Liquid phase cyclohexene oxidation over vanadia based catalysts with tert-butyl hydroperoxide: Epoxidation versus allylic oxidation

Article abstract of DOI:10.1016/j.molcata.2014.07.002

VO₂-SiO₂ based catalysts with V contents between 5 and 20 wt.% were prepared from inorganic precursors via the sol-gel process and subsequently dried, calcined and reduced at 673 K. Structural characterization of these materials was carried out with X-rays diffraction (XRD), ICP, N ₂ adsorption-desorption at 77 K, UV-vis diffused reflectance spectroscopy (DR UV-vis), FTIR and pyridine adsorption followed by FTIR. Their catalytic activities in the cyclohexene epoxidation with TBHP as oxidant and heptane as solvent were also examined. Results of XRD and DR UV-visible spectroscopy revealed that VO₂ species are well dispersed on silica. BET analysis showed that the surface area decreases from 390 to 22 m² g^-1 with V content. The results of pyridine adsorption followed by FTIR indicated that the 5 wt.% VO₂-SiO₂ catalyst displays low acid densities. Experimental results pointed out that the 5 wt.% VO ₂-SiO₂ catalysts offer excellent activity. The obtained cyclohexene conversion and selectivity toward epoxide are 21% and 84% respectively. A reaction mechanism explaining clearly epoxidation versus allylic oxidation is proposed.

Full text of DOI:10.1016/j.molcata.2014.07.002

Accepted Manuscript
Title: Liquid phase cyclohexene oxidation over vanadia based
catalysts with tert-butyl hydroperoxide: Epoxidation versus
allylic oxidation
Author: Sanaa El-Korso Ilyes Khaldi Sumeya Bedrane
´ ´
Abderrahim Choukchou-Braham Frederic Thibault-Starzyk
Redouane Bachir
PII:
S1381-1169(14)00294-5
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2014.07.002
MOLCAA 9184
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
21-3-2014
28-6-2014
2-7-2014
Please cite this article as: S. El-Korso, I. Khaldi, S. Bedrane, A. Choukchou-
Braham, F. Thibault-Starzyk, R. Bachir, Liquid phase cyclohexene oxidation
over vanadia based catalysts with tert-butyl hydroperoxide: Epoxidation
versus allylic oxidation, Journal of Molecular Catalysis A: Chemical (2014),
http://dx.doi.org/10.1016/j.molcata.2014.07.002
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Highlights
 We have epoxidation in the presence of redox and monomeric vanadium sites.
 Allylic oxidation was obtained with acidic and/or polymeric vanadium sites.
 This reaction is heterogeneous and vanadium leaching is not observed.
 The results indicate that this material is stable for 4 cycles of the reaction.
1
Page 1 of 20

Liquid phase cyclohexene oxidation over vanadia based catalysts with tert-butyl
hydroperoxide: Epoxidation versus allylic oxidation
Sanaa EL-KORSO1, Ilyes KHALDI1, Sumeya BEDRANE1, Abderrahim CHOUKCHOU-
BRAHAM1*, Frédéric THIBAULT-STARZYK2 and Redouane BACHIR1
1
Laboratoire de Catalyse et Synthèse en Chimie Organique, Faculté des sciences, University
of Tlemcen 1300, Algeria. (* cba@mail.univ-tlemcen.dz)
2 Laboratoire Catalyse et Spectrochimie, ENSICAEN Université de Caen CNRS, 6 Boulevard
Maréchal Juin, 14050 Caen Cedex, France.
Abstract
VO2–SiO2 based catalysts with V contents between 5 and 20wt.% were prepared from
inorganic precursors via the sol gel process and subsequently dried, calcined and reduced at
673 K. Structural characterization of these materials was carried out with X-rays diffraction
(XRD), ICP, N2 adsorption-desorption at 77 K, UV-vis diffused reflectance spectroscopy (DR
UV-vis), FTIR and pyridine adsorption followed by FTIR. Their catalytic activities in the
cyclohexene epoxidation with TBHP as oxidant and heptane as solvent were also examined.
