Synthesis and characterization of gold nanoparticles supported on thiol functionalized chitosan for solvent-free oxidation of cyclohexene with molecular oxygen

Article abstract of DOI:10.1016/j.molcata.2013.08.030

The selective liquid phase oxidation of cyclohexene to 2-cyclohexe-1-one and 1,2-cyclohexanediol has been investigated over gold nanoparticles (GNPs) with molecular oxygen in a solvent-free condition. The gold nanoparticles were immobilized on thiolated chitosan derivative (TChD), by grafting thiol groups on the support. The catalyst was characterized by XPS, N₂ adsorption/desorption, TEM, FT-IR and UV-vis spectroscopy. TEM results show that the majority of Au particles have diameters in the range of 3-6 nm. X-ray photoelectron spectroscopy (XPS) revealed the coexistence of both oxidized and metallic gold species on the surface of TChD. The results show that the catalytic performance of GNPs/TChD is quite remarkable and the catalytic activity over recycled catalyst remains at a high level after at least 4 cycles. Activity tests were carried out in an autoclave at 80 C without any solvent. In order to obtain maximum conversion, the reaction parameters such as reaction temperature and time were optimized. Under optimized conditions, a maximum of 87% conversion and 70% selectivity was achieved with the GNPs/TChD catalyst.

Full text of DOI:10.1016/j.molcata.2013.08.030

Accepted Manuscript
Title: Synthesis and characterization of gold nanoparticles
supported on thiol functionalized chitosan for solvent-free
oxidation of cyclohexene with molecular oxygen
Author: Mehran Ghiaci Nasim Dorostkar M. Victoria
´
Martınez-Huerta J.L.G. Fierro Parisa Moshiri
PII:
S1381-1169(13)00322-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.08.030
MOLCAA 8851
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
20-4-2013
28-8-2013
30-8-2013
´
Please cite this article as: M. Ghiaci, N. Dorostkar, M.V. Martınez-Huerta,
J.L.G. Fierro, P. Moshiri, Synthesis and characterization of gold nanoparticles
supported on thiol functionalized chitosan for solvent-free oxidation of cyclohexene
with molecular oxygen, Journal of Molecular Catalysis A: Chemical (2013),
http://dx.doi.org/10.1016/j.molcata.2013.08.030
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Synthesis and characterization of gold nanoparticles supported on thiol
functionalized chitosan for solvent-free oxidation of cyclohexene with
molecular oxygen
Mehran Ghiacia,, Nasim Dorostkara, M. Victoria Martínez-Huertab, J.L.G. Fierrob, Parisa
Moshiria
aDepartment of Chemistry, Isfahan University of Technology, Isfahan, 8415683111, Iran
bInstitute of Catalysts and Petroleochemistry, CSIC, Marie Curie 2, 28049 Madrid, Spain.
Abstract: The selective liquid phase oxidation of cyclohexene to 2-cyclohexe-1-one and
1,2-cyclohexanediol has been investigated over gold nanoparticles (GNPs) with
molecular oxygen in a solvent-free condition. The gold nanoparticles were immobilized
on thiolated chitosan derivative (TChD), by grafting thiol groups on the support. The
catalyst was characterized by XPS, N2 adsorption/desorption, TEM, FT-IR and UV-
Visible spectroscopy. TEM results show that the majority of Au particles have diameters
in the range of 3-6 nm. X-ray photoelectron spectroscopy (XPS) revealed the coexistence
of both oxidized and metallic gold species on the surface of TChD. The results show that
the catalytic performance of GNPs/TChD is quite remarkable and the catalytic activity
over recycled catalyst remains at a high level after at least 4 cycles. Activity tests were
carried out in an autoclave at 80 ºC without any solvent. In order to obtain maximum
conversion, the reaction parameters such as reaction temperature and time were
optimized. Under optimized conditions, a maximum of 87% conversion and 70%
selectivity was achieved with the GNPs/TChD catalyst.
 Corresponding author.
E-mail address: mghiaci@cc.iut.ac.ir (M. Ghiaci).
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Key words: Gold nanoparticles, modified chitosan, cyclohexene, oxidation, solvent free.
