Characterization and catalytic properties of molybdenum oxide catalysts supported on ZrO2-γ-Al2O3 for ammoxidation of toluene

Article abstract of DOI:10.1039/c4ra07435a

Molybdenum oxide catalysts with MoO₃ loadings ranging from 6.6 to 25 wt% supported on ZrO₂-γ-Al₂O₃ (1:1 wt%) mixed oxide were prepared by a wet impregnation method. The catalytic behavior of catalysts in the toluene ammoxidation reaction was investigated in a lab-scale tube reactor at 400 °C. The catalytic performance of MoO ₃/ZrO₂-γ-Al₂O₃ was dependent on the catalyst compositions and reaction temperature. MoO₃ (20.0 wt%) ZrO₂-γ-Al₂O₃ exhibited a good toluene oxidation; over this catalyst, the selectivity to benzonitrile reached 67.0% with a toluene conversion of 68.5% at 400 °C, while the selectivity to benzaldhyde was 24.4% with a toluene conversion of 68.5% at 400 °C. The catalysts were characterized by various techniques, such as N₂ sorption, FTIR, SEM and XRD. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra07435a

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Characterization and catalytic properties of
molybdenum oxide catalysts supported on ZrO2–
Cite this: RSC Adv., 2014, 4, 37679
g-Al2O3 for ammoxidation of toluene
a
Abbas Teimouri, Bahareh Najari,a Alireza Najafi Chermahini,b Hossein Salavatia
*
and Mahmoud Fazel-Najafabadic
Molybdenum oxide catalysts with MoO3 loadings ranging from 6.6 to 25 wt% supported on ZrO2–g-Al2O3
(1 : 1 wt%) mixed oxide were prepared by a wet impregnation method. The catalytic behavior of catalysts in
the toluene ammoxidation reaction was investigated in a lab-scale tube reactor at 400 ꢀC. The catalytic
performance of MoO3/ZrO2–g-Al2O3 was dependent on the catalyst compositions and reaction
temperature. MoO3 (20.0 wt%) ZrO2–g-Al2O3 exhibited a good toluene oxidation; over this catalyst, the
selectivity to benzonitrile reached 67.0% with a toluene conversion of 68.5% at 400 ꢀC, while the
selectivity to benzaldhyde was 24.4% with a toluene conversion of 68.5% at 400 ꢀC. The catalysts were
characterized by various techniques, such as N2 sorption, FTIR, SEM and XRD.
Received 22nd July 2014
Accepted 1st August 2014
DOI: 10.1039/c4ra07435a
www.rsc.org/advances
materials have been employed as catalysts in various catalytic
applications.28,29 The advantages of Al2O3–ZrO2 as a catalyst
Introduction
support include moderate surface area, higher thermal stability
and medium acidity. The ammoxidation of toluene25,31–53 and
other alkyl aromatics30,52–63 over various supported metal oxide
catalysts has been extensively studied. Iron,64–66 MoO3/MgF2,67
MoO3/ZrO2,68,69 V2O5/ZrO2–g-Al2O3,70,71 V2O5/g-Al2O3,72 Mo–V–P/
g-Al2O3,73 vanadium-containing catalysts,74,75 Fe2O3-based cata-
lysts,76 ZrO2–g-Al2O3,77 and SiO2-supported molybdate catalyst,78
have also been used for the preparation of aromatic nitriles.
In the present study, we report the synthesis of benzonitrile
by the vapor phase ammoxidation over highly dispersed
molybdena catalysts supported on ZrO2–g-Al2O3 mixed oxide, as
shown in Scheme 1. The catalysts were characterized by X-ray
diffraction (XRD), SEM, BET specic surface area and temper-
ature programmed desorption of N2.
