Investigation of the effect of VO: X/ZrO2 structure on the catalytic activity in cyclohexene epoxidation

Article abstract of DOI:10.1039/c6ra20251a

Vanadium oxide catalysts were prepared by impregnation of ammonium metavanadate on monoclinic zirconia with a metal loading equal to 5 wt%. Further thermal treatment led to V₂O₅/ZrO₂ upon calcination at 400 °C and VO₂/ZrO₂ upon calcination at 400 °C, followed by reduction at the same temperature. The third compound underwent further heat treatment at 800 °C, under argon. The effect of high temperature treatment on the structure of the materials as well as on the catalytic properties of the vanadate species was investigated in the epoxidation of cyclohexene, with TBHP as the oxidant and heptane as the solvent. Structural characterization of these materials was carried out using ICP-OES, X-ray diffraction (XRD), N₂ adsorption-desorption at 77 K, UV-Vis diffuse reflectance spectroscopy (DR UV-Vis), Fourier-Transform Infrared Spectroscopy (FTIR) and pyridine adsorption followed by FTIR. The coexistence of a solid solution with polymeric vanadium species in the case of 5VZr-2 and 5VZr-3 with the formation of ZrV₂O₇ in the case of 5VZr-3, resulted in good catalytic performance, with up to 80% epoxide selectivity and 32% cyclohexene conversion. In the case of 5VZr-1, the appearance of the crystalline V₂O₅ phase decreased the catalytic activity in the cyclohexene reaction.

Full text of DOI:10.1039/c6ra20251a

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Investigation of the effect of VO /ZrO structure on
x
2
the catalytic activity in cyclohexene epoxidation
Cite this: RSC Adv., 2016, 6, 110375
Sanaa El-Korso,* Sumeya Bedrane, Abderrahim Choukchou-Braham
and Redouane Bachir
Vanadium oxide catalysts were prepared by impregnation of ammonium metavanadate on monoclinic
2 5 2
zirconia with a metal loading equal to 5 wt%. Further thermal treatment led to V O /ZrO upon
ꢀ
ꢀ
calcination at 400 C and VO
2
/ZrO
2
upon calcination at 400 C, followed by reduction at the same
ꢀ
temperature. The third compound underwent further heat treatment at 800 C, under argon. The effect
of high temperature treatment on the structure of the materials as well as on the catalytic properties of
the vanadate species was investigated in the epoxidation of cyclohexene, with TBHP as the oxidant and
heptane as the solvent. Structural characterization of these materials was carried out using ICP-OES, X-
2
ray diffraction (XRD), N adsorption–desorption at 77 K, UV-Vis diffuse reflectance spectroscopy (DR UV-
Vis), Fourier-Transform Infrared Spectroscopy (FTIR) and pyridine adsorption followed by FTIR. The
coexistence of a solid solution with polymeric vanadium species in the case of 5VZr-2 and 5VZr-3 with
Received 11th August 2016
Accepted 12th November 2016
the formation of ZrV
epoxide selectivity and 32% cyclohexene conversion. In the case of 5VZr-1, the appearance of the
crystalline V phase decreased the catalytic activity in the cyclohexene reaction.
2 7
O in the case of 5VZr-3, resulted in good catalytic performance, with up to 80%
DOI: 10.1039/c6ra20251a
www.rsc.org/advances
2 5
O
Structural characterization of supported vanadate species is of
key importance, because the variation in the catalytic activity
Alkene epoxidation is a very useful reaction in industrial with the support can be partly attributed to the structure of
1
. Introduction
15
organic synthesis. The resultant epoxides are essential precur- vanadate species dispersed on the surface of various supports.
