic nature of the support, one should believe the slight increase in the surface area.
3.3. Immobilization of gold nanoparticles on modified bentonite and modified silica
MEDPT@CP-bentonite (0.5 g) was dispersed in dry n-hexane (100 mL) and stirred
-2
3
for 24 h under nitrogen. Then 300 µl of aqueous solution of AuCl (concentration = 1x10
M) was added to the mixture during 12 h. The mixture was stirred for another 24 h under
nitrogen, and then the solid was separated by centrifugation, washed with ethanol and ether.
Finally, the solid was dried at room temperature to furnish the corresponding MEDPT@CP-
bentonite-Au(III) sample. Then the as-prepared sample was treated in a stream of hydrogen at
230 ºC for 3 h to yield MEDPT@CP-bentonite-Au(0) catalyst. No diffractions were detected
for Au NP species from powder XRD patterns after H reduction which indicates Au loading
2
is too low or the nanoparticles are too small in the catalyst. Inductively coupled
plasma/atomic emission spectroscopy (ICP-AES) analysis of the obtained catalyst showed
that the Au weight percentage in the catalyst was 6.72%.
To immobilize the gold nanoparticles on the SiO
.0 g of the support was dispersed in dry n-hexane (100 mL) and stirred for 24 h at room
temperature under nitrogen. Then 300 µl of an aqueous solution of AuCl (concentration =
2
-pA-Acrylate-Thioamide support,
1
3
1
1

-2
1
x10 M) was added to the mixture during 12 h. The mixture was stirred for another 24 h
under nitrogen, and then the solid was separated by centrifugation, washed with ethanol and
ether. In this way, a new catalyst containing gold i.e., SiO -pA-Acrylate-Thioamide-Au (III)
was prepared. Similar to the procedure used for reduction of Au(III) in MEDPT@CP-
bentonite-Au(III), we treated the SiO -pA-Acrylate-Thioamide-Au(III) sample in a stream of
at 230 ºC for 3 h to prepare SiO -pA-Acrylate-Thioamide-Au(0) catalyst. Inductively
2
2
H
2
2
coupled plasma/atomic emission spectroscopy (ICP-AES) analysis of the obtained catalyst
showed that the Au weight percentage in the catalyst was 1.02%.
The TEM images (Fig. 3) show the distribution of Au nanoparticles in the range of 5-
10 nm size on the surface of modified silica, and in comparison with the Au nanoparticles on
the modified bentonite (Fig. 4), the size of nanoparticles are smaller by at least a factor of
five.
Finally, the SEM photographs of the both catalysts are shown in Fig. 5. By comparing
the surface area of Na-bentonite with the surface area of modified bentonite i.e.,
MEDPT@CP-bentonite catalyst, it has lost about 80% of its surface. The silica by
modification has lost about 20% of its surface. What could this mean? This means that for
modification of bentonite more carbon is added to the bentonite than that added to silica for
modification. It should also be mentioned that the organic moieties used for modification of
bentonite are naturally much more hydrophobic. Therefore, one would expect that bentonite
becomes more hydrophobic and fluffier!
3
3
.4. Catalytic performances
.4.1. Effect of catalyst and temperature
Oxidation of cyclohexene was carried out over MEDPT@CP-bentonite-Au (0), and
2
SiO -pA-Acrylate-Thioamide-Au (0) catalysts. In fact cyclohexene was chosen because this
molecule could be oxidized in two positions, both C=C bond and allylic positions; of course
oxidation of allylic positions are more important. In this reaction we also used oxygen as a
green and cheap oxidant. Moreover, because oxidation reaction was conducted in absence of
solvent the importance of the reaction has doubled. The results summarized in Table 2,
2
indicate that the conversion and selectivity of cyclohexene oxidation with the SiO -pA-
Acrylate-Thioamide-Au (0) catalyst containing 1.02 wt% Au was much higher than that with
the MEDPT@CP-bentonite-Au (0) catalyst containing 6.72 wt% Au. As an overall view, the
1
2

