Preparation and characterization of V-Ag-O catalysts for the selective oxidation of toluene

Article abstract of DOI:10.1016/j.apcata.2010.02.023

V-Ag-O complex oxide catalysts with relatively high surface areas of 13-21 m²/g could be prepared by the heterogeneous azeotropic distillation (HAD) method. Specifically, V₂O₅ and AgNO₃ were dissolved in aqueous solution of H₂O₂, followed by evaporation and drying in n-butanol at 353 K. Silver vanadates with highly dispersed nano silver particles in the layered structures of VO_x might be formed during the preparation process, which were then turned into Ag_0.68V₂O₅ and metallic silver during the reaction of selective oxidation of toluene at 573 K. Characterizations with microcalorimetric adsorption of NH₃, temperature programmed reduction and isopropanol probe reactions showed that the V-Ag-O catalysts exhibited weaker surface acidity but stronger redox ability than the VO_x, and therefore the better performance for the selective oxidation of toluene to benzaldehyde and benzoic acid. In addition, the V-Ag-O catalysts prepared by the HAD method exhibited much higher activity than its counterpart prepared by the co-precipitation for the conversion of isopropanol and the selective oxidation of toluene to benzaldehyde and benzoic acid in air.

Full text of DOI:10.1016/j.apcata.2010.02.023

Applied Catalysis A: General 379 (2010) 7–14
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Preparation and characterization of V-Ag-O catalysts for the selective
oxidation of toluene
a
a
a
b
a,∗
Mingwei Xue , Hui Chen , Huiliang Zhang , Aline Auroux , Jianyi Shen
a
Lab of Mesoscopic Chemistry, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, China
Institut de Recherches sur la Catalyse et l’Environnement de Lyon, UMR 5256, CNRS-Université Lyon 1, 2 Avenue Albert Einstein, 69626 Villeurbanne Cedex, France
b
a r t i c l e i n f o
a b s t r a c t
2
Article history:
V-Ag-O complex oxide catalysts with relatively high surface areas of 13–21 m /g could be prepared by
the heterogeneous azeotropic distillation (HAD) method. Specifically, V2O5 and AgNO3 were dissolved
in aqueous solution of H2O2, followed by evaporation and drying in n-butanol at 353 K. Silver vanadates
with highly dispersed nano silver particles in the layered structures of VOx might be formed during
the preparation process, which were then turned into Ag0.68V2O5 and metallic silver during the reac-
tion of selective oxidation of toluene at 573 K. Characterizations with microcalorimetric adsorption of
NH3, temperature programmed reduction and isopropanol probe reactions showed that the V-Ag-O cat-
alysts exhibited weaker surface acidity but stronger redox ability than the VOx, and therefore the better
performance for the selective oxidation of toluene to benzaldehyde and benzoic acid. In addition, the
V-Ag-O catalysts prepared by the HAD method exhibited much higher activity than its counterpart pre-
pared by the co-precipitation for the conversion of isopropanol and the selective oxidation of toluene to
benzaldehyde and benzoic acid in air.
Received 27 November 2009
Received in revised form 18 January 2010
Accepted 12 February 2010
Available online 26 February 2010
Keywords:
Heterogeneous azeotropic distillation
drying
Silver vanadates
Surface acidity
Surface redox property
Selective oxidation of toluene
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
zaldehyde and benzoic acid were low over the V O5. Supports and
2
promoters, such as SiO , TiO , K and Ag have been used to modify
2
2
Caprolactam is a raw material for the synthesis of nylon 66, an
the surface properties of vanadia catalysts, in order to improve the
catalytic behavior for the selective oxidation of toluene [9,10,15].
important engineering plastic. In the SNIA process [1–3], toluene is
a starting material for the synthesis of caprolactam. The first step
of the process is the oxidation of toluene to benzoic acid in liq-
uid phase through the free radical mechanism. Such free radical
reaction converts toluene mainly into benzoic acid. Only trace of
benzaldehyde is formed during the process. However, benzalde-
hyde is more costly than benzoic acid since it is an important raw
material for the synthesis of many other valuable chemicals such
as flavors, medicines and pesticides [4]. Thus, the catalytic oxida-
tion of toluene to benzaldehyde and benzoic acid in gas phase to
increase the selectivity to benzaldehyde has received considerable
attention [5–19]. However, there is no commercial plant available
currently for the production of benzaldehyde and benzoic acid from
the oxidation of toluene by air in gas phase due to the low activity
and/or selectivity.
Our earlier work [20] showed that the addition of Ag and Ni to V O5
2
greatly decreased the surface acidity and increased redox ability of
the catalysts leading to the increased conversion of toluene or the
selectivity to benzaldehyde.