sors in the synthesis of various important substances such as
The predominating opinion is that active vanadium-based
plasticizers to perfumes and epoxy resins. The design of catalysts possess a molecularly dispersed vanadium oxide phase
1,2
3
catalytic systems for complete oxidation of hydrocarbons is an which is typically more active than the bulk V O . It has been
2 5
3
important issue in environmental catalysis. Many studies have demonstrated that the strong inuence of the support on the
revealed the high catalytic activity of various supported vana- activity and selectivity is mainly due to the presence of the active
3–6
16
dium oxides in a number of oxidation reactions. In fact, the species V–O-support. Deo and Wachs suggested the V–O-support
choice of the support to be used depends on the reaction, bridging bond as a factor controlling both reactivity and reduc-
3,5
because the catalytic activity depends not only on the number of ibility of supported vanadium oxide catalysts. The same authors
active sites but also on their distribution on the surface and also noted that the strength of V–O-support bond is a fundamental
7
on the vanadium loading in vanadium oxide-based materials.
For example, VO
such as CeO , TiO
for oxidative dehydrogenation (ODH) reactions, compared to 5 wt% in accordance to earlier studies.
those supported on irreducible supports, such as Al O and thermal treatment on their structural properties was investi-
criterion of the oxidative activity of supported metal oxide catalysts.
In the present study, various zirconia-supported VO systems
show signicantly higher activity were prepared by impregnation. The V loading was chosen to be
x
catalysts supported on reducible supports,
, and ZrO
x
2
2
2
17,18
The inuence of
2
3
8–12
SiO2.
Today, the new trend is moving towards the use of gated. Their catalytic performances were tested in the cyclo-
heterogeneous catalysts in the selective epoxidation of olens. hexene epoxidation by tert-butylhydroperoxide.
On the one hand; molecular sieve catalysts, containing redox
metal ions incorporated in the framework, have been widely
13
used, on the other, reducible supports for vanadia catalysts, 2. Experimental
such as ZrO
2
have received growing attention due to their
2
.1. Synthesis
.1.1 Raw materials. The following chemicals were used in
the preparation of 5 wt% V/ZrO , namely zirconium(IV) oxide
14
thermal stability, and their redox and catalytic features as well.
2
2
(
2 4 3
ZrO ; Prolabo), ammonium metavanadate (NH VO ; Strem
Laboratoire de Catalyse et Synth `e se en Chimie Organique, Facult ´e des Sciences,
Universit ´e de Tlemcen, Algeria. E-mail: sana.korso@mail.univ-tlemcen.dz
chemicals, 99%), nitric acid (Aldrich), cyclohexene (Aldrich
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ꢀ
9
9%), hydroperoxide tertio-butyl TBHP (Aldrich 70%) and degassed at 150 C, for 15 min, to remove the physisorbed frac-
heptane (Fluka 99%). tion. Finally, the adsorbed pyridine was followed by evacuation
.1.2 Synthesis of the system 5wt% VO /ZrO . The for 15 min, at different temperatures. The IR spectrum thus ob-
2
x
2
impregnation procedure was used to prepare the 5 wt% VO / tained was used to calculate the number of acid sites on the
x
19
ZrO
the ZrO
metavanadate (NH
quantity of 40 mL of nitric acid was added to 0.57 g of ammo-
nium metavanadate and stirred for 2 h, and then 4.75 g of ZrO
2
catalysts used in present study. Prior to impregnation, sample, by measuring the peak area of the adsorbed pyridine.