1. Introduction
Gold nanoparticles (GNPs), particularly with dimension less than 8 nm [1-5],
have been widely used to catalyze many chemical transformations such as selective
oxidation of alcohols and hydrocarbons, hydrogenation, dehydrogenation and redox
reactions [6-11]. The unique catalytic properties of GNPs in comparison with bulk gold
are closely related to their particle size and morphology which provide large surface to
volume ratio with respect to the bulk system. However, GNPs are nearly unstable and
difficult to separate from the reaction medium when they are used as a catalyst. By
immobilizing them on a solid support, the nanoparticles can be effectively protected
from aggregation and as a result of this strategy, improvement in selectivity and stability
were obtained for many important chemical reactions such as, low temperature oxidation
of CO [9, 12-15], oxidation of alcohols [9, 16-19], direct oxidation of hydrogen to
hydrogen peroxide [20, 21], epoxidation of alkenes [9, 12, 22-24], oxidation of olefins [4,
9, 25], oxidation of alkanes [9, 26, 27], oxidation of cyclohexane and cyclohexene [9, 27-
35]. Extensive studies on the oxidation of cyclohexene have been carried out due to the
potential uses of the products, including 2-cyclohexene-1-ol, 2-cyclohexene-1-one and
many other chemical intermediates [36]. Cyclohexene oxidation over the Au/C catalyst
with oxygen gave a good conversion (approximately 50%) along with many notable
products [34]. To improve the conversion and selectivity of Au as catalyst, choice of a
support with good physicochemical properties and affinities is very important and plays
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crucial role for the overall stability of the supported catalyst. Therefore, we chose
modified chitosan (MCh) to improve the efficiency of the gold catalyst for cyclohexene
oxidation using oxygen as oxidant, and to explore the effect of composition of catalysts
on oxidation of cyclohexene.
The Chitosan biopolymer (Ch) and its derivatives are nowadays widely used
for different purposes. As a naturally-occurring biopolymer, chitosan has many
advantages, such as biocompatibility, biodegradability, non-toxicity, etc., which make it
an environmental friendly substrate [37, 38]. Various chemical groups of Ch especially
its amino groups which can be grafted by different ligands are responsible for site-
selective chemical modification, such as Schiff base formation, N-acylation and reductive
alkylation [39-41]. These modifications favor the continuous growth of Ch derivatives
for various applications fields [40].
One of the chemical modifications of Ch involves grafting thiol groups in the
polymer structure. Sulfur-containing covalent and noncovalent linkers (in particular,
thiol, -SH) have attracted individual attention. Thiol groups due to the soft character of -
SH group [42] that lead to the high affinity to gold [42-46], were successfully employed
as a linker for anchoring GNPs to the support.
In this paper, we are reporting a new MCh which was synthesized by N-alkylation
of Ch with methyl acrylate in the first step, and successively by 1,2-ethanedithiol. We
have shown that the synthesized thiolated chitosan derivative (TChD) is a very effective
ligand for anchoring GNPs. The resulting GNPs/TChD catalyst was compared with
GNPs/Ch. The GNPs/TChD catalyst showed a better catalytic performance than
3
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GNPs/Ch and its related conversion was the highest reported conversion in solvent-free
condition and using molecular oxygen as a cheap oxidant for oxidation of cyclohexene.
2. Experimental Section
2.1. Materials and physical measurements
All chemicals and solvents used in the synthesis were of reagent grade and were
used without further purification. Chitosan (Sigma Aldrich medium molecular weight,
75-85% deacetylated) was used as the polymeric component. Methyl acrylate (MA),
auric chloride (AuCl3) and 1,2-ethanedithiol were purchased from Merck chemical
company.
The Fourier transform infrared spectroscopy (FT-IR) analysis was performed
using
a
Fourier transform infrared spectrometer (Jasco FT/IR-680 plus
spectrophotometer). The powders were blended with IR-grade KBr in an agate mortar
and pressed into tablets. The electronic spectra of solid samples were taken on a JASCO
V-550 UV–vis spectrophotometer in the range of 200 to 800 nm, using the diffuse
reflectance technique (DR UV–vis spectroscopy).
Particle size and morphology were evaluated from the transmission electron
microscopy (TEM) images obtained in a JEM 2100F microscope operated with an
accelerating voltage of 200 kV. The standard procedure involved dispersing 4 mg of the
sample in ethanol in an ultrasonic bath for 15 min. The sample was then placed on a Cu
carbon grid where the liquid phase was evaporated. The average diameter was calculated
by measuring the diameters of no less than 500 randomly selected metal particles.
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XP spectra were obtained with a VG Escalab 200R spectrometer equipped with a
hemispherical electron analyzer (constant pass energy of 50 eV) and a non-
monochromatic (hν = 1253.6 eV) X-ray source, powered at 120 W. Wide scan data
were collected using pass energy of 100 eV whereas high energy resolution spectra
were recorded using pass energy of 20 eV. As the samples experienced variable
degrees of charging the binding energy scale was referenced to adventitious carbon
contamination using the C1s line at 284.8 eV. The samples were analyzed after a
base pressure of 2 x 10-9 mbar was reached within the analysis camber. Data
processing was performed with the ‘‘XPS peak’’ software. The spectra were
decomposed with the least squares fitting routine provided with the software, with
Gauss/Lorentz lines (90G/10L) and after subtracting a Shirley background. Atomic
ratios were calculated from the peak areas using sensitivity factors provided with
the data system [47].
Elemental CHNS analysis was performed on a Perkin Elmer Carbon Hydrogen
Nitrogen (CHN), 2400 Series II Elemental Analyzer.