Much work has been done to prepare the V-Ag-O complex
oxides for lithium batteries and catalysts. The V-Ag-O complex
oxides were usually prepared by co-precipitation and solid state
synthesis at high temperatures with low surface areas (about
2
1 m /g) [11,21]. Generally speaking, the increase of surface area
increased the number of active sites of a catalyst, leading to the
increased catalytic activity.
In our previous work, high surface area mesoporous vanadium
oxides (VOx) were prepared by the heterogeneous azeotropic distil-
lation (HAD) method using n-butanol to decrease the capillary force
during drying. The mesoporous VOx thus prepared might possess
During the past decades, studies have been continuing for the
selective oxidation of toluene to benzaldehyde and benzoic acid.
2
surface area of about 180 m /g and exhibited much higher activity
Pure vanadium oxide (V O5) was tested for the partial oxidation of
than traditional V O5 for the selective oxidation of toluene to ben-
2
2
toluene, but the conversion of toluene and the selectivity to ben-
zaldehyde and benzoic acid [22]. In this work, the same method
was used to prepare the V-Ag-O complex oxides with surface areas
much higher than 1 m /g that were tested for the selective oxida-
2
tion of toluene. The morphologies and structures of these V-Ag-O
catalysts were characterized by X-ray diffraction (XRD), Fourier
transform infrared spectroscopy (FTIR) and transmission electron
∗ Corresponding author. Tel.: +86 25 83594305; fax: +86 25 83594305.
E-mail addresses: aline.auroux@ircelyon.univ-lyon1.fr (A. Auroux),
jyshen@nju.edu.cn (J. Shen).
0
926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.02.023

8
M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
microscopy (TEM). The surface acidic and redox properties of the
catalysts were determined by the techniques of microcalorimet-
ric adsorption of ammonia, temperature programmed reduction
The reaction of selective oxidation of toluene was performed by
using a U-tube fixed-bed microreactor (Ø 12) loaded with a sample
of about 0.5 g with 20–40 meshes. The reactants were fed into the
reactor by bubbling air through a glass saturator filled with toluene
maintained at 330 K. The reaction was performed at 573–613 K with
a total space velocity of 8.9–14.4 L/(g h) and air/toluene = 5 (v/v).
Products were analyzed after 1 h on-stream when the activity was
stabilized, by an online gas chromatograph. The organic compounds
were separated by an FFAP capillary column and detected by an FID
while COx was detected by using a Hayesep D packed column and a
TCD. The reaction was performed at different temperatures with 4 h
on-stream at a given temperature. The reaction temperature was
increased step-wise and then decreased to the first one in order
to see the effect of temperature and the stability of a catalyst. The
carbon balance was calculated to be between 95-103% according to
the amount of toluene introduced into the reactor and the amount
of toluene, benzaldehyde, benzoic acid and COx detected in the tail
gas.
(
TPR) and isopropanol probe reactions, and were correlated with
the catalytic behavior for the selective oxidation of toluene to ben-
zaldehyde and benzoic acid. A co-precipitated V-Ag-O (V/Ag = 2)
catalyst was also prepared without the use of n-butanol for the
comparison.
2
. Experimental
2.1. Preparation of catalysts
Desired amounts of V O5 and AgNO3 were dissolved in 300 ml
2
aqueous solution containing 10% H O . The resultant solution was
2
2
evaporated at 353 K for 15 h, and a brown precipitate was formed.
Then, 100 ml n-butanol was added and evaporated again at 353 K
for 20 h. A brown powder was obtained and dried at 393 K over
night (about 12 h). The samples were pelletized, crushed and sieved
to 20–40 meshes for further uses.
3. Results and discussion
2.2. Characterization of catalysts
3.1. Textural and structural properties
The surface areas were measured by N2 adsorption at the tem-
Table 1 presents the surface areas of the V-Ag-O complex oxides
perature of liquid N2 employing the BET method. The phases
present in the catalysts were determined by XRD using the X’TRA
diffractometer with a Cu K␣ radiation source (ꢀ = 0.15418 nm) and
a graphite monochromator. The applied voltage and current were
after evacuation at 673 K and reaction for the selective oxidation of
toluene at 573 K, respectively. The V-Ag-O oxides prepared by the
heterogeneous azeotropic distillation (HAD) method with different
2
V/Ag ratios possessed surface areas of 13–21 m /g after the evac-
4
0 kV and 40 mA, respectively. The FTIR spectra were recorded with
uation at 673 K or the reaction at 573 K, while the one prepared
a Brucker Vector 22 FTIR spectrophotometer. The samples were
mixed with KBr and pressed into self-supporting wafers for the
FTIR measurements. TEM measurements were carried out using a
JEOL electron microscope (JEM-2010), with an accelerating voltage
of 200 keV.
by the simple co-precipitation method had the surface area of only
2
∼
1 m /g. Thus, the HAD method greatly increased the surface areas
of V-Ag-O complex oxides, which did not seem to decrease during
the reaction of selective oxidation of toluene at 573 K.