Results of XRD and DR UV-visible spectroscopy revealed that VO2 species are well
dispersed on silica. BET analysis showed that the surface area decreases from 390 to 22 m2 g-1
with V content. The results of pyridine adsorption followed by FTIR indicated that the
5wt.%VO2-SiO2 catalyst displays low acid densities. Experimental results pointed out that the
5wt.%VO2-SiO2 catalysts offers excellent activity. The obtained cyclohexene conversion and
selectivity towards epoxide are 21 % and 84 % respectively. A reaction mechanism
explaining clearly epoxidation versus allylic oxidation is proposed.
Keywords: VOx-SiO2, Cyclohexene, Epoxidation versus allylic oxidation, Leaching.
1. Introduction
During the last fifteen years, fine-chemical industry has pointed out that olefin epoxidation is
one of the most important reactions in industrial organic synthesis [1]. Among olefins,
cyclohexene is one of the most studied compounds. Cyclohexene oxide (epoxycyclohexane)
is a useful monomer in polymerisation and coating industries. It is used in the synthesis of
alicyclic target materials including pesticides, pharmaceuticals, perfumery and dye stuffs [2, 3].
2
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However, in the epoxidation of alkenes, several side reactions can take place, i.e. oxidation in
the allylic positions, ring-opening of the epoxide by hydrolysis or solvolysis, epoxide
rearrangement or even total breakdown of the C=C double bond. Cyclohexene is one of the
most difficult cases, as the first two situations, namely allylic oxidation and epoxide ring-
opening, can occur extensively [4].
It is well known that among the most active homogeneous and heterogeneous catalysts for
alkenes and alkanes oxidation, are those based on transition metals of groups 4 (Ti), 5 (V and
Nb) and 6 (Mo) [5]. In the case of heterogeneous catalysts, the preparation conditions play a
crucial role in obtaining high metal dispersion on the support, which in turn, generates highly
active and selective catalysts towards oxidation reactions [6 - 9].
Among the supported metals used in cyclohexene oxidation, titanium is probably the most
widely used. The couple TiO2-SiO2 has been known for a long time as an efficient catalyst in
liquid-phase oxidation reactions, when using hydrogen peroxide or alkylhydroperoxides as
oxygen donors [10 - 19]. Besides titanium, vanadium has been used in homogeneous [20 - 22]
and heterogeneous [23 - 30] oxidation reactions of cyclohexene. Most heterogeneous
vanadium catalysts are supported catalysts prepared by impregnation. Thereafter, the sol-gel
method has been used for the preparation of vanadia-silica catalysts which have been tested in
reactions such as selective catalytic reduction of nitric oxide with ammonia [31], epoxidation
of allyl alcohol [32], and oxidation of alkenes [33].
Many spectroscopic techniques have been employed to study supported vanadia catalysts in
order to establish the effect of the support on vanadia dispersion and the type of vanadium
species present [34 - 37]. Wachs et al. [35, 37] and Weckhuysen and Keller [38] described the
possible structures of supported vanadium species and their effects on the catalytic activity.
Different species may be present on SiO2 and Al2O3 supports: isolated VO4 species,
polymeric VO4 species, crystalline V2O5 and mixed oxide with support [38].
As noted above, the epoxidation of cyclohexene [39 - 44] is rivaled by allylic oxidation that
forms mainly cyclohexene2-one and cyclohexene2-ol [45-48].
In the present work, we report the sol–gel synthesis of X wt.%VO2–SiO2 catalysts with V
contents from 5 to 20 wt.%, calcined and reduced at 673 K. The prepared catalysts are
characterized by physico-chemical techniques, such as, ICP, infrared spectral analysis using
pyridine as a probe molecule, X-rays diffraction (XRD), specific surface area determination
(BET) and UV-Vis DR Spectroscopy, to understand their behavior, and their performances as
solid catalysts for epoxidation of cyclohexene, using tert-butyl hydroperoxyde (TBHP) as
oxidant. The results of this work and those already published [49] are focused on the evidence
of active sites and the comprehension of the mechanism of epoxidation versus allylic
oxidation.
