Preparation and characterization of MTiX for the catalytic oxidation of cyclohexane

Article abstract of DOI:10.1039/c4ra02241f

MTiX (M = Cr and V) samples, with 0, 5, 10 and 20 atomic percent, were synthesised by a sol-gel method and calcined at 400 °C in air. The XRD analysis results indicated that the materials presented a crystalline structure with the presence of TiO₂

Full text of DOI:10.1039/c4ra02241f

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Preparation and characterization of MTiX for the
catalytic oxidation of cyclohexane†
Cite this: RSC Adv., 2014, 4, 22374
a
b
c
c
A. Bellifa, A. Choukchou-Braham,* C. Kappenstein and L. Pirault-Roy
MTiX (M ¼ Cr and V) samples, with 0, 5, 10 and 20 atomic percent, were synthesised by a sol–gel method
ꢀ
and calcined at 400 C in air. The XRD analysis results indicated that the materials presented a crystalline
2
structure with the presence of TiO anatase forms. BET analysis showed that the surface area varied with
the variation of metal content. EDX analysis showed a heterogeneous distribution of samples. The results
of pyridine adsorption followed by FT-IR indicated that Brønsted and Lewis acid sites are present on the
surface of catalysts. Liquid phase oxidation of cyclohexane was carried out under milder reaction
conditions over MTiX catalysts using tert-butyl hydroperoxide (TBHP) as oxidant, acetic acid and
acetonitrile as solvents. The results indicated that conversion depends on metal content and solvent
polarity. Chromium is selective for cyclohexanol. The best result was obtained with VTi20 with 35% of
conversion and 92% of olone selectivity.
Received 14th March 2014
Accepted 7th May 2014
DOI: 10.1039/c4ra02241f
www.rsc.org/advances
Creation of monomeric species on the surface of anatase
Introduction
(
TiO ) is important for high performance of the DeNOx reac-
2
5
Sol–gel procedures are among the most important and versatile tion. However, there have been few studies clearly elucidating
methods used for the preparation of inorganic and hybrid the roles of the surface chromate species and the supports in
1
6
organic–inorganic materials. The process has been known for oxidation reactions.
00 years, but in the last 20 years, it has become an attractive
2 5 2
Catalysts based on V O –TiO continue to be a research
1
and intensive research area for the preparation of glass, subject of intense focus, owing to the diversity of catalytic prop-
ceramics, heterogeneous catalysts and several other highly erties, such as those encountered in chemical industry and in air
7–14
homogeneous materials. Recently, the sol–gel technique has pollution control.
Vanadium oxide is deposited on different
offered new approaches for the synthesis of porous materials, as inert surfaces, in order to provide a desirable homogeneous
well as simple and mixed oxides. Each step of the process can be dispersion, mechanical strength as well as thermal stability
well and easily controlled and modied in order to obtain a leading to a continuous catalytic activity, even at elevated
2–4
13,15,16
specic product displaying the best catalytic characteristics.
temperatures.
Extensive studies have been devoted to such
Transition metals are known for their redox properties and catalysts regarding the dispersion, surface structure, oxidation
their capacity to catalyse the oxidation of hydrocarbons. Chro- states and reducibility of supported vanadia species under
mium-based catalysts have been widely examined for polymer- different conditions; these properties have been correlated with
13–16
ization,
hydrogenation–dehydrogenation,
isomerization, the performance in selective oxidation reactions.
In the last two decades, considerable applied as well as
aromatization, partial oxidation, and DeNOx reaction because
of the peculiar characteristics of chromium oxide species on the fundamental research efforts have been made in the activation
surface of the support, including the oxidation state, coordi- and functionalization of hydrocarbons. The oxidation of cyclo-
nation environment, and degree of polymerization. This versa- hexane represents a typical example for this type of reactions
tility of supported chromium oxide catalysts has led to extensive which have become the theme of several researches these last
17–23
fundamental studies on the parameters controlling the molec- years.
The cyclohexanol–cyclohexanone mixture, called
ular structure of chromium oxide on the catalyst surface.
“olone”, is the main product of the cyclohexane oxidation
reaction: 10 tonnes of this mixture are yearly produced, and
6
then further converted to adipic acid leading nally to nylon.