support was calcined at 673 K for 4 h. Ammonium
VO ) was used as a vanadium precursor. A 2.3. Catalytic experiments
2
4
3
The catalytic epoxidation of cyclohexene with tert-Butyl hydro-
peroxyde TBHP (Aldrich, 70 wt% in H O) as oxidant, was carried
2
2
were added to this solution. The resulting mixture was stirred
out in a two neck glass round-bottom ask, equipped with
a magnetic stirrer and a reux condenser. First, TBHP was stirred
with heptane as solvent, in order to perform a transfer from water
to organic phase. Typically, 25 mL of heptane and 38.45 mmol
ꢀ
for 24 h, at room temperature, and subsequently dried at 60 C
ꢀ
for one night and at 120 C for another night. The resulting
ꢀ
solid was calcined under owing air at 400 C, for 4 h. The
calcined sample (noted 5VZr-1) was further reduced under
(5.5 mL) of oxidant (TBHP) were mixed in a closed Erlenmeyer
ꢀ
hydrogen ow, at 400 C, for 4 h. The reduced material (noted
ask, and then magnetically stirred for 24 h. Next, the organic
5
VZr-2) exhibited a green color, indicating the presence of
phase was separated from the aqueous phase. In order to control
the phase transfer, the concentration of the remaining TBHP in
the aqueous phase was determined by iodometric titration. More
than 90% of the initial TBHP was transferred into the organic
phase to be used in the catalytic reaction. The amount of
20
reduced vanadium (+4) species. Finally, the reduced 5VZr-2
sample underwent another thermal treatment under argon
ꢁ1
ꢀ

ow (28 mL min ), at 800 C, for 4 h. It will be noted 5VZr-3.
2
9 mmol (3 mL) of cyclohexene with 0.1 g of catalyst and the
2
.2. Catalyst characterization
TBHP–heptane admixture were all mixed in a magnetic stirrer-
glass reactor, at 65 C for 6 h. The reaction products were
All catalysts were characterized by X-rays powder diffraction
ꢀ
(
1
XRD) using a Bruker D5005 diffractometer (Cu Ka radiation, l ¼
analyzed by gas chromatography (GC), using a SCHIMADZU 14-B
gas chromatograph equipped with an Agile HP-FFAP capillary
column. A ame ionization detector (FID) was also used and 0.5
mL of the sample was analyzed. Prior the GC analysis, the
remaining TBHP was decomposed by introducing an excess of
triphenylphosphine (Aldrich). In parallel, and in order to control
the remaining TBHP in the reaction mixture, an iodometric
titration was performed at the end of the reaction, aer 6 h
ꢀ
ꢀ
ꢀ
.54060 A) in the range of 2q ¼ 10–80 , with a step of 0.02 and an
acquisition time of 1 s. Elemental analysis was performed using
atomic absorption with a Perkin Elmer Analyst 300 Atomic
Absorption spectrometer. The prepared samples were analyzed
by thermal analysis (TGA/DTA) using an SDT Q600 instrument.
The 20–30 mg sample mass was introduced into a platinum
crucible, which is supported by the beam of a balance in the
oven. The analysis was performed in air with a temperature ramp
of 5 K min in the range 298–1473 K. XRD, ATG and elemental
analysis were carried out in the Institute of Chemistry of
Mediums and Materials of Poitiers (IC2MP) at the Faculty of
Science, Poitiers University, France. The BET surface areas were
determined from N adsorption/desorption isotherms at 77 K,
2
using a Quantachrom instrument. Prior to absorption measure-
ments, the sample was outgassed at 300 C, for 3 h, under 30%
nitrogen in helium ow. The FTIR spectra of the solid samples
were recorded, using an Agilent Technologies Cary 60 series FTIR
spectrometer, with ATR accessories, in a measuring range of 400–
(results reported as % TBHP consumption in Table 3).
ꢁ1
The catalytic performances were reported in terms of the
cyclohexene conversion, selectivity towards products and turn-
over frequency (TOF). They were calculated by means of the
expressions below:
Conversionð%Þ ¼
ꢀ
moles of initial substrate ꢁ moles of residual substrate
ꢂ 100
moles of initial substrate
ꢁ1
moles of individual product
4
000 cm . The UV-visible spectra (200–800 nm) of these samples
Selectivityð%Þ ¼
ꢂ 100
moles of total products
were measured on a Perkin Elmer UV-Vis spectrometer equipped
with a Cary 4/5 diffuse reection sphere. The baseline was set to
zero with the help of an MgO reference. The Scanning Electron
Microscopy (SEM) with EDX (Energy Dispersive X-ray) analysis
was performed using a Hitachi TM-1000 apparatus, at an accel-
eration voltage of 150 kV. FTIR spectroscopy analysis of pyridine
adsorption was carried out on a NEXUS-Nicolet spectrometer,
with a 4 cm resolution and a 4000–400 cm scanning range.