3
Page 3 of 20

2. Experimental
2.1. Synthesis
2.1.1. Starting materials
The following chemicals were employed for the preparation of X wt.%VO2-SiO2 materials:
Tetraethylorthosilicate (C8H20O4Si, Aldrich 98 %), ammonium metavanadate (NH4VO3,
Strem chemicals 99 %), chloridric acid (Aldrich 37 %), ethanol (Aldrich 98 %).
2.1.2. Synthesis of SiO2
The preparation was carried out in a 100 mL beaker. 16.22 mL (0.091 mol) of
tetraethylorthosilicate was dissolved in 13 mL of ethanol. The mixture was stirred for 24 h,
and then 4.5 mL of HCl was added to this solution. The resulting gel was then stirred for 24 h.
This gel was first dried at 333 K for one night, next at 393 K for another night and then
calcined in air at 673 K for 4 h.
2.1.3. Synthesis of the system X wt.%VO2-SiO2
Typical procedure for the preparation of 5wt.%VO2-SiO2: the mixed oxide was prepared by
an acidic-catalyzed sol-gel process [6]. A quantity of 0.46 g of ammonium metavanadate
(NH4VO3) was dissolved in the mixture of 16.22 mL of tetraethylorthosilicate and 13 mL of
ethanol. The mixture was stirred for 24 h, and then 4.5 mL of HCl was added to this solution.
The resulting gel was then stirred for 24 h. This gel was first dried at 333 K for one night,
next at 393 K for another night and then calcined in air at 673 K for 4 h, and finally reduced,
under hydrogen flow, at 673 K for 4 h. The materials obtained have a green color which
indicates the presence of reduced vanadium (IV) species [33].
2.2. Materials characterization
The chemical composition of the samples was determined by inductively coupled plasma
emission spectroscopy (ICP) using a Varian ICP OES.
The BET surface area was determined from N2 adsorption isotherms at 77 K using a
Quantachrom NOVA 1000 instrument. Prior to adsorption measurements, the sample was
outgases at 673 K for 3 h.
The UV-visible spectra (200 - 800 nm) of these samples were collected on an UV/Vis
spectrometer Perkin Elmer with integration sphere. The baseline was recorded using MgO as
a reference.
4
Page 4 of 20

The FTIR spectra of the solid samples were recorded using an Agilent Technologies Cary 60
series FTIR spectrometer, with ATR accessories, and a measuring range of 4000–400 cm-1.
FTIR analysis of pyridine adsorption was carried out on a NEXUS- Nicolet spectrometer with
a 4 cm-1 resolution and a 4000–400 cm-1 scanning range. The spectrometer was equipped with
MCT detector cooled by liquid nitrogen. 18-20 mg of the catalyst sample was pressed into a
self-supported wafer of approximately 2 cm in diameter, then evacuated at 623 K for 4 h, and
finally cooled down to 293 K. A known amount of pyridine was then introduced in the cell at
room temperature and the wafer was degassed at 423 K for 15 min to remove the physisorbed
fraction. Finally pyridine adsorbed was followed by evacuation for 15 min at different
temperature. The IR spectrum thus obtained was used to calculate the amount of acid sites on
the sample by measuring the peak area of pyridine adsorbed.