Currently, in industrial oxidation processes, the conversion of
cyclohexane is lower than 3.9%; however the selectivity of
a
Laboratoire de Mat ´e riaux, Applications et Environnement, Facult ´e des Sciences et
Technologie, Universit ´e de Mascara, Algeria
b
Laboratoire de Catalyse et Synth `e se en Chimie Organique, Facult ´e des Sciences,
24
Universite de Tlemcen, Algeria. E-mail: cba@mail.univ-tlemcen.dz
´
cyclohexanol and cyclohexanone is approximately 78%. In
c
IC2MP UMR-CNRS 7285, Institute of Chemistry of Mediums and Materials of Poitiers, industrial processes, the aerobic cyclohexane oxidation is
Faculty of Science, 86022 Poitiers, France
operated at 415–435 K and 1–2 MPa with Mn and Co naph-
thenates as catalysts.
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4ra02241f
1
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In spite of the good catalytic activity obtained when using the
RSC Advances
The samples of MTiX were prepared in the same way as that
homogeneous catalysis, the disadvantages of these processes adopted for TiO , with the addition of an aqueous solution of
2
remain in the difficulty and cost to separate the catalyst from chromium nitrate or vanadium(V) oxytripropoxide before add-
the reactional mixture, and the pollution caused by the rejec- ing the nitric acid solution to get the nal metal contents of 5,
tions. Therefore, the development of effective recyclable solid 10 and 20%. The samples were called MTi5, MTi10 and MTi20
catalysts could offer some advantages. Leaching in the liquid for 5, 10 and 20 of atomic percent, respectively.
phase can be avoided, provided that anhydrous tert-butyl
hydroperoxide be used. However, the use of aqueous solutions
of H (30%) or tert-butyl hydroperoxide (70%) will lead to
considerable metal leaching.
Catalyst characterization
2 2
O
Micromeritics Tristar 3000 instrument was used to measure the
adsorption–desorption isotherms at liquid nitrogen tempera-
ture (77 K) to determine the specic area and calculate the mean
pore size using BJH methods. The samples were outgassed at
19
Several studies have been carried out to optimize powerful
active and selective catalysts for the oxidation reaction of
25–33
cyclohexane.
2 5 2
The modied mixed oxide Sb–V O /TiO has
3
63 K for 1 h then at 623 K for 10 h under a ow of 30% nitrogen
in helium.
The crystalline phases of the samples were identied by X-ray
been used in the oxidative dehydrogenation of propane by
34
Stelzer et al. A conversion of approximately 25% has been
reported for materials containing 10 wt% vanadium. The
conversion decreases when the vanadium amount decreases.
The propene selectivity is about 43%, whereas for COx it is
about 50%. In the present work, we have also studied cyclo-
hexane oxidation with hydrogen peroxide using 20 wt% V O –
powder diffraction (XRD) using Bruker D8 Advance and D5005
diffractometers with Cu Ka radiation (lKa1 ¼ 0.15406 nm). The
diffractograms were obtained under the following conditions:
dwell time: 2 s; step: 0.04 2q; divergence slit: 0.3 . The crys-
talline phases were identied by comparison with Powder
Diffraction Files (PDF) standards from ICDD.
ꢀ
ꢀ
2
5
35
2
TiO catalysts, prepared by sol–gel route. The results showed a
low conversion. The use of acetic acid as solvent and acetone as
initiator presented an approximate 8% conversion into cyclo-
To characterize the physical and chemical alterations of
doped TiO materials, thermo gravimetric analysis (TGA) and
2
hexanol, with 76% selectivity. Recently Pt/oxide (Al
2 3 2
O , TiO and
differential thermal analysis (DTA) were performed on calcined
powders (15 to 30 mg), from room temperature to 1000 C,
using a Thermal Analyst STD 2960 TA Instrument. The samples
were heated at a rate of 10 C min for measurements under
dry air ow.
The acidity of samples was characterized by pyridine
adsorption followed by IR spectroscopy. The IR spectra were
recorded on Nicolet Magna IR spectrometer using a thin wafer
ZrO ) catalysts have been used for the catalytic oxidation of
2
ꢀ
36
cyclohexane by tert-butyl hydroperoxide. The results showed
the catalytic performance of Pt/Al O as being very high in terms
2
3
ꢀ
ꢁ1
of turnover frequency.
In this investigation, MTiX (M ¼ Cr and V) catalyst was
synthesized by sol–gel process. XRD, BET, EDX and IR were
employed to investigate the structural and textural properties of
the catalysts. Then, MTiX was used for the liquid oxidation of
cyclohexane under mild conditions. The catalytic activity was
carried on by varying the metal content of the as well as the
reaction solvent.