The nitrogen adsorption isotherm recorded at -196 ºC using a Quantasorb
(Quantachrome, USA) equipment. Before nitrogen adsorption, the samples were
degassed at 110 ºC for 3 h to a residual vacuum of 10-3 mbar. The specific areas
were calculated by the Brunauer–Emmett–Teller (BET) method using a value of
0.162 nm2 for the cross section of adsorbed nitrogen molecule.
2.2. Reaction of methyl acrylate with Ch
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Firstly, N-carboxyethyl chitosan methyl ester (CChME) was prepared
successfully by the Michael addition as depicted by Scheme 1(A). Briefly, chitosan (2
g) was added to 50 mL methanol, and it was stirred overnight at 50 ºC to be dispersed
completely. The mixture was transferred to a 100 mL three-necked round-bottom flask
fitted with a dropping funnel. Methyl acrylate (48 mmol; 4.3 mL) was dissolved in
methanol (5 mL), and added to the reaction mixture dropwise over 10 min through
dropping funnel under nitrogen atmosphere. The reaction was monitored using FT-IR
spectroscopy. The unreacted methyl acrylate in the reaction mixture was removed
completely by washing with methanol in a Soxhlet apparatus.
2.3. Grafting dithioethane to CChME
Once prepared, CChME was grafted with dithioethane. The procedure
consisted in dispersing CChME in chloroform in a round bottom flask that was
equipped with a reflux condenser. 1,2-ethanedithiol (36 mmol; 3 mL) was added to the
reaction mixture, which was refluxed for a period of 24 h. After the completion of the
reaction, the product was rinsed several times with pure ethanol to remove unreacted 1,2-
ethanedithiol and dried under vacuum at 30 ºC overnight to obtain thiolated chitosan
derivative (TChD) (Scheme 1(B)). The results of the CHNS analysis, FT-IR and XPS
spectroscopy (sections 3.1.1, 3.1.2 and 3.1.4, respectively) confirmed the presence of the
thiol groups on the support.
2.4. Synthesis of GNPs on TChD
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Next step consisted in the preparation of GNPs deposited on the TChD
substrate. In a typical experiment, 2 g of the TChD were dispersed into 500 mL of
distillated water (containing 2 wt% acetic acid), to which 5 mL of 10-2 M aqueous
solution of AuCl3 was added in 10 steps to the suspension during 24 h, under vigorous
stirring for loading of 0.5 wt% of Au (III). The resulting product was separated via
centrifugation, and washed few times with distilled water. The product was dried under
vacuum at room temperature before characterization and catalytic activity measurements.
In order to prepare 1 wt% of Au (III), 10 mL of aqueous solution of AuCl3 (10-2 M) was
added to suspension and all of the above processes was repeated again. To obtain the
GNPs on the support, after addition of a given amount of aqueous metal salt (AuCl3) to
the suspension, an aqueous NaBH4 solution (molar ratio of NaBH4:Au = 10:1) was
slowly added under vigorous stirring at room temperature, and left overnight; a bright red
color was observed immediately after addition of reductant to the solution. Then the
catalyst was separated by filtration, washed with distilled water, left overnight at room
temperature and dried in oven at 90 ºC.
Chemical reduction of Au (III) loaded on TChD with excess NaBH4 results in
formation of interdendrite-Au clusters (Scheme 1(C)) on TChD. Evidence for this
reduction comes from the immediate change in solution color. It is noteworthy to mention
that the same procedure was followed for Ch as a support without any modification to
compare its catalytic activity with the new synthesized catalyst. The importance of
modifying Ch on trapping the Au ions will be discussed in the section 3.1.3.
2.5. The selective oxidation of cyclohexene by molecular oxygen
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In a typical experiment, 20 mL of cyclohexene and 50 mg of the solid catalyst were
loaded in a titanium batch reactor. Proper pressure was provided with oxygen capsule
(99.98%) and desired temperature selected and applied to the reaction mixture. After
purging with O2, the reactor was heated to 80 ºC and the O2 pressure was adjusted to 10
bar. After a desired time (8 or 16 h), the reactor was cooled down to 30 ºC and the
reaction mixture was separated from the catalyst by centrifugation and analyzed by gas
chromatography (Trace GC Ultra) which was fitted with a HP-5 capillary column (25
m0.32 mm) and a flame ionization detector (FID). Initial temperature = 60 ºC, 1 min;
final temperature = 250 ºC, heating rate = 10 ºC/min, temperature of injector = 200 ºC,
temperature of detector = 280 ºC.
The reaction products of oxidation were determined by a GC–MS QP5050
SHIMADZU with CBP20 (25m) capillary column and a FID detector. The conversion
and selectivity were determined using undecane as an internal standard in all reactions.