2.3. Catalytic experiments
The catalytic epoxidation of cyclohexene with tertiobutylhydroperoxyde TBHP (Aldrich, 70
wt. % in H2O) as an oxidant was carried out in a two neck glass round-bottom flask equipped
with a magnetic stirrer and a reflux condenser. First, TBHP was stirred with heptane as
solvent in order to perform a phase transfer from water to organic phase. Typically, 25 mL of
heptane and 38.45 mmol (5.5 mL) of oxidant (TBHP) were mixed in a closed Erlenmeyer
flask and magnetically stirred for 24 h. The organic phase was then separated from the
aqueous phase. To control the phase transfer, the concentration of the remaining TBHP in the
aqueous phase was determined by iodometric titration. Less than 10 % of the initial TBHP
remained in the aqueous phase. Then, 29 mmol (3 mL) of cyclohexene, 0.1 g of catalyst and
the TBHP-heptane mixture were mixed in the magnetic stirrer-glass reactor at 338 K during 6 h.
The reaction products were identified by comparison with authentic products and the course
of reactions was followed by gas chromatography (GC), using a SCHIMADZU 14-B gas
chromatograph equipped with Agile HP-FFAP capillary column. A flame ionization detector
(FID) was used and 0.5 µL of the sample was analyzed. Before the GC analysis, the
remaining TBHP was decomposed by introducing an excess of triphenylphosphine (Aldrich).
On the other hand, to control the remaining TBHP, an iodometric titration was performed at
the end of the reaction (after 6 h) by analyzing the organic phase.
5
Page 5 of 20

3. Results and discussion
3.1. Catalyst characterization
Table1 shows that experimental vanadium loadings, measured by ICP for two samples, 5 and
15 wt.%VO2, are very close to the theoretical values.
Nitrogen adsorption–desorption revealed a type-I adsorption isotherm (cf. ESI; Fig. S1†)
corresponding to a microporous structure according to the IUPAC [52]. The corresponding
textural parameters are also presented in Table 1. The X wt.%VO2-SiO2 mixed oxides surface
area decreases from 390 to 22 m2 g-1 when the VO2 content rises from 5 to 20 wt.%. The
decrease in surface area, average pore diameter and pore volume with an increasing vanadium
loading is attributed to the blocking of micro-pores (covering effect) [2, 50].
The XRD analysis (cf. ESI; Fig. S2†) showed that all catalysts based on silica have
amorphous nature of silica and the peak of VO2 phase does not appear. This implies that
either VO2 is well dispersed on silica with a crystal size below the detection limit of the
method [50] or a completely amorphous phase was formed on the sample [51].
Table 1 Vanadium weight percentage determined by ICP and BET surface area results
wt.% of V
SBET
/m2 g-1
390
Pore volume
/cm3g-1
0.19
Pore size
Material
measured by ICP
/Å
20
18
13
25
5%VO2-SiO2
10%VO2-SiO2
15%VO2- SiO2
20%VO2- SiO2
4.83
/
14.80
/
177
0.10
77
0.02
22
0.03
The UV-Vis spectra of the samples are shown in Fig. 1. All samples present two bands
located at around 260 and 380 nm. The first UV-Vis band of vanadium species in the range
240-280 nm should be assigned to charge transfer transitions, involving oxygen and vanadium
(IV) in tetrahedral coordination, present in isolated species [53, 54]. The band at 380 nm is
attributed to octahedral V4+ species [53]. However, the band in the absorption range of 500-
850 nm, attributed to d-d transition of V4+ [55], is not observed at low vanadium loadings.
According to Luan et al. [56], Dutoit et al. [57] and also Golińska et al. [58], the intensity of
d-d transitions of V4+ recorded in the range of 500–850 nm is generally 10 to 30 times lower
than that of CT transitions. This may explain the lack of UV–vis bands, characteristic of V4+
species, whereas the intensity of the last band increases with increasing vanadium content
(Fig. 1) [59]. The samples 15 and 20 wt.%VO2-SiO2 show even the presence of new
6
Page 6 of 20

polymeric species such as those in tetrahedral environment (~320 nm) [60] and in octahedral
environment (~480 nm) [53, 60]. In conclusion, increasing the vanadium content leads to the
formation of polymeric species that coexist with isolated vanadyl species [59].
