Influence of nanoparticles oxidation state in gold based catalysts on the product selectivity in liquid phase oxidation of cyclohexene

Article abstract of DOI:10.1016/j.molcata.2013.03.008

Supported gold catalysts Au/TiO₂ and Au/ZrO₂ were prepared and used for oxidation of cyclohexene with TBHP. These catalysts were characterized by ICP, TEM and diffuse-reflectance UV-vis. The catalytic tests were carried out in liquid phase, at 80 C and at atmospheric pressure. The effects of support and thermal treatment (oxidation state of Au nanoparticles) on catalytic performance were studied. Gold in the catalysts that have been dried is deposited as Au^δ+. These catalysts have a high selectivity towards alkene oxides, 81% of 2-cyclohexene-1-ol for Au/TiO ₂ and 85% of 2-cyclohexene-1-one for Au/ZrO₂.

Full text of DOI:10.1016/j.molcata.2013.03.008

Accepted Manuscript
Title: Influence of nanoparticles oxidation state in gold based
catalysts on the product selectivity in liquid phase oxidation of
cyclohexene
Author: <ce:author id="aut0005"> N. Ameur<ce:author
id="aut0010"> S. Bedrane<ce:author id="aut0015"> R.
Bachir<ce:author id="aut0020"> A. Choukchou-Braham
PII:
S1381-1169(13)00109-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2013.03.008
MOLCAA 8700
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
17-5-2012
28-2-2013
3-3-2013
Please cite this article as: N. Ameur, S. Bedrane, R. Bachir, A. Choukchou-Braham,
Influence of nanoparticles oxidation state in gold based catalysts on the product
selectivity in liquid phase oxidation of cyclohexene, Journal of Molecular Catalysis
A: Chemical (2013), http://dx.doi.org/10.1016/j.molcata.2013.03.008
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*Graphical Abstract (for review)
Page 1 of 14

Influence of nanoparticles oxidation state in gold based catalysts on the product
selectivity in liquid phase oxidation of cyclohexene
N. Ameur, S. Bedrane, R. Bachir, A. Choukchou-Braham
Laboratory of Catalysis and Synthesis in Organic Chemistry,
Université de Tlemcen, Algeria
BP 119 Imama 13000 Tlemcen Algeria
Tel & Fax 213 43 213198
Abstract:
Supported gold catalysts Au/TiO2 and Au/ZrO2 were prepared and used for oxidation of cyclohexene with
TBHP. These catalysts were characterized by ICP, TEM and Diffuse-Reflectance UV-Visible. The catalytic tests
were carried out in liquid phase, at 80°C and at atmospheric pressure.
The effects of support and thermal treatment (oxidation state of Au nanoparticles) on catalytic performance were
studied. Gold in the catalysts that have been dried is deposited as Au+. These catalysts have a high selectivity
towards alkene oxides, 81% of 2-cyclohexene-1-ol for Au/TiO2 and 85% of 2-cyclohexene-1-one for Au/ZrO2
Key Words: Gold, Nanoparticles, Oxidation, Cyclohexene, Selectivity
1. Introduction
Catalytic oxidation of olefins into value added oxygenated derivative is very important in chemical industry [1].
The oxidation of cyclohexene can give rise to a number of products (scheme 1) depending on the catalysts and
reaction conditions. The 2-cyclohexene-1-ol and 2-cyclohexene-1-one, are used in the manufacture of high value
pharmaceutical chemicals [2]
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Scheme 1: Products resulting from the cyclohexene oxidation.
In the last decades, an increasing interest has been directed to the catalytic potential of gold catalysts.
Gold-based catalysts are used in different reactions of carbone oxidation, such as oxidation of CO [3-14],
oxidation of sugar [15], oxidation of benzylacohol [17-20], oxidation of aliphatic alcohols [21-23], oxidation of
polyols [16, 24, 25], oxidation of glycerol [26, 27] epoxidation of trans-stilbene [28,29], epoxidation of styrene [30,31]
oxidation of cyclohexane [32-34].