ꢁ
2
(16 mm in diameter, 10–15 mg cm ) activated in situ in the IR
ꢁ
3
ꢀ
cell under secondary vacuum (10 Pa) at 200 C for 2 h. Pyri-
dine was adsorbed on the samples at 150 C. The IR spectra
were recorded at room temperature aer activation, and pyri-
ꢀ
ꢁ
3
dine thermo desorption under vacuum (10 Pa) for 1 h at
ꢀ
1
50 C. The total amounts of Lewis and Brønsted acid sites
Experimental
Synthesis of samples
accessible to pyridine were quantied by estimating the
difference between P150 (spectrum of catalyst aer pyridine
adsorption–desorption) and Pref (spectrum of catalyst before
pyridine adsorption).
Starting materials. The following chemicals were employed
for the preparation of MTiX: titanium butoxide Ti(OC
4
3
H
H
9
7
)
)
4
(
(
Aldrich, 97%), vanadium(V) oxytripropoxide OV(OC
3
Catalytic reactions
Aldrich, 98%) and chromium(III) nitrate nanohydrate
$9H O (Aldrich, 99%).
Cr(NO
3
)
3
2
First, TBHP was stirred with cyclohexane in order to perform a
The following chemicals were used for catalytic reactions: phase transfer from water to cyclohexane. In a typical reaction,
tertiobutyl-hydroperoxide TBHP (Aldrich, 70 wt% in water), 64 mmol of cyclohexane and 64 mmol of TBHP as oxidant were
cyclohexane (Prolabo, 98%).
mixed in a closed Erlenmeyer ask and magnetically stirred for
Synthesis procedure. The samples were synthetized by sol– 24 h. The organic phase was then separated from the aqueous
3
7
gel method. The preparation of TiO
1
2
was carried out in a phase.
Cyclohexane oxidation reactions were carried out in a 250
00 mL beaker. Titanium butoxide was dissolved in absolute
ethanol. The mixture was stirred for 30 min and then a solution mL three-necked ask, placed in a temperature equilibrated oil
of ethanol and nitric acid was added dropwise in the mixture. bath and tted with a reux condenser. The solvent (40 mL) was
ꢀ
ꢀ
The white gel obtained was dried in a sand bath at 60 C for 6 h, added to the TBHP–cyclohexane mixture. At 70 C, a quantity of
ꢀ
then in an oven at 120 C for a night. Next, the sample was 30 mg of catalyst (MTiX) was subsequently added to the reaction
calcined in a muffle furnace at 400 C for 4 h, under air mixture (time zero). The products were analyzed by gas chro-
ꢀ
atmosphere.
matography (GC), taking aliquots (0.5 mL) at different reaction
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times. A Varian CP-3800 gas chromatograph equipped with a Table 1 Surface area, porous volume, pore size and acidity of MTiX
a
samples
CP-WAX 52 CB column and a ame ionization detector (FID)
were used. Before GC analysis, the remaining TBHP was
decomposed by introducing an excess of triphenylphosphine.
S
BET
Pore volume Average pore L.A
B.A
(mmol g )
2
ꢁ1
3
ꢁ1
ꢁ1
ꢁ1
Sample (m g
)
(cm g
)
size (nm)
(mmol g
)
TiO
CrTi5
CrTi10 230
CrTi20 152
VTi5
VTi10 129
2
68
180
0.06
0.22
0.19
0.10
0.07
0.11
0.10
3.6
5.0
3.3
2.6
3.5
3.5
4.0
197
239
28
16
177
—
0
0
0
74
0
—
Results and discussion
Characterization
76
The N2 adsorption–desorption isotherms of the different
samples are reported in Fig. 1. The isotherm prole for all
samples corresponds to a type IV with the presence of a
hysteresis loop indicating the existence of mesopores in
VTi20
98
23
0
a
ꢀ
Samples calcined at 400 C; B.A: Brønsted acidity; L.A: Lewis acidity.
the samples. A sharp increase in the adsorption volume of N
was observed in the P/P range of 0.4–0.8 and can be attrib-
2
0
uted to the capillary condensation, indicating a good
homogeneity of the samples. The values of the specic
surface, pore volume and pore size are shown in Table 1. The
specic surface area increases with increasing content of
metal from 5 to 10%. MTi20 displays smaller specic surface
area than MTi10.
IR spectra of pyridine adsorption (Fig. 2 and 3) show the
ꢁ1
presence of Lewis acid sites (1455 cm ) in all samples. MTi5
displays the highest Lewis acid quantity. We note the presence
ꢁ
1
ꢁ1
of Brønsted acid sites (1545 cm and 1637 cm ) only for
CrTi20 (Table 1) (cf. ESI, Fig. S1†). As the metal content
decreases, the number of Lewis acid sites goes down while that
38
of Brønsted acid sites rises.