Fig. 1 shows the XRD patterns for the V-Ag-O samples dried
at 393 K. The XRD pattern of VOx exhibited a series of diffrac-
TPR was performed by using a quartz U-tube reactor loaded with
about 50 mg of a sample. A mixture of N2 and H2 (5.13% H2 by vol-
ume) was used and the flow rate was maintained at 40 ml/min. The
hydrogen consumption was monitored using a thermal conductiv-
ity detector (TCD). The reducing gas was first passed through the
reference arm of the TCD before entering the reactor. The reactor
exit was directed through a trap filled with Mg(ClO4)2 (to remove
water) and then to the second arm of the TCD. The temperature was
raised at a programmed rate of 10 K/min from 303 to 1173 K.
Microcalorimetric adsorption of NH3 was carried out on a Tian-
Calvet heat-flux apparatus. The microcalorimeter was connected to
a gas handling and volumetric adsorption system, equipped with a
Baratron capacitance manometer (MKS, USA) for precision pressure
measurement. The differential heat of adsorption versus adsorbate
coverage was obtained by measuring the heats evolved when doses
of a gas (1–10 ␮mol) were admitted sequentially onto the catalyst
until the surface was saturated by the adsorbate. Ammonia with
a purity of 99.99% was used. Before microcalorimetric measure-
ments, the samples were typically evacuated at 673 K for 1 h. The
microcalorimetric adsorption was performed at 423 K.
◦
◦
◦
◦
tion peaks at 6.3 , 12.6 , 19.0 and 31.9 , indicating the layered
structure. The layer distance of VOx was about 1.34 nm, calculated
from the diffraction peaks. The structure of VOx may be layered
V O5·nC H10O [22]. The diffraction peaks for the V-Ag-O with dif-
2
4
ferent V/Ag ratios were weak, suggesting the small crystallites of or
◦
◦
non-crystalline oxides. Three broad diffraction peaks at 26.0 , 29.4
◦
and 50.5 were detected for the V-Ag-O samples with V/Ag = 3 and
. The peak at 26.0 might belong to a vanadia species, which inten-
◦
4
sity decreased with content of Ag and disappeared for the V-Ag-O
◦
samples with V/Ag = 2 and 1.5. The peak at 50.5 was almost not
changed with the increase of Ag content, and it might be assigned
to the diffraction peak of some vanadia or silver vanadate species.
◦
The peak at 29.4 remained, but weakened and broadened with the
increase of content of Ag. It might also be ascribed to some silver
vanadium complex oxide.
Table 1
Surface areas of the V-Ag-O catalysts.
2.3. Measurements of catalytic reactions
2
Catalyst
Surface area (m /g)
Evacuated at 673 K
Reacted at 573 K
The probe reaction was carried out in a fixed-bed glass tube reac-
VOx
183
21
15
13
17
19
1
148a
19
13
15
13
21
1
tor (Ø 12). About 100 mg of a sample was loaded for the reaction.
Isopropanol was introduced to the reaction zone by bubbling air
V/Ag = 1
V/Ag = 1.5
V/Ag = 2
V/Ag = 3
V/Ag = 4
(
60 ml/min) through a glass saturator filled with isopropanol main-
tained at 295 K. Isopropanol and reaction products were analyzed
by an online gas chromatograph, using a PEG 20 M packed column
connected to an FID. The reaction was performed at 433 and 453 K
with a total space velocity of 12.7 L/(g h) and air/isopropanol = 20
b
V/Ag = 2
a
Reacted at 593 K.
Prepared by the co-precipitation method and calcined at 673 K.
b
(
v/v).

M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
9
◦
◦
and Ag V O (29.4 and 50.5 ). After calcination at 573 K, the peak
2
◦
4
11
◦
◦
at 26.0 almost disappeared and the peaks at 29.4 and 50.5 were
weakened, suggesting the reaction of VOx with Ag V O11, prob-
2
4
ably for the formation of Ag0.68V O5 with poor crystalline since
2
the XRD peaks of Ag0.68V O5 became stronger later with the fur-
2
ther increase of calcination temperatures. In fact, after calcination
◦
◦
at 598 K, two peaks at 30.6 and 49.8 belonging to Ag0.68V O5
2
appeared which intensities were increased significantly after cal-
cination at 623 K. Calcination at 773 K resulted in the sample with
clear and intensive X-ray diffraction peaks. The main phase might
◦
be attributed to Ag0.68V O5 with the strongest peak at 30.6 . The
2
Ag V O remained for the sample after calcination at 773 K with
2
4
11
◦
◦
◦
◦
◦
the peaks at 28.9 , 29.4 , 30.6 and 50.5 . The peak at 27.8 might
be assigned to the crystalline VO2 (M). After calcination at 873 K,
the sample contained the main phase of Ag0.68V O5 with a small
2
amount of VO2 (M), while the phase of Ag V O disappeared.