3. Results and Discussion
3.1. Characterization of the synthesized catalyst
3.1.1. Elemental analysis data and calculation of deacetylation degree of Ch
There are many methods to determine the degree of deacetylation (DD) of Ch
such as FT-IR, titration, 1H-NMR, elemental analysis, etc [48-50]. The elemental analysis
results of original Ch and TChD are shown in Table 1, in which the theoretical values are
calculated according to the molecular formula under the condition of 85 % deacetylation.
Through the elemental analysis data and according to the following equation (1) [50] the
85 % DD value of Ch was obtained.
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C/N - 5.145
DD = (1-
) × 100
(1)
6.861 - 5.145
Scheme 1. Preparation process for synthesis of catalyst. A) Reaction of methyl acrylate
with Ch. B) Grafting dithioethane to CChME C) Formation of interdendrite-Au clusters
on TChD.
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The carbon/nitrogen ratio (C/N, w/w) would vary from 5.145 to 6.861 for completely N-
deacetylated chitosan and fully N-acetylated chitin, respectively. The experimental data
of CHNS analysis is more similar to A [C6H9O4 (NC10S4H18O2)0.85 (NHCOCH3)0.15] than
B [C6H9O4 (NHC5S2H9O)0.85 (NHCOCH3)0.15]. B is the product when the amino
groups of Ch reacted with methyl acrylate just once while A is the product obtained
when doubly Michael addition of amino group with methyl acrylate occurred.
Therefore, according to Scheme 1, nearly all of the deacetylated groups (amine-free
groups) in the polymer were to a large extent doubly reacted with methyl acrylate.
Table 1.
Elemental analysis (wt. %) of Ch and TChD.
Experimental value
Compound
Theoretical value
Molecular formula A Molecular formula B
C% N% H% S%
C/N DD%
C%
N% H% S%
C%
-
N% H% S%
Ch
45.19 8.37 6.75
-
-
-
-
-
-
-
-
5.4
-
85
-
TChD
42.28 3.02 5.79 26.1 42.39 3.34 5.94 26
43.2 4.8
6.17 18.55
3.1.2. FT- IR data of the synthesized catalyst
Infrared wave numbers (cm-1) of significant stretching vibrations are helpful for
identification of the synthesized catalyst (Fig. 1). In Fig. 1(a), Ch spectrum, the broad
band in the region of 3200-3500 cm-1 is attributed to inter- and intra- molecular
hydrogen bonded –NH and –OH stretching vibrations. The characteristic peaks at 1654,
1597 and 1375 cm-1 were assigned to the amine and amide groups [40, 46], the key
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absorption bands of Ch, respectively. The absorption band at 1152 cm-1 was attributed to
the asymmetric stretching of the C–O–C bridges. Bands at 1075 and 1030 cm-1 were
assigned to the skeletal vibration of C–O stretching. The increasing intensity of the
peak at 2890 cm-1 which is ascribed to C-H stretching, confirms the alkylation of the
Ch backbone (Fig. 1b). Compared with the Ch spectrum, CChME in Fig. 1(b) shows
strong absorption band at 1729 cm-1 due to the stretching vibration of the ester groups
which were formed by the Michael addition reaction of Ch with methyl acrylate. The new
peak at 2567 cm-1 in Fig. 1(c), was attributed to the thiol stretching vibration [51-53] in
TChD, which confirms the presence of the thiol groups in the MCh.
The appearance of GNPs on TChD could be detected from FT-IR spectroscopy
through disappearance of the –SH vibration at 2567 cm-1 as a consequence of the reaction
between sulfur and gold nanoparticles (Fig. 1(d)). Other characterization techniques such
as UV-Vis spectroscopy and TEM that will be discussed in the next sections were used to
detect the appearance of GNPs on the support.
3.1.3. DR UV-Vis spectroscopy and Specific surface area
UV-vis spectra were recorded with aim to reveal changes in the chemical
structure of chitosan upon modification and also to provide further evidence on the
morphology of Au species. Two absorption bands in the wavelength regions 200-380
nm (zone I) and 400-600 nm (zone II) can be distinguished (Figure 2). In zone I (Fig.
2(a)), Ch shows a strong absorption around 290 nm together with a shoulder at
about 370 nm. TChD sample displays a similar bands structure, although it is more
intense (Fig. 2(b)). The absorption spectrum for the Au loaded GNP/TChD appears
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at 519 nm (Fig. 2(c)) that is quite similar to its Au-free TChD counterpart, except
that the shoulder at about 370 nm became more intense. It is likely that the increase
in intensity of the shoulder at 370 nm is
Wavenumber [cm-1]
Fig 1. FT-IR spectra of a) Ch, b) CChME, c) TChD,
d) GNPs prepared on the TChD, the zoomed region
is related to the thiol S-H stretching vibrations (pink
color) on TChD and elimination of it (red color) as
a result of the reaction between sulfur and gold
nanoparticles in GNP/TChD.