Fig. 2 IR adsorption spectrum of pyridine for CrTiX; evacuation at
ꢀ
1
50 C.
Fig. 3 IR adsorption spectrum of pyridine for VTiX; evacuation at
ꢀ
1
50 C.
X-ray diffraction (XRD) patterns of the samples calcined at
ꢀ
4
00 C are shown in Fig. 4. The XRD patterns of the samples
show the crystalline structure of pure anatase phase. No corre-
sponding peaks to rutile, brookite, chromium and vanadium
ꢀ
containing phases were observed. The peak at 44 of 2q is
attributed to Kanthal phase.
ꢀ
For the samples calcined at 400 C, the curves of TGA–TDA
Fig. 1 Isotherms for adsorption of nitrogen on MTiX samples.
analysis (cf. ESI, Fig. S2 and S3†) display one endothermic peak
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EDX analysis, focusing on different zones of the sample
CrTi20 (Fig. 5), shows four emission peaks, two for titanium
(around 4500 eV and 5000 eV) and two for chromium (around
5400 eV and 6000 eV). In this work, we present the EDX of one
zone only for the CrTi20 sample. The content ratio between
titanium and chromium in various zones is evaluated from their
relative intensities (Table 2). The results showed that the ratio is
almost the same for the three zones. As a conclusion, EDX
analysis evidenced a homogeneous distribution of titanium and
chromium in the sample of CrTi20.
For VTi20, the EDX analysis shows that the most intense
vanadium emission peak, at about 5000 eV, superimposes a
titanium peak (Fig. 6). Nevertheless, the relative intensity of this
peak, as compared to that of pure titanium emission at 4500 eV,
is not constant. Indeed, for the three zones, the ratio between
vanadium and titanium peak intensities is not constant. This
last result leads to a heterogeneous distribution of titanium and
vanadium in the VTi20 sample.
Catalytic tests
Kinetic studies of the catalytic oxidation of cyclohexane were
performed on MTiX samples. The desired products are cyclo-
hexanol and cyclohexanone, but other products did also
40,41
form.
For this study, we focused on the selectivity towards
olone only (cyclohexanol + cyclohexanone). Two parameters
have been followed during the catalytic oxidation of
Table 2 Percentage atom of Ti and Cr, calculated from their intensi-
Fig. 4 X-ray diffraction (XRD) patterns of MTiX; a: anatase.
ties in CrTi20
Zone
Ti
1
2
3
ꢀ
in the range 0–200 C, corresponding to desorption of water
%
73
27
2.7
77
23
3.3
76
24
3.1
previously adsorbed on the surface, with a weight loss between 5 %Cr
and 7%. In the range of 0–400 C, a mass loss of 3.6%, 9–11% Ti/Cr
ꢀ
and 3–4% was found for TiO
2
, CrTiX and VTiX, respectively. On
ꢀ
the other hand, the total weight loss observed between 400 C
ꢀ
and 800 C was less than 1% for VTiX and 0.6, 0.94 and 1.45%
for CrTi5, CrTi10 and CrTi20, respectively. This last result can
be attributed to the presence of residual hydroxyl groups due to
the increase of metal content which can be removed by a
39
temperature increase. No exothermic peak was observed, and
this characterizes the anatase to rutile transition.
Fig. 5 EDX analysis of zone 3 of the CrTi20 sample.
Fig. 6 EDX analysis of different zones of the VTi20 sample.
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a
Table 3 Cyclohexane oxidation with MTiX
Without
Acetonitrile
Acetic acid
Catalyst
Cy-ol (mmol)
Cy-one (mmol)
Cy-ol (mmol)
Cy-one mmol)
Cy-ol (mmol)
Cy-one (mmol)
CrTi5
CrTi10
CrTi20
VTi5
VTi10
VTi20
4.84
5.80
5.52
8.17
2.95
11.86
0.00
0.00
0.00
4.65
1.80
8.73
2.87
3.55
4.04
3.20
2.37
3.67
0.00
0.00
0.70
3.81
0.87
0.92
5.47
4.91
4.86
7.26
3.70
4.22
0.23
0.00
0.50
4.70
2.84
3.46
a
ꢀ
6
C H
12 (64 mmol), TBHP (64 mmol), T ¼ 70 C, t ¼ 6 h; Cy-ol: cyclohexanol; Cy-one: cyclohexanone.
cyclohexane by TBHP, namely the nature of solvent and the
When using acetic acid as solvent, the olone selectivity
metal content. Cyclohexane was rst used as reagent and increases while the metal content increases, whereas the
solvent, but no conversion was noticed for the control reaction conversion decreases with increasing metal content.
aer the addition of TBHP, in complete absence of the catalyst.