2
4
11
These XRD results seemed to suggest roughly a process of phase
changes during the calcination: The main phase might be Ag V O
2
4
11
with some VOx for the V-Ag-O (V/Ag = 3) dried at 393 K (both were
poorly crystallized). With the increase of calcination temperatures,
the solid reaction between Ag V O and VOx occurred, resulting in
2
4
11
the formation of crystalline Ag0.68V O5. Meanwhile, the un-reacted
2
VOx turned to crystalline VO2 (M).
Fig. 3 displays the XRD patterns for the V-Ag-O catalysts after
the reaction of selective oxidation of toluene at 573 K. The origi-
nal VOx became V O with very weak diffraction peaks. Obvious
Fig. 1. XRD patterns of V-Ag-O samples dried at 393 K.
6
13
In order to understand the phases and phase changes in the
V-Ag-O samples, the V-Ag-O (V/Ag = 3) was calcined in N at differ-
Ag0.68V O5 phase was detected for all the catalysts containing Ag.
2
2
No other phases were detected for the V-Ag-O with V/Ag = 4 and 3.
ent temperatures (573–873 K). The XRD patterns of these calcined
samples were shown in Fig. 2. With the increase of calcination
temperature, the diffraction peaks changed obviously. Before the
However, since the V/Ag atomic ratio of Ag0.68V O5 was 2.94, lower
2
than 4, there must be some vanadia species (V O or VO ) in the V-
6
13
2
Ag-O with V/Ag = 4 that were not detected by XRD due to the small
crystallites or non-crystalline grains. With the increase of Ag con-
tent, diffraction peaks of metallic Ag were detected for the catalysts
◦
◦
calcinations, the sample seemed to contained VOx (26.0 and 50.5 )
with V/Ag = 2 and 1.5, besides the Ag0.68V O5. Different from the V-
2
Ag-O catalysts prepared by the co-precipitation method, in which
Ag V O , Ag V O11-y, Ag1.2V O existed besides a small amount
2
4
11
2
4
3
8
Fig. 3. XRD patterns of V-Ag-O catalysts after the reaction of selective oxidation
of toluene at 573 K. The phases were assigned according to JCPDS numbers and
a reference: Ag (04-0783), Ag0.68V2O5 (74-2407), Ag2V4O11 [28], Ag2V4O11−y [28],
Ag1.2V3O8 (23-0647) and VO2 (82-0661). (*) A sample prepared by co-precipitation
after the same reaction at 613 K was added for comparison.
Fig. 2. XRD patterns of V-Ag-O (V/Ag = 3) calcinated in N2 at different temperatures.
The phases were assigned according to JCPDS numbers and a reference: Ag0.68V2O5
(
74-2407), Ag2V4O11 [28] and VO2 (82-0661).

1
0
M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
at 997 and 921 cm 1 were observed for VOx after the reaction,
−
which could be assigned to V = O vibrations of V O13, considering
6
the XRD result above. The complex oxides with high Ag contents
(
9
V/Ag = 2, 1.5 and 1) displayed two strong absorption peaks at
−
1
80 and 900 cm , which might be assigned to V O vibrations
V O5
in Ag0.68V O5 [24] since XRD showed the phases of Ag
2
0.68
2
and metallic Ag for the samples after the reaction at 573 K. It was
reported [11] that the V-Ag-O complex oxide with V/Ag = 2.16 con-
tained the main phases of Ag0.68V O5 and Ag V O5, and exhibited
2
0.8
2
−
1
two strong absorption peaks at 980 and 900 cm . The cata-
lysts with low Ag contents (V/Ag = 3 and 4) showed three bands
around 984, 940–950 and 908–910 cm . The peaks around 984
and 908–910 cm might be attributed to Ag
they were close to the bands 980 and 900 cm for the samples with
−
1
−
1
V O5, too, since
2
0
.68
−
1
−
1
V/Ag lower than 2. The peaks around 940–950 cm were difficult
−
1
to assign, but they seemed to be turned into the band at 900 cm
with the increase of silver content. Thus, they might be assigned to
O vibrations in VOx affected by the presence of silver. The FTIR
Fig. 4. FTIR spectra of V-Ag-O samples dried at 393 K.
V
spectrum of the V-Ag-O prepared by co-precipitation was quite dif-
ferent from those of the V-Ag-O prepared by the HAD method due
to the different phase compositions [20].
of metallic Ag [20], the V-Ag-O catalysts prepared with the HAD
method in this work contained only Ag0.68V O5 and metallic Ag,
2
The morphology of V-Ag-O complex oxides was established
using TEM analysis. The results are presented in Fig. 6. The V-Ag-
O with V/Ag = 3 dried at 393 K exhibit rods of vanadium oxides
with layered structures and highly dispersed silver nano particles
of 2–5 nm in diameters (Fig. 6(a and b)), similar to that reported
by Sharma et al. [25]. These silver particles could not be detected
by XRD due to their small sizes. Such layered structures of rods
with highly dispersed silver nano particles remained after the sam-
ple was evacuated at 673 K (Fig. 6(c and d)). After the reaction of
selective oxidation of toluene at 573 K, the rods remained while the
silver nano particles disappeared. Considering that the only phase
after the reaction of selective oxidation of toluene.