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responsible for the the presence of small Aun clusters (1 < n < 10) [54]. Moreover,
the wide band in zone II is usually attributed to the surface plasmon resonance
(SPR) of gold nanoparticles which has been proven to be very sensitive to the size,
shape, composition and dielectric environment of metal nanoparticles [55-56]. SPR is the
optical phenomenon which was observed in transmission when the collective oscillation
of conduction electrons (free electrons) on the surface of metal nanoparticles was
stimulated by incident light. At the frequency of incident photons, these electrons are set
into resonant oscillation. SPR in nanostructures is called localized surface plasmon
resonance (LSPR) [58-60]. The color change from yellow in the bulk to red in the
nanometer range (13 nm spherical GNPs with bright red color [61]) is the striking
evidence for this optical phenomenon The position and intensity of the plasmon peak
is affected by the dielectric constant of the surrounding environment [57,58, 62].
According to bibliography, GNPs were produced in the presence of Ch by two
methods. The method that produces a more uniform distribution of GNPs consisted in
adding the gold chloride to Ch solution and used excess of NaBH4 as the reductant. In
the other method for producing GNPs, Ch itself was used as a reductant at high
temperature [63-68].
As reported in the literature [69,70], the rate of the nucleation can be
affected considerably by the presence of thiols or polymeric thiols. It means that
nanoparticle nucleation is accelerated by the presence of polar polymeric thiols,
yielding more small particles rather than fewer large ones. Also with larger
thiol/gold mole ratio, smaller average core sizes were resulted. With knowledge to
this point that TChD consist of a thiol groups which bind tightly to surface of gold
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particles can concluded that TChD could be as a controller of nucleation as well as
stabilizer. By adding NaBH4 as a reducing agent, colloidal nucleation is achieved and
begins to grow. The solution changed to a bright red color due to the formation of GNPs.
After centrifugation, the resultant precipitate was washed several times with deionized
water and dried in oven overnight at 100 ºC.
Fig 2. UV-Vis spectra of a) Ch, b) TChD, c) GNP/TChD.
The specific surface area was determined using Brunauer-Emmett-Teller (BET)
method. The surface area of the TChD was determined and compared to Ch. The surface
area of Ch was 3 m2/g. After modification of Ch, its surface area did not change
significantly and became 2.8 m2/g. After loading metal on TChD, the surface area
decreased drastically to 0.4 m2/g.
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3.1.4. Transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy
(XPS).
Successful preparation of the GNPs is evidenced by TEM images (Figures 3a-
3d). Fig. 3(a) shows the image of the modified polymeric support before adding gold (III)
chloride solution to it. As shown in Fig. 3(b) after adding Au (III) salt to the support, the
image does not show any particle in a nanometric size which probably means that Au
(III) ions were distributed in the polymeric matrix as Au (III) complexes coordinated by
different ligands, and they could not be seen because of their atomic size. By adding the
reducing agent ie., NaBH4, metal ions appear to be reduced and start aggregating to
some extent. The presence of GNPs on the surface of the polymer was evidenced by
detection of dark spots (Fig.3(c)) in comparison with the previous images (Figs 3(a) and
3(b)). In general, the particles were spherical in shape and no aggregation could be
identified by TEM images. According to the Fig. 4 the average particle size of the gold
nanoparticles was 5.5 ± 2.3 nm and more than 70% of them were in the range of 3-6 nm.
In Fig. 3(d), TEM image of the catalyst after the reaction shows aggregation of the
nanoparticles that occurred in some regions while in the other regions small
nanoparticles still remained.
The surface composition and oxidation states of the metal catalyst were investigated by
XPS. Figure 5(a) shows the Au 4f core-level spectrum of the GNPs/TChD sample. Each
component of the Au 4f doublet is asymmetric on the high binding energy side indicating
the presence of two different electronic environments of than 3-4 nm [72, 73]. Because
only 29% of Au particles show sizes above ca. 5 nm, in which no final state effects are
expected to appear, and the most intense peak (Au 4f7/2 component) of the Au 4f doublet
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appears at a BE of 84.5 eV, This BE value can be associated to metallic Au species. On
the basis of the above reasoning, ionic Au (III) species [74,75] are considered to be
responsible for the binding energy of the Au 4f7/2 component of the less intense doublet
at 86.0 eV in Fig. 5(a).
a
b
c
d
Fig 3. TEM images of a) TChD, b) Au(III) on the TChD,
c) GNP/TChD, d) GNP/TChD after reaction.
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Fig 4. Size distribution of GNP found in the TChD.
Photoelectron spectroscopy also confirmed the presence of S-C bonds of the
HS-C2H5 moieties. The high resolution S 2p of the sample is shown in Fig. 6. This
spectrum
a
b
Fig. 5. A4f core-level spectra of: a) GNPs/TChD b) GNPs/Ch.