silica supported catalyst displayed 55% higher cyclohexene oxidation than bentonite
supported catalyst. As TEM images of the two catalysts show the size of nanogolds in silica
supported catalyst are smaller than the nonogolds in the bentonite basis catalyst by a factor of
five, and also the silica supported catalyst has a much higher surface area, and clearly these
characteristic properties have a large effect on conversion and selectivity.
The reaction temperature has also an impact on both activity and selectivity
2
especially on SiO -pA-Acrylate-Thioamide-Au (0) catalyst. It should be reminded that the
boiling point of cyclohexene is 83 ºC at atmospheric pressure, and we have done the reaction
under the pressure of oxygen (PO2 = 8-13 bar). Therefore, at 80 ºC one would expect that
cyclohexene was in the liquid phase in the reaction conditions, and its contact with the
catalyst might be in optimal condition. As a matter of fact, by increasing the reaction
temperature to 80 ºC, selectivity to cyclohexene-2-one has increased to 97%. However, the
MEDPT@CP-bentonite-Au (0) catalyst was not efficient in this regard! By increasing the
temperature to 95 ºC we did not lose the activity of the catalyst because of high pressure of
oxygen that increased the boiling point of cyclohexene. Interestingly, like our previous work
in oxidation of cyclohexene [14], the optimum temperature for oxidation 80 ºC was chosen,
because of energy saving!
3.4.2. Effect of oxygen pressure
The efficient operating pressure for both catalysts has been obtained. As a matter of
2
fact, the two catalysts have different optimal pressure. The best operation pressure for SiO -
pA-Acrylate-Thioamide-Au (0), and MEDPT@CP-bentonite-Au (0) catalysts was 13 and 8
bars, respectively. It is interesting that these two catalysts have different optimum pressure.
As a matter of fact, the bentonite supported catalyst has a much lower surface area, and it will
be saturated with oxygen at lower pressure, and there would be less room for substrate.
Therefore, by increasing the pressure of oxygen to higher values one should expect a
decrease in conversion in comparison with the silica supported catalyst (Table 3).
3.4.3. Effect of reaction time
Table 4 shows the effect of time on the cyclohexene oxidation with molecular oxygen
over the two catalysts. Clearly, 8 h was chosen as the best time for the oxidation reaction over
silica supported catalyst, and 10 h was the best for bentonite supported catalyst. Again this
has something to do with surface area. The catalyst that has more surface area has more
1
3

2
chance to convert the cyclohexene to products in a shorter time. Also the SiO -pA-Acrylate-
Thioamide-Au (0) catalyst shows much more selectivity to 2-cyclohexene-1-one in different
condition than the MEDPT@CP-bentonite-Au (0) catalyst.
3.4.4. Effect of amount of catalyst
To study the effect of amount of catalyst, the oxidation reaction was carried out at 80
ºC, oxygen pressure, 17 bar and the reaction time of 8 h. From the results summarized in
Table 5, it is observed that catalyst loading of 50 mg is appropriated to run the reaction with
2
SiO -pA-Acrylate-Thioamide-Au (0) catalyst. However, in the case of MEDPT@CP-
bentonite-Au (0) catalyst by using 100 mg of the catalyst the conversion increased to some
extent, but because of the low selectivity of the catalyst we did not bother to show more
curiosity. We demonstrated in our previous work [13] that 1,2-dichloroethane is the best
solvent for the reaction. Thus, using this solvent, conversion and selectivity to ketone reached
90% and 55%, respectively. Comparing these data with solvent-free conditions, a significant
decrease in selectivity was observed under solvent condition.
3.4.5. Reusability of the catalyst
One of the main advantages of heterogeneous catalysts over homogeneous is that the
former can be recovered and reused. The GNPs/TChD catalyst could be easily separated from
the reaction mixture by centrifugation and then wash it with organic solvent and subsequently
with water. The results of reusability are presented in Table 6. Data showed that the
selectivity of their used catalyst remained unchanged after four cycles, and then the catalytic
performance decreased gradually. As a matter of fact, oxidation in the liquid phase is very
often subject of leaching phenomenon and the question about the true nature of the catalytic
reaction (homogeneous or heterogeneous) is a matter of debate. To answer this question, the
catalyst was separated from the reaction mixture and then the reaction was continued without
the heterogeneous catalyst according to the method described by Sheldon et al. [21]. The
results show that the amount of leached metal is very low and its contribution to the total
activity of the catalyst in the cyclohexene oxidation reaction is probably negligible.
1
4