However, the literature reports few studies on the use of gold catalysts for the oxidation of cyclohexene,
except those of Zhen-Yu Cai and al. about Au/OMS-2(La-OMS-2) [38] and Au/HNTs [39]. They report that
conversion does not exceed 50% and selectivity to 2-cyclohexene-1-ol and 2-cyclohexene-2-one is between 30
and 49%.
In addition, supported gold nanoparticles on C [ ] and CNTs [40] showed a remarkable catalytic
1
activity for this reaction.
The objective of this work is to study the influence of gold nanoparticles oxidation state, on the
oxidation of cyclohexene by TBHP. TiO2 and ZrO2 oxides are used as a supports because they have comparable
acidities but different reducibility features.
2. Experimental
2.1 Catalysts Preparation
A suspension of support (TiO2 or ZrO2) in distilled water is introduced into a three-necked flask and heated to
80° C. Then, a solution of gold salt, HAuCl4.3H2O, (10 g / L) is mixed with urea (0.9g). All is closed and stirred
2
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for 16 h at the same temperature in the dark. At the end, the solid is separated by centrifugation and washed with
distilled water many times.
After each washing, a test with (AgNO3) is carried out to verify the presence of chlorides. Generally, after the
first wash, no trace of chloride is found. A second qualitative test was also carried out on the wash solution by
NaBH4. No changes in the wash solution color indicate that the total amount of the introduced gold is deposited
on oxides. The resulting solids are dried at 120 ° C overnight (dried catalyst). A part of the dried samples are
then reduced by hydrogen at 300 ° C (reduced catalyst), and are stored in a vacuum desiccator, protected from
light [34,38,41,42]
Catalysts characterization
2.2
The effective contents of Au and Cl in the catalysts were determined by ICP chemical analysis. TEM
photographs were obtained from a JOEL JEM-100CXII instrument, operating at 120KV with a resolution of 0.35
nm. Diffuse Reflectance UV–Vis spectroscopy measurements were carried out at room temperature with
Lambda 800 UV/VIS Spectrometer in the range of 200–800 nm. This setup was equipped with a diffuse
reflectance accessory set to collect the diffuse reflected light only.
2.3 Catalytic tests
2.4 Catalytic tests were performed using 0.1 g of catalyst, 4 mL of cyclohexene and 5.5 mL of tert-butyl
hydroperoxide [TBHP (70%) in water]. The oxidation of cyclohexene was carried out at 70 ° C for 6 hours under
atmospheric pressure.
The consumption of TBHP was determinate by iodometric titration, and the Products were analyzed by gas
chromatography (GC SCHIMADZU 14-B) with a capillary column "HP-FFAP" and an FID detector.
The oxidation activity is expressed in TON, calculated at the end of the reaction (6h) as follow:
n0: initial mole number of cyclohexene, C%: Cyclohexene conversion, mcat: amount of catalyst, %Au: gold
loading,
D: gold accessibility is calculated considering the hypothesis according to which the particles are regarded as
cubes with one face in contact with the support and the five others are accessible [43].
3. Results and Discussion:
3.1 Catalysts characterization.
3
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The catalysts metal loading analysis shows that the majority of the dissolved gold is deposited onto the
oxides (Table 1).
Table 1: Analysis of the catalysts.
Sample
Nominal content (wt%)
d(nm)
3.4
Au
0.9
Cl
Au/TiO2
Au/ZrO2
0.02
0.8
<0.02
6.0
The characterization by TEM of reduced Au/TiO2 (Fig.1a) reveals the presence of gold nanoparticles, uniformly
distributed on the surface of TiO2, with an average size of 3.4 nm. The histogram of the particle size distribution
has a Gaussian shape (Fig.1b)
.
a
b
Figure 1: TEM characterization of reduced Au/TiO2 (a) TEM image (b) Size distribution histogram
For reduced Au/ZrO2 (Fig.2.a), the gold nanoparticles are irregularly distributed on the surface of ZrO2, with an
average size of about 6.0 nm. The histogram of size distribution is bimodal. It indicates the presence of particle
sizes of 2-3 nm and 7-8 nm (Fig.2b).