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shows the most intense S 2p3/2 component of the S 2p doublet (spin-orbit splitting of 1.2
eV) at a binding energy of 163.4 eV. Although there are claims in literature that the
interaction between the mercapto group and gold colloids is through a true chemical
bond [76], the binding energy of S 2p3/2 peak recorded here at 163.4 eV is indicative of
the presence of covalent C-S bond of the grafting ethanethiol molecule but in no case
originated from S-Au linkages [77-79]. Notwithstanding, as the ethanethiol moieties
still remain adsorbed on the catalyst surface under the ultrahigh vacuum conditions
under which the electron spectrometer operates during analysis, it can be inferred
that the thiol group interacts strongly chemisorbed on the surface. It is interesting to
note that by impregnating the Au (III) ions on the unmodified Ch and reducing them by
NaBH4, the catalyst contains only Au (0) as revealed by the binding energy of Au 4f7/2
component of the Au 4f doublet at 84.0 eV. According to Fig. 5(b), the observed binding
energy of Au 4f7/2 component of the Au 4f doublet of gold species on the Ch without any
modifying group at 84.0 eV is conclusive that metallic gold is the only Au species.
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Fig. 6. S2p core-level spectrum of the GNP/TChD.
Table 2 compiles the surface atomic ratios of gold-based samples. These data
indicate that surface composition changes markedly upon Ch modification. At a
first glance, it is clear that the value of S/C ratio is much higher for the TChD than
for the non-modified Ch counterpart. The extent of Au loading on the surface of
TChD has become nearly 6 times more than while using Ch alone as a support. This
result suggests that catalytic performance of support will increase in parallel by
modifying it with the thiol ligands. As opposite, the atomic ratio of N/C in Au/TChD
decreased as a consequence of increasing C backbone over the surface of Ch upon
modification with methyl acrylate and 1,2-ethanethiol. Similar reasoning applies for
the decreasing O/C atomic ratio in the Au/TChD sample.
19
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Table 2.
Surface atomic ratios of gold-based samples
Entry
sample
Ch-Au
O/C at
0.381
0.205
S/C at
0.004
0.357
N/C at
0.061
0.026
Au/C at
0.0020
0.0117
1
2
TChD-Au
3.2. Catalytic performances
3.2.1. Effect of catalyst composition and temperature
Oxidation of cyclohexene was carried out over different Au-loaded catalysts.
The results summarized in Table 3 indicate that the conversion of cyclohexene
oxidation with the catalyst containing 1 wt% Au(III) was higher than with the one
containing 0.5 wt % Au (III) although no significant changes were observed in the
selectivity of the two catalysts.
The effect of different oxidation states of Au on the performance is also
shown in Table 3. Au (0) displayed 16 % higher cyclohexene conversion than Au (III)
(entries 5, 6). However, if the higher conversion of Au(0) is not considered, at 80 ºC
the Au(III) catalyst showed better selectivity. As the extent of Au adsorption on the
surface of TChD was higher than on Ch (Table 2), one would expect a higher
activity for GNPs/TChD catalyst than for the Au/Ch one. Such a trend on the
oxidation of cyclohexene is confirmed with data collected in Table 3. The conversion
of cyclohexene on GNPs/TChD (Table 3, entry 6) was about 1.5 times higher than on
the GNPs/Ch catalyst (Table 3, entry 3). Moreover, a blank experiment with the Au-
20
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free TChD sample revealed negligible conversion of cyclohexene (Table 3, entry 2).
The influence of the reaction temperature on the catalytic performance of GNPs on
TChD was also studied. Table 3 summarizes results of the cyclohexene oxidation at
60, 80, 100 and 120 ºC (entries 6-10).
Table 3.
Effect of the type of catalyst and temperature on the oxidation of cyclohexene
Entry
Catalyst
Temperature
Conversion
Selectivity (%)
diol
(ºC)
80
(%)
ketone
-
others
1
Blank
No reaction
2
TChD without Aua
1% wt Au(0)/Chb
80
6.2
57
51
64
80
67
57
38
68
69
57
65
70
66
62
33
2
30
30
15
8
3
80
1
4
0.5 % wt Au(III)/TChDb
1 % wt Au(III)/TChDb
1 % wt Au(0)/TChDb
1 % wtAu(0)/TChDb
1 % wt Au(0)/TChDb
1 % wt Au(0)/TChDb
80
28
27
14
11
33
37
5
80
6
80
18
23
5
7
60
9
100
120
10
30
a reaction condition: 0.05 g of catalyst, 20 mL of cyclohexene, PO2: 10 bar, reaction time: 16 h.
b reaction condition: 0.05 g of catalyst, 20 mL of cyclohexene, PO2: 10 bar, reaction time: 8 h.