Ammoxidation of alkyl aromatics such as toluene to their cor-
responding nitriles has been the subject of numerous studies in
recent times, because the nitriles are very useful organic inter-
mediates to prepare a good number of industrially important
chemicals.1,2 The ammoxidation reaction generally refers to the
one-step formation of nitrile compounds in a single step by the
oxidation of simple olens, aromatics and heteroaromatics in
the presence of oxygen and ammonia in the gas phase.3–5 Sup-
ported molybdenum oxide catalysts are well known and widely
investigated as they represent an important group of catalysts
for the heterogeneous oxidation and ammoxidation of
hydrocarbons.6–17
Because pure MoO3 is relatively volatile, molybdena is
almost always used in the presence of a second oxide, on an
oxide support such as Al2O3, TiO2, ZrO2, SiO2 and
MgO.6,11,12,14,18–23
The desirable inherent properties of alumina and zirconia
supports can be explored by combination of both supports in a
mixed oxide. The ZrO2–g-Al2O3 supported catalysts have been
found to show better catalytic properties than catalysts sup-
ported on pure oxides.24,25 The combination of Al2O3 and ZrO2
provides greater mechanical strength, resulting in improved
resistance to attrition.26,27 In recent times, ZrO2–g-Al2O3 based
Experimental section
Materials and instruments
Toluene and other agents were purchased from Merck and
Aldrich and used without further purication. Products were
characterized by spectroscopy data (FTIR, 1H NMR and 13C
NMR spectra). NMR spectra were recorded on a Bruker 400 Ultra
aChemistry Department, Payame Noor University, 81395-671, 19395-3697, Tehran,
Isfahan, I. R. of Iran. E-mail: a_teimouri@pnu.ac.ir; a_teimoory@yahoo.com; Fax:
+98 31 33521802; Tel: +98 31 33521804
bDepartment of Chemistry, Isfahan University of Technology, Isfahan, 841543111, Iran
cMechanical Engineering Department, Payame Noor University, 19395-3697, Tehran,
I. R. of Iran
Scheme 1 Catalytic ammoxidation of toluene to benzonitrile.
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shield NMR and DMSO-d6 was used as the solvent. Mass
Spectra were recorded on a Shimadzu Gas Chromatograph Mass
Spectrometer GCMS-QP5050A/Q P5000 apparatus.
Results and discussion
XRD analysis
The samples were analyzed by X-ray diffraction (XRD) using
Philips X'PERT MPD X-ray diffractometer (XRD) with Cu Kaꢀ
X-ray diffraction (XRD) patterns of the catalysts were obtained
˚
using Cu Ka radiation (l ¼ 1.5405 A). Crystallite size of the
ꢀ
˚
(1.5405 A). Date sets were collected over the range of 5–90 with
a step size of 0.02ꢀ and a count rate of 3.0ꢀ minꢁ1. The structural
morphology of the samples was evaluated using scanning
electron microscope (SEM, JEOL, JSM-6300, Tokyo, Japan). A
JASCO FT/IR-680 PLUS spectrometer was applied to record IR
spectra using KBr pellets. The BET specic surface areas and
BJH pore size distribution of the samples were determined by
adsorption–desorption of nitrogen at liquid nitrogen tempera-
ture using a Series BEL SORP 18.
crystalline phase was determined from the peak of maximum
intensity by using Scherrer formula,79 with a shape factor (K) of
0.9, which can be described as: crystallite size ¼ Kl/W cos q,
where W ¼ Wb ꢁ Ws and Wb is the broadened prole width of
experimental sample and Ws is the standard prole width of
reference silicon sample.
Fig. 1A shows the XRD patterns of MoO3, MoO3/ZrO2 and
MoO3/g-Al2O3 samples. The peaks presented at 2q ¼ 20–30ꢀ are
attributed to the pure MoO3, Fig. 1A(a). The XRD pattern of
MoO3/ZrO2 showed peaks at 2q ¼ 30, 50 and 60, which were
obviously the characteristics of the tetragonal ZrO2. The X-ray
diffraction pattern of MoO3/g-Al2O3 exhibit broad peaks at 2q ¼
45 and 66ꢀ, which were attributed to g-Al2O3, Fig. 1A(c).