Fig. 1 Diffuse-reflectance UV-Vis spectra deconvoluted bands
IR spectra of the sample diluted in KBr matrix were deconvoluted using the peakFit software
in order to determine more accurately band frequencies and compare our solids to literature
data. Deconvoluted infrared spectra of VO2-SiO2 materials in the 1500–600 cm-1 region are
reproduced in Fig. 2. For 5 and 10 wt.%VO2-SiO2, absorption bands are observed at 1060 cm-1
and the shoulder at 1190 cm-1, due to the asymmetric stretching vibrations of the three
dimensional Si–O–Si network [53]. The band at 930 cm-1 is generally assigned to a Si–O–V
stretch mode [50, 53] and it is also postulated that both Si–O–V and V–O–V bridges can
7
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contribute to this band [38, 61]. The band at 793 cm−1 is assigned to symmetric stretching
vibrations of the three dimensional Si–O–Si network [39, 45, 53, 55]. The absorption bands
between 550 and 670 cm−1 are attributed to the rocking modes of the V–O–V bonds [60, 62-
63]. Concerning 15 and 20 wt.%VO2-SiO2, in addition to the bands previously described,
another band appeared as a shoulder at ~1024 cm-1. It is usually assigned to the V=O
stretching vibration in polyvanadate [63].
Fig. 2 Deconvoluted bands of room temperature IR spectra with ATR of the X wt.% VO2–
SiO2 materials
8
Page 8 of 20

Pyridine adsorption was performed on the surface of 5, 10, 15 and 20 wt.%VO2-SiO2
catalysts, and followed by IR spectroscopy. The bands of Brønsted acid sites (1538 cm-1) and
Lewis acid sites (1448 cm-1) are characteristic of pyridine adsorption on the catalyst
surface [64]. After pyridine adsorption on 5 wt.%VO2-SiO2 catalyst, the IR-spectra
exhibited both Lewis acid site bands and Brønsted acid site bands (Fig. 3). However, the IR-
spectra of 10, 15 and 20 wt.%VO2-SiO2, shows only the Lewis acid sites adsorption. The
quantification of these sites is shown in Table 2. The decrease in the number of Lewis acid
sites is due to the coverage of these sites by the increasing amount of V in the catalyst. So, the
total acidity is found to decrease at higher loadings [65]. This is explained by the stacking of
the vanadyl species on the surface of silica which leads to a decrease in the number of
adsorption sites exposed.
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2spectresousvideIIà300°C15min1,7.10-6torr
0,24
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2spectresousvideIIà250°C15min1,9.10-6torr
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2spectresousvideII1,6.10-6torrà200°C15min
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2spectresousvideIIà150°c15min1,1.10-5torr
0,22
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2SpectresousvideIIà100°C15min1,1.10-5torr
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2spectresousvideIIà50°C15min1,2.10-5torr
Résultatdesoustraction:Résultatdesoustraction:5%VO2-SiO2spectresousvideIIà350°C15min1,8.10-6torr
0,20
323 K
0,18
0,16
0,14
0,12
e
c
n
a
b
r
0,10
0,08
0,06
0,04
0,02
0,00
-0,02
-0,04
o
s
b
A
623 K
1600
1580
1560
1540
1520
1500
1480
1460
1440
1420
Nombred'onde(cm-1)
Fig. 3 Infrared spectra of pyridine adsorbed on 5 wt.%VO2-SiO2 at different temperatures
(323 – 623 K, with a step of 50 K)
Table 2 X wt.%VO2-SiO2 catalysts acidity at 423 K estimated by pyridine adsorption
followed by infrared spectroscopy
Lewis acid sites
Brønsted acid sites
Catalyst
/μmol g-1
/μmol g-1
SiO2
0
70
4
0
10
0
5%VO2-SiO2
10%VO2-SiO2
15%VO2- SiO2
20%VO2- SiO2
6
0
10
0
9
Page 9 of 20

3.2. Catalytic Activity Results
The oxidation of cyclohexene with TBHP in the presence of vanadium mixed oxides resulted
in the formation of several products, as shown in Table 3. Through GC analysis, 1,2-
cyclohexanediol was not detected in the reaction product. In the absence of catalyst,
cyclohexene gave only 12 % conversion; no epoxide was found. Since the SiO2 catalytic test
gave a similar result to the one with no catalyst, it can be said that the support was not active
[66]. From the results in Table 3, we note that the conversion of cyclohexene increases when
the vanadia loading rises to 15 wt.%, and decreases at higher vanadia loadings. The decrease
in conversion of the catalysts beyond 15 wt.% is probably due to the transformation of
isolated vanadium active sites into polymeric ones because of the agglomeration of vanadium
oxide [67]. As shown in table 3, all the catalysts give epoxide as the major product, while
selectivity decreases due to the appearance of the polymeric vanadium species (Fig. 1 and Fig.