The reaction temperature has a great impact on both activity and selectivity. By
increasing the reaction temperature to 80 ºC, selectivity and conversion increased
significantly, however if the reaction is performed at higher temperature (>80ºC), both
the conversion and selectivity decreased slightly. These results suggest that at higher
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temperatures, the Au nanoparticles tend to aggregate on the surface of TChD. This
inference is supported by a TEM image of GNPs on TChD (Figure 7).
Fig 7. TEM image of GNPs on TChD after
performing the reaction at 120oC.
3.2.2. Effect of oxygen pressure
The optimal operation pressure has been identified. As shown in Fig. 8, the
increase of oxygen pressure increased the conversion of cyclohexene, and such
increase of the conversion occurred in the range of oxygen pressure 5-10 bar with of
GNPs/TChD catalyst. Product distribution did not change more and it remained
nearly constant upon increasing oxygen pressure.
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Fig 8. Effect of the oxygen pressure on the oxidation of cyclohexene.
Reaction conditions: amount of catalyst: 50 mg, reaction temperature:
80ºC, reaction time: 8 h under the solvent free condition.
3.2.3. Effect of reaction time
Fig. 9 shows the effect of time on the cyclohexene oxidation with molecular
oxygen over the GNPs/TChD catalyst. By increasing the time of the reaction from 4 to 8
h, the cyclohexene conversion and selectivity to 2-cyclohexene-1-one was increased, and
almost kept unchanged after this time.
3.2.4. Effect of amount of catalyst and solvent effect
To study the effect of amount of GNPs/TChD, the oxidation reaction was carried
out at 80 ºC, oxygen pressure: 17 bar and the reaction time of 8 h. From the results
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summarized in Table 4, it is seen that catalyst loading of 50 mg is appropriated to run
the reaction. With increasing the amount of the catalyst a slight decrease in the
conversion was observed.
Fig 9. Effect of the reaction time on the oxidation of cyclohexene. Reaction
conditions: amount of catalyst: 50 mg, reaction temperature: 80ºC, oxygen
pressure: 17 bar under the solvent free condition.
The solvent effect was studied with 1,2-dichloroethane because we
demonstrated in our previous work [80] 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
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conditions, a significant decrease in selectivity was observed under solvent
condition.
Table 4.
Effect of the amount of the catalyst on the oxidation of cyclohexene.a
Entry
Amount of catalyst (mg)
Conversion (%)
Selectivity (%)
Ketone
65
Diol
others
1
2
3
4
10
65
77
87
40
23
28
27
21
12
3
30
69
50
70
3
100
45
24
a All reactions were done with 20 mL cyclohexene, at 80 °C, reaction time = 8 h, and the pressure
of oxygen was 17 bar.
3.3. 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
5. Data showed that the selectivity of the reused 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. [81]. The results show that the
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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.
Schiff-base metal complexes supported on polymeric material such as
chloromethylated polystyrene supported tridentate Schiff-base metal complexes (PS–
DA–M) (M= Cu2+, Co2+, Ni2+ and Mn+2) [31], PAMAMSA-Mn (II) metallodendrimers
based on polyamidoamine (PAMAM) [82], CrMCM-41 [32], Au/HNTs [29] and Au/La-
OMS-2 [30] are some examples of catalysts which have been used for oxidation of
cyclohexene under solvent-free condition with molecular oxygen. The data in Table 5
compares the results obtained over GNPs/TChD and other catalysts used for this reaction.
As shown in Table 5, the catalyst used in the present study might be one of the best
catalysts in respect of oxidation of cyclohexene to 2-cyclohexene-1-one.
4. Conclusion
In this research, we have successfully prepared TChD as a new support for
stabilizing of GNPs and preventing their aggregation. The size of these nanoparticles was
in the range of 5.5 ± 2.3 nm. These particles were well dispersed and had an excellent
activity toward the oxidation of cyclohexene. The conversion and selectivity of this
reaction were 87% and 70% at the optimum conditions under the solvent free condition
with molecular oxygen.
Acknowledgement
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Thanks are due to the Research Council of Isfahan University of Technology and
Center of Excellency in the Chemistry Department of Isfahan University of Technology
for supporting this work.
Table 5.
Catalytic activities of GNPs/TChD and some previously reported catalysts in the cyclohexene oxidation.