Fig. 4 shows FTIR spectra for the V-Ag-O complex oxides dried at
3
93 K. The VOx exhibited the typically spectrum of V O5 xero-gel.
2
−1
The peak at 1007 cm was due to the V O double bond vibration,
while the peaks at 763 and 520 cm could be assigned to asymmet-
−1
−
1
ric and symmetric vibrations of V–O–V [23]. The peak at 1007 cm
−1
shifted to 980 and 960 cm for the samples with V/Ag = 4 and 2,
+
respectively, indicating the effect of Ag . In addition, a new peak
−
1
at 920 cm appeared, which might be ascribed to silver vanadate
−1
species. The peak at 1007 cm disappeared for the V-Ag-O with
V/Ag = 1.5, indicating the complete formation of silver vanadate
species.
detected by XRD was Ag0.68V O5 here, the rods observed in Fig. 6(e
2
and f) might be now Ag0.68V O5 with silver cations homogeneously
2
FTIR spectra for the V-Ag-O catalysts after the reaction of selec-
tive oxidation of toluene at 573 K are presented in Fig. 5. Two peaks
distributed between the layers of VOx octahedra. On the other hand,
the rods disappeared for the V-Ag-O sample with V/Ag = 1.5 after
the reaction of selective oxidation of toluene at 573 K. XRD showed
Ag0.68V O5 and metallic Ag in this sample after the reaction. Thus,
2
the layered structure of rods might be destroyed by the presence
of large amount of metallic silver. Accordingly, spherical particles
of metallic silver with Ag0.68V O5 might be formed in the V-Ag-O
2
with V/Ag = 1.5 (Fig. 6(h)).
3.2. Surface acidic and redox properties
Fig. 7 shows the TPR profiles for the V-Ag-O samples evacuated
at 673 K. The VOx exhibited one reduction peak at 786 K. With the
addition of Ag, two main peaks around 669–686 and 779 K were
observed for the V-Ag-O samples with V/Ag = 3 and 2. Considering
the formation of silver vanadates in the V-Ag-O samples (see XRD
results above), the current TPR profiles indicated that the reduction
of silver vanadates might be much easier than VOx. The reduction
process of V-Ag-O complex oxides has been studied carefully by
the combined techniques of TPR and XRD [20,26]. According to the
previous results [20,26], the peaks around 669–686 K for the V-
Ag-O samples with V/Ag = 3 and 2 might be the reduction of silver
vanadates (Ag V O and Ag0.68V O5) to V O and metallic silver,
2
4
11
2
2
4
while those with peak temperatures higher than 779 K might be
attributed to further reduction to V O . With the increase of silver
2
3
content, a reduction peak around 594–600 K occurred for the V-Ag-
O sample with V/Ag = 1.5 and 1, which might be attributed to the
reduction of AgVO [26]. In addition, the V-Ag-O samples prepared
3
by the HAD method might be easier to reduce than that prepared
by the co-precipitation method, since the former exhibited lower
TPR peaks.
Fig. 5. FTIR spectra of V-Ag-O catalysts after the reaction of selective oxidation of
toluene at 573 K. (*) A sample prepared by co-precipitation after the same reaction
at 613 K was added for comparison.

M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
11
Fig. 6. TEM images for the V-Ag-O (V/Ag = 3) after drying at 393 K (a and b), evacuation at 673 K (c and d) and reaction at 573 K (e and f), as well as for the V-Ag-O (V/Ag = 1.5)
after the reaction at 573 K (g and h). The reaction was performed at 573 K for the selective oxidation of toluene.
Fig. 8 shows the results of microcalorimetric adsorption of NH3
at 423 K on the VOx and V-Ag-O catalysts. The VOx exhibited rel-
atively strong surface acidity with the initial heat of 135 kJ/mol
and coverage of 650 ␮mol/g. Addition of silver decreased the initial
heat, indicating the decreased surface acidity. The surface acidity
was further weakened when more silver was added. The initial heat
and coverage were only 35 kJ/mol and 100 ␮mol/g, respectively, for
the adsorption of ammonia on the V-Ag-O sample with V/Ag = 2.
The probe reaction of isopropanol in air was employed to char-
acterize the surface acidic and redox properties of the metal oxide
catalysts. Table 2 shows the results. The conversion of isopropanol
was high on VOx (62%) with the high selectivity to propylene and

1
2
M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
Table 2
Conversion of isopropanol in air over the V-Ag-O catalysts at different temperatures
with the total space velocity of 12.7 L/(g h) and air/isopropanol = 20 v/v).