2). Intermediate species adsorb on both monomeric or polymeric vanadium V-O-V promotes
another reaction pathway and thereby reducing selectivity to epoxide [68].
Table 3 Cyclohexene oxidation over X wt.%VO2-SiO2 catalysts
TBHP
Conversion
/%
Selectivity /%
Catalysts
Consumption
/%
Epoxide Enone One Enol Ol Diol
5%VO2-SiO2
10%VO2-SiO2
15%VO2-SiO2
20 %VO2-SiO2
21
36
46
39
84
77
63
58
/
12
8
4
7
/
/
/
/
/
/
/
/
/
24
48
45
61
8
2
4
35
38
/
Reaction conditions: 29 mmol cyclohexene, 38.45 mmol TBHP, 25 mL Heptane, 0.1 g catalyst, 6 h, reaction
temperature 338 K
The oxidation of cyclohexene with TBHP in the presence of vanadium mixed oxides 5
wt.%VO2-SiO2, 5 wt.%VO2-Al2O3 and 5 wt.%VO2-TiO2 was studied elsewhere [49]. High
cyclohexene conversion is observed in the case of 5 wt.%VO2-SiO2 (Table 4) due to highest
surface area.
10
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Table 4 Cyclohexene oxidation vs. catalysts physico-chemical features [49].
Conversion
Selectivity /%
TBHP
Consumption
/%
SBET
Lewis acid
sites
Catalysts
/%
/m2 g-1
Epoxide Allylic
oxidation
/μmol g-1
70
5%VO2-SiO2
5%VO2-Al2O3
5%VO2-TiO2
21
18
13
84
/
16
100
78
24
20
57
390
368
140
599
22
306
These results show three different behaviors: (i) VO2-SiO2 directs the reaction towards the
epoxidation with a selectivity of 84 % ; this is due to the presence of a weak Lewis acidity and
a predominant redox character of vanadium oxide, (ii) VO2-Al2O3 directs the reaction towards
the allylic oxidation, because of the strong Lewis acidity of alumina [9, 42], (iii) for VO2-
TiO2, the major products of the allylic oxidation are obtained; due to the strong Lewis acid
sites of the catalyst, but also a 22 % selectivity towards epoxide is observed due to the strong
interaction between vanadium and titanium [62]. In conclusion, the catalyst 5wt.%VO2-SiO2
has a higher catalytic activity in the cyclohexene epoxidation reaction due to the redox
character of the vanadium oxide; which directs the reaction towards epoxidation. However,
increasing the strength of Lewis acid sites leads to allylic oxidation. This reaction involves
two paths. The first one is a direct epoxidation in the presence of redox catalysts which leads
to the epoxide that becomes a diol in presence of water and acidic medium. The second one is
an allylic oxidation, which in the presence of Lewis acid sites gives cyclohexenol,
cyclohexenone, cyclohexanone and cyclohexanol, as shown on the following catalytic scheme
(Fig. 4).
