Chemistry Letters 2000
527
vanadium species were tetrahedral V5+ and Mg V O (diamag-
was produced. The ESR spectrum of the carbon dioxide-treated
catalyst was similar to that of the fresh catalyst. However, it
3
2
8
netic), respectively. It is supposed that a slight anion vacancy
4+
4+
existed in the Mg V O structure and that V ions were highly
dispersed in the V ions.
exhibited weaker resonance of the V hyperfine structure, and
3
2
8
5
+
2+
the spectrum due to Mn still remained (Figure 1(e)), indicat-
The ESR spectrum of the catalyst after the dehydrogena-
tion reaction with ethylbenzene in the presence of carbon diox-
ide (Figure 1(b)) was similar to that of the fresh catalyst,
although the signal intensities decreased and another six-line
appeared due to Mn2 contained within MgO as an impurity. In
contrast, the ESR spectrum after the reaction in the absence of
carbon dioxide showed new eight-lines with a smaller hyperfine
ing that carbon dioxide oxidizes the reduced vanadium species
to a higher oxidation state, although the catalyst cannot com-
pletely return to the state of the fresh catalyst.
To our knowledge, such an oxidation capability of carbon
dioxide over vanadium oxide species has never been observed.
According to thermochemical calculation (∆G at 800-900 K),
oxidation of lower valent vanadium oxides to V O is highly
+
2
5
4
+
4+
5+
coupling constant superimposed on the V signals (Figure
(c)). Sharma et al. reported that this eight-line spectrum was
difficult (V →V ; ∆G=156.5 kJ/atom O at 800 K). However,
above findings strongly suggest that carbon dioxide can oxidize
the lower valency state-vanadium species. During the dehydro-
1
2
+
9
assigned to the V ion in octahedral symmetry. This result
shows that the oxidation state of the vanadium species in the
presence of carbon dioxide was kept higher than that in the
absence of carbon dioxide.
5+
genation of ethylbenzene, vanadium species, V is reduced to
4+
V
or much lower oxidation state; consequently, the catalytic
activity of dehydrogenation decreased. Lower valency state
vanadium species on MgO exhibited lower activity for the
dehydrogenation of ethylbenzene, since the dehydrogenation of
ethylbenzene did occur after the run 6 h in argon atmosphere.
Under carbon dioxide atmosphere, however, the vanadium
species is kept at a higher oxidation state (Scheme 1), thus the
higher ethylbenzene conversion and styrene yield are achieved.
The oxidation of lower valency state vanadium species with
carbon dioxide could be assisted by the support magnesium
oxide. Therefore, even if the bulk lower valency state vanadi-
um oxides could not be oxidized to higher valency state with
carbon dioxide, complex oxides of magnesium and vanadium or
vanadium oxide on MgO could be oxidized to higher valency
state with carbon dioxide.
In order to confirm the oxidation of the reduced vanadium
species by carbon dioxide, two ESR spectra were measured as
follows. Figure 1(d) shows the ESR spectrum of the catalyst
reduced with hydrogen at 550 °C for 6 h. A sharp eight-line
2+
spectrum due probably to V was observed for the reduced cat-
alyst as well as the catalyst after the reaction with ethylbenzene
in the absence of carbon dioxide (Figure 1(c)). These results
indicate that the reduction of vanadium species proceeds during
the reaction with ethylbenzene in the absence of carbon diox-
ide. Subsequently, the eight-line feature of the hydrogen-
reduced catalyst disappeared after treatment with carbon diox-
ide at 550 °C for 6 h. During the reaction, carbon monoxide
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