The spectrometer was equipped with a liquid nitrogen cooled
MCT-detector. A mass ranging from 18 to 20 mg of the catalyst
À
Á
nbr of epoxide molecules
ꢁ1
TOF h
¼
nbr V sites ꢂ time
3
. Results and discussion
ꢁ1
ꢁ1
3.1. Physico-chemical characterization
3.1.1 ICP-OES. The 5VZr-2 sample, with a theoretical
sample was pressed into a self-supported wafer of approximately vanadium content of 5 wt%, was analyzed by ICP-OES. The
ꢀ
2
cm in diameter, then evacuated at 400 C for 4 h, and nally results showed that vanadium which was actually deposited on
ꢀ
cooled down to 20 C. A known amount of pyridine was then zirconium is 4.27 wt%, indicating a 14% loss of vanadium
introduced into the cell, at room temperature, and the wafer was during the synthesis procedure.
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Table 1 Textural characteristics, as determined by N
desorption for different samples
2
adsorption/
3
2
.1.2 Powder XRD analysis. X-rays diffractograms of ZrO ,
5
VZr-1, 5VZr-2 and 5VZr-3 are shown in Fig. 1.
ZrO
2
exhibits a monoclinic structure [ICDD pattern 037-
Temperature
treatment ( C)
S
BET
Pore volume
Pore size
(A)
1
484], with an average crystallite size of 30 nm. The deposi-
ꢀ
2
ꢁ1
3
ꢁ1
ꢀ
Material
ZrO
(m g
)
(cm g
)
tion of vanadium oxide on the support and the heat treatment
do not affect the structure of the material, therefore the
monoclinic structure is conserved. However, in the case of
VZr-1, the crystalline phase of V O appears [ICDD pattern
2 5
41-1426], with an average crystallite size of 45 nm. Moreover,
2
400
400
400
800
<10
<10
<10
ꢃ0
0.02
0.02
0.02
—
173
94
87
5
5
5
VZr-1
VZr-2
VZr-3
5
0
—
the diffraction peaks of 5VZr-2 and 5VZr-3 diffraction show
a slight shi towards smaller angles compared to the ICDD
reference (zoom on the two most intense peaks). This shi
suggests the formation of a solid solution resulting from the
3
.1.4 TGA/DTA analysis. Fig. 2 illustrates the TGA/DTA
analysis of 5VZr-2 material before any thermal treatment
(dried sample). An endothermic peak, observed between room
temperature and 200 C, can be attributed to water removal.
Small endothermic peak, beyond 600 C, may be imputed to
phase transformation, leading to the formation of ZrV
4
+
distortion of the ZrO lattice by insertion of V cations.
2
ꢀ
At 800 C, 5VZr-3 diffractogram presents new low-intensity
ꢀ
22
A
ꢀ
peaks at 2q ¼ 20.8, 23, 25 and 27 , corresponding to the
ꢀ
formation of a new crystalline phase ZrV
2
O
7
[ICDD pattern
22,23
2
O
7
.
0
16-422]. This new crystalline phase, which appears at high
ꢀ
22,24
Over 950 C, further phase transitions may have occurred.
temperatures, may originate from the intimate contact
between dispersed surface vanadium oxide species and the
The total weight loss of the sample was found equal to about 2%
in the temperature range 25 to 1200 C, which corresponds
perfectly to what was expected theoretically (transformation of
ꢀ
1
6,21
zirconia support.
.1.3 Surface area measurements. Table 1 show that all
3
ammonium metavanadate to V
.1.5 DR UV-Vis spectroscopy. Over the past few years, this
technique has been increasingly involved in the character-
2 5
O ).
materials have a very low surface area and porosity that
decreases even further upon high temperature treatment.