X-ray diffraction patterns of the catalysts with different
loadings of molybdena catalysts supported on Al2O3–ZrO2 and
Catalyst preparation
The MoO3/ZrO2 and MoO3/g-Al2O3 catalysts were prepared by
impregnation of g-Al2O3 or ZrO2 with a 2 M oxalic solution of
ammonium heptamolybdate. The mixture was le in an open
vessel with stirring at 60 ꢀC for 24 h to evaporate the excess
water. The precursor was dried at 100 ꢀC for 12 h and calcined at
ꢀ
calcined at 450 C are shown in Fig. 1B. The catalysts showed
characteristic peaks at 2q ¼ 32.8, 37.3, 45.9, 62.3 and 66.09ꢀ that
were related to the support g-alumina. The sharp diffraction
lines at 2q ¼ 30.4, 51.0 and 60.2ꢀ corresponded to the tetragonal
ZrO2 phase. Loading of molybdenum species led to the
appearance of new peaks at 2q ¼ 12.8, 23.9, 22.2, 25.7, 28.5, 34.2
and 39.4ꢀ.
ꢀ
500 C for 6 h before use.
A series of MoO3/ZrO2/g-Al2O3 catalysts with MoO3 loadings
in the range of 6.6–25.0 wt% were prepared by wet impregnation
method. To impregnate MoO3, the calculated amount of
ammonium heptamolybdate was dissolved in 30–40 ml doubly
distilled water and reux at 85–90 ꢀC for 5 h. Then, a few drops
of dilute NH4OH were added to make the solution clear and
keep the pH constant (pH ¼ 8). Aer impregnation, the reaction
mixture was added to a 50 ml Pyrex ask. The mixture was
irradiated in the water bath of the ultrasonic at 20 kHz for 1 h
within the temperature range of 25–30 ꢀC. Then the catalysts
were dried at 85–90 ꢀC for about 4 h and calcined at 500 ꢀC for
6 h before use.
FT-IR analysis
The FTIR spectrum for MoO3 is presented for the range, 350–
4000 cmꢁ1 in Fig. 2A(a). The bands at 991, 870, and 491 cmꢁ1
,
were assigned to the Mo]O stretching mode, the Mo–O–Mo
Ammoxidation of toluene
A stainless steel cylindrical micro reactor (i.d. 4.8 cm; a reactor
length of 8.55 cm; volume 150 cm3), was charged with toluene
(3 ml), 20 mg catalyst and a magnetic stirring bar. The autoclave
was purged and lled with NH3 until the pressure reached 0.75
MPa. Then O2 was introduced until the total pressure reached to
1.25 MPa. The reaction mixture was stirred at a controlled
temperature (400 ꢀC for 2 h). Aer the reaction, the mixture was
ltered. The ltrate was analyzed by GC-MS and GC using
benzonitrile as an internal standard. For recycling tests, the
catalyst was ltered aer the reaction, washed with acetone
three times and then with doubly distilled water several times.
Then, it was dried at 110 ꢀC, calcined at 400 ꢀC for 4 h, and then
used for the next run.
Conversion and selectivity were dened as follows
ꢂ C (mol%) ¼ (mol toluene reacted/mol toluene in the feed) ꢃ 100
ꢂ Si (mol%) ¼ (mol i formed/mol toluene reacted) ꢃ 100
where i ¼ B, BA, BN.
Fig. 1 (A) XRD patterns of (a) MoO3; (b) MoO3/ZrO2; (c) MoO3/g-
Al2O3. (B) XRD patterns of MoO3/ZrO2–g-Al2O3 catalysts with different
MoO3 loadings: (a) 6.6 wt%; (b) 12.5 wt%; (c) 20.0 wt%; (d) 25.0 wt%.
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Fig. 2 (A) FTIR spectra of the (a) MoO3; (b) MoO3/ZrO2; (c) MoO3/g-Al2O3. (B) FTIR spectra of the MoO3/ZrO2–g-Al2O3 catalysts with different
MoO3 loadings: (a) 6.6 wt%; (b) 12.5 wt%; (c) 20.0 wt%; (d) 25.0 wt%.