(
Catalyst
T (K) Conversion (%) Selectivity (%)
Propene Diisopropyl ether Acetone
VOx
433
53
62
90
53
88
36
12
11
0
4
V/Ag = 4
V/Ag = 3
V/Ag = 2
433
53
13
65
15
39
44
28
41
34
4
433
53
12
61
15
36
51
35
35
29
4
433
53
5
24
8
17
20
18
73
65
4
V/Ag = 1.5 433
53
3
11
5
13
17
17
78
69
4
V/Ag = 1
433
53
3
14
9
17
25
29
66
54
4
V/Ag = 2a
453
2
18
14
68
a
Prepared by the co-precipitation method and calcined at 673 K. All the catalysts
were pretreated in N2 at 673 K for 2 h before the reaction.
the conversion of isopropanol was much higher over the sam-
ple prepared by the HAD method, due to the high surface area,
than the one prepared by the co-precipitation method. Thus, the
relative strengths of surface acidity and redox ability might be sim-
ilar for the two samples, but both the surface acidity and redox
ability might be significantly enhanced for the sample prepared
by the HAD method as compared to the one prepared by the co-
precipitation.
Fig. 7. TPR profiles of V-Ag-O catalysts prepared by HAD and co-precipitation (*)
methods.
diisopropyl ether. Since it is known that isopropanol is dehydrated
to propylene and diisopropyl ether on acidic sites and to acetone
in air on redox sites, this result indicated that the VOx exhib-
ited quite strong surface acidity and relatively weak redox ability.
With the addition of Ag, the conversion of isopropanol as well
as the selectivity to propylene and diisopropyl ether was signifi-
cantly decreased, implying the decreased surface acidity. However,
the selectivity to acetone increased greatly, suggesting the greatly
enhanced redox ability upon the addition of silver. When more sil-
ver was added, the selectivity to acetone was further increased.
The selectivity to acetone might be higher than 65% for the V-Ag-O
samples with V/Ag = 2, 1.5 and 1. These results are consistent with
those of microcalorimetric adsorption of ammonia and TPR.
Two V-Ag-O samples with V/Ag = 2 were compared in Table 2,
which were prepared with different methods. The selectivity to ace-
tone was similar (65–68%) for the two samples at 453 K. However,
3.3. Selective oxidation of toluene
Table 3 presents the results of selective oxidation of toluene over
the VOx and V-Ag-O catalysts. The conversion of toluene was low
2%) over VOx at 573 K with 48% and 31% selectivity to benzaldehyde
(
and benzoic acid, respectively. With the addition of Ag, the conver-
sion of toluene was increased to 6–9%, indicating the significantly
increased catalytic activity. The total selectivity to benzaldehyde
and benzoic acid was increased to over 90% at 573 K. Characteriza-
tions showed the greatly decreased surface acidity and increased
redox ability for the V-Ag-O catalysts as compared to VOx, which
might be the reasons for the increased conversion of toluene and
selectivity to benzaldehyde and benzoic acid. It is generally true
that the increase of redox ability of VOx and V-Ag-O increases the
Table 3
Selective oxidation of toluene over the V-Ag-O catalysts at different temperatures
(
with the total space velocity of 8.9 L/(g h) and air/toluene = 5 v/v).
Catalyst
T (K)
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
Total
VOx
573
2
4
48
45
31
32
79
77
593
V/Ag = 4
V/Ag = 3
573
573
6
9
37
48
57
45
94
93
V/Ag = 2
573
8
17
30
56
48
26
38
40
30
94
88
56
5
6
93
13
V/Ag = 1.5
V/Ag = 1
V/Ag = 2
573
573
613
9
7
5
61
60
92
31
31
0
93
91
92
a
a
Prepared by the co-precipitation method and calcined at 673 K. All the catalysts
Fig. 8. Differential heats versus coverage for NH3 adsorption at 423 K on VOx and
V-Ag-O catalysts evacuated at 673 K.
were heated from room temperature to the reaction temperature under the flow of
reactants (air/toluene = 5).

M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
13
conversion of toluene and the increase of surface acidity favors the
selectivity to benzoic acid due to the strong adsorption of toluene
and benzaldehyde over acidic surfaces [7,13,22,27]. Thus, more
benzoic acid was produced over the V-Ag-O (V/Ag = 2) prepared by
the HAD method as compared to that prepared by co-precipitation,
since the former possessed relatively stronger surface acidity [20].