The catalytic activity of CrTiX is not correlated to the specic
In the presence of TiO alone, using acetonitrile as solvent, a surface but is directly related to the amount of chromium
2
conversion of about 10% with a selectivity to cyclohexanol of present. Conversion is in good correlation with the increase in
about 03% were found. No formation of cyclohexane was chromium content, except in the case of acetic acid as solvent.
obtained.
On the other hand, the variation of conversion depends on the
The results of cyclohexane oxidation are shown in Tables 3 reaction medium. Catalytic activity is enhanced in the presence
and 4. Cyclohexanol and traces of cyclohexanone were observed, of a polar solvent. Therefore conversion varies in the following
with CrTiX as catalysts. However, a mixture of cyclohexanol and order: acetonitrile > acetic acid > cyclohexane (without solvent).
cyclohexanone (olone) was formed with VTiX as catalysts. The
In the case of VTiX, conversion remains constant with high
ol/one ratio as larger than 1 for all samples, and therefore the vanadium content and is favored for small quantities. We found
cyclohexanol produced was not totally transformed into cyclo- that the best conversion is obtained with acetic acid as solvent.
hexanone. The olone quantity was higher with VTiX than with Indeed, the conversion varies in the following order: acetic acid
CrTiX.
> cyclohexane $ acetonitrile. In general, VTiX samples are more
The best olone quantity was obtained without solvent. The active than those of CrTiX. The best conversion (83%) is
olone selectivity decreased when the chromium content obtained with VTi5 in presence of acetic acid as solvent, and the
increased from 5 to 20%, whereas an excess of vanadium best olone selectivity (92%) is obtained with VTi20, without
content led to higher olone selectivity. However, the conversion solvent. The best catalyst is VTi20 with 35% of conversion and
increased with increases in chromium content and decreased 92% of olone selectivity.
when vanadium content increased.
The reactivity of the catalysts in liquid phase oxidation
In the presence of acetonitrile, it is clear that the olone reactions depends on the polarity, the protic/aprotic character
selectivity remains constant for both CrTiX and VTiX. On the of the solvent and the hydrophobic–hydrophilic nature of the
other hand, the conversion increased as chromium content catalyst associated with the nature of the oxidant. This is such a
increased, whereas it decreased with increasing vanadium complex mixture that it is difficult to give a reliable explanation.
content. In our previous study, we found a low conversion when We note that if acetic acid is used as solvent, per acid is formed;
3
5
using VTi20 with H
2
O
2
as oxidant.
it enables a better reactivity, as already obtained for the
conversion values of MTi5. We can also explain our results by a
competition between the adsorption of reactants and desorp-
tion of products. In the case of VTi20, we have good “olone”
selectivity in the free solvent because the products formed are
easily desorbed.
a
Table 4 Variation of conversion and olone selectivity
Without
Acetonitrile
Acetic acid
Conv.
(%)
Olone
(%)
Conv.
(%)
Olone
(%)
Conv.
(%)
Olone
(%)
Catalyst
Conclusions
CrTi5
CrTi10
CrTi20
VTi5
VTi10
VTi20
20.1
42.9
44.3
62.8
35.8
34.9
37.8
21.1
19.5
31.9
20.8
92.1
41.3
45.4
64.3
46.7
40.3
37.5
11.6
13.1
12.3
25.1
13.4
20.5
51.2
46.2
21.6
83.1
38.9
38.8
17.4
16.6
38.8
22.5
26.3
31.1
The catalyst MTiX (M ¼ Cr and V; X ¼ 0, 5, 10 and 20%) was
ꢀ
prepared by the sol–gel method and calcined at 400 C. XRD
analysis showed that materials present a crystalline structure
with the presence of TiO anatase phase. The specic surface of
2
samples increased with the metal content but decreased for an
excessive content. The EDX analysis evidenced a heterogeneous
distribution of titanium and vanadium in the VTi20 sample.
a
12 (64 mmol), TBHP (64 mmol), T ¼ 70 ꢀC, t ¼ 6 h; conv.:
6
C H
conversion.
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However, a homogeneous distribution of titanium and chro- 18 M. P. de Almeida, L. Martins, S. Carabineiro, T. Lauterbach,
mium has been reported in the CrTi20 sample.
F. Rominger, A. Hashmi, J. Pombeiro and J. Figueiredo,
Regarding cyclohexane oxidation, the results showed that all
Catal. Sci. Technol., 2013, 3, 3056.
catalysts were active, with and without solvent. The presence of 19 U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R. S. Cruz,
chromium leads to the formation of cyclohexanol only, whereas
M. C. Guerreiro, D. Mandelli, E. V. Spinac ´e and E. L. Pires,
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Appl. Catal., A, 2001, 211, 1.
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