stretching mode, and MoO3 vibration mode, respectively. For OH bands of alumina hydroxyls. The band at 3764 cmꢁ1 was
MoO3/ZrO2 catalyst in Fig. 2A(b), FTIR band at 989 cmꢁ1 was assigned to basic hydroxyl groups bound to a single tetra-
due to the Mo]O stretching mode of the molybdenum oxide hedrally coordinated aluminum atom, while the band at 3642
complex bonded to the ZrO2 surface. Molybdenum oxides cmꢁ1 was due to bridged OH groups shared by an octahe-
species was stabilized through multiple Mo–O–Zr bonds drally and tetrahedrally coordinated aluminum cation. The
between each molybdenum oxide species and the zirconia appearance of the band around 1050 cmꢁ1 was typical for g-
surface. The FTIR spectrum for MoO3/g-Al2O3 catalyst is shown alumina due to Al–O vibration mode. On the other hand,
in Fig. 2A(c). The band at 899 cmꢁ1 characterized the stretching bands that appeared at 1632 and 2350 cmꢁ1 were related to
mode of the Mo]O bond in surface-bound Mo species. These physisorbed water and OH group free from the interaction of
species could be either isolated tetrahedral or octahedral poly- H bonding respectively. There was a strong absorption band
molybdate species.
at 417 cmꢁ1 which could be attributed to the tetragonal
FT-IR spectra of the supports and molybdenum catalysts zirconia. At higher MoO3 loading, the bands due to micro-
are shown in Fig. 2B. A broad band in the range of 350–4000 crystallites MoO3 appeared at 518, 734, and 878 cmꢁ1. The
cmꢁ1 appeared for all catalysts related to the MoO3 species increase in the intensity of this band with MoO3 loading
(including the vibrations of Mo–O, bridging oxygen corre- indicated the growth of polymolybdate species. These bands
sponding to Mo–O–Mo). The bands at 878, 734 and 497 cmꢁ1 were associated with Mo–O–Al and Mo]O bond vibration in
corresponding to the polymolybdates species. The spectrum aluminum molybdate and crystalline MoO3 phases,
in Fig. 2B exhibited bands at 3450–3760 cmꢁ1, typical of the n respectively.
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availability of active MoO3 compound. With the increase of the
reaction temperature, the toluene conversion and the selectivity
to benzonitrile and benzaldehyde were increased, while the
selectivity to benzene was decreased. Over this catalyst, the
selectivity to benzonitrile reached to 67.0% with the toluene
conversion of 68.5% at 400 ꢀC, while the selectivity to benzo-
nitrile was 58.6% with the toluene conversion of 64.6% at
300 ꢀC. A signicant drop in surface area occurred when
molybdena loading was increased from 6.6 to 20.0 wt%. Such a
decrease might be due to either the blockage of some pores of
ZrO2/g-Al2O3 by mixed oxides formed from the decomposition
of molybdate or the solid-state reaction between the supporting
oxides and the dispersed active oxides.80–82 Table 2 shows the
BET surface area values of the catalysts. The surface area and
pore volume of the MoO3/ZrO2–g-Al catalysts were in the range
of 48–116 m2 gꢁ1 and 0.50–0.62 cm3 gꢁ1, respectively. A gradual
decrease in surface area was observed for the catalysts with an
increase in the loading of molybdena supported on ZrO2/g-
Al2O3, but an increase in the average pore diameters. This
phenomenon might be due to two reasons. One refers to MoO3
particles deposited in the pores of ZrO2–g-Al2O3 and the
blocked part of the small pores.
In addition, the Mo surface density values are measured and
data are presented in Table 2. The surface density is dened as
the number of Mo atoms per square nanometer BET surface
area (Mo atoms nmꢁ2). Surface areas decreased only slightly
with increasing MoO3 loading; therefore, the Mo surface density
increased almost linearly with increasing MoO3 concentration.
In addition it has been noted that for Mo/Al2O3 system,
formation of a polymolybdate monolayer on Al2O3 at surface
densities of 4.8 Mo nmꢁ2 occur.83 As you can see, the loading
6.6% MoO3 leads to formation of a molybdate monolayer on
mixed oxide surface.
SEM analysis
Fig. 3 shows SEM micrographs of catalysts obtained from MoO3
loadings ranging from 6.6 to 25.0 wt%. Alumina displayed an
irregular texture and accumulated aggregates with a variety of
particles size. This indicated that the introduction of ZrO2 into
Al2O3 largely changed the morphology of the support compos-
ites. It can be seen from Fig. 3 that the particles seemed to
aggregate to form microspheres. SEM shows a regular texture
with small, uniform and dispersed particles.
