SELECTIVE OXIDATION OF n-BUTANE ON OXIDIZED (VO)2P2O7
389
It is noteworthy that the increase of selectivity on ox- amount of oxygen could be stored in the (VO)2P2O7 lattice
idized sample is mainly correlated with a decrease of se- before the formation of a definite crystalline VOPO4 phase.
lectivity to CO2 (the formation of which is virtually sup- Lo´pez-Granados et al. (41) already proposed the presence
pressed) and not to CO. This could be interpreted by the of interstitial oxygen in the pyrophosphate lattice. Oxygen
fact that the direct oxidation of butane to CO2 is inhib- storage within a few surface layers has been also invoked in
– –
ited on oxidized catalysts. In a previous study (19), we have oxygen-treated V P O catalysts by Schuurman et al. (14).
shown that this is the preferential route to COx formation. Finally the use of other techniques appears to be neces-
However, Igarashi et al. (35) concluded that the consecu- sary to gain more information on the nature of V5+ species
tive overoxidation of MA is limiting the selectivity when the which are most likely responsible for the strong increase
conversion increases. Therefore, the enhancement in MA of selectivity to MA. This will be the aim of a forthcoming
selectivity could also be explained by some inhibition of the publication based on electrical conductivity, Raman spec-
MA overoxidation. According to Schuurman and Gleaves troscopy, NMR, and XPS characterization.
(15), the MA molecule is more weakly adsorbed on the oxi-
ACKNOWLEDGMENTS
dized pyrophosphate and therefore will desorb more easily.
TGA, UV–VIS, and NMR spectroscopies have shown
that the pyrophosphate samples are slowly oxidized at
500ꢀC under oxygen. According to the estimation based
We are indebted to Dr. A. Tuel for his assistance in the NMR analysis.
We thank Dr. G. Coudurier for helpful discussions on the UV–VIS spectra.
We are also grateful to Mr. V. Martin for his assistance in the TGA–DTA
on 31P NMR spin echo mapping spectra, the total amount experiments.
of V5+ reaches about 13% after an oxidation treatment for
24 h. Assuming 8.4 ꢄmol/m2 of vanadium on the surface of
(VO)2P2O7 (36) and considering a specific surface of 5 m2/g,
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It may be added that the presence of V5+ within a signif-
icant number of sublayers also means that a corresponding