favoured as time-on-stream increases (Fig. 8). XRD analysis
was carried out on ZrO -XC and ZrO -A1C after reaction, in
as, in the present case, to those which ultimately lead to a
xerogel or an aerogel sample. It could be speculated for
instance that for ZrO -A1C (and ZrO -A2C) the defective side
terminations and the crystal planes are such that the undis-
sociative adsorption mode is favoured over the dissociative;
2
2
order to check whether the unstable behaviour of the former
is related to crystal phase modiÐcations occurring during the
catalytic run. For both the samples tetragonal-to-monoclinic
conversion occurred to some extent during the reaction. This
is probably due to water by-product, which is known to
induce such a phase transformation in zirconia.27,28 The
extent of the monoclinic phase formation during the reaction
is however comparable for the two catalysts: from 0 to 15%
for ZrO -XC and from 9 to 30% for ZrO -A1C. Accordingly,
2
2
whereas the latter mode is favoured on the ZrO -XC surface.
2
The resulting increase of hydroxylation on ZrO -XC during
2
the reaction would then enhance the BrÔnsted acidity and
progressively shift the reaction mechanism from E2 to E1; as
a consequence, the 2-alkene becomes progressively favoured
over the desired 1-alkene.
2
2
structural factors cannot be invoked for explaining the di†er-
ent behaviour of the two catalysts with time-on-stream.
Acknowledgements
Instead, the occurrence of surface conditioning on ZrO -XC
2
Thanks are due to Universita di Cagliari for Ðnancial support.
`
(
but not on ZrO -A1C) should be considered. Changes in the
2
activity during butan-1-ol dehydration on aluminas, phos-
phates and AlPO4 catalysts have been reported.29,30 It was
assumed that interconversion of Lewis to BrÔnsted sites
occurred as a consequence of the adsorption of water mol-
ecules formed during the reaction.
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1
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6
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25
26
27
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2
2
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32 on the activation conditions. However, the surface features
could well be sensitive to other preparative conditions, such
Paper a908992f
1854
Phys. Chem. Chem. Phys., 2000, 2, 1847È1854