3
6
,25–27
ization of vanadia catalysts.
In fact, the ligand-to-metal
charge transfer (LMCT) transitions of vanadium appear in
the 200–500 nm region and the d–d transition bands in the
4
+
28
6
00–800 nm region, due to d–d electron transfer of V . The
energy of oxygen-to-vanadium charge transfer absorption
band, which is correlated with the minimum diffuse reec-
tance, is strongly inuenced by the number of ligands
surrounding the central vanadium ion. It can therefore be
used to obtain very useful information on the coordination of
vanadium ions in different surface species, in supported
2
9,30
vanadia catalysts.
The UV-Vis spectra of the samples are shown in Fig. 3, and
may be interpreted as follows:
(
a) The bands around 220 nm are most likely due to charge
4
+
31,32
transfer between Zr and oxygen.
b) The band of vanadium species, observed in the 300–
50 nm region, should be assigned to the charge-transfer
(
3
Fig. 1 X-rays diffraction (XRD) patterns of the ZrO
ZrO systems.
2
and 5wt% V
O
x y
/
Fig. 2 Thermal gravimetric analysis of the 5VZr-2 sample, before
thermal treatment.
2
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Fig. 3 Diffuse-reflectance UV-Vis deconvoluted spectra of the 5 wt%
V
x
y
O /ZrO
2
materials.
x y 2
Fig. 4 FTIR-ATR deconvoluted spectra of 5 wt% V O /ZrO materials,
at room temperature.
transitions involving oxygen and vanadium species in tetrahedral
ꢁ
1
33–35
(d) in the case of 5VZr-1, a new band centered at 911 cm is
observed, it is attributed to the crystal phase of the vanadium
coordination, present as polymeric species.
The band in the
4
00–500 nm region is attributed to octahedral vanadium poly-
7,43
33,36,37
oxide (V
e) in the case of 5VZr-3, the band at 867 cm may be due to
the presence of ZrV O phase, detected by XRD.
The results of the FTIR spectroscopy are correlated with
those of UV-Visible and XRD analyses.
.1.7 Measurement of surface acidity. Pyridine adsorption
was performed on the surface of 5VZr-2 catalyst, and fol-
lowed by IR spectroscopy. The bands corresponding to
2 5
O ),
meric species.
ꢁ1
(
(
c) In the case of 5VZr-1, the band at ꢃ570 nm is due to the
33,38,39
2
7
2 5
presence of crystallized V O .
(
d) In the case of 5VZr-2 and 5VZr-3, the band in the 600–
4
+
7
00 nm region is due to the V species d–d transition.
e) In the case of 5VZr-3, the existence of a band at 765 nm
should be due to the presence of ZrV O , evidenced by XRD.
3
(
2
7
26
Gao et al. found that in the case of 4% V O /ZrO , the
2
5
2
ꢁ1
Brønsted acid sites (1538 cm ) and Lewis acid sites (1448
cm ) are characteristic of pyridine adsorption on the cata-
polymerized surface VO
4
species are predominant.
ꢁ1
3
.1.6 FTIR-ATR absorption. Deconvoluted infrared spectra
were determined using the peakFit deconvolution soware.
Deconvoluted infrared spectra of 5 wt% V /ZrO materials in
4
4
lyst surface. Aer pyridine adsorption on 5VZr-2 catalyst,
the IR-spectra exhibited both Lewis acid site bands and
Brønsted acid site bands (Fig. 5). The quantication of these
sites is shown in Table 2. The number of Lewis acid sites
decreases with increasing evacuation temperature. Elassal
x
O
y
2
ꢁ
1
the 1500–500 cm region are reported in Fig. 4. Aer decon-
volution, the following adsorption bands are revealed:
ꢁ
1
(
a) between 530 and 650 cm , attributed to the rocking
7
33,40,41
et al. found small amounts of Lewis acid sites in catalysts 4,
modes of the V–O–V bonds,
1
3,45
ꢁ
1
2 5 2 2
5 and 6% V O /ZrO . ZrO itself has very weak acidity.