Cycles
Catalyst
PO2 (bar) Temperature
(oC)
Reaction
time (h)
Conversion
(%)
Ketone
Ref
selectivity (%)
1
2
3
4
5
6
7
1
4
1
4
1
5
1
4
GNPs/TChDa
GNPs/TChDa
GNPs/TChDa
GNPs/TChDa
GNPs/TChDa
GNPs/TChDa
GNPs/TChDa
Au/HNTsb
17
17
17
17
17
17
17
4
80
80
80
80
80
80
80
80
80
80
80
70
70
70
70
8
8
87
87
70
70
This work
This work
This work
This work
This work
This work
This work
29
8
87
70
8
85
69
8
73
65
8
72
63
8
72
62
12
12
24
24
10
10
24
24
29.5
29.5
48
49
Au/HNTsb
4
49
29
Au/La-OMS-2c
Au/La-OMS-2c
PS–DA–Cud
PS–DA–Cud
CrMCM-41f
CrMCM-41f
4
44
30
4
44
49
30
1
51.9
41.6
52.2
41.5
40.9
32.5
71.2
74.2
31
1
32
1
32
1
32
a Reaction was done with 50 mg of catalyst and 20 mL of cyclohexene.
b Reaction was done with 200 mg of catalyst and 20 mL of cyclohexene.
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c Reaction was done with 200 mg of catalyst and 20 mL of cyclohexene.
d Reaction was done with 2 mg of catalyst and 2 mL of cyclohexene.
f Reaction was done with 20 mg of catalyst and 1 g of cyclohexene.
References
[1]
[2]
T. Ishida, M. Haruta, Angew. Chem. Int. Ed. 46 (2007) 7154-7156.
C. Della Pina, E. Falletta, L. Prati, M. Rossi,Chem. Soc. Rev. 37 (2008) 2077-
2095.
[3]
[4]
R. Zanella, E.V. Basiuk, P. Santiago, V.A. Basiuk, E. Mireles, I. Puente Lee,
J.M. Saniger, J. Phys. Chem. 109 (2005) 16290-16295.
M. Turner, V. B. Golovko, O. P. H. Vaughan, P. Abdulkin, A. Berenguer-
Murcia, M. S. Tikhov, B.F.G. Johnson, R.M. Lambert, Nature. 454 (2008) 981-
984.
[5]
M. Comotti, C. Della Pina, R. Matarrese, M. Rossi, Angew. Chem. Int. Ed. 43
(2004) 5812-5815.
[6]
[7]
[8]
[9]
[10]
A. Arcadi, S. Di Giuseppe, Curr. Org. Chem. 8 (2004) 795-812.
B.N. Zope, D.D, Hibbitts, M. Neurock, R.J. Davis, Science. 330 (2010) 74-78.
L.F. Chen, J.C. Hu, R. Richards, J. Am. Chem. Soc. 131 (2009) 914-915.
C. Della Pina, E. Falletta, M. Rossi, Chem. Soc. Rev. 41 (2012) 350-369.
A.A. Herzing, C.J. Kiely, A.F. Carley, P. Landon, G.J. Hutchings, Science. 321
(2008) 1331-1335.
[11]
K. Idegami, M. Chikae, K. Kerman, N. Nagatani, T. Yuhi, T. Endo, E. Tamiya,
Electroanal. 20 (2008) 14-21.
28
Page 28 of 36

[12]
[13]
[14]
[15]
D. Gajan, K. Guillois, P. Delichère, J.M. Basset, J.P. Candy, V. Caps, C. Copéret,
A. Lesage, L. Emsley, J. Am. Chem. Soc. 131 (2009) 14667-14669.
M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M.J. Genet, B. Delmon. J.
Catalys. 144 (1993) 175-192.
C.J. Jia, Y. Liu, H. Bongard, F. Schüth, J. Am. Chem. Soc. 132 (2010) 1520-
1522.
S.A.C. Carabineiro, N. Bogdanchikov, A. Pestryakov, P.B. Tavares, L.S.G.
Fernandes, J.L. Figueiredo, Nanoscale Res. Lett. 6 (2011) 435-441.
A. Murugadoss, H. Sakuraia, J. Mol. Catal. A: Chem. 341 (2011) 1-6.
A. Abad, A. Corma, H. Garcia, Pure Appl. Chem. 79 (2007) 1847-1854.
T. Mitsudome, A. Noujima, T. Mizugaki, K. Jitsukawa, K. Kaneda, Adv. Synth.
Catal. 351 (2009) 1890-1896.
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
L.C. Wang, L. He, Q. Liu, Y.M. Liu, M. Chen, Y. Cao, H.Y. He, K.N. Fan, Appl.
Catal. A: Gen. 344 (2008) 150-157.
R. Zeis, T. Lei, K. Sieradzki, J. Snyder, J. Erlebache, J. Catal. 253 (2008) 132-
138.
Y. Yang, G. Yang, Y. Huang, H. Bai, X.A. Lu, Colloid Surface A. 340 (2009)
50-55.
V. Mendez, K. Guillois, S. Daniele, A. Tuel, V. Caps, Dalton Trans. 39 (2010)
8457-8463.
F. Zhang, X. Zhao, C. Feng, B. Li, T. Chen, W. Lu, X. Lei, S. Xu, ACS Catal. 1
(2011) 232-237.
K. Guillois, L. Burel, A. Tuel, V. Caps, Appl. Catal., A. 415–416 (2012) 1-9.
29
Page 29 of 36