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
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