(
b) around 710–750 cm , representing stretching vibration
28
When vanadium is inserted in the ZrO lattice, in order to
form the solid solution, the acidity of the oxide remains
2
of the V–O–V bonds,
ꢁ1
(
c) at 1000 cm , being the asymmetric stretching vibration
of the V]O bond in the polyvanadate,
42,43
unchanged.
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V
x
O
y
/ZrO
2
catalysts are summarized in Table 3. The highest
performances were obtained over the reduced samples 5VZr-2
and 5VZr-3, due to the presence of the solid solution in both
ꢁ1
samples. 5VZr-3 is a better catalyst with a TOF ¼ 49 h , this
may be explained by the presence of ZrV
treatment at 800 C. Several studies have shown the impor-
tance of polymeric vanadium species for the catalytic activity
2
O
7
upon thermal
ꢀ
3
,6,26,58,60
in oxidation reactions over VO
as the formation of ZrV phase.
with different preparation methods,
showed that bulk ZrV O is quite selective. This suggests that
x
/ZrO
2
6,61
systems,
However, a later study of
as well
1
2 7
O
6
,58,62
the same system
2
7
some other factors, rather than V–O–Zr sites (since ZrV O is
2
7
exclusively composed of V–O–Zr functionalities), contribute to
the catalytic activity in the oxidation reaction. In our case, one
may suggest that the presence of reduced polymeric vanadium
species in the solid solution is responsible for the increase in
the catalytic activity and selectivity towards epoxide. This may
be indicative of the relevance of the V–O–Zr bond, which is
present in both zirconia-supported vanadium oxide and
Fig. 5 Infrared spectra of pyridine adsorption on 5VZr-2, at different
temperatures (from 50 to 350 C, in steps of 50 C).
ꢀ
ꢀ
Table 2 Acidity of 5VZr-2 catalysts at different temperatures, esti-
mated by pyridine adsorption and followed by infrared spectroscopy
1
6
ZrV O phase. The lower activity of 5VZr-1 must be ascribed
2
7
to the presence of considerable amounts of crystalline V
2 5
O
Temperature
Lewis acid
sites (mmol g )
ꢀ
ꢁ1
(
C)
which decreases the number of surface polymeric
3
,16,30,56,61
sites.
5
1
1
2
2
3
0
29
26
15
9
4
2
17
Finally, we discussed elsewhere the reaction mechanism
for cyclohexene oxidation and we suggested that the
epoxide production pathway is favored by redox features
and by the presence of monomeric or polymeric vanadium
00
50
00
50
00
18
sites.
x y 2
To our knowledge, no heterogeneous 5 wt% V O /ZrO
catalysts were used in the cyclohexene epoxidation reaction. The
closest results to our work are summarized in Table 4. It can be
observed that our catalysts (Table 3) present the best perfor-
3.2. Catalytic activity results
Generally, the cyclohexene oxidation reaction leads mainly to mances in terms of epoxide selectivity and conversion. Only M.
63
the formation of cyclohexene oxide (epoxide) and cyclohexane Sharbatdaran et al. reported an interesting conversion of 80%
diols, besides cyclohexenol (Enol), cyclohexenone (Enone), with a selectivity of 61% towards epoxide. However, it is worth
4
6–52
cyclohexanol (Ol) and cyclohexanone (One) (Fig. 6).
It is well established that the VO
active species has silica zirconia molybdate which is much more complex than our
a signicant impact on the catalytic performance of vanadium- catalysts.
underlining that these authors used an inorganic polymer of
x
5
3,54
supported catalysts,
well as acidic sites.
due to the presence of surface redox as
Using GC analysis in all experiments,
1
7,55,56
3
.3. Leaching and stability
alone, a very low Leaching was carried out to see whether the passage of the
conversion (2%) was observed and no epoxide catalyst into the organic liquid phase occurs or not. For this,
no 1,2-cyclohexanediol was detected in the reaction prod-
1
7,32,57
ucts.
