64
X.-J. Yang et al. / Journal of Catalysis 253 (2008) 57–65
of Nb species with Te–Mo and Te–V entities. It is notewor-
thy that the effectiveness of the ultrasonic treatment can vary
among catalysts with different elemental compositions.
5
. Conclusion
Scheme 1. Reaction network for the propane partial oxidation over Mo–V–Te–
Nb) catalysts [14].
(
To summarize, we fabricated the Mo–V–Te–Nb MMOs us-
ing TeO2 and H6TeO6 as Te sources through ultrasonic and
hydrothermal treatments. The results demonstrate that the use
of TeO2 in catalyst preparation can be successful by enhanc-
ing the dispersion of TeO2 in the preparation medium, and
that the TeO2-derived Mo–V–Te–Nb catalyst can be superior
to the H6TeO6-derived counterpart in the partial oxidation of
propane to AA. Over the TeO2-derived A2 catalyst, an AA yield
propylene occurs on the Mo/Te sites, and (iii) the oxidation of
acrolein to AA occurs on the Mo/Nb sites [48]. The function of
the Te sites for the abstraction of an allylic hydrogen (α-H)
4+
from propylene was first proposed by Millet et al. [28] and
Grasselli [45], whereas that of Mo6 sites for oxygen-insertion
into the π-allylic intermediates was suggested by Grasselli [45].
Application of ultrasonic treatment in catalyst fabrication en-
hanced the dispersion of the Te constituent and, consequently,
the presence of Te on the surface (Table 1); such a situation is
considered beneficial for AA formation.
+
−1
−1
of 33.4 mol% and AA formation rate of 22.3 µmol g min
◦
were recorded at 380 C, which is comparable to the perfor-
mance of the best MMO catalysts reported in the literature. The
results of characterization show that the improved dispersion
As suggested by Grasselli [45], the overoxidation of prod-
ucts on MMO catalysts can be suppressed by having the active
sites spatially isolated by the Nb species. According to Ueda
et al. [43,44], the Nb species can promote rapid desorption of
the desired products so as to prevent further oxidation. The
ultrasonic treatment adopted after the addition of Nb during
catalyst fabrication can enhance the interaction of Nb with the
other constituents, leading to increased site isolation. In addi-
tion, the enhanced dispersion of Nb species also can lead to
shorter prism particles (Fig. 6a) and better exposure of the ac-
tive (100) plane [27,31,49]. Guliants et al. [22] measured the
consumption rate of propane and the formation rate of propy-
lene and AA, and found that these rates were dependent on the
surface concentration of V rather than on that of Mo and Te
of TeO in preparation media can result in (i) enhanced pres-
2
ence of the orthorhombic Te2M20O57 (M = Mo, V, Nb) and
hexagonal Te0.33MO3.33 (M = Mo, V, Nb) phases, (ii) surface
enrichment of Te, (iii) increase in Mo–O–Te and V–O–Te enti-
ties, and (iv) better site isolation due to enhanced dispersion of
Nb species. The redox and surface acidic properties of the cata-
lysts fabricated with ultrasonic treatment also were found to be
beneficial for the generation of AA.
Acknowledgments
Financial support was provided by Hong Kong Baptist Uni-
versity (FRG/06-07/I-08). The research activities conducted
at NJU were financed by the NSFC (20673052) and JSNSF
(BK2006112). X.J.Y. thanks Dr. Na Zhang for her kind help
and fruitful discussions.
(
as determined by low-energy ion scattering). We observed that
within a certain range of overall surface V concentration (as
determined by XPS), the TORAA increased with a rise in sur-
face V content over the H6TeO6-derived catalysts, whereas the
TORAA remained almost unchanged over the TeO2-derived cat-
alysts (Fig. 8). Védrine et al. suggested that the acid sites of
medium strength in high density are responsible for good cat-
alytic performance [31]. The results of NH3 calorimetric mea-
surement revealed that catalysts A2 and B3 displayed surface
acidity of relatively milder strength (Fig. 7). Clearly, catalyst
surface acidity can be modified by changing the Mo and Te con-
stituents, and catalyst activity and selectivity can be improved
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