In Table 7, we have compared the result obtained over previously reported catalysts
for oxidation of cyclohexene. Supporting different Schiff-base metal complexes on
polymeric materials such as chloromethylated polystyrene (PS-DA-M; M = Cu2+, Co2+,
2
+
Ni2+, and Mn2+) [22], polyamidoamine (PAMAM-Mn ) [23], CrMCM-41 [24], Au/HNTs
25], Au/La-OMS-2 [26], and our previously reported nanogolds over modified chitosan [14]
are some examples of catalysts which have been used for oxidation of cyclohexene under
solvent free conditions with molecular oxygen. As shown in the Table 7, the catalyst SiO
[
2
-
pA-Acrylate-Thioamide-Au (0), reported in this work has shown an excellent selectivity to 2-
cyclohexene-1-one.
4. Conclusion
In this work we have prepared two supports with identical anchoring ligands for gold
nanoparticles and compared their reactivity and selectivity for oxidation of cyclohexene
under solvent-free conditions and using molecular oxygen as the greenest oxidant. The size of
gold nanoparticles on the SiO
2
-pA-Acrylate-Thioamide-Au (0) catalyst was in the range of 5-
10 nm. These nanoparticles were well dispersed and have shown a good reactivity (92%) and
selectivity (97%) to 2-Cyclohexene-1-one.
Acknowledgments:
Thanks are due to the Iranian Nanotechnology Initiative and the Research Council of
Isfahan University of Technology and Centre of Excellence in the Chemistry Department of
Isfahan University of Technology for supporting of this work.
1
5

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1
7

Figure Captions
Scheme 1. Synthesis of s,s′-bis(2-mercaptoethyl)-3,3′-(dodecylazanediyl) dipropanethioate
(MEDPT).
Scheme 2. Model of sorption of surfactant on the clay surface, and preparation of Au
nanoparticles.
1
8

Scheme 3. Modification of silica and preparation of Au nanoparticles.
Figure 1. FT-IR spectra of bentonite (a) raw bentonite, (b) monolayer bentonite and (c)
bilayer bentonite [KBr pill].
1
9

2 2 2
Figure 2. FT-IR spectra of (a) Silica gel, (b) SiO -pA, (c) SiO -pA-Acrylate and SiO -pA-
Acrylate-Thioamide [KBr pill].
Figure 3. Gold nanoparticles immobilized on the modified silica.
2
0

2
1

2
Figure 5. SEM images of (a) MEDPT@CP-bentonite-Au (0), and (b) SiO -pA-Acrylate-
Thioamide-Au (0) catalysts.
2
2

Tables
Table 1. BET analysis for modification of different supports.
Entry
Support
Surface area
2
(
m /g)
1
2
3
4
5
6
7
Raw Bentonite
Na-Bentonite
63
124
26
CP-Bentonite
MEDPT@CP-bentonite
Silica gel
25
480
375
378
SiO
2
-pA-Acrylate
SiO
2
-pA-Acrylate-Thioamide
Table 2. Oxidation of cyclohexene over MEDPT@CP-bentonite-Au (0) and
SiO
2
-pA-Acrylate-Thioamide-Au (0) at various temperatures.
Catalyst
selectivity
Cy-ol%
Temperature
Conversion
%
Cy-one%
Others%
Entry
0
C
Au- Bentonite
Au- Silica
Au-Bentonite
Au-Silica
52
94
53
92
37
87
51
78
63
97
45
29
0
34
3
39
20
22
3
0
16
1
2
3
95
80
65
Au-Bentonite
Au-Silica
74
10
4
Table 3. Effect of the oxygen pressure on the oxidation of cyclohexene.
Pressure
Conversion
%
selectivity
Cy-ol%
Cy-one%
Others%
Entry
Catalyst
(bar)
Au- Bentonite
Au- Silica
Au-Bentonite
Au-Silica
Au-Bentonite
Au-Silica
37
87
38
90
40
69
53
51
69
47
81
46
73
63
30
9
28
7
32
8
19
22
25
12
22
19
3
1
2
3
4
15
13
10
8
Au-Bentonite
Au-Silica
34
35
52
58
7
2
3