BET analysis
Fig. 4 shows the N2 adsorption–desorption isotherms. Surface
area was calculated by applying the BET equation to the
isotherm.27 The samples were degassed under vacuum at 120 ꢀC
for 4 h, prior to adsorption measurement, to evacuate the
physisorbed moisture.
The effects of catalyst composition and reaction temperature
on the toluene conversion and product distribution for toluene
oxidation over MoO3/ZrO2–g-Al2O3 are illustrated in Table 1. As
shown, the toluene conversion reached a maximum over the
catalyst with the MoO3 loading of 20.0 wt% under each reaction
temperature, while the selectivity to the main products uctu-
ated with the increase of MoO3 loading.
The MoO3/ZrO2/g-Al2O3 catalysts with different MoO3
contents were evaluated for the ammoxidation of toluene. The
ammoxidation of toluene resulted in the formation of benzo-
nitrile as the major product, while benzene and benzaldehyde
were formed in very low amounts. The catalysts with the low
loading of MoO3, up to 6.6 wt%, showed moderate activity and
when the loading was increased to 20.0 wt%, a substantial
increase in activity was observed. The catalyst with 20.0 wt%
MoO3 exhibited the highest activity. With further increase in the
active content to 25 wt%, the ammoxidation activity was
decreased marginally. The low catalytic activity of 6.6–20.0 wt%
MoO3/ZrO2/g-Al2O3 catalysts might be because of the less
The other relates to the morphology of composite supports
changed from big blocks into small particles (observed from
SEM image), thereby forming more inter pores between the
particles. All samples were mesoporous, with N2 adsorption–
desorption isotherms of type IV according to the IUPAC classi-
cation. Such isotherms are shown in Fig. 4, which shows the
case of a bare support taken as a representative example for 20.0
wt% MoO3/ZrO2–g-Al2O3 catalyst.
One of the most important advantages of heterogeneous
catalysis over the homogeneous counterpart is the possibility of
reusing the catalyst by simple ltration, without loss of activity.
The recovery and reusability of the catalyst were investigated in
the product formation. Aer completion of the reaction, the
catalyst was separated by ltration, washed rst 3 times with 5 ml
acetone and then with doubly distilled water several times, dried
at 110 ꢀC and calcined at 400 ꢀC for 4 h. Then the recovered
catalyst was used in the next run. The results of three consecutive
runs showed that the catalyst could be reused several times
without any signicant loss of its activity (see Fig. 5).
It is generally accepted that the reaction proceeds by the
adsorption of toluene on the catalyst surface through the
formation of a p-complex with a Lewis site of the catalyst;
furthermore, we should consider H abstraction of a benzylic H-
atom to form a methylene-like species with parallel formation
Fig. 3 SEM images of the molybdate supported catalysts (a) 6.6%
wtMoO3/ZrO2–g-Al2O3, (b) 12.5 wt% MoO3/ZrO2–g-Al2O3, (c) 20.0
wt% MoO3/ZrO2–g-Al2O3, (d) 25.0 wt% MoO3/ZrO2–g-Al2O3.
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Fig. 4 N2 adsorption–desorption isotherm of the MoO3 (20.0 wt%)/ZrO2–g-Al2O3.