11
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OH
H2O
O
Cyclohexane epoxide
OH
OH
Cyclohexane-1,2-diol
0.1 g VO2-MxOy
TBHP, Heptane
Reflux, 6h
O
Cyclohexene
Oxydation
Cyclohex-2-enol
Cyclohex-2-enone
O
Cyclohexanone
Fig. 4 Cyclohexene oxidation reaction scheme following two pathways
As it was found that 5wt.%VO2-SiO2 is the best catalyst, affording a high conversion of
cyclohexene and a high epoxide selectivity under the optimized reaction conditions (i.e.
reaction temperature 338 K, reaction time 6 h, 0.1 g of catalyst and a cyclohexene:TBHP
molar ratio of 1:1.3), it was selected as the standard catalyst to study the influence of various
parameters, such as cyclohexene to TBHP molar ratio and catalyst stability in the
cyclohexene oxidation reaction.
3.2.1. Effect of cyclohexene/TBHP molar ratio
The concentration of TBHP is also another important parameter affecting the cyclohexene
oxide selectivity. While keeping the substrate (cyclohexene) amount constant, the effect of
TBHP quantity was studied over a wide range of cyclohexene to TBHP molar ratio, i.e. from
0.5 to 2, with 5 wt.%VO2-SiO2 catalyst.
Table 6 Effect of cyclohexene/TBHP molar ratio on the cyclohexene epoxidation reaction
with TBHP, for 5 wt.%VO2-SiO2
cyclohexene
/TBHP
molar ratio
TBHP
Consumption
Conversion
/%
Selectivity /%
Epoxide Enone One Enol Ol Diol
%
40
24
35
35
1/2
1/1.3
1/1
39
21
18
14
82
84
77
85
3
/
12
12
9
3
4
9
6
/
/
/
/
/
/
/
/
5
5
1/0.5
4
Reaction conditions: 29 mmol cyclohexene, 25 mL Heptane, 0.1 g catalyst, 6 h, reaction temperature 338 K
12
Page 12 of 20

Table 6 represents the cyclohexene conversion and product selectivity at various
cyclohexene/TBHP molar ratios in the presence of 5wt.% VO2-SiO2. As shown, the
cyclohexene conversion decreases while the cyclohexene/TBHP molar ratio increases and the
epoxide selectivity remains almost unchanged. Anand et al. [70] attributed the decreasing
conversion of cyclohexene when the cyclohexene/TBHP molar ratio increases to the fact that
cyclohexene molecules occupy most of the active centers over the catalyst surface leaving less
space for TBHP. In our case, when the cyclohexene/TBHP molar ratio increases, there is a
competition between cyclohexene and TBHP; that is the more cyclohexene there is, the less
TBHP is adsorbed on the active sites. Therefore conversion decreases. However, each time a
TBHP molecule is adsorbed, it reacts with the cyclohexene adsorbed on an adjacent site to
form the epoxide; selectivity remains constant.
3.2.2. Mechanism
The reaction mechanism of cyclohexene oxidation reaction is widely discussed in the
literature where several intermediate complexes are proposed without a single mechanism to
secure the agreement of all authors [12, 71-74]. Using O2 or H2O2 as oxidant, a radical
mechanism leading to the formation of a hydroperoxide reaction intermediate was evidenced
[4, 13]. In the case of TBHP, on the basis of various studies reported in the literature [46, 75],
we propose the following mechanism occurring in two pathways from the intermediate (A)
formed between the catalyst and cyclohexene in the presence of TBHP (Fig. 5). tBuO-,
formed throughout the reaction in parallel to intermediate A attacks either on the vanadium
atom or on the C6 ring leading to the different paths:
 Path 1 is a reaction producing epoxide which is favored by the redox and monomeric
vanadium sites, but requires the presence of acidic sites;
 In path 2, the reactions occur with catalysts which have polymeric vanadium sites and
Lewis acidic sites. This path, it is divided into two parts: the first one to form an
unsaturated alcohol which undergoes a new oxidation reaction that gives cyclohexenone.
The second one leads to the production of cyclohexanone by acidic transposition.