Without catalyst and with ZrO
2
3
2,49,58,59
was produced.
The results of the catalytic tests of reactions of cyclohexene in the presence of the 5VZr-2 cata-
cyclohexene oxidation by TBHP at 70 C, over different 5 wt% lyst were studied. Aer 6 h of reaction time, the organic
phase and the solid phase were separated by ltration. The
ꢀ
reaction of the organic phase was started again for 2 hours,
adding an amount of each reactant and heating to reux
temperature. The results of this reaction showed that no
epoxide was produced; similar results were reported by
6
8
67
Brutchey et al. and Farzaneh et al. In addition, it was
noted that the solution remains clear in the presence of
6
9
TBHP and cyclohexene. It can be stated from these results
that no catalyst leaching occurs, and the reaction is really
heterogeneous.
The results of the stability of 5VZr-2 catalyst during the
Fig. 6 Different possible products of cyclohexene oxidation reaction. epoxidation reaction of cyclohexene are shown in Table 5. They
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a
Table 3 Oxidation of cyclohexene, using different supported catalysts
Activity
Selectivity (%)
Epoxide
TBHP
consumption (%)
ꢁ1
Catalysts
Without
Conv. (%)
TOF (h
)
Enone
One
Enol
Ol
Diol
02
—
21
28
32
00
00
20
42
49
88
—
—
5
12
—
—
—
—
12
8
50
50
37
ZrO
2
—
51
80
82
—
6
—
—
43
15
15
—
—
—
—
5VZr-1
5VZr-2
5VZr-3
—
3
a
ꢀ
Reaction conditions: cyclohexene 29 mmol, TBHP 38.45 mmol, heptane 25 mL, 0.1 g catalyst, reaction time 6 h, reaction temperature 65 C.
Table 4 Comparison of present work results with those reported in literature
Activity
Conversion
(%)
TOF
(h
Epoxide
selectivity (%)
ꢁ1
Catalyst
Substrate
)
Ref.
64
32
63
18
24
65
66
47
a-ZrP$Fe(Salen)
Cyclohexene, TBHP as oxidant, benzene
as solvent, 80 C, 5 h, 0.066 g catalyst
13
21
ꢀ
Au/ZrO
SZ1–Mo
VO /CeO
2
Cyclohexene, TBHP as oxidant, reux,
29
18
ꢀ
6
h, 70 C, 0.1 g catalyst
Cyclohexene, TBHP as oxidant, CH
solvent, reux, 8 h, 0.05 g catalyst
Cyclohexene, TBHP as oxidant, heptane
as solvent, 65 C, 6 h, 0.100 g catalyst
Cyclohexene, H
solvent, 90 C, 24 h, 0.040 g catalyst
Cyclohexene, TBHP as oxidant, 60 C, 7 h,
3
CN as
80
61
2
2
45
17.1
77
ꢀ
2
ZrO –SiO
2
2
2
O
2
as oxidant, MeOH as
28
—
0
ꢀ
ꢀ
2
ZrO –SiO
<1
<1
0
.020 g catalyst
Cyclohexene, TBHP as oxidant, 70 C,
2 h, 0.025 g catalyst
ꢀ
[
VO(hacen)]–Y
27.51
36.47
1
VO
VO
VO
2
2
2
–SiO
–TiO
–Al
–TiO
2
Cyclohexene, TBHP as oxidant, heptane
as solvent, 65 C, 6 h, 0.100 g catalyst
21
13
17
46
83
21
0
ꢀ
2
2
O
3
2
V
2
O
5
Cyclohexene, TBHP as oxidant, heptane
as solvent, 80 C, 6 h, 0.100 g catalyst
Cyclohexene, TBHP as oxidant, heptane
as solvent, 70 C, 6 h, 0.100 g catalyst
Cyclohexene, TBHP as oxidant,
chloroform as solvent, 65 C, 6 h, 0.200 g
75
46
50
67
ꢀ
Vanadium–
chromium–bentonite
V-MCM-41
32
27
69
92
ꢀ
ꢀ
catalyst
indicate that this material is stable during 2 cycles of the reac-
tion. A slight decrease in the catalytic performances occurs from
the 3 one.
