8
86, 753, 734 and 673 cm21, probably assigned to V–O–V, V–
O–Cr and Cr–O–Cr stretching vibrations of the monoclinic
CrVO -I structure. An increase in the P content resulted in a
blue shift of the bands of CrVO -I, indicating that some V–O
bonds are replaced by P–O. CrV0.95 (623),
(843) and CrPO (973) showed no absorption
4
4
0.05 4
P O
CrV0.75
P
0.25
O
4
4
bands probably due to the amorphous structures, because of too
low calcination temperatures for their crystallization.
In the oxidation of 3-picoline over CrV1 2 x
products were nicotinic acid (NA) and pyridine-3-carbaldehyde
3-Ald), as well as CO as a by-product. A small amount of
x 4
P O , the main
(
2
pyridine (Pyr) was formed probably by decarboxylation of
nicotinic acid. Mass balance calculated between 3-picoline
conversion and the product yields was above 95% in every
reaction. CrV0.95
nicotinic acid and pyridine-3-carbaldehyde, followed by
(843), although their surface areas were lower
(823). When x exceeded 0.05, the yields gradually
decreased with increasing x, and finally CrPO (973) showed a
0.05 4
P O (843) showed the highest yields of
0.02 4
CrV0.98P O
than CrVO
4
4
0.05 4
Fig. 3 Oxidation of 3-picoline over CrV0.95P O (843).
sudden decline in the activity, resulting in no formation of
nicotinic acid. Yields of the products per surface area of
only anatase TiO
transformed to rutile TiO
the exothermic reaction.
(843) is thermally stable and a promising
catalyst for the oxidation reaction. CrV0.95 (843) showed
again the highest activity in the oxidation of both 2- and
-picolines. 2-Picoline afforded pyridine-2-carbaldehyde with a
yield of 50.2% at a conversion of 69.2% and no picolinic acid
was formed at 573 K. 4-Picoline afforded pyridine-4-carbalde-
hyde (yield, 8.9%), isonicolinic acid (yield, 84.3%) at a
conversion of 98.7% at 598 K.
2
was effective as the support and is possibly
, resulting in the deactivation, during
On the other hand,
CrV0.95
P
0.05
O
4
calcined at 623 and 843 K are compared in Fig.
(623) showed extremely low values, but
(843) showed the highest values among the
2
2
0.05 4
. CrV0.95P O
CrV0.95
P
0.05
O
4
0.05 4
CrV0.95P O
2
21
catalysts tested. The surface area was 103.6 m g-cat for
CrV0.95
CrV0.95
0.05 4
P O
2
21
P
P
0.05
O
O
4
(623)
(843). Nonetheless of such significant discrep-
-I
and
21.1
m
g-cat
for
0.05
4
4
ancy in the surface area, the effect of crystallization of CrVO
4
structure by calcination was clearly observed. A similar effect
of the calcination temperature was observed over
CrV0.75P O . These critical features can be explained by the
0.25 4
structure change from amorphous to crystalline as observed by
XRD (Fig. 1(c) and (d)). Interestingly, it is concluded that
4
monoclinic CrVO -I-based structure is essential for the se-
lective oxidation of 3-picoline and its activity is enhanced by the
incorporation of P in the V-site in the crystal structure.
Increasing temperature is favorable for the selective oxida-
tion of 3-picoline to nicotinic acid and the optimum temperature
was found to be 633 K (Fig. 3). Above 633 K, the reaction
temperature could not be controlled probably due to increasing
combustion, resulting in decreasing selectivity. The activity of
NH
3
-TPD showed an increase in the number of acid sites by
-I. The favorable
replacing V with small amount of P in CrVO
4
effect of water addition and the results of DRIFTS measurement
16
of pyridine adsorbed on the catalyst in the presence of water
suggest that Brønsted acid sites are the active species.
Moreover, all the V species are present as VO
4
tetrahedra in the
-I structure and considered to act as active sites via
their redox properties. It is concluded that picolines can be
selectively oxidized on the CrV0.95 catalyst having
14
CrVO
4
0.05 4
P O
Brønsted acid sites assisted by the redox properties. This
thermally stable crystalline structure seems to be favorable as
the active phase for the oxidation reaction with gaseous oxygen
that tends to form hot spots in the catalyst bed.
CrV0.95
than that of Cr0.5Al0.5VO
yields of nicotinic acid, 78.4%, and pyridine 3-carbaldehyde,
.4%, at a conversion of 92.6% were found at 633 K. It was
reported that V /TiO catalyzed the selective 3-picoline
oxidation at 520–550 K and afforded nicotinic acid with a
0.05 4
P O (843) for nicotinic acid production was higher
8
4
previously reported. The highest
5
2
O
5
2
Notes and references
2,3
selectivity above 90% at a conversion above 90%. However
1
2
S. Jaras and S. T. Lundin, J. Appl. Chem. Biotechnol., 1977, 27, 499.
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6
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8
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9
J. P. Attfield, J. Solid. State Chem., 1987, 67, 58.
1
1
0 R. D. Shannon, Acta. Crystallogr., Sect. A, 1976, 32, 751.
1 E. Baudrin, S. Denis, F. Orsini, L. Seguin, M. Touboul and J-M.
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1
1
2 M. Touboul and K. Melghit, J. Mater. Chem., 1995, 5, 147.
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1
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1
5 E. J. Baran, J. Mater. Sci., 1998, 33, 2479.
Fig. 2 Oxidation of 3-picoline over the CrV0.95
P
0.05
O
4
catalysts. Reaction
(623).
16 S. J. Puttock and C. H. Rochester, J. Chem. Soc., Faraday Trans. 1,
1986, 82, 2773.
temperature: 623 K. (a) CrV0.95 (843) and (b) CrV0.98P O
P
0.05
O
4
0.02 4
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