J. Bilde et al. / Catalysis Today 192 (2012) 10–15
15
catalysts particles during the decomposition process. However,
when the decomposition was conducted at low temperature in
air–ozone mixture, the formation of carbon was avoided and active
and selective catalysts were obtained. This preparation technique
should be applicable to other preparation of oxide materials from
oxalate precursors and even other precursors like acetylacetonate
or acetate and open new preparation routes for catalysts. The
increased activity and selectivity to ACN observed for the catalyst
prepared with this new method has been ascribed to the synthesis
of a catalyst with a higher content of isolated vanadium species in
the bulk as shown by EPR that may also be present at the surface
and to a higher reduction level of the vanadium species.
With the co-precipitation method at pH 5.5 it has been reported
that V/Al/O mixed phase was formed by a condensation reac-
tion of tetrahedral polymeric species [VOx]n and the aluminum
hydroxide ions ([Al2(OH)2(OH2)4]4+ [8]. A study by X-ray absorp-
tion spectroscopy (XAS) of the same type of catalysts showed that
a part of the V cations changed in reaction conditions their coor-
dination to octahedral but no data was reported on the isolation
or polymerization of such species [5]. The change in coordination
was confirmed later by a study combining of valence-to-core X-rays
emission spectroscopy and XAS, which proposed that V cations had
a mixed octahedral or square pyramidal oxygen local environment
in the activated catalysts [9]. However none of the studies reported
on the nuclearity of the V species in the catalysts, which is shown in
this study to be important both for the activity and the selectivity
of the catalysts
To conclude, theses catalysts are promising due to their out-
standing high productivity. However compared to other catalytic
systems working under propane-lean conditions, the obtained
ACN yield is lower and the reaction temperature and ammonia
to propane ratio needed for an optimum productivity are high.
The catalysts consume as much ammonia as the other multi-
component efficient catalysts and therefore no cost reduction can
be established; furthermore under the conditions of catalysis used,
the by-product formed is N2O and no N2, which requires further
treatment. Further improvements of the catalystic performance
might be possible in particular by modifying the catalyst composi-
tion and maybe by modifying the reaction conditions with propane
rich feeds.
Fig. 8. ESR spectra at 298 ◦C of the VAlO-0.25 (a) and VAlO-OX (b) catalysts after
catalytic testing.
However, the double integral of the EPR signal for the catalyst pre-
pared via the oxalate method Ioxa is much more higher than the
double integral of the EPR signal for the sample prepared by co-
precipitation Ipre, indicating that the latter sample contained less
amount of V4+ after use in propane ammoxidation (Ioxa/Ipre = 2.3).
The relative areas of the two superimposed signals, Iiso and Ipoly
,
showed that proportionally the sample prepared by the oxalate
method contained more isolated species than the sample pre-
pared via precipitation (Iiso/Ipoly = 0.50 for the oxalate method and
Iiso/Ipoly = 0.11 for the co-precipitated sample).
If we postulate that the isolation of vanadium sites are key for
a high selectivity to acrylonitrile, these results would explain why
the solid prepared with oxalate method was more selective than
the one prepared using the precipitation method. This assumes
however that the surface and bulk composition are the same
but we have shown in a previous study that surface composition
determined from XPS data corresponded relatively well to the
bulk composition for these catalysts [7]. It is also possible that the
better selectivity might be related to the higher reduction level
of V that would help avoiding over-oxidation of acrylonitrile or
intermediate products.
Acknowledgment
4. Conclusion
This work has been financed by the French agency, Agence
Nationale de la Recherche, Program EFC Environmentally, project
AMMOXAN reference ANR-08-EFC-02-01.
The results described in this paper confirm the high efficiency
of the V/Al/O catalysts for propane ammoxidation. The catalysts
characterized by a V/Al around 0.30–0.35 were the most efficient.
We speculate at present time that it corresponds to the higher
vanadium surface content of isolated or low nuclearity clusters of
vanadyl species. We confirm that the mixed oxides are nitrided
in the reaction conditions. This nitridation seems to be crucial for
their efficiency although it mainly depends on the reaction condi-
tions and not of the catalyst composition. Propene was shown to
be the main primary product from propane ammoxidation on the
V/Al/O catalysts. It is further transformed to acrylonitrile and/or
acetonitrile. CO and CO2 should be formed from propene but also
in minor amount from propane and/or nitriles. This leads to a reac-
tion pathway rather similar to that observed for multi-component
catalysts either based on antimonates or molybdates.
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