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a
b
Figure 2: TEM characterization of reduced Au/ZrO2 (a) TEM image (b) Size distribution histogram
Diffuse-Reflectance UV-Visible Spectra of TiO2 oxide and Au/TiO2 (dried and reduced catalysts) are shown in
(Fig.3). TiO2 spectrum (Fig.3a) has a broadband between 210 nm and 340 nm. This band is characteristic of the
anatase phase [44] It corresponds to transitions of Ti4+ ions in a tetrahedral and an octahedral environment
[45-
.
47]
The spectrum of dried Au/TiO2 catalyst (Fig.3b) shows that the introduction of gold leads to a decrease of the
spectrum intensity compared to TiO2 alone, particularly the characteristic bands of anatase phase for TiO2 (200
nm-350 nm). This is due to deposition of gold particles. It seems that these particles cover the surface of TiO2 ,
i.e. Ti4+ sites, and reduce the contact of UV radiation with the support [35, 36]
Figure 3: Diffuse-Reflectance UV-Visible spectrum (a) TiO2, (b) dried Au/TiO2 and (c) reduced Au/TiO2.
5
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The comparison of the deconvoluted UV-vis spectra of TiO2 and dried Au/TiO2 (Fig 4) reveals the presence of
three new bands at 213 nm, 231 nm and 328 nm in the case of dried Au/TiO2. They are characteristic of Au3+,
Au+ and Auδ+, respectively [48]. However, no change in the range (400-800 nm) of the dried Au/TiO2 spectrum
is shown. This indicates that this catalyst contains only oxidized species (Au3 +, Au +, Au + δ) and no reduced ones
(Au0).
Figure 4: Deconvoluted bands of Diffuse-Reflectance UV-Visible spectra of (a) TiO2, and (b) dried Au/TiO2
The Diffuse-Reflectance UV-Visible spectrum of the reduced Au/TiO2 catalyst (Fig.3c) shows a band between
200-350 nm which is substantially identical to that of TiO2. However, a new large band appears with a maximum
at 537 nm. This band is characteristic of the reduced gold nanoparticles plasmon resonance. It is influenced by
the size, the shape of gold nanoparticles and their environment (support) [49,50]. This result indicates that all
gold particles were reduced after treatment in H2 at 300 °C.
Diffuse-Reflectance UV-Visible spectra of ZrO2 oxide and Au/ZrO2 (dried and reduced catalysts) are shown in
(Fig.5). The spectrum of ZrO2 (Fig.5a) shows a characteristic band of the tetragonal phase at 235.32 nm. It is due
to the electronic transition between Zr4+ and oxygen [51,52]
On the Au/ZrO2 dried catalyst spectrum (Fig.5b), the support band disappears and a characteristic band of
oxidized gold species Au3+ appears at 257.24 nm [48].
On the reduced catalyst spectrum (Fig.5c), it is clearly seen that the characteristic band of the oxidized species
Au3+ disappears. The characteristic band of the support (ZrO2) reappears with a slight shift from 235.32 nm to
227 nm. This shift is related to metal-support interactions [53]. In addition, a characteristic band of reduced gold
6
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nanoparticles plasmon resonance with spherical shapes appears at 540.37 nm. Thus, just like Au/TiO2, all the
gold particles were reduced after treatment in H2 at 300 °C.
Figure 5: Diffuse-Reflectance UV-Visible spectrum (a) ZrO2, (b) dried Au/ZrO2 and (c) reduced Au/ZrO2.
The characterization of the samples, dried and reduced by Diffuse-Reflectance UV-Visible spectroscopy, shows
that gold is deposited in an oxidation state (Aun+); they remain with this oxidation state even after drying. Their
reduction to Au0 is done after heat treatment in H2 at 300 ° C.
3.2 Cyclohexene Oxidation
The reaction of cyclohexene oxidation can give six products (Scheme 1): 2-cyclohexene-1-ol (enol); 2 -
cyclohexene-1-one (enone); cyclohexene epoxide (PO); cyclohexanol (ol); cyclohexanone (one) and
cyclohexene-1, 2-diol. It is worth mentioning that in all the reactions conducted, the cyclohexene-1,2-diol was
never detected.
The results of activity and selectivity of the four catalysts are summarized in (Table 2). The TONs of the dried
catalysts are respectively 217 and 1090 for Au/TiO2 and Au/ZrO2 Reduction induces an increase of TON to 913
.
in the case of Au/TiO2, and a decrease it in the case of Au/ZrO2.