Table 4. Effect of the reaction time on the oxidation of cyclohexene.
Conversion
%
selectivity
Cy-ol%
Entry
Catalyst
Time (h)
Cy-one%
Others%
1
Au- Bentonite
Au- Silica
Au-Bentonite
16
10
56
93
53
48
74
63
31
9
34
21
17
3
2
Au-Silica
Au-Bentonite
Au-Silica
Au-Bentonite
Au-Silica
92
28
92
17
80
81
52
97
71
97
7
29
3
27
3
12
19
0
2
0
3
4
8
6
Reaction conditions: cyclohexene 10 mL; catalyst 0.05 g 80 °C. Au-Bentonite in 8 bar; Au-Silica in 13 bar.
;
Table 5. Effect of the amount of the catalyst on the oxidation of cyclohexene.
amount of catalyst
selectivity
Cy-nol%
Conversion
%
Entry
Cy-one%
Others%
(g)
1
Without catalyst
0
5
0
0
0
2
MEDPT-Bentonite without gold
35
28
37
(
0.05)
Modified – Silica without gold
0.05)
7
68
63
10
34
22
3
(
3
4
Au-Bentonite (0.05)
53
Au-Silica (0.05)
Au- Bentonite (0.10)
Au-Silica (0.03)
92
68
78
97
54
83
3
25
7
0
21
10
0
0
Reaction conditions: Cyclohexene 10 mL; Au-Bentonite, 8 bar; 10 h; 80 C; Au- Silica, 13 bar, 8 h, 80 C.
Table 6. Recycling of the catalysts.
Cycles
Catalyst
Conversion
%
TOF
selectivity
Cy-ol%
Cy-one%
Others%
3
3
3
3
3
1
2
3
4
5
Au- Bentonite
Au- Silica
Au-Bentonite
Au-Silica
53
92
52
90
50
90
41
89
-
3.65*10
6.15*10
3.58*10
6.02*10
3.44*10
6.02*10
2.83*10
5.95*10
-
63
97
58
95
53
91
56
91
-
34
3
33
3
35
4
32
3
-
3
0
10
2
12
5
12
6
-
Au-Bentonite
3
3
3
Au-Silica
Au- Bentonite
Au- Silica
Au-Bentonite
Au-Silica
3
85
5.68*10
90
2
8
0
0
Reaction conditions: Cyclohexene 10 mL; Au-Bentonite 0.05, 8 bar; 10 h; 80 C Au-Silica 0.05, 13 bar; 8 h; 80 C.
2
4

Table 7. Catalyst activities of Au-silica, Au-bentonite and some previously reported catalysts in the
cyclohexene oxidation.
Conversion
%)
Ketone selectivity
(%)
Cycles
Catalyst
P
O2 (bar)
Temperature (ºC)
Reaction time (h)
Ref
(
a
a
1
2
1
4
1
4
1
5
1
4
1
4
1
5
GNPs/TChD
GNPs/TChD
1
1
7
7
80
80
80
80
80
80
70
70
70
70
80
80
80
80
8
87
72
70
62
14
8
14
b
Au/HNTs
4
12
12
24
24
10
10
24
24
10
10
8
29.5
29.5
48
49
25
b
Au/HNTs
4
4
4
1
1
1
1
8
8
49
25
c
c
Au/La-OMS-2
44
26
Au/La-OMS-2
44
49
26
22
d
PS-DA-Du
51.9
41.6
52.2
41.5
53
40.9
32.5
71.2
74.2
63
d
PS-DA-Du
22
e
Cr-MCM-41
24
e
Cr-MCM-41
24
f
Au-Bentonite
Au-Bentonite
In this work
In this work
In this work
In this work
f
41
56
f
Au-Silica
1
3
3
92
97
f
Au-Silica
1
8
85
90
a
b
c
d
e
f
Reaction was done with 50 mg catalyst and 20 mL cyclohexene.
Reaction was done with 200 mg catalyst and 20 mL cyclohexene.
Reaction was done with 200 mg catalyst and 20 mL cyclohexene.
Reaction was done with 2 mg catalyst and 2 mL cyclohexene.
Reaction was done with 20 mg catalyst and 1 g cyclohexene.
Reaction was done with 50 mg catalyst and 10 mL cyclohexene
2
5