The V-Ag-O catalysts with V/Ag = 1–4 seemed to exhibit the
similar conversion of toluene (6–9%) and total selectivity to
benzaldehyde and benzoic acid (>90%) at 573 K. However, the
selectivity to benzaldehyde and benzoic acid was different for the
catalysts with different V/Ag ratios. While the catalysts with less
silver (V/Ag= 4 and 3) produced more benzoic acid than benzalde-
hyde, the catalysts with more silver (V/Ag = 2, 1.5 and 1) produced
more benzaldehyde than benzoic acid. This might be due to that
the catalysts with more silver exhibited weaker surface acidity,
but stronger redox ability. Since XRD showed the only phase of
(58%) and benzoic acid (35%) over the V-Ag-O with V/Ag = 1.5 at
593 K.
4. Summary and conclusions
(
1) V-Ag-O complex oxide catalysts with relatively high surface
2
areas of 13–21 m /g could be prepared by the heterogeneous
azeotropic distillation (HAD) method. In this method, V O5 and
2
AgNO3 were dissolved in aqueous solution of H O , followed
2
2
by evaporation and drying in n-butanol at 353 K. Silver vana-
dates with highly dispersed nano silver particles in the layered
structures of VOx were formed during the preparation process.
After heating in N2 at 873 K, the sample with V/Ag = 3 con-
tained the main phase of Ag0.68V O5, which was also the only
2
phase for the sample after the reaction of selective oxidation of
toluene at 573 K. In addition, the relatively high surface areas
Ag0.68V O5 in the V-Ag-O catalysts with V/Ag = 4 and 3, but both
2
2
of 13–21 m /g remained for the V-Ag-O samples after the reac-
Ag0.68V O5 and metallic Ag in the V-Ag-O catalysts with V/Ag < 2,
2
tion at 573 K. The V-Ag-O catalysts with high contents of silver
after the reaction at 573 K, the metallic silver in the catalysts might
not be just a spectator. The presence of metallic silver in conjunc-
(
V/Ag < 3) contained the phases of Ag0.68V O5 and metallic sil-
2
ver after the reaction of selective oxidation of toluene at 573 K.
tion with Ag0.68V O5 seemed to favor the selective oxidation of
2
The presence of metallic silver and Ag0.68V O5 together seemed
2
toluene to benzaldehyde.
to favor the selective oxidation of toluene to benzaldehyde and
benzoic acid.
Two V-Ag-O samples with V/Ag = 2 prepared with different
methods were compared in Table 3 for the selective oxidation of
toluene. It is apparent that the catalyst prepared with the HAD
method exhibited much higher conversion of toluene than the
one prepared by co-precipitation, probably due to that the former
possessed significantly higher surface area. The total selectivity
to benzaldehyde and benzoic acid was similar for the two cata-
lysts. However, the selectivity to benzaldehyde was much higher
on the co-precipitated catalyst than on the one prepared by the
HAD method. This might be due to the higher surface area and rel-
atively stronger surface acidity of the catalyst prepared by the HAD
method.
Table 4 gives the data for the selective oxidation of toluene
collected at different space velocity and temperatures over some
V-Ag-O catalysts. Comparing the data in Tables 3 and 4, the increase
of space velocity from 8.9 to 14.7 L/(g h) did not seem to decrease
the conversion of toluene significantly. However, the selectivity to
benzaldehyde seemed to increase with the increase of space veloc-
ity. In addition, when the reaction was performed at the relatively
high space velocity (14.7 L/(g h)), the total selectivity to benzalde-
hyde and benzoic acid remained high (decreased only slightly) with
the increase of reaction temperature and the conversion of toluene.
By optimizing the catalysts and reaction conditions, good per-
formance for the selective oxidation of toluene to benzaldehyde
and benzoic acid could be obtained. For example, the conver-
sion of toluene reached 14% with 93% selectivity to benzaldehyde
(
2) Characterizations with microcalorimetric adsorption of NH3,
TPR and isopropanol probe reactions showed that the V-Ag-
O catalysts exhibited weaker surface acidity but stronger redox
ability than the VOx. On the other hand, the relative strengths
of surface acidity and redox ability might be similar for the
V-Ag-O samples (with the same V/Ag ratio) prepared by the
HAD and co-precipitation methods, but both the surface acid-
ity and redox ability might be significantly enhanced for the
sample prepared by the HAD method, due to the significantly
increased surface area. Thus, the V-Ag-O with V/Ag = 2 prepared
by the HAD method exhibited much higher activity than its
counterpart prepared by the co-precipitation for the conver-
sion of isopropanol and the selective oxidation of toluene to
benzaldehyde and benzoic acid in air.
(
3) In the series of catalysts prepared by the HAD method, the
addition of Ag into VOx significantly increased the conver-
sion of toluene and selectivity to benzaldehyde and benzoic
acid, due to the decreased surface acidity and enhanced redox
ability. Addition of more silver seemed to lead to the produc-
tion of more benzaldehyde. Excellent performance of selective
oxidation of toluene to benzaldehyde and benzoic acid could
be obtained over the V-Ag-O catalysts prepared by the HAD
method. For example, the conversion of toluene reached 14%
with 93% selectivity to benzaldehyde (58%) and benzoic acid
(35%) over the V-Ag-O with V/Ag = 1.5 at 593 K.