4
. Conclusions
rd
A series of zirconia-supported vanadium oxide catalysts were
prepared by impregnation process, with different temperature
ꢀ
treatments. 5VZr-1 was obtained upon calcination at 400 C of
Table 5 Results of the stability of 5VZr-2 during the epoxidation
reaction of cyclohexene with TBHP
ꢀ
dried V/ZrO
2
, 5VZr-2 was obtained upon reduction at 400 C of
a
5
5
VZr-1, and 5VZr-3 was obtained upon thermal treatment of
VZr-2 under argon ow, at 800 C. Characterization of these
ꢀ
Conversion
(%)
Epoxide
selectivity (%)
TOF
(h
TBHP
ꢁ
1
Cycle
)
consumption (%) catalysts showed the presence of crystalline V O and a small
2 5
amount of Lewis acid sites on 5VZr-1. This system has proven
to be an efficient catalyst for the selective epoxidation of
cyclohexene with TBHP as oxidant and heptane as solvent (TOF
st
1
28
24
15
12
80
78
78
55
42
35
22
12
50
30
17
17
nd
2
rd
3
ꢁ1
th
¼ 20 h ). The epoxidation catalytic performances can be
4
ꢁ1
exalted (TOF ¼ 42 h ) when a solid solution is formed with
polymeric vanadium species (detected by UV-Vis, FTIR and
XRD). These performances can be even higher (TOF ¼ 49 hꢁ ) in
a
Reaction conditions: cyclohexene 29 mmol, TBHP 38.45 mmol, heptane
5 mL, 0.1 g catalyst, reaction time 6 h, reaction temperature 65 C.
ꢀ
2
1
110380 | RSC Adv., 2016, 6, 110375–110382
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the presence of ZrV
2
O
7
in crystalline form (conrmed by XRD). 23 R. P ´e rez-Hern ´a ndez, D. Mendoza-Anaya, M. E. Fern ´a ndez
This may be indicative of the relevance of the V–O–Zr bond,
which is present in zirconia-supported vanadium oxide as well
and A. G ´o mez-Cort ´e s, J. Mol. Catal. A: Chem., 2008, 281,
200–206.
as in the ZrV O phase. Meanwhile, the presence of the crys- 24 C. Pallazi, L. Oliva, M. Signoretto and G. Strukul, J. Catal.,
2
7
talline V
2
O
5
phase decreases the epoxide selectivity.
2000, 194, 286–293.
25 X. Gao, M. A. Banares and I. E. Wachs, J. Catal., 1999, 188,
3
25–331.
Acknowledgements
2
6 X. Gao, J.-M. Jehng and I. E. Wach, J. Catal., 2002, 209, 43–50.
The authors would like to thank the General Directorate for 27 D. I. Enache, E. Bordes-Richard, A. Ensuque and F. Bozon-
Scientic Research and Technological Development (DGRSDT)
Verduraz, Appl. Catal., A, 2004, 278, 93–102.
as well as the Thematic Agency for Research in Science and 28 J. Liu, Z. Zhao, C. Xu, A. Duan and G. Jiang, J. Rare Earths,
Technology (ATRST) for the nancial support to the project rtn-
2010, 28, 198–204.
tr010 Pro Theme/56/2015.
29 G. C. Bond and J. C. Vedrine, Catal. Today, 1996, 27, 437–455.
3
0 K. V. R. Chary, C. P. Kumar, D. Naresh, T. Bhaskar and
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1 D. Prasetyoko, Z. Ramli, S. Endud and H. Nur, J. Mol. Catal.
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