13
Page 13 of 20

O
H
O
H)
O
V
O
V
V
O
OH
+
M
O
O
O
M
M
V
O
+
t-BuOH
O
M
H
H
(A)
t-BuO-
t-BuO-
O
O
M
V
O
H
t-Bu-O-O-H
O
OH
Oxidation
O
Fig. 5 Proposed mechanism for the cyclohexene epoxidation reaction by TBHP
3.2.3. Leaching and stability
Leaching was carried out in order to observe if the passage of the catalyst into the organic
phase occurs or not. For this purpose, cyclohexene reaction, in the presence of 5wt.%VO2-
SiO2, was studied. After 6 hours of reaction, the organic phase and the solid phase were
separated. A reaction was afterwards carried out with the organic phase for two hours, adding
an amount of each reagent and heating to reflux temperature. The results of this reaction
showed no production of epoxide, in agreement with the results reported by Farzaneh et al.
[72], and Brutchey et al. [76]. In addition, the solution remains clear in the presence of TBHP
14
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and cyclohexene [2]. These results show that we have no leaching of vanadium active species,
as previously reported by Leus et al. [77]. Moreover, a catalyst analysis was carried out before
and after catalytic test. These analyses showed a vanadium loading of 4.83 and 4.81 %
respectively, indicating a relative vanadium loss of 0.4 % that may be negligible.
The results of the 5wt.%VO2-SiO2 catalyst stability upon cyclohexene epoxidation are shown
in Table 7.
Table 7: 5 wt.%VO2-SiO2 catalyst stability upon cyclohexene epoxidation with TBHP
Cycle
Conversion
/%
Epoxide Selectivity
/%
TBHP Consumption
/%
1st
2nd
3rd
4th
21
29
23
18
28
84
87
74
79
85
24
30
28
20
38
5%V2O5-SiO2
Reaction conditions: 29 mmol cyclohexene, 38.45 mmol TBHP, 25 mL Heptane, 0.1 g catalyst, 6 h, reaction
temperature 338 K
Results indicate that this catalyst is stable upon 4 reaction cycles. Indeed, the conversion of
cyclohexene decreases from the third cycle because of the mass loss due to the filtration
(0.0875 g in the 3rd cycle and 0.0663 g in the 4th cycle) (see Table 5).
4. Conclusion
The Xwt.% VO2–SiO2 mixed oxide materials were prepared by the sol–gel method. These
materials were calcined and reduced at 673 K. The materials obtained showed a green color
indicating the presence of reduced vanadium (IV) species. The XRD results revealed that VO2
is well dispersed on amorphous silica. The mixed oxides microporous surface area and acidic
sites decrease with increasing vanadia loading. The samples UV-Vis spectra show that
vanadyl species are incorporated in the support in a monomeric form. When the vanadium
content increases, the polymeric vanadium species appear. This system has proven to be an
efficient catalyst for the selective epoxidation of cyclohexene with TBHP as oxidant and
heptane as solvent. A maximum selectivity for cyclohexene epoxidation (84 %) was observed
for the 5wt.%VO2–SiO2 catalyst at 338 K. This material is stable upon 4 reaction cycles. This
reaction is really heterogeneous and no vanadium leaching was observed.
Finally, from the catalytic results reported in this paper, we suggest that the cyclohexene
oxidation reaction involves two paths. The first one is a direct epoxidation, in the presence of
15
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redox catalysts, leading to epoxide formation. The second one is an allylic oxidation in the
presence of Lewis acid catalysts to give cyclohexenol, cyclohexenone, cyclohexanone and
cyclohexanol.
Acknowledgments
The authors acknowledge the facilities and the scientific and technical assistance of LCS,
ENSICAEN. They especially thank Mrs. Valérie Ruaux for her help. They also thank Mr.
Mohamed Benabdallah from LCSCO for the fruitful discussion and the reaction mechanism
proposal.
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O
O
O
O
V
O
i
T
O
O
OH
O
Page 20 of 20