Acknowledgements
Table 4
Selective oxidation of toluene over the V-Ag-O catalysts at different temperatures
(
with the total space velocity of 14.7 L/(g h) and air/toluene = 5 v/v).
Financial supports from NSFC (20233040 and 20673055) and
MSTC (2005CB221400) are acknowledged.
Catalyst
V/Ag = 3
T (K)
Conversion (%)
Selectivity (%)
Benzaldehyde
Benzoic acid
Total
References
573
9
10
13
62
55
40
33
40
55
95
95
95
5
5
83
93
[
[
[
[
[
[
1] A. Sabatini, G. Sloli, CA Patent 817 920 (1969) Snia Viscosa.
2] A. Antoni, R. Vincenzo, CA Patent 773 590 (1967) Snia Viscosa.
3] P.P. Rossi, M. Catoni, US Patent 4 349 473 (1980) Snia Viscosa.
4] http://www.qrx.cn.
5] X. Ge, H. Zhang, J. Fan, Chin. J. Catal. 19 (1998) 42–46.
6] W. Kuang, Y. Fan, K. Chen, Y. Chen, J. Catal. 186 (1999) 310–317.
V/Ag = 2
573
8
11
13
61
56
52
33
35
38
94
91
90
5
5
83
93
V/Ag = 1.5
573
9
12
14
66
60
58
29
34
35
95
94
93
[7] G. Centi, S. Perathoner, S. Tonini, Catal. Today 61 (2000) 211–221.
[
8] D.A. Bulushev, L. Kiwi-Minsker, V.I. Zaikovskii, O.B. Lapina, A.A. Ivanov, S.I.
Reshetnikov, A. Renken, Appl. Catal. A 202 (2000) 243–250.
5
5
83
93
[
9] S. Larrondo, A. Barbaro, B. Irigoyen, N. Amadeo, Catal. Today 24 (2001) 179–187.
The catalysts were heated from room temperature to the reaction temperature
under the flow of reactants (air/toluene = 5).
[10] F. Konietzni, H.W. Zanthoff, W.F. Maier, J. Catal. 188 (1999) 154–164.
[11] H. Zhang, W. Zhong, X. Duan, X. Fu, J. Catal. 129 (1991) 426–437.

1
4
M. Xue et al. / Applied Catalysis A: General 379 (2010) 7–14
[
12] H. Zhang, W. Zhong, X. Ge, X. Fu, CN Patent. 92107558.8 (1992) Nanjing Uni-
[21] K.J. Takeuchi, R.A. Leising, M.J. Palazzo, A.C. Marschilok, E.S. Takeuchi, J. Power
Sources 119–121 (2003) 973–978.
versity.
[
[
[
13] A. Brückner, Appl. Catal. A 200 (2000) 287–297.
14] N. Shimizu, N. Saito, M. Ueshima, Stud. Surf. Sci. Catal. 44 (1988) 131–138.
15] D.A. Bulushev, L. Kiwi-Minsker, V.I. Zaikovskii, A. Renken, J. Catal. 193 (2000)
[22] M. Xue, H. Chen, J. Ge, J. Shen, Microporous Mesoporous Mater. 131 (2010)
37–44.
[23] J.H. Harrelda, B. Dunn, L.F. Nazar, Int. J. Inorg. Mater. 1 (1999) 135–146.
[24] R.A. Leising, C. William, E.S. Thiebolt, Takeuchi III, Inorg. Chem. 33 (1994)
5733–5740.
[25] S. Sharma, M. Panthöfer, M. Jansen, A. Ramanan, Mater. Chem. Phys. 91 (2005)
257–260.
[26] X. Ge, H. Zhang, J. Solid State Chem. 141 (1998) 186–190.
[27] L. Kiwi-Minsker, D.A. Bulushev, F. Rainone, A. Renken, J. Mol. Catal. A 184 (2002)
223–235.
1
45–153.
[
[
16] A. Bottino, G. Capannelli, A. Comite, R.D. Felice, Catal. Today 99 (2005) 171–177.
17] M. Ponzi1, C. Duschatzky, A. Carrascull, E. Ponzi, Appl. Catal. A 169 (1998)
3
73–379.
18] F. Konietzni, U. Kolb, U. Dingerdissen, W.F. Maier, J. Catal. 176 (1998)
27–535.
[
5
[
[
19] M.H. Zahedi-Niaki, S.M.J. Zaidi, S. Kaliaguine, Appl. Catal. A 196 (2000) 9–24.
20] M. Xue, J. Ge, H. Zhang, J. Shen, Appl. Catal. A 330 (2007) 117–126.
[28] R.A. Leising, E.S. Takeuchi, Chem. Mater. 6 (1994) 489–495.