Table 1 Effect of MoO3 content in the support on the catalyst activity and product selectivity of different MoO3/ZrO2–g-Al2O3 catalysts for
toluene ammoxidation
Product selectivity (%)
MoO3 loading
6.6%
T (ꢀC)
Toluene conversion (%)
Benzonitrile
Benzene
Benzaldehyde
200
300
400
200
300
400
200
300
400
200
300
400
28.6
44.5
44.2
32.5
48.4
56.6
44.2
64.6
68.5
30.3
43.7
48.6
40.7
46.2
63.5
46.3
50.4
66.4
40.3
58.6
68
#1
1.3
2
#1
1.3
2
1.3
2
#1
#1
1.3
2
20.5
23.7
24.4
20.5
23.7
24.4
23.7
24.4
24.4
20.5
23.7
24.2
12.5%
20.0%
25.0%
35.9
47.5
67.6
of water, partial oxidation, N-insertion and subsequent rear- the so formed benzonitrile, which was followed by oxidative
rangements of the chemisorbed activated surface species, reconstruction of the catalyst surface (Fig. 6). A similar mech-
which was converted to an adsorbed imine, and desorption of anism has been proposed for this reaction.84
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Table 2 Nitrogen adsorption characteristics of molybdate supported catalysts
BET surface
area (m2g-1)
Surface density
Pore volume
Average pore
diameter (nm)
Catalyst
Mo/nm2
(cm3 gꢁ1
)
6.6% MoO3/ZrO2–g-Al2O3
12.5% MoO3/ZrO2–g-Al2O3
20.0%MoO3/ZrO2–g-Al2O3
25.0% MoO3/ZrO2–g-Al2O3
116.26
78.13
66.32
48.46
2.37
6.69
12.61
21.57
0.78
0.74
0.68
0.64
6.22
6.23
6.25
6.34
to the increase in the number of sites on the active molybdena
phase, which could be increased with the increase in molyb-
dena content on the surface of the support. The surface prop-
erties and catalytic activity results of 20.0 wt% MoO3 supported
onmixed oxide alumina–zirconia catalysts have been compared
in Table 2. It clearly shows that molybdena was well dispersed
on MoO3/ZrO2–g-Al2O3 support, with more acidic sites per m2
surface of the support.
Thus, it can be inferred that 20.0 wt% MoO3/ZrO2–g-Al2O3
catalyst can be more active in ammoxidation reaction
compared to a time when it is supported on alumina–zirconia
catalysts.
Fig.
formation.
5 The results obtained from catalyst reuse in the product
This was also reected in the catalytic activity of these cata-
lysts. Conversion of toluene to benzonitrile was increased
continuously with molybdena loading up to 20.0 wt%. It indi-
cated that the moderate and weak acidic sites played an
important role in the ammoxidation of toluene.
The results of ammoxidation of toluene on various MoO3/
ZrO2–g-Al2O3 catalysts at 400 ꢀC are plotted in Fig. 7. The
conversion and selectivity were increased with an increase in
MoO3 loading up to 20.0 wt% and beyond this loading, the
activity was decreased slightly due to the formation of MoO3-
crystallites on the surface of ZrO2–g-Al2O3 support. The increase
in the ammoxidation activity of the catalysts might be attributed
Fig. 7 Ammoxidation of toluene over various MoO3/ZrO2–Al2O3
catalysts (reaction temperature of 400 ꢀC).
Fig. 6 A plausible mechanism for toluene ammoxidation over MoO3/ZrO2–g-Al2O3 catalysts.
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(1C), 129.16 (2C), 132.05 (2C), 132.82 (1C); ESI MS (m/z):
103.08 (M+).
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The catalyst MoO3/ZrO2–g-Al2O3 exhibited excellent catalytic
performance in toluene ammoxidation with benzonitrile as the
main product. The Al2O3–ZrO2 binary oxide was found to be an
interesting support to investigate the dispersion of molyb-
denum oxide and catalytic properties. The catalytic perfor-
mance of MoO3/ZrO2–g-Al2O3 was dependent on the catalyst
compositions and reaction temperature. Increasing the MoO3
loading from 6.6 to 25.0 wt% enhanced the activity of the
catalyst. Above 20.0 wt%, however, it led to inactivity and
performance failure of the catalyst. Over this catalyst, the
selectivity to benzonitrile reached 67.0% with the toluene
conversion of 68.5% at 400 ꢀC, while the selectivity to benzo-
nitriꢀle was 58.6% with the toluene conversion of 64.6% at
300 C.
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Conflict of interest
The authors declare no competing nancial interest.
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Acknowledgements
The authors would like to thank the Iranian National Science
Foundation (INSF) for nancial support of this work. Supports
from Isfahan research council at Payame Noor University and
the help from Isfahan University of technology are gratefully
acknowledged.
26 A. Sahibed-Dine, B. Bouanis, K. Nohair and M. Bensitel,
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