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Table 2: Activity and selectivity of gold catalysts in cyclohexene oxidation.
(S: selectivity), C%: Conversion of cyclohexene, TBHP%: conversion of TBHP
TON(10-3)
%
Sample
C% TBHP % SPO%
SONE
0
SOL%
0
SENOL% SENONE%
Au/TiO2
dried
2.77
17.7
58.4
39.5
28.6
6.2
53.3
30.0
34.7
1
20.2
9
18
81.1
Au/TiO2
reduced
9.13
10.90
7.90
23.2
22
0
0
0
34.6
Au/ZrO2
dried
3.2
2.5
30.9
85.2
Au/ZrO2
reduced
18
20.7
30.4
Comparing the selectivities on dried and reduced catalysts, it is clear that with dried Au/TiO2, only cyclohexene-
1-ol (81.1%) and 2-cyclohexen-1- one (18%) are obtained, whereas with reduced Au/TiO2, the cyclohexene-1-ol
disappears completely and the other oxidized products are produced with comparable amounts: 2-cyclohexen-1-
one (34.6%), cyclohexene epoxide (20.2%), cyclohexanol (22%) and cyclohexanone (23.2%).
Dried Au/ZrO2 is very selective towards 2-cyclohexen-1-one (85%), while reduced Au/ZrO2 induces (as in the
case of reduced Au/TiO2) selectivity toward all products with comparable amounts [2-cyclohexen-1-one
(30.4%), cyclohexene epoxide (18%), cyclohexanol (30.9%) and cyclohexanone (20.7%)].
The results of activity show that gold particles behave differently, depending on the oxide used as support.
Indeed, the TON increased 3 times when switching from dried Au/TiO2 to reduced Au/TiO2, whereas Au/ZrO2
reduction induces a decrease of the TON. It seems that a part of gold particles on TiO2 (probably very small
particles) need a reduction to be activated. However, on ZrO2 the majority of gold particles are activated just
after drying.
Moreover, the results let us say that selectivities depend more on the oxidation states of gold nanoparticles than
on the nature of the oxide. Indeed, whatever the support is, dried catalysts, containing Au3+, Au+ and Auδ+ but
not Au0 produce exclusively 2-cyclohexen-1-one and /or cyclohexene-1-ol. However, for reduced catalysts,
which contain Au0 but not Au3+, Au+ and Auδ+, the reaction products are: 2-cyclohexen-1-one, cyclohexene
epoxide, cyclohexanol and cyclohexanone.
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In order to explain how the major products are formed, we propose to see the different paths of cyclohexene
oxidation according to the literature [38, 53-57] .
Scheme 2 shows that the oxidation of cyclohexene begins first by the formation of cyclohexene hydroperoxide. .
After the formation of this hydroperoxide, the final oxidation may be carried out either:
i) through the intervention of a second molecule of cyclohexene (path I and II),
ii) or by disintegration of the hydroperoxide (paths III and IV).
Cyclohexanol can be produced through the hydration of cyclohexene (path V).
Scheme 2: Mechanism of cyclohexene oxidation.
Based on these reaction paths, we can say that whatever the support is (TiO2 or ZrO2), Au3+, Au+ and Auδ+ lead
to the disintegration of the hydroperoxide of cyclohexene (path III and IV) to give predominantly either 2-
cyclohexene-1-ol in the case of dried Au/TiO2 or 2-cyclohexene-1-one in the case of dried Au/ZrO2. On the other
hand, Au0 nanoparticles lead to the intervention of a second molecule of cyclohexene (path I and II).
4. Conclusion
Au/TiO2 and Au/ZrO2 catalysts have shown high efficiency for cyclohexene oxidation. The activity seems to
depend both on the oxidizing state of gold nanoparticles and on the nature of the support. Au0 nanoparticles are
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and 2-cyclohexene-1-ol
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Highlights
Au/TiO2 and Au/ZrO2 are used for oxidation of cyclohexene
Effect of oxidation states of Au nanoparticles on catalytic performance are studied
+
Gold in the catalysts that have been only dried is deposited as Au
+
Au have a high selectivity